Functional coatings based on MWCNT-Metal oxide nanocomposite for solar energy harvester application.

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.