Niobium Segregation in Niobium-Doped Titanium ... - ACS Publications

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Niobium Segregation in Niobium-Doped Titanium Dioxide (Rutile) Armand J. Atanacio,†,‡ Tadeusz Bak,‡ and Janusz Nowotny*,‡ †

Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia Solar Energy Technologies, University of Western Sydney, Penrith, NSW 2751, Australia

‡

ABSTRACT: This work determined the effect of niobium segregation on the surface and near-surface composition of Nb-doped TiO2, containing 0.18 atom % Nb and 0.018 atom % Nb, after annealing in the gas phase of controlled oxygen activity. The segregation-induced concentration profiles were determined using a range of analytical techniques of different depth resolution, including secondary ion mass spectrometry (SIMS), X-ray photoelectron spectroscopy (XPS), Rutherford backscattering (RBS), and proton-induced x-ray emission (PIXE). The XPS analysis of the 0.18 atom % Nb specimen annealed at 1273 K in oxidizing conditions and showed segregation-induced niobium surface enrichment of 2.83 atom % and 2.35 atom % in p(O2) = 75 kPa and p(O2) = 10 Pa, respectively. However, annealing at 1273 K in strong reducing conditions, p(O2) = 10−10 Pa, leads to depletion of the surface layer in niobium to the level of 0.05 atom % (desegregation). The results of SIMS, RBS, and XPS are consistent. The derived theoretical model, explaining the effect of oxygen activity on niobium segregation, considers contribution from three driving forces of segregation, including (i) strain relaxation, (ii) the formation of a lowdimensional surface structure, and the (iii) electric field associated with the surface charge. The established effect of oxygen activity on niobium segregation/desegregation may be used as a technology for imposition of (i) controlled surface composition that is required to achieve enhanced performance of TiO2 in solar-to-chemical energy conversion and (ii) chemically induced electric field required for charge separation.

1. INTRODUCTION Titanium dioxide, TiO2, is a candidate material for the conversion of solar energy into the chemical energy to generate hydrogen fuel via water oxidation.1−3 Its performance, which is determined by photoreactivity of TiO2 with water, is closely related to surface properties. Therefore, the performance of TiO2 in solar energy conversion may be enhanced by a modification of these properties. Consequently, intensive research is needed for better understanding of the effect of processing on surface properties and the impact of surface properties on performance. A major step forward in the surface science of titanium dioxide has been made by Diebold,4 who reported its surface properties, including structural properties, electronic structure, defect disorder, and reactivity with inorganic and organic molecules. The surface properties of solids are substantially different from those of the bulk phase as a result of segregation. The difference concerns chemical composition, structure, and the related functional properties.5−7 To date, the theory of segregation has been derived for metals and alloys.8−10 However, the picture of segregation for nonstoichiometric compounds is more complex. For metal oxides, for example, the segregation-induced enrichment depends on oxygen activity in the gas phase environment.11 Moreover, unlike metallic solids, the surface charge compensation in oxides occurs over a long distance from the surface. The thickness of the surface layer exhibiting the surface charge in oxide semiconductors is approximately 50−100 nm, compared to 1 nm in metals. Also the thickness of the surface layer enriched by segregation © 2014 American Chemical Society

in oxides, that is affected by the surface charge, is substantially larger than that in metallic solids.11 Knowledge of the effect of segregation on surface properties is essential for correct understanding of the functional properties of solids, such as reactivity and photoreactivity. The present work shows that the effect of oxygen activity on segregation may be used for tailoring surface composition in order to impose the specific properties that are needed to achieve desired performance. The determination of reproducible data on surface properties of metal oxides is difficult. Moreover, the experimental approach in using the surface sensitive tools for surface analysis of metal oxides is more complex than this is the case for metals.4 Substantial progress in this matter was made by Hirschwald12 who considered the application of a wide range of surface sensitive tools in the determination of reproducible data on surface properties of metal oxides. So far, the reported data on segregation in oxides are scarce and often conflicting. It has been documented that segregationinduced concentration gradients in metal oxides are welldefined only when the studied specimen is equilibrated in the gas phase of controlled oxygen activity and subsequently cooled down from equilibrium temperature to the temperature of the analysis in a controlled manner.11 Received: November 10, 2013 Revised: April 28, 2014 Published: April 29, 2014 11174

dx.doi.org/10.1021/jp4110536 | J. Phys. Chem. C 2014, 118, 11174−11185

The Journal of Physical Chemistry C

Article

Table 1. Defect Disorder of Pure TiO2 and the Related Defect Equilibria, Equilibrium Constants, Charge Neutralities, the Related Effect of Oxygen on the Concentration of Electronic Charge Carriers and the Associated Thermodynamic Functions20,21 (Where n and p Denote the Concentration of Electrons and Electron Holes, Respectively, ΔH° and ΔS° Denote the Enthalpy and Entropy Terms, Respectively, and p(O2) is Oxygen Activity) defect reaction

V O••

1

OOx

2

x Ti Ti + 2OOx ⇔ Ti••• + 3e′ + O2 i

3

x Ti Ti

4 5

⇔

+ 2e′ + 1/2O2

2OOx

Ti•••• i

equilibrium constant

charge neutrality

2[V O••]

K1 = [V O••]n2p(O2 )1/2

n=

3 K 2 = [Ti••• i ]n p(O2 )

n = 3[Ti••• i ] 4[Ti•••• ] i

∂log n ∂log p(O2)

ΔHO (kJ/mol)

ΔSO [J/(mol K)]

− 1/6

493.1

106.5

− 1/4

879.2

190.8

− 1/5

1025.8

238.3

K3 = [Ti•••• ]n 4p(O2 ) i

n=

• x O2 ⇔ V ′′′′ Ti + 4h + 2OO

′′′′ 4 K4 = [V Ti ]p p(O2 )−1

′′′′ p = 4[V Ti ]

