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A New Method to Synthesize S-Doped TiO2 with Highly Efficient and Stable Indoor Sunlight Photocatalytic Performance Mingshan Zhu, Chunyang Zhai, Liqun Qun, Cheng Lu, Andrew S Paton, Yukou Du, and M. Cynthia Goh ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01137 • Publication Date (Web): 02 Nov 2015 Downloaded from http://pubs.acs.org on November 12, 2015

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ACS Sustainable Chemistry & Engineering

A New Method to Synthesize S-Doped TiO2 with Stable and Highly Efficient Photocatalytic Performance under Indoor Sunlight Irradiation Mingshan Zhu †, Chunyang Zhai ‡, Liqun Qiu †, Cheng Lu †,*, Andrew S. Paton †, Yukou Du ‡, and M. Cynthia Goh †,* † ‡

Department of Chemistry, University of Toronto, M5S3H6, Canada.

College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China.

ABSTRACT

In this paper, we report a new, low-cost and facile solvothermal

approach to synthesize visible-light-active S-doped TiO2 (S-TiO2) by using dimethyl sulfoxide (DMSO) as both the S source and the solvent. Energy-dispersive X-ray (EDX) and X-ray photoelectron spectroscopy (XPS) solidly confirmed the presence of S element in the final product. The as-prepared S-TiO2 nanoparticles exhibited excellent and long-term stable photocatalytic performance for the degradation of organic pollutants under visible and indoor sunlight illumination. The catalyst still kept high photoactivity even after several months of exposure to the indoor sunlight irradiation. This result suggests a new approach to achieve stable and highly efficient solar light driven photocatalysts for water purification.

KEYWORDS: photocatalyst; S-doped TiO2; visible light; indoor sunlight; long-term 1

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stability

INTRODUCTION To cope with the growing pollution of our hydrosphere, a variety of technologies, including heterogeneous semiconductor photocatalytic oxidation, have been developed for wastewater treatment.1-4 Titanium dioxide (TiO2), as the most widely studied semiconductor photocatalyst, has thus far been explored to meet the requirements of water purification.1-4 However, the band gap of pure TiO2 is ca. 3.2 eV, which means that it can only show activity under UV irradiation. It is well known that UV light accounts for no more than 5% of the total solar energy, which is a small amount compared to the 45% of energy in the visible region.4 Hence, in order to effectively

utilize

solar

radiation,

it

is

desirable

to

develop

efficient

visible-light-driven photocatalysts for remediating the growing pollution in our hydrosphere. Since the Asahi group first reported the visible-light photocatalytic activity of nitrogen doped TiO2,5 more research has been focused on the modification of TiO2 with non-metal or/and metal ion doping.3-5 In these cases, the dopant is often incorporated as an anion or cation to take the place of Ti or/and oxygen in the lattice of TiO2, resulting in bandgap narrowing in TiO2 nanostructures and showing high visible-light photocatalytic activity. Among these dopants, sulfur (S) doping has received particular attention, owing to its highly thermal stability and significant enhancement in visible light driven photocatalytic activity,6-13 where the TiS2, thiourea, CS2, etc. were often used as the S source.6-13 However, these precursors are either 2


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expensive or highly toxic. Dimethyl sulfoxide (DMSO), an important polar aprotic compound, is widely used as a solvent in various organic and inorganic syntheses. Recently, some researchers found that DMSO can slowly release S2– ions into solution for synthesis of S-based semiconductors under the facile one-pot solvothermal condition, resulting in highly crystalline structures of Cu2S, CdS, ZnS and NiS.14-17 Compared to the other S sources, DMSO is low-cost and easy to operate. We therefore are inspired to explore a new route to synthesize S-doped TiO2 (S-TiO2) by using DMSO as the S source.

