A chemical bath deposition route to facet-controlled Ag3PO4 thin films with improved visible light photocatalytic activity
Graphical abstract
Introduction
Thin films of inorganic semiconductors attract intense research interest because of their diverse functionalities as photocatalysts, heterogeneous catalysts, electrodes, and electrosensors [1]. The morphology control of deposited crystals is of great importance as an effective method for optimizing the functionality of semiconductor films [1], [2], [3], [4]. The exposure of reactive crystal facets makes possible the enhancement of photocatalytic and photoelectric performances via the increase of surface reactivity [5], [6], [7]. Many sophisticated techniques including chemical vapor deposition [8], [9], pulsed laser deposition [10], physical vacuum deposition [11], molecular beam epitaxy, and sputtering are developed for the deposition of facet-oriented semiconductor crystals. However these methods suffer from serious drawbacks like the use of expensive instruments, harsh deposition condition, limited reproducibility, and so on [12]. Alternatively intense research efforts are devoted for economic chemistry-based deposition techniques such as electrodeposition, chemical bath deposition (CBD), successive ionic layer adsorption and reaction, and sol-gel method [5], [13], [14], [15], [16]. In one instance, the facet-tailored growth of ZnO crystals can be achieved with CBD technique [5], [14], [15], [16]. Additionally there is a report about the facet-controlled growth of Cu2O nanocrystals via electrodeposition method [13]. The selective exposure of {110} facets in the resulting rhombic dodecahedral Cu2O crystal is quite effective in enhancing its photocatalytic activity, which is attributable to the high density of surface-exposed copper ions in these facets [17]. In these experiments, surface capping agents like polymers, ions, and molecules are employed for the selective control of crystal growth habit via preferential adsorption of capping agents on specific crystallographic planes [13], [17]. Such a use of capping agents however results in the limited exposure of highly reactive facets, which requires additional complex processes to remove the surface-adsorbed capping molecules [18], [19]. In contrast to these reports, there is a report about the capping agent-free synthesis of facet-controlled Ag3PO4 nanocrystal with rhombic dodecahedral shape [20]. The resulting exposure of high energetic {110} planes is quite effective in optimizing the visible light photocatalytic activity of Ag3PO4 for O2 evolution and the degradation of volatile organic compounds (VOC) [20]. However, we are unaware of report about the facet-controlled deposition of Ag3PO4 thin film, even though the fabrication of thin film is very useful for the practical application of photocatalyst.
In the present work, a facile CBD technique is developed for the facet-controlled growth of Ag3PO4 thin films on various substrates. The fine-control of deposition condition allows us to tailor the crystal shape of the deposited Ag3PO4 from cube with exposed {100} facets to rhombic dodecahedron with exposed {110} facets. The photocatalytic activity and crystal morphology of the obtained Ag3PO4 thin films are investigated together with their structural and physicochemical properties. This CBD method is also applied for the fabrication of Ag3PO4–ZnO hybrid films through the successive coating process, since the hybridization with other semiconducting material is quite useful in exploring novel efficient photocatalyst materials [21], [22], [23], [24], [25].
Section snippets
Materials
Silver nitrate, silver acetate, methylene blue (MB), ammonium hydroxide (28%), triethanolamine (99%, TEA), ammonium hydrogen phosphate, and sodium hydrogen phosphate were purchased from Sigma-Aldrich, and used without further purification. Several substrates such as indium doped tin oxide coated glass (ITO), silicon wafer, and stainless steel (SS) were used for the deposition of Ag3PO4 films.
Deposition of Ag3PO4 thin films
Chemical bath for the deposition of Ag3PO4 was prepared from an aqueous solution of silver precursor
XRD analysis
Fig. 1 represents powder XRD patterns of Ag3PO4 thin films deposited on glass, SS, Si, and ITO substrates. Like bulk Ag3PO4 materials, all the obtained Ag3PO4 films display typical Bragg reflections of body-centered cubic structured Ag3PO4 (JCPDS no. 06-0505) as well as several XRD peaks originating from the substrates. Both the bulk Ag3PO4 materials prepared with the baths A and B exhibit noticeable difference in the relative intensities of the XRD peaks of Ag3PO4 phase depending on their
Conclusion
In this study, we are able to develop a facile, economic, and reproducible CBD route to facet-controlled thin films of Ag3PO4 and its hybrid with metal oxide. A fine-control of bath temperature, precursor, substrate, and solution pH allows us to tailor the crystal shape of Ag3PO4 from cube with the surface {100} facets to rhombic dodecahedron with the surface {110} facets. Additionally, the tuning of [Ag+]/[phosphate] ratio makes possible the maximal loading of Ag3PO4 crystals per the unit area
Acknowledgments
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. NRF-2014R1A2A1A10052809), by Korea Ministry of Environment (MOE) as ‘Business Ideas Commercialization Program (RE20150914)’, and by the National Research Council of Science and Technology through the Degree & Research Center program (DRC-14-3-KBSI). Dr. J. L. Gunjakar was supported by RP-Grant 2014 of Ewha Womans University.
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