Concurrent growth, structural and photocatalytic

Scripta Materialia 142 (2018) 143–147

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Concurrent growth, structural and photocatalytic properties of hybridized C, N co-doped TiO2 mixed phase over g-C3N4 nanostructured Mohamad Azuwa Mohamed a,⁎, Juhana Jaafar b,⁎, M.F. M. Zain c, Lorna Jeffery Minggu a, Mohammad B. Kassim a,d, Mohd Nur Ikhmal Salehmin a, Mohamad Saufi Rosmi e, W.N. W. Salleh b, Mohd Hafiz Dzarfan Othman b a

Solar Hydrogen Group, Fuel Cell Institute (SELFUEL), Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia Advanced Membrane Technology Research Centre, Universiti Teknologi Malaysia, 81310 Skudai, Johor Bahru, Malaysia c Sustainable Construction Materials and Building Systems (SUCOMBS) Research Group, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600 Bangi, Malaysia d School of Chemical Sciences & Food Technology, Faculty of Science & Technology, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia e Department of Chemistry, Faculty of Science and Mathematics, Universiti Pendidikan Sultan Idris, 35900 Tanjung Malim, Perak, Malaysia b

a r t i c l e

i n f o

Article history: Received 10 June 2017 Received in revised form 27 August 2017 Accepted 28 August 2017

Keywords: Sol-gel synthesis G-C3N4 Doping Heterojunction Photocatalysis

a b s t r a c t A concurrent and facile sol-gel assisted low temperature calcination approach to homogeneous growth of TiO2 mixed phase nanoparticles over g-C3N4 for designing visible-light-driven photocatalyst is demonstrated in this study. The structural and morphological studies revealed a well-interconnected g-C3N4/TiO2 mixed phase heterojunction photocatalyst was achieved through a sol-gel process and calcination at 400 °C. The well-interconnected g-C3N4/TiO2 mixed phase heterojunction photocatalyst has strong visible light absorption capability due to the presence of an in-situ nitrogen and carbon dopants. The noticeably increased in the visible-light-photocatalytic activity performance is ascertained mainly due to the improvement of electron-hole separation and charge carrier migration. © 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Recently, a non-metal graphitic carbon nitride (g-C3N4) has been proposed as a solution to overcome the issues related to the use of TiO2 as a photocatalyst [1–3]. This non-metal semiconductor has attracted a wide attention owing to its narrower band gap of 2.73 eV relative to TiO2 and hence, can be applied as a visible light driven photocatalyst. It is also featured to have a high stability and favorable electronic structure [4]. However, the key issues related to a fast charge recombination rate and poor conductivity need to be addressed to improve the g-C3N4 photocatalytic performance [5]. By combining TiO2 and g-C3N4, researchers have been able to produce a composite that has a higher photocatalytic activity compared to a pristine TiO2 or gC3-N4 [6–9]. Moreover, g-C3N4 provides higher surface area which is desirable as a template for TiO2 and more active sites for adsorption and reaction [10–12]. Previously, researchers have been synthesizing a composite of TiO2 and g-C3N4 by thermal treatment [13] or by heating an ethanol solution of titanium tetrachloride with C3N4 [14]. A multi-heterojunction of gC3N4 loaded a-TiO2/c-TiO2 nanocomposite was also proposed by using sequential gas-phase and wet-chemical synthesis techniques [15]. ⁎ Corresponding authors. E-mail addresses: [email protected], [email protected] (M.A. Mohamed), [email protected], [email protected] (J. Jaafar).

http://dx.doi.org/10.1016/j.scriptamat.2017.08.044 1359-6462/© 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

