ZnS quantum dots impregnated-mesoporous TiO 2 nanospheres for enhanced visible light induced photocatalytic application †

ZnS quantum dots were impregnated on the surface of TiO 2 mesospheres by a soft template-assisted solvothermal approach. XRD and elemental analysis con ﬁ rmed the presence of ZnS in the TiO 2 nanostructures. Morphological analysis showed that the ZnS quantum dots were ﬁ rmly immobilized on the TiO 2 mesospheres, which improved electron and hole pair separation at the TiO 2 /ZnS interface. The photocatalytic activity of the mesoporous nanostructures was assessed by photodegradation of methylene blue (MB) as a model pollutant under visible light irradiation. Impregnation with ZnS quantum dots enhanced reaction activity remarkably compared with mesoporous TiO 2 . The maximum degradation e ﬃ ciency was observed for 0.025 M of ZnS impregnated on TiO 2 . The MB-related absorption peak completely disappeared after 32 min of irradiation. Photo-charge scavenger analysis indicated that hydroxyl radicals played a pivotal role in the photodegradation mechanism. The mesoporous photocatalyst was stable and can be used repeatedly under visible irradiation.


Introduction
2][3] The search continues for an economic and efficient method to remove such pollutants from wastewater and reduce negative environmental impacts.Heterogeneous photocatalysis is a popular technique with great potential in degradation of organic pollutants, and has proven to be effective in environmental remediation. 46][7] The organic compounds are decomposed into the less toxic materials CO 2 and water. 2,8TiO 2 is one of the best photocatalysts for the self-cleaning environmental applications because of its relatively high efficiency, low cost and its availability.TiO 2 has been widely used as a photocatalyst for removal of hazardous organic substances and as an electrode material for dye-sensitized solar cells 9,10 because of its strong oxidizing and reducing ability under UV light irradiation.
Mesoporous TiO 2 is an interesting structure for photocatalytic applications because of its continuous particle framework, which is benecial for catalyst recovery when compared with nanoparticles. 11The photocatalytic activity of mesoporous TiO 2 is affected by larger surface area that increases the reaction rate, while the existence of amorphous phases promotes eÀ and h + recombination, which in turn decreases photocatalytic activity.Additionally, in the photocatalytic eld, the pores of nanostructures can serve as channels for charge carriers to penetrate the interior and reduce the recombination rate, thus enhancing the activity. 12To exploit its excellent photocatalytic performance, much effort has been devoted to synthesis of mesoporous TiO 2 in recent years.Attempts have been made to control the crystallization and to make more crystalline material with maintenance of mesoscale order. 13Da Silva et al. 14 prepared truncated bipyramidal Wulff-shape mesoporous TiO 2 by nonaqueous sol-gel synthesis, and this exhibited superior photocatalytic activity.Liu et al. 15 synthesized spindle-shaped mesoporous TiO 2 using an aqueous peroxotitanium solution with polyacrylamide.7][18] Several strategies have been adopted for preparation of such visible light active photocatalysts.6][27][28][29] In particular, ZnS has been studied in many applications as it is an environmentally friendly material and is constituted by earthabundant elements. 30ZnS is an important II-VI semiconductor exhibiting a wide direct optical band gap (3.6 eV), making it a very attractive material for optical applications, especially in its nanocrystalline form.Photocatalytically active TiO 2 /ZnS composites show better photostability and activity than do their individual components.Vaclav et al. 31 synthesized a TiO 2 /ZnS nanocomposite by homogeneous hydrolysis in an aqueous solution of thioacetamide.The prepared composite showed better photocatalytic activity compared with bare TiO 2 and ZnS nanoparticles.Xiaodan et al. 32 prepared photoactive ZnS/TiO 2 nanocubes via a microemulsion-mediated solvothermal method.The photocatalytic activity of ZnS/TiO 2 composites was enhanced compared with pure anatase TiO 2 under visible light irradiation.Srinivasa Rao et al. 33 synthesized a TiO 2 /ZnS photo-anode on uorine-doped tin oxide (FTO), which accumulated a large number of photo-injected electrons in the conduction band (CB) and achieved lower recombination rate compared with bare TiO 2 .Franco et al. 34 synthesized a distinct nanocrystalline TiO 2 -capped ZnS using a chemical vapour deposition method.The TiO 2 -capped ZnS increased the catalyst photoactivity compared with bare TiO 2 .However, all these reports are related to the TiO 2 nanoparticles and ZnSany interaction between the ZnS nanoparticles and TiO 2 nanoparticles is not discussed.Therefore, research is required into the ZnS included TiO 2 mesoporous network, with the aim of to extending charge separation and ultrafast degradation under visible light irradiation.To date, no research has been reported on mesoporous TiO 2 /ZnS nanospheres.
The present study describes a so template route to synthesize ZnS quantum dot impregnated mesoporous TiO 2 spheres with enhanced photocatalytic activity.The effect of metal sulphide concentration on the phase and morphology was investigated.The functional properties of mesoporous TiO 2 spheres were investigated using X-ray diffraction (XRD), eldemission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS).The photocatalytic activity of the synthesized materials was characterized by quantifying the rate of methylene blue (MB) degradation in the aqueous suspension under visible light irradiation.The photostability and photocatalytic mechanism of mesoporous TiO 2 is proposed.

