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Enhancement of the photocatalytic activity of N-doped TiO2 nanograss array films by low-temperature sulfur doping
Materials Science in Semiconductor Processing 108 (2020) 104872
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Enhancement of the photocatalytic activity of N-doped TiO2 nanograss array films by low-temperature sulfur doping Te Hu a, Jiancheng Yan b, Yifei Hu b, Tongyang Liu b, Sujun Guan c, Yun Lu d, Liang Hao b, e, f, *, Touwen Fan a, **, Dongchu Chen a a
College of Materials Science and Energy Engineering, Foshan University, No. 18 Jiangwan 1 Road, Foshan, 528000, PR China College of Mechanical Engineering, Tianjin University of Science & Technology, No. 1038, Dagu Nanlu, Hexi District, Tianjin, 300222, PR China Department of Physics, Tokyo University of Science, No. 1-3 Kagurazaka, Shinjuku-ku, Tokyo, 162-8601, Japan d Graduate School of Science and Engineering, Chiba University, No. 1-33, Yayoi-cho, Inage-ku, Chiba, 263-8522, Japan e Tianjin International Joint Research and Development Center of Low-carbon Green Process Equipment, PR China f Tianjin Key Lab of Integrated Design and On-line Monitoring for Light Industry & Food Machinery and Equipment, Tianjin, PR China b c
A R T I C L E I N F O
A B S T R A C T
Keywords: TiO2 nanograss array films Doping Low temperature Chemical dissociation Photocatalytic activity
Moderate in-situ chemical oxidation of Ti meshes was used to prepare nitrogen-doped (N-doped) anatase TiO2 nano-grass array films with urea as the nitrogen source. Further sulfur doping through immersion of the films in a thiourea solution at a relatively low temperature of 353 K was subsequently performed to enhance the visiblelight-driven photocatalytic activity of the films. SEM images confirmed that the loose microstructure of the nanograss arrays became compact after N-doping or N, S co-doping. N-doping and N, S co-doping decreased the band gap values of the TiO2 films and increased the effective visible light utilization. XPS analysis revealed that nitrogen mainly existed in interstitial form. Subsequent sulfur ions appeared as S6þ, S4þ, and S2 , with S6þ and S4þ replacing Ti4þ, and S2 replacing O2 , respectively. With an increase in the urea treatment time, the sulfur ions were oxidized, achieving higher valence states. S6þ substitution resulted in a charge imbalance, which could only be neutralized by hydroxide ions. The imbalance made it easier for water that had settled on the TiO2 films to chemically dissociate, and thereby generating hydroxyl radicals. OH� radicals as well as Ο�2 radicals played a significant role in the degradation of RhB dye. The RhB decolorization activity achieved by the N, S co-doped samples was 48 times that of the undoped samples.
1. Introduction Titanium dioxide (TiO2) doped with non-metallic elements, partic ularly nitrogen [1] and sulfur [2,3], has showed an outstanding visible-light response within a wide wavelength range and excellent visible-light-driven photocatalytic activity. It is generally known that doping of TiO2 could introduce additional electronic states above the valence band edge of pure TiO2 [4]. The additional electronic states result in a red shift in the absorption wavelength within the visible re gion of the spectrum, which leads to visible-light-driven photocatalytic activity [5]. Research studies on nitrogen and sulfur co-doping of TiO2 to improve visible-light-driven photocatalytic activity have been widely conducted [6–10]. However, N or/and S doping is almost always
performed under high temperature conditions so that N or/and S atoms can easily diffuse into the lattice of TiO2 [11–14]. Studies on nitrogen or sulfur doping of TiO2 at low temperatures have not been reported so far. In addition, the nature of the doping form and the competition between nitrogen and sulfur; the influence of nitrogen and sulfur doping on visible light absorption; the generation, separation, and recombination of photogenerated electron-hole pairs; and the surface chemical disso ciation ability are unclear. In this study, we prepared N-doped and N, S co-doped TiO2 nanograss array films on a Ti wire mesh with moderate in-situ chemical oxidation and facile solution immersion at a low temperature of 353 K. The competition and interaction of nitrogen and sulfur ions was inves tigated. The influence of nitrogen and sulfur doping on visible light
* Corresponding author. College of Mechanical Engineering, Tianjin University of Science & Technology, No. 1038, Dagu Nanlu, Hexi District, Tianjin, 300222, PR China. ** Corresponding author. E-mail address: [email protected] (L. Hao). https://doi.org/10.1016/j.mssp.2019.104872 Received 10 May 2019; Received in revised form 22 November 2019; Accepted 2 December 2019 Available online 5 December 2019 1369-8001/© 2019 Elsevier Ltd. All rights reserved.
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absorption, absorption properties of the films, and generation of active oxidizing radicals was qualitatively analyzed.
cycle evaluation was investigated under the same conditions. 3. Results and discussion
2. Experimental
Fig. 1 shows the XRD patterns of the calcined samples. From Fig. 1, it can be seen that the intensities of the diffraction peaks of titanium correspond to different crystal planes. On the other hand, the diffraction peaks of anatase TiO2 were rather weak, which could be ascribed to the small thickness of the formed anatase TiO2 films on the Ti wire mesh. The weak diffraction peak at about 25.3� of 2θ is ascribed to the crystal plane (101) of anatase TiO2 according to JCPDS No. 21–1272. It suggests that anatase TiO2 grew along the direction preferentially. In other words, the anatase crystals were exposed with (101) facets. Moreover, a slight shift of 2θ corresponding to the diffraction peak was also confirmed, which should be ascribed to N and S doping. The Raman spectra of the calcined samples are shown in Fig. 2. Five intense peaks at 149, 200, 400, 519 and 637 cm 1 were detected. They are consistent with the Eg, Eg, B1g, A1g þ B1g, and Eg vibrational modes of the anatase TiO2 phase [16,17]. Compared to single crystal TiO2 [18], anatase TiO2 exhibited a Raman band shift towards a higher wavenumber. The shift might be related to the decrease in crystal size. Meanwhile, the increase in intensities of the peaks could be ascribed to an increase in particle size. Two additional peaks at 248 and 284 cm 1 were detected for the U-80-96-S sample. They are attributed to the first order scatterings of non-stoichiometric Ti–N or Ti–S bonding [19,20]. It should be noted,
