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Intergrowth of Cocatalysts with Host Photocatalysts for Improved Solar-to-Hydrogen Conversion Zhixiao Qin, Yubin Chen, Xixi Wang, Xu Guo, and Liejin Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b09943 • Publication Date (Web): 29 Dec 2015 Downloaded from http://pubs.acs.org on December 30, 2015

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ACS Applied Materials & Interfaces

Intergrowth of Cocatalysts with Host Photocatalysts for Improved Solar-to-Hydrogen Conversion Zhixiao Qin, Yubin Chen*, Xixi Wang, Xu Guo and Liejin Guo* International Research Center for Renewable Energy, State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Shaanxi 710049, P. R. China. ABSTRACT: In the field of photocatalytic hydrogen generation, cocatalysts play a vital role on the enhanced properties. Delicate control of the physicochemical properties of cocatalysts and systematic optimization of the coupling between cocatalysts and host photocatalysts are essential. Herein, a simple one-step hydrothermal method was proposed to synthesize noble-metal-free NiSx/CdS photocatalysts for the first time. Time-dependent growth studies revealed that NiSx cocatalysts and CdS host photocatalysts were intergrown with each other in the one-step hydrothermal process. Compared with NiSx@CdS photocatalysts prepared by the common two-step method, the intergrowth effect induced close contact between NiSx and CdS, as well as smaller size and better dispersity of NiSx nanoparticles. These specific characters of NiSx/CdS finally resulted in the efficient charge separation and rapid surface reaction, giving rise to the significantly improved photocatalytic activity with the apparent quantum efficiency at 420 nm as high as 60.4%. To our knowledge, this value is the highest efficiency for NiSx modified CdS photocatalysts, and is among the best efficiencies for visible-light-driven photocatalysts. It is believed that the present work can provide a general guidance to 1

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develop efficient heterostructured cocatalyst/photocatalyst system for hydrogen generation. KEYWORDS: cocatalyst, photocatalyst, hydrogen generation, one-step, intergrowth INTRODUCTION Photocatalytic hydrogen generation from water over semiconductors as a promising route for the conversion and storage of solar energy has been widely studied during the past few decades.1-7 In the various aspects of photocatalytic hydrogen generation, cocatalysts played a vital role on the enhanced photocatalytic properties.8 Cocatalysts can effectively capture the photogenerated electrons/holes to suppress the charge recombination, provide delicately designated sites for surface redox reaction, and decrease the reaction overpotential, which are crucial to the efficient photocatalytic hydrogen generation.9-14 Recent studies has shown that photocatalytic activities were closely related with the physicochemical properties of cocatalysts (such as: particle size,15 particle distribution,16 and chemical states17), as well as the contact between cocatalysts and host photocatalysts.18,19 In an ideal cocatalyst/photocatalyst heterostructured system,4 cocatalysts with optimal particle size and uniform dispersity should be intimately coupled with host photocatalysts, giving rise to the efficient charge transfer and surface redox reaction.20 In general, cocatalysts are always combined with host photocatalysts by a two-step route. Host photocatalysts are initially prepared, and cocatalysts are loaded via a subsequent physical/chemical deposition.21 The two-step

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route is a time-consuming process. Meanwhile, in some cases, large amount of cocatalysts can tend to aggregate together, which is detrimental to the charge separation and surface reaction. As a consequence, development of alternative loading method for cocatalysts and detailed discussion on the matching principle between cocatalysts and host photocatalysts should be carried out. Cadmium sulfide with the bandgap of ca. 2.4 eV and appropriate conduction band potential has been extensively investigated as a visible-light-driven photocatalyst for hydrogen generation.22-30 High solar-to-hydrogen conversion yields were reported for the CdS-based photocatalysts.19,21 Therefore, CdS should be an ideal model photocatalyst to examine the specific characters of cocatalysts. Pt and PdS are proved to be effective cocatalysts for CdS.21,31 Unfortunately, the high price and low reserve of noble metals increase the prohibitive costs of photocatalytic hydrogen generation. Hence, seeking efficient and low-cost cocatalysts is of great importance for the large-scale application of photocatalytic hydrogen production.32-34 It has recently been demonstrated that a series of transition metal sulfide MxS (M = Mo, W, Ni) could significantly promote the photocatalytic hydrogen generation of CdS.35-38 In particular, NiS@CdS showed superior performance. Apparent quantum efficiency (AQE) of 51.3% at 420 nm could be reached from lactic acid sacrificial solution.39 However, the previously reported NiS@CdS photocatalysts were prepared by a two-step hydrothermal method. CdS was firstly hydrothermally synthesized, and then NiS nanoparticles were loaded by the second hydrothermal process. As discussed above, large amount of NiS nanoparticles could aggregate together through the two-step 3



