Synergetic effect of polyoxoniobate and NiS as cocatalysts for enhanced photocatalytic H₂ evolution on Cd_0.65Zn_0.35S

Abstract

Composite photocatalyst K₇HNb₆O₁₉–NiS/Cd_0.65Zn_0.35S is demonstrated to be highly active for H₂ evolution under visible light due to the introduction of K₇HNb₆O₁₉ and NiS as cocatalysts. In the absence of noble metals, a high quantum yield of 42% was achieved at 420 nm.

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Conversion of solar energy to chemical energy by producing hydrogen fuel from water splitting has been one of the attractive strategies to address the environmental crises and energy shortage issues.¹ Since the first discovery by Fujishima and Honda in 1972,² semiconductor photocatalysts for water splitting have been extensively studied. Great efforts have been devoted to developing strategies for improving the efficiency of the photocatalysts. One commonly used method for controlling the band structure is forming solid solution between wide and narrow band gap semiconductors.³ Cd_1−xZn_xS is one of the most extensively studied sulfides solid solution due to its controllable band structure and excellent performance under visible light irradiation.⁴ However, the efficiency of photocatalytic H₂ evolution for these solid solutions is far from satisfying level owing to the rapid recombination of photo-generated electrons and holes in photocatalysts. Therefore, it is highly desirable to optimize the photocatalytic system, aiming not only to promote the separation of photogenerated carriers, but also to facilitate their transport pathways. One strategy is to modify the surface of photocatalysts with co-catalysts (normally noble-metals such as Pt and Pd) by forming Schottky junctions and lowering the activation potentials for H₂ evolution. To reduce the cost of photocatalytic H₂ production, it is necessary to explore alternative inexpensive co-catalysts. Recently, some transition-metal compounds such as MoS₂, NiS have been found to be potential co-catalysts for water reduction.⁵

Polyoxometalates (POMs), a class of molecular metal-oxo cluster compounds based mainly on Mo, W, V, Nb and Ta elements, exhibit remarkably rich redox and photochemical properties.⁶ Among POMs, polyoxoniobates are of particular interest for photocatalysis because some dense niobates have been used for overall water-splitting photocatalysts. Recently, several molecular photocatalytic systems based on polyoxoniobates and polytantalotungstates have been studied for water splitting into H₂.⁷ However, their large band gaps make them only UV-active and have to be assisted by noble metal Pt cocatalyst. In addition, the low yield of these crystalline compounds limits their practical application. Our recent researches found that POMs could act as electron acceptors to promote the power conversion efficiency of semiconductor thin-film solar cells.⁸ However, POMs are rarely used in semiconductor-based photocatalytic H₂ production systems.⁹ Therefore, we attempt to introduce the effect of POMs on the photocatalytic H₂ evolution systems of semiconductors.

Herein, we report the synthesis and evaluation of a low-cost and stable K₇HNb₆O₁₉–NiS/Cd_0.65Zn_0.35S photocatalytic system. It is shown that the activity of Cd_0.65Zn_0.35S is enhanced by the presence of the polyoxoniobate K₇HNb₆O₁₉ (Linqvist structure,¹⁰ denoted as Nb₆) and NiS as cocatalysts. The H₂ evolution rate is increased to 3.4 times that of Cd_0.65Zn_0.35S. With the optimization of the concentration of the sacrificial reagents, a high quantum yield of 42% at 420 nm was achieved without any noble-metal co-catalyst. The high photocatalytic activity arises from the positive synergetic effect between Nb₆ and NiS in the cocatalysts, which serve as electron collectors and active adsorption sites.

X-ray diffraction (XRD) patterns of Cd_0.65Zn_0.35S samples exhibit hexagonal phase as shown in Fig. S1.† The successive shifts of the XRD patterns, compared with pure CdS, indicate that the crystals obtained were not a mixture of ZnS and CdS, but Cd_1−xZn_xS solid solution.^4a The weak signals of NiS in the Nb₆–NiS/Cd_0.65Zn_0.35S sample indicate that NiS could possibly adopt a rhombohedral structure.^5b No characteristic diffraction peaks associated with Nb₆ was observed, suggesting that Nb₆ is highly dispersed on the surface of Cd_0.65Zn_0.35S sample.¹¹ The UV-visible diffuse reflectance spectra of Cd_0.65Zn_0.35S and Nb₆–NiS/Cd_0.65Zn_0.35S were displayed in Fig. S2.† The Cd_0.65Zn_0.35S sample has an absorption edge at about 540 nm, with an energy gap estimated to be 2.46 eV. What's more, an enhanced absorption was observed in the spectrum of Nb₆–NiS/Cd_0.65Zn_0.35S in the visible light region from 500 nm to 800 nm, which is attributed to the NiS loaded on Cd_0.65Zn_0.35S.¹²

