Synthesis and characterization of zinc oxide nanorods sensitized by metal chalcogenides for photoelectrochemical application
Zinc oxide (ZnO) is considered as a promising oxide semiconductor for photoelectrochemical application. Despite its bandgap that is not ideal for sunlight absorption, this metal oxide is very efficient in absorbing UV light. It is well-known that UV light takes up only a small fraction (below 10%...
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Zinc oxide (ZnO) is considered as a promising oxide semiconductor for
photoelectrochemical application. Despite its bandgap that is not ideal for sunlight
absorption, this metal oxide is very efficient in absorbing UV light. It is well-known
that UV light takes up only a small fraction (below 10%) of the sun's energy compared
to visible light (about 44%). To solve this problem, heterostructures of the
nanocomposite semiconductors such as cadmium sulfide/zinc oxide, silver
sulfide/zinc oxide and silver sulfide/cadmium sulfide/zinc oxide, are proposed to be
the alternative conversion medium, which are sustainable and could possibly harvest
larger spectrum of sunlight. A variety of parameters for selective modification can be
applied while conducting the synthesis process, thus ensuring that the overall
photoresponse of these nanocomposites can be increased. These include depositing
sensitised materials on the surface of ZnO nanorod arrays, which could result in the
modification of the electronic interface and facilitating charge carrier transfer between
the attached substance and the host semiconductor. In this study, pure ZnO
nanoparticles as seed layer was prepared utilising the sol-gel spin coating technique.
At the same time, ZnO nanorods, CdS/ZnO shell-core, Ag2S/ZnO quantum dot-core,
and Ag2S/CdS/ZnO quantum dot-shell-core were prepared through the hydrothermal
method. In addition, considering the effect of various parameters on hydrothermal
formation of CdS/ZnO nanocomposite, the synthesis was carried out with variation of
growth temperature (70 oC to 110 oC), growth time (30 min to 120 min), concentration
of CdCl2 (0.001 M to 0.010 M), concentration of CS (NH2)2 (0.01 M to 0.10 M), pH
(3.5 to 7.5) and annealing temperature (100 oC to 500 oC). During optimisation
process, the formation of ZnO nanorods and CdS were noticed when the colour of the
samples changed from colourless to white for ZnO, and yellowish for CdS/ZnO
sample. The powder X-ray diffraction (XRD) analysis verified that the samples
synthesised has hexagonal phase with the cauliflower-like nanostructure was observed
in the field emission scanning electron microscopy (FESEM) as well as the red shifted
absorbance spectra from the analysis of the UV-visible spectrophotometry. On the
other hand, the transmission electron microscopy (TEM) provided the estimated average for shell thickness of the CdS/ZnO heterostructure shell-core, followed by
determination of lattice fringe (d-spacing) from high-resolution transmission electron
microscopy (HRTEM).
Furthermore, the hydrothermal technique was also used in the synthesis of Ag2S/ZnO
nanocomposite. The effect of changing deposition parameters such as growth time (15
min to 35 min), growth temperature (50 oC to 90 oC), concentration of AgNO3 (0.001
M and 0.01 M), concentrations of thiourea, CS(NH2)2 (0.01 M to 0.10 M), pH (3 to 7)
and annealing temperatures (100 °C to 500 °C) were studied. While conducting
optimisation process, the formation of ZnO nanorods and Ag2S was noticed as the
colour of the samples changed from colourless to white for ZnO and brown for the
Ag2S/ZnO sample. XRD, FESEM and UV-visible spectrophotometry analyses
showed that the samples synthesised has a monoclinic phase, nanoparticles like
morphology and blue shifted on the absorption edge of Ag2S. The average particle
size for the Ag2S/ZnO heterostructure quantum dot-core was determined using the
data from TEM analysis; meanwhile, the lattice fringe (d-spacing) was taken from the
HRTEM analysis. The ternary-structure, Ag2S/CdS/ZnO, was hydrothermally
synthesised mainly by varying three parameters; growth temperature and time, as well
as the annealing temperature of Ag2S on CdS/ZnO/ITO. The quantum confinement
effect is clearly explained by the blue shift on the absorption edge of Ag2S
demonstrated in Ag2S/ZnO and Ag2S/CdS/ZnO. Brus formula was employed to
calculate the particle size of the Ag2S nanoparticles in the nanostructured Ag2S/ZnO
and Ag2S/CdS/ZnO. This was achieved from the UV-visible data, which was in fair
agreement with that determined from the HRTEM images.
