Layered cesium copper titanate for photocatalytic hydrogen production

https://doi.org/10.1016/j.apcatb.2018.01.039Get rights and content

Highlights

  • Layered Cs0.68Ti1.830.17O4 was for the first time doped with Cu2+.

  • During photocatalytic hydrogen production, Cu2+ cations are reduced to metallic Cu.

  • In-situ formation of Cu nanoparticles as co-catalyst could be directly observed.

  • Cu co-catalyst on Cs0.68Ti1.830.17O4 lead to H2 evolution of up to 5 mmol/h.

  • Structural changes upon Cu removal were observed and thoroughly characterized.

Abstract

Layered cesium copper titanate as well as the unmodified cesium titanate Cs0.68Ti1.830.17O4 (□: vacancy) were synthesized by a solution-based approach. The insertion of small amounts of copper into the vacancies of Cs0.68Ti1.830.17O4 led to a significant red shift of the band gap energy from 3.4 eV to 2.9 eV. During photocatalytic H2 production experiments, a steady increase in the evolution rate was detected, which can be referred to the in-situ reduction of incorporated copper ions to metallic Cu. The reduced copper ions leach out of the lattice to the catalyst surface and act as co-catalyst for H2 formation, considerably exceeding the activity achieved with Cs0.68Ti1.830.17O4 modified with 0.075 wt.-% of Rh as co-catalyst. The use of diffuse reflectance spectroscopy enabled a direct measurement of the copper nanoparticle formation by following their rising plasmon resonance at operating conditions. Characterization by X-ray diffraction (XRD) revealed a significant change in the crystal structure upon photocatalysis.

Graphical abstract

Copper-doped layered cesium titanates show in-situ photodeposition of metallic copper, revealed by in-situ localized surface plasmon resonance (LSPR) spectroscopy and followed by increasing photocatalytic hydrogen production.

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Introduction

The demand for hydrogen as an alternative fuel has grown steadily in recent decades [1]. Photocatalytic active materials for solar hydrogen production represent one promising solution for this increasing request. Nevertheless, most semiconductor materials suffer from large band gaps and fast electron hole recombination, and therefore of low quantum efficiency [[2], [3]]. Two methods are commonly used to decrease the electron hole recombination: i) utilization of methanol [4], ethanol [5] or triethanolamine [6] as sacrificial agents acting as hole scavengers; and ii) deposition of noble metal particles like Rh [[7], [8]], Au [[9], [10]] and Pt [[11], [12]] to establish Schottky contacts for electron trapping. In general, these two methods are applied together to achieve higher activities. However, these techniques are not cost effective because expensive critical metals as well as valuable chemical compounds are used.

Alternatively to critical noble metals, earth-abundant metals or metal oxides like Cu [[13], [14], [15]] and NiOx [[16], [17]] are deposited as co-catalysts for hydrogen generation. Investigations by Domen et al. [18] in 1998 initiated a revival of copper species as hydrogen evolution co-catalyst. Different publications characterize the active Cu surface component as Cu2O or Cu/Cu2O (core/shell) systems primarily investigated on TiO2 [[13], [19], [20]]. The presence of Cu(I) species is verified by X-ray photoelectron spectroscopy (XPS) measurements [[13], [19]]. Nevertheless, most of the commonly used characterization methods like XPS, X-ray diffraction (XRD), absorption spectroscopy, chemisorption, physisorption, transmission and scanning electron microscopy (TEM, SEM) provide only characterization data of the catalyst or co-catalyst status ex-situ before and after the photocatalytic process. In contrast, localized surface plasmon resonance (LSPR) spectroscopy enables an in-situ observation of the formation of the plasmon active co-catalyst during photocatalytic H2 production and gives first insights of the actually operating co-catalyst [21].

The layered cesium titanate Cs0.68Ti1.830.17O4 (□: vacancy) consists of a lepidocrocite γ-FeOOH-type layered structure [22] and represents a good starting material for further modifications with regard to the incorporation of cations [[23], [24], [25]] and anions [26] for band gap reduction, due to its open layered structure and the theoretical amount of 0.17 equivalents of vacancies distributed in the titanium oxide sheets, which are spatially separated by cesium cations. This material enables a simple insertion of, for example, Mg, Fe, Co, Ni, Cu or Zn ions into the vacancies of the titanate sheets. Up to now, the cation modification and the synthesis of the base material have been mainly performed by solid state reaction (SSR) [[23], [24], [27]]. However, SSR requires high calcination temperatures [27] and long calcination times [28], resulting in a highly crystalline material but characterized by large particle sizes. A main drawback of the SSR for cation doping can be an inhomogeneous distribution of the inserted metal cation through the catalyst material because of the slow diffusion rate of ions in solids during the calcination period. Already in 2001 Sumida et al. presented an alternative synthesis of Cs0.68Ti1.830.17O4 by employing the polymerized complex (PC) method applying citrate ions and ethylene glycol as complexing agent [28]. Cation modification of materials prepared via solution-based routes can minimize the inhomogeneity issue because of the improved dispersion of the metal ions. Additionally, a solution-based method like the PC-route allows the synthesis of mixed metal oxides at lower calcination temperatures in shorter reaction times, as shown in this work.

