self - organized tio2 nanotube 
 layers have attracted remarkable 
 attention within the past 15 years due their unique architecture , 
 high surface area , semiconductive properties , and biocompatibility . in addition , they are produced by a low - cost electrochemical anodization 
 of ti substrates in suitable electrolytes containing fluorides . all 
 these features enabled utilization of tio2 nanotube layers 
 in a wide number of applications as photocatalyst , anode 
 of dye - sensitized solar cells ( dssc ) and perovskite 
 solar cells ( psc ) , gas sensors , and biomedical materials , among others . 
 in all these applications , tio2 nanotube layers have shown 
 superior performance compared to other tio2 nanostructures . 
 in parallel , important efforts have been carried out to tune the nanotube 
 aspect ratio and to improve the nanotube ordering and crystallinity . annealing treatment of amorphous as - synthesized tio2 nanotube layers leads to their crystallization into anatase 
 ( > 280 c ) , a combination of anatase and rutile ( > 450 c ) , 
 or rutile ( > 550 c ) . the anatase nanotubular 
 structure has shown to be more favorable 
 than rutile for photoelectrochemically assisted applications , such 
 as photocatalysis and dssc . thus , the stability of anatase nanotubular structure is highly desired , 
 and numerous efforts have been focused on this target , especially 
 at high temperatures . the introduction of alloying elements as nb or c was reported 
 to induce a shift of the anatase to rutile transition ( further noted 
 as art ) to higher temperature and increased thermal resistance against 
 collapse . however , the main disadvantage of alloying is the formation 
 of undesired secondary impurity phases , e.g. , ti  nb2o5 . the art threshold depends 
 on whether the nanotube layers are attached or separated from the 
 ti substrate . high temperature stability ( up to 700 c ) of tio2 nanotube arrays , preserving the nanotubular integrity and 
 anatase structure , was reported for free - standing tio2 nanotube 
 arrays . for tio2 nanotube 
 layers attached to ti substrate , anatase structure and no structural 
 collapse is maintained either by a previous solvothermal 
 treatment or previous annealing at lower 
 temperature . the highest published temperature 
 without nanotube collapse ( 1048 c ) was reached during 
 the flame annealing process . however , 
 such flame high temperature processing led to undesired transition 
 to rutile structure and a significant uptake of carbon from the flame . 
 despite numerous efforts focused onto the high temperature stability 
 of tio2 nanotube layers , the temperature working window 
 is still restricted . another constrain for applications of tio2 nanotube layers is their limited chemical stability in harsh 
 acidic environments , where nanotube layers undergo chemical dissolution . the improvement of the thermal , chemical , and eventually also mechanical 
 properties of tio2 nanotube layers would enable their utilization 
 in previously nonimaginable working environments and surely interesting 
 expansion of their application range . in principle , addition of a 
 thin continuous coating of an appropriate secondary material ( with 
 excellent thermal and chemical stability ) within nanotubes should 
 significantly alter also their stabilities . so far , however , no such 
 treatment has been shown . to date , the atomic layer deposition ( ald ) 
 technique is the only method that enables homogeneous , continuous , 
 and conformal coating of secondary materials into tio2 nanotube 
 layers . deposition of al2o3 and zno coating by ald into tio2 nanotube layers has been reported 
 yielding interesting synergic effects . the resulting composite heterostructures 
 revealed significant improvement of their photovoltaic and photocatalytic 
 performance due to enhanced charge separation induced by coatings 
 of secondary materials . therefore , in the present work we investigated 
 the thermal , chemical , 
 and mechanical properties of self - organized tio2 nanotube 
