Template-Free, Solid-State Synthesis of Hierarchically Macroporous S-Doped TiO2 Nano-Photocatalysts for Efficient Water Remediation.

Nanosized sulfur-doped titanium dioxide emerged as an attractive photocatalyst in various environmental remediation applications, yet most synthesis methods require hazardous sulfurizing agents and intricate synthesis procedures. Herein, we present a facile, sustainable, and environmentally friendly preparation process for the production of visible-light-active meso-macroporous sulfur-doped anatase TiO2 (S-TiO2) nanoparticles for the first time. This strategy encompasses solventless mixing of titanium salt and surfeit yet nontoxic abundant elemental sulfur under continuous ball milling and moderate thermoannealing. The characterizations of as-obtained S-TiO2 nanoparticles showed enhanced physicochemical properties including distinctive surface features composed of hierarchical hollow macroporous channels having nanostructured mesoporous core walls. The annealing temperature was observed to control the structure and extent of sulfur doping in TiO2. Upon insertion of a sulfur atom into the TiO2 lattice, the band gap energy of S-TiO2 was significantly lowered, facilitating the enhanced photochemical activity. Owing to the effective S doping (1.7-2.8 atom %), and the interconnected hollow meso-macroporous nanostructure, the resulting nanosized S-TiO2 exhibited unique adsorption properties and superior photocatalytic efficiency for the rapid degradation of hazardous organic dyes and phenols for water remediation. The presented strategy holds high potential to provide rapid production of a hierarchical and highly porous S-TiO2 photocatalyst on a large scale for various environmental remediation and other myriad photochemical applications.

Introduction

Light-induced oxidative catalytic treatment for water remediation is considered as one of the most effective and viable measures to resolve the ever-growing water pollution.[1,2] Hence, the search for highly active and economical photocatalysts for effective remediation of wastewater is still ongoing and considered to be the most sustainable approach.[3,4] Over the past few decades, the use of nanostructured materials, especially semiconductor types such as TiO2, ZnS, CdS, CeO2, Fe2O3, GaN, Bi2S3, and many more, has attracted tremendous attention as potential photocatalysts because of their distinctive band gap energies and because of charge-transfer processes harnessing these nanomaterials to effectively harvest light energy for the oxidative decomposition of various toxic contaminants.[5−10]

Among these heterogeneous photocatalysts, titanium dioxide (TiO2) or titania has emerged as one of the extensively studied semiconductor photocatalysts in various environmental remediation applications.[11−18] The choice of TiO2 over other existing semiconductor photocatalysts was mainly due its robust properties such as high photo-oxidation capacity, unique band gap energy, high charge-transfer capability, long-term photo- and thermal stability against corrosion, diverse and hierarchical morphology, economic production, etc.[19] However, the practical applications of TiO2 in visible-light-assisted chemical reactions are limited because of its poor visible light absorption capacity (Eg ≈ 3.2 eV).[20,21] Hence, to utilize TiO2 photocatalyst under visible light, a wide array of strategies have been developed that include oxygen substitution by nonmetals and heteroatoms such as nitrogen (N), sulfur (S), fluorine (F), etc. and proven to be an effective way to modify and enhance the structural and physicochemical parameters of TiO2-based nano-photocatalysts.[22−26] Among these, the inclusion of a sulfur atom can significantly amend the electronic properties of TiO2, primarily owing to the larger ionic radius of “S” compared to those of other heteroatoms, which can induce a band gap narrowing effect on TiO2 and harness it as a highly active visible-light photocatalyst.[27−29] So far, numerous methodologies have been reported to synthesize S-TiO2 photocatalysts, such as a solvothermal,[29,30] hydrothermal,[31,32] sol–gel,[33] wet-chemical,[34] coprecipitation,[35] chemical vapor deposition (CVD),[36] and supercritical methods.[25] However, most of these methods frequently use highly intricate procedures, various toxic sulfurizing agents, and expensive surfactants and templates that invariably restrict the large-scale and sustainable production of S-doped TiO2 for large-scale water treatment.

