Synthesis and characterization of ZnS with controlled amount of S vacancies for photocatalytic H2 production under visible light

Controlling amount of intrinsic S vacancies was achieved in ZnS spheres which were synthesized by a hydrothermal method using Zn and S powders in concentrated NaOH solution with NaBH4 added as reducing agent. These S vacancies efficiently extend absorption spectra of ZnS to visible region. Their photocatalytic activities for H2 production under visible light were evaluated by gas chromatograph, and the midgap states of ZnS introduced by S vacancies were examined by density functional calculations. Our study reveals that the concentration of S vacancies in the ZnS samples can be controlled by varying the amount of the reducing agent NaBH4 in the synthesis, and the prepared ZnS samples exhibit photocatalytic activity for H2 production under visible-light irradiation without loading noble metal. This photocatalytic activity of ZnS increases steadily with increasing the concentration of S vacancies until the latter reaches an optimum value. Our density functional calculations show that S vacancies generate midgap defect states in ZnS, which lead to visible-light absorption and responded.

I n solving the global energy need and environmental pollution, hydrogen has attracted much attention for its potential to replace fossil fuels. Currently, however, pro-duction of hydrogen is mostly based on fossil fuels and on a process requiring high energy consumption 1,2 . Since the report of photocatalytic activity of TiO 2 for hydrogen production, photocatalysis has become a desirable approach toward producing hydrogen with clean, environmentally friendly and economical process. In the past few decades, numerous photocatalysts have been found to exhibit high activities for water splitting [3][4][5][6] . These photocatalysts are mostly active only under UV light, which accounts for only 4% of the total sunlight. For practical applications, therefore, photocatalysts for hydrogen production need to operate under visible light.
ZnS is one of the most widely investigated photocatalysts because it rapidly generates electron-hole pairs under photoexcitation, and exhibits a relatively high activity for H 2 production under UV light 6,7 . ZnS has a hexagonal structure and forms nanosheets or nanorods with large specific surface area [7][8][9] . Nevertheless, ZnS still has a substantially negative potential for excited electrons, and it does not respond to visible light due to its large band gap (,3.6 eV) 10 . Attempts have been made to extend the optical absorption of ZnS into the visible region by doping it with transition-metal ions (Au, Ni, Cu) [10][11][12] . However, these composite materials were differently to synthesize and hard to evaluate their intrinsic property of materials. Surface defects can also enhance light harvesting in photocatalytic materials 13 , and can also serve as adsorption sites where a charge transfer to the adsorbed species can prevent the recombination of photogenerated electrons and holes (namely, photogenerated charge carriers). However, when present in excessive amount, defects can act as traps for charge carriers leading to the recombination of photogenerated electrons and holes and hence decreasing the photocatalytic activity [14][15][16][17] . Therefore, controlling the amount of defects is great important to photocatalytic reaction. Vacancies are another kind of intrinsic defects in crystals, which are easily formed in quasi-two-dimensional materials because the exposed atoms on their surface can escape from the lattice hence affecting their physical and chemical properties 18,19 . Theoretical investigations have reported that S vacancies can decrease the band gap of ZnS, and the introduction of S vacancies is harder than Zn vacancies in ZnS crystals 20,21 . McCloy et al. introduced different vacancies into cubic ZnS crystals by chemical vapor deposition under various atmospheres and studied the change in their lattice parameters 22 . To our knowledge, there has been no study on S vacancies in ZnS with an aim for photocatalytic applications.
In the present work we first controlled amount of S vacancies in ZnS spheres, and characterized the ZnS samples with S vacancies by various experimental techniques and density functional calculations. The ZnS samples with controllable S vacancies were obtained via adding NaBH 4 as a reducing agent in NaOH solution, and found to possess good photocatalytic activities for H 2 production without loading noble metal under visible light irradiation. The probable cause for the visible-light photocatalytic activity of these ZnS materials was explored.

Results
XRD and SEM characterizations. The XRD profiles for the ZnS samples synthesized using NaBH 4 as reducing agent are given in Fig. 1, which is coordinated with that of ZnS ref reported by Zhang et al 10 . These patterns have a strong and sharp peak at 2h 5 27.2u, which can be indexed as the (100) reflection of wurtzite ZnS, and the broader peak is the overlapping peaks of wurtzite (002), sphalerite (111), and wurtzite (101) 10,23 . These suggest that the {001} facet of wurtzite crystals is exposed, which is confirmed by SEM images (see below). The peaks of ZnS decrease with increasing the amount of NaBH 4 (hence the amount of S vacancies), because S vacancies diminish the crystallinity of ZnS crystals.