Scheme 1. Schematic illustration of the formation process of S-TiO2 powders. Specifically in this paper, we report a new, low-cost and facile method for the synthesis of S-TiO2 through a solvothermal method, as shown in Scheme 1. Titanium butoxide was used as the Ti source and DMSO served as both the S source and the solvent during the reaction. The resulting powders were then calcined at 500 °C to further crystallize the structure and to remove any surface attached organic species. Energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) solidly confirmed the presence of the sulfur element in the final product,

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suggesting the generation of S-TiO2. More importantly, these S-TiO2 nanoparticles exhibited more excellent and long-term stable photocatalytic performance for the degradation of organic pollutants under indoor sunlight illumination than commercial P25 TiO2. The catalyst still kept high photoactivity even after several months of exposure to the indoor sunlight irradiation. This result suggests a new approach to achieve stable and highly efficient solar light driven photocatalysts for water purification.

EXPERIMENTAL SECTION Materials. Dimethyl sulfoxide (DMSO, certified ACS, Fisher Scientific), Titanium (IV) butoxide (97%, Sigma-Aldrich), All other chemicals were purchased from Sigma-Aldrich without further purification before use. Milli-Q water was used throughout our experiments. Synthesis of the S doped TiO2 (S-TiO2) nanoparticles. To synthesize the S-doped TiO2 powders, titanium butoxide (5.1 g, 0.015 mol) was added into the 50 mL DMSO solvent under stirring. The solution was continuous stirred at room temperature for 30 min, then transferred into a 100 mL Teflon autoclave and held at 180 °C for 18 h. After that, the precipitates were collected by centrifugation, washed with water and ethanol, and then dried in an oven at 60 °C for 12 h. After that, the powder was calcined at 500 °C for 4 h, resulting in a light yellow S-TiO2 sample. Photocatalytic performance. For catalytic experiments, 20 mg of samples were dispersed in a 10 mL rhodamine B (RhB, 22.5 mg L−1) or methylene blue (MB, 20 mg L−1) solution, wherein a 20 mL cuvette was used as the reactor. A 150 W xenon arc 4


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lamp installed in a laboratory lamp housing system (LS 150 Xenon Arc Lamp Source, Abet Technologies) was employed as the light source. The light passed through a polyester Lee type-226 cut off filter (400nm) before entering the reactor. The reaction system was kept for 30 min in dark to achieve an equilibrium adsorption state before visible-light irradiation. The photodegradation of RhB and MB was investigated by measuring the real-time UV−vis absorption of RhB and MB at 554 nm and 665 nm, respectively. Aliquot of the reaction solution (0.25 mL) was taken out from the reaction system for the real-time sampling. The pollutants relative concentrations (C/C0) variation were used to evaluate the photocatalytic activities, where C was the concentration of RhB or MB at a real-time t, and C0 was the concentration in the RhB or MB solution before it was kept in dark. The integrated light intensity was measured to be ca. 30 mW cm–2 by a visible–light radiometer (model: PM200, Thorlabs GmbH). In order to investigate the wavelength-dependent photocatalytic performance, the incident light was passed the assigned bandpass filter (365 ±15 nm, 400±15 nm and 465 ±15 nm) before entering the reactor. In these cases, 50 mg of samples were dispersed into 40 mL of RhB or MB aqueous solution (10 mg L-1) for the catalytic experiments. The average intensity of the incident light was ca. 4.4 mW cm–2. To investigate the long term photocatalytic activity, 20 mg of S-TiO2 samples were dispersed in two cuvettes containing 10 mL RhB (13.5 mg L−1) and MB (15 mg L−1) solution, respectively. The reaction system was kept beside the laboratory window without stirring. The indoor sunlight (>350 nm) irradiated the reactor from 5