The X-ray photoelectron spectra (XPS) were carried out to determine the surface chemical compositions and electronic state of the as-prepared samples. The survey scan spectrum (Fig. S3a†) confirms the presence of Cd, Zn, S, Ni, Nb and O in the composite sample, which indicates the successful synthesis of the Nb₆–NiS/Cd_0.65Zn_0.35S composites. The peak located at 1023.3 eV in Fig. S3b† corresponds to the Zn 2p3/2 level, and the two sharp peaks at 405.2 and 412.0 eV are attributed to the Cd 3d5/2 and Cd 3d3/2 level, respectively. Meanwhile, the binding energy values for Ni 2p and Nb 3d (Fig. S3b†) are in agreement with the data reported in the literature.^5b,13 In addition, the two subpeaks at 161.2 and 162.4 eV can be assigned to S 2p in Cd_0.65Zn_0.35S and NiS, respectively (Fig. S3a†).^5b The XPS results confirm that Nb₆ and NiS were successfully loaded on the surface of Cd_0.65Zn_0.35S and no valence change was found after they were introduced into the composite photocatalysts. However, slight binding energy shifts for Nb₆–NiS/Cd_0.65Zn_0.35S, compared with pure Cd_1−xZn_xS,^4c might derive from the electronic interactions among Nb₆, NiS and Cd_0.65Zn_0.35S.

The SEM image reveals that the Cd_0.65Zn_0.35S nanoparticles are in the range of 110–230 nm (Fig. 1a). As shown in Fig 1b, aggregated NiS nanoparticles were loaded on the surface of Cd_0.65Zn_0.35S. And Nb₆ was dispersed on the surface of Cd_0.65Zn_0.35S and NiS nanoparticles (Fig 1c). The presence of NiS on the Cd_0.65Zn_0.35S surface is further evidenced by the HRTEM images (Fig. S4†). Although the HRTEM image does not show lattice fringes corresponding to Nb₆, the existence of Nb is evidenced from the energy-dispersive X-ray spectrometry (Fig 1d). It is suggested that amorphous state Nb₆ exists in the as-prepared Nb₆–NiS/Cd_0.65Zn_0.35S. These results are in accordance with the XRD and XPS analyses.

Fig. 1 SEM images of Cd_0.65Zn_0.35S (a), NiS/Cd_0.65Zn_0.35S (b), Nb₆–NiS/Cd_0.65Zn_0.35S (c) and EDS (d).

Fig. 2 shows the rate of H₂ evolution on the pristine Cd_0.65Zn_0.35S solid solution and Nb₆–NiS/Cd_0.65Zn_0.35S photocatalysts, together with that on NiS/Cd_0.65Zn_0.35S, Nb₆/Cd_0.65Zn_0.35S, Nb₆–Pt/Cd_0.65Zn_0.35S, NiS and Nb₆ for comparison. As seen in Fig. 2, no H₂ was detected when NiS or Nb₆ was alone used as the photocatalyst, suggesting that both NiS and Nb₆ are inactive for photocatalytic H₂ evolution under visible light. Cd_0.65Zn_0.35S alone shows activity in photocatalytic H₂ evolution with a rate of 982 μmol h⁻¹ g⁻¹. After loading 1 wt% of NiS on Cd_0.65Zn_0.35S, the rate of H₂ evolution on NiS/Cd_0.65Zn_0.35S is increased to 1.29 mmol h⁻¹ g⁻¹. When 1 wt% Nb₆ is further introduced into NiS/Cd_0.65Zn_0.35S, the H₂ evolution rate is remarkably increased to 3.34 mmol h⁻¹ g⁻¹, which is about 3.4 times that obtained on the pure Cd_0.65Zn_0.35S sample and 2.6 times that of NiS/Cd_0.65Zn_0.35S. However, as for the Nb₆/Cd_0.65Zn_0.35S system in the absence of NiS, the photocatalytic H₂ production activity decreased compared with that of pure Cd_0.65Zn_0.35S sample owing to the lack of active sites for H₂ evolution. The excellent photocatalytic activity of Nb₆–NiS/Cd_0.65Zn_0.35S is even higher than that on Nb₆–Pt/Cd_0.65Zn_0.35S. These results indicate that three components Nb₆, NiS and Cd_0.65Zn_0.35S are well matched in the photocatalyst system with Nb₆ and NiS as electron collectors and active adsorption sites. Photocatalytic activities of NiS/Cd_0.65Zn_0.35S photocatalysts with different NiS contents have also been investigated and the sample with 1 wt% NiS gave the highest H₂ production. Therefore, 1 wt% NiS was applied in all Nb₆–NiS/Cd_0.65Zn_0.35S composites.