The morphologies of the prepared ZnS samples are shown in Fig. 2. The nanosheets of microspheres become crimped gradually with increasing the amount of NaBH 4 , because S vacancies enhance the stress in the nanosheets of ZnS. The morphology of the microsphere turns into a blossom as the amount of NaBH 4 increases to 0.03 mol, in which nanosheets are already unfolded and broken. The morphologies of the microspheres are gradually ruptured, crisp and unfolded as the amount of NaBH 4 is increased, which may decrease the specific surface areas of the samples reducing the photocatalytic activity.
Optical absorption. The UV-Vis diffuse reflectance spectra of the ZnS samples obtained under different conditions are presented in Fig. 3, with their colour changes presented in the inset. It is clearly seen that ZnS ref is a typical direct gap semiconductor with no absorption in the visible light region. The ZnS samples, which obtained by the reaction between Zn and S powder without adding NaBH 4 (0 mol), exhibited visible light absorption, but one cannot control their visible-light absorption and their photocatalytic activities are low. NaBH 4 is a strongly reducing agent, and can reduce Zn 21 ions thereby creating S vacancies due to the charge balance requirement. Fig. 3 shows the absorption spectra measured for ZnS ref and ZnS prepared without using NaBH 4 as well as those prepared using NaBH 4 . The amount of NaBH 4 used for synthesis clearly affects the visible-light absorption; the larger the amount of NaBH 4 , the stronger the absorption in the visible light.
Being a source of H 2 , NaBH 4 can reduce H 2 O to generate hydrogen. However, under the high-concentration NaOH solution used in our synthesis, the concentration of H 1 in the solution is negligible so that reduction of H 2 O by H 2 is negligible. Thus, H 2 reduces Zn 21 ions of the ZnS crystal lattice thereby forming Zn 0 . To keep the charge balance, the amount of S atoms in the ZnS crystal lattice should decrease leading to S vacancies. With increasing the amount of NaBH 4 , more S vacancies are formed resulting in a darker colour of the samples (see the inset of Fig. 3). Therefore, visible-light absorption of ZnS crystals can be tuned by the concentration of S vacancies, which in turn can be controlled by varying the amount of NaBH 4 in the synthesis process.  24 . When Zn and S powders were used for the synthesis without adding NaBH 4 , the resulting ZnS sample weakly absorbs visible light and a lower binding energy for both Zn 2p 3/2 and Zn 2p 1/2 . This is attributed to the presence of some S vacancies 20 . With increasing the amount of NaBH 4 during the synthesis, the binding energy for the Zn 2p 3/2 and Zn 2p 1/2 peaks of the ZnS samples shift gradually to a lower energy. This indicates that, to a certain degree, the concentration of S vacancies can be controlled by varying the amount of NaBH 4 used. As depicted in Fig. 5, the Zn 2p 3/2 peaks can be decomposed into two Gaussian peaks. With increasing the amount of NaBH 4 from 0 to 0.03 mol, the major Zn 2p 3/2 peaks change from 1020.8 eV to 1018.9 eV, and the minor Zn 2p 3/2 peaks from 1019.1 eV to 1017.4 eV. At the same time, the major peaks of Zn 2p 1/2 change from 1043.8 eV to 1041.8 eV, and the minor Zn 2p 1/2 from 1042.1 eV to 1039.9 eV. All these peaks are lower in energy than that of ZnS ref . It is reasonable to assign the major peaks to the Zn 21 ions away from the S vacancy sites, and the minor peaks to those Zn 21 ions adjacent to the S vacancy sites, and that the presence of Zn 0 atoms around the S vacancy sites (see below) lowers the binding energies of the Zn 21 ions. EDS results also proved that absence of S element in ZnS samples (Fig S6, ESI { ). The XPS results of S element in different samples were also investigated, and peaks of S 2p were all shifted to higher energy (Fig S7, ESI { ).
