Article pubs.acs.org/JPCC
The Impact of Niobium Surface Segregation on Charge Separation in Niobium-Doped Titanium Dioxide L. R. Sheppard,*,† T. Dittrich,‡ and M. K. Nowotny† †
Solar Energy Technologies Research Group, School of Computing, Engineering and Mathematics, University of Western Sydney, Locked Bag 1797, Penrith, NSW 2751, Australia ‡ Helmholtz-Zentrum Berlin für Materialien und Energie, Institute of Heterogeneous Materials, Hahn-Meitner-Platz 1, D-14109 Berlin, Germany ABSTRACT: The present investigation has studied the impact of Nb surface segregation on charge carrier separation in polycrystalline Nb-doped (0.65 at % Nb) TiO2. By use of secondary ion mass spectrometry, the effect of applied processing conditions (temperature and oxygen activity) on the segregation of Nb has been obtained and correlated against surface photovoltage data obtained using surface photovoltage spectroscopy (SPS). It has been observed that within the range of the applied processing conditions, the segregation of Nb toward or away from the surface can be achieved. The corresponding SPS results demonstrate that, when Nb is removed from the surface and near-surface regions of Nb-TiO2, charge separation increases, as reflected by an increase in the surface photovoltage. After consideration of the impact of oxygen activity on the defect disorder of Nb-TiO2, it is concluded that the application of very low oxygen activities (p(O2) in the vicinity of 10−15 Pa) at temperatures of 1473 K and above leads to the removal of Nb from the surface and near-surface region. This provides additional upward band bending and is believed to be responsible for the improvement in charge separation. long lasting photoelectrode materials.5−9 However, despite these many attempts energy conversion efficiencies have remained close to 1% for TiO2-based systems, with higher performance only obtained at the expense of reduced chemical stability/photoelectrode lifetime.10 The exception to this is the work by Khan et al.8 who have reported energy conversion efficiency values of 8.35%. However this outstanding result has yet to be independently confirmed. The ability of a photoelectrode to efficiently convert incident solar energy into hydrogen gas depends on a number of processes that are related to its material properties.11 One such process is charge separation, which is essential for ensuring that the photoelectrons and photoholes generated during the absorption of light have sufficient lifetime to perform useful work. When charge separation is poor, the recombination of these charge carriers can impact heavily upon energy conversion efficiency, even when light absorption is strong. In the present investigation, compositional gradients resulting from segregation are investigated for their impact on charge separation. In principle, such compositional gradients are accompanied by electric fields that can influence the migration of electrical charge. Since Nb segregation in TiO2 has been shown to be strongly influenced by processing parameters such as temperature and oxygen activity,12 it is expected that
1. INTRODUCTION All over the world, concern over the consequences of global warming and climate change is growing. Inter-related with energy security, these global issues are now regular topics of international debate. The core problem relates to how energy can be sourced and utilized without detrimentally impacting upon the short- and long-term living standards of every inhabitant on Earth.1,2 The generation of hydrogen fuel from solar driven watersplitting has particular promise as a process for supplying sustainable and environmentally friendly fuel.3 This process involves two electrodes that are immersed in an aqueous electrolyte and form a photoelectrochemical cell. At least one of the two electrodes is a photoelectrode, where incident sunlight is collected and converted into electronic charge carriers that flow through the cell and ultimately participate in redox reactions that liberate hydrogen and oxygen gases. In principle, solar-hydrogen could be produced wherever sunlight and water are available. Similar to current-day fossil fuel usage, solarhydrogen could also be utilized to provide both vehicle propulsion and electricity generation but without the corresponding carbon emissions.3 While the generation of solar-hydrogen from water splitting has great promise, considerable barriers must be overcome to achieve generation efficiencies that permit commercial viability. Since Fujishima and Honda first demonstrated successful photoelectrochemical water-splitting with rutile TiO2 in 1972,4 many efforts have been made to develop high performing and © 2012 American Chemical Society
Received: July 2, 2012 Revised: August 31, 2012 Published: September 5, 2012 20923
