Nano Research & Applications

Keywords

Solvothermal method; In-situ synthesis; TiO₂-ZnS hierarchical nanostructure; Photocatalytic activity

Introduction

Rapid growth of organic dye-manufacturing and dyeconsuming industries which generate wastewaters always draws a special attention to the materials researchers due to the serious environmental concern [1-3]. Generally, organic-dyes are intractable compounds that are very hard to remove by biological treatment [2-4]. However, the nanostructured Metal Oxide Semiconductor (MOS) as photocatalyst could efficiently and economically eliminate such toxic organic compounds by decomposing into the less toxic materials like CO₂ and water [5]. In photocatalysis, highly reactive oxygen species such as hydroxyl and superoxide radicals are generated to react with dyes and decompose them in the presence of a semiconductor and ultraviolet or visible light [5-7]. Among the various MOS, titanium dioxide has been extensively studied over the past few decades due to its potential applications in different fields. From the first discovery of hydrogen production by TiO₂ photoanode under UV irradiation, lots of research on TiO₂ has been performed, which has broadly expanded the applications of TiO₂ in the various fields like photovoltaic cells, dye-sensitized solar cells, photocatalysis, environmental remediation as well as photoelectrochemical sensors [4,5]. Moreover, TiO₂ is widely used as photocatalysts for removal of hazardous organic compounds as well as the self-cleaning environmental applications due to its strong oxidizing and reducing ability under UV light, relatively high efficiency, low cost and availability [8]. The photocatalytic activity of TiO₂ is improved by increasing surface area which provides more active sites for photocatalytic reactions. In this regard, nanostructuring of MOS is the renowned method to increase the surface area of the materials [9,10]. Da Silva et al. [11] reported Wulff-shape mesoporous TiO₂ for enhanced photocatalytic activity. Liu et al. [12] also reported spindle-shaped mesoporous TiO₂ as a superior photocatalyst.

However, rapid photogenerated charge carrier recombination in TiO₂ limits its photocatalytic efficiency. Metal doping and coupling of TiO₂ with other suitable semiconducting metal oxides/sulphides are widely used approaches to improve the photogenerated charge separation and photocatalytic activity [11-14]. In this regard, semiconducting metal sulphides especially ZnS has been broadly studied due to its environmental friendliness and earth abundance [15]. ZnS is an important semiconductor material having a wide direct optical band gap of 3.6 eV [16]. Only UV light is required to promote excited electrons to the conduction band of ZnS [16,17]. This type of material with high band gap energy may act as a good electron trapper due to its vacant conduction band [12]. Moreover, coupling of TiO₂ with ZnS enhances the photocatalytic activity of the nanocomposite compared to pristine TiO₂ and pristine ZnS by forming type-II heterojunction between TiO₂ and ZnS [13]. In this regard, Vaclav et al. [14] synthesized TiO₂/ZnS nanocomposite and reported better photocatalytic activity compared to bare TiO₂ and ZnS nanoparticles. Moreover, Xiaodan et al. [18] prepared ZnS/TiO₂ nanocubes via solvothermal method showed the enhanced photocatalytic activity of the composite compared to pristine anatase TiO₂ under visible light exposure. Franco et al. [19] also synthesized nanocrystalline TiO₂-capped ZnS via chemical vapor deposition technique and reported the increased photoactivity of TiO₂- capped ZnS compared to bare TiO₂. However, most of the reports available in the literatures are related to the two steps synthesis of TiO₂-ZnS nanocomposite which is time consuming and relatively high cost [20-22]. To the best of our knowledge, there are no reports available on single step in-situ synthesis of hierarchical TiO₂-ZnS nanostructure (TiO₂ sphere wrapped with ZnS nanorods) in the literatures.

In this work, titanium oxysulfate and zinc nitrate hexahydrate were used as precursor materials for TiO₂ and ZnS, respectively whereas DMF and distilled water as solvents. The reaction mixture was transferred into a 150 ml Teflon-lined stainless steels autoclave and kept in an oven at 150°C for 6 hours. The precipitate was cooled down to room temperature and washed several times with distilled water. Then, the sample was cured at 500°C for 2 hours in an electrical furnace. Pure TiO₂ was synthesized with the same procedure as adopted for TiO₂-ZnS composite using titanium oxysulfate as a precursor for comparison. Finally, structural, materials and optical properties of the synthesize materials were correlated with the photocatalytic activity.

Experiment

Precursor materials

Zinc nitrate hexahydrate (ZN, Merck, 97% purity), Titanium oxysulfate (TIOS, Sigma Aldrich, 99% purity), DMF (Merck, 98% purity) and Distilled water.

