Simultaneous Ni nanoparticles decoration and Ni doping of CdS nanorods for synergistically promoting photocatalytic H₂ evolution

Highlights

• An ALD-reduction method is developed to introduce Ni doping and Ni NPs.

• Ni doping narrows the band gap of CdS and improves charge separation.

• The size of Ni NPs is precisely tailored by vary the number of NiO ALD cycle.

• The Ni doping and Ni NPs synergistically enhance the H₂ production of CdS.

• The Ni480/Ni-doped CdS NRs reachs an apparent quantum efficiency of 37.5% at 420 nm.

Abstract

Heteroatom doping and loading of co-catalysts are two efficient tactics to boost the photocatalytic activity of semiconductors. Combining these two strategies in one photocatalyst system has great potential to further enhance the catalytic performance. Herein, a facile and controllable atomic layer deposition (ALD)-reduction approach is used to simultaneously introduce Ni doping and Ni nanoparticles (NPs) with tunable size on CdS nanorods for boosting its photocatalytic activity. The Ni doping and Ni NPs synergistically enhance the H₂ production of CdS nanorods (NRs), achieving an optimized rate of 20.6 mmol·g⁻¹·h⁻¹ ~1.3- and 28.6-fold higher than CdS NRs supported Ni NPs with similar size and pristine CdS NRs, respectively. The Ni NPs/Ni doped CdS NRs exhibits an apparent quantum efficiency (AQE) of 37.5% at 420 nm, outperforming most previously reported Ni doped and Ni NPs decorated CdS catalysts. The outstanding photocatalytic activity of the Ni NPs/Ni doped CdS NRs can be ascribed to synergism of the Ni doping and uniformly dispersed Ni co-catalyst with appropriate size, which promote carrier separation of semiconductor and surface hydrogen evolution kinetics on nanocatalyst surface. This work provides a promising pathway to integrate heteroatom doping and loading of co-catalysts strategies in one photocatalyst system for synergistically promoting photocatalytic performance.

Introduction

As an appealing approach to utilize solar energy, photocatalytic water splitting to hydrogen fuel by utilizing semiconductor nanomaterials has drawn increasing attention because of its huge potential for facing growing environmental and energy challenges [1], [2], [3]. During the past several decades, various type of semiconductor photocatalysts (such as metal oxides [4], [5], [6], [7], sulfides [2], [8], [9], [10], and oxynitrides [11], [12]) have been designed and developed for solar-driven H₂ production through water splitting. Among them, CdS based semiconductor nanomaterials are some of the most widely studied for visible light driven H₂ production due to its narrow bandgap and proper band structure position [13], [14], [15], [16], [17]. However, pure CdS semiconductors suffer from fast recombination of photoinduced carriers and poor photostability, which seriously limits its photocatalytic efficiency and H₂-production rate. Hence, the main challenge for CdS photocatalysis is to separate and extract the photoinduced charges by competing with their recombination [15], [18].

Heteroatom doping [2], [19], [20] and loading of co-catalysts [8], [15], [21] are two efficient approaches to promote carrier separation of semiconductor via regulating the electronic structure and surface redox reaction kinetics, respectively. Metal doping can create impurity levels between the conduction band (CB) and valence band (VB) of CdS via the d-orbitals hybridization between the metallic impurities and CdS, resulting in improved light absorption and regulated electronic structure [22], [23], [24]. For example, recent experimental and theory studies revealed that substitutional Ni doping can not only decrease the band gap of CdS for extending its visible light absorption range, but also facilitate photogenerated charge separation by forming shallow surface states to trap the photogenerated electrons [25], [26]. As an alternative way, loading co-catalysts on semiconductors can promote redox reactions by providing more active reaction sites, which in turn help to inhibit the electron-hole recombination and reverse reactions [27]. Traditionally, in order to effectively extract the photoinduced charges to the surface for reactions, noble metals (e.g., Pt, Rh, and Pd) [28] are generally used as the co-catalysts due to their high activities for the photoreaction. However, due to the high price and rarity of noble metals, low-cost and highly efficient alternatives are sought, such as non-noble metals, graphene [29], [30], [31], sulphide [32], [33], [34], [35]. Particularly, nickel has recently attracted increasing attention as a promising cheap metal cocatalyst for photocatalytic reactions [8], [15], [36], [37], [38], [39]. The performance of Ni co-catalyst was significantly affected by its size and the interaction between the semiconductor and Ni nanoparticles. Fu et al. reported a Ni-CdS photocatalytic system for photocatalytic hydrogen production by mixing CdS nanorods with size-controlled colloidal Ni nanoparticles (NPs) [36]. They found that the Ni NPs size had great influence on the hydrogen evolution activity. Very recently, Amirav et al. [40] systematically examined the photocatalytic performance of Ni decorated CdSe@CdS nanorods with different Ni NPs sizes to reveal the size dependency. It is suggested that the Ni size effect on charge transfer could be ascribed to the interaction of the Schottky barrier and Coulomb blockade charging energy between Ni and CdS. Despite intensive efforts and much progress in separately modulating the metal doping and co-catalysts in CdS for improved photocatalytic activity, there have few reports on deliberate integration of them in one material system. It is very desirable for the simultaneous introduction of metal doping and co-catalysts with a controlled size and good dispersion in one CdS based nanomaterial system, which is expected to achieve the optimal electronic structure and surface redox reaction kinetics for further promoted photocatalytic performance.

Herein, we synthesized Ni NPs/Ni doped CdS NRs photocatalyst by a facile two steps method. That is, controlled size and well dispersed NiO NPs were firstly grown on the surface of CdS NRs by ALD technology. Next, the NiO/CdS NRs were treated under a 5% H₂/Ar atmosphere at 350 °C for 2 h, which reduced the NiO NPs to metallic Ni NPs and synchronously enabled a part of Ni species diffusing into the CdS NRs, resulting in simultaneous introduction of uniformly dispersed Ni co-catalysts and Ni doping in CdS NRs. The synergistic effect of the Ni doping and uniformly dispersed Ni co-catalyst allow for Ni NPs/Ni doped CdS NRs to exhibit superb photocatalytic for H₂ production, surpassing those of similar catalytic systems, namely either Ni doped or Ni NPs decorated CdS catalysts. In addition, the size of Ni NPs (controlled by varying ALD cycle numbers) has great influence in hydrogen production rate on Ni NPs/Ni doped CdS NRs. The 480Ni NPs/Ni-doped CdS NRs produced by 480ALD cycles exhibit the fastest H₂-generation rate of 20.6 mmol g⁻¹ h⁻¹, which is 28.6 folds faster than that of CdS nanorods. The AQE at 420 nm can be up to 37.5%, outperforming most previously reported Ni doped and Ni NPs decorated CdS catalysts. The excellent photocatalytic performance was mainly attributed to extended light absorption, efficient carriers separation and charge transfer across the Ni NPs-Ni doped CdS interface, as well as excellent water reduction activity of optimized Ni NPs.