Experimental method
All chemicals were purchased from Wako Chemicals (Japan) and used without further purication.Three steps were followed for preparation of mesospheres TiO 2 /ZnS mesoporous nanostructures.
Formation of titania glycolate spheres 1 mL of titanium tetraisopropoxide was added to 50 mL ethylene glycol.The solution was stirred at room temperature, and then added to an acetone bath (150 mL) containing trace water.The solution was stirred for 2 h to form a white suspension, which was collected by centrifugation, thoroughly washed with distilled water and ethanol to remove impurities, and dried at 100 C for 10 h.

Formation of mesoporous TiO 2 spheres
The titania glycolate spheres were dispersed in an equal volume of water and ethanol (30 mL), and stirred for 2 h.The white solution was transferred to a 100 mL Teon-lined stainless steel autoclave, and heated at 150 C for 12 h.The resulting product was collected and annealed at 300 C for 2 h.

Formation of mesoporous TiO 2 /ZnS nanocomposites
The mesoporous TiO 2 spheres were dispersed in 50 mL of water and different mole concentrations of zinc acetate (0.025, 0.050, 0.075 and 0.1 M) and thioacetamide (0.025, 0.050, 0.075 and 0.1 M) were added.Pyridine was added as a capping ligand.The reaction was stirred for 12 h.The white solution was transferred to a 100 mL Teon-lined stainless steel autoclave, and heated at 150 C for 15 h.The resulting product was collected and dried at 100 C for 10 h.The sample were termed as Ti for pure mesoporous TiO 2 , TiZ-1 for 0.025 M of ZnS, TiZ-2 for 0.050 M of ZnS, TiZ-3 for 0.075 M of ZnS and TiZ-4 for 0.1 M of ZnS, respectively.Characterization Surface morphologies were observed using JEOL JSM 7001F eld-emission scanning electron microscopy (FESEM).Transmission electron microscopy (TEM) images were recorded using a JEOL JEM 2100F microscope at an accelerating voltage of 200 kV.Crystalline phases were obtained by X-ray diffraction (XRD), using a Rigaku diffractometer (RINT-2200, Japan, CuK a radiation) with a 0.02 s À1 scan rate.Raman spectra were obtained using a JASCO NR-1800 spectrometer.Ultraviolet-visible (UVvis) absorption spectra were measured using a Shimadzu 3100 PC spectrophotometer (Japan).X-ray photoelectron spectroscopy (XPS) were recorded by a Shimadzu ESCA 3400.