Synthesis of hierarchical TiO₂-ZnS nanostructure

For the preparation of TiO₂-ZnS nanostructure (TZS), 1.10 g Zn (NO₃)₂.6H₂O (3 mmol), 2.64 g TiO (SO₄)₂ (7 mmol) were taken in 85 ml DMF followed by the addition of 5 ml H₂O and the resultant mixture was stirred for 30 min. Then the turbid solution was stirred and heated on a magnetic stirrer at 90°C to obtain clear homogeneous solution. The resultant clear solution was cooled down to room temperature and then poured it into a 150 ml Teflon-lined stainless-steel autoclave and kept in an oven at 150°C for 6 hours. The precipitate was cooled down to room temperature and washed several times with distilled water. Finally, the sample was cured at 500°C for 2 hours in an electrical furnace. Pure TiO₂ was synthesized with the same procedure as adopted for TiO₂-ZnS composite using titanium oxysulfate as a precursor material.

Characterization

X-Ray Diffraction (XRD) study of the sample was performed by Rigaku Smart Lab using CuKa radiation (1.5406°A) operating at 9 kW in the diffraction angle (2θ), 20 to 800. The surface morphology of the materials was investigated by FESEM (ZEISS, SUPRA^™35VP) study. TEM measurements were carried out by Tecnai G2 30ST (FEI) electron microscope operating at 300 kV from the dispersed material by ultrasonication onto 300 mesh carbon coated cupper grid. TEM/HRTEM and TEM-EDS were performed to analyses the particle size, crystal phase and content of elements. UV-Vis-NIR spectrophotometer (Shimadzu UV-PC-3100; photometric accuracy: transmission ± 0.3%, wavelength resolution, 0.10 nm) was used to measure UV-Vis absorption spectra of the sample. Band Gap Energy (BGE) of the samples was calculated from the respective absorption spectrum. PerkinElmer (LS55) spectrofluorometer was employed to measure the photoluminescence property of the sample at room temperature. X-ray Photoelectron Spectra (XPS) of the sample was done by employing PHI Versaprobe II Scanning XPS microprobe surface analysis system using Al-Ka X-rays (hn, 1486.6 eV; DE, 0.7 eV at room temperature). The energy scale of the spectrometer was calibrated with pure (Ag) samples and the pressure in the XPS analysis chamber was better than 5×10^-10 mbar. The chemical state of each element present in the nanocomposite was determined by taking position of (C1s) peak as standard (with the binding energy of 284.5 eV).

Measurement of photocatalytic activity

Photocatalytic Activity (PA) of the nanocomposite towards degradation of Rhodamine B (Rh-B) was studied in a custombuilt stainless-steel UV (wavelength, 254 nm) curing chamber. The total solution in 5 ml capacity cuvate containing the solution of dye (10^-5 M, Co) was placed in UV chamber. The detailed PA measurement set up has already been reported anywhere [19-22]. The PA of the synthesized materials was performed under UV light irradiation. For this purpose, a 300 W highpressure xenon lamp (PLSSXE300UV, Beijing Trust tech. Co. Ltd., wavelength ranges from 300 to 800 nm with total intensity of ca. 200 mW/cm2) have been used to carry out the experiment. For this experiment, ̴ 5 ml dye solution took out a certain time of interval and UV-visible absorption spectrum was recorded to finish the remnant Concentration (C) of the dye with the help of a calibration curve of the dye solutions. The calibration curve was drawn by plotting dye concentration against absorbance at 554 nm peak wavelength of Rh-B solutions obeying Lambert Bayer’s law. The PA of the catalyst was analysed by plotting ln (Co/C) (dye concentration; Co, initial and C, remnant) versus irradiation time. The rate constants of decomposition reaction (considering pseudo first order reaction kinetics) were calculated from the plots.

Results and discussion

Phase structure

The X-ray diffraction patterns (Figure 1) of the samples were recorded in the 2θ range 20° to 80° to investigate the crystallinity and crystal phase of TiO₂ and ZnS. From the X-ray diffraction peaks it was found that both the TiO₂ and ZnS in the TZS sample crystalizes as anatase TiO₂ and cubic ZnS, respectively. The diffraction peaks (2θ degree) centered at 25°, 38°, 48°, 54°, 63° and 75° corresponded to the (101), (112), (200), (105), (204) and (215), respectively confirms the anatase phase of TiO₂ (JCPDS card no. 21-1272). Moreover, the diffraction peaks (2ϴ) located at 28° and 56° corresponded to the (111) and (311) planes arise mainly due to the formation of cubic ZnS (JCPDS card no. 05-0566) in TZS sample. The crystallite size of the samples was calculated with the help of Debye- Scherrer's equation (eqn. 1) [14].

Figure 1: XRD patterns of pristine TiO₂ (TO) and TiO₂-ZnS (TZS) samples.

Where, k is proportionality constant (0.9), λ is wavelength of X-ray (1.5406 Å), β denoted as FWHM (Full Width at Half Maximum) of the peak of maximum intensity in radians, ϴ is diffraction angle and D is crystallite size.

