Nanoparticles and Sintered Compacts for Enhanced Thermoelectric

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Chemical Tuning of TiO Nanoparticles and Sintered Compacts for Enhanced Thermoelectric Properties Chengyan Liu, Lei Miao, Jianhua Zhou, Rong Huang, Craig A.J. Fisher , and Sakae Tanemura J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp401132g • Publication Date (Web): 07 May 2013 Downloaded from http://pubs.acs.org on May 23, 2013

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The Journal of Physical Chemistry

Chemical Tuning of TiO2 Nanoparticles and Sintered Compacts for Enhanced Thermoelectric Properties

Chengyan Liu,†,‡ Lei Miao,*,† Jianhua Zhou,† Rong Huang,¶ Craig A. J. Fisher,§ and Sakae Tanemura†,§,¤ †

Key Laboratory of Renewable Energy and Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou, 510640, P. R. China ‡

¶

Graduate University of Chinese Academy of Sciences, Beijing, 100049, P. R. China

Key Laboratory of Polarized Materials and Devices, Ministry of Education, East China Normal University, Shanghai 200062, P. R. China §

Japan Fine Ceramics Center, 2-4-1 Mutsuno, Atsuta-ku, Nagoya, 456-8587, Japan ¤

Powder Technology Laboratory, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya, 466-8555, Japan *

Corresponding Author: Lei Miao

Mailing Address: Key Laboratory of Renewable Energy and Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou, 510640, P. R. China Tel: +86-20-87035351 Fax: +86-20-87035351 E-mail: [email protected]

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ABSTRACT A novel, fast combustion method for synthesizing anatase TiO2 nanoparticles (average diameter ∼14 nm) co-doped with N and Nb in a single step is reported. XRD, STEM-EDX, and XPS measurements confirm that Nb ions are incorporated into the tetragonal lattice on Ti sites while N ions occupy O sites, and likely also interstitial sites. Sintering of pellets of co-doped powders under reducing conditions produced polycrystalline samples with the rutile structure. Chemically tuned samples have power factors up to 9.87×10-4 Wm-1K-2, seven times higher than that of pure TiO2 sintered under the same conditions. In addition, the thermal conductivity is considerably lower at 2.6∼4.0 Wm-1K-1 as a result of greater grain-boundary and point-defect scattering. The figure of merit, ZT, is improved to 0.35 at 700 oC, which is the highest value reported for a TiO2 material to date, and is comparable with the highest values of any n-type thermoelectric oxide. Our material also exhibits good thermal stability in a pure N2 atmosphere, and is an excellent candidate for thermoelectric power generators. Consequently, the combustion technique represents a promising new strategy for preparing foreign-atom-doped metal oxides; the chemical tuning approach, combination of foreign-atom-doped nanoparticle synthesis and optimized sintering process, can be applied to prepare superior thermoelectric materials. KEYWORDS: combustion synthesis, co-doped nanoparticles, chemically tuned TiO2, thermoelectric properties. 2


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INTRODUCTION Thanks to the rapid increase in global energy demand, thermoelectric materials, which can potentially generate large amounts of electrical energy from industrial waste heat and other abundant heat resources (e.g., solar thermal systems1), are becoming increasingly attractive. Unfortunately, at present they are confined to niche markets because of their low conversion efficiencies, which are generally evaluated in terms of the dimensionless figure of merit, ZT = S2σT/κ, where S, σ , T and κ are the Seebeck coefficient, electrical conductivity, absolute temperature and thermal conductivity of the material, respectively.2 In addition, most conventional alloy-based thermoelectric materials, such as Bi2Te3 and PbTe,3 suffer from poor thermal instability, high toxicity and high cost. Transition metal oxide ceramics, in contrast, are (generally speaking) free of these disadvantages, and have the added advantage of being stable at high temperatures (> 500 oC). Transition metal oxides are thus promising candidates for the core component in thermoelectric power generators. Titanium dioxide, TiO2, is one of the most widely used and intensively studied transition metal oxides, with potential applications in photocatalysis,4 dye-sensitized solar cells,5 lithium ion batteries,6 and display panels and other optical devices,7 which take advantage of its tunable semiconducting properties. TiO2 also displays tantalizing thermoelectric behavior due to the large Seebeck coefficient.8 If the other thermoelectric properties, namely electrical 3



