nowadays , one - dimensional nanostructural materials ( nanorods , nanowires , nanobelts and nanotubes ) have attracted heightened attention because of their potential applications in nanodevices and functional materials . continuous effort has been devoted to the investigation on the crystalline phases of the layered transition metal dichalcogenides , because of their unusual crystal structure and unique physical and chemical properties . in analogy to graphite , nanoparticles of many inorganic compounds such as mos2 , ws2 are not stable against folding , and can adopt nanotubular and fullerene - like structures . as transition metal sulfides , mos2 has been the subject of significant research for applications including nonaqueous lithium batteries , catalytic hydride sulfurization of petroleum and wear resistance . for each of these applications , the important processes occur either at the surface or at the exposed edges of the mos2 layers . mos2 is a layered semiconductor ( band gap = 1.2 ev , indirect ) that resists oxidation even in moist air at temperatures up to 85 c . recent studies have suggested that reducing the size of the mos2 crystals can improve their lubrication properties in bearings , o - rings or other heavy  wear applications . inorganic syntheses using organic surfactants are successfully applied for the preparation of various nanostructural materials . various methods of preparation includes chemical vapor deposition , sol  gel processing , spray pyrolysis , co - precipitation , sonochemical synthesis , metathesis reactions , two step electrochemical synthesis etc are having been in practice [ 9 - 12 ] . tenne and co - workers   first reported the production of fullerene - like mos2 nanotubes via the gas phase reaction between moo3x and h2s in a reducing atmosphere at elevated temperature ( 8001000 c ) . rao and co - workers obtained mos2 nanotubes by simple heating mos3 in a stream of hydrogen under high temperature . synthesized fibers and tubules of mos2 by the thermal decomposition of ammonium thiomolybdate precursors at 450 c . mos2 nanotubes   were made by subliming 2h  mos2 powder in presence of h2s at 1300 c . recently mos2 nanorods   were synthesized using -moo3 nanorods with a mixture of h2s  h2 ( 15 vol% of h2s ) for 4 h at several temperatures . however , almost all the methods to obtain mos2 nanotubes and nanorods utilize high temperature gas - solid reaction under reducing gas atmosphere . synthesized mos2 nanorods and nanospheres by hydrothermal method using ammonium molybdate with na2s in presence of nh2oh  hcl as oxidizing agent . hydrothermal synthesis is becoming popular for environmental reason , since water is used as reaction solvent than organics . this method has been widely used to prepare nanostructures due to its simplicity , high efficiency and low cost . in this article , as a part of our recent efforts on the investigation of 1-d materials , we report a simple low temperature hydrothermal method for preparation of mos2 nanofiber bundles with morphology control . we examined the effect of precursors ( ammonium molybdate , sodium sulphide and citric acid ) concentration , duration and the temperature on the morphology of the product . the hydrothermal process was carried out like our previous report   to prepare mos2 nanofiber bundles . in a typical synthesis of mos2 nanofibers , sodium sulfide and h2s gas were used as sulfur sources in order to investigate the source effect on the morphology of the product . in the first trail 1.235 g ammonium heptamolybdate tetrahydrate ( ( nh4)6mo7o24  4h2o ) was put into 25 ml distilled water under continuous stirring . after 10 min , 0.5 g citric acid monohydrate ( c6h8o7  h2o ) was added to the above solution . to this solution mixture , 0.312 g sodium sulfide crystals were added . the initial greenish black color was changed to red after 1h stirring on a magnetic stirrer . in the subsequent trail , 0.5 g ammonium heptamolybdate tetrahydrate was put into 25 ml distilled water under continuous stirring . gas is passed in to the warmed acidified with dilute hcl solution for about 58 min . the sealed 25 ml teflon lined autoclaves containing above solution mixture were placed for hydrothermal treatment at 180 c for several hours . the autoclaves were cooled and the resulted black solid was retrieved from the solution by centrifugation , washed with distilled water followed by ethanol to remove the ions possibly remaining in the end product , and finally dried in air . powder x - ray diffraction data were recorded on philips xpert pro x - ray diffractometer with graphite monochromatized cu k radiation ( ( = 1.541  ) . the fourier transform infrared spectrum of the samples was collected using nicollet ftir spectrometer . scanning electron micrograph images were taken with jeol ( jsm840 a ) scanning electron microscope . x - ray photoelectron spectroscopy was carried out on an esca-3 mark ii x - ray photoelectron spectrometer , ( vg