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Effect of adsorption properties of phosphorus-doped TiO2 nanotubes on photocatalytic NO removal
Journal of Colloid and Interface Science 553 (2019) 647–654
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Regular Article
Effect of adsorption properties of phosphorus-doped TiO2 nanotubes on photocatalytic NO removal Rui Huang a, Shule Zhang a,⇑, Jie Ding a, Yahan Meng a, Qin Zhong a,⇑, Deshuang Kong b, Changjun Gu b a b
School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China Jiangsu Gaochun Ceramics Co., Ltd, Nanjing, China
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 12 April 2019 Revised 18 June 2019 Accepted 18 June 2019 Available online 19 June 2019
A series of P doped TiO2 nanotubes (P-TNTs) catalysts were prepared for photocatalytic oxidation (PCO) of NO. Compared with TiO2 nanotubes, P-TNTs catalysts exhibited excellent catalytic activity. We confirmed that P was doped into TiO2 nanotubes lattice by XRD, Raman and TEM characterization. It was found that the enhanced PCO performance of NO was not attributed to the photoelectric properties of the P-TiO2 from UV–vis, Mott-Schottky and transient photocurrent response results. Therefore, we focused on the studies of catalyst reaction process and reactant adsorption characteristics. The results of transient reactions showed that the catalytic reaction proceeded via a mixed mechanism of Langmuir-Hinshelwood (LH) and Eley-Rideal (E-R) on the P-TiO2 catalyst. In addition, P doping was beneficial to the adsorption of H2O2 and NO, which is ascribed to the improved PCO performance of NO. Ó 2019 Published by Elsevier Inc.
Keywords: P doped TiO2 nanotubes PCO of NO Mixed mechanism Adsorption
1. Introduction Nitrogen oxide (NOx) emission is a main source of atmospheric contamination since it can easily cause acid rain, photochemical smog and ozone depletion [1]. The selective catalytic reduction (SCR) is the most common method to remove NO by using V2O5/WO3-TiO2 catalyst [2]. However, the SCR technology requires
⇑ Corresponding authors. E-mail addresses: (Q. Zhong).
[email protected]
https://doi.org/10.1016/j.jcis.2019.06.063 0021-9797/Ó 2019 Published by Elsevier Inc.
(S.
Zhang),
[email protected]
a high reaction temperature (300–400 °C) and thus is only used in large coal-fired power plant. For small and medium-sized industrial boilers, the SCR technology cannot be used, because most exhaust gas is below 150 °C or even at room temperature. For the process of NO oxidized into higher nitrogen oxides, lowtemperature flue gas (<150 °C), O3 [3] or H2O2 [4,5] is often used in the industry, while the main oxidation product NO2 is difficult to be absorbed by wet flue gas desulfurization [6]. Photocatalytic oxidation (PCO) of NO by H2O2 is a promising technology with many advantages including simple operation, mild reaction conditions, high efficiency and low temperature
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[7,8]. Furthermore, the oxidation product of NO is NO 3 , which is easily absorbed [9]. Ou et al. prepared the BiVO4 to remove the NO using H2O2 by PCO [5,7]. Duan et al. investigated the photocatalytic removal of NO by TiO2-supported Ag nanoparticles [10]. Ma et al. studied the photocatalytic denitrification of g-C3N4-TiO2 composite in air [11]. These studies focused on the potentials of the valence/conduction band of photocatalysts and the qualitative and quantitative analysis of photocatalytically active species such as superoxide radicals and hydroxyl radicals on the catalysts. However, few studies focused on the reaction process mechanism of photocatalyst for PCO of NO. Titanium dioxide (TiO2) used as photocatalyst was first reported by Frank and Bard [12] which had attracted much attention from researchers because of its high efficiency, nontoxicity, low cost and photochemical stability [13]. In order to further improve the catalytic performance, TiO2 is modified in various methods. For instance, in terms of morphological structure, there are titanium dioxide nanosheet [14], titanium dioxide nanoball [15], titanium dioxide nanowire [16], titanium dioxide nanotube [17,18] and nanofibers prepared by electrospinning technology [19–22]. In terms of ion doping, non-metal doping has been widely reported benefited from the smaller band-gap, stronger light absorption, and better nitrification resistance [23]. In the non-metal cationdoped TiO2 catalysts, phosphorus-doped TiO2 (P-TiO2) has caused great attention due to its high photocatalytic activity compared to pure TiO2 and P25. Up to now, P-TiO2 as a photocatalyst has been widely investigated in dye degradation and CO2 reduction [24–27]. Although the adsorption properties of the catalyst surface have been studied [28], there is no study about P-TiO2 used in PCO of NO. In this study, we prepared TiO2 and P-TiO2 nanotubes (P-TNTs) and found that P-TiO2 improved the PCO of NO. We also confirmed the P doped into TiO2 nanotubes lattice. It was found that the enhanced PCO performance of NO was not attributed to the photoelectric properties of the P-TiO2 from UV–vis, Mott-Schottky and transient photocurrent response results. Therefore, this paper focused on the studies of catalyst reaction process and reactant adsorption characteristics. The results of transient reactions showed that the catalytic reaction proceeded via a mixed mechanism of L-H and E-R on the P-TiO2 catalyst. In addition, P doping was beneficial to the adsorption of H2O2 and NO, which improved the PCO performance of NO.
2. Experimental 2.1. Catalyst preparation TiO2 nanotubes were prepared following a literature procedure [29]. 5 g TiO2 (Degussa P25) powder was dispersed in 175 mL of NaOH (10 M) solution, and the mixture solution was transferred into a Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained at 130 °C for 24 h, then was cooled to room temperature naturally. After that, the precipitates were collected and washed with 0.1 M HCl and deionized water until the pH value of the system reached neutral. The sample was then dried at 80 °C overnight to obtain the TiO2 nanotubes precursor. P-TiO2 nanotubes were fabricated by a facile wet chemical method [29]. A certain amount of sodium hydrogen phosphite (Na2HPO4) was dissolved in 60 mL water. 0.5 g TiO2 nanotubes precursor was added to the solution and dissolved by ultrasound for 1 h. The mixture was magnetically stirred overnight followed by drying at 80 °C for 12 h. Finally, the product was calcined at 500 °C in a muffle furnace for 3 h. These phosphorus-doped titania nanotubes were denoted as P-TNTs. The catalysts of P-TNTs coupled with 0 wt%, 2.5 wt%, 5 wt%, 7.5 wt% phosphorus were
obtained, and denoted as TNTs, 2.5P-TNTs, 5P-TNTs, 7.5P-TNTs, respectively. 2.2. Catalytic activity tests Experiments were conducted at room temperature and atmospheric pressure in a fixed bed continuous flow reactor (a tubular quartz reactor with d = 10 mm). The mixture of reactant gases (400 ppm NO, 10% O2, N2 balance) was fed into the reactor with a total flow rate of 100 mL/min. A 350 W Xe lamp was placed parallel to the catalyst portion. 30 wt% H2O2 solution was injected into the reactor with a flow rate of 0.023 mL/min. The system was assumed to be stable once the outlet NO concentration became equal to the inlet NO concentration. After keeping the system stable for 60 min, the lamp was turned on and the experiment started. The gas products were analyzed by in situ FT-IR gas analyzer SERVOPRO 4900. The NO conversion is defined by Eq. (1).
NO conversion ¼
NOðinletÞ NOðoutletÞ 100=% NOðinletÞ
ð1Þ
2.3. Characterization Produced ions in the solution were analyzed by Ion Chromatography (IC, Dionex ICS90), X-ray diffraction (XRD) patterns were recorded by using a Beijing Purkinjie General Instrument XD-3 Xray diffractometer, Raman by using the Raman spectroscopy (DRX, Thermo Fisher, USA). The micromorphology of the catalyst was examined by using a JEOL JEM-2100 transmission electron microscope (TEM), Fourier transform infrared (FT-IR) spectrometer recorded on an IS10 FT-IR spectrometer (Nicolet, USA). UV–vis diffuse reflectance spectra (DRS) were obtained by using a UV–vis spectrophotometer (Shimadzu UV-2550). Thermogravimetric analysis (TG) was performed on a thermogravimetric analyzer (TA SDT Q600). The Photoelectrochemical test was performed on a CHI 760D electrochemical workstation (Chenhua Instrument, Shanghai, China) using a standard three-electrode cell with a working electrode, a Pt wire as the counter electrode, and a standard Ag/AgCl in saturated KCl as the reference electrode. An aqueous solution of 0.5 mol/L Na2SO4 (pH 6.8) was used as an electrolyte and purged with N2 before each experiment. The working electrode was prepared by dip-coating: 10 mg photocatalyst was added into 1 mL ethyl alcohol and 50 lL Nafion solution to form slurry, and then dip-coated onto a 1 cm 1 cm FTO glass electrode. Subsequently, the films were dried at 80 °C overnight and calcined at 180 °C for 2 h at N2 atmosphere. The variations of photoinduced current density with time (i–t curve) was measured with a 350 W Xe light as the light source and 0.5 mol/L Na2SO4 as the electrolyte. The NO-TPD measurements were performed on an automated chemisorption analyzer (Quantachrome Instruments). Firstly, the temperature was preheated at 300 °C for 30 min, which treated the substance adsorbed on the surface of the catalyst. Then the temperature drops to 50 °C. After that, NO was introduced for 30 min for adsorption, then switched to N2 and purged to the baseline for stability. Finally, the temperature starts to rise according to the program. 3. Results and discussion 3.1. Catalytic activity tests The NO removal efficiency of P-TNTs were shown in Fig. 1a. The NO removal efficiency of all the P-TNTs catalysts were higher than that of TNTs. The NO removal efficiency decreased in this following
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Fig. 1. (a) NO removal efficiency for different catalysts in UV/H2O2 system, (b) The result of ion chromatography after transient reaction.
order: 5P-TNTs > 7.5P-TNTs > 2.5P-TNTs > TNTs. Therefore, the P doping could improve the NO removal efficiency, and 5P-TNTs was the best catalyst. In the catalytic activity tests, it was found that the NO2 content of the outlet also reduced, compared with the inlet. Therefore, the NO2 was not the main product of NO oxidation. In order to verify the product, Ion Chromatography (IC) was used to clarify the reaction product [30]. After the experiment was completed, 1 mL solution was removed from the collected solution (4 mL) and diluted by 100 times. Then the diluted solution was injected into IC. As shown in Fig. 1b, the qualitative IC analysis result of 5P-TNTs catalyst shows that the main product in the solution was NO 3 in this PCO of NO.
