Enhanced photocatalytic H2 evolution on ultrathin Cd0.5Zn0.5S nanosheets without a hole scavenger: Combined analysis of surface reaction kinetics and energy-level alignment

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Highlights

  • Cd0.5Zn0.5S nanosheets were employed for H2 evolution without a hole scavenger.

  • NaOH boosts hole scavenging and lessens Cd0.5Zn0.5S photocorrosion.

  • OH adsorption is a rate-limiting step in photocatalytic H2 generation.

  • The 'photocatalyst-electrolyte' strategy is applicable to other chalcogenides.

Abstract

The surface photocatalytic water splitting kinetics of ultrathin Cd0.5Zn0.5S nanosheets without hole scavengers is critically examined, focusing on the effects of the pH and NaOH as an electrolyte. In aqueous NaOH, hole scavenging from Cd0.5Zn0.5S is enhanced, which reduces catalyst photocorrosion. Based on the Langmuir-Hinshelwood model, OH adsorption on Cd0.5Zn0.5S is the rate-limiting step of the whole process, and the reaction rate increases with increase in pH and temperature. The hydrogen evolution reaction (HER) activity of Cd0.5Zn0.5S reaches 12.44 and ∼ 20 mmol‧g−1‧h−1 at 303.15 K and 313.15 K (6 M NaOH, 0.5 h), respectively, two orders of magnitude higher than that in pure water. At high pH, direct OH oxidation is favored, enhancing water splitting and indicating that water oxidation shifts to a ∙OH production pathway with activation energy approximately of 23.73 kJ‧ mol−1. Promisingly, this strategy to improve H2 production rates is applicable to other chalcogenide photocatalysts (CdS, Cd0.5Mn0.5S, ZnSe and Cd0.5Zn0.5Se).

Introduction

Solar energy conversion coupled with photocatalytic water splitting has aroused considerable attention because of its potential low cost and simplicity compared with fossil-fuel-based H2 generation. Over the past few decades, numerous materials for photocatalytic H2 evolution have been developed, but most require hole scavengers [1], [2]. Crucially, the basic reaction pathway of the H2 evolution half reaction has been determined, and, based on this mechanism, when using sacrificial agents, the overall process is energetically downhill, and, thus, the photon energy cannot be stored [3], [4], [5].

Recently, light-driven stoichiometric water splitting into H2 and O2 has been demonstrated over graphitic carbon nitride (g-C3N4) and titanate-based photocatalysts [6], [7], [8]. As alternatives, metal chalcogenide are another class of promising photocatalyst with particularly high prospect. In fact, CdxZn1-xS (0 < x < 1) and others materials show very high activities for the isolated H2 evolution half-reaction (HER), and they also have an appropriate band alignment. Therefore, they have potential for use in related redox reactions [9]. However, water oxidation over CdxZn1-xS involves multiple hole-transfer processes, and the detrimental oxidation of lattice S2- occurs in pure water (so-called photocorrosion) [10], which is one reason why analogous chalcogenides nanocrystals (e.g., CdS and Cd0.5Mn0.5S) have been rarely reported for H2 evolution from water without a hole scavenger, despite the impressive advances in HER catalysts. To date, methods such as co-catalyst loading and surface heterojunction engineering have been used to prevent the self-corrosion of chalcogenide nanocrystals in pure water, but these methods have limited effects because of the contact between the catalyst and water (especially oxygen-rich water), as well as the slow hole extraction at the liquid–surface interface [11]. Therefore, the construction of core–shell structures by coating CdxZn1-xS with an anti-photocorrosion layer (e.g., Cr2O3 and Al2O3) has been explored as a strategy to prevent self-corrosion, but this reduces the intrinsic catalytic activity and decreases the surface reaction efficiency because of the blocked active sites [12], [13].