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9:00 am to 3:00 pm during a sunny day. The light intensity of the indoor sunlight was 5 ~ 35 mW cm–2. When the dyes were degraded completely, the concentrated RhB and MB solutions were added in the reactor to reach the initial concentration for a new cycle. The cycling experiment was repeated for over three-month period to evaluate the long term photocatalytic stability under indoor sunlight irradiation. The recycled photocatalysts were then collected to further determine the photocatalytic activity. In details, 20 mg of catalyst powders were dispersed in two cuvettes containing 10 mL RhB (13.5 mg L−1) and MB (15 mg L−1) solution, respectively. The solutions were irradiated under simulate indoor sunlight (λ>350 nm, 50 mW cm-2) for 24 minutes. Apparatus and measurements. Transmission electron microscopy (TEM) studies were conducted on a TECNAI-20 electron microscope operating at an accelerating voltage of 200 kV. Scanning electron microscope (SEM, S–4700) was used to determine the morphology of the as-prepared composite samples. The energy dispersive X-ray (EDX) analysis was conducted with a Horiba EMAX X-act energy dispersive spectroscope that was attached to the S-4700 system. The X–ray diffraction (XRD) measurements were performed on a PANalytical X' Pert PRO MRD system with Cu Ka radiation (k =1.54056 Å) operated at 40 kV and 30 mA. UV-vis diffuse reflectance spectra were obtained on an UV-vis spectrophotometer (Hitachi, Model U-3900) using BaSO4 as the reference. X-ray photoelectron spectroscopy (XPS) was performed on an ESCALab220i-XL electron spectrometer from VG Scientific using 300 W Al Kα radiation. The binding energies were referenced to the C1s line at 284.8 6


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eV from adventitious carbon. All of these measurements were carried out at room temperature. BET surface areas were measured on Autosorb-1 (Quantachrome Inc.) using N2 adsorbent at 77K.

RESULTS AND DISCUSSION The morphology of the as-synthesized S-TiO2 nanoparticles was analyzed by scanning electron microscope (SEM) and transmission electron microscope (TEM). As shown in Figure 1a and 1b the S-TiO2 nanoparticles appeared to be small spherical particles with the average size ca 9.1 nm (See insert of the Figure 1b, the corresponding size histogram of S-TiO2 nanoparticles counted from the TEM image). The X-ray powder diffraction (XRD) pattern of the S-TiO2 powders was carried out to investigate the crystalline phase of the as-prepared sample. Figure 2 clearly reveals the peaks at 25.5°, 38.0°, 48.1°, 54.2°, 55.2°, 62.8°, 69.1°, 70.3°, and 75.3° which were assigned to the diffraction of the (101), (004), (200), (105), (211), (204), (116), (220), and (215) crystal planes, respectively, of anatase TiO2 (JCPDS No. 21-1272).13 As there are no diffraction peaks due to the rutile phase observed in the spectrum, we conclude that the as-prepared S-TiO2 was in a purely anatase structure. Furthermore, the average grain size determined from the Scherrer equation (D = 0.9λ/βcosθ) was about 9.5 nm based on (101) diffraction peak,13 which also consisted with the TEM image observation.

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Figure 1. (a) SEM and (b) TEM images of the as-prepared S-TiO2 nanoparticles. The insert in image b is S-TiO2 size distribution histogram deduced from the TEM image. The UV–Vis diffuse reflectance spectra are used to analyze the optical properties and the bandgap energy of the samples. Figure 3A shows the results from the reflectance measurements of S-TiO2 and commercial P25 TiO2 nanoparticles. Compared to the P25 TiO2 (curve b, the absorption edge at ca. 398 nm), the absorption edge of S-TiO2 red–shifted to ca. 435 nm (curve a), and the corresponding UV-Vis spectrum shows a trailing absorption from 400 nm to 550 nm. The photographic images (insets in Figure 3A) also show a distinct colour difference between the as-prepared S-TiO2 nanoparticles (light yellow) and commercial P25 TiO2 nanoparticles (white). Extrapolation of the reflectance was used to obtain the band gap energy of the samples. The bandgap energy of S-TiO2 and P25 TiO2 were ca. 2.85 eV and 3.1 eV, respectively, as shown in Figure 3B. The “tail-like” feature and bandgap narrowing were attributed to the introduction of S atoms in the lattice of TiO2. The formation of doping states can reduce the electron transition energy from the valence to conduction band and thus lead to a red-shift of the absorption edge.