3.2. Characterization of structure, morphology and photoelectric properties 3.2.1. XRD analysis The XRD patterns of TNTs and P-TNTs are shown in Fig. 2a. The TNTs sample calcined at 500 °C is mainly composed of the relative spikes of anatase (PDF#21-1272). The P-TNTs catalysts calcined under 500 °C was also the only single phase of TiO2 anatase structure. It can be seen from Fig. 2a that the XRD peak intensity of the TiO2 (1 0 1) crystal face of the P-TNTs catalysts was lower than that of TNTs. Fig. 2b shows that the XRD peak of the TiO2 (1 0 1) crystal plane. As the P doping increased to 5%, a shift of the XRD peaks towards higher diffraction angles was observed. This phenomenon
was caused by the reduction in P-TNTs lattice constant, which could be attributed to the smaller ionic radius of P5+ than that of Ti4+. Therefore, the change in XRD peak intensity and peak position suggested that P was doped into the TiO2 lattice. 3.2.2. Raman analysis Fig. 3 displays the Raman spectroscopy of TNTs and P-TNTs catalysts. For the TNTs, five fundamental peaks located at 141 cm1 (Eg), 199 cm1 (Eg), 398 cm1 (B1g), 513 cm1 (A1g) and 639 cm1 (Eg), which can be ascribed to anatase TiO2 [31]. This was consistent with the results obtained by XRD. In addition, the Raman intensity gradually decreases with increasing P doping content. As the P doping content was 5%, the Raman intensity was the lowest. This observation could be attributed to the formation of TiAOAP with asymmetric vibration. When the P doping content was increased to 7.5%, the Raman intensity of 7.5P-TNTs increases. It was possible that the excessive P elements lead to the formation of PO3 4 by self-bonding, which was confirmed by FT-IR (Fig. S1). The formation of PO3 4 inhibited the P into the TiO2 lattice, which might be responsible for the reduced catalytic activity of 7.5PTNTs. 3.2.3. TEM analysis Fig. 4 shows the TEM and HRTEM images of TNTs and 5P-TNTs. As shown in Fig. 4a, the TNTs displayed a tubular structure. The tube diameter and the length were 7 nm and 100 nm. However,
Fig. 2. XRD patterns images of different wright ratios of P-TNTs.
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can be found from the Fig. 5(a) that the band gaps of TNTs (3.26 eV) and 5P-TNTs (3.22 eV) showed the most differences, which indicated that the P doping had little effect on the band gap. According to the literature [29], the P doping could cause the band gap to be narrow. However, it was found by TEM images (Fig. 4(c)) that the nanotube structure in 5P-TNTs reduced. The band gap of the nanoparticle catalyst was wider than that of the nanotube catalyst [29]. Therefore, these two factors had little effect on the P-doped band gap.
Fig. 3. Raman spectra of TNTs, 2.5P-TNTs, 5P-TNTs, 7.5P-TNTs.
for the 5P-TNTs (Fig. 4c), the length of nanotube became shorter which could be attributed to the agglomeration of the nanotubes calcined at 500 °C. According to Fig. 4b, the lattice spacing of TNTs was determined to be 0.380 nm which was assigned to (0 1 0) plane of anatase phase spacing. In Fig. 4d, part of 5P-TNTs interplanar spacing was narrower than that of TiO2 (0 1 0). The ionic radius of P5+ and Ti4+ and O2 are 0.0380, 0.0605 and 0.14 nm, respectively. According to the Bragg’s law, the lattice spacing becomes narrower when the ionic radius of the doping element was smaller than that of the substituted ion. Therefore, this observation could confirm P as cation doping into TNTs [32].
3.2.4. UV–vis analysis Fig. 5 shows the UV–vis spectrum of TNTs and P-TNTs catalysts and their band gap. Compared with TNTs, the UV–vis absorption of P-TNT showed a small red shift. The band gaps of TNTs and P-TNTs were obtained by the formula of Ahv = K(hv 8Eg)1/2. The band gaps of the catalysts were not very different from each other. It
3.2.5. Surface redox potential: Mott-Schottky Surface redox reaction is important to photocatalytic activity. The reduction and oxidation abilities of e and h+ are strongly dependent on the positions of conduction band (CB) and valence band (VB). The CB position can be calculated from Mott-Schottky (MS) using the impedance-potential method [33]. As shown in the Fig. 6, both TNTs and 5P-TNTs photoelectrodes showed positive slopes, indicating that these two samples were n-type semiconductors. Furthermore, extrapolating the chart to 1/C2 = 0, the Fermi level (EF) value of TNTs and 5P-TNTs was estimated to be 0.37 and 0.38 V vs. SCE (equivalent to 0.168 and 0.178 eV vs. NHE) respectively. It was well known that CB potential (ECB) of n-type semiconductor was very close to EF (0–0.2 eV is more negative) [34]. Here, the voltage difference between CB and the flat potential was set at 0.1 eV [9]. Therefore, the ECB of TNTs and 5PTNTs were 0.268 eV and 0.278 eV vs. NHE, respectively. The VB positions were estimated by the following relation: EVB = ECB + Eg. Thus, the VB potentials of TNTs and 5P-TNTs were 2.992 eV and 2.942 eV, respectively. The difference in potential between CB and VB of TNTs and 5P-TNTs was small. Therefore, the redox property of P-TNTs has no effect on the increase of NO conversion compared with TNTs. 3.2.6. Transient photocurrent response The transient photocurrent response measurement is an important analytical technique to evaluate the charge transfer efficiency
Fig. 4. TEM images of (a) TNTs, (c) 5P-TNTs, HRTEM images of (b) TNTs, (d) P-TNTs.
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Fig. 5. UV–vis diffuse reflection spectra patterns (a) and band gap energy (b) of all sample.
3.3. Catalyst reaction process and reactant adsorption characteristics From the characteristics of the UV–vis and surface redox properties, there was almost no difference between TNTs and P-TNTs catalysts, and electrons and holes separation efficiency of TNTs were superior to that of P-TNTs. Given the reaction system was a gas-liquid-solid three-phase reaction, it was necessary to study the reaction process to investigate the cause of the increase of catalytic activity for P-TiO2 catalysts.
Fig. 6. Mott-Schottky plots of TNTs and 5P-TNTs.
of photocatalysts [35]. Generally, the higher photocurrent represents the more electrons and higher holes separation efficiency, which was beneficial to photocatalytic activity. Fig. 7 showed the transient photocurrent response of different catalysts. As shown in Fig. 7, the order of photocurrent density decreased in this following order: TNTs > 5P-TNTs > 5P-TNTs > 2.5P-TNTs. The low photocurrent response of P-TNTs might be related to the P sites or the form of impurity level, which leads to accelerating the recombination of electrons and holes. Therefore, the separation of electrons and holes were not the reason for the increase of PTNTs catalytic activity.
Fig. 7. Transient photocurrent response of different catalysts.
3.3.1. Transient reactions In this study, we designed transient reactions to investigate the reaction model to determine the reaction process of the H2O2 and NO on the catalyst. The transient reaction shows that H2O2 and catalysts were first introduced into the reactor. After H2O2 reached saturated adsorption, NO was continuously introduced instantaneously, and the change of the NO concentration of the reactor outlet was monitored continuously. In the experiment, the reaction temperature was kept at 70 °C to reduce the effect of mass transfer in the surface liquid membrane of the catalyst and to accurately determine the intrinsic properties of catalytic reaction. Fig. 8a showed the transient reaction between H2O2 and NO on the PTNTs. Under the condition of light on and light off, after the NO was instantaneously introduced, the NO conversion of P-TNTs underwent a process of continuous increase, then continuous decrease, and finally stabilization. In the range of 3300–4500 s, the NO conversion of P-TNTs decreased after the NO conversion reached a higher level, and then it gradually got the highest level. This phenomenon might be related to the reaction between NO and free radicals generated on the catalyst surface under UV light. Fig. 8b shows the hypothetical E-R mechanism (reaction between NO in the gas phase and adsorbed H2O2 on the catalyst). If the catalytic reaction of P-TNTs followed E-R mechanism, the NO conversion would be highest as the NO was introduced instantaneously into the reactor, because the amount of H2O2 adsorption and radicals is the maximum on the catalyst. As the reaction proceeds, the NO conversion will gradually decrease and finally stabilize. Fig. 8c shows the hypothetical L-H mechanism (reaction between adsorbed NO and adsorbed H2O2 on the catalyst). If the catalytic reaction of P-TNTs followed L-H mechanism, the NO conversion would gradually increase and finally stabilize because NO and H2O2 compete for adsorption as the NO was introduced instantaneously into the reactor. It was worth noting that the actual NO conversion of P-TNTs was neither E-R nor L-H, but somewhere in between. Although this study cannot determine which mechanism was dominant, it was clear that the adsorption of H2O2 and NO on the P-TNTs catalyst had an effect on the catalytic reaction.
