Influence of the Ferroelectric Nature of Lithium Niobate to Drive

Sep 5, 2012 - b was investigated over powders of lithium niobate or lithium niobate doped with ... drive photocatalytic oxidation (PCO) to remove the ...
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Influence of the Ferroelectric Nature of Lithium Niobate to Drive Photocatalytic Dye Decolorization under Artificial Solar Light Matt Stock† and Steve Dunn*,‡ †

Nanotechnology Centre, Cranfield University, MK43 0AL, England Centre for Materials Research, School of Engineering and Materials Science, Queen Mary University of London, E1 4NS, England

‡

S Supporting Information *

ABSTRACT: Photocatalytic decolorization of acid black 1 (a.k.a. amido black 10B) and rhodamine b was investigated over powders of lithium niobate or lithium niobate doped with iron to form ntype material doped or p-type magnesium doped lithium niobate. In all cases, photostimulation was performed using simulated solar illumination. The rate of decolorization of the dye solutions was found to be fastest over p-type material than with the undoped powder, with the n-type proving least effective. The change in rate was attributed to changes in the majority carrier, associated with the dopant altering the ratio of reactive species (holes and electrons). We also show that the surface depolarization field associated with a ferroelectric material alters the surface chemistry by changing the Stern and inner Helmholtz plane due to the interaction of catalyst surface charge and the polar nature of solvated species. The spatial separation of REDOX reactions in ferroelectric powders positively influences the proportion of reactions being driven to completion, enabling a high rate of decolorization despite the wide band gap (3.7 eV) of the catalyst. Finally, our results indicate that the band structure of lithium niobate enables single carrier oxidation of water to occur through an altered reaction mechanism when compared to a typical system such as titania, further enhancing the rate of decolorization. Our results give further evidence that ferroelectric materials provide an interesting alternative to nonferroelectric materials as photocatalysts.

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INTRODUCTION Pollution produced by industrial processes poses a significant risk to the environment. The release of nonfixed dyes, particularly in the textile sector, is an example of this with estimates that up to 15% of total world dye production is eluted in wastewater.1 The dyes cause eutrophication and aesthetic pollution, and many dyes and their degradation products have been shown to be carcinogenic. The problems posed by pollution have led to increasingly stringent international regulations and demand for effective and environmentally friendly methods to deal with organic and inorganic waste. One extensively studied technique to achieve pollution control is heterogeneous photocatalysis. In the generic case, semiconductor particles are suspended in aqueous dye solutions and irradiated with super band gap light. Photogenerated carriers are formed in the semiconductor, electrons in the conduction band, and holes in the valence band. The carriers react with water or oxygen that is adsorbed on the catalyst to drive photocatalytic oxidation (PCO) to remove the dye.2,3 Under sub band gap light, excitation of electrons in adsorbed dye molecules can also drive a separate but less effective mechanism called photoassisted oxidation (PAO) over some semiconductors.3−5 The exact mechanism for the decolorization and eventual mineralization of dye molecules is the subject of debate in the literature. However, it seems likely that, as dyes can be cationic or anionic when in solution and the exact nature of the initiator species is dependent on the environment of the photocatalysis, there will be a variety of pathways by which a dye molecule is degraded during the photocatalytic process.6 © 2012 American Chemical Society

While semiconductor systems are a promising method of waste treatment, several issues limit their wider application: (i) an artificial UV light source is required to drive reactions as only approximately 4% of the solar spectrum can excite carriers in typical semiconductor photocatalysts due to large intrinsic band gaps,7 (ii) a high rate of electron−hole recombination at the surface results in decreased efficiency,8 and (iii) reduction and oxidation processes occur in close proximity at the surface, allowing back reactions to revert a significant proportion of products back to reactants. These problems mean that continued development of heterogeneous photocatalysis requires a new approach. One route to achieve this is to investigate new types of photocatalyst materials of which ferroelectrics are a viable and increasingly well-studied candidate. Ferroelectrics, which are increasingly being considered as wide band gap semiconductors, produce photogenerated carriers under super band gap illumination, behaving as a semiconductor.9 This enables the material to drive photocatalytic reactions.10−16 Ferroelectrics possess a spontaneous polarization or surface polarization of the ferroelectric material below the Curie temperature which has been measured to range between 0.1 and 78 μC cm−2.10 This polarization is stable under a wide range of conditions that include environmental, thermal, and chemically aggressive environments. Using the accepted Received: May 29, 2012 Revised: September 5, 2012 Published: September 5, 2012 20854

