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Adsorption-assisted photocatalytic activity of nitrogen and sulfur codoped TiO2 under visible light irradiation† Junho Chung,a Jae Woo Chung*b and Seung-Yeop Kwak*a Applying post thermal treatment on the doped TiO2 at high temperature is mostly regarded as an indispensable process, although it has negative effects on the photocatalytic activity of doped TiO2. Herein, we synthesized the N- and S-codoped TiO2 (NSTs) with an anatase phase using a simple solvothermal treatment and investigated their visible light photocatalytic activity associated with the thermal behavior of dopants in NSTs. We found that the as-synthesized NST (NST-As) has better visible light photocatalytic activity and adsorptivity than the commercially available P25 and the thermally treated NSTs. The S dopants effectively assist the surface reaction by adsorbing cations of organic dyes on the NST-As surface. The N dopants increase the absorbance at visible light region of NST-As by

Received 21st April 2015, Accepted 4th June 2015

forming a delocalized state at the band gap of NST-As. However, the photocatalytic activity of NSTs

DOI: 10.1039/c5cp02322j

transformed from sulfide to sulfate during the thermal treatment and N dopants move out during the

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crystallization of TiO2. The adsorption-assisted photocatalytic activity of NST-As under visible light irradiation is an attractive feature for environmental and photonic technologies.

gradually weakens with the post thermal treatment, because S dopants on the NST-As surface are

1 Introduction Over the past several decades, photocatalysts have been extensively studied as promising materials for various applications such as solar energy conversion, sensing, air and water purification, and stain prevention.1,2 Among them, titanium dioxide (TiO2) has significantly attracted attention due to its outstanding photocatalytic activity, chemical and photochemical stability, non-toxicity, and low cost.3,4 TiO2, however, is typically inactive under visible light irradiation because its band gap energy is large (3.2 eV for the anatase phase). Given that visible light constitutes 45% of the sunlight, a lack of absorbance in this region is a critical problem for practical use of TiO2.5 Hence, many efforts have been made to improve the visible light photocatalytic activity of TiO2.6 a

Department of Materials Science and Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-744, Korea. E-mail: [email protected]; Fax: +82-2-885-9671; Tel: +82-2-880-8365 b Department of Organic Materials and Fiber Engineering, Soongsil University, 369 Sangdo-ro, Dongjak-gu, Seoul 156-743, Korea. E-mail: [email protected]; Fax: +82-2-817-8346; Tel: +82-2-828-7047 † Electronic supplementary information (ESI) available: XPS spectra of nitrogen and sulfur dopants of NST-250 and NST-350, absorbance in the visible light region and band gap energies of P25, NST-As, NST-350, and NST-400, UV-Vis spectral changes of rhodamine B solution under visible light irradiation and in the presence of NST-350 and NST-400, relative concentrations of methylene blue in a dark room and under visible light irradiation; and relative atomic ratios of sulfur dopants in the NSTs, measured by ICP-AES. See DOI: 10.1039/c5cp02322j

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One approach for improving the visible light photocatalytic activity of TiO2 is incorporating dopants, such as transition metals (Fe, Mo, Ru, Os, Re, V, and Rh)7–14 or non-metals (B, C, N, O, F, and S).15–27 These dopants enhance the visible light absorbance by forming a delocalized state or intra-band states within the band gap and promoting the participation of charge carriers in photocatalytic reaction by capturing charge carriers.16 However, the transition metal dopants can act as recombination centers that hinder the photocatalytic reaction, so the quantity of the doped elements must be strictly limited not to reduce the photocatalytic activity.10–14,16 Unlike the transition metal dopants, non-metal dopants show less recombination between holes and electrons.12,18 The elements N and S, in particular, act as efficient dopants because they effectively reduce the band gap energy of TiO2 by forming a delocalized state in the TiO2 band gap without promoting charge carrier recombination, in comparison to other non-metal dopants.28,29 N- and/or S-doped TiO2 can be easily synthesized using wet chemistry methods, such as sol–gel or solvothermal processes.28 Typically, the as-synthesized N- and/or S-doped TiO2 do not show the visible light photocatalytic activity. Thus, to improve their photocatalytic activity under visible light, most researchers apply a thermal post-treatment at high temperatures (4350 1C) to the as-synthesized doped TiO2. Unfortunately, the N- and/or S-dopants that are present within TiO2 easily move to TiO2 surface during the thermal treatment, resulting in a significant

