Dual Mechanism of Indium Incorporation into TiO - American

Article pubs.acs.org/JPCC

Dual Mechanism of Indium Incorporation into TiO2 (Rutile) Janusz Nowotny,* Tadeusz Bak, and Mohammad A. Alim Solar Energy Technologies, School of Computing, Engineering and Mathematics, University of Western Sydney, Penrith, NSW 2751, Australia ABSTRACT: This work considers the effect of indium on defect disorder and semiconducting properties of TiO2 (rutile). It is shown that the electrical conductivity and thermoelectric power data of indium-doped TiO2 can be best described by a dual mechanism of indium incorporation into the TiO2 lattice, including both the titanium sites, leading to the formation of acceptors and interstitial sites, leading to the formation of donors. The concentration ratio of these defects depends on the distance from the surface. The proposed defect disorder model of In-doped TiO2 is considered in terms of the surface layer enriched predominantly in indium located in interstitial sites and the bulk phase involving mainly indium located in the titanium sublattice.

• Defect disorder is the best approach in explanation of semiconducting properties of nonstoichiometric oxides.8 • The key performance-related properties (KPPs) of TiO2 as photocatalyst, such as electronic structure, charge transport, and the concentration of surface active sites, are defect-related.9 Therefore, the performance of TiO2 in solar-to-chemical energy conversion may be maximized in a controlled manner by optimization of the KPPs using defect engineering.9 Consequently, explanation of the observed effects of indium on the photocatalytic performance of TiO2 requires better understanding of the defect disorder and the related semiconducting properties of In-doped TiO2. So far, little is known in this matter. The effect of indium on enhanced photocatalytic performance of TiO2 has been mainly considered in terms of band gap reduction.10−12 However, this simplified approach does not take into account the local properties of the surface layer, which are entirely different from those of the bulk phase as a result of segregation.13 The local properties of the surface layer and it defect disorder have a crucial effect on light absorption and the light-induced reactivity. According to Kofstad,8 the incorporation of trivalent ions, such as indium, into the bulk phase of the TiO2 lattice leads to the formation of acceptor energy levels as a result of a substitutional mechanism. The strongest evidence of the substitution mechanism of indium incorporation is the recently established effect of indium on the electrical properties of TiO2, including electrical conductivity and thermoelectric power.14 It has been shown the indium results in a shift of the n−p transition point toward lower oxygen activity.

1. INTRODUCTION TiO2 is a promising candidate material for photocatalytic and photoelectrochemical energy conversion.1,2 The main stream of research, which aims at the formation of TiO2-based semiconductors with enhanced performance in solar energy conversion, consists in the incorporation of foreign ions into the TiO2 lattice (doping).1,3,4 The ultimate aim of the research is the modification of the performance-related properties, such as band gap, surface composition, flat band potential and charge transport. The present work is focused on the determination of the effect of indium on defect disorder of TiO2. In this work we show that the effect of indium on properties of TiO2 should be considered in terms of a dual mechanism of indium incorporation into the TiO2 lattice, involving both titanium sites (leading to the formation of acceptors) and interstitial sites (resulting in the formation of donors). It is also shown that the donor-type indium ions have a tendency to segregate to the surface leading to the formation of a chemically induced electric field. The reported effect of indium on the semiconducting properties of TiO2 should be considered in terms of a competition between these two mechanisms. These effects are of crucial importance in the interpretation of the performance of In-doped TiO2 as photoelectrode for solar water splitting and solar water purification. 2. LITERATURE OVERVIEW Chandra Babu et al.5 reported that indium doping results in an enhanced performance of TiO2 as photoanode in photoelectrochemical hydrogen generation by water splitting. Also Wang et al.6 and Karakitsou and Verykios7 reported that indium incorporation leads to enhanced photocatalytic activity of TiO2. Defect disorder seems to be the right platform to explain the observed effect of indium on both photocatalytic and photoelectrochemical properties for the following reasons: © XXXX American Chemical Society

