Shape Effect and Shape Control of Polycrystalline Semiconductor

PERSPECTIVE pubs.acs.org/JPCL

Shape Effect and Shape Control of Polycrystalline Semiconductor Electrodes for Use in Photoelectrochemical Cells Kyoung-Shin Choi* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907

ABSTRACT This Perspective discusses the morphological considerations involved in the construction of efficient polycrystalline semiconductor electrodes for use in photoelectrochemical cells. The important morphological features that can affect the energetics and kinetics of semiconductor electrodes are described. Select electrochemical synthesis approaches useful in gaining a better understanding of shape-dependent properties of photoelectrodes and the construction of desirable polycrystalline architectures are also briefly examined. This Perspective will provide a useful basis for optimizing electrode morphologies in order to enhance desired photoelectrochemical properties.

P

hotoeffects at the semiconductor/liquid interfaces have been utilized to construct photoelectrochemical cells that can generate electricity (e.g., dye-sensitized solar cell) or chemical fuels (e.g., water splitting, CO2 reduction).1-4 In order to increase efficiency-to-production cost ratio, construction of photoelectrochemical cells based on polycrystalline semiconductor electrodes has been investigated extensively due to lowered production costs compared to those of single-crystal electrodes. When a semiconductor material is produced as a polycrystalline electrode, morphological details of the electrode such as size, shape, and connectivity of the particles have a significant impact on the interfacial energetics, kinetics, and charge-transport properties. Therefore, understanding and controlling the morphological aspects of polycrystalline electrodes is critical in producing highly efficient devices. To date, morphology control of polycrystalline photoelectrodes has mostly focused on increasing surface areas via production of nanocrystalline or nanoporous electrodes.4-8 For example, in the construction of dye-sensitized solar cells, the use of a nanoporous TiO2 electrode significantly enhanced the loading of dye molecules and therefore the efficiency of the cell.4 It has also been demonstrated that producing nanostructured electrodes is highly beneficial for reducing electron-hole recombination in a material that has an extremely short minority carrier diffusion length such as R-Fe2O3.5 However, there are many morphological features of polycrystalline electrodes other than surface area that can have a direct impact on the efficiency and stability of photoelectrodes. A better understanding of morphology-dependent properties will significantly enhance our ability to optimize electrode structures and construct high-performance polycrystalline semiconductor electrodes. In order to best study morphology-dependent properties, electrodes with systematically varying morphologies should be produced using similar

r 2010 American Chemical Society

synthesis conditions, so that no factors other than morphology are changed significantly (e.g., crystallinity, doping level, band gap energy). This will ensure that any differences observed in properties are truly due to the difference in morphology.

In order to best study morphologydependent properties, electrodes with systematically varying morphologies should be produced using similar synthesis conditions, so that no factors other than morphology are changed significantly. In this Perspective, we will briefly describe the key morphological features that can affect the energetic and kinetic factors of photoelectrodes and examine desirable polycrystalline photoelectrode architectures. Then, we will discuss recent progress made in electrochemical synthesis that has the capability of altering the morphological features of semiconductor electrode materials in a systematic manner. This will provide a useful basis for considering shape-dependent properties when designing polycrystalline photoelectrode Received Date: May 13, 2010 Accepted Date: July 1, 2010 Published on Web Date: July 07, 2010

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DOI: 10.1021/jz100629n |J. Phys. Chem. Lett. 2010, 1, 2244–2250


Figure 2. A schematic diagram showing two semiconductor electrodes with the same bulk Fermi level immersed in the same electrolyte, creating different barrier heights (VB) at the semiconductor/liquid junction when the change in VH causes a change in the flat band potential (Ufb). Uredox represents the potential of the redox couple in the electrolyte.