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Fig. 9. NO-TPD of TNTs and P-TNTs catalysts.
ment of NO adsorption by P doping can be attributed to the active sites of P doping. Compared with 2.5P-TNTs and 5P-TNTs, the TPD peak area of 7.5P-TNTs increased in the range of 100–400 °C, while the TPD peak area of NO x above 400 °C decreased significantly. The decreased catalytic activity of 7.5P-TNTs indicated that the strong adsorption state NO x produced by NO adsorption could improve the PCO of NO. It was worth noting that the TPD peak area about NO x of 5P-TNTs was lower than that of 2.5P-TNTs, and this TPD peak shifted toward lower temperatures. Considering the highest catalytic activity of 5P-TNTs, this observation could imply the two following possibilities: (1) Not only the NO x was the reactive species, but also the adsorption and activation of the reactant H2O2 played an important role in the reaction. (2) The decrease of binding energy between NOx and catalyst might improve the catalytic activity.
Fig. 8. (a) Transient reaction between H2O2 and NO on the P-TNTs; (b) The hypothetical E-R mechanism on catalyst. (c) The hypothetical L-H mechanism on catalyst.
3.3.2. Study on NO adsorption The adsorption behavior of NO on the TNTs and P-TNTs catalysts was determined by NO-TPD analysis, and the results are shown in Fig. 9. Several desorption peaks could be detected in the NO-TPD profiles. In Fig. 9, the TPD peaks in the range of 100– 200 °C were ascribed to the physisorbed NO [36]. The TPD peaks in the range of 300–400 °C were assigned to NO2 desorption arising from the decomposition of bidentate and/or monodentate nitrates species [37]. The TPD peaks in the range of 400–600 °C were attributed to NO x species [38]. As the P doping amount was 2.5%, the TPD peak area of 2.5P-TNTs was larger than that of TNTs, indicating that P doping was beneficial to the NO adsorption. Therefore, combining with the results that the catalytic activity of 2.5P-TNTs was higher than that of TNTs, NO adsorption could promote PCO performance of NO for P-TNTs. The surface area decreased in this following order: 2.5P-TNTs (166.8 m2/g) > TNTs (166.1 m2/g) > 5P-TNTs (113.2 m2/g) > 7.5P-TNTs (111.2 m2/g). Therefore, the improve-
3.3.3. Study on H2O2 adsorption According to literature, the H2O2 desorption on the catalyst could be confirmed by TG mothed [39]. Fig. 10 shows the H2O2 desorption on the catalysts by TG. It was reported in the literature that the decomposition temperature of H2O2 is between 100 and 400 °C. It can be seen from Fig. 10 that the amount of H2O2 adsorption decreased in this following order: 5P-TNTs > 7.5P-TNTs > 2.5PTNTs > TNTs, which was consistent with the NO removal efficiency of these catalysts. The improvement of H2O2 adsorption with P doping might be attributed to the polarization between P atom and OAH bond of H2O2 molecular [40]. Combined with transient reactions, NO-TPD and TG, it could be confirmed that P doping improved the NO and H2O2 adsorption, which results in the enhancement of PCO of NO. 3.4. The stability of the samples To investigate the stability of the photocatalysts, we used XPS and TEM to characterize the catalyst before and after the reaction (Fig. S2). Fig. S2(a) and (b) shows the XPS spectrum of 5P-TiO2 before and after the NO removal test. There was only one peak at 133.5 eV in P 2p XPS spectrum due to P5+ before and after the NO removal test. It indicates that P doping exists in the form of P5+ and chemical environment of P doping is stable [41,42]. The TEM of 5P-TiO2 before and after the reaction was shown in Fig. S2(c) and (d). There was little change between Fig. S2(c), (d) and Fig. 4. Therefore, the P doping TiO2 catalyst had no change in morphology after the photocatalytic reaction. In addition, we also tested the stability of 5P-TiO2 for NO removal under UV light (Fig. 11). As shown in Fig. 11, the catalyst still had a high photocatalytic oxidation capacity and it was reusable during the four cycles.
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Achievements into a Special Fund Project (BA2016055 and BA2017095), Top-notch Academic Programs Project of Jiangsu Higher Education Institutions. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.06.063. References
Fig. 10. The H2O2 desorption on the catalysts by TG.
Fig. 11. The stability tests of 5P-TiO2 for NO removal under UV light.
Therefore, P-TiO2 has stability both in terms of catalyst properties and reaction. 4. Conclusions In summary, P-TNTs was successfully prepared by hydrothermal and wet chemical method. The P-TNTs catalysts can significantly enhance NO oxidation in H2O2/UV system, and the 5PTNTs shows the highest efficiency, with a value of 90%. We confirmed that P doped into TiO2 nanotubes lattice by XRD, Raman and TEM. The UV–vis and Mott-Schottky results showed that the photoelectric properties of the P-TiO2 were not the main reasons for the increase of PCO performance of NO. The results of transient reactions showed that the catalytic reaction proceeded via a mixed mechanism of L-H and E-R on the P-TiO2 catalyst. In addition, P doping was beneficial to the adsorption of H2O2 and NO, which improved the PCO performance of NO. In the field of PCO of NO, this study provides a better understanding of the P species role and offers a new method to design the catalysts in terms of reactant adsorption. Acknowledgments This work was financially supported by the Key Project of Chinese National Programs for Research and Development (2016YFC0203800), the National Natural Science Foundation of China (51578288), Jiangsu Province Scientific and Technological
[1] L.B. Kreuzer, C.K. Patel, Nitric oxide air pollution: detection by optoacoustic spectroscopy, Science 173 (1971) 45–47. [2] O. Kröcher, M. Elsener, Chemical deactivation of V2O5/WO3–TiO2 SCR catalysts by additives and impurities from fuels, lubrication oils, and urea solution: I. Catalytic studies, Appl. Catal. B Environ. 77 (2008) 215–227. [3] C. Han, S. Zhang, L. Guo, Y. Zeng, X. Li, Z. Shi, Z. Yi, B. Zhang, Z. Qin, Ehanced catalytic ozonation of NO over black-TiO2 catalyst under inadequate ozone (O3/NO molar ratio = 0.6), Chem. Eng. Res. Des. 136 (2018) 219–229. [4] Y. Wang, S. Zhang, Z. Qin, Y. Zeng, X. Chen, Q. Wang, L. Guo, One-step hydrothermal synthesis of novel 3D BiFeWOx/Bi2WO6 composite with superior visible-light photocatalytic activity, Green Chem. 20 (2018) 3014– 3023. [5] M. Ou, S. Wan, Q. Zhong, S. Zhang, Y. Song, L. Guo, W. Cai, Y. Xu, Hierarchical Zscheme photocatalyst of g-C3N4@Ag/BiVO4 (040) with enhanced visible-lightinduced photocatalytic oxidation performance, Appl. Catal. B Environ. 221 (2018) 97–107. [6] L. Chen, J.W. Lin, C.L. Yang, Absorption of NO2 in a packed tower with Na2SO3 aqueous solution, Environ. Prog. 21 (2010) 225–230. [7] O. Man, Z. Qin, S. Zhang, H. Nie, Z. Lv, C. Wei, Graphene-decorated 3D BiVO4 superstructure: Highly reactive (040) facets formation and enhanced visiblelight-induced photocatalytic oxidation of NO in gas phase, Appl. Catal. B Environ. 