Nanostructured charge transfer complex of CuTCNQF₄ for efficient photo-removal of hexavalent chromium

Abstract

The high toxicity of hexavalent chromium warrants the development of efficient catalysts that could reduce chromium into relatively non-toxic trivalent chromium species. Pristine charge transfer complexes of the MTCNQ family (M = Cu or Ag; TCNQ = 7,7,8,8-tetracyanoquinodimethane) have previously failed to catalyse the reduction of Cr⁶⁺ to Cr³⁺. We demonstrate that due to the outstanding electron transfer properties of one of the fluorinated derivatives of MTCNQ, i.e., 7,7,8,8-tetracyano-2,3,5,6-tetraflouroquinodimethane (CuTCNQF₄), it is able to catalyse the reduction of hexavalent chromium in aqueous solution at room temperature. We further demonstrate that the semiconducting nature of these organic charge transfer complexes allows CuTCNQF₄ to act as an outstanding material for the reductive photo-removal of hexavalent chromium under UV photoexcitation conditions. Such materials are likely to play an important role in photoactive electron transfer reactions.

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Introduction

Metal–organic semiconducting materials of the MTCNQ family (M = Cu or Ag; TCNQ = 7,7,8,8-tetracyanoquinodimethane) have attracted considerable attention over the last decade due to their potential applications in data and energy storage,^1,2 sensors,^3,4 electric devices,^3–5 catalyst surfaces,^6–9 and antibacterial materials.¹⁰ Fluorinated analogues of TCNQ, such as 7,7,8,8-tetracyano-2,3,5,6-tetraflouroquinodimethane (TCNQF₄) have shown a significant improvement in the stability of the anion and dianion radicals in acetonitrile, while further improving their electron accepting properties, thereby enhancing the potential applicability of TCNQF₄-based semiconducting materials.^6,11,12 The synthesis and structural characterization of MTCNQF₄ crystals have shown their outstanding optical, electrical, and magnetic properties^13–20 compared to their non-fluorinated analogues. However, it is surprising that the applications of TCNQF₄-based semiconducting materials have remained limited.^11,12 Recently, the potential of CuTCNQF₄ and AgTCNQF₄ for electron transfer reactions between ferrocyanide and thiosulfate was investigated, and revealed that the balance between the injection of charge to the catalyst surface and the ejection of charge from the surface to the solution is an important factor in promoting this reaction.⁶ Similarly, CuTCNQF₄ and AgTCNQF₄ have also been used to fabricate field emission¹⁷ and switching devices.²¹

Hexavalent chromium, a carcinogen and mutagen, is a serious pollutant found in many industrial effluents, including metal plating, wood processing, pigment manufacturing and leather tanning.^22,23 Chromium usually exists in two oxidation states viz. trivalent (Cr³⁺) and hexavalent (Cr⁶⁺) in aqueous solutions. While Cr³⁺ is minimally toxic to the environment and humans (it is even known as an essential nutrient for humans);²⁴ Cr⁶⁺ remains extremely toxic to humans.²⁵ It is also well-known that in comparison to Cr⁶⁺ that can easily migrate through groundwater and soil, Cr³⁺ can be readily precipitated in the form of Cr(OH)₃ in aqueous solutions at neutral pH.²⁶ This further negates the toxic effect of trivalent chromium in the environment. Therefore, it is of utmost importance to find effective ways to remove Cr⁶⁺ from aqueous solutions.

A facile approach for the removal of hexavalent chromium may involve the reduction of Cr⁶⁺ to Cr³⁺ followed by its precipitation to Cr(OH)₃ in combination with other separation methods.²⁷ Several strategies have been developed to reduce Cr⁶⁺ to Cr³⁺ in aqueous solution, including treatment with sulphur compounds or iron salts^28,29 under acidic conditions followed by precipitation with an alkali, electrochemical reduction,³⁰ photocatalytic reduction using semiconducting materials either by directly using the semiconductor for reduction or using the semiconductor as an adsorbent material followed by catalytic conversion in the presence of light illumination conditions^31,32 and more recently bacteria^33,34 and marine planktons.³⁵ Among these strategies, the use of photocatalytic processes, wherein a semiconducting material is employed in combination with additional electron donors such as organic acids, is finding particular attention due to improved efficiencies.^9,11,27,36 Although the use of inorganic semiconductors as photocatalysts is well-established,^27,36 recent investigations have started to reveal that organic semiconductors decorated with noble metals can also effectively reduce Cr⁶⁺ to Cr³⁺.⁹ However, those studies found that pristine organic semiconductors without noble metals do not have enough driving force to catalyse this reaction. This is challenging, as noble metals typically add significant costs to the catalytic processes, which make their use impractical for environmental remediation.