Photocatalytic studies
The photocatalytic activity of the synthesized samples was evaluated at room temperature under a xenon light source (MAX-303, Asahi Spectra) as a source of visible-light irradiation.In a typical reaction, the dye concentration was xed at 10 ppm and a known dosage (50 mg L À1 ) of photocatalyst was added to the dye solution.The suspension was stirred to achieve an absorption-desorption equilibrium state of the solution, which was kept in the dark before light irradiation. 35,36The reaction mixture was irradiated with stirring under the halogen lamp positioned at 21 cm above the reaction mixture.The reaction vessel consisted of an external jacket for water circulation to maintain the reaction mixture at room temperature.At regular time intervals, 3 mL of the suspension was collected, centrifuged, and analyzed using a UV-vis spectrometer.The MB degradation was estimated from the decrease in the intensity of the associated characteristic band absorption at 664 nm.The photodegradation percentage of MB was calculated using the following equation: 37 where C 0 and C t are the concentrations of MB at time 0 and t (s), respectively, and t is the irradiation time.

Result and discussion
X-ray diffraction measurements were performed to investigate the crystal structure of the TiO 2 and TiO 2 /ZnS nanostructures (Fig. 1).All the diffraction peaks corresponded to anatase phase TiO 2 and were in good agreement with standard JCPDS card no.Raman spectra of the mesoporous TiO 2 and TiO 2 /ZnS mesoporous nanostructures are shown in Fig. 2. The peaks at 143.8, 395.4,9][40] When ZnS was in mesoporous TiO 2 , the peaks were shied to 149.4,404.5, 520.7 and 643.7 cm À1 .The shi of the peak position and the decrease of the peak intensities indicated that ZnS quantum dots inuenced TiO 2 .Hence, signicant peak shi was observed. 32PS was performed to further analyze the chemical states of elements in the as-prepared TiO 2 and TiO 2 /ZnS mesoporous nanostructures.Fig. 3 and 4 show the high resolution XPS spectra of Ti 2p, Zn 2p, S 2s and O 1s states, respectively.In Fig. 3(a), the high resolution Ti 2p spectra presented two peaks  at binding energies of 458.46 eV (Ti 2p 3/2 ) and 464.42 eV (Ti 2p 1/2 ), which were assigned to Ti 4+ in anatase titanium. 41,42The separation between the Ti 2p 3/2 and Ti 2p 1/2 was 5.9 eV, consistent with the reported value of TiO 2 . 43The binding energies of sample TiZ-1 were shied to 458.91 eV and 464.68 eV from 458.46 eV and 464.42 eV compared with mesoporous TiO 2 .Furthermore, increasing the concentration of ZnS caused the peaks to shi to 459.14 eV and 464.73 eV for sample TiZ-2, 459.24 eV and 464.96 eV for sample TiZ-3 and 459.35 eV and 465.05 eV for sample TiZ-4.The binding energy of Ti 2p shied to higher energy with increasing ZnS concentration. 44he Zn 2p 3/2 and 2p 1/2 peaks were located at 1022.57and 1045.52 eV, respectively [Fig.3(b)], illustrating formation of ZnS. 45 The difference between the two binding energies was 22.95 eV, which is in good agreement with the standard value of 22.97 eV. 46The position of peaks in sample TiZ-4 shied from 1022.57eV to 1022.96 eV and from 1045.52 eV to 1046.05 eV compared with sample TiZ-1.As the ionic radius of Zn 2+ is slightly larger than that of Ti 4+ , substituting the Zn atom in the TiO 2 crystal structure could slightly distort the anatase crystal. 47,48These observations indicated possible diffusion of Zn in mesoporous TiO 2 .The peak at 225.81 eV corresponded to S 2s state [Fig.3(c)], consistent with the reported value. 49A similar shi of peak from 225.81 eV to 226.75 eV was observed with increasing Zn content.According to the high-resolution scan spectra, the binding energies of Ti 2p, Zn 2p, S 2s and O 1s shied to higher values as the concentration of Zn increased in the composites, suggesting changes in the chemical environment.Binding energy was dependent on shielding effect caused by the electron density around atoms.Hence, the     typical FESEM images of the titania glycolate spheres, which exhibited an average size of 400-500 nm with smooth surfaces.Porous TiO 2 spheres were formed aer solvothermal treatment as shown in Fig. 5(b).A TEM image of the porous TiO 2 spheres is shown in Fig. 5(c).In addition, the presence of lattice fringes of mesoporous TiO 2 can be clearly observed in the HRTEM image, as shown in Fig. 5(d).Lattice fringe spacing is 0.35 nm and is in good agreement with the (101) lattice plane of anatase TiO 2 .When the ZnS concentration was 0.025 M, ZnS quantum dot was impregnated on the mesoporous TiO 2 , as shown in Fig. 6(a).As the ZnS concentration increased to 0.050, 0.075 and 1.0 M, the morphology of the products became densely covered mesoporous TiO 2 .The morphology of TiZ-1 was further analyzed by TEM, as shown in Fig. 6(c).The surface of the spheres was rough with surface decoration.At higher concentration, the surface of TiO2 was densely covered with ZnS nanoparticles.Fig. 6(d) displays the representative HRTEM image of the TiO2/ZnS mesospheres, with the white dashed line corresponding to ZnS quantum dots and the yellow dashed line corresponding to anatase TiO 2 .The lattice fringe spacing of TiO2 (101) and ZnS (111) is 0.35 nm and 0.31 nm, respectively.The HRTEM images make evident that the TiO 2 nanocrystals are in close contact with the ZnS nanocrystals.The formation of a heterojunction (pink dashed line) enhanced transport of photogenerated electrons and holes between TiO 2 and ZnS.The ZnS nanoparticles were about 2-5 nm for TiZ-1, 2-6 nm for TiZ-2, 2-6 nm for TiZ-3 and 3-6 nm for TiZ-4, as shown in Fig. S1.† High resolution TEM images showed the good crystalline nature.Fig. 10 shows the elemental mapping of the Ti, Zn, O and S in the TiZ-1 sample.It is clearly evident that the Zn signal originated in a similar spatial area to that of the corresponding Ti signal.From the elemental mapping, it is conrmed that a composite distribution was formed in TiO 2 /ZnS mesoporous nanostructure.

