ITO Electrode in a

J. Phys. Chem. C 2008, 112, 9937–9942

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Roles of MgO Coating Layer on Mesoporous TiO2/ITO Electrode in a Photoelectrochemical Cell for Water Splitting Shin-Tae Bae,† Hyunho Shin,*,‡ Jin Young Kim,§ Hyun Suk Jung,| and Kug Sun Hong† School of Materials Science and Engineering, Seoul National UniVersity, Shillim-dong, San 56-1, Gwanak-gu, Seoul, 151-744, Korea, Department of Ceramic Engineering, Kangnung National UniVersity, Jibyun-dong, Kangnung, Gangwon-do 210-702, Korea, Chemical and Bioscience Center, National Renewable Energy Laboratory, Golden, Colorado 80401, and School of AdVanced Materials Engineering, Kookmin UniVersity, Jungneung-dong, Sungbuk-gu, Seoul 136-702, Korea ReceiVed: March 12, 2008; ReVised Manuscript ReceiVed: April 11, 2008

A MgO layer was spin coated on a mesoporous TiO2/ITO electrode for application to the working electrode of a water splitting photoelectrochemical cell. The role of MgO has two faces: (1) blocking the back electron transfer from TiO2 to electrolyte (positive influence) and (2) blocking the hole escape from TiO2 to electrolyte (negative influence). The competition between the positive and negative influences of the MgO layer results in maximal performance of the cell at an optimum effective thickness of the MgO layer. Further, the mechanism of light interference of the electrolyte/MgO/TiO2 system also contributes to the performance maximization with regard to the MgO layer thickness. Under a bias voltage, however, the role of blocking the hole escape prevails the blocking of the back electron transfer as well as the influence of the light interference; thereby the increase of MgO thickness diminishes the performance of the cell. 1. Introduction Hydrogen production via photocatalytic water splitting has received much attention due to not only its scientific interest, but also its potential industrial benefits as an energy source. In particular, n-type TiO2 has been applied extensively as a photocatalyst1–4 since the first demonstration by Fujishima and Honda.5 Hydrogen production using photoelectrochemical cells (PECs) composed of working (e.g., TiO2/ITO) and counter (e.g., Pt) electrodes has advantages over the use of photocatalytic nanopowders (e.g., TiO2) in that PECs do not need any oxygen scavengers since the hydrogen molecules are generated at the counter electrode, which is separated from the working electrode. One more important benefit of the PECs is that, in cases where a potential difference exists between two electrodes, hydrogen production can be enhanced as the electrons and holes are easily separated by the applied field between the electrodes. One of the most important technical issues in PEC-based photocatalytic water splitting is the design of the working electrode. In this regard, coating of TiO2 nanoparticles with wide bandgap materials such as MgO, CaCO3, and ZnO has the potential of improving cell efficiency because the overcoating materials retard the back electron transfer from TiO2 to electrolyte (retardation of the charge recombination) when TiO2 electrode is applied to dyesensitized solar cells (DSSCs).6–17 Similar phenomenon is anticipated to take place in photocatalytic water splitting cells as well, but systematic investigation with the analysis of the roles of the overcoating materials has been sparse. Here we report the detailed roles of the MgO coating layer on mesoporous TiO2/ITO to improve the performance of water splitting photoelectrochemical * Corresponding author. Tel: +82-33-640-2484. Fax: +82-33-640-2244. E-mail: [email protected]. † Seoul National University. ‡ Kangnung National University. § National Renewable Energy Laboratory. | Kookmin University.

