In order for the future energy needs of humanity to be adequately and sustainably met, alternative energy techniques such as artificial photosynthesis need to be made more efficient and therefore ...commercially viable. On a grand scale, the energies coming to and leaving from the earth are balanced. With the fast increasing waste heat produced by human activities, the balance may be shifted to threaten the ecosystem in which we reside. To avoid such dire consequences, it is necessary to power human activities using energy derived from the incoming source, which is predominantly solar irradiation. Indeed, most life on the surface of the earth is supported, directly or indirectly, by photosynthesis that harvests solar energy and stores it in chemical bonds for redistribution. Being able to mimic the process and perform it at high efficiencies using low-cost materials has significant implications. Such an understanding is a major intellectual driving force that motivates research by us and many others. From a thermodynamic perspective, the key energy conversion step in natural photosynthesis happens in the light reactions, where H2O splits to give O2 and reactive protons. The capability of carrying out direct sunlight-driven water splitting with high efficiency is therefore fundamentally important. We are particularly interested in doing so using inorganic semiconductor materials because they offer the promise of durability and low cost. In this Account, we share our recent efforts in bringing semiconductor-based water splitting reactions closer to reality. More specifically, we focus on earth-abundant oxide semiconductors such as Fe2O3 and work on improving the performance of these materials as photoelectrodes for photoelectrochemical reactions. Using hematite (α-Fe2O3) as an example, we examine how the main problems that limit the performance, namely, the short hole collection distance, poor light absorption near the band edge, and mismatch of the band edge energetics with those of water redox reactions, can in principle be addressed by adding nanoscale charge collectors, forming buried junctions, and including additional light absorbers. These results highlight the power of forming homo- or heterojunctions at the nanoscale, which permits us to engineer the band structures of semiconductors to the specific application of water splitting. The key enabling factor is our ability to synthesize materials with precise control over the dimensions, crystallinity, and, most importantly, the interface quality at the nanoscale. While being able to tailor specific properties on a simple, earth-abundant device is not straightforward, the approaches we report here take significant steps towards efficient artificial photosynthesis, an energy harvesting technique necessary for the well-being of humanity.
We report the highest external quantum efficiency measured on hematite (α-Fe2O3) without intentional doping in a water-splitting environment: 46% at λ = 400 nm. This result was enabled by the ...introduction of TiSi2 nanonets, which are highly conductive and have suitably high surface areas. The nanonets serve a dual role as a structural support and an efficient charge collector, allowing for maximum photon-to-charge conversion. Without the addition of any oxygen-evolving catalysts, we obtained photocurrents of 1.6 and 2.7 mA/cm2 at 1.23 and 1.53 V vs RHE, respectively. These results highlight the importance of charge transport in semiconductor-based water splitting, particularly for materials whose performance is limited by poor charge diffusion. Our design introduces material components to provide a dedicated charge-transport pathway, alleviating the reliance on the materials’ intrinsic properties, and therefore has the potential to greatly broaden where and how various existing materials can be used in energy-related applications.
In this work, the operation of n- and p-type field-effect transistors (FETs) on the same WSe2 flake is realized,and a complementary logic inverter is demonstrated. The p-FET is fabricated by ...contacting WSe2 with a high work function metal, Pt, which facilities hole injection at the source contact. The n-FET is realized by utilizing selective surface charge transfer doping with potassium to form degenerately doped n+ contacts for electron injection. An ON/OFF current ratio of >104 is achieved for both n- and p-FETs with similar ON current densities. A dc voltage gain of >12 is measured for the complementary WSe2 inverter. This work presents an important advance toward realization of complementary logic devices based on layered chalcogenide semiconductors for electronic applications.