Interestingly, the stability of chalcogenide nanocrystals is strongly correlated with the efficiency of the removal of surface holes in the reaction system (hole scavenging) [14], [15]. In addition, the kinetics of water splitting is determined by whether the water oxidation half-reaction (WOR, which is a hole-receiving process) can proceed efficiently [16], [17], and various studies have identified a correlation between photocatalyst stability and the surface reaction rate. In other words, the enhancement of the surface reaction kinetics, especially with regard to efficient WOR process (efficient hole receiving process), could inhibit the competing surface lattice S2- oxidation. However, it is difficult to enhance the hole-dependent WOR process without using hole a scavenger, and, thus, the use of scavenger-free systems has been rarely investigated in the field of photocatalysis.

In addition, the rate and product distribution of photochemical reactions are largely determined by the reaction environment. For instance, MacFarlane et al. revealed that H2O2 synthesis on a MnOx photoanode requires the use of a butyl ammonium bisulfate electrolyte [18].Besides, Sayama’s group reported that the use of a Na2CO3 electrolyte could reduce the onset potential of BiVO4 while achieving the selective value-added production of H2O2 rather than cheap O2 [19]. ClO4, CO32−, HPO32−/H2PO32−, and HCO3 have been demonstrated to be conducive to the two-electron WOR process [20], [21]. Therefore, a rational question is whether there is any synergy between CdxZn1-xS and the electrolyte ions in the water splitting reaction. In particular, if the WOR could be regulated via synergistic effects between the reaction medium and the catalyst, the surface reaction kinetics of CdxZn1-xS could be enhanced significantly.

In this paper, we propose a surface reaction enhancement strategy for chalcogenides and present an explanation that is consistent with the classical reaction kinetics and thermodynamic theory. Specifically, ultrathin Cd0.5Zn0.5S nanosheets were employed for water splitting, and NaOH was used as the electrolyte. NaOH is frequently used for the electrolysis of water because of its higher ionic conductivity (approximately 10 S‧m−1 compared with that of 0.05 S‧m−1 pure water), which reduces overpotential losses. Such a “two birds with one stone” strategy could enhance the surface reaction kinetics and alleviate Cd2+ leakage from Cd0.5Zn0.5S without needing any cocatalyst or sacrificial agent. We found that the HER activity exhibited by Cd0.5Zn0.5S was improved by two orders of magnitude compared to that in pure water simply by regulating the reaction conditions, and, in the presence of NaOH, the WOR pathway kinetics was shifted to favor ‧OH production. Importantly, this strategy was found to be suitable for other chalcogenides photocatalysts (e.g., CdS, ZnSe, Cd0.5Zn0.5Se and Cd0.5Mn0.5S). This proof-of-concept demonstration of a “photocatalyst–electrolyte” effect on water splitting provides novel insights into photocatalytic solar energy conversion.

Section snippets

Synthesis of ultrathin Cd0.5Zn0.5S nanosheets

First, 2 mmol Zn(NO3)2·6H2O was dissolved in a mixture of 10 mL H2O and 50 mL diethylenetriamine (DETA). Subsequently, 250 mg thiourea was added, and the solution was heated at 160 °C for 16 h. Next, the produced sediment (ZnS(DETA)0.5) was centrifuged and dried for further use.

Secondly, 89.3 mg CdCl2·2.5H2O was dissolved in 70 mL ethyl alcohol, and 100 mg ZnS(DETA)0.5 was dispersed into the solution by ultrasonic treatment for 30 min. The reaction mixture was maintained at 160 °C for 4 h, and,

Physical characterization

As shown in Scheme 1, ultrathin Cd0.5Zn0.5S nanosheets were synthesized through the cation exchange [24], [25]. First, the inorganic–organic hybrid ZnS(DETA)0.5 was synthesized as the template substrate for the Cd0.5Zn0.5S nanosheets. The X-ray diffraction (XRD) patterns and transmission microscopy (TEM) images in Figs. S2 and S3, respectively, reveal that the orthorhombic ZnS(DETA)0.5 formed smooth flake-like particles, and no lattice fringes were observed. In addition, the component elements

Conclusion

A “photocatalyst–electrolyte” effect on chalcogenide catalysts was observed in which the activity exhibited by Cd0.5Zn0.5S could be enhanced by two orders of magnitude simply by regulating the reaction medium and temperature. The increased OH concentration and reaction temperature enhanced reactant adsorption and water dissociation in the water splitting rate-limiting step, thus facilitating H2 evolution significantly. In addition, the enhanced surface reaction significantly reduced the

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

Thanks National Natural Science Foundation of China (No. 21676213, 22078261 and 11974276), the China Postdoctoral Science Foundation (No. 2016M600809), Natural Science Basic Research Plan in Shaanxi Province of China (No. 2020JM-422, 2018JM5020) for the financial support of this work. The authors also would like thank Shiyanjia lab (www.shiyanjia.com) for the support of ICP-OES test.