− 1/5

354.5

−202.1

nil ⇔ e′ + h•

K i = np

n=p

0

222.1

44.6

+

⇔

+ 4e′ + O2

ln K = [(ΔS°)/R] − [(ΔH°)/RT].

equilibrium constants for the formation of intrinsic defects in TiO2, which are shown in Table 1.21 These data and the associated charge neutrality conditions have been used in derivation of defect disorder diagrams that represents the effect of oxygen activity on the concentration of both electronic and ionic defects. The defect disorder diagram for pure TiO2 at 1273 K is shown in Figure 1.20 The reactions in Table 1 and the

The present work is part of a research program to determine the effect of segregation of both acceptor- and donor-type ions on surface composition of TiO2 and its solid solutions. The aim of the research is to use the phenomenon of segregation as a technology in the formation of electrodes, photoelectrodes, and photocatalysts with desired properties, including (i) chemical composition of surface and subsurface layers, (ii) semiconducting properties of the surface layers, and (iii) a chemically induced electric field that is related to the concentration gradient within the surface and near-to-surface layers. This field aims at separation of the light-induced charge carriers and reduction of the recombination-related energy losses in solar energy conversion. The reported studies on the effect of niobium on the properties of TiO2 (rutile) have been focused on the determination of bulk properties, such as electrical conductivity.13,14 So far, little is known on surface properties of Nb2O5−TiO2 solid solutions, such as surface versus bulk composition and its impact on reactivity and photoreactivity with water. The present work examines the effect of niobium segregation on the surface and near-surface composition of Nbdoped TiO2. It is important to realize that the surface layer is not autonomous, and its properties are closely related to the bulk phase on one side and the gas phase on the other side. Therefore, the specific aim of this work is to determine the effect of oxygen activity on the segregation-induced concentration gradients within the surface layer. The experimental part of this work is preceded by sections considering the basic properties of TiO2, the effect of niobium on properties of rutile and definitions of the related basic terms. Since the effect of niobium on properties of TiO2 can be best described using the theory of defect chemistry, the introductory sections consider the defect disorder of both pure and Nb-doped TiO2.

2. DEFINITION OF TERMS 2.1. Defect Disorder and the Related Semiconducting Properties of TiO2. There is a common perception that the predominant ionic defects in TiO2 are oxygen vacancies, which are responsible for n-type charge transport.15 However, the most recent progress in semiconducting properties of TiO2 (rutile) indicates that rutile is an amphoteric semiconductor exhibiting both n- and p-type properties in reducing and oxidizing conditions, respectively.16−19 It has also been documented that the semiconducting properties of TiO2 are closely related to its defect disorder.20,21 An accumulation of empirical data on defect-related properties of TiO2 has resulted in the determination of

Figure 1. Defect diagram showing the effect of oxygen activity on the concentration of both electronic and ionic defects for pure TiO2.

diagram in Figure 1 are described using the Kröger−Vink notation.22 The diagram allows the following points to be made. (i) The predominant ionic defects in TiO2 are oxygen vacancies, which are compensated mainly by electrons in reducing conditions and titanium vacancies in oxidizing conditions. (ii) In oxidizing conditions, TiO2 exhibits n−p transition. (iii) In strongly oxidizing conditions, TiO2 is a ptype semiconductor. (iv) In strongly reducing conditions, TiO2 is an n-type semiconductor. 11175



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equilibrium concentrations of titanium vacancies corresponding to the n−p transition point that is represented in Figure 4.

In equilibrium, the properties of TiO2 are determined by the conditions of the equilibrium (oxygen activity and temperature). The data in Table 1 may be used for the determination of the effect of temperature on the concentration of both electronic charge carriers that is shown in Figure 2. As seen,

Figure 4. Equilibrium concentrations of titanium vacancies corresponding to the n−p transition point.19 Figure 2. Arrhenius plot of the concentration of electronic charge carriers in pure TiO2, according to the defect-related data in Table 1.

Figures 1 and 4 describe the TiO2/O2 system in gas/solid equilibrium. This is the case when the kinetic factor allows the equilibrium state to be reached relatively fast. This is possible for fast lattice species, such as oxygen vacancies and titanium interstitials.18 This is not the case for titanium vacancies, which exhibit an extremely slow diffusion rate.19 It is essential to note at this point that the data in Table 1, and the associated diagram shown in Figure 1, are related to the bulk and are not valid for the surface layer, which exhibits an entirely different defect disorder. Even if the surface layer remains in equilibrium (with the gas phase on one side and the bulk phase on the other side), its properties are entirely different from the bulk as a result of the excess of surface energy. The present work is an attempt to understand the effect of segregation on the chemistry and the related defect chemistry of the surface layers of Nb-doped TiO2, which is crucial for proper interpretation of photocatalytic and photoelectrochemical performance of this oxide system. 2.2. Effect of Niobium on Defect-Related Properties of TiO2. As seen in Figure 1, the concentration of electronic charge carriers assumes a minimum at the n−p transition point. The charge neutrality in the vicinity of the n−p transition point is governed by the following ionic charge compensation:

cooling of n-type TiO2 results in the formation of p-type TiO2 (the temperature of the n−p transition point depends on oxygen activity). The diagram in Figure 2 is valid for the TiO2/ O2 system that is cooled slowly enough so that it remains in equilibrium during cooling. However, rapid cooling of n-TiO2 from equilibrium to lower temperatures results in quenching the n-type TiO2, which then is a metastable compound. The diagram in Figure 1 has been verified experimentally and excellent agreement has been revealed. Figure 3 represents the electrical conductivity data reported by Balachandran and Eror23 (○) along the theoretical dependence derived using the theoretical model (continuous lines). As seen, there is an agreement between model and the experimental data. The data in Table 1 may also be used for the determination of

′′′′ [V O••] = 2[V Ti ]

(1)

