Facile Synthesis of N- and S-Incorporated Nanocrystalline TiO2 and

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Facile Synthesis of N- and S-Incorporated Nanocrystalline TiO2 and Direct Solar-Light-Driven Photocatalytic Activity Brundabana Naik,† K. M. Parida,*,† and Chinnakonda S. Gopinath*,‡ Colloids and Materials Chemistry Department, Institute of Minerals and Materials Technology (CSIR), Bhubaneswar 751 013, Orissa, India, and Catalysis DiVision, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, India ReceiVed: September 1, 2010; ReVised Manuscript ReceiVed: September 30, 2010

Sulfur- and nitrogen-incorporated mesoporous TiO2 (SNT) nanocomposites have been synthesized by a template-free homogeneous coprecipitation technique. The above nanocomposites have been thoroughly characterized by physicochemical and spectroscopy methods to explore the structural, electronic, and optical properties. The photocatalytic activities of the catalysts were evaluated for the degradation of methyl orange and phenol under direct solar light. SNT shows about a 2-fold higher photocatalytic activity than singly N-doped or S-doped mesoporous TiO2 and 3-fold higher than Degussa P25. The higher activity might be attributed to the synergetic interaction of sulfate and nitrogen with the TiO2 lattice. N-Ti-O and O-Ti-N-O environments are responsible for a red shift, and the sulfate group on TiO2 acts as a cocatalyst, for increasing surface acidity as well as for sustaining the redox cycles for high stability. 1. Introduction Visible-light-induced photocatalysis is currently a prime research area because of its potential application in clean and renewable energy as well as pollution abatement. Among semiconductor photocatalysts, TiO2 is the best one in terms of stability, oxidative power, nontoxicity, low cost, and efficiency for degradation of hazardous pollutants.1,2 In photocatalytic applications, anatase is generally the preferred TiO2 polymorph; however, the large band gap (3.2 eV) restricts its activity to the UV range. To induce visible light absorption of TiO2, the region of absorbance must be shifted from the UV to the visible spectral range. With this view, doping of pure TiO2 has been undertaken by a number of research groups3-20 in the last decade. Modification by noble/transition metals has been regarded as the first generation strategy and by nonmetals as the second strategy.3 Cation doping into the TiO2 lattice causes thermal instability and the metal centers can act as electron traps, which reduces the photocatalytic efficiency.4,5 Anion doping has shown great potential in introducing bathochromism, and intensive efforts have been taken to synthesize anion-doped titania toward visible-light-active photocatalysts.6-13 Recent emphasis has been placed on codoped systems, that is, those involving combinations of cations and anions14 or two anions together within the oxide lattice, in which the dramatic enhancement of photocatalytic behavior has been reported.15-20 A few groups have studied N and S codoped TiO2, where a significant improvement in activity arises mainly from a red shift in the absorption edge and also by modification of textural characteristics, such as high surface area.15-19 Further, Sathish et al11,19 suggest that codoping of S and N improves the photocatalytic activity marginally when compared to TiO2-xNx. However, no detailed investigations have been made so far to * To whom correspondence should be addressed. Tel: +91-674-2581636, ext. 425 (K.M.P.), 0091-20-2590 2043 (C.S.P.). Fax: +91-674-2581637 (K.M.P.), 0091-20-2590 2633 (C.S.P.). E-mail: [email protected] (K.M.P.), [email protected] (C.S.P.). † Institute of Minerals and Materials Technology (CSIR). ‡ National Chemical Laboratory.

explain the higher photocatalytic activity associated with S- and N-incorporated titania. A synergetic interaction of B and N with titania in B and N codoped TiO2 lattice systems has been explained by Liu et al.21 Although some results have been reported on S and N codoped TiO2 by various methods,16,19,22,23 still many important points are to be addressed, especially the reasons for enhanced photocatalytic activity. In the present work, we have synthesized S- and Nincorporated TiO2 nanocomposites (SNT) by a mild and soft chemical one-pot homogeneous coprecipitation route. Because no organic or inorganic templates have been used, the method is very simple, cost-effective, and environmental friendly. The solar-light-induced photocatalytic activity has been tested by the degradation of phenol and methyl orange (MO). The dramatic enhancement in photocatalytic activity under solar light has been explained on the basis of the synergetic electronic interaction of N and S with the TiO2 lattice and the different roles of S and N. 2. Experimental Section 2.1. Sample Preparation. The hydrate of titanium oxysulfate sulfuric acid complex (TiOSO4 · xH2SO4 · xH2O) (Sigma Aldrich), thiourea (Qualigen), and other chemicals from SD fine chemicals (ethanol, phenol, and MO dye used in the reactions) were used as such and without further purification. SNT nanocomposites were prepared through a soft chemical route by a homogeneous coprecipitation reaction of TiOSO4 · xH2SO4 · xH2O, thiourea, ethanol, and water. Particularly for the synthesis of the best performing SNT-2, a detailed preparation procedure is given below. First, 7.6 g of thiourea was dissolved in 69 mL of ethanol. To that solution, 13.8 g of TiOSO4 · xH2SO4 · xH2O and 77.4 mL of water was added under ice cold conditions. The weight ratio of TiOSO4/thiourea/ethanol/water was 13.8: 7.6:55.2:77.4. The molar composition of TiOSO4 to thiourea was varied as 1:1, 1:2, 1:3, and the samples are named as SNT1, SNT-2, and SNT-3, respectively, throughout the paper. The final solutions have been stirred for 3-4 h. The resulting homogeneous (transparent) solutions were heated at 80 °C for