However, to date there is no report on a concurrent synthesis of well-interconnected g-C3N4 and C, N co-doped TiO2 (anatase/rutile) mixed phase. It has been shown that a mixed phase of anatase/rutile TiO2 can improve the charge carrier separation and consequently reduce the electron recombination [16–18]. Therefore, the present study proposes a simple concurrent growth and highly scalable method for producing a photocatalyst consisted of a homogeneous and wellinterconnected g-C3N4 and TiO2 (anatase/rutile) mixed phase with enhanced photocatalytic properties. Details of the synthesis, characterization and photocatalytic properties evaluation are explained in Section S1-S4 (Supplementary data). The occurrence, phase and crystallinity of pristine g-C3N4 and gC3N4/TiO2 are shown in Fig. 1(a). The pristine g-C3N4 exhibited two significant diffraction peaks at 2θ = 13.1–13.4° (100) and 27.4–27.6° (002), which were attributed to the in-planar repeat period for the hole-to-hole distance among the N-bridged tri-s-triazine units [19], and the typical inter-planar stacking of the conjugated aromatic sheets that indicated the peak characteristic of g-C3N4, respectively [3,12,15, 20]. On the other hand, the g-C3N4/TiO2 exhibited diffraction patterns of TiO2 mixed phase (anatase/rutile) [18] at [(011), (004), (020)] and [(110), (101), (211), (130)], which were attributed to the anatase and rutile phases, respectively. Notably, the broad peak at 2θ = 24–30° in g-C3N4/TiO2 was due to the characteristic diffraction peaks of g-C3N4

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Fig. 1. (a) XRD pattern of g-C3N4 and g-C3N4/TiO2 [inset is its FESEM images, respectively]. (b) FTIR spectra of for g-C3N4 and g-C3N4/TiO2. (c) HRTEM image of pristine g-C3N4 [(ci) inset is its TEM image] and (d) HRTEM image g-C3N4/TiO2 sample [(di) inset is its TEM image]. (e) Schematic illustration of the growth titania mixed phase nanoparticles over g-C3-N4 structure in g-C3-N4/TiO2 sample via sol-gel assisted calcination temperature approach.

(27.4–27.6°) that overlapped with the diffraction peaks of anatase (011) and rutile (110) phases. In addition, the weak diffraction peak observed at 2θ = 11.5° might be ascribed to the (100) crystal plane of g-C3N4 which has been shifted to lower 2θ due to interaction with TiO2 crystallites [15,21]. Further evidence of the presence of g-C3N4 in the samples could be shown by the FTIR spectra of the samples as shown in Fig. 1(b). The peaks in the region of 800–807 and 1230–1630 cm−1 in the spectra of g-C3N4 and g-C3N4/TiO2 indicated the existence of g-C3N4. The sharp band at around 800–807 cm−1 corresponds to the characteristic outof-plane bending vibration of triazine/heptazine rings system [12,22, 23]. The strong absorption bands ranging from 1230 to 1630 cm−1 correspond to the skeletal stretching of the tri-s-triazine heterocycle motif [23]. The broad bands in the range of the 3100–3400 cm−1 were attributed to the presence of adsorbed H2O molecules in TiO2 and N\\H vibration in g-C3N4 [12]. In addition, the deformation mode of N\\H can be observed at 885 cm−1. Moreover, the g-C3N4/TiO2 spectra were mostly shifted compared to the spectra of pristine g-C3N4 that implies the existence of an interfacial interaction between g-C3N4 layers and TiO2, which would induce a synergistic effect between the two materials to improve the photocatalytic efficiency [19]. The structural and morphology of the samples were further examined using FESEM, TEM and HRTEM techniques. The pristine g-C3N4

[Fig. 1(ai)] consists of a 2D sheet bulk-structured morphology which is not flat but back-folded, especially at the edge. The spontaneous growth of TiO2 nanoparticles can be demonstrated by the uniform and unfolded morphology at the edge of g-C3N4 sheet [Fig. 1(aii)]. The different morphologies for observed the pristine g-C3N4 and g-C3N4/TiO2 supported our suggestion that TiO2 is an efficient promoter for synthesizing the 2D sheet g-C3N4 with fewer layers. In addition, the TEM image [Fig. 1(ci)] revealed that the pristine g-C3N4 exhibited a porous structure (red arrow) and similar observation can also be seen in the HRTEM image [Fig. 1(c)]. Remarkably, the porous structure of g-C3-N4/TiO2 becomes smaller and constricted due to the growth and formation of TiO2 nanoparticles within the g-C3N4 pore structure [Fig. 1(di)]. This justification was further strengthened with the nitrogen adsorption/desorption measurement where the average pore size and the pore volume of the g-C3-N4/TiO2 sample were lower than pristine g-C3N4 (Supplementary data S5). Furthermore, the HRTEM image of g-C3N4/TiO2 sample [Fig. 1(d)] shows a well-dispersed TiO2 nanoparticles (red arrow) over the g-C3N4 structure with a clear lattice fringe spacing of 0.35 nm and 0.36 nm, indicating the presence of (011) and (110) crystal planes of anatase and rutile phases TiO2, respectively. This observation was consistent with the XRD analysis. In addition, there is no exact shape of TiO2 nanoparticles growth was observed within the g-C3N4 pore structure. It might be due to the conglutinated of TiO2 nanoparticles