Similarly, the nano-photocatalysts composed of an ordered macroporous structure were proven to be highly advantageous due to their unique morphological and structural characteristics.[37,38] Such a hierarchical structural feature often possesses high surface area and plentiful exposed active sites for the effective reactant adsorption and subsequent photoinduced reactions. Moreover, the nanostructured materials having hollow channelized macroporous–mesoporous network cores can further facilitate the effective diffusion and adsorption of large-sized pollutant molecules.[38−40] Similarly, these hierarchical nano-photocatalysts exhibit superior light-harvesting ability owing to their enhanced multilight scattering and reflection.[41,42] Therefore, if harnessed with these fascinating properties, the macroporous S-TiO2 nano-photocatalysts could exhibit superior photocatalytic performance when compared to that of the pristine TiO2 for effective water remediation. Hence, the development of a facile and scalable synthesis method is the key to the commercialization of such macroporous S-TiO2 nano-photocatalysts for water remediation and other environmental applications.

In the present study, we report a newer synthetic strategy that reveals in situ sulfur atom infusion into TiO2 using abundant elemental sulfur as a sulfuring agent. The presented methodology resulted in a unique S-TiO2 nanostructure composed of macroporous channels with mesoporous cores for the first time. Owing to its environmental benignity, reproducibility, and low-cost, this strategy holds high potential for large-scale production. A series of experiments and characterizations were performed to investigate the temperature effect on the structure and properties of the resultant S-TiO2 nanostructures. The as-synthesized macroporous S-TiO2 nanoparticles were further studied as visible-light-active nano-photocatalysts for the photodegradation of organic dyes such as methylene blue (MB) and methyl orange (MO). Similarly, the degradation of toxic phenols such as 4-nitrophenol (4-NP) was also accomplished. It was demonstrated that owing to its unique surface features and enhanced photoresponse, the synthesized porous S-TiO2 nano-photocatalysts exhibit a superior visible-light-driven photocatalytic activity and greater efficiency for sustainable water remediation.

Experimental Section

Preparation of S-TiO2 Nano-Photocatalysts

The phase-specific hierarchical meso–macroporous S-TiO2 nano-photocatalysts were prepared by an environmentally friendly solventless template-free approach (Scheme ). In this typical solid-state procedure, 0.6 g of titanium hydroxide and 1.0 g of elemental sulfur were ball-milled using an IKA ULTRA-TURRAX Tube Drive control homogenizer at 4000 rpm for 40 min using ten balls, each weighing 509.3 mg. The tube is purged with nitrogen intermittently during the continuous ball milling. Subsequently, the resulting solid powder mix was subjected to thermal treatment using a tube furnace at different temperatures such as 400, 500, and 600 °C for 2 h at a heating rate of 5 °C min–1 under an inert atmosphere that produces pale yellow nanosized S-TiO2 powders. For comparison, a similar strategy was employed to synthesize pure TiO2 nanoparticles at 500 °C, in the absence of elemental sulfur. The resulting nano-photocatalysts were directly used and do not require any further purification or washing procedures.

Scheme 1

Schematic Representation of the Synthesis Process to Prepare Macroporous S-TiO2 Nano-Photocatalysts