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3. BRIEF LITERATURE REVIEW A number of studies on solute segregation in TiO2 have been reported in the literature.17−27 While these cover a broad selection of dopants and applied processing conditions, segregation in TiO2 remains a difficult phenomenon to understand and control. This is compounded by the lack of data on segregation kinetics which hampers efforts to use the attainment of equilibrium or steady state conditions as a yardstick for achieving reproducible outcomes. What has been established is that segregation in TiO2 is sensitive to both temperature and the ambient oxygen activity.19,22 As an example, Bernasik et al.19 has shown for Fe-doped TiO2 that high oxygen activity leads to the surface becoming enriched in Fe. In contrast, the application of reducing conditions lead to the complete absence of Fe surface enrichment. The opposite trend has been reported by Gulino et al.22 for Sb segregation; annealing under reducing conditions was more favorable for Sb segregation than heat treatment in air. Ruiz et al.24,25 have demonstrated the effect of temperature on solute segregation for Nb- and Cr-doped TiO2 thin films. In both cases, temperature has been observed to favor segregation; however, at such moderate temperatures, 873 and 1173 K, this may have more to do with diffusion kinetics rather than driving force. Regardless of the reason, the promotion of Nb segregation by increased temperature has been theoretically and experimentally demonstrated.17,18 Microstructure is another factor that has also been shown to influence segregation.20,21 Several SPS studies of TiO2 have been undertaken.14,28−30 From these, the technique has been used to identify surface and bulk electronic states within the rutile band gap,14 assess charge separation mechanisms at the Pd-porphyrin/TiO2 interface,29 and provide evidence for the existence of p-type TiO2.30 As a spectroscopic technique, it is typical for incident light to monochromatically scan a given wavelength range and the obtained surface photovoltage spectrum is related to the excitation of various electronic states within the band gap. The current investigation is unique in that illumination is at a fixed photon energy that exceeds the band gap energy of TiO2, and as such, the excitation of electronic states within the band gap is negligible compared to the excitation of states across the band gap. Consequently, the measured surface photovoltage can be related to the overall ability of the processed specimens to separate photogenerated charge carriers.
such high temperature processing could be utilized for manipulating charge separation. Hence, the aim of the present investigation is to establish the relationship between hightemperature processing, Nb segregation, and charge separation. For this investigation, charge separation will be assessed using surface photovoltage spectroscopy (SPS).13,14
2. POSTULATION OF THE PROBLEM Segregation is a mass transport phenomenon where ionic defects present within the bulk of a material migrate toward interfaces (surfaces and grain boundaries) under the driving force of lattice energy and/or strain energy minimization.15 Segregation occurs at elevated temperatures where ions are free to migrate and results in the enrichment of the surface in defects and the establishment of compositional gradients within the near-surface region. Associated with these compositional gradients are electrical potential gradients whose strength can be considerable when the degree of surface enrichment is high.11 Because of the ability of segregation to influence both the composition of a surface and its ability to transfer charge, segregation has the potential to play a determining role over the functional properties exhibited by any material subjected to high temperatures. To achieve high performance from a photoelectrode during the photoelectrochemical splitting of water, incident sunlight must be efficiently converted to photogenerated electrons and holes.3 These charge carriers must then be effectively separated and remain separated long enough to perform useful work. In the absence of an electric field, such charge separation is usually hindered. Unlike the electric fields that naturally exist at the interface between the photoelectrode and the electrolyte,16 segregationinduced electric fields result from compositional differences within the surface and near-surface region of the semiconductor. Hence, they are highly influenced by the applied processing regime. To verify that segregation can be exploited to engineer higher performing photoelectrode materials for solar-driven water splitting, it is necessary to understand how processing can ensure that: • The polarity of the segregation-induced electric field is compatible with the driving of minority charge carriers toward the electrolyte/semiconductor interface. • The segregation-induced electric field applies across a sufficiently deep region to influence carriers immediately subsequent to their generation, in addition to those carriers that may migrate into the electric field from deeper in the bulk. In the present investigation, SPS will be used to study the charge separation capabilities of a number of different segregation profiles in Nb-doped TiO2. This technique exploits a Kelvin Probe arrangement where the contact potential difference between a vibrating inert electrode and the specimen surface is measured.13 During illumination, the surface potential of the specimen is altered due to the generation and separation of photogenerated charge carriers. This change in potential is considered the surface photovoltage and its magnitude reflects the extent of charge separation. By combining this technique with secondary ion mass spectrometry (SIMS), which will characterize the composition of the surface and near-surface region, an initial link between high temperature processing and charge separation can be established.