cells. The MgO has been selected as it is one of the most intensively investigated coating materials in existing works for DSSCs.11–17 2. Experimental Section TiO2 working electrode was first prepared by the spin-coating method. To prepare the coating slurry, the mixture of commercial TiO2 nanoparticles (average diameter of 21 nm, P25, Degussa, Germany), acetylacetone (0.3 cc, Aldrich, USA), absolute ethanol (10 cc, Hayman Ltd., UK) and distilled water (10 cc) was ball-milled for 12 h. The transparent conducting glass substrate (indium-tin oxide, Samsung SDI, Korea) was spin-coated by the prepared TiO2 slurry at 3000 rpm for 30 s. After drying at 150 °C for 5 min, the coated electrodes were heat treated at 450 °C for 1 h in air to remove organic matter. MgO sol was then prepared for the coating layer. Mgmethoxide (6 wt % solution in methanol, Aldrich, USA) was selected as a precursor material. Dry methanol (99.9%, Samchun, Korea) and diethanolamine (98.5%, Aldrich, USA) were used as solvent and stabilizer, respectively.18 The TiO2/ ITO/glass substrate was spin-coated by the prepared MgO sol (at 3000 rpm for 30 s), followed by thermal treatment at 450 °C for 30 min in air. A series of MgO-coated TiO2/ITO/ glass substrate electrodes with varying doses of MgO spin coating and the associated thermal treatment was prepared (the number of spin coating and the thermal treatment was varied from 0 to 3). The area of the MgO/TiO2 coating layer on ITO/glass substrate was approximately 2 × 2 cm2. A Pt-plate (0.5 × 4 cm2) and the 0.1 M KOH aqueous solution were used as the counter electrode and the electrolyte, respectively. The reactor with fused silica window was gastight, and the two electrodes (working and counter) were separated by a Nafion membrane (Nafion115, Fuel Cell Store, USA). The concentration ratio of [Mg]/[Ti] in the MgO-coated TiO2/ ITO/glass electrode was quantified by induction coupled plasma (Model Optima 4300DV, Perkin-Elmer, USA). The crystallinity

10.1021/jp8021562 CCC: $40.75  2008 American Chemical Society Published on Web 06/07/2008

9938 J. Phys. Chem. C, Vol. 112, No. 26, 2008

Figure 1. XRD patterns from (a) sol-derived MgO after drying and annealing at 450 °C for 30 min in air and (b) MgO reference powder.

Bae et al.

Figure 3. Change in amount of hydrogen production as a function of cell operation time up to 10 h for varying [Mg]/[Ti] ratio of the MgOcoated TiO2/ITO electrode.

(Figure 1). For this purpose, the MgO sol used to spin coat the TiO2 nanoparticle layer on the ITO/glass substrate was separately dried and thermally annealed under the same condition as the fabrication process of the MgO-coated TiO2/ITO/glass structure. For XRD characterization, 20 wt % of silicon powder was mixed with the sample as an internal standard. Included in Figure 1 is the XRD pattern from the commercial reference MgO powder (99.9%, Aldrich, USA). The integrated peak intensity ratio, IMgO/ISi at the maximum peak intensity positions of respective phase is 3.70 for the MgO reference powder in contrast with 2.44 for the specimen derived from the sol. Using this approach, the crystallinity of the sol-derived MgO is only 66% (2.44/3.70 × 100) with reference to the MgO reference; the rest of MgO is amorphous (34%). The crystallite size of the sol-derived MgO is calculated to be fairly fine, i.e., 8.2 nm based on the Scherrer equation19 Figure 2. Microstructures of MgO-coated TiO2/ITO electrodes on glass substrate after annealing at 450 °C for 1 h in air. Number of spin coating was (a) 0, (b) 1, (c) 2, and (d) 3.

and particle size of the MgO product derived from the MgO sol itself was characterized by X-ray diffraction (Model M18XHF-SRA, MACSCIENCE, Tokyo, Japan) using KR radiation at an operation power of 8 kW. The cross sectional view of the fabricated TiO2 nanoparticle/ITO electrode on the glass substrate was observed using a field-emission scanning electron microscope (Model JSM-6330F, JEOL, Japan). The amounts of hydrogen production under UV-illumination from a 450 W high-pressure mercury lamp (50 mW/ cm2, peak wavelength ∼365 nm, Korea Ultra Violet Co., Korea) were measured using a gas chromatograph (Model DS6200, Donam Instruments, Inc., Korea) equipped with a thermal conductivity detector (TCD). Photocurrents were measured using a picoammeter (Model 6487 Picoammeter/ Voltage Source, Keithley Instruments, Inc., USA) under UVillumination from a 450 W high-pressure mercury lamp. Impedance of the PEC cells by employing standard threeelectrode configuration under UV-illumination from a 450 W high-pressure mercury lamp and reversely biased opencircuit voltage were also measured by a potentiostat (Model CHI 608C, CH Instruments, USA). 3. Results and Discussion 3.1. Characteristics of MgO-Coated TiO2/ITO Electrode. The crystallinity and particle size of the MgO product itself derived from the MgO sol were first characterized by XRD

