semiconductor nanostructures have been attracting worldwide attention due to their exceptional electrical , optical , and magnetic properties , and their potential applications in nanoscale electronics , photonics , and functional materials as well [ 1 - 3 ] . among them , tin sulfide ( sns ) has sparked intensive interest for its semiconducting and optical properties . vi group semiconductors , exhibits both the p- and n - type conduction , has an energy band gap of about 1.3 ev . normally , sns is composed of double layers of tightly bound sn  s atoms and the bonding between layers are extremely weak van der vaals forces , which has an orthorhombic structure . additionally , sns has the advantage of its constituent elements being abundant in nature and not posing any health and environmental hazards . therefore , sns has a big potential to be used as solar absorber in a thin film solar cell and near - infrared detector , as photovoltaic materials , and as a holographic recording medium . therefore , single crystalline sns nanowires reported in this paper are expected to offer enhanced properties . therefore , it is important to investigate practical synthesis routes for novel sns nanostructures , especially in single crystalline . crystalline tin sulfides have been prepared by a variety of methods , such as direct vapor transport method , stoichiometric composition technique , physical vapor transport method , and bridgman  stockbarger technique . in recent years , thin films of sns have been investigated widely due to their applications in photovoltaic and photoelectrochemical solar cells . sns thin films have been prepared by spray pyrolytic deposition , electrochemical deposition , chemical vapor deposition , and chemical bath deposition . to our knowledge , panda et al .   has reported surfactant - assisted synthesis of sns nanowires grown on tin foils and sns nanorods were reported by biswas et al . . we had used the anodic aluminum oxide ( aao ) template synthesized from some metal sulfide nanowire arrays   and in this paper , we have presented single crystalline sns nanowires prepared by template - assisted electrochemical deposition . the highly ordered porous aao films were prepared by anodizing an aluminum foil ( 99.999% ) in an acid solution using a two - step anodizing process , which could be seen in the refs . [ 19 - 22 ] . a three - electrode cell was used for pulse electrochemical deposition : a saturated calomel electrode ( sce ) as the reference electrode , an aao template with aluminum substrate as the working electrode ( cathode ) , and a platinum sheet as the contrary electrode ( anode ) . the potential applied to the cathode was pulsed - form , its  on  potential von was 10 v and  off  potential voff was 0 v , both  on  time and  off  time were 10 s in all the voltage conditions . after deposition experiment , the deposited sample was washed softly in pure water , and naturally dried in air . all the chemicals used were analytical grade reagents and the water used was deionized distilled water . the phase purity of as - synthesized product was examined by x - ray diffraction ( xrd ) using rigaku rint-2000 diffractometer with monochromatized cukradiation (  = 0.15405 nm ) . the nano / microstructure of the sns product was further observed by transmission electron microscope ( tem ) and field - emission scanning electron microscope ( fesem ) with an energy dispersive spectrometer ( eds ) analysis attachment , which were performed on a hitachi model h-800 ( 200 kv ) and a field - emission microscope ( s-4800 , 15 kv ) , respectively . the high resolution transmission electron microscope ( hrtem ) image and the corresponding selected area electron diffraction ( saed ) pattern were taken by a jeol-2010 tem with an accelerating voltage of 200 kv . vis  nir absorption spectra were measured at room temperature with a cary 5000 uv  vis  nir spectrometer . the typical xrd pattern of the sample which etched for 5 h is shown in fig the peaks at about 30.44 , 32.06 , 54.42 , and 64.14 were assigned to ( 101 ) , ( 040 ) , ( 240 ) , and ( 212 ) diffraction of orthorhombic sns , respectively . all the peaks can be indexed to orthorhombic sns with lattice constants : a = 4.34  , b = 11.14  , and c = 3.96  , which is in good agreement with the values from the standard card ( jcpds no . it can be seen that the major peak ( 101 ) is strongly dominating other peaks indicating the preferred orientation . the sharp and narrow ( 101 ) peaks indicate that the nanowires are highly crystalline and consist of only a single compositional phase . xrd patterns of the sns nanowire arrays after etching time of 5 h figure 2a shows a typical sem micrograph of the aao template , anodized using 0.3 m h2so4electrolyte at 0 c and a voltage of 20 v. it was found that the nanopores of the aao template are uniform and highly ordered with average diameters of 50  4 nm and the interpore distance is about 30 nm . in addition , the varied diameters and lengths of nanopores can be obtained by adjusting the varied acid , anodizing time , and anodizing temperature . figure 2b  d show the sem images of sns nanowires grown in aao template . these photographs indicate that the nanowires are uniformly distributed , highly ordered , and parallel to each other . figure 2b , c is a planform , from which we can find several clusters of nanowires . the clusters may result from the situation that the nanowires do not cover by the framework of aao template but stand free incompletely . when the alumina on the top of aao template is dissolved away , the nanowires that embedded in the template are released gradually and incline to agglutinate together . figure 2b also shows that sns nanowires are abundant , uniform , and highly ordered in the large area . figure 2d reveals a cross - section where the alumina matrix of the aao template has been partially dissolved away . it can be seen that the nanowires deposited inside the nanochannel of the aao template are parallel , tidily aligned , and uniformly distributed . it is correlative to that the aao template had an array of densely parallel nanoholes arranged in a hexagonal form . from these figures it can be estimated that the length of the nanowires is about several tens of micrometers . sem images of aao template and sns nanowire arrays.atypical sem image of aao template.bandcthe top view in a low magnification.dsem image of a typical cross - section tem images of sns nanowires formed within the aao template are shown in fig . 