Yongjing Lin

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.

We report the highest external quantum efficiency measured on hematite (α-Fe(2)O(3)) without intentional doping in a water-splitting environment: 46% at λ = 400 nm. This result was enabled by the introduction of TiSi(2) 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/cm(2) 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.

As the most commonly encountered form of iron oxide in nature, hematite is a semiconducting crystal with an almost ideal bandgap for solar water splitting. Compelled by this unique property and other advantages, including its abundance in the Earth's crust and its stability under harsh chemical conditions, researchers have studied hematite for several decades. In this perspective, we provide a concise overview of the challenges that have prevented us from actualizing the full potentials of this promising material. Particular attention is paid to the importance of efficient charge transport, the successful realization of which is expected to result in reduced charge recombination and increased quantum efficiencies. We also present a general strategy of forming heteronanostructures to help meet the charge transport challenge. The strategy is introduced within the context of two material platforms, webbed nanonets and vertically aligned transparent conductive nanotubes. Time-resolved photoconductivity measurements verify the hypothesis that the addition of conductive components indeed increases charge lifetimes. Because the heteronanostructure approach is highly versatile, it has the potential to address other issues of hematite as well and promises new opportunities for the development of efficient energy conversion using this inexpensive and stable material.

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.

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 >10(4) 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.