Surface areas of ZnS. An important factor affecting the photocatalytic activity of a ZnS sample is its surface area (Table 1). Fig S2 (ESI { ) shows the typical isotherms of N 2 adsorption onto the ZnS sample obtained using 0.01 mol of NaBH 4 , with the pore size distributions in the inset. These isotherms exhibit type IV isotherms with hysteresis loops according to the IUPAC classification 25 , which implies the presence of mesopores. The pore size distribution curves (the inset in Fig S2) indicate that the sample possesses mesopores and macropores in a wide size range of 10 to over 90 nm. Use of NaBH 4 can increase the BET of the sample to a certain degree. The ZnS sample obtained with 0.01 mol of NaBH 4 possesses a relatively large specific surface area, while use of a greater amount of NaBH 4 decreases by destroying the microspheres.  Photocatalytic production of H 2 . The ability of the ZnS samples (0.5 g) for photocatalytic hydrogen generations under visible light (l . 420 nm) irradiation was evaluated by using 100 mL aqueous solution containing Na 2 S (0.25 M) and Na 2 SO 3 (0.5 M) as sacrificial agents. The H 2 production of the ZnS sample (obtained with 0.01 mol of NaBH 4 ) as a function of the irradiation time is presented in Fig. 6A, which shows that the activity of the sample has no decrease after 7 h irradiation (The XPS results of this sample, before and after photocatalytic reaction, were also show in Fig S4, ESI { ). The hydrogen production rates of the ZnS samples synthesized using different amounts of NaBH 4 (0-0.03 mol), displayed in Fig. 6B, show that the production rate increases until the amount of NaBH 4 reaches 0.01 mol but drops abruptly when it is beyond 0.01 mol. This means that the S vacancies enhance the visible light absorption resulting in an enhanced high photocatalytic activity. However, when present in excess amount, vacancies act as   PL spectra. The photocatalytic activity is enhanced when photogenerated electron-hole pairs are efficiently separated. PL emission is an effective way of estimating the ability of a sample to separate these photogenerated carriers. The PL spectra of different ZnS samples are presented in Fig. 7, which were carried out on a Hitachi F-4500 fluorescence spectrophotometer at room temperature and obtained with excitation wavelength at 300 nm. It is observed that ZnS ref has the highest intensity of photoluminescence emission, indicating that the photogenerated carriers are quickly recombined. The PL intensity of the ZnS samples decreases with increasing the amount of NaBH 4 reaches 0.01 mol, but decreases when it is beyond 0.01 mol. It clearly indicates that the S vacancies can become centers for the recombination of photogenerated electrons and holes when present in excessive amount. This finding is consistent with the results of the photocatalytic activities of these ZnS samples. The examination of the relaxed structures around the S vacancies show that one of the four ZnS 3 pyramids surrounding the S vacancy becomes strongly pyramidal (with %S-Zn-S<95 0 ) while the remaining three ZnS 3 pyramids do not undergo a strong change in shape (with %S-Zn-S<105 0 ). In ZnS each S 22 ion is surrounded by four Zn 21 ions to form a SZn 4 tetrahedron (Fig. 9A), and each Zn 21 ion surrounded by four S 22 ions to form a ZnS 4 tetrahedron. To a first approximation, the bonding in ZnS can be described in terms of the sp 3 hybridization for the Zn and S atoms so that each Zn-S bond results from a bonding combination of a filled sp 3 orbital of S 22 (i.e., a sp 3 lone pair) with an empty sp 3 orbital of Zn 21 (Fig. 9B). When an S atom is removed from a Zn 4 tetrahedron, each of the four Zn atoms surrounding the S vacancy becomes a ZnS 3 pyramid with one sp 3 dangling bond. If the structure around the vacancy site is not relaxed, then the interactions between the four dangling bonds lead to the midgap states 1a and 1t (1a , 1t) of the Zn 4 tetrahedron lying approximately in the middle of the gap between the valence band maximum (VBM) and conduction band minimum (CBM) (left of   . When the structure around the vacancy site is relaxed, the energy gap between the 1a and 1t levels becomes large with the 1t level splitting (right of Fig. 9C) due to the symmetry lowering. As a consequence, the 1a level comes close to the VBM and becomes doubly filled, while the three empty levels come close to the CBM. In the relaxed structure around an S vacancy, one ZnS 3 pyramid that becomes strongly pyramidal (%S-Zn-S<95 0 ) has its dangling bond doubly filled becoming a lone pair (Fig. 9D). Then the optical excitations associated with the filled defect level close the VBM and the empty defect levels below the CBM are responsible for the visible-light absorption of the ZnS samples with S vacancies. Furthermore, trapping of photogenerated electrons and holes by these defect states helps slow down their recombination, thereby enhancing the photocatalytic activities of ZnS samples with S vacancies. In summary, aimed at finding photocatalytic ZnS systems for H 2 production operating under visible-light irradiation, we synthesized a new ZnS samples with controlled amount of S vacancies by the hydrothermal reaction of zinc and sulfur powders with NaBH 4 added as a reducing agent. The concentration of S vacancies in the ZnS samples can be easily controlled by varying the amount of NaBH 4 . The as-prepared ZnS samples exhibit enhanced visible-light absorption with increasing the concentration of S vacancies. The photocatalytic activities of these samples for H 2 production, evaluated under visible light irradiation, show that the photocatalytic activity increases with increasing the concentration of S vacancies until it reaches the optimum value, but decreases sharply when it goes beyond the optimum value because, when present in excessive amount, S vacancies destroy both crystal structure and morphologies of the samples and also play as recombination sites of photogenerated electrons and holes. Our density functional analysis reveals that S vacancies generate midgap defect states in ZnS, which are responsible for visible-light absorption and also act as trap centers for electrons and holes improving the separation efficiency of photogenerated charge carries. This work provides a novel method to induce and control S vacancies in crystals for enhancing their photocatalytic H 2 production under visible light.