 layers uniformly coated with al2o3 layers of 
 different nominal thicknesses : 1 , 10 , and 42 nm . these coatings were 
 carried out by atomic layer deposition ( ald ) using different number 
 of identical deposition cycles . after the al2o3 coating , the tio2 nanotube layers were annealed at temperature 
 up to 870 c for 1 h to evaluate their thermal stability . the 
 resulting crystal structure and composition were analyzed through 
 x - ray diffraction ( xrd ) and scanning transmission electron microscopy 
 ( stem ) . mechanical properties ( hardness ) were characterized by nanoindentation 
 measurements using an atomic force microscope . the chemical stability 
 was tested by soaking the al2o3-coated tio2 nanotube layers into h3po4 solutions 
 of different concentrations for 48 h at laboratory temperature and 
 for an additional 8 h in solutions with temperature of 60 c . self - organized 
 tio2 nanotube layers with a thickness 
 of 20 m and a nanotube diameter of 110 nm ( aspect 
 ratio 180 ) were fabricated via anodization of ti foils using 
 a previously published approach . prior 
 to anodization , the ti foils ( sigma - aldrich , 0.127 mm thick , 99.7% 
 purity ) were degreased by sonication in isopropanol and acetone , then 
 rinsed with isopropanol , and dried in air . the electrochemical setup 
 consisted of a two - electrode configuration using a platinum foil as 
 the counter electrode , while ti foils ( working electrodes ) were pressed 
 against an o - ring of the electrochemical cell , leaving 1 cm open to the electrolyte . a high - voltage potentiostat ( pgu-200 v , 
 ips elektroniklabor gmbh ) was employed to carry out the electrochemical 
 experiments at room temperature . ethylene glycol containing 1.5 vol 
 % deionized water and 176 mm nh4f was used as electrolyte . electrolytes were aged before the first use for 15 h by anodization 
 of blank ti foils at 60 v under the same conditions for the anodization 
 experiments ti foils were anodized for 
 4 h after sweeping the potential from 0 to 60 v with a sweeping rate 
 of 1 v / s . the tio2 nanotube layers 
 by were coated with al2o3 by atomic layer deposition 
 tool ( thermal ald , 
 tfs 200 , beneq ) . this technique based on sequential and self - limiting 
 gas  surface reactions allows conformal deposition of various 
 coatings within tio2 nanotube layers with a nanometer scale 
 accurate thickness , as shown previously . . grade , 99.999+% ) and 
 deionized water ( 18 m ) were used as aluminum and oxygen precursors , 
 respectively . under these deposition conditions , one growth ald cycle 
 was defined by the following sequence : tma pulse ( 1 s)n2 purge ( 3 s)h2o pulse ( 1 s)n2 purge ( 3 s ) . all processes were carried out at a temperature 
 of 200 c and using n2 ( 99.9999% ) as carrier gas at 
 a flow rate of 400 standard cubic centimeters per minute ( sccm ) . al2o3 deposition was carried out running 8 , 88 , and 
 366 ald cycles , leading to coatings of different nominal thicknesses : 
 1 , 10 , and 42 nm , respectively . the number of cycles required for 
 the different al2o3 thicknesses was estimated 
 from the growth per cycle value of the al2o3 process at 200 c ( 1.1  / cycle ) . the thicknesses 
 of al2o3 coatings were confirmed by variable 
 angle spectroscopic ellipsometry ( vase ellipsometer , j.a . woollam , 
 co. , inc . ) of al2o3 coatings on si wafers . upon the al2o3 coating process the annealing process was carried out in a muffle oven in 
 an air atmosphere applying a heating rate of 15 c / min , until 
 the target temperature ( 870 c ) was reached . the annealing process 
 proceeded at such temperature for 1 h. afterward , the layers were 
 allowed to naturally cool down . the morphology of the tio2 nanotube layers was characterized 
 by a field - emission sem ( fe - sem jeol jsm 7500f ) and a scanning transmission 
 electron microscope ( stem , fei tecnai f20 x - twin ) fitted with a high 
 angle annular dark field ( haadf ) detector and operating at 200 kv . the cross - sectional views were obtained from mechanically bent samples . 