2.1. Preparation of N-doped TiO2 nanograss array films Moderate in-situ chemical oxidation was used to prepare N-doped TiO2 nanograss array films on a Ti wire mesh with a mesh number of 100 (wire diameter: 0.1 mm, purity: 99.8 wt%, Kangwei Co., Ltd., China). The preparation process was as follows. First, pieces of Ti wire mesh, each with a dimension of 60 mm � 50 mm, were successively cleaned with hydrochloric acid, acetone and distilled water for 5 min in an ul trasonic cleaner. The washed pieces of the Ti wire mesh were then dried in a vacuum drying oven at 333 K. Second, chemical oxidation was performed in a sealed vessel containing the five pieces of cleaned Ti wire mesh, 147 mL H2O2 solution (30 wt%, Reagent Co., Ltd, China), 3 mL HNO3 solution (65–68 wt%, Sinopharm Chemical Reagent Co., Ltd., China), and 0.909 g urea powder (purity: 99 wt%, Macklin Co., Ltd., China). The temperature of chemical oxidation was set as 353 K, and the oxidation was allowed to proceed for 48, 72 and 96 h. Finally, the asprepared samples were calcined at 723 K for 60 min with a heating rate of 5 K/min to transform amorphous TiO2 to anatase [15]. The above calcined samples were labeled as U-80-x, where x refers to heating holding time. For comparison, an undoped 80-x sample was also pre pared by the same preparation method, but without urea powder. 2.2. Preparation of S, N co-doped TiO2 nanograss array films To achieve further sulfur doping, before calcination, the U-80-x samples were placed in a sealed glass pot containing a 150 mL thiourea (purity: 99 wt%, Alfa Aesar Co., Ltd., China) solution with a concen tration of one mol/L. The sealed pot was placed in a drying oven at 353 K for 24 h. After that, the samples were heated at 723 K for 1 h. The above calcined samples were named U-80-x-S. 2.3. Characterization The crystal types of the samples were analyzed by X-ray diffraction (XRD) (D8 Advance, Bruker, Germany). The phase types were confirmed using laser confocal Raman spectroscopy (inVia, Renishaw, England). The surface morphology was observed by scanning electron microscopy (SEM) (Sigma 300, Zeiss, Germany). Light absorption tests were con ducted using UV–Vis diffuse reflectance spectroscopy (UV 3600Plus, Shimadzu, Japan). The surface elements and their chemical valences were analyzed using X-ray photoelectron spectroscopy (XPS) (Escalab 250Xi, Thermo Fisher Scientific, USA). The hydroxyl radicals (OH�) and superoxide radicals (Ο�2 ) were examined by electron paramagnetic resonance (EPR) spectroscopy (A300, Bruker, Germany). 2.4. Photocatalytic activity evaluation Rhodamine B (RhB) was chosen as the target degradation dye in the evaluation of the visible-light-responsive photocatalytic activity of TiO2 nanograss array films. First, sample pieces with dimensions of 30 mm � 25 mm were laid on the bottom of 50 mL glass beakers. Second, a 20 mL RhB solution with an initial concentration of about 10 mg/L was poured into the beakers containing the samples. For the RhB molecules to reach an adsorption-desorption equilibrium, the beakers containing the RhB aqueous solution and the samples were kept in darkness for 30 min. To remove ultraviolet light, an optical filter was inserted between the Xe lamp and the beakers to cut off light with a wavelength less than 420 nm. The optical power density of the visible light arriving at the beakers was measured to be approximately 45 mW/cm2. The decolorization of the RhB aqueous solution was monitored by a UV–Vis spectrophotometer (DR3900, Hach, USA) every 30 min. The photocatalytic activity in the
Fig. 1. XRD patterns of the calcined samples: (a)–(f) correspond to samples 80–48, U-80-48, U-80-72, U-80-96, U-80-48-S, U-80-72-S, and U-80-96-S, respectively. 2
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has been confirmed by other researchers [15]. The anatase TiO2 phase exhibited a morphology similar to that of a nanograss array. Fig. 4a shows the UV–Vis absorption spectra of the calcined samples. The band gap values were obtained with the Kubelka-Munk function [23]. Urea treatment and thiourea treatment changed the absorption threshold. Fig. 4b represents the corresponding Tauc plots [24–26] of these samples. From these plots, we could obtain the band gap values as shown in Table 1. We found that urea treatment decreased the band gap values, and further thiourea treatment reduced the band gap even further, except for U-80-96-S sample. This is related to N- and S-doping [1]. It is common knowledge that the band gap of anatase TiO2 is about 3.2 eV. Urea treatment decreased the band gap to 2.66–2.51 eV in a treatment-time dependent manner, i.e., with an increase in treatment time from 48 to 96 h, the band gap decreased further. We could conclude, from these results, that N-doping through urea treatment extended the visible light absorption of the as-prepared samples. With further thiourea treatment, the visible light absorption of the asprepared samples improved, and their band gap values further decreased, except that of sample U-80-96-S. In addition, the band gap values of the N, S co-doped samples increased as the thiourea treatment time increased. The unexpected increase in band gap should be ascribed to the doping state of sulfur. Generally, S-doping through thiourea treatment should be an efficient method for improving the visible light absorption of TiO2 nanograss array films. Fig. 5 shows the XPS spectra of Ti 2p and O1s of the calcined samples. In the Ti 2p spectra (Fig. 5a), two peaks located at 459.0 eV and 464.7 eV were detected for the undoped 80-48 sample. They were the typical peaks of Ti 2p3/2 and 2p1/2 [27]. Meanwhile, a negative shift of 0.1–0.2 eV was detected after the urea and thiourea treatment, indicating that the N-doping and S, N co-doping that resulted from urea and thiourea treatment might have affected the electronic density of the states around Ti. For samples U-80-96 and U-80-96-S, an obvious positive shift of 0.9–1.1 eV was also noted. Fig. 5b shows the O 1s spectra of these samples. The XPS peaks of O 1s could be perfectly divided into two peaks by fitting. For the undoped 80-48 sample, two peaks at 530.4 and 531.4
Fig. 2. Raman spectra of the calcined samples.
however, that the Raman shift of 248 cm 1 has been confirmed as belonging to the brookite phase [19,21,22]. Figs. 1 and 2 revealed that anatase TiO2 formed and a urea or thiourea treatment has no impact on the crystal type of the formed TiO2 films. Fig. 3 shows the surface SEM images of all the samples. Films with a thickness of about 5 μm covering the Ti wires looked like cracked tree bark (Fig. 3b). Although peeling of oxide films was observed, most of the films were so firm that they were not disrupted when an ultrasonic cleaner was used to remove the contaminants on their surfaces. The films look like grass arrays with nano-sized stems (Fig. 3c). The micro structure of the grass arrays is rather loose; as result, more active sites favoring photocatalytic redox are exposed. With urea treatment, the microstructure of the TiO2 nanograss array became compact (Fig. 3d–f). With further thiourea treatment, the TiO2 nanograss array films became even more compact (Fig. 3g–j). From Figs. 1–3, we could confirm that the anatase TiO2 phase formed on the surface of the Ti wire mesh. This
Fig. 3. SEM images of the calcined samples: (a–c) 80–48, (d–j) U-80-48, U-80-72, U-80-96, U-80-48-S, U-80-72-S, and U-80-96-S. 3
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Fig. 4. UV–Vis absorbance spectra (a) and the corresponding Tauc plots (b) of the calcined samples.
to Ti — O bonds and OH species. A great shift in binding energy cor responding to the Ti — O bonds was observed for samples U-80-96 and U-80-96-S. Peaks at around 457.4 eV corresponding to the Ti 2p3/2 of Ti3þ [29] were not discovered, which indicates that Ti3þ did not form on the surface of the TiO2 nanograss array films. Therefore, we could safely conclude that Ti exists in the form of Ti4þ. It is worth noting that the peak intensity corresponding to the OH species (531.3–533.2 eV) reduced with an increase in urea treatment time. On the other hand, thiourea treatment increased the peak intensity corresponding to OH for samples U-80-72 and U-80-96. The XPS spectra of N 1s for the calcined samples are shown in Fig. 6. For the urea-treated samples (Fig. 6a), one peak located at 399.8 or 400.0 eV was detected. It is reported that XPS peaks ranging from 398 to 402 eV should be assigned to the interstitial N [30,31]. Peaks ~ 397 eV suggest the formation of N — Ti – O linkages, indicating the substitution of the N ion for O ion [32]. Therefore, we could confirm that N-doping was complete, and nitrogen mainly existed in an interstitial form. With an increase in processing time, the corresponding XPS peak reduced,
Table 1 Absorption threshold and band gap of the calcined samples. Sample No.
