Mesoporous Cd_xZn_1-xS with abundant surface defects for efficient photocatalytic hydrogen production

Abstract

The practical application of photocatalytic water splitting for hydrogen evolution hinges on the development of high-efficient and low-cost photocatalysts. Defects engineering has emerged as a promising strategy to enhance photocatalytic activity effectively. Herein, a facile and versatile co-precipitation method is proposed to fabricate mesoporous Cd-Zn-S solid solutions (E-Cd_xZn_1-xS) with abundant surface defects by the inorganic salts formed in the reaction system as self-template. Compared with Cd-Zn-S solid solutions (W-Cd_0.65Zn_0.35S) prepared by the traditional co-precipitation method, the enhanced specific surface area and abundant surface defects endow E-Cd_0.65Zn_0.35S with more accessible active sites and effective separation of electron-hole pairs for the photocatalytic water splitting reaction. The E-Cd_0.65Zn_0.35S solid solution exhibits hydrogen evolution rate of 5.2 mmol h⁻¹ g⁻¹ without loading noble metal as cocatalyst under visible light, which is 1.13 times higher than that of W-Cd_0.65Zn_0.35S sample. The present work provides a simple, low-cost and prospective strategy for the synthesis of defective Cd-Zn-S solid solutions, and it also delivers guidance to design and develop the advanced visible-light photocatalyst in the future.

Introduction

Photocatalysis has been widely considered as a promising and versatile technique to confront the energy crisis and environmental pollution. Many important applications based on photocatalysis have been developed such as pollutant degradation, chemical reactions, CO₂ reduction and hydrogen production [[1], [2], [3], [4]]. Among them, producing hydrogen (H₂) under the visible light is regarded as one of the greenest and sustainable means to realize energy conversion and develop energy carrier. Thus, the efficient photocatalytic systems for hydrogen production via photo-driving water splitting have attracted tremendous attention.

So far, various photocatalysts have been designed and synthesized to achieve efficient photocatalytic hydrogen production, including metal oxides (e.g. TiO₂ [5], ZnO [6], CuO [7],), sulfides (e.g. CdS [8,9], ZnS [10], NiS [11],), metal-free photocatalysts (e.g. g-C₃N₄ [12],), and solid solutions (e.g. Cd-Zn-S [13,14], Zn-In-S [15], Mn-Cd-S [16],). Among them, CdS is considered as one of the most efficient photocatalysts for H₂ evolution owing to its low cost, suitable bandgap, and excellent reactivity. However, several major problems caused by the inherent properties of metal sulfides still exist, such as poor stability (photocorrosion) and rapid photogenerated carrier recombination. Hence, the development of alternative photocatalysts with high active and stability is highly desired. Zn-Cd-S solid solution is a promising ternary metal sulfide for photocatalytic hydrogen production due to its excellent activity, high stability and adjustable band gap by changing the Cd/Zn ratio, and the synthesized methods of Zn–Cd–S solid solutions have been widely explored. For example, Xing et al. [17], prepared Cd_1-xZn_xS solid solutions by the coprecipitation and calcination method using Zn(Ac)₂, Cd(Ac)₂ and NaS₂ as the precursors. Wang et al. [18], reported the synthesis of size-controlled Zn_1-xCd_xS solid solutions by using NaOH to adjust the pH at room-temperature. Shen et al. [19], synthesized Zn_1-xCd_xS solid solutions with the enhanced H₂ evolution activity by the coprecipitation and subsequent hydrothermal process. Biomolecule-assisted hydrothermal synthesis [[20], [21]] was also used to fabricate Zn–Cd–S solid solutions. In addition, some efforts have been devoted to tailor the physicochemical properties of Cd-Zn-S solid solutions by metal doping [22], coupling cocatalyst [23], heterophase junctions [24], and so on.

Recently, defects engineering has been regarded as an effective strategy to enhance photocatalytic activity [25],. The introduction of defects sites (e.g. vacancies and doping defects) into the photocatalysts has important impacts on the photocatalytic performance and the reasons are as follows: (1) the formation of electrons trap induced by defects can suppress the recombination of charge; (2) the improvement of the absorption of visible light can be achieved by defects; (3) defects can serve as active sites to effectively enhance photocatalytic activity. Zhang and coworkers [26], prepared Zn-Cd-S solid solutions with surface defects and proved that sulfur vacancies could greatly improve H₂ production activity. Although numerous Zn_xCd_1-xS solid solutions and Cd_xZn_1-xS-based photocatalysts have impressive performance in photocatalytic H₂ evolution, the engineering of abundant surface sulfur vacancies in porous Cd_xZn_1-xS materials by a simple and scalable method to further enhance the catalytic performance is still lacking and full of challenge.

In this work, mesoporous Cd_xZn_1-xS solid solutions (E-Cd_xZn_1-xS) with abundant surface defects were fabricated by inorganic self-template co-precipitation method at room temperature. The inorganic salt of NaNO₃ generated simultaneously with Cd-Zn-S solid solutions in the reaction system was used as the pore-making and defect-protecting agent. The removal of NaNO₃ can not only lead to the formation of mesoporous structure but also expose more surface defects for Cd_xZn_1-xS. Benefitting from the synergistic effect of well-defined mesoporous structure and abundant surface defects, E-Cd_0.65Zn_0.35S possesses better visible-light photocatalytic H₂ evolution efficiency compared with the sample W-Cd_0.65Zn_0.35S obtained by traditional co-precipitation method. Our work presents an enabling and versatile strategy for the facile synthesis of efficient mesoporous catalysts with enhanced photocatalytic H₂ evolution activity.