Dr. Nicolas M. Gaillard Profile

Dr. Nicolas M. Gaillard

Assistant Researcher at the Thin Films Lab at Hawaii Natural Energy Institute

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Publications (3)

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Proceedings Article | 16 September 2013 Paper

Development of metal tungstate alloys for photoelectrochemical water splitting

D. Prasher, M. Chong, Y. Chang, P. Sarker, M. Huda, N. Gaillard

Proceedings Volume 8822, 88220E (2013) https://doi.org/10.1117/12.2026464

KEYWORDS: Copper, Particles, Scanning electron microscopy, Water splitting, Solar energy, Metals, Photocatalytic water splitting, Transmission electron microscopy, X-ray diffraction, Spherical lenses

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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.

Proceedings Article | 25 August 2010 Paper

Status of research on tungsten oxide-based photoelectrochemical devices at the University of Hawai'i

N. Gaillard, Y. Chang, J. Kaneshiro, A. Deangelis, E. Miller

Proceedings Volume 7770, 77700V (2010) https://doi.org/10.1117/12.860970

KEYWORDS: Tungsten, Oxides, Thin films, Silicon, Nanoparticles, Electrodes, Oxygen, Ruthenium, Scanning electron microscopy, Photomicroscopy

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For more than a decade, the Hawaii Natural Energy Institute has conducted research on photoelectrochemical (PEC) technologies and achieved major milestones, including the fabrication of high-performance photoactive thin film materials and the development of innovative device integrations (hybrid-photo-electrode). In this paper, we focus our discussion on tungsten oxide-based materials, one of our two principal topics of research in this field. After a description of pure WO₃ physical, chemical and energetic properties we present our latest results on tungsten oxide PEC properties improvement. In our general approach, each component of the PEC electrode is addressed, from the absorber (bulk) to the surface energetics (near-surface) and catalysis (surface). Recently, progresses have been made on surface treatment for catalytic purposes as well as on PEC materials integration. In the case of catalytic treatment, our studies show that reactive sputtering technique is suitable to form high quality RuO₂ thin films and nanoparticles. Tests conducted on RuO2 thin films pointed out an oxygen evolution reaction potential as low as 0.2 V. When used as an anode in 2- electrode configuration, RuO2 thin films lead to a photocurrent onset potential reduction as low as 500 mV for p-type PEC materials (CGSe₂ and a-SiC, so far tested) when compared to platinum. In the case of RuO2 nanoparticles, a photocurrent density increase of approx. 20% was observed on treated tungsten oxide films. Finally, we present a new integration scheme to increase photocurrent density using highly textured substrates (HTS). In our approach, HTS were obtained by anisotropic etching of [100] silicon substrates in KOH solution. Initial results indicated a very good coverage of WO₃ onto the silicon pyramids and a photocurrent doubling is observed when compared to WO3 deposited on flat silicon substrates.

Proceedings Article | 25 August 2010 Paper

Surface modification of a-SiC photoelectrodes for photocurrent enhancement

Ilvydas Matulionis, Jian Hu, Feng Zhu, Josh Gallon, Nicolas Gaillard, Todd Deutsch, Eric Miller, Arun Madan

Proceedings Volume 7770, 77700Z (2010) https://doi.org/10.1117/12.860669

KEYWORDS: Nanoparticles, Titanium, Amorphous silicon, Interfaces, Hydrogen, Plasma enhanced chemical vapor deposition, Oxides, Semiconductors, Thin films, Silicon

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Photoelectrochemical (PEC) water dissociation into hydrogen and oxygen at a semiconductor-liquid interface offers an environmentally benign approach to hydrogen production. We have developed an integrated PEC device using hydrogenated amorphous silicon carbide (a-SiC or a-SiC:H) material as photoelectrode in conjunction with an amorphous silicon (a-Si) tandem photovoltaic device. Such a "hybrid PV/a-SiC" PEC cell produces photocurrent of about 1.3 mA/cm² in a short-circuit configuration and is durable in a pH2 electrolyte. On the other hand, the aforementioned structure finished with ITO contacts and measured as a solid-state device features a current density of 5 mA/cm², indicating a potential solar-to-hydrogen (STH) conversion efficiency of about 6% in the hybrid PV/a-SiC PEC cell. The much lower photocurrent measured in the hybrid PEC cell suggests that there exists an interfacial barrier between the a-SiC and electrolyte, which hinders the photocurrent extraction. In order to mitigate against the interfacial barrier and hence improve the photo-generated charge carrier transport through the a-SiC/electrolyte interface, we have explored several surface modification techniques, namely the use of metallic nano-particles (such as platinum or palladium) and the growth of an additional thin layer (a-SiNx, carbon-rich a-SiC, a-SiF, etc.) on the top of a-SiC by PECVD. In the latter case, it is observed that the addition of a thin PECVD-fabricated layer does not significantly improve the photocurrent, presumably due to a poor band alignment at the a-SiC/electrolyte interface. The use of lower work function nanoparticles like titanium has led to promising results in terms of photocurrent enhancement and an a nodic shift in the onset potential.

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