Million-fold Increase of the Conductivity in TiO

Communication pubs.acs.org/cm

Million-fold Increase of the Conductivity in TiO2 Rutile through 3% Niobium Incorporation Girija Sahasrabudhe, Jason Krizan, Susanna L. Bergman, Robert J. Cava, and Jeffrey Schwartz* Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States S Supporting Information *

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obtain a phase-pure rutile structure. Magnetization was measured for powders (80−100 mg), which were enclosed in plastic wrap. Magnetic susceptibility was measured as a function of temperature with a 1 T applied field. Given possible electrode applications for Ti1−xNbxO2,2,11 room temperature measurements of conductivity vs the mole fraction of Nb were made on high density pellets, and an unusual trend was observed (Figure 1a): Conductivity increased dramatically from 10−4 ohm−1 cm−1 for x = 0.0005 to 40 ohm−1 cm−1 for x = 0.03, but it then decreased; for 0.03 ≤ x ≤ 0.9 conductivity dropped to the range 40−0.017 ohm−1 cm−1. The conductivity of both low Nb content material, Ti0.95 Nb0.05O2, and high Nb content material, Ti0.1Nb0.9O2, increased with temperature (Figure 1b,d); significantly, the plot of log ρ vs 1/T is not linear for either case (Supporting Information Figure 1) in any temperature range, which is inconsistent with ideal semiconducting behavior.14,16,18 Instead, a linear plot was measured for log σ vs (1/T)1/4 (Figure 1c,e) for both stoichiometries, indicating that the 3-dimensional variable range hopping model for conduction in strongly disordered systems with localized charge carrier states in the band gap19 provides a good description of the system. Hall and hot probe measurements show that electrons are the majority carriers in Ti1−xNbxO2 for all Nb contents. Magnetic susceptibility (χ) of Ti1−xNbxO2 (x = 0.05, 0.1, 0.3, 0.5, 0.75, 0.9) was measured at 1 T applied field from 10 to 300 K; magnetic susceptibility for diamagnetic TiO2 was measured as a control. Curie−Weiss fits of Ti1−xNbxO2 were linear only between 10 and 60 K with Curie−Weiss temperature (Θ) between −4 and +1 K, consistent with paramagnetic behavior.20 These studies indicate the presence of small fractions of localized magnetic states (Supporting Information Figure 2). The mole fraction of these paramagnetically active species (y) was derived from Curie−Weiss fits (Figure 2; Supporting Information Table 2). Paramagnetism is due to localized Ti(III) and Nb(IV) states, but it exists even for x = 0, which implicates oxygen vacancies that are created by thermal treatment under hydrogen. These vacancies are present at 0.1 (Figure 3) in agreement with a previous report.21 Nb and Ti oxidation state assignments were corroborated with spectra for