4. EXPERIMENTAL PROCEDURE Specimen Preparation. The 0.65 at % Nb-doped TiO2 specimens were prepared from sol−gel-derived Nb-doped powder according to a previously reported procedure.31 In this procedure, Nb-TiO2 was precipitated from the mixture of Nb2Cl5 in ethanol and high-purity Ti-isopropoxide (Sigma Aldrich) in ethanol. This precipitate was subsequently ground with agate mortar and pestle and calcined in a horizontal tube furnace at 1173 K in flowing oxygen (100 mL/min) for 2.5 h. After cooling, the powder was uniaxially pressed at 60 MPa into disks of 20 mm diameter and ∼4 mm thick. These disks were subsequently isostatically pressed at 400 MPa before being sintered at 1773 K for 5 h in air. After this densification step, the specimens were polished to a high mirror finish using 65, 15, 6, 3, and 1 μm lapping and polishing pads (Struers Laboforce and Rotopol systems). Between each polishing grade, the specimens were thoroughly cleaned to ensure pad 20924
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contamination was avoided. Once the final polish had been completed, the disks were cut into brick shaped specimens with approximately 3 mm thicknesses using a precision diamond saw (Struers Accutom). Finally, each individual specimen was annealed for 50 h at 1173, 1473, or 1673 K in either a controlled atmosphere of pure oxygen (Coregas, 99.9% purity) at 100 mL/min or 1%H2/99%Ar (Coregas) at 100 mL/min. This respectively achieved an oxygen activity of p(O2) = 75 kPa or p(O2) = 10−15 kPa as determined using a zirconia-based oxygen probe. At this point the specimens were ready for surface compositional analysis using SIMS and characterization using SPS. SIMS Determination. The 93Nb segregation profiles were determined in each specimen using SIMS (IMS 5f, Cameca, France). A Cs+ primary ion beam of 7.5 kV accelerating voltage and ∼5 nA ion current was used to raster the surface of the specimen over an area of 250 μm × 250 μm with a circular analysis area of 55 μm diameter. The sampled elements included 93Nb, 133Cs, 133Cs16O, and 133Cs48Ti. To avoid effects associated with cutting the specimen, analysis was performed centrally on all specimens. The sputter rate (averaged over the duration of sputtering) was calculated from the depth of the sputter crater formed after an especially long analysis. The crater itself was profiled using a stylus profilometer (KLA Tencor Alpha-Step IQ), and the sputter rate was determined to be 0.0231 nm/s. SPS Characterization. SPS measurements were performed at room temperature in an atmosphere of pure argon (BOC Gases, p(O2) = 10 Pa) using a Kelvin Probe arrangement with a vibrating gold mesh reference electrode (Delta Phi, Besocke). The samples were illuminated with monochromatic light of wavelength λ = 310 nm, supplied by a 1000-W xenon arc lamp (Müller Elektronic, Germany) and quartz prism monochromator for an extended “light on” duration between brief periods of darkness (“light off”). Changes to the measured contact potential difference between the specimen and the reference electrode represent negative changes in the surface photovoltage during periods of illumination and darkness.
Figure 1. Typical SIMS intensity profile, obtained for the Nb-TiO2 specimen annealed at 1173 K for 50 h in oxygen activity equal to 75 kPa.
