Elsevier

Journal of Catalysis

Volume 350, June 2017, Pages 197-202
Journal of Catalysis

Facile fabrication of bitter-gourd-shaped copper (II) tungstate thin films for improved photocatalytic water splitting

https://doi.org/10.1016/j.jcat.2017.04.008Get rights and content

Highlights

  • Hydrothermal synthesis of bitter-gourd shaped CuWO4 on FTO.

  • The CuWO4 films showed a superior photocurrent density 0.6 mA/cm2 at 1.23 V.

  • Increase in the valence band edge position in the CuWO4 compared to WO3.

Abstract

Copper (II) tungstate (CuWO4) has better light absorption and selectivity for a photochemical water-splitting reaction than those of its binary oxide counterpart. In this work, we report a facile single-step hydrothermal method to grow CuWO4 with a bitter gourd shape directly on a transparent conductive substrate for the first time. CuWO4 was synthesized via the condensation of a stable aqueous precursor solution of peroxopolytungstic acid and a copper precursor. The proposed method uniformly deposited CuWO4 on fluorine-doped tin oxide with strong adhesion. The obtained CuWO4 films showed a superior photocurrent density of 0.6 mA/cm2 at 1.23 V vs. a reversible hydrogen electrode in a 0.1 M Na2SO4 electrolyte, which was relatively higher than the values recently reported for CuWO4 photoanodes. The optimized CuWO4 exhibited an incident photon-to-current conversion efficiency of 30% at 350 nm for the photoelectrochemical oxidation of water. An X-ray photoelectron spectroscopy valence band edge analysis revealed an increase in the valence band edge position in the CuWO4, along with a decrease in the band gap compared to those in WO3. The procedure proposed here provides a promising approach for the design of an efficient CuWO4 photoanode for water splitting.

Introduction

A new viable approach is required for the production of alternative fuels because of the depletion of fossil fuel reserves [1]. As an alternate fuel, hydrogen (H2) is regarded as one of the most promising clean energy carriers of the future, with zero emissions [2]. Among various methods of producing H2, photoelectrochemical (PEC) water splitting is the most advanced and greenest method. The solar energy incident on the earth’s surface is an inexhaustible and sustainable resource, which can potentially be harvested to produce H2 through water splitting to meet our rising need for clean and renewable energy. Hence, this method offers the possibility of generating an essentially unlimited supply of clean-burning hydrogen fuel using sunlight and water [3], [4]. It is of the utmost fundamental and technological importance to fabricate the photoelectrodes and photocatalysts employed in PEC water splitting using earth-abundant materials [5].

Photoelectrochemical water splitting based on semiconductor materials is a promising and environmentally friendly approach to hydrogen generation. In particular, employing metal oxide semiconductors for solar water splitting has become the most researched field because of their advantages, which include earth abundance, low cost, low toxicity, and relatively high stability in aqueous environments [6]. Various types of metal oxide semiconductors such as titanium dioxide (TiO2) [7], ZnO [8], hematite (a-Fe2O3) [9], tungsten trioxide (WO3) [10], and BiVO4 [11] have been investigated for solar water-splitting applications. Although these materials show significant photoelectrochemical activity, some of them have limited utility because their band gap energies are poorly suited to capturing visible light photons, in which the bulk of solar spectrum energy lies. They are also prone to photocorrosion and have a short carrier path length and high charge carrier recombination [12], [13], [14]. Even though enormous effort has been invested in this area of research, the development of a high-efficiency water splitting PEC cell for solar-to-hydrogen fuel conversion remains a challenge. Therefore, it is important to research ways to exploit small band gap and stable semiconductors for efficient water splitting.

CuWO4 is an n-type ternary metal oxide semiconductor and an attractive and promising photoanode for solar water splitting. It has a favorable band gap of 2.2–2.4 eV, leading to >10% theoretical solar-to-hydrogen (STH) conversion efficiency. It is also stable in aqueous media and has an environmentally benign composition [15], [16], [17]. CuWO4 exhibits a small band gap property for water splitting in a neutral pH electrolyte because its hybridized Cu3d and O2p orbitals make the main contribution to the valence band maxima [15]. In other words, compared with WO3, CuWO4 possesses a narrow band gap due to the increase in the valence band maximum through the interaction of Cu3d and O2p orbitals and provides increased light harvesting [15]. Hence, CuWO4 absorbs a large part of the visible spectrum, which is crucial for photoelectrochemical water oxidation reaction catalysts that are to obtain high photo-to-chemical conversion efficiency. Usually, CuWO4 photoanodes are fabricated via multiple steps such as sol–gel synthesis and spin coating [18], drop-casting [19], [20], electrodeposition [15], seed-layer-assisted growth [21], sputtering [16], or the use of a WO3 support layer [17], [19], which are complex and time-consuming. Despite such efforts, the photocurrents achieved are relatively low. Therefore, fabricating CuWO4 photoanodes for efficient water splitting is crucial. In view of this, we report for the first time a single-step hydrothermal method for the fabrication of CuWO4 with a bitter-gourd shape directly on a fluorine-doped tin oxide (FTO) substrate. The proposed method uniformly deposits CuWO4 on FTO with strong adhesion. The electrode exhibits an unprecedented photocurrent of 0.6 mA/cm2 at 1.23 V versus a reversible hydrogen electrode (RHE) under simulated sunlight without an added catalyst, which is one of the best results reported so far.