Absorption threshold (nm)
Band gap (eV)
80–48 U-80-48 U-80-48-S 80–72 U-80-72 U-80-72-S 80–96 U-80-96 U-80-96-S
425 466 539 431 470 481 429 494 479
2.92 2.66 2.30 2.88 2.64 2.58 2.89 2.51 2.59
eV were detected; they were ascribed to the lattice oxygen in Ref. [22] and surface hydroxyl species [28], respectively. For U-80-48 sample, three peaks located at 530.3, 531.3, and 532.6 eV were found. The first peak was attributed to Ti — O bonds, and the last two were attributed to OH species. For U-80-48-S sample, the two peaks were also attributed
Fig. 5. XPS spectra of the calcined samples: (a) Ti 2p and (b) O 1s. 4
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Fig. 6. XPS spectra of N 1s for: (a) the N-doped and (b) the N, S co-doped samples.
replaced Ti4þ [33]. Meanwhile, Zhang et al. believed that the peak should be attributed to S4þ [34]. The second one is attributed to S2 , which could correspond to Ti–S [33–35]. This also suggests that sub stitution of oxygen by sulfur in the lattice of anatase TiO2 occurred. The last one located at 159.2 eV should be attributed to the Ti–S bond, which means that S2 replaced O2 [36]. For U-80-96-S sample, only one peak located at 170.0 eV was detected. It is generally believed that the peak should be ascribed to S6þ [37]. A positive shift of binding energy cor responding to the S6þ XPS peak suggests the electronic density decreased around S ions. From the above analysis, we could conclude that the sulfur in TiO2 films mainly exists in the form of substitutional S2 ions and as a small amount of S4þ and S6þ ions in sample U-80-48-S. When urea treatment was extended to 72 h (U-80-72-S), the doping amount of S4þ/S6þ was roughly equivalent to that of S2 . With a further increase in the treatment time to 96 h, S4þ and S2 ions disappeared, and only substitutional S6þ ions remained. In other words, the sulfur ions were oxidized from S2 to S4þ and finally S6þ as urea treatment increased. The valence state change also affected the optical absorption capacity and band gap values of the films. The ionic form of sulfur is dependent on the preparation method. However, the substitution of Ti4þ by S6þ is more chemically favorable than that of Ti4þ by S4þ and O2 by S2 . This is because S2 (1.7 Å) has a larger radius than O2 (1.22 Å) [38]. When S6þ ions replace Ti4þ ions in the lattice of TiO2, a charge imbalance is created. This would result in extra positive charges on the films that could only be neutralized by hydroxide ions [39]. The EPR signals of the OH� radicals in the surfaces of the calcined samples under visible light irradiation were recorded, and with the re sults depicted in Fig. 8. The characteristic quartet peaks of DMPO- OH� adducts with an intensity ratio of 1:2:2:1 were observed for all the samples. From Fig. 8a, the intensity of the EPR signals of sample 80–48 are similar to that of sample U-80-48. On the other hand, the intensity of the EPR signals of sample U-80-48-S is much greater than that of sample U-80-48. In other words, N-doping has little impact on the generation of OH� radicals, while S-doping has a considerable impact. The identical results shown in Fig. S1 were also obtained for other N-doped and N, S co-doped samples. They suggest that S-doping rather than N-doping could result in the generation of more OH� radicals. The influence of urea treatment time on the production of OH� radicals was also studied, and our findings are shown in Fig. 8b. Although the doping amounts of nitrogen or sulfur decreased (Figs. 6 and 7), the generated OH� radicals
which suggests that the doping amount of nitrogen became smaller. In other words, nitrogen ions diffused out from the lattice of TiO2. After further thiourea treatment (Fig. 6b), the intensities and binding energy corresponding to the peaks of N 1s seem to be invariant compared to those before thiourea treatment. This indicates that further thiourea treatment has no impact on nitrogen doping. Fig. 7 shows the XPS spectra of S 2p of the urea and thiourea-treated samples. For samples U-80-48-S and U-80-72-S, three peaks located at 168.8, 164.5, and 159.2 eV of binding energy were detected. The first peak is believed to be due to the 2p3/2 of S4þ and S6þ, which could be attributed to the linking of S — O [33]. Furthermore, S4þ and S6þ
Fig. 7. S 2p XPS spectra of the calcined samples: (A) (B) (C) correspond to U80-48-S, U-80-72-S, and U-80-96-S, respectively. 5
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Fig. 8. EPR signals of OH� radicals for the calcined samples under visible irradiation.
increased contrarily. Fig. 9 shows the EPR spectra of DMPO-Ο�2 signals on the surfaces of the calcined samples. From Fig. 9a, the intensity of the DMPO-Ο�2 sig nals of U-80-48 sample is much greater than that of sample 80–48. On the other hand, the difference in intensity between samples U-80-48 and U-80-48-S is not obvious. This suggests that N-doping greatly promoted the generation of Ο�2 radicals. S-doping, however, had a negligible in fluence on the generation of Ο�2 radicals. Similar results were likewise obtained for the other samples (Fig. S2). For N, S co-doped samples, the intensity of the DMPO-Ο�2 signals for U-80-72-S sample was the greatest although the doping amount of nitrogen was smaller than that of U-8048. Fig. 10 shows the EPR signals of the OH� radicals of the calcined samples in darkness. The EPR signals of the OH� radicals for N, S codoped samples have a greater intensity than those for the undoped and N-doped samples. Generally, without light irradiation, holes required for the generation of OH� radicals are not formed. Water could physically settle on TiO2 films, and some molecules might chemically dissociate to form Hþ ions and OH groups. Chemically dissociated OH groups attach to the surface of TiO2 films, especially N, S co-doped TiO2 films. This is supported by the XPS results shown in Fig. 5b. The chemically dissociated OH groups were captured by the DMPO trap ping reagent, and that produced the EPR signals of the DMPO- OH�. As described above, the substitution of Ti4þ with S6þ would produce extra positive charges that could only be neutralized by OH groups. Sample U-80-96-S showing the greatest EPR signal intensity possesses the largest ratio of S6þ ions. Therefore, we confirmed that water can easily dissociate on N, S co-doped TiO2 films. The photodegradation of the RhB aqueous solution during photo catalytic degradation evaluation was recorded (Fig. 11a). For undoped
Fig. 10. EPR signal of OH� radicals of the calcined samples in darkness.