193 (2016) 160–169. [8] Y. Zhou, X. Zhang, Q. Zhang, F. Dong, F. Wang, Z. Xiong, Role of graphene on the band structure and interfacial interaction of Bi2WO6/graphene composites with enhanced photocatalytic oxidation of NO, J. Mater. Chem. A 2 (2014) 16623–16631. [9] Y. Wang, S. Zhang, Y. Zeng, O. Man, Z. Qin, Photocatalytic oxidation of NO over TiO2-Graphene catalyst by UV/H2O2 process and enhanced mechanism analysis, J. Mol. Catal. A: Chem. 423 (2016) 339–346. [10] Yanyan Duan, Jianmin Luo, Shaochen Zhou, Xinyou Mao, M. Wajid Shah, TiO2supported Ag nanoclusters with enhanced visible light activity for the photocatalytic removal of NO, Appl. Catal. B Environ. 234 (2018) 206–212. [11] J. Ma, C. Wang, H. Hong, Enhanced photocatalytic oxidation of NO over g-C3N4TiO2 under UV and visible light, Appl. Catal. B Environ. 184 (2016) 28–34. [12] S.N. Frank, A.J. Bard, Heterogeneous photocatalytic oxidation of cyanide ion in aqueous solutions at titanium dioxide powder, Cheminform 8 (1977) 303–304. [13] N. Negishi, K. Takeuchi, T. Ibusuki, Surface structure of the TiO2 thin film photocatalyst, J. Mater. Sci. 33 (1998) 5789–5794. [14] N.O. Gopal, B.M. Hussain, TiO2 Nano-flakes with high activity obtained from phosphorus doped TiO2 nanoparticles by hydrothermal method, Ceram. Int. 44 (2018) 22129–22134. [15] W. Hong, K. Cai, J. Liu, X. Zhang, Y. Song, Synthesis of nanosphere TiO2 with flower-like micro-composition and its application for the selective catalytic reduction of NO with NH3 at low temperature, RSC Adv. 6 (2016) 84294. [16] N.T.Q. Hoa, Solvothermal synthesis and photocatalytic activity of Co-doped TiO2 nanowires, Electrochim. Acta 52 (2015) 3630–3640. [17] P. Mazierski, J. Nadolna, W. Lisowski, M.J. Winiarski, M. Gazda, M. Nischk, T. Klimczuk, A. Zaleska-Medynska, Effect of irradiation intensity and initial pollutant concentration on gas phase photocatalytic activity of TiO2 nanotube arrays, Catalysis Today. 284, (2017), 19-26. [18] S. Xie, K. Yang, Y. Lao, P. He, Q. Wang, Heterostructured ZnFe2O4/TiO2 nanotube arrays with remarkable visible-light photoelectrocatalytic performance and stability, J. Colloid Interface Sci. 493 (2017) 198–205. [19] L. Jiang, H. Tu, Y. Lu, Y. Wu, J. Tian, X. Shi, Q. Wang, Y. Zhan, Z. Huang, H. Deng, Spherical and rodlike inorganic nanoparticle regulated the orientation of carbon nanotubes in polymer nanofibers, Chem. Phys. Lett. 650 (2016) 82–87. [20] S. Xin, Z. Zeng, X. Zhou, W. Luo, X. Shi, Q. Wang, H. Deng, Y. Du, Recyclable Saccharomyces cerevisiae loaded nanofibrous mats with sandwich structure constructing via bio-electrospraying for heavy metal removal, J. Hazard. Mater. 324 (2017) 365–372. [21] Z. Zhou, W. Xiao, X. Shi, B. Ding, Q. Wang, Y. Zhan, H. Deng, Y. Du, Pore volume and distribution regulation of highly nanoporous titanium dioxide nanofibers and their photovoltaic properties, J. Colloid Interface Sci. 490 (2017) 74–83. [22] Y. Yi, H. Tu, X. Zhou, R. Liu, Y. Wu, D. Li, Q. Wang, X. Shi, H. Deng, Acrylic acidgrafted pre-plasma nanofibers for efficient removal of oil pollution from aquatic environment, J. Hazard. Mater. 371 (2019) 165–174. [23] S. Zhang, L.I. Hongyu, Z. Qin, Promotional effect of F-doped V2O5–WO3/TiO2 catalyst for NH3-SCR of NO at low-temperature, Appl. Catal. A Gen. 435–436 (2012) 156–162. [24] C. Minglei, T. Leiming, L. Xiaowei, B. Yi, L. Man, Y. Hong, Z. Mingqiang, Z. Zhicheng, S. Yan, W. Mingkui, Phosphorus-doped TiO2-B nanowire arrays
654
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
R. Huang et al. / Journal of Colloid and Interface Science 553 (2019) 647–654 boosting robust pseudocapacitive properties for lithium storage, J. Power Sour, 396, (2018), 327-334. J. Chen, Z. Liu, Y. Wu, Y. Li, J. Zhao, X. Zhu, P. Na, Sodium hypophosphite induced a simultaneous P doping and hollowing process of TiO2 spherical nanostructures with enhanced photocatalytic activity, Chem. Commun. 54 (2018) 1972–1975. D. Méndez, R. Vargas, C. Borrás, S. Blanco, J. Mostany, B.R. Scharifker, A rotating disk study of the photocatalytic oxidation of p-nitrophenol on phosphorusmodified TiO2 photocatalyst, Appl. Catal. B Environ. 166–167 (2015) 529–534. K. Wang, Y. Jing, L. Liu, H. Lin, F. Jin, Hierarchical P-doped TiO2 nanotubes array@Ti plate: Towards advanced CO2 photocatalytic reduction catalysts, Ceram. Int. 42 (2016) 16405–16411. Y. Ou, C. Zhu, Y. Zhao, L. Wang, S. Xie, Q. Wang, Adsorption mechanism of magnetically separable Fe3O4/graphene oxide hybrids, Appl. Surf. Sci. 355 (2015) 562–569. A.S.A.P.U. Ramesh, M. Palla, B. Wang, Z. Guo, C.H.E.N. Rakesh, H. Daniel, Phosphorus-doped titania nanotubes with enhanced photocatalytic activity, J. Photochem. Photobiol. A Chem. 225 (2011) 81–87. H. Nie, M. Ou, Q. Zhong, S. Zhang, L. Yu, Efficient visible-light photocatalytic oxidation of gaseous NO with graphitic carbon nitride (g–C3N4) activated by the alkaline hydrothermal treatment and mechanism analysis, J. Hazard. Mater. 300 (2015) 598–606. P. Falaras, H.L. Goff, M.C. Bernard, A. Xagas, Characterization by resonance Raman spectroscopy of sol–gel TiO2 films sensitized by the Ru(PPh3)2(dcbipy) Cl2 complex for solar cells application, Sol. Energy Mater. Sol. Cells 64 (2000) 167–184. S. Zhang, D. Jiang, M. Ou, Z. Sun, Q. Zhong, Y. Wang, S. Liu, Q. Ji, Photocatalytic removal of NO from coal-fired flue gas by H2O2/UV reaction over TiS catalyst, J. Alloys Comp. 691, (2017), 1005-1017. M. Ye, J. Gong, Y. Lai, C. Lin, Z. Lin, High-efficiency photoelectrocatalytic hydrogen generation enabled by palladium quantum dots-sensitized TiO2 nanotube arrays, J. Am. Chem. Soc. 134 (2012) 15720–15723.
[34] A. Ishikawa, T. Takata, J.N. Kondo, M. Hara, H. Kobayashi, K. Domen, Oxysulfide Sm(2)Ti(2)S(2)O(5) as a stable photocatalyst for water oxidation and reduction under visible light irradiation (lambda < or = 650 nm), J. Am. Chem. Soc. 124 (2002) 13547–13553. [35] X. Li, H. Shi, T. Wang, Y. Zhang, S. Zuo, S. Luo, Y. Chao, Photocatalytic removal of NO by Z-scheme mineral based heterojunction intermediated by carbon quantum dots, Appl. Surf. Sci. 456 (2018) 835–844. [36] N. Fang, J. Guo, S. Song, H. Luo, Y. Chu, J. Li, Enhancement of low–temperature activity and sulfur resistance of Fe0.3Mn0.5Zr0.2 catalyst for NO removal by NH3–SCR, Chem. Eng. J. 325 (2017) 114–123. [37] L.I. Junhua, J. Hao, F.U. Lixin, T. Zhu, Z. Liu, X. Cui, Cooperation of Pt/Al2O3 and In/Al2O3 catalysts for NO reduction by propene in lean burn condition, Appl. Catal. A Gen. 265 (2004) 43–52. [38] P. Yue, L. Junhua, H. Xu, L. Xiang, S. Wenkang, S. Xiaoxu, W. Dezhi, H. Jiming, Deactivation mechanism of potassium on the V2O5/CeO2 catalysts for SCR reaction: acidity, reducibility and adsorbed-NOx, Environ. Sci. Technol. 48 (2014) 4515–4520. [39] S. Yan, S. Zhang, Y. Zhao, L.I. Xianming, Y. Zhang, H. Zhang, J. Wang, F.U. Jianqiong, Adsorption Mechanisms of TMPDO and 4-NOH-TMPD in HTS/HO System and Effects on the Stabilization of Ti-OOH, Chem. J. Chin. Univ. (2016). [40] M.T. Baei, A.V. Moradi, P. Torabi, M. Moghimi, Adsorption properties of H2O2 trapped inside a boron phosphide nanotube, Monatshefte für Chemie – Chem. Month. 143 (2012) 37–41. [41] Y. Zhang, W. Fu, H. Yang, S. Liu, P. Sun, M. Yuan, D. Ma, W. Zhao, Y. Sui, M. Li, Y. Li, Synthesis and characterization of P-doped TiO2 nanotubes, Thin Solid Films 518 (2009) 99–103. [42] C. Sotelo-Vazquez, N. Noor, A. Kafizas, R. Quesada-Cabrera, D.O. Scanlon, A. Taylor, J.R. Durrant, I.P. Parkin, Multifunctional P-doped TiO2 films: a new approach to self-cleaning, transparent conducting oxide materials, Chem. Mater. 27 (2015) 3234–3242.