Section snippets

Experimental

CuWO4 thin films were grown directly on fluorine-doped tin oxide glass (FTO) using a single-step hydrothermal method. First, 0.815 g of sodium tungstate dihydrate (Sigma Aldrich ≥99.0%) was added to 93.5 ml of de-ionized water and stirred until it dissolved. Then, 2.5 mL of concentrated HCl (Sigma Aldrich) was added to the tungsten-containing solution and stirred for 1 h to obtain a yellow gelatinous precipitate of tungstic acid. After 1 h, 4.5 mL of H2O2 (Sigma Aldrich, 30% in water) was added to

Results and discussion

A facile and single-step hydrothermal method was developed to grow a bitter-gourd-shaped CuWO4 nanostructure on a FTO substrate. The nature and stability of the precursor solution were crucial for the hydrothermal deposition of the CuWO4 on the FTO surface [10]. Therefore, we first prepared a stable peroxopolytungstic acid (PTA) solution from an aqueous sodium tungstate solution. In this process, tungstic acid is formed when HCl is added to an aqueous solution of sodium tungstate and is then

Conclusions

In conclusion, we successfully fabricated CuWO4 with a bitter gourd shape on FTO substrates using a hydrothermal method. The synthesis procedure involved is simple and rapid. The uniform deposition of CuWO4 was achieved without any empty surface regions and with good adhesion. The maximum photocurrent density of 0.6 mA/cm2 at 1.23 V vs. RHE was recorded at 1 sun of illumination in 0.1 M Na2SO4 without catalysts. This value is very competitive with many reported values using CuWO4 photoanodes. The

Acknowledgments

This research was supported by the Basic Science Program through the National Research Foundation (NRF-2015R1A2A2A01003790) funded by the Ministry of Science, ICT, and Future Planning, Republic of Korea. A fellowship awarded to S.K. was funded by the BK-plus program.

References (24)

  • C. Herrero et al.

    Coord. Chem. Rev.

    (2008)
  • H.P. Maruska et al.

    Sol. Energy Mater.

    (1979)
  • A. Kudo et al.

    Chem. Soc. Rev.

    (2009)
  • N.S. Lewis

    Science

    (2007)
  • M.G. Walter et al.

    Chem. Rev.

    (2010)
  • D. Eisenberg et al.

    J. Am. Chem. Soc.

    (2014)
  • R. van de Krol et al.

    J. Mater. Chem.

    (2008)
  • A. Fujishima et al.

    Nature

    (1972)
  • X. Yang et al.

    Nano Lett.

    (2009)
  • S.C. Warren et al.

    Nat. Mater.

    (2013)
  • S.S. Kalanur et al.

    J. Mater. Chem. A

    (2013)
  • N. Aiga et al.

    J. Phys. Chem. C

    (2013)
  • Cited by (31)

    • Reduced CuWO<inf>4</inf> photocatalysts for photocatalytic non-oxidative coupling of methane reaction

      2024, Colloids and Surfaces A: Physicochemical and Engineering Aspects
    • Z-Scheme CuWO<inf>4</inf>/BiOCl photocatalysts with oxygen vacancy as electron mediator for boosted photocatalytic degradation of norfloxacin

      2022, Surfaces and Interfaces
      Citation Excerpt :

      Tauc's equation [36] was utilized for estimating the bandgap of the samples. Since BiOCl and CuWO4 belong to the indirect bandgap semiconductor [17,37], the bandgap energies of Vo-BiOCl and CuWO4 were 3.05 and 2.17 eV, respectively, near the reported values [36,38]. Due to the limitation of the Generalized Gradient Approximation (GGA), the experimental values of this were greater than all calculated values [39].

    View all citing articles on Scopus
    View full text