the normalization of the 93Nb yield against the 133Cs48Ti yield can provide an estimate for the concentration of 93Nb as indicated by the following relation C Nb =
I(93 Nb) I( Cs48Ti) 133
(1)
While this approach provides an estimation of concentration, the reliability of this estimate depends upon a number of different variables, and concentration calibration curves are necessary to reliably quantify concentration.33 However, this estimation does accurately represent changes in concentration during depth profiling with SIMS, which is sufficient for the purposes of the present investigation. The SIMS depth profiles for each specimen, in terms of CNb, are summarized according to the oxygen activity of the annealing atmosphere in Figure 2. In both figures, a polished specimen, which has not undergone heat treatment, is included as a reference. As seen in these figures, temperature and oxygen activity have had clear influences on the nature and extent of Nb surface segregation. In Figure 2, under the application of high oxygen activity, the driving force for Nb segregation is substantial as indicated by the clear enrichment of the surface in Nb. The extent of surface enrichment also noticeably decreases as the temperature is increased which indicates that the driving force for segregation is temperature dependent. It is also seen that with increased temperature, the depth of the region affected by segregation (the near-surface region) becomes narrower. However, since the time made available for segregation is fixed in this study, this observation may reflect the temperature dependence of segregation kinetics rather than segregation driving force. Further investigation is required.
5. RESULTS SIMS Results. An example of a typical intensity profile obtained from SIMS is displayed as Figure 1. As seen in this figure, the intensity yield for each studied element varies as a function of sputtering time. While the intensity yields measured initially (t = 0 s) do originate from the specimens surface, some time is required for the Cs primary beam to establish steady state sputtering conditions. As this process can be highly variable, the initial stage of SIMS analysis lacks reliability and must be excluded from consideration. Unfortunately, there is no exact approach for identifying when this uncertainty becomes tolerable without disregarding a large volume of data. Hence, in the present investigation, the first 50s of analysis time has been excluded based on the typical time needed for the 133Cs yield to complete approximately 50% of its initial stabilizing drop. This is indicated in Figure 1 as the “assumed surface”. Finally, by using the determined sputter rate, sputtering time can be calibrated for depth. This enables all observed intensity yields in the present investigation to become depth profiles, which are henceforth referred to throughout the remainder of this report. It has been shown that the intensity yields obtained from SIMS data for stable lattice elements can be used to estimate the concentration of impurity species of interest.32 Specifically, 20925
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Figure 3. Comparison of SIMS depth profiles for specimens annealed in high and low oxygen activity (p(O2) = 75 kPa and 10−15 Pa, respectively) at 1173 K (left) and 1673 K (right).
of low oxygen activity. Comparable behavior has been observed in Nb-doped TiO2 single crystals under similar processing conditions, and this phenomenon is considered to represent a situation where the surface activity of Nb is dramatically altered as a result of the ambient oxygen activity.12 In Figure 2, it is also seen that apart from the specimen treated at 1473 K in low oxygen activity, all specimens, including the polished reference specimen, attained equivalent bulk intensity levels. This indicates that the bulk composition of each specimen was not significantly influenced by the applied processing conditions. At present, it is not entirely clear why the specimen treated at 1473 K and low oxygen activity does not also display this characteristic. Yet, since it does appear to be a simple vertical shifting of data across the entire profile, it could be simply attributed to an inconsistency in the setup procedure of the SIMS. SPS Results. The SPS results obtained for each specimen have been displayed with the corresponding SIMS depth profiles at 1173, 1473, and 1673 K in parts a, b, and c of Figure 4, respectively. From this set of figures, several pertinent features are clear. In all cases the surface photovoltage is positive, which indicates that photogenerated electrons are migrating away from the surface during illumination and that photoholes are migrating toward it.14 Since Nb-doped TiO2 is an established n-type semiconductor,31 this behavior confirms the presence of a depletion layer at the surface of all specimens. The displayed surface photovoltage also becomes increasingly positive as the annealing temperature is increased. Out of concern for the influence of processing, at any studied temperature the application of low oxygen activity during annealing has consistently led to a greater surface photovoltage. This indicates that charge separation has been promoted by the application of low oxygen activity during annealing. Since the observed surface photovoltage is remaining positive throughout, such processing is promoting the function of Nb-TiO2 as a photoanode material. While the oxygen activity applied during processing is having the most noticeable effect on the surface photovoltage, temperature is also playing a role. Specifically, increasing the temperature of processing is having the same influence as decreasing the oxygen activity, leading to an increasingly positive surface photovoltage.
Figure 2. (a) SIMS depth profiles obtained for each specimen after annealing in high oxygen activity at (a) p(O2) = 75 kPa and (b) p(O2) = ∼10−15 Pa.