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by the presence of amorphous carbon matrixes that acts as a glue linking with the TiO2 mixed phase and subsequently improved the thermal stability and crystallinity of the TiO2 mixed phase nanoparticles [21]. These observations and the explanations on the FESEM and EDS results (Supplementary data S6), it is strongly suggested that a heterojunction between TiO2 (anatase/rutile) mixed phase and g-C3N4 was formed resulting in a homogeneous and well interconnected structures. In addition, the well interconnected structure reduced the barriers formed on the interface of heterojunction, which inevitably blocked the migration of charge carriers that leads to high rate of photo-generated electron-hole-pairs recombination [6]. Based on the aforementioned findings, the growth process of TiO2 nanoparticles over g-C3N4 structure prior to formation of g-C3N4/TiO2 heterojunction can be deliberated as follows. As the urea molecules are trapped in the solid lattice, they can only vibrate in place and the near neighbours limit these vibrational motions. Urea is known as a polar molecular solid. As we dissolved the urea in water, the urea molecules can move and rotate as well as vibrate freely. Thus, a good interaction between urea molecules and titanium butoxide precursor was established during hydrolysis-polymerization of TiO2 in an aqueous urea solution. The dissolved titanium butoxide precursor was slowly added into the aqueous urea solution to promote the development of Ti-O-Ti chain and subsequently polymerized to form a three-dimensional oxide network. Due to the electronegativity of the nitrogen atom in urea molecules, hydrogen bonds were formed between the titanium butoxide precursor molecules at both N\\H sites of the urea molecule. In addition, there was an additional interaction between the C_O group and the titanium butoxide precursor since there is a respectable polarity on C_O bond. Both N\\H and C_O sites would be responsible for the nitrogen and carbon doping which was shown in accordance with the XPS analysis (Supplementary data S7). Subsequently, the condensation (oxolation and olation) could proceed simultaneously during nucleation and growth leading to the formation of gel in the form of amorphous hydrous oxide [24]. Later, the process was then followed by polycondensation by the removal alcohol and/or water molecules from the gel to form small particles known as xerogel (xero means dry) [25]. The closely packed first order small particles were produced via a three-dimensional gel skeleton. Prior to this, the aqueous urea solution had simultaneously undergone a recrystallization in the sol-gel system. The transition of the amorphous C, N co-doped TiO2 into crystalline form was induced by heat treatment and calcination at 400 °C. At this moment, the thermal condensation of the recrystallized urea would lead to the formation of g-C3N4, which was in accordance to our experimental results. On the contrary, it is commonly accepted that the g-C3N4 could successfully be formed by a direct thermal condensation at 550 °C [12]. A previous study suggested that the OH-rich group on the TiO2 was responsible for the formation of g-C3N4 at a lower temperature [26]. Therefore, the presence of OH-rich group on the hydrous amorphous C, N co-doped TiO2 particles has contributed to the lowering of the thermal condensation temperature for the formation of g-C3N4. The details of the direct conversion of urea into graphitic carbon nitride under OH-rich TiO2 has been reported in the previous study [26]. As shown in Fig. 1(e), the titanol groups (Ti-OH) are used to attacked the carbonyl of isocyanic acid (Eq. (2)), to form Ti-NH2 groups, which subsequently react with cyanic acid to generate cyanamide (Eq. (3)) and finally yield g-C3N4 (Eq. (4)) [27]. These reactions occurred spontaneously during the high-temperature treatment. Simultaneously, the rearrangement of atoms (Ti, O, C and N) resulted in the substitution of a fraction of oxygen atoms in TiO2 lattice with carbon and nitrogen atoms. In addition, it is interesting to note that the formation and arrangement of crystalline C, N-codoped TiO2 was suspected to occur inside the pores of g-C3N4 structure and exhibited a good agreement with TEM/HRTEM and porosity analysis. The photocatalytic properties of the samples were shown in Fig. 2(a). There was evidence of a direct photolysis of methylene blue (MB) that suggested the MB molecule was considerably stable and

