Photochemistry on a polarisable semi-conductor: what do we understand today?
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The continued development of ferroelectric materials into more and more consumer led applications has been at the forefront of recent ferroelectric material research. It is, however, possible to view a ferroelectric as a wide band gap semi-conductor that can sustain a surface charge density. This charge density arises from the movement of ions in the crystal lattice and the need to compensate for this charge. When viewing ferroelectrics as polarisable semi-conductors a large number of new interactions are possible. One such is the use of super band gap illumination to generate electron–hole pairs. These photogenerated carriers can then perform local electrochemistry. What is most interesting for ferroelectric materials is that the REDOX chemistry can be chosen by selectively modifying the domain structure of the ferroelectric—we can perform oxidation and reduction on the surface of the same material at spatially separate locations, or use the material to drive photoexcited carriers apart. This means we can separate the REDOX products or produce patterns of photogenerated material in places we have predetermined. This review aims to introduce the background research that has led to the current understanding as well a highlight some of the current areas that require further development.
KeywordsDomain Wall BaTiO3 LiNbO3 Grain Boundary Barium Titanate
The review is intended to contextualise the current understanding of the interactions of the surface of a ferroelectric under non-equilibrium conditions in the light of excellent work undertaken last century. It shows where the new understanding is knitting together a number of diverse research topics into one thread of understanding. This current work is not intended to give a deep expose on any one topic rather highlight some of the seminal works in topical areas and bind them together. Over the past 5 years there has been an increase in the interest in the use of ferroelectric materials as photocatalysts, or their use in photochemistry. This is due to the ability of a ferroelectric material to be patterned at length scales that range from tens of nanometres to many centimetres. The different regions that have been patterned on the ferroelectric surface are both stable (with only a few exceptions) and exhibit different REDOX chemistry that is determined by the polarisation of the ferroelectric. The possibility of using such materials in a host of optoelectronic devices is also covered due to the effective electron–hole separation that can be achieved.
This review will cover the historical perspective of the semi-conducting nature of ferroelectric materials which is the natural pre-cursor to investigating the photochemistry, and then focus on the new developments of patterned surfaces being used for selected chemical reactions. It will conclude by looking at some of the possible future implications, and applications of the results. Work focussing on the semi-conductor nature of a ferroelectric has shed light on the properties and phenomena occurring on the surface of the ferroelectric material under the non-equilibrium conditions of irradiation by super band gap irradiation. It also gives some indications as to what is happening at the interface between a poled ferroelectric material and its wider environment. Prior to the publication by Kalinin et al. in 2002 , where a novel paradigm for the assembly of 3-D nanoparticles on a ferroelectric surface, was demonstrated there had been some investigations detailing the photo induced interactions of ferroelectric surfaces and the wider environment. Most of the recent research has been focussing on the nature and reasons behind the preferential growth of nanoparticles on domain-patterned ferroelectric materials. The developed nanoparticles, and nanoparticle patterns, have huge application in the field of bio-medicine, optoelectronic devices, photonics, product labelling and authentication.
Strong electromechanical coupling, a high dielectric constant and the unique property of sustaining spontaneous polarisation below the Curie temperature make ferroelectrics extremely fascinating materials and constitute the basis of wide technological applicability [2, 3, 4, 5, 6]. It is true to say that in one form or another ferroelectric materials have become ubiquitous for the society of today. After the discovery of ferroelectricity in Rochelle Salt by Valasek  in 1920, ferroelectric materials have emerged as a very promising material group with wide applications such as sensors, actuators, transducers, electronic and electroptical materials and numerous novel applications including microelectromechanical systems [8, 9, 10]. Applications range from the use of BaTiO3 in capacitors required for almost every electronic device to improvement of diesel engine performance through accurate control of injection performance. On a more research focused level a new and novel application of ferroelectrics is to facilitate the growth of self-assembling 3D nanostructures.