In Figure 2b, it is seen that the application of low oxygen activity has resulted in the suppression of Nb segregation altogether, and with increased temperature, the extent to which Nb enrichment is replaced by Nb depletion increases. As seen in this figure, the depth of the near-surface region affected by this process varies considerably, and at 1673 K, the affected region is almost 50 nm deep, while at 1473 K, the same region extends only 10−15 nm. It is important to note that, for this sample, an error occurred in the setup of the SIMS instrument that resulted in the vertical exaggeration of the 93Nb/133Cs48Ti depth profile. For this reason, the bulk yield for this sample appears richer in Nb than the remaining samples. This error does not affect the change in proportionality between 93Nb and 133 Cs48Ti yields however. At 1173 K, almost no affected region exists at all. As with the behavior displayed by the oxidized specimens, it is not obvious what is responsible for this kinetic behavior, and further investigation is required. A comparison of Nb segregation behavior under high and low applied oxygen activity is made in Figure 3 at 1173 and 1673 K. As seen at 1173 K, Nb segregation has become almost entirely suppressed under the application of low oxygen activity but remains promoted when exposed to high oxygen activity. When the temperature was increased to 1673 K, the effect of atmosphere becomes more dramatic, with the surface now becoming clearly depleted in Nb in response to the application 20926
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Figure 5. Summary of the observed surface photovoltage behavior for each studied Nb-TiO2 specimen.
20−25% of their photovoltage during the remaining 1200 s of dark measurement. In comparison, the remaining specimens lose less than 10% of their photovoltage over the same duration. What is also apparent in the specimens that discharge slowly is an abrupt increase in surface photovoltage immediately subsequent to the removal of light. This feature has been highlighted in Figure 4c but is also present in the surface photovoltage profiles of specimens in parts a and b of Figure 4. This particular behavior is suggestive of hole trapping and will be considered further.
6. DISCUSSION When comparing the results obtained using SIMS and SPS, it is clear that the applied processing conditions have had a pronounced effect on both the Nb segregation behavior and the ability of Nb-TiO2 to separate photogenerated charge carriers during superband gap illumination (hv = 4 eV). In particular, temperature and oxygen activity have been observed to influence both phenomena. Temperature and oxygen activity have also been shown to have a pronounced effect on the manner in which Nb is incorporated into the TiO2 lattice and the semiconducting behavior that results.31,34 As demonstrated by Sheppard et al.31,34 Nb dopant incorporation can proceed via two different mechanisms that depend upon the ambient oxygen activity and have different consequences for the prevailing defect disorder and related semiconducting properties. Under oxidizing conditions, incorporated Nb is compensated ionically by titanium vacancies according to the following chemical equilibria and charge neutrality condition (expressed using Kröger−Vink notation31,34,35):
Figure 4. Observed surface photovoltage behavior with corresponding SIMS depth profile for polycrystalline Nb-TiO2 specimens annealed at (a) 1173 K, (b) 1473 K, and (c) 1673 K in high and low oxygen activity.
Finally, the processing procedure has also influenced the sensitivity of the specimens to changes in light intensity. As observed in Figure 5, the two specimens annealed in low oxygen activity at 1673 and 1473 K display greater responsiveness to the introduction and removal of UV illumination than the remaining specimens. When illumination commences, only these two specimens manage to reach their maximum surface photovoltage in the allotted time period, as indicated by the stabilization of the surface photovoltage for a period of time before light is removed. Similarly, when illumination is removed, these two specimens also discharge more rapidly than the other specimens, losing approximately
⁗ 2Nb2 O5 ↔ 4Nb•Ti + V Ti + 10OOx
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(2)
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when Nb is enriched at the surface points away from a significant change in the concentration of recombination centers and toward another possible explanation. An alternative situation resulting from the application of very low oxygen activity at 1473 and 1673 K could be analogous to the formation of a p−n junction, where the resulting surface becomes less n-type relative the bulk due to the depletion of the donor dopant. This scenario would yield an electric field oriented such the migration of photoelectrons away from the surface would receive assistance and thereby reduce recombination losses. In the case where Nb is enriched at the surface, the converse would occur. Concerning the observed discharge behavior upon the removal of UV illumination, the depletion of Nb at the surface is seen to demonstrate a markedly more rapid response (see Figure 5). This suggests that either Nb or a compensating ionic defect is behaving as an effective trapping center for photogenerated charge carriers within the near surface region. This will require investigation of the trapping rates of defects within undoped and Nb-doped TiO2.