10.1021/jp1083345  2010 American Chemical Society Published on Web 10/25/2010

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5 h for complete precipitation. The final precipitate was collected after washing repeatedly with distilled water and hot water, followed by ethanol so that all the impurities and physically adsorbed sulfate species could be removed completely. The sample was dried at 80 °C for 10 h in an air oven and ground to powders. The powders that were calcined at 500 °C for 3 h exhibit the highest activity. Nonetheless, the calcination temperatures were also varied between 300 and 700 °C and explored for the changes in properties. For comparison studies, NT has also been prepared by employing urea in the place of thiourea while maintaining the water and ethanol ratio constant. The weight ratio of the reactants TiOSO4/urea/ethanol/water is 13.8:3:55.2:77.4, and the resulting solution was stirred for 3-4 h, followed by aging at 80 °C for 5 h. Other steps were that of the routine synthesis of SNT materials. Similarly, ST was prepared by hydrolysis in the presence of Na2SO4. The weight ratio of TiOSO4/Na2SO4/ethanol/water was maintained at 13.8:7.1:55.2:77.4, and the solution was stirred, followed by aging at 80 °C for 5 h. The final precipitate was filtered and dried. The powders were calcined at 500 °C and named as ST. 2.2. Characterization. The structure and phase identification was carried out by powder X-ray diffraction (XRD, Philips 1710, Cu KR radiation). Diffraction patterns were recorded between 5 and 80° with a step width of 0.02° s-1. Surface morphologies as well as their relationship to each other on the atomic scale were observed through a transmission electron microscope (TEM) (FEI, TECNAI G2 20, TWIN) operating at 200 kV. The samples for electron microscopy were prepared by dispersing the powder in ethanol and coating a very dilute suspension on carbon-coated Cu grids. TEM images were recorded by using a Gatan CCD camera. N2 adsorption-desorption isotherms were measured at 77 K on an ASAP-2020 system from which the surface area (BET method), the pore size (Barrett-Joyner-Halenda (BJH) model), and pore volume (VP) were calculated. Optical absorbance was measured by UV-visible diffuse reflectance spectra (Shimadzu UV 2450). FTIR spectra were determined from a Varian 800 FTIR spectrophotometer using KBr pellets. The electronic structure aspects of SNTs were investigated by X-ray photoelectron spectroscopy (XPS) (Kratos Axis 165 with a dual anode (Mg and Al) apparatus) using a nonmonochromatized Mg KR source. All the binding energies (BEs) were calibrated by using the adventitious carbon (C1S ) 285 eV) as a reference. The BEs of the samples were reproducible within (0.1 eV. Photoluminescence (PL) studies have been carried out with an excitation wavelength of 380 nm. Temperature-programmed desorption (TPD) studies were performed using a CHEMBET-3000 (Quantachrome, U.S.A.) instrument in the temperature range of 313-1073 K. About 25-30 mg of the powder sample was taken in a quartz “U” tube and degassed at 523 K for 1 h in a N2 flow to remove any physisorbed species. After the sample was cooled to room temperature, NH3 was adsorbed and the temperature ramped at a heating rate of 10 K/min to record the TPD profile. 2.3. Photocatalytic Activity Measurement. For a typical photocatalytic experiment, the catalyst (50 mg) was added to an aqueous solution of MO (50 ppm, 50 mL) in a 100 mL quartz reactor. Photocatalysis experiments were carried out under solar light for 240 min. Before any irradiation, dark adsorption experiments were carried out for 1 h under continuous stirring. Samples taken at regular intervals were centrifuged, and the clear solution was analyzed by a UV/vis spectrophotometer at λmax ) 464 nm. The absorption spectrum of MO and molecular structure are shown in the Supporting Information (Figure S1).

