Decomposition of dimethyl methylphosphonate vapor on ultrathin-film titania photocatalytic light absorber
Introduction
Security threat by possible use of chemical war agents (CWAs) by terrorists or in battlefield, and environmental pollution by industrial toxic chemicals have been growing global concerns (Knudson, 2001; Alexander and Klein, 2006; MatouŠEk, 2006; Tetyana et al., 2006; Talmage et al., 2007). Photocatalytic detoxification using solar energy represents an environmentally friendly and cost-efficient strategy to degrade these toxic chemicals (Kabra et al., 2004; Robertson et al., 2005; Blanco-Galvez et al., 2006; Dalrymple et al., 2010; Thakur et al., 2010; Ahmed and Haider, 2018). Titania (TiO2) has been the most frequently employed photocatalyst in detoxification processes due to its inherent environmental friendliness and low-cost, together with the potential of complete mineralization of organic toxic chemicals. The current photocatalytic processes, however, are highly inefficient, particularly in the case of solar activation. This inefficiency arises because the charge carrier (electrons/holes, e−/h+) extraction efficiency in photocatalysts is far too low (Kitano et al., 2007; Habisreutinger et al., 2013; Liu and Chen, 2014; Ma et al., 2014). The charge carriers are generated by light irradiation to initiate the catalytic reactions, but their recombination rates are much faster than utilization, which highly limits the activity of photocatalysts. Catalyst materials and design strategies that have potential to enhance e−/h+ extraction efficiency, light absorption, and the resultant catalytic activity and efficiency in photocatalytic detoxification are highly desired.
Nanostructured TiO2-based catalysts such as nanowires, nanotubes or nanosheets (Mor et al., 2006; Kamat, 2007; Jun et al., 2012; Liang et al., 2012; Liu et al., 2015) have been studied to promote efficiency in photocatalysis. The nano-sized TiO2 catalysts shorten the diffusion length, disfavor the electron and hole recombination and enhance multi-electron transfer for photocatalytic reactions (Linsebigler et al., 1995; Aprile et al., 2008; Fujishima et al., 2008). Although an enhancement in catalytic performance of nanostructured TiO2-based materials has been observed, the photocatalytic efficiency is still too low to meet practical applications, mainly due to lack of sufficient light adsorption in nanostructured photocatalyst materials. The decrease in catalyst particle size leads to a significant reduction in light absorption and generation of charger carriers. The intrinsic trade-off between optical absorption and charge carrier extraction efficiency, i.e., a light absorber should be thick enough to absorb the light allowable by its band gap but thin enough to allow charge carrier extraction for catalytic reactions, therefore, will be conquered.
One promising scheme to enhance the optical absorption without sacrificing the catalytic activity/selectivity in nanostructured catalysts would be to employ ultra-thin film super light absorbers on the catalysts. For instance, planar thin film interference can enhance the optical absorption within ultra-thin films (Dotan et al., 2013; Kats et al., 2013). This mechanism could significantly reduce the required thickness of the light absorbers. For instance, over 60% resonance absorption was successfully demonstrated with a 1.5-nm-thick Ge film (Song et al., 2014) and even a monolayer of MoS2(Janisch et al., 2016) on predesigned nanocavities, paving the way for high-efficiency ultrathin-film energy conversion materials, structures and devices (Xia et al., 2017), including photocatalytic systems. In particular, the boosted optical absorption in planar ultra-thin photocatalytic Ti-doped α-Fe2O3 films (e.g. ∼26 nm) was successfully used for efficient water splitting (Dotan et al., 2013). Our previous work has designed the ultrathin-film photocatalytic light absorber (UFPLA), made by sequential deposition of an aluminum (Al) reflector and a TiO2 thin film (<30 nm) on a glass substrate, which is effective to resolve the intrinsic trade-off between optical absorption and charge carrier extraction efficiency. The TiO2/Al films introduced the active sites, and manipulated the phase of the reflective partial waves to realize the destructive interference. Therefore, the light absorption capability of the TiO2 thin film was maximized, significantly increasing catalytic efficiency by generating much more surface charge carriers. For example, the UFPLA structures significantly improve activity and selectivity in photocatalytic carbon dioxide (CO2) reduction with water to oxygenated hydrocarbons (Song et al., 2018). In comparison to the benchmark photocatalyst (Aeroxide®P25), the CO2 reduction rate was enhanced up to a factor of 1145 times (Song et al., 2018).
In this work, we employed the UFPLA structures with TiO2 thin-films for the photocatalytic detoxification of CWAs. Due to toxicity of CWAs, the dimethyl methylphophonate (DMMP) was used as the simulant in this study. The performance of UFPLA with different TiO2 film thicknesses in photocatalytic decomposition of DMMP vapor was studied and compared to that of Aeroxide®P25 catalyst. The effects of reactant (i.e. DMMP, water and oxygen, respectively) partial pressure and reaction temperature on DMMP decomposition were measured. The kinetic data were described by both Langmuir-Hinshelwood and Elay-Redial models, and the former gave a better fitting.
Section snippets
Materials
Dimethyl methylphosphonate (DMMP, 97% purity) and Aeroxide® P25 were purchased from Alfa Aesar. Air (ultrapure), nitrogen (N2, ultrapure) Argon (Ar, ultrapure) and helium (He, ultrapure) were purchased from Airgas. Deionized (DI) water (H2O) was used throughout the experiments.
UFPLA preparation
The UFPLA, comprised of an Al reflector and a TiO2 thin film, was synthesized using the method reported in our previous work (Song et al., 2018). Typically, the Al reflector layer (150-nm thick) was deposited on a glass
Structure and optical absorption of UFPLA structures
The physicochemical property of TiO2 thin film in TiO2/Al UFPLA structures as well as the Aeroxide®P25 control sample have been examined and the results was described in detail in our previous publication (Song et al., 2018). For completeness, we provide a brief discussion here and the characterization data are presented in Section S1 in the Supporting Information. The surface of TiO2 thin film is uniform, following the pattern of Al layer, independent on the TiO2 film thickness (Figure S1a-b).
Conclusions
The ultrathin-film photocatalytic light absorber (UFPLA) were studied for the first time for the photocatalytic DMMP decomposition. The UFPLA structure is comprised of a TiO2 thin-film and an Al reflector, sequentially deposited on a supportive glass substrate. The nanocavity effect of the structure improves optical absorption in ultra-thin TiO2 films, and thus maintains strong light absorption and charge carrier extraction efficiency for efficient photocatalytic DMMP decomposition. Compared to
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This material is based upon work supported by, or in part by, the U. S. Army Research Laboratory and the U. S. Army Research Office under contract/grant number: W911NF-17-1-0363. The authors gratefully acknowledge financial support from National Science Foundation (NSF-CBET 1264599 and NSF-CBET 1351384).
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