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loss of dopants.18,30 Besides, the thermal post-treatment causes a decrease in the specific surface area of TiO2. Such thermal post-treatment inevitably deteriorates the visible light photocatalytic activity of the N- and/or S-doped TiO2.30,31 The high cost and energy consumption during these steps also restrict their practical utility.32,33 Several attempts have been made to develop a visible light active doped TiO2 without deteriorating the properties by a high-temperature thermal post-treatment.15,34–36 Zhao et al.34 reported that N-doped TiO2 treated at low temperatures (200–250 1C) could display a high visible light photocatalytic activity despite its low crystallinity. Changseok et al.35 reported that the photocatalytic activity of an S-doped TiO2 film could be higher after a low-temperature thermal treatment than after a high-temperature thermal treatment. Several studies have also reported the doped TiO2 with a high visible light photocatalytic activity via a simple solvothermal treatment even without the post thermal treatment.15,26 Nevertheless, it is still in discussion about how the post thermal treatment at low temperatures (or the absence of the post thermal treatment) can provide the N and/or S doped TiO2 with excellent visible light photocatalytic activity. Therefore, it is very important to elucidate the effect of the post thermal treatment temperature on the chemical and physical states of the dopants in the TiO2 and concurrently on the photocatalytic activity of the N and/or S doped TiO2. Herein, the N- and S-codoped TiO2 (NSTs) were solvothermally synthesized. The as-synthesized NST (NST-As) was thermally treated at different temperatures (200, 250, 300, 350, and 400 1C). Interestingly, the NST-As, i.e., non-post thermally treated NST, showed excellent visible light photocatalytic activity compared to other NSTs that underwent the post thermal treatment. To identify the high performance of NST-As, we rationalized the thermal behavior of the N and S dopants and discussed about the effects of the dopants. It was found that such a high photocatalytic activity of the NST-As was attributed to the enhanced surface reaction and solution bulk reaction rates; this originated from the synergetic adsorption–photodecomposition introduced by the S dopant on the TiO2 surface, and the high solution bulk reactivity by the N dopant which decreased the band gap energy. To the best of our knowledge, this is the first report to elucidate the origin of the high visible light photocatalytic activity observed in as-synthesized N- and S-codoped TiO2. This makes N- and S-codoped TiO2 tremendously attractive as an eco-friendly, cost-effective, high-performance photocatalyst for applications ranging from water purification involving adsorption and decomposition of organic pollutants, to protection of humans from hazardous materials such as chemical warfare agents.

2 Experimental section 2.1

Materials

Titanium(IV) isopropoxide (TTIP, Ti(OPri)4), 2,4-pentanedione (AcAc, C5H8O2), thiourea, and absolute ethanol were all purchased from Sigma-Aldrich and were used as received without

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further purification. Highly deionized water with a resistivity of 18.0 MO cm1 was used throughout the experiments. 2.2

Preparation of N-, and S-codoped TiO2

0.02 mol of TTIP and 0.04 mol of AcAc were dissolved in 30 mL of absolute ethanol. As a dopant source, 0.02 mol of thiourea was sufficiently dissolved in 70 mL of absolute ethanol. The solutions were mixed under vigorous stirring, followed by the addition of deionized water (2 mL) to the mixed solution. After 10 minutes, the solution was transferred to a 170 mL Teflonlined stainless-steel autoclave, and the solvothermal treatment was performed at 115 1C for 12 h. The resulting yellow powder was collected and washed several times with deionized water and methanol by centrifugation. The washed sample was dried in a vacuum oven at 50 1C for 24 h. The as-synthesized N- and S-codoped TiO2 samples were individually thermally treated at 200, 250, 300, 350, and 400 1C for 2 h (the rate of the temperature increase was 5 1C min1) in air. These N- and S-codoped TiO2 samples were denoted as NST-As and NST-x (where x indicates the thermal treatment temperature). 2.3