Naik et al. For photodegradation of phenol, the catalyst (100 mg) was added to an aqueous solution of phenol (10-4 M, 40 mL) in a quartz reactor. The photocatalysis experiments were carried out under solar light for 360 min. Prior to irradiation, dark adsorption experiments were carried out for 1 h under continuous stirring. Residual phenol concentrations were analyzed periodically by a Varian Cary UV-vis spectrophotometer. Before spectrophotometric analysis, the color was developed by addition of 2.5 mL of 0.5 N ammonium hydroxide solution, followed by phosphate buffer to maintain the pH in the range of 7.7-7.9. After the pH adjustment, 1 mL of 4-aminoantipyrene (Merck, 98%) and 1 mL of potassium ferricyanide (Aldrich, 99.5%) were added to develop the red color. It was then analyzed with a spectrophotometer at 504 nm. All photocatalytic experiments in this investigation were observed within 3% deviation when repeated. The intensity of the solar light was measured using a LT Lutron LX-101A digital light meter. The sensor was always set in the position of maximum intensity, and the solar light intensity was measured for every hour between 09.00 and 13.00 h. The average light intensity was around 104 000 lx, which was nearly constant during the experiments. 3. Results and Discussion 3.1. Structural Features. TEM analysis (Figure 1) was performed to examine the size, shape, and orientation of individual nanocrystallites that aggregate to form SNT nanocomposites. Figure 1a shows the TEM image of SNT-2 particles. It consists of intergrown fundamental particles with a rough and uneven surface (Figure 1a). The intergrowth of small primary particles results in aggregates with significant extraframework void space, which is consistent with the textural mesoporosity observed in N2 adsorption isotherms, and it is due to the wormhole mesoporous structure. The selected area electron diffraction (SAED) pattern taken from these nanocrystalline particles shown in Figure 1b clearly reveals the presence of (101), (004), (200), and (105) concentric diffraction rings of anatase TiO2 phase, which indicates the polycrystalline nature of TiO2-x-yNxSy, consistent with the XRD data. The high intense (101) feature highlighting the TiO2 crystallites is preferentially oriented along the (101) plane. The high-resolution lattice image shown in Figure 1c confirmed that the sample is composed of aggregates of crystalline titania nanoparticles with a d-spacing (interplanar spacing) of 0.35 nm, which corresponds to the (101) plane of anatase TiO2 phase24,25 (strongest intensity in SAED). Figure S2 (Supporting Information) indicates the crystalline frameworks of SNT-2, suggesting a very good crystallinity. Such high anatase crystallinity in the mesoporous TiO2 is highly desirable to photocatalysis. The EDX spectrum (Figure 1d) from an arbitrary region displays the presence of N and S in the TiO2 sample. The EDX was taken at several random spots on the specimen, and all of these spectra show the presence of N, S, O, and Ti, which clearly indicates that N and S are well distributed in the TiO2 lattice. Figure 1e shows the particle size distribution for SNT2. The distribution could be fitted well by a Gaussian function. The nanocrystallite size distribution of SNT samples is found to be a narrow one with an average particle size of 5.2 ( 0.2 nm with a standard deviation of σ ) 0.6 ( 0.2 nm, which is consistent with values calculated from the XRD data. These values are obtained using several TEM micrographs and the IMAGEJ analysis package. The similarity of the particle and crystallite sizes calculated from TEM and XRD indicates no agglomeration of the crystallites.

Sunlight-Driven Photocatalysis with TiO2-x-yNxSy

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Figure 1. (a) Representative TEM image of SNT nanocomposites exhibiting wormholes [SNT-2] and (b) corresponding selected area diffraction pattern. (c) High-resolution TEM image of individual nanocrystals and (d) EDX spectrum from an arbitrary region showing the presence of N and S in the sample. The large amount of C and some Cu in the EDX results arises from the carbon-coated Cu grid. (e) The histogram displaying the crystallite size distribution. It is fitted with a Gaussian distribution to determine the average crystallite size and the standard deviation of the size distribution.

Figure 2a shows the XRD patterns of SNT nanocomposites (SNT-1, SNT-2, and SNT-3). The XRD patterns of SNT-1, SNT-2, and SNT-3 samples at 500 °C are similar, containing only the anatase phase. Figure 2b indicates the PXRD patterns of NT, ST (calcined at 500 °C), and SNT (calcined at 300, 400, 500, 600, and 700 °C). It is clear from the diffraction patterns that calcination of the samples up to 700 °C does not change the anatase phase significantly. Generally, the rutile phase develops above 600 °C for pure TiO2.24,25 However, as the S and N are doped into the TiO2 system, these species apparently have a significant role to inhibit the phase transformation. We have also observed in our previous study that SO42species are responsible for inhibition of phase transformation.26-28 The crystallite sizes (D) of SNT nanocomposites

were determined by employing the Scherrer formula (eq 1) and are listed in Table 1

