ZnO-M xO y heterostructures (M=Co, Mn, Ni, or In) are fabricated via hydrothermal synthesis method. X-ray diffraction and Fourier-transform infrared spectroscopy analyses endorse the successive formation of the various heterostructures. Field Emission Scanning electron microscope and Brunauer-Emmett-Teller (BET) surface area studies confirm the porous nature of the heterostructures obtained. The band gaps of various heterostructures are calculated that, 3.1, 2.71, 2.64, and 2.19 eV for ZnO-NiO, ZnO-In 2O 3, ZnO-Co 3O 4, and ZnO-MnO 2, respectively. The photocatalytic activities of the fabricated heterostructures are investigated through the degradation of phenol under direct sunlight irradiation. The results show that the photocatalytic activity is affected by the conduction band (CB) and valence band (VB) positions rather than surface area of ZnO-M xO y heterostructure nanocomposites.
Water pollution has become one of the serious problems that threaten human life. Photocatalytic degradation is a promising green technology to remove organic and inorganic pollutants from the water. A lot of metal oxide semiconductors have been explored for photocatalytic pollutant degradation [
The ingredients were procured through different commercial suppliers as follows: Zn(NO3)2∙6H2O (Alfa Aesar, 99%), Ni(NO3)2∙6H2O (Sigma Aldrich, 99.99%, Co(NO3)2∙6H2O (Loba Chemie, 99.8%), In(NO3)∙xH2O (Sigma-Aldrich, 99.99%), Mn(NO3)∙4H2O (Alfa Aesar, 98%) and NaOH (Sigma-Aldrich, 97%). Ultra-pure water (18. 2 MΩ∙cm) was used in the experiments.
The synthesis of ZnO/MxOy nanocomposites were carried out by using 15 ml 0.1M Zn(NO3)2∙6H2O mixed with 15 ml 0.1M along with a desired metal nitrate (Ni(NO3)2∙6H2O, Co(NO3)2∙6H2O, Mn(NO3)2∙4H2O or In(NO3)∙xH2O). The mixed solution was ultrasonicated thoroughly and mixed for 30 minutes to get the homogenous mixture. Later, about 24 ml of the suspension was transferred into a 30 ml capacity Teflon liner and closed tightly. The percent fill was kept at 80% in all experiments. The Teflon liner was inserted into SS316 Stainless Steel autoclave and heated up to a particular temperature for a required period of time (
The prepared heterostructures were characterized using Rigaku Smart Lab-II X-ray diffractometer (XRD) with CuKα radiation (λ = 1.540598 Å), Carl Zeiss MERLIN Compact field emission scanning electron microscopy (FE-SEM), JASCO 460 plus FTIR spectrometer, a Shimadzu UV-2450 spectrophotometer (UV-Vis DRS), NETZSCH, Germany, Model STA 2500 Regulus, thermo-gravimetric analysis (TGA), and BELSORP MINI 2, BEL, Japan, Brunauer-Emmett-Teller (BET) surface area measurements.
The photocatalytic properties of as synthesized ZnO-MxOy nanocomposites were carried out for the study of phenol degradation. The degradation was performed under solar light irradiation to the phenol sample with the catalyst ZnO-MxOy nanocomposites. The phenol samples with different concentrations, like 8, 16, 24 and 32 ppm, were prepared and the optimum phenol degradation concentration was investigated for the different catalysts. In typical procedure, the degradation
ZnO/MxOy composite | Ratio | Temperature (˚C) | Hydrothermal reaction time duration (hr) |
---|---|---|---|
ZnO/MnO2 | 1:1 | 180 | 5 |
ZnO/NiO | 1:1 | 160 | 5 |
ZnO/Co3O4 | 1:1 | 150 | 6 |
ZnO/In2O3 | 1:1 | 180 | 8 |
was carried out by preparing 100 ml of phenol solution with above mentioned different concentration in a 250 cm3 conical flasks. Later, 0.5 g of the catalysts such as ZnO-MxOy nanocomposites were added to individual flasks and closed tightly with rubber cork. The degradation reaction was carried out for the period of 3 h (from 11 am to 2 am in a sunny day in April 2018 in Mysuru, India) under solar light irradiation. After the completion of degradation reaction, the samples were filtered and the percentage of phenol degradation was investigated by performing chemical oxygen demand (COD) analysis.
composite [
The FTIR analysis of heterostructured ZnO-MxOy nanocomposites was performed.
The carbonate peaks are prominent in all the composites and it is again attributed to the absorption of CO2 from the atmosphere. The finger print region of metal oxygen bond shows the characteristic absorption bands in the wavenumber region 400 to 600 cm−1 [
of the heterostructured nanocomposites and agree well with XRD results.