8


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Figure 2. XRD pattern of the as-prepared S-TiO2 nanoparticles.

Figure 3. (A) The UV–Vis diffuse reflectance spectra and (B) plots of (αhν)2 vs. photon energy of S-TiO2 (a) and P25 TiO2 (b) nanoparticles. The inserts in image A are the photographs of the S-TiO2 (a) and commercial P25 TiO2 (b) nanoparticles. Table 1. Ti, O and S contents of as-prepared S-TiO2 were determined by EDX and XPS study. Element

EDX (atom %)

XPS (atom %)

Ti

32.07

31.17

O

66.24

66.83

S

1.69

2.00

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Figure 4. EDX elemental analysis of of the as-prepared S-TiO2 nanoparticles. To verify the existence of S, the EDX and XPS spectra of S-TiO2 were applied and shown in Figure 4 and Figure 5. First, from the EDX spectrum (Figure 4), Ti, O, and S elements were observed in the as-prepared samples, suggesting the formation of S-doped TiO2. Moreover, the XPS survey spectrum clearly shows the O 1s, Ti 2p, S 2p and C 1s core levels (Figure 5). Specifically, the O 1s XPS spectrum can be resolved into two peaks at ca. 529.9 and 531.7 eV, which are ascribed to Ti–O and surface OH species, respectively.18 The Ti 2p in S-TiO2 displays two peaks centered at 458.8 and 464.4 eV, which can be ascribed to the binding energy of Ti 2p3/2 and Ti 2p1/2, respectively.18 The presence of S was confirmed by a peak at 168.5 eV. This peak can be further deconvoluted into two peaks at 168.5 eV and 169.7 eV, which can be assigned to S 2p3/2 and S 2p1/2 respectively.11,13 Generally, the peak at ~168.5 eV is assigned to the S6+ state, with a appearance of S 2p3/2 peak about twice the intensity or area higher than that of S 2p1/2 peak.11 Hence, the S element might be S6+ in the lattice of S-TiO2. This is also similar to previous literature reports.7,11,12 The C 1s XPS spectrum showed one peak at 284.8 eV and a shoulder at around 288.8 eV, which are assigned to C-C bonds and C-O bonds19, respectively. This is possibly due to the 10


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environmental species detected during the measurement. However, we did not observe the formation of C-Ti bonds because of the missing of the bands at ~ 282 eV, suggesting element C was not doped into the crystalline lattice20. The semiquantitative analysis of the as-synthesized S-TiO2 by EDX and XPS are summarized in Table 1. First, the results indicate that the atomic ratio between O and Ti is similar to the theoretical stoichiometric atomic ratio, indicating the formation of TiO2. The S content in S-TiO2 is about 2 atom% determined by EDX and XPS (Table 1). All the above results prove the successful generation of an S doped TiO2 sample.

Figure 5. (a) XPS survey spectrum of S-TiO2 nanoparticles, (b) Ti 2p, (c) O 1s, (d) S 2p and (e) C 1s signals taken from S-TiO2. The above analyses of the S-TiO2 suggest that the S element was successfully introduced into the lattice of TiO2, resulting a distinct absorption in the visible light range. This visible light response enables the S–TiO2 nanoparticles a potential to utilize visible light in solar spectrum for catalytic degradation of organic pollutants. We chose two typical dyes molecules of rhodamine B (RhB) and methylene blue (MB) 11



