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See detailFunctional coatings based on MWCNT-Metal oxide nanocomposite for solar energy harvester application.
Prasadam, Vasu Prasad UL

Doctoral thesis (2021)

Global energy demand propelled humankind in search of clean and renewable energy sources. Among them, solar energy outstands all the available renewable sources. In this context, concentrated solar ... [more ▼]

Global energy demand propelled humankind in search of clean and renewable energy sources. Among them, solar energy outstands all the available renewable sources. In this context, concentrated solar thermal technology (CST) and hydrogen storage via solar water splitting significant feature contributions in global power generation. Nevertheless, the major challenge in CST technology is achieving a high solar absorption selectivity with thermal stability above 923 K. Whereas the limited chemical stability and low performance remain significant challenges in solar water-splitting technology. We address these technologies' critical issues through multiwalled carbon nanotubes (MWCNT)-metal oxide hybrid materials. MWCNTs are known for their high solar absorption, thermal and electrical conductivity. While metal oxides such as VO2, Al-doped ZnO are known for their infrared reflecting properties with high transparency in the visible region. TiO2 and ZnO have appropriate band positions for water splitting reactions. Here, combining CNTs and metal oxides at the nanoscale leads to unique properties not present in individual constituents. We fabricate the MWCNT-metal oxide through the hybrid chemical vapour deposition-atomic layer deposition (CVD-ALD) process. Here the CVD is implemented to grow MWCNTs, while ALD is used to produce conformal metal oxide shells on the 3D porous MWCNT structures. The MWCNT-VO2 nanostructures performed in this study feature a solar selectivity modulation across the semiconductor-metal transition temperature of VO2, i.e., 67˚C. The thermally induced optical modulation was investigated as a function of the morphology of VO2 phase. The grown VO2 nanoparticles on MWCNT illustrate an enhancement in the spectral emissivity across the SMT temperature. A contrasting optical modulation is displayed by the continuous VO2 layer on MWCNT. Aluminium doped zinc oxide (AZO) layer (4.7 at %) illustrated solar absorbance of 0.96 and thermal emittance of 0.6. The limited thermal stability of the engineered MWCNT-AZO was enhanced by the deposition of a thin Al2O3 layer at the MWCNT-AZO interface. A core-double shell structure, i.e., CNT-Al2O3-AZO, withstands thermal treatment at 1000 K for 72 h. Solar water splitting study on MWCNT-TiO2 and MWCNT-ZnO nanostructures revealed a significant performance improvement relative to the respective oxides. For MWCNT-TiO2 core-shell structure, an enhancement of photocurrent by 400 % was observed relative to planar Si-TiO2. While in MWCNT-ZnO core-shell structure, similar results as CNT-TiO2 is observed but with higher photocurrent density because of better electrical properties of ZnO. We observed an increase of 458 % of the photocurrent density relative to Si-ZnO. The difference in performance between Si-ZnO/TiO2 and MWCNT-ZnO/TiO2 was associated with the diminished electron-hole recombination, efficient electron collection and increased relative surface in the core-shell structure. [less ▲]

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