"Solar can potentially provide all the electricity and fuel we need to power the planet," said Gray. "The Holy Grail of solar research is to use sunlight efficiently and directly to 'split' water into its elemental constituents – hydrogen and oxygen – and then use the hydrogen as a clean fuel."

According to Gray, more energy from sunlight strikes the Earth in one hour than all of the energy consumed on the planet in one year. He believes that the single biggest challenge is reducing costs so that a large-scale shift away from coal, natural gas and other non-renewable sources of electricity makes economic sense. With the average cost of photovoltaic energy at 35 to 50 cents per kilowatt-hour, it’s expensive compared to coal and natural gas, which cost around 5 to 6 cents per kilowatt-hour.

That said, solar energy has the advantages of being clean and renewable so it may not need to match the cost of conventional energy sources, says Gray. He believes the breakthrough for solar energy will come when scientists reduce the costs to about 10 cents per kilowatt-hour.

"Once it reaches that level, large numbers of consumers will start to buy in, driving the per-kilowatt price down even further," he said. "I believe we are at least 10 years away from photovoltaics being competitive with more traditional forms of energy."

Challenges include developing cheap solar cells that don’t deteriorate and reduced the amounts of toxic materials used in cell manufacture. The generation of clean fuels at costs that can compete with oil and coal is also needed.

"The pressure is on chemists to make hydrogen from something other than natural gas or coal," said Gray. "We’ve got to start making it from sunlight and water."

Paul Alivisatos of the University of California at Berkeley also stressed the importance of R&D at the ACS Meeting.

"Our energy destiny is in our own hands – we’re not forced to look at a future in which we are starved for energy or destroying the planet," he said. "There is a way in which we can create a sustainable energy future. The answer for getting there is to let the scientists go at it and attack this problem with the appropriate scale of effort."

Having done the calculations, Alivisatos believes that there is nothing in principle to prevent the creation of an artificial material that is robust, stable and made out of abundant materials and that could use the energy of the sun to make a fuel directly.

"We just don’t know how yet," he said. "We haven’t mounted a scale of effort that’s proportionate to the scale of the problem. It’s going to take many scientists working together to actually achieve this but it’s completely doable."

While Alivisatos thinks that biofuels is an important area of research, he explained how switchgrass – the most efficient growing plant – grows at about 0.25% power efficiency from photon to biomass.

"If we could make a material that goes directly from photon to fuel – all the way direcly to long chain alcohol – at 1% power efficiency and if we could do that on an area of 100 million acres (a little bit more than one quarter of US agricultural land, although it wouldn’t be on agricultural land) then we could replace all the gasoline use in the US," he said. "And if we can do that at 5% then we could replace all the energy use of the US. The NREL [water-splitting solar] cell can operate somewhere between 10 to 15% so you see why scientists have had their imagination captured by this."

Gray is also a fan of the NREL (US National Renewable Energy Laboratory) cell. At the Chemical Bonding Center, a collaboration between Caltech and MIT, Gray says the principal goal is to "split water with sunlight into hydrogen fuel and oxygen or to use components of split water – namely electrons and protons – to combine with carbon dioxide to make alcohol fuel". And one of the team’s targets is to make "something as good" as the US National Renewable Energy Laboratory (NREL) cell – a water-splitting device that contains layers of GaInP and GaAs semiconductors coupled with a platinum electrode to evolve oxygen.

"It splits water with 12.5% efficiency, which is more than 10 times the efficiency of natural photosynthesis,” explained Gray, "but it doesn’t contain Earth-abundant materials, it can’t be scaled up and it's extremely expensive."

With that in mind, the team hopes to make a simple nanoscale water-splitting device built entirely from Earth-abundant materials, so that it could be scaled up. "The elements we’re interested in are iron, cobalt and manganese," said Gray.

The researchers are developing a device comprising a photovoltaic membrane built of silicon and zinc oxide nanorods, with a catalytic cathode at one end of the membrane assembly for evolving hydrogen. Although the team has made much progress in developing cobalt catalysts to replace platinum at the cathode, the catalytic anode is proving more tricky.

"The big challenge we have is the catalytic anode where the holes that we get from our nanorod assembly will interface with a catalyst to evolve oxygen from water," said Gray. "We don’t have anything that works at the present time. We have some ruthenium compounds that work marginally."

In phase II of the Chemical Bonding Center, Gray hopes to see the involvement of researchers from across the US and from Switzerland.