In the current study, we report for the first time, the ability of a pristine organic semiconductor based on CuTCNQF₄ in promoting the catalytic reduction of toxic Cr⁶⁺ to its non-toxic Cr³⁺ counterpart. The influence of reaction conditions including solution pH, electron donors, temperature, and UV light irradiation on the reduction efficiency is evaluated.

Materials and methods

Materials

Copper foil (99.99% purity) was procured from Chem Supply. Acetonitrile, concentrated sulphuric acid, nitric acid, methanol, acetic acid, and sodium dichromate (Na₂Cr₂O₇) were purchased from Ajax Finechem. Tetrafluoro-1,4-benzoquinone was purchased from Sigma-Aldrich. All chemicals were used as received. TCNQF₄ was synthesised in house. The copper foil was treated with dilute nitric acid, thoroughly rinsed with deionized water (MilliQ) and dried with a flow of N₂ gas prior to use.

Synthesis of TCNQF₄

The TCNQF₄ synthesis was performed using a protocol reported previously.³⁷ A suspension containing 5 mM of compound [1] in 150 mL of pyridine was added to 10 mM of TiCl₄ (Scheme 1). This reaction mixture was slowly heated under a nitrogen atmosphere. To this heated mixture, 100 mM of malononitrile in 20 mL of dry pyridine was added and the reaction was refluxed for 15 h. At the end of 15 h, the reaction mixture was concentrated to half its volume. The solid product obtained after cooling was filtered, washed and dried in a vacuum oven. The product containing TCNQF₄ was purified by recrystallization (methanol and acetonitrile) yielding a brown solid (TCNQF₄) with 87% yield. Elemental analysis for C₁₂F₄N₄: calculated: C – 52.19, N – 20.29; actual: C – 52.20, N – 20.29; FTIR using KBr disc, ν 3012 cm⁻¹ (C–H aromatic), 2226 cm⁻¹ (C≡N), 1665 cm⁻¹ (C=C ring) and 1460 cm⁻¹ (C–CN stretching); MALDI-TOF (m/z): [M⁺] calculated for C₁₂F₄N₄: 276.0059, actual: 276.0057.

Scheme 1 Synthesis of TCNQF₄.

Synthesis of CuTCNQF₄

Copper foil (2 × 0.5 cm²) was immersed in 6 mL of a 2 mM TCNQF₄ solution at 45 °C for 3 h. The surface of the copper foil turned purple in colour indicating the formation of CuTCNQF₄. The sample was rinsed several times with deionized water and dried in a stream of N₂ gas prior to characterization.

Characterization of CuTCNQF₄

The crystal structures and elemental composition of CuTCNQF₄ grown on copper foil were studied by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) using an EDX-equipped Phillips XL30 SEM instrument. Fourier transform infrared spectroscopy (FTIR) was performed on a PerkinElmer D100 spectrometer in attenuated total reflectance mode, while Raman spectroscopy was performed using a PerkinElmer Raman Station 200F. The Raman spectra were background corrected using a smoothing free algorithm developed in house.³⁸ X-ray photoelectron measurements (XPS) were carried out using a Thermo K-Alpha XPS instrument (Al Kα radiation, photon energy of 1486.6 eV). The C 1s, Cu 2p, N 1s and F 1s core level spectra were recorded with an overall resolution of 0.1 eV. The core level spectra were background corrected using the Shirley algorithm and chemically distinct species were resolved using a nonlinear least squares fitting procedure. UV-Vis reflectance measurements were carried out using a CRAIC Technologies microspectrophotometer with a 15× objective. The measurements were obtained from a 50 μm × 50 μm area.

