Niobium Surface Segregation in Polycrystalline Niobium-Doped

Jan 15, 2013 - ABSTRACT: In the current investigation, Nb surface segregation has been ... Segregation is a mass transport process whereby ionic defec...
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Niobium Surface Segregation in Polycrystalline Niobium-Doped Titanium Dioxide L. R. Sheppard* Solar Energy Technologies Research Group, School of Computing, Engineering and Mathematics, University of Western Sydney, Locked Bag 1797, Penrith, NSW 2751, Australia ABSTRACT: In the current investigation, Nb surface segregation has been studied in polycrystalline Nb-doped TiO2 (0.65 at %) at two extremes of oxygen activity (p(O2) = 101 kPa and p(O2) = 10−15 Pa) over the temperature range of 1173 to 1673 K. The aim has been to establish the effect of changes in ambient oxygen activity and temperature on surface Nb enrichment. Using X-ray photoelectron spectroscopy and secondary ion mass spectrometry, the concentration of Nb at the surface has been determined along with Nb depth profiles. It has been found that the ambient oxygen activity during processing at elevated temperatures has a strong influence on the activity of Nb at the surface and the extent of Nb segregation. Specifically, it was found that the application of low oxygen activity during processing (p(O2) = 10−15 Pa) substantially increases the surface activity of Nb, resulting in the removal of Nb from the surface and near-surface region. In contrast, processing under conditions of high oxygen activity (p(O2) = 101 kPa) was found to promote the enrichment of Nb in the surface due to an apparent drop in the surface activity of Nb. Temperature was observed to have a weaker influence on Nb segregation than oxygen activity, but increased temperature was observed to clearly decrease the surface concentration of Nb. The obtained results indicate that the driving force for Nb segregation can be “tuned” through manipulation of p(O2) and temperature and promise to deliver a mechanism for controlling both the composition of the surface and the near-surface region.

1. INTRODUCTION The photoelectrochemical splitting of water into hydrogen and oxygen gases using rutile TiO2 was first demonstrated in 1972 by Fujishima and Honda.1 At the time, this experiment promised to deliver a route that would enable hydrogen to become an inexpensive and abundant alternative fuel. However, as time has proceeded, the absence of carbon emission during generation and usage has become an added virtue of hydrogen fuel due to the growing linkage between fossil fuel consumption and climate change.2 The most significant challenge associated with solar-driven water splitting is the very low-energy conversion efficiencies (ECEs) that are achieved when corrosion-resistant photoelectrode materials are used (such as TiO2).2 When photoelectrode materials are selected on the basis of high photosensitivity over corrosion resistance, relatively high ECEs (>10%) can be achieved but at the detriment of cost and stable performance. Hence, the attainment of a commercially viable photoelectrochemical cell for hydrogen production remains allusive. As a low-cost semiconductor with outstanding corrosion resistance, TiO2 in different configurations has received considerable attention as a candidate photoelectrode material.3−7 Additionally, its tendency for high degrees of nonstoichiometry has enabled TiO2 to display broad ranging functional properties through the alteration of its defect disorder.8,9 Through controlled processing, the surface and © 2013 American Chemical Society

bulk of TiO2 may also be manipulated in such a fashion that compositional gradients may be imposed.10 These compositional gradients are accompanied by electric fields, whose strength and orientation may be modified through processing. The influence of compositional gradients on charge separation during UV illumination was recently demonstrated by Sheppard et al.11using surface photovoltage spectroscopy. Hence, the controlled manipulation of compositional gradients within the near-surface region of TiO2 offers a novel approach to performance improvement. In the present work, compositional gradients have been imposed in the near-surface region of Nb-doped TiO2 through controlled processing and the exploitation of solute segregation. The aim of this investigation is to identify how temperature and oxygen activity, when used as processing variables, can influence the extent and nature of the segregation-induced compositional variation. To study these changes in the surface and near surface compositions with the greatest accuracy, X-ray photoelectron spectroscopy (XPS) has been combined with secondary ion mass spectrometry (SIMS). Received: November 19, 2012 Revised: January 11, 2013 Published: January 15, 2013 3407

