KEYWORDS: Copper, Particles, Scanning electron microscopy, Water splitting, Solar energy, Metals, Photocatalytic water splitting, Transmission electron microscopy, X-ray diffraction, Spherical lenses
In the present paper, we report our efforts on the development of metal tungstate alloys for efficient and economical photoelectrochemical water splitting. As suggested by density functional theory (DFT), the addition of copper to the host tungsten trioxide improves the visible light absorption. Past studies at the Hawaii Natural Energy Institute have demonstrated that water splitting with co-sputtered and spray-deposited CuWO4 with 2.2 eV band gap was feasible, although the efficiency of the process was severely limited by charge carrier recombination. Density functional theory calculation showed that CuWO4 contains unfilled mid-gap states and high electron effective mass. To improve transport properties of CuWO4, we hypothesized that copper tungstate (CuWO4) hollow nanospheres could improve holes transfer to the electrolyte and reduce recombination, improving the water splitting efficiency. Nanospheres were synthesized by sonochemical technique in which the precursors used were copper acetate, ammonium meta-tungstate and thiourea (used as a fuel to complete the reaction). All chemicals undergo a high-energy sonication by using ethylene glycol as a solvent. Preliminary linear scan voltammetry (LSV) performed for annealed CuWO4 under front side and back side simulated AM-1.5 illumination demonstrated that the CuWO4 hollow nanospheres were photoactive. Subsequent scanning (SEM) and transmission (TEM) electron microscopy studies revealed the clear formation of nano sized hollow spherical shaped CuWO4 particles. X-ray diffraction analysis showed a clear formation of triclinic CuWO4 structure during the sonochemical process.
Efficient photo-electrochemical (PEC) splitting of water to hydrogen usually requires photoelectrodes to have certain
electronic properties. Unfortunately, at present available semiconductors do not meet all these criteria. So, a thorough
understanding of band-engineering for mixed alloys is necessary to successfully design these photoelectrodes. Among
the semiconductors, transition metal oxides are of particular interest due to their low cost and relatively high stability in
aqueous media. Here, we will present a theoretical study of delafossite-alloys for PEC photo-electrodes. Previous studies
have indicated that the group IIIA delafossite family (CuMO2, M = Al, Ga, In) do not exhibit direct band gaps. Their
fundamental band gaps are significantly smaller than their reported optical band gaps. On the other hand group IIIB
delafossite family (CuMO2, M = Sc, Y, La) in general show direct band gaps and, except for CuLaO2, band gaps are
above 3.00 eV. However, both of these two families exhibit p-type conductivity. We will show that by appropriate
alloying of these two delafossite-families we can tune their band gaps and other opto-electronic properties. These types
of alloying are desirable, as these introduce no localized impurity states in the band gap due to isovalent alloying. Also,
the electronic effective masses can be lowered by selective doping of main group elements. Finally, it will be discussed
that, lowering the symmetry constraints of these alloys would enhance their optical absorption properties. We'll also
discuss that alloying with other 3d metal elements may decrease the band gap, but would increase the effective masses of
the photo-electrons.
We combine first-principles density functional theory, material synthesis and characterization, and photoelectrochemical
(PEC) measurements to explore methods to effectively reduce the band gap of ZnO for the application of PEC water
splitting. We find that the band gap reduction of ZnO can be achieved by N and Cu incorporation into ZnO. We have
successfully synthesized ZnO:N thin films with various reduced band gaps by reactive RF magnetron sputtering. We
further demonstrate that heavy Cu-incorporation lead to both p-type doping and band gap significantly reduced ZnO thin
films. The p-type conductivity in our ZnO:Cu films is clearly revealed by Mott-Schottky plots. The band gap reduction
and photoresponse with visible light for N- and Cu-incorporated ZnO thin films are demonstrated.
Conference Committee Involvement (1)
Wide Bandgap Materials, Devices, and Applications IV
15 August 2019 | San Diego, California, United States
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