Reduction of Cr⁶⁺

The catalysis experiments were carried out by placing CuTCNQF₄ at the bottom of a 50 mL beaker containing a total 20 mL reaction volume with 10 mM of Cr⁶⁺ and 1 mL of electron donor (either dried methanol or acetic acid). The pH of the mixture was adjusted using 1 M H₂SO₄ before the reaction. CuTCNQF₄ grown on 1 × 0.5 cm² of foil was used for electron donor and pH-dependent catalytic experiments. For dark and UV irradiation (200 mW cm⁻² UV with a λ_max of 254 nm obtained from Edmund Optics) experiments, the Cu foil size on which CuTCNQF₄ was grown was increased to 2 × 0.5 cm² to increase the surface for light absorption. The sample was placed 2 cm away from the light source. The reaction mixture was continuously stirred at 250 rpm during the experiment. Samples were collected at different time points and examined using a Cary 50 UV-Vis spectrophotometer to determine the reduction of Cr⁶⁺ to Cr³⁺ by measuring the absorbance changes between 300 and 500 nm (characteristic absorption maxima λ_max = 352 nm). For reusability experiments, the CuTCNQF₄ catalyst grown on Cu foil was removed from the reaction vessel after completion of the reaction, followed by washing the catalyst surface three times with deionized water, before being used for the subsequent cycle for up to 10 cycles.

Results and discussion

The synthesis of TCNQF₄ derivative [2] from precursor tetrafluoro-1,4-benzoquinone [1] was performed using a previously described protocol (Scheme S1†).³⁷ Briefly, the quinones present in [1] were reacted with malononitrile to form TCNQF₄ [2], which was purified by precipitation and recrystallization from methanol and acetonitrile, and obtained as a crystalline high-temperature melting solid with 87% yield. The structure of TCNQF₄ derivative [2] was confirmed by mass spectroscopy, elemental analysis (see Materials and methods) and FTIR.

The fabrication of CuTCNQF₄ on copper foil proceeds through a one electron transfer reaction between copper metal and TCNQF₄ in acetonitrile solution. During this process, a simple reduction of TCNQF₄⁰ dissolved in acetonitrile by Cu⁰ as the electron donor results in the formation of Cu⁺TCNQF₄⁻ crystals on the surface of the Cu foil. This is similar to what was observed in the case of CuTCNQ crystals formed on a copper foil.^5–9,15 Illustrated in Fig. 1 are the SEM images of CuTCNQF₄ crystals synthesised on both sides of a copper foil through a spontaneous crystallisation process in acetonitrile. These crystals show cuboidal morphology with an edge length of approximately 3 μm. The reaction time was optimised to obtain a uniform coating of the crystals throughout the surface of the copper foil, wherein a reaction time of 3 h yielded complete coverage of the copper foil with CuTCNQF₄ crystals, while using 2 mM TCNQF₄ at 45 °C.

Fig. 1 (a) Low and (b) higher magnification SEM images of pristine CuTCNQF₄ crystals synthesised on Cu foil in acetonitrile.

An EDX spectrum (Fig. 2a) obtained from the CuTCNQF₄ crystals shows characteristic C Kα, N Kα, and F Kα energy lines at 0.277, 0.392 and 0.677 keV, respectively, which are attributed to TCNQF₄ in the sample. An additional characteristic energy line for Cu Lα at 0.930 keV suggests the formation of CuTCNQF₄ crystals. The calculated atomic ratios of C : F : N : Cu from the EDX measurements were 12 : 4 : 4 : 1, which is consistent with the expected stoichiometric ratio of the CuTCNQF₄ elemental composition.¹⁷ Raman spectroscopy is an ideal technique to distinguish between neutral (TCNQF₄⁰) and reduced TCNQF₄ (TCNQF₄⁻). Illustrated in Fig. 2b are background-corrected Raman spectra³⁸ obtained from pristine TCNQF₄ powder and CuTCNQF₄. The pristine TCNQF₄ powder shows characteristic signatures at 2226 cm⁻¹ (C≡N), 1665 cm⁻¹ (C=C ring) and 1460 cm⁻¹ (C–CN stretching), confirming its neutral state (TCNQF₄⁰).^6,15,17 Following the reaction of TCNQF₄ with Cu, the C≡N (2200 cm⁻¹) and C=C (1640 cm⁻¹) bands shift to lower wavenumbers, confirming that the neutral TCNQF₄ has converted to its reduced state (TCNQF₄⁻), signifying CuTCNQF₄ formation.^6,15,17 Furthermore, an important signature suggesting the formation of the TCNQF₄⁻ radical is the splitting of the C≡N stretch, which indicates that the coordination between Cu⁺ and TCNQF₄⁻ is through the CN group.^6,15,17 Additional peak shifts of the C–CN, C–C, and C–F stretching provides evidence for the presence of the TCNQF₄⁻ radical.^6,15,17 The formation of CuTCNQF₄ was also confirmed through the presence of characteristic FTIR bands at 2215, 1497 and 1223 cm⁻¹ (Fig. S1, ESI†).^6,15 The presence of the 1497 cm⁻¹ peak corresponding to the C=C ring stretch, which is found at higher energy for TCNQF₄, clearly suggests that the TCNQF₄ units in CuTCNQF₄ are present in their reduced form. In addition, the shift in the C–F out of plane bending at 1206 cm⁻¹ also confirms the formation of CuTCNQF₄.^15,39

