10.1117/2.1201703.006874 Nanoengineered lone-pair active photocatalysts for more efficient water splitting Louis Piper, G¨ okhan Ersan, Sarbajit Banerjee, and David Watson A whole family of promising nanoengineered photocatalysts have been identified to enable the generation of solar fuels, hydrogen, and oxygen via artificial photosynthesis. In artificial photosynthesis, sunlight, water, and engineered cat- alysts are used to generate solar fuels. The ability to simply store solar energy in the form of fuel (e.g., oxygen and hydrogen) is highly appealing, particularly compared with more complicated configurations that are based on solar cells and lithium ion bat- teries. Splitting water into its constituent parts using the energy from UV radiation was first realized in the 1970s, during the peak of an oil crisis. In this approach, platinum was used to initi- ate the hydrogen evolution reaction and titanium dioxide (TiO 2 ) was used to oxidize water. 1 Ideally, it would be possible to use visible light for water split- ting. The realization of efficient water splitting using this fre- quency range remains elusive, however. The problems largely stem from the challenge of initiating the oxygen-evolution re- action (i.e., OER, the oxidation of water into oxygen gas and protons). 2 For this process, charge (in the form of holes) is re- quired at energies that are difficult to obtain from photogener- ated carriers in oxide-based photocatalysts (without relying on the application of a large bias). Metal oxides such as TiO 2 are preferred for this purpose because they are inexpensive and are stable in aqueous solutions. 2 However, TiO 2 has a large band gap (3eV) with a deep oxygen 2p-derived valence band. Thus, only 5% of sunlight can generate electron-hole pairs, and of these, the hole energies lie approximately 2eV below the OER. A major research effort over the last decade has focused on engineering metal-oxide heterojunctions that enable hole trans- fer at energies closer to the oxygen-evolving (H 2 O=O 2 ) redox potential. 3 Figure 1. Our design of a lone-pair active nanowire hybrid system for water splitting. The top image shows the two operations performed by the nanowire (NW). First, photogenerated holes from the photoactive quantum dots (QDs) are transferred to the dark catalyst for the oxi- dation of water. The remaining electrons are then involved in evolving hydrogen. This process is shown in closer detail in the bottom image. The energy alignment between the QDs and NWs enables the photo- generated holes to transfer to the NWs faster than the electrons, thereby facilitating efficient charge separation. The transferred holes have en- ergies close to the oxygen-evolving (H 2 O=O 2 ) redox. This figure was generated as a result of a ‘graphic design as a research tool’ project between Louis Piper and G¨ okhan Ersan at Binghamton University. Continued on next page