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route, possibly leading to poor contact with CdS. Recently, our group introduced a one-step hydrothermal method for the synthesis of heterostructured photocatalysts.40 The one-step process led to high dispersity of individual nanoparticles and intimate contact between different components, which resulted in superior photocatalytic properties compared to heterostructured photocatalysts prepared by the common two-step method. These results inspire us. If cocatalysts and host photocatalysts are synthesized by a one-step route, two phases can be intergrown with each other.41 Strong coupling between cocatalysts and host photocatalysts, together with favorable physicochemical properties of cocatalysts for hydrogen generation can be expected. In the present study, NiSx/CdS photocatalysts were synthesized for the first time via a simple one-step hydrothermal method. Compared with NiSx@CdS photocatalysts prepared by the common two-step method, NiSx/CdS photocatalysts showed apparently superior hydrogen generation. Time-dependent growth studies revealed that NiSx cocatalysts and CdS host photocatalysts were intergrown with each other in the one-step hydrothermal process, which resulted in the efficient charge separation and rapid surface reaction for hydrogen generation. EXPERIMENTAL SECTION Material Synthesis All chemicals were of analytical grade and were used without further purification. NiSx/CdS photocatalysts were synthesized via a simple one-step hydrothermal method. In a typical procedure, 0.02 mol of Cd(NO3)2·4H2O and appropriate amount of Ni(OAc)2·4H2O were firstly added into 40 mL of deionized water under stirring. The 4


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Ni/Cd molar ratios were respectively 0.032, 0.064, 0.08, 0.096, 0.128, and 0.16. After the suspension was stirred uniformly, 40 mL of NaOH solution (OH-/(Cd2++Ni2+) molar ratio was 6) was added slowly with strong magnetic stirring. Subsequently, the suspension was stirred continuously for another 20 min and excess amount of thioacetamide (S2-/(Cd2++Ni2+) molar ratio was 4) was added. The suspension was then transferred into a 110 mL Teflon-lined stainless steel autoclave and heated at 150 °C for 48 h. After the autoclave cooled naturally in air, the resulting precipitate was thoroughly washed with ethanol and deionized water, and finally dried at 80 °C under vacuum for 12 h. Pure CdS or NiSx was also prepared in the same way like NiSx/CdS with only 0.02 mol of Cd(NO3)2·4H2O or 0.02 mol of Ni(OAc)2·4H2O as the cation precursor for the hydrothermal reaction. NiSx@CdS photocatalysts were synthesized via a two-step hydrothermal method for comparison. CdS photocatalysts were firstly synthesized by the above hydrothermal route. Subsequently, 0.02 mol of as-prepared CdS powders were added to 40 mL of deionized water under strong stirring, and appropriate amount of Ni(OAc)2·4H2O was added (Ni/Cd molar ratios were respectively 0.032, 0.064, 0.08, 0.096, 0.128, and 0.16). After the suspension was stirred uniformly, 40 mL of NaOH solution (OH-/Ni2+ molar ratio was 6) was added slowly under strong stirring. Subsequently, the suspension was stirred continuously for another 20 min and excess amount of thioacetamide (S2-/Ni2+ molar ratio was 4) was added. Then the second hydrothermal treatment was applied for the synthesis of NiSx@CdS sample at 150 °C for 48 h. The following process was the same as that for NiSx/CdS prepared by the 5