(3)
where Nb•Ti represents an ionized Nb atom substitutionally occupying a regular cation site, V′Ti′′′ represents a fully ionized titanium vacancy, and OxO represents an neutrally charged oxygen ion occupying a regular oxygen site. Under reducing conditions, Sheppard et al.31,34 have similarly shown that Nb incorporation involves electronic compensation by electrons according to the following chemical equilibria and charge neutrality condition Nb2 O5 ↔ 2Nb•Ti + 2e′ + 4OOx + [Nb•Ti ] = n
1 O2(g) 2
(4) (5)
here n represents the concentration of electrons. According to Sheppard et al.31,34 the transition between ionic and electronic Nb compensation occurs around an oxygen activity of ∼1 Pa, but this varies with temperature. At very low oxygen activities it is possible to enter a defect regime that is no longer dominated by the dopant but instead behaves intrinsically. In this case, the predominant ionic defects are oxygen vacancies whose formation is governed by the following chemical equilibria and simplified charge neutrality condition31 OOx ↔ V O•• + 2e′ + [VO••] = n
1 O2(g) 2
7. CONCLUSIONS This investigation has studied the impact of segregationinduced composition gradients on the ability of Nb-TiO2 to separate photogenerated charge carriers during UV illumination. From the obtained SIMS depth profiles, it is clear that the applied processing conditions, in particular the oxygen activity, dictates whether the Nb dopant is accumulated or depleted from the surface and near-surface region. As a result of these imposed differences in processing, charge separation has been observed to be strongly influenced, as indicated by the measured surface photovoltage. Specifically, it has been observed that charge separation is favored by processing NbTiO2 in very low oxygen activity (p(O2) equal to ∼10−15 Pa) and high temperature where the surface is depleted in Nb and more representative of undoped TiO2. To account for this behavior, it is speculated that by removing the donor dopant from the surface, a solid-state junction can be formed where the surface is weakly n-type relative to the bulk. The formation of this junction provides additional driving force for the migration of photoelectrons away from the surface which helps to facilitate greater charge separation. The removal of Nb from the surface and near-surface region also supports a rapid response to illumination, in contrast to the alternative situation where the donor enriched surface leads to the apparent trapping of photogenerated charge carriers at the surface during the removal of light.
(6) (7)
where V•• O represents a fully ionized oxygen vacancy. In the present investigation, the applied processing conditions span a broad range of oxygen activity. Correspondingly, at least two of the above defect regimes are represented and potentially all three. On the basis of the high-temperature results of Baumard and Tani36 for those specimens annealed in p(O2) equal to 75 kPa, it is clear that Nb is incorporated according the ionic compensation mode, i.e., Nb is compensated by the formation of titanium vacancies. For those specimens annealed at p(O2) equal to ∼10−15 Pa, it is equally clear that Nb incorporation via the electronic compensation mode prevails. However, due to the particularly high temperature applied to some of these specimens (1473 and 1673 K annealed in p(O2) equal to ∼10−15 Pa), it is possible that the imposed defect disorder is predominated by oxygen vacancies (as per eq 6) rather than electronically compensated Nb (as per eq 4). Segregation-related changes in the Nb concentration at the surface and near-surface will need to be considered in terms of these different defect regimes. As seen in Figure 5, the highest surface photovoltage result obtained in this investigation was achieved by processing in very low oxygen activity at 1673 K. The second highest result was obtained under the same ambient atmosphere but at 1473 K. In both cases, SIMS profiles have revealed that the surface has become depleted in Nb (see Figure 4), suggesting that the removal of Nb from the surface and near-surface region of NbTiO2 improves charge separation. On first inspection, this result suggests that incorporated Nb is behaving as an efficient recombination center and that the enrichment of Nb in the surface and near-surface region is detrimentally impacting upon charge separation. However, due to the close proximity of Nb•Ti to the conduction band edge,
