Graphene and its derivatives have emerged as potential materials for several technological applications including sunlight-driven water splitting reactions. This review critically addresses the latest achievements concerning the use of graphene as a player in the design of hybrid-photoelectrodes for photoelectrochemical cells. Insights about the charge carrier dynamics of graphene-based photocatalysts which include metal oxides and non-metal oxide semiconductors are also discussed. The concepts underpinning the continued progress in the field of graphene/photoelectrodes, including different graphene structures, architecture as well as the possible mechanisms for hydrogen and oxygen reactions are also presented. Despite several reports having demonstrated the potential of graphene-based photocatalysts, the achieved performance remains far from the targeted benchmark efficiency for commercial application. This review also highlights the challenges and opportunities related to graphene application in photoelectrochemical cells for future directions in the field. Graphene and its derivatives have emerged as potential materials for several technological application including sunlight-driven water splitting reactions. The scientific and industrial community efforts over the last decades have been spurred to develop carbon-free technologies. In this sense, artificial photosynthesis rises as one of the most elegant, sustainable, and renewable approaches to produce high energy density carriers, as molecules, driven by solar energy.Our society is facing a drastic environmental crisis associated with a significant dependence on non-renewable energy sources which release an uncontrollable amount of CO2, ZnO, α-Fe2O3, WO3, BiVO4, WO3, Cu2O, CdS, CZTS and MoS2 are the most studied materials associated with their potential for photoelectrochemical (PEC) applications. However, significative drawbacks present in those materials such as in TiO2,1 WO3 2 and ZnO3 related to their large band gap enabling them to absorb only small portion of the solar spectrum have prevented the technology development. Another common limitation reported is the surface recombination that reduces the material performance and can be exemplified by BiVO4 4 and α-Fe2O3.5 Cu-based materials have shown great performance in comparison with other cited system, however, it suffers with serious problem of chemical stability due to the photocorrosion process, shortening its lifetime as PEC component. To boost the PEC cell performance using photocatalysts, several strategies have been pursued, for instance, extending the absorption spectra towards infrared, improve charge carrier kinetics and increase the active sites at the surface including the doping with anions or cations,6–8 surface coupling with metals or semiconductors9,10 and film morphology design .Several semiconducting materials such as metal oxides, nitrides or sulphides have been synthesized and their performance continuously evaluated as photoelectrodes. Materials such as TiO11–13 Since the pioneer report of graphene it become the most popular compound in the carbon-based family,14 mainly attributed to the presence of a singular sp2 carbon network that provides a privileged surface area, high conductivity and excellent electron mobility, ideal for applications in solar energy conversion.In this sense, carbon-based materials in nanometric scale have called the attention of many researchers in the field due to their properties and potential to improve optical and electronic of those mentioned semiconductors. It has been proved that the coupling between a semiconductor with carbon-based material can efficiently enhance the PEC activity by suppressing recombination, extending the excitation wavelength, or increasing the surface-active sites.In view of these highlighted characteristics, its application as photocatalyst modifier for helping them overcome the already discussed limitation on PEC cell was naturally incorporated in the field routine investigation. In fact, graphene and its derivatives such as reduced graphene oxide (rGO) enabled the photoelectrodes enhances the optical, conductive, and chemical properties of different materials, while the utilization of a single-layer graphene (SLG) optimizes their stability, electrical, and redox properties. Moreover, graphene quantum dots (GQDs) band gap and Fermi level can be modified as function of their particle size proving that their combination with semiconducting electrodes boost the efficient interfacial charge carrier transfer.3N4) material has also attracted widespread attention due to low cost, appropriate electronic band structure (Eg ∼ 2.7 eV) corresponding to 460 nm in the visible range and excellent photochemical stability.15 However, only few studies have been dedicated to study its combination with graphene (or its derivatives) in photoelectrodes. The latest available reports show that graphene sheets can act as a support for the g-C3N4 growth as well as charge acceptor, thus improving the electronic transport of the film.15–24 Moreover, the graphene addition can lead to enhanced electrochemical active surface area, electron diffusion length and total exciton lifetime as recently demonstrated.25It is worth to mention that graphitic carbon nitride  of the real effect of graphene and its derivatives as modifier for PEC applications.Although, the research involving graphene-modified photoelectrode has reached the top in the last few years, not many insightful comprehensive review papers have been published. For this reason, this review summarizes and discusses the latest advances achieved in the field and highlights the existing limitations and how we could overcome them.1.1Split water in the presence of a photocatalyst under sunlight to produce hydrogen represents a sustainable and attractive manner to store solar energy. A photoelectrochemical cell (PEC) is a device specially designed for this purpose in a very elegant way. In this set-up, two electrodes  are able to capture the solar energy (photons) and transform it into the chemical energy required to activate the water redox reaction. +)–electron (e−) pairs recombines, while another fraction is separated. After the separation is achieved, the electrons are collected by the back contact while the holes migrate to the photoanode surface to evolve the oxygen evolution reaction (OER); this phenomenon can be observed for the photocathodes in which the holes are collected while electrons are transferred to the interface to carry out the hydrogen evolution reaction (HER); when both are interconnected oxidation and reduction can be done simultaneously and can utilize light energy more efficiently. The mechanism will be explained in detail later in this section.When a PEC cell is illuminated, electrons located in the semiconductor valence band (VB) are excited and promoted into the conduction band (CB) of the photocatalyst. A portion of these hole (hEg) must be larger than 1.23 eV but it needs to be small enough to absorb the major part of the solar spectrum, being the ideal value around 1.6–2.0 eV.30–32 It is important to mention that even if the semiconductor is capable to absorb efficiently sunlight, the band edge energies need to be positioned correctly to preference the OER and HER.The Ideally, the CB must be placed at higher energy than the HER potential while the VB must be located at lower energy than the OER potential; Ef) of the semiconductor equals to the electrolyte redox potential.Despite the fact that the semiconductor properties dictate their potential use in efficient solar water splitting PEC system, as the photoelectrodes are immersed in water, it is imperative to comprehend the thermodynamic and kinetic parameters involved in the solid–liquid junction which is created by the interaction between the semiconductor and the electrolyte, known as semiconductor–electrolyte interface (SEI). When the electrode is immersed into an electrolyte solution, charge transfer reactions occur until thermodynamic equilibrium is reached and the Fermi level energy  is referred to the condition when a semiconductor in contact with another material is not polarized in the interfacial region. At this condition, the CB and VB are flat and no electric field acts on the charge carriers. More details about SEI and how the charge transfer processes at this interface are affected by film structure and preparation methods can be found elsewhere.32,35 It is important to note that the described phenomena occur under dark conditions; if the cell is illuminated, light induced processes will take place, and the electrode/electrolyte dynamics will be modified.The Ef increases due to the internal photovoltage. This process interrupts the thermodynamic equilibrium in the depletion region previously produced under dark conditions and the creation of a quasi-Fermi energy level is necessary to describe the electrochemical potential of the photo-generated electrons and holes  is a measure of the ratio of the photocurrent These two analyses, ABPE and IPCE, were proposed in literature as the most reliable way to compare performances of materials designed by different synthesis methodology.ηSTH), described by 2 per s corresponds to the H2 rate production and Pincident the total incident light power.Another alternative to calculate the total efficiency of solar radiant energy transformed into chemical hydrogen energy by a parameter known as solar to hydrogen efficiency  and stability for commercial application still far to be achieved. In recent years, graphene appeared as a promising material with positive impact when added during the fabrication of photoelectrodes. However, the role of graphene and how the properties are improved remain unclear. The next section will emphasize the main graphene properties which make it suitable for certain applications before introducing a discussion regarding its role in the photoelectrodes in PEC.1.238 gas sensors,8 electronic device design,39 transistors, fuel cells,40 photovoltaic devices,41 photocatalytic hydrogen generation, etc.Graphene is a metal-free material formed by a 2D carbon monolayer with a closely packed hexagonal lattice structure. Compared to graphite and carbon nanotubes, graphene exhibits much greater surface area and exceptional conductivity making it a potential candidate for different uses. Graphene has been reported in several applications such as a biosensors,2 hybridized carbons with a band gap nearly to zero. The electrical conductivity of graphene is related to its carbon network, which is 60 times higher than carbon nanotubes conductivity.39 Graphene also possesses a high density of surface-active sites and it is thermally stable.42,43 Contemplating its remarkable electron mobility, conductivity and stability, graphene is an excellent candidate to improve the efficiency of PEC systems. It has been shown that the insertion of graphene helps to improve the charge separation and increase the electrode surface area. In parallel, the new interface between the graphene and the semiconductor facilitates the hole/electron extraction, enhancing charge transfer reactions. Moreover, graphene, employed as a scaffold, creates a 2D conductive support path for charge transport and collection to enhance the electron transport properties of semiconductors.26,44Graphene is composed by sp44,45 It is worth mentioning that some reports have demonstrated that chemical bond formation and the Schottky junction between the semiconductor and the graphene can modify the semiconductor band gap and enhance the light absorption.46,47Since the graphene owns a higher work function compared to other semiconductors, it can act as an electron bridge to accelerate the electron transfer