Material Characterization

The resulting macroporous S-TiO2 and TiO2 nano-photocatalysts were broadly characterized by employing several advanced techniques. The crystalline structure was elucidated by X-ray diffraction (XRD) (Philips X’Pert Pro) with a scintillation counter and Cu Kα radiation (λ = 1.54 A°) reflection mode, a voltage of 35 kV, and a current of 40 mA, and scanning was performed at a diffraction angle ranging between 5 and 80°. The average crystallite size of the S-TiO2 nanoparticles was estimated from the broadening of the diffraction peak using the Debye–Scherrer formula, D = Kλ/β cos θ, where D is the crystallite size (nm), K is the Scherrer constant, λ is the wavelength of the X-ray source, β is the full width at half-maximum, and θ is the Bragg angle. The surface morphologies were investigated by a scanning electron microscope (SEM, Zeiss-1540 XB) and a transmission electron microscope (TEM/HRTEM, FEI Tecnai-G20). Energy-dispersive X-ray analysis (EDX) was used to determine the extent of sulfur doping and respective elemental mapping in the S-TiO2 nanoparticles. X-ray photoelectron spectroscopy (XPS) analysis was performed on a Surface Science Laboratories, Inc. SSX-100 with an Al Kα X-ray emission source. The Brunauer–Emmett–Teller (BET) surface areas and BJH pore size distributions were measured by N2 adsorption using a Quantachrome Autosorb gas-sorption system pretreated at 100 °C, and samples were analyzed on an ASPS 2010 gas adsorption analyzer (Micromeritics). Room-temperature Raman spectra were measured with a LabRam HR Horiba Scientific confocal Raman microscope with an excitation line of 633 nm. The UV–visible diffuse reflectance (UV-DRS) spectra were obtained using a UV–vis–NIR spectrophotometer (Shimadzu, Japan, Model UV-3600) over a wavelength range of 200–800 nm. The concentrations of dyes and phenol solution were measured by a Cary 500 UV–vis spectrophotometer (Varian, Palo Alto, CA).

Photocatalytic Studies

Preweighed portions of the as-prepared nano-photocatalysts (10 mg), respective dyes, and nitrophenol aqueous solution (1 × 10–4 mol L–1) were added into a reaction flask with simultaneous shaking. Later, this dispersion was left in the dark for 30 min to attain the adsorption–desorption equilibrium. Subsequently, this dispersion was exposed to visible light generated from a 300 W metal halide lamp (OSRAM, Germany). The photocatalytic reactions were monitored by analyzing the aliquots taken at deferent time intervals using a UV–vis spectrometer. The changes in the characteristic absorbance peaks of the respective dyes were used to validate the photocatalytic reactions.

The photocatalytic degradation efficiency was estimated by the following equationwhere D is the nano-photocatalyst degradation efficiency and C0 and C represent the concentrations of dyes before irradiation and at a tested time interval, respectively.

Similarly, the photodegradation reaction rate constant (k) was obtained by employing the following equation

Recyclability and Reactive Oxygen Species (ROS) Tests

The photocatalyst stability was evaluated by repeated photocatalytic reactions, i.e., recycling under the set conditions. Between subsequent cycles, the photocatalyst was recovered by centrifugation and repeatedly washed using deionized water and ethanol before the next cycle. The morphological and structural features of the recycled catalysts were revealed by SEM and XRD studies. Similarly, the reactive oxygen species (ROS) generated during the photocatalytic reactions were monitored by scavenging experiments. The different types of scavengers, namely, p-benzoquinone (BQ), t-butanol (t-BuOH-TBA) and silver nitrate (AgNO3-SN) were used to trap the reactive oxygen species such as superoxide anion radical (•O2–), h+ (holes) scavenger and •OH scavenger, respectively. Typically, 10 mg of photocatalyst was mixed into 50 mL of MB (10 ppm) solution, then 20 mg of scavenger was added, and the reaction mixture was subjected to photocatalytic reactions similar to S-TiO2, as described in the photocatalytic reaction study.