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would not degrade without the presence of photocatalyst. In addition, the g-C3N4/TiO2 sample has surpassed the photocatalytic activity of pristine g-C3N4 and TiO2-P25 with the degradation percentage of 84.1, 9.16, and 23.0%, respectively. The highest photocatalytic activity in gC3N4/TiO2 sample was due to the excellent visible light absorption capability for the electron-holes generation for the degradation of MB molecules in aqueous solution. As shown in Fig. 2(b), the photocatalytic activity of g-C3N4/TiO2 sample was sustained as high 83% after four cycles under the same condition. The lower photocatalytic activity of TiO2-P25 sample was due to the requirement that it can only be effectively activated under UV light irradiation. Our findings appear to be well substantiated by the photocurrent response analysis as shown in Fig. 2(c). On the basic of a commonly recognized relationship, it is suggested that a higher photocurrent response indicates a higher electron-holes separation efficiency [28]. Thus, a higher photocatalytic activity would be suspected in the g-C3N4/TiO2 sample. As can be seen in Fig. 2(c), the g-C3-N4/TiO2 sample exhibited the highest photocurrent density of 29 μA/cm2, followed by TiO2-P25 and finally the pristine g-C3-N4 sample with the photocurrent density of 19 and 8.5 μA/cm2, respectively (recorded at 1.0 V versus RHE). The highest photocurrent density value for g-C3N4/TiO2 sample indicated an excellent photoinduced electron-hole separation as compared to the TiO2-P25 and pristine g-C3N4 samples. In addition, it was observed that both g-C3-N4/TiO2 and TiO2-P25 samples exhibited fast and stable photoresponse over time as compared to pristine g-C3N4 sample [Fig. 2(d)]. The well-interconnected heterojunction formation between gC3N4 and TiO2 promotes the separation of electron–hole pairs through interface carrier-transfer pathways, as well as extending the light-response range by coupling with suitable electronic structures [29]. In addition, the synergetic effect between anatase and rutile has contributed to the high photocatalytic activity due to an excellent charge transfer process [30,31]. The band gap alignment of the g-C3N4/TiO2 sample is illustrated in Fig. 2(e) to further understand its electron-holes separation. The overall valence band (VB) energy level of the g-C3N4/TiO2 sample is more positive than the oxidation potential of the surface adsorbed H2O or OH– to produce •OH radicals (V vs. NHE (H2O/•OH) = 1.99 V) [32,33]. Therefore, the photo generated holes in VB of g-C3N4/TiO2 sample could directly oxidize organic substances or water to their radicals [34]. On top of that, the overall conduction band (CB) energy level of g-C3N4/ TiO2 sample is more negative than the reduction potential of dissolved oxygen (V vs. NHE (O2/O2•−) = −0.33 V) [32], which was responsible for the formation of superoxide radical anions. The electrons rich CB could continue to react with superoxide radical anions to form hydrogen peroxide and finally, generated hydroxyl radicals [35]. Since the CB energy level of g-C3N4 is more negative than anatase and rutile, the electron will transfer from g-C3N4 to anatase and rutile accordingly, before being transferred to the surface molecular oxygen to generate superoxide anion radicals. The charge transfer from anatase to rutile was accordance with the previous study [36]. The CB position of the anatase phase was slight negative compared to the CB position of rutile with a minimum difference of as low as 0.085 V. The migration of photo-excited electron from g-C3N4 to anatase and rutile would lead to the reduction of electron-holes recombination rate. This would lead to the extension of the positive holes half-life and thus, a high photocatalytic activity was predicted for the sample g-C3N4/TiO2. In addition, the photo-generated positive holes in the VB of g-C3N4 (1.58 V) was insufficient for a direct oxidation of water to hydroxyl radical (1.99 V). This is might the reason why the photocatalytic activity of pristine g-C3N4 was the lowest. Consequently, some of the photo-generated positive holes in VB position of anatase and rutile were transferred to the VB energy of g-C3N4, which in turn was able to degrade the MB molecules directly in the aqueous solution. At the same time, some of the photogenerated positive holes in VB position of anatase could be transferred to VB position of the rutile and g-C3N4. Subsequently, the photo-generated positive holes from anatase to rutile would continuously migrate