D ) Kλ/β cos θ

(1)

where λ is the wavelength of the X-ray (Cu KR), β is the full width at half-maximum of the diffraction peak, K is a shape factor (0.94), and θ is the angle of diffraction. It has been observed that the anatase phase of TiO2 shows better photocatalytic activity than the rutile phase.29 As the SNT samples contain only anatase phase, that may be one reason for the higher photocatalytic activity. There are other several reasons for the increase in photocatalytic activity, which are

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Figure 2. XRD patterns of (a) (i) SNT-1, (ii) SNT-2, and (iii) SNT-3 nanocomposites and (b) (i) NT, (ii) ST, and (iii) SNT-2 calcined at 300, (iv) 400, (v) 500, (vi) 600, and (vii) 700 °C.

Figure 3. N2 adsorption-desorption isotherms of SNT nanocomposites SNT-1, SNT-2, and SNT-3. The inset indicates the pore size distribution curve of SNT nanocomposites.

discussed later. The crystallite sizes lie between 5 and 6 nm, as calculated by the Scherrer’s formula. The SNT-1, SNT-2, and SNT-3 samples have the crystallite sizes of 5.6, 5.2, and 5.2 nm, respectively, which are well consistent with the TEM studies. 3.2. Surface Area and Pore Structures. The specific surface area of the samples is measured using the BET method by N2 adsorption and desorption at 77 K. Table 1 shows the physicochemical properties of SNT nanocomposites, NT, and ST catalysts. Figure 3a,b shows the nitrogen adsorption-desorption isotherms of SNT nanocomposites (SNT-1, SNT-2, and SNT3) and a comparison between NT, ST, and SNT-2. SNT-1 and SNT-2 display a type IV isotherm with an inflection of nitrogenadsorbed volume at P/P0 ) 0.47 (H2 hysteresis loop according to IUPAC nomenclature), indicating the presence of mesoporosity. Though the SNT-3 sample also exhibits a type IV isotherm, the hysteresis loop indicates H3 type, suggesting the

presence of irregular pores. However, in all these SNT samples, the mesoporosity is not as in typical mesoporous materials, such as MCM-41 and SBA-15,30 where intraparticle mesopores of a definite geometry exist. Here, there is the presence of an interparticle wormhole mesoporosity that presumably formed due to the aggregation of smaller particles. However, the isotherm can be described on the basis that larger pores in the system show hysteresis at higher relative pressures during desorption. SNT samples display a surface area between 120 and 140 m2/g after calcination at 400 °C. The higher surface area of SNT compared with that of NT (79 m2/g) and ST (104 m2/g) (Table 1 and Figure 3b) may be attributed to the formation of sulfate ions, which has a great impact on increasing the surface area. Some of our previous studies revealed that modification by sulfate increases the surface area of materials to a great extent.20,22-24 However, a large amount of sulfate ions causes deactivation of the catalyst. Further sulfate works better

TABLE 1: Textural Properties, Absorption Band Edge, and N and S Contents of SNT Nanocomposites sample code

average crystallite size (nm)a

BET surface area (m2/g)b

pore size (nm)b

pore volume (mL/g)b

energy gap in eVc

N content (atom %)d

NT ST SNT-1 SNT-2 SNT-3

12 10 5.6 5.2 5.2

79 104 121 132 143

4.5 4.8 3.9 4.6 4.9

0.06 0.10 0.11 0.13 0.15

2.9 3.0 2.8 2.5 2.4

0.76

a

d

Calculated from TEM and XRD. Calculated from the XPS study.

b

Calculated from N2 adsorption-desorption isotherms.

c

0.56 0.91 0.94

S content (atom %)d 2.3 2.0 2.5 4.5

Calculated from the UV-vis DRS study.


Figure 4. UV-visible diffuse reflectance spectra of SNT nanocomposites, Degussa P25, and NT and ST nanoparticles. The inset denotes the specific absorption band edges calculated from DRS studies.