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Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Regular Article
Effect of adsorption properties of phosphorus-doped TiO2 nanotubes on photocatalytic NO removal Rui Huang a, Shule Zhang a,⇑, Jie Ding a, Yahan Meng a, Qin Zhong a,⇑, Deshuang Kong b, Changjun Gu b a b
School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China Jiangsu Gaochun Ceramics Co., Ltd, Nanjing, China
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 12 April 2019 Revised 18 June 2019 Accepted 18 June 2019 Available online 19 June 2019
A series of P doped TiO2 nanotubes (P-TNTs) catalysts were prepared for photocatalytic oxidation (PCO) of NO. Compared with TiO2 nanotubes, P-TNTs catalysts exhibited excellent catalytic activity. We confirmed that P was doped into TiO2 nanotubes lattice by XRD, Raman and TEM characterization. It was found that the enhanced PCO performance of NO was not attributed to the photoelectric properties of the P-TiO2 from UV–vis, Mott-Schottky and transient photocurrent response results. Therefore, we focused on the studies of catalyst reaction process and reactant adsorption characteristics. The results of transient reactions showed that the catalytic reaction proceeded via a mixed mechanism of Langmuir-Hinshelwood (LH) and Eley-Rideal (E-R) on the P-TiO2 catalyst. In addition, P doping was beneficial to the adsorption of H2O2 and NO, which is ascribed to the improved PCO performance of NO. Ó 2019 Published by Elsevier Inc.
Keywords: P doped TiO2 nanotubes PCO of NO Mixed mechanism Adsorption
1. Introduction Nitrogen oxide (NOx) emission is a main source of atmospheric contamination since it can easily cause acid rain, photochemical smog and ozone depletion [1]. The selective catalytic reduction (SCR) is the most common method to remove NO by using V2O5/WO3-TiO2 catalyst [2]. However, the SCR technology requires
⇑ Corresponding authors. E-mail addresses: (Q. Zhong).
[email protected]
https://doi.org/10.1016/j.jcis.2019.06.063 0021-9797/Ó 2019 Published by Elsevier Inc.
(S.
Zhang),
[email protected]
a high reaction temperature (300–400 °C) and thus is only used in large coal-fired power plant. For small and medium-sized industrial boilers, the SCR technology cannot be used, because most exhaust gas is below 150 °C or even at room temperature. For the process of NO oxidized into higher nitrogen oxides, lowtemperature flue gas (<150 °C), O3 [3] or H2O2 [4,5] is often used in the industry, while the main oxidation product NO2 is difficult to be absorbed by wet flue gas desulfurization [6]. Photocatalytic oxidation (PCO) of NO by H2O2 is a promising technology with many advantages including simple operation, mild reaction conditions, high efficiency and low temperature
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[7,8]. Furthermore, the oxidation product of NO is NO 3 , which is easily absorbed [9]. Ou et al. prepared the BiVO4 to remove the NO using H2O2 by PCO [5,7]. Duan et al. investigated the photocatalytic removal of NO by TiO2-supported Ag nanoparticles [10]. Ma et al. studied the photocatalytic denitrification of g-C3N4-TiO2 composite in air [11]. These studies focused on the potentials of the valence/conduction band of photocatalysts and the qualitative and quantitative analysis of photocatalytically active species such as superoxide radicals and hydroxyl radicals on the catalysts. However, few studies focused on the reaction process mechanism of photocatalyst for PCO of NO. Titanium dioxide (TiO2) used as photocatalyst was first reported by Frank and Bard [12] which had attracted much attention from researchers because of its high efficiency, nontoxicity, low cost and photochemical stability [13]. In order to further improve the catalytic performance, TiO2 is modified in various methods. For instance, in terms of morphological structure, there are titanium dioxide nanosheet [14], titanium dioxide nanoball [15], titanium dioxide nanowire [16], titanium dioxide nanotube [17,18] and nanofibers prepared by electrospinning technology [19–22]. In terms of ion doping, non-metal doping has been widely reported benefited from the smaller band-gap, stronger light absorption, and better nitrification resistance [23]. In the non-metal cationdoped TiO2 catalysts, phosphorus-doped TiO2 (P-TiO2) has caused great attention due to its high photocatalytic activity compared to pure TiO2 and P25. Up to now, P-TiO2 as a photocatalyst has been widely investigated in dye degradation and CO2 reduction [24–27]. Although the adsorption properties of the catalyst surface have been studied [28], there is no study about P-TiO2 used in PCO of NO. In this study, we prepared TiO2 and P-TiO2 nanotubes (P-TNTs) and found that P-TiO2 improved the PCO of NO. We also confirmed the P doped into TiO2 nanotubes lattice. It was found that the enhanced PCO performance of NO was not attributed to the photoelectric properties of the P-TiO2 from UV–vis, Mott-Schottky and transient photocurrent response results. Therefore, this paper focused on the studies of catalyst reaction process and reactant adsorption characteristics. The results of transient reactions showed that the catalytic reaction proceeded via a mixed mechanism of L-H and E-R on the P-TiO2 catalyst. In addition, P doping was beneficial to the adsorption of H2O2 and NO, which improved the PCO performance of NO.
2. Experimental 2.1. Catalyst preparation TiO2 nanotubes were prepared following a literature procedure [29]. 5 g TiO2 (Degussa P25) powder was dispersed in 175 mL of NaOH (10 M) solution, and the mixture solution was transferred into a Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained at 130 °C for 24 h, then was cooled to room temperature naturally. After that, the precipitates were collected and washed with 0.1 M HCl and deionized water until the pH value of the system reached neutral. The sample was then dried at 80 °C overnight to obtain the TiO2 nanotubes precursor. P-TiO2 nanotubes were fabricated by a facile wet chemical method [29]. A certain amount of sodium hydrogen phosphite (Na2HPO4) was dissolved in 60 mL water. 0.5 g TiO2 nanotubes precursor was added to the solution and dissolved by ultrasound for 1 h. The mixture was magnetically stirred overnight followed by drying at 80 °C for 12 h. Finally, the product was calcined at 500 °C in a muffle furnace for 3 h. These phosphorus-doped titania nanotubes were denoted as P-TNTs. The catalysts of P-TNTs coupled with 0 wt%, 2.5 wt%, 5 wt%, 7.5 wt% phosphorus were
obtained, and denoted as TNTs, 2.5P-TNTs, 5P-TNTs, 7.5P-TNTs, respectively. 2.2. Catalytic activity tests Experiments were conducted at room temperature and atmospheric pressure in a fixed bed continuous flow reactor (a tubular quartz reactor with d = 10 mm). The mixture of reactant gases (400 ppm NO, 10% O2, N2 balance) was fed into the reactor with a total flow rate of 100 mL/min. A 350 W Xe lamp was placed parallel to the catalyst portion. 30 wt% H2O2 solution was injected into the reactor with a flow rate of 0.023 mL/min. The system was assumed to be stable once the outlet NO concentration became equal to the inlet NO concentration. After keeping the system stable for 60 min, the lamp was turned on and the experiment started. The gas products were analyzed by in situ FT-IR gas analyzer SERVOPRO 4900. The NO conversion is defined by Eq. (1).
NO conversion ¼
NOðinletÞ NOðoutletÞ 100=% NOðinletÞ
ð1Þ
2.3. Characterization Produced ions in the solution were analyzed by Ion Chromatography (IC, Dionex ICS90), X-ray diffraction (XRD) patterns were recorded by using a Beijing Purkinjie General Instrument XD-3 Xray diffractometer, Raman by using the Raman spectroscopy (DRX, Thermo Fisher, USA). The micromorphology of the catalyst was examined by using a JEOL JEM-2100 transmission electron microscope (TEM), Fourier transform infrared (FT-IR) spectrometer recorded on an IS10 FT-IR spectrometer (Nicolet, USA). UV–vis diffuse reflectance spectra (DRS) were obtained by using a UV–vis spectrophotometer (Shimadzu UV-2550). Thermogravimetric analysis (TG) was performed on a thermogravimetric analyzer (TA SDT Q600). The Photoelectrochemical test was performed on a CHI 760D electrochemical workstation (Chenhua Instrument, Shanghai, China) using a standard three-electrode cell with a working electrode, a Pt wire as the counter electrode, and a standard Ag/AgCl in saturated KCl as the reference electrode. An aqueous solution of 0.5 mol/L Na2SO4 (pH 6.8) was used as an electrolyte and purged with N2 before each experiment. The working electrode was prepared by dip-coating: 10 mg photocatalyst was added into 1 mL ethyl alcohol and 50 lL Nafion solution to form slurry, and then dip-coated onto a 1 cm 1 cm FTO glass electrode. Subsequently, the films were dried at 80 °C overnight and calcined at 180 °C for 2 h at N2 atmosphere. The variations of photoinduced current density with time (i–t curve) was measured with a 350 W Xe light as the light source and 0.5 mol/L Na2SO4 as the electrolyte. The NO-TPD measurements were performed on an automated chemisorption analyzer (Quantachrome Instruments). Firstly, the temperature was preheated at 300 °C for 30 min, which treated the substance adsorbed on the surface of the catalyst. Then the temperature drops to 50 °C. After that, NO was introduced for 30 min for adsorption, then switched to N2 and purged to the baseline for stability. Finally, the temperature starts to rise according to the program. 3. Results and discussion 3.1. Catalytic activity tests The NO removal efficiency of P-TNTs were shown in Fig. 1a. The NO removal efficiency of all the P-TNTs catalysts were higher than that of TNTs. The NO removal efficiency decreased in this following
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Fig. 1. (a) NO removal efficiency for different catalysts in UV/H2O2 system, (b) The result of ion chromatography after transient reaction.
order: 5P-TNTs > 7.5P-TNTs > 2.5P-TNTs > TNTs. Therefore, the P doping could improve the NO removal efficiency, and 5P-TNTs was the best catalyst. In the catalytic activity tests, it was found that the NO2 content of the outlet also reduced, compared with the inlet. Therefore, the NO2 was not the main product of NO oxidation. In order to verify the product, Ion Chromatography (IC) was used to clarify the reaction product [30]. After the experiment was completed, 1 mL solution was removed from the collected solution (4 mL) and diluted by 100 times. Then the diluted solution was injected into IC. As shown in Fig. 1b, the qualitative IC analysis result of 5P-TNTs catalyst shows that the main product in the solution was NO 3 in this PCO of NO.