from the semiconductor CB to graphene, achieving a better charge separation.48–50 Due to this contrast, graphene derivatives represent an interesting and challenging field to be studied.However, regardless of its benefits, graphene poor dispersibility significantly reduce its electrical conductivity, surface area, optical transparency and diminish the mass transfer kinetics.51 Usually, graphene and its derivatives are synthesized via graphene oxide (GO) which can be obtained easily by graphite oxidation and subsequently exfoliation.52 GO consists of a graphene structure modified with different functional oxygen groups such as carboxylic, hydroxyl and epoxide which offer some attractive advantages for photoactive materials.50One of the main challenges in the synthesis and application of graphene sheets is to overcome the strong cohesive van der Waals energy of the p-stacked layers in graphite.2 carbon sites and a large bandgap between the s-states of its sp3-bonded carbons.53,54 This sp2/sp3 ratio can be modified by chemical reduction in order to produce reduced graphene oxide (rGO) which has brought the attention due to its remarkable conductivity and uniform incorporation in semiconductor electrodes. It has been proved that GO is able to drive the water splitting reaction by itself due to the band gap adjustment as a function of its oxidation degree.55 In fact, a recent work shows that an rGO/GO electrode achieves an enhanced response compared to GO, which the authors relate to the band gap tunning  technique, a more expensive method. Different reports have shown that rGO improves the optical, conductive, and chemical properties of different photoanodes and photocathodes.28,57 associated with their particle size, usually less than 10 nm, provide highly active reaction sites at the electrode. Additionally, GQDs have demonstrated excellent chemical stability, low cytotoxicity and high luminescence.28 One of the main advantages of GQDs is related to their electronic structure which can be modulated according to the particle size, conjugating them with polyaromatic rings to enlarge the π-conjugated sp2-carbon network or by introducing an intermediate n-orbital between π and π* orbitals via functionalization with electron-donating chemical groups.58 It has been shown that GQDs modification in TiO2 electrodes decreases the system impedance and improves the charge transport kinetics.57 Indeed, in some cases GQDs have been employed to tune the band gap to enhance the light absorption in the visible spectrum.59Another graphene derivate that is drawing a lot of attention are the graphene quantum dots (GQDs). The quantum-confinement and edge effects60,61 Furthermore, SLG is chemically inert in air and in electrochemical media.62,63 As other graphene derivatives, the SLG has demonstrated an intrinsic photoresponse.In contrast, the use of overlayers and underlayers to improve the charge carrier dynamics is a solid strategy that has been largely explored. The deposition of a single layer of graphene (SLG) has shown better properties than rGO or amorphous carbon for PEC applications. Due its transparency, SLG only absorbs 2% of the solar spectrum, not interfering with the electrode absorption. Moreover, this layer conserves the high mobility of the charge carriers in graphene.Although all aforementioned, each photoelectrode material presents unique properties and characteristics that need to be analysed in detail prior graphene incorporation to maximize its efficiency. The next sections disclosure the performance of graphene/semiconductor photoelectrodes supported by different studies published in the last five years.2Eg range of these semiconductors brings the possibility of being used as absorber to arise higher efficiencies. In terms of costs, most of metal oxides are composed by earth abundant elements which make their fabrication affordable. Unfortunately, one of the main limitations is associated to their relatively poor carrier kinetics which is reflected on the photoelectrode performance. Over the last decades, several efforts have been focused on creating new methodologies for produce stable and efficient photoanodes/photocathodes. Recently graphene (and its derivatives) started to be considered an attractive material to incorporate into metal oxides systems to evolve the HER and OER reactions.For PEC applications, metal oxides semiconductors offer many advantages as photoelectrodes. When used as photoanodes, they are not susceptible to oxidation reactions. Additionally, the wide In this section, the recent developments of graphene/metal oxides as photoelectrode materials for water splitting will be highlighted. The graphene influence on the metal oxide photoanodes, photocathodes and heterojunction photoelectrodes reflected in different aspects  during the water splitting process will be described as discussed in this section.2.164 that preclude their use as photoanodes by not reaching their maximum photocurrent density.64,65 It is noted that the key limiting factor of the poor PEC performance of many metal oxides is the high recombination rate of the charge carriers. Key strategies to address improvements for their use as stable and efficient photoanodes have been devoted. Researchers seek to include SLG, rGO,66 nitrogen-doped graphene67 and zero-dimensional GQDs68 as catalysts and/or supports in a wide range of photoanodes. In this section, a brief description of different graphene-based materials and their use as a “metal-free” carbon electrocatalyst materials69 on metal oxide photoanodes will be presented. Despite possessing unique and promising properties for water oxidation, many of metal oxides present intrinsic drawbacks70 In this way, SLG has high transparency (97.7%) at visible light (λ = 550 nm); the Fermi level band alignment between SLG and the photoactive material makes it convenient for use in photoelectrochemical applications.71 It is expected that the SLG presence would not lead to a change in the photoanode optical properties due to its very thin layer and high transparency.72 Since α-Fe2O3 typically suffer from undesirable surface states,73 the addition of SLG as a passivation layer leads to an impressive improvement in the PEC performance of hematite photoanodes. The passivation effect of the surface states after SLG incorporation on the top of the photoanode, resulted in an increased lifetime of the photogenerated holes.70 Additionally, as a consequence of the strong interaction between SLG and the photoanode, it was observed an efficient charge carrier separation and better hole accumulation at the photoanode surface,70,74 increasing the water oxidation efficiency.70 Although much higher photocurrent can be obtained after the deposition of SLG on the top of metal-oxides films, it is important to highlight that in this case different methods for SLG deposition should be taken into account for better interaction of the semiconductor with the graphene material, in such a way that not only the photoanode surface would be benefited by the SLG presence, but the whole composite.The incorporation of graphene with no or negligible defects in the form of a large area sheet is highly required to maintain or enhance its electrical conductivity, electron mobility and carrier density.66 It has been shown that when incorporated, rGO can broaden the absorption spectrum of many metal oxides that poorly absorb visible light.75 In general, it has been discussed that this may occur due to the orthogonalization of light in porous structure of carbon materials. The high porosity of rGO-based materials can lead to multiple reflection of light inside the film, improving the light absorption.76,77Differently of SLG, the reduced graphene oxide (rGO) can lead to a significant improvement in the light absorption upon its addition on photoelectrocatalyst system. In this trend, the incorporation of rGO has been widely explored.78 Besides the light absorbance improvements, owing to its excellent electronic properties, the presence of rGO on top of the photoanodes can lead to an increase of the charge separation efficiency. As a consequence, a lower bulk recombination is obtained, which makes rGO as an effective electron capturing and transferring material.79,80 The onset potential shift suggest that the role of rGO is to increase the carrier transport and electron collection efficiency.The two-dimensional rGO nanosheets present high electron mobility and π-electron conjugation.76 in which a portion of hot electrons generated in metal nanoparticles can be accepted and shuttled via rGO sheets either by UV or visible light, minimizing charge recombination. Despite the incorporation of rGO has provided excellent improvements in photocurrent density of many graphene-based photoanodes, this material still contains a considerable amount of heteroatoms,81 as well as an incomplete restoration of a perfect carbon lattice,82 turning other graphene derivatives more attractive.The application of rGO with plasmonic nanoparticles in order to enhance the PEC performance of conventional semiconductors has also been reported,83 Graphene electronic properties can be tuned by doping it with heteroatoms, opening a new venue to further improve the performance of the metal oxides.84 By reacting epoxy, hydroxyl and carbonyl groups present in the rGO with molecules containing heteroatoms like N, P and S, these atoms can replace some of the carbon sites in the graphene framework structure, introducing defects. These intentionally added defects lead to improved chemical stability, as well as surface area and electron transfer.84,85 By doping with heteroatoms, the doped-graphene material can be tailored to act as electron donor or acceptor, depending on the electronegativity of the dopant with respect to carbon atoms. Such defects are responsible for opening the Eg in graphene-based materials, offering additional fine tuning of the band alignment.Besides the simple addition of graphene aiming better PEC performances by passivation effect, another approach is to make graphene properties tuneable.86 The efficient visible light harvesting resulted in the improvement in photocurrent density. The enhancement was attributed to the increased separation rate of electron–hole pairs as well as electron transfer at the surface, leading to a strongly chemical coupling. As consequence, a negative shift in the onset potential which was attributed to the negatively shifted Fermi level was noticed. The resulted positively charged carbon atoms turned them able to adsorb OH− species, resulting the more favourable O2 evolution at the photoanode surface.In this context, it has been shown that sulfur doping greatly broad the photoanode light absorption.Jabs), efficiency of charge carrier separation (ηsep) and injection (ηinj), different types of graphene derivatives can lead to different PEC behaviours.90 This trend can be simplified in the following equation:Since the photocurrent density is described as a function of the absorbed photocurrent , high aqueous solubility, robust chemical inertness and low cytotoxicity91 open up new avenues and shed light on a novel branch of advanced nanostructured materials for solar-driven water splitting. For instance, by modulating the emission wavelength of the GQDs and matching it with the absorption edge of the semiconductor  can input novel valuable properties to graphene-based materials. By co-doping them with different atoms, SNGQDs can originate two or more excitation wavelength-independent PL and broad the absorption in the visible region of light with multiple absorption edges,e SNGQDs  can be oe SNGQDs .88 to deposit GQDs on TiO2 photoanodes, which led to a considerable difference in charge transfer and PEC performance. By