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Fig. 2. (a) Photocatalytic activity for the degradation of MB under visible light irradiation, (b) photodegradation stability over g-C3N4/TiO2 sample, (c) photocurrent density versus applied potential (V vs RHE), (d) photocurrent density versus time collected at 1.0 V vs RHE. (e) Schematic drawing illustrating the band structure alignment and the electron-holes separation, transport and photodegradation of MB under visible light irradiation. Details on band gap edge determination can be found in (Supplementary data S8 and S9).

towards VB position of g-C3N4. Without a heterojunction formation with g-C3N4, all charge carriers would accumulate on rutile and thus, no improvement in photocatalytic activity would be observed [37]. Therefore, it is important to note that the unique heterojunction formation between TiO2 mixed phase and g-C3N4 has significantly improved the photocatalytic activity with high oxidative capacity under visible light irradiation. In summary, the homogeneous growth of TiO2 mixed phase (anatase/rutile) nanoparticles over g-C3N4 structure in g-C3-N4/TiO2 sample was successfully prepared via a facile sol-gel assisted low temperature calcination approach. The urea has not only thermally condensed into polymeric g-C3-N4 but also acted as the carbon and nitrogen sources in the formation of the TiO2 (anatase/rutile) mixed phase lattice structure. The concurrent preparation of g-C3N4/TiO2 sample was triggered by the lower temperature heat treatment due to the presence of the OH-rich group on the TiO2. The formation of a heterojunction structure between the TiO2 mixed phase nanoparticles and g-C3N4 led to a very promising strategy to improve the photocatalytic activity by enhancing the charge carrier separation and hence, reduced the recombination rate. In addition, the potential application of this photocatalyst can be extended in the field of solar fuel generation, solar cell, and as a nanofiller for the membrane technology. We acknowledge the financial supports from the Ministry of Higher Education, Malaysia for HICOE 4J184 and 4J185; UTMF

R.J130000.7746.4J231, Flagship Q.J130000.2446.03G31, FRGS/1/2017/ TK10/UKM/01/3 and LEP 2.0/14/UKM/TH/01/3 grants. The authors would also like to acknowledge technical and management support from Research Management Centre (RMC) from Universiti Teknologi Malaysia and Centre for Research and Instrumentation (CRIM) from Universiti Kebangsaan Malaysia. The first author also would like to thank Universiti Kebangsaan Malaysia for PhD scholarship under the Skim Zamalah Yayasan Canselor 2016. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.scriptamat.2017.08.044. References [1] W. Iqbal, C. Dong, M. Xing, X. Tan, J. Zhang, Catal. Sci. Technol. (2017). [2] S. Yuan, Q. Zhang, B. Xu, S. Liu, J. Wang, J. Xie, M. Zhang, T. Ohno, Catal. Sci. Technol. (2017). [3] F. Dong, Y. Sun, L. Wu, M. Fu, Z. Wu, Catal. Sci. Technol. 2 (2012) 1332. [4] X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domen, M. Antonietti, Nat. Mater. 8 (2009) 76–80. [5] Y. Li, J. Wang, Y. Yang, Y. Zhang, D. He, Q. An, G. Cao, J. Hazard. Mater. 292 (2015) 79–89. [6] X. Chen, J. Wei, R. Hou, Y. Liang, Z. Xie, Y. Zhu, X. Zhang, H. Wang, Appl. Catal. B Environ. 188 (2016) 342–350.

M.A. Mohamed et al. / Scripta Materialia 142 (2018) 143–147 [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]