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conductors that are in contact with the ferroelectric.27 There is a small but significant body of work investigating the photocatalytic properties of LiNbO3 that indicate the suitability of the material to split water and act as a photocatalyst. The published work in the area divides into two main bodies. The first considers the influence of the domain structure of the catalyst and the influence on the catalytic process.28 The second group of publications has not considered the ferroelectric nature of the material but has demonstrated that water can be split in a photocatalytic process using LiNbO3 as a single-phase material29 or multiphase material.30,31 In all cases, there is agreement that LiNbO3 is able to generate hydrogen and oxygen under suitable illumination from a suitable source. In this work, we investigate powders of lithium niobate that in bulk samples possess a strong polarization (65−78 μC cm−2), a Curie point of 1210 °C,32 and band gap of 3.7 eV33 as a photocatalyst to decolorize dye molecules under simulated solar irradiation. We attribute the rapid rate of dye decolorization to separation of carriers in the material due to the ferroelectric nature of the catalyst and a one-electron, onephoton mechanism to promote photocatalysis due to the band position of the material used.

notation from previous literature, the surface at which the polarization produces a positive potential is termed C+ face, and the surface where the polarization produces a negative potential is termed the C− face. Depolarization fields acting to screen the surface potential draw electrons to the positive C+ face and holes to the negative C− face.17 This forms space charge regions at both surfaces, inducing downward band bending at the C+ face and upward bending at the C− face. It should be noted that this band bending is largely irrelevant of the environment in which the ferroelectric material is placed. The effect of the polarization in ferroelectric materials has been shown to produce properties that may be beneficial for photocatalytic processes, and recently the benefits of ferroelectric materials have been observed in mechano or modified mechano catalytic processes termed pyroelectric catalysis.18 In this paper, the catalyst first produces OH• by oxidizing water and the super oxide anion by reducing oxygen, these species subsequently drive further chemical processes. Such a process is dependent, as are the interesting features of photocatalysis over ferroelectric materials, on the separation of electrons and holes at the C+ and C− face due to the dipole of the ferroelectric. This can increase the lifetime of generated carriers, with evidence of extended carrier lifetime being provided by the long photoluminescence of up to 9 μs in lithium niobate.19 The separation of carriers also causes REDOX reactions at ferroelectric surfaces to be spatially separated. It has been shown that the length scales for spatial separation range from nanometers to millimeters and that there is no appreciable scaling impact on the photochemistry.20 In other words, a small pattern of polarization behaves in a manner similar to a large pattern of polarization. These patterns of polarization are defined as ferroelectric domains. This domain pattern causes reduction to occur at the C+ face and oxidation at the C− face, which can help suppress the rate of backward reactions.21,22 Other potentially useful properties include: (i) The charge at the surface modifies the bond angles of adsorbed species. For example, linear carbon dioxide has been found to have its bonds bent when adsorbed by lithium niobate23 or niobium-doped PZT.24 This will delocalize electron density and alter the reactivity of any adsorbed species. (ii) Species chemisorbed at the surface of ferroelectric materials are bound for an order of magnitude longer compared to nonpolar systems. (iii) Polarized barium titanate thin films adsorb more gaseous ethanol molecules at both the C+ and C− face compared to when the film had been depolarized.25 To drive heterogeneous photocatalysis, the photocatalyst material is typically prepared as powder due to the increased surface area this provides. To date, the majority of work on ferroelectric photocatalysis has been undertaken on thin film or sintered compacts of ferroelectric materials. However, when tested for a variety of materials properties and applications, ferroelectrics in powdered form have been shown to display properties similar to those of bulk samples. Recently, it has been shown that barium titanate with particles sizes of 1−5 μm can drive spatially separated REDOX reactions at the surface in a manner identical to the bulk material.26 Heterostructured powder photocatalysts have also been prepared comprising a nanostructured titania shell surrounding microcrystalline barium titanate cores. Photocatalytic hydrogen production was shown to be greater than over either nanostructured titania or barium titanate alone and has generated an increasing interest and awareness of the interaction with the dipole inherent in a ferroelectric material and traditional semi-

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EXPERIMENTAL SECTION Titanium dioxide 325 mesh powder was purchased from Sigma Aldrich and used as provided. Lithium niobate and 5% magnesium-doped lithium niobate were provided by MTI Corporation.34 Iron-doped lithium niobate (0.05%) was provided by Photox Optical Systems.35 Powders were prepared by grinding using a mortar and pestle. Rhodamine b (99.99%) and acid black 1 (99.99%) were provided by Sigma Aldrich. The dyes were used as provided with no further purification. Photoillumination experiments were carried out using a solar simulator (1000 W full spectrum Newport Solar Simulator with a 4 × 4 in. beam size) with an output equivalent to 1.4 suns (top of reaction vessel) and 1.0 suns (bottom of reaction vessel) at air mass 1.5 to provide visible light. The emission spectrum of the light source is shown in Figure 1.