Photocatalysis
The photocatalytic activities of the mesoporous TiO 2 and TiO 2 / ZnS quantum dots were evaluated by examination of MB dye degradation under visible light irradiation.The photocatalytic activities of prepared samples were tested by examining the degradation of organic pollutants (MB) as a function of time.The decrease in relative concentration of the MB was estimated This journal is © The Royal Society of Chemistry 2017 by measuring the relative intensity of the peak at 664 nm from the optical absorbance spectra.Fig. 11(a) shows the timedependent UV absorption spectra of mesoporous TiO 2 catalyst, which completely decomposed with irradiation time of 60 min.The effect of addition of different concentrations of TiO 2 as TiZ-1, TiZ-2, TiZ-3 and TiZ-4 is shown in Fig. 11(b-e).higher photocatalytic activity.Thus, it can be concluded that introduction of a very small amount of ZnS to the TiO 2 surface resulted in improved performance of dye degradation.
Fig. 12(a) shows the effect of mesoporous TiO 2 and TiO 2 /ZnS nanocomposite on MB degradation.MB decolonization in the absence of catalyst was also evaluated.Less than 10% of the MB in the solution disappeared aer 60 min of photolysis.The degradation rate of mesoporous TiO 2 /ZnS catalysts decreased with increasing ZnS content from 0.025 M to 0.1 M. Sample TiZ-1 had the highest activity of all samples.As the photocatalytic reaction is dependent on the surface atomic arrangement at the interface between the catalyst surface and organic pollutants, 54,55 the optimum content of ZnS is an important factor in the photocatalytic activity of the TiO 2 /ZnS photocatalyst. of ZnS caused the number of ZnS nuclei to increase and the nanoparticles were grown completely on the mesoporous spheres.Thus, ZnS concentration has an important role in formation of heterojunctions between TiO 2 /ZnS nanocomposites.It results in inhibition of electron/hole pair recombination. 56,57Average pore size distribution was calculated using the Barrett-Joyner-Halenda (BJH) method and the values were 9.79, 9.31, 8.98, 8.88 and 6.82 nm for Ti, TiZ-1, TiZ-2, TiZ-3 and TiZ-4, respectively.Such pore size analysis revealed that higher concentration of ZnS reduces the pore size of mesoporous spheres by occupying pores of the network.This signicantly suppresses the interaction of organic pollutants with the photocatalyst.
To elucidate the photocatalytic process under visible light, active species generated during the reaction were identied by free radical and hole scavenging experiments.Hydroxyl radicals (cOH), holes (h + ) and superoxide anions (O 2 À c) are possible active species in photodegradation of organic pollutants. 37,44To detect the active species during the photocatalytic reaction, benzoic acid (BA), the sodium salt of ethylenediamine tetraacetate (EDTA) and potassium persulphate (K 2 S 2 O 8 ) were introduced into the catalyst solution as scavengers, respectively, of hydroxyl radicals, holes and superoxide radical anion. 41,47ig. 13(a) presents the photodegradation of MB catalysed by TiO 2 /ZnS (TiZ-1) in the presence of these various scavengers under visible light illumination.Compared with the scavenger-free system, the dye degradation efficiency in the presence of O 2 À c scavenger was 91.93%.In contrast, the reaction with the addition of h + scavenger EDTA, was almost inhibited with 38% of MB degradation aer 32 min.To further determine the degradation mechanism, another experiment was performed using the BA scavenger.The photocatalytic activity was greatly reduced in the presence of the O 2 À c scavengers, with 54% MB degraded in 32 min.These results strongly suggest that hydroxyl radicals, holes and superoxide radical anions all contribute to photodegradation, but that the hole is a key intermediate as trappingit totally inhibited photodegradation.It can be concluded that hole (h + ) radical is the major oxidative species responsible for photooxidative conversion of MB. 58 The stability of a photocatalyst is important for practical applications, thus the TiO 2 /ZnS composite photocatalyst was recycled under the same conditions.Fig. 13(b) shows the reusability of TiZ-1 photocatalyst for degradation of MB examined over three cycles of 32 min under visible light irradiation.Aer the photocatalysis experiments, the catalyst was separated from the reaction mixture by centrifugation and the concentration of the dye solution was adjusted to its initial value.Photocatalysts were reused for three cycles and the obtained degradation values were 90.90, 90.64 and 89.92% for the rst, second and third cycles, respectively.The photocatalytic efficiency of the TiO 2 /ZnS composites did not decline signicantly, suggesting that the catalyst has good stability and sustainability.Fig. S2 † shows the XRD patterns of the TiO 2 /ZnS composites before and aer four runs of photocatalytic activity under the visible light irradiation for degradation of MB.It can be clearly observed that the phase and structure of the TiO 2 /ZnS composite were unchanged aer the photocatalytic cycles; suggesting that the sample was stable under the present photocatalytic degradation process.In addition, the photocatalytic structural stabilities and photocatalytic loss of TiO 2 /ZnS composites were investigated using XPS spectra, as shown in Fig. S2.† The binding energies of Zn 2p, Ti 2p, O 1s and S 2p of the recycled TiO 2 /ZnS showed no peak-shi compared with those of the fresh sample, inferring that the chemical states of Zn, Ti, O and S elements in TiO 2 /ZnS did not change during the reaction process.