J. Ma, C. Wang, H. He, Appl. Catal. B Environ. 184 (2016) 28–34. J. Su, L. Zhu, G. Chen, Appl. Catal. B Environ. 186 (2016) 127–135. J. Su, L. Zhu, P. Geng, G. Chen, J. Hazard. Mater. 316 (2016) 159–168. F. Dong, L. Wu, Y. Sun, M. Fu, Z. Wu, S.C. Lee, J. Mater. Chem. 21 (2011) 15171. Y. Zhang, J. Liu, G. Wu, W. Chen, Nano 4 (2012) 5300. F. Dong, Z. Wang, Y. Sun, W.-K. Ho, H. Zhang, J. Colloid Interface Sci. 401 (2013) 70–79. H. Yan, H. Yang, J. Alloys Compd. 509 (2011) L26–L29. N. Yang, G. Li, W. Wang, X. Yang, W.F. Zhang, J. Phys. Chem. Solids 72 (2011) 1319–1324. A.P. Singh, P. Arora, S. Basu, B.R. Mehta, Int. J. Hydrog. Energy 41 (2015) 5617–5628. M.A. Mohamed, W.N. Wan Salleh, J. Jaafar, N. Yusof, J. Technol. 70 (2014) 65–70. M.A. Mohamed, W.N.W. Salleh, J. Jaafar, A.F. Ismail, J. Sol-Gel Sci. Technol. 74 (2015) 513–520. M.A. Mohamed, W.N.W. Salleh, J. Jaafar, A.F. Ismail, N.A.M. Nor, Mater. Chem. Phys. 162 (2015) 113–123. L. Gu, J. Wang, Z. Zou, X. Han, J. Hazard. Mater. 268 (2014) 216–223. M. Fu, J. Liao, F. Dong, H. Li, H. Liu, J. Nanomater. 2014 (2014) 1–8. Y. Wu, S. Chen, J. Zhao, X. Yue, W. Deng, Y. Li, C. Wang, J. Environ. Chem. Eng. 4 (2016) 797–807. X. Zou, G.-D. Li, Y. Wang, J. Zhao, C. Yan, M. Guo, L. Li, J. Chen, Chem. Commun. 47 (2011) 1066–1068. X.S. Zhang, J.Y. Hu, H. Jiang, Chem. Eng. J. 256 (2014) 230–237. S. Mahshid, M. Askari, M.S. Ghamsari, J. Mater. Process. Technol. 189 (2007) 296–300.

147

[25] S. Bagheri, Z.A. Mohd Hir, A.T. Yousefi, S.B. Abdul Hamid, Microporous Mesoporous Mater. 218 (2015) 206–222. [26] X. Zou, G. Li, Y. Wang, J. Zhao, C. Yan, M. Guo, L. Li, J. Chen, Chem. Commun. 47 (2011) 1066–1068. [27] J. Zhu, P. Xiao, H. Li, S.A.C. Carabineiro, ACS Appl. Mater. Interfaces 6 (2014) 16449–16465. [28] Y. Wu, L. Tao, J. Zhao, X. Yue, W. Deng, Y. Li, C. Wang, Res. Chem. Intermed. 42 (2015) 3609–3624. [29] W. Li, Z. Wu, J. Wang, A. a Elzatahry, D. Zhao, Chem. Mater. 26 (2014) 287–298. [30] M. Pelaez, N.T. Nolan, S.C. Pillai, M.K. Seery, P. Falaras, A.G. Kontos, P.S.M. Dunlop, J.W.J. Hamilton, J.A. Byrne, K. O'Shea, M.H. Entezari, D.D. Dionysiou, Appl. Catal. B Environ. 125 (2012) 331–349. [31] D.O. Scanlon, C.W. Dunnill, J. Buckeridge, S.A. Shevlin, A.J. Logsdail, S.M. Woodley, C.R. a Catlow, M.J. Powell, R.G. Palgrave, I.P. Parkin, G.W. Watson, T.W. Keal, P. Sherwood, A. Walsh, A. a Sokol, Nat. Mater. 12 (2013) 798–801. [32] N. Boonprakob, N. Wetchakun, S. Phanichphant, D. Waxler, P. Sherrell, A. Nattestad, J. Chen, B. Inceesungvorn, J. Colloid Interface Sci. 417 (2014) 402–409. [33] S. Sun, M. Sun, Y. Fang, Y. Wang, H. Wang, RSC Adv. 6 (2016) 13063–13071. [34] J. Wen, X. Li, W. Liu, Y. Fang, J. Xie, Y. Xu, Chin. J. Catal. 36 (2015) 2049–2070. [35] M.A. Mohamed, W.N.W. Salleh, J. Jaafar, M.S. Rosmi, Z.A.Mohd. Hir, M. Abd Mutalib, A.F. Ismail, M. Tanemura, Appl. Surf. Sci. 393 (2017) 46–59. [36] S. Shen, X. Wang, T. Chen, Z. Feng, C. Li, J. Phys. Chem. C 118 (2014) 12661–12668. [37] R. Marschall, Adv. Funct. Mater. 24 (2014) 2421–2440.