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as targeting objects to evaluate the photocatalytic activities of the as-prepared S-TiO2. Figure 6 shows the molecular structure of the above two dyes molecules. Photo-degradations of RhB and MB under visible-light irradiation were performed as the photoreaction probes to evaluate the photocatalytic activity of S-TiO2, and the results are shown in Figure 7. For comparison, commercial P25 TiO2 was also used at the same experimental conditions. There was negligible degradation of RhB and MB pollutants when no catalysts were used after 120 min of visible-light irradiation. Around 82.8% and 63.8% of RhB and MB molecules degraded in 120 min when P25 TiO2 was used. In contrast, when our S-TiO2 nanoparticles were used as the photocatalyst, the photoactivity was significantly improved, and the RhB and MB molecules were degraded nearly 97% and 100% under visible-light irradiation in 120 minutes. Moreover, the BET surface area of the as-prepared S-TiO2 is ca. 124.24 m2 g-1, which is larger than the P25 TiO2 (generally is 35~65 m2 g-1, from Sigma-Aldrich). The larger surface area might also play a role to improve the catalytic activity. Furthermore, we also synthesized S-doped TiO2 following the previously reported work by using thiourea as S source through the precipitation method (details see Supporting Information)7, and the obtained samples was named as S-TiO2-P. However, there were only 62.3% and 72.4% of RhB and MB molecules degraded respectively in 120 min over S-TiO2-P sample at the same experiment conditions. (Figure S1). The results clearly suggest by using DMSO as both S source and solvent is a better method to fabricate high efficient visible-light active S-TiO2 photocatalyst over the other reported work. 12


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Figure 6. Molecular structures of the RhB and MB molecules.

Figure 7. (a and d) Photocatalytic activities, and (b and e) kinetic linear simulation curves for the degradation of RhB (a and b) and MB (d and e) pollutants without catalysts and with commercial P25 TiO2 and S-TiO2 catalysts under visible light (λ>400 nm) irradiation. The real-time absorption spectra of RhB (c) and MB (f) solution during the photodegradation process over S-TiO2 under visible-light illumination from 0 min to 120 min. As plotted in Figure 7b and 7e, there is a nice linear correlation between ln(C/C0) and the reaction time (t), indicating that the decomposition of RhB and MB over TiO2 photocatalysts follows the first-order kinetics. The rate constants of the catalytic degradation of RhB and MB over commercial TiO2 were 0.014 min-1 and 0.008 min-1, 13



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respectively, while those of S-TiO2 were 0.028 min-1 and 0.043 min-1. The photocatalytic efficiency of S-TiO2 was improved ca. 2 and 5 times compared with the P25 TiO2 for the degradation of RhB and MB molecules, respectively. The reason of the improved catalytic performance of the as-prepared S-TiO2 is attributed to the S element introduced into the lattice of TiO2, which narrows the bandgap of the TiO2, resulting in enhanced visible light absorption of solar energy. The solvothermal temperature controls the DMSO decomposition, and further controls the S-doping ratio in the TiO2 crystalline lattice. As DMSO is non-degradable below 150 oC, and decomposes at boiling point 189 oC, possibly leading to the explosion,17 the solvothermal reaction was controlled between 140 oC and 180 oC. We did not observe S-doping at 140 0C possibly due to the DMSO non-degradation. Figure S2 shows the amount of S doping in TiO2 was ca. 0.73 when the solvothermal treatment temperature was 160 °C (S-TiO2-160). Figure S3 shows RhB and MB molecules were degraded nearly 71.6% and 87.3% under visible-light irradiation within 120 min by using S-TiO2-160 photocatalysts. S-TiO2-160 shows the poor photocatalytic activities compared with S-TiO2 synthesized at 180 °C solvothermal treatment, possibly due to the lower S-doping in the lattice. Calcination temperature determines the crystallinity of the formed TiO2, and the Figure S4 shows the XRD patterns of the as-prepared S-TiO2 calcined at 400 °C, 500 °C and 600 °C. The S-TiO2 presented a pure anatase structure at 400 °C and 500oC calcination, and a mixture of anatase and rutile phases at 600 oC. Figure 8 shows the photocatalytic degradation of RhB and MB pollutants by using S-TiO2 14


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calcined at different temperature. The catalytic performance for S-TiO2 calcined at 400 oC and 500 oC was similar, but decreased when the calcination temperature was increased to 600 °C. Apparently, the catalytic performance is correlated with the weighting percent of the active photocatalytic component anatase structure. High temperature (600 oC) calcination promotes the transition from anatase to rutile structure, deteriorating the catalytic performance accordingly.