Results and Discussion

Nano-Photocatalyst Characterization

The formation of TiO2 can be achieved by the dehydration of Ti(OH)4, as shown in eq . Subsequently, sulfur above its vaporization temperature can replace the oxygen atom from the TiO2 crystal lattice to form S-doped TiO2 nanoparticles (eq ).[43,44]The crystalline structure and phase orientation resulting in S-TiO2 nanoparticles prepared at different annealing temperatures are investigated by XRD diffraction analysis, as shown in Figure . Both pristine TiO2 and S-TiO2 nanoparticles show characteristic peaks at 25.25, 38.02, 48.13, 54.00, 55.19, 62.86, 68.92, 70.50, and 75.22°, which can be indexed to the (101), (004), (200), (105), (221), (204), (116), (220), and (215) planes of the anatase TiO2 (JCPDS 21-1272), respectively.[45] The XRD pattern features are identical for both TiO2 and S-TiO2, thus indicating that the insertion of the S species inside the TiO2 does not alter its inherent lattice structure under the adopted preparatory conditions. Moreover, the XRD results confirm that the presented synthesis strategy forms exclusive anatase TiO2 phases, which is considered to be a better photocatalytic performer than its rutile phase counterpart.[46] Also, it was evident that the crystallinity of S-TiO2 nanoparticles increased with the calcination temperature without altering the characteristic anatase phase. The average crystallite size of the S-TiO2 nanoparticles showed a trivial increment with the annealing temperature as estimated using the Debye–Scherrer equation,[47] as presented in Table . It was also noted that the higher annealing temperature leads to a slightly increased particle size of the resulting S-TiO2 nanoparticles. These results underline that the annealing temperature has a size-controlling effect on these in situ-formed nanoparticles is trivial.

Figure 1

XRD pattern of as-prepared TiO2 and S-TiO2 at different temperatures.

Table 1

Crystallite Size, Sulfur Content, and Band Gap of TiO2 and S-TiO2 Nanoparticlesd

sample	average Dp (nm)a	S content (atom %)b	band gap (eV)c	surface area (m2 g–1)
TiO2	16.5	0	3.14	13.27
S-TiO2 400	16.1	1.7	3.00	53.37
S-TiO2 500	18.4	2.8	2.89	67.75
S-TiO2 600	20.8	2.4	2.80	63.51

As determined by XRD.

XPS.

UV-DRS.

N2 adsorption (BET).

The morphological features of as-synthesized S-TiO2 nanoparticles were examined by SEM and TEM. The SEM micrograph of S-TiO2 formed after thermal annealing at 500 °C (Figure a–c) clearly shows that the distinctly spread spherical macrospores were laterally arranged to have diameters ranging from 0.5 to 1 μm. Similarly, it can be seen that the broken sidewalls of the macropores are composed of S-TiO2 nanoparticles and further indicate that the internal cylindrical arrangement has a one-dimensional (1D) macroporous structure (Figure c). These unique 1D macrospores are representing ultralong (ca. 20 μm) ordered channels with one end closed and having uniformly distributed S-TiO2 assemblies. Unlike previously reported studies, no surfactants or templates were used in the present study to create the macroporous S-TiO2 structures.[48] It is believed that the meso–macroporous morphology could emerge when thermal treatment of titanium hydroxide and elemental sulfur undergoes a controlled dehydration and sulfur sublimation, which lead to the formation of mesoporous S-TiO2 nanoparticles. Subsequently, the growth of these mesoporous assemblies continues to form macroporous-patterned interconnected skeletons.

Figure 2

SEM images (a–c) and mixed elemental mapping of O–S (d) of S-TiO2 500.

Thus, it was expected that such unique cavelike channels could facilitate fast diffusion and adsorption of toxic organic pollutants. Further, the mixed element mappings of O–S (Figure d) and Ti–S (Figure S1) clearly indicated the successful and uniform sulfur doping in the TiO2 lattice. Moreover, the TEM images revealed that the as-formed S-TiO2 nanoparticles are tightly aggregated having an average size within the 20 nm range (Figure a). The high-resolution TEM (HRTEM) image shown in Figure b depicts the exposing of distinctive lattice fringes, confirming the high crystallinity of the S-TiO2 nanoparticles. Further, the spacings between the lattices were estimated to be 0.36 nm, which matches well with the distance between the characteristic (101) crystal planes of anatase TiO2. For S-TiO2, the elemental mapping and the EDS spectra (Figure S2a–d) indicated the exclusive presence of Ti, O, and S elements, which exclude the possibility of any foreign element contamination. For comparison, the morphological features of S-TiO2 nanoparticles prepared at 400, 600 °C are presented in Figure S3. It was also observed that the thermoannealing conditions have a significant impact on the overall morphology and the pore structures of the resulting S-TiO2. At low temperatures, macrospores are not well grown and poorly crystallized (Figure S3a).