TiO2 films (80-x), the decolorization of the RhB solution was no more than 10% within 150 min of visible light irradiation. As for the N-doped samples (U-80-x), the decolorization increased to 30–80% within the same irradiation time. However, the decolorization of the RhB solution due to N, S co-doped sample (U-80-x-S) was the highest; in particular, with sample U-80-96-S, the decolorization increased to 90%. Fig. 11b shows the rate constants for RhB degradation under visible-light irra diation. For the undoped samples 80–48, 80–72, and 80–96, the rate
Fig. 9. EPR signals of Ο�2 radicals for the calcined samples under visible light irradiation. 6
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Fig. 11. (a) Concentration evolution of the RhB solution and (b) corresponding plot of Ln (C0/C) versus irradiation time.
constants were 9.77 � 10 4, 5.73 � 10 4, and 3.92 � 10 4, respectively. For the N-doped samples U-80-48, U-80-72, and U-80-96, the rate con stants were 3.65 � 10 3, 7.12 � 10 3, and 12.43 � 10 3, respectively. The RhB decolorization activity of these corresponding samples increased by 2.73, 11.42, and 30.7 times through nitrogen doping. For the N, S co-doped samples U-80-48-S, U-80-72-S, and U-80-96-S, the rate constants increased to 1.32 � 10 2, 1.14 � 10 2, and 1.93 � 10 2, respectively. Sulfur doping increased the RhB decolorization activity of the corresponding N-doped samples by 262%, 60%, and 56%. Fig. 12 shows the photocatalytic degradation tests of RhB, with sample U-80-96S showing the highest photocatalytic activity. It could be observed that the RhB decolorization activity hardly decreased, which could ensure the long service life of the films. Through low-temperature chemical oxidation, TiO2 nanograss array films on a Ti wire mesh were prepared. The microstructure of the loose nanograss array favors photocatalytic redox. With urea and thiourea treatment, N-doping and N, S co-doping were achieved. N-doping decreased the band gap of the undoped samples and N, S co-doping further reduced the band gap of the N-doped samples. The visible light absorption of TiO2 films was enhanced, and the effective utilization of visible light increased. This explains why the visible-light-responsive photocatalytic performance of these TiO2 films increased. The substi tution of Ti4þ with S6þ produced extra positive charges, which could only be neutralized by hydroxide ions. Compared to the S2 and S4þ, S6þ seems to be more beneficial for photocatalytic activity enhancement. Moreover, the increase in the amounts of OH� and Ο�2 radicals owing to N-doping and N, S co-doping can also account for the enhanced photo catalytic activity of these TiO2 films.
Fig. 12. Recycle photodegradation evaluation of the RhB solution under visible light.
charges that made it easier for water to chemically dissociate into Hþ and OH . The RhB decolorization capacity achieved by the N, S codoping was 48 times that of the undoped sample. Author statement
4. Conclusions
Te Hu: Characterization and photocatalytic activity evaluation. Jiancheng Yan: Material preparation. Yifei Hu and Tongyang Liu: Software, data analysis. Sujun Guan and Yun Lu: data analysis and discussion. Liang Hao and Touwen Fan: Conceptualization, methodology, and writing of manuscript. Dongchu Chen: supervision.
N-doped anatase TiO2 nanograss array films were prepared by moderate in-situ chemical oxidation on a Ti wire mesh. Sulfur doping (Sdoping) of N-doped TiO2 films was realized by thiourea solution im mersion at a low temperature of 353 K. Nitrogen doping (N-doping) and S-doping enhanced the visible light absorption of these TiO2 films to a certain degree and decreased their band gap values. Dopant nitrogen ions mainly existed in interstitial form. With an increase in urea-treated time, the doping amount of nitrogen in TiO2 films decreased. The sub sequent S-doping had a negligible effect on the dopant nitrogen. Dopant sulfur ions in TiO2 films appeared in three states: S6þ, S4þ, and S2 . The first two kinds of ions could replace Ti4þ while the last one replaced O2 . As the urea treatment time increased, the ions were gradually oxidized. Both OH� and Ο�2 radicals should play a significant role in the degra dation of RhB under visible light irradiation. N-doping and S-doping, especially S6þ-doping, resulted in the generation of extra positive
Declaration of competing interest The authors declared that they have no conflicts of interest to this work. Acknowledgments The National Natural Science Foundation of China of China (No. 51404170) and the Key Project of Department of Education of 7
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Guangdong Province (No. 2016GCZX008) supported this work. Te Hu and Jiancheng Yan equally contributed to this work.
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Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.mssp.2019.104872. References [1] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Visible-light photocatalysis in nitrogen-doped titanium oxides, Science 293 (2001) 269–271. [2] T. Umebayashi, T. Yamaki, S. Tanaka, K. Asai, Visible light-induced degradation of methylene blue on S-doped TiO2, Chem. Lett. 32 (2003) 330–331. [3] T. Tachikawa, S. Tojo, K. Kawai, M. Endo, M. Fujitsuka, T. Ohno, K. Nishijima, Z. Miyamoto, T. Majima, Photocatalytic oxidation reactivity of holes in the sulfurand carbon-doped TiO2 powders studied by time-resolved diffuse reflectance spectroscopy, J. Phys. Chem. B 108 (2004) 19299–19306. [4] X. Chen, C. Burda, The electronic origin of the visible-light absorption properties of C-, N-, and S-doped TiO2 nanomaterials, J. Am. Chem. Soc. 130 (2008) 5018–5019. [5] W. Wang, M.O. Tad�e, Z. Shao, Nitrogen - doped simple and complex oxides for photocatalysis: a review, Prog. Mater. Sci. 92 (2018) 33–63. [6] G. Zhang, X. Ding, F. He, X. Yu, J. Zhou, Y. Hu, J. Xie, Preparation and photocatalytic properties of TiO2 - montmorillonite doped with nitrogen and sulfur, J. Phys. Chem. Solids 69 (2008) 1102–1106. [7] Y. Komai, K. Okitsu, R. Nishimura, N. Ohtsu, G. Miyamoto, T. Furuhara, S. Semboshi, Y. Mizukoshi, N. Masahashi, Visible light response of nitrogen and sulfur co-doped TiO2 photocatalysts fabricated by anodic oxidation, Catal. Today 164 (2011) 399–403. [8] N. Todorova, T. Vaimakis, D. Petrakis, S. Hishita, N. Boukos, T. Giannakopoulou, M. Giannouri, S. Antiohos, D. Papageorgiou, E. Chaniotakis, C. Trapalis, N and N, S-doped TiO2 photocatalysts and their activity in NOx oxidation, Catal. Today 209 (2013) 41–46. [9] S.M. El-Sheikh, G. Zhang, H.M. El-Hosainy, A.A. Ismail, K.E. O’Shea, P. Falaras, A. G. Kontos, D.D. Dionysiou, High performance sulfur, nitrogen and carbon doped mesoporous anatase-brookite TiO2 photocatalyst for the removal of microcystin-LR under visible light irradiation, J. Hazard Mater. 