Energy harvested directly from sunlight offers a desirable approach toward fulfilling, with minimal environmental impact, the need for clean energy. Solar energy is a decentralized and inexhaustible natural resource, with the magnitude of the available solar power striking the earth’s surface at any one instant equal to 130 million 500 MW power plants.1 However, several important goals need to be met to fully utilize solar energy for the global energy demand. First, the means for solar energy conversion, storage, and distribution should be environmentally benign, i.e. protecting ecosystems instead of steadily weakening them. The next important goal is to provide a stable, constant energy flux. Due to the daily and seasonal variability in renewable energy sources such as sunlight, energy harvested from the sun needs to be efficiently converted into chemical fuel that can be stored, transported, and used upon demand. The biggest challenge is whether or not these goals can be met in a costeffective way on the terawatt scale.2

http://pubs.acs.org/doi/pdf/10.1021/cr1002326

Solar Water Splitting Cells

Identifying and building a sustainable energy system are perhaps two of the most critical issues that today's society must address. Replacing our current energy carrier mix with a sustainable fuel is one of the key pieces in that system. Hydrogen as an energy carrier, primarily derived from water, can address issues of sustainability, environmental emissions, and energy security. Issues relating to hydrogen production pathways are addressed here. Future energy systems require money and energy to build. Given that the United States has a finite supply of both, hard decisions must be made about the path forward, and this path must be followed with a sustained and focused effort.

http://science.sciencemag.org/content/sci/305/5686/972.full.pdf

Sustainable Hydrogen Production

Organometallic chemistry on silicon and germanium surfaces.

We describe the development of solar water-splitting cells comprising earth-abundant elements that operate in near-neutral pH conditions, both with and without connecting wires. The cells consist of a triple junction, amorphous silicon photovoltaic interfaced to hydrogen- and oxygen-evolving catalysts made from an alloy of earth-abundant metals and a cobalt|borate catalyst, respectively. The devices described here carry out the solar-driven water-splitting reaction at efficiencies of 4.7% for a wired configuration and 2.5% for a wireless configuration when illuminated with 1 sun (100 milliwatts per square centimeter) of air mass 1.5 simulated sunlight. Fuel-forming catalysts interfaced with light-harvesting semiconductors afford a pathway to direct solar-to-fuels conversion that captures many of the basic functional elements of a leaf.

https://science.sciencemag.org/content/sci/early/2011/09/28/science.1209816.full.pdf

Wireless Solar Water Splitting Using Silicon-Based Semiconductors and Earth-Abundant Catalysts

To convert the energy of sunlight into chemical energy, the leaf splits water via the photosynthetic process to produce molecular oxygen and hydrogen, which is in a form of separated protons and electrons. The primary steps of natural photosynthesis involve the absorption of sunlight and its conversion into spatially separated electron–hole pairs. The holes of this wireless current are captured by the oxygen evolving complex (OEC) of photosystem II (PSII) to oxidize water to oxygen. The electrons and protons produced as a byproduct of the OEC reaction are captured by ferrodoxin of photosystem I. With the aid of ferrodoxin–NADP+ reductase, they are used to produce hydrogen in the form of NADPH. For a synthetic material to realize the solar energy conversion function of the leaf, the light-absorbing material must capture a solar photon to generate a wireless current that is harnessed by catalysts, which drive the four electron/hole fuel-forming water-splitting reaction under benign conditions and under 1 su...