Fig. 2 (a) EDX spectrum of CuTCNQF₄ crystals; (b) Raman spectra of pristine TCNQF₄ and CuTCNQF₄ crystals.

Further evidence for the formation of CuTCNQF₄ is evident from XPS measurements, wherein the Cu 2p core level XPS spectrum shows two characteristic 2p_3/2 and 2p_1/2 splitting components at 931.6 eV and 951.5 eV, respectively (Fig. 3). The absence of signatures corresponding to shake-up satellites suggests that the Cu species in CuTCNQF₄ is present as Cu⁺, which corroborates well with the values reported in the literature.^17,40,41 The N 1s core level XPS spectrum provides information about the involvement of the CN functional group during the formation of CuTCNQF₄.^5,7–9 In comparison to pristine TCNQF₄ (Fig. S2, ESI†), the N 1s core level spectrum of CuTCNQF₄ shifts to a lower binding energy (BE) of 398.2 eV with an additional shake-up peak at a higher BE of 401.6 eV.¹⁸ This additional higher BE feature is due to the π–π conjugated system, a feature commonly observed in TCNQ and its derivatives.⁴² The BEs of C 1s and F 1s core level XPS spectra are also consistent with the reported values of TCNQF₄.^6,15,17

Fig. 3 XPS spectra showing Cu 2p, N 1s, C 1s and F 1s core levels obtained from CuTCNQF₄ crystals grown on a copper foil.

It is well-known that there is a considerable activation energy barrier during the reduction of Cr⁶⁺, even when a large driving potential is available during the reaction between Cr⁶⁺ and an electron donor. For instance, the reduction potentials (versus the standard hydrogen electrode) of the two-half equations, i.e. Cr⁶⁺ reduction and methanol oxidation, are 1.36 V and 0.02 V, respectively (E⁰ values).⁴³ Therefore, even with a large driving potential of 1.34 V, the reaction does not proceed spontaneously and requires an efficient catalyst and/or high temperatures. This is probably the reason why non-fluorinated CuTCNQ could not catalyse Cr⁶⁺ reduction, even at high temperatures.⁹ However, when CuTCNQ was converted to CuTCNQ–metal hybrids, although Cr⁶⁺ could be reduced at ambient temperatures, elevated temperatures accelerated Cr⁶⁺ reduction.⁹ Given that the fluorinated analogues of TCNQ have superior electron accepting properties over TCNQ, in the current study, we assessed the performance of CuTCNQF₄ as a potential catalyst for the reduction of hexavalent chromium in aqueous solutions at room temperature. In a typical reaction, the catalyst was immersed in an aqueous solution containing Cr⁶⁺ ions in the presence of acetic acid and methanol as electron donors, and held at a stable temperature of 25 °C. The UV-Vis spectra of the dichromate ions in the presence and absence of electron donors were measured as a function of time. It is apparent that while the reaction proceeds only in the presence of electron donors (Fig. 4a) due to ligand-to-metal charge transfer of dichromate ions;^9,44 it is very interesting that pristine CuTCNQF₄ could in fact catalyse Cr⁶⁺ reduction even at 25 °C. This is a significant advantage over non-fluorinated CuTCNQ, which remained inactive even at high temperatures for this particular reaction.⁹ Assuming that the reaction follows a pseudo-first-order kinetic model (the reaction proceeds in the presence of excess donor concentration), plotting the ln(A_t/A₀) versus time (where A_t is the absorbance at time t and A₀ is the absorbance at time zero) allows the determination of the rate constants. The relative rate constants for the reduction of Cr⁶⁺ at pH 2.0 in the presence of two electron donors, viz. methanol and acetic acid, were 2.5 × 10⁻³ min⁻¹ and 2.0 × 10⁻³ min⁻¹, respectively (Table S1, ESI†). This suggests that methanol has a higher driving force over acetic acid in the presence of the catalyst and was therefore the choice of electron donor for subsequent studies. It is noteworthy that the reaction rates largely depend on the reaction conditions; therefore these rates are calculated for comparative purposes under similar reaction conditions.^6,9,11

Fig. 4 Plot of ln(A_t/A₀) versus time for the CuTCNQF₄ mediated reduction of Cr⁶⁺ (a) in the presence of different electron donors at pH 2.0 and 25 °C; (b) at different pH values in the presence of methanol at 25 °C and (c) at different temperatures in the presence of methanol and at pH 2.0.

