Sa Zhou

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.

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.

Si/TiSi2 Heteronanostructures as High-Capacity Anode Material for Li Ion Batteries

We synthesized a unique heteronanostructure consisting of two-dimensional TiSi(2) nanonets and particulate Si coating. The high conductivity and the structural integrity of the TiSi(2) nanonet core were proven as great merits to permit reproducible Li(+) insertion and extraction into and from the Si coating. This heteronanostructure was tested as the anode material for Li(+) storage. At a charge/discharge rate of 8400 mA/g, we measured specific capacities >1000 mAh/g. Only an average of 0.1% capacity fade per cycle was observed between the 20th and the 100th cycles. The combined high capacity, long capacity life, and fast charge/discharge rate represent one of the best anode materials that have been reported. The remarkable performance was enabled by the capability to preserve the crystalline TiSi2 core during the charge/discharge process. This achievement demonstrates the potency of this novel heteronanostructure design as an electrode material for energy storage.

Comparing One- and Two-Dimensional Heteronanostructures As Silicon-Based Lithium Ion Battery Anode Materials

The performance of advanced energy conversion and storage devices, such as solar cells, supercapacitors, and lithium (Li) ion batteries, is intimately connected to the electrode design at the nanoscale. To enable significant developments in these research fields, we need detailed information about how the properties of the electrode materials depend on their dimensions and morphologies. This information is currently unavailable, as previous studies have mostly focused on understanding one type of morphology at a time. Here, we report a systematic study to compare the performance of nanostructures enabled by two platforms, one-dimensional nanowires and two-dimensional nanonets. The nanowires and nanonets shared the same composition (titanium disilicide) and similar sizes. Within the framework of Li ion battery applications, they exhibited different stabilities upon lithiation and delithiation (at a rate of 6 A/g), the nanonets-based nanostructures maintaining 90% and the nanowires-based ones 80% of their initial stable capacities after 100 cycles of repeated charge and discharge. The superior stability of the nanonets was ascribed to the two-dimensional connectivity, which afforded better structural stability than nanowires. Information generated by this study should contribute to the design of electrode materials and thereby enable broader applications of complex nanostructures for energy conversion and storage.

A nanonet-enabled Li ion battery cathode material with high power rate, high capacity, and long cycle lifetime.

The performance of advanced energy conversion and storage devices, including solar cells and batteries, is intimately connected to the electrode designs at the nanoscale. Consider a rechargeable Li ion battery, a prevalent energy storage technology, as an example. Among other factors, the electrode material design at the nanoscale is key to realizing the goal of measuring fast ionic diffusion and high electronic conductivity, the inherent properties that determine power rates, and good stability upon repeated charge and discharge, which is critical to the sustainable high capacities. Here we show that such a goal can be achieved by forming heteronanostructures on a radically new platform we discovered, TiSi(2) nanonets. In addition to the benefits of high surface area, good electrical conductivity, and superb mechanical strength offered by the nanonet, the design also takes advantage of how TiSi(2) reacts with O(2) upon heating. The resulting TiSi(2)/V(2)O(5) nanostructures exhibit a specific capacity of 350 Ah/kg, a power rate up to 14.5 kW/kg, and 78.7% capacity retention after 9800 cycles of charge and discharge. These figures indicate that a cathode material significantly better than V(2)O(5) of other morphologies is produced.

Spontaneous Growth of Highly Conductive Two‐Dimensional Single‐Crystalline TiSi2 Nanonets