onducting metal oxides enjoy applications in photovoltaics, energy systems, and catalysts.1−3 Given that titanium dioxide is inexpensive, chemically stable, and a wide band gap semiconductor,4−9 it is commonly doped, notably with Nb, to improve its electrical conductivity3,6−8,10 and photocatalytic properties.2,11,12 Single crystal Nb-doped anatase has room temperature conductivity as high as 20 ohm−1 cm−1;13 Nb-doped single crystal rutile displays best reported room temperature conductivities of 102−103 ohm−1 cm−1.14,15 Polycrystalline Nb-doped anatase and rutile have comparable, lower bulk conductivities of 0.25−5 ohm−1 cm−1.1,3,6 Treatment under hydrogen increases the bulk conductivity of Nbdoped rutile,6 and various levels of Nb incorporation, up to 20%, have been suggested to be optimal.1−3,6−8,11,13−17 Here, in a study specifically designed to find the optimal doping conditions relevant to polycrystalline electrode material, we were surprised to find that the conductivity of polycrystalline Nb-doped rutile prepared under our conditions reached a sharp maximum (40 ohm−1 cm−1) at only about 3 mol % Nb content. The conductivity actually decreased above 5% Nb incorporation, reaching a value of only 0.017 ohm−1 cm−1 at 90% incorporation. We propose a general model to explain how fundamental electronic and structural properties of rutile TiO2 correlate with increasing Nb incorporation. Our results corroborate the gross behavior reported by Sakata,17,18 but we find an optimal doping level of 3%, with 6 orders of magnitude increase in the conductivity on doping. In order to develop our model it was necessary to characterize rutile materials from 0.05% to 90% Nb incorporation not only by powder X-ray diffraction (pXRD), but also by X-ray photoelectron (XPS) and UV−vis diffuse reflectance spectroscopies, and by magnetic susceptibility, conductivity and Kelvin probe measurements. Polycrystalline Ti1−xNbxO2, 0.0005 ≤ x ≤ 0.9, was synthesized by mixing reagent grade Nb2O5 and TiO2 (Alfa Aesar, >99.5% and >99.9%, respectively) in the elemental ratios of xNb:(1−x)Ti. Each oxide mixture was thoroughly ground using a mortar and pestle, and was then treated at 1000 °C in a tube furnace under flowing 5% H2 in argon. Grinding and heating processes were repeated until phase-pure powder X-ray diffraction (pXRD) rutile patterns were obtained for each mixture. Polycrystals were analyzed by X-ray photoelectron spectroscopy (XPS) to determine the oxidation states of Nb and Ti for each doping level of Nb−TiO2. Highly sintered pellets (packing fraction >0.9) were prepared by pressing powder samples in a 0.25″ die using a hydraulic press (1 ton) and then heating at 1400 °C under flowing argon for 5 h. These pellets were further treated at 1000 °C under 5% H2/Ar to © XXXX American Chemical Society

Received: May 19, 2016

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DOI: 10.1021/acs.chemmater.6b02031 Chem. Mater. XXXX, XXX, XXX−XXX

Communication

Chemistry of Materials

Figure 3. X-ray photoelectron spectra showing Nb(V), Nb(IV), Ti(IV) and Ti(III) in Ti1−xNbxO2; raw data (black), fit data (red), major species (blue), and minor species (green). For x = 0.03, see Supporting Information Figure 3e,f.

Figure 1. (a) Room temperature conductivity measurements for Ti1−xNbxO2. Conductivity values for each x are averages measured for three independent pellets; variations in each value indicated are less than the diameter of the circles shown, and are listed in Supporting Information Table 1. (b−e) Temperature dependent conductivities of Ti1−xNbxO2 (x = 0.05 and 0.9).

Figure 4. Structural characterization X-ray diffraction data (a) and unit cell parameters plotted (b, c) for Nb-incorporated rutile (d).

Figure 2. Mole fraction of all paramagnetic species (y) vs x in Ti1−xNbxO2; y = Nb(IV) + Ti(III). At x = 0, paramagnetism results from oxygen vacancies only (see arrow).

found that a followed Vegard’s law for 0.0005 ≤ x ≤ 0.9, but that c was linear only for x < 0.1, as reported previously.16 Interestingly, the major change in the effect of Nb doping on c was found for x > 0.1, commensurate with measured changes in the XP spectra: for 0.1 ≤ x ≤ 0.75, there was little change in c, above which range it decreased. This change in structural response suggests the inception of metal−metal pairing along the c axis24,25 as reported for related rutiles such as NbO2 and VO2.20,26−29 Work function (WF) measurements were made by Kelvin probe on Ti1−xNbxO2 pellets. The WF for pure TiO2 was measured to be 4.97 eV, whereas for Ti0.95Nb0.05O2 and

reagent grade Nb2O5, NbO2 and TiO2 (Supporting Information Figure 3), which show Nb(V), Nb(V)/Nb(IV) and Ti(IV) respectively.1,3,22,23 Phase-pure XRD patterns were recorded for the compounds in the solid solution series (Figure 4a,d; Supporting Information Figure 4 and Table 4) and were analyzed by Topas. Peaks moved systematically to smaller values for 2θ with increasing content of Nb in TiO23,7,8,11 consistent with the larger ionic radii for Nb(V), Nb(IV) and Ti(III) compared to Ti(IV).6−9,11,24 When the tetragonal unit cell parameters a and c for Ti1−xNbxO2 were plotted against x (Figures 4b,c), it was B