Other reaction conditions such as the pH and temperature may also have a significant impact on the reaction rates. Hence, the catalytic conversion of Cr⁶⁺ was also performed at different pH values in the presence of methanol as an electron donor. Fig. 4b shows that at 25 °C, the reaction proceeds only under acidic pH conditions; such that while the reaction rates do not improve at pH values lower than 2.0, they dramatically drop at and above pH 3.0. Therefore, subsequent experiments were carried out at pH 2 in the presence of methanol. The role of pH in improving the reduction reaction is due to either an increase in oxidising ability of Cr⁶⁺ and/or the availability of more catalytically active sites through a reduction in the amount of Cr(OH)₃ deposited on the catalyst surface.^11,27 Fig. 4c shows the influence of temperature on the catalytic reduction of Cr⁶⁺, wherein while CuTCNQF₄ was catalytically active at ambient temperatures, an increase in temperature from 25 to 50 °C did not significantly improve the reaction rates. This suggests that the outstanding electron transport properties of CuTCNQF₄ may allow it to act as an efficient catalyst for even room-temperature reduction of hexavalent chromium.

The photocatalytic activity of traditional inorganic semiconductors such as TiO₂, SnO₂ and ZnO is well-known.^45,46 Given that CuTCNQF₄ also possesses semiconducting properties, we explored the possibility of improving the rate of Cr⁶⁺ reduction through photo-excitation. The absorption characteristics of CuTCNQF₄ and pristine TCNQF₄ suggested that while similar to traditional oxides,⁴⁵ these molecules primarily absorb in the UV region, they also have absorbance across the visible region of the electromagnetic spectrum (Fig. S3, ESI†).

Illustrated in Fig. 5 is the effect of UV light irradiation on the reduction of Cr⁶⁺. To assess the influence of UV light irradiation, the experiments were first carried out in the dark in the presence and absence of the catalyst. Under dark conditions, although the reaction proceeds faster in the presence of the catalyst, the reaction rate is not remarkably improved. In contrast, when the same reaction is carried out during UV light irradiation, the CuTCNQF₄-mediated reduction of Cr⁶⁺ becomes significantly faster, such that the reaction rate is increased over an order of magnitude from 3.4 × 10⁻³ min⁻¹ to 3.6 × 10⁻² min⁻¹ in the presence of CuTCNQF₄ (reaction rates for all reactions are outlined in Table S1, ESI†). Notably, although the non-fluorinated analogue (CuTCNQ) was unable to promote the reduction of Cr⁶⁺,⁹ it is very interesting that the use of a fluorinated analogue (CuTCNQF₄) in the current case could not only actively promote the catalytic reduction of Cr⁶⁺ even at room temperature, but the reaction rates could also be significantly enhanced by UV-light irradiation.

Fig. 5 Plot of ln(A_t/A₀) versus time for CuTCNQF₄-mediated photo-reduction of Cr⁶⁺ at pH 2 and 25 °C in the presence of methanol as an electron donor.

The reusability of the CuTCNQF₄ catalyst was also evaluated by employing the same catalyst for 10 consecutive catalytic reactions, wherein the surface of the catalyst was washed several times with deionised water between two reactions. The efficiency of the Cr⁶⁺ reduction was maintained for at least 10 cycles with no loss of efficiency (Fig. S4, ESI†). In addition, the stability of the catalyst was also ascertained using SEM and Raman spectroscopy. No structural change was observed to the catalyst surface as seen from the SEM images (Fig. S5, ESI†). The Raman spectra obtained before and after the catalytic reaction showed no change in the chemical composition (Fig. S6, ESI†) further demonstrating the high stability of CuTCNQF₄.