Figure 1. Emisson spectrum of the solar simulator used in experiments. The vertical line separates super and sub band gap light.

A Pyrex beaker (250 mL) was used to house the reaction and loaded with dye solution (40 mL, 10 ppm), lithium niobate powder (1.5 g L−1), and a stirrer bar. A quartz disk was used as a lid, allowing transmission of incident light into the vessel. The suspensions were stirred for 30 min under dark conditions prior to illumination. At predetermined intervals samples (2 mL) were collected. 20855

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Suspended powder was removed from liquid samples using a centurion scientific C2 centrifuge. A lambda 7 spectrophotometer was used to collect absorption spectra. The peak absorption of the dyes rhodamine b (525 nm) and acid black 1 (620 nm) were recorded and used to plot degradation using eq 1. rate of decolorization = C /C0

Table 1. Adsorption of Acid Black 1 and Rhodamine b by Lithium Niobate and Titanium Dioxide Powders under Dark Conditionsa

(1)

Piezoforce microscopy was performed using a modified DI3000 scanning probe microscope and Pt/Ir coated tips.36−38

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powder

spontaneous polarization (μC cm−2)

surface area (m2 g−1)

acid black 1b

rhodamine bb

LiNbO3 Fe:LiNbO3 Mg:LiNbO3 TiO2

78 72 65 −

1.4 0.8 1.3 10

10.2 14.1 15.2 11.5

10.9 16.5 16.3 12.7

a Dye removal % is not scaled for surface area. b% of dye removed during equilibrium phase of 30 minutes dark absorption.

RESULTS AND DISCUSSION Samples of dye exposed to solar simulated light in the absence of any photocatalyst or samples of dye with photocatalyst suspended in them in the absence of solar simulated light did not show any appreciable decolorization, ruling out reactions occurring by mechano catalytic or direct reactions with light, such as photolysis of the dye molecules in the presence of incoming light. This control experiment indicated that the decolorization of the dye molecules was as a result of the photocatalytic interaction between the catalyst and dye as indicated in previous studies showing water splitting for LiNbO3. In all cases, the systems were held at thermal and optical equilibrium. It was calculated that approximately 1.4 mW cm−2 of super band gap light was provided to lithium niobate powder (band gap 3.7 eV) when under illumination, a complete incident energy of 1.4 × 10−3 W cm−2 from the solar simulator into the reaction vessel. Examples of typical SEM micrographs of the powder as used are available in the Supporting Information, Figure S1. An example of a typical XRD pattern is indexed in Figure S2, available in the Supporting Information. Piezo-force microscopy (PFM) was performed on grains of powdered lithium niobate, magensium-doped lithium niobate, and iron-doped lithium niobate that had been deposited onto a glass slide with carbon tape. In all cases, the powders exhibited a similar domain structure that had features ranging from domains of 10's of nanometers to large 100 nm domain patterns. It was not possible to discern any difference in the domain structure of the lithium niobate powders from the PFM images. Ferroelectric materials use two distinct mechanisms to screen surface charge associated with domains and the remnant polarization found at the surface: internal using mobile internal carriers and external using the dipole or charge of chemisorbed molecules/ions in the Stern layer.39,40 Lithium niobate is wellknown to use a dominant external screening process due to the low number of carriers present in the crystal41 and as such can strongly influence the structure and nature of the Stern layer at interfaces. Samples of the catalyst were suspended in dye solutions under dark conditions. Adsorption/desorption proccesses reached equilibrium after 30 min, and the amount of dye adsorbed was determined (Table 1) using UV−vis analysis. The titanium dioxide powder has a larger surface area (10 m2 g−1)42 when compared to the measured value of the undoped lithium niobate powder (1.4 m2 g−1). However, the titanium dioxide powder adsorbed only 1 or 2% more dye from solution during the equilibrium phase than the undoped ferroelectric material. This provides evidence that the polar nature of lithium niobate powder that is associated with the ferroelectric depolarization field of the material has enabled a higher loading of dye molecules on the catalyst per unit surface area than for a