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2. BRIEF LITERATURE REVIEW Segregation is a mass transport process whereby ionic defects present within the bulk of a material migrate toward interfaces (such as free surfaces and grain boundaries) under the driving force of lattice or strain energy minimization.12 This process occurs at elevated temperatures where ions can migrate more freely and results in the establishment of compositional gradients within the near-surface regions of a material. Associated with these compositional gradients are electrical potential gradients whose strength can be considerable when the degree of surface enrichment is high. Because of the ability of segregation to influence the composition of a surface, segregation has the potential to determine the functional properties exhibited by a material that has been subjected to high temperatures during its processing. Whereas the study of segregation in metallic systems is richly represented in the literature,13−15 the study of segregation in metal oxides is comparatively limited. This is related to the difficulties associated with the preparation and investigation of well-defined metal oxide surfaces.16 Despite these difficulties, a number of studies of solute segregation have been reported in TiO2.17−26 These reports cover a broad range of dopants and from these it is clear that the extent of segregation, irrespective of the dopant involved, is strongly affected by temperature and oxygen activity. This has been shown clearly by Bernasik et al.19 for Fe-doped TiO2, who observed a clear surface enrichment in Fe by annealing for 20h at 1400 K in oxygen. Under reducing but otherwise identical conditions, these authors reported an almost complete absence of Fe surface enrichment. In contrast, Gulino et al.22 have concluded that reducing conditions, as achieved through vacuum annealing, favor the segregation of Sb when compared with annealing in air. The effect of temperature has also been clearly demonstrated by Ruiz et al.24,25 for both Cr and Nb segregation in sol−gel derived polycrystalline TiO2 thin films. In these reports, the segregation of either dopant is favored by increasing the annealing temperature. However, given the relatively low temperatures involved, 873 and 1173 K, it is possible that this result is due to increased mass transport kinetics rather than increased segregation driving force. Nevertheless, the conclusion that segregation of Nb is favored by increased temperature is also supported theoretically and experimentally by Ikeda et al.17,18 Microstructure has also been demonstrated to influence segregation behavior.20,21 The current author has also made preliminary investigations of Nb surface segregation in polycrystalline Nb-TiO2.10,16 In both investigations, Nb surface segregation was observed to be favored by increased oxygen activity. However, the time made available for segregation was only 10 h, and it was not certain that gas/solid equilibrium was established. As a result, it was unclear whether the observed behavior was a reflection of the material itself or the specific processing history. The current author has also recently established a link between segregation and charge separation in Nb-doped TiO2,11 which provides a promising new route toward improving ECE through improved quantum efficiency. In this particular investigation, it was observed that the depletion of Nb from the surface of Nb-TiO2 lead to improved charge separation. However, Nb segregation was only qualitatively assessed, and so further investigation is required to better understand the observed segregation phenomena. SIMS is a particularly convenient method for obtaining compositional information about a material as a function of

depth from the material’s surface. However, this technique does introduce some specific uncertainties which must be managed. In particular, upon the initiation of analysis, a short period of time is required for the stabilization of sputtering. As a result, SIMS cannot provide reliable information about the composition of the actual surface. Hence, for a complete picture of the surface, near surface and bulk compositions, SIMS results should be combined with a more sensitive surface technique. In the present investigation, this issue has been addressed through the use of XPS, which is a reliable technique for the assessment of surface compositions, but cannot yield information for depths exceeding ∼10 nm. The combination of SIMS and XPS in the present investigation is expected to yield a clear compositional profile that extends from the surface to the bulk and may be the first such report for TiO2.

3. EXPERIMENTAL PROCEDURE We prepared 0.65 at % Nb-doped TiO2 specimens from sol−gel derived Nb-doped powder according to a previously reported procedure.27 In this procedure, Nb-TiO2 was precipitated from the mixture of Nb2 Cl5 in ethanol and high-purity Tiisopropoxide (Sigma Aldrich) in ethanol. This precipitate was subsequently ground with an 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 then 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 that pad contamination was avoided. Once the final polish had been completed, the disks were cut into brick-shaped specimens with ∼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, which yielded an oxygen partial pressure of ∼10−15 Pa as determined using a zirconia-based oxygen probe. At this point the specimens were ready for surface compositional analysis using SIMS and XPS. Secondary Ion Mass Spectrometry Determination. The 93 Nb segregation profiles were determined in each specimen using SIMS (SIMS-IMS 4fE7, 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, the 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. X-ray Photoelectron Spectroscopy. The surface composition of all Nb-TiO2 samples was determined by XPS analysis, conducted using a Thermo Scientific ESCALAB250Xi instrument. The X-ray source consisted of monochromatic Al Kα Xrays (1486.68 eV) with an operating power of 164 W, and the photoelectron takeoff angle was 90°. The pass energy was 100 eV for survey scans and 20 eV for narrow region scans. The spot size 3408

