Efficient and clean fuel synthesis methods have been actively researched in recent years in response to fossil fuel depletion and global warming concerns. By absorbing solar energy with a semiconducting photoelectrode, efficiency enhancement can be achieved for clean fuel production, such as in the case of electrochemically synthesizing hydrogen from water. One can also configure a solar cell with liquid phase photoelectrochemical redox reactions, which has the advantage of active cooling with liquid coolant circulation. To achieve these tasks, however, the photoelectrochemical anode (photoanode) must be able to withstand oxidative biases and harsh pH environments. This stability requirement has typically limited semiconductor selection to wide bandgap materials, such as TiO2 (bandgap ~ 3 eV) and Fe2O3 (bandgap 2.3 eV). These materials are known to be oxidatively stable semiconductors but very limited in photoabsorption efficiency. Ideally, small bandgap semiconductors (e.g. Si) are desirable photoanodes because of their efficient photoabsorption properties. Such materials are, however, easily oxidized at the potentials required of photoanodes, resulting in either insulating oxide surface passivation or dissolution. In this work, we protect the small bandgap Si with a thin, pinhole-free layer of atomic-layer-deposited TiO2 (ALD-TiO2) to create a stable and efficient photoanode. In the case of water oxidation, an ultrathin, optically transparent, Ir layer was deposited on the top to serve as a water oxidation catalyst and electron transfer mediator. This nanocomposite photoanode is stable during continuous water oxidation for periods greater than 24 hours in 1M NaOH, an alkaline solution that would very rapidly oxidizes an unprotected Si electrode. With AM1.5G illumination, a 550 mV photovoltage was obtained, and the onset of water oxidation occurred at ~200 mV below the thermodynamic equilibrium potential, with saturation current density approaching the 36 mA/cm<SUP>2</SUP> theoretical limit for AM1.5G illumination of a Si photoanode. The photovoltage approaches the open circuit voltages of the best Si solar cells, while the saturation current density is four to five times larger than the current state-of-the-art Fe2O3 electrodes. Electron transport through the protective oxide coating on the Si occurs by tunneling for the thin (2-3 nm) TiO2 films prepared by ALD. A photoelectrochemical solar cell using the ALD-TiO2 protected photoanode was also demonstrated with ferri/ferro-cyanide solutions as the redox medium. Large open circuit voltages and short circuit currents have been obtained. This ALD-TiO2 passivation approach is quite general and may be applied to other semiconductor anodes used in water splitting or in other electrochemical reactions.