1. Introduction Since the discovery of CdS/Cu₂S heterojunction solar cells in 1954 ^[1],Cu_2-xS have been the focus of intense interests as an important semiconductor in photovoltaic cells. Cu_2-xS as materials,particularly in its thin film form,maintain transmittance in the infrared,low reflectance (<10%) in the visible,and relatively high reflectance (> 15%) in the near-infrared region,which are ideal characteristics for solar energy adsorption ^[2]. Copper sulfide can exist as seven solid phases with various stoichiometries,Cu_2-xS,(0 < x < 1),including Cu-rich Cu₂S (chalcocide),Cu_1.97S (djurleite),Cu_1.8S (digenite),Cu_1.75S (anilite) Cu_1.12S (yarrowite),Cu_1.06S (talnakhite) and CuS (covellite) ^{[3, 4]}. Nair and coworkers ^[5] reported that all of Cu_2-xS compounds exhibited a p-type semiconductor characteristic with copper vacancy defect as acceptors,which is the most important reason for their applications in the photovoltaic devices. Moreover,the electrical resistance,band gap and structure of Cu_2-xS exhibit stoichiometry dependence ^[3]. The compositional controlling of copper sulfide nanocrystals is an important way to tune the optoelectrical properties of Cu_2-xS-based materials ^[6]. So far,Cu_xS nanomaterials with different structures like nanorods,flakes,disks,spheres,wires,platelets,tubules,nanoribbons,flower-like structures,and urchinlike structures have been successfully synthesized to investigate their properties and to expand their applications ^{[7, 8, 9, 10, 11, 12, 13]}. Chen and his colleagues synthesized uniform CuS nanotubes (NTs) with a diameter of 220 nm with photocatalytic activity under visible light ^[4]. Chen and Hu synthesized 500-800 nm flower-like CuS as an efficient 980 nm laser-driven photothermal agent for ablation of cancer cells ^[10]. Furthermore,uniform small sized CuS nanocrystals (CuS NCs) NCs have been rarely reported. Most of the small CuS NCs in the reported literature were capped with long hydrocarbon molecules,such as oleic acid (OA) and oleylamine (OAm). However,the existence of such bulky hydrophobic molecules created a barrier around each NC,which greatly impacts their photoelectrical and catalytic properties. He and his coworkers synthesized uniform,small sized CuS NCs prepared via a facile sol-gel method without using template and complicated post-treatment ^[14]. Moreover,a variety of methods were employed to deposit Cu_2-xS thin films,such as chemical vapor deposition,thermal coevaporation ^[15],photochemical deposition ^[16],electrochemical method ^[17] and chemical bath deposition (CBD) ^{[18, 19, 20, 21]}. Among these,the chemical bath deposition is an attractive technique for the deposition of semiconductors as it does not require sophisticated instrumentation like vacuum systems or other expensive equipment. With the CBD method,electrical conductivity of the substrate is not a necessary requirement. Self-assembled monolayers (SAMs) with varying functionalities have been successfully utilized to direct the assembly of metal chalcogenide nanocrystals ^[20]. The key point of the success of selective deposition is that organic,functionalize groups of SAMs can control crystal heterogeneous nucleation and growth ^[22].

Usually,complexing agents are used to enhance the uniformity and quality of thin films in the chemical bath deposition technique ^{[23, 24]}. Moreover,some physical and chemical properties of materials are dependent on the complexing agents ^[25]. Singh et al.^[26] reported the influence of complex agent on both conversion efficiency and stability to degradation of bath deposited CdS photoelectrode under PEC conditions. Cheng investigated the structural,electrical,and optical properties of the Ag-In-S ternary system semiconductor using the method of CBD. The results showed that the complexing agents in the solution played important roles in the nucleation and growth of thin films ^[27]. In addition,the absorbed ligands can change the growth kinetics and surface energies of different crystal faces,which will lead to anisotropic growth of low symmetry nanostructures and further produce nanoparticles of different morphology,such as nanorods,nanodisks and nanowires ^[28].