The incorporation of niobium into the TiO2 lattice in oxidizing conditions leads to the formation of titanium vacancies:13,14,24 ′′′′ 2Nb2 O5 → 4Nb•Ti + V Ti + 10OOX

(2)

Consequently, the related defect disorder is governed by the following ionic charge compensation: ′′′′ 4[V Ti ] = [Nb•Ti ]

(3)

With eq 3 as well as the eqs 4 and 5 in Table 1 taken into account, the relationship between the concentration of electronic charge carriers and oxygen activity in this regime may be represented by the following expression:

Figure 3. Effect of oxygen activity on the electrical conductivity for pure TiO2; continuous lines represent the theoretical model described in Table 1 and ○ represent the experimental data reported by Balachandran and Eror.23 11176


The Journal of Physical Chemistry C ⎛ [Nb• ] ⎞1/4 Ti n = K i⎜ ⎟ p(O2 )−1/4 ⎝ 4K4 ⎠

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layer, leading to the formation of a low-dimensional surface structure. In summary, the derived defect disorder allows the following points to be made. (i) Reducing conditions: pure TiO2 in reducing conditions is an n-type semiconductor. Its predominant defects are oxygen vacancies and are compensated by electrons. Addition of niobium results in an increase of the concentration of electrons as a result of electronic charge compensation. (ii) Oxidising conditions: pure TiO2 in oxidizing conditions is a n- or p-type semiconductor (depending on oxygen activity), and its predominant defects are oxygen and titanium vacancies. The incorporation of niobium results in the formation of titanium vacancies, which lead to ionic charge compensation. The effect of oxygen activity on the electrical conductivity for pure and Nb-doped TiO2 in oxidizing and reducing conditions is shown in Figure 5. As seen, the effect is substantial. It appears

(4)

where n is the concentration of electrons, Ki is the intrinsic electronic equilibrium constant, and K4 is the equilibrium constant of the reaction between TiO2 and oxygen resulting in the formation of titanium vacancies. The incorporation mechanism of niobium and the associated electrical effects in reducing conditions are different. In this case, the incorporation of niobium results in the formation of quasi-free electrons (required for charge compensation) and evolution of oxygen gas (required to satisfy the mass conservation law): Nb2 O5 → 2Nb•Ti + 2e′ + 4OOX + 1/2O2

(5)

The resulting defect disorder is governed by electronic charge compensation of niobium: n = [Nb•Ti ]

(6)

The charge neutrality expressed by eq 6 indicates that the charge transport in this regime is controlled by the concentration of niobium incorporated into the TiO2 lattice. In this case, the associated concentration of electrons is essentially independent of the p(O2). In strongly reducing conditions, the concentration of oxygen vacancies becomes much larger than that of niobium ions. Therefore, the effect of niobium may be ignored. Then the simplified charge neutrality assumes the following form: 2[V O••] = n

(7)

The defect disorder of Nb-doped TiO2 and the related charge neutrality conditions were first derived by Baumard and Tani.13,14 The photoemission studies of Morris et al.25 indicated that at high oxygen activity only 0.58% of niobium ions are compensated by electrons, and the remaining 99.42% Nb are compensated by titanium vacancies. The mechanism of niobium incorporation represented by the equilibria 2 and 5 are in agreement with the electrical properties of Baumard and Tani.13,14 However, there are several bits of evidence indicating that the incorporation mechanism of both intrinsic and extrinsic ions into the surface layer includes both substitution and interstitial positions: (i) it has been shown that indium incorporates into the bulk and the surface layer of TiO2, according to the substitution and interstitial mechanism, respectively.26 (ii) Deren et al.27 observed that lithium incorporates into nickel sites of the bulk phase of NiO and interstitial sites at the surface. (iii) Several ions of ionic radii similar to that of niobium, diffuse in the TiO2 lattice according to an interstitialcy mechanism involving the cooperative migration of tetravalent titanium interstitial ions and the foreign ions.28 The related diffusion data indicate that the presence of niobium ions in interstitial sites is a logical consequence of this transport mechanism. Therefore, one may expect that in addition to the proposed substitution mechanism (2) and (5), which is valid for the bulk phase, niobium is incorporated into the surface layer of TiO2 according to the following mechanism: Nb2 O5 ↔ 2Nb••••• + i

5 O2 + 10e′ 2

Figure 5. Arrhenius-type plot representing the effect of oxygen activity on the electrical conductivity data for pure and Nb-doped TiO2.

that oxygen activity has a substantial effect on segregation as well. It is important to note that the effect of segregation on properties may be considered in terms of defect disorder only when the dopant forms a solid solution with the oxide matrix. However, when the segregation-induced enrichment surpasses a certain critical value, then the oxide lattice undergoes a local structural transition leading to the formation of low-dimensional surface structures, which exhibit extraordinary properties.11,26 2.3. Diffusion Coefficient. The expressions (1)−(6) are valid when niobium is incorporated into the TiO2 lattice,

(8)

In this case, the presence of niobium in interstitial positions at the surface results in a structural deformation of the surface 11177