It is, however, interesting to note that ferroelectric materials were among some of the first materials to be shown to have photoelectric or photochemical properties. The studies on ferroelectric materials followed on from the early work that resulted in the observation that paint containing particles of TiO2 blistered or discoloured under sunlight. It was in 1938 that the first publication showed that TiO2 was photoactive . This was then followed by interesting work that resulted in the Mashio et al. publishing a paper which showed the photoxidation performance of TiO2 . The powerful oxidation performance of the photogenerated hole was highlighted in this work and led to the significant effort to develop the use of TiO2 in photocatalytic and photochemical systems. During the oil crisis in the late 1960s and early 1970s a number of groups picked up on the photostability of semi-conductors as a possible means of generating useful energy, either in the form of an electric current or chemical energy. The result of this work was the Nature paper published in 1972 by Fujishama and Honda  which showed the use of semi-conductors in photogenerated cells. It was only a matter of a few years later when the photochemistry of BaTiO3 was reported in a paper by Nasby and Quinn , who showed a pH dependence on flat-band potential and that BaTiO3 shows excellent photostability and good photocurrents. This seminal paper left a number of unanswered questions that were a result of the ferroelectric nature of BaTiO3 and the influence that has on the surface photochemistry due to selective movement of photoinduced carriers. The interest in the surface photochemistry of ferroelectrics was largely left untouched for a number of years until around 2000 when Giocondi and Rohrer  published a paper highlighting the variation in surface photochemistry for BaTiO3. This paper related the variation in the band bending associated with the domain polarisation of the ferroelectric, this was the first time such a relationship was established for surface photochemistry. It should be noted that although there was little work investigating the surface photochemistry of semi-conductors there was substantial work developing theories to relate the semi-conductor properties of ferroelectric materials .
Synthesis of devices with micro and nanoscale dimensions is a focus of intense scientific and technological interest. This is highlighted by the boom in nanotechnology and microtechnology research from all corners of the world. In the UK alone there is now a dedicated Microsystems and Nanotechnology network and associated ring fenced funding.
Using the photo-oxidation/photoreduction reactions to direct the assembly of nanoparticles on selected regions of the ferroelectric surface.
Using analogous tests it has been shown that the deposition of Rh, Pd, Au, Co, Ni, Fe can been achieved on PZT that has been grown via sol–gel. BaTiO3 was produced as a single crystal, thin film and polycrystals  and exhibited similar results. There is also now a growing body of literature showing that similar reactions can occur on LiNbO3, although there is not the same level of understanding for this material as the perovskite systems. In all these cases there has been a focus on the reduction of a metal salt to produce metallic products. If an examination of the band structure for PZT and BaTiO3 is performed then it is possible to see why reduction of certain metal salts is possible. The conduction band of the materials PZT and BaTiO3 sit between −0.5 and −1 V when compared to the standard hydrogen electrode. The discrepancy of the exact location of the conduction band is due to variations in processing conditions, stoichiometry and experimental techniques. However, what has been determined is that salts with a reduction potential greater than the chemical potential of the conduction band will be reduced when the surface of the ferroelectric is under super band gap photoillumination. All of the metals that have been produced so far fit this criterion. The selective deposition process is attributed to the various phenomena taking place on the surface of ferroelectric thin films. In this review we will focus on the various properties and phenomena occurring on the surface of ferroelectric material.
Ferroelectricity and piezoelectricity
Ferroelectricity is a phenomenon in which a crystal exhibits two or more stable configurations in the absence of an electric field and can be shifted from one to another state by the application of electric field. The most studied and frequently applied ferroelectric materials are those with perovskite-type structures with general formula ABO3, which A and B are cations such as Pb2+/Ba2+/Sr2+, Zr/Ti and O is oxygen anion O2−.
Piezoelectricity is a phenomenon in which a crystal can be polarised by the application of mechanical stress. Conversely when an electric field is applied across a piezoelectric crystal, it will change its dimensions to either expand or contract. Therefore, application of pressure to a piezoelectric crystal between two electrodes causes a charge to flow in one direction. The sign of piezoelectric charge or the direction of strain is the same as the direction of applied mechanical and electrical fields respectively.
Domain and domain boundaries
In semiconducting ferroelectric thin films the coupling between polarisation and space charges leads to the formation of charge double layers at 90° domain walls, which are decorated by defects such as dopants, oxygen vacancies . There is no such charge layer formation in 180° walls.