One of the effects of crystal shape on the properties of photoelectrodes comes from the fact that different crystal faces create different barrier heights at the semiconductor/liquid interface.2 This is because each crystal face has distinctively different atomic arrangements, which results in a difference in surface charge. The difference in surface charge affects the Helmholtz double-layer structure by changing the point of zero zeta potential (pHpzzp), resulting in a difference in the potential drop across the Helmholtz layer (VH) (eq 1)11

Figure 1. (A) Single-crystal electrodes with (100) and (111) terminations. The solid lines in the electrode schematically represent (100) planes. Notice that the difference in crystal face exposed at the interface of these electrodes is due to the difference in crystal orientation relative to the substrate. (B) Polycrystalline electrodes with (100) planes exposed at the interface. The three electrodes shown here have different orientations, but all of them expose (100) planes at the interface because all individual crystals consisting these electrodes have the same shapes. (C) If a material has a cubic crystal structure, polycrystalline electrodes composed of only cubic (left) and octahedral (right) crystals expose only (100) and (111) planes at the electrode/electrolyte interface, respectively. (D) If a material has a hexagonal crystal structure, polycrystalline electrodes composed of hexagonal plates and hexagonal rods can increase the areas of (0001) and (01-10) planes exposed at the interface, respectively.

V H ¼ 0:059ðpHpzzp - pHÞ

The change in VH in turn affects the flat band potential (Ufb), as shown in eq 2, where φSC is the work function of the semiconductor and 4.5 is the scale factor relating the Hþ/H2 redox level to vacuum. The flat band potential, which is a critical parameter characterizing the semiconductor/liquid interface, is the electrode potential that eliminates the space charge layer and makes the semiconductor bands flat11

architectures. Thoughtfully designed photoelectrodes will be able to enhance our insight into shape-dependent properties and lead to optimal photoelectrochemical performance. Face-Dependent Photoelectrochemical Properties. It is well established from studies on single-crystal semiconductor electrodes (photoelectrodes) that photoelectrochemical properties vary significantly depending on which crystal face is exposed at the interface.2,9,10 They are often referred to as orientation-dependent properties since the orientation of the crystal determines which crystal face is exposed at the interface (or parallel to the substrate) when a monolithic singlecrystal electrode is prepared (Figure 1A). For polycrystalline electrodes composed of multiple crystals with polyhedral shapes, shapes instead of orientations of individual crystals determine which atomic planes are exposed at the interface (Figure 1B). Therefore, if polycrystalline electrodes with individual particles of uniform shapes are constructed, face-dependent properties can be studied using polycrystalline electrodes. For example, if a material has a cubic crystal structure, it is possible to expose only (100) planes and (111) planes at the interface by making an electrode composed of only cubic-shaped crystals and octahedral crystals, respectively (Figure 1C). If a material has a hexagonal structure, an electrode composed of hexagonal plates will increase the area of (0001) planes, while an electrode composed of hexagonal rods will increase the area of (01-10) planes (Figure 1D).


ð1Þ

U fb ðNHEÞ ¼ φSC þ V H - 4:5

ð2Þ

As the band bending or barrier height (VB) formed at the semiconductor/liquid interface is determined by the difference between the flatband potential of the semiconductor electrode and the Fermi level of the electrolyte (Ured) (eq 3), eqs 1-3 thus indicate that a semiconductor with a given bulk Fermi level (φSC) which is immersed in an electrolyte with an identical redox potential (Uredox) can still create different barrier heights (VB), depending on which crystal face is exposed at the semiconductor/liquid junction (Figure 2). Since the quantum yield of the photocurrent is directly related to the width of the space charge layer determined by the barrier height, which crystal face is exposed can have a direct impact on the efficiency of the photoelectrochemical cell.11 V B ¼ U redox - U fb

ð3Þ

It is also expected that different crystal faces will generate different surface states in terms of both concentrations and positions as their surface atomic arrangements and terminations are different. The presence of surface states not only affects surface recombination but also plays an important role in interfacial charge-transfer kinetics when the redox levels of the solution species do not overlap with energy levels of the

