Introduction

In the modern scientific era the materials in the nanoscale have gained more attention than their bulk form owing to their peculiar physical and chemical properties. In the semiconductor industry nanocrystalline ZnO has many applications for numerous fields, such as lasers, sensors, solar cells and field emission devices due to its higher band gap and larger excitation binding energy (60 MeV). Among the organic pollutants, brilliant green (BG) creates an impact on its enriched outreaching to the environment through effluent of textiles and paint industries [2-9]. Brilliant green can create so many toxic effects on the human beings and animals; therefore its degradation needs more attention. As a result of nontoxic nature and low coast, ZnO is an alternate to TiO₂ in the degradation of organic pollutants [10-12]. Doping that does essential incorporation of ions of particular elements into the host material to tailor its properties is a widely accepted technique in the semiconductor industry. Further, the morphology control through doping is another major task as the size and shape can influence various properties of the prepared products. Doping ZnO with tellurium (Te) can tailor the luminescence properties of ZnO by passivation of oxygen defects. Already few works are available on the Te doped ZnO film [13-16]. Previously we have studied the photocatalytic properties of ZnO on cerium doping. In the present work, we have attempted to tailor the photocatalytic properties of ZnO by incorporating Te in the lattices of Zn2+. For that tellurium in different proportions (0.1, 0.15, 0.2, and 0.25 M) are doped into ZnO through a simple chemical precipitation method. The prepared products are analysed for their structural, optical, morphological, and photocatalytic properties.

Materials and Methods

All the chemicals used in this study are of AR grade with 99% purity (E.Merck) and used without further purification. Sample preparation and dilutions were made of ultrapure water. Zinc nitrate hydrate [Zn(NO)₃.6H₂O], sodium telluride [Na₂Te] and potassium hydroxide (KOH) were used as precursors.

For the synthesis of Te-doped ZnO, 7.4 g (0.5 M) of zinc nitrate hydrate dissolved in 50 ml of deionized water was stirred vigorously by magnetic stirrer and sodium telluride of preferred mole (0.1, 0.15, 0.2, and 0.25 M) prepared in 20 ml aqueous was mixed drop wise. Then, 6.2 g (2.5 M) of potassium hydroxide in 50 ml of deionized water was added drop by drop to the above mixture. The entire was stirred magnetically at 60 ºC until a white precipitate was formed. The obtained dispersions were purified

by dialysis against de-ionized water and ethanol several times to remove impurities. The purified products were dried in hot air oven at 100 °C for 6 h to evaporate water and organic materials to the maximum extent. The dried powders were pulverized to fine powders using agate mortar for further characterizations. A similar method of preparation without the addition of tellurium was used to synthesize undoped ZnO Nano crystals.

For the purpose of studying the photocatlytic activity of ZnO, 0.2 g of ZnO was added to a quartz photoreactor containing 100 ml of a 1 mg/l brilliant green (BG) aqueous solution. After stirring for 120 min in the dark in order to reach the absorption equilibrium, the mixture was irradiated with sunlight with intensity fluctuation of 950 ± 25 Wm-2. The residual BG in the aqueous solution was analyzed by checking the absorbance at 624 nm in the UV-Vis absorption spectra. To determine the percentage of degradation of MB, the samples were collected at regular intervals (for every 30 min), filtered and centrifuged to remove the nanophotocatalyst particles that exist as undissolved particles in the sample and studied using UV-Vis absorption. The degradation percentage of the dye in the presence and absence of ZnO nanopartices can be calculated from the following equation [18].

Where C₀ is the initial concentration of the dye and C_t is the concentration of dye after irradiation in selected time interval. The same procedure was adopted for tellurium doped ZnO nanoparticles.

Figure 1A: X-ray diffraction patterns of ZnO and various levels of Te doped ZnO nanosheets.

Figure 1B: X-ray diffraction patterns of ZnO and Te doped ZnO nanosheets enlarged view of (100), (002) and (101) peaks.

Table 1: XRD derived parameters of undoped and Te doped ZnO nanoparticles.

