Toward Solar Fuels: Photocatalytic Conversion of Carbon Dioxide to Hydrocarbons Somnath C. Roy, † Oomman K. Varghese, † Maggie Paulose, and Craig A. Grimes* Department of Electrical Engineering, and Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802. † These authors contributed equally to this work. H ydrocarbon fuels are currently the most important source of energy due to their ready availability, sta- bility, and high energy density (33 GJ/m 3 for gasoline). 1 In the United States, in 2008, hy- drocarbon fuels (petroleum, natural gas, and coal) provided more than 80% of the energy. 1 Advantages such as being freely provided by Nature, high energy content, and ease of transportation and storage con- tinue to drive our hydrocarbon depen- dence. Unfortunately, hydrocarbon fuel combustion carries with it the significant drawback of environmental pollution, to in- clude carbon dioxide emissions, 2 and the associated impact on health that is gener- ally not reflected in their purchase price. 3 While volcano eruptions can demon- strate climate altering affects almost imme- diately, another less dramatic path to alter- ing the earth’s climate is to change the heat trapping properties of the atmosphere. It is well-established that higher atmospheric carbon dioxide (CO 2 ) levels result in an at- mosphere that better retains heat. As such, atmospheric CO 2 plays a significant role in global heating appearing to act as the boundary condition, that is, the rule setter, as to what climates are available to the planet. 4 At the current rate of increase in at- mospheric CO 2 concentration, many cli- mate simulations suggest that the average global temperature will increase some 6 °C long before the end of the current century. 5 In a seminal work, Hansen and co-workers 6,7 use paleoclimate data to show that an aver- age global temperature change of 6 °C is sufficient to swing earth’s climate from one of glacial conditions to one supporting an ice-free Antarctica. A global cooling trend that began 50 million years ago is tied to falling CO 2 levels, with the planet being nearly ice-free until atmospheric CO 2 con- centrations drop below 450  100 ppm. A planet so hot as to make Antarctica ice-free must be warm indeed, presenting the pos- sibility that vast regions of arable land that now sustain the worlds population may likely be given over to desert. 8 With a 2008 atmospheric CO 2 level of 385 ppm, business as usual with respect to hydrocarbon fuel use poses a suite of disconcerting issues. 9 It has been suggested that CO 2 col- lected from emission sources may be stored in geological formations including oil and gas reservoirs, unminable coal seams, and deep sea reservoirs. 10 The strategy is the fol- lowing: CO 2 emitted from point sources, such as coal-fired power plants, is trapped, purified, and then compressed for transpor- tation to storage sites where it is injected into one geological formation or another. Of course, each of these stepsOseparation, purification, compression, transportation, and storageOrequires additional energy. To maintain a constant power output given carbon capture and storage (CCS), studies have estimated that CCS equipped power plants would require an additional fuel in- put of 25 to 80%. 11-14 House and co- workers 15 report that the energy cost of capturing and storing 80% of the CO 2 *Address correspondence to cgrimes@engr.psu.edu. Published online February 8, 2010. 10.1021/nn9015423 © 2010 American Chemical Society ABSTRACT The past several decades have seen a significant rise in atmospheric carbon dioxide levels resulting from the combustion of hydrocarbon fuels. A solar energy based technology to recycle carbon dioxide into readily transportable hydrocarbon fuel (i.e., a solar fuel) would help reduce atmospheric CO 2 levels and partly fulfill energy demands within the present hydrocarbon based fuel infrastructure. We review the present status of carbon dioxide conversion techniques, with particular attention to a recently developed photocatalytic process to convert carbon dioxide and water vapor into hydrocarbon fuels using sunlight. KEYWORDS: carbon dioxide recycling · global warming · photocatalytic conversion · TiO 2 nanotube array · electrochemical anodization · hydrocarbon fuels · gas chromatography REVIEW www.acsnano.org VOL. 4 ▪ NO. 3 ▪ 1259–1278 ▪ 2010 1259