using spin coating as deposition method, GQDs behave as a photosensitizer donating electrons to the CB of the semiconductor. On the other hand, if the amount of GQDs deposited on a specific photoelectrode is not well controlled, as occurred when deposited by electrodeposition, their excess causes blockage of electron transfer, in a way that instead of facilitating electron transfer to the semiconductor conduction band  is added, a competition of light absorption between graphene and the semiconductor and light blocking effects12,116,121,122 are noticeable.Despite Cu–O–C bonds have been successfully demonstrated by some authors, optical measurements are primordial to understand the role of graphene and its derivatives into the photocathode electronic structure. Absorption measurements of photocathodes modified by graphene,116,118,119,121 demonstrated to be a promising strategy to avoid the electrode photocorrosion. Since graphene and its derivatives can act as electron acceptors, they suppress the redox activity of the copper-based oxides photocathodes. J) divided by the photocurrent density at t = 0 s (J0); it is observed that, in all systems, the stability improved significantly compared to the bare system. Some authors117–119 affirm that the synergistic interaction between graphene and Cu photocathodes inhibit the catalyst self photocorrosion as well as improved the photocurrent density.Furthermore, to overcome the main drawback faced by Cu-based photocathodes, the photocorrosion, diverse strategies such as the passivation layers, crystal structure/morphology modification and cocatalyst addition have been employed to improve stability. Considering this approach, photocathode modification with graphene-based materials2O layer. For this reason, they deposited an additional layer of TiO2 119 or amorphous molybdenum sulfide (a-MoSx)122 which improved the stability percent to 67% and 40% respectively. These studies bring the possibility of combining graphene with other semiconducting oxides to improve PEC systems. And in fact, the research in this field is pointing towards a combination of materials to succeeded in improved OER and HER reactions.By analysing the stability , two sysvs. RHE, the longest test data reported is 20 minutes which suggest the electrode photostability is not enough for scale-up. Even if several authors affirm that graphene electron mobility and its behaviour as electron acceptor boost the transfer of photogenerated electrons from photocathode to the electrolyte,12,116,121 it must be taken into account that Cu based materials by themselves can achieve remarkable performance during limited time and, for this reason, it is necessary to focus on resolve their stability drawbacks to then create strategies focused on charge transfer kinetics.Although the chronoamperometric measurements have been performed at 0 V 123 determined that the GO modification on Cu2O/C/NiCoB films increases the photocathode surface area, suggesting that GO addition induce the formation of abundant catalytic sites facilitating the charge transfer during the water reduction reaction. Also, it is necessary to understand how the electrocatalytic effect of graphene and its derivates influences the charge carrier dynamics of the photocathodes which is observed in the J–V curves.118,120Despite all mentioned above provides valuable information to create new strategies for photocathode design, some issues are still elusive. Recently, Yu and collaboratorsEven if the quantity reported studies are still limited, the recent studies have demonstrated the importance of the graphene deposition technique; if an optimal deposition is achieved, graphene and its derivatives can create chemical bonds between the photocathode and their carbon network facilitating the charge carrier dynamics and the stability without blocking the photocathode light absorption. Although the clear improvement in the charge carrier dynamics that graphene brings to the photocathode performance, this fact is not fully understood and, in some cases, it is important to consider if graphene can be the best material to be used as passivation layer or can be combined with other semiconductors to enhance the metal oxide photocathode activity.2.3124–129 Several findings published to date confirm that drastic limitations of photoelectrodes, such as elevated charge carriers recombination rates and propensity of undergoing photocorrosion, can be mitigated by the proper connection of semiconductors, in which the formed intimate contact, referred as heterojunction, favours the spatial separation of photoexcited electrons and holes as a result of suitable band energy levels alignments.124,125,130–132 And in fact, we might consider that the combination of graphene with photoanodes and photocathodes discussed previously, is an heterojunction itself.The possibility of improving the performance of water splitting photoelectrodes by the integration of two or more semiconductors with different optical and/or physical characteristics has been constantly reviewed.124,133Since effective heterojunctions drive electrons and holes in opposite directions, especially under the influence of an applied bias, they minimize the surface/interface charge accumulation that provokes photocorrosion. This electric field built across the framework intensifies the charge carriers migration, excelling the electron–hole separation and, consequently, reducing their recombination at specific points of the crystalline structure.127,134In general, graphene and its derivatives are incorporated in photoelectrode heterojunctions as solid-state mediators to intensify the charge shuttle between the components, due to their aforementioned inherent electronic properties. Besides, owing to its high specific surface area, graphene contributes to the photoelectrocatalytic reactions, as it provides extra adsorption and catalytic sites.125 Furthermore, compared to GO, rGO possess a reduced number of oxygen-containing groups that can act as deep recombination centres.Among the graphene-related rational designs recently employed in heterostructured water splitting photoelectrodes, rGO is the most recurrent because it presents striking advantages over pure graphene and GQD, as mentioned before.In fact, based on evidences from recent publications, this balanced content of pendent functional groups on the reduced graphene oxide layers is the responsible for the effective linking between the semiconductors in heterojunctioned photoelectrodes, resulting in the broader usage of rGO as mediator compared to the other graphene-like structures. As the connected semiconductors also contain reactive surface functional groups, they interact with those pendent on the rGO sheets, creating bonds that ensure an effective contact between the heterostructure components.89,125,134–136The lack of functional groups in pure graphene tends to preclude a stable and concomitant connection of the graphene sheets with two semiconductors that might possess very distinct surface properties. On the other side, non-reduced GO and graphene quantum dots have higher densities of these oxygenated functional groups. As these groups act as charge carrier trap centres, they may impair the charge transport within the layer and, consequently, the charge shuttle between the photocatalysts and the overall photocatalytic efficiency.As a consequence, most of the research investigations employ effective routes in which the connection of the semiconductors is guaranteed by their interaction with GO containing high density of oxygenated groups, followed by a reduction procedure to eliminate the excess of these recombination centres.79,92,132,137–139 A clear example is schemed in 140 where it is noticeable that the addition of rGO had a paramount contribution to the activity of the photoanode, as the charge transfer at the graphene-free interface was hindered by recombination centres at the intrinsically discontinued heterojunction interface. In contact with the oxide semiconductor, rGO nanosheet creates a Schottky junction which favours the transport of charge carriers from one oxide semiconductor to the other, lessening the recombination of electron–hole pairs.125 Also, it has been suggested that graphene layer reduces the interfacial charge transfer resistance.141 Moreover, some authors affirms that rGO tune the electrical connection between both semiconductors.142As a recent showcase, this strategy was adopted for a water splitting photoanode in which rGO was employed as the solid-state charge carrier mediator two oxide semiconductors (SC/rGO/SC).143–145Heterojunctions formed by earth abundant non-oxide semiconductors can also be improved by the rGO incorporation as a bridged solid-state mediator, forming type II heterostructure.125,143,145,146 As an example, a substantial boost in the photocurrent was observed for the TiO2/rGO/MoS2 photocathode . To design the photoelectrode, an intimate contact between ZnO nanorods (NRs) and Cu2O nanocubes was created, then rGO sheets wrapped the whole set as cocatalyst 150 where, additional to a protective layer, the formation of p–n junction and a cocatalyst deposition were indispensable for raising a promising performance.4/rGO nanocomposite performance can be interesting, the photocurrent values obtained by other methodologies are close from this benchmark.In case of the photoanodes, the incorporation of rGO into different systems has shown the best results. The photocurrent values obtained by the rGO/heterojunction represent a good strategy to optimize the photoanodes. In contrast, although Mo:BiVOin situ or operando with advanced techniques to amplify the comprehension of the mechanisms and species involved.Despite impressive photocurrent values have been obtained by the addition of many types of graphene and its derivatives in metal oxide semiconductors, the nature of the active sites remains elusive, leading to a superfluous comprehension of the role of the graphene. Therefore, future challenges should bring together 3Non-oxide semiconductors are considered as attractive candidates for PEC solar fuel production due to their small band gap which permits capturing a wide range of the solar spectrum. Materials such as transition metal sulfides, selenides or silicon based photoelectrodes have been explored to evolve the OER or HER by themselves or combined with other semiconductors. However, during operation, few drawbacks associated with their physical, chemical and electronic properties have inhibited their performance to obtain high efficiencies. In this sense, graphene incorporation has been employed to overcome the problems associated with chemical and PEC corrosion, charge transport and separation deficits with remarkable results. In this section, the influence of graphene (and its derivatives) insertion on the PEC performance of non-oxide semiconductors in the past 5 years will be summarized. Due to the extensive nature of the non-oxide semiconductors and their peculiarities, this section has been further divided according their chemical structure.3.1Eg compared to metal oxides.151 However, the PEC performance using TMCs-based as photoelectrodes is still limited because of the fast charge carrier recombination. In addition, photocorrosion is actually a common problem for most TMCs.152 Besides a plethora of morphologies , introduction of noble metal nanoparticles (e.g. Pt156 and Au157), formation of heterojunctions  are normally considered potential candidates for PEC water splitting. The VB of TMCs normally consists of 3p orbitals of S (or Se), which result in a more negative VB and a narrower oS2 ref. ), the in3.1.1.Eg = 2.4 eV) and size-dependent electronic and optical properties.159 However, CdS is well-known by