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established earlier.40 In both reports,41,42 Sheppard et al. claim that oxygen activity of the gas mixture involving 99% argon and 1% hydrogen in the range 1173−1673 K remains at the level of p(O2) = 10−15 Pa. Since this claim is in obvious conflict with the mass action law, the data reported by Sheppard et al.41,42 must be in question. Former work of the authors has shown that the segregationinduced enrichment of the surface layer In-doped TiO2 results in the formation of a low-dimensional surface structure that exhibits outstanding properties.26 The formation of surface structure has been also observed for pure TiO2 by Tao et al.43 3.2. Unresolved Problems. The driving force for niobium segregation to the surface of TiO2 has been considered in terms of ionic size mismatch between the larger niobium ions (0.07 nm) and smaller titanium ions (0.068 nm).25 However, the ionic size alone does not explain the observed effects of oxygen activity on surface composition of Nb-doped TiO2. In other words, the effect of oxygen on surface composition of Nbdoped TiO2 should be considered in terms of several driving forces of segregation. Therefore, there is a need to derive a theoretical model of niobium segregation in TiO2, which is consistent with the reported data on the effect of oxygen activity on surface composition. In this matter, the following questions can be formulated: (i) What are the segregation driving forces that explain the effect of oxygen activity on niobium segregation in TiO2? (ii) What is the effect of defect disorder on segregation of niobium in TiO2? In order to address these questions, one should collect segregation-related data that must be well-defined in terms of the following key requirements: (i) segregation-induced concentration profiles should be determined for specimens equilibrated in the gas phase of controlled oxygen activity. (ii) Specimens equilibrated at elevated temperatures should be subsequently cooled down in the gas phase of well-defined oxygen activity. So far, such data is not available. Furthermore, correct interpretation of the segregation-induced concentration profiles requires the determination of dopant enrichment at different distances from the surface. This can be achieved by applying surface sensitive tools of different depth resolution. The commonly used surface sensitive tool, X-ray photoelectron spectroscopy (XPS), informs only of an average chemical composition within a certain thickness, which depends on the applied angle of incidence (4−6 nm). However, this technique cannot provide depth profiles with sufficiently high depth resolution. The surface analysis technique that can determine the concentration-related intensity profiles versus depth is secondary ion mass spectrometry (SIMS). However, the determination of quantitative depth profiles, in terms of the chemical composition, requires awkward calibrations of the SIMS intensity data and matrix effects. Both SIMS and XPS applied independently provide complementary information. An alternative technique of larger depth resolution is Rutherford backscattering (RBS).44 This technique, which provides information on elemental composition, is less surface sensitive than XPS and SIMS. Finally, the analytical technique, which is predominantly bulk-sensitive in elemental analysis is proton-induced X-ray emission (PIXE).35 Therefore, the most reliable approach in the determination of surface versus bulk chemical composition is by applying all of these analytical techniques (the related penetration depths are represented schematically in Figure 6).

forming a homogeneous solid solution. This is the case when the following conditions are met: (i) the Nb/Ti ratio is within the solubility limit. In accordance with Eror, the solubility limit of niobium in TiO2 is 8 atom %.29 (ii) The kinetic factor allows the formation of solid solution. Quantitative analysis of this factor requires knowledge of the diffusion coefficient of niobium in the TiO2 lattice. The related self-diffusion coefficient can be expressed as follows:30 D Nb = (4.7 ± 0.4) × 10−7 ⎧ −244 ± 9 kJ/mol ⎫ 2 −1 ⎬[m s ] exp⎨ ⎩ ⎭ RT

(9)

Knowledge of this diffusion coefficient allows selection of the processing conditions (time and temperature) required for niobium incorporation into the TiO 2 lattice and its homogeneous distribution.

3. BRIEF LITERATURE OVERVIEW 3.1. Effect of Niobium on Bulk vs Surface Properties of TiO2. The incorporation of niobium into the TiO2 lattice results in a replacement of tetravalent titanium ions with pentavalent ions leading to the formation of donor-type centers. The Nb-related energy levels are located just below the bottom of the conduction band.25 Ionization of these centers results in increased concentration of quasi-free electrons and enhanced charge transport.13,14 According to Yang et al.,31 incorporation of niobium into TiO2 results in band gap reduction from 3.1 eV for pure TiO2 to 2.9 eV for Nb-doped TiO2 (5 atom % Nb). This effect suggests that the Nb-related donor states are 0.2 eV below the bottom of the conduction band. According to Morris et al.,25 these states are located 0.02−0.03 eV below the conduction band minimum. Several attempts have been made to determine niobium segregation in Nb-doped TiO2.31−36 However, the reported data is conflicting in terms of the niobium enrichment factor and its temperature dependence. Analysis of these reports indicates that the studied specimens are not well-defined in terms of high temperature processing and subsequent cooling conditions. The observed discrepancies are commonly related to the following. Lack of Equilibrium. The reported data correspond to the kinetic regime rather than thermodynamic equilibrium. Oxygen Activity. High-temperature annealing is not welldefined in terms of oxygen activity of the surrounding gas phase. Cooling. The cooling process is not well-defined in terms of the cooling rate and the gas phase environment during cooling. Awareness is growing that the key factor in the formation of segregation-induced concentration gradients of solutes in metal oxides is oxygen activity.37,38 The effect of oxygen activity on the segregation-induced enrichment in oxides was reported by Black and Kingery37 for both Cr-doped MgO and Fe-doped MgO, Sikora et al.38 for Cr-doped CoO, and Bernasik et al. for Fe-doped TiO2.39 Application of XPS and SIMS in surface analysis of Nbdoped TiO2 annealed in controlled oxygen activity has shown that annealing in oxidizing and reducing conditions results in a strong enrichment and depletion of the surface in niobium, respectively.40 Recently Sheppard et al.41,42 reported the effect of oxygen activity on niobium segregation in Nb-doped TiO2. Their SIMS- and XPS-related study aim to reinvestigate the effects 11178