Over the years there have been extensive and exhaustive studies that have focused on the formation and static properties of domains in bulk crystalline ferroelectrics by polarising optical microscopy, etching and surface decoration [41, 42]. These methods have a disadvantage in that they have a low spatial resolution of the order of 1 μm. Under certain circumstances resolution below 10 nm can be achieved by SPM . Domain related topographic features are characterised by contact and intermittent mode [43, 44, 45, 46, 47, 48] and electrostatic properties above the surface by electrostatic force microscopy [49, 50].
Ferroelectrics: insulators or semiconductors?
Traditionally ferroelectrics have been regarded as insulators following the Devonshire–Ginsburg–Landau theory . The band gap of ferroelectric materials is large, e.g. for PZT type materials it is around 3.2–3.7 eV [52, 53, 54], and for BaTiO3 it is around 3.2 eV. Variations in the band gap, especially for PZT and other complex perovskite materials, arise due to variations in processing conditions and other external factors and as such there is yet to be a definitive Eg that is widely accepter for PZT. Under equilibrium situations the materials can be treated as insulators, however, under the influence of sufficiently high energy radiation the non-equilibrium that develops follows basic semi-conductor theory. Furthermore the large concentration of structural defects acting as doping centres, the presence of Schottky barriers at contacts, and large leakage currents are characteristic properties of a semiconducting material. Therefore these materials can also be considered, and treated, as wide band gap semiconductors . A non-doped PZT film, i.e. as close as possible to stoichiometric composition is a p-type semiconductor according to defect chemistry. Indeed the chemistry of the PZT produces some interesting effects; due to the oxygen vacancy accumulation an oxygen deficient layer, which is about 30 nm thick, n-type conductivity arises in the region close to the surface, whereas bulk remains p-type . According to Mihara et al.  the n-type layer appears during the crystallisation annealing of films at high temperatures in the range 500–700 °C because of the volatilisation of lead oxide from the film composition.
Polarisation and screening
In ferroelectric materials the displacement of a body centred cation, e.g. in a cubic perovskite BaTiO3 the displacement of Ti3+, gives rise to a dipole in the unit cell. The interactions between dipoles in unit cells cause polarisation alignment resulting in a polarisation bound charge at the surface of the block of ceramic. This polarisation charge affects surface topography, chemical reactivity, optical and electronic properties of ferroelectric surfaces and interfaces, and this is clearly demonstrated by phenomena such as ferroelectric electron emission [58, 59], polarisation dependent work function , metal photodeposition [32, 61].
The first suggestion about the existence of surface layer was given by Kanzig  on the basis of experiments performed on BaTiO3. It was observed that a strong electric field of 104–106 V/cm and a low dielectric constant could be observed in the volume of the surface layer. It was then shown that the surface layer is not a foreign film but is a surface region of the crystal in which dielectric saturation and piezoelectric compression occur because of strong electric field . Through detailed investigation of properties and nature of the surface layer Treibwasser  modelled it as a Schottky barrier.
The evaluated values for saturation polarisation Ps, static dielectric constant εst hole concentration p(T), apparent built-in potential Vbi′, effective space charge density in the depleted layer Neff, potential barrier ϕB and width of the surface layer δ (with permission from Ref. 65)
p(T) (1017 cm−3)
Neff (average) (1020 cm−3)
ϕ B 0 (average) (eV)
δ (average) (nm)
The quality of the ferroelectric film is an important factor which contributes to the thickness of the surface layer. As defect density has an impact on the photochemical properties  of the ferroelectric material that has been processed in different ways could be expected to show some differing photochemical effects. When a sample has been made by processes such as sol–gel  then a very different defect density and structure through out the thickness of the thin film exists compared to MBE grown material. The interaction between the density and nature of defects, in the material and the internal depolarisation field leads to the development of the space charge layer. The depth to which the space charge layer penetrates is determined by how many mobile carriers are available to screen the depolarisation field. Therefore a material with a large spontaneous polarisation and few defects will generate a wide space charge layer, increasing the defect density, and with that the number of available carriers, reduces the width of the space charge layer . If the ferroelectric is processed using low temperature sol–gel type mechanisms then there are different defect structures compared to material that has been processed at higher temperatures using standard ceramic processing techniques.