Kinetic study
The rate of photocatalytic reduction of the nanocomposites can be described by pseudo-rst-order kinetics, so plots of ln(C 0 /C t ) versus irradiation time for the adsorption and degradation of MB on TiO 2 /ZnS nanocomposites were examined.The ln(C 0 /C t ) curves versus irradiation time were linear, indicative of good correlation to rst-order kinetics (Fig. 14(a)).The apparent rate constant K was calculated to be 0.0428, 0.1199, 0.0675, 0.0591 and 0.0526 min À1 for Ti, TiZ-1, TiZ-2, TiZ-3 and TiZ-4, respectively.K value decreased with addition of ZnS at concentrations from 0.025 M to 0.1 M (0.1199 min À1 to 0.0526 min À1 ).The K for the TiO 2 /ZnS sample was 0.1199 min À1 , 2.5 times higher than that of pure TiO 2 (0.0428 min À1 ).The kinetic data obtained using the pseudo-rst order model such as apparent rate constants (K app ), corresponding correlation coefficients (R 2 ) and maximum dye degradation in the presence of TiO 2 /ZnS nanostructures are presented in Table 1.Introduction of ZnS quantum dots into the mesoporous TiO 2 was in favour of high increased photocatalytic activity and suppressed the recombination of photogenerated electron/hole pairs. 59,60chematic representation of photocatalytic activity of mesoporous TiO 2 /ZnS nanocomposites is shown in Fig. 14(b).When the light is irradiated, the visible light provides the photons required to generate electron and hole pairs.The conduction band (CB) of TiO 2 lies at a more positive potential than that of ZnS, while the valence band (VB) of ZnS is more negative than that of TiO 2 .From the energy band diagram, it was found that an electron from the bottom of the CB of ZnS quickly transferred to the CB of TiO 2 .Meanwhile, the photogenerated hole transfer could take place from the VB of TiO 2 to the VB of ZnS, suggesting that the photogenerated electrons and holes were efficiently separated. 61band structure facilitates separation of the excited electrons and hole pairs, and facilitates redox reactions where electrons reduce dissolved molecular oxygen to produce superoxide radical anions (cO 2 À ), while holes oxidize H 2 O molecular to yield hydroxyl radicals (HOc) on the TiO 2 /ZnS surfaces.Organic dye pollutants (MB) are eventually oxidized by these highly active species to CO 2 and H 2 O products. 62