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decrease the intensity of the low-energy X-rays excited with large cross sections. The PIXE spectra were processed using GUPIXWIN to determine the concentration of elements in the sample. 4.2.2. Secondary Ion Mass Spectrometry (SIMS). The surface versus bulk depth profiles were determined by SIMS (Cameca IMS 5f). A Cs+ primary ion beam of 5 keV net impact energy and 5 nA beam current rastered the area of approximately 250 × 250 μm. Aperture settings were applied to limit the analysis of secondary ions to a 55 μm circular area within the rastered region to avoid contributions from crater edge effects. The crater depth, measured by a stylus profilometer (Alpha Step 2000, KLA Tencor), was used to determine the average sputter rate (0.012 nm/s). In order to remove the surface charge that is built-up during sputtering, gold coating is needed to dissipate the charge. The rationale for gold coating may be summarized as follows. (i) Even if niobium is entirely incorporated, the resulting electrical conductivity is not high enough to prevent the charging effects during sputtering. (ii) The effect of Nb on electrical conductivity depends on oxygen activity in the oxide lattice as represented by eqs 2 and 5. This means that the Nb-doped TiO2 equilibrated in oxidizing conditions is an insulator. Figure 5 shows that niobium results in increased conductivity only when the specimen is reduced. As seen, the electrical conductivity of Nb-doped TiO2 in oxidizing conditions is much lower than that of pure TiO2 in reducing conditions. Therefore, Nb-doped TiO2 annealed in oxidizing conditions requires gold coating. 4.2.3. X-ray Photoelectron Spectroscopy (XPS). The XPS analysis was performed using a Thermo Scientific ESCALAB 250 xi instrument. The sample was excited with a monochromatic Al Kα source of 1486.6 eV X-ray energy operated at 15 kV and 160 W. The X-ray spot was approximately 0.5 mm in diameter, and the takeoff angle of 90° relative to the sample surface was used for all analyses, which results in an analysis depth of approximately 6 nm. An energy pass filter of 100 eV was used for survey scans and 20 eV for elemental region scans. A 20 s argon sputtering etch was required prior to analysis of as-polished samples to remove surface carbon likely remaining from the polishing/lubrication medium used. The Ar+ beam energy was 3 keV. The related etching rate (approximately 0.4 nm/s) has been calibrated using a Ta2O5/Ta reference sample. The relative surface concentrations of the different species were determined by integrating their respective XPS peak areas. 4.2.4. Rutherford Backscattering (RBS). The RBS analysis was performed on a 2MV tandem accelerator. He+ ions of 2 MeV energy were used at normal angle of incidence to the sample surface with backscattered ions being detected at an angle of 160°. A collimated 3 mm diameter beam of approximately 10 nA beam current was used for analysis, and a charge of 40 μC was acquired for each sample. The backscattered ions were detected using a silicon surface-barriercharged particle detector (4 mm diameter active area) and energy analyzed using a multichannel analyzer. The RBS spectra were processed using SIMNRA. 4.2.5. Verification of Solid Solution Formation. In addition to the determination of the diffusion coefficient of niobium in the TiO2 lattice (see section 2.3), we have performed the following experimental tests. We determined the electrical conductivity of Nb-doped TiO2 during both oxidation and reduction experiments in the temperature range of 1073−1273

Figure 6. Schematic representation comparing the approximate penetration depth of SIMS, XPS, RBS, and PIXE analysis techniques.

4. EXPERIMENTAL SECTION 4.1. Specimens and Procedure. The Nb-doped TiO2 polycrystalline specimens of nominal concentrations of 0.2 atom % Nb and 0.02 at % Nb were prepared using the sol−gel method. The basic reactants included titanium isopropoxide and NbCl5 of appropriate proportions. The resulting solid solutions were ground to a fine powder and calcined at 1173 K. The powder was then pressed into pellets using uniaxial pressing (50 MPa) and subsequently pressed isostatically under 400 MPa. These pellets were sintered at 1773 K for 5 h in air. The resulting polycrystalline specimens were fully dense with a grain size in the range of 4−10 μm. The selected content of niobium was substantially below the solubility limit that is 8 atom % Nb.29 The surface of the specimens was then ground and polished. The resulting as-polished specimens were used as a reference. The as-polished specimens were subsequently annealed at 1273 K for 24 h in order to induce segregation. After annealing, the specimen was rapidly removed from the hot zone of the furnace to the room temperature zone while still remaining in the same gas phase composition. The removal aimed at quenching the segregation equilibrium. Our X-ray diffraction (XRD) analysis revealed that the specimens consist of a single rutile phase. The specimens were annealed in the gas phase of controlled oxygen activity, which was monitored during the entire time of annealing using an electrochemical oxygen probe based on yttria-stabilized zirconia. The controlled oxygen activity gas phases used during annealing were the following. (i) Pure oxygen: its oxygen activity determined electrochemically is p(O2) = 75 kPa. (ii) Pure argon: the related oxygen activity is p(O2) = 10 Pa. (iii) Argon−hydrogen mixture: involving 1% hydrogen and 99% argon. The oxygen activity of this mixture at 1273 K is p(O2) = 10−10 Pa. (iv) The gas flow velocity during all experiments was 100 mL/min. 4.2. Bulk vs Surface Chemical Analysis. 4.2.1. ProtonInduced X-ray Emission (PIXE). The PIXE technique determined the elemental composition of the bulk phase. PIXE was performed with a 2 MV tandem accelerator using ion beam of 2.6 MeV protons at a normal angle of incidence to the sample. The X-rays were detected at an angle of 45 deg from normal with a Si (Li) detector fitted with a 25 μm thick Be window. A 10 μC charge was acquired for each sample. A pinhole filter (1700 μm thick acrylic with 2% hole area) fitted with an additional 4 μm thick mylar film was utilized to further 11179



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5.2. Secondary Ion Mass Spectrometry. The SIMS spectra are represented in the form of the intensity ratio of niobium to TiO species. These spectra were determined for the specimen of Nb-doped TiO2 containing 0.18 and 0.018 atom % Nb after polishing as well as after annealing, respectively, at 1273 K in the gas phase of controlled oxygen activity. The data for as-polished specimen is considered as the reference data reflective of bulk composition. The spectra for the specimen containing 0.18 atom % Nb are shown in Figure 8a along with the spectrum of gold. The data

K. If the specimens were heterogeneous, including two reacting phases, the conductivity should differ from run to run. The data in Figure 5 shows that the electrical conductivity exhibit outstanding reproducibility within both oxidation and reduction experiments. Moreover, these data are in perfect agreement with the theoretical defect disorder model.20 The electrical conductivity data are in perfect agreement with the defect model derived for Nb-doped TiO2.