280 (2014) 723–733. � [10] A.J. Albrbar, V. Djoki�c, A. Bjelajac, J. Kova�c, J. Cirkovi� c, M. Mitri�c, D. Jana�ckovi�c, R. Petrovi�c, Visible-light active mesoporous, nanocrystalline N, S-doped and codoped titania photocatalysts synthesized by non-hydrolytic sol-gel route, Ceram. Int. 42 (2016) 16718–16728. [11] Y. Guo, T. Guo, J. Chen, J. Wei, L. Bai, X. Ye, Z. Ding, W. Xu, Z. Zhou, Synthesis of C-N-S co-doped TiO2 mischcrystal with an isobandgap characteristic and its photocatalytic activity under visible light, Catal. Sci. Technol. 8 (2018) 4108–4121. [12] A. koltsakidou, M. Antonopoulou, E. Evgenidou, I. Konstantinou, A.E. Giannakas, M. Papadaki, D. Bikiaris, D.A. Lambropoulou, Photocatalytic removal of fluorouracil using TiO2-P25 and N/S doped TiO2 catalysts: a kinetic and mechanistic study, Sci. Total Environ. 578 (2017) 257–267. [13] Q. Xiang, J. Yu, M. Jaroniec, Nitrogen and sulfur co-doped TiO2 nanosheets with exposed {001} facets: synthesis, characterization and visible-light photocatalytic activity, Phys. Chem. Chem. Phys. 13 (2011) 4853–4861. [14] P.V.R.K. Ramacharyulu, D.B. Nimbalkar, J. Praveen Kumar, G.K. Prasad, S. Ke, Ndoped, S-doped TiO2 nanocatalysts: synthesis, characterization and photocatalytic activity in the presence of sunlight, RSC Adv. 5 (2015) 37096–37101. [15] J. Wu, Photodegradation of rhodamine B in water assisted by titania nanorod thin films subjected to various thermal treatments, Environ. Sci. Technol. 41 (2007) 1723–1728. [16] S. Javed, M. Islam, M. Mujahid, Synthesis and characterization of TiO2 quantum dots by sol gel flux condensation method, Ceram. Int. 45 (2019) 2676–2679. [17] C. Jiang, K. Lee, C.M.A. Parlett, M.K. Bayazit, C.C. Lau, Q. Ruan, S.J.A. Moniz, A. F. Lee, J. Tang, Size-controlled TiO2 nanoparticles on porous hosts for enhanced photocatalytic hydrogen production, Appl. Catal. Gen. 521 (2016) 133–139.
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Enhancement of the photocatalytic activity of N-doped TiO2 nanograss array films by low-temperature sulfur doping Te Hu a, Jiancheng Yan b, Yifei Hu b, Tongyang Liu b, Sujun Guan c, Yun Lu d, Liang Hao b, e, f, *, Touwen Fan a, **, Dongchu Chen a a
College of Materials Science and Energy Engineering, Foshan University, No. 18 Jiangwan 1 Road, Foshan, 528000, PR China College of Mechanical Engineering, Tianjin University of Science & Technology, No. 1038, Dagu Nanlu, Hexi District, Tianjin, 300222, PR China Department of Physics, Tokyo University of Science, No. 1-3 Kagurazaka, Shinjuku-ku, Tokyo, 162-8601, Japan d Graduate School of Science and Engineering, Chiba University, No. 1-33, Yayoi-cho, Inage-ku, Chiba, 263-8522, Japan e Tianjin International Joint Research and Development Center of Low-carbon Green Process Equipment, PR China f Tianjin Key Lab of Integrated Design and On-line Monitoring for Light Industry & Food Machinery and Equipment, Tianjin, PR China b c
A R T I C L E I N F O
A B S T R A C T
Keywords: TiO2 nanograss array films Doping Low temperature Chemical dissociation Photocatalytic activity
Moderate in-situ chemical oxidation of Ti meshes was used to prepare nitrogen-doped (N-doped) anatase TiO2 nano-grass array films with urea as the nitrogen source. Further sulfur doping through immersion of the films in a thiourea solution at a relatively low temperature of 353 K was subsequently performed to enhance the visiblelight-driven photocatalytic activity of the films. SEM images confirmed that the loose microstructure of the nanograss arrays became compact after N-doping or N, S co-doping. N-doping and N, S co-doping decreased the band gap values of the TiO2 films and increased the effective visible light utilization. XPS analysis revealed that nitrogen mainly existed in interstitial form. Subsequent sulfur ions appeared as S6þ, S4þ, and S2 , with S6þ and S4þ replacing Ti4þ, and S2 replacing O2 , respectively. With an increase in the urea treatment time, the sulfur ions were oxidized, achieving higher valence states. S6þ substitution resulted in a charge imbalance, which could only be neutralized by hydroxide ions. The imbalance made it easier for water that had settled on the TiO2 films to chemically dissociate, and thereby generating hydroxyl radicals. OH� radicals as well as Ο�2 radicals played a significant role in the degradation of RhB dye. The RhB decolorization activity achieved by the N, S co-doped samples was 48 times that of the undoped samples.
1. Introduction Titanium dioxide (TiO2) doped with non-metallic elements, partic ularly nitrogen [1] and sulfur [2,3], has showed an outstanding visible-light response within a wide wavelength range and excellent visible-light-driven photocatalytic activity. It is generally known that doping of TiO2 could introduce additional electronic states above the valence band edge of pure TiO2 [4]. The additional electronic states result in a red shift in the absorption wavelength within the visible re gion of the spectrum, which leads to visible-light-driven photocatalytic activity [5]. Research studies on nitrogen and sulfur co-doping of TiO2 to improve visible-light-driven photocatalytic activity have been widely conducted [6–10]. However, N or/and S doping is almost always
performed under high temperature conditions so that N or/and S atoms can easily diffuse into the lattice of TiO2 [11–14]. Studies on nitrogen or sulfur doping of TiO2 at low temperatures have not been reported so far. In addition, the nature of the doping form and the competition between nitrogen and sulfur; the influence of nitrogen and sulfur doping on visible light absorption; the generation, separation, and recombination of photogenerated electron-hole pairs; and the surface chemical disso ciation ability are unclear. In this study, we prepared N-doped and N, S co-doped TiO2 nanograss array films on a Ti wire mesh with moderate in-situ chemical oxidation and facile solution immersion at a low temperature of 353 K. The competition and interaction of nitrogen and sulfur ions was inves tigated. The influence of nitrogen and sulfur doping on visible light
* Corresponding author. College of Mechanical Engineering, Tianjin University of Science & Technology, No. 1038, Dagu Nanlu, Hexi District, Tianjin, 300222, PR China. ** Corresponding author. E-mail address: [email protected] (L. Hao). https://doi.org/10.1016/j.mssp.2019.104872 Received 10 May 2019; Received in revised form 22 November 2019; Accepted 2 December 2019 Available online 5 December 2019 1369-8001/© 2019 Elsevier Ltd. All rights reserved.