Figure 3

TEM (a) and HRTEM (b) images of S-TiO2 500.

Similarly, at higher temperatures, the sample is well crystallized but most of the pores are blocked by freely grown particles (Figure S3b). Overall, the precise control over the annealing temperature was believed to be an imperative parameter for the generation of such unique 1D macrochannel morphology. The surface chemical compositions, binding energies, and valence states of the resulting S-TiO2 elements were investigated by XPS analyses, as shown in Figure . The atomic concentration of the S atom in S-TiO2 prepared at different temperatures was estimated from XPS analysis and is presented in Table . The highest S doping was achieved at an annealing temperature of 500 °C.

Figure 4

XPS survey spectra of as-obtained S-TiO2 500 (a) and Ti 2p (b), O 1s (c), and S 2p (d) signals taken from the S-TiO2 500 sample.

Figure a provides the XPS survey spectra of the S-TiO2 500 °C sample that clearly indicates the typical elemental peaks of Ti 2p, O 1s, and S 2p; further, the absence any other element peak confirms the contamination-free in situ evolution of S-TiO2 macroporous nanoassemblies. In Figure b, the core-level XPS spectra shows two doublet peaks at 459.7 (Ti 2p1/2) and 465.8 (Ti 2p3/2) eV, demonstrating the characteristics of Ti(IV) species. Similarly, a peak at 531.9 eV was characteristic of oxygen atom infused into the crystal lattice of S-TiO2 (Figure c). Figure d shows two peaks for the S 2p region centered at about 161.3 (S 2p1/2) and 165.3 (S 2p3/2) eV, which are ascribed to the spin–orbit S2– states. Moreover, the absence of the S6+ XPS peak at around 169.0 eV confirms that all S atoms are present in the state of S2–. This observation support the formation of the Ti–S bond and signifies the infusion of a few sulfur atoms into the TiO2 crystal lattice, thereby stimulating the band gap narrowing in S-TiO2.[27]

The microstructure of S-TiO2 nanoparticles was further investigated by Raman scattering spectra, as illustrated in Figure a. The as-obtained nanoparticles exhibit four typical Raman peaks at around 147 cm–1 (Eg), 392 cm–1 (B1g), 508 cm–1 (A1g), and 632 cm–1 (Eg), representing the characteristic Raman scattering for TiO2.[49] The absence of any other sulfur- or titanium-related peak confirms that S-TiO2 has similar Raman scattering to that of TiO2, which is well supported by XRD results shown in Figure . However, an enlarged image of the Eg Raman band at 147 cm–1 shown in the inset of Figure a indicates a slight peak shift of undoped TiO2 toward a higher side compared to that of S-TiO2. Such shift could be ascribed to an S atom insertion during the Ti–S bond formation, which could alter the resulting force constant of the Eg vibration mode in contrast to the Ti–O bonds.[50] Hence, Raman results indicate that the insertion of a sulfur atom can alter the TiO2 microstructure to a certain extent. No significant effect of the annealing temperature (400 and 600 °C) was observed in the respective Raman spectrum (Figure S4).

Figure 5

Raman spectra of TiO2 and S-TiO2 500 (a), N2 adsorption and desorption curves (inset is the BJH pore volume distribution curve) of S-TiO2 500 (b), UV-DRS spectra of TiO2 and different S-TiO2 nanoparticles (c), and photoluminescence emission spectra of TiO2 and S-TiO2 500 (d).

