Dunwei Wang

Germanium nanowire field-effect transistors with SiO2 and high-κ HfO2 gate dielectrics

Single-crystal Ge nanowires are synthesized by a low-temperature (275 °C) chemical vapor deposition (CVD) method. Boron doped p-type GeNW field-effect transistors (FETs) with back-gates and thin SiO2 (10 nm) gate insulators are constructed. Hole mobility higher than 600 cm2/V s is observed in these devices, suggesting high quality and excellent electrical properties of as-grown Ge wires. In addition, integration of high-κ HfO2 (12 nm) gate dielectric into nanowire FETs with top-gates is accomplished with promising device characteristics obtained. The nanowire synthesis and device fabrication steps are all performed below 400 °C, opening a possibility of building three-dimensional electronics with CVD-derived Ge nanowires.

Hematite-based solar water splitting: challenges and opportunities

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 Earths 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.

Growth of p-Type Hematite by Atomic Layer Deposition and Its Utilization for Improved Solar Water Splitting

Mg-doped hematite (α-Fe(2)O(3)) was synthesized by atomic layer deposition (ALD). The resulting material was identified as p-type with a hole concentration of ca. 1.7 × 10(15) 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.

Hematite‐Based Water Splitting with Low Turn‐On Voltages

Sunlight-driven photoelectrochemical (PEC) water splitting offers promise as a method for effective solar-energy harvesting and storage. To transform the reaction into economically competitive technology, we need materials that can absorb sunlight broadly, transfer the energy to excited charges at high efficiencies, and catalyze specific reduction and oxidation reactions. Furthermore, the materials should be inexpensive and stable against photocorrosion. To date, an ideal material that satisfies all of these considerations remains elusive. This challenge can, in principle, be addressed by combining various material components, each purposedesigned to offer desired properties with respect to photovoltage generation, charge transport, and catalytic activity. For example, it has recently been shown that the performance of hematite (a-Fe2O3)-based water splitting can indeed be improved by introducing dedicated charge collectors, buried homoand heterojunctions, and oxygen-evolution catalysts. Hematite was chosen as a prototypical system for these proof-of-concept demonstrations because it is an earth-abundant material with great promise for high-efficiency, low-cost water splitting. To realize the potential of hematite, however, we still need to address a key issue concerning its low photovoltage (Vph, typically 0.4 V), which is unreasonably low given that the bandgap of hematite is 2.0 eV. For successful integration with a small-bandgap photocathode, the photovoltage generated at the photoanode needs to be significantly higher so that a total (combined) photovoltage of 1.61 V (or greater, with a minimum overpotential of 0.38 V) is produced. Herein we show that this issue may be addressed by modifying the hematite surface. When decorated with an amorphous NiFeOx layer (Figure 1), hematite produces photovoltages as high as 0.61 V, which enable the observation of turn-on voltages (Von) as low as 0.62 V (versus the reversible hydrogen electrode, RHE) without the need for a second absorber (unless otherwise noted, all electrochemical potentials reported herein are relative to RHE). When a second absorber, Si, was added, a record-low turn-on voltage of 0.32 V was measured. The basis for our approach is illustrated schematically in Figure 2. The fundamental reason for the observed limited photovoltage generation by hematite lies in the relatively positive positions of its valenceand conduction-band edges. However, even within these limits, the Vph value of 0.6–0.8 V calculated for reported flat-band potentials (Vfb) of 0.4–0.6 V has not been reached.We understand the cause of this discrepancy to be a partial Fermi level pinning effect. That is, owing to the existence of surface states, a nonnegligible potential drop takes place within the Helmholtz layer (hH, Figure 2a). [22] The effect is manifested as a more positive Von value, since a significant portion of the applied potential is used to overcome the overpotential hH (Figure 2c). Appropriate surface modification enables the hH to be minimized or eliminated (Figure 2b) and a less positive Von value to be measured (Figure 2d). The effect of the NiFeOx overlayer was profound: it led to a Von shift from approximately 1.0 V to approximately 0.6 V (Figure 1b). Although the apparent effect of the cathodic Von shift is similar to the effect of reducing the kinetic over[*] C. Du, Dr. X. Yang, Dr. M. T. Mayer, H. Hoyt, J. Xie, Dr. G. McMahon, G. Bischoping, Prof. Dr. D. Wang Department of Chemistry Merkert Chemistry Center, Boston College 2609 Beacon Street, Chestnut Hill, MA, 20467 (USA) E-mail: dunwei.wang@bc.edu Homepage: http://www2.bc.edu/dunwei-wang [] These authors contributed equally to this work.

Water Splitting by Tungsten Oxide Prepared by Atomic Layer Deposition and Decorated with an Oxygen-Evolving Catalyst†