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semiconductor bands.12,13 They can also affect the flat band potential and barrier height via Fermi level pinning if they accumulate excess charges at the electrode surface.2,3 The surface atomic arrangements also affect the adsorption of reactant ions or molecules that participate in desired photon-induced reactions (e.g., Hþ, OH-, H2O for photoelectrolysis of water), resulting in differences in interfacial chargetransfer processes or surface catalytic behavior for photoinduced reactions.14 Similarly, the interaction between the semiconductor electrode and catalysts deposited on the semiconductor surface to aid photon-induced reactions (e.g., H2 evolution or O2 evolution catalysts) will also differ depending on which planes are exposed at the interface. Another important factor that can be influenced by the shape of polycrystalline semiconductor electrodes is the tendency for (photo)corrosion.15 Cathodic and anodic corrosion of semiconductor electrodes involves changes in oxidation states and bonding of cationic or anionic species at the semiconductor surface. It often involves participation of solution species (e.g., Hþ, OH-) to complete the reaction. Therefore, the stability of the photoelectrodes against corrosion can also depend on atomic arrangement on the electrode surface and the types and local concentrations of solution species present at the semiconductor/electrolyte interface. In addition to the effects directly resulting from the different atomic arrangement of crystal faces, there are other additional differences that can be induced during the synthesis of crystals with different shapes. Doping types and the doping levels of semiconductors are often determined by defects or imperfections of the lattice (anionic or cationic vacancies) if no impurities are intentionally introduced during synthesis. If a certain crystal face is more vulnerable to a certain type of defect, growing semiconductor crystals with that face exposed during synthesis will increase the defect concentration, which will affect the doping level. In this case, although shape itself does not have a direct influence on the bulk Fermi level of a semiconductor, the process of growing a material with a different shape may result in a change in the Fermi level. Other Morphological Considerations. As mentioned previously, production of nanocrystalline or nanoporous electrodes has been extensively studied as a way of enhancing interface areas of photoelectrodes. However, if increasing surface area involves a significant increase in grain boundary areas that interfere with efficient charge-carrier transport processes within the electrode, it may not be advantageous for increasing the overall efficiency of the electrode. Many recent studies have demonstrated that increasing surface area via the construction of nanoscale one-dimensional units (e.g., rod, fiber, wire, tube) is beneficial by ensuring good transport properties of majority carriers along the longitudinal direction while providing a short pathway for minority carriers to reach the interface (Figure 3A).16-19 Each of these one-dimensional units can be a single crystal or an aggregation of nanocrystals forming an overall one-dimensional shape. Polycrystalline morphologies based on dendritic architectures are also of particular interest as they naturally develop a


Figure 3. (A) Polycrystalline electrodes composed of one-dimensional rods (left) and tubes (right). Transport directions of electrons and holes are shown for an n-type semiconductor. (B) Increasing surface areas of faceted crystals typically involve producing nanocrystals with an increase in nucleation density (top). A dendritic branching pattern can increase surface areas without increasing nucleation density (bottom).

network with atomic-level continuity, which results in superior charge-transport properties (Figure 3B).20,21 Since multiple dendritic branches developed from a single nucleus can effectively increase surface area without the need to increase nucleation density, dendritic growth can be exploited to build photoelectrode architectures that can simultaneously achieve high surface area and good charge-transport properties. Furthermore, unusual surface topology induced during dendritic growth (spikes, tip-splitting, and curved surfaces) may create additional positive effects on the catalytic activity of the electrode surface. For a semiconductor that possesses a significant anisotropy in the mobility of charge carriers, the orientation of crystals relative to the substrate can also affect the charge-transport properties and therefore becomes an important factor to consider in constructing photoelectrode architectures.5 Shape Control of Photoelectrodes via Electrochemical Synthesis. In order to improve electrode performance based on a solid understanding of interface-property relationships, it is necessary to have a synthesis method that can produce a semiconductor electrode with systematically varying interfacial structures. Electrochemical synthesis is one of the most ideal methods to achieve this goal due to several intrinsic advantages. First, the solution-based nature of electrochemical synthesis allows for the manipulation of many synthesis variables (e.g., pH, additives, types of solvents, and temperature) that markedly affect the growth direction and growth rates and, therefore, the final shape of materials deposited. In addition, the deposition potential and current serve as additional and powerful synthesis variables that can finely control the nucleation and growth processes. In typical solution synthesis, the concentration of reactants (i.e., ions or molecules that feed the crystal) affects both the growth rate and mass-transport rate. As such, it is difficult to manipulate the growth rate independently from the mass-transport rate. However, the material's growth rate in electrochemical