its instability and low photocurrent output. Different reports160–163 have demonstrated the graphene (and its derivatives) efficiency as electron acceptor and transporter. Upon light absorption, electrons from the CB of the semiconductor are quickly transferred to the graphene network, and then to the back contact. As observed for metal oxide photoelectrodes, an excess graphene content leads to a deterioration of the PEC performance, attributed to the “shielding effect”.42 Despite a photocatalytic and PEC improvement when graphene (and its derivatives) acts as electron acceptor, the enhanced photoresponse are still in μA cm−2. For this reason, additional to the graphene modification, some authors have decided to integrate the graphene/non-oxide semiconductor materials with other oxide semiconductors to overcome the charge collection deficit.Cadmium sulfide (CdS) is one of the most studied transition metal sulfides due to its appropriate band gap energy (e.g. TiO2 and ZnO) to extend the absorption edge into visible light region and improve photoelectrochemical activity.164,165 As observed for metal oxide heterojunctions, graphene sheets (and its derivatives) can also be wrapped between two semiconductors to achieve better charge separation. Several reports155,166–168 have shown that a graphene interlayer (mostly rGO) between the semiconductor leads a photocurrent density improvement associated with a decrease in the recombination. However, as CdS acts as absorber layer and it is directly exposed to the electrolyte solution, the photocurrent decay of the systems can be related to Cd photocorrosion which has not been addressed by the authors.Due to its band gap, CdS been combined with wide band gap semiconductors (Eg = 3.6 eV), which means that it cannot be photoexcited by visible light irradiation. A particular case where rGO is used as support for the growth of ZnS/Cd NRs open the possibility of designing new organized heterojunctions with attractive properties.156 Additionally to the photocurrent improvement associated with graphene as charge transfer mediator,158,160,169 the ZnS role as charge collector and the CdS absorption capability, the authors could not reproduce a similar pattern on the heterojunction without rGO addition.For wide band gap transition metal sulfide semiconductors, graphene and its derivatives can also improve photogenerated carrier separation and transport. Zinc sulfide (ZnS) is a wide band gap semiconductor (e.g. autoclave and an oven) and can achieve a high-yield and large-scale production. During the hydrothermal synthesis, the GO, employed as precursor, is reduced into the reactor to obtain rGO which usually present structural defects, such as vacancies, heptagon and pentagon rings, edge effects and residual functional groups.179 Although these defects significantly affect electronic and chemical properties of the rGO sheets, the recent reports (66 reports  showed t3.1.2.2), molybdenum disulfide (MoS2) and tungsten disulfide (WS2) are typical representatives. Benefited from their low cost, non-toxicity, large surface area, high stability and environmentally friendly characters, the layered transition metal disulfides with narrow Eg (∼2.1 eV) make them good semiconductors with visible-light-responsive ability.180,181 Although great achievements have been realized on 2D transition metal sulfide photoelectrodes, the incorporation of graphene or its derivatives can also promote the rapid transfer of carriers, inhibiting the electron–hole pairs recombination and extending the lifetime of photogenerated charge carriers.180,181The combination of two-dimensional (2D) transition metal sulfide layers with graphene or its derivatives for photocatalysis and PEC applications has also been reported and some examples are summarized in 2 is also a typical layered transition-metal dichalcogenide with three stacked atomic layers (S–Mo–S) held together by van der Waals forces. This material has attracted the scientific community attention due to the presence of highly reactive edge sites showing efficient photocatalytic and PEC activities.182 In addition, 2D MoS2 nanosheets have appropriate Eg for solar absorption that can be tuned within the range 1.2–2.0 eV depending on thickness and lateral size.160 Similar to CdS, the MoS2 nanosheets can also extend the light absorption of other semiconductors (e.g. TiO2 and ZnO) in the visible region.183,184 Coupling graphene and its derivatives with MoS2 is also one of the strategies widely used to circumvent the problem of high charge carrier recombination rate.159,185 Ternary heterojunction nanocomposites composed by MoS2, graphene (or its derivatives)  have evidenced a similar trend: when graphene (or its derivatives) is added onto MoS2 no significative enhancement is observed; however, when MoS2/graphene is deposited on top of other semiconductor (SC/MoS2/graphene) an important photoresponse is noticeable. The authors attributed this enhancement to the electron transfer from the CB of ZnO to the CB of MoS2 and then to graphene. Unfortunately, some reports do not show the response of SC/MoS2 so it is complicated to define what part of this improvement is due to graphene insertion.MoS2 constitute the active sites for many important reactions.185 Numerous efforts have been devoted to maximize the exposure of active edge sites and improve the PEC performances. In this context, the graphene sheets (and its derivatives) can serve as a support for the growth of MoS2 and accelerate the interfacial electron transfer.191 Among the synthesis techniques, CVD offers unique advantages for achieving uniform graphene films on substrates in a controllable manner and with less structural defects. The edge-rich MoS2 grown on the edge-oriented 3D graphene glass synthesized by CVD can achieve the optimized charge transport along the 2D vector plane from MoS2 layers to graphene. In short, the 3D-graphene/E-MoS2 photoelectrode exhibited strong and broadband absorption, efficient exciton separation and good electron transport. In contrast with the metal oxide systems, the use of CVD technique to get a uniform substrate coating to promote the charge transport instead of covering a porous layer, represent a promising strategy to enhance the photoelectrode performance.The edges of the layered MoS3.1.3.Eg not accessible to binary transition metal sulfides.197,198 It has been noticed that solvothermal synthesis that in some cases rGO insertion can shift the absorption edge of the sulfide and decrease the band gap.198 Additionally, the rGO avoid the formation of undesirable phases which inhibit the performance. Due to their excellent electrical conductivity, the rGO sheets also acted as electron collectors and transporters to efficiently suppress the recombination of photoinduced electron–hole pairs.A new category known as ternary transition metal sulfides has also been widely used in PEC water splitting because these semiconductors exhibit high absorption coefficient over a wide spectral range. The use of two different metals allows access to the 2) transition metal sulfide QDs  and lead (Pb), has gained attention in recent years. There has been a slow but steady and conscious increase in the use of friendly environmentally materials. The ternary (ABSnS2 ref.  and AgIn. 2 ref. ) have be2) transition metal sulfide, the ZnIn2S4 of the (AB2S4) family has attracted attention due to its narrow Eg (2.34–2.48 eV), high chemical stability and high surface area.201,202 Similar to the situation of other frequently studied semiconductors, the PEC performance of ZnIn2S4 can also be enhanced with the incorporation of graphene (and its derivatives).203,204 Analyzing the information in −2,205 a hole scavenger was added to the electrolyte solution, indicating that the electrode is still facing hole injection deficiencies. Also, the high cost of indium prevents the large-scale use of these semiconductors in practical applications.As the ternary  hybrids as PEC electrodes.207 Several roles of graphene (and its derivatives) have also been emphasized such as charge acceptors and separators or as protective, catalytic and supporting layers.208 In other case, graphene (and its derivatives) sheets can act as conductive scaffold which enhances the charge carrier transfer, reducing both charge transfer resistance and carrier recombination.192,209–211 Interestingly, transition metal selenides have shown better responses compared to the sulfides (2 (CIGS) photocathodes after rGO modification, in which rGO as protective and binder layers improves the photocurrent density and stability.211Transition metal selenides are also considered good candidates for PEC water splitting. These materials have a similar structure and properties that the transition metal sulfides. They not only share some similarities but also differences. For example, the transition metal selenides have better electrical conductivity than the transition metal sulfides.sulfides , represee.g. hydrothermal and solvothermal) have been more used to synthesize graphene-based TMCs nanocomposites. These one-step methods produce nanoparticles with defined morphology and homogeneously distributed on the surface of the rGO sheets. Although these methods produce rGO sheets with structural defects and residual functional groups, their attributes are still comparable to graphene sheets. These residual oxygen functionalities facilitate better attachment of nanoparticles to the rGO sheets.212 Besides, these methods can achieve high-yield, large-scale and low-cost production.66 Graphene and its derivatives as protective layers have also been reported in the PEC systems.183 Among the materials, SLG is the most used as protective layers because it has much less defects and less functional groups on surface and edges. CVD is one of the techniques that has been most used to synthesize SLG sheets in a more reproductive way, with well-known composition and more accurate properties. However, expensive and tedious procedures and inability to accomplish large scale production limit commercial deployment.200,212 When rGO acts as protective layer, the type and residual amount of functional groups on the rGO surface must be controlled by selecting various oxidation and reduction agents, as well as adjusting the reduction time.211In summary, among the methodologies, the chemical methods  calculations and 3.2219 Its CB edge position (−0.5 VNHE at pH = 0) is sufficiently more negative than the reduction potential of protons for hydrogen production (H+/H2).185 This semiconductor shows a great potential for solar hydrogen production. However, there are some limitations of utilizing Si as photoelectrodes in a PEC cell. Because of large reflection of conventional planar Si wafers, various textured Si based photoelectrodes has been studied, such as Si nanowires array,220 Si pyramid array221 and Si nanopore array.222 The increased surface area in these textured photoelectrodes causes undesirable charge carrier recombination and accelerates surface photocorrosion/passivation during the electrochemical reactions.219 An alternative approach to reduce this problem is the use of suitable protective layers on the electrode. Due to their high optical transmittance, chemical inertness, good electrical conductivity and large surface area, graphene and its derivatives can be easily applied to semiconductor surfaces over large areas.223 These materials, specifically SLG, can be used as protective coating layer to improve the photocorrosion/passivation resistance of Si-based photoelectrodes.223 Among the techniques employed, CVD is one of the most used to synthesize SLG sheets on the Si photoelectrodes and their high PEC performances are summarized in 41 and reflect the excellent role of SLG as passivation layer which remains a relatively stable current during 30 h  is one of the most studied semiconductors in the field of PEC water splitting as a light absorber thanks to its suitable band gap (1.12 eV).ing 30 h . Some re229 In these circumstances, rGO acts as an acceptor of the electrons generated in the Si NWs photocathode that improves the separation of the photogenerated charge carriers. Consequently, the enhancement of the photoelectrochemical performance of the SiNWs/rGO20 photocathode may be greatly attributed to the high electrical conductivity and the boosted charge transfer rate.Graphene and its derivatives could also be used as catalysts for a solar-driven hydrogen evolution reaction on Si-photocathodes. 