D)

0.9λ β cos θ

(1)

where D is the effective particle size, λ is the wavelength of X-ray, β is the peak broadening (full width at half-maximum) at the diffraction angle. The cross sectional view of the fabricated TiO2 nanoparticle/ ITO electrode on the glass substrate is shown in Figure 2 together with the MgO-coated TiO2 nanoparticle/ITO electrode. The ITO layer is approximately 180 nm in thickness and the TiO2 mesoporous layer is 400 nm (Figure 2a). When the MgO layer is spin coated once (Figure 2b), no apparent change in the morphology of the TiO2 mesoporous layer is observed, indicating that most of the MgO sol in the electrode was soaked into the mesoporous TiO2 layer to overcoat the individual TiO2 nanoparticles. When spin coated twice (Figure 2c), a less porous region in the upper portion of the formerly TiO2 mesoporous layer is observed: this region is believed to be rich in MgO. For the case of spin coating for three times (Figure 2d), the less porous region believed to be rich in MgO is more remarkable than the case when spin coated twice (Figure 3c). Finally, the concentration ratio of [Mg]/[Ti] in the prepared MgO-coated TiO2/ITO/glass electrode by spin coating varying times was quantified by ICP with reference to the concentration of indium. [Mg]/[Ti] ratios in the prepared electrodes were, 0, 0.19, 0.35, and 0.44, for the number of coating of 0, 1, 2, and 3, respectively. Indicates that the first spin coating increases the amount of MgO in the electrode ([Mg]/[Ti] ) 0.19) most efficiently, whereas the amount of MgO coating found purely at the second ([Mg]/[Ti] ) 0.16) and the third ([Mg]/[Ti] ) 0.09) spin coating processes decreases. This result is consistent

MgO Coating Layer

J. Phys. Chem. C, Vol. 112, No. 26, 2008 9939

Figure 4. Change in photocurrent density and amount of hydrogen production as a function of [Mg]/[Ti] ratio: (a) under no bias voltage and (b) under the presence of bias voltage of 0.5 V.

with the microstructural observation in that the MgO sol is most efficiently soaked to the mesoporous TiO2 layer in the first spin coating (Figure 2b) and the degree of soaking decreases as the number of spin coating increases (Figure 2c,d). 3.2. Influence of MgO-Coating on the Cell Performance. The prepared (MgO)/TiO2/ITO/glass specimens with such microstructures (Figure 2) were applied as the working electrode of the photoelectrochemical cell. The change in hydrogen production amount as a function of the operation time of the cell is shown in Figure 3 for varying [Mg]/[Ti] ratio (varying number of spin coating). No bias voltage was applied. Up to 10 h, MgO coating improves the hydrogen production regardless of the [Mg]/[Ti] ratio (number of spin coating). However, note that the cell performance is the best of all when [Mg]/[Ti] )0.35 (spin coated twice). For the comparison of the cell performance as a function of [Mg]/[Ti] ratio, photocurrent density and the amount of hydrogen production at the operation time of 10 h are shown in Figure 4a for the case when no bias voltage is applied. Photocurrent density is maximal when [Mg]/[Ti] ) 0.35 (MgO is coated twice), which is consistent with the [Mg]/[Ti] ratio where the amount of hydrogen production is maximal. However, when the bias voltage of 0.5 V is applied (Figure 4b), both the photocurrent density and the amount of produced hydrogen decrease monotonically as the number of MgO spin coating increases. 3.3. Roles of MgO Coating under No Bias Voltage. On the basis of the observation of maximal photocurrent density at an optimal dose of MgO overcoating material (Figure 4a), sources influencing the photocurrent density have been analyzed hereinafter. The photocurrent-influencing-mechanisms by the application of the MgO overcoating layer include: (1) the change in the impedance of TiO2/electrolyte interface, (2) the change in charge carrier tunneling through the MgO layer, and (3) the change in photon density arriving at TiO2 by passing through the MgO layer. As a first step to uncover the sources influencing the photocurrent by the application of the MgO layer, internal resistance (impedance) of the photoelectrochemical cell has been investigated; the result of this analysis is shown in Figure 5. The frequency at the top of the semicircle in the Nyquist plot is the characteristic frequency ω defined as ω ) (RC)-1, where R and C are resistance and capacitance at an electrochemical interface, respectively. In the PEC with no MgO coating ([Mg]/ [Ti])0), ωa (∼104 Hz), ωb (1.7 Hz), and ωc (0.17 Hz) are noted, although the half-circle for ω1 is not apparent in the scale adopted in Figure 5.