The artificial leaf

Despite the fact that H-terminated, HF-etched Si crystals are the starting point for construction of most contemporary electronic devices,1 little is known about the chemical reactions of H-terminated Si surfaces under ambient temperature and pressure.2,3 Functionalization of Si without partial oxidation and/or formation of electrical defects is potentially important in fabricating improved electronic devices4,5 as well as in measurement of charge transfer rate constants at semiconductor/ liquid contacts.6 One recently described approach involves the reaction of HF-etched Si(111) with olefins and organic diacyl peroxides, in which formation of a self-assembled (near)monolayer of Si-alkyls was hypothesized.2 We report here an alternative strategy to functionalize HF-etched Si surfaces involving halogenation and subsequent reaction with alkyl Grignard or alkyl lithium reagents. We report vibrational spectroscopic and temperature programmed desorption data which confirm that the alkyl groups are bonded covalently to the Si surface, and we demonstrate that such overlayer formation can impede the undesirable oxidation of Si in a variety of environments while providing surfaces of high electrical quality. The H-terminated Si surface7 was first exposed to PCl5 for 20-60 min at 80-100 °C, in chlorobenzene with benzoyl peroxide as the radical initiator.8,9 Upon chlorination, the XP survey spectra (Figure 1) showed peaks at 270.2 ( 0.4 binding electron volts, BeV, (Cl 2s) and 199.3 ( 0.4 BeV (Cl 2p), indicating that this procedure yielded Cl on the surface. The high-resolution XP spectrum of the Si 2p peak of this surface displayed, in addition to the substrate Si signal, an additional peak located at 0.98 ( 0.12 BeV higher in binding energy (Figure 2) whose position and intensity was consistent with the formation of a surface Si-Cl bond.10 Auger electron spectra

Alkylation of Si Surfaces Using a Two-Step Halogenation/Grignard Route

Non-vacuum processing of CuIn1−xGaxSe2 solar cells on rigid and flexible substrates using nanoparticle precursor inks

High-efficiency integrated multijunction photovoltaic/electrolysis systems for hydrogen production

A two-step procedure, involving radical-initiated chlorination of the Si surface with PCl5 followed by reaction of the chlorinated surface with alkyl-Grignard or alkyl-lithium reagents, has been developed to functionalize crystalline (111)-oriented H-terminated Si surfaces. The surface chemistry that accompanies these reaction steps has been investigated using X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), temperature programmed desorption spectroscopy (TPDS), high-resolution electron energy loss spectroscopy (HREELS), infrared (IR) spectroscopy in both glancing transmission (TIR) and attenuated total multiple internal reflection (ATR) modes, ellipsometry, and contact angle goniometry. The XPS data show the appearance of the Cl signal after exposure to PCl5 and show its removal, and concomitant appearance of a C 1s signal, after the alkylation step. Auger electron spectra, in combination with TPD spectroscopy, demonstrate the presence of Cl after the chlorination process and it...

Spectroscopic Studies of the Modification of Crystalline Si(111) Surfaces with Covalently-Attached Alkyl Chains Using a Chlorination/Alkylation Method

A two-step chlorination/alkylation technique used to introduce alkyl groups, —C n H 2n+1 (n=1-6), functionally onto single-crystal, ( 111 )-oriented, n-type Si surfaces. H-terminated Si photoanodes were unstable under illumination in contact with an aqueous 0.35 M K 4 Fe(CN) 6 -0.05 MK 3 Fe(CN) 6 electrolyte. Such electrodes displayed low open-circuit voltages and exhibited a pronounced time-dependent deterioration in their current density vs potential characteristics due to anodic oxidation. In contrast, Si surfaces functionalized with —CH 3 and —C 2 H 5 groups displayed significant improvements in stability while displaying excellent electrochemical properties when used as photoelectrodes in the aqueous Fe(CN) 6 3−/4− electrolyte.

Ashish Bansal

Papers

Alkylation of Si Surfaces Using a Two-Step Halogenation/Grignard Route

Non-vacuum processing of CuIn1−xGaxSe2 solar cells on rigid and flexible substrates using nanoparticle precursor inks

High-efficiency integrated multijunction photovoltaic/electrolysis systems for hydrogen production

Spectroscopic Studies of the Modification of Crystalline Si(111) Surfaces with Covalently-Attached Alkyl Chains Using a Chlorination/Alkylation Method

Stabilization of Si photoanodes in aqueous electrolytes through surface alkylation