when it is dispersed on a mesoporous framework. Hence, for this reason, ST shows lesser activity, and it will be discussed later. It should be mentioned that the pore size distribution of SNT-3 is irregular compared with that of SNT-1 and SNT-2. This may be due to higher amounts of sulfate species. The inset in Figure 3a shows the pore size distribution plots calculated using the BJH equation from the adsorption branch of the isotherm. The pore size distribution measurement indicates that the SNTs have a pronounced mesoporosity of a narrow pore size distribution with an average pore diameter between 3 and 5 nm. The wormhole-like channel motif is a potentially important structural feature for catalytic reactivity, in part, because channel branching within the framework can facilitate access to reactive sites on the framework wall.31 Indeed, the above feature enhances the effective utilization of charge carriers because the diffusion of charge carriers is to a small distance. 3.3. Spectroscopic Investigations. To study the optical response of doped TiO2 nanoparticles, their UV-vis diffuse reflectance spectra were measured. As shown in Figure 4, a noticeable shift in the absorption edge to the visible light region was observed for the SNT nanocomposites in comparison to NT, ST, and Degussa P25. The inset in Figure 4 represents the specific absorption cutoff for SNT nanocomposites as compared to Degussa P25. The band-gap energies of SNT nanocomposites and Degussa P25 are given in Table 1. The results clearly indicate the remarkable absorption shift for SNT-2 and SNT-3 samples, having a band gap of 2.5 and 2.4 eV, respectively. The remarkable red shift may be assigned to the synergetic effect of S and N codoping. This is also further supported by the mixing of N 2p with O 2p states of TiO2 and sulfate, which may form a localized state above the valence band of TiO2 and narrow the energy gap.6,11,19 Light absorbance of the SNT sample in the visible light region is of great importance for its practical application because it could be even activated by sunlight. The bonding of sulfur and nitrogen in SNT nanocomposites was examined by FTIR studies. Figure 5 shows the FTIR spectrum of SNT samples, NT, and Degussa P25. The broad peaks in the range of 3000-3400 and at 1630 cm-1 originated from surface-adsorbed water and surface hydroxyl groups are an important factor affecting the photocatalytic activity.32,33 The greater the number of surface hydroxyl groups, the faster the photocatalytic reaction.34 The large peak observed at 1630 cm-1 for NT and SNT samples, compared with Degussa P25, indicates


Figure 5. FTIR spectra of SNT nanocomposites (SNT-1, SNT-2, SNT3), NT, and Degussa P25.

a large number of surface hydroxyl groups and hence more photocatalytic activity. The absorption peaks at 1220, 1130, 1050, and 985 cm-1 are the characteristic peaks of a bidentate C2V SO42- coordinated to Ti4+.16-18 Besides that, a peak around 1400 cm-1 could be attributed to the surface-adsorbed nitrogen species in the TiO2 network.35 To investigate the electronic environment and oxidation state of S and N in SNT nanocomposites, XPS analysis has been performed. Figure 6 shows the XPS spectra of SNT samples for (a) N 1s and (b) S 2p core levels. The core-level peak for N 1s was found to be a broad peak centered at 400 eV (Figure 6a). Nonetheless, deconvolution of N 1s spectra of SNT-2 (and SNT-1) suggest two kinds of nitrogen environments present in the catalyst. The BE at 398.6 eV refers to N- species substituted for O2- in the TiO2 lattice.11,19,36,37 This substitutional N doping is most effective and mainly responsible for the visible light absorption by reducing the band gap by 0.13 eV, forming a localized N 2p state just above the valence band.11 The other peak at 401 eV may be due to the oxidized N, such as the Ti-N-O environment in the TiO2 lattice. The oxidized Ti-N-O environment has also some important roles in the catalytic activity of TiO2.36,38,39 Figure 6b shows the S 2p corelevel peak around 168.5 eV. The above peak corresponds to SO42- ions on TiO2 forming bidentate linkages with Ti4+ ions. The S 2p core level on SNT-3 displays a relatively higher BE at 169 eV, compared with SNT-1 and SNT-2 and the standard BE of pure SO42-.19,40-42 It should be mentioned here that there is no sulfur doping, as it would lead to an S 2p feature below 162 eV, which is not observed in SNT nanocomposites. It should also be noted that the N 1s BE is also higher on SNT-3, highlighting the electronic nature of the SNT-3 samples, which is significantly different from that of SNT-1 and SNT-2. This may be attributed to the stronger binding of sulfate to the titania substrate. A general bidentate bonding of sulfate to Ti4+ is shown in Scheme 1. As we suggested earlier, because of the substitutional N- replacing O2- in the TiO2 lattice, a charge imbalance is expected. This is likely compensated by the reduction of required Ti4+ to Ti3+ species. Indeed, a significant amount of Ti in the 3+ oxidation state was observed in Ti 2p XPS peaks of SNT-1 (about 40%) and SNT-2 (about 10%), as shown in Figure 7. The absence of Ti3+ species in SNT-3 supports the above rational argument (substitutional N- doping causes formation of Ti3+ in our catalysts). There are many reports regarding

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Figure 7. XPS figures showing Ti 2p XPS peaks of SNT-1, SNT-2, and SNT-3 catalysts. The inset illustrates Ti 2p deconvolution spectra of SNT-1 showing the Ti3+ peak.