Figure 5. Impedance spectra of the cells using the MgO-coated TiO2/ ITO electrode for varying [Mg]/[Ti] ratio. The ordinate and abscissa are the imaginary and real parts of the impedance, respectively.

Note that in DSSCs, the characteristic frequencies of 103-105 (ω1), 1-103 (ω2), and 0.1-1 (ω3) Hz, from left to right, are related to the charge transport at the ITO/TiO2 or Pt/electrolyte interface (Z1), TiO2/electrolyte interface (Z2), and the Nernstian diffusion in the electrolyte (Z3), respectively.20,21 In DSSCs, adsorbed dye molecules additionally exist and electrolyte is different from the water-splitting PECs. However, note that the TiO2/dye or TiO2-MgO/dye interfacial resistance is not detected in the impedance analysis of a DSSCs. Further, the alteration of the characteristic frequency ω3 of the Nernstian diffusion by the change in electrode type is not more than the order of difference. Indeed, the measured values of ωa (∼104 Hz), ωb (1.7 Hz), and ωc (0.17 Hz) in the current work are well matched to ω1 (103-105 Hz), ω2 (1-103 Hz), and ω3 (0.1-1 Hz) of DSSCs. Thus, the current study refers to the existing characteristic frequencies of DSSCs with the same electrode structure11–17 as the current work to interpret the characteristic frequencies of the water-splitting PEC. In the current work, the change in ωa is negligible as the resistance at ITO/TiO2 or Pt/electrolyte interface is anticipated to be the same regardless of the number of MgO spin coating ([Mg]/[Ti] ratio); such phenomena are not apparent in the scale adopted in Figure 5. However, ωb (resistance at TiO2/electrolyte interface) increases monotonically as the effective MgO thickness increases, although the full half-circle in the Z′-Z′′ domain could not be completed for the MgO-coated cases due to the overly high impedance of the cell (Z′ and Z′′ are the real and imaginary parts of the impedance Z, respectively). Therefore, the increased resistance between TiO2 and the electrolyte by the role of MgO interphase indicates that the electron backward reaction with electrolyte (a major source of charge recombination) is significantly retarded. At an open circuit, the charge-separated electrons are accumulated in TiO2, and thus react suitably with electrolyte. Thus, an increased open circuit voltage (VOC) indicates a retarded back

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Bae et al. guarantee that β is around unity in a water-splitting PEC. However, the response time itself obtained by eq 2 is a good indicator to describe the characteristic recovery time when the system is displaced from an illuminated steady state at open circuit to the equilibrium dark state. Thus, a higher response time certainly indicates the retarded back electron transfer. As seen in Figure 6b, indeed, the response time increases with increased dose of MgO coating, which supports the view that the application of MgO retards the back electron reaction with electrolyte. Second, the change in the degree of charge carrier tunneling by the application of the MgO overcoating layer has been analyzed as a potential source to influence the photocurrent. The only way for the charge carriers to migrate through the insulating MgO layer is to resort to quantum tunneling. The probability Px of finding the charge carrier at a tunneling distance x is given by23

[

Px ) A2 exp -

Figure 6. (a) Change in open circuit voltage (VOC) with time when illumination was turned off at time of zero, and (b) change in response time as a function of decaying VOC for varying [Mg]/[Ti] ratio of MgOcoated TiO2/ITO electrode. NSC in the inset of (a) denote “number of spin coating”.