traditional nonpolar material. In effect the dye has been more strongly absorbed on the ferroelectric catalyst surface. Adsorption of dye molecules at the surface of n-type lithium niobate43 and p-type lithium niobate powder44,45 was also investigated. The doped lithium niobate materials both had surface areas similar to the undoped lithium niobate. However, the doped powders adsorbed measurably more dye molecules than the undoped lithium niobate or the titania. As there are no significant changes in morphology or domain structures and the remnant polarization, which indicates the strength of the surface charage, is similar for all samples, the change in adsorption must be attributed to the effect of doping upon the structure of the double layer. The greater number of free carriers in the doped lithium niobate powder will cause internal screening of the surface potential to increase, resulting in a reduction in external screening. It should be noted that although there are more carriers in the doped lithium niobate it is still a highly insulating material as evidenced by the ability to generate a ferroelectric hysteresis loop and measure the remnant polarization of the material found in many previous publications and text books. In addition to the doping altering the screening, there will also be changes to the scattering of carriers in the system as mobile species will encounter possible scattering and recombination centers in the LiNbO3. The scattering of carriers will alter the availability of charge carriers at the surface and will have an influence on the surface photochemistry. The reduction in external screening might be expected to cause fewer dye molecules to be adsorbed. However, we show that a higher amount of dye molecules4 to 5% moreare absorbed for the doped materials. This indicates that there is a change in the composition of the Stern layer as in all cases the surface of the ferroelectric must be completely screened. It is well-known that a tight Stern layer forms on the surface of ferroelectric material when in an aqueous environment,46,47 and we propose that a well-bound Stern layer forms in our system. Molecules of acid black 1 or rhodamine b are significantly larger than the water molecules which make up the bulk of the dye solutions. In the case of the doped materials, this Stern layer consists of a significant proportion of dye molecules. For the undoped catalyst, the Stern layer consists of smaller water molecules which are able to provide the necessary higher level of screening through the molecular dipole or by forming positive (OH3+) or negative (OH−) hydroxyl species after dissociative adsorption. These small ions are able to pack densely on the surface and provide the necessary screening of surface charge. When the larger dye molecules are used to screen the surface charge, they are unable to pack so densely on the surface due to steric constraints and so are unable to 20856

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provide the level of screening required when internal screening is weak due to a low number of available carriers. Traditional photocatalysts such as titanium dioxide and zinc oxide are able to drive dye decolorization under sub band gap illumination via photoassisted oxidation (PAO). For PAO reactions to occur, the conduction band potential of the photocatalyst must be below the LUMO of the dye molecule. As lithium niobate’s conduction band potential lies above the LUMO of rhodamine b and acid black 1 (Figure 2), PAO

Figure 2. Band potentials of lithium niobate vs NHE. The potentials of the HOMO and LUMO of acid black 1 and rhodamine b are also shown.

reactions are unable to take place. This was confirmed by illuminating suspensions of lithium niobate in rhodamine b solution with an incandescent bulb emitting no light exceeding the lithium niobate band gap. After 48 h illumination the dye solution had not been detectably decolorized. The decolorization of dye solutions under simulated solar illumination decolorization over lithium niobate powder was, therefore, driven by photogenerated carriers formed by absorption of super band gap photons in the lithium niobate. Under solar simulated illumination, suspensions of doped and undoped lithium niobate powders drove decolorization of both acid black 1 and rhodamine b (Figure 3). Photocatalytic degradation of dyes typically follows first-order kinetics; however, over p-type lithium niobate powder and undoped lithium niobate powder, the rate of reactions does not appear to follow this scheme. After illumination was commenced the reaction rate initially increased with time. This may be a result of carriers excited in the powders internally screening the surface charge. This would cause dye molecules adsorbed in the stern layer by the surface charge to be released into solution, increasing the dye concentration. As a result the degradation reaction appears slower until internal screening has reached equilibrium. The rate of decolorization increased over p-type material and decreased using n-type material in comparison to undoped lithium niobate. Acid black 1 was decolorized faster than rhodamine b by n-type powder or lithium niobate powder. The p-type powder decolorized rhodamine b at a faster rate than acid black 1 (Table 2). It should be noted that contrary to a traditional semiconductor such as titania the REDOX couple on lithium niobate is spatially separated. This implies that reduction and oxidation are occurring independently and that the full REDOX couple can proceed via a number of pathways that include all species found in the double layer. The rate of decolorization of solutions of acid black 1 or rhodamine b increased over p-type material and decreased over n-type material in comparison to lithuim niobate powder (Figure 3). Evidence has shown the decolorization rate over