It was found that the calculated average crystallite size (D) of pristine TiO₂ was ~ 52 nm whereas the same for TiO₂ and ZnS were ~ 43 nm and ~ 36 nm, respectively in TZS sample.

Morphology and Microstructure

FESEM Study

Surface morphology of the synthesized pristine TiO₂ and TZS samples were analysed with the help of FESEM study (Figure 2).

Figure 2: FESEM images of the samples synthesized via solvothermal technique by tuning the precursor solutions composition: (a) pristine TiO₂ (b) TZS (Ti: Zn = 8:2, mol ratio) (c) TZS (Ti: Zn = 6:4, mol ratio) and (d) TZS (Ti: Zn = 7:3, mol ratio). Insets of (a) and (d) represents the higher magnified FESEM images.

The samples were synthesized by solvothermal technique followed by curing at 500°C under air atmosphere to obtain the hierarchical sphere like morphology. The FESEM image shows the formation of spherical TiO₂. Size of TiO₂ spheres was calculated from the FESEM image and it was in the range of ~ 0.9 to ~ 2.0 μm. It is seemed to be appeared the hierarchical sphere like morphology of the TZS sample after changing the amount of precursor solutions in the reaction mixture. It was found that the TiO₂ spheres were uniformly wrapped with ZnS nanorods at a particular solution composition (Ti: Zn = 7:3, mol ratio) whereas at the other solution compositions (Ti: Zn = 8:2, and Ti: Zn = 6:4, mol ratio), ZnS nanorods were found to be sparsely distributed on the surface of the TiO₂ spheres. Length and width of the ZnS nanorods were also calculated from the FESEM image these were ~ 400 nm and ~ 60 nm, respectively. TiO₂ spheres wrapped with ZnS nanorods in TZS sample would be beneficial to improve the photocatalytic activity of the sample.

TEM study

The microstructural properties of TZS sample was investigated by TEM study (Figure 3). Figure 3a,b show the bright field TEM images of the sample. From the bright field TEM images, it is clearly seen that the TiO₂ spheres are wrapped with ZnS nanorods. This observation was further confirmed from the FESEM images of the sample. It was confirmed from the HRTEM image that the sample was fully crystallised with anatase TiO₂ (a-TiO₂) and cubic ZnS (c-ZnS) at the experimental curing temperature of 500°C [5]. This observation was further confirmed from XRD study. Moreover, the TEM EDS plot showed the presence of titanium, zinc, sulphur and oxygen elements in the sample.

Figure 3: TEM microstructural studies of TZS sample cured at 500°C: (a-b) Bright field TEM images, (c) HRTEM image and (d) EDS plot.

XPS Study

The XPS analysis was performed to observe the oxidation states of the constitute elements present in TZS sample. The strong binding energy peaks (Figure 4) centered at 1044.2 eV and 1021.3 eV, corresponded to the core levels of Zn2p1/2 and Zn2p3/2, respectively confirmed the presence of Zn²⁺ ions in the sample [23,24]. Figure 4b shows the binding energy curve of Ti2p with two intense peaks located at ~ 458.5 eV and 464.4 eV, corresponded to core levels of Ti2p3/2 and Ti2p1/2, respectively. The binding energy difference between the two peaks was calculated and found to be ~ 5.9 eV, confirmed the existence of Ti⁴⁺ in the TZS sample [10]. Figure 4c shows the broad binding energy peaks of O1s. The broad nature of the O1s peak could indicate the presence of Olattice, Odefect (surface oxygen vacancies) and Ohydroxyl [17]. The presence of oxygen defects in the sample could help to improve the photocatalytic activity by enhancing light absorption [18]. Moreover, the presence of sulphur ion in the sample was confirmed from the binding energy peak centered at 163.7 eV (S2p1/2) and 161.8 eV (S2p3/2) corresponded to the S-interstitial state and S2-, respectively [25].

Figure 1: XPS biding energy curves of TZS sample: (a) Zn2p, (b) Ti2p, (c) O1s and (d) S2p.

Photo Luminescence (PL) study

The PL spectral study of pristine TiO₂ and TZS samples was performed and shown in Figure 5. The PL spectra of pristine TiO₂ and TiO₂-ZnS nanocomposites were recorded at an excitation wavelength of 325 nm. The PL spectra of both TiO₂ and TZS exhibit a strong emission peak centered at ~ 397 nm which could be ascribed to the band edge emission of anatase TiO₂ semiconductor [26]. The presence of a sharp emission peak centered at 425 nm can be recognized to the recombination of self-trapped excitons in anatase TiO₂ [27]. The shoulder peaks centered at 451 and 468 nm are related to the transition of an electron from the shallow level of oxygen vacancies to the valence band [27]. Moreover, a broad and intense peak at 486 nm appeared due to the presence of Ti⁴⁺ ions adjacent to oxygen vacancies (intra gap surface states) [28]. On the other hand, TZS nanocomposite exhibited a weak emission peak at 397 nm compared to pristine TiO₂ due to the interaction between TiO₂ and ZnS [27]. This interaction could facilitate the photogenerated charge carrier separation vis-à-vis enhance the photocatalytic activity [27].