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one-step hydrothermal method. Electrode Fabrication NiSx/CdS, NiSx@CdS, CdS, and NiSx films were prepared by electrophoretic deposition (EPD) of NiSx/CdS, NiSx@CdS, CdS, and NiSx photocatalysts onto fluorine-doped tin oxide (FTO) coated glass substrates. 50 mg of prepared photocatalysts were firstly dispersed in 30 mL of acetone for EPD. In a typical EPD process, two pieces of FTO glass were immersed into the suspension solution with a separation distance of 1 cm, and a dc voltage of 50 V was applied for 3-4 min. The glass electrodes were then rinsed with ethanol, dried at 50 °C under vacuum for 12 h, and ready for electrochemical experiments. Characterization X-ray diffraction (XRD) patterns of as-prepared photocatalysts were obtained in a PANalytical X’pert MPD Pro X-ray diffractometer with Cu-Kα irradiation at 40 kV and 40 mA. Ultraviolet visible (UV-vis) absorption spectra of the samples were measured on a UV-visible spectrophotometer (HITACHI U-4100). Transmission electron microscope (TEM) images, the energy-dispersive X-ray (EDS) spectra and TEM-EDS mapping were conducted using a FEI Tecnai G2 F30 S-Twin microscope with an X-ray energy dispersive spectrometer. Scanning electron microscope (SEM) images were obtained by a JEOL JSM-6700F microscope. The X-ray photoelectron spectroscopy (XPS) measurements were conducted on a Axis Ultra, Kratos (UK) multifunctional spectrometer using monochromatic Al Ka radiation. Binding energies were calibrated relative to the C 1s peak (284.8 eV) from adventitious carbon 6


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adsorbed on the surface of the samples. Photocatalytic Measurement The photocatalytic reaction for hydrogen generation was performed in a side irradiation Pyrex cell. 0.1 g of as-prepared photocatalysts was well dispersed into 200 mL of aqueous solution containing 30 vol% lactic acid with stirring. Before irradiation, nitrogen was purged through the cell for 15 min to remove air in the dark. The temperature for all reaction was kept at 35 °C. A 300 W Xe-lamp equipped with a 420 nm cut-off filter was used to provide the visible light irradiation. The amount of H2 evolved was determined with a TCD gas chromatograph. The average hydrogen production rates were calculated based on the hydrogen generation in the first 5 h reaction. The apparent quantum efficiency (AQE) could be calculated according to Equation (1).

AQE(%) =

The number of evolved hydrogen molecules × 2 ×100% The number of incident photons

(1)

Electrochemical Measurement Electrochemical measurements were carried out using a CHI 760D electrochemical workstation in a three-electrode system. The as-prepared films were used as the work electrode, a platinum foil was used as the counter electrode and a saturated calomel electrode was used as the reference electrode. A 500 W xenon lamp coupled with an AM 1.5 filter was used as the light source. Photocurrent density and electrochemical impedance spectroscopy (EIS) were measured in the 0.5 M Na2SO3 aqueous solution as the electrolyte. Photocurrent density was measured at -0.3 V vs Hg/Hg2Cl2. EIS measurement was done at open circuit potential. The amplitude of the sinusoidal wave 7



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was 5.0 mV and the frequency range examined was from 100 KHz to 0.1 Hz. The current-voltage curves of the NiSx, Pt, and FTO electrodes were measured in 0.1 M KCl aqueous solution containing 30 vol% lactic acid as the electrolyte. RESULTS AND DISCUSSION As shown in Figure 1a, the photocatalytic activities of NiSx/CdS and NiSx@CdS with different Ni/Cd molar ratios were investigated. The hydrogen production rate of single CdS was rather low, and NiSx coupling could greatly improve the photocatalytic property. Single NiSx exhibited no apparent hydrogen, indicating the character of NiSx as cocatalyst. For both NiSx/CdS and NiSx@CdS photocatalysts, with the increased amount of NiSx, the hydrogen production rate initially increased and then underwent a decrease due to the shielding effect of excess cocatalysts.42,43 Compared with NiSx@CdS prepared by the two-step hydrothermal method, NiSx/CdS prepared by the one-step hydrothermal method owned higher photocatalytic activity regardless of the Ni/Cd molar ratio. In particular, NiSx/CdS sample with the Ni/Cd molar ratio of 0.08 showed the highest hydrogen production rate of 2.86 mmol/h, corresponding to the apparent quantum efficiency (AQE) up to 60.4 % at 420 nm. In the previous study, Xu’s group has reported a two-step hydrothermal method to synthesize NiS@CdS photocatalysts for hydrogen production and achieved an AQE of 51.3% at 420 nm, which was the highest AQE previously reported for NiS@CdS photocatalyst.39 Herein, a facile one-step route was proposed to prepare NiSx/CdS samples for the first time, and a higher AQE of 60.4% was reached for photocatalytic hydrogen production. To our knowledge, this value is the highest AQE for NiSx modified CdS photocatalysts, 8


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and is among the best AQEs for visible-light-driven photocatalysts. In order to exclude the influence of Ni/Cd molar ratio, NiSx/CdS and NiSx@CdS samples with the Ni/Cd molar ratio of 0.08 were chosen for detailed comparison in the following work.