Conclusion
Assessment of photocatalytic degradation of organic compounds using the TiO 2 /ZnS mesoporous nanostructures revealed a remarkably higher reaction rate under visible light irradiation compared with that of pure mesoporous TiO 2 .Impregnation of ZnS quantum dots was conrmed by XPS and elemental analysis.
The investigation of photocatalytic activity indicated that the TiO 2 /ZnS nanocomposites possessed higher photocatalytic activity compared with mesoporous TiO 2 for degradation of MB under visible light irradiation.The maximum degradation efficiency was observed for 0.025 M of ZnS, where the MB related absorption peak completely disappeared aer 32 min of irradiation.Photogenerated holes (h + ) over TiO 2 /ZnS supported photocatalysts in the photodegradation of organic pollutants.Aer four cycles of reuse, the catalyst showed signicant capacity for dye degradation.

Fig. 4
Fig. 4 XPS spectra of O 1s state of mesoporous samples.

Fig. 11 (
Fig. 11(b) shows a rapid decrease in the initial absorbance of the peak, which disappeared completely aer 32 min of irradiation.As the concentration of Zn was increased to 0.050, 0.075 and 0.1 M, photodegradation time increased to 40, 44 and 52 min, as shown in Fig. 11(c)-(e).Pure TiO 2 (Ti) exhibited the lowest photocatalytic activity, with TiO 2 /ZnS composite showing

Fig. 13 (
Fig. 13 (a) Effect of MB degradation over TiO 2 /ZnS in the presence of various scavengers and (b) reusability of sample TiZ-1 under visible light irradiation.

Fig. 14 (
Fig. 14 (a) Plots of ln(C 0 /C t ) as a function of time (min) for the photodegradation of MB over the TiO 2 /ZnS nanocomposites, and (b) photocatalytic mechanism of TiO 2 /ZnS nanocomposites.

Table 1
Observed pseudo-first-order rate constants, R 2 values, maximum degradation (%) and time required for maximum degradation of TiO 2 /ZnS nanocomposites