Matthew B. Starr

Ferroelectric Polarization-Enhanced Photoelectrochemical Water Splitting in TiO2–BaTiO3 Core–Shell Nanowire Photoanodes

The performances of heterojunction-based electronic devices are extremely sensitive to the interfacial electronic band structure. Here we report a largely enhanced performance of photoelectrochemical (PEC) photoanodes by ferroelectric polarization-endowed band engineering on the basis of TiO2/BaTiO3 core/shell nanowires (NWs). Through a one-step hydrothermal process, a uniform, epitaxial, and spontaneously poled barium titanate (BTO) layer was created on single crystalline TiO2 NWs. Compared to pristine TiO2 NWs, the 5 nm BTO-coated TiO2 NWs achieved 67% photocurrent density enhancement. By numerically calculating the potential distribution across the TiO2/BTO/electrolyte heterojunction and systematically investigating the light absorption, charge injection and separation properties of TiO2 and TiO2/BTO NWs, the PEC performance gain was proved to be a result of the increased charge separation efficiency induced by the ferroelectric polarization of the BTO shell. The ferroelectric polarization could be switched by external electric field poling and yielded PEC performance gain or loss based on the direction of the polarization. This study evidence that the piezotronic effect (ferroelectric or piezoelectric potential-induced band structure engineering) holds great promises in improving the performance of PEC photoelectrodes in addition to chemistry and structure optimization.

Interface Engineering by Piezoelectric Potential in ZnO-Based Photoelectrochemical Anode

Through a process of photoelectrochemical (PEC) water splitting, we demonstrated an effective strategy for engineering the barrier height of a heterogeneous semiconductor interface by piezoelectric polarization, known as the piezotronic effect. A consistent enhancement or reduction of photocurrent was observed when tensile or compressive strains were applied to the ZnO anode, respectively. The photocurrent variation is attributed to a changed barrier height at the ZnO/ITO interface, which is a result of the remnant piezoelectric potential across the interface due to a nonideal free charge distribution in the ITO electrode. In our system, ∼1.5 mV barrier height change per 0.1% applied strain was identified, and 0.21% tensile strain yielded a ∼10% improvement of the maximum PEC efficiency. The remnant piezopotential is dictated by the screening length of the materials in contact with piezoelectric component. The difference between this time-independent remnant piezopotential effect and time-dependent piezoelectric effect is also studied in details.

Growth of Titanium Dioxide Nanorods in 3D-Confined Spaces

Three-dimensional (3D) nanowire (NW) networks are promising architectures for effectively translating the extraordinary properties of one-dimensional objects into a 3D space. However, to uniformly grow NWs in a 3D confined space is a serious challenge due to the coupling between crystal growth and precursor concentration that is often dictated by the mass flow characteristic of vapor or liquid phase reactants within the high-aspect ratio submicrometer channels in current strategies. We report a pulsed chemical vapor deposition (CVD) process that successfully addressed this issue and grew TiO(2) nanorods uniformly covering the entire inner surface of highly confined nanochannels. We propose a mechanism for the anisotropic growth of anatase TiO(2) based on the surface-reaction-limited CVD process. This strategy would lead to the realization of NW-based 3D nanoarchitectures from various functional materials for the applications of sensors, solar cells, catalysts, energy storage systems, and so forth.

Band Structure Engineering at Heterojunction Interfaces via the Piezotronic Effect

Engineering the electronic band structure using the piezopotential is an important aspect of piezotronics, which describes the coupling between the piezoelectric property and semiconducting behavior and functionalities. The time-independent band structure change under short-circuit condition is believed to be due to the remnant piezopotential present at the interface, a result of the finite charge-screening depth at the interface. A series of materials, including metals, semiconductors and electrolytes, are selected to investigate the interfacial band structure engineered by remnant piezopotential when they are in contact with a strained piezoelectric semiconductor. The remnant piezopotential at the interface can switch the junction between Ohmic and Schottky characters, enhance charge combination/separation, regulate barrier height, and modulate reaction kinetics. The difference between the regular time-dependent, pulse-type piezopotential and constant remnant piezopotential is also discussed in detail using a ZnO-based photoelectrochemical anode as an example. The piezotronic effect offers a new pathway for engineering the interface band structure without altering the interface structure or chemical composition, which is promising for improving the performance of many electronics, optoelectronics, and photovoltaic devices.