 because of the rupture of the nanotube layers via this bending , it 
 was possible to visualize nanotubes within the layers and coatings 
 within nanotubes in various directions and nanotube layer depths . average values and standard 
 deviations were calculated from at least three different locations 
 with a high number of measurements ( n > 100 ) . diffraction analyses of the al2o3-coated 
 tio2 nanotube layers carried out using x - ray diffractometer 
 ( xrd , d8 advance , bruker axe ) using cu k radiation with secondary 
 graphite monochromator and na(tl)i scintillation detector . nanoindentation 
 measurements were performed to analyze the mechanical 
 properties ( hardness ) of the tio2 nanotube layers . they 
 were determined by an atomic force microscope ( afm , solver next , nt 
 mdt ) equipped with a nanoindentation head ns01ntf and a berkovich 
 type of tip ( three - sided pyramid geometry with a parameter of static 
 stiffness , k = 10.2  0.3 kn / m ) . the nanotube 
 layers were measured in longitudinal direction for compressive force 
 of 0.5 mn , loaded for 100 s. fused silica sio2 was used 
 as a calibration sample ( hardness , h = 9.5  
 0.5 gpa by iso 9450 - 76 ) . the penetration depth of the tip was up to 
 a maximum 10% of the total thickness of the nanotube layer the hardness 
 was evaluated at 30 different areas of each nanotube layer 
 to ensure statistically relevant data set / significant results . the chemical stability of al2o3-coated tio2 nanotube layers was analyzed by soaking in h3po4 solutions of different concentrations : 25 , 50 , 70 , and 85 
 wt % ( prepared from 85 wt % h3po4 , penta ) . tio2 nanotube layers were soaked in these solutions for 48 h , 
 including a thermal treatment for 8 h by which solutions were heated 
 up at 60 c to further study the nanotube chemical stability 
 under warm acidic conditions . before the subsequent sem analysis highly ordered tio2 nanotube layers , with 
 a thickness of 20 m and an average diameter value of 
 110 nm ( aspect ratio 180 ) , were prepared by anodization 
 of ti foils as described in detail in the experimental 
 section . as - prepared amorphous tio2 nanotube layers were coated with al2o3 of different nominal 
 thicknesses , namely 1 , 10 , and 42 nm by ald , as verified by sem and 
 ellipsometric measurements ( 1.1  0.2 , 10  0.5 , and 44 
  2.1 nm ) . freshly coated nanotube layers were annealed at 870 
 c for 1 h along with reference uncoated tio2 nanotube 
 layers . figure 1 shows 
 sem images of the tio2 nanotube layers with and without 
 al2o3 coating annealed at 870 c . uncoated 
 tio2 nanotube layers ( figure 1a ) collapsed during the annealing process 
 into a pillar nanostructure ( figure 1b ) . when coated , the nanotube layers were preserved 
 after the annealing process , regardless of the thickness of the al2o3 , as apparent for coatings of either 1 nm ( figure 1c , d ) , or 10 nm ( figure 1e , f ) . it is quite 
 fascinating that even 1 nm thin al2o3 coating 
 can built a very thermally robust cage all over tio2 nanotubes 
 with some 2040 nm thick tube walls . sem top - view images of 
 uncoated tio2 nanotube layer 
 ( a ) before and ( b ) after annealing ; al2o3 coated 
 ( 1 nm ) tio2 nanotube layer ( c ) before and ( d ) after annealing ; 
 al2o3 coated ( 10 nm ) tio2 nanotube 
 layer ( e ) before and ( f ) after annealing . the annealing was carried 
 out at 870 c for 1 h. insets : magnification of the corresponding 
 sem images . all the scale bars denote 100 nm . figure 2 shows 
 representative 
 stem - haadf images of the nanotube body ( separated from the annealed 
 al2o3 coated ( 10 nm ) tio2 nanotube 
 layer by mechanical bending of the layers followed by sonication in 
 methanol ) at a low ( a ) and at a high magnification ( b ) . especially 
 from figure 2b , the 
 interface between the tio2 wall and al2o3 coating is well distinguishable . there are actually two interfaces 
 between the tio2 wall and al2o3 coating , 
 as the al2o3 coating is deposited inside ( interior 
 coating ) and outside ( exterior coating ) the tio2 tube walls . this feature is in accordance with our previous ald work , where we showed very good uniformity of al2o3 coatings on the amorphous tubes and absence 
 of any pinholes in the coating . as apparent from figure 2 , al2o3 coatings remained continuous and pinhole - free even after annealing , 
 during which thermally induced crystallization of tio2 tube 
 walls occurred . some delamination of the coating seen at the outer 
 and inner interface between tio2 wall and al2o3 coating ( especially at figure 2b ) stems most likely from the stress that 
 these layers are exposed to during the preparation of specimens for 
 sem and stem observation , which includes mechanical rupture of layers . these roughening and delamination events have no detrimental effect 
 on coated nanotube layers that were not submitted for sem and stem 
 and that completely survived soaking in h3po4 solutions ( described later in text ) . representative stem - haadf 
 images of ( a ) a fragment of al2o3 coated ( 10 
 nm ) tio2 nanotube and ( b ) the 
 corresponding higher magnification of the nanotube wall . interfaces 
 between individual parts of the tubes are distinguished by solid lines 
 and appropriate description . it is 
 generally accepted that the annealing process influences crystal structure , 
 phase transition , and structural integrity of the tio2 nanotube 
 layers . it has also been accepted that amorphous as - prepared 
 tio2 nanotube layers crystallize into anatase above 280 
 c in air . the anatase to rutile 
 transition ( art ) has been reported at different temperatures , most 
 usually in the range of temperature between 500 and 600 c , depending 
 on the nanotube dimensions ( diameter , thickness , and composition ) . annealing at temperatures higher 600 c leads to the 
 coexistence of anatase and rutile structures , while total conversion 
 to rutile structure takes place above 800 c . figure 3a shows the 
 xrd pattern obtained for reference uncoated tio2 nanotube 
 layers annealed for 1 h at either 400 or 870 c , respectively . 