5. RESULTS AND DISCUSSION 5.1. Proton-Induced X-ray Emission. The PIXE spectrum for the as-polished specimen is shown in Figure 7,

Figure 7. PIXE X-ray yield vs energy spectra for Nb-doped TiO2 (0.18 atom %) as-polished specimen.

including the inset for the niobium-related peak. In accordance with this analysis, the bulk concentration of niobium is 0.18 atom %. The PIXE analysis shows that the concentration of niobium in the second specimen is 0.018 atom %. The results are shown in Table 2. As seen, niobium concentration is practically the same for the specimens annealed in different oxygen activities. This data indicates that the effect of niobium evaporation can be ignored. The concentration of niobium resulting from the PIXE analysis has been assumed as the reference concentration related to the bulk phase.

Figure 8. SIMS depth profile for Nb-doped TiO2, including specimens containing (a) 0.18 atom % and (b) 0.02 atom % Nb, in terms of the intensity ratio of Nb/TiO secondary ion yield.

Table 2. Collection of PIXE, SIMS, XPS, and RBS Data on Niobium Concentration and the Related Enrichment Factors concentration of niobium (atom %) enrichment factor, f = [Nb]surface/[Nb]PIXE bulk Nb (atom %)

p(O2) [Pa]

PIXE

XPS

RBS

0.18

as polished

0.180 ± 0.004

100 × 103

0.179 ± 0.004

10

0.177 ± 0.004

1 × 10−10

0.181 ± 0.004

0.17 ± 0.03 f XPS = 0.9 2.83 ± 0.6 f XPS = 15.7 2.35 ± 0.5 f XPS = 13.1 0.05 ± 0.01 f XPS = 0.3

0.19 ± 0.04 f RBS = 1.1 0.31 ± 0.06 f RBS = 1.7 0.32 ± 0.06 f RBS = 1.8 0.16 ± 0.03 f RBS = 0.9

as polished 100 × 103

0.018 ± 0.001

0.018

10

11180

SIMS (I

Nb

/I

TiO)

0.19 4.1 f SIMS 2.3 f SIMS 0.16 f SIMS 0.03 0.07 f SIMS 0.04 f SIMS

= 21.6 = 12.1 = 0.84

= 2.33 = 1.33



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factors f = 15.7 and f = 13.1, respectively. On the other hand, the concentration of niobium in the surface layer of the specimen annealed in strongly reducing conditions is 0.05 atom %. The related enrichment factor ( f = 0.3) indicates that the surface is impoverished in niobium. The XPS data are in a good agreement with the SIMS data in terms of the effect of oxygen activity on surface enrichment with niobium. 5.4. Rutherford Backscattering. Figure 10 represents the RBS spectra for the Nb-doped TiO2 (0.18 atom % Nb)

related to nonequilibrium sputtering (associated with the gold cover and demarcated by the vertical line) is not taken into account. Therefore, only the part of the spectra on the right side of the vertical line is considered as meaningful for the oxide lattice.45 Consequently, the depth is referred to as the vertical line that is marked as d = 0. The depth profiles shown in Figure 8 indicate that (i) the highest intensity ratio of Nb/TiO at the surface (d = 0) is observed for the specimen annealed at p(O2) = 75 kPa. The related enrichment factor, in terms of ion intensities, is f = 21.6. This intensity ratio converges with the spectrum for the as-polished specimen (reflecting bulk composition) at d = 30 nm. The latter data can be considered as the segregation-induced penetration depth of niobium at 1273 K. (ii) The intensity ratio of Nb/TiO for the specimen annealed at p(O2) = 10 Pa at d = 0 is slightly lower. The related enrichment factor is f = 12.1. As seen, the segregation-induced enrichment is reduced to the bulk level at d = 30 nm. (iii) The depth profile of the Nb/TiO ratio for the specimen annealed at p(O2) = 10−10 Pa indicates that the surface layer is impoverished in niobium. The related enrichment factor, which is f = 0.84, is reflective of the reduced concentration of niobium at the surface. The data for the Nb-TiO2 specimen involving 0.018 atom % Nb is consistent with that for 0.18 atom % Nb. 5.3. X-ray Photoelectron Spectroscopy. Figure 9 represents the XPS spectra for the Nb-doped TiO2 specimen

Figure 10. RBS yield vs energy spectra for Nb-doped TiO2 (0.18 atom % Nb) specimens: (a) as-polished (reference), (b) annealed in the gas phase of p(O2) = 100 kPa, (c) annealed in the gas phase of 10 Pa, and (d) annealed in the gas phase of 10−10 Pa.

specimens. Each spectrum shows the full backscattered energy range for the specimens annealed in the gas phase of different oxygen activity as well as the reference specimen. The data points represent the experimental values, and the solid lines represent the simulated data fits obtained with SIMNRA (the backscattered energy steps at 740, 1440, and 1690 keV correspond to oxygen, titanium, and niobium, respectively). The insert plots provide an expanded view of the energy region related to niobium. The region of interest (shaded areas) corresponds to the depth of 61 nm. As seen in Figure 10a, the niobium concentration for the as-polished specimen is 0.19 atom %. This concentration is well-consistent with the bulk content of niobium determined by PIXE (0.18 atom %). As seen in Figure 10 (panels b and c), the niobium-related peaks in the shaded areas indicate that niobium concentration for the specimens annealed at p(O2) = 75 kPa and p(O2) = 10 Pa is elevated to the level of 0.31 and 0.32 atom %, respectively. On

Figure 9. XPS intensity vs binding energy spectra for Nb-doped TiO2 (0.18 atom % Nb), including the survey scan for the (a) as-polished (reference) specimen and elemental region scans for the (b) aspolished specimen, (c) annealed in the gas phase of p(O2) = 75 kPa and (c) p(O2) = 10 Pa and (d) p(O2) = 10−10 Pa.

containing 0.18 atom % Nb, including the reference (aspolished) specimen and the annealed specimens. The related concentration data are shown in Table 2. As seen, the specimen annealed in oxygen and in argon exhibit surface enrichment 11181