The crystalline phase and particle size of pure and Te-doped ZnO nanoparticles were analyzed by X-ray diffraction (XRD) measurement which was carried out at room temperature by using XPERT-PRO diffractometer system (scan step of 0.05° (2θ), counting time of 10.16s per data point) equipped with a Cu tube for generating Cu K radiation (k = 1.5406 Å); as an incident beam in the 2-theta mode over the range of 10°–80°, operated at 40 kV and 30 mA. The photoluminescence (PL) emission spectra of the samples were recorded with a Spectroflurometer (Jobin Yvon, FLUOROLOG–FL3-11). The functional groups were determined by SHIMADZU-8400 Fourier-transform infrared spectrometer in which the IR spectra were recorded by diluting the milled powders in KBr and in the wavelength between 4000 and 400 cm-1 was used to assess the presence of functional groups in pure and Te-doped ZnO. The morphological analysis was performed by HITACHI S-4700 field emission scanning electron microscope (FESEM). Energy-dispersive spectrum (EDS) analysis of the products was performed during

It has been seen that the bond length value of ZnO increases on low level of Te doping (≤ 0.15 M) and decreases at higher doping concentrations ( ≥ 0.15 M).

The room temperature PL emission spectra of ZnO and ZnO: Te with 320 nm excitation are shown in Figure 2. All the samples exhibit an UV and four visible emissions. The obtained UV emission at 390 nm is attributed to the near band edge emission of ZnO, originating from the excitonic transitions between the electrons in the conduction bands and the holes in the valence bands [20]. The appearance of blue emission at 437 nm originates from zinc interstitial. The blue green emission centered at 487 nm is due to a radiative transition of an electron from the deep donal level of Zn_i to an acceptor level of neutral V_zn [21].

Figure 2: PL emission spectra of ZnO and various levels of Te doped ZnO nanosheets.

Figure 3: FT-IR spectra of ZnO and various levels of Te doped ZnO nanosheets.

Figure 4: FESEM images of (a, b) ZnO, corresponding particle size distribution histograms (c, d) Te (0.2 M) doped ZnO nanosheets, corresponding thickness of the nanosheets distribution histograms and (c) corresponding EDX pattern.

Figure 5A: Time dependent UV-Vis absorption spectra of the photo catalytic degradation of BG in the presence of ZnO nanoparticles.

Table 2: The effect of BG dye degradation by ZnO and Te (0.2 M) doped ZnO nanosheets

Figure 5B: Time dependent UV-Vis absorption spectra of the photo catalytic degradation of BG in the presence of tellurium (0.2 M) doped ZnO nanosheets.

Figure 6: Kinetic study of photo degradation of BG in the presence of ZnO and tellurium (0.2 M) doped ZnO nanosheets.

The intense red emission at 643 nm can be ascribed to oxygen-related defects [22]. Significantly, the defect related emissions of the host material can be tuned by dopant material. In the present case, the PL intensity of ZnO strongly depends on the concentration of Te. At the initial stage, in the absence of dopant, emission peaks have higher intensity, however, on increasing the concentration of doping the intensity reduction can be seen with the broadening of the FWHM as a function of Te content. This strongly suggests that the density of oxygen defects responsible for red emissions could be controlled by doping of Te in the host material ZnO. The same type of passivation of oxygen defects on Te doping was reported by other researcher [15].

 To know the influence of Te doping on the Zn-O bonding, FT-IR spectra were recorded for undoped and doped ZnO in the range of 4000- 400 cm-1. The obtained spectra are presented in Figure 3. The appearance of absorption peaks around 3414 and 1630 cm-1 can be ascribed to O-H stretching and bending vibrations, respectively[23].  The broad band absorbed around 2368 cm-1 can be related to the O = C = O vibration of CO₂ molecule exist in air [24]. The presence of nitrate peaks at around 883 cm-1 can be originated from zinc nitrate used as zinc source. The broad absorption feature positioned at 431 cm-1 is due to stretching vibration of Zn-O [25]. On doping, stretching vibrations of Zn-O are shifted to lower and higher energy regions as a result of change in bond length. Such a change in bond length of Zn-O already has been discussed in the XRD section.

