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Insights into the Controllable Chemical Composition of Metal Oxide Nanowires and Graphene Aerogels


The design and synthesis of materials that absorb visible light and create fuel to store solar energy is a pursuit that has captivated chemists for decades. In order to take part in solar water splitting, i.e. the production of hydrogen and oxygen gas from water and sunlight, electrode materials must fit specific requirements in terms of their electronic structure. Zinc oxide (ZnO) and titanium dioxide (TiO2) are both of interest for their ability to produce oxygen from photogenerated holes, but their band gaps are too large to capture a significant portion of the solar spectrum. We address this challenge by modifying the crystal structures of ZnO and TiO2 to make lower band gap materials. Furthermore, we use nanowires as the synthetic template for these materials because they provide a large semiconductor-liquid interfacial area.

ZnO nanowires can be alloyed with In3+, Fe3+ and other trivalent metal ions to form a unique structure with the formula M2O3(ZnO)n, also known as MZO. We synthesize indium zinc oxide (IZO) and indium iron zinc oxide (IFZO) nanowires and study their crystal structure using atomically-resolved transmission electron microscopy (TEM), among other methods. We elucidate a structural model for MZO that resolves inconsistencies in the existing literature, based on the identification of the zigzag layer as an inversion domain boundary. These nanowires are shown to have a lower band gap than ZnO and produce photocurrent under visible light illumination.

The solid-state diffusion reaction to form ternary titanates is also studied by TEM. TiO2 nanowires are coated with metal oxides by a variety of deposition methods, and then converted to MTiO3 at high temperatures, where M is a divalent transition metal ion such as Mn2+, Co2+, or Ni2+. When Co3O4 particles attached to TiO2 nanowires are annealed for a short time, we observe the formation of a CoO(111)/TiO2 (010) interface. If the nanowires are instead coated with Co(NO3)2 salt and then annealed briefly, then isolated pockets of MTiO3 are formed on the nanowire surface. This structure retains the conductive channel in the center of the nanowire, which can be useful for charge separation. Longer annealing times result in segmented nanowires; the segments formed from a Ni-coated nanowire are bounded by TiO2(01-1) twin planes and NiTiO3{100}/TiO2{03-1} interfaces.

An alternative strategy for storing solar energy takes advantage of the capacitance between a semiconductor surface and adsorbed ions in solution. This type of energy storage device is called an electric double layer capacitor (EDLC). Graphene-based aerogels, which are porous materials composed of few-layer graphitic sheets, have the potential for higher surface area and higher conductivity than standard carbon aerogels. These properties make graphene-based aerogels a good material candidate for EDLC electrodes.

Graphene oxide (GO) is the precursor material for the synthesis of a graphene-based aerogel, and it has been widely studied. Yet its hydrothermal gelation is still not fully understood, due to the high pressure reaction conditions and the non-uniform nature of GO. We demonstrate a number of changes that occur to the GO sheets during gelation: wrinkling, formation of a densified monolith, deoxygenation, increasing thermal stability, and color change. Plotting the time evolution of all these properties shows that they are simultaneous and likely of common origin. Possible mechanisms for gelation are explored.

Graphene aerogels are synthesized by vapor phase thermal reduction of GO aerogels at temperatures up to 1600 °C. Further deoxygenation is observed in the aerogel during thermal reduction, along with enhanced crystallinity and an associated change in the electronic structure. When graphene aerogels are exposed to high-temperature boron oxide vapor, they are converted to boron nitride (BN) aerogels. The structure of the BN aerogel is investigated and shown to be similar in nanoscale morphology to the precursor graphene aerogel, with largely turbostratic stacking between the atomic layers. BN aerogels are superhydrophilic and thermally stable, allowing them to adsorb oil and then be regenerated by burning in air.

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