Spongy ball-like copper oxide nanostructure modified by reduced graphene oxide for enhanced photocatalytic hydrogen production

https://doi.org/10.1016/j.materresbull.2020.111026Get rights and content

Highlights

  • Synthesis of highly crystalline CuO and CuO/rGO photocatalysts by hydrothermal route.

  • SEM study shows that the spongy ball -like morphology of CuO/rGO composite.

  • CuO/rGO composite shows the superior photocatalytic hydrogen production activity than CuO.

Abstract

In this study, spongy ball-like CuO and CuO/rGO photocatalysts were successfully synthesized by a simple hydrothermal method and further studied for photocatalytic hydrogen production activity. The CuO and CuO/rGO photocatalysts were analyzed by various structural, morphological, and optical characterization techniques. X-ray diffraction and Raman spectroscopy studies revealed that the CuO/rGO composite was formed without the presence of any secondary phase or impurities. The optical results demonstrated that the band gap lies in the visible range of the solar spectrum. The comparative study revealed that the CuO/rGO composite displays superior photocatalytic hydrogen production activity in comparison to CuO. Also, the CuO/rGO composite shows a hydrogen production rate of 19.2 mmol h−1 g−1 which was nearly 3 times higher than that of the CuO photocatalyst (6.8 mmol h−1 g−1).

Introduction

In the recent years, environmental pollution has posed a serious problem, which is generated by the burning of fossil fuels. Heavy industrialization has many adverse effects, and researchers are striving to create environmentally benign and sustainable energy resources. Hydrogen (H2) is a clean and resourceful energy source owing to properties like zero emissions and high combustion energy. Moreover, hydrogen fuel demonstrates a high heating rate of 141.8 MJ kg−1, which is three times higher than that of gasoline; therefore, it has the potential to fulfill the world's increasing demand for energy. A simple and effective method to harvest H2 gas is the photocatalytic water-splitting reaction [[1], [2], [3]]. Currently, 95 % of hydrogen is acquired from fossil fuels through steam reforming of natural gas, coal, and crude oil in unsustainable processes since it requires a high pressure and temperature, and also because these are energy intensive technologies. Moreover, they are generated from nonrenewable feed stock and have a substantial carbon footmark [[4], [5], [6]]. The remaining 5% comes from the reforming of biofuels or from electrolysis. H2 production requires a high energy input, since hydrogen production reactions are endothermic. For effective generation of H2, simple systems such as semiconductor materials and water are required. It has several advantages such as simple synthesis methods, eco-friendliness, cost effectiveness, and absence of secondary pollutants or byproducts. The important criteria for the selection of effective semiconductor materials (photocatalysts) are an effective absorption of light energy which is equivalent to or higher than the band gap of the semiconductor and that the bandgap of the material should lie between the redox potential of the reaction species [7,8]. Upon illumination, the photocatalyst material creates electron-hole pairs, which participate in the redox reaction and lead to the production of H2 by the reduction of H+ in the aqueous solution from photo-created electrons [9].

Different semiconductor materials are widely used for hydrogen production, such as TiO2, CuO, doped TiO2, and ZnO owing to their excellent chemical and physical properties [10,11]. However, the main drawbacks of these materials, i.e., the wide band gap, large recombination of photo-generated charge carriers, also activated in ultraviolet light illumination [12]. The sun releases different types of radiation, such as UV, visible, and IR, but the UV radiation region (wavelength from 100 nm to 400 nm) is limited, hence, it cannot utilize the observable region of the solar band. CuO is an important p-type metal oxide exhibiting a narrow band gap (2.1–2.4 eV) [13]. CuO is broadly used in many applications such as solar energy conversion, magnetic storage media, photocatalysis, and gas sensors [[11], [12], [13], [14]]. The various advantages of CuO, such as high chemical stability, nontoxic properties, and better photosensitivity enable its usefulness in many applications. CuO has been synthesized by different methods such as chemical bath method, electrodeposition, and SILAR etc. [15]. Carbon-based materials such as graphene derivatives, carbon nanotubes, and carbon nitrides are used to control the crystal structure, size, and morphology of the particles, which enhance the activity of the catalyst-support composite structure. In photocatalytic applications, graphene derivatives are commonly used owing to their higher electron conductivity and large surface area [16,17]. Moreover, it is advantageous to minimize the recombination of charge carriers and to avoid the aggregation of the photocatalysts [18]. Further, it has been reported that during photocatalytic reaction, GO also loses some oxygen functional groups, leading to bandgap reduction and increased conductivity [19]. Therefore, GO and rGO were preferred here as the catalyst support for the CuO catalyst.