Fig. 2 Photocatalytic hydrogen evolution over different photocatalysts. Reaction condition: 0.2 g catalyst; 200 ml aqueous solution containing 0.1 M Na₂S and 0.1 M Na₂SO₃; light source: 300 W Xe lamp (λ > 400 nm).

To optimize the photocatalytic conditions for the highest hydrogen yield, the amount of Nb₆ loaded on NiS/Cd_0.65Zn_0.35S have been investigated. The rate of H₂ evolution on Nb₆–NiS/Cd_0.65Zn_0.35S samples with different amount of Nb₆ were shown in Fig. S5.† After loading only 0.5 wt% of Nb₆ on NiS/Cd_0.65Zn_0.35S, the rate of H₂ evolution is increased from 1.29 mmol h⁻¹ g⁻¹ to 2.01 mmol h⁻¹ g⁻¹. With the increase of the amount of Nb₆, the rate of H₂ evolution on Nb₆–NiS/Cd_0.65Zn_0.35S is increased further and achieves a maximum of 4.16 mmol h⁻¹ g⁻¹ when the loading amount of Nb₆ on NiS/Cd_0.65Zn_0.35S is 2 wt%. However, when the amount of Nb₆ is above 2 wt%, the hydrogen evolution rate of Nb₆–NiS/Cd_0.65Zn_0.35S decreases. It is supposed that excessive Nb₆ will become the recombination centers for photogenerated carriers as electron trap filling sites.⁸

In order to promote the reducing half reaction in water splitting process, the effect of the concentration of sacrificial reagent was also studied. As shown in Fig. S6,† the remarkably improved H₂ evolution activity (7.06 mmol h⁻¹ g⁻¹) was achieved on Nb₆ (2 wt%)–NiS/Cd_0.65Zn_0.35S photocatalyst when the concentration of the sacrificial reagents increases from 0.1 M Na₂S/0.1 M Na₂SO₃ to 0.25 M Na₂S/0.35 M Na₂SO₃. However, further increasing the concentration of sacrificial reagents hardly improved the activity. The apparent quantum yield (QY) using the optimized photocatalyst was 42% at 420 nm, which is close to the highest efficiency for Cd_1−xZn_xS-based photocatalysts ever reported.¹⁴

A tentative mechanism proposed for the high H₂ production activity of the Nb₆–NiS/Cd_0.65Zn_0.35S sample is illustrated in Fig. 3. Under visible light illumination, the valence band (VB) electrons of Cd_0.65Zn_0.35S are excited to the conduction band (CB), creating holes in the VB. The CB electrons of Cd_0.65Zn_0.35S can be injected into Nb₆ because the Nb₆O₁₉⁸⁻/Nb₆O₁₉⁹⁻ redox potential (−0.38 eV) is lower than the CB of Cd_0.65Zn_0.35S. Then NiS is proposed to be able to trap the electrons, forming at first the intermediate HNiS (reaction (1)), and then electrochemically release H₂ (reaction (2)),¹² though NiS occupies much lower CB position compared to the reduction potential of H⁺/H₂.