Characterization

The crystal structure and sizes of the NSTs were investigated using X-ray diffraction methods (XRD, New D8 Advance) over the 2y range of 20–801 using Cu Ka radiation as the X-ray source (l = 0.154 nm). The quantitative crystallite sizes were obtained using the Scherrer equation (F = Kl/b cos y), where F is the crystallite size, l is the wavelength of X-ray radiation, K is usually taken as 0.89, b is the full width at half-maximum intensity (FWHM), and y is the diffraction angle of the (101) peak for the anatase phase. The morphology was observed using high-resolution transmission electron microscopy (HR-TEM, JEM-3010) at a 300 kV electron acceleration voltage. The specific surface area was characterized using nitrogen adsorption–desorption isotherm measurements fitted to the Brunauer–Emmett–Teller (BET) equation. X-ray photoelectron spectroscopy (XPS, AXIS-Hsi) was performed using monochromatic Mg Ka radiation as the X-ray source. All binding energies were calibrated to the C1s peak at 284.5 eV. Prior to conducting the XPS measurements, the surfaces of the NSTs were argon-etched for 10 minutes at a sputtering rate of 0.3 nm min1 to verify the chemical states and quantities of the dopants inside the NSTs. The atomic quantities of dopants in the NSTs, especially S, after thermal treatment were determined using inductively coupled plasma atomic emission spectroscopy (ICP-AES, Optima 4300DV) employing an argon plasma source operated at 6000 K. The visible light absorbance spectra of the NSTs were measured using UV-Vis diffuse reflectance spectroscopy (UV-DRS, Cary 5000). The measured wavelength range was 350–800 nm and the scan speed was 600 nm min1. 2.4

Photocatalytic activity evaluation

The visible light photocatalytic activities of the NSTs were evaluated by measuring the degree of rhodamine B (RhB) and methylene blue (MB) degradation in an aqueous solution under visible light irradiation. RhB was selected as a model compound because it is a common poisonous contaminant present in

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industrial wastewater.37 MB was selected because it strongly adsorbs onto the surfaces of metal oxides.38 Prior to conducting the measurements, the NST powders were dispersed in water by sonication for 30 minutes and UV light was applied to the NST aqueous solution to remove residual organic by-products from the NSTs. The UV light irradiating chamber was made up of four lamps producing 15 W UV-A and a quartz glass beaker. The temperature of the chamber was 50 1C and the distance between each lamp and the beaker was 120 mm. The visible light photocatalytic activities of the NSTs were examined using a visible light photochemical reactor consisting of one commercial 400 W halogen spotlight and a quartz glass beaker. The samples were composed of 150 mL of deionized water, 60 mg of NSTs, and 1.5 mg of RhB or MB. The reactor temperature was maintained at 36  1 1C using four air coolers, and the distance between the lamp and the beaker was 200 mm. The reproducibility of NST-As in RhB solution was evaluated over 4 cycles under the same visible light irradiation conditions. The adsorptivity was evaluated under the conditions used to measure the photocatalytic activity, except for that the quartz glass beaker was wrapped in aluminum foil to block the visible light. As a reference, the photocatalytic activity of the commercial TiO2 (P25) was examined under the experimental conditions used to evaluate the photocatalytic activities of the NSTs.

3 Results and discussion 3.1

Crystal structures and specific surface areas of the NSTs

X-ray diffraction (XRD) measurements were performed to investigate the crystal structures of the NSTs. As shown in Fig. 1, all NSTs showed peaks at 2y = 25.4, 38.0, 48.1, 54.5, and 62.7 corresponding to an anatase TiO2 without rutile and brookite phases. NST-As exhibited weak, broad anatase TiO2(101) and (200) peaks, which indicates that the anatase crystals did not grow extensively. This trend was maintained under the thermal treatment at 300 1C. However, thermal treatment at temperatures exceeding 350 1C resulted in a marked increase in the anatase characteristic