2192/FeNiCoOx photoanode.230 The role of the rGO in the photoelectrode was attributed to the efficient charge separation at the interface (Si/graphene) and the defects in the graphene layer that provided electrochemically active sites for water oxidation. Other records also emphasize that the changes of the surface functional groups on graphene and its derivatives227 and the different graphene deposition techniques on the Si substrate224,226 can significantly influence the performance of PEC water splitting.Generally, an integrated Si photoelectrode system for PEC water splitting requires a Si absorber, a protective layer and a catalytic layer.In contrast, rGO/sulfide photoanodes synthesized by solvothermal methodologies have shown good PEC response. For these systems, graphene (and its derivatives) is employed not only as an electron acceptor and transporter but also as growth support, allowing the formation of optimized structures. As it can be noticed, the systems that have raised the maximal current are graphene/heterojunction nanocomposites, indicating that graphene by itself does not resolve the photoelectrodes limitations.Eonset. Furthermore, some factors directly affect the final performance of these photoelectrodes, including synthesis methodology, graphene content in the composite, semiconductor morphology, interfacial interaction and contact area between graphene and semiconductor.42In case on non-oxide photoelectrodes, the incorporation of graphene sheets in the semiconductor matrix increases the photocurrent density value, without changing the 4The introduction of graphene in semiconducting photoelectrodes aiming the production of hydrogen and oxygen has become a potential strategy to design the next generation of photocatalysts. In this review, we have summarized the most recent accomplishments where graphene structures were combined with different semiconductor electrodes  in PEC applications. In all cases discussed, the insertion of graphene or other graphene derivatives has always led to an improvement in the performance and/or stability of the electrode. Regardless the positive and synergic effects, the mechanisms that drive the charge carrier dynamics within the system and how they operate are not fully understood, bringing new opportunities to be explored in the field.In view of charge carrier dynamics, graphene and its derivatives addition led to a more efficient charge transport through their layers compared to the photoelectrodes where grain boundaries and other defects act as charge carrier traps are present. Although for a single layer graphene, both electron and hole transport are possible and efficient, most samples are not pure or single layer graphene. Graphene oxide, doped graphene, reduced graphene oxide and other derivatives tend to benefit of one carrier to another. And even applying sophisticated techniques, it is hard to separate the effect on the electron or hole dynamics.Many reports have demonstrated that graphene can also passivate the surface states in oxides and non-oxides photoelectrodes leading a significant enhancement performance and stability. Here, passivation also comes from the possibility to modify graphene edges and basal planes. Nevertheless, even if graphene has shown remarkable performance as protective layer, some reports has shown that the deposition methodologies bring inherent problems to accomplish this role.Among graphene derivatives, rGO is the most employed probably associated with its facile synthesis and low-cost precursors. As graphene, rGO can acts as a passivation layer, excels the photoexcited charge carrier separation and, consequently, reduce their recombination at specific points of the electrode. An interesting application on rGO widely reported for non-oxide photoelectrodes consist in using it as growth supporting to create ordered structures with improved properties.Incorporating GQDs on metal oxide photoelectrodes represents a promising and not extensively studied field. In addition to the charge separation/transport and stability gains, the coupled system formed by the photoelectrodes and GQDs also showed a superior light absorption compared to the non-coupled electrode which can be related to a higher photocurrent value. However, the amount of GQDs needs to be carefully controlled since this excess block the active sites and inhibits the OER/HER at the surface. In fact, this is an issue commonly found for other carbonaceous materials when incorporated in the electrodes.in situ/operando techniques to characterize graphene-bases photocatalysts is extremely valuable because they can provide fundamental aspects for sharpening our understanding on the nature of active sites, reaction mechanisms, and charge carrier dynamics. These techniques could also assist us to elucidate the chemical states and electronic structures, especially at the interface semiconductor–electrolyte.Despite some studies present promising results in the field, some issues are still elusive; several reports affirm that a Schottky junction is formed between the semiconductor and the graphene but, to our knowledge, the junction nature between the semiconductors and graphene is still under debate and not fully understood. Otherwise, the usage of Notwithstanding all the advantages presented in this review, it is important to point that better strategies to synthesize graphene coupled to photoelectrodes need to be developed. Despite all methodologies reported, the desired performance depends on a successful graphene addition or transfer to the substrate. For this reason, surface engineering plays an important role to further improve the PEC systems for water splitting.The path ahead for graphene and its derivatives in photoelectrodes towards water splitting reactions is bright, but challenging, and many efforts in the chemistry and physics of these systems are still needed. The full understanding of how graphene operates may help us to tailor the properties of the semiconducting materials, to adjust the synergy and to gain benefits from all parts. We believe that for many years it will be a fascinating research area because of the many possibilities and opportunities.There are no conflicts to declare.


Our results on In–Si-grapheneprovide a materials system for controlled self-assembly and heteroatomicanchoring of single atoms and few-atom nanoclusters on graphene.Single atoms andfew-atom nanoclusters are of high interest incatalysis and plasmonics, but pathways for their fabrication and placementremain scarce. We report here the self-assembly of room-temperature-stablesingle indium (In) atoms and few-atom In clusters (2–6 atoms)that are anchored to substitutional silicon (Si) impurity atoms insuspended monolayer graphene membranes. Using atomically resolvedscanning transmission electron microscopy (STEM), we find that thesymmetry of the In structures is critically determined by the three-or fourfold coordination of the Si “anchors”. All structuresare produced without electron-beam induced materials modification.In turn, when activated by electron beam irradiation in the STEM,we observe  These have suggested single atomsand few-atom clusters to be of high application potential, particularlyin heterogeneous catalysis 16 Controlled placement of individual impurity atomswithin the graphene lattice via the controlled movement of the STEMelectron beam (e-beam) has been demonstrated,19 mostly relyingon ubiquitous substitutional Si impurities inherent to chemical vapordeposited (CVD) graphene,20 which howeveralso can be deterministically introduced.21 This STEM-based manipulation has been expanded toward the studyof homoatomic Si clusters in graphene.26 Recently, the e-beam driven merging of such Si clusterswith a single Pt atom toward heteroatomic Pt–Si structureson graphene was also reported.27 A shortcomingof the STEM manipulation approach is, however, the difficulty in extendingit to larger-scale fabrication. A materials system that intrinsicallyand readily allows single-atom and few-atom cluster formation andanchoring on graphene membranes in a self-assembled fashion has remainedelusive to date.An emerging pathway toward both controllable fabricationand concurrentmicroscopic imaging of small atomic arrangements is the applicationof aberration-corrected scanning transmission electron microscopy(STEM) to atoms that are placed onto or implanted into suspended two-dimensional(2D) membranes such as graphene.28 to observe the In–Si structuresincluding probing their structural dynamics and formation mechanisms in situ via e-beam induced structural rearrangements.30 We find that the original coordination of Si in the graphene latticeis critical for the atomic arrangement of the In single atoms andclusters. While single In atoms and threefold symmetric In clustersform on threefold coordinated Si atoms, fourfold symmetric clustersare found on fourfold coordinated Si atoms. Within the fourfold andthreefold symmetric clusters, In atoms can be located on top of eithercarbon atoms or hexagon centers. Hexagon-centered fourfold symmetricIn6 clusters can dynamically transform into three differentstructures during e-beam irradiation: a chain with six In atoms, anIn dimer located on a pentagon center, and a dimer located on a hexagoncenter. Unlike the fourfold symmetric clusters which are immobileunder electron irradiation,20 the C-centeredthreefold symmetric In5 clusters can move within the graphenelattice due to the electron irradiation induced movement of the underlyingthreefold coordinated Si site. These threefold clusters can also transformto anchored single In atoms.We report here the self-assembly of room-temperature-stablesingleIn atoms (1 atom) and few-atom In clusters (2–6 atoms) andtheir concurrent anchoring onto individual substitutional Si dopantatoms in graphene membranes. We employ atomically resolved and element-sensitiveSTEM31 or adventitious carboncontamination.32 Self-assembled anchoringon graphene via single foreign impurity atoms in the graphene34 (such as on Si impurities here) has been theoretically suggested35 but not experimentally verified, in particular, not on the atomicscale. Toward filling this gap, our experimental observations hereevidence a large diversity of room-temperature-stable, heteroatomicallyanchored single atoms and nanoclusters in the In–Si-graphenesystem. Our work thereby contributes a materials system toward thedevelopment of a rational toolbox for the controlled self-assemblyand heteroatomic anchoring of single atoms and few-atom clusters ongraphene.Typically foreign single atomsand nanoclusters on graphene havebeen found to preferentially reside at high-reactivity sites in thegraphene such as lattice defects–9 mbar), which comprises the microscope and directlycoupled preparation chambers including laser annealing and an in situ evaporation chamber. We note that CVD graphene intrinsicallyincludes a small number of substitutional Si heteroatoms, as well-documentedin prior literature.37 The sample is then first irradiated