Layered cesium copper titanate for photocatalytic hydrogen production
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.
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 Cs0.68Ti1.83□0.17O4 (□: 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 Cs0.68Ti1.83□0.17O4 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 Cs0.68Ti1.83□0.17O4 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)
- et al.
Photoassisted hydrogen production from a water-ethanol solution: a comparison of activities of Au/TiO2 and Pt/TiO2
J. Photochem. Photobiol. A: Chem.
(1995) - et al.
Photochemical hydrogen evolution from aqueous triethanolamine solutions sensitized by binaphthol-modified titanium(IV) oxide under visible-light irradiation
J. Photochem. Photobiol. A: Chem.
(2003) - et al.
Kinetics and mechanism of glycerol photo-oxidation and photo-reforming reactions in aqueous TiO2 and Pt/TiO2 suspensions
Catal. Today
(2013) - et al.
Photodecomposition of water over Pt/TiO2 catalysts
Chem. Phys. Lett.
(1980) - et al.
Photocatalytic hydrogen production over CuO-modified titania
J. Colloid Interface Sci.
(2011) - et al.
Promotion effect of nanosized Pt, RuO2 and NiOx loading on visible light-driven photocatalysts K4Ce2M10O30 (M = Ta Nb) for hydrogen evolution from water decomposition
Sci. Technol. Adv. Mater.
(2007) - et al.
Cu-modified TiO2photocatalysts for decomposition of acetic acid with simultaneous formation of C-1-C-3 hydrocarbons and hydrogen
Appl. Catal. B:Environ.
(2013) - et al.
Exfoliation and thermal transformations of Nb-substituted layered titanates
J. Solid State Chem.
(2011) - et al.
Ni/Mgo catalyst prepared using citric acid for hydrogenation of carbon dioxide
Appl. Catal. A
(1997) - et al.
Three-dimensional/one-dimensional transition in the Cs+ sublattice of the mixed valence CsTi8O16 hollandite: structures at 297 and 673 K
J. Solid State Chem.
(1991)
Location of Cs ions in a hollandite-related superstructure
J. Solid State Chem.
Water splitting on semiconductor catalysts under visible-light irradiation
ChemSusChem
Inorganic materials as catalysts for photochemical splitting of water
Chem. Mater.
Heterogeneous photocatalyst materials for water splitting
Chem. Soc. Rev.
Identification of the active species in photochemical hole scavenging reactions of methanol on TiO2
J. Phys. Chem. Lett.
Noble-metal/Cr2O3 Core/Shell nanoparticles as a cocatalyst for photocatalytic overall water splitting
Angew. Chem. Int. Ed.
Photolysis of water over metallized powdered titanium dioxide
J. Chem. Soc. Faraday Trans.
Low energy photosynthesis of gold-titania catalysts
Photochem. Photobiol. Sci.
Plasmonic Photocatalyst for H2 evolution in photocatalytic water splitting
J. Phys. Chem. C
Non-noble metal Cu-loaded TiO2 for enhanced photocatalytic H2 production
Nanoscale
CuOx-TiO2Photocatalysts for H2 production from ethanol and glycerol solutions
J. Phys. Chem. A
Photocatalytic water splitting into H2 and O2 over various tantalatephotocatalysts
Catal. Today
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