Figure 8. Photocatalytic activities of various S-TiO2 by different calcinations temperature for the degradation of RhB and MB under visible light irradiation with 120 min. Different bandpass filters were used to study the irradiation light wavelength dependence during the photocatalytic degradation of RhB and MB pollutants, and the results were shown in Figure 9. Commercial available P25 TiO2 was used as a reference for comparison. In UV band when 365±15 nm bandpass filter was used, the as-prepared S-TiO2 and P25 TiO2 showed the similar photocatalytic activities. In visible band 400±15 nm, the photocatalytic activity of S-TiO2 was more than two times higher than that of P25 TiO2. There was very weak dyes degradation on both 15



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S-TiO2 and P25 TiO2 when the longer wavelength bandpass filter 465±15 nm was used. These results first are consistent with the bandgap analyzation as discussed in Figure 3; furthermore, by comparing the photocatalytic activity of P25 TiO2 and S-TiO2, the activity similarity at 365±15 nm band and apparent increase at 400±15 nm for S-TiO2 suggest that the main reason for enhancing catalytic activities were attributed to S element introduced into the lattice of TiO2 other than the increased surface area. In summary, S-TiO2 has superior photocatalytic performance than P25 TiO2 to degrade the organic pollutants.

Figure 9. Wavelength dependence for photocatalytic degradation of RhB and MB by using commercial P25 TiO2 and S-TiO2 as photocatalysts. The light exposure time was 4 hours. Besides catalytic performance, stability is another important factor for high quality catalysts with practical applications. Moreover, in a real environment, sunlight is the most economical and renewable energy source to explore for the application of 16


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photocatalysts. Considering the above issues, the stability of S–TiO2 photocatalyst was investigated by recycling degradation of RhB and MB pollutants under indoor sunlight (λ>350 nm) irradiation. At the end of each cycle, the organic pollutants were resupplied to the initial concentration for the next run. It should be noted that these experiments were carried out without stirring or extra light irradiation. Figure 10 shows the catalyst still kept a high catalytic capability to degrade both RhB and MB over more than 40 cycles in a 3 month test span by using S-TiO2 as the photocatalyst. We estimated the number of the bonded surface hydroxyl group on S-TiO2 was the same as that of P25 TiO2, which is about 9.5 1019/g (equals 1.58 104 mol/g) as previously reported, 21 and all of surface bonded –OH groups acted as active sites for catalytic reaction. As a result, if we regard the target organic molecule as a catalysant, the turnover number for RhB and MB catalytic degradation is 3.66 and 6.08, respectively. To further compare the photocatalytic performance between S-TiO2 and P25 TiO2, we investigated the catalytic activity for the degradation of RhB and MB pollutants before and after the long term irradiation. All the photodegradation reactions were carried out under simulated indoor sunlight (λ>350 nm) irradiation for 24 min. As shown in Figure S5, the fresh catalysts S-TiO2 and P25 TiO2 used before the long-term irradiation treatment both exhibited a high photoactivity, and the RhB and MB were degraded nearly 98.5% and 100%, 94.6 % and 94.4 %, respectively. P25 TiO2 showed significant activity drop after cycling 20 times under simulate indoor sunlight irradiation, and the photocatalytic degradation of RhB and MB was 17



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reduced to 77.2 % and 75 %. On the other hand, the S-TiO2 still displayed high catalytic activity even after long-term indoor sunlight irradiation. Figure S5A shows that 96.2 % of RhB and and 93.1 % MB were degraded in 24 minutes after S-TiO2 were recycled 40 times. These results solidly suggest that as-synthesized S-TiO2 could be employed as a long term stable and efficient catalyst for the water purification under solar light illumination in practical use.