in UHV by 600 mW of laser powerat 445 nm wavelength, allowing the removal of adventitious hydrocarbonadsorbates from the graphene that stem from membrane fabrication andambient air exposure.39 Subsequently, without vacuumbreak, In is evaporated onto the graphene membrane using a custom-builtpreparation chamber (base pressure ∼10–9 mbar).30 After in situ deposition ofIn, the sample is transferred into the microscope, without a vacuumbreak, and once more irradiated in situ by a similar600 mW laser before STEM imaging. Due to the low melting point (∼160°C) but low vapor pressure of In,40 the second laser irradiation critically promotes the diffusion ofIn atoms over the graphene surface.The experiments we carried outto deposit indium single atoms andfew-atom clusters on graphene are schematically shown in Z∼1.6 for isolated atoms and roughly linear with specimen thickness fora given Z).28 Furtherelemental identification is made via simultaneously recorded electronenergy loss spectroscopy (EELS) data. Further experimental detailscan be found in the During STEM imaging,medium and high angle annular dark field (MAADFand HAADF) signals are acquired simultaneously. These allow the discriminationof imaged elements based on signal intensity  are found on the graphene after In deposition and the second laserirradiation. The key feature of interest we observe in these largeclean graphene areas are the small bright spots in the MAADF imagesin 6 e– nm–2 s–1 (b), 0.18 × 106 e– nm–2 s–1 (d)) the In structures appear to be unperturbedby STEM imaging. We verify the structure of these In clusters on grapheneat higher magnification in the next sections. Imaging at higher magnificationsis associated with higher electron dose rates, however, which drivesstructural modifications of the In structures by energy transfer fromthe scanning e-beam,30 but at the same time it alsoallows us to in situ probe their structural dynamics.We stress that the single In atoms and In clusters are alreadypresent before extended STEM imaging, i.e., they are generally notproduced by the electron irradiation, but have self-assembled duringIn evaporation and the second laser anneal prior to imaging. In thefields of view in M- and Si L-edges show that the cluster consists of In and Si atoms reveals the self-assemblyprocessof the In–Si structures (here emulated by the energy inputfrom the e-beam). This sequence has been acquired at a higher magnification(and thus higher electron dose rate of 20.83 × 106 e– nm–2 s–1) than 30 In particular, 9 e– nm–2 s–1), Inatoms have become trapped at this Si atom to form an intermediateSi-anchored In structure (9 e/nm2), additional In atoms have become trapped to eventuallyform a fourfold symmetric In cluster anchored to the fourfold Si atomin the graphene lattice (further details on the structure below).This observation of In cluster formation suggests that a single centralSi atom is present in the observed In cluster structures. Notably,no In reservoir is present within the field of view around the Siatom during cluster formation in 27Toward addressing this question, the HAADF image sequencein tructure 4. After 41) and become trapped during encounters with the substitutional Si,thereby self-assembling into the observed Si-anchored In clusterson the graphene (with some Si atoms remaining “empty”from our processing). We suggest that this facile self-assembly relieson the significant mobility42 of In atomson the graphene membrane at room temperature (resulting from In’slow melting point (∼160 °C) and low vapor pressure overa wide temperature range40) and the higherreactivity of the Si site compared to the relatively inert graphenelattice.35 The observation in 43 For the as-produced clusters imaged at largerfields of view After thisfirst assessment of the overall nature of the anchoredclusters, we now turn to detailed atomic structure analysis aidedby density functional theory (DFT)-based structure modeling. In 4 cluster. The DFT-relaxedmodel matches the experimental data well . Those result in a muchpoorer match, further corroborating that the bright spots on HAADFimages are In. Besides these fourfold symmetric In clusters, we consideredanother In structure with four In and two Si atoms . This structure is similar to thatshown in The second observed structure is also fourfold symmetric 3b, but twell see 3b,f,h (el see SI S4. Those6 clusterunder electron irradiation (dose rate 46.88 × 106 e– nm–2 s–1) areshown in SI Video 2). In 9 e– nm–2) into an In chain where the In atoms arealigned along the armchair direction of the graphene lattice, withall six In atoms retained. After a further 42.0 s (∼1.96 ×109 e– nm–2), all Inatoms of the cluster disappear from the field of view. Notably, acentral fourfold symmetric substitutional Si impurity in the graphenelattice remains visible, again corroborating the role of substitutionalSi as a central “anchor” for the In clusters on graphene., two In atoms are again captured by the Si atom(similar as in the cluster formation sequence in 9 e– nm–2); these two In atoms rotate by 90° to overlaythe graphene hexagon centers. However, they return back to their initialposition by again rotating 90° after 125.8 s (∼5.90 ×109 e– nm–2). Thisstructure finally attracts four additional In atoms after 167.8 s(∼7.86 × 109 e– nm–2) to form the hexagon-centered fourfold In6 cluster, identicalto that observed in the first frame of the sequence.The dynamics of a hexagon-centered fourfold In5 cluster . We suggestthis to be related to In diffusion through defects or grain boundariesin the CVD graphene, that enable some In atoms to reach the graphenemembrane’s opposite side under our conditions. The hexagon-centeredthreefold In3 structure notably includes two Si atoms,one of which is substitutional in the graphene lattice and one whichis part of the In cluster. As shown in In addition to indium clusters formed on fourfoldcoordinated Siimpurities, we observe threefold symmetric In clusters as well assingle In atoms anchored on threefold coordinated Si impurities, asshown in detail in ster see 5a,b and ster see 5g,h. Not5 cluster. Notably, although four In atoms are removed,the remaining single In atom keeps its position on the Si atom underelectron irradiation . As shown in SI Figure S6d, the atomic structure shown in Under e-beam exposure at higher magnifications, the In atomsinthese threefold clusters can be readily ejected by the focused electronbeam see .HAADF-Stion see . In addin see SI S6. As sh5, in the hexagon-centeredthreefold In3 cluster only the anchoring Si impurity remainsin the graphene lattice after removal of three In atoms and one Siatom . Unlike C-centeredthreefold InSI Figure S9), albeit these arrangements are never as stable asin the case of threefold Si anchoring. We also note that when thecoordination of the Si atom in the graphene lattice is changed dueto e-beam exposure, trapping of different structures on the same Siatom is observed. For instance, in SI Figure S10 a threefold symmetric In cluster is first trapped by a threefoldcoordinated Si atom, then again removed, after which the Si atom switchesinto fourfold coordination, which eventually enables the trappingof an In2 dimer.We note that, intermittently,short-term capture of single In atomsby fourfold Si impurities is also observed (–2) and barethreefold coordinated Si (0.89/1000 nm–2) anchoringheteroatoms in the graphene lattice are roughly similar for our samples.In descending order, we find the most often observed  Si-anchored In structures to be as follows: hexagon-centeredfourfold symmetric In6 clusters (0.37/1000 nm–2), single In atom anchored on Si (0.32/1000 nm–2), hexagon-centered In3 threefold symmetric clusters (0.22/1000nm–2), C-centered fourfold symmetric In4 (0.20/1000 nm–2), C-centered threefold symmetricIn5 clusters (0.15/1000 nm–2), In2 dimers (0.12/1000 nm–2), and In6 chains (0.07/1000 nm–2). These statistics underlinethat both In single atoms and few-atom clusters are well stabilizedby the Si-graphene system. Notably we find neither single In atomsnor few-atom In clusters that have nucleated without Si “anchors”,such as on the empty graphene basal plane or at single vacancy ortopological defects in the graphene.After having identified the variousobserved structures of theIn–Si-graphene system, we can semiquantitatively elucidatetheir relative formation probabilities by measuring their areal observationdensities when imaged at wide fields of view . Note that observed areal densities of barefourfold coordinated Si  anchored on the Si impurities andnot larger particle formation. However, since DFT calculations addressonly the static stability of the structures, this points to kineticeffects in the formation of the observed structures. These effectsmight be related to limited In supply  or dynamic attachment/detachmentbarrier differences between In–Si and In–In, respectively.The latter possibility is supported by the fact that for most of ourobserved structures, the majority of In atoms are on the Si’sfirst coordination shell , while purely In-bound In atoms remain limited in ourexperimentally observed structures.The structure of the observedSi-anchored In nanoclusters is notrelated to the body-centered-tetragonal bulk structure of In particles,which we have previously observed for much larger In particles ongraphene with average particles size of ∼5 nm.within the In clusters (3 clusters are not observed to change their position during e-beamirradiation (see SI Video 3), we show in 5 threefold symmetric In cluster duringirradiation . Fromits initial position, the cluster moves after 4.2 s (∼0.05× 109 e– nm–2)with respect to the graphene lattice. The movement of the clusteris due to the migration of the Si impurity to a neighboring C sitealong the zigzag direction of graphene, presumably via the same bondinversion mechanism as Si impurity manipulation.20 The initial positions of the In atoms are marked by redcircles in the DFT-relaxed model corresponding to the structure acquiredat 4.2 s (∼0.05 × 109 e– nm–2). (Note that the Si atom is not visible in the modeldue to the In atom located on top of it.) The change in the locationof the Si changes the alignment of the In atoms with respect to thegraphene lattice, and thus the entire cluster migrates to preserveits bonding. The movement of the cluster continues after 8.4 s (∼0.10× 109 e– nm–2).In the HAADF image, an additional In contrast appears below the cluster(same effect is visible in the frame at 21.0 s at a dose of ∼0.25× 109 e– nm–2).We believe this to be because the cluster rotated before the scanwas finished, which suggested acquiring STEM images at a higher scanspeed. Although we were not able to resolve the rotation directionof the cluster, additional In contrast does appear on the HAADF imageat 8.4 s (∼0.10 × 109 e– nm–2) suggesting that the cluster rotates 60° anticlockwisearound the center . Until the last image frame acquired after 54.5 s (∼0.64× 109 e– nm–2),the Si atom and the cluster have jumped five times along the zigzagdirection of the graphene lattice , we not only observe cluster formation and structural dynamics clusters 2, 4, 5 b48 and their resulting atom-decomposed densitiesof states  indicatethat the Si–In structures observed here on graphene are allapproximately one- or two-electron deficient with a large local chargepolarization and, as such, may be promising candidates for applicationscreening, e.g., for single-site catalysis.4 Notably in this context, the rational anchoringapproach presented here for In single atoms and In nanoclusters viaSi anchors in the graphene lattice may also be extendable to otheranchored metals as well as other solid supports including widely usedcarbon supports in heterogeneous catalysis. While our work here