Interstitial carbon doped of setaria viridis-like Znln₂S₄ hollow tubes for efficient the performance of photocatalytic hydrogen production

Highlights

• The unique C/ZIS hollow tube was fabricated by simply solvothermal method.

• C/ZIS hollow tube showed high 1241.94 μmol g⁻¹ h⁻¹ H₂ production performance.

• Excellent photocatalytic activity can be attributed to structure and carbon doping.

Abstract

In the photocatalytic water splitting hydrogen production system, the key to efficient use of solar energy is to choose a suitable photocatalyst. As an important ternary sulfide, ZnIn₂S₄ (ZIS) has attracted wide attention because of its narrow band gap (E_g = 2.3–2.8 eV) and wide light absorption range. However, further modification was still needed. Therefore, in this work, the unique C/ZIS hollow tubes with nano-flakes were prepared by a simple solvothermal method. To a large extent, this unique structure increased the utilization of light and active sites. Moreover, the dissolution of PAN during the solvothermal process caused the carbon element to be uniformly doped into the hollow tube framework. After a series of characterization results, C/ZIS-3.0 has the best hydrogen release rate (1241.94 μmol g⁻¹ h⁻¹) and good cyclability under visible light irradiation (λ ≥ 420 nm). And its unique morphology and possible catalytic mechanism were further discussed.

Introduction

In recent years, with the rapid development of social economy, environmental pollution and energy shortage have become two urgent problems for human beings to achieve sustainable development [[1], [2], [3], [4], [5], [6]]. At the same time, the excessive consumption of fossil fuels such as petroleum has led to the discharge of a large number of pollutants, aggravated environmental pollution, and seriously affected people's lives and health. The development and use of solar energy are an effective way to solve this problem. Among them, photocatalytic hydrogen production technology can convert solar energy into hydrogen energy. Hydrogen energy is considered to be an ideal energy carrier with high energy density, clean and environmental protection [[7], [8], [9], [10], [11], [12]]. Since the 1970s Japanese scientists used TiO₂ photocatalytic decomposition of water to produce hydrogen and oxygen, this field has become the focus of research at home and abroad [13]. The use of solar energy for water splitting to produce hydrogen is considered to be an effective means of obtaining hydrogen energy in the future. In the past decades, people have done a lot of work in developing new semiconductor photocatalysts. And hundreds of semiconductor materials have been studied, such as sulfides [[14], [15], [16], [17], [18]] oxides [[19], [20], [21], [22]] and nitrides [[23], [24], [25], [26]], etc.

The following three requirements should be considered when choosing a photocatalyst for photolysis of water to produce hydrogen: (1) effective absorption of visible light; (2) appropriate band gap; (3) whether it meets the thermodynamic requirements of photolysis of water. In various catalyst systems, sulfide is considered as a good material for photolysis of water to produce hydrogen. Among the common sulfide photocatalysts, the ternary sulfur compound ZnIn₂S₄ (ZIS) is viewed a promising candidate for the water splitting to produce hydrogen due to its proper band gap and relatively high photocatalytic activity [[27], [28], [29], [30], [31]]. As early as 2003, Li et al. [32] discovered that ZIS can be used to photolysis of water, which aroused strong interest. For practical application and industrialization, it still has space for progress. More and more studies have been conducted to further increase the ability to release hydrogen. Common methods include the following three: (1) Coupling with precious metal cocatalyst [33] and some semiconductor materials such as Ni(OH)₂, CuInS₂, In₂O₃ and so on [[34], [35], [36]]; (2) The introduction of vacancies will also significantly promote the physical and chemical properties of ZIS. Recently, Shi [37] and colleagues constructed ZIS nanosheets with high zinc vacancies. It is worth noting that ZIS with high zinc vacancies can reduce the activation energy of carrier transport, extend the light response range, and provide more active sites, that is, “three birds with one stone” to enhance the photocatalytic performance; (3) Hollow structure semiconductors have many advantages in photocatalyst design. Thus, Xie et al. [38] synthesized g-C₃N₄/ZIS hollow sphere structure. This composite material can not only increase the light absorption capacity, but also create more active sites. The formation of a heterogeneous structure can also increase the efficiency of photoelectron migration and separation, so the serious photogenerated electron recombination of the two materials can be weakened.

According to literature surveys, the effect of element doping on the photocatalytic performance of ZIS has not been studied yet. Therefore, the innovative point of this work was to combine the two methods of element doping and special morphological structure. The unique hollow tube with ZIS nanoflake was synthesized by a simple one-step solvothermal method, and the carbon element was uniformly retained in the framework (Scheme 1). A series of characterization methods were used to confirm that compared with ordinary ZIS particles, this unique hollow tube structure can not only achieve multiple reflections of incident light, thereby enhancing the light absorption capacity, and generating more active sites than ZIS microspheres. It can also reduce the band gap width of ZIS and improve photocatalytic activity.