The FESEM image of ZnO exhibits that the particles are having spherical morphology with homogeneous size distribution (Figure 4a). The corresponding histogram exhibits the average particle size as 23 nm (Figure 4b). The influence of Te doping on the morphology of ZnO is clearly seen from the Figure 4c. As shown in figure, the doped product is in the form of nanosheets stack together. These nano-sheets are showing regular hexagonal structure of thickness 27 nm (Figure 4d). The EDS analysis of Te doped ZnO (Figure 4e) reveals that Zn. O, and Te are the constituents of the prepared materialIn the evaluation of photo catalytic activity of ZnO and ZnO: Te against Brilliant green (BG), 100 ml of BG aqueous solution (1 mg/L) was taken in a quartz photo reactor. Further, 0.2 g of ZnO was added into the quartz photo reactor. Before irradiation, the reaction mixture was stirred in dark for 30 min to achieve the absorption- desorption equilibrium between the catalyst and dye molecules. The Sunlight with intensity fluctuation of 950 + 25 wm-2 was used as an irradiation source. During the course of light irradiation, 5 ml of sample was collected at regular intervals (every15 min), filtered and centrifuged to remove the undissolved photocatalyst. The filtrate was analyzed by UV-Vis spectrometer at P_max = 624 nm and the photodegradations were calculated by equation (1).

The photocatalytic activity is based on the reactive nature of an electron-hole pair generated in the semiconductor nanoparticles. Under illumination by light of energy greater than the semiconductor band gap, electron is excited to the conduction band and electron in the conduction band migrates to the lattice surface. If no recombination takes place, these charge carriers can react with adsorbed molecules.

Figures 5(a) and 5(b) exhibit the change in absorbtion pattern of BG exposed to Sunlight for varies irradiation times in the presence of ZnO and ZnO: Te, respectively. The percentage of degradation was calculated using the equation 1 and the results are given in Table 2. From the table, it is clear that after 120 min of light irradiation, 75 % of dye was degraded in the presence of ZnO. However, for ZnO: Te, the degradation rate was increased to 86% at the same 120 min of light irradiation. This result revealed that Te doped ZnO exhibited higher photocatalytic activity than pure ZnO. Further, the rate constant values for dye degradation for the catalysts were calculated using the first order rate eqution [17].the samples and from the slope of the graph, rate constant values were calculated. The value of rate constant for undoped ZnO was found to be 0.01235 min-1, whereas, for ZnO: Te, it was increased to 0.02111 min-1. The increased k value suggests the improved photocatalytic activity of ZnO on Te doping. The possible mechanism behind the improved photocatalytic activity of Te doped ZnO is explained as follows.

When light of energy greater than forbidden gap is irradiated, electrons from the valance band can make a quantum jump to conduction band of ZnO. The presence of dopant Te4+ within the crystal matrix or on the surface of ZnO can trap the photogenerated electrons or holes and subsequently transfer the same to adsorbed oxygen and hydroxyl ions to generate super oxide radicals (O₂•ˉ) and hydroxide radicals (•OH), respectively. This behavior will reduce the electron-hole recombination and generates more and more free radicals responsible for the degradation of BG. In this way ZnO doped with Te reveals higher activity to the degradation of the dye brilliant green.

Conclusion

 In summary, high quality nanocrystals of pure and Te doped ZnO have been synthesized through a simple chemical precipitation approach. The XRD patterns suggest the formation of wurtzite ZnO nanocrystals with a size of 24 nm. The sizes of the samples could be controlled up to the concentration of 0.2 M of tellurium and were found to be independent at 0.25 M of doping. The PL spectra of the doped products show the intensity quenched red emissions suggesting the passivation of oxygen defects. The change in the ZnO bond length on doping has been confirmed by FT-IR analysis. The FESEM analysis of the products reveals a change in morphology of ZnO from spherical particles to nanosheets on Te doping. The EDS pattern of Te-doped ZnO exhibits the presence of Zn, O, and Te as expected. The doping of Te showed an enhancement of the photocatalytic activity of ZnO against brilliant green (BG). The enhanced photocatalytic activity of Te doped ZnO suggesting its usage as a scavenger against the pollutant brilliant green which is being discharged from the textiles industries.

 References

Keywords

diffraction, photoluminescence, degradation, brilliant green,photocatalytic