Figure 1 (a) Photocatalytic hydrogen generation over NiSx/CdS and NiSx@CdS with different Ni/Cd molar ratios; (b) Photocatalytic hydrogen generation over NiSx/CdS prepared with different hydrothermal time.

Reaction conditions: 0.1g of

photocatalysts; 200 ml of aqueous solution containing 30 vol% lactic acid; 300 W Xe lamp equipped with a cut-off filter (λ ≥ 420 nm). Long-time photocatalytic test was carried out to investigate the stabilities of NiSx/CdS and NiSx@CdS samples. As displayed in Figure S1, over the 20 hours’ reaction, neither NiSx/CdS nor NiSx@CdS photocatalyst showed apparent decrease in

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the photocatalytic activity, indicating their good stabilities. Since both NiSx cocatalysts and CdS host photocatalysts were synthesized in the one-step hydrothermal process, the photocatalytic properties of NiSx/CdS samples should be influenced by the hydrothermal time. As shown in Figure 1b, with the prolonging hydrothermal time, the hydrogen generation rate of NiSx/CdS photocatalysts firstly increased and subsequently decreased. The NiSx/CdS sample hydrothermally treated for 48 h owned the best performance. The reduced photocatalytic activity of NiSx/CdS sample treated for more than 48 h should result from the aggregation of NiSx nanoparticles, as a poor NiSx dispersion could be observed in Figure S2. Phase identification of as-prepared samples was done by X-ray diffraction (XRD). As shown in Figure 2, the diffraction peaks of hydrothermally synthesized CdS could be well assigned to hexagonal CdS (JCPDS no. 77-2306). The diffraction peaks of NiSx sample observed at 2θ values of 30.15°, 34.67°, 45.92°, and 53.55° could be indexed to (100), (101), (102) and (110) planes of hexagonal NiS (α-NiS, JCPDS no. 75-0613).36 Furthermore, it was noted that NiSx sample owned other phases, including rhombohedral NiS (β-NiS, JCPDS no. 86-2281) and cubic Ni3S4 (JCPDS no.47-1739). However, the main diffraction peaks of NiSx sample corresponded to hexagonal NiS, which indicated that NiSx sample had massive hexagonal NiS, as well as less rhombohedral NiS and cubic Ni3S4. For NiSx/CdS and NiSx@CdS photocatalysts, the crystal structure of CdS did not change with the adding of NiSx, and no obvious NiSx peaks were observed, possibly owing to the low amount of NiSx.36,39,44

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Figure 2 XRD patterns of CdS, NiSx, NiSx/CdS, and NiSx@CdS samples. In order to elucidate the growth mechanism of NiSx cocatalysts and the matching principle between NiSx cocatalysts and CdS host photocatalysts in NiSx/CdS and NiSx@CdS, time-dependent morphologies of NiSx/CdS-t, NiSx@CdS-t, CdS-t, and NiSx-t samples were characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM), where t represented the hydrothermal time for the products. In the present study, the hydrothermal time of 0, 6, and 48 h was chosen for analysis due to the apparently varied photocatalytic properties of NiSx/CdS-0, NiSx/CdS-6, and NiSx/CdS-48. Figure 3 shows SEM and TEM images of various NiSx/CdS-t samples synthesized by one-step hydrothermal process. As observed in Figure 3a and 3d, plenty of nanosheets and nanoflakes were stacked together before hydrothermal treatment. SEM and TEM images of single CdS-0 (Figure S3a and S3b) and NiSx-0 (Figure S3c and S3d) demonstrated that nanosheets and nanoflakes in NiSx/CdS-0 corresponded to CdS and NiSx respectively. XRD patterns of CdS-0 and NiSx-0 revealed that hexagonal CdS with low crystallinity and amorphous NiSx were obtained (Figure S4).