 in line with literature , the former exhibits pure anatase crystal 
 phase identified by typical anatase peaks associated with planes ( 101 ) , 
 ( 004 ) , ( 105 ) , and ( 211 ) , with a dominant orientation ( 101 ) . the latter 
 reveals pure rutile crystal phase with well - defined diffraction peaks 
 of planes ( 110 ) , ( 011 ) , ( 111 ) , ( 211 ) , and ( 220 ) . the intensity of 
 the peak at 2 = 27.4 indicates a preferred orientation 
 along the ( 110 ) direction ; no trace of anatase polymorphic phase is 
 detected . xrd patterns of ( a ) uncoated tio2 nanotube layers annealed 
 at 870 and 400 c for 1 h ; ( b ) al2o3-coated 
 tio2 nanotube layers with different coating thicknesses 
 ( 1 , 10 , and 42 nm ) annealed at 870 c for 1 h ; and ( c ) al2o3-coated ( 1 and 10 nm ) tio2 nanotube 
 layers preannealed ( 400 c , 1 h ) and second annealing at 870 
 c . a = anatase , r = rutile , and t = titanium substrate . the xrd patterns , obtained from 
 al2o3-coated 
 tio2 nanotube layers annealed at 870 c for 1 h , are 
 shown in figure 3b . therein the coexistence of anatase and rutile structures can be clearly 
 seen , in contrast to the uncoated nanotube layers ( figure 3a ) where rutile was exclusively 
 formed at this temperature , in line with the previous work . the incomplete art of coated tio2 nanotube layers annealed at 870 c stems from the hindered 
 surface reconstruction of the tio2 due to al2o3 coating and also the impact of al as the 
 phase transformation inhibitor . in contrast , 
 for uncoated nanotube layers ( annealed at 870 c ) the surface s 
 reconstruction can easily take place ( no space constrains are present ) , 
 allowing for the mass flow and rearrangements that yield complete 
 rutile conversion , expected at this temperature . another evidence for limited reconstruction of coated tio2 nanotube layers annealed at 870 c is the fact that 
 they do not collapse ( sinter ) , which they would otherwise do without coating . in the case of thermally 
 stable coated nanotube layers , the coating acts in similar fashion 
 as it does for nanoparticles and nanorods that can be annealed , when 
 coated , at high temperatures without undergoing sintering events . quantification of the content of each crystal phase and an average 
 of the corresponding crystallite size are given in table 1 . the anatase : rutile ratio was found 
 to be dependent on the al2o3 coating . the nanotube 
 layer with the thickest 
 al2o3 coating ( 42 nm ) exhibited dominant rutile 
 structure ( 62% ) with peaks corresponding to the planes ( 110 ) , ( 001 ) , 
 ( 111 ) , ( 211 ) , and ( 220 ) and only one minor anatase peak corresponding 
 to ( 101 ) plane . in clear contrast , the anatase content for 10 and 
 1 nm thin al2o3 coating was found to increase 
 up to 73% and 83% , respectively , on account of rutile . in addition , 
 identical rutile peaks were revealed for them as for the nanotube 
 layer with thickest al2o3 coating ( 42 nm ) . it 
 is noteworthy that the 1 nm thin al2o3 coated 
 nanotube layer revealed anatase peaks with preferential orientation 
 along the ( 004 ) plane , instead of usual ( 101 ) plane . who associated a particular thermal stability of the nanotube layers 
 to such anatase ( 004 ) plane . based on the xrd results in figure 3b , there is obvious 
 retardation of the anatase to rutile transition ( further noted as 
 art ) with decreasing thickness of al2o3 coatings 
 ( 1 and 10 nm ) . in order to give a physical description of the 
 results , factors 
 affecting the art first , we should consider 
 the influence of the number of oxygen vacancies within the tio2 on the art temperature . reported that the larger is the number of oxygen vacancies within 
 tio2 , the lower is its art temperature , or the art does 
 not proceed at all and tio2 remains in anatase form . the 
 defects ( in this case oxygen vacancies ) provide a low energy mass 
 transport route and lower such art temperature . the number of oxygen vacancies within tio2 is 
 also strongly influenced by the annealing atmosphere . previous works 