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absorption, absorption properties of the films, and generation of active oxidizing radicals was qualitatively analyzed.
cycle evaluation was investigated under the same conditions. 3. Results and discussion
2. Experimental
Fig. 1 shows the XRD patterns of the calcined samples. From Fig. 1, it can be seen that the intensities of the diffraction peaks of titanium correspond to different crystal planes. On the other hand, the diffraction peaks of anatase TiO2 were rather weak, which could be ascribed to the small thickness of the formed anatase TiO2 films on the Ti wire mesh. The weak diffraction peak at about 25.3� of 2θ is ascribed to the crystal plane (101) of anatase TiO2 according to JCPDS No. 21–1272. It suggests that anatase TiO2 grew along the direction preferentially. In other words, the anatase crystals were exposed with (101) facets. Moreover, a slight shift of 2θ corresponding to the diffraction peak was also confirmed, which should be ascribed to N and S doping. The Raman spectra of the calcined samples are shown in Fig. 2. Five intense peaks at 149, 200, 400, 519 and 637 cm 1 were detected. They are consistent with the Eg, Eg, B1g, A1g þ B1g, and Eg vibrational modes of the anatase TiO2 phase [16,17]. Compared to single crystal TiO2 [18], anatase TiO2 exhibited a Raman band shift towards a higher wavenumber. The shift might be related to the decrease in crystal size. Meanwhile, the increase in intensities of the peaks could be ascribed to an increase in particle size. Two additional peaks at 248 and 284 cm 1 were detected for the U-80-96-S sample. They are attributed to the first order scatterings of non-stoichiometric Ti–N or Ti–S bonding [19,20]. It should be noted,
2.1. Preparation of N-doped TiO2 nanograss array films Moderate in-situ chemical oxidation was used to prepare N-doped TiO2 nanograss array films on a Ti wire mesh with a mesh number of 100 (wire diameter: 0.1 mm, purity: 99.8 wt%, Kangwei Co., Ltd., China). The preparation process was as follows. First, pieces of Ti wire mesh, each with a dimension of 60 mm � 50 mm, were successively cleaned with hydrochloric acid, acetone and distilled water for 5 min in an ul trasonic cleaner. The washed pieces of the Ti wire mesh were then dried in a vacuum drying oven at 333 K. Second, chemical oxidation was performed in a sealed vessel containing the five pieces of cleaned Ti wire mesh, 147 mL H2O2 solution (30 wt%, Reagent Co., Ltd, China), 3 mL HNO3 solution (65–68 wt%, Sinopharm Chemical Reagent Co., Ltd., China), and 0.909 g urea powder (purity: 99 wt%, Macklin Co., Ltd., China). The temperature of chemical oxidation was set as 353 K, and the oxidation was allowed to proceed for 48, 72 and 96 h. Finally, the asprepared samples were calcined at 723 K for 60 min with a heating rate of 5 K/min to transform amorphous TiO2 to anatase [15]. The above calcined samples were labeled as U-80-x, where x refers to heating holding time. For comparison, an undoped 80-x sample was also pre pared by the same preparation method, but without urea powder. 2.2. Preparation of S, N co-doped TiO2 nanograss array films To achieve further sulfur doping, before calcination, the U-80-x samples were placed in a sealed glass pot containing a 150 mL thiourea (purity: 99 wt%, Alfa Aesar Co., Ltd., China) solution with a concen tration of one mol/L. The sealed pot was placed in a drying oven at 353 K for 24 h. After that, the samples were heated at 723 K for 1 h. The above calcined samples were named U-80-x-S. 2.3. Characterization The crystal types of the samples were analyzed by X-ray diffraction (XRD) (D8 Advance, Bruker, Germany). The phase types were confirmed using laser confocal Raman spectroscopy (inVia, Renishaw, England). The surface morphology was observed by scanning electron microscopy (SEM) (Sigma 300, Zeiss, Germany). Light absorption tests were con ducted using UV–Vis diffuse reflectance spectroscopy (UV 3600Plus, Shimadzu, Japan). The surface elements and their chemical valences were analyzed using X-ray photoelectron spectroscopy (XPS) (Escalab 250Xi, Thermo Fisher Scientific, USA). The hydroxyl radicals (OH�) and superoxide radicals (Ο�2 ) were examined by electron paramagnetic resonance (EPR) spectroscopy (A300, Bruker, Germany). 2.4. Photocatalytic activity evaluation Rhodamine B (RhB) was chosen as the target degradation dye in the evaluation of the visible-light-responsive photocatalytic activity of TiO2 nanograss array films. First, sample pieces with dimensions of 30 mm � 25 mm were laid on the bottom of 50 mL glass beakers. Second, a 20 mL RhB solution with an initial concentration of about 10 mg/L was poured into the beakers containing the samples. For the RhB molecules to reach an adsorption-desorption equilibrium, the beakers containing the RhB aqueous solution and the samples were kept in darkness for 30 min. To remove ultraviolet light, an optical filter was inserted between the Xe lamp and the beakers to cut off light with a wavelength less than 420 nm. The optical power density of the visible light arriving at the beakers was measured to be approximately 45 mW/cm2. The decolorization of the RhB aqueous solution was monitored by a UV–Vis spectrophotometer (DR3900, Hach, USA) every 30 min. The photocatalytic activity in the
Fig. 1. XRD patterns of the calcined samples: (a)–(f) correspond to samples 80–48, U-80-48, U-80-72, U-80-96, U-80-48-S, U-80-72-S, and U-80-96-S, respectively. 2
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has been confirmed by other researchers [15]. The anatase TiO2 phase exhibited a morphology similar to that of a nanograss array. Fig. 4a shows the UV–Vis absorption spectra of the calcined samples. The band gap values were obtained with the Kubelka-Munk function [23]. Urea treatment and thiourea treatment changed the absorption threshold. Fig. 4b represents the corresponding Tauc plots [24–26] of these samples. From these plots, we could obtain the band gap values as shown in Table 1. We found that urea treatment decreased the band gap values, and further thiourea treatment reduced the band gap even further, except for U-80-96-S sample. This is related to N- and S-doping [1]. It is common knowledge that the band gap of anatase TiO2 is about 3.2 eV. Urea treatment decreased the band gap to 2.66–2.51 eV in a treatment-time dependent manner, i.e., with an increase in treatment time from 48 to 96 h, the band gap decreased further. We could conclude, from these results, that N-doping through urea treatment extended the visible light absorption of the as-prepared samples. With further thiourea treatment, the visible light absorption of the asprepared samples improved, and their band gap values further decreased, except that of sample U-80-96-S. In addition, the band gap values of the N, S co-doped samples increased as the thiourea treatment time increased. The unexpected increase in band gap should be ascribed to the doping state of sulfur. Generally, S-doping through thiourea treatment should be an efficient method for improving the visible light absorption of TiO2 nanograss array films. Fig. 5 shows the XPS spectra of Ti 2p and O1s of the calcined samples. In the Ti 2p spectra (Fig. 5a), two peaks located at 459.0 eV and 464.7 eV were detected for the undoped 80-48 sample. They were the typical peaks of Ti 2p3/2 and 2p1/2 [27]. Meanwhile, a negative shift of 0.1–0.2 eV was detected after the urea and thiourea treatment, indicating that the N-doping and S, N co-doping that resulted from urea and thiourea treatment might have affected the electronic density of the states around Ti. For samples U-80-96 and U-80-96-S, an obvious positive shift of 0.9–1.1 eV was also noted. Fig. 5b shows the O 1s spectra of these samples. The XPS peaks of O 1s could be perfectly divided into two peaks by fitting. For the undoped 80-48 sample, two peaks at 530.4 and 531.4
Fig. 2. Raman spectra of the calcined samples.