3.2. Characterization of structure, morphology and photoelectric properties 3.2.1. XRD analysis The XRD patterns of TNTs and P-TNTs are shown in Fig. 2a. The TNTs sample calcined at 500 °C is mainly composed of the relative spikes of anatase (PDF#21-1272). The P-TNTs catalysts calcined under 500 °C was also the only single phase of TiO2 anatase structure. It can be seen from Fig. 2a that the XRD peak intensity of the TiO2 (1 0 1) crystal face of the P-TNTs catalysts was lower than that of TNTs. Fig. 2b shows that the XRD peak of the TiO2 (1 0 1) crystal plane. As the P doping increased to 5%, a shift of the XRD peaks towards higher diffraction angles was observed. This phenomenon
was caused by the reduction in P-TNTs lattice constant, which could be attributed to the smaller ionic radius of P5+ than that of Ti4+. Therefore, the change in XRD peak intensity and peak position suggested that P was doped into the TiO2 lattice. 3.2.2. Raman analysis Fig. 3 displays the Raman spectroscopy of TNTs and P-TNTs catalysts. For the TNTs, five fundamental peaks located at 141 cm1 (Eg), 199 cm1 (Eg), 398 cm1 (B1g), 513 cm1 (A1g) and 639 cm1 (Eg), which can be ascribed to anatase TiO2 [31]. This was consistent with the results obtained by XRD. In addition, the Raman intensity gradually decreases with increasing P doping content. As the P doping content was 5%, the Raman intensity was the lowest. This observation could be attributed to the formation of TiAOAP with asymmetric vibration. When the P doping content was increased to 7.5%, the Raman intensity of 7.5P-TNTs increases. It was possible that the excessive P elements lead to the formation of PO3 4 by self-bonding, which was confirmed by FT-IR (Fig. S1). The formation of PO3 4 inhibited the P into the TiO2 lattice, which might be responsible for the reduced catalytic activity of 7.5PTNTs. 3.2.3. TEM analysis Fig. 4 shows the TEM and HRTEM images of TNTs and 5P-TNTs. As shown in Fig. 4a, the TNTs displayed a tubular structure. The tube diameter and the length were 7 nm and 100 nm. However,
Fig. 2. XRD patterns images of different wright ratios of P-TNTs.
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can be found from the Fig. 5(a) that the band gaps of TNTs (3.26 eV) and 5P-TNTs (3.22 eV) showed the most differences, which indicated that the P doping had little effect on the band gap. According to the literature [29], the P doping could cause the band gap to be narrow. However, it was found by TEM images (Fig. 4(c)) that the nanotube structure in 5P-TNTs reduced. The band gap of the nanoparticle catalyst was wider than that of the nanotube catalyst [29]. Therefore, these two factors had little effect on the P-doped band gap.
Fig. 3. Raman spectra of TNTs, 2.5P-TNTs, 5P-TNTs, 7.5P-TNTs.
for the 5P-TNTs (Fig. 4c), the length of nanotube became shorter which could be attributed to the agglomeration of the nanotubes calcined at 500 °C. According to Fig. 4b, the lattice spacing of TNTs was determined to be 0.380 nm which was assigned to (0 1 0) plane of anatase phase spacing. In Fig. 4d, part of 5P-TNTs interplanar spacing was narrower than that of TiO2 (0 1 0). The ionic radius of P5+ and Ti4+ and O2 are 0.0380, 0.0605 and 0.14 nm, respectively. According to the Bragg’s law, the lattice spacing becomes narrower when the ionic radius of the doping element was smaller than that of the substituted ion. Therefore, this observation could confirm P as cation doping into TNTs [32].
3.2.4. UV–vis analysis Fig. 5 shows the UV–vis spectrum of TNTs and P-TNTs catalysts and their band gap. Compared with TNTs, the UV–vis absorption of P-TNT showed a small red shift. The band gaps of TNTs and P-TNTs were obtained by the formula of Ahv = K(hv 8Eg)1/2. The band gaps of the catalysts were not very different from each other. It
3.2.5. Surface redox potential: Mott-Schottky Surface redox reaction is important to photocatalytic activity. The reduction and oxidation abilities of e and h+ are strongly dependent on the positions of conduction band (CB) and valence band (VB). The CB position can be calculated from Mott-Schottky (MS) using the impedance-potential method [33]. As shown in the Fig. 6, both TNTs and 5P-TNTs photoelectrodes showed positive slopes, indicating that these two samples were n-type semiconductors. Furthermore, extrapolating the chart to 1/C2 = 0, the Fermi level (EF) value of TNTs and 5P-TNTs was estimated to be 0.37 and 0.38 V vs. SCE (equivalent to 0.168 and 0.178 eV vs. NHE) respectively. It was well known that CB potential (ECB) of n-type semiconductor was very close to EF (0–0.2 eV is more negative) [34]. Here, the voltage difference between CB and the flat potential was set at 0.1 eV [9]. Therefore, the ECB of TNTs and 5PTNTs were 0.268 eV and 0.278 eV vs. NHE, respectively. The VB positions were estimated by the following relation: EVB = ECB + Eg. Thus, the VB potentials of TNTs and 5P-TNTs were 2.992 eV and 2.942 eV, respectively. The difference in potential between CB and VB of TNTs and 5P-TNTs was small. Therefore, the redox property of P-TNTs has no effect on the increase of NO conversion compared with TNTs. 3.2.6. Transient photocurrent response The transient photocurrent response measurement is an important analytical technique to evaluate the charge transfer efficiency
Fig. 4. TEM images of (a) TNTs, (c) 5P-TNTs, HRTEM images of (b) TNTs, (d) P-TNTs.
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Fig. 5. UV–vis diffuse reflection spectra patterns (a) and band gap energy (b) of all sample.
3.3. Catalyst reaction process and reactant adsorption characteristics From the characteristics of the UV–vis and surface redox properties, there was almost no difference between TNTs and P-TNTs catalysts, and electrons and holes separation efficiency of TNTs were superior to that of P-TNTs. Given the reaction system was a gas-liquid-solid three-phase reaction, it was necessary to study the reaction process to investigate the cause of the increase of catalytic activity for P-TiO2 catalysts.
Fig. 6. Mott-Schottky plots of TNTs and 5P-TNTs.
of photocatalysts [35]. Generally, the higher photocurrent represents the more electrons and higher holes separation efficiency, which was beneficial to photocatalytic activity. Fig. 7 showed the transient photocurrent response of different catalysts. As shown in Fig. 7, the order of photocurrent density decreased in this following order: TNTs > 5P-TNTs > 5P-TNTs > 2.5P-TNTs. The low photocurrent response of P-TNTs might be related to the P sites or the form of impurity level, which leads to accelerating the recombination of electrons and holes. Therefore, the separation of electrons and holes were not the reason for the increase of PTNTs catalytic activity.
Fig. 7. Transient photocurrent response of different catalysts.
3.3.1. Transient reactions In this study, we designed transient reactions to investigate the reaction model to determine the reaction process of the H2O2 and NO on the catalyst. The transient reaction shows that H2O2 and catalysts were first introduced into the reactor. After H2O2 reached saturated adsorption, NO was continuously introduced instantaneously, and the change of the NO concentration of the reactor outlet was monitored continuously. In the experiment, the reaction temperature was kept at 70 °C to reduce the effect of mass transfer in the surface liquid membrane of the catalyst and to accurately determine the intrinsic properties of catalytic reaction. Fig. 8a showed the transient reaction between H2O2 and NO on the PTNTs. Under the condition of light on and light off, after the NO was instantaneously introduced, the NO conversion of P-TNTs underwent a process of continuous increase, then continuous decrease, and finally stabilization. In the range of 3300–4500 s, the NO conversion of P-TNTs decreased after the NO conversion reached a higher level, and then it gradually got the highest level. This phenomenon might be related to the reaction between NO and free radicals generated on the catalyst surface under UV light. Fig. 8b shows the hypothetical E-R mechanism (reaction between NO in the gas phase and adsorbed H2O2 on the catalyst). If the catalytic reaction of P-TNTs followed E-R mechanism, the NO conversion would be highest as the NO was introduced instantaneously into the reactor, because the amount of H2O2 adsorption and radicals is the maximum on the catalyst. As the reaction proceeds, the NO conversion will gradually decrease and finally stabilize. Fig. 8c shows the hypothetical L-H mechanism (reaction between adsorbed NO and adsorbed H2O2 on the catalyst). If the catalytic reaction of P-TNTs followed L-H mechanism, the NO conversion would gradually increase and finally stabilize because NO and H2O2 compete for adsorption as the NO was introduced instantaneously into the reactor. It was worth noting that the actual NO conversion of P-TNTs was neither E-R nor L-H, but somewhere in between. Although this study cannot determine which mechanism was dominant, it was clear that the adsorption of H2O2 and NO on the P-TNTs catalyst had an effect on the catalytic reaction.