Piezopotential-Driven Redox Reactions at the Surface of Piezoelectric Materials†

Themanipulation of charge-carrier conduction characteristics is a critical attribute governing the operation and efficiency of photovoltaic, catalytic, and other energy-converting systems that are based on electrochemical principles. This manipulation is often accomplished through the application of electrical-potential gradients by an external power supply and/or the creation of electronic-state discontinuities by heterojunction-interface engineering. For example, in electrochemical systems, the transport of charge across chemical phases is governed by the energy and density of electronic states within the disparate phases as well as any existing bias between said phases. Piezoelectric materials have long been used as a source of bias and mechanical displacement, relying on their mechanical to electrical coupling character for applications in sensors, actuators, and energy harvesters. In contrast to this historical precedent, contemporary integration of piezoelectric materials in semiconductor heterostructures capitalizes on the capability of the piezoelectric potential to manipulate charge carriers (i.e., piezotronics). For instance, the straining of a piezoelectric element in order to change its semiconducting properties has recently been investigated in zinc oxide nanomaterials, which have opened the doors to strain-gated logic operations and new possibilities for microelectronic circuitry elements. Straining effects in piezoelectric photoelectrochemical cells have also been shown to result in performance enhancements through manipulation of interface energetics. In principle, the piezoelectric modulation of charge carrier energetics should extend beyond the bounds of the buried electronic interfaces explored to date, thus allowing the direct enhancement or suppression of electrochemical processes that occur at the interface of a piezoelectric material and a solution (i.e., piezocatalysis). Preliminary experiments have shown an evolution of H2 and O2 from mechanically agitated piezoelectric BaTiO3 and ZnO microstructures in an aqueous sonication bath. In order to elucidate the intriguing piezocatalytic phenomenon, we report a systematic study of the piezoelectric-potentialdriven electrochemical H2 evolution process that takes place at the electrodes located on the surface of the material. The results compliment the general trends expected from the combinatorial assemblage of piezoelectricity and electrochemistry. The H2 evolution rates were dependent upon the oscillation frequency and amplitude of the piezoelectric material, in accordance with the combination of the direct piezoelectric effect and electrochemical reactions. A study of the piezocatalytic effect was conducted on a single-crystalline ferroelectric Pb(Mg1/3Nb2/3)O3-32PbTiO3 (PMN-PT) cantilever in a sealed chamber (see Figure S1 in the Supporting Information). The voltage output of the PMN-PT slab was first characterized in air (Figure 1a). The cantilever was mechanically oscillated with a fixed frequency and amplitude. When the piezoelectric cantilever was transitioned to the deionized water environment, the voltage amplitude decreased while the mechanical oscillation frequency and amplitude remained constant. In order to encompass the entire piezocatalytic system into a cohesive entity, an analogous circuit was constructed (Figure 1b). In this circuit, opposite and iteratively alternating sides of the piezoelectric material serve as both the working and counter electrodes, respectively. A strained piezoelectric material can be considered a charged capacitor, and thus the change in measured voltage is correlated with a change in piezoelectricity-induced surface charge (DQp). When a strained piezoelectric material is placed within an aqueous medium of finite conductivity and polarizability, its piezoelectricity-induced surface charge can be depleted through two primary pathways: Faradic (If) and capacitive (Ic=dCdVd/dt, in which Cd and Vd are the double layer capacitance and voltage drop across the double layer, respectively) currents:

Nanometre-thick single-crystalline nanosheets grown at the water-air interface.

To date, the preparation of free-standing 2D nanomaterials has been largely limited to the exfoliation of van der Waals solids. The lack of a robust mechanism for the bottom-up synthesis of 2D nanomaterials from non-layered materials has become an obstacle to further explore the physical properties and advanced applications of 2D nanomaterials. Here we demonstrate that surfactant monolayers can serve as soft templates guiding the nucleation and growth of 2D nanomaterials in large area beyond the limitation of van der Waals solids. One- to 2-nm-thick, single-crystalline free-standing ZnO nanosheets with sizes up to tens of micrometres are synthesized at the water–air interface. In this process, the packing density of surfactant monolayers adapts to the sub-phase metal ions and guides the epitaxial growth of nanosheets. It is thus named adaptive ionic layer epitaxy (AILE). The electronic properties of ZnO nanosheets and AILE of other materials are also investigated.

Fundamental Analysis of Piezocatalysis Process on the Surfaces of Strained Piezoelectric Materials

Recently, the strain state of a piezoelectric electrode has been found to impact the electrochemical activity taking place between the piezoelectric material and its solution environment. This effect, dubbed piezocatalysis, is prominent in piezoelectric materials because the strain state and electronic state of these materials are strongly coupled. Herein we develop a general theoretical analysis of the piezocatalysis process utilizing well-established piezoelectric, semiconductor, molecular orbital and electrochemistry frameworks. The analysis shows good agreement with experimental results, reproducing the time-dependent voltage drop and H2 production behaviors of an oscillating piezoelectric Pb(Mg1/3Nb2/3)O3-32PbTiO3 (PMN-PT) cantilever in deionized water environment. This study provides general guidance for future experiments utilizing different piezoelectric materials, such as ZnO, BaTiO3, PbTiO3, and PMN-PT. Our analysis indicates a high piezoelectric coupling coefficient and a low electrical conductivity are desired for enabling high electrochemical activity; whereas electrical permittivity must be optimized to balance piezoelectric and capacitive effects.

Piezotronic-Enhanced Photoelectrochemical Reactions in Ni(OH)2-Decorated ZnO Photoanodes

Controlling the interface electronic band structure in heterostructures is essential for developing highly efficient photoelectrochemical (PEC) photoanodes. Here, we presented an enhanced oxygen evolution reaction (OER) by introducing the piezotronics concept, i.e., piezoelectric polarization (Ppz)-induced band engineering. In a Ni(OH)2-decorated ZnO photoanode system, appreciably improved photocurrent density of sulfite (SO3(2-)) and hydroxyl (OH(-)) oxidation reactions were obtained by physically deflecting the photoanode. Both theoretical and experimental results suggested that the performance enhancement was a result of the piezoelectric Ppz-endowed enlargement of the built-in electric field at the ZnO/Ni(OH)2 interface, which could drive an additional amount of photoexcited charges from ZnO toward the interface for OER. This strategy demonstrates a new route for improving the performance of inexpensive catalysts-based solar-to-fuel production.