In addition, Figure b shows the N2 adsorption–desorption isotherms of S-TiO2 500, indicating a type IV isotherm and an H1 hysteresis loop, which designates the evolution of the inhomogeneous mesoporous structure. Similarly, the corresponding BJH pore size distribution curve (inset of Figure b) exhibits a broad peak around 18 nm, which clearly indicates the mesoporous nature of S-TiO2 (Table S1). The tabulated BET surface areas for TiO2 and S-TiO2 nanoparticles (Table ) indicated that upon sulfur doping the BET surface areas of S-TiO2 samples were significantly increased over pristine TiO2 nanoparticles (Figure S5). At higher temperatures, owing to the higher crystallinity, a slight decrease in the surface area was observed. Further, the total pore volumes for the resulting S-TiO2 and pristine TiO2 nanoparticles were measured to be between 0.019 and 0.280 cm3 g–1, respectively. Hence, owing to this enhanced surface features (both the specific surface area and pore diameters), the S-TiO2 nanoparticles are expected to show better interfacial contacts with reactant molecules compared to those of pristine TiO2.

The UV–vis diffuse reflectance (UV–vis DRS) converted absorbance spectra of TiO2 and S-TiO2 nanoparticles were recorded to understand the visible light response of synthesized materials and are shown in Figure c. It was observed that the insertion of a sulfur atom exhibits significant light absorption in the visible region mainly due to an active contribution of p state of sulfur in lowering the band gap after mixing with O 2p states of TiO2.[27] Thus, sulfur doping was observed to enhance the optical properties of the resulting S-TiO2. Moreover, the changes in optical behavior were also confirmed by a change in the color of the resulting S-TiO2 powder (pale yellow) compared to that of TiO2 (gray-white), as shown in the inset of Figure c. The band gap energy was measured from absorbance spectra derived from the transformation based on the Kubelka–Munk equation and is tabulated in Table . The S-TiO2 nanoparticles annealed at 500 °C (2.80 eV) showed band gap narrowing compared to TiO2 nanoparticles (3.14 eV), which is well enough to absorb light in the visible region. Further, it was evident that the lower annealing temperature (400 °C) showed a small band gap narrowing compared to S-TiO2 annealed at higher temperatures, probably due to the increased crystallinity with temperature.

Similarly, the efficiency of charge carrier separation and transportation of the as-prepared macroporous S-TiO2 materials were analyzed by photoluminescence (PL) spectroscopy (Figure d). It was observed that both TiO2 and S-TiO2 show PL peaks that are closely centered, which confers that the fluorescence has arisen from the TiO2 interior recombination of electron–hole pairs primarily accredited to charge transfer from the Ti3+ state to the oxygenated state of the TiO68– complex.[51] However, efficient quenching of the PL intensity of S-TiO2 500 compared to that of TiO2 suggests that the S insertion into the TiO2 lattice can dramatically subdue the charge carrier recombinations.[52] Consequently, the photocatalytic activity of the resulting photocatalysts may be enhanced because of its high charge carrier separation efficiency.

Photocatalytic Degradation

The aforementioned characterizations evidently reveal that in the resulting S-TiO2 samples the S atom is successfully infused into the crystal lattice TiO2, ensuring a distinct enhancement of the surface properties and, notably, the absorption ability in the visible-light region. Such visible-light-harvesting ability enables S-TiO2 to be utilized as a nano-photocatalyst for the degradation of harmful organic pollutants under solar light. Hence, we selected three types of common water pollutants, i.e., cationic dye (methylene blue—MB), anionic dye (methyl orange—MO), and phenol (4-nitrophenol—4-NP), as the targeting molecules. The molecular structures of these molecules are presented in Figure S6. The photocatalytic degradation efficiencies of the S-TiO2 and TiO2 nanoparticles were studied by monitoring the photo-oxidation behavior of MB, MO, and 4-NP.