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peak intensities. This indicates that the NST crystallite size and crystallinity increased significantly. The quantitative crystallite sizes of the NSTs were calculated using the Scherrer equation based on the anatase peak (101). As listed in Table 1, NST-350 and -400 displayed crystals that were twice the size of the crystals observed in the case of NST-As, -200, -250, and -300. These results were further confirmed by characterizing the morphology and crystal structure of the NSTs using high-resolution transmission electron microscopy (HR-TEM, see Fig. 2). Fig. 2 clearly reveals that the NSTs were composed of agglomerated small crystals and the anatase (101) plane was dominant in the nano-size crystals. NST-350 and NST-400, in particular, exhibited clear crystal structures compared to the NSTs that were thermally treated at temperatures below 300 1C, which is in good agreement with the XRD results. From N2 adsorption and desorption analyses, we found that the surface areas of NSTs were larger than the surface area of P25 (Table 1). Some studies reported that thiourea or sulfate ions promote the dispersion of TiO2 and increase the surface area of TiO2.27,39 Thus, the high surface area of the NSTs was thought to be attributed to the high dispersion of TiO2 by thiourea, during the solvothermal process. However, post thermal treatment of the NSTs negatively influenced the specific surface areas. The surface areas of the NSTs gradually decreased with thermal treatment at 200–300 1C, and NST-350 and NST-400 showed dramatic decreases in their surface areas. In general, at low temperatures, the growth of TiO2 crystals is mainly dominated by an interface nucleation,31 so the small decrease in surface areas of NST-200, -250, and -300 was thought to be attributed to the restrictive agglomeration between TiO2 nanocrystals via the interfacial crystallization. On the other hand, TiO2 nanocrystals were drastically merged into larger particles at high temperature by a rapid crystallization rate, so the gross agglomeration between TiO2 nanocrystals occurred at 350 1C to 400 1C. As a result, the NST surface area decreased considerably with the overall crystallization of anatase crystals at high temperatures exceeding 350 1C. These results supported the XRD results, which showed a distinct increase in the crystal sizes of NST-350 and NST-400. 3.2

Fig. 1 XRD patterns of (A) NST-As, (B) NST-200, (C) NST-250, (D) NST-300, (E) NST-350, and (F) NST-400.

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Thermal behavior of the NST dopants

The effects of the thermal post-treatment on the quantities and chemical states of the dopants in the NSTs were investigated using X-ray photoelectron spectroscopy (XPS) analysis. The chemical states of the dopants inside the NSTs were explored by etching the NST surfaces with argon. Ar+ etching is an efficient method for removing the top few nanometers of the surface shell to facilitate the observation of dopants at inner sites within the TiO2. Fig. 3(A) and (B) show the chemical states of the N dopants at the surfaces and inside the NSTs, respectively. In Fig. 3(A), all NSTs show the N1s binding energy peaks beyond 400 eV, suggesting that the N dopants on the TiO2 surface were bound to oxygen atoms, i.e., the formation of Ti–N–O and/or Ti–O–N linkages, regardless of the thermal treatment temperature.18 In the case of the N dopants inside NST-As (after Ar+ etching of the NST-As), an additional peak at a the lower binding energy (396 eV) was observed as well as the binding energy peak

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Table 1

PCCP Crystal sizes and surface areas of the NSTs and P25

Sample

P25

NST-As

NST-200

NST-250

NST-300

NST-350

NST-400

Crystal size (nm) Surface area (m2 g1)

— 51.4

4.21 291.5

3.85 253.9

4.77 242.3

4.62 232.7

6.54 207.2

8.45 121.3

Fig. 2 HR-TEM images of (A) NST-As, (B) NST-200, (C) NST-350, and (D) NST-400. The inset data were obtained from a FFT analysis (scale bar = 5 nm).

beyond 400 eV. This means that some of the N dopants replaced the oxygen atom and were bound to three titanium atoms inside NST-As, instead of binding with oxygen atoms. However, the peaks at 396 eV corresponding N dopants that replaced the oxygen atom inside NST-As disappeared at higher thermal posttreatment temperatures (see NST-300 and NST-400 in Fig. 3(B)). In addition, the quantitative atomic concentrations of the N dopants, evaluated from the XPS results, showed that the amount of N dopants in both areas of the NST gradually