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Figure 3b and 3e exhibited the formation of polyhedral nanocrystals in NiSx/CdS-6 products after 6 hour hydrothermal treatment. Similar morphology of hydrothermally treated CdS sample (Figure S3e) revealed that the polyhedral nanocrystals were comprised of CdS. Besides, several NiSx nanoparticles dispersed on the surface of polyhedral CdS nanocrystals were observed in the SEM and TEM images of NiSx/CdS-6. The average particle size of NiSx in NiSx/CdS-6 sample was determined to be 8.2 nm (Figure S5a), and the average particle size of CdS in NiSx/CdS-6 sample was determined to be 100 nm (Figure S6a and S6b). After 48 hour hydrothermal process, numerous NiSx nanoparticles uniformly deposited on the surface of CdS nanocrystals could be detected (Figure 3c and 3f). Compared to NiSx/CdS-6, NiSx nanoparticles grew up from 8.2 nm to 14.7 nm (Figure S5b), and the increased CdS particle size was also observed in NiSx/CdS-48 (Figure S6c and S6d). The morphology of typical NiSx nanoparticle in NiSx/CdS-48 (marked by white circle) was enlarged in Figure 3l. Close contact between NiSx nanoparticle and CdS nanocrystal could be observed. The lattice spacing of 0.296 nm could be assigned to the (100) plane of hexagonal or rhombohedral NiS (α(β)-NiS). The energy-dispersive X-ray (EDS) spectrum confirmed the existence of S, Cd, and Ni elements in NiSx/CdS-48 sample (Figure 3g). The weak Ni signals should result from the high dispersity of NiSx nanoparticles. The elemental mapping of NiSx/CdS-48 sample (Figure 3h-k) revealed that individual NiSx nanoparticle was successfully distributed on the surface of CdS polyhedral nanocrystal. Only the signals of Cd and S could be observed in the CdS nanocrystal, which indicated that Ni ions were not incorporated 12


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into CdS lattice.

Figure 3 (a) SEM image of NiSx/CdS-0; (b) SEM image of NiSx/CdS-6; (c) SEM image of NiSx/CdS-48; (d) TEM image of NiSx/CdS-0; (e) TEM image of NiSx/CdS-6; (f) TEM image of NiSx/CdS-48; (g) EDS spectrum of NiSx/CdS-48; (h-k) TEM-EDS mapping of NiSx/CdS-48; (l) HRTEM image of NiSx/CdS-48. As a comparison, the morphologies of various NiSx@CdS-t samples synthesized by two-step hydrothermal process were examined. As shown in Figure 4a and 4d, CdS polyhedral nanocrystals prepared by the first step of hydrothermal treatment were covered with NiSx nanoflakes in NiSx@CdS-0. After 6 hour hydrothermal process, aggregated NiSx nanoparticles were formed on the surface of CdS polyhedral nanocrystals, leading to poor dispersity of NiSx nanoparticles (Figure 4b and 4e). The average particle size of NiSx in NiSx@CdS-6 sample was determined to be 18.1 nm 13



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(Figure S7a). When the hydrothermal time was further extended to 48 hours, the agglomeration of NiSx nanoparticles became more serious (Figure 4c and 4f), and the average particle size of NiSx was increased to 26.8 nm (Figure S7b). However, the morphology and size distribution of CdS nanocrystals in NiSx@CdS samples were not apparently changed after the second step of hydrothermal treatment (Figure 4a and S8). The agglomeration of NiSx nanoparticles was further evidenced in the HRTEM image of the integrated NiSx@CdS-48 sample (Figure 4l). It was observed that the lattice spacing of 0.197 nm could be attributed to the (102) plane of hexagonal NiS (α-NiS). The EDS spectrum confirmed that NiSx@CdS-48 sample consisted of S, Cd, and Ni elements (Figure 4g). The elemental mapping of NiSx@CdS-48 nanoparticles (Figure 4h-k) revealed that aggregated NiSx nanoparticles were deposited on the surface of CdS polyhedral nanocrystal. The absence of Ni signals in the CdS nanocrystal indicated that Ni ions were not doped into CdS.