3a , b. figure 3a shows that the sns nanowires cross and overlap with each other , and fig . it s diameter is approximately equal to that of the nanochannels of the employed aao template . these nanowires are uniformly distributed , which indicates that the alumina matrix is dissolved completely . the nanostructure of the sns nanowires was further investigated with saed . the saed pattern ( fig . 3c ) taken from a single nanowire , indicates that the sns nanowires are a good single crystalline . the hrtem image of a single sns nanowire is given in fig . the lattice fringes of the sns are clear and uniform , and additionally it confirmed that these single crystalline sns nanowires are of high quality . 3d is 0.295 nm , corresponding to the value of ( 101 ) planes of the orthorhombic sns nanowires . tem images of aao template and sns nanowire arrays.athe sample was etched for 10 h.bthe sns nanowires with a diameter of about 50 nm.cthe saed pattern taken from the nanowires in ( b).dthe hrtem image of the sns nanowires in ( b ) energy dispersive spectrometer ( eds ) analysis reveals that the product is composed of stannum and sulfur , and the ratio of the s atom and sn atom is 1:0.985 , which just accords with the stoichiometric ratio of sns . the representative optical absorption spectrum of the sns nanowires synthesized by template - assisted electrochemical deposition is shown in fig . this figure indicated that the sns nanowires have high absorption in the range of ultraviolet , and the absorption coefficient is above the 70% . the absorption reduces rapidly with the increase in the wavelength , and the absorption is very small or becomes zero when the wavelength is above 800 nm . the absorption coefficient  , of sns nanowires , was calculated from the average absorption index ( a ) as . the spectral behavior of absorption coefficient as a function of energy , hv , is shown in fig . sns nanowires have high absorption coefficient ( > 10 cm ) in the wavelength range from 400 to 800 nm . optical properties of sns nanowires : ais the optical absorption spectrum andbis the dependence of absorption coefficient on photon energy to determine the energy band gap , eg , and the type of optical transition responsible for this intense optical absorption , and the absorption spectrum was analyzed using the equation for the near - edge absorption . we can see n = 2 and 2/3 for direct allowed and forbidden transitions , respectively , and n = 1/2 and 1/3 for indirect allowed and forbidden transitions , respectively . the curve has a good straight line fit with higher energy range above the absorption edge , indicating a direct optical transition near the absorption edge . 5 , the direct energy gap eg of the sample has been calculated as 1.59 ev , which is higher than the literature value of sns bulk or films . the increased band gap values of sns nanowires which was compared to the bulk material , do not manifest quantum size effects . the absence of size quantization effects may be attributed to the very small bohr radius for the sns . and it is well - known that bohr radius of sns should be smaller than 7 nm . the nanowires diameter is far greater than the bohr radius and it can be suggested that the increased band gap values do not manifest quantum size effects . the energy band gap at 1.59 ev detected in our previous study may be attributed to the surface effect of the carriers in the semiconductor nanowires . the lattice distortion inducing a smaller lattice constant or surface lattice defects will lead to a size dependent enlargement of the band gap , which results in a blue shift in the absorbance onset , as observed in this work . the low - toxicity sns nanowire arrays have been successfully synthesized using the template - assisted pulsed electrochemical deposition in the aao template . the xrd pattern indicates that the nanowires are composed of sns phase and have a highly preferential ( 101 ) orientation . the tem images show that the diameter of the sns nanowires is about 50 nm and length up to several tens of micrometers . eds result indicates that the ratio of s atom and sn atom in our samples is 1:0.985 , which just accords with the stoichiometric ratio of sns . the synthesized sns nanowires exhibit strong absorption in the visible and near - infrared spectral region . the direct energy gapegof the sns nanowires has been calculated as 1.59 ev , and this experimental optical band gap value is the evidence for the quantum confinement of the sns nanowires . this work was partially supported by the national outstanding youth science foundation of china ( no . 50825101 ) , and the national natural science foundation of china ( no . 50671087 ) . the correspondence author ( d. l. peng )
the sns nanowire arrays have been successfully synthesized by the template - assisted pulsed electrochemical deposition in the porous anodized aluminum oxide template . the investigation results showed that the as - synthesized nanowires are single crystalline structures and they have a highly preferential orientation . 
 the ordered sns nanowire arrays are uniform with a diameter of 50 nm and a length up to several tens of micrometers . 
 the synthesized sns nanowires exhibit strong absorption in visible and near - infrared spectral region and the direct energy gapegof sns nanowires is 1.59 ev .