Simple nanostructures (e.g. nanowires) form complex nanomaterials when connected by single-crystalline junctions. These nanomaterials offer better mechanical strength and superior charge transport while preserving unique properties associated with the small-dimension nanostructure. Tremendous research interest has focused on this new class of materials, especially in the field of electronics and energy applications. The synthesis of these materials is challenging because of their combined features of low dimensionality and high complexity; the former requires growth suppression whereas the latter demands growth enhancement. Here we report our success in growing single-crystalline two-dimensional (2D) networks of TiSi2, a free-standing structure that is micrometers wide and long but only approximately 15 nm thick; beams of these networks are nanobelts 25 nm wide. This new structure can serve as a testing grounds for probing a host of intriguing properties and applications, and the synthesis should inspire work on the growth of complex nanostructures in general. We were motivated to study TiSi2 by its properties and potential applications as well as its unique crystal structures. TiSi2 is an excellent electronic material as it is one of the most conductive silicides (resistivity 10 mWcm). TiSi2 was also recently shown to be a good photocatalyst for splitting H2O by absorbing visible light, which is a promising approach toward solar-generated H2 as a clean energy carrier. [8] The improved charge-transport properties offered by complex nanoscale structures of TiSi2 are desirable for nanoelectronics and solar energy harvesting. Their chemical synthesis is thus appealing; however, the synthetic conditions required by the two key features of complex nanostructures, low dimensionality and complexity, seem to contradict each other. Growth of onedimensional (1D) features involves promoting additions of atoms or molecules in one direction while constraining those in all other directions; this is often achieved either by surface passivation to increase the energies of sidewall deposition (such as solution-phase synthesis) or by the introduction of impurities to lower the energies of deposition for the selected directions (most notably the vapor–liquid–solid mechanism). Complex crystal structures, on the other hand, require controlled growth in more than one direction. The challenge in making 2D complex nanostructures is even greater as it demands more stringent controls over the complexity to limit the overall structure within two dimensions. Indeed, successful chemical syntheses of complex nanostructures have been mainly limited to 3D ones, and demonstrations of 2D nanocrystals are less frequent. Yang et al., for instance, have reported a simple comblike ZnO nanostructure. Multicrystalline nanosheets of tetragonal TiO2 were also reported. [16,17] The complex TiSi2 nanostructures of orthorhombic symmetry that we present herein represent a far more complex 2D structure than those previously achieved. Two-dimensional TiSi2 nanonets spontaneously formed as products of the simple reaction of TiCl4 and SiH4 in a H2-rich environment at moderate temperatures ( 675 8C) without any catalyst. The unique structure is shown in the scanning electron micrograph (SEM, Figure 1a). At relatively low magnifications, the high-yield products pack to resemble tree leaves, except that each sheet is composed of nanometer-scale beams (Figure 1a, inset). The structure is better seen under transmission electron microscope (TEM, Figure 1b). Within each of the 2D structures are nanobelts 25 nm wide and 15 nm thick, all linked together by single-crystalline junctions at 908 angles. To our knowledge such structures have not yet been encountered in nanomaterials; we henceforth designate them as nanonets (NNs) to emphasize the networklike characteristics. We provide evidence that these structures are indeed two dimensional with a series of tilted TEM pictures along with analogous 2D sketches (Figure 1c–e). A similar series of tilted images was also obtained using SEM (see Figure S1 in the Supporting Information). In addition, we observed that the 2D NNs bend and roll up when pushed by a STM tip during TEM characterizations (Figure 5b); this further verifies the 2D nature and suggests that the structures are highly flexible as a result of the thinness. High-resolution imaging and electron diffraction (ED) patterns of different regions of a representative NN reveal that the entire structure is single crystalline, including the 908 joints and the middle and ends of any given nanobelt (Figure 2). We suggest that the beams are nanobelts based on two main observations: 1) Loose ends often bend on the supporting films used for TEM, which is characteristic of nanobelts (Figure 1b); and 2) the thickness of the beams in a NN ( 15 nm) is obviously less than the width ( 25 nm), as [*] S. Zhou, Dr. X. Liu, Y. Lin, Dr. D. Wang Department of Chemistry Merkert Chemistry Center, Boston College 2609 Beacon Street, Chestnut Hill, MA 02467 (USA) Fax: (+1)617-552-2705 E-mail: dunwei.wang@bc.edu Homepage: http://www2.bc.edu/~dwang [] These authors contributed equally to this work.

Unique lithiation and delithiation processes of nanostructured metal silicides.

We report that TiSi(2) nanonet exhibits considerable activities in the reversible lithiation and delithiation processes, although bulk-sized titanium silicide is known to be inactive when used as an electrode material for lithium ion batteries. The detailed mechanism of this unique process was studied using electrochemical techniques including the electrochemical impedance spectroscopy (EIS) method. By systematic characterizations of the Nyquist plots and comparisons with the microstructure examinations, we identified the main reason for the activities as the layered crystal structure that is found stable only in TiSi(2) nanonets. The layer structure is characterized by the existence of a Si-only layer, which exhibits reactivity when exposed to lithium ions. Control studies where TiSi(2) nanowires and TiSi(2)/Si heteronanostructures were involved, respectively, were performed. Similar to bulk TiSi(2), TiSi(2) nanowires show limited reactivity in lithium ion insertion and deinsertion; the EIS characteristics of TiSi(2)/Si heteronanostructures, on the other hand, are distinctly different from those of TiSi(2) nanonets. The result supports our proposed TiSi(2) nanonet lithiation mechanism. This discovery highlights the uniqueness of nanoscale materials and will likely broaden the spectrum of electrode material choices for electrochemical energy storage.

Understanding the growth mechanism of titanium disilicide nanonets.

The titanium disicilicate (TiSi(2)) nanonet is a material with a unique two-dimensional morphology and has proven beneficial for energy conversion and storage applications. Detailed knowledge about how the nanonet grows may have important implications for understanding seedless nanostructure synthesis, in general, but is presently missing. Here, we report our recent efforts toward correcting this deficiency. We show that the TiSi(2) nanonet growth is sensitive to the nature of the receiving substrates. High-yield nanonets are only obtained on those exhibiting no or low reactivities with Si. This result indicates that Si-containing clusters deposited on the substrate surfaces play an important role in the nanonet synthesis, and we suggest they serve to initiate the growth. The morphological complexity of the nanonet depends on the precursor concentrations but not on the growth durations. More TiCl(4) results in nanonets with more complex structures. We understand that once a beam of a TiSi(2) nanonet is formed, its sidewalls are resistant to branch formation. Instead, the tip of a beam is where a branch forms. This process is driven by the reactions between Ti- and Si-containing species. Building on this understanding, we demonstrate the creation of second-generation nanonets.