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Figure 4 (a) SEM image of NiSx@CdS-0; (b) SEM image of NiSx@CdS-6; (c) SEM image of NiSx@CdS-48; (d) TEM image of NiSx@CdS-0; (e) TEM image of NiSx@CdS-6; (f) TEM image of NiSx@CdS-48; (g) EDS spectrum of NiSx@CdS-48; (h-k) TEM-EDS mapping of NiSx@CdS-48; (l) HRTEM image of NiSx@CdS-48. As a consequence, the formation process of NiSx/CdS and NiSx@CdS samples prepared by two different routes can be illustrated in Figure 5. In the one-step hydrothermal synthesis of NiSx/CdS, Cd2+, Ni2+ and S2- ions were subsequently added into NaOH aqueous solution with magnetic stirring. The first nucleation involved the precipitation of Cd2+and Ni2+ cations by OH- ions to form Cd(OH)2 and Ni(OH)2. By adding S2- ions, the transition from Cd(OH)2 and Ni(OH)2 to CdS nanosheets and NiSx nanoflakes were collected instantly. After hydrothermal treatment for 6 hours, CdS nanosheets and NiSx nanoflakes were converted to CdS polyhedral nanocrystals 15



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and NiSx nanoparticles respectively. As the hydrothermal process was further extended to 48 hours, the particle sizes of both CdS and NiSx were apparently increased. In the one-step hydrothermal process, NiSx phase and CdS phase were intergrown with each other, which led to well dispersed NiSx nanoparticles on the surface of CdS nanocrystals, as well as close contact between two phases. In the two-step hydrothermal synthesis of NiSx@CdS, CdS nanoparticles were initially synthesized by the first step of hydrothermal process. Then as-prepared CdS nanoparticles, together with Ni2+ and S2- ions were added into NaOH aqueous solution with stirring at room temperature. The precipitation of Ni2+ cations by OH- ions to form Ni(OH)2 and the subsequent transition from Ni(OH)2 to NiSx nanoflakes by adding S2- ions were collected instantly. In the second step of hydrothermal process, NiSx nanoflakes grew up to NiSx nanoparticles, and the particle size was increased with the prolonging hydrothermal time. In the two-step hydrothermal process, NiSx cocatalysts were separately formed. The agglomeration of NiSx nanoparticles could not be avoidable, which led to reduced contacts between NiSx cocatalysts and CdS host photocatalysts.

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Figure 5 Schematic diagram of the formation process for (a) NiSx/CdS prepared by a one-step hydrothermal method and (b) NiSx@CdS prepared by a common two-step hydrothermal method. To analyze the chemical states of nickel in NiSx, NiSx/CdS and NiSx@CdS, XPS measurements were carried out. As seen in Figure 6, the binding energies of Ni 2p3/2 for NiSx sample were located at 852.5 eV and 855.5 eV, revealing that both NiS and Ni3S4 were achieved.38 This phenomenon was consistent with the XRD result. The binding energies of Ni 2p3/2 for NiSx@CdS sample were the same as those for NiSx, indicating their similar electronic states. However, NiSx/CdS sample exhibited slightly lower binding energies of Ni 2p3/2 compared to single NiSx. The binding energy shift should be ascribed to the electronic interaction between NiSx and CdS, which resulted from the intimate contact via the one-step hydrothermal synthesis.45,46 Nevertheless, the binding energies of Cd 3d for all samples were almost the same (Figure S9), probably due to the low loading amount of NiSx, which could not supply enough interaction to alter the Cd-S bond strength. Therefore, it was demonstrated that one-step hydrothermal treatment could result in close contact between NiSx cocatalysts and CdS host photocatalysts, which were crucial to the efficient charge transfer. However, the XPS result indicated that the contact between NiSx and CdS could be poor in NiSx@CdS prepared by two-step hydrothermal method, since NiSx cocatalysts were separately fabricated in the second step.