When dealing with materials at the nanoscale there is an interest in how the dimensions affect the fundamental properties of the material. This is also true for a ferroelectric and there has been some detailed work investigating the impact of cross sectional thickness on various parameters. Of particular interest here is the impact on the depolarisation of the ferroelectric and the influence that reducing the thickness of the thin film has. In a short review Scott  discusses finite size effects. It is well known that the spontaneous polarisation it at a maximum at the surface of the ferroelectric and decays to zero towards the centre. A key question is what is the dimension required for the spontaneous polarisation to decay and do nanostructured ferroelectrics allow this?
The influence of size on a ferroelectric when treated as a semi-conductor, can be seen in Table 1. The surface layer can vary from 33 to 3 nm depending on the composition (these values are for various PZT compositions). It is routine to make ferroelectric samples that are 10–100 nm in the z-direction. This would mean that for many ferroelectric compositions there would be an interaction between the SCL at both interfaces, as a result of the depolarisation field decaying across the thickness of the film. As a thin film ferroelectric would normally be grown on a conducting substrate there will be pinning of the Fermi levels of electrode and ferroelectric. In essence there is likely to be a strong influence of the depolarisation field on the surface photochemistry in thin film ferroelectric systems.
The impact of an interface between a traditional semiconductor and solution or secondary phase has long since been established . After the interface has come to equilibrium there has been a movement of free carriers that is the result of drift due to local concentration gradients. The band bending that is found in the semiconductor is a direct result of the drift of mobile carriers across the interface. There are, however, some fundamental differences when dealing with a material that has an internal field due to displacement of ions in the crystal lattice, in other words a ferroelectric material. These materials, while exhibiting n or p-type behaviour do not generate bent bands in the same way as a traditional semiconductor—the need to screen the spontaneous depolarisation is too high and so the band bending is determined by this need.
It is generally true that for perovskites such as BaTiO3 and PbTiO3 as well as the more complex systems such as PbZrxTi(1−x)O3 (PZT) that Ev is the top of the valence band and is associated with oxygen 2p orbitals and Ec is bottom of conduction band associated with titanium, or A/B 3d orbitals. Ef is the Fermi level and Ebb is the energy associated with band bending that is introduced either due to the field developed due to crystal distortion associated with ferroelectric behaviour or contact with. The density of states for the ferroelectric materials varies with the composition and also the processing parameters of the materials.
Electrostatic properties of the ferroelectric surface
Completely unscreened surface σs = 0.
Partially screened surface σpol ≥ σs.
Completely screened surface σpol = −σs.
Over-screened surface σpol ≺ −σs.
The sign and magnitude of surface potential on a ferroelectric surface are not only dependent on polarisation charges, but they depend on the intricate balance between the polarisation and screening charges. An extreme case could be surface polarisation reversal due to the sorption of highly polarised long chain hydrocarbons on the surface of a ferroelectric akin to the behaviour of detergent molecules in an oil and water mix. In the case of the ferroelectric the balance is due to the availability of carriers (from within the sample) and the variation in the space charge layer that forms. The exact shape and dimensions of the space charge layer depend on a number of factors that are material and in some cases sample specific. It has been shown that photo deposition of silver can occur on both positive and negative domains on PZT when it is  orientated, but that silver only deposits on PZT  when it is positively charged . This is due to variations in the density of states and also in the variation in the remnant polarisation of the sample, which impacts the shape of the space charge layer and screening processes. It is clear from Fig. 10 that above the unscreened surface the surface potential varies linearly and reciprocally with domain size whereas the electric field is domain size independent. Above the completely screened surface the electric field varies linearly and reciprocally with domain size and the surface potential is domain size independent.
Electronically active grain boundaries
The grain boundary (GB) is a region which separates crystallographically coherent areas in crystalline solids. GBs act as barriers for the charge transport of carriers in materials showing ionic/electronic conduction. The GB effect has been studied for a variety of n-type semiconducting ceramics [77, 78]. For n-type semiconducting ceramics negatively charged GB states are compensated by positively charged donor centres in the bulk close to the GB. This creates a depletion of the negatively charged mobile carriers in a layer of the width dGB symmetrically surrounding all grain boundaries, and leads the formation of depletion layers. From the electronic point of view these depletion layers can be described as two Schottky diodes back to back. In perovskite type alkaline earth titanates a highly resistive GB layer has been observed [79, 80, 81]. In acceptor, p-type, doped alkaline earth titanates the GB states are electrically active and are positively charged due to donor centres. These are compensated by negative space charged acceptor dopants from the bulk close to the GB. The positive charge in the GB creates a depletion region of mobile positively charged carriers, holes and oxygen vacancies. This depletion space charge layer can be considered as two back to back Schottky barriers. This model was introduced by Chiang and Tagaki .