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Fig. 9. NO-TPD of TNTs and P-TNTs catalysts.
ment of NO adsorption by P doping can be attributed to the active sites of P doping. Compared with 2.5P-TNTs and 5P-TNTs, the TPD peak area of 7.5P-TNTs increased in the range of 100–400 °C, while the TPD peak area of NO x above 400 °C decreased significantly. The decreased catalytic activity of 7.5P-TNTs indicated that the strong adsorption state NO x produced by NO adsorption could improve the PCO of NO. It was worth noting that the TPD peak area about NO x of 5P-TNTs was lower than that of 2.5P-TNTs, and this TPD peak shifted toward lower temperatures. Considering the highest catalytic activity of 5P-TNTs, this observation could imply the two following possibilities: (1) Not only the NO x was the reactive species, but also the adsorption and activation of the reactant H2O2 played an important role in the reaction. (2) The decrease of binding energy between NOx and catalyst might improve the catalytic activity.
Fig. 8. (a) Transient reaction between H2O2 and NO on the P-TNTs; (b) The hypothetical E-R mechanism on catalyst. (c) The hypothetical L-H mechanism on catalyst.
3.3.2. Study on NO adsorption The adsorption behavior of NO on the TNTs and P-TNTs catalysts was determined by NO-TPD analysis, and the results are shown in Fig. 9. Several desorption peaks could be detected in the NO-TPD profiles. In Fig. 9, the TPD peaks in the range of 100– 200 °C were ascribed to the physisorbed NO [36]. The TPD peaks in the range of 300–400 °C were assigned to NO2 desorption arising from the decomposition of bidentate and/or monodentate nitrates species [37]. The TPD peaks in the range of 400–600 °C were attributed to NO x species [38]. As the P doping amount was 2.5%, the TPD peak area of 2.5P-TNTs was larger than that of TNTs, indicating that P doping was beneficial to the NO adsorption. Therefore, combining with the results that the catalytic activity of 2.5P-TNTs was higher than that of TNTs, NO adsorption could promote PCO performance of NO for P-TNTs. The surface area decreased in this following order: 2.5P-TNTs (166.8 m2/g) > TNTs (166.1 m2/g) > 5P-TNTs (113.2 m2/g) > 7.5P-TNTs (111.2 m2/g). Therefore, the improve-
3.3.3. Study on H2O2 adsorption According to literature, the H2O2 desorption on the catalyst could be confirmed by TG mothed [39]. Fig. 10 shows the H2O2 desorption on the catalysts by TG. It was reported in the literature that the decomposition temperature of H2O2 is between 100 and 400 °C. It can be seen from Fig. 10 that the amount of H2O2 adsorption decreased in this following order: 5P-TNTs > 7.5P-TNTs > 2.5PTNTs > TNTs, which was consistent with the NO removal efficiency of these catalysts. The improvement of H2O2 adsorption with P doping might be attributed to the polarization between P atom and OAH bond of H2O2 molecular [40]. Combined with transient reactions, NO-TPD and TG, it could be confirmed that P doping improved the NO and H2O2 adsorption, which results in the enhancement of PCO of NO. 3.4. The stability of the samples To investigate the stability of the photocatalysts, we used XPS and TEM to characterize the catalyst before and after the reaction (Fig. S2). Fig. S2(a) and (b) shows the XPS spectrum of 5P-TiO2 before and after the NO removal test. There was only one peak at 133.5 eV in P 2p XPS spectrum due to P5+ before and after the NO removal test. It indicates that P doping exists in the form of P5+ and chemical environment of P doping is stable [41,42]. The TEM of 5P-TiO2 before and after the reaction was shown in Fig. S2(c) and (d). There was little change between Fig. S2(c), (d) and Fig. 4. Therefore, the P doping TiO2 catalyst had no change in morphology after the photocatalytic reaction. In addition, we also tested the stability of 5P-TiO2 for NO removal under UV light (Fig. 11). As shown in Fig. 11, the catalyst still had a high photocatalytic oxidation capacity and it was reusable during the four cycles.
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Achievements into a Special Fund Project (BA2016055 and BA2017095), Top-notch Academic Programs Project of Jiangsu Higher Education Institutions. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.06.063. References
Fig. 10. The H2O2 desorption on the catalysts by TG.
Fig. 11. The stability tests of 5P-TiO2 for NO removal under UV light.
Therefore, P-TiO2 has stability both in terms of catalyst properties and reaction. 4. Conclusions In summary, P-TNTs was successfully prepared by hydrothermal and wet chemical method. The P-TNTs catalysts can significantly enhance NO oxidation in H2O2/UV system, and the 5PTNTs shows the highest efficiency, with a value of 90%. We confirmed that P doped into TiO2 nanotubes lattice by XRD, Raman and TEM. The UV–vis and Mott-Schottky results showed that the photoelectric properties of the P-TiO2 were not the main reasons for the increase of PCO performance of NO. The results of transient reactions showed that the catalytic reaction proceeded via a mixed mechanism of L-H and E-R on the P-TiO2 catalyst. In addition, P doping was beneficial to the adsorption of H2O2 and NO, which improved the PCO performance of NO. In the field of PCO of NO, this study provides a better understanding of the P species role and offers a new method to design the catalysts in terms of reactant adsorption. Acknowledgments This work was financially supported by the Key Project of Chinese National Programs for Research and Development (2016YFC0203800), the National Natural Science Foundation of China (51578288), Jiangsu Province Scientific and Technological
[1] L.B. Kreuzer, C.K. Patel, Nitric oxide air pollution: detection by optoacoustic spectroscopy, Science 173 (1971) 45–47. [2] O. Kröcher, M. Elsener, Chemical deactivation of V2O5/WO3–TiO2 SCR catalysts by additives and impurities from fuels, lubrication oils, and urea solution: I. Catalytic studies, Appl. Catal. B Environ. 77 (2008) 215–227. [3] C. Han, S. Zhang, L. Guo, Y. Zeng, X. Li, Z. Shi, Z. Yi, B. Zhang, Z. Qin, Ehanced catalytic ozonation of NO over black-TiO2 catalyst under inadequate ozone (O3/NO molar ratio = 0.6), Chem. Eng. Res. Des. 136 (2018) 219–229. [4] Y. Wang, S. Zhang, Z. Qin, Y. Zeng, X. Chen, Q. Wang, L. Guo, One-step hydrothermal synthesis of novel 3D BiFeWOx/Bi2WO6 composite with superior visible-light photocatalytic activity, Green Chem. 20 (2018) 3014– 3023. [5] M. Ou, S. Wan, Q. Zhong, S. Zhang, Y. Song, L. Guo, W. Cai, Y. Xu, Hierarchical Zscheme photocatalyst of g-C3N4@Ag/BiVO4 (040) with enhanced visible-lightinduced photocatalytic oxidation performance, Appl. Catal. B Environ. 221 (2018) 97–107. [6] L. Chen, J.W. Lin, C.L. Yang, Absorption of NO2 in a packed tower with Na2SO3 aqueous solution, Environ. Prog. 21 (2010) 225–230. [7] O. Man, Z. Qin, S. Zhang, H. Nie, Z. Lv, C. Wei, Graphene-decorated 3D BiVO4 superstructure: Highly reactive (040) facets formation and enhanced visiblelight-induced photocatalytic oxidation of NO in gas phase, Appl. Catal. B Environ. 