No matter whether a film or other nanostructure of Cu_2-xS,the difficulties in composition control and product impurity levels are the common challenges of methods mentioned above. In the present work,Cu_2-xS (x = 0,1) thin films were successfully synthesized on the self-assembled,monolayer substrate with various concentrations of complexing agents in the solution using the chemical bath deposition (CBD) method. The effects of the concentration of the complexing agent on crystal structure,optical properties and photo-electrochemical response of nanostructured Cu_2-xS thin films were investigated by XRD,UV-vis spectrum and computer controlled potentiostat (CHI 660d) with a threeelectrode system. The growth mechanisms of Cu_2-xS nanocrystals with different EDTA concentrations are proposed and discussed as well. This paper also demonstrates some of the existing uncertainties about the physical-chemical properties of Cu_2-xS with different chemical stoichiometries. 2. Experimental 2.1. Preparation of Cu_2-xS thin films

The procedure for preparation of SAMs with -NH₂ terminal groups has been detailed in our previous work ^[29]. To ensure successful preparation of SAMs,the static contact angles of water on SAMs were 50-52° (-NH₂ terminal group),in agreement with the literature ^[30].

The Cu_2-xS thin films were prepared by a simple chemical bath deposition method. The functionalized substrates were immersed in a prepared precursor solution consisting of 10 mmol/L CuSO₄·5H₂O (copper source),10 mmol/L Na₂S₂O₃ (sulfur source) and EDTA (complexing agent) with different molar ratio w = 0.5,1.5 and 3 (molar ratio between EDTA and CuSO₄·5H₂O),which were labeled as S1,S2 and S3,respectively. The solution temperature was maintained at 70 ℃ using a thermostatically controlled water bath. The pH of the bath solution was adjusted to 2.3 by adding H₂SO₄ solution (1 mol/L). After deposition,the deposited films were rinsed in deionized water and ultrosonicated to remove any leftover Cu_2-xS precipitates,and finally,dried with nitrogen gas. 2.2. Characterization

The structure and the phase composition were analyzed by X’pert PRO X-ray diffraction (XRD,Netherlands) with Cu Kα radiation at the scanning speed of 1.2°/min. The chemical states of the elements on the films were determined using a PHI5702 multifunctional X-ray photoelectron spectroscope (XPS,USA). The XPS analysis was conducted at 400W and pass energy of 29.35 eV,using AlKα (1486.6 eV) radiation as the excitation source and the binding energy of contaminated carbon (C 1s = 284.6 eV) as reference. Field emission scanning electron microscopy (FESEM,JSM-5600LV,Japan) and transmission electron microscopy (TEM) were used to study the surface morphology of the thin films. The optical absorption spectra were obtained with a UV-vis spectrophotometer (U-3010,Japan) within the wavelength range of 300- 1000 nm. A computer controlled potentiostat (CHI 660d) was used for all PEC experiments. The PEC responses of the samples were measured in an electrochemical cell with a three-electrode system,in which Cu_2-xS thin films,a platinum wire and saturation mercury electrode were used as the working electrode,the counter electrode and reference electrode,respectively. A 125W mercury lamp was used as the light source. The electrolyte,aqueous HClO₄ (0.1 mol/L) solutions,was freshly prepared using double deionized water. Then,the electrolyte was put into an ultrasonic bath for 30 min before each experiment in order to decrease the gas solutes in the electrolytes. 3. Results and discussion 3.1. Films formed on APTS SAMs

The nucleation and growth of thin films are affected by the degree of super saturation that can be controlled by varying solution conditions. In this research,the effects of EDTA concentration on the nucleation and growth of Cu_2-xS films were examined in detail. Fig. 1 shows the growth rate of Cu_2-xS films on the substrate at different EDTA:Cu²⁺ ratios. It shows that after the induction period,the film growth is initially linear with time followed by a nonlinear region of a much lower growth rate. The decrease of the EDTA concentration resulted in a higher growth rate and shorter inducing time. The thickness of samples S1,S2 and S3 on the substrate with APTS SAMs nearly reached a maximum after 6 h,9 h and 10 h in the solution at 70℃,respectively.