Relative rates of deposition of Ag clusters on a highly heterogeneous PZT surface (with permission from Ref. 85)
Ferroelectric photocurrents and surface photochemistry
Conduction in solids is dependent upon the mobilities and concentrations of both ionic defects (interstitial ions and vacancies) and electronic charge carriers (electrons and holes) through the bulk and along and across interfaces such as grain boundaries, electrode contacts and surfaces . It has been shown by Maier  in 1993, that both conductive mechanisms: ionic and electronic, may be modelled using analogous methods and that both mechanisms, and their dependence on environmental parameters such as temperature and partial pressure, participate in the overall reactivity of many materials, including ferroelectrics.
Ionic conduction mechanisms rely on the transport of interstitial ions and vacancies which are also subject to a minimum energy requirement that is referred to as free activation energy. It has been shown by Franklin  that the enthalpies of vacancy formation in elemental solids are approximately proportional to their absolute melting points with results ranging from a fraction of one eV (e.g. Ar, Xe) to over 2 eV (Mo), whilst the free enthalpy change caused by the introduction of an oxygen defect is in the order of 1 eV, or 100 kJ mol−1 .
Defect concentrations can be manipulated via thermal excitation or doping and the introduction of intermediate donor or acceptor impurity levels within the band gap can influence the level of energy required for electron and hole transitions across the gap . These intermediate states can behave as non-radiative Schockley Hall Read (SHR) generation-recombination centres or Auger type traps.
While SHR generation-recombination enables electrons and holes to recombine and thus navigate the potential barrier to produce an additional current flow mechanism via thermal exchange, recombination at deep Auger trap sites is non-radiative and reduces carrier densities whilst trapping the energy within deep lying energy bands. However, Auger rates decrease exponentially with increasing bandgap values and become more or less inconsequential for bandgaps above ~1.5 eV . It should also be noted however that heavy doping results in a narrowing of the bandgap.
In all photochemical reactions the kinetics of the process plays an important role. While the thermodynamics of the system may be favourable the rate at which the products are formed is determined by the complex interaction of the surface and near boundary layers. As has been discussed earlier this is very complex for a ferroelectric surface that is screened by counter ions as the inherent REDOX couple (e.g. reduction on a c+ surface) requires the participation of M+ species in solution. The development of a screening layer that is composed of species that are effectively of an opposite charge to those required for the reaction means that any REDOX chemistry must occur through either local tunnelling or a disruption of the local double layer interface . What is interesting is that although there are a number of inhibiting features to the REDOX chemistry at the surface of a ferroelectric there is also a strong driving force—the internal dipole—that enables the reactions to proceed. It has been shown that reduction of highly charged cations with large negative reduction potentials can be achieved on ferroelectric surfaces, an example of which is the reduction of Al3+ to Al0 on LiNbO3.
For surface electrochemistry to take place the standard reaction potential (U0), corresponding to the sum of the energy requirements for the transfer of the participating defect species, must be exceeded to accommodate the fugacities of the products and the overvoltages of the electrodes. For semiconducting electrodes, the Fermi level of the electrode corresponds to its surface potential. Thus, the flat-band potential (Ufb) of the electrode, which is itself sensitive to the pH of the electrolyte at an approximate rate of 0.059 V per pH unit , and to absorption of certain solutes [96, 97] must exceed the required reaction potential and will define the ‘activation’ or ‘switch on’ potential of the reaction.
In general therefore, photolysis may be optimised via manipulation of both the chemical and electrical electrode surface potentials.
Ferroelectric photocurrents and the anomalously high photovoltage
It is well know that for a standard semiconductor the photovoltage cannot exceed the band gap of the semiconductor. In practice several features combine to mean that the photovoltage is lower than the band gap of the semi-conductor, these include parasitic losses, and losses associated with local defects in the semi-conductor. The only exception has been semiconductor heterostructures that have been textured to give a series of p–n junctions which break this fundamental rule. One group of materials that break this rule when treated homogeneously are ferroelectric materials. Anomalously high photovoltages (APV) have been reported from ferroelectric Rochelle salt crystal as early as 1939 by Brady and Moore . It has been shown that APV’s in the order of 40 V can be obtained from BaTiO3  under ideal circumstances which far exceed the band gap of 3.2 eV. The source of the APV comes from an interaction of phase transition, internal dipole and interaction with the incoming radiation.