Figure 3. Decolorization curves of (a) rhodamine b and (b) acid black 1 under simulated solar light using lithium niobate, iron-doped lithium niobate, or magensium-doped lithium niobate powder as the catalyst.

Table 2. Decolorization of acid black 1 and Rhodamine b Solutions after 60 min Simulated Solar Illumination over Lithium Niobate Powders powder

acid black 1

rhodamine b

Fe:LiNbO3 LiNbO3 Mg:LiNbO3

6.5 17.2 33.1

0.0 12.0 34.2

semiconductor systems to be related to the absorption of dye at the photocatalysts surface48,49 and the number of available photoexcited carriers able to perform chemistry. As discussed previously, both p-type and n-type lithium niobate powder adsorbed more acid black 1 and rhodamine b molecules than undoped material. The quantity of dye adsorbed by p-type and n-type powder was found to be similar (Table 1). This suggests that adsorption and composition of the double layer was not the cause of the difference in decolorization rates over the catalyst powders. If this was the case then we would have expected a strong correlation between absorption of dye and rate of decolorization which was not evident. Decolorization reactions over semiconductor materials are proportional to the amount of carriers formed. In our experiments, the emitted spectrum of the simulated solar light source was equivalent to AM 1.5, providing lithium niobate with 1.4 × 10−3 W cm−2 of super band gap light . UV− vis analysis shows that for all catalysts the loading was sufficient to absorb all super band gap light. Doping with iron increases the absorption of lithium niobate as the material absorbs nearinfrared and red visible light in addition to super band gap light to excite electrons into the conduction band. Details of the 20857

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A majority carrier of electrons increases the proportion of reactions at C+ sites, while a majority carrier of holes increases the proportion of reactions occurring at C− sites where selective REDOX chemistry is driven. The trend of fastest decolorizaiton over p-type then undoped and finally n-type indicates reactions proceed most rapidly at the C− sites where oxidation is forced to occur. The adsorption of reactive species at the surface of lithium niobate is affected by the surface potential. At C+ sites anions or the negative dipole of a molecule are found most tightly bound to the surface, enabling screening of the surface charge, while the opposite occurs on the C− surfaces (Figure 5). Differences in the orientation of

absorption and optical properties of the materials can be found in Peithmann et al.50 Iron-doped lithium niobate has a visible red color, while pure or magnessium doped is transparent to visible light. Although the iron-doped lithium niobate will generate more active carriers, due to enhanced absorption, we find that the rate at which decolorization occurred decreased (Figure 3) when compared to the other catalysts used. This shows that the change in decolorization rates over the doped powders was not a result of a change in the number of photogenerated carriers produced. Instead, the majority carrier type, electrons in the ntype material and holes in the p-type material, is influencing the rate of decolorization. Another factor infulencing the reaction rate over the p-type and n-type material in comparison to lithium niobate may be the effect of dopants upon impurity scattering and, hence, the rate of recombinaton of excited carriers. As the rate of reaction over p-type material increased compared to undoped powder and the rate of reaction decreased over n-type material in comparison to undoped powder, this suggests doping with magnesium or iron has different effects upon carrier recombinaton or that the effect of dopants upon impurity scattering in these cases was less significant than the effect upon the majority carrier type in determining the activity of the powders. Photoexcited holes oxidize water or the hydroxyl ion to form the hydroxyl radical, and electrons reduce oxygen to form the superoxide anion radical (Figure 4). Reactions driven by hole

Figure 5. Orientation of the superoxide anion radical and hydroxyl radical formed at the C+ or C− face of lithium niobate.