Figure 6. (a) N 1s core-level peak of SNT-1, SNT-2, and SNT-3 catalysts. The inset illustrates N 1s deconvolution spectra of SNT-2 showing a nitride peak at 398 eV. (b) S 2p core-level peaks of SNT-1, SNT-2, and SNT-3 catalysts.

experimental and theoretical studies about the nature of Ti3+.43-47 Despite all these, the exact nature of charge compensation is still unclear and is debatable. Another controversial issue on the nature of Ti3+ species is its localized or delocalized electronic character.46 In the localized case, the extra electron should reside on a few Ti ions or even on a single Ti and the corresponding energy level is usually in the band gap. Instead, the delocalized electronic state is a CB state where the extra electron is shared by a finite fraction of Ti ions in the sample. The character of the Ti3+ states has a key role in electron transport. As it has been discussed, the charge compensation mechanism involved

does not require any formation of undercoordinated Ti ions because no oxygen or Ti is removed. Because N- doping produces fully coordinated 6-fold Ti3+ ions by reduction of Ti4+, it is likely that the extra electrons hop from one regular Ti3+ to the next neighbor with very small activation barriers (nearly 0.07 eV).47 The atomic contents of N and S in SNT were calculated from XPS peak areas and are given in Table 1. They were found to be 0.56, 0.91, and 0.94 atom % of N and 1.98, 2.5, and 4.5 atom % of S for SNT-1, SNT-2, and SNT-3, respectively. Despite the larger N content, that no Ti3+ or N- was observed on SNT-3 is attributed to a large content of sulfate ions. 3.4. NH3-TPD Studies. Figure 8 shows the ammonia TPD profiles of the NT and SNT-2 samples. It indicates a broad distribution of desorption peaks in the temperature range of 100-600 °C with at least two peak maximas at 180 and 450 °C. Ammonia desorption in the ranges of 100-250, 280-330, and 380-500 °C is normally attributed to NH3 chemisorbed to weak, medium, and strong acid sites, respectively.48,49 A large number of strong acid sites present on SNT-2 are clearly evident from the integrated intensity of desorbed ammonia. The number of acid sites of SNT-2 samples was found to be 0.7 mmol/g, whereas that of NT was only 0.1 mmol/g. Although preparation of both NT and SNT involves the TiOSO4 as a starting material, the large amount of strong acidity with SNT-2 is mostly due to the sulfate species. It is likely that thiourea might be playing an active role in retaining the sulfate on SNT nanocomposites. Indeed, the presence of sulfate was already demonstrated by

SCHEME 1: Mechanism of Light Absorption and Charge Transfer in the Catalyst


Figure 8. TPD profiles of SNT-2 and NT nanocomposites.

Figure 9. Photocatalytic degradation of methyl orange over SNT-1, SNT-2, SNT-3, NT, ST, and Degussa P25. C ) concentration; C0 ) initial concentration.

XPS studies. There is no significant desorption of ammonia above 650 °C, hinting at the dissociation of sulfate above 600 °C. 3.5. Photocatalytic Activity. Photodegradation of MO and phenol was employed to evaluate the photocatalytic activities of SNT nanocomposites because both the molecules are photostable and cannot be photodegraded in the absence of any photocatalyst under light irradiation. Figure 9 shows the results of MO degradation with a variety of photocatalysts. Adsorption experiments were performed on all catalysts in the absence of light for 1 h. About 20% MO adsorption was detected on SNT2. Figure S3 (see the Supporting Information) shows a fast decrease in MO content in the presence of SNT-2 under solar light within 120 min. The time course degradation study of MO for each 20 min interval for SNT-2 catalyst is shown in Figure S3 (Supporting Information). SNT catalysts are showing a high transition in the band gap toward the visible region and higher activity compared to NT and ST. Of all the catalysts prepared, SNT-2 gave the best activity under solar light irradiation. According to these results (Figure 9), the order of photocatalytic activity of different catalysts after 120 min was as follows: Degussa P25 < ST ∼ NT < SNT-3 < SNT-1 < SNT-2. The


Figure 10. Photocatalytic degradation of phenol over SNT-1, SNT-2, SNT-3, NT, ST, and Degussa P25. C ) concentration; C0 ) initial concentration.