Figure 5: PL spectra of TiO₂ and TZS nanocomposites.

UV-Visible absorption study

The optical properties of the samples were characterized with the help of UV-Vis spectrophotometer and shown in Figure 6. It was found that the anatase TiO₂ exhibited a wide absorption band in the range from 200 to 385 nm whereas TiO₂-ZnS composite showed a UV absorption band in the range from 200 to 338 nm. The UV-Vis spectrum of TiO₂-ZnS composite showed blue shift compared to pristine TiO₂. This observation could be attributed to the interaction of ZnS with TiO₂ [17,18,29]. The optical band gap energies of the samples were estimated from the corresponding absorption spectra using Tauc equation [22]. It was noted that the calculated band gap energies are ~3.3 ± 0.03 and 3.56 ± 0.01 eV for pristine TiO₂ and TiO₂-ZnS nanocomposite, respectively. The widening of the band gap energy in the TiO₂-ZnS composite compared to pristine TiO₂ beneficiates the efficient charge separation vis-a-vis improved photocatalytic activity [18,29].

Figure 6: UV-Vis absorption and optical band gap energy of pristine TiO₂ and TiO₂-ZnS composites.

Photocatalytic Performance

Photocatalytic Activity (PA) of pristine TiO₂ (TO) and TZS composite was evaluated by measuring of organic dye (Rhodamine B, RhB) degradation under UV light irradiation. The PA of synthesized samples was examined by the degradation of organic pollutants (RhB) as a function of time. It was noted that the reduction of concentration of the RhB was measured from the relative intensity of characteristic peak at 554 nm from the UV absorbance spectra. The time dependent UV absorption spectra (Figure 7) of TO and TZS shows the decomposition of RhB with irradiation time of 50 min. Under the UV irradiation time of 50 min the TZS sample almost decomposed the RhB dye. The TZS composite exhibited higher PA compared to TO. This observation could be attributed to the formation of heterojunction between TiO₂ and ZnS [30]. The photocatalytic reduction rate of the samples can be designated by pseudo-first-order kinetics. So, the plots of ln (C0/Ct) versus irradiation time were examined. It was found that the ln (C0/Ct) curves versus irradiation time were linear, suggested pseudo first-order kinetics [5]. The rate constant K was calculated and it was found 0.0175 min^-1 and 0.0241 min^-1 for TO and TZS composite, respectively. The K value for the TZS sample was ~1.5 times higher than that of TO. Thus, it is concluded that the presence of ZnS onto the TiO₂ surface resulted the improved PA towards photocatalytic organic dye degradation under UV light irradiation.

Figure 7: UV absorption spectra for photocatalytic dye degradation (a,b) and rate constant curves of rhodamine B (c) for pristine TiO₂ and TiO₂-ZnS nanocomposites.

Conclusions

For the first time, we successfully synthesize the hierarchical TiO₂-ZnS nanostructure (TiO₂ sphere wrapped with ZnS nanorods) by single step solvothermal process for photocatalytic degradation of rhodamine B under UV light irradiation. Titanium oxysulfate and zinc nitrate hexahydrate were used as precursor materials for TiO₂ and ZnS, respectively whereas DMF and distilled water as solvents. The reaction mixture was transferred into a 150 ml Teflon-lined stainless steels autoclave and kept in an oven at 150°C for 6 hours. The precipitate was cooled down to room temperature and washed several times with distilled water. Then, the sample was cured at 500°C for 2 hours in an electrical furnace. Pure TiO₂ was synthesized with the same procedure as adopted for TiO₂-ZnS composite using titanium oxysulphate as a precursor for comparison. Finally, structural, materials and optical properties of the synthesize materials were correlated with the photocatalytic activity. It was noted that hierarchical TiO₂-ZnS sample showed around 1.5 times higher photocatalytic activity towards rhodamine B degradation compared to pristine TiO₂ under UV light irradiation. This work could make an avenue for the synthesis of other multicomponent efficient photocatalyst for environmental remediation.

Acknowledgement

The authors are grateful to Director, CSIR-CGCRI, Kolkata for his kind support and encouragement. The authors also acknowledge the help rendered by Nanostructured Materials Division and Electron Microscopy Section for microstructural characterizations. The work had been done as an associated research work of LSRB-DRDO Sponsored project (GAP0623).

Conflict of interest

The authors declare that they have no conflict of interest.

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