disappeared as the thermal treatment temperature increased. Thus, it was thought that the N dopants shifted out of the NST particles, rather than accumulating at the NST surface. In Fig. 3(C) and (D), S dopants inside and on the surface of NST-As show the binding energy peaks at 162 eV and 165 eV, respectively. The binding energy of 162 eV observed inside NST-As was regarded as the sulfide (S2) peak, indicating that S dopants replaced the oxygen atoms of TiO2 and bonded to three titanium atoms. The binding energy of 165 eV observed at the surface of NST-As was probably ascribed to the insufficiently bonded S dopants to three titanium atoms; this was neither the sulfide (S2) nor the sulfate (S6+) peak observed in most studies of typical S-doped TiO2. Typically, it is known that S dopants on the TiO2 surface are hard to be perfectly bound to the three titanium atoms due to the unstable nature of the surface and the large ionic radius of sulfur.23 Thus, the lack of bonding to titanium atoms with low electronegativity can lead to the increase in the binding energy of 2p electrons of S dopants at the surface of NST-As. When the thermal treatment temperature was higher than 300 1C, the binding energy of 162 eV completely disappeared inside TiO2 (Fig. 3(D)). In addition, Fig. 3(C) exhibits a peak at around 165 eV disappearing at 200 1C, and the peak at around 170 eV corresponding to a sulfate group was rapidly developed at 300 1C and above. We note that the quantities of S dopants at the surface of NST-As markedly increased, while those inside NST-As decreased at high temperatures above 350 1C (see Table 2). These results suggest that the sulfide-formed S dopants inside TiO2 moved to the surface of TiO2 and changed into the sulfate-formed S dopants, with the increase in the thermal treatment temperature. These results were further confirmed by ICP-AES measurements. The ICP-AES results showed that the relative quantities of S, i.e., the total atomic ratio of the NSTs (S/Ti), remained constant irrespective of the thermal

Fig. 3 XPS spectra of (A) nitrogen (surface), (B) nitrogen (inside), (C) sulfur (surface), and (D) sulfur (inside). The sample names are NST-As (1st line), NST-200 (2nd line), NST-300 (3rd line), and NST-400 (4th line).

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Table 2 Atomic ratios of the nitrogen (N/Ti at%) and sulfur dopants (S/Ti at%) at the surface and inside of the NSTs, obtained from the XPS and ICP-AES results

Sample

NST-As NST-200 NST-250 NST-300 NST-350 NST-400 a

N/Ti (surface) 11.49 6.29 N/Ti (inside)a S/Ti (surface)a 1.26 a 1.25 S/Ti (inside) 0.99 S/Ti (total)b

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a b

9.42 8.58 — 1.27 1.00

8.8 7.16 — 0.73 0.97

9.29 8.17 1.16 — 1.06

6.14 2.21 2.02 — 1.05

8.67 2.56 3.75 — 1.07

The atomic ratios of N and S dopants obtained from the XPS results. The atomic ratio of S dopants obtained from the ICP-AES results.

treatment temperature (Table 2). These results support the fact that most of the S dopants inside TiO2 moved toward the TiO2 surface at high temperatures, rather than escaping from the TiO2 as in N dopants. 3.3

Visible light absorbance of the NSTs

The visible light absorbance spectra of the NSTs and P25 were obtained using UV-Vis diffuse reflectance spectroscopy (UV-DRS). As shown in Fig. 4, all NSTs exhibited higher visible light absorbance than P25. In particular, NST-As showed a noticeable red shift in the visible light region, whereas NST-350 and NST-400 showed smaller red shifts. Although NST-200, -250, and -300 apparently showed higher absorbance in the visible light region, compared to NST-As, this may be ascribed to the presence of the carbonized organic residue in NST-200, -250, and -300 by mild thermal treatment.34 Thus, the band gap energies of NST-As, -350, and -400 were calculated using the Kubelka–Munk equation as follows FðRÞ ¼

ð1  RÞ2 : 2R

(1)

where R is the reflectance. The band gap energy was determined by extrapolating the linear portion of the graph. Their band gap energies (NST-As = 3.00 eV, NST-350 = 3.23 eV, and NST-400 = 3.23 eV) were lower than that of commercially available P25 (3.33 eV, see Fig. S1, ESI†). It has been reported that N dopants

Fig. 4 Absorbance spectra of the NSTs and P25 in the visible light region.