scientific , uk ) using al k radiation ( 1486.6 ev ) as the exciting source . the morphology of the samples was observed using ( h-800em japan hitachi ) transmission electron microscope ( tem ) equipped with eds ( kevex sigma tm quasar , usa ) to measure the elements contained in the samples . the hydrothermal process was carried out like our previous report   to prepare mos2 nanofiber bundles . in a typical synthesis of mos2 nanofibers , sodium sulfide and h2s gas were used as sulfur sources in order to investigate the source effect on the morphology of the product . in the first trail 1.235 g ammonium heptamolybdate tetrahydrate ( ( nh4)6mo7o24  4h2o ) was put into 25 ml distilled water under continuous stirring . after 10 min , 0.5 g citric acid monohydrate ( c6h8o7  h2o ) was added to the above solution . to this solution mixture , 0.312 g sodium sulfide crystals were added . the initial greenish black color was changed to red after 1h stirring on a magnetic stirrer . in the subsequent trail , 0.5 g ammonium heptamolybdate tetrahydrate was put into 25 ml distilled water under continuous stirring . gas is passed in to the warmed acidified with dilute hcl solution for about 58 min . the sealed 25 ml teflon lined autoclaves containing above solution mixture were placed for hydrothermal treatment at 180 c for several hours . the autoclaves were cooled and the resulted black solid was retrieved from the solution by centrifugation , washed with distilled water followed by ethanol to remove the ions possibly remaining in the end product , and finally dried in air . powder x - ray diffraction data were recorded on philips xpert pro x - ray diffractometer with graphite monochromatized cu k radiation ( ( = 1.541  ) . the fourier transform infrared spectrum of the samples was collected using nicollet ftir spectrometer . scanning electron micrograph images were taken with jeol ( jsm840 a ) scanning electron microscope . x - ray photoelectron spectroscopy was carried out on an esca-3 mark ii x - ray photoelectron spectrometer , ( vg scientific , uk ) using al k radiation ( 1486.6 ev ) as the exciting source . the morphology of the samples was observed using ( h-800em japan hitachi ) transmission electron microscope ( tem ) equipped with eds ( kevex sigma tm quasar , usa ) to measure the elements contained in the samples . figure 1a , b shows the pxrd patterns of the samples prepared using sodium sulfide and h2 as sulfur sources in acidic medium at 180 c for 48 and 24 h , respectively . it shows only a broad , weak envelope beginning at 2 = 30 and continuing out above 60 , the two maxima approximately located at the 100 and 110 positions of bulk 2h  mos2 . the pxrd patterns resemble that of the single - layer mos2 prepared by exfoliation method . all the diffractions can be readily indexed as hexagonal 2h  mos2 [ space group p63/mmc ( 194 ) ] with lattice constants a = 3.16116  and c = 12.2985  , identical to the reported data in the jcpds cards ( 371492 ) . at 180 c the absence of the 002diffraction at 2 = 14.4 suggests that absence of stacking , thus the sample should be an aggregate of single sheets of mos2 . powder xrd pattern of mos2 nanofiber bundles prepared at ( a ) 180 c for 48 h using na2s as sulfur source with 0.5 g citric acid . ( c ) 180 c for 48 h using na2s as sulfur source without citric acid . ( d ) 180 c for 48 h using na2s as sulfur source with 1 g citric acid chemical analysis using eds indicates the presence of mo and s ( elements of mos2 ) , and no other elements existed as observed in fig . furthermore , the quantification of the peaks shows that the atomic ratio of s to mo is 1.92 , which is very close to the stoichiometric mos2 . from the ir spectrum ( fig . 3 ) , it is observed that the band at 480 cm corresponds to as(mo o vibrations and they indicate that oxygen is present in similar types of coordination as in moo3 . we assign the band at 941 cm to the as(mo  o ) vibrations of terminal ( mo = o ) group and 693 cm to (mo  o ) vibrations . the frequency of the vibration is shifted to the lower wavenumber compare to its standard value in crystalline moo3 , due to the presence of reduced mo centers and sulfur as a ligand . eds pattern of mos2 nanofiber bundles prepared at 180 c for 48 h ftir spectrum of mos2 nanofiber bundles prepared at 180 c for 48 h x - ray photoelectron spectra of mo(3d ) and s(2p ) core level regions of as - prepared nanoparticles are shown in fig . mo(3d ) peaks in as - prepared nanoparticles were deconvoluted into sets of spin - orbit doublet . accordingly , the high intense mo(3d5/2 ) peaks at 229.1 and low intense mo(3d3/2 ) peak at 232.1 ev could be attributed to mos2 . whereas lower binding energy shoulder peak at 226.25 ev is because of s(2s ) . xps pattern of mos2 nanofiber bundles prepared at 180 c for 48 h xps data of mo(3d ) and s(2p ) whm = full width half maximum figure 4b shows s(2p ) doublet peak ( 2p3/2 and 2p1/2 ) at 161.25 and 162.94 ev respectively in which 