electron transfer. In Figure 6a, indeed, increased VOC is observed by the increased dose of MgO. Further, the retarded back electron transfer should yield an increased lifetime, which is defined for the exponential decay following a small variation of the Fermi level, for example from a steady state current at IO to that at IO + ∆IO by a small perturbation of the illumination over a steady state. Despite the precise definition of the lifetime, Zaban et al.22 provided a method to obtain the lifetime by a large perturbation, which monitors the transient of VOC during the relaxation from the illuminated quasiequilibrium state to the dark equilibrium. The response time is obtained by the reciprocal of the derivative of the decay curve (Figure 6a) normalized by the thermal voltage

τn ) -

( )

kBT dVOC e dt

-1

(2)

where kB is the Boltzmann constant, e is the charge of electron, T is absolute temperature, and t is time. When the rate of charge recombination with electrolyte is linear with respect to electron concentration, that is β is unity in the equation U ) krnβ (U is the rate of recombination, kr is recombination rate constant, and n is the electron concentration, and β is the nonlinearity constant), the response time turns out to be the lifetime. In DSSCs, β was shown to be about 1.5, implying that eq 2 can be a good approximation to the lifetime.22 In the current work, however, the mechanism of charge recombination with electrolyte (recombination with protons in electrolyte) is different from DSSCs (recombination with electrolyte-oxidized species rather than oxidized dye), and thereby there is a limitation to interpret the obtained response time as the lifetime; there is no

2√2m(V - E) x p

]

(3)

where m is the mass of charge carrier, V is the height of the confining potential applied by the MgO layer, E is the energy of the charge carrier confined in the potential well V, and A is the orthonormalization constant of the eigenfunction. As charge carriers are mostly in ground-state at room temperature, V - E ≈ V. By roughly taking the confining potential V as half the band gap energy of MgO, and mass of the charge carrier as the free electron mass, the ratio Px/Po can be calculated, where Po is the probability of finding charge carrier at TiO2 surface. The fraction of charge carrier at tunneling distance of x with reference to the surface of TiO2 (x ) 0) can be estimated by the term Px/Po: this fraction decays to almost zero at the distance x of less than 2 nm. Assuming other reasonable values of V and m in the same order do not yield any apparent change in the tunneling distance x. The tunneling distance of at the best mark, 2 nm, indicates that if a MgO layer of a few tens of nm in thickness (see refs 11 and 12) is perfectly sealing the surfaces of the TiO2 particles, practically no photocurrent will flow because the charge carriers cannot migrate through the overcoating layer. Thus, the applied MgO layer is believed to have an open structure (for instance, the presence of uncovered TiO2, or a porous MgO structure as in the case of topotactic MgO in refs 11 and 12) so that charges can migrate. By combining this result with the cell impedance analysis (Figure 5), the role of the MgO layer is indeed profound in that it serves as a barrier for the back transfer of the charge-separated electrons to the electrolyte (retardation of charge recombination) to improve the cell performance, while it should allow the escape of the charge-separated holes from TiO2 to the electrolyte. A similar intriguing role is also believed to be operating when insulating-layer-coated TiO2 nanoparticles are applied to the electrode of DSSCs.6–17 In DSSCs, electron injection from dye molecules to TiO2 is achieved by the tunneling through the insulating layer while the back electron transfer from TiO2 to the electrolyte is efficiently retarded by the coated insulating layer. A yet to be solved question concerns how the insulating layer blocks the back electron transfer (in water splitting cells and DSSCs), while allowing the electron injection from dye to TiO2 (in DSSCs) and the hole escape to electrolyte (in a water splitting cells and DSSCs). The current work identifies for the first time these two faces of the roles of the coated insulating layer; the detailed roles of the interesting coating layer present an open topic for further investigation. Finally, the change in photon density arriving at the TiO2 surfaces by the applied MgO coating layer has been analyzed

MgO Coating Layer

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Figure 7. Change in calculated transmittance of 3.2 eV photon to TiO2 as a function of the effective thickness of the MgO layer in electrolyte/ MgO/TiO2 three planar layers system under the assumption of no light absorption. The dotted lines indicate the roughly estimated effective thickness of the MgO layer in the current work.

as a source to influence the photocurrent density. The photon density arriving at TiO2 is influenced by both the light absorption of the MgO layer itself and light reflection due to the light interference of electrolyte/MgO/TiO2 system. As for the light absorption by the MgO layer, its absorption coefficient is only 1.5 cm-1 when the photon energy is equivalent to the band gap of TiO2, i.e., 3.2 eV,24 which energy is required for the photoexcitation (charge separation). According to Lambert’s law,19 intensity of light I after passing through the distance x is given by