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Figure 6 Ni 2p XPS spectra of NiSx/CdS, NiSx@CdS and NiSx. UV-vis absorption spectra of CdS, NiSx, NiSx/CdS and NiSx@CdS are shown in Figure 7a. Pure NiSx displayed complete absorption in the observed wavelength range, while pure CdS had sharp absorption edge at around 540 nm, corresponding to the band gap of 2.4 eV.47 NiSx/CdS and NiSx@CdS exhibited strong absorption in the region of 540-750 nm from NiSx, and the absorption edges corresponding to CdS were close to that of pure CdS. It was reported that a red shift of the absorption edge could be observed for Ni-doped CdS compared to single CdS.48 In our work, the accurate bandgaps of different samples were estimated from the tangent line in the plot of (αhν)2 versus hν (Figure 7b), where α is the optical absorption coefficient and hυ is the photon energy. The consistent bandgaps were obtained for CdS, NiSx/CdS and NiSx@CdS samples, indicating that nickel ions were not incorporated into CdS lattices to modify the bandgaps of CdS in NiSx/CdS and NiSx@CdS samples.49 Owing to the different absorption features, varied colors of as-prepared products could be achieved (Figure S10).

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Figure 7 (a) UV-vis absorption spectra of CdS, NiSx, NiSx/CdS and NiSx@CdS; (b) The plots of (αhν)2 versus hν for CdS, NiSx/CdS and NiSx@CdS. The charge transfer behavior for the different samples was investigated through the transient photocurrent experiments. The photoelectrodes for photoelectrochemical tests were fabricated by electrophoretic deposition (EPD) of photocatalysts onto fluorine-doped tin oxide (FTO) coated glass substrates. As shown in Figure 8a, pure CdS showed the lowest photocurrent, which could be attributed to the fast recombination of photogenerated charges. NiSx cocatalysts could significantly enhance the photocurrent of CdS host photocatalysts. Compared to NiSx@CdS, NiSx/CdS exhibited much higher photocurrent density, indicating that more efficient charge transfer was achieved. In addition, The electrochemical impedance 19



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spectroscopy (EIS) Nyquist plots (Figure 8b) showed that the impedance of pure CdS was the highest, and NiSx/CdS owned the smallest impedance, which further proved that most efficient charge transfer was achieved for NiSx/CdS.

Figure 8 (a) Transient photocurrent responses of NiSx/CdS, NiSx@CdS and CdS photoelectrodes; (b) EIS Nyquist plots of NiSx/CdS, NiSx@CdS and CdS photoelectrodes. A 500 W xenon lamp coupled with an AM 1.5 filter was used as the light source. Photocurrent density and EIS were measured in the 0.5 M Na2SO3 aqueous solution as the electrolyte. Photocurrent density was measured at -0.3 V vs Hg/Hg2Cl2. EIS measurement was done at open circuit potential. The amplitude of the sinusoidal wave was 5.0 mV and the frequency range examined was from 100 KHz to 0.1 Hz. As discussed above, well dispersed NiSx nanoparticles, together with close contact between NiSx and CdS could be obtained for NiSx/CdS prepared by the one-step route 20


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(Figure 9). The close contact could promote the efficient charge transfer from CdS to NiSx. The uniform dispersity of NiSx nanoparticles could provide more contacts with CdS to facilitate the charge transfer, and avoid the charge recombination in the crystal boundaries of cocatalysts. Meanwhile, the NiSx particle size in NiSx/CdS was smaller than that in NiSx@CdS. The smaller NiSx particle size could accelerate the charge transfer from NiSx cocatalyst to the reaction solution, lowering the possibilities of charge recombination in individual NiSx nanoparticle. Therefore, the superior efficiency of charge separation for NiSx/CdS should be ascribed to the close contact between NiSx and CdS, as well as better dispersity and smaller size of NiSx nanoparticles. On the contrary, agglomeration of NiSx nanoparticles, bigger NiSx particle size, and poor contact between NiSx and CdS were observed for NiSx@CdS prepared by the two-step route, which led to the lower efficiency of charge separation. Figure S11 shows the action spectrum for hydrogen evolution in 30 vol% lactic acid aqueous solution over NiSx/CdS photocatalysts. The onset of the action spectrum agreed well with the absorption edge, which revealed that the visible-light response for hydrogen evolution was due to the bandgap transition of CdS host photocatalysts.50 NiSx acted as cocatalysts to improve the activity and could not generate hydrogen alone. In order to improve the charge separation for efficient photocatalytic hydrogen production, sacrificial electron donors were often added to consume the photogenerated holes. Herein, different sacrificial electron donors were investigated, and lactic acid was demonstrated to be the optimal one for NiSx/CdS photocatalysts (Figure S12). Lactic acid in the reaction solution served as the 21



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sacrificial electron donor, and the oxidation of lactic acid to pyruvic acid was the only pathway for the consumption of photogenerated holes.39 In addition, the photocatalytic activities of NiSx/CdS from the aqueous solution with different concentrations of lactic acid were also studied. As shown in Figure S13, the aqueous solution with 30 vol% lactic acid was the best one.