Figure a demonstrate the enhanced photocatalytic efficiency for S-TiO2 500 toward the degradation of MB as a gradual decrease in the intensity of characteristic absorption peaks of MB (λ = 668 nm) accompanied by substantial discoloration with increase in the irradiation time was observed when compared with that of pristine TiO2 (Figure S7). It was also noted that the S-TiO2 500 nano-photocatalyst completely degrades the MB at a faster rate in 60 min with a degradation efficiency of 98%, whereas only 67% degradation was achieved using pristine TiO2 (Figure b). Similarly, other S-doped samples S-TiO2 400 and S-TiO2 600 show degradation efficiencies of 82 and 90%, respectively. Such enhanced efficiency of S-TiO2 500 photocatalysts could be ascribed due to the hierarchical macroporous channels that may facilitate the diffusion of the dye and have maximum reach to the available active sites for the possible reaction to occur. Furthermore, the photocatalytic reaction kinetics of the MB dye degradation was investigated by employing a pseudo-first-order kinetic model that enables us to calculate the photocatalysis reaction rate constant (k) under visible light (Figure c). This kinetic study reveals “k” values to be 0.019, 0.038, 0.75, and 0.10 min–1 for the samples of TiO2, S-TiO2 400, S-TiO2 500, and S-TiO2 600, respectively. Among these samples, the S-TiO2 500 nano-photocatalysts exhibit almost 40 times higher degradation efficiency than that of pristine TiO2 and around 20 times than that of S-TiO2 400.

Figure 6

(a) UV–visible absorbance spectra vs photoreaction time and discoloration of MB dye (inset) in the presence of S-TiO2 500. (b) Comparison of the photocatalytic activities of TiO2 and S-TiO2 (400, 500, 600) for the degradation of MB in the aqueous solution. (c) Corresponding first-order kinetics plots. (d) Effect of a series of scavengers on the degradation efficiency of S-TiO2 500 for MB (illumination time t = 80 min).

To further shed light on the reaction mechanism involved in the photocatalytic degradation of MB, the trapping experiments were performed, and the results are displayed in Figure d. The photocatalytic efficiency of S-TiO2 500 was recorded upon the addition of AgNO3, t-BuOH, and benzoquinone (BQ), which were used to trap the reactive species (ROS), namely, h+, •OH, and •O2– radicals, respectively, generated during the photodegradation of MB.[43] It was observed that the particular ROS elimination slows down the photocatalyst activity, demonstrating that the analogous ROS is critical in the photocatalytic reaction. Specifically, when BQ was used, pronounced quenching of the photocatalytic reaction was observed and the degradation efficiency was recorded to be 42.8%, whereas t-BuOH and AgNO3 show decrease in efficiency 64.5 and 83.1%, respectively. These results indicated that the •O2– radical remains the dominant active species and •OH and h+ species play a minor role in the overall visible-light-driven photocatalytic mechanism. Furthermore, owing to its importance in practical applications, the photocatalyst photostability and recyclability were also tested for the S-TiO2 500 sample under repetitive photocatalytic cycles (Figure a).

Figure 7

Recyclability of the S-TiO2 photocatalyst for the photocatalytic reaction (a), XRD patterns of S-TiO2 500 before and after the recycling test (b), and morphology of S-TiO2 500 after the recycling test (c).

A marginal decrease in the photocatalytic activity was noted even after four cycles of MB degradation under identical experimental conditions. Therefore, S-TiO2 nano-photocatalysts can be essentially realistic in an ecofriendly way as a regenerative photocatalyst with excellent efficiency for water remediation. Also, no significant changes in the crystal structure and morphology were observed in XRD and SEM analyses performed on the recycled S-TiO2 500, indicating the significant structural and crystalline stability (Figure b,c). Moreover, a very trivial reduction in the sulfur content was observed in the photocatalyst sample after the recycling test (Figure S8). This result indicates the structural integrity of S-TiO2 and indicates that the sulfur atom remains in the lattice structure of S-TiO2 and does not undergo oxidation under extended light exposure.