Received: November 9, 2014 Revised: December 13, 2014

A

DOI: 10.1021/jp5112197 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C While the acceptor-type mechanism of indium incorporation into TiO2 is obvious, the recently reported segregation data suggest that the mechanism of indium incorporation into the surface layer of TiO2, leading to the formation of a lowdimensional surface structure, is entirely different.13 According to Atanacio et al.,13 surface segregation of indium may be best explained assuming the interstitial mechanism of indium incorporation. The presence of indium in interstitial sites has already been reported by Peterson and Sasaki15 who considered that the transport of indium in the rutile structure occurs according to the interstitialcy mechanism. Also, Nakamura et al.16 have observed that implantation of indium into the TiO2 (rutile) leads to indium incorporation into titanium sites (approximately 80%) and the remaining ions enter interstitial sites (the ratio between these sites depends on the implantation dose, the energy of the implanted ions and the structural disorder caused by implantation). Nakamura et al. have shown that annealing results in the transfer of interstitial indium ions into substitution sites, which are preferred energetically. Nakamura et al. also have observed that a small ratio of interstitially located indium ions remains in the interstitial sites after annealing. The effect of the implantation-induced defect disorder on the mechanism of indium incorporation was considered by Nakamura et al. in terms of a tendency of indium ions to form defect clusters, such as indium−titanium vacancy pair, indium-Schottky defect complex, indium−oxygen divacancy complex and indium−titanium mixed dumbbell.16 One may expect some analogy between the defect disorder of indium-doped TiO2 (rutile) and that of aluminum- and nitrogen-doped TiO2 concerning the mechanism of incorporation of trivalent ions into the rutile structure. Escudero et al.17,18 have shown that, while aluminum ions are predominantly incorporated into titanium sites of TiO2, aluminum can also be incorporated into octahedral interstitial sites of rutile. The evidence of Al located in interstitial sites have been obtained using XRD and NMR studies. The double mechanism of aluminum incorporation into TiO2 has been confirmed by absorption spectroscopy studies Tsai et al.19 Shieh et al.20 and Wang et al.21 have shown that also nitrogen is incorporated into TiO2 according to dual mechanism involving both substitution and interstitial sites. Taking into account that the electrical conductivity and thermoelectric power are predominantly the bulk-related properties and the segregation data should be considered as surface-related, the following question should be formulated: What is the effect of indium on defect disorder of TiO2 for the bulk phase and the surface layer? The present work is an attempt in addressing this question.

Table 1. Kröger-Vink Notation of Defects for TiO2 and InDoped TiO222 description

Kröger-Vink notation

Ti4+ Ti ion in the titanium lattice site Ti3+ Ti ion in the titanium lattice site titanium vacancy Ti3+ i in the interstitial site Ti4+ i in the interstitial site O2− O ion in the oxygen lattice site oxygen vacancy O−O ion in oxygen lattice site (quasi-free electron hole) In3+ Ti in the titanium lattice site In3+ i in the interstitial site

TixTi e′ V′Ti′′′ Ti••• i Ti•••• i OxO V•• O h• In′Ti In••• i

accumulation of empirical data on defect-related properties of TiO2 resulted in the determination of equilibrium constants for the formation of the predominant intrinsic defects in TiO2.25 These equilibrium constants are shown in Table 2.25 Most of the intrinsic ionic defects are formed as a result of interactions between the TiO2 surface and oxygen in the gas phase. Therefore, oxygen activity is the crucial parameter in considering the effect of the gas phase on defect disorder, involving mainly four ionic defects in addition to the electronic defects. The equilibrium constants in Table 2 and the related simplified charge neutrality conditions (that are valid under the assumption that the specific defect reactions dominate the disorder) may be used for the determination of the effect of oxygen activity on the concentration of electrons. In general terms, this dependence may be expressed by the following relation: log[def] 1 = mdef log p(O2 )

(1)

where [def] is the concentration of specific defects and 1/mdef is the exponent of oxygen activity that is related to these defects. The parameter 1/mdef may be used in derivation of Brouwer-type defect diagrams, which considers defect disorder within narrow regimes of oxygen activity corresponding to the validity of the simplified charge neutrality conditions. The slopes for electrons related to the intrinsic equilibria 1−5 are shown in Table 2. While the Brouwer-type diagram is the first approach in the characterization of the effect of oxygen activity on the defect models within narrow ranges of oxygen activity,12 defect disorder is best described by full charge neutrality. The charge neutrality condition for TiO2 and its solid solutions (with donor- and acceptor-type dopants) assumes the following form:

3. DEFECT CHEMISTRY OF TiO2 This section reports the basic terms that will be used in the considerations of the experimental data in terms of defect disorder of pure and In-doped TiO2 and the related semiconducting properties. The defect disorder in this work is derived using the Kröger-Vink notation that is shown in Table 1.22 3.1. Defect Chemistry of TiO2. There is a common perception that TiO2 is an n-type semiconductor and its predominant defects are oxygen vacancies.8 This perception is correct when TiO2 is equilibrated (at elevated temperatures) in reducing and strongly reducing conditions. However, recent studies show that defect disorder of pure TiO2 is more complex and exhibits both n- and p-type properties. 9,23,24 An

••• •••• 2[V •• ] + p + [D•] O ] + 3[Tii ] + 4[Tii

= n + 4[V ′′′′ Ti ] + [A′]

(2)

•

where [D ] and [A′] denote the concentrations of extrinsic ions in the cation (or oxygen) sublattice, corresponding to singly ionized donor- and acceptor-type foreign ions, respectively. Quantitative relationships between the concentrations of ionic and electronic defects and the key variables, such as oxygen activity, may be obtained by the combination of the charge neutrality condition, expressed by eq 3, and equilibrium constants of the predominant defect equilibria, which are defined in Table 2:25 B


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Table 2. Basic Defect Equilibria in TiO2 (n and p Denote the Concentration of Electronic and Electron Holes, Respectively)25 defect reaction 1

OOx

2

V •• O

simplified charge neutrality

equilibrium constant

2[V •• O]

(∂ log n)/[∂ log p(O2)]

ΔHO (kJ/mol)

−(1/6)

493.1

106.5

ΔSO [J/(mol·K)]

2 1/2 K1 = [V •• O ]n p(O2 )

n=

x Ti Ti + 2OOx ⇔ Ti••• + 3e′ + O2 i

3 K 2 = [Ti••• i ]n p(O2 )

n = 3[Ti••• i ]

−(1/4)

879.2

190.8

3

x Ti Ti + 2OOx ⇔ Ti•••• + 4e′ + O2 i

K3 = [Ti•••• ]n 4p(O2 ) i

n = 4[Ti•••• ] i

−(1/5)

1025.8

238.3

4

• x O2 ⇔ V ′′′′ Ti + 4h + 2OO

4 −1 K4 = [V ′′′′ Ti ]p p(O2 )

p = 4[V ′′′′ Ti ]

−(1/5)

354.5

−202.1

5

nil ⇔ e′ + h•

K i = np

n=p

0

222.1

44.6

⇔

+ 2e′ + 1/2O2

ln K = [(ΔS°)/R ] − [(ΔH °)/RT ]

4K4K i−4Pn8 + n5 − ([D•] − [A′])n 4 − K in3 − 2K1P−1/2n2 − 3K 2P−1n − 4K3P−1 = 0

(3)

where

P = p(O2 )

(4)

and K1, K2, K3, K4, and Ki are defined in Table 2. Equation 3 may be used for derivation of defect disorder diagrams for the thermodynamically reversible defects, including titanium vacancies. The related concentration of electronic charge carriers for pure TiO2 at 1073 K as a function of oxygen activity is represented in Figure 1a. Eq 3, representing defect disorder of TiO2, is consistent with the data of electrical conductivity24,25 (Figure 1b), thermoelectric power26 (Figure 1c), and oxygen deficit in TiO2−x26−32 (Figure 1d). The full charge neutrality expressed by eq 2, and the effect of oxygen activity on the concentration of electrons, expressed by eq 3, may be used in derivation of a full defect diagram of TiO2 that is shown in Figure 2. This diagram allows the following points to be made: • The ionic defect disorder of TiO2 involves mainly oxygen vacancies that are compensated by titanium vacancies. • The properties of TiO2 may be considered in terms of the following three regimes: ◦ The predominant electronic defects are electrons, then TiO2 exhibits n-type properties. ◦ The concentration of electrons and electron holes are comparable, then TiO2 is in the vicinity of the n−p transition point. ◦ The predominant electronic defects are electron holes, then TiO2 exhibits p-type properties. The diagram in Figure 2, which describes the TiO2/O2 system in the gas/solid equilibrium represents the defect disorder for a real system only when the kinetic factor allows the equilibrium state to be reached. It appears that the kinetics factor should be considered independently for fast defects (oxygen vacancies and titanium interstitials) on one hand and slow defects (titanium vacancies) on the other hand. It has been documented that the diffusion rate of the latter species is so low that imposition of their equilibrium concentration requires prolonged annealing at elevated temperatures.33 The data in Table 2 and the related diagram in Figure 2 describe the defect disorder in the bulk phase. At this point it is essential to note that the defect disorder for the surface layer is entirely different.13 Even if the surface layer remains in equilibrium with the gas phase and the bulk phase, its defectrelated properties are different from that in the bulk as a result of the excess of surface energy. The resulting difference in the properties of the surface layer should be considered in terms of

