Heterojunction p-n-p Cu2O/S-TiO2/CuO: Synthesis and application to photocatalytic conversion of CO2 to methane
Graphical abstract
Introduction
The dramatic increase in atmospheric CO2 concentration has become a matter of global concern [1], [2], motivating the scientific community to investigate techniques for capturing, sequestering, or recycling the gas. The use of sunlight to drive the photocatalytic conversion of CO2 into a hydrocarbon fuel compatible with the current energy infrastructure is a particularly appealing prospect [3], [4], [5] and has motivated our current study.
A number of materials have been tested for use in the photocatalytic conversion of CO2 [6], [7], [8], [9], in particular n-type anatase TiO2 [10], [11], [12] due to its excellent photocorrosion stability and charge transport properties. However TiO2 absorbs only a small fraction of the solar spectrum energy [13], and the TiO2 conduction band is higher than the CO/CO2 potential [7], [14]. To overcome the limitation of restricted light absorption, tremendous researches have been investigated to narrow down the TiO2 band gap, in turn extending its light absorption with harvesting of moderated terrestrial light absorption resulting in improved photocatalytic performance. Such strategies involve metals and non-metals doping of TiO2 [15], [16], [17], loading of noble metal co-catalysts e.g. Pt, Au, Pd on TiO2 [18], [19], graphene based TiO2 materials [20], coupling of TiO2 with low band gap materials such as CZTS, CuxO etc. [21], [22], [23], and nanostructured photocatalysts [5], [6], [7], [8]. All these approaches are aimed towards the worthy objectives of improved photocatalytic performance. According to author’s opinion amongst these approaches, noble metals loading is an efficient but pricey way to improve photocatalytic activity, whereas the doping strategy is a cost-effective approach with overall modest photocatalytic performance. Moreover coupling of TiO2 with an appropriate narrow band gap materials also results in an efficient heterojunctioned material with broadened visible light absorption and hence improved photocatalytic performance.
In the present work, we have developed a new synthesis approach with a key aim of merging both non-metal doped TiO2, seeking to lower its band gap while maintaining its excellent intrinsic properties, in combination with semiconductors that enhance broad-spectrum light absorption while promoting needed charge-transfer processes [15], [16]. The low band gap material we selected is copper (II) oxide, a p-type semiconductor with a band gap energy of Eg = 1.35–1.6 eV [23], [24], a relatively inexpensive oxide semiconductor with appropriate band edge positions for photoreduction of CO2. Based on our developed strategy, a new readily synthesized material, Cu2O/S-TiO2 (Sulfur doped TiO2) micro-blocks decorated with vertically aligned CuO nanowires, hereafter referred to as Cu2O/S-TiO2/CuO is fabricated and the resultant material is utilized, at ambient temperature, in the photocatalytic conversion of CO2 and water vapor to hydrocarbon fuels, finding primarily methane. The characterization (XRD, XPS, and PL investigations) of such novel architecture indicates that it consists of a p-n-p heterojunction. As our interest is in achieving an inexpensive, industrially scalable photocatalyst we specifically avoid the use of relatively expensive metal co-catalysts such as Pt or Pd. Our described material architecture offers several benefits including: (1) favorable shifts in the semiconductor band edges to promote CO2 reduction; (2) broad-spectrum light absorption and (3) efficient separation of photogenerated charges through the formation of p-n-p heterojunctions.
The synthesis strategy of Cu2O/S-TiO2/CuO is shown in Fig. 1. CuS dendrites, which serve as the Cu and S doping sources of subsequent structures, are synthesized by anodization of Cu foil in a sulfide-based aqueous electrolyte. The CuS dendrites are then immersed in a TiO2 precursor (titanium (IV) isopropoxide, TTIP), air dried, and then annealed at various temperatures in air. During drying TTIP reacts with air moisture forming Ti(OH)4 [25], [26], [27].Ti(OR)4 + 4H2O → Ti(OH)4 + 4ROHWhere R is ethyl, i-propyl, n-butyl, etc.
Annealing causes the Ti(OH)4 layer to first crack, then subsequently crystallize, and promotes diffusion of the copper ions from the CuS dendrites to the surface where they react with oxygen forming CuO nanowires [28], [29]. Further, with annealing sulfur ions diffused through the Ti(OH)4 micro-blocks resulting in a spatially varying bandgap, akin to graded junction diodes.
Section snippets
Materials
Copper substrate (Cu foil, 99.9%, Nilaco), Titanium Foil (Ti, 99.7%, Sigma Aldrich), Sodium Sulfide (Na2S, Sigma Aldrich), Titanium (IV) isopropoxide (TTIP, Ti[OCH(CH3)2]4, 97%, Sigma Aldrich), Carbon paper (C, CNL energy, 420 μm), Acetone, Ethanol, Deionized water.
Preparation of Cu2O/S-TiO2/CuO photocatalyst
Before anodization the Cu foil was cleaned with acetone and ethanol, followed by a deionized water rinse. The anodization was performed using a two-electrode cell with Cu foil as the working electrode and carbon paper (2 cm × 3 cm × 0.042
Characterization of Cu2O/S-doped TiO2/CuO photocatalyst
FESEM surface and cross sectional images of the CuS dendrites synthesized on the Cu foil are shown in Fig. 2a,b; during the annealing process the dendrites hold the Ti-precursor film to the substrate. Fig. 2c is a top-view image of a CuS dendrite film that has been dip-coated in TTIP and allowed to be dried; the formation of Ti(OH)4 micro-blocks is apparent. Fig. 2d is a FESEM image showing what little of the Ti-precursor film has attached to an untreated Cu foil sample. Vertically aligned CuO
Conclusions
In summary, we describe the facile synthesis of a unique p-n-p heterojunction material architecture, tandem Cu2O/S-TiO2 micro-blocks covered with CuO nanowires, that show excellent performance in the photocatalytic conversion of CO2 and water vapor to methane. Given that this synthesis strategy is general, and that the design parameters can be systematically varied and optimized, we believe this report will be of significant service to the materials and chemistry communities.
Acknowledgements
The authors gratefully acknowledges the support by the DGIST R&D Program of the Ministry of Education, Science and Technology of Korea (17-BD-0404 & 17-01-HRLA-01) and by Basic Science research program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (2013R1A1A008678 & 2014K1A3A1A47067086). This research was also supported by the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by
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