I ) Ioe-βx

(4)

where Io is the initial light intensity, and β is the absorption coefficient. The absorption coefficient of 1.5 cm-1 of MgO indicates that, even after passing through the thickness of 100 nm, less than 3% of photon is absorbed by the MgO layer. Thus, in the current work, the change in the thickness of the MgO layer would not yield apparent change in light absorption by the MgO layer itself. As for the light reflection by the light interference, for the simplicity of the analysis, an effective planar three layer system is assumed, i.e., electrolyte/MgO/TiO2, along the direction of light travel. The fraction of the light reflected by the three layer system R is given by25

R) r1 )

n0 - n1 n0 + n1

r21 + r22 + 2r1r2 cos ∆1 1 + r21r22 + 2r1r2 cos ∆1 r2 )

n1 - n2 n1 + n2

∆1 )

(5) 4π n d (6) λ 1 1

where subscripts 0, 1, and 2 denote electrolyte, MgO, and TiO2, respectively, no, n1, and n2 are refractive index of electrolyte (1.35 by assuming water), MgO (1.8), and TiO2 (3.0), respectively, d1 is the thickness of the MgO layer, and λ is the wavelength of the traveling photon with the band gap energy of TiO2, i.e., 3.2 eV (387.5 nm). The fraction of the light arriving at TiO2, transmittance percentage T ()100 × (1-R)) by assuming no absorption), is illustrated in Figure 7 as a function of the effective thickness of MgO. By applying the MgO layer, the amount of photon arrival to TiO2 increases from 85.5 to 98.8% at the effective MgO thickness of 53.8 nm. The effective thickness of the MgO layer was hardly measurable in the current work due to the soaking of the MgO sol into the mesoporous TiO2 layer. Further, the MgO layer itself

may be porous as aforementioned. Nevertheless, the effective thickness of the MgO layer has been roughly estimated by assuming that the MgO/TiO2 layer forms a dense structure without pores. The total thickness of the MgO/TiO2 coating layers was similar for the cases of [Mg]/[Ti] ) 0, 0.19, and 0.35 (436 nm), while it was 510 nm when [Mg]/[Ti] ) 0.44 (Figure 1). The densities of TiO2 and MgO were assumed to be 4.25 and 3.60 g/cm3, respectively.26 The effective thickness of MgO calculated based on these values is estimated to be 0, 32.7, 56.7, and 80.7 nm, for [Mg]/[Ti] ) 0, 0.19, 0.35 and 0.44, respectively (these calculated effective thicknesses of MgO in the current work are included in Figure 7 as dotted lines). The roughly estimated effective thickness of MgO when [Mg]/[Ti] ) 0.35 (56.7 nm) is well correlated to the thickness position where the light arrival to TiO2 is maximized (53.8 nm) in Figure 7. Thus, it is believed that the light interference mechanism is certainly contributing to the photocurrent maximization (Figure 4a) at a certain effective thickness of MgO layer when [Mg]/ [Ti] ) 0.35 herein (spin coated twice). The mechanism of the retardation of the back electron transfer by the MgO layer cannot explain the decay of the photocurrent after the effective optimal thickness of MgO (Figure 4a) because the increase of MgO thickness can only monotonically increase the retardation of the back electron transfer: no reason appears to explain why the degree of hole escape (tunneling) from TiO2 to electrolyte is maximized at an optimal thickness of MgO. Indeed the light interference-based minimized reflection (maximized transmission; Figure 7) is associated with the maximization of the photocurrent at optimal effective thickness of MgO (Figure 4a). Such a maximization of the cell performance at a certain amount of overcoating material has been scarcely reported in existing works on either water-splitting cells or DSSCs which employed the overcoating material in the TiO2/ ITO electrode. In the cases of DSSCs, the cell performance was rather saturated after a certain amount of overcoating material within the investigated range.6,7 An overly high dose of the overcoating materials would certainly degrade the performance of DSSCs as well due to the mechanism of light interference (Figure 7). The present work identifies for the first time this light interference mechanism for controlling the photocurrent of the water splitting cells and DSSCs. 3.4. Roles of MgO Coating under a Bias Voltage. When bias voltage is applied, the magnitude of the photocurrent density at a given effective thickness of MgO (Figure 4b) is higher than in cases when no bias voltage is applied (Figure 4a): this difference can be explained by the decreased Schottky contact barrier at the interface of the ITO/TiO2. When bias voltage is applied (Figure 4b), the photocurrent density monotonically decreases with the increase of the MgO layer thickness. The role of the MgO in photon delivery to TiO2 via the light interference is the same as in the case of applied voltage; thus, this role cannot be the reason for the decreasing photocurrent in Figure 4b. Under the circumstance of the decreased Schottky contact barrier, the bias voltage drifts the charge-separated electrons toward the ITO direction while drifting the charge-separated holes toward the electrolyte. In these drift processes under the bias voltage, the mechanism of blocking the back electron transfer to electrolyte by the MgO layer (positive influence of MgO) would no longer be operating because electrons are already drifted toward the ITO direction. As a result, the presence of MgO will only act as a barrier for the hole escape from TiO2 to the electrolyte. Thus, only negative influence of MgO on the photocurrent will appear under the bias voltage.