 clearly indicated that dry annealing atmospheres such as ar or co led to more 
 oxygen vacancies in tio2 than those performed in o2 or air . thus , the art was enhanced in oxygen - free atmospheres 
 and resulted into larger rutile crystallites . second , it is important to define the fundamental reasoning 
 behind 
 the art origin , which has been subject of controversy . tio2 nanotube interface , where ti metal 
 would be directly thermally oxidized into rutile structure . the presence 
 of oxygen vacancies in between the ti substrate and tio2 nanotube layer was believed to induce the art at such interface , 
 spreading toward the whole nanotube walls in the course of time , as 
 reported by zhu et al . in contrast , yu 
 et al . proposed that art does not stem 
 from metal ti , but from the anatase ( created at lower temperatures , 
 while ramping up the temperature ) at the interface between tio2 nanotubes and ti substrate , which converts to rutile at temperatures 
 600 c . in addition , works on annealing and crystallization 
 of free - standing nanotube layers ( i.e. , nanotubes 
 were detached from the ti substrate before annealing ) reported both 
 the preservation of the anatase structure in the nanotube walls at 
 temperatures higher than 600 c and much higher triggering art 
 temperature . those results would point on a significant role of metal 
 ti substrate  tio2 nanotube interface on the art . there is a clear link between the experimental results obtained 
 in this work and the literature about factors affecting the art . according to figure 3 , the anatase : rutile ratio is clearly dependent on the al2o3 coating thickness . that larger number of oxygen vacancies 
 promotes the art , it is clear that the al2o3 coating within our tio2 nanotube layers influences the 
 number of oxygen vacancies as it possesses a barrier against the oxygen 
 diffusion . for example , the thickest 
 al2o3 coating ( 42 nm ) hinders the oxygen diffusion 
 most significantly from all used coatings in this work , leads to highest 
 number of oxygen vacancies within tio2 nanotubes , and boosts 
 the art process that ends up with the highest rutile content . in contrast , 
 the oxygen diffusion into tio2 takes place more easily 
 through thinner al2o3 coatings ( 1 and 10 nm ) , 
 resulting in a lower number of oxygen vacancies , retarding the art 
 within tio2 nanotubes . in addition , the largest rutile 
 crystal size , calculated by the scherrer equation ( table 1 ) , corresponds to the thickest 
 al2o3 coating , which also corroborates previously 
 published findings on art and size of rutile crystals . to get a complete picture about the art , we also fully explored 
 anatase tio2 nanotube layers ( annealed at 400 c for 
 1 h ) , shown in figure 3a , for al2o3 coating . we coated these nanotube 
 layers with 1 and 10 nm of al2o3 by ald , before 
 undergoing a second thermal treatment at 870 c for 1 h. first , 
 the figure 3c shows 
 that the al2o3-coated ( 10 nm ) tio2 nanotube layer consisted of 100% rutile , while in the al2o3-coated ( 1 nm ) tio2 nanotube layer rutile 
 content was only 26% . these results confirm the active role of the 
 al2o3 coating for tio2 crystal structure 
 and are in line with the results and theories discussed in figure 3b . second , the significantly 
 different crystal structure of the both investigated types of al2o3 ( 10 nm ) tio2 nanotube layer were 
 revealed . the formerly annealed al2o3 coated 
 ( 10 nm ) tio2 nanotube layer ( fully anatase comprised ) underwent 
 a complete art and was 100% rutile comprised ( see figure 3c ) . in contrast , the initially 
 amorphous tio2 nanotube layer revealed a predominant anatase 
 content of 74% ( see figure 3b ) . this comparison clearly confirms that the tio2 structure influences the art and that it is clearly promoted 
 for the tio2 nanotube layers annealed to anatase before 
 art and ald coating . in other words , the lack of coating induces during 
 the thermal annealing to 400 c more oxygen vacancies in the 