193 (2016) 160–169. [8] Y. Zhou, X. Zhang, Q. Zhang, F. Dong, F. Wang, Z. Xiong, Role of graphene on the band structure and interfacial interaction of Bi2WO6/graphene composites with enhanced photocatalytic oxidation of NO, J. Mater. Chem. A 2 (2014) 16623–16631. [9] Y. Wang, S. Zhang, Y. Zeng, O. Man, Z. Qin, Photocatalytic oxidation of NO over TiO2-Graphene catalyst by UV/H2O2 process and enhanced mechanism analysis, J. Mol. Catal. A: Chem. 423 (2016) 339–346. [10] Yanyan Duan, Jianmin Luo, Shaochen Zhou, Xinyou Mao, M. Wajid Shah, TiO2supported Ag nanoclusters with enhanced visible light activity for the photocatalytic removal of NO, Appl. Catal. B Environ. 234 (2018) 206–212. [11] J. Ma, C. Wang, H. Hong, Enhanced photocatalytic oxidation of NO over g-C3N4TiO2 under UV and visible light, Appl. Catal. B Environ. 184 (2016) 28–34. [12] S.N. Frank, A.J. Bard, Heterogeneous photocatalytic oxidation of cyanide ion in aqueous solutions at titanium dioxide powder, Cheminform 8 (1977) 303–304. [13] N. Negishi, K. Takeuchi, T. Ibusuki, Surface structure of the TiO2 thin film photocatalyst, J. Mater. Sci. 33 (1998) 5789–5794. [14] N.O. Gopal, B.M. Hussain, TiO2 Nano-flakes with high activity obtained from phosphorus doped TiO2 nanoparticles by hydrothermal method, Ceram. Int. 44 (2018) 22129–22134. [15] W. Hong, K. Cai, J. Liu, X. Zhang, Y. Song, Synthesis of nanosphere TiO2 with flower-like micro-composition and its application for the selective catalytic reduction of NO with NH3 at low temperature, RSC Adv. 6 (2016) 84294. [16] N.T.Q. Hoa, Solvothermal synthesis and photocatalytic activity of Co-doped TiO2 nanowires, Electrochim. Acta 52 (2015) 3630–3640. [17] P. Mazierski, J. Nadolna, W. Lisowski, M.J. Winiarski, M. Gazda, M. Nischk, T. Klimczuk, A. Zaleska-Medynska, Effect of irradiation intensity and initial pollutant concentration on gas phase photocatalytic activity of TiO2 nanotube arrays, Catalysis Today. 284, (2017), 19-26. [18] S. Xie, K. Yang, Y. Lao, P. He, Q. Wang, Heterostructured ZnFe2O4/TiO2 nanotube arrays with remarkable visible-light photoelectrocatalytic performance and stability, J. Colloid Interface Sci. 493 (2017) 198–205. [19] L. Jiang, H. Tu, Y. Lu, Y. Wu, J. Tian, X. Shi, Q. Wang, Y. Zhan, Z. Huang, H. Deng, Spherical and rodlike inorganic nanoparticle regulated the orientation of carbon nanotubes in polymer nanofibers, Chem. Phys. Lett. 650 (2016) 82–87. [20] S. Xin, Z. Zeng, X. Zhou, W. Luo, X. Shi, Q. Wang, H. Deng, Y. Du, Recyclable Saccharomyces cerevisiae loaded nanofibrous mats with sandwich structure constructing via bio-electrospraying for heavy metal removal, J. Hazard. Mater. 324 (2017) 365–372. [21] Z. Zhou, W. Xiao, X. Shi, B. Ding, Q. Wang, Y. Zhan, H. Deng, Y. Du, Pore volume and distribution regulation of highly nanoporous titanium dioxide nanofibers and their photovoltaic properties, J. Colloid Interface Sci. 490 (2017) 74–83. [22] Y. Yi, H. Tu, X. Zhou, R. Liu, Y. Wu, D. Li, Q. Wang, X. Shi, H. Deng, Acrylic acidgrafted pre-plasma nanofibers for efficient removal of oil pollution from aquatic environment, J. Hazard. Mater. 371 (2019) 165–174. [23] S. Zhang, L.I. Hongyu, Z. Qin, Promotional effect of F-doped V2O5–WO3/TiO2 catalyst for NH3-SCR of NO at low-temperature, Appl. Catal. A Gen. 435–436 (2012) 156–162. [24] C. Minglei, T. Leiming, L. Xiaowei, B. Yi, L. Man, Y. Hong, Z. Mingqiang, Z. Zhicheng, S. Yan, W. Mingkui, Phosphorus-doped TiO2-B nanowire arrays
654
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
R. Huang et al. / Journal of Colloid and Interface Science 553 (2019) 647–654 boosting robust pseudocapacitive properties for lithium storage, J. Power Sour, 396, (2018), 327-334. J. Chen, Z. Liu, Y. Wu, Y. Li, J. Zhao, X. Zhu, P. Na, Sodium hypophosphite induced a simultaneous P doping and hollowing process of TiO2 spherical nanostructures with enhanced photocatalytic activity, Chem. Commun. 54 (2018) 1972–1975. D. Méndez, R. Vargas, C. Borrás, S. Blanco, J. Mostany, B.R. Scharifker, A rotating disk study of the photocatalytic oxidation of p-nitrophenol on phosphorusmodified TiO2 photocatalyst, Appl. Catal. B Environ. 166–167 (2015) 529–534. K. Wang, Y. Jing, L. Liu, H. Lin, F. Jin, Hierarchical P-doped TiO2 nanotubes array@Ti plate: Towards advanced CO2 photocatalytic reduction catalysts, Ceram. Int. 42 (2016) 16405–16411. Y. Ou, C. Zhu, Y. Zhao, L. Wang, S. Xie, Q. Wang, Adsorption mechanism of magnetically separable Fe3O4/graphene oxide hybrids, Appl. Surf. Sci. 355 (2015) 562–569. A.S.A.P.U. Ramesh, M. Palla, B. Wang, Z. Guo, C.H.E.N. Rakesh, H. Daniel, Phosphorus-doped titania nanotubes with enhanced photocatalytic activity, J. Photochem. Photobiol. A Chem. 225 (2011) 81–87. H. Nie, M. Ou, Q. Zhong, S. Zhang, L. Yu, Efficient visible-light photocatalytic oxidation of gaseous NO with graphitic carbon nitride (g–C3N4) activated by the alkaline hydrothermal treatment and mechanism analysis, J. Hazard. Mater. 300 (2015) 598–606. P. Falaras, H.L. Goff, M.C. Bernard, A. Xagas, Characterization by resonance Raman spectroscopy of sol–gel TiO2 films sensitized by the Ru(PPh3)2(dcbipy) Cl2 complex for solar cells application, Sol. Energy Mater. Sol. Cells 64 (2000) 167–184. S. Zhang, D. Jiang, M. Ou, Z. Sun, Q. Zhong, Y. Wang, S. Liu, Q. Ji, Photocatalytic removal of NO from coal-fired flue gas by H2O2/UV reaction over TiS catalyst, J. Alloys Comp. 691, (2017), 1005-1017. M. Ye, J. Gong, Y. Lai, C. Lin, Z. Lin, High-efficiency photoelectrocatalytic hydrogen generation enabled by palladium quantum dots-sensitized TiO2 nanotube arrays, J. Am. Chem. Soc. 134 (2012) 15720–15723.
[34] A. Ishikawa, T. Takata, J.N. Kondo, M. Hara, H. Kobayashi, K. Domen, Oxysulfide Sm(2)Ti(2)S(2)O(5) as a stable photocatalyst for water oxidation and reduction under visible light irradiation (lambda < or = 650 nm), J. Am. Chem. Soc. 124 (2002) 13547–13553. [35] X. Li, H. Shi, T. Wang, Y. Zhang, S. Zuo, S. Luo, Y. Chao, Photocatalytic removal of NO by Z-scheme mineral based heterojunction intermediated by carbon quantum dots, Appl. Surf. Sci. 456 (2018) 835–844. [36] N. Fang, J. Guo, S. Song, H. Luo, Y. Chu, J. Li, Enhancement of low–temperature activity and sulfur resistance of Fe0.3Mn0.5Zr0.2 catalyst for NO removal by NH3–SCR, Chem. Eng. J. 325 (2017) 114–123. [37] L.I. Junhua, J. Hao, F.U. Lixin, T. Zhu, Z. Liu, X. Cui, Cooperation of Pt/Al2O3 and In/Al2O3 catalysts for NO reduction by propene in lean burn condition, Appl. Catal. A Gen. 265 (2004) 43–52. [38] P. Yue, L. Junhua, H. Xu, L. Xiang, S. Wenkang, S. Xiaoxu, W. Dezhi, H. Jiming, Deactivation mechanism of potassium on the V2O5/CeO2 catalysts for SCR reaction: acidity, reducibility and adsorbed-NOx, Environ. Sci. Technol. 48 (2014) 4515–4520. [39] S. Yan, S. Zhang, Y. Zhao, L.I. Xianming, Y. Zhang, H. Zhang, J. Wang, F.U. Jianqiong, Adsorption Mechanisms of TMPDO and 4-NOH-TMPD in HTS/HO System and Effects on the Stabilization of Ti-OOH, Chem. J. Chin. Univ. (2016). [40] M.T. Baei, A.V. Moradi, P. Torabi, M. Moghimi, Adsorption properties of H2O2 trapped inside a boron phosphide nanotube, Monatshefte für Chemie – Chem. Month. 143 (2012) 37–41. [41] Y. Zhang, W. Fu, H. Yang, S. Liu, P. Sun, M. Yuan, D. Ma, W. Zhao, Y. Sui, M. Li, Y. Li, Synthesis and characterization of P-doped TiO2 nanotubes, Thin Solid Films 518 (2009) 99–103. [42] C. Sotelo-Vazquez, N. Noor, A. Kafizas, R. Quesada-Cabrera, D.O. Scanlon, A. Taylor, J.R. Durrant, I.P. Parkin, Multifunctional P-doped TiO2 films: a new approach to self-cleaning, transparent conducting oxide materials, Chem. Mater. 27 (2015) 3234–3242.