Figure 1. Effect of oxygen activity on (a) concentration of electronic charge carriers, (b) electrical conductivity, (c) thermoelectric power, and (d) deviation from stoichiometry for TiO2 at 1073 K.23−32

the local concentration of defects, which results in the formation of low-dimensional surface structures. The defect disorder of the grain boundary layer of pure TiO2 has been assessed by comparative studies of the electrical properties for both single crystal and polycrystalline specimens.9 These studies indicate the following: • Grain boundaries are enriched in donor-type defects. • The concentration of electrons at grain boundaries is markedly larger than that in the bulk phase. The effect of C


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Figure 2. Effect of oxygen activity on the concentration of electronic and ionic defects in TiO2 at 1073 K.25

oxygen activity on the enrichment effect indicates that grain boundaries are enriched in titanium interstitials rather than oxygen vacancies.35 The analysis of the data on defect-related properties that has been collected so far14 for In-doped TiO2 indicates that the interstitial mechanism of indium incorporation should be taken into account, along the substitution mechanism, in considering the effect of indium on defect disorder and defect-related properties of In-doped TiO2. Both TiO2 and In-doped TiO2 are amphoteric semiconductors. Therefore, their n−p transition points can be used in considering the effect of indium on semiconducting properties and the related defect disorder. The effect of oxygen on the n−p transition point, which has been determined for both pure TiO2 and In-doped TiO2 with a high accuracy,14,16,34 will be the key quantity is assessing the effect of indium on defect disorder of In-doped TiO2. 3.2. Defect Chemistry of In-Doped TiO2. Assuming that indium incorporation into TiO2 is limited to the substitution mechanism leading to the formation of acceptor-type centers, InTi ′ , the defect disorder of In-doped TiO2, may be represented qualitatively in Figure 3 in terms of a Brouwer-type diagram within strongly reducing, reducing, and oxidizing conditions. The expressions for the concentration of the associated defects, derived using the usually acceptable models for trivalent ions,13 are shown in the lower part of Figure 3. However, as it results from the literature overview the properties of In-doped TiO2 cannot be explained solely using the substitutional mechanism. Therefore, there is a need to consider alternative mechanism in terms of their consistency to the experimental data. Assuming (for the sake of simplicity) that the predominant intrinsic defects in the bulk phase of pure rutile are oxygen vacancies and electrons, which are compensated by titanium vacancies and electron holes, the lattice charge neutrality may be expressed as •• 4[V ′′′′ Ti ] + n = p + 2[V O ]

Figure 3. Brouwer-type defect diagram for In-doped TiO2 within strongly reducing, reducing, and oxidizing conditions (upper part) and the expressions for the concentration of the associated defects (lower part). ••• •• [In′Ti] + 4[V ′′′′ Ti ] + n = p + 3[In i ] + 2[V O ]

(6)

Let us now consider several defect disorder models involving only two types of the predominant defects that exhibit opposite charge. The related defect equilibria may be governed by simplified charge neutrality conditions for all combinations of defects in the condition 6, including both mechanisms of indium incorporation and the intrinsic defects shown in Table 2: In2O3 +

Taking into account that defect disorder of In-doped TiO2 involves indium in both titanium and interstitial sites, the charge neutrality requires that the following condition is met: D

(7)

[In′Ti] = p

(8)

In2O3 ↔ 2In′Ti + 3OOx + V •• O

(9)

[In′Ti] = 2[V •• O]

(10)

2In2O3 ↔ 3In′Ti + 6OOx + In••• i

(11)

3[In••• i ]

(12)

• O2 ↔ 2OOx + V ′′′′ Ti + 4h

(13)

4[V ′′′′ Ti ] = p

(14)

•• nil ↔ V ′′′′ Ti + 2V O

(15)

•• 2[V ′′′′ Ti ] = [V O ]

(16)

x 2In2O3 ↔ 4In••• + 3V ′′′′ i Ti + 6OO

(17)

[In′Ti] =

(5)

1 O2 ↔ 2In′Ti + 4OOx + 2h• 2


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The Journal of Physical Chemistry C ••• 4[V ′′′′ Ti ] = 3[In i ]

