Facile in situ fabrication of Cu₂O@Cu metal-semiconductor heterostructured nanorods for efficient visible-light driven CO₂ reduction

Highlights

• A simple, low-cost and efficient method was developed to fabricate catalysts.

• Cu₂O@Cu nanorods show excellent light harvesting and good separation of carriers.

• Cu₂O@Cu catalysts achieve an apparent quantum efficiency of 2.4% for CH₄ and C₂H₄.

• Unique Cu₂O@Cu catalysts show superior stability for photocatalytic CO₂ reduction.

Abstract

Cuprous oxide (Cu₂O) is known to be a promising photocatalyst for CO₂ reduction into solar fuels under visible-light irradiation. However, the issues of fast recombination of photogenerated carriers and photocorrosion severely limit its photocatalytic (PC) performance. Herein, we report a unique design of one-dimensional (1D) Cu₂O@Cu metal-semiconductor heterostructured nanorods via a simple in situ reduction method for efficient CO₂ reduction to hydrocarbons fuels. The well-defined 1D Cu₂O nanorod arrays ensure excellent visible-light harvesting capability, and the in situ fabricated Cu₂O@Cu heterostructure endows the catalyst with enhanced conductivity as well as highly improved separation and transfer efficiency of photogenerated carriers. Consequently, the optimized Cu₂O@Cu heterostructure achieves an apparent quantum efficiency of 2.40% for CH₄ and C₂H₄ and as high as 92% activity retained after four PC cycles. Furthermore, the CO₂ reduction performance was further improved when applied a low external bias. This study not only provides a novel, low-cost, and efficient strategy to address the stability and activity issues of Cu₂O, but also sheds light on the development of active and robust photocatalysts for energy conversion and storage.

Introduction

As the rapid development of economy and overconsumption of fossil resources, the concentration of carbon dioxide (CO₂) in atmosphere quickly increases, resulting in “greenhouse effect”, which has become a common concern for all mankind [1], [2]. Efficient conversion of CO₂ into chemical fuels is one of the most effective ways to ease the energy crisis and reduce CO₂ emission [3], [4]. As such, exploring new technologies for efficient CO₂ conversion has attracted much attention. Semiconductor-based photocatalysis is a promising approach due to its economy, simple, and cleanliness [5], [6]. Since photocatalytic CO₂ reduction with the semiconductor powders suspension was reported by Fujishima [7], titanium dioxide (TiO₂) has been intensely investigated owing to its economy and high stability [6], [8]. Unfortunately, this semiconductor can only utilize ultraviolet (UV) light due to its large bandgap (~3.2 eV), which restricts its photocatalytic performance [9], [10], [11]. Therefore, tremendous efforts have been made to develop novel visible-light active photocatalysts since visible-light is abundant in the solar spectrum. So far, many semiconductors have been reported as visible-light responsive photocatalysts, such as graphitic-C₃N₄ [12], WO₃ [13], BiVO₄ [14], [15], Cu₂O [16], [17], and CdS [18], etc. Among these catalysts, Cu₂O is considered to be one of the most promising candidates for CO₂ reduction. As a p-type semiconductor, Cu₂O has a direct bandgap of about 2.0 eV, which guarantees efficient visible-light absorption [19]. What’s more, this semiconductor possesses favourable energy band positions, where the conduction band is more negative than that needed for the CO₂ reduction, making it possible for CO₂ reduction to hydrocarbon fuels [19]. However, Cu₂O has its own disadvantages. The high recombination rate of photogenerated electrons and holes results in a low CO₂ reduction efficiency [20]. Besides, photocorrosion is also one of the most important factors that limits its application for CO₂ photoreduction [21].

To overcome these limitations, many strategies have been reported to enhance the activity and stability of Cu₂O for photocatalytic CO₂ reduction, including nanostructuralization [22], semiconductor-semiconductor heterojunction [23], metal nanoparticle deposition [24], [25], etc. In addition, one-dimensional (1D) nanostructures, including nanowires, nanorods, and nanotubes, are highly attractive building blocks [26]. First, 1D geometry facilitates the charge transfer and reduces the recombination of photogenerated carriers by providing a significantly shortened carrier transfer distance [27]. Second, the well-defined nanostructures of 1D arrays can effectively harvest visible-light owing to the multiple light scattering and absorption in the gaps [27]. Hence, we have developed 1D carbon-decorated Cu₂O nanorods [28] and Cl-doped Cu₂O nanorods photocatalysts for efficent photocatalytic CO₂ reduction [1]. On the other hand, metal-semiconductor heterostructure has been identified as a effective strategy to simultaneously solve the aforementioned problems, resulted from an efficient spatial separation of photogenerated electron-hole pairs [29].

Based on the above viewpoints, herein, we couple metal-semiconductor heterostructure with one-dimensional nanorod arrays by a simple in situ method, fabricating a unique 1D Cu₂O@Cu heterostructured nanorod arrays for efficient CO₂ reduction to CH₄ and C₂H₄ under visible-light. Benefiting from the 1D Cu₂O nanorod arrays and in situ fabricated metal-semiconductor heterostructure, our novel 1D Cu₂O@Cu heterostructured catalysts exhibit the advantages of excellent visible-light absorption capacity, high carrier concentration, fast charge transfer ability with efficient separation of photogenerated charge carriers, and good structural integration. Consequently, the unique Cu₂O@Cu heterostructured catalyst achieves high apparent quantum efficiency of 2.40% for CH₄ and C₂H₄ and as much as 92% activity retained after four photocatalytic cycles. Furthermore, the yields of CH₄ and C₂H₄ for the Cu₂O@Cu heterostructured catalyst are 4 and 1.46 times as high as that of Cu₂O under a small bias. To the best of our knowledge, this study is the first time to synthesize unique Cu₂O@Cu heterostructure with 1D nanostructure arrays by in situ method for efficient CO₂ reduction under visible-light irradiation.