photocatalytic activity of SNT (89%) increases about 3-fold higher than that of Degussa P25 (31%) and about 2-fold than that of NT (51%) or ST (50%) under solar light. To understand the visible light absorption effect of the MO solution and the small resultant photosensitization of the semiconductor, phenol degradation was also evaluated as a model reaction. Although the rate of degradation of phenol was slightly less than that of MO, SNT-2 catalysts once again showed higher photocatalytic activity than all other catalysts (Figure 10), clearly demonstrating the effect of incorporation of S and N together in TiO2, toward visible light photocatalytic activity. All experiments were conducted under natural pH conditions, and the pH was monitored for the experiment using the most active photocatalyst. When the photocatalyst was added to the aqueous solution of phenol, the initial solution was at pH ) 3.5. During the irradiation, the pH value decreased to 2.5 and, after 240 min, the pH reverted back to 3.6. The decrease in pH of the solution during the photocatalytic experiment is due to the formation of acidic products derived from the degradation of phenol. The rise in pH after the experiment is seemingly due to the mineralization of the phenol. To explore the stability of the catalyst, prolonged recycling studies were performed on SNT-2. Even after five cycles, no remarkable change in the phenol degradation activity was detected (Figure 11). Formation of carbonate species, due to mineralization of phenol to carbon dioxide, during photocatalytic oxidation generally deactivates the catalyst.50 Therefore, a small deactivation on recycling of the catalysts may be seen as a result of the accumulation of intermediates, such as carbonates, formed during the degradation. The activity of the used catalyst after five cycles has been tested again after calcination at 400 °C for 3 h to remove any surface contamination. Almost the same activity was restored as that of fresh catalyst after the above calcination. This confirms that the gradual decrease in degradation activity is mainly due to the accumulation of the reaction intermediates, such as carbonates, on the recycled samples. The higher visible-light-induced photocatalytic activity is attributed to several reasons, as discussed below. The smaller particle size of the titania with high crystallinity is an important factor that enhances the charge diffusion to the surface of the catalyst and helps for its effective utilization to improve photocatalytic activity. It has also been investigated that

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Naik et al. catalyst contributing to the visible light absorption, and the O-Ti-O-S or N-Ti-O-S environment on the photocatalyst surface acts as an effective cocatalyst. The detailed process of charge transfer is described in Scheme 1. The photoexcited electrons in the CB migrate to the most electropositive S atom in the sulfate species to reduce the system from the S6+ to the S4+ state. The electrons are then transferred to the surfaceadsorbed oxygen molecule to retain the system as S6+ and produce superoxide species, followed by hydrogen peroxide as an intermediate product that subsequently decomposes to a hydroxyl radical. In this process, sulfate species undergo a redox reaction and are regenerated, indicating the likely reason for stable activity. The hydroxyl radicals are mainly responsible for the degradation

SNT + hν f SNT + e- + h-

(2)

OH- + h+ f OH•

(3)

O2 + e- f O2

(4)

+ O2 + H f HO2

(5)

2HO2 f H2O2 + O2

(6)

H2O2 + e- f OH• + OH-

(7)

Figure 11. Phenol degradation activity measured with recycled catalysts (1-5 cycles) and calcined after the 5th cycle.

crystallite sizes greatly affect the catalytic activity. The smaller the crystallite size, the higher is the photocatalytic activity in anatase TiO2.51-55 Present SNT nanocomposites display a crystallite size around 5-6 nm, and this could be a main reason for higher photocatalytic activity. We have also mentioned that the SNT-2 nanocomposite exhibits the lowest crystallite size and hence the highest photocatalytic activity. However, the significant differences obtained in the photocatalytic activities cannot be attributed only to the size and composition. Another factor that could influence significantly the photocatalytic activity is the surface area and porosity of the catalysts.56,57 As we have discussed in an earlier section, SNT nanocomposites have a high surface area, wormhole mesoporosity with a narrow pore size distribution, and large pore volume. The surface areas of SNT nanocomposites are 121-143 m2/g, whereas those of the singly doped NT, ST, and Degussa P25 are 79, 104, and 51 m2/g, respectively. As it has been discussed earlier, modification by bidentate sulfate ions enhances the surface area to a great extent, inhibits the phase transformation at rather high temperature, and develops acidity on the TiO2 surface. Though the SNT-3 sample has the highest surface area, the irregular pore size distribution (H3 type) and large sulfate content might be decreasing the catalytic activity. Along with surface area, the electronic structure and surface properties, such as surface states and their concentration, are also important parameters that are affected by doping a semiconductor with alio-valent ions. Besides all these, the most important cause for enhancement of photocatalytic activity is the shifting of absorbance toward the visible light region (up to 550 nm). The increased photoactivity of SNT nanocomposites for their synergism is well-explained based on our experimental data. The concept of cocatalyst is very important for the development of highly efficient photocatalysts, especially for photocatalytic water splitting. The use of Ag, Au, Pt, RuO2, and NiO as effective cocatalysts58,59 in photocatalytic water splitting is known; a mixed oxide of rhodium and chromium has been shown to be a new cocatalyst with good activity in visible light on a solid solution of ZnO and GaN.60 The cocatalyst present during photocatalysis strongly promotes the effective transfer of photoinduced carriers by acting as an antenna to collect the charge carriers formed by Fermi level equilibration upon light excitation, thereby enhancing the efficiency of the chargetransfer process.61 In the present system, it seems that the N-Ti-O or Ti-N-O environment acts as the effective main