however, that the Raman shift of 248 cm 1 has been confirmed as belonging to the brookite phase [19,21,22]. Figs. 1 and 2 revealed that anatase TiO2 formed and a urea or thiourea treatment has no impact on the crystal type of the formed TiO2 films. Fig. 3 shows the surface SEM images of all the samples. Films with a thickness of about 5 μm covering the Ti wires looked like cracked tree bark (Fig. 3b). Although peeling of oxide films was observed, most of the films were so firm that they were not disrupted when an ultrasonic cleaner was used to remove the contaminants on their surfaces. The films look like grass arrays with nano-sized stems (Fig. 3c). The micro structure of the grass arrays is rather loose; as result, more active sites favoring photocatalytic redox are exposed. With urea treatment, the microstructure of the TiO2 nanograss array became compact (Fig. 3d–f). With further thiourea treatment, the TiO2 nanograss array films became even more compact (Fig. 3g–j). From Figs. 1–3, we could confirm that the anatase TiO2 phase formed on the surface of the Ti wire mesh. This
Fig. 3. SEM images of the calcined samples: (a–c) 80–48, (d–j) U-80-48, U-80-72, U-80-96, U-80-48-S, U-80-72-S, and U-80-96-S. 3
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Fig. 4. UV–Vis absorbance spectra (a) and the corresponding Tauc plots (b) of the calcined samples.
to Ti — O bonds and OH species. A great shift in binding energy cor responding to the Ti — O bonds was observed for samples U-80-96 and U-80-96-S. Peaks at around 457.4 eV corresponding to the Ti 2p3/2 of Ti3þ [29] were not discovered, which indicates that Ti3þ did not form on the surface of the TiO2 nanograss array films. Therefore, we could safely conclude that Ti exists in the form of Ti4þ. It is worth noting that the peak intensity corresponding to the OH species (531.3–533.2 eV) reduced with an increase in urea treatment time. On the other hand, thiourea treatment increased the peak intensity corresponding to OH for samples U-80-72 and U-80-96. The XPS spectra of N 1s for the calcined samples are shown in Fig. 6. For the urea-treated samples (Fig. 6a), one peak located at 399.8 or 400.0 eV was detected. It is reported that XPS peaks ranging from 398 to 402 eV should be assigned to the interstitial N [30,31]. Peaks ~ 397 eV suggest the formation of N — Ti – O linkages, indicating the substitution of the N ion for O ion [32]. Therefore, we could confirm that N-doping was complete, and nitrogen mainly existed in an interstitial form. With an increase in processing time, the corresponding XPS peak reduced,
Table 1 Absorption threshold and band gap of the calcined samples. Sample No.
Absorption threshold (nm)
Band gap (eV)
80–48 U-80-48 U-80-48-S 80–72 U-80-72 U-80-72-S 80–96 U-80-96 U-80-96-S
425 466 539 431 470 481 429 494 479
2.92 2.66 2.30 2.88 2.64 2.58 2.89 2.51 2.59
eV were detected; they were ascribed to the lattice oxygen in Ref. [22] and surface hydroxyl species [28], respectively. For U-80-48 sample, three peaks located at 530.3, 531.3, and 532.6 eV were found. The first peak was attributed to Ti — O bonds, and the last two were attributed to OH species. For U-80-48-S sample, the two peaks were also attributed
Fig. 5. XPS spectra of the calcined samples: (a) Ti 2p and (b) O 1s. 4
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Fig. 6. XPS spectra of N 1s for: (a) the N-doped and (b) the N, S co-doped samples.
replaced Ti4þ [33]. Meanwhile, Zhang et al. believed that the peak should be attributed to S4þ [34]. The second one is attributed to S2 , which could correspond to Ti–S [33–35]. This also suggests that sub stitution of oxygen by sulfur in the lattice of anatase TiO2 occurred. The last one located at 159.2 eV should be attributed to the Ti–S bond, which means that S2 replaced O2 [36]. For U-80-96-S sample, only one peak located at 170.0 eV was detected. It is generally believed that the peak should be ascribed to S6þ [37]. A positive shift of binding energy cor responding to the S6þ XPS peak suggests the electronic density decreased around S ions. From the above analysis, we could conclude that the sulfur in TiO2 films mainly exists in the form of substitutional S2 ions and as a small amount of S4þ and S6þ ions in sample U-80-48-S. When urea treatment was extended to 72 h (U-80-72-S), the doping amount of S4þ/S6þ was roughly equivalent to that of S2 . With a further increase in the treatment time to 96 h, S4þ and S2 ions disappeared, and only substitutional S6þ ions remained. In other words, the sulfur ions were oxidized from S2 to S4þ and finally S6þ as urea treatment increased. The valence state change also affected the optical absorption capacity and band gap values of the films. The ionic form of sulfur is dependent on the preparation method. However, the substitution of Ti4þ by S6þ is more chemically favorable than that of Ti4þ by S4þ and O2 by S2 . This is because S2 (1.7 Å) has a larger radius than O2 (1.22 Å) [38]. When S6þ ions replace Ti4þ ions in the lattice of TiO2, a charge imbalance is created. This would result in extra positive charges on the films that could only be neutralized by hydroxide ions [39]. The EPR signals of the OH� radicals in the surfaces of the calcined samples under visible light irradiation were recorded, and with the re sults depicted in Fig. 8. The characteristic quartet peaks of DMPO- OH� adducts with an intensity ratio of 1:2:2:1 were observed for all the samples. From Fig. 8a, the intensity of the EPR signals of sample 80–48 are similar to that of sample U-80-48. On the other hand, the intensity of the EPR signals of sample U-80-48-S is much greater than that of sample U-80-48. In other words, N-doping has little impact on the generation of OH� radicals, while S-doping has a considerable impact. The identical results shown in Fig. S1 were also obtained for other N-doped and N, S co-doped samples. They suggest that S-doping rather than N-doping could result in the generation of more OH� radicals. The influence of urea treatment time on the production of OH� radicals was also studied, and our findings are shown in Fig. 8b. Although the doping amounts of nitrogen or sulfur decreased (Figs. 6 and 7), the generated OH� radicals
which suggests that the doping amount of nitrogen became smaller. In other words, nitrogen ions diffused out from the lattice of TiO2. After further thiourea treatment (Fig. 6b), the intensities and binding energy corresponding to the peaks of N 1s seem to be invariant compared to those before thiourea treatment. This indicates that further thiourea treatment has no impact on nitrogen doping. Fig. 7 shows the XPS spectra of S 2p of the urea and thiourea-treated samples. For samples U-80-48-S and U-80-72-S, three peaks located at 168.8, 164.5, and 159.2 eV of binding energy were detected. The first peak is believed to be due to the 2p3/2 of S4þ and S6þ, which could be attributed to the linking of S — O [33]. Furthermore, S4þ and S6þ
Fig. 7. S 2p XPS spectra of the calcined samples: (A) (B) (C) correspond to U80-48-S, U-80-72-S, and U-80-96-S, respectively. 5
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Fig. 8. EPR signals of OH� radicals for the calcined samples under visible irradiation.