 tio2 nanotube layers than it does when coatings are present 
 during this annealing step . the mechanical 
 integrity of the 
 tio2 nanotube layers is of significant importance , especially 
 for synthesis of devices based on flow - through membranes utilizing 
 nanotube layers opened on both sides . even though some nanoindentation analyses of the tio2 nanotube arrays were already carried out , nanotube layers modified with additional coatings , as in the present 
 case , have not yet been analyzed . figure 4 shows hardness of tio2 nanotube 
 layers with al2o3 coatings of different thicknesses 
 as well as two reference nanotube layers . if not denoted otherwise , 
 all nanotube layers were annealed at 870 c for 1 h as the last 
 processing step . first , the uncoated amorphous ( i.e. , did not undergo annealing ) tio2 nanotube layer displayed lower hardness value than the annealed 
 uncoated counterpart , fully rutile structure comprised . that was expected 
 as the crystal structure has ( as a rule of thumb ) higher hardness 
 than amorphous mass of the same compound . second , the annealed al2o3-coated tio2 nanotube layers exhibited 
 larger hardness with the increasing al2o3 coating 
 thickness . this can be ascribed to increasing content of rutile ( table 1 ) and to an increasing 
 al2o3 mass within the nanotubes . , 
 all nanotube layers were annealed at 870 c for 1 h. even though rutile and anatase are similar in structure , 
 the reason 
 to rutile to be more mechanically robust than anatase is that its 
 octahedra shares four edges instead four corners ( anatase case ) , which 
 leads to the formation of chains arranged subsequently in a 4-fold 
 arrangement . this also explains why the al2o3-coated ( 1 nm ) tio2 coated layer ( which 
 has mainly anatase structure as shown in figure 3b ) has lower hardness than uncoated annealed 
 layers ( completely rutile based , also shown in figure 3a ) . the hardness values presented here 
 for high aspect ratio ( 180 ) 
 nanotube layers were larger than those found in the literature that 
 reports typical hardness in the range from 94 mpa to 3.5 gpa . however , it is difficult to establish a comparison , principally 
 because the published reports show results for exclusively uncoated 
 and lower aspect ratio tio2 nanotubes layers with thicknesses 
 from 625 nm to 8.5 m ( compared to 20 m in the 
 present case ) . moreover , the use of different indenter tips ( vickers 
 tips for microhardness , berkovitch tips for nanohardness , cube corner 
 tips for nanohardness , etc . ) entails different hardness values . nevertheless , 
 it can be concluded that both the annealing treatment and the al2o3 ald coating resulted in a substantial enhancement 
 of the mechanical properties of tio2 nanotube layer . finally , the chemical stability 
 of al2o3-coated tio2 nanotube layers 
 was investigated in strongly acidic environment , namely in h3po4 solutions with different concentration . such stability 
 is significant for the nanotube layers to sustain in environments , 
 where up to now they could not preserve their morphological integrity . for example , in various biological environments with low ph , the knowledge 
 about the stability threshold is important . as it can be seen in figure 5 , even the thinnest 
 coating ( 1 nm al2o3 ) completely preserved the 
 tio2 nanotube layers from degradation in concentrated h3po4 . in line with that no degradation was observed 
 for any of the thicker al2o3 coatings : 10 and 
 42 nm ( data not shown here ) . the soaking tests were performed on the 
 40 h time scale for all nanotube layers . this expands the already wide range 
 of environments , where ald al2o3 coatings are 
 stable , in addition to published stability results of these coatings 
 in various acidic ( h2so4 , hno3 , hcl ) 
 and alkaline ( koh ) environments and water . to make the h3po4 environment 
 even more harsh , the h3po4 solutions were heated 
 up to 60 c , and soaking was carried out for an additional 8 