Figure 10. Long-term (3 months) continuous photocatalytic degradation of RhB and MB pollutants by using S-TiO2 catalysts (20 mg) under indoor sunlight (λ>350 nm) irradiation. The high photocatalytic performance of S-TiO2 is first owing to S doping in the Ti-O-Ti crystalline structure, resulting in a red-shift of the absorption edge and narrowed bandgap, and leading to the visible light photosensitivity. Furthermore, XPS results (Figure 5) suggest sulphur is in +6 state, hence when S atom takes the place of Ti, it forms SO4 tetrahedral unit in the lattice. Visible light illumination on the photocatalyst forms the photogenerated pairs. Compared with photogenerated holes 18


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(TiO2+), (SO2+) holes are expected to have higher oxidation potential and can easily decompose any unreacted intermediate species adsorbed on the surface. This strong surface self-cleaning property leads to long-term photocatlytic stability. CONCLUSION In conclusion, visible-light-sensitive S-doped TiO2 with high photocatalytic performance and long-term stability was synthesized through a facile solvothermal method, in which DMSO acted as both the solvent and the S source. The existence of S in the lattice of TiO2 resulted in a narrowing of the S-TiO2 bandgap and therefore providing a visible-light catalytic response. Compared with commercial P25

TiO2,

S–TiO2 showed evidently enhanced photoactivity for the degradation of RhB and MB under visible light irradiation. Excitingly, S-TiO2 displayed excellent catalytic stability after long-term indoor sunlight irradiation. This work provides a new method for developing stable and efficient visible-light-driven photocatalysts to degrade organic pollutants. These materials would also be expected to have promising applications in solar cells, water splitting, and other light harvesting systems. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website Preparation of S-doped TiO2 by using thiourea as S source (S-TiO2-P) and corresponding photocatalytic performance, EDX spectrum and photocatalytic

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activities of S-TiO2-160, XRD patterns of S-TiO2 annealed at different temperature, and photocatalytic stability of S-TiO2 and P25 TiO2

AUTHOR INFORMATION Corresponding Author *

Tel./Fax: +1-416-9784526.

E-mail:

[email protected]

(C.

Lu);

[email protected] (M. C. Goh). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes: The authors declare no competing financial interest.

ACKNOWLEDGMENT Funding for this work was provided by the Natural Sciences and Engineering Research Council of Canada.

REFERENCES (1) Chen, X.; Mao, S. S. Titanium dioxide nanomaterials: synthesis, properties, modifications, and applications. Chem. Rev. 2007, 107, 2891-2959. (2) Kapilashrami, M.; Zhang, Y.; Liu, Y.-S.; Hagfeldt, A.; Guo J. Probing the optical property and electronic structure of TiO2 nanomaterials for renewable energy applications. Chem. Rev. 2014, 114, 9662 -9707. (3) Fujishima, A.; Zhang, X.; Tryk, D. A.; TiO2 photocatalysis and related surface phenomena. Surf. Sci. Rep. 2008, 63, 515-582. 20