(18)

nil ↔ e′ + h•

(19)

n=p

(20)

OOx ↔ V •• O + 2e′ +

1 O2 2

n = 2[V •• O]

In2O3 → 2In••• + i n=

Taking into account the formula 27, surface versus bulk concentration of indium in interstitial sites may be considered within one of the following two scenarios: • The bulk phase and the surface layer exhibit the rutile structure. In this case, indium distribution in the surface layer as well as in the bulk phase is described by the equilibrium constant K10. Then the enrichment of the surface layer in indium located in interstitial sites leads to impoverishment in indium ions located in titanium sites. • The surface layer exhibits a low-dimensional surface structure. Then indium distribution in the structure is entirely different than that in the bulk phase. One should expect that the related equilibrium constant K10 for the surface layer is a function of the distance from the surface. The incorporation of indium into TiO2 also has an effect on the transport kinetics that is considered below. 3.3. Mobility of Intrinsic Defects. The defect disorder model derived before9 is based on the defect-related experimental data determined in the gas/solid equilibrium. The studies of the equilibration kinetics of TiO2 using the measurements of the electrical conductivity indicate that a stable value can be achieved at elevated temperatures (1000− 1300 K) can be achieved within 1 h. However, the studies of oxidation kinetics during prolonged period of time indicates that the equilibration involves two stages; the fast kinetics regime determined by fast defects, such as oxygen vacancies and titanium interstitials, and the slow kinetics regime determined by titanium vacancies.33 In the latter case, the equilibration time at 1000−1300 K is approximately 4000−6000 h. The difference has an impact on formation of TiO2 with reproducible properties.

(21) (22)

3 O2 + 6e′ 2

3[In ••• i ]

(23) (24)

The reactions 13, 15, 19, and 21 describe the formation of intrinsic defects. The reactions representing the incorporation of indium into titanium sites and interstitial sites, which are governed by the electronic charge compensation (reactions 7 and 23, respectively) result in a fixed concentration of electronic charge carriers that are independent of oxygen activity. The reaction 7, for example, is associated with oxygen incorporation, while the reaction 23 results in oxygen release. Therefore, these two reactions are expected to dominate at high and low oxygen activity, respectively. In this context, the terms high and low oxygen activities are determined by specific equilibrium constants and indium concentration, rather than any specific absolute values. The equilibria 9 and 17 represent indium incorporation into titanium sites and interstitial sites, which are governed by ionic charge compensation. In these two cases the change in the concentration of electrons may be expressed by the following respective expressions: ⎧ 2K1 ⎫1/2 ⎬ p(O2 )−1/4 n=⎨ ⎩ [In′Ti] ⎭

(25)

⎧ 2[In••• ⎫1/2 i ] ⎬ p(O2 )−1/4 n = K i⎨ ⎩ 4K4 ⎭

(26)

4. EXPERIMENTAL SECTION The polycrystalline specimens of pure and indium-doped TiO2 used in this work were processed by the sol−gel technique.13 The conditions of the final annealing time, which aimed at homogeneous distribution of indium, were determined based on the diffusion data of indium in TiO2.34 The electrical conductivity and thermoelectric power were determined simultaneously at 1273 K using the High-Temperature Seebeck Probe.23 The oxygen activity in the gas phase was imposed using the argon−oxygen mixture and the argon−hydrogen mixture in order to achieve oxidizing and strongly reducing environment, respectively. The oxygen activity was determined by a zirconia-based electrochemical sensor. The experimental procedure included monitoring of the electrical properties during isothermal increase and decrease of oxygen activity (oxidation and reduction runs, respectively). The n−p transition points were determined in the following manner: • The electrical conductivity was plotted versus oxygen activity within the n−p transition. Then the minimum of conductivity, σmin, was found by interpolation. In the first approximation σmin corresponds to the n−p transition point. • The thermoelectric power data was plotted versus oxygen activity in the vicinity of the n−p transition. This plot was used for exact determination of the oxygen activity corresponding to S = 0. This approach is most reliable in the determination of the n−p transition point since thermoelectric power is determined by concentrations of the electronic charge carriers.

where Ki, K1, and K4 are defined in Table 2. As seen, in both cases the concentration of electrons is the same function of oxygen activity. Therefore, the effect of oxygen activity on the electrical conductivity is the same for the mechanisms 9 and 17. The equilibrium constant of the reaction 11 may be expressed as