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the surface of TiO2, and the effect of oxygen activity on surface vs bulk defect disorder. 5.6.1. Experimental Data. The effect of oxygen activity on the segregation-induced enrichment factor of niobium in Nbdoped TiO2 is summarized in Figure 11 and Table 2. As seen,

the other hand, annealing in strongly reducing conditions, p(O2) = 10−10 Pa (Figure 10d), results in a slight impoverishment of niobium to the level of 0.16 atom % (the related concentration data are in Table 2). While the RBS data are less surface sensitive than those related to XPS and SIMS, these results are well-consistent. 5.5. Segregation Driving Forces. The strain relaxationinduced driving force of segregation that is related to ionic mismatch, explains the segregation-induced surface enrichment observed in an oxidizing environment. However, with the effect of oxygen activity on surface composition in both oxidizing and reducing environments, it is essential to take into account the involvement of three driving forces of segregation acting independently: (i) strain relaxation, (ii) electric field between the surface layer and segregating species, and (iii) formation of low-dimensional surface structure. 5.5.1. Strain Relaxation. The strain relaxation term is related to the misfit between the ionic radii of the matrix and the solute.5 The strain relaxation component for Nb-doped TiO2 is due to the difference between the ionic radii of titanium (Ti4+) and niobium (Nb5+), which are 0.068 and 0.07 nm, respectively. Therefore, the strain energy component of the segregation driving force explains enrichment of the surface in niobium. In accordance with Morris et al.,25 the strain energy is the predominant driving force of surface segregation in oxide solutions, such as Nb-doped TiO2. 5.5.2. Electric Field. The electric field, which is formed by the surface charge, may lead to enhanced transport of charge species toward, or outward from, the surface. The surface charge at elevated temperatures may result from segregation of intrinsic defects, leading to the formation of concentration gradients in the surface layer. These defects are formed at the gas/solid interface and are quenched at the interface. In the case of TiO2, a negative surface charge is likely to be imposed by titanium vacancies, which are formed in oxidizing conditions very fast but remain at the surface, owing to their extremely low diffusion rate in the TiO2 lattice.19 5.5.3. Formation of Low-Dimensional Surface Structures. There has been an accumulation of data, indicating that the excess of surface energy leads to the formation of lowdimensional surface structures, which differ from the bulk phase in terms of chemical composition, structure, and electronic structure.11 It has been shown for example that the outermost surface layer of CoO is covered by a spinel-type thin surface layer that is formed in the conditions corresponding to the stability range of CoO.46 Recent studies of Tao et al.43 have shown that the surface of pure TiO2 exhibits a two-dimensional structure, which is distinctively different from that of the bulk phase. This structure, which is confined to the topmost surface layer with some relaxations of atoms in the lower layers, has a band gap of 2.1 eV compared to 3.0 eV in the bulk phase. Similar low-dimensional surface structures have also been reported for oxide solid solutions.11 It has been shown that the outermost surface layer of In-doped TiO2 is stabilized by an In2TiO5-type surface structure.26,47 The tendency of oxides to form such structures is an additional driving force for segregation of the lattice components needed to achieve a specific surface composition that is thermodynamically stable. 5.6. Theoretical Models. This section considers the obtained experimental data in terms of several theoretical models that take into account the established defect disorder of TiO2, possible mechanisms of niobium incorporation into the TiO2 lattice, several driving forces of niobium segregation to

Figure 11. Effect of oxygen activity on the segregation-induced enrichment factor of niobium, reflective of different analytical techniques.

the enrichment at p(O2) = 75 kPa related to SIMS ( f = 21.6) is larger than that for XPS (f = 15.7) and substantially larger than that for RBS (f = 1.7). These data are consistent with depth resolutions of the respective techniques. The depletion data observed in the strongly reducing environment, p(O2) = 10−10 Pa, as shown by XPS (f = 0.3), SIMS (f = 0.84), and RBS (f = 0.9), are distinctively below the related reference levels. 5.6.2. Oxidizing Conditions. While the dominant driving force for segregation is strain relaxation associated with the ionic radii misfit between niobium and titanium, niobium segregation is also enhanced by the presence of titanium vacancies, which are formed at the TiO2/O2 interface and trapped at the surface.19 The segregation-induced enrichment of the surface in niobium results in the formation of an Nb2TiO7-type surface structure, which is charged negatively (compared to the bulk phase). The concentration gradients associated with segregation are represented in Figure 12, showing a strong gradient of titanium vacancies in the initial stage (Figure 12a), the surface structure formed in equilibrium (Figure 12b), and the associated distribution of the electrical potential (Figure 12c). The bulk phase of Nb-doped TiO2 may be considered as an ideal solid solution involving niobium ions distributed randomly. However, niobium enrichment of the surface layer results in substantial interactions between the defects within this layer, leading to the formation of larger defect aggregates.48 It is therefore reasonable to expect that the following defect complexes are formed: ′′′′ ′′′′ x 4Nb•Ti + V Ti → {(Nb•Ti )4 V Ti }

(10)

These complexes are the precursors of a new low dimensional surface structure, which is formed when the concentration of defects surpasses a critical concentration. 5.6.3. Reducing Conditions. The picture in reducing conditions is entirely different. In this case, the surface is enriched in oxygen vacancies, which have a tendency to form a Ti8O15-type surface structure, which is positively charged in 11182