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was 500 μm. The background vacuum in the analysis chamber was better than 2 × 10−7 Pa. All binding energies were calibrated to the C 1s peak at 284.8 eV for adventitious surface hydrocarbon contamination. The obtained spectra were resolved into Gaussian−Lorentian components after background subtraction using the Shirley fitting routine. The use of argon etching to remove carbonaceous surface contamination was avoided to protect the underlying composition of the surface and nearsurface region.

• During processing under high oxygen activity (p(O2) = 101 kPa), over the range of 1173−1673 K, increased Nb surface concentration is favored by decreasing the annealing temperature. • The Nb surface concentration is reduced to almost zero by applying very low oxygen activity (p(O2) = 10−15 Pa) during annealing at elevated temperatures. • During processing under very low oxygen activity (p(O2) = 10−15 Pa), over the range of 1173−1673 K, increased Nb surface concentration is favored by decreasing the annealing temperature. However, under the studied temperature range, the effect of temperature is marginal. Secondary Ion Mass Spectrometry Results. SIMS depth profiles were obtained for each specimen to identify changes in the concentration of Nb with depth from the specimen surface. Figure 4 illustrates a typical SIMS depth profile, where several elements of interest are traced as a function of depth. In this profile, which corresponds to the specimen annealed at 1173 K in p(O2) equal to 101 kPa, it is seen that the intensities of both matrix elements, 133Cs48Ti and 133Cs16O, vary substantially over the initial 10 nm before establishing reasonably stable levels for the remaining ∼20 nm of analysis. In part, this initial deviation from the steady-state level is due to the stabilization of sputtering by 133Cs. However, because the 133Cs intensity is comparatively stabilized throughout this region, the observed variations can be considered to reflect changes in the actual composition. In other depth profiles, the initial 133Cs intensity is less stable, and so the first 50 s (or first ∼1 nm) of analysis has been ignored throughout all SIMS depth profiles for the sake of consistency. As indicated in Figure 4, the SIMS technique delivers data of a semiquantitative nature, and the actual concentrations of particular species cannot be directly acquired without specially prepared reference specimens.10 Nevertheless, the concentrations of dopant species can be approximated by normalizing the intensity of the dopant against the intensity of a stable lattice element34 as shown below:

4. RESULTS X-ray Photoelectron Spectroscopy Results. XPS survey spectra and narrow scan spectra were obtained for all Nb-TiO2 specimens and have been, respectively, displayed as Figures 1 and

Figure 1. XPS survey spectra obtained for the surface of 0.65 at % Nbdoped TiO2 that had been annealed at 1173 K for 50 h in p(O2) equal to 101 kPa.

2 for a typical sample. From the survey spectra (Figure 1), photoelectron peaks have been revealed for Ti 2p3/2 (458.8 eV), O 1s (530.1 eV), Nb 3d (207.2 eV), and C 1s (285.0 eV). Upon closer inspection of each identified peak (Figure 2), additional photoelectron peaks have been identified for Nb 3d (207.2 and 209.9 eV), O 1s (530.1 and 531.9 eV), and C 1s (285.0, 286.6, 288.0, and 289.0 eV). From comparison with literature reports, the values obtained for these additional peaks are consistent with the presence of Nb 3d5/2, Nb 3d3/2, TiO2 lattice oxygen (530.1 eV), oxidized hydrocarbon (531.9 eV), and adventitious carbon contamination (285.0, 286.6, 288.0, 289.0 eV).28−31 The peak position of Ti2p3/2 at 458.8 eV is also very close to the value reported for Ti4+ (458.7 eV).30,32 The presence of Nb 3d 5/2 and Nb 3d3/2 peaks indicates a spin−orbital splitting of 2.7 eV and, at least within the surface and near-surface region, niobium has been substitutionally incorporated as Nb5+. On the basis of these XPS results the surface concentration of Nb was found to vary from 0.02 at % through to 4.96 at % depending on the oxygen activity during processing (see Figure 3). Table 1 summarizes these results and some general behavior becomes evident when the Nb surface concentration of the “polished only” specimen is used as a point of reference: • The Nb surface concentration is substantially increased by applying high oxygen activity (p(O2) = 101 kPa) during annealing at elevated temperatures. However, the observed results remain comfortably within the reported solubility limits of Nb in rutile TiO2.33