K10 =

3 [In••• i ][In′Ti]

[In2O3]2

(27)

Consequently, the concentrations of indium ions located in titanium sites and interstitial sites depend on each other, if defect disorder corresponds to the gas/solid equilibrium. This is not the case when defect disorder is imposed by a nonequilibrium process, such as implantation. The implantation-induced structural damage leads to random distribution of indium in the rutile structure. According to Nakamura et al.,16 the fraction of interstitial indium occupancy increases with the implanted indium dose. However, increasing the dose above a certain critical limit (2 × 1015 /cm2), which is considered as the “a maximum of the lattice disorder saturation dose”, results in random distribution of almost all indium atoms. E


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6. EFFECT OF INDIUM ON THE n−p TRANSITION LINE The n−p transition lines, representing the oxygen activity corresponding to the conditions n = p (S = 0) are shown in Figure 5 as a function of temperature for the following specific

Details of the applied experimental procedures were reported before.14 The effect of indium on the n−p transition point was determined from a comparison of the minimum of the electrical conductivity (σmin) and zero value of thermoelectric power (S = 0), plotted as a function of oxygen activity, for both high purity TiO2 and In-doped TiO2. In addition to the measurements of the electrical conductivity, the studied specimens have been characterized using XPS spectra that were reported by Atanacio et al.13

5. RESULTS AND DISCUSSION The applied procedure results in the experimental data, which are represented in the following forms: • Isothermal plots of electrical conductivity as a function of oxygen activity, which are represented in the upper part of Figure 4. These plots may be used in the

Figure 5. Effect of temperature on the n-p transition lines according to (1) theoretical model, 8 (2) experimental data for pure TiO2 polycrystalline,19 (3) In-doped TiO2 polycrystalline,14 (4) according to the theoretical model for acceptor-doped TiO2,25 (5) pure TiO2 single crystal,8 and (6) Cr-doped TiO2 (1 at% Cr).36

cases: (1) The theoretical model derived from eq 4 for pure TiO2 ([A′] = 0, [D•] = 0). (2) Experimental data for pure TiO2 (SC)exp. (3) Experimental data for pure polycrystalline TiO2 (PC)exp. (4) Experimental data for polycrystalline In-doped TiO2 (PC)exp. (5) Theoretical model derived from eq 4 for acceptor-doped TiO2 at [A′] = 0.4 at%. (6) The data point related to Cr-doped TiO2.36 1. Theoretical model. This n−p transition line is determined by the theoretical model that was derived using a range of the experimental data for TiO2 single crystal, including electrical conductivity, thermoelectric power, coulometric titration, and thermogravimetry. Therefore, the model does not involve the effects related to grain boundaries. 2. Experimental data for pure TiO2 single crystal. This line is obviously in a close vicinity of the theoretical line #1, since the model is mainly based on this data. 3. Experimental data for high purity polycrystalline TiO2. The observed shift of the n−p transition line toward larger oxygen activities is induced by the donor-type defects at grain boundaries, such as oxygen vacancies and titanium interstitials. However, since the defect disorder at grain boundaries is entirely different than that in the bulk phase, the shift in this case cannot be described by the theoretical model that is derived for the bulk phase. 4. Experimental data for polycrystalline In-doped TiO2. This line is shifted to larger oxygen activities compared to the curve #4 representing the theoretical model. This shift is reflective of the departure of the experimental data from the theoretical model. 5. Theoretical model derived for acceptor-doped TiO2 (0.4 at% A′). This line, which is determined using the theoretical model derived by eq 4 for an acceptor-doped TiO2, indicates the expected n−p transition for In-doped TiO2.

Figure 4. Effect of oxygen activity on the electrical conductivity (upper part) and thermoelectric power (lower part) for both pure TiO2 (dashed lines) and In-doped TiO2 (continuous lines) at 1123 K.14

determination of approximate value of the n−p transition point corresponding to the minimum of the electrical conductivity, which corresponds to the condition σn = σp. • Isothermal plots of thermoelectric power as a function of oxygen activity, which are shown in the lower part of Figure 4. These plots may be used for the determination of the exact value of the n−p transition point corresponding to the condition S = 0. As seen in Figure 4, representing the effect of oxygen activity on both electrical conductivity and thermoelectric power, addition of indium leads to a shift of the n−p transition point toward lower oxygen activity. This effect indicates that indium incorporation results in the formation of acceptors leading to a decrease in the concentration of electrons. The color of pure TiO2 depends on annealing conditions (white and black in oxidizing and reducing gas phase). The color of In-doped TiO2 is yellowish-brown. F