Later, whilst researching barium titanate and lithium niobate, respectively, both Chynoweth  in 1955 and Chen  in 1967 recorded weak but steady, underlying photocurrents in each ferroelectric crystal when under illumination with no external bias and heated through temperatures rising from below, to well above, their individual Curie temperatures. Open circuit voltages were eventually measured in single domain barium titanate crystal  and found to be fractional relative to the ~3.15 eV band gap . Similarly less than band gap photovoltages were also observed in unpoled ceramic barium titanate . These photovoltages were linked to the presence of potential variations within the crystal, supporting Kanzig’s  in 1955 proposal that there exists temperature independent, space charge or barrier layers at crystal surfaces induced by extrinsic ionic conduction at impurity concentrations nearing 1018 cm−3. According to Kanzig, in ferroelectric crystals fields of 105 or 106 V cm−3 are enough to generate saturation polarisation within this surface layer, creating a potential difference of approximately 1 V between surface and bulk, and the resulting piezoelectric strain is structurally inelastic, prevailing despite bulk phase changes resulting from above Curie temperatures.
In contrast, Glass et al.  reported high photovoltages in doped single crystal LiNbO3 and Brody later reported greater than band gap photovoltages of several hundred volts per centimetre in poled ceramic BaTiO3 + 5 wt% CaTiO3, Pb(Zr0.65, Ti0.35)O3 and other ferroelectric ceramics [107, 108, 109] which were shown to be linearly proportional to both the sample length and the remnant polarisation and were observed only below the Curie temperature. Ionue et al.  recorded photovoltages three times larger than the bandgap for Lead Strontium Zirconate Titanate with directed polarisation fields in opposite directions to the photocurrent.
Hanson et al.  published work showing that the local reactivity on ferroelectric surfaces depends on relative densities of polarisation charges, adsorbed charges and surface states as well as the polarisation itself. Photochemical reduction of silver ions on the surface of a domain-patterned LiNbO3 template resulted in silver decoration of 180° domain walls rather than of positive domains as would be expected. This effect was explained by Hanson et al. as a combination of inhomogeneous distribution of the electric field in the vicinity of the domain wall and low conductivity of the template material. Dunn et al.  have proposed that extreme band bending due to a build up of charge at the surface, can bring the flat-band region close enough to the surface to allow carriers to tunnel through the space charge region and thus reach active reaction sites.
Indeed the interest in ferroelectric photochemistry is now moving away from perovskite materials such as BaTiO3 and PZT and growing to cover a wider spectrum of ferroelectric materials. New areas of study include polymeric ferroelectric systems such as poly(vinylidene fluoride)  and more widely LiNbO3. What is particularly interesting in the case of LiNbO3 is the variation in the details about the published results. When the work relating to perovskite materials is reviewed there are no publications that demonstrate homogenous wire like structures that are being formed on the ferroelectric surface. In all cases the material that formed on the surface has done so from discrete nucleation sites. The reasons for this are that in materials such as PZT and BaTiO3 the spontaneous polarisation is among the largest for ferroelectric materials.
Although ferroelectric conduction can be modelled as wide bandgap semi-conduction from the perspective of generalised charge concentrations and mobilities, it should be noted that influences of the spontaneous polarisation of a ferroelectric function to either counteract or enhance different aspects of the material’s properties. Whilst the bound structural charge produces strong depolarising fields across grains, the free charge gradients are enhanced across the bulk in line with domain polarities.
Surface reactivity is reliant upon properties inherent in the whole system and so the overall effects of charge gradients at boundaries, interfaces and therefore ultimately on reaction surfaces must be considered.
In this review we also focussed on the new developments of patterned surfaces being used for selected chemical reactions and possible future implications. The state of the art approach to the directed assembly of complex nanostructures (oxide substrates, metal nanoparticles and organic and biological molecules) has been discussed which opens new avenues to nanodevice integration.
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