I(93 Nb) ∝ C Nb I(133Cs 48Ti)

(1)

where CNb is the approximated concentration of 93Nb, and I(93Nb), and I(133Cs48Ti) are, respectively, the intensities of 93Nb and 133Cs48Ti obtained during SIMS analysis. Whereas this approach may only approximate the concentration of Nb, it is an accurate reflection of changes in the concentration of Nb with depth. On the basis of this approach, the Nb segregation profiles of each specimen have been identified and are summarized in Figures 5a,b. On the basis of these results, Nb surface segregation is clearly favored by the application of high oxygen activity during processing at elevated temperature (see Figure 5a). Furthermore, over the studied temperature range, Nb surface segregation decreases in strength as the temperature is increased. In contrast, under the application of very low oxygen activity during processing at elevated temperatures, Nb becomes depleted from the surface of Nb-doped TiO2 (see Figure 5b). As the annealing temperature is increased, this depletion is seen to become more pronounced, and at 1673 K the shape of the depth profile is altered considerably from the profiles observed after annealing at 1173 and 1473 K. Consequently, the near-surface region that is affected by Nb segregation at 1673 K is comparatively deep, greater than 45 nm as compared with 15 nm or less for the other samples. 3409

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Figure 2. XPS narrow scan spectra of (a) niobium, (b) titanium, (c) oxygen, and (d) carbon photoelectron peaks obtained from the surface of 0.65 at % Nb-doped TiO2 that had been annealed at 1173 K for 50 h in p(O2) equal to 101 kPa.

Table 1. Summary of Annealing Conditions and Resultant Nb Surface Concentrations As Determined Using XPS temperature (K) 1173 1473 1673 1173 1473 1673

oxygen activity (Pa) polished only 101 000 101 000 101 000 10−15 10−15 10−15

time (h)

Nb concentration (at %)

50 50 50 50 50 50

0.21 4.96 3.69 3.09 0.05 0.03 0.02

From this Figure, it is clear that the surface enrichment/ depletion of Nb observed by the SIMS depth profiles consistently reflects the surface Nb concentration behavior observed with the XPS measurements. Furthermore, it is clear that changes in the oxygen activity during processing have had a significant impact on the Nb segregation behavior and that the extent of dependence varies with temperature. As summarized in Figure 5a, the surface of Nb-TiO2 becomes enriched in Nb during processing under high oxygen activity (p(O2) = 101 kPa), yet as the annealing temperature is increased from 1173 to 1673 K, the extent of enrichment marginally decreases. From Figure 5b, the Nb surface concentration is becoming depleted from the surface due to processing under low oxygen activity (p(O2) = 10−15 Pa), and as the temperature is increased over the studied range, the extent of depletion is increasing. This observed

Figure 3. Summary of the Nb surface concentration obtained using XPS for Nb-doped TiO2 that has been processed under controlled conditions of temperature and oxygen activity.

5. DISCUSSION In Figure 5, the effect of oxygen activity on the behavior of Nb segregation is indicated by the obtained depth profiles and surface Nb concentrations ([Nb]Surf) of each studied specimen. 3410

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Figure 6. Schematic illustration of the relationship between Nb activity and Nb concentration during the re-establishment of equilibrium.

point in the bulk, the activity of Nb will be defined by the interactions between the Nb ion and the surrounding lattice.35 For the purposes of simplicity, this particular activity may be considered to represent the standard state of Nb in Nb-TiO2, a°Nb, and would not change markedly unless the bulk composition of the lattice was dramatically altered. When a condition of equilibrium exists between the surface and the bulk of the specimen (i.e., after a long period of time, t = ∞), the activity of Nb at the surface would equal a°Nb and the resultant depth profile of Nb concentration would only reflect changes in the activity coefficient, f, with depth from the surface, in accordance with the definition of activity