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reaction 15, leading in consequence to a decrease in the concentration of oxygen vacancies:

6. Experimental data for Cr-doped TiO2 (1 at% Cr). Taking into account that chromium incorporated into the TiO2 lattice results in acceptor energy levels31,32 this data point is consistent with the theoretical model represented by the curves #4 and #5 for pure and acceptor-doped TiO2. For clarity, Figure 6 represents only the n−p transition line determined by the experimental data for In-doped TiO2 (0.4 at

[V •• O] =

K15 [V ′′′′ Ti ]

(28)

where K15 is the equilibrium constant of reaction 15. Therefore, assuming that the preferred indium sites at grain boundaries are interstitial positions, these grain boundaries are impoverished in oxygen vacancies and enriched in titanium vacancies. This, as a consequence, leads to weak links in ionic charge transport that is reflected by a decrease in the ionic conductivity component that has been observed experimentally.13 The additional evidence of indium incorporation according to the mechanism (reaction 16) arises from the study of indium segregation in In-doped TiO2. It has been namely shown that the predominant driving force of indium segregation results from an electric field between positively charged segregating indium ions and titanium vacancies at the surface.12 An alternative approach, which explains the decrease of the ionic conductivity for In-doped TiO2, is the formation of defect associates between indium ions incorporated into titanium sites and oxygen vacancies leading to the formation of defect complexes:37 x 2In′Ti + V •• O ↔ {2InTiVO}

Figure 6. Effect of temperature on the n−p transition lines representing the discrepancy between the experimental data and the theoretical model for In-doped TiO2 (0.4 at% In).

(29)

The mechanism (29) has been considered for Al-doped TiO2 (anatase).37 In summary, the present work provides several bits of evidence that part of indium is incorporated into substitution sites, leading to the formation of acceptors (Figure 1) and the remaining part enters into interstitial sites leading to the formation of donors. The established effect of indium segregation in In-doped TiO2 indicates that the ratio of the indium ions in interstitial sites to the indium ions in the titanium sites at the surface is entirely different than that in the bulk phase. The available bulk- and surface-related data indicates that indium is incorporated into the bulk phase and the surface layer mainly as an acceptor and donor, respectively.

%) and the theoretical line calculated from the model represented by eq 4 for acceptor-doped TiO2 (0.4 at%). As seen, the discrepancy between these two lines is substantial. On one hand the shift of the minimum of the electrical conductivity and the zero value thermoelectric power due to indium incorporation (Figure 1) indicates that indium is incorporated into titanium sites leading to the formation of acceptor-type energy levels. However, the acceptor-type behavior of indium observed by the experimental data (dashed line) is weaker than expected. The difference between the two is particularly large at lower temperatures. The discrepancy between the experimental line and the theoretical line for A′doped TiO2 suggests that some of the indium atoms are incorporated into interstitial sites leading to the formation of donors. This conclusion is consistent with the effect of indium on ionic conduction.

8. SURFACE VERSUS BULK It has been shown that the surface layer of pure TiO2 is enriched in donor-type defects such as titanium interstitials and oxygen vacancies.37 Their formation is represented in Table 2. Ionization of these defects results in the concentration of electrons at the surface being much higher than that in the bulk phase.38 Addition of indium results in some complications. The collected experimental data indicates the following: (1) Indium is predominantly incorporated into the bulk phase of TiO2 according to the substitution mechanism leading to the formation of acceptor energy levels; (2) The segregation data indicate that the surface layer of In-doped TiO2 is enriched in indium. Its effect on surface properties may be considered in terms of the following scenarios: ◦ The segregation driving force is the electrostatic field formed by negatively charged titanium vacancies located at the surface and positively charged indium ions in interstitial sites, which are formed according to the equilibria 17 and 23. The interactions between these defects leads to the formation of defect complexes:

7. EFFECT OF INDIUM ON IONIC CONDUCTION The effect of indium incorporation into titanium sites of TiO2, forming acceptors, has been attested by the measurements of the electrical properties, which are shown in Figure 1. According to the reaction represented by eq 9, the generation of acceptors as a result of indium incorporated into titanium sites leads to the formation of oxygen vacancies. Since these defects are highly mobile (see section 3.3), their formation is expected to result in an increase of the ionic conductivity component. According to the obtained experimental data for ionic charge transport, this is not the case.14 On the contrary, it has been observed that the incorporation of indium results in a substantial decrease of ionic conductivity.14 As seen from eq 17, the incorporation of some indium ions into interstitial sites results in the formation of titanium vacancies. This reaction affects the Schottky-type defect disorder, represented by the

