Theoretical and experimental investigation of highly photocatalytic performance of CuInZnS nanoporous structure for removing the NO gas

Highlights

• Band gap and absorption ability of CuInZnS are influenced by band-edge Cu-3d and In-5s electronic state.

• Photocatalytic performance of CuInZnS is influenced by the ratio of the constituent elements.

• Photocatalytic activity with a high removal ratio of 47.71% is achieved by the compound with 0.04 wt% Cu.

Abstract

Solid solutions composed of I–III–VI₂ (I = Cu, Ag; III = Al, Ga, In; VI = S, Se, Te) ternary chalcopyrites and II-VI (II = Zn; VI = S, Se, Te) binary zincblendes are a group of important photocatalyst for water splitting applications. We here investigate the photocatalytic performance of CuInS₂-ZnS (CIZS) nanospheres with nanoporous structures for degrading the NO poisonous gas. It is well known that the performance of a photocatalyst is determined by not only the intrinsic material properties but also the morphology of a real sample. Therefore, we first predict the fundamental material properties of CIZS for the photocatalytic application using first-principle theoretical approaches. Our results indicate that CIZS can form high crystalline structures and that its band gap and optical absorption ability are sensitively influenced by band-edge Cu-3d and In-5s electronic states. We then synthesize CIZS nanospheres with nanoporous structures, which can generate large reaction surfaces, enhance transport of photon-excited carriers, and avoid particle polymerization. It is found that the photocatalytic performance of CIZS is sensitively influenced by the mixing ratio of the constituent elements, and a high removal ratio of 47.71% is achieved by the compound with 0.04 wt% Cu.

Introduction

Environmental issues caused by organic contaminants and noxious gases have attracted increasing attention, and, particularly, the removing of the NO poisonous gas is currently an intensively studied topic. Among various technologies to solve this problem, photocatalytic degradation of NO under visible-light irradiation is considered to be a very promising one [1], [2], [3], [4]. A central issue for this approach is to find suitable photocatalysts, which should be highly efficient, environment-friendly, and of low fabrication cost.

Roughly speaking, three major categories of photocatalysts have been widely studied for degrading air pollutants. First, TiO₂ in the form of nanoparticles is the most intensively studied system so far [5]. The efficiency of these materials is, however, largely restricted by their bandgap (E_g = 3.2 eV), which is too wide for an effective utilization of visible-light [6]. In addition, TiO₂ nanoparticles tend to aggregate, resulting in a reduction of surface area and thereby a degradation of photocatalytic efficiency [6]. Second, binary CdS, CdSe, and PbS also have high photocatalytic performances. Their practical applications are, however, strongly limited because of the toxic constituent elements such as Hg, Cd, and Pb [7]. Finally, noble metal doping, copolymerization, and semiconductor coupling have been proposed as alternative efficient approaches with, however, some restrictions for their practical applications [8], [9], [10], [11], [12], [13], [14], [15]. For example, although MoS₂-g-C₃N₄ nanocomposites have a good photocatalytic performance, its wide usage is hindered by the high cost associated with its fabrication process (i.e., constructing of a heterostructure with MoS₂) [16].

Recently, I–III–VI₂ (I = Cu, Ag; III = Al, Ga, In; VI = S, Se, Te) ternary chalcopyrites (e.g., AgInS₂ [17], [18], [19], [20], [21], [22], [23] and CuInS₂ [24], [25], [26], [27]) are widely investigated for their possibilities as alternative candidates. One great advantage of those materials is that their electronic and optical properties can be readily optimized or tuned due to their structural complexity and flexibility [28], [29]. Furthermore, those materials can be mixed with binary zincblendes (e.g., ZnS and ZnSe), leading to ternary-binary solid solutions with an even greater space for materials designing and they often have improved performances. For example, the fundamental property of optical band gap, which is relevant to various optoelectronic applications including the photocatalytic, is adjustable between 1.02 eV and 3.49 eV for the Ag(Cu)InS₂-ZnS solid solution [30]. For the photocatalytic properties, the CuInS₂-ZnS (CIZS hereafter) compounds were found to have a high activity for water splitting under visible-light irradiation [31], [32], [33], [34]. Another advantage of those systems is that all their constituent elements can be non-toxic, earth-abundant, and very cheap.

Considering the great advantages of the ternary-binary compounds for water splitting, they are very promising to be advanced photocatalysts for degrading poisonous gases as well. Related reports are, however, unavailable so far. In this paper, we combine theoretical and experimental approaches to study the photocatalytic performance of CIZS for degrading NO poisonous gas under visible-light irradiation. Since the performance of a photocatalyst material is closely related to its fundamental electronic and optical properties, we first carry out corresponding theoretical predictions using first-principle approaches. The photocatalytic performance of a real sample is also strongly influenced by the morphology. Therefore, we synthesize CIZS nanospheres with nanoporous structures in order to generate large reaction surfaces, enhance the transport ability of photon-excited carriers, and avoid particle polymerization. Our CIZS samples have a high ratio of 47.71% for degrading NO, indicating a great potential for industrial applications.