Chang et al. [20] fabricated an optimized wire mesh/Ag-Bi2WO6 photocatalyst in order to cover the absorption wavelength range from UV light to visible light region. Kumar et al. [21] prepared CuO, Cu2O, rGO-CuO, and rGO-Cu2O composites and studied their photocatalytic, antibacterial, and supercapacitive properties. Bosta et al. [22] synthesized a copper oxide/reduced graphene oxide (CuO/rGO) nanocomposite (NC) via a simple and green hydrothermal method and investigated the photocatalytic properties of ortho and para nitrophenols. Zhang et al. [23] manufactured CuO/Cu2O nanowire arrays (NWAs) on a Cu foil substrate through a simple thermal process and then grafted with reduced graphene oxide (rGO) nanosheets via self-assembly and studied the photocatalytic CO2 reduction.

In the present study, hydrothermally synthesized CuO was modified using rGO nanosheets to improve the photocatalytic activity and photostability of CuO. In the case of the CuO/rGO composite, rGO acts as an electron acceptor for photogenerated electrons extracted from CuO due, which increases the charge separation efficiency of CuO with a limited self-photoreduction process. As results of this process, the hydrogen production activity was increased for CuO/rGO composite as compared to CuO from 6.8 mmol h−1 g−1 to 19.2 mmol h−1 g−1.

Section snippets

Materials

Graphite powder, potassium permanganate (KMnO4), hydrogen peroxide, sodium borohydride (NaBH4), hydrochloric acid, sulfuric acid, copper nitrate Cu(NO3)2. 6H2O, ammonia, all chemicals used without further purification.

Synthesis of CuO and CuO/rGO composite

The CuO photocatalyst was prepared by a chemical, cost-effective hydrothermal method. In this synthesis, copper nitrate 0.1 M Cu(NO3)2 was used as the copper source and dissolved in 100 mL of di-ionized (DI) water. Aqueous ammonia was mixed to control the pH of the precursor

Results and discussion

XRD was used to determine the crystal structure and confirm the phase in the synthesized photocatalysts CuO and CuO/rGO. Fig. 1(a), Fig. 1(b), and Fig. 1(c) display the XRD patterns of rGO, CuO, and CuO/rGO composite, which confirm the nanocrystalline nature of these photocatalyst. Fig. 1(a) shows the XRD pattern of rGO, which contains strong diffraction peaks at 25.62° (200 plane), which confirm the formation of rGO [29]. From the XRD pattern of CuO (Fig. 1(b)), it was clear that no additional

Photocatalytic H2 production measurements

A comparative study of the photocatalytic hydrogen production activity of CuO and CuO/rGO composite samples was conducted under full solar wavelength light radiation in the presence of methanol (20 %) using Pt nanoparticles as co-catalysts. The continuous hydrogen production graph and rate of H2 production for different samples are shown in Fig. 6(a). For both catalysts, the hydrogen production rate increases with irradiation time. CuO exhibits a slower hydrogen production rate than the CuO/rGO

Conclusions

In this research article, CuO and CuO/rGO composite were fabricated using a hydrothermal method. Raman and XPS analyses confirmed the formation of the CuO/rGO composite. This study shows that reduced graphene nanosheets act as electron acceptors, thus the photocatalytic activity for hydrogen production of CuO/rGO composite in visible light is increased 3 times compared to that of CuO with an enhancement in photostability. This may be due to the p-n junction formed between the CuO/rGO composite.

Authorship statement

All persons who meet authorship criteria are listed as authors, and all authors certify that they have participated sufficiently in the work to take public responsibility for the content, including participation in the concept, design, analysis, writing, or revision of the manuscript. Furthermore, each author certifies that this material or similar material has not been and will not be submitted to or published in any other publication before its appearance in the Materials Research Bulletin.

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgments

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. NRF-2019R1A5A8080290).

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