The solar energy that reaches the earth in one hour is more than enough to supply the demand of all humans for a year. The most direct application of solar energy is photovoltaic (PV) technology, as evidenced by the increased attention and the $10 billion/year market size.  However, since photovoltaic cells only generate energy during the day, storage has been increasingly important if we want to move toward a cleaner world. Two of the ways to store solar energy is to convert solar energy to hydrogen fuel or chemical fuel (carbon monoxide), as shown in the following chemical reactions:
|2H2O → 2H + 2e-||(1)|
|CO2 + 2H2O → CH4 + 2O2||(2)|
Eq. (1) shows the catalysis reaction on water, which splits water into hydrogen and oxygen. Splitting water with solar energy is also called photocatalysis. Hydrogen is a promising fuel as it is copmletely clean, i.e. it doesn't create any greenhouse gas or other toxic chemicals after it is burned.  On the other hand, eq. (2) shows the production of methane. Although methane is a fossil fuel, it produces much less carbon dioxide than coal or any other fossil fuels. Furthermore, if we can capture the CO2 emission from burning methane, we can fuel this reaction even more.
Under normal conditions, the rate of water splitting into hydrogen and oxygen is very low. This is why a catalyst is required to accelerate the rate of the reaction. Water splitting has been researched for a long time since the Honda-Fujishima effect, which involved the use of a TiO2 electrode.  That said, the amount of hydrogen produced from TiO2 has been little. A review of various water splitting technologies in Chemistry Letters showed that NiO/NaTaO3 (nitrogen oxide/sodium tantalite) under UV light irradiation is a highly efficient way to split water. The conduction band level is a lot higher than the reduction potential of H2O, which generates voltage. Furthermore, the photocatalytic activity increased even more with doping of LA (lanthanoid) ions. 
In 1979, Honda and its coworkers reduced carbon dioxide to organic compounds, including methane, with the use of sunlight.  Since then there has been numerous research in reducing carbon dioxide, especially as the topic of sustainability has caught attention. Titania has been considered one of the best candidate for photocatalytic processes due to its oxidation properties, charget transport properties, and corrosion resistance. Research in Pennsylvania State University has furthered the progress of carbon dioxide conversion rate by the following strategies: (i) increase the surface area of titania by using nanotube arrays; (ii) modify the band gap to utilize the visible portion of the solar spectrium since that is where the bulk of solar energy lies; (iii) apply other catalysts such as Cu and Pt on the surface of the nanotube array to adsorb the reactants and help the redox process. 
Although the results are promising, current water splitting technologies still do not generate enough hydrogen relative to the cost of the catalysts and experimentation. A library of photocatalyst materials will be helpful for future researchers to clarify the various factors that affect photocatalytic properties and as well as to design better photocatalysts. On the other hand, although research has demonstrated a high photocatalytic rate of the conversion of carbon dioxide to methane, the researchers have yet to verify their hypothesis of the 5 intermediate chemical reactions that happen during the conversion process, as well as the role of OH radicals and O2 in the possible reactions. That said, results are promising and if we can pair this technology up with a technique that can capture most of the CO2 that are generated from burning fossil fuels, we will be able to provide clean energy without relying on unstable renewable energy sources such as solar or wind, as their availablility changes drastically across different time periods.
© Chun-Kai Kao. The author grants permission to copy, distribute, and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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