Preparation of ZnS. Samples of ZnS with S vacancies were synthesized by mixing Zn powder and sublimed sulfur in 50 ml of 21 M NaOH solution. After the suspension was cooled to room temperature, a various amount of NaBH 4 (0-0.03 mol) was added to control the amount of S vacancies in ZnS crystals. After constant stirring for 2 h, the mixed solution was transferred into a 120 ml sealed Teflon-lined autoclave and was heated at 230uC for at least 12 h. The autoclave was slowly cooled down to room temperature, and the obtained sample was washed with distilled water, and was finally dried at 40uC overnight. For the purpose of comparison, we prepared a reference ZnS sample following the method proposed by Zhang et al. 10 with slight modification 10 , which will be hereafter referred to as ZnS ref ; a certain amount of ZnO was dissolved in 50 ml of 21 M NaOH solution, and Na 2 S?9H 2 O was added after the solution cooled down. The suspension was transferred into a 120 mL sealed Teflonlined autoclave and followed by hydrothermal treatment at 230uC for at least 12 h. The sample was also collected after washing and drying.
Sample characterizations. XRD patterns were recorded on a Bruker AXS D8 Advance powder diffractometer (Cu Ka X-ray tube, l 5 0.154056 nm). The morphologies of the samples were obtained using SEM (Hitachi S-4800). The chemical energy-dispersive X-ray spectrum (EDS) were examined by an energy dispersive X-ray spectrometer equipped in the SEM machine. The surface areas of the as-prepared samples were measured by the BET method using nitrogen adsorptiondesorption isotherms on a Micromeritics ASAP 2020 apparatus at liquid nitrogen temperature. UV-Vis diffuse reflectance spectra were obtained for dry-pressed disk samples by using a Shimadzu UV 2550 recording spectrophotometer equipped with an integrating sphere, and BaSO 4 was used as a reference. PL spectra were carried out on a Hitachi F-4500 fluorescence spectrophotometer at room temperature and the excitation wavelength was 300 nm.
Details of calculations. Our density functional calculations employed the projector augmented wave method as implemented in the Vienna ab initio simulation package [26][27][28][29] , the generalized gradient approximation of PBE for exchange and correlation corrections 30    Photocatalytic activity for H 2 prodution. A top irradiation vessel connected to a glass-enclosed gas-circulation system was used to evaluate the photocatalytic hydrogen evolution of the ZnS samples. In a typical photocatalytic experiment, 0.5 g sample was suspended in 100 mL aqueous solution containing Na 2 S?9H 2 O (0.35 M) and Na 2 SO 3 (0.25 M) as sacrificial agents. The temperature was maintained at 5uC. The visible light source was obtained from a 300 W Xe arc lamp (PLS-SXE 300, Beijing Trusttech Co. Ltd.) equipped with an ultraviolet cutoff filter (l . 420 nm). The amount of H 2 produced was determined with a gas chromatograph (Varian GC-3800) equipped with thermal conductivity detector.  The VB and CB refer to valence and conduction bands, respectively. Here, for simplicity, it was assumed that the unrelaxed Zn 4 tetrahedron has a Td symmetry, and the relaxed Zn 4 tetrahedron a C 3 symmetry. www.nature.com/scientificreports