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(4) Asahi, R.; Morikawa, T.; Irie, H.; Ohwaki, T. Nitrogen-doped titanium dioxide as visible-light-sensitive photocatalyst: designs, developments, and prospects. Chem. Rev. 2014, 114, 9824-9852. (5) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Visible-light photocatalysis in nitrogen-doped titanium oxides. Science 2001, 293, 269-271. (6) Umebayashi, T.; Yamaki, T.; Itoh, H.; Asai, K. Band gap narrowing of titanium dioxide by sulfur doping. Appl. Phys. Lett. 2002, 81, 454-456. (7) Ohno, T.; Akiyoshi, M.; Umebayashi, T.; Asai, K.; Mitsui, T.; Matsumura, M. Preparation of S-doped TiO2 photocatalysts and their photocatalytic activities under visible light. Appl. Catal. A: Gen. 2004, 265, 115-121. (8) Ho, W.; Yu, J. C.; Lee, S. Low-temperature hydrothermal synthesis of S-doped TiO2 with visible light photocatalytic activity. J. Solid State Chem. 2006, 179, 1171-1176. (9) Li, H.; Zhang, X.; Huo, Y.; Zhu, J. Supercritical preparation of a highly active S-doped TiO2 photocatalyst for methylene blue mineralization. Environ. Sci. Technol. 2007, 41, 4410-4414. (10) Chen, X.; Burda, C. The electronic origin of the visible-light absorption properties of C-, N- and S-doped TiO2 nanomaterials. J. Am. Chem. Soc. 2008, 130, 5018-5019. (11) Periyat, P.; Pillai, S. C.; McCormack, D. E.; Colreavy, J.; Hinder, S. J. Improved high-temperature stability and sun-light-driven photocatalytic activity of sulfur-doped anatase TiO2. J. Phys. Chem. C 2008, 112, 7644-7652. 21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(12) Tian, G.; Pan, K.; Fu, H.; Jing, L.; Zhou, W. Enhanced photocatalytic activity of S-doped TiO2–ZrO2 nanoparticles under visible-light irradiation. J. Hazardous Mater. 2009, 166, 939-944. (13) Naik, B.; Parida, K. M.; Gopinath, C. S. Facile synthesis of N- and S-incorporated nanocrystalline TiO2 and direct solar-light-driven photocatalytic activity. J. Phys. Chem. C 2010, 114, 19473-19482. (14) Wu, Z.; Pan, C.; Yao, Z.; Zhao, Q.; Xie, Y. Large-scale synthesis of single-crystal double-fold snowflake Cu2S dendrites. Cryst. Growth Des. 2006, 6, 1717-1719. (15) Cao, A.; Liu, Z.; Chu, S.; Wu, M.; Ye, Z.; Cai, Z.; Chang, Y.; Wang, S.; Gong, Q.; Liu, Y. A facile one-step method to produce graphene–CdS quantum dot nanocomposites as promising optoelectronic materials. Adv. Mater. 2010, 22, 103-106. (16) Li, Y.; Liu, Y.; Shen, W.; Yang, Y.; Wen, Y.; Wang, M. Graphene–ZnS quantum dot nanocomposites produced by solvothermal route. Mater. Lett. 2011, 65, 2518-2521. (17) Wang, W.; Wang, S.-Y.; Gao, Y.-L.; Wang, K.-Y.; Liu, M. Nickel sulfide nanotubes formed by a directional infiltration self-assembly route in AAO templates. Mater. Sci. Engineering: B 2006, 133, 167-171. (18) Zhai, C.; Zhu, M.; Lu, Y.; Ren, F.; Wang, C.; Du, Y.; Yang, P. Reduced graphene oxide modified highly ordered TiO2 nanotube arrays photoelectrode with enhanced photoelectrocatalytic performance under visible-light irradiation. Phys. Chem. Chem. Phys. 2014, 16, 14800-14807. 22


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(19) Yang, X.; Cao, C.; Hohn, K.; Erickson, L.; Maghirang, R.; Hamal, D.; Klabunde, K. Highly visible-light active C- and V-doped TiO2 for degradation of acetaldehyde. J. Catal. 2007, 252, 296–302. (20) Magnuson, M.; Lewin, E.; Hultman, L.; Jansson, U., Electronic structure and chemical bonding of nanocrystalline-TiC/amorphous-C nanocomposites. Phys. Rev. B 2009, 80, 235108. (21) Takahashi, J.; Itoh, H.; Motai, S.; Shimada, S., Dye adsorption behavior of anatase- and rutile-type TiO2 nanoparticles modified by various heat-treatments. J. Mater. Sci. 2003, 38, 1695-1702.

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For Table of Contents Use Only Title: A New Method to Synthesize S-Doped TiO2 with Stable and Highly Efficient Photocatalytic Performance under Indoor Sunlight Irradiation Authors: Mingshan Zhu, Chunyang Zhai, Liqun Qiu, Cheng Lu, Andrew Paton, Yukou Du, and M. Cynthia Goh

A new facile method for the synthesis of S-doped TiO2 with excellent and long-term stable photocatalytic performance under indoor sunlight irradiation

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New Method to Synthesize S-Doped TiO2 ... - ACS Publications

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