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dominantly reduce the band gap energy of TiO2 in the N-and S- codoped system.27,40 Because a relatively higher amount of N dopants was doped in the inner oxygen sites and the surface of NST-As, compared to NST-350 and -400 (see Table 2), the NSTAs had the lowest band gap energy and the highest visible light absorbance among NSTs. In the case of NST-350 and -400, although N dopants moved out from the TiO2 at high temperatures, NST-350 and -400 showed lower band gap energy compared to P25 because N and S dopants were still located on the surface of NST-350 and -400 (see Table 2). Generally, a high photocatalytic activity under the visible light irradiation requires a band gap energy of around 2.0 eV, which is much lower than the band gap energy of the NST-As. Nevertheless, the UV-DRS spectra of NST-As showed a continuous curve until its absorbance reached zero at 660 nm corresponding to around 1.9 eV, instead of showing a rapid decrease in absorbance across the visible light region. The continuous absorbance spectra for NST-As could be explained by the broad size distribution of the small anatase nanocrystals. A broad crystal size distribution can provide TiO2 with various band gaps, because the band gap energy of anatase TiO2 nanocrystals is strongly dependent on the crystal size in a few nanometer scales.41,42 Accordingly, NST-As can lead to the overlap of various absorbance spectra of nanocrystals, resulting in the long-ranged continuous visible light absorbance spectra. On the other hand, NST-350 and -400 had relatively large crystal sizes, which were not affected by the quantum effect and showed a sharp decrease of absorbance in the visible light region. Thus, the longrange continuous absorbance of NST-As till the visible light region suggests that the NST-As acts as an efficient visible light active photocatalyst. 3.4

Photocatalytic activity

The photocatalytic activities of the NSTs and P25 were evaluated by measuring the decrease in rhodamine B (RhB) in an aqueous solution under visible light irradiation. To ensure that the decrease in RhB is mainly induced by the photocatalytic activity of the NSTs, the change in the magnitude of the RhB concentration was measured under the dark conditions in the presence of the NST solutions, prior to conducting the photocatalytic activity evaluation tests. As shown in Fig. 5(A), the RhB concentration did not show a noticeable change under the dark conditions, with the exception of NST-As that showed a 13% decrease in the quantity of RhB. In contrast, the concentration of RhB dramatically decreased under the visible light irradiation (Fig. 5(B)). This reveals that RhB molecules are predominantly degraded by the photocatalytic decomposition under the visible light irradiation. The photocatalytic activities of the NSTs and P25 were determined by directly calculating the magnitude of the RhB decrease. The degradation rate was calculated using the pseudofirst-order equation as follows   C0 Ka t ¼ ln : (2) C where Ka is the rate constant, t is the reaction time, C0 is the initial concentration of RhB, and C is the interval concentration

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Fig. 5 Relative concentration of (A) rhodamine B in a dark room and (B) under the visible light irradiation and (C) photocatalytic activity of NSTs and P25 under the visible light irradiation, and (D) the reproducibility evaluation results of NST-As.

of RhB. The RhB degradation rates of NSTs and P25 under visible light irradiation are presented in Fig. 5(C). The RhB degradation rate of NST-As was 1.793 h1, which is 15.3 times the rate measured in the presence of P25 (0.117 h1). NST-200, -250, and -300 also showed three times higher visible light photocatalytic activities, compared to the activity observed in the presence of P25. On the other hand, NST-350 and -400 showed a relatively low activity compared to P25. Typically, two photodegradation pathways are available for rhodamine B: a solution bulk reaction and a surface reaction.43 The electrons in the valence band of TiO2 that are excited under the light irradiation formed hydroxyl radicals capable of promoting the solution bulk reaction, and the hydroxyl radicals directly photodecompose RhB in the aqueous solution. On the other hand, the surface reaction is induced by electrons in the RhB molecules. In this reaction, RhB with the excited electrons is first adsorbed onto the TiO2 surface under visible light irradiation. The excited electrons are injected into the conduction band of TiO2 and generate hydroxyl radicals. Finally, the hydroxyl radicals provoke the destruction of the RhB molecules, which is called deethylation. These photodegradation routes could be distinguished by analyzing the shape change in the UV-Vis absorbance spectra. When RhB molecules are decomposed via the solution bulk reaction, the absorbance of RhB is simply decreased without a peak shift. On the contrary, RhB molecules that are deethylated by the surface reaction are accompanied by a shift in the UV-Vis peak from 554 to 496 nm. As shown in Fig. 6, the absorbance spectra of the NST-As, -200, -250, and -300 solution showed a