demonstratesthe viability of self-assembly of Si-anchored In single atoms andnanoclusters on graphene under UHV conditions with stability at roomtemperature, the presented In–Si-graphene structures stillneed to prove their usability for actual catalysis or plasmonics applications.Such application screening will require, e.g., improving the selectivityof the preparation route toward few-atom structures  and assessing the stability, possible phase transformations,and activity of the In–Si-graphene structures under relevant,e.g., oxidative or reductive, reaction conditions in future operando type work.Finally, large-supercell semilocal DFT calculations6 clusters, single In atoms anchored on Si,and hexagon-centered threefold symmetric In3 clusters.Notably, the original coordination of the Si determines the atomicarrangements of the In structures: While single In atoms and threefoldsymmetric In clusters form on threefold coordinated Si impurities,fourfold symmetric clusters are found on fourfold coordinated Si impurities.Due to energy transfer from the scanning e-beam, in higher-dose-rateclose-up imaging we observe in situ the formation,structural changes, and translation dynamics of the Si-anchored Instructures on graphene: The hexagon-centered In6 fourfoldsymmetric clusters transform into three different structures duringe-beam irradiation, including In chains and dimers. Unlike the fourfoldsymmetric clusters, the C-centered In5 threefold symmetricclusters can move under e-beam irradiation along the zigzag directionof graphene lattice and also transform to single In atoms anchoredon the Si. The observed Si-anchored In structures on graphene arepromising for future application screening in, e.g., catalysis. Combined,our atomic-scale observations provide a materials system and rationalframework toward the controlled self-assembly and heteroatomic anchoringof single metal atoms and few-atom clusters on graphene.We report the self-assemblyand anchoring of single In atoms andfew-atom In clusters onto substitutional Si impurity atoms in suspendedmonolayer graphene membranes. A variety of structure types that arestable at room temperature are found from our facile fabrication routewithout the requirement for e-beam induced materials modification.Most frequently observed structures are hexagon-centered fourfoldsymmetric In–9 mbar) using concurrent high-angleannular dark field (HAADF) and medium-angle annular dark field (MAADF)detectors with collection angles of 80–300 mrad and 60–80mrad, respectively. The EELS experiments were carried out by a GatanPEELS 666 spectrometer retrofitted with an Andor iXon 897 electron-multiplyingcharge-coupled device camera. The energy dispersion, the beam current,and the EELS collection semiangle were 0.5–1 eV per channel,30 pA, and 35 mrad, respectively.49 TheSTEM is equipped with a custom-made sample loading and transfer systemto enable direct transfer of samples from various preparation chambersinto the STEM without exposure to ambient.51STEMimages were acquiredwith a Nion UltraSTEM100 operated at a 60 kV accelerating voltagein UHV  was used to clean the graphenesurfaces. Laser irradiation of the sample held in a transfer arm wasperformed through a viewport in both STEM and UHV sample preparationchambers. In the experiments, the laser was operated at 10% duty cyclereducing the laser power to 600 mW, which does not induce structuraldamage in graphene but is sufficient for cleaning.in situ evaporationof In was achieved using a custom-builtpreparation chamber (base pressure ∼10–9 mbar)coupled to the STEM. The evaporation source was a Knudsen cell withIn pellets , which were heated to 700°C. The resulting In flux was then directed at the graphene sample,which was not intentionally heated. Nominally deposited In thicknesswas monitored using a quartz microbalance and kept to ∼10 nm.Substrates used for In deposition were commercial CVD graphene (GrapheneaInc.) suspended on perforated silicon nitride grids (Ted Pella Inc.).Subsequent to in situ In evaporation, a second in situ laser cleaning step was applied to the sample.The 52 chromatic aberrationcoefficient of 1 mm, a spherical aberration coefficient of 1 μm,and energy spread of 0.48 eV. HAADF and MAADF detector angle rangesare set to the experimental range of 80–300 mrad and 60–80mrad, respectively.HAADF and MAADF image simulationswere carried out using the QSTEM software with parameters correspondingto the experiments:28 in some cases after applying Wiener filtering.The parameters used for double Gaussian filtering are σ1 = 0.25,σ2 = 0.20, weight = 0.3  in the LCAO mode54 with the grid spacing of 0.2 Å and a 5× 5 × 1 k-point mesh so that maximum forceswere <0.02 eV Å–1.55 For details of large-supercell density of states calculations, Wannier-functiondecomposed in order to assess charge transferability and performedusing the linear-scaling package ONETEP;48 see SI Figures S13–S14.Density functional theory (DFT) simulationswere carried out using the grid-based projector-augmented wave (GPAW)software package


III complexes with neutral bisguanidine ligand units. Further one‐electron oxidation is ligand‐centred in the case of Co–bisguanidine complexes with fully alkylated guanidino groups, giving CoIII complexes with radical monocationic bisguanidine ligands. On the other hand, the hydrogen‐bond strengthening upon oxidation of the Co–bisguanidine complex with partially alkylated guanidino groups initiates metal reduction (CoIII→CoII) and two‐electron oxidation of the guanidine ligand, providing the first example for the stimulation of redox‐induced electron transfer by interligand hydrogen bonding.Octahedrally coordinated cobalt(II) complexes with a redox‐active bisguanidine ligand and acac co‐ligands were synthesized and their redox chemistry analysed in detail. The N−H functions in a bisguanidine ligand with partially alkylated guanidino groups form N−H⋅⋅⋅O hydrogen bonds with the acac co‐ligands, thereby massively influencing the redox chemistry. For all complexes, the first one‐electron oxidation is metal‐centred, leading to Co III complex with a neutral redox‐active bisguanidine ligand, leading to metal reduction (CoIII→CoII) and two‐electron oxidation of the bisguanidine ligand. In the absence of interligand hydrogen bonding, oxidation leads to a CoIII complex with a radical monocationic guanidine ligand.The strengthening of interligand hydrogen bonding triggers a redox‐induced intramolecular electron transfer (RIET) upon oxidation of a Co III complex preferred at low temperature into a high‐spin CoII complex favoured at higher temperature.III into a higher‐energetic CoII complex by light at low temperature (similar to the light‐induced excited spin state trapping (LIESST) effect discovered by Gütlich et al.III complex by light (similar to reverse‐LIESST).‐benzodioxole (L1), 5,6‐bis‐2,2‐dimethyl‐‐benzodioxole (L2), and 5,6‐bis‐2,2‐dimethyl‐‐benzodioxole (L3) sketched in Figure II complexes with one of the bisguanidines L1–L3 as ligand and two acetylacetonato (acac) co‐ligands led to stable complexes that were fully characterized. We present the first example for the initiation of RIET by hydrogen‐bond enforcement, enriching the arsenal of IET stimulants by a biomimetic, innovative alternative.Herein, we report the first IET in a cobalt complex with a redox‐active bisguanidine ligand, being triggered in an unprecedented way by interligand hydrogen bonding. We used the three bisguanidines 5,6‐bis2(L1)], [Co(acac)2(L2)], and [Co(acac)2(L3)] 2(L3)] (see Lewis structure in Scheme +) salt Fc(PF6), and two electron oxidation either also with Fc(PF6) or with the stronger oxidant NO(SbF6). The complexes were isolated and fully characterized in all three redox states. The first one‐electron oxidation is metal‐centred (CoII→CoIII) for all complexes. Further one‐electron oxidation of the complexes with L1 or L2 is ligand‐centred, giving CoIII complexes with radical monocationic ligand L1.+ or L2.+. On the other hand, oxidation of the monocationic complex [Co(acac)2(L3)]+ to the dication [Co(acac)2(L3)]2+ is coupled with IET (RIET) leading to cobalt reduction (CoIII→CoII). RIET is triggered by the increase of the interligand N−H⋅⋅⋅O hydrogen‐bond strength upon two‐electron oxidation of the ligand. Hence detailed analysis clearly shows that the dication [Co(acac)2(L3)]2+ is a high‐spin CoII complex with dicationic guanidine ligand. In principle, a further one‐electron oxidation is imaginable, leading to a tricationic CoIII complex with dicationic guanidine ligand. Indeed, cyclic voltammetry measurements indicate the possibility of reversible three‐electron oxidation (see below). However, it was not possible to isolate these ultimately oxidized complexes in pure form in experiments that relied on NO(SbF6) as oxidizing reagent. According to the experiments, partial degradation leads to cleavage of the bond between the metal and the dicationic guanidine in significant amount. In the following, the electronic structures of the neutral, as well as the singly and doubly oxidized complexes will be evaluated in detail. The discussion concentrates on the comparison between complexes of the ligands L1 and L3. The experimental results obtained for the complexes with L2 showed similar behaviour as the complexes of L1.The three neutral Coo(acac)2L], [Co(aco(acac)2L],  2(L1)]+/ [Co(acac)2(L1)]0, E1/2=−0.03 V for [Co(acac)2(L1)]2+/ [Co(acac)2(L1)]+, and E1/2=0.41 V for [Co(acac)2(L1)]3+/ [Co(acac)2(L1)]2+. Also, three redox processes are found for [Co(acac)2(L2)], at E1/2=−0.54 V for the redox couple [Co(acac)2(L2)]+/[Co(acac)2(L2)]0, E1/2=−0.21 V for [Co(acac)2(L2)]2+/[Co(acac)2(L2)]+, and E1/2=0.32 V for [Co(acac)2(L2)]3+/[Co(acac)2(L2)]2+. The potential for the first one‐electron oxidation is almost equal for [Co(acac)2(L1)] (E1/2=−0.56 V) and [Co(acac)2(L2)] (E1/2=−0.54 V), and significantly lower than the potentials required to oxidize the free ligands L1 (E1/2=−0.25 V for L1.+/L10) or L2 (E1/2=−0.46 V for L2.+/L20). These results clearly indicate that the first one‐electron oxidation is metal‐centred (CoII→CoIII), as expressed by the Lewis structures in Scheme −1) a small feature is visible on the high‐potential side of the first oxidation wave for all three complexes, vanishing at slower scan rates. The disappearance of this feature at slow scan rates and the similar behaviour of all three compounds argue against the presence of a compound with different composition in the solutions. The change from high‐spin CoII to low‐spin CoIII brings about massive structural and electronic changes, and therefore it might be possible that the presence of an intermediate complex (e.g. a CoIII complex with different spin multiplicity that relaxes to the low‐spin CoIII ground state) causes the additional weak features at high scan rates.In the cyclic voltammogram recorded for 2+/[Co(acac)2(L1)]+ and −0.21 V for [Co(acac)2(L2)]2+/[Co(acac)2(L2)]+, are both higher than the potentials for one‐electron oxidation of the free ligands. Such an increase could easily be explained by the bonding to the Lewis‐acidic metal. Moreover, the difference in the E1/2 values of 0.18 V is almost equal to the difference of 0.21 V for the free ligands. On these grounds, the second one‐electron oxidation is assigned to a ligand‐centred oxidation process, leading to CoIII complexes with radical monocationic guanidine ligands  are clearly potentially separated (ΔE1/2=0.44 V for L1 and 0.53 V for L2), comparing with ΔE1/2=0.14 and 0.08 V for the free ligands L1 and L2, respectively.The s Scheme . The thi2(L3)] is also included in Figure E1/2=−0.53 V (Eox=−0.46 V), E1/2=−0.19 V (Eox=−0.09 V), and E1/2=+0.11 V (Eox=+0.17 V), assigned to the couples [Co(acac)2(L3)]+/[Co(acac)2(L3)]0, [Co(acac)2(L3)]2+/[Co(acac)2(L3)]+, and [Co(acac)2(L3)]3+/[Co(acac)2(L3)]2+, respectively. The potential of the first one‐electron process is close to those measured for the analogue complexes with L1 and L2, and lower than the potential measured for the free ligand L3; therefore it is assigned to a metal‐centred oxidation (CoII→CoIII).The cyclic voltammogram of 2+/[Co(acac)2(L3)]+ is large (0.22 V), possibly due to the significant structural changes induced by the RIET process. From the synthesis of stable salts of the monocation [Co(acac)2L3]+ and dication [Co(acac)2L3]2+ by chemical oxidation of the neutral compound (see below) an inherent instability of the redox states could be excluded. The CV measurements suggest the third one‐electron redox process, leading to a CoIII complex with dicationic guanidine ligand, to be reversible. However, it was not possible to synthesize salts of these complexes in pure form.The second and third one‐electron redox events then belong to ligand‐centred oxidation. We will see that the second redox event is accompanied by a RIET process 2(L1)]. For [Co(acac)2(L2)], g⊥=7.53 and g∥=2.00 add up to an effective g value is 3.84. Values of g⊥=6.66 and g∥=2.68 are obtained for [Co(acac)2(L3)], resulting in an effective g value of 4.00. Hyperfine coupling to the nuclear spin of the 59Co nucleus (I=7/2) should produce splitting into eight lines. Indeed, eight lines are visible for [Co(acac)2(L2)] ], the others being buried. The coupling constants for [Co(acac)2(L1)] (98 G) and for [Co(acac)2(L2)] (95 G) are very similar. For [Co(acac)2(L3)], eight sharp lines show, but also additional lines due to coupling with other nuclei. With 72 G, the A value is slightly smaller, presumably due to the structural peculiarities produced by the two hydrogen bonds (see structure below).2(L1)](PF6) in CH2Cl2, indicating that [Co(acac)2(L1)]+ is a diamagnetic CoIII complex with neutral ligand in the whole temperature region 6–300 K. Complementary with these results, signals appeared in the 1H NMR spectrum of the compound . The EPR spectrum of [Co(acac)2(L1)](PF6)2 in a frozen CH2Cl2 solution at 6 K shows a sharp signal due to a ligand‐centred radical with a g value near 2 (1.997). At 293 K, a clear hyperfine structure due to isotropic hyperfine coupling with the 59Co nucleus is visible, resulting in line splitting with a hyperfine‐coupling constant A=97 G ](PF6)2 . Hence, [Co(acac)2(L1)]2+ and [Co(acac)2(L2)]2+ are CoIII complexes with radical monocationic ligands, in line with the results from cyclic voltammetry and the Lewis structures in Scheme No EPR signal was detected for [Co(acac)G Figure . Similar2L](PF6) in2(L3)](PF6)2, confirming the absence of an organic radical and the formulation as a CoII complex with dicationic ligand unit L32+. In similarity to the neutral complexes, the absence of clear signals could be rationalized by signal broadening due to fast relaxation at room temperature. Interestingly, a weak and broad signal that might be assignable to an organic radical appeared in the 6 K EPR spectrum, together with weak, broad signals in the region 550–3000 G . In principal, we could not completely exclude that these signals arise from the presence of a minor impurity, but their absence in the room temperature spectra seems to be not in accordance with this explanation. Another possibility is a change in the electronic structure of the [Co(acac)2(L3)]2+ complex at very low temperature, that is also motivated by the calculated small energy difference between two redox isomers (see discussion below).On the other hand, no sharp signal appears in the room‐temperature EPR spectrum of +, [Co(acac)2(L2)]+, and [Co(acac)2(L3)]+ are also free of strong bands, arguing for metal‐centred oxidation (CoII→CoIII), in line with the results from cyclic voltammetry and EPR spectroscopy. The spectra recorded for the dications [Co(acac)2(L1)]2+ and [Co(acac)2(L2)]2+ obtained upon two‐electron oxidation contain a band at 364 nm and a broad absorption in the visible region with an absorption maximum at ca. 554 nm for [Co(acac)2(L1)]2+ and 503 nm for [Co(acac)2(L2)]2+, indicating the presence of a CoIII complex with radical monocationic ligand. On the other hand, bands at 295 and 450 nm in the spectrum of [Co(acac)2(L3)]2+ in CH2Cl2  in the visible region, being responsible for the red colour of the compounds. The visible regions in the spectra of the monocations [Co(acac)2L]+, [Co2(L1)] is displayed in Figure 2(L2)] is shown in the SI . In both compounds, the bisguanidine ligand binds with its two imino N atoms to the octahedrally coordinated CoII atom. The Co−N bond lengths measure 2.135(2)/2.149(2) Å for [Co(acac)2(L1)] and 2.085(2)/2.245(2) for [Co(acac)2(L2)]. Due to combined σ‐ and π‐contributions to the metal−guanidine bonding,2(L1)] and [Co(acac)2(L2)], respectively.2(L1)] to the monocation [Co(acac)2(L1)]+ and the dication [Co(acac)2(L1)]2+  to a low‐spin CoIII complex upon oxidation. The decrease of the Co−N bond lengths (by 0.183/0.189 Å) and Co−O bond lengths (by 0.163/0.187 Å) after two‐electron oxidation add up to an average bond length decrease of 0.180 Å, being similar to the value reported for VT between CoII and CoIII complexes with two oxolene ligand units.II complexes with reduced, neutral L1, measuring in average 1.321 Å, are elongated to an average value of 1.382 Å upon two‐electron oxidation. The N−C bonds connecting the guanidino groups to the C6 ring decrease from 1.408 Å before to 1.362 Å after two‐electron oxidation. In summary, the crystal structures fully support the Lewis structures in Scheme The Co−N and Co−O bond lengths decrease significantly from neutral [Co(acac)2L] to the 2L] to the 2(L3)], and [Co(acac)2(L3)](PF6)2. The Co–N distances slightly increase, from average 2.150 Å before to 2.175 Å after oxidation, and the Co–O distances slightly decrease, from average 2.083 Å before to 2.043 Å after oxidation, but these values remain in the region typical for CoII (Table 2(L3)]2+ is best described as a CoII complex with dicationic ligand unit L32+. To obtain structural information about the hydrogen atoms within the hydrogen bonds, the positions of the relevant hydrogens in [Co(acac)2(L3)]2+ and [Co(acac)2(L3)]2+ were taken from difference Fourier maps and freely refined. The experimental NH⋅⋅⋅O distances decrease from 2.22(6)/2.15(6) Å in [Co(acac)2(L3)] to 2.020(2) Å in [Co(acac)2(L3)]2+. The more reliable distances between the N and O atoms within the N−H⋅⋅⋅O hydrogen bonds decrease from 3.033(9)/2.966(8) Å in [Co(acac)2(L3)] to 2.762(2) Å in [Co(acac)2(L3)]2+ (see SI), a value that is quite small in comparison to previously reported interligand N−H⋅⋅⋅O hydrogen bonds.(PF6)2 and [Co(acac)2(L3)](PF6)2 to confirm the differences in the electronic structure ](PF6)2, the χ T value measures 0.181 cm3 K mol−1 at 300 K and slightly increases to 0.276 cm3 K mol−1 at 40 K, being close to the theoretical value of 0.375 cm3 K mol−1 obtained from the Curie law for a compound with one unpaired electron (S=1/2). Hence, the data are in line with the presence of low‐spin CoIII and L1.+.e Figure . For [Coχ T value of ca. 2.5 cm3 K mol−1 was found for [Co(acac)2(L3)](PF6)2 at 300 K. This large value fully supports the formulation as a high‐spin CoII complex (S=3/2) with dicationic ligand, L32+ and an unquenched orbital contribution. Due to significant spin‐orbit coupling (4T1g ground term in hO symmetry), it is higher than the spin‐only value for high‐spin CoII (S=3/2) of 1.876 cm3 K mol−1 predicted by the Curie law. The value decreases almost linearly with decreasing temperature until ca. 50 K  EPR spectrum of a frozen CH2Cl2 solution , and by the calculated small energy difference between two redox isomers (see below). However, it is not possible to reach a final conclusion on this point.A much higher omplexes.52, 53Quantum‐chemical calculations. Finally, B3LYP/def2‐TZVP calculations were carried out, the solvent effect being included by single‐point calculations with the conductor‐like screening model (COSMO). The calculated structures are in pleasing agreement with the experimentally derived structures in the solid state (see SI for details). In Figure 2(L1)]2+ and [Co(acac)2(L3)]2+. The calculations for the quartet state of [Co(acac)2(L1)]2+ did not converge to a CoII complex with dicationic ligand L12+, but instead to an intermediate‐spin CoIII complex with radical monocationic ligand L1.+  is preferred by ΔE=85 kJ mol−1 at ϵr=37.5 with respect to this intermediate‐spin CoIII redox isomer.II redox isomer of [Co(acac)2(L3)]2+ (S=3/2) with dicationic ligand, L32+, the N−H⋅⋅⋅O hydrogen bond lengths measure 1.824 Å, being significantly shorter than in the calculated CoIII redox isomer with radical monocationic L3.+ unit (2.061 Å). Additionally, the calculated distances between the N and the O atom in the N−H⋅⋅⋅O hydrogen bonds in the neutral and the dicationic complexes of L3 are similar to those found in the corresponding crystal structures. Hence experimental values of 3.033/2.966 and 2.762 Å were found for [Co(acac)2(L3)] and [Co(acac)2(L3)]2+, respectively, being close to the calculated values of 2.905/2.835 and 2.788 Å (see SI). The decrease of the hydrogen‐bond lengths signals a stronger interligand interaction, leading to a stabilization of the CoII redox isomer by the two strong interligand hydrogen bonds. Consequently, the calculations predict the otherwise highly unfavourable quartet spin state (being high‐energy states for complexes of L1 and L2) to be even slightly preferred over the doublet state (by 2 kJ mol−1) at ϵr=37.5 (CH3CN solution); without solvent effect the doublet state (CoIII redox isomer) is slightly preferred, but by not more than 10 kJ mol−1.In the calculated Co2(L1)]2+ in its S=3/2 state with free L3 to give [Co(acac)2(L3)]2+ in the S=3/2 ground state and free L1, a ΔG value (at 298 K) of −76 kJ mol−1 was obtained, underlining the massive stabilization of the S=3/2 state (CoII redox isomer) with respect to the S=1/2 state (CoIII redox isomer) for the complex with L3 by hydrogen‐bonding.The Gibbs free energy for a hypothetical ligand exchange reaction was calculated to further highlight the impact of hydrogen bonding Figure . For theIII complex with the neutral guanidine ligand leads to metal reduction and two‐electron oxidation of the guanidine ligand, resulting in a CoII complex with dicationic guanidine ligand unit. The results of this study demonstrate the possibility to alter the outcome of redox processes and to initiate intramolecular electron transfer (IET) by introducing intramolecular hydrogen‐bond interactions. Thereby, they pave the way to a sophisticated control of redox and IET processes. Nature extensively uses the weakening or strengthening of hydrogen bonding up to the point of proton transfer to trigger electron transfer,The first mononuclear, octahedrally coordinated cobalt complexes with redox‐active bisguanidine ligands were synthesized and their redox chemistry was studied. Three different redox‐active bisguanidines were used in this study, two of them with fully alkylated and one with partially alkylated guanidino groups. Interligand N−H⋅⋅⋅O hydrogen bonds between two guanidino N−H functions and oxygen atoms of the acetylacetonate (acac) co‐ligands are established in the Co–bisguanidine complex with partially alkylated guanidino groups. The increase of the hydrogen‐bond strength upon oxidation of this guanidine ligand triggers a redox‐induced electron transfer (RIET) that is absent in the complexes with the guanidine ligands having fully alkylated guanidino groups. Hence, one‐electron oxidation of the monocationic CoThe authors declare no conflict of interest.As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.SupplementaryClick here for additional data file.