Synthesis of core-shell nanostructured Cr₂O₃/C@TiO₂ for photocatalytic hydrogen production

Abstract

In this study, the Cr₂O₃/C@TiO₂ composite was synthesized via the calcination of yolk–shell MIL-101@TiO₂. The composite presented core–shell structure, where Cr-doped TiO₂ and Cr₂O₃/C were the shell and core, respectively. The introduction of Cr³⁺ and Cr₂O₃/C, which were derived from the calcination of MIL-101, in the composite enhanced its visible light absorbing ability and lowered the recombination rate of the photogenerated electrons and holes. The large surface area of the Cr₂O₃/C@TiO₂ composite provided numerous active sites for the photoreduction reaction. Consequently, the photocatalytic performance of the composite for the production of H₂ was better than that of pure TiO₂. Under the irradiation of a 300 W Xe arc lamp, the H₂ production rate of the Cr₂O₃/C@TiO₂ composite that was calcined at 500 °C was 446 μmol h⁻¹ g⁻¹, which was approximately four times higher than that of pristine TiO₂ nanoparticles. Moreover, the composite exhibited the high H₂ production rate of 25.5 μmol h⁻¹ g⁻¹ under visible light irradiation (λ > 420 nm). The high photocatalytic performance of Cr₂O₃/C@TiO₂ could be attributed to its wide visible light photoresponse range and efficient separation of photogenerated electrons and holes. This paper offers some insights into the design of a novel efficient photocatalyst for water-splitting applications.

Introduction

The amount of sun energy that reaches the earth every second is approximately 1.465 × 10¹⁴ J, which is the equivalent of the 10-day workload of a power plant. Although it is difficult to entirely convert solar energy into useful energy, improving energy conversion efficiency is extremely important for the development of mankind. Converting solar energy into H₂ fuel via photocatalysis is considered to be a promising method for alleviating the worsening worldwide energy crisis, and could also solve the global environmental problems [1, 2, 3]. Semiconductor materials have been attracting researchers’ attention ever since Fujishima and Honda published their paper on the use of the TiO₂ electrode for splitting water [⁴]. To date, semiconductor materials (e.g., CuO [⁵], ZnO [⁶], SnO₂ [⁷], graphitic carbon nitride [8, 9, 10, 11], CdS [12, 13], and BiVO₄ [14, 15]) have been used as photocatalysts for photocatalytic H₂ production systems. Compared to other semiconductor photocatalytic materials, TiO₂ is more advantageous owing to its robust chemical stability, low cost, and non-toxicity [16, 17]. However, its high photogenerated electron-hole pairs recombination rate and low absorption capability for visible light, which was attributed to its wide bandgap (∼3.2 eV), significantly limit the use of TiO₂ for photocatalytic applications [¹⁸].

Many scholars have reported improving the photocatalytic activity of TiO₂ via heteroatom-doping [¹⁹], surface modification [20, 21], the loading co-catalysts [22, 23, 24], and the formation of heterojunctions [25, 26, 27, 28]. Among these methods, the transition-metal cation modification of TiO₂ is an effective strategy for enhancing the visible light absorption ability and improving the photocatalytic properties of TiO₂ [29, 30]. Cr₂O₃ has been considered to be an efficient visible-light-inducing photocatalytic material owing to its narrow bandgap, and thus, it has been explored as a dopant for TiO₂ [31, 32]. Irie et al. [³³] doped Cr³⁺ into TiO₂ powder using the impregnation method to improve its photocatalytic properties. Jun and Lee [³⁴] synthesized Cr-doped TiO₂ films and demonstrated that their different band gaps depended on the sputtering parameters. Wang et al. [³⁵] synthesized Cr and Co co-doped crystal TiO₂ powder using the sol-gel method, studied its sonocatalytic activity for the degradation of azo fuchsine, and determined that the sonocatalytic activity of Cr and Co co-doped TiO₂ powder for the degradation of azo fuchsine exceeded that of pristine TiO₂. All the above-mentioned studies confirmed that Cr doping and Cr₂O₃ modification could improve the photocatalytic activity of TiO₂.

Metal–organic frameworks (MOFs) have received considerable attention in many research areas owing to their unique structure, which involves high surface areas and numerous micropores [36, 37]. Using MOFs as precursors and templates is promising for obtaining porous materials with well-tunable structure and high surface area [38, 39]. The high surface area, large porosity, and tailorability of MOFs could be inherited by their derivatives, and therefore, MOFs-derived materials present great potential for catalytic applications. To date, most studies have been focusing on the preparation of porous carbon [40, 41], metallic oxide or C/metallic oxide composites [42, 43, 44] via the calcination of MOFs under different atmospheres. Using MOF-derived C/metallic oxide porous materials and TiO₂ as cores and shells, respectively, to design novel photocatalysts could be a promising strategy for improving the photocatalytic efficiency of TiO₂ by improving the diffusion efficiency of the reactants and providing more active sites for the catalytic reactions. In this study, Cr₂O₃/C@TiO₂ composites were successfully prepared using Cr₂O₃/C that was derived from MIL-101 (Cr₃O(OH)(H₂O)₂[C₆H₄(CO₂)₂], which is one of the most stable MOFs) as the core. MIL-101 was used as the template and also as the C and Cr source. The synthesis process of the Cr₂O₃/C@TiO₂ composite is illustrated in Scheme 1. First, MIL-101 octahedra were synthesized using terephthalic acid (BDC) and Cr³⁺ as the organic ligand and central metal ion, respectively, under hydrothermal conditions. Then, the prepared MIL-101 particles were used as the core to form MIL-101@TiO₂ (MT) core-shell structures via the hydrothermal method using TiF₄ as the titanium precursor. Lastly, the prepared core-shell MT particles were carbonized under N₂ atmosphere at 500 °C for 5 h, and the core-shell Cr₂O₃/C@TiO₂ particles were obtained.