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more significant peak shift from l = 554 nm to l = 496 nm, while a small peak shift in the case of the P25 solution was observed. These results indicate that the surface reaction rates of NST-As, -200, -250, and -300 were much larger than that obtained from P25. In particular, the maximum peak intensity at 496 nm appeared in the NST-As solution after 2 h of visible light irradiation, whereas 4 h were required for the NST-200 solution and 6 h were required for the NST-250 and -300 solutions. This reveals that the surface reaction rate increased in the order of NST-As 4 NST-200 4 NST-250 E NST-300. On the other hand, NST-350 and -400 showed a small peak shift, which indicates that the surface reaction occurred slowly (see Fig. S2, ESI†). The degree of solution bulk reaction rate could also be analyzed by observing the change in the intensity of the absorbance at 496 nm, after the surface reaction is finished. UV-Vis spectra of NST-As solution, as shown in Fig. 6(B), exhibit that the absorbance at 496 nm decreased dramatically within 2 h after the 2 h initial rapid surface reaction. These results indicate that NST-As efficiently decompose RhB under visible light irradiation via both the fast surface reaction and the solution bulk reaction. Furthermore, NST-As showed excellent reproducibility over 4 cycles of the photodecomposition process. Over the 4 cycles, most of the RhB was rapidly decomposed after 2 h of visible light irradiation (Fig. 5(D) and Table S2, ESI†). NST-200, -250, -300 also showed faster solution bulk reactions compared to P25. The rapid solution bulk reaction observed in NST-As, -200, -250, and -300 was thought to be attributed to the high visible light absorbance. On the contrary, NST-350 and -400 showed the low solution bulk reaction properties, due to their

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Fig. 6 UV-Vis spectral changes of a rhodamine B solution under visible light irradiation and in the presence of (A) P25, (B) NST-As, (C) NST-200, (D) NST-250, and (E) NST-300.

low visible light absorbance by the loss of N dopants from TiO2 (see Fig. 4 and Fig. S2, ESI†). It is quite interesting that the surface reaction rate of NST was dependent on the thermal post treatment temperature. There are several studies reporting that the deviations of the crystallinity, crystal structure, specific surface area, loaded metals, quantity and chemical state of dopants on the surface of TiO2 significantly affect the surface reaction rate.44 We note that such variables in NST-As, -200, -250, and -300 were indistinguishable, except for the transformation of the chemical structure of S dopants on the surface of NST-As from sulfide to sulfate with the increase post thermal treatment temperature at 200 and 300 1C (see Fig. 3C and Fig. S1C, ESI†). This suggests that the S dopants on the surface of NST-As play an important role in the destruction of RhB. Thus, we further investigated the effect of the S dopants on the surface reaction of NST-As through the adsorptivity and the photocatalytic activity tests using a different organic dye, methylene blue (MB). MB can adsorb to metal oxide surfaces,38 especially to the surfaces of S-modified substrates, through the formation of strong

sulfur–sulfur interactions between the S on the substrate and the ring sulfur atom of MB.45 Moreover, it is known that the C–S+QC functional group in MB predominantly interacts with anionic groups on the TiO2 surface, rather than with ammonium cations in MB.46 Thus, the quantitative analysis of the change in the surface reaction rate of NSTs using MB molecules can clearly show the effect of S dopants on the enhanced surface reaction of NST-As observed in RhB solution. Fig. 7(A) shows the relative concentrations of the removed MB in NSTs and P25 solution. MB adsorption and degradation tests were carried out under the conditions used for the RhB removal test and under visible light irradiation. Unlike the RhB adsorption test results, Fig. 7(A) shows that MB was rapidly adsorbed to the surface of NST-As, -200, -250, and -300 within 3 h (black bars in Fig. 7(A)). In particular, NST-As adsorbed the largest quantity of MB among NSTs, where the MB removal ratio of NST-As reached 80%. This was attributed to the strong interaction of S dopants on the surface of NST-As with sulfur cations of MB, by the formation of the sulfur–sulfur interactions. In contrast, the higher thermal treatment temperature resulted in a lower amount of MB adsorbed onto the NSTs.

Fig. 7 (A) The relative concentrations of the methylene blue removed by P25 or NSTs after 3 h, and (B) the relative concentration of methylene blue in the NST-As solution. The experiments were conducted under visible light irradiation or under visible light-blocked conditions.