increased contrarily. Fig. 9 shows the EPR spectra of DMPO-Ο�2 signals on the surfaces of the calcined samples. From Fig. 9a, the intensity of the DMPO-Ο�2 sig nals of U-80-48 sample is much greater than that of sample 80–48. On the other hand, the difference in intensity between samples U-80-48 and U-80-48-S is not obvious. This suggests that N-doping greatly promoted the generation of Ο�2 radicals. S-doping, however, had a negligible in fluence on the generation of Ο�2 radicals. Similar results were likewise obtained for the other samples (Fig. S2). For N, S co-doped samples, the intensity of the DMPO-Ο�2 signals for U-80-72-S sample was the greatest although the doping amount of nitrogen was smaller than that of U-8048. Fig. 10 shows the EPR signals of the OH� radicals of the calcined samples in darkness. The EPR signals of the OH� radicals for N, S codoped samples have a greater intensity than those for the undoped and N-doped samples. Generally, without light irradiation, holes required for the generation of OH� radicals are not formed. Water could physically settle on TiO2 films, and some molecules might chemically dissociate to form Hþ ions and OH groups. Chemically dissociated OH groups attach to the surface of TiO2 films, especially N, S co-doped TiO2 films. This is supported by the XPS results shown in Fig. 5b. The chemically dissociated OH groups were captured by the DMPO trap ping reagent, and that produced the EPR signals of the DMPO- OH�. As described above, the substitution of Ti4þ with S6þ would produce extra positive charges that could only be neutralized by OH groups. Sample U-80-96-S showing the greatest EPR signal intensity possesses the largest ratio of S6þ ions. Therefore, we confirmed that water can easily dissociate on N, S co-doped TiO2 films. The photodegradation of the RhB aqueous solution during photo catalytic degradation evaluation was recorded (Fig. 11a). For undoped
Fig. 10. EPR signal of OH� radicals of the calcined samples in darkness.
TiO2 films (80-x), the decolorization of the RhB solution was no more than 10% within 150 min of visible light irradiation. As for the N-doped samples (U-80-x), the decolorization increased to 30–80% within the same irradiation time. However, the decolorization of the RhB solution due to N, S co-doped sample (U-80-x-S) was the highest; in particular, with sample U-80-96-S, the decolorization increased to 90%. Fig. 11b shows the rate constants for RhB degradation under visible-light irra diation. For the undoped samples 80–48, 80–72, and 80–96, the rate
Fig. 9. EPR signals of Ο�2 radicals for the calcined samples under visible light irradiation. 6
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Materials Science in Semiconductor Processing 108 (2020) 104872
Fig. 11. (a) Concentration evolution of the RhB solution and (b) corresponding plot of Ln (C0/C) versus irradiation time.
constants were 9.77 � 10 4, 5.73 � 10 4, and 3.92 � 10 4, respectively. For the N-doped samples U-80-48, U-80-72, and U-80-96, the rate con stants were 3.65 � 10 3, 7.12 � 10 3, and 12.43 � 10 3, respectively. The RhB decolorization activity of these corresponding samples increased by 2.73, 11.42, and 30.7 times through nitrogen doping. For the N, S co-doped samples U-80-48-S, U-80-72-S, and U-80-96-S, the rate constants increased to 1.32 � 10 2, 1.14 � 10 2, and 1.93 � 10 2, respectively. Sulfur doping increased the RhB decolorization activity of the corresponding N-doped samples by 262%, 60%, and 56%. Fig. 12 shows the photocatalytic degradation tests of RhB, with sample U-80-96S showing the highest photocatalytic activity. It could be observed that the RhB decolorization activity hardly decreased, which could ensure the long service life of the films. Through low-temperature chemical oxidation, TiO2 nanograss array films on a Ti wire mesh were prepared. The microstructure of the loose nanograss array favors photocatalytic redox. With urea and thiourea treatment, N-doping and N, S co-doping were achieved. N-doping decreased the band gap of the undoped samples and N, S co-doping further reduced the band gap of the N-doped samples. The visible light absorption of TiO2 films was enhanced, and the effective utilization of visible light increased. This explains why the visible-light-responsive photocatalytic performance of these TiO2 films increased. The substi tution of Ti4þ with S6þ produced extra positive charges, which could only be neutralized by hydroxide ions. Compared to the S2 and S4þ, S6þ seems to be more beneficial for photocatalytic activity enhancement. Moreover, the increase in the amounts of OH� and Ο�2 radicals owing to N-doping and N, S co-doping can also account for the enhanced photo catalytic activity of these TiO2 films.
Fig. 12. Recycle photodegradation evaluation of the RhB solution under visible light.
charges that made it easier for water to chemically dissociate into Hþ and OH . The RhB decolorization capacity achieved by the N, S codoping was 48 times that of the undoped sample. Author statement
4. Conclusions
Te Hu: Characterization and photocatalytic activity evaluation. Jiancheng Yan: Material preparation. Yifei Hu and Tongyang Liu: Software, data analysis. Sujun Guan and Yun Lu: data analysis and discussion. Liang Hao and Touwen Fan: Conceptualization, methodology, and writing of manuscript. Dongchu Chen: supervision.
N-doped anatase TiO2 nanograss array films were prepared by moderate in-situ chemical oxidation on a Ti wire mesh. Sulfur doping (Sdoping) of N-doped TiO2 films was realized by thiourea solution im mersion at a low temperature of 353 K. Nitrogen doping (N-doping) and S-doping enhanced the visible light absorption of these TiO2 films to a certain degree and decreased their band gap values. Dopant nitrogen ions mainly existed in interstitial form. With an increase in urea-treated time, the doping amount of nitrogen in TiO2 films decreased. The sub sequent S-doping had a negligible effect on the dopant nitrogen. Dopant sulfur ions in TiO2 films appeared in three states: S6þ, S4þ, and S2 . The first two kinds of ions could replace Ti4þ while the last one replaced O2 . As the urea treatment time increased, the ions were gradually oxidized. Both OH� and Ο�2 radicals should play a significant role in the degra dation of RhB under visible light irradiation. N-doping and S-doping, especially S6þ-doping, resulted in the generation of extra positive
Declaration of competing interest The authors declared that they have no conflicts of interest to this work. Acknowledgments The National Natural Science Foundation of China of China (No. 51404170) and the Key Project of Department of Education of 7
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Guangdong Province (No. 2016GCZX008) supported this work. Te Hu and Jiancheng Yan equally contributed to this work.
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