Fig. 2 presents the XRD patterns of samples S1,S2,and S3. The as-prepared samples consisting of copper sulfide nanocrystals were confirmed by XRD pattern. The peaks of S1 (Fig. 2a) can match the orthorhombic Cu₂S crystal phase with a = 11.73Å ,b = 27.1Å and c = 22.67Å ,which matches the JCPDS reference file for Cu₂S [02-1294]. Therefore,at w = 0.5,it consists of well-defined diffraction peaks of cuprous sulfide,indicating that pure Cu₂S thin film is formed onto the Si substrate. Samples S2 and S3,however,show the diffraction pattern of hexagonal CuS (No. 01-1281) crystal phase with a = 3.800Å and a = 16.33Å for S2 (Fig. 2b) and a = 3.787Å and c = 16.44Å for S3 (Fig. 2c). The board peak at 56° should be assigned to the Si substrate. The diffraction pattern also revealed that the crystallinity is slightly preferred for S3,as the peak intensity is higher compared with the other two samples. It is attributed to slow precipitation of the CuS molecules (as shown in Fig. 1),which is caused by relatively high concentration of EDTA complex. XPS spectrums of Cu 2p for the samples (S1,S2 and S3) are shown in Fig. 3. In Fig. 3a,the binding energies of the observed photoelectron peaks of Cu 2p3/2,2p1/2 for Cu2S were 932.8 eV and 952.7 eV,respectively. In Fig. 3b and c,the binding energies of the observed photoelectron peaks of Cu 2p3/2,2p1/2 for CuS were 932.3 eV and 952.2 eV,and were well consistent with the standard reference XPS spectrum of Cu 2p in CuS. It can be calculated that the Cu/S ratio is 1.9922 and 0.9835 for Cu2S and CuS,respectivel.

3.2. SEM and TEM analysis The morphology and size of as-prepared copper sulfide at different concentrations of EDTA have been studied using FESEM and TEM as shown in Figs. 4,5. The molar ratio between EDTA and CuSO₄·5H₂O affects greatly the shape and size of Cu_2-xS nanocrystals. The FESEM and TEM images of Cu_2-xS nanocrystals reveal the morphology and size change with the increase of EDTA:CuSO₄·5H₂O ratio from 0.5:1 to 3:1. At w = 0.5,the thin film consists of sphere-like nanoparticles with a diameter of 30~50 nm (as shown in Fig. 4a and 5a). While at w = 1.5 and 3,the morphologies of these two thin films are similar and consist with sheet-like nanoparticles standing perpendicularly on the substrates (Figs. 4b,c and 5c,b). This is attributed to the different growth mechanisms of Cu_2-xS crystal caused by EDTA concentrations. In the CBD technique,the formation of the nanoparticle is divided into two parts: the nucleation event and the growth process. The nucleation and growth rate of the particle are two of the deposition parameters that influence the surface morphology in polycrystalline thin film. At low concentration of EDTA,[Cu(S₂O₃)_n](^2n-1)- is formed as a major complex. Due to the low formation constant of [Cu(S₂O₃)_n](^2n-1)- (lgK = 1010.78),it is much easier to release copper ions compared with Cu-EDTA (lgK = 1018.8). Therefore,there are excess free copper(I) ions and sulfur ions in the solution. The nucleation process dominated during the whole reaction process and had a strong influence on the size of the final sample,and finally,forming small sphere-like nanoparticles. At higher concentration of EDTA,the XRD pattern indicated the formation of hexagonal CuS. The c/a ratio of the asprepared hexagonal CuS (S2 and S3) is 4.29 and 4.34,which is greater than the ideal hexagonal value (1.633),implying that the {0 0 0 1} facets have a surface energy lower than those of the {1 0 1 0} and {1 1 2 0} facets ^[31]. Only the appearance of crystal facets is helpful to limit the exposure of other higher energy surfaces,which might therefore reduce the exposure of the {1 0 1 0} and {1 1 2 0} facets with relatively higher surface energy of CuS. As a result,reducing the total energy of the nanocrystals will result in the formation of the stable,sheet-like nanoparticles. Their selected area electronic diffraction pattern indicated that all samples were polycrystalline nanoparticles. Moreover,the crystallinity increased with increasing the concentration of EDTA,which is in reasonable agreement with the XRD results.
3.3. Nucleation and growth mechanism of thin films

The deposition process is based on slow release of Cu²⁺ and S^2- ions. The free copper and sulfide ion must be maintained at low concentration in order to avoid the process of immediate precipitation. In the present case,sulfide ions are generated in the bath from sodium thiosulfate,as shown below:

Na₂S₂O₃ is reducing agent by virtue of the half cell reaction ^[32]. In the acidic medium of pH 2.3,sodium thiosulfate gradually release sulfide ions upon hydrolytic decomposition ^[33]. The copper salt is dissolved in deionized distilled water to form [Cu(H₂O)₆]²⁺ ions,and by adding the ligand to this solution leads to the formation of complexes by successive displacement of water molecules. However,there exists an equilibrium between the Cu(II) and Cu(I) oxidation states ^[5];