Communication


for pure TiO2 rutile.31 Surprisingly, further incorporation of Nb into the rutile lattice has a detrimental effect on material conductivity. These results resolve ambiguities for the dopingdependent behavior of this system: As the Nb content increases, the probability of Nb···Nb proximity increases, and such proximity leads to Nb−Nb bonding as reflected in changes in the c parameter of the tetragonal unit cell. Because conductivity of Ti1‑xNbxO2 (x ≤ 0.1) is attributed to promotion of an electron from formally Nb(IV) into states near the conduction band of the mixed metal oxide (which also yields the observed Nb[V]), Nb−Nb pairing cannot contribute to such electron promotion (Figure 6). Hence conductivity does not rise appreciably as more Nb(IV) is incorporated beyond a pairing threshold in the rutile lattice. Work function measurements accord well with this model: Ti0.95Nb0.05O2 has a lower work function than pure TiO2 due to its high charge carrier density, and it has a lower work function than Ti0.25Nb0.75O2 in which Nb−Nb pairing dominates. Although thin films of Nb-doped TiO2 prepared under nonoxidizing32−34 or reducing35,36 conditions can show conductivity appreciably higher than that which we report here, our method may prove useful for device applications given its ease and scalability of preparation. With regard to possible applications,2,3,6,8,11 we note that the conductivity and phase purity of our Ti1−xNbxO2 pellets remained unaltered after exposure to ambient conditions for over 1 year.

Ti0.25Nb0.75O2 the WF was 4.88 and 5.13 eV, respectively. We also note significant changes in the UV−vis absorption spectra as a function of Nb incorporation. At low Nb values, we see strong absorbance in the visible range, which is not present for pure TiO2, yet an absorption edge remains (Figure 5). As the content of Nb increases, this absorption edge eventually disappears, but absorbance in the visible range remains.

Figure 5. Room temperature UV−vis absorption spectra for Ti1−xNbxO2 compared with pure TiO2 (x = 0.000, black; 0.005, red; 0.030, blue). At low x, a band edge (ca. 3 eV) and plasma absorption (1−3 eV) are observed; at high x, plasma absorption dominates.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b02031. Complete experimental procedures; tables of conductivity data, unit cell parameters, and Curie−Weiss correlations; powder patterns (22) for all compositions listed in a table of unit cell parameters; resistivity vs temperature, magnetization, and UV spectral plots; XPS spectra of reference compounds and of Ti0.1Nb0.9O2 (PDF).

Figure 6. Cartoon showing the band gap and population of gap states for Ti1−xNbxO2 rutile as a function of x. At low values, electrons in states related to dilute Nb atoms can populate the conduction band through thermal excitation; at high x, these electrons are sequestered in Nb−Nb bonds, forming dimers along c and are no longer available for excitation to the conduction band.

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The model we propose is based on our structural, conductivity, and magnetism data for x ≤ 0.1, and is summarized by the cartoon in Figure 6. Simply heating Nb2O5 and TiO2 to 1000 °C does not yield a mixed metal rutile structure. Instead, we propose that initial reaction of these mixed oxide with hydrogen yields Nb(IV), which is incorporated into the TiO2 rutile structure and is converted back to formal Nb(V) by electron population of new states in the band gap. At low temperature (10−100 K) there is only minor occupation of such states near the conduction band,30 resulting in only miniscule increase in conductivity; at higher temperatures (150−300 K), more extensive population of such states occurs, yielding a highly conductive material. That such populated states exist is shown by the resulting deep blue color of the material23 and the measurement of Nb only in the (V) oxidation state. Under these conditions, the 3D hopping model for conduction dominates,4 and the magnetic susceptibility falls to zero (Supporting Information Figure 2d). We have shown that the structural changes that accompany increasing Nb content correlate with changes in bulk conductivity of Nb-doped TiO2. We found that incorporating only about 3% Nb gives rise to a rutile with conductivity of 40 ohm−1 cm−1, which is to be compared with 10−7 ohm−1 cm−1

AUTHOR INFORMATION

Corresponding Author

*J. Schwartz. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Science Foundation Princeton MRSEC (DMR-1420541) and the Princeton Institute for the Science and Technology of Materials for support of this research. They also thank Prof. Annabella Selloni for helpful discussions.

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REFERENCES

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Million-fold Increase of the Conductivity in TiO

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