When sunlight is used as direct energy input, water can be split into hydrogen and oxygen at conversion efficiencies similar to those of solar cells. This process offers a method for energy storage to address the problem that the sun does not shine continuously, and is a particularly appealing approach to solar-energy harvesting. Notwithstanding the intense research efforts, progress in this area is extremely slow. Efficient and inexpensive water splitting remains elusive. A key reason for the sluggish progress is the lack of suitable materials. The “ideal” material must absorb strongly in the visible range, be efficient in separating charges using the absorbed photons, and be effective in collecting and transporting charges for the chemical processes. Such a material has yet to be found. The difficulties in finding a suitable material stem from the competing nature of intrinsic material properties (e.g., optical depth, charge diffusion distance, and width of the depletion region, among others), which leaves limited opportunity for tunability. We recently demonstrated that heteronanostructures, a type of nanoscale material consisting of multiple components that complement each other, have a combination of properties which are not available in singlecomponent materials. For instance, we can add chargetransport components to oxide semiconductors to solve the issue of low conductivity that oxide semiconductors generally suffer. In a similar fashion, one can add an effective catalyst to address the challenge that oxygen evolution is complex and tends to be the rate-limiting step. These new materials will likely lead to significant improvement in solar watersplitting efficiencies. The success of a heteronanostructure design relies on the ability to produce high-quality components with interfaces of low defect density, and on the availability of various components. Here we show that crystalline WO3 can be synthesized by the atomic layer deposition (ALD) method in the true ALD regime. When coated with a novel Mn-based catalyst, the resulting WO3 survives soaking in H2O at pH 7 and produces oxygen by splitting H2O under illumination. We choose ALD to prepare WO3 because of the following advantages: 1) a high degree of control over the resulting materials; 2) excellent step coverage to yield conformal coatings; and 3) process versatility to tailor the composition of the deposit. WO3 was studied because it is one of the most researched compounds for water splitting. The widely available literature makes it easy to compare our results with existing reports and thus allows us to test the power of the heteronanostructure design. To avoid the production of corrosive byproducts during the ALD process and to ensure the reaction occurs in the true ALD regime, we used (tBuN)2(Me2N)2W as tungsten precursor and H2O as oxygen precursor, as described in the Experimental Section (see Supporting Information for more details). Our first goal was to verify that the growth indeed takes place in the ALD regime. The dependence of the growth rate on the precursor pulse times and on the substrate temperature unambiguously confirms this. In addition, the excellent linear dependence of the deposition thickness on the number of precursor pulses supports the ALD growth mechanism and shows the extent of control we can achieve (see Supporting Information). That a long H2O pulse time is necessary to initiate growth is a key finding of this work. Despite intentional strengthening of the oxidative conditions, as-grown WO3 exhibited a tinted color, indicating the existence of oxygen deficiencies, which was then corrected by an annealing step in O2 at 550 8C. The crystalline nature of the product is manifested in the highresolution (HR) TEM image in Figure 1a. We also synthesized WO3 on two-dimensional TiSi2 nanonets. [18,19] The uniformity and good coverage around the nanonet branches show that this deposition technique is suitable for the creation of heteronanostructures. Ready dissolution of WO3 in aqueous solutions with pH 4 is a significant challenge that impedes its widespread use. We sought to solve this problem by coating WO3 with an Mnbased catalyst. Derived from the Brudvig–Crabtree catalyst, this coating was prepared by thermally decomposing [(H2O)(terpy)Mn(O)2Mn(H2O)(terpy)](NO3)3 (terpy= 2,2’:6’,2’’terpyridine). Similar to the oxo-bridged dimanganese catalyst, the thermal decomposition product exhibits good [*] R. Liu, Y. Lin, S. W. Sheehan, Prof. Dr. D. Wang Department of Chemistry, Merkert Chemistry Center Boston College 2609 Beacon St., Chestnut Hill, MA 02467 (USA) Fax: (+1)617-552-2705 E-mail: dunwei.wang@bc.edu Homepage: http://www2.bc.edu/~dwang

Hematite/Si Nanowire Dual-Absorber System for Photoelectrochemical Water Splitting at Low Applied Potentials

Hematite (α-Fe(2)O(3)) was grown on vertically aligned Si nanowires (NWs) using atomic layer deposition to form a dual-absorber system. Si NWs absorb photons that are transparent to hematite (600 nm < λ < 1100 nm) and convert the energy into additional photovoltage to assist photoelectrochemical (PEC) water splitting by hematite. Compared with hematite-only photoelectrodes, those with Si NWs exhibited a photocurrent turn-on potential as low as 0.6 V vs RHE. This result represents one of the lowest turn-on potentials observed for hematite-based PEC water splitting systems. It addresses a critical challenge of using hematite for PEC water splitting, namely, the fact that the band-edge positions are too positive for high-efficiency water splitting.

Forming Heterojunctions at the Nanoscale for Improved Photoelectrochemical Water Splitting by Semiconductor Materials: Case Studies on Hematite

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.

Piezoresistance of carbon nanotubes on deformable thin-film membranes

Carbon nanotubes have interesting electromechanical properties that may enable a new class of nanoscale mechanical sensors. We fabricated two-terminal nanotube devices on silicon nitride membranes, measured their electronic transport versus strain, and estimated their band gaps and the strain-induced changes in them. We found band-gap increases and decreases among both semiconducting and small-gap semiconducting (SGS) tubes. The SGS band gaps exceeded the predicted curvature-induced gaps for their diameter. Some of the band-gap changes for both types of tubes exceeded the predicted maxima. These anomalies are likely caused by interaction with the rough silicon nitride surface.

TiO2/TiSi2 Heterostructures for High-Efficiency Photoelectrochemical H2O Splitting

A TiO(2)/TiSi(2) complex heteronanostructure was synthesized to improve the efficiencies of TiO(2) in photosplitting H(2)O. Photoactive TiO(2) served to convert incident photons into separated charges, and the supporting TiSi(2) nanonet acted as an efficient conductor to transport separated charges. The structural complexity of TiSi(2) 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 TiO(2) growth was further explored to extend the absorption to the visible range by incorporating W into TiO(2), and 0.83% efficiency was measured under simulated solar lights.

Enabling unassisted solar water splitting by iron oxide and silicon

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.