The relative stability of Cu(I) and Cu(II) depends upon the nature of anion or ligand in the bath. In this technique,the thiosulfate ions can also be used as a complexing agent. Copper is known to form at least three consecutive complex compounds with the thiosulfate: [Cu(S₂O₃)_n]^-,[Cu(S₂O₃)_n]^3-,[Cu(S₂O₃)_n]^{5- [33]}. Therefore,there exists the following equilibrium,

At w = 0.5,due to lower concentration of EDTA,Cu(II) in cupric sulfide tend to be reduced and combined with thiosulfate to form anionic thiosulfatecopper(I) complex compound,[Cu(S₂O₃)_n]^-. The dissociation of this gives Cu(I) ions,[Cu(S₂O₃)_n]^-⇔Cu+ (aq.) + n(S₂O₃)^{2- [34]}. The cuprous ions then combine with the sulfide ions,to form insoluble Cu₂S. However,at w = 1.5 and higher ratios up to 3,due to the excessive concentration of EDTA,Cu(II) firstly combines with the EDTA and forms the Cu(II)-EDTA complex. Then,the EDTA complex breaks down slowly with temperature releasing the Cu(II) ions,which will react finally with sulfide ion to form a CuS thin film. Therefore,the control of the crystal structure of the Cu_2-xS has been realized by utilizing different concentrations of complexing agent during chemical bath deposition. 3.4. Optical and photoelectrochemical properties

UV-vis absorptions of Cu_2-xS thin films with different concentrations of EDTA were investigated to examine the effects of the morphology and structure on the optical properties. As shown in Fig. 6,the absorption edge of the thin film varies with composition ‘‘x’’. The absorption spectrum of sphere-like Cu₂S nanocrystals (w = 0.5),exhibits a well-defined absorption feature at 365 nm and has a broad absorption between 450 and 650 nm,which is also the most important reason for their applications in solar energy adsorption and photovoltaic devices ^[35]. On the other hand,the absorbance never reaches zero intensity,but rises for longer wavelengths again,which is thought to be caused by the free-carrier intraband absorbance. Therefore,samples prepared were assigned to Cu₂S nanocrytals. It shows that the spectrum of CuS thin film (w = 1.5 and 3) has a strong absorption around 396 and 400 nm (Fig. 6b and c),which has red-shift compared with Cu₂S thin film due to the quantum size effect,and reaches a minimum around 550 nm,but again does not decrease to zero intensity. It rises for longer wavelengths stronger than the Cu₂S,which is attributed to free-carrier absorbance ^[8].

The photoelectrochemical (PEC) responses of the samples were measured to examine the possibility of their application in solar energy cells ^{[26, 36]}. In Fig. 7,instantaneous photoresponse of the three samples were featured with the rise and decay curves of photocurrent (Isc) during successive exposure of the films to illuminated and dark conditions. It can be seen that all samples have very intense photocurrent responses when the light was regularly switched ‘‘on’’ and ‘‘off’’,a series of almost identical electric signals can be obtained and hold the stable photocurrent. For the Cu_2-xS thin films with different w values (0.5,1.5,3),Isc were ca. 0.204,0.084,and 0.09 A/m²,respectively. The Cu₂S (w = 0.5) shows the higher value of generated photocurrents compared with that of CuS (w = 1.5 and 3). On the one hand,the Cu₂S is appealing as an absorber layer in thin film photovoltaics due to the nearly ideal bandgap. On the other hand,this was attributed to the high density of the copper vacancies of Cu₂S as it was proposed that the density of the copper vacancies increased with the increment of 2-x value for the Cu_2-xS thin films ^[37].

In this study,the Cu_2-xS (x = 0,1) semiconductor films were successfully deposited using chemical bath deposition. The structural,optical and photoelectrochemical properties of Cu_2-xS thin films were investigated. With an increase of the concentration of the complexing agent EDTA in the reaction solution,it was found that the compositions of the Cu_2-xS nanocrystals were varied from Cu₂S (chalcocide) to CuS (covellite). Moreover,the high concentration of EDTA led to better crystallinity and sheetlikemorphology. A UV-vis absorption analysis illustrated that the maximum in the absorption spectra of Cu_2-xS (x = 0,1)films shifts to longer wavelength with an increase of the concentration of EDTA in the solution. According to experimental results,all samples showed a photo-enhancement effect. The photocurrent density of Cu₂S thin film reached 0.204 A/m² at the external potential of -0.6 V versus a Pt electrode,which showed that Cu₂S thin film can be used as a good photo-absorber in PEC applications.