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synthesis can be set by deposition current independently from the concentration of the reactants. Therefore, electrodeposition can create various combinations of growth rate and mass-transport rate, thus allowing for more systematic control over mass-transport-limited growth. After electrochemical synthesis, semiconductor crystals with varying shapes grown on the conducting substrates can easily be analyzed electrochemically and photoelectrochemically to investigate morphology-dependent properties with no further electrode preparation. Figure 4. Polycrystalline Cu2O electrodes composed of (A) cubic and (B) octahedral crystals exposing only (100) and (111) faces at the interface, respectively. Reproduced from ref 32 with permission. Copyright 2004, Wiley-VCH Verlag GmbH & Co. KGaA.

Electrochemical synthesis is one of the most ideal methods to improve electrode performance based on a solid understanding of interfaceproperty relationships. Traditionally, electrochemical synthesis has been used mainly for metal plating, but there has been significant progress in utilizing electrochemical synthesis to produce a variety of semiconductor electrodes with controlled morphologies (e.g., metal oxides, sulfides, selenides, and tellurides).22-27 A few examples are given below to demonstrate how electrodeposition can be exploited to deposit semiconductor electrodes with morphologies that are beneficial for investigating shape-dependent photoelectrochemical properties and/or to enhance desired photoelectrochemical properties. In general, materials grown near equilibrium conditions have polyhedral shapes with smooth and well-developed facets because the shapes in this growth regime are driven to achieve a minimum total surface free energy.28,29 In electrodeposition, growth near equilibrium is obtained when crystals are grown with a low deposition overpotential. The deposition overpotential is defined as the difference between the applied deposition potential and the equilibrium redox potential of the species being deposited. The deposition overpotential affects the deposition current exponentially, and therefore, a low deposition overpotential results in a slow crystal growth.30 As a result, semiconductor electrodes prepared with a low deposition overpotential generally possess well-faceted polyhedral crystals. In order to modify shapes of faceted crystals, the relative growth rates along various crystallographic directions must be controlled.31 One of the most effective ways to achieve this is through the use of additives in the growth medium that preferentially adsorb on a certain set of crystallographic planes or step edges of the crystal via electrostatic or covalent interactions.31-36 Such preferential adsorption of additives slows the growth perpendicular to the bound plane and results in modification of the final crystal shape. This concept has been combined with electrodeposition to produce polycrystalline electrodes containing various uniformly controlled polyhedral crystals (Figure 4).32 This allows for regulating the


Figure 5. (A) Polycrystalline electrodes composed of dendritic zinc crystals, where all crystals are physically connected through “stems” (reproduced from ref 20). (B) Polycrystalline electrodes composed of dendritic Cu2O crystals, where intricate dendritic branching patterns increase surface areas (reproduced from ref 21).

types and ratios of crystal faces exposed at the interface and makes it possible to study face-dependent photoelectrochemical properties using polycrystalline electrodes. When crystallization occurs with a fast growth rate at conditions far from equilibrium, the growth of crystals can become limited by mass transport, which results in a dendritically branched morphology.37,38 This morphology is not energetically stable due to its higher surface area and surface roughness compared to well-faceted polyhedral shapes. However, the extraordinary network and connectivity between the crystals developed during dendritic growth can be exploited to simultaneously achieve high surface areas and good charge-transport properties. One example shown in Figure 5A is an electrochemically generated dendritic pattern of zinc crystals, where numerous nanocrystals are physically connected through the “stems”. As a result, surface area is enhanced while ensuring good electrical continuity between the nanocrystals.20 Another example shown in Figure 5B is an electrode composed of dendritically branched Cu2O crystals. In this case, the complex branching pattern effectively increases the surface area and the surface coverage of the substrate, although the nucleation density is extremely low.21 This resulted in a significantly enhanced photocurrent compared to that generated by a Cu2O electrode composed of micrometersized Cu2O crystals produced from a similar deposition