Particle size distributions of as prepared nanocomposites are shown in
The DRS UV-Vis spectra of as-synthesized ZnO-MxOy nanocomposites were recorded to determine their light absorption characteristics and
The thermal analysis of as synthesized ZnO-MxOy nanocomposites was carried out to get information about change in composites properties as function of temperature.
in the weight at 105˚C, attributed to the evaporation of any residual water. Further decrease in the weight loss up to 290˚C was due to the evaporation of CO2. The presence of CO2 was evident from the FTIR analysis as well as the sudden loss in the weight% at temperature 290˚C to 350˚C was due to the decomposition of Co3O4 which in turn form Co3O4 and O2. The total weight loss of ZnO-Co3O4 was ~15% and it shows high thermal stability. TGA curve at the temperature range between 40˚C - 180˚C and 190˚C - 375˚C of ZnO-NiO composite shows the weight loss occurred due to evaporation of water and CO2 respectively. Again the sudden increase in the weight at temperature range between 380˚C - 390˚C occurred due to the re-adsorption of CO2 with the influence of fluid N2. Further the sudden decrease in the weight loss at temperature between 440˚C - 520˚C was due to the decomposition of composites. The TGA curve of ZnO-MnO2 composite shows the curve with gradual decrease in the weight loss at temperature 40˚C - 150˚C due to evaporation of water molecules. Further the weight loss at temperature 420˚C - 550˚C was due to the thermal decomposition of ZnO-MnO2 to MnO2, and also the total weight loss observed was only ~8%, which depicts the higher thermal stability. The TGA of ZnO-In2O3 composite shows that the gradual decrease of weight loss at 40˚C - 120˚C and 150˚C - 240˚C indicate the evaporation of water and CO2 respectively. Further the sudden decrease in the weight loss at 280˚C - 330˚C was due to the decomposition of composite, with a larger weight loss (35%).
The N2 sorption isotherms of all samples can be classified as type IV with H3 hysteresis loops (
The photocatalytic activities of the as prepared nanocomposites were performed by using phenol as a model organic pollutant in an aqueous solution for the degradation process.
degradation in different concentrations of various composites under sunlight irradiation for the period of 3 hours. The order of phenol degradation follows the trends: ZnO-Co3O4 > ZnO-In2O3 > ZnO-MnO2 > ZnO-NiO. Apparently, the order of phenol degradation does not follow the order of surface area, but it may occur because of the chemical composition of composites. The photocatalytic degradation was maximum by using ZnO-Co3O4, where 97% of 8 ppm of phenol have been degraded followed with 95% degradation by ZnO-In2O3, 84% degradation by ZnO-MnO2 and 29% degradation by ZnO-NiO respectively. The degradation efficiency decreased when the concentration of phenol increased from 8 ppm to 32 ppm, which may be due to phenol molecules around the photocatalytic active sites resulted in inhibiting the penetration of light to the surface of the catalyst. Hence, the generation of relative amount of ?OH and ˙ O 2 − on the surface of the catalyst decreased [
The band gaps and band positions were calculated and the band structure diagrams of the different nanocomposites were presented in
helpful in understanding the mechanism of phenol degradation. The enhancement in the photocatalytic activity of ZnO-Co3O4 and ZnO-In2O3 can be attributed to the small band gap of ZnO-In2O3 (2.71 eV) and ZnO-Co3O4 (~2.64 eV) which results in improving the visible light absorption, and the enhancement in the charge separation occurs because of coupling ZnO with Co3O4 and In2O3 separately [
Although the band gap of ZnO-MnO2 (2.19 eV) is smaller than ZnO-In2O3, ZnO-Co3O4 and ZnO-NiO shows lower photocatalytic activity and the reason is charge separation efficiency of ZnO-MnO2 is very low when compared with ZnO-Co3O4 and ZnO-In2O3. This process is indicated clearly in
ZnO-Co3O4, ZnO-In2O3, ZnO-MnO2, and ZnO-NiO heterostructures were fabricated via facile hydrothermal route. The successive formation of heterostructures and their purity were confirmed by XRD analysis. FT-IR analysis further verified that fabricated heterostructures are metal oxides and no other impurities. SEM and BET studies revealed that the heterostructures obtained exhibit porous nature. TGA analysis shows that the highest and lowest thermal stability were exhibited by ZnO-MnO2 and ZnO-In2O3 respectively. The band gap of the prepared heterostructures follows the trend: ZnO-NiO (3.1 eV) > ZnO-In2O3 (2.71 eV) > ZnO-Co3O4 (2.64 eV) > ZnO-MnO2 (2.19 eV). The photocatalytic activity was influenced by the electronic structure and the band gap of heterostructure rather than its surface area. The photocatalytic activity follows the order: Co3O4-ZnO > In2O3-ZnO > ZnO-MnO2 > ZnO-NiO. The low charge separation efficiency of ZnO-MnO2 and its inappropriate CB edge position are the reasons for lower photocatalytic activity than Co3O4-ZnO and ZnO-In2O3 with higher band gaps.
Authors wish to acknowledge the financial support and the laboratory facilities to carry out this work under UGC-UPE project, Govt. of India. FE-SEM work was carried out at Mangalore University, India.
The authors declare no conflicts of interest regarding the publication of this paper.
Nayan, M.B., Jagadish, K., Abhilash, M.R., Namratha, K. and Srikantaswamy, S. (2019) Comparative Study on the Effects of Surface Area, Conduction Band and Valence Band Positions on the Photocatalytic Activity of ZnO-MxOy Heterostructures. Journal of Water Resource and Protection, 11, 357-370. https://doi.org/10.4236/jwarp.2019.113021