NiS + e⁻ + H₂O ↔ HNiS + OH⁻ (1)

HNiS + e⁻ + H₂O ↔ NiS + H₂ + OH⁻ (2)

Fig. 3 Proposed charge transfer and separation process over the band-structure controlled Nb₆–NiS/Cd_0.65Zn_0.35S photocatalyst.

The process is similar to the cathodic H₂ evolution reactions in electrolysis. Meanwhile, the photogenerated holes will migrate to the surface to react with the sacrificial agents. The realization of such a high efficiency might be due to a synergetic effect between Nb₆ and NiS on Cd_0.65Zn_0.35S, including the suppression of charge recombination, improvement of interfacial charge transfer, and an increase in the number of active adsorption sites and photocatalytic reaction centers.

Metal sulfides are usually not stable during the photocatalytic reaction and subjected to the photocorrosion. Fig. S7† shows the stability test of Nb₆–NiS/Cd_0.65Zn_0.35S samples for hydrogen evolution. The photocatalytic activity of Nb₆–NiS/Cd_0.65Zn_0.35S did not show any deactivation in the first 6 h and then decreased slightly after 8 h in one continuous reaction (12 h). The decreased activity could be ascribed to the reduced concentration of sacrificial reagents. After a continuous reaction for 12 h, the photocatalyst was centrifuged and then stored overnight. The next day the photocatalyst was redispersed in a new reaction solution with fresh sacrificial reagents. The photocatalytic activity of Nb₆–NiS/Cd_0.65Zn_0.35S was recovered in the third cycle and even higher than the first cycle, which might be due to the reorganization of the active sites on the surface of the photocatalysts.¹⁵ A similar activity was obtained as for the fourth cycle, which implied the composite photocatalyst Nb₆–NiS/Cd_0.65Zn_0.35S has an enhanced photostability.

In order to understand the effect of Nb₆ and NiS in the photocatalytic activity, the surface photovoltage spectra (SPS) measurements¹⁶ and photoluminescent (PL) measurements were performed on the powder samples of the Cd_0.65Zn_0.35S, NiS/Cd_0.65Zn_0.35S and Nb₆–NiS/Cd_0.65Zn_0.35S. As shown in Fig. S8,† the weak photovoltaic response band of Cd_0.65Zn_0.35S appears at 300–550 nm, which can be mainly assigned to the electron transition from the valence band to the conduction band. By contrast, the photovoltaic response band of NiS/Cd_0.65Zn_0.35S became stronger. The strongest photovoltaic response of Nb₆–NiS/Cd_0.65Zn_0.35S suggests that the positive synergetic effect between Nb₆ and NiS can promote the separation and transfer of photoinduced electron and holes in Cd_0.65Zn_0.35S. In addition, the fluorescence of NiS/Cd_0.65Zn_0.35S and Nb₆–NiS/Cd_0.65Zn_0.35S is clearly quenched compared to that of Cd_0.65Zn_0.35S (Fig. S9†). The photoinduced electron transfer from CdS to NiS and/or Nb₆ should be responsible for the fluorescence quenching. Therefore, the introduction of Nb₆ and NiS in the composite photocatalyst could effectively suppress the charge recombination, leaving more photoinduced electron to form reactive species, which in turn results in high photocatalytic H₂ evolution rate.

In conclusion, the enhanced performance of photocatalytic H₂ evolution was achieved under visible light over Nb₆–NiS/Cd_0.65Zn_0.35S using a feasible coprecipitation-surface loading strategy. The as-prepared Nb₆–NiS/Cd_0.65Zn_0.35S sample reached a high H₂ evolution rate of 7.59 mmol h⁻¹ g⁻¹ with 2 wt% Nb₆ and 1% NiS loaded and the QY of 42% at 420 nm. This work represents a significant advance in preparation of Cd_1−xZn_xS based composite catalysts for photocatalytic H₂ evolution by using POMs and transition-metal sulfides as cocatalysts.

Acknowledgements

This project is financially supported by the Natural Science Foundation of China (Grant no. 21001021, 21273031 and 21361027), Jilin Provincial Science and Technology Development Foundation (Grant no. 201201068 and 20140101120JC).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Details of the synthesis, measurements and additional figures, XRD, XPS, SPS, PL, IR, diffuse reflectivity and transient photocurrent response data. See DOI: 10.1039/c4ra01827c

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