2p3/2 and 2p1/2 orbital peaks coincide with each other . it is to be noted that the peak positions of s(2p ) and mo(3d5/2 ) for mos2 is in good agreement with he results on mos2 . the chemical shifts of mo(3d5/2 ) and s(2p ) in mos2 nanoparticle is + 1.6 and 2.2 ev in comparison to elemental mo and s . therefore there occurs some partial electron transfer from molybdenum to sulfur . as a result , molybdenum becomes positively charged as shown by the increase in binding energy and sulfur becomes negatively charged as shown by the decrease in binding energy . mos2nanofiber of about 120300 nm in diameter and 2025 m in length are identified from sem images ( fig . they are well aligned and occur in - group of bundles with very high aspect ratio . it can be seen that the nanofiber self assembled into bundles and the nanofiber in a bundles had the same growth direction . the morphology and structure of individual mos2 nanorods have been characterized in further detail using tem , which are shown in fig . 6 . it can be seen that the nanorods are straight and uniform along their entire length and thickness of about up to 300 nm . figure 6d is the high resolution tem image of the mos2 nanorods and this image shows solid nanorods are uniform in thickness . sem images of mos2 nanofiber bundles prepared at 180 c for 48 h tem images of mos2 nanofiber bundles prepared at 180 c for 48 h the reduction of transition metal ions by surfactant is useful for the production of ultra fine particles of metal sulfides . unfortunately , molybdic compound reduction chemistry is quite complex and the nature of the products strongly depends on the reaction conditions . for example , afanasiev et al . show that the reduction of aqueous ammonium molybdate at different temperatures and conditions yields binary sulfides such as mos5.6 , mos6 , and mos3 as well as binary oxides such as moo3 and moo2 . in fact , the mechanism differs markedly in aqueous and in nonaqueous solutions . in order to disclose the transition process from ( nh4)6mo7o24  4h2o to mos2 , the role of citric acid [ 30 - 32 ] on phase of products was investigated . the possible reaction mechanism can be written as 
 ( 1 ) step 1 is a sulfureted reaction , in which an intermediate phase ( nh4)2mos4 is produced , and step 2 is a reduction reaction under hydrothermal conditions . thus by the addition of 1 g of citric acid or without citric acid , sulfureted reaction and reduction reaction do not react completely so that the products consist of much moo3 phase as shown in the fig . 1c , d. therefore during the formation of mos2 nanofiber bundles , citric acid plays a very prominent role . figure 7a , b gives the sem images of the product prepared with and without citric acid . sem images of mos2 nanofiber bundles prepared at ( a ) 180 c for 48 h with 1 g citric acid and ( b ) without citric acid the morphology of the product prepared at 180 c for 24 h ( fig . 8) using h2 resembles with the morphology of the product prepared at 180 c for 48 h ( fig . the thickness of the fiber is found to be about 300 nm and length is about 12 m . from the sem images , we observed that the length of all fibers is found to be same with sharp ends having voids in each mos2 bundles . sem images of mos2 nanofiber bundles prepared at 180 c for 24 h using h2s as sulfur source the possible reaction mechanism can be written when h2s as sulfur source is(2)(3 ) step 3 is a reduction reaction , in which an intermediate phase mocl4 is produced , and step 4 is a sulfureted reaction under hydrothermal conditions . figure 9a , b shows the sem images of mos2 prepared at 180 c for 16 and 20 h. when the duration is increased to 48 h , nanorods ( fig . the thickness of the nanorods is found to be in the range of 200600 nm . sem images of mos2 nanofiber bundles prepared at 180 c h using h2s for ( a ) 16 h ( b ) 20 h and ( c ) 48 h nanofiber of mos2bundles with 150300 nm in diameters and 2025 m in length have been successfully synthesized by a simple low temperature hydrothermal method . pxrd pattern indicated the amorphous mos2was made up of single layer and layer stacking had not taken place . the nanofiber self assembled into bundles and the nanofiber bundle had the same growth direction . sem study reveals the formation of bundles of well - aligned mos2nanofiber with very high aspect ratio . since the mos2nanomaterials are often very difficult to prepare and the search for more simple routine is quite valuable with regard to energy - saving and less pollution . it can be demonstrated that the optimum conditions for preparing mos2nanofiber are at 180 c for 48 h using citric acid and 180 c for 24 h by passing h2s in presence of hcl . under these conditions the author g. t. chandrappa thankful to the department of science and technology , nsti phase - ii , new delhi , government of india for financial support to carryout this research work . we also thank the central facility , dept . of physics and dept . of metallurgy ,