9942 J. Phys. Chem. C, Vol. 112, No. 26, 2008 3.5. Further Discussions. From the harmful effect of the MgO layer under the bias voltage, it is believed that when no bias voltage exists, the positive influence of MgO (blocking the back electron transfer) and the negative influence of MgO (retarding the hole escape from TiO2 to electrolyte) are competing with each other. Under this assumption, the maximal performance of the photocatalytic water-splitting cell would result at the optimum effective thickness of the MgO overcoating layer (approximately 56.7 nm when [Mg]/[Ti] ) 0.35 in the current work). In fact, no evidence of the harmful effect of the MgO layer exists in the literature. Thus, without observing the diminished water splitting performance by the applied MgO layer under the bias voltage in the current work, the competition between the positive and negative influences could hardly have been deduced. As mentioned previously, in addition to this competition process, the mechanism of light interference also contributes to yield the maximal cell performance at a certain effective thickness of MgO. In the cases of DSSCs employing the overcoated nanoparticle electrode, the competition between the positive effect (blocking back electron transfer) and the negative effect (blocking electron injection to TiO2 from dye and the hole escape from TiO2 to the electrolyte), and the light interference would also yield the performance maximization at an optimal effective thickness of the overcoating layer. Under the presence of the bias voltages in water splitting cells, only the negative influence, i.e., the blocking of hole transfer to the electrolyte, operates to monotonically decrease the photocurrent with the thickness increase of MgO. The influence of blocking the hole transfer certainly prevails the influence of light interference, since the increase of MgO thickness monotonically decreases the overall water splitting performance under the bias voltage. 4. Conclusions A mesoporous TiO2/ITO/glass electrode was prepared by spin coating of TiO2 slurry, followed by annealing at 450 °C. A MgO sol was further spin coated on the mesoporous TiO2 layer and annealed at 350 °C. A series of (MgO)/TiO2/ITO/glass structures with varying effective thickness of MgO (varying [Mg]/[Ti] ratio) was applied to working electrodes of photoelectrochemical cells for water splitting. The role of MgO with regard to the water splitting performance has two faces: (1) blocking the back electron transfer from TiO2 to electrolyte (positive influence) to improve the performance and (2) blocking the hole transfer from TiO2 to electrolyte (negative influence) to degrade the performance. The competition between the positive and negative influences of the MgO layer results in maximal performance of the photoelectrochemical cell at an optimum effective thickness of the MgO overcoating layer. Further, the light interference of the electrolyte/MgO/TiO2 system also contributes to the performance maximization with regard to the effective thickness of the MgO overcoating layer. Under the presence of bias