Figure 4. SIMS depth profile analysis obtained for 0.65 at % Nb doped TiO2 annealed at 1173 K in p(O2) equal to 101 kPa for 50 h.

behavior demonstrates that the activity of Nb at the surface of Nb-TiO2, aNb, is significantly altered due to the change in oxygen activity and that the driving force for segregation is altered as a result. To explain these observations further, Figure 6 schematically illustrates the relationship between Nb concentration and Nb activity in the surface and near-surface region of Nb-TiO2. At a

Figure 5. Summary of SIMS depth profiles for 93Nb in Nb-doped TiO2 annealed in (a) high oxygen activity (p(O2) = 101 kPa) and (b) low oxygen activity (p(O2) = 10−15 Pa) at different annealing temperatures. 3411

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aNb = f [Nb]

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activity of Nb but to a lesser extent than oxygen activity. As readily controllable processing parameters, the results obtained from this investigation demonstrate a mechanism for manipulating the composition of both the surface and near surface regions of Nb-TiO2 and possibly for other TiO2-based systems as well.

(2)

However, if an abrupt change is made to a processing variable such as temperature or p(O2), then a change in the activity of Nb at the surface may result. This is indicated in Figure 6 at t = 0. The deviation between the surface Nb activity at this moment and the bulk Nb activity represents a difference in free energy between the surface and the bulk and provides a driving force for Nb segregation. As time proceeds, Nb migrates between the surface and the bulk under the action of this driving force such that the activity of Nb at the surface is shifted toward a°Nb. At t = ∞, the Nb activity has resumed a constant value throughout the solid, and the driving force for segregation has been removed. The surface and bulk are now at equilibrium but display differences in composition. By applying the above concept to the Nb segregation behavior displayed in Figure 5, several observations and conclusions can be drawn: 1) Irrespective of the applied oxygen activity, the bulk 93 Nb/133Cs48Ti intensity ratio for all specimens is trending toward the same value, 0.25. Whereas this is not a direct measure of activity, this supports the assumption that a°Nb may be considered to be stable under the presently applied processing regimes. The alternative possibility that a°Nb has shifted and been exactly compensated by a shift in f such that the concentration of Nb remains constant is considered to be remote. 2) As a result of the application of processing conditions of high oxygen activity (p(O2) = 101 kPa), the surface activity of Nb drops substantially, leading to the strong enrichment of the surface in Nb; [Nb]Surf ranges from 3.09 to 4.96 at %. 3) As a result of the application of processing conditions of low oxygen activity (p(O2) = 10−15 Pa), the surface activity of Nb increases substantially, leading to the surface becoming depleted of Nb; [Nb]Surf ranges from 0.02 to 0.05 at %. Physically, this depletion is likely the result of Nb evaporation. 4) As a result of increasing temperature (over the range 1173−1673 K), the surface activity of Nb increases marginally, leading to a slight but obvious depletion of Nb at the surface. Identifying the exact mechanism for the change in the surface activity of Nb with changing p(O2) is made difficult by the challenges associated with identifying the actual phase present at the immediate surface. This will be the subject of future investigation.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +61 2 4570 1745. Fax: +61 2 4570 1369. E-mail: l. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Australian Research Council’s Discovery Project funding scheme (project number DP0987084). Also acknowledged is the support of Dr. Bin Gong at the University of New South Wales Mark Wainwright Analytical Centre for performing the XPS analysis and the support of Dr. David Nelson for SIMS analysis (University of Western Sydney).

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REFERENCES

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6. CONCLUSIONS In the current investigation, Nb surface segregation in polycrystalline Nb-doped TiO2 (0.65 at % Nb) was studied as a function of oxygen activity and temperature. Using surface composition data obtained with XPS and Nb depth profiles obtained with SIMS, it was found that oxygen activity plays a strong role in determining the activity of Nb at the surface, and in turn, the segregation of Nb toward the free surface. Specifically, the application of strongly reducing conditions (p(O2) = 10−15 Pa) significantly increases the surface activity of Nb, resulting in the depletion of Nb from the surface and near-surface region. In contrast, the application of strongly oxidizing conditions (p(O2) = 101 kPa) leads to the surface becoming highly enriched in Nb. Temperature has also been observed to influence the surface 3412

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