In this paper, the properties of layered cesium copper titanate for photocatalytic H2 generation are presented and discussed. The in-situ formation of metallic Cu as co-catalyst on the photocatalyst surface is followed by Cu LSPR, and an explicit effect on the long-term hydrogen evolution is observed. Structural changes after H2 production are investigated in detail, leading to new insights into the tested photocatalyst.

Section snippets

Catalyst preparation

Cesium copper titanates were synthesized by a complex-based process similar to the PC method [[29], [30], [31]]. The photocatalysts were synthesized by an aqueous citrate-based method [[30], [31], [32]]. The used quantities of the metal compounds correspond to the stoichiometric composition of Cs0.68Ti1.83O4. Basing on this formula relative to cesium a 3 at.-% excess of Ti was used in the synthesis in order to obtain complete phase purity. In the synthesis of Cu containing cesium titanates,

Results and discussion

During incorporation of additional elements into the crystal structure of a metal oxide, the overall charge neutrality of the host material has to be kept. For the compensation of the Cu substitution, in principle a decrease in the titanium and cesium content or an increase in the vacancy amount may occur. X-ray fluorescence analysis for element determination was performed in order to clarify the accurate stoichiometric composition of the cesium copper titanates. According to the stoichiometry

Conclusions

The paper demonstrates that Cu2+ ions incorporated into the layered cesium titanate Cs0.68Ti1.830.17O4 with lepidocrocite structure by the citrate route act as a pre-inserted precursor allowing an easy in-situ formation of an active, cheap and abundant co-catalyst for photocatalytic H2 production.

During photocatalysis, Cs0.64Ti1.79Cu0.1O4 is subjected to a change in crystal structure due to the exchange of cesium ions by protons. Furthermore, the band gap energy changes since the band gap

Acknowledgements

R.M. gratefully acknowledges funding in the Emmy-Noether program (MA 5392/3-1) of the German Research Foundation DFG. M.W. acknowledges financial support by the DFG within the priority program SPP 1613 (MW 1116/28-1).

References (45)

  • L.A. Bursill et al.

    Location of Cs ions in a hollandite-related superstructure

    J. Solid State Chem.

    (1984)
  • R.M. Navarro Yerga et al.

    Water splitting on semiconductor catalysts under visible-light irradiation

    ChemSusChem

    (2009)
  • F.E. Osterloh

    Inorganic materials as catalysts for photochemical splitting of water

    Chem. Mater.

    (2008)
  • A. Kudo et al.

    Heterogeneous photocatalyst materials for water splitting

    Chem. Soc. Rev.

    (2009)
  • M. Shen et al.

    Identification of the active species in photochemical hole scavenging reactions of methanol on TiO2

    J. Phys. Chem. Lett.

    (2001)
  • K. Maeda et al.

    Noble-metal/Cr2O3 Core/Shell nanoparticles as a cocatalyst for photocatalytic overall water splitting

    Angew. Chem. Int. Ed.

    (2006)
  • K. Yamaguti et al.

    Photolysis of water over metallized powdered titanium dioxide

    J. Chem. Soc. Faraday Trans.

    (1985)
  • R. Kydd et al.

    Low energy photosynthesis of gold-titania catalysts

    Photochem. Photobiol. Sci.

    (2007)
  • J. Chen et al.

    Plasmonic Photocatalyst for H2 evolution in photocatalytic water splitting

    J. Phys. Chem. C

    (2011)
  • W.J. Foo et al.

    Non-noble metal Cu-loaded TiO2 for enhanced photocatalytic H2 production

    Nanoscale

    (2013)
  • V. Gombac et al.

    CuOx-TiO2Photocatalysts for H2 production from ethanol and glycerol solutions

    J. Phys. Chem. A

    (2010)
  • A. Kudo et al.

    Photocatalytic water splitting into H2 and O2 over various tantalatephotocatalysts

    Catal. Today

    (2003)
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