Bae et al. voltage, however, the role of blocking the hole escape (negative influence) prevails the blocking of the back electron transfer (positive influence) as well as the influence of the light interference; thereby the increase of MgO thickness diminishes the performance of the photoelectrochemical cell. Acknowledgment. This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MOST) (R01-2007-000-11075-0). References and Notes (1) Khan, S. U. M.; Al-Shahry, M.; Ingler, W. B. Science 2002, 297, 2243. (2) Bolton, J. R. Sol. Energy 1996, 57, 37. (3) Aroutiounian,; V, M.; Arakelyan, V. M.; Shahnazaryan; G, E. Sol. Energy Mater. Sol. Cells 2005, 89, 153. (4) Bak, T.; Nowotny, J.; Rekas, M.; Sorrell, C. C. Int. J. Hydrogen Energy 2002, 27, 991. (5) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (6) Lee, S.; Kim, J. Y.; Youn, S. H.; Park, M.; Hong, K. S.; Jung, H. S.; Lee, J. K.; Shin, H. Langmuir 2007, 23, 11907. (7) Lee, S.; Kim, J. Y.; Hong, K. S.; Jung, H. S.; Lee, J. K.; Shin, H. Sol. Energy Mater. Sol. Cells 2006, 90, 2405. (8) Wang, Z.-S.; Huang, C.-H.; Huang, Y.-Y.; Hou, Y.-J.; Xie, P.-H.; Zhang, B.-W.; Cheng, H.-M. Chem. Mater. 2001, 13, 678. (9) Kumara, G. R. A.; Okuya, M.; Murakami, K.; Kaneko, S.; Jayaweera, V. V.; Tennakone, K. J. Photochem. Photobio. A 2004, 164, 183. (10) Taguchi, T.; Zhang, X. T.; Sutanto, I.; Tokuhiro, K.; Rao, T. N.; Watanabe, H.; Nakamori, T.; Uragami, M.; Fujishima, A. Chem. Commun. 2003, 19, 2480. (11) Jung, H. S.; Lee, J. K.; Nastasi, M.; Lee, S.; Kim, J. Y.; Park, J. S.; Hong, K. S.; Shin, H. Langmuir 2005, 21, 10332. (12) Jung, H. S.; Lee, J. K.; Nastasi, M.; Kim, J. R.; Lee, S.; Kim, J. Y.; Park, J. S.; Hong, K. S.; Shin, H. Appl. Phys. Lett. 2006, 88, 013107. (13) Tennakone, K.; Bandara, J.; Bandaranayake, P. K. M.; Kumara, G. R. A.; Konno, A. Jpn. J. Appl. Phys., Part 2 Lett. 2001, 40 (7B), L732. (14) Perera, S.; Senadeera, R.; Tennakone, K.; Ito, S.; Kitamura, T.; Wada, Y.; Yanagida, S. Bull. Chem. Soc. Jpn. 2003, 76 (3), 659. (15) Taguchi, T.; Zhang, X.-t.; Sutanto, I.; Tokuhiro, K.-i.; Rao, T. N.; Watanabe, H.; Nakamori, T. Chem. Comm. 2003, 2003 (19), 2480. (16) Kumara, G. R. A.; Tennakone, K.; Okuya, M.; Kaneko, S. Electrochem. Soc. 2002, 2002, 1053. (17) Senevirathna, M. K. I.; Pitigala, P. K. D. D. P.; Premalal, E. V. A.; Tennakone, K.; Kumara, G. R. A.; Konno, A. Sol. Energy Mater. Sol. Cells 2007, 91 (6), 544. (18) Kim, J. Y.; Jung, H. S.; Hong, K. S. J. Am. Ceram. Soc. 2005, 88, 784. (19) Cullity, B. D. Elements of x-ray diffraction; Addison-Wesley: Reading, MA, 1978. (20) Hoshikawa, T.; Yamada, M.; Kikuchi, R.; Eguchi, K. J. Electrochem. Soc. 2005, 152, 68. (21) Kern, R.; Sastrawan, R.; Ferber, J.; Stangl, R.; Luther, J. Electrochim. Acta 2002, 47, 4213. (22) Zaban, A.; Greenshtein, M.; Bisquert, J. ChemPhysChem 2003, 4, 859. (23) Eisberg, R.; Resnick, R. Quantum physics of atoms, molecules, solids, nuclei, and particles; John Wiley and Sons, Inc.: New York, 1974. (24) Molnar, J. P.; Hartman, C. D. Phys. ReV. 1950, 79, 1015. (25) Anders, H. Thin Films in Optics; The Focal Press: London, 1965. (26) Mat-Web–Material Property Data. http://www.matweb.com. (accessed March 1, 2008).

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