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condition. Similar or even more intricate dendritic patterns may be obtained for other metals and semiconductors by investigating various nonequilibrium growth conditions, which will offer various new polycrystalline architectures desirable for photoelectrode construction. Electrochemical synthesis has also been used in preparing materials with various one-dimensional morphologies, including rods, wires, and fibers. One of the most widely used electrochemical methods to produce uniform rod-type materials involves the use of porous templates such as porous anodic aluminum oxide (AAO). When a conducting layer is attached or deposited at the one end of the AAO, it can be used as the working electrode, and materials grow from the conducting bottom, filling up the pores to form rod-shaped materials. The AAO templates can be easily removed by an alkaline aqueous solution. Various semiconductors (e.g., ZnO, V2O5, Bi2Te3, CdS, CdSe/CdTe) have been prepared as rodshaped crystals or arrays of rods by this method.39-45 If a material has an anisotropic crystal structure (e.g., ZnO and CdSe with wurtzite structure), wire- or fiber-type morphologies can also be prepared without the use of hard templates by creating conditions that selectively enhance longitudinal growth.46 Another interesting and useful one-dimensional morphology is a nanotube array. The most common method for preparing semiconductor nanotube arrays involves electrochemical anodization of a metal foil in acidic fluoride-containing electrolytes. This process was first developed to form AAO,47 but to date, nanotube arrays of various oxides (Fe2O3, Nb2O5, Ta2O5, TiO2, WO3, and ZrO2) have been prepared from corresponding metal sheets by anodization.48-55 Tube length, pore width, and wall thickness, as well as the uniformity of these dimensions, can all be controlled using various experimental conditions (electrolyte composition and concentration, pH, temperature, voltage, duration, and thickness of the original metal layer). The resulting nanotube arrays have been utilized for construction of various devices for use in solar energy conversion.18,53-55 The anodization process can be combined with other chemical treatments to produce a diversity of materials other than oxides that are suitable as photoelectrode materials (e.g., formation of nitrides via heat treatment of oxides under ammonia flow).56 To date, few systematic studies on morphology-property relationships of polycrystalline photoelectrodes have been carried out because the synthesis methods commonly used to prepare polycrystalline photoelectrodes have a limited ability to systematically alter the morphological features of polycrystalline electrodes. However, the increasing number of reports that illustrate the importance of morphology control in enhancing the efficiency of photoelectrodes warrants a more methodical investigation in this area. Electrochemical synthesis can serve as a useful tool in the production of various model systems, elucidating general morphologyproperty relationships. By electrically, electrochemically, and photoelectrochemically analyzing a range of systematically varying polycrystalline structures, it would be possible to identify parameters and properties of the semiconductor electrodes that are affected by changes in morphological features. This understanding will provide useful guidelines


for designing and constructing high-performance polycrystalline photoelectrode materials using various synthesis methods.

The increasing number of reports that illustrate the importance of morphology control in enhancing the efficiency of photoelectrodes warrants a more methodical investigation in this area. AUTHOR INFORMATION Corresponding Author: *E-mail: [email protected]. Tel: 1-765-494-0049. Fax: 1-765494-0239.

Biographies Kyoung-Shin Choi received her B.S. and M.S. degrees from Seoul National University in South Korea in 1993 and 1995, respectively, and a Ph.D. degree from Michigan State University in 2000. She then spent two years at the University of California, Santa Barbara, as a postdoctoral researcher (2000-2002) and joined the chemistry faculty at Purdue University as an assistant professor in 2002, where she is currently an associate professor. She was a visiting scholar at the National Renewable Energy Laboratory (NREL) during the Fall of 2008. Her current research combines solid-state chemistry, electrochemistry, and materials chemistry in order to address materialsrelated issues of electrode materials for use in electrochemical and photoelectrochemical devices. Her specific research interest lies in the construction of multicomponent composite electrodes (e.g., photoelectrode/catalyst) with optimum architectures via precise and rational morphology control. She is currently serving as one of the 2011 volume organizers for Materials Research Society (MRS) Bulletin and the 2011 chair elect of the ACS Division of Inorganic Chemistry, Solid State Chemistry subdivision.

ACKNOWLEDGMENT This work was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through Grant DE-FG02-05ER15752.

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Shape Effect and Shape Control of Polycrystalline Semiconductor

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