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conditions, eq 11 assumes the form represented by eq 7. In this condition, the positive surface charge is responsible for removal of niobium from the surface layer. Consequently, the surface layer in a strongly reducing environment is mainly enriched in oxygen vacancies. Strong interactions between these defects are expected to result in the formation of a lowdimensional surface structure, which is similar to Magneli-type structures.49 5.7. Depth-Resolved XPS Analysis in Strongly Reducing Conditions. The effect of annealing in a strongly reducing environment, which results in depletion of the surface layer in niobium, is important due to its potential application. Namely, the phenomenon of segregation/desegregation may be used in surface engineering. Therefore, it seems worthwhile to verify the observed SIMS depth profile by using an alternative approach, such as XPS. XPS analysis was performed at two depths on the 0.18 atom % specimen after 30 min of annealing at 1273 K. The first analysis was done at the surface and again after a surface layer of 20 nm thick was removed by Ar+ sputtering. As seen in Figure 14, a substantial effect of

Figure 12. Theoretical model representing the concentration gradients of niobium and titanium vacancies during different stages of segregation in the oxidizing environment, including (a) the initial stage, (b) the final stage, and (c) the electrical potential distribution.

comparison to the bulk. The resulting electric field is the predominant driving force for the transport of positively charged niobium ions from the surface toward the bulk, which results in a depletion of the surface layer. The concentration gradients of defects in Nb-doped TiO2, which develop during reduction and the associated electrical potential barrier, are shown in Figure 13.

Figure 14. Results of XPS analysis of the 0.18 atom % Nb-doped TiO2 specimen surface after 30 min annealing at 1273 K and after subsequent removal of 20 nm thick surface layer with argon sputtering.

desegregation is already observed after only 30 min of annealing. These data show that the concentration of niobium is reduced from the initial level of 0.18 atom % (the bulk level) to 0.1 and 0.06 atom % at depths of 20 and 6 nm from the surface, respectively, indicating that desegregation is induced by the low oxygen activity relatively fast. 5.8. Effect of Evaporation. The effect of oxygen activity on segregation-induced surface concentration of niobium is well-defined only when the effect of niobium evaporation is absent. This effect, if any, was assessed using the measurements of electrical conductivity, which are very sensitive to chemical composition. Figure 15 represents the reproducibility of the electrical conductivity versus oxygen activity data for two experimental runs of oxidation and reduction; each run was performed after a prolonged annealing. As seen, there is excellent reproducibility between the two runs, indicating that the evaporation effect of either Nb or Ti can be ignored. These data are in agreement with the thermodynamic data reported by Sheldon and Gilles50 and Kamegashira et al.51

Figure 13. Model representing the concentration gradients of niobium and titanium vacancies during different stages of segregation in reducing environment including (a) the initial stage, (b) the final stage, and (c) the electrical potential distribution.

The models represented in Figures 12 and 13 may be considered in terms of defect disorder, and the related charge neutrality condition for the Nb-doped TiO2 lattice in the surface layer and the bulk phase. The condition requires that negatively charged defects (titanium vacancies and electrons) are compensated by positively charged defects (niobium ions in titanium sites and oxygen vacancies): ′′′′ 4[V Ti ] + n = [Nb•Ti ] + 2[V O••]

(11)

The increase of oxygen activity results in an imposition of enhanced concentration of titanium vacancies at the surface. Then eq 11 assumes a simple form represented by eq 3. In this case, the negative surface charge is the driving force of niobium segregation. On the other hand, in reducing 11183



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6. CONCLUSIONS It is shown that oxygen activity has a crucial effect on segregation and the associated surface versus bulk composition of Nb-doped TiO2. The surface layer may either be enriched or depleted in niobium as a result of segregation or desegregation, respectively. Assessment of this effect indicates that segregation for Nb-doped TiO2 must be considered in terms of several driving forces that are closely related to oxygen activity and the associated defect disorder in the bulk and within the surface layer. It is concluded that the established effect of oxygen on segregation-induced surface composition for Nb-doped TiO2 may be used as a technology for surface engineering of oxide semiconductors in order to achieve desired performance in solar energy conversion.

■

Figure 15. Electrical conductivity vs oxygen activity for Nb-doped TiO2 single crystal showing the reproducibility of data at 1298 K during oxidation and subsequent reduction; as seen, the data exhibits outstanding reproducibility even if the annealing time between oxidation and reduction was 140 h (the numbers correspond to the time interval between oxidation and reduction).

Corresponding Author

*E-mail: [email protected]. Tel: 61-2-4284-7829. Fax: 61-4620-3711. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

5.9. Surface Potential. The electrical potential gradient, which is formed as a result of segregation, has an effect on separation of light-induced electronic charge carriers. The segregation-induced enrichment/depletion of the surface in niobium is substantial. The electrical potential gradient component associated with niobium segregation can be determined from the Boltzmann relation:52 CS = C B( −ez Ψ/kT )

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Thanks are due to Dr. Mihail Ionescu and Mr. Ed Stelcer for their assistance with RBS and PIXE analysis, Dr. David Nelson for SIMS analysis, and Dr. Bill Gong for XPS analysis.

■

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where CS and CB denote the surface and bulk concentration, respectively, e is the elementary charge, ψ is electrical potential, z is the valence of segregating species, k is the Boltzmann constant, and T is the absolute temperature. The effect of segregation/desegregation-induced enrichment, determined by XPS and SIMS, and the related electric field are shown in Table 3. The sign + and − are reflective of the field polarity.

XPS

SIMS V (m)

f

V (m)

15.7 13.1 0.3

−2.9 × 106 −2.7 × 106 1.2 × 106

21.6 12.1 0.84

−3.2 × 106 −2.6 × 106 1.8 × 105

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Table 3. Enrichment (Depletion) Coefficients of the Surface Layer in Niobium and the Related Electric Fields f

AUTHOR INFORMATION

5.10. Impact in Application. It has been documented that the conversion efficiency of solar energy into chemical energy is determined by several key performance-related properties, including (i) charge transport, (ii) electronic structure and band gap, (iii) flat band potential, and (iv) reactivity-related surface active sites.32 All these properties may be affected by segregation in the surface layer, within the light penetration distance. Therefore, the phenomenon of segregation may be used as a technology in the modification of surface and nearsurface properties in a controlled manner in order to achieve desired performance. 11184



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