Conclusions

In summary, we have shown the fabrication of a nanostructured CuTCNQF₄ organic charge transfer complex on copper foil by employing a facile redox reaction in acetonitrile. While non-fluorinated CuTCNQ is known to be inactive towards the reduction of hexavalent chromium, for the first time, its fluorinated derivative CuTCNQF₄ is demonstrated to catalytically promote this reaction in aqueous solutions at room temperature. Importantly, under UV photo-excitation conditions, CuTCNQF₄ could enhance the rate of this reaction by an order of magnitude over that in the dark. Given the ability of CuTCNQF₄ in promoting catalytic reactions that are not successfully catalysed by its non-fluorinated analogues, the outcomes presented here open up a new avenue for employing fluorinated MTCNQ type organic semiconductors for reductive photocatalysis applications.

Acknowledgements

D. D. L. thanks RMIT University for financial support. R. R. thanks RMIT University for a Vice Chancellor’s Research Fellowship. V. B. and S. V. B. acknowledge the Australian Research Council for ARC Future Fellowships (FT140101285, FT110100152). V. B. also acknowledges the generous support of the Ian Potter Foundation for establishing an Ian Potter NanoBioSensing Facility at RMIT University. The authors acknowledge the support from the RMIT Microscopy and Microanalysis Facility (RMMF) for technical assistance and providing access to characterization facilities.