Figure 9 Schematic diagram of photocatalytic hydrogen generation over NiSx/CdS and NiSx@CdS. Besides the effect of charge separation, photocatalytic property is also restricted by the surface reaction rate. As displayed in Figure S14, the overpotential of NiSx electrode (-0.06 V vs RHE) for hydrogen evolution was comparable to that of Pt electrode (-0.03 V vs RHE), which revealed that NiSx acted as an effective cocatalyst for proton reduction. The better dispersity and smaller size of NiSx nanoparticles in NiSx/CdS compared to those in NiSx@CdS could give rise to the larger specific surface area of cocatalysts for proton reduction, leading to higher surface reaction rate. Our recent study revealed that NiSx samples with varied NiS/Ni3S4 molar ratios could be prepared by controlling the sulfur source. The properties of NiSx as cocatalysts for hydrogen production and their HER (hydrogen evolution reaction) activities were 22


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influenced by the NiS/Ni3S4 molar ratios. In particular, the best activity could be obtained with the NiS/Ni3S4 molar ratio of 1.0. The detailed reasons are currently under investigation. CONCLUSIONS In summary, noble-metal-free NiSx/CdS photocatalysts were successfully fabricated for the first time via a one-step hydrothermal method. Compared with NiSx@CdS photocatalysts prepared by the common two-step method, NiSx/CdS photocatalysts showed superior photocatalytic hydrogen generation, with the apparent quantum efficiency at 420 nm as high as 60.4%. To our knowledge, this value is the highest efficiency for NiSx modified CdS photocatalysts, and is among the best efficiencies for visible-light-driven photocatalysts. Time-dependent growth studies revealed that NiSx cocatalysts and CdS host photocatalysts were intergrown with each other in the one-step hydrothermal process, which led to close contact between NiSx and CdS, as well as smaller size and better dispersity of NiSx nanoparticles on the surface of CdS host photocatalysts. These specific characters of NiSx/CdS finally resulted in the efficient charge separation and rapid surface reaction, giving rise to the significantly improved photocatalytic activity. It is considered that the present work can provide a general guidance to develop efficient heterostructured cocatalyst/photocatalyst system for hydrogen generation. ACKNOWLEDGMENT The authors thank the financial support from the National Natural Science Foundation 23



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of China (Nos. 51236007 and 51323011), the grant support from the China Postdoctoral Science Foundation (No. 2014M560768), and the China Fundamental Research Funds for the Central Universities. ASSOCIATED CONTENT Supporting Information Long-time photocatalytic test of NiSx/CdS and NiSx@CdS; SEM images of NiSx/CdS-48 and NiSx/CdS-72; SEM and TEM images of CdS-0, CdS-48 and NiSx-0; XRD patterns of CdS-0 and NiSx-0; TEM images of NiSx/CdS-6 and NiSx/CdS-48 and the size distribution of NiSx nanoparticles; SEM images of NiSx/CdS-6 and NiSx/CdS-48; TEM images of NiSx@CdS-6 and NiSx@CdS-48 and the size distribution of NiSx nanoparticles; SEM images of NiSx@CdS-6 and NiSx@CdS-48; Cd 3d XPS spectra of NiSx/CdS, NiSx@CdS, and CdS samples; Photograph of NiSx, NiSx/CdS, NiSx@CdS, and CdS samples; The action spectrum for hydrogen evolution in 30 vol% lactic acid aqueous solution over NiSx/CdS photocatalysts and their UV-vis absorption spectrum; Photocatalytic activities of NiSx/CdS under different sacrificial systems; Photocatalytic activities of NiSx/CdS in the aqueous solution with different concentrations of lactic acid; Current-voltage curves of the NiSx, Pt, and FTO electrodes. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author 24


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Intergrowth of Cocatalysts with Host Photocatalysts ... - ACS Publications

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