 h ( in total 48 h ) . since again no visible changes were observed , soaking 
 experiments were terminated afterward . reference uncoated layers ( namely 
 as - anodized amorphous and annealed ( 400 c for 1 h ) nanotube 
 layers ) did not survive these conditions . in order to determine the 
 chemical threshold conditions for these reference uncoated layers , 
 lower h3po4 concentrations had to be used . the 
 stability threshold was revealed to be 10 wt % ( figure 5e ) and 40 wt % ( figure 5f ) on the scale of 24 h for the amorphous 
 and annealed case , respectively , without any heating . all in all , 
 the results presented in figure 5 for al2o3-coated nanotube layers 
 confirm the outstanding enhancement of the chemical stability of tio2 nanotube layers provided by uniform al2o3 coatings . sem top - view images of annealed al2o3-coated 
 ( 1 nm ) tio2 nanotube layers before ( a ) and after soaking 
 in h3po4 solutions with different concentrations : 
 ( b ) 50 wt % , ( c ) 70 wt % , and ( d ) 85 wt % in total for 48 h ( last 
 8 h at 60 c ) . sem top - view images of reference uncoated amorphous 
 ( e ) and anatase ( f ) tio2 nanotube layers after soaking 
 in h3po4 solutions of 10 and 40 wt % , respectively , 
 for 24 h. all the scale bars denote 100 nm . thus , the present results , especially for the thinnest al2o3 coating ( 1 nm ) , are very useful and promising 
 for practical 
 applications of the nanotube layers . as - treated tio2 nanotube 
 layers : ( i ) maintain anatase structure ( more favorable for photovoltaics 
 and photocatalysis than rutile ) in the tubes over a very broad temperature 
 range , ( ii ) possess significantly improved charge separation on the 
 interface with various electrolytes ( especially because electrons 
 can tunnel to tio2 via al2o3 coatings 
 thinner than 2 nm ) , ( iii ) possess 
 strong mechanical integrity , and ( iv ) provide extremely good stability 
 in strongly acidic environments . all these features pave favorable 
 way for the functionalization of tio2 nanotube layers by 
 secondary materials . it is foreseen that additional materials , such 
 us various oxides , nitrides , sulfides , etc . thermal 
 and chemical stability of al2o3-coated tio2 nanotube layers can extend the utilization of nanotube layers 
 for catalytic applications and sensing of gases ( such as co , nox , ch3ch2oh , h2 , and o2 ) at high temperatures and/or in harsh acidic 
 environment , so far unfeasible for uncoated tio2 nanotube 
 layers counterparts . in parallel , membranes composed of ultrahigh 
 aspect ratio tio2 nanotube layers that have been used for 
 photocatalytic or flow - through experiments may be prone to mechanical instabilities . thus , they could greatly 
 benefit from a thin al2o3 coating to become 
 mechanically more robust . in 
 this work , effects of al2o3 coating produced 
 by ald on the crystal structure , mechanical , and chemical properties 
 of tio2 nanotube layers were explored . noteworthy improvement 
 of the thermal stability upon annealing in air was revealed up to 
 temperatures of 870 c , even with an extremely thin al2o3 coating ( 1 nm ) . in contrast to uncoated tio2 nanotube layers ( 100 vol % rutile ) , a high fraction of anatase structure 
 ( 83 vol % ) was determined for al2o3-coated ( 1 
 nm ) tio2 nanotube layers upon annealing at 870 c , 
 which is highly desired due to its optical and electronic properties 
 for photovoltaic and photocatalytic applications . an enhanced hardness 
 was revealed for al2o3-coated tio2 nanotube layers with a positive impact on the mechanical properties 
 of nanotube layers . in addition , al2o3 coatings 
 provided to the tio2 nanotube layers extremely good stability 
 in extremely acidic environments of h3po4 solutions 
 with different concentrations . all in all , self - organized tio2 nanotube layers coated with thin al2o3 coatings yield superior thermal , chemical , and mechanical stabilities 
 that will extend their application range to previously nonimaginable 
 working environments .