Notes and references

1. H. L. Peng, C. B. Ran, X. C. Yu, R. Zhang and Z. F. Liu, Adv. Mater., 2005, 17, 459–464.
2. C. Ran, H. Peng, W. Zhou, X. Yu and Z. Liu, J. Phys. Chem. B, 2005, 109, 22486–22490.
3. R. Ramanathan, A. E. Kandjani, S. Walia, S. Balendhran, S. K. Bhargava, K. Kalantar-zadeh and V. Bansal, RSC Adv., 2013, 3, 17654–17658.
4. R. Ramanathan, S. Walia, A. E. Kandjani, S. Balendran, M. Mohammadtaheri, S. K. Bhargava, K. Kalantar-zadeh and V. Bansal, Langmuir, 2015, 31, 1581–1587.
5. A. Pearson, R. Ramanathan, A. P. O’Mullane and V. Bansal, Adv. Funct. Mater., 2014, 24, 7570–7579.
6. M. Mahajan, S. K. Bhargava and A. P. O’Mullane, RSC Adv., 2013, 3, 4440–4446.
7. A. Pearson, A. P. O’Mullane, V. Bansal and S. K. Bhargava, Inorg. Chem., 2011, 50, 1705–1712.
8. A. Pearson, A. P. O’Mullane, S. K. Bhargava and V. Bansal, Inorg. Chem., 2012, 51, 8791–8801.
9. A. Pearson and A. P. O’Mullane, ChemPlusChem, 2013, 78, 1343–1348.
10. Z. M. Davoudi, A. E. Kandjani, A. I. Bhatt, I. L. Kyratzis, A. P. O’Mullane and V. Bansal, Adv. Funct. Mater., 2014, 24, 1047–1053.
11. M. Mohammadtaheri, R. Ramanathan and V. Bansal, Catal. Today, 2016 DOI:10.1016/j.cattod.2015.1011.1017.
12. A. Nafady, A. P. O’Mullane and A. M. Bond, Coord. Chem. Rev., 2014, 268, 101–142.
13. D. A. Dixon, J. C. Calabrese and J. S. Miller, J. Phys. Chem., 1989, 93, 2284–2291.
14. T. H. Le, A. Nafady, N. T. Vo, R. W. Elliott, T. A. Hudson, R. Robson, B. F. Abrahams, L. L. Martin and A. M. Bond, Inorg. Chem., 2014, 53, 3230–3242.
15. M. Mahajan, S. K. Bhargava and A. P. O’Mullane, Electrochim. Acta, 2013, 101, 186–195.
16. S. A. O’Kane, R. Clérac, H. Zhao, X. Ouyang, J. R. Galán-Mascarós, R. Heintz and K. R. Dunbar, J. Solid State Chem., 2000, 152, 159–173.
17. C. Ouyang, Y. Guo, H. Liu, Y. Zhao, G. Li, Y. Li, Y. Song and Y. Li, J. Mater. Chem. C, 2009, 113, 7044–7051.
18. J. Wang, W. Xu, J. Wu, G. Yu, X. Zhou and S. Xu, J. Mater. Chem. C, 2014, 2, 2010–2018.
19. J. Wang, W. Xu, J. Zhang and S. Xu, J. Mater. Chem. C, 2014, 118, 24752–24760.
20. K. Xiao, M. Yoon, A. J. Rondinone, E. A. Payzant and D. B. Geohegan, J. Am. Chem. Soc., 2012, 134, 14353–14361.
21. R. S. Potember, T. O. Poehler, A. Rappa, D. O. Cowan and A. N. Bloch, Synth. Met., 1982, 4, 371–380.
22. N. Spanos, S. Slavov, C. Kordulis and A. Lycourghiotis, Colloids Surf., A, 1995, 97, 109–117.
23. P. R. Wittbrodt and C. D. Palmer, Environ. Sci. Technol., 1995, 29, 255–263.
24. L. E. Eary and D. Rai, Environ. Sci. Technol., 1988, 22, 972–977.
25. X. Wang, J. Wang, L. Chang, Q. Ding, H. Liu and X. Jiang, RSC Adv., 2012, 2, 12315–12321.
26. H. Gu, S. B. Rapole, J. Sharma, Y. Huang, D. Cao, H. A. Colorado, Z. Luo, N. Haldolaarachchige, D. P. Young, B. Walters, S. Wei and Z. Guo, RSC Adv., 2012, 2, 11007–11018.
27. M. Valari, A. Antoniadis, D. Mantzavinos and I. Poulios, Catal. Today, 2015, 252, 190–194.
28. S. S. Chen, C. Y. Cheng, C. W. Li, P. H. Chai and Y. M. Chang, J. Hazard. Mater., 2007, 142, 362–367.
29. M. Gheju and A. Iovi, J. Hazard. Mater., 2006, 135, 66–73.
30. C. Barrera-Díaz, F. Ureña-Nuñez, E. Campos, M. Palomar-Pardavé and M. Romero-Romo, Radiat. Phys. Chem., 2003, 67, 657–663.
31. R. Abe, J. Photochem. Photobiol., C, 2010, 11, 179–209.
32. F. Zheng, X. Lin, H. Yu, S. Li and X. Huang, Sens. Actuators, B, 2016, 226, 500–505.
33. K. H. Cheung and J.-D. Gu, Int. Biodeterior. Biodegrad., 2007, 59, 8–15.
34. R. Singh, A. Kumar, A. Kirrolia, R. Kumar, N. Yadav, N. R. Bishnoi and R. K. Lohchab, Bioresour. Technol., 2011, 102, 677–682.
35. S.-X. Li, F.-Y. Zheng, H.-S. Hong, N.-S. Deng and L.-X. Lin, Mar. Environ. Res., 2009, 67, 199–206.
36. S. C. Xu, S. S. Pan, Y. Xu, Y. Y. Luo, Y. X. Zhang and G. H. Li, J. Hazard. Mater., 2015, 283, 7–13.
37. N. Martin, R. Behnisch and M. Hanack, J. Org. Chem., 1989, 54, 2563–2568.
38. A. E. Kandjani, M. J. Griffin, R. Ramanathan, S. J. Ippolito, S. K. Bhargava and V. Bansal, J. Raman Spectrosc., 2013, 44, 608–621.
39. T. Le, A. O’Mullane, L. Martin and A. Bond, J. Solid State Electrochem., 2011, 15, 2293–2304.
40. S. R. Anderson, M. Mohammadtaheri, D. Kumar, A. P. O’Mullane, M. R. Field, R. Ramanathan and V. Bansal, Adv. Mater. Int., 2016, 3, 1500632.
41. R. Ramanathan, M. R. Field, A. P. O’Mullane, P. M. Smooker, S. K. Bhargava and V. Bansal, Nanoscale, 2013, 5, 2300–2306.
42. J. M. Lindquist and J. C. Hemminger, J. Phys. Chem., 1988, 92, 1394–1396.
43. C. G. Zoski, Handbook of electrochemistry, Elsevier, Amsterdam, 2007.
44. C. E. Barrera-Díaz, V. Lugo-Lugo and B. Bilyeu, J. Hazard. Mater., 2012, 223–224, 1–12.
45. A. Henglein, Chem. Rev., 1989, 89, 1861–1873.
46. R. Ramanathan and V. Bansal, RSC Adv., 2015, 5, 1424–1429.

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Footnote

† Electronic supplementary information (ESI) available: FTIR and XPS spectra, and catalytic reaction rates. See DOI: 10.1039/c6ra02636b

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