Results showed that due to its narrow bandgap, Ag2S is able to harvest more light
compared to CdS. In addition, the Ag2S coated photoelectrodes appeared dark brown
compared to the white ZnO and bright yellow CdS. Therefore, compared to CdS, it
can be interpreted that Ag2S could possibly extend the ZnO range of absorption from
UV to also cover a larger visible region of the solar spectrum. In comparison with ZnO
NRs/ITO PEC electrode with a band gap of 3.15 eV, CdS/ZnO/ITO, Ag2S/ZnO/ITO
and Ag2S/CdS/ZnO/ITO showed much smaller band gaps of 2.35 eV, 1.75 eV and
1.50 eV respectively. This cascading bands gap alignment decreases the chance for
electron-hole recombination and enhance the efficiency of electrons collection.
The photoelectrochemical performance of the bare ZnO nanorods, CdS/ZnO,
Ag2S/ZnO, and Ag2S/CdS/ZnO was inspected under the illumination of a visible light.
The enhancement of photoconversion efficiency was observed to be 3.43% for
CdS/ZnO, 4.08% for Ag2S/ZnO and 6.05% for Ag2S/CdS/ZnO compared to pure ZnO
nanorods sample of only 0.42%. |
| format |
Thesis |
| author |
Alshabbani, Araa Mebdir Holi |
| spellingShingle |
Alshabbani, Araa Mebdir Holi Synthesis and characterization of zinc oxide nanorods sensitized by metal chalcogenides for photoelectrochemical application |
| author_facet |
Alshabbani, Araa Mebdir Holi |
| author_sort |
Alshabbani, Araa Mebdir Holi |
| title |
Synthesis and characterization of zinc oxide nanorods sensitized by metal chalcogenides for photoelectrochemical application |
| title_short |
Synthesis and characterization of zinc oxide nanorods sensitized by metal chalcogenides for photoelectrochemical application |
| title_full |
Synthesis and characterization of zinc oxide nanorods sensitized by metal chalcogenides for photoelectrochemical application |
| title_fullStr |
Synthesis and characterization of zinc oxide nanorods sensitized by metal chalcogenides for photoelectrochemical application |
| title_full_unstemmed |
Synthesis and characterization of zinc oxide nanorods sensitized by metal chalcogenides for photoelectrochemical application |
| title_sort |
synthesis and characterization of zinc oxide nanorods sensitized by metal chalcogenides for photoelectrochemical application |
| publishDate |
2017 |
| url |
http://ethesis.upm.edu.my/id/eprint/9760/1/ITMA%202017%201%20-%20T.pdf |
| _version_ |
1819311013843959808 |
| spelling |
oai:ethesis.upm.edu.my:9760 http://ethesis.upm.edu.my/id/eprint/9760/ Synthesis and characterization of zinc oxide nanorods sensitized by metal chalcogenides for photoelectrochemical application Alshabbani, Araa Mebdir Holi Zinc oxide (ZnO) is considered as a promising oxide semiconductor for photoelectrochemical application. Despite its bandgap that is not ideal for sunlight absorption, this metal oxide is very efficient in absorbing UV light. It is well-known that UV light takes up only a small fraction (below 10%) of the sun's energy compared to visible light (about 44%). To solve this problem, heterostructures of the nanocomposite semiconductors such as cadmium sulfide/zinc oxide, silver sulfide/zinc oxide and silver sulfide/cadmium sulfide/zinc oxide, are proposed to be the alternative conversion medium, which are sustainable and could possibly harvest larger spectrum of sunlight. A variety of parameters for selective modification can be applied while conducting the synthesis process, thus ensuring that the overall photoresponse of these nanocomposites can be increased. These include depositing sensitised materials on the surface of ZnO nanorod arrays, which could result in the modification of the electronic interface and facilitating charge carrier transfer between the attached substance and the host semiconductor. In this study, pure ZnO nanoparticles as seed layer was prepared utilising the sol-gel spin coating technique. At the same time, ZnO nanorods, CdS/ZnO shell-core, Ag2S/ZnO quantum dot-core, and Ag2S/CdS/ZnO quantum dot-shell-core were prepared through the hydrothermal method. In addition, considering the effect of various parameters on hydrothermal formation