••• x 4In••• + 3V ′′′′ i Ti ↔ {(In i )4 (V⁗ Ti)3 }

G

(30)


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The Journal of Physical Chemistry C ◦ When the enrichment of the surface layer surpasses a certain critical level, a low-dimensional surface structure, In2TiO5-type, is formed at the outermost surface layer.13 The proposed model of indium distribution for In-dopedTiO2 is represented schematically in Figure 7.

Figure 8. XPS spectra for In-doped TiO2 after different processing conditions (see the text).

10. CONCLUSIONS Indium, which is preferably a trivalent ion is commonly considered an acceptor-type dopant in TiO2, if incorporated into titanium sites (substitution mechanism). However, there has been an accumulation of several bits of evidence indicating that indium is also incorporated into the TiO2 lattice according to the interstitial mechanism leading to the formation of donors. The ratio between these two indium species (acceptors and donors), which is affected by oxygen activity, should be considered as a function of the distance from the interface, such as the external surface and the grain boundary. In summary, indium incorporates into the rutile structure according to a dual mechanism, which is influenced by the distance from the interface and processing conditions, especially oxygen activity. The first consists in indium entering titanium sites and leading to the formation of acceptor-type energy levels. This mechanism seems to be the predominant one since the observed shift of the n−p transition line toward lower oxygen activity may only be explained by this mechanism. However, in order to explain the other semiconducting properties of Indoped TiO2, it is essential to assume that indium is also incorporated into interstitial sites leading to donor-type defects. These defects, which are the minority-type defects in the bulk phase, are responsible for a substantial drop of ionic conductivity component and deviation of the n−p transition line from the theoretically predicted for purely acceptor-doped TiO2. The effect of indium on the properties of the surface layer is different. First of all the concentration of indium in this layer is substantially enhanced as a result of segregation. The preferred mechanism of indium incorporation in this layer is the interstitial mechanism. Quantitative assessment of the effect of indium on surface properties of TiO2 requires the determination of a defect-related property for the surface layer that can be used for derivation of defect disorder model for this layer. So far, however, such experimental data is not available.

Figure 7. Theoretical model representing surface vs bulk distribution of defects and the formation of a low-dimensional surface structure.

The thickness of the surface structure in the outermost layer remains within one atomic layer that is approximately 0.1−0.2 nm thick. However, the thickness of the sublayer involving the segregation-induced concentration gradients remains in the range 10−20 nm.13

9. SURFACE SPECTROSCOPY STUDIES The XPS spectra have been determined for In-doped TiO2 (0.4 at% In), including the following specimens: • As-polished. The aim of polishing was to remove the surface layer modified during annealing. Therefore, the composition of the surface layer for this specimen is the same as that in the bulk phase. • Annealed in oxygen, p(O2) = 75 kPa, at 1273 K. • Annealed in oxygen, p(O2) = 10 Pa, at 1273 K. The XPS spectra are shown in Figure 8. The recorded spectrum for the as-polished specimen (Figure 8a) confirms that the indium content in the bulk phase is 0.4 at %. The binding energy of the indium peak in this case is 444.86 eV. Annealing in pure oxygen results in segregation-induced surface enrichment to the level of 16.67 at % In; however, the position of the indium peak in this case corresponds to 444.77 eV. Annealing the specimen in reduced conditions, p(O2) = 10 Pa, results in segregation-induced surface enrichment to the level of 2.61 at % In; however, the position of the indium peak in this case remains at the 444.77 eV level. As seen, the difference between the energy levels for the as-polished specimen (444.86 eV) and the specimens after annealing (444.77 eV) is substantially below the resolution level that is 0.5 eV. Therefore, the energy shifts that are associated with different indium species, if any, are below the resolution level.

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*Tel.: 61-2-4284-7829. Fax: 61-4620-3711. E-mail: j.nowotny@ uws.edu.au. H


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The Journal of Physical Chemistry C Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The support of the University of Western Sydney through the FIF Grant is acknowledged.

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Dual Mechanism of Indium Incorporation into TiO - American

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