Coralline-like Ni₂P decorated novel tetrapod-bundle Cd_0.9Zn_0.1S ZB/WZ homojunctions for highly efficient visible-light photocatalytic hydrogen evolution

Abstract

In this study, Ni₂P-Cd_0.9Zn_0.1S (NPCZS) composites were synthesized by coupling tetrapod bundle Cd_0.9Zn_0.1S (CZS) and coralline-like Ni₂P (NP) via a simple calcination method. CZS shows outstanding activity in photocatalytic hydrogen evolution (1.31 mmol h⁻¹), owing to its unique morphology and heterophase homojunctions (ZB/WZ), which accelerate the separation and transfer of photogenerated charges. After coupling with NP, the photoactivity of NPCZS was enhanced, and the maximum hydrogen evolution rate of 1.88 mmol h⁻¹ was reached at a NP content of 12 wt%, which was 1.43 times higher than that of pure CZS. The experimental results of the photocatalytic activity, viz. photoluminescence spectra, surface photovoltage spectra, and electrochemical test showed that the enhanced photoactivity of NPCZS should be attributed to the synergistic effects of the novel tetrapod-bundle morphology, heterophase homojunctions, and decoration of the NP co-catalyst. Moreover, the as-prepared NPCZS composites exhibited excellent photostability and recyclability. Herein, we propose a possible mechanism for the enhanced photocatalytic activity.

Introduction

Today, the implementation of eco-friendly methods for energy production is vital because the exhaustion of fossil fuels and environmental pollution are becoming increasingly prominent problems [1, 2, 3, 4, 5, 6]. Photocatalytic splitting of water into hydrogen gas (H₂) over semiconductors is one of the most promising approaches to overcome the challenges of energy and environment [7, 8, 9, 10, 11].

So far, numerous semiconductors have been extensively investigated as photocatalysts. Among them, metal sulfides have attracted considerable attention attributed to their suitable band structure and effective performance in the photocatalytic hydrogen evolution reaction (HER) under visible-light irradiation [12, 13]. The catalytic activities of materials are closely related to their morphology [14, 15, 16]. The morphology influences the separation and transfer of photogenerated charges and band gap structure, impacting the photocatalytic activity of the material. Consequently, various morphologies of metal sulfides, such as nanorods [17, 18], hollow nanorods [19], nanowires [16, 20], quantum dots [21, 22] and nanosheets [23], have been investigated in the field of the photocatalytic HER. However, tetrapod metal sulfides with cubic zinc blende (ZB) and hexagonal wurtzite (WZ) structures have seldom been reported. Therefore, we synthesized a tetrapod bundle Cd_0.9Zn_0.1S (CZS) with a ZB phase as the center and a WZ phase as the arms, based on the pioneering research of Xue et al. [24]. In addition, the arms had a new morphology compared to those of previous reports, which was a bundle structure composed of numerous nanorods. Moreover, the tetrapod bundle showed extremely high photocatalytic HER activity. Based on previous research, we considered that the high catalytic performance of the composite might be due to the formation of an electrostatic field between the ZB/WZ heterophase and its unique morphology, which could accelerate the separation and transfer of photogenerated charges [15, 25]. Further, we studied the photocatalytic HER activity of the composite under different morphologies by destroying its tetrapod-bundle structure to various degrees and proposed a possible mechanism for its enhanced activity.

Generally, adding a co-catalyst to the photocatalyst can effectively improve its performance in the photocatalytic HER. Noble metals as co-catalysts can attract and trap photogenerated electrons easily and act as the active center of a H⁺ reduction reaction [26]; numerous kinds of noble metals and their related compounds have been widely investigated, such as Pt [27], Pd [28], Ru [29], Ag [30], and PdS [31]. However, noble metals are scarce and high-cost, which limit their large-scale commercial applications. Thereby, it is necessary and urgent to develop low-cost and high-efficiency co-catalysts using abundant elements. Effective HER electrocatalysts can act as co-catalysts in the photocatalytic HER. In this regard, transition-metal phosphides (TMPs), such as CoP [32], Co₂P [33], MoP [34], and Cu₃P [35] have higher potential as electrocatalysts due to their low cost, outstanding catalytic activity, and stability for the HER [36, 37]. Researchers have confirmed that the above electrocatalysts can effectively enhance the photocatalytic HER activity as co-catalysts [38, 39, 40, 41, 42]. Consequently, we believe that this strategy for finding photocatalytic co-catalysts from TMPs is feasible. Compared to other TMP co-catalysts mentioned above, Ni₂P (NP) has many attractive advantages, such as high photocatalytic HER activity and stronger stability [43, 44]. Additionally, the reported morphologies of NP are only nanoparticles [44, 45]; there are no reports of other morphologies. Based on the above analysis, we prepared a new coralline-like NP via a hydrothermal process and used it as a co-catalyst; to the best of our knowledge, this morphology is the first reported.

In this study, Ni₂P-Cd_0.9Zn_0.1S (NPCtZS) photocatalysts were successfully synthesized and used in the photocatalytic HER under visible-light irradiation. Specifically, CZS and NP showed novel tetrapod-bundle and coralline-like morphologies, respectively. The morphology, structure, optical properties, and electrochemical characterizations of the NPCZS photocatalysts were studied in detail. The tetrapod-bundle CZS showed outstanding photocatalytic HER activity, which was further promoted after loading the NP co-catalyst. The possible mechanisms for the photocatalytic activity over pristine CZS and NPCZS composites are proposed. This work demonstrates that the morphology of the catalyst exerts a strong influence on the photocatalytic HER activity, and the coralline-like NP is an effective co-catalyst.