reactive species alter the reactivity of a compound. A combination of changing the majority carrier type and hence the predisposition to oxidation or reduction and the compostion and orientation of molecules at the reactive site is responsible for the changes in rate of decolorization over the three lithium niobate powders tested. Acid black 1 was decolorized at a faster rate than rhodamine b by lithium niobate and doped lithium niobate powders (Table 2), except in the case of the Mg:LiNbO3 samples where there differences in decolorization rate fell within experimental tolerances. Comparison of acid black 1 and rhodamine b after 60 min illumination showed that, compared to undoped powder, the difference between decolorization of the two dyes was greatest using the n-type powder. This suggests the decolorization reactions over p-type material are less structurally selective than undoped material, while selectivity of reactions increased over n-type material. This relationship between the rate of decolorization and catalyst structure further indicates that the predominant reaction pathway is through an initial oxidation step as the reactive species that drive the reaction most quickly are associated with the highest concentration of photoexcited holes. We should also consider that rhodmine b forms a postive ion and acid black 1 a negative ion. Therefore, we would expect the extent and nature of the decoration of the surface of the catalyst with dye to differ given that the solvated dyes will be attracted to different regions of the catalyst. We show that in all, like for like, cases the decolorization of acid black is quicker than rhodamnie b. This indicates that there is a significant step in forming an oxidized initiator from the C− surface, which in the case of the rhodamine b will be decorated with dye. This will have the effect of reducing the number of available photogenerated holes to initiate oxidation and has a damaging influence on the complete REDOX cycle. The rhadamine b dye will selectively absorb incident photons on the C− regions reducing the number of carriers formed in the lithium niobate. As we have shown that carriers abosorbed in the dye will not initiate a reaction, these absorbed photons are effectively lost. Investigation of photoreduction of silver ions at the surface of lithium niobate has shown that when a high concetration of reactant and high intensity of super band gap light is used

Figure 4. Proposed mechansim or formation of reactive species at the surface of LiNbO3. Electrons reduce oxygen to form the superoxide anion radical at the C+ face, and hole carriers oxidize water or hydroxyl ions to form the hydroxyl radical at the C− face.

carriers produce a greater rate of decolorization than those driven by electrons because (i) the amount of oxygen adsorbed in the aqueous solutions will be low due to the low solubility of oxygen at the temperatues of solution found during the experiment, typically in the range of 50−60 °C, meaning availability of oxygen molecules in the double layer would be limited. Water molecules and hydroxyl species are abundant and (ii) the super oxide anion radical and the hydroxyl radical will decolorize dye molecules via a different reaction pathway. 20858

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reactions occur preferentially at the boundary of C+ domains.15 The absortion of photons by the catalyst is important to drive the reactions forward, and with selective absorption of the dye on the surface of the catalyst interfering with the path of photons into the catalyst a variation in performance of decolorization with the different dyes due to the binding to the surface of the catalyst is seen.