of organic dyes through its successive attacks by formation of several intermediate products. The synergistic effects of creating visible light absorption and then fast diffusion of charge carriers to the surfaces are responsible for the high visible light photocatalytic activity for SNT nanocomposites. The formation of the Ti-O-S environment leads to the construction of a favorable surface structure that facilitates the separation and transfer of charge carriers, thereby promoting the photocatalytic activity. Along with visible light absorption, bulk diffusion, and surface charge transfer, inhibition of electron-hole recombination is necessary for a material to be an efficient photocatalyst.3,21,62 To determine the electron-hole recombination characteristics, PL studies have been undertaken. It is known that the PL emission results from the recombination of excited electrons and holes. Thus, the lower PL intensity indicates a lower recombination rate.21,63,64 Figure 12 shows the PL spectra of NT, ST, and SNT-2. It can be observed that the PL intensity of the SNT-2 sample is significantly lower than that of the NT or ST samples. This result indicates that the recombination of charge carriers is effectively suppressed after incorporating S and N in the TiO2 system. The presence of sulfate ions on the surface of the N-doped TiO2 sample favors the migration of photoproduced electrons, thus improving the electron-hole separation. 4. Conclusions SNT nanocomposites with excellent visible light activity have been successfully synthesized through a soft chemical homogeneous coprecipitation route. The crystallite sizes of SNT nanocomposites are around 5-6 nm with the anatase phase, and they exhibit wormhole-like mesoporosity. High visible-light-driven photocatalytic activity is mainly due to


Figure 12. Photoluminescence spectra of ST, NT, and SNT-2.

the narrowing of the band gap. High surface area, small crystallite size, wormhole mesoporosity with a narrow pore size distribution, and large pore volume are also the determining factors for the enhanced photocatalytic activity. Besides all of the above facts, the electronic environment of the SNT nanocomposites having N-Ti-O and O-Ti-NO environments is mainly responsible for visible light absorption. The presence of Ti3+ states produced through substitutional N-doping has also a leading role for high photocatalytic activity. Sulfate species anchored on TiO2 as a cocatalyst contribute to an enhancement of surface acidity, separation and transfer of charge carriers, increase in surface area, inhibition of phase transformation, and hence contribute to an overall increase in the photocatalytic activity. Acknowledgment. The support and permission of Prof. B. K. Mishra, Director, Institute of Minerals and Materials Technology (CSIR), Bhubaneswar, is greatly acknowledged. B.N. is grateful to CSIR, New Delhi, for awarding him an SRF. Supporting Information Available: The Supporting Information contains structures and absorption spectra of MO dye, the crystalline framework of SNT-2, and time courses of the photocatalytic degradation of MO on SNT in each 20 min interval. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Fujishima, A.; Honda, K. Nature 1972, 238, 37–38. (2) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69–96. (3) Chen, X.; Mao, S. S. Chem. ReV. 2007, 107, 2891–2959. (4) Choi, W.; Termin, A.; Hoffmann, M. R. Angew. Chem. 1994, 106, 1148–1149. (5) Wang, C.; Bahnemann, D. W.; Dohrmann, J. K. Chem. Commun. 2000, 1539–1540. (6) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269–271. (7) Khan, S. U. M.; Al-Shahry, M.; Ingler, W. B. Science 2002, 5590, 2243–2245. (8) In, S.; Orlov, A.; Berg, R.; Garcia, F.; Pedrosa-Jimenez, S.; Tikhov, M. S.; Wright, D. S.; Lambert, R. M. J. Am. Chem. Soc. 2007, 129, 13790– 13791. (9) Yang, K.; Dai, Y.; Huang, B.; Han, S. J. Phys. Chem. B 2006, 110, 24011–24014. (10) Yu, J. C.; Ho, W.; Yu, J.; Yip, H.; Wong, P. K.; Zhao, J. EnViron. Sci. Technol. 2005, 39, 1175–1179. (11) Sathish, M.; Viswanathan, B.; Viswanath, R. P.; Gopinath, C. S. Chem. Mater. 2005, 17, 6349–6353. (12) Lin, L.; Lin, W.; Xie, J. L.; Zhu, Y. X.; Zhao, B. Y.; Xie, Y. C. Appl. Catal., B 2007, 75, 52–58. (13) Yang, K.; Dai, Y.; Huang, B. J. Phys. Chem. C 2007, 111, 12086– 12090.

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JP1083345

Facile Synthesis of N- and S-Incorporated Nanocrystalline TiO2 and

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