of CdS/ZnO nanocomposite, the synthesis was carried out with variation of growth temperature (70 oC to 110 oC), growth time (30 min to 120 min), concentration of CdCl2 (0.001 M to 0.010 M), concentration of CS (NH2)2 (0.01 M to 0.10 M), pH (3.5 to 7.5) and annealing temperature (100 oC to 500 oC). During optimisation process, the formation of ZnO nanorods and CdS were noticed when the colour of the samples changed from colourless to white for ZnO, and yellowish for CdS/ZnO sample. The powder X-ray diffraction (XRD) analysis verified that the samples synthesised has hexagonal phase with the cauliflower-like nanostructure was observed in the field emission scanning electron microscopy (FESEM) as well as the red shifted absorbance spectra from the analysis of the UV-visible spectrophotometry. On the other hand, the transmission electron microscopy (TEM) provided the estimated average for shell thickness of the CdS/ZnO heterostructure shell-core, followed by determination of lattice fringe (d-spacing) from high-resolution transmission electron microscopy (HRTEM). Furthermore, the hydrothermal technique was also used in the synthesis of Ag2S/ZnO nanocomposite. The effect of changing deposition parameters such as growth time (15 min to 35 min), growth temperature (50 oC to 90 oC), concentration of AgNO3 (0.001 M and 0.01 M), concentrations of thiourea, CS(NH2)2 (0.01 M to 0.10 M), pH (3 to 7) and annealing temperatures (100 °C to 500 °C) were studied. While conducting optimisation process, the formation of ZnO nanorods and Ag2S was noticed as the colour of the samples changed from colourless to white for ZnO and brown for the Ag2S/ZnO sample. XRD, FESEM and UV-visible spectrophotometry analyses showed that the samples synthesised has a monoclinic phase, nanoparticles like morphology and blue shifted on the absorption edge of Ag2S. The average particle size for the Ag2S/ZnO heterostructure quantum dot-core was determined using the data from TEM analysis; meanwhile, the lattice fringe (d-spacing) was taken from the HRTEM analysis. The ternary-structure, Ag2S/CdS/ZnO, was hydrothermally synthesised mainly by varying three parameters; growth temperature and time, as well as the annealing temperature of Ag2S on CdS/ZnO/ITO. The quantum confinement effect is clearly explained by the blue shift on the absorption edge of Ag2S demonstrated in Ag2S/ZnO and Ag2S/CdS/ZnO. Brus formula was employed to calculate the particle size of the Ag2S nanoparticles in the nanostructured Ag2S/ZnO and Ag2S/CdS/ZnO. This was achieved from the UV-visible data, which was in fair agreement with that determined from the HRTEM images. Results showed that due to its narrow bandgap, Ag2S is able to harvest more light compared to CdS. In addition, the Ag2S coated photoelectrodes appeared dark brown compared to the white ZnO and bright yellow CdS. Therefore, compared to CdS, it can be interpreted that Ag2S could possibly extend the ZnO range of absorption from UV to also cover a larger visible region of the solar spectrum. In comparison with ZnO NRs/ITO PEC electrode with a band gap of 3.15 eV, CdS/ZnO/ITO, Ag2S/ZnO/ITO and Ag2S/CdS/ZnO/ITO showed much smaller band gaps of 2.35 eV, 1.75 eV and 1.50 eV respectively. This cascading bands gap alignment decreases the chance for electron-hole recombination and enhance the efficiency of electrons collection. The photoelectrochemical performance of the bare ZnO nanorods, CdS/ZnO, Ag2S/ZnO, and Ag2S/CdS/ZnO was inspected under the illumination of a visible light. The enhancement of photoconversion efficiency was observed to be 3.43% for CdS/ZnO, 4.08% for Ag2S/ZnO and 6.05% for Ag2S/CdS/ZnO compared to pure ZnO nanorods sample of only 0.42%. 2017-05 Thesis NonPeerReviewed application/pdf en http://ethesis.upm.edu.my/id/eprint/9760/1/ITMA%202017%201%20-%20T.pdf Alshabbani, Araa Mebdir Holi (2017) Synthesis and characterization of zinc oxide nanorods sensitized by metal chalcogenides for photoelectrochemical application. PhD thesis, Universiti Putra Malaysia. (ITMA 2017 1). |
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13.4562235 |