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This can be explained by the transformation of sulfide-formed S dopants into the sulfate form that cannot easily interact with MB molecules. Fig. 7B shows that the amount of MB molecules much rapidly decreased under visible light irradiation. The further decrease of MB in NST-As solution could be due to the rapid adsorption-assisted surface reaction on the NST-As surface via the strong sulfur–sulfur interactions. To observe the degree of MB decomposition by the surface reaction of NST-As, the chemical states and the quantities of S dopants on the surface of NST-As, in MB solution under visible light irradiation, were observed by XPS analysis. As shown in Fig. 8(A) and (B), the quantity of S groups at the surface of NST-As was doubled after 3 h of MB adsorptivity evaluation. However, the quantity of S groups on the NST-As surface, where MB molecules were adsorbed noticeably, decreased after 3 h of visible light irradiation (Fig. 8(C)). Houas et al. suggested that the ring sulfur of MB was transformed to other sulfur species, such as the –SO3H group, with decomposition of benzene ring in MB by the surface reaction of MB in TiO2 solution.46 In this case, MB molecules lose their adsorption property to TiO2. Thus, the lower quantity of S on the NST-As surface under visible light irradiation suggests that the MB adsorbed onto the S dopants was desorbed through the adsorptionassisted photodecomposition of MB by NST-As. In particular, the photodecomposition of MB having a strong adsorptivity was much faster than that of RhB. The combined results of MB degradation tests, under the dark or visible light conditions, evidently showed that the enhanced surface reaction of NST-As observed in RhB solution resulted from the S dopants (sulfide form) on the surface of NST-As via the adsorption-assisted surface reaction. The effects of the N and S dopants on the visible light photocatalytic activity of the NST-As are illustrated in Fig. 9. The S dopants on the NST-As surface effectively enhanced the surface reaction with cationic organic dyes, by promoting the adsorption onto the surface of NST-As. The cationic organic dyes are first adsorbed at the NST-As surface by strong interactions with the S dopants. When the electrons in the organic dye that are adsorbed at the surface of

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Fig. 9 Schematic illustration of the suggested effects of nitrogen and sulfur dopants on the enhanced visible light photocatalytic activity of TiO2.

NST-As are excited by visible light and the excited electrons move to the NST-As surface. Subsequently, the electrons produce hydroxyl radicals, which facilitate the surface reaction, and cationic organic dyes strongly bound to the S dopants are rapidly photodecomposed by the generated hydroxyl radicals. The N dopants in the NST-As form a delocalized state in the band gap, which leads to the enhanced solution bulk reaction by increasing visible light absorbance of NST-As. As a result, NST-As showed an excellent visible light photocatalytic activity even with a relatively low crystallinity. However, the thermal treatment causes a decrease in the surface reaction and solution bulk reaction rates, due to the transformation of S dopants from sulfide to sulfate and the loss of N dopants.

4 Conclusions In summary, we synthesized the N- and S-codoped TiO2 (NSTs) using a low-temperature solvothermal process and investigated the effect of post thermal treatment on the photocatalytic activity of NSTs, in conjunction with the thermal behavior of the dopants in NSTs. We found that the as-synthesized NST had a superior visible light photocatalytic activity than the commercially available P25 and the thermally treated NSTs. This was attributed to the enhanced surface and solution bulk reactions due to the strong adsorptive interaction of S dopants with cationic organic dyes and the high visible light absorbance by N dopants. The thermal posttreatment rather brought about the change in the chemical structure of S dopants and the loss of N dopants, resulting in the reduction of the photocatalytic activity of NSTs. This is the first report that discloses the origin of high visible light photocatalytic activity of the as-synthesized NST, and our results showed that the as-synthesized NST is quite attractive as a highly active photocatalyst under visible light environments for applications ranging from waste water treatment to solar energy conversion.

Acknowledgements Fig. 8 Chemical state and atomic ratio of the sulfur groups (S/Ti at%) present on (A) the NST-As surface, and (B) the NST-As surface after 3 h of the methylene blue adsorption process, and (C) the NST-As surface after 3 h of the methylene blue adsorption–photodecomposition process.

Phys. Chem. Chem. Phys.

This research was supported by the Defense Acquisition Program Administration (DAPA) and by the Agency for Defense Development (ADD), Republic of Korea.

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Phys. Chem. Chem. Phys.

Adsorption-assisted photocatalytic activity of nitrogen and sulfur codoped TiO2 under visible light irradiation.

Applying post thermal treatment on the doped TiO2 at high temperature is mostly regarded as an indispensable process, although it has negative effects...
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