References (60)

  • L. Yu et al.

    Inhibition of photocorrosion and photoactivity enhancement for ZnO via specific hollow ZnO core/ZnS shell structure

    Appl. Catal.B: Environ.

    (2015)
  • R. Williams et al.

    Zinc sulfide surface chemistry: an electrokinetic study

    J Colloid. Interf. Sci.

    (1985)
  • Q. Zhang et al.

    Surface ionization and complexation at the sphalerite/water interface: I. computation of electrical double-layer properties of sphalerite in a simple electrolyte

    J Colloid. Interf. Sci.

    (1995)
  • J. Zhensheng et al.

    Investigation of the functions of CdS surface composite layer and Pt on treated Pt/CdS for photocatalytic dehydrogenation of aqueous alcohol solutions

    J. Mol. Catal.

    (1989)
  • R. Shen et al.

    Nanostructured CdS for efficient photocatalytic H2 evolution: a review纳米结构硫化镉光催化分解水产氢综述

    Sci. China Mater.

    (2020)
  • L. Lin, T. Hisatomi, S. Chen, T. Takata, K. omen, Visible-Light-Driven Photocatalytic Water Splitting: Recent Progress...
  • Z. Wang et al.

    Recent developments in heterogeneous photocatalysts for solar-driven overall water splitting

    Chem. Soc. Rev.

    (2019)
  • M.J. Berr et al.

    Hole scavenger redox potentials determine quantum efficiency and stability of Pt-decorated CdS nanorods for photocatalytic hydrogen generation

    Appl. Phys. Lett.

    (2012)
  • Q. Wang et al.

    Particulate photocatalysts for light-driven water splitting: mechanisms, challenges, and design strategies

    Chem. Rev.

    (2020)
  • T. Takata et al.

    Photocatalytic water splitting with a quantum efficiency of almost unity

    Nature

    (2020)
  • L. Lin et al.

    Molecular-level insights on the reactive facet of carbon nitride single crystals photocatalysing overall water splitting

    Nat. Catal.

    (2020)
  • S. Chen et al.

    Semiconductor-based photocatalysts for photocatalytic and photoelectrochemical water splitting: will we stop with photocorrosion?

    J. Mater. Chem. A

    (2020)
  • C.M. Wolff et al.

    All-in-one visible-light-driven water splitting by combining nanoparticulate and molecular co-catalysts on CdS nanorods

    Nat. Energy

    (2018)
  • B. Weng, M. Qi, C. Han, Z. Tang, Yi. Xu, Photocorrosion Inhibition of Semiconductor-Based Photocatalysts: Basic...
  • X. Ning et al.

    Photocorrosion inhibition of CdS-based catalysts for photocatalytic overall water splitting

    Nanoscale

    (2020)
  • R. Shi et al.

    Interstitial P-doped CdS with long-lived photogenerated electrons for photocatalytic water splitting without sacrificial agents

    Adv. Mater.

    (2018)
  • D. Wang et al.

    Identifying the key obstacle in photocatalytic oxygen evolution on rutile TiO2

    Nat. Catal.

    (2018)
  • T. Simon et al.

    Redox shuttle mechanism enhances photocatalytic H2 generation on Ni-decorated CdS nanorods

    Nat. Mater.

    (2014)
  • A. Izgorodin et al.

    Low overpotential water oxidation to hydrogen peroxide on a MnOx catalyst

    Energ. Environ. Sci.

    (2012)
  • K. Fuku et al.

    Efficient oxidative hydrogen peroxide production and accumulation in photoelectrochemical water splitting using a tungsten trioxide/bismuth vanadate photoanode

    Chem. Commun.

    (2016)
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