(15) Inoue, Y.; Niiyama, T.; Asai, Y.; Sato, K. J. Chem. Soc., Chem. Commun. 1992, 579−580. (16) Dunn, S.; Jones, P. M.; Gallardo, D. E. J. Am. Chem. Soc. 2007, 129, 8724−8728. (17) Sun, Y.; Eller, B. S.; Nemanich, R. J. J. Appl. Phys. 2011, 110, 084303. (18) Gutmann, E.; Benke, A.; Gerth, K.; Böttcher, H.; Mehner, E.; Klein, C.; Krause-Buchholz, U.; Bergmann, U.; Pompe, W.; Meyer, D. C. J. Phys. Chem. C 2012, 116, 5383−5393. (19) Harhira, A.; Guilbert, L.; Bourson, P.; Rinnert, H. Phys. Status Solidi C 2007, 4, 926−929. (20) Kalinin, S. V.; Bonnell, D. A.; Alvarez, T.; Lei, X.; Hu, Z.; Ferris, J. H.; Zhang, Q.; Dunn, S. Nano Lett. 2002, 2, 589−593. (21) Giocondi, J. L.; Rohrer, G. S. J. Phys. Chem. B 2001, 105, 8275− 8277. (22) Giocondi, J. L.; Rohrer, G. S. Chem. Mater. 2001, 13, 241−242. (23) Cabrera, A. L.; Vargas, F.; Albers, J. J. Surf. Sci. 1995, 336, 280− 286. (24) Ramos-Moore, E.; Lederman, D.; Cabrera, A. L. Appl. Surf. Sci. 2011, 258, 1181−1183. (25) Li, D.; Zhao, M. H.; Garra, J.; Kolpak, A. M.; Rappe, A. M.; Bonnell, D. A.; Vohs, J. M. Nat. Mater. 2008, 7, 473−477. (26) Giocondi, J. L.; Rohrer, G. S. Top. Catal. 2008, 49, 18−23. (27) Li, L.; Rohrer, G. S.; Salvador, P. A. J. Am. Ceram. Soc. 2012, 95, 1414−1420. (28) Park, Y. -.; Kim, J. -.; Yang, W. Surf. Interface Anal. 2012, 44, 759−762. (29) Zielińska, B.; Borowiak-Palen, E.; Kalenzuk, R. J. J. Phys. Chem. Solids 2008, 69, 236−242. (30) Saito, K.; Koga, K.; Kudo, A. Dalton Trans. 2011, 40, 3909− 3913. (31) Chen, Q.; Yang, Y.; Yin, Z.; Li, J.; Liang, S. Trans. Nonferrous Met. Soc. China 2004, 14, 798−801. (32) Karapetyan, K.; Kteyan, A.; Vardanyan, R. Solid State Commun. 2006, 140, 474−475, 476. (33) Dhar, A.; Mansingh, A. J. Appl. Phys. 1990, 68, 5804−5809. (34) MTI Corporation, 860 South 19th Street, Richmond, CA 94804, USA. (35) Photox Optical Systems, 88 Carsick Hill Road, Sheffield S10 3LX, United Kingdom. (36) Marshall, J. M.; Dunn, S.; Whatmore, R. W. Integr. Ferroelectr. 2004, 61, 223−230. (37) Dunn, S. Integr. Ferroelectr. 2003, 59, 1505−1512. (38) Dunn, S.; Shaw, C. P.; Huang, Z.; Whatmore, R. W. Nanotechnology 2002, 13, 456−459. (39) Lines, M. E.; Glass, A. M. Principles and Applications of Ferroelectrics and Related Materials; Clarendon Press: Oxford, 2001. (40) Eng, L. M. Nanotechnology 1999, 10, 405−411. (41) Hanson, J. N.; Rodriguez, B. J.; Nemanich, R. J.; Gruverman, A. Nanotechnology 2006, 17, 4946−4949. (42) Velegraki, T.; Kalogerakis, M.; Charalabaki, M.; Poulious, I. Proc. 9th Int. Conf. Environ. Sci. Technol. 2005, A−1163. (43) Orlowski, R.; Krätzig, E. Solid State Commun. 1978, 27, 1351− 1354. (44) Volk, T.; Wöhlecke, M. Lithium Niobate: Defects, Photorefraction and Ferroelectric Switching; Springer: Berlin, 2008. (45) Xu, J.; Zhang, G.; Li, F.; Zhang, X.; Sun, Q.; Liu, S.; Song, F.; Kong, Y.; Chen, X.; Qiao, H.; Yao, J.; Lijuan, Z. Opt. Lett. 2000, 25, 129−131. (46) Dunn, S.; Cullen, D.; Abad-Garcia, E.; Bertoni, C.; Carter, R.; Howorth, D.; Whatmore, R. W. Appl. Phys. Lett. 2004, 85, 3537−3539. (47) Jones, P. M.; Dunn, S. J. Phys. D 2009, 42. (48) Tanaka, K.; Padermpole, K.; Hisanaga, T. Water Res. 2000, 34, 327−333. (49) Chun, H.; Yizhong, W.; Hongxiao, T. Appl. Catal., B 2001, 35, 95. (50) Peithmann, K.; Wiebrock, A.; Buse, K. Appl. Phys. B: Lasers Opt. 1999, 68, 777.

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CONCLUSIONS We provide evidence that lithium niobate is a reactive photocatalyst under simulated solar irradiation and that the majority carrier and decoration of the surface of the catalyst with the dye influence the overall REDOX performance of the system. By relating the change in majority carrier concentration we show that the reaction is dependent on a limiting oxidative step and that increasing the number of photoexcited holes increases the decolorization rate of the dyes. A relationship between the rate of decolorization and the Stern layer and the influence on the molecules held within the Stern layer is also shown with all the factors being related to the ferroelectric nature of the lithium niobate.

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ASSOCIATED CONTENT

S Supporting Information *

Showing the XRD pattern of a typical sample and micrographs taken by SEM of the samples used. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support from the EPSRC is acknowledged. This work was funded by a DTA supported studentship at Cranfield University

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REFERENCES

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dx.doi.org/10.1021/jp305217z | J. Phys. Chem. C 2012, 116, 20854−20859