Studies on hydrothermally synthesized zinc oxide nanorods films for visible light photocatalysis | |
| Author | Mahmood, Mohammad Abbas |
| Call Number | AIT Diss. no.ISE-12-14 |
| Subject(s) | Zinc oxide Nanostructures |
| Note | A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Nanotechnology, School of Engineering and Technology |
| Publisher | Asian Institute of Technology |
| Series Statement | Dissertation ; no. ISE-12-14 |
| Abstract | Photocatalysis involves a wide band gap semiconductor material which when irradiated with light of certain high frequency creates electron-hole pairs. Both of these photogenerated charge carriers have high redox potentials and decompose/mineralize any organic material adsorb at the photocatalyst. This characteristic has attracted a great deal of researcher’s interest for application of the process for amelioration of the environment with the help of ambient solar light. Among the list of wide band gap metal oxide semiconductors, TiO2(band gap ≈3.2 eV) in its nanoparticulate powder form has been conceded as the closest to an ideal photocatalyst due to its stability, low cost, and efficacy to degrade wide range of organic contaminants. The material, however, has limitations of low photocatalytic efficiency under solar light. Zinc oxide (ZnO) with comparable band gap (3.37 eV) could be a promising alternate material in this regards. In addition to higher optical absorption than TiO2, ZnO could be synthesized in the form of nanorod films on solid substrates at low temperatures. In this research work, we have focused on low temperature hydrothermal synthesis of ZnO nanorod films on glass substrates and their application for photocatalytic degradation of an organic dye in aqueous medium with visible/solar light.Enhancement in the visible light photocatalytic efficiency of ZnO has been reportedly achieved through doping. To anticipate the limited control over doping, we were able to incorporate high concentration defects in ZnO nanocrystals through fast crystallization under microwave irradiation. ZnO nanocrystallites, obtained through microwave assisted hydrothermal synthesis process, exhibited higher visible light photocatalytic activity than the conventionally hydrolyzed nanoparticles as well as manganese (Mn) doped nanoparticles (ZnO:Mn2+). The enhancement in the photocatalytic activity was attributed to the higher defect concentration in the as-synthesized nanocrystallites which was confirmed from their optical absorption and photoluminescence spectra.Microwave assisted hydrolysis could be an effective and convenient alternate method to introduce defects in ZnO nanocrystallites which enhances visible light photocatalytic activity of ZnO nanocrystals.Initially, we have concentrated upon the conventional hydrothermal synthesis of ZnO nanorod arrays for their enhanced visible light photocatalytic degradation of 5 μM aqueous methylene blue (MB) solution. Influence of all the synthesis parameters, like; concentration ivof the precursor solution, relative concentration of hexamine and zinc nitrate, growth time, growth temperature, pre-growth seeding, post growth annealing, contaminant concentration, pH of the contaminant suspension, and light source on the photocatalytic activity were studied. Results of each of the experiments have been recorded in terms of the kinetic curves of the photocatalytic degradation of a test contaminant (methylene blue) irradiated with artificial and ambient solar light. Maximum photocatalytic activities were obtained when ZnO nanorods were synthesized with equimolarconcentration of 10 mM hexamine and zinc nitrate grown for 15 hours, hydrolyzed at 90oC. Photocatalytic activity could be further enhanced by annealing the nanorod arrays at 250oC in air. Further, the decoloration rate was observed to drop with increased dye concentration, while the activity increased exponentially with pH value of the contaminant suspension. Also, photocatalytic degradation was observed to take place much faster under ambient solar irradiation than under white light from a lamp. It was also found that application ofvoltage of 0.1 V to the nanorod film grown on a conducting surface increase the photocatalytic activity.Seeding is an important step in hydrothermal synthesis of ZnO nanorods to provide nucleation sites for the growth ensures adhesion of the as-synthesized films to the substrates. Herein,we introducedspray pyrolysis as a simple and efficient technique for the deposition of ZnO seeding film on glass. The method involves the in situ formation of ZnO nanocrystals by spraying aqueous solution of zinc acetate on hot substrate (~400oC) whereby ZnO nanocrystallites with narrow size distribution (4-8 nm) are formed. Majority of the crystals were found to have their polar (0001) facet oriented perpendicular to the substrate surface. Upon hydrothermal growth of ZnO nanorods in an equimolar solution of zinc nitrate and hexamine, it was observed that seeding by spray pyrolysis led to the formation of dense ZnO nanorod arrays that were well aligned and strongly attached to the glass substrates.While ZnO nanorod film due to higher surface area exhibited an enhanced photocatalytic activity than the nanoparticulate film deposited on a glass substrate, we applied the microwave assisted synthesis technique to the growth of ZnO nanorod films. Synthesis parameters like concentration of the precursor solution, growth time, and the applied microwave power have been optimized. It was found that the technique gives 4-5 times faster growth than what has been observed inthe case of conventional heating system. vExperiments showed that ZnO nanorod films grown through microwave irradiation for 5 h were nearly 20% more active than the one synthesized through conventional heating for 20 h. The reason could evidently be the inclusion of higher concentration of crystal defects resulted with the fast crystallization process under microwave irradiation. Coupling of ZnO nanorod films with zinc stannate (ZTO), accomplished through thermal treatment (in an autoclave) of pre-synthesized nanorod films in aqueous mixed solution of NaOH and SnCl4, was considered to further enhance the photocatalytic activity. Effects of several parameters like temperature, time, NaOH/SnCl4molar ratio, and Sn4+concentration were investigated. Maximumincrease in the photocatalytic activity was obtained for 2-3 h heat treatment at temperature of 170-180oC, NaOH/SnCl4molar ration of 10/1, and using Sn4+concentration of 2 mM. ZTO coating of ZnO nanorod arrays was also being attempted at ambient pressure under microwave irradiations. The process, however, resulted in the transformation of the ZnO nanorod films into films of zinc hydroxystannate (ZHS). |
| Year | 2011 |
| Corresponding Series Added Entry | Asian Institute of Technology. Dissertation ; no. ISE-12-14 |
| Type | Dissertation |
| School | School of Engineering and Technology (SET) |
| Department | Department of Industrial Systems Engineering (DISE) |
| Academic Program/FoS | Industrial Systems Engineering (ISE) |
| Chairperson(s) | Dutta, Joydeep; |
| Examination Committee(s) | Anal, Anil Kumar ;Chiu, Gregory L.F; |
| Scholarship Donor(s) | Link programbetween Asian Institute of Technology,Thailand and University of Engineering andTechnology Peshawar,Pakistan; |
| Degree | Thesis (Ph. D.) - Asian Institute of Technology, 2011 |