Photoelectrochemical (PEC) water splitting promises a solution to the problem of large-scale solar energy storage. However, its development has been impeded by the poor performance of photoanodes, ...particularly in their capability for photovoltage generation. Many examples employing photovoltaic modules to correct the deficiency for unassisted solar water splitting have been reported to-date. Here we show that, by using the prototypical photoanode material of haematite as a study tool, structural disorders on or near the surfaces are important causes of the low photovoltages. We develop a facile re-growth strategy to reduce surface disorders and as a consequence, a turn-on voltage of 0.45 V (versus reversible hydrogen electrode) is achieved. This result permits us to construct a photoelectrochemical device with a haematite photoanode and Si photocathode to split water at an overall efficiency of 0.91%, with NiFeOx and TiO2/Pt overlayers, respectively.
An amorphous Si thin film with TiO2 encapsulation layer is demonstrated as a highly promising and stable photocathode for solar hydrogen production. With platinum as prototypical cocatalyst, a ...photocurrent onset potential of 0.93 V vs RHE and saturation photocurrent of 11.6 mA/cm2 are measured. Importantly, the a-Si photocathodes exhibit impressive photocurrent of ∼6.1 mA/cm2 at a large positive bias of 0.8 V vs RHE, which is the highest for all reported photocathodes at such positive potential. Ni–Mo alloy is demonstrated as an alternative low-cost catalyst with onset potential and saturation current similar to those obtained with platinum. This low-cost photocathode with high photovoltage and current is a highly promising photocathode for solar hydrogen production.
Mg-doped hematite (α-Fe2O3) was synthesized by atomic layer deposition (ALD). The resulting material was identified as p-type with a hole concentration of ca. 1.7 × 1015 cm–3. When grown on n-type ...hematite, the p-type layer was found to create a built-in field that could be used to assist photoelectrochemical water splitting reactions. A nominal 200 mV turn-on voltage shift toward the cathodic direction was measured, which is comparable to what has been measured using water oxidation catalysts. This result suggests that it is possible to achieve desired energetics for solar water splitting directly on metal oxides through advanced material preparations. Similar approaches may be used to mitigate problems caused by energy mismatch between water redox potentials and the band edges of hematite and many other low-cost metal oxides, enabling practical solar water splitting as a means for solar energy storage.
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► We summarize research on using nanoscale semiconductors for solar water splitting. ► Covers an area that has not been reviewed previously in a concise fashion. ► A comprehensive ...comparison is provided in the form of a table for the first time.
Recent efforts on solar water splitting by nanoscale semiconductor materials is reviewed. We show that innovations in materials’ morphologies can improve charge transport and thereby increase overall power conversion efficiencies. Nanostructures of varying complexities, from one-dimensional nanotubes, nanowires, and nanorods, to two-dimensional films and nanonets, and three-dimensional porous structures have been reported to exhibit superior performance. We also summarize recent successes in advancing the field by heterogeneous nanostructures, which make it possible to achieve combined functionalities not observed with single-component materials.
A TiO2/TiSi2 complex heteronanostructure was synthesized to improve the efficiencies of TiO2 in photosplitting H2O. Photoactive TiO2 served to convert incident photons into separated charges, and the ...supporting TiSi2 nanonet acted as an efficient conductor to transport separated charges. The structural complexity of TiSi2 also provided a framework of high surface area to enhance photoabsorption. 16.7% peak conversion efficiency was obtained when measured under monochromic UV illuminations. The TiO2 growth was further explored to extend the absorption to the visible range by incorporating W into TiO2, and 0.83% efficiency was measured under simulated solar lights.
The role of TiO2 thin films deposited by atomic layer deposition on p-InP photocathodes used for solar hydrogen generation was examined. It was found that, in addition to its previously reported ...corrosion protection role, the large valence band offset between TiO2 and InP creates an energy barrier for holes reaching the surface. Also, the conduction band of TiO2 is well-aligned with that of InP. The combination of these two effects creates an electron-selective contact with low interface recombination. Under simulated solar illumination in HClO4 aqueous electrolyte, an onset potential of >800 mV vs RHE was achieved, which is the highest yet reported for an InP photocathode.
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