In addition to MB, further photodegradation studies using the best performing S-TiO2 500 nano-photocatalyst toward the degradation of another model dye molecule methyl orange (MO) and toxic phenol molecule 4-nitrophenol (4-NP) were accomplished and are displayed in Figure a,b. As can be seen, a gradual decrease in the intensity of the characteristic absorption peaks of MO and 4-NP, accompanied by substantial discoloration with an increase in the irradiation time, was observed. Furthermore, the time-dependent absorption spectra imply that the S-doped TiO2 nanoparticles show a faster dye degradation rate compared to that of pristine TiO2, indicating enhanced photocatalytic activity. The photocatalytic degradation efficiency and reaction kinetic plots indicated that S-TiO2 500 showed the highest activity among the pristine TiO2 and S-TiO2 nanoparticles examined (Figures S9 and S10). The higher photocatalytic rate for the S-TiO2 500 photocatalyst can be attributed to its higher level of S doping that enabled the optimum optical band gap and improved surface properties. The resulting S-TiO2 500 shows enhanced visible-light-induced photocatalytic performance when compared with that of previously reported S-TiO2 and other nonmetal-doped TiO2 photocatalysts (Table S1).

Figure 8

Comparison of the photocatalytic activities of TiO2 and S-TiO2 (400, 500, 600) for the degradation of MO (a) and 4-NP (b) in the aqueous solution with respective dye discoloration (inset).

As confirmed from various characterization results, the S atom was successfully incorporated into the TiO2 crystal lattice through “O” replacement, which altered the surface properties of the latter very significantly. This observation indicates that the atomic replacement of oxygen by sulfur improves the surface properties and visible-light activity of S-TiO2, which can promote the organic dye adsorption capability and subsequent photodegradation. In brief, mechanistically, the photocatalytic degradation of organics involves the activity of charge carriers that are generated upon photoexcitation of S-TiO2 nanoparticles. In pristine TiO2, the orbital positioning of Ti 3d and O 2p was attributed to the conduction band (CB) and valence band (VB), respectively, and occupied by photogenerated charge carriers. After substitution of S for an O atom into the TiO2 crystal structure, new S 3p states are formed just above the O 2p, therefore yielding band gap narrowing through the shallow acceptor level under the conduction band of the S 3p orbital. Hence, such a mixing of the S 3p states with VB widens the VB state toward CB, which validates the band gap narrowing effect after S doping, as shown in Scheme . It is rationalized that the photogenerated charge carriers (h+, e–) facilitate the formation of various reactive oxygen species (•OH, O2•–, etc.) that lead to the oxidative degradation of organic dyes and toxic phenol molecules, as illustrated in Scheme .[53,54]

Scheme 2

Schematics Representing the Plausible Photodegradation Mechanism Using S-TiO2 Nano-Photocatalysts under Visible-Light Irradiation

Conclusions

A series of nanosized anatase S-TiO2 particles having a unique meso–macroporous architecture were prepared using a template-free, facile, scalable method for the first time. This environmentally friendly method uses surfeit yet nontoxic elemental sulfur as an effective sulfurizing agent. The reaction temperature was observed to control the size, S doping, and surface properties of the resulting S-TiO2 nanoparticles. The highest S-doping level was achieved at 500 °C, demonstrating band gap narrowing and exhibiting enhanced visible-light-responsive optical properties compared to those of pristine TiO2. Moreover, the unique one-dimensional hollow macroporous channels having a mesoporous core nanostructure favor easy transportation cum adsorption of relatively large organic toxic molecules and also facilitate multilight scattering/reflection, expediting the effective harvesting of the excited light. Hence, these surface-modified S-TiO2 nanoparticles displayed remarkable photocatalytic activity and effective degradation of dyes and toxic phenols was successfully accomplished. S-TiO2 500 displays outstanding photocatalytic efficiency, which could be endorsed by the cumulative effect of narrowed band gap energy, high photogenerated charge separation, enhanced crystallinity, and increased surface area. The present study provides a newer pathway to produces high-efficiency visible-light-active nano-photocatalysts using a scalable, economical, and sustainable method. Owing to their exceptional light-harvesting ability, the resulting S-TiO2 nanoparticles hold high potential for numerous other environmental and energy applications.