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conductivity and thermal conductivity, could be improved, TiO2 would make an excellent thermoelectric material because of its non-toxicity, good chemical and thermal stability, and natural abundance. Recently, Dynys et al. investigated the thermoelectric properties of undoped and doped (Ti0.75Sn0.25)O2, and demonstrated that the electrical conductivity was enhanced by three orders of magnitude when Nb2O5 or Ta2O5 are added, while the thermal conductivity was reduced to below 4 W/mK.9 Portehault et al. prepared percolated nanocomposites using substoichiometric TiO2 nanoparticles embedded in a carbon matrix.10 These nanocomposites have low electrical resistivity (2×10-4 Ωm) and reduced thermal conductivity (1 W/mK) with respect to bulk materials. Xu et al. reported that Al doping can lead to phonon scattering at grain boundary interfaces and consequently decreases thermal conductivity of TiO2.11 Other studies have also focused on the thermoelectric properties of TiO2-based materials.12-19 These show that the properties of TiO2-based materials depend strongly on the preparation process, particle size, and microstructure, making their behavior difficult to predict using theoretical methods such as first-principles calculations. For TiO2 to be competitive as a thermoelectric material, ZT needs to be increased because most values reported so far are below 0.2. Like other transition metal oxides, TiO2 possesses lower electrical conductivity and higher thermal conductivity compared with thermoelectric alloys. Doping is the most common way of tailoring electronic properties. For 4


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instance, uniform porous conducting architectures based on Nb-doped TiO2 can be made because replacement of a small amount of Ti with Nb drastically increases the electrical conductivity of TiO2.20 Moreover, a reducing atmosphere is more favorable for increasing the electrical conductivity of Nb-doped TiO2 than oxidizing atmospheres, including air, owing to the difference in oxygen activity.21 However, current methods of preparing foreign-atom-doped TiO2, including sol-gel processes6,22 and hydrothermal and solvothermal reactions,22,23 involve numerous time-consuming steps, which adds to the cost and complexity and may compromise the reproducibility of the materials. Apart from doping, crystalline defects are also known to affect the electronic properties of TiO2, resulting in absorption of visible light and relatively high electrical conductivities; highly defective materials are thus of intense interest for use in photovoltaic24 and photocatalytic25 applications. Chemical tuning by combining both doping with foreign atoms and formation of large numbers of intrinsic defects (i.e., vacancies and interstitials) is a powerful approach for improving the electrical conductivity of oxide materials. To be a good thermoelectric material, the Seebeck coefficient, which depends inversely on carrier concentration, also needs to be kept at a reasonable level. Generally, the decrease in the Seebeck coefficient brought about by an increase in number of charge carriers should be outweighed by the increase in electrical conductivity, contributing to a higher power factor. Chemical tuning can also be used to reduce the thermal conductivity. For 5



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example, both doping26 and generation of intrinsic defects27 can increase the scattering of phonons with relative short wavelengths, which is usually the dominant process at high temperatures. In addition, nanoparticle technology enables the preparation of materials with high concentrations of grain boundaries, which can scatter phonons with relatively long wavelengths.28 Consequently, in order to prepare a good thermoelectric material using a bottom-up approach, precursors with sufficiently small particle sizes need to be prepared, which provides high concentrations of grain boundaries. Appropriate sintering processes also should be chosen to minimize the negative effect of grain boundaries on the electrical conductivity, since grain boundaries could degrade transport of charge carriers by scattering in reasonably conductive systems.29 The aim of the present study is to enhance the n-type thermoelectric properties of TiO2 using as straightforward a synthesis method as possible. Herein combustion synthesis is reported for the first time, and attractive route for achieving this because it is fast and direct, with nanopowders obtained in a single step. In this study urea was used as the fuel to prepare anatase TiO2 nanoparticles, as well as the source of N. NbCl5 was used as the source of Nb. This synthesis process provides precursors with sufficiently small particle sizes, which is the key point in bottom-up approaches. After combustion, powders were pressed into bars or pellets and sintered, in some cases under strongly reducing conditions by addition of Ti powder. 6


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In short, chemical tuning was carried out to enhance the thermoelectric properties by controlling three factors: (1) the amounts of N and Nb co-doping; (2) the grain boundary density; and (3) the concentration of intrinsic defects. As a result, the figure of merit, ZT, is improved to 0.35 at 700 oC, which is the highest value reported for a TiO2 material to date, and is comparable with the highest values of any n-type thermoelectric oxides such as SrTiO3 (ZT = 0.37 at 1000K, film),30 In2O3 (ZT = 0.3 at 1000K)31 and ZnO (ZT = 0.44 at 1000K).32 The enhanced thermoelectric performance and advantages of TiO2 mentioned above suggest that the chemically tuned TiO2 is an excellent candidate for thermoelectric application. Consequently, the chemical tuning approach, combination of foreign-atom-doped nanoparticle synthesis and optimized sintering process, can be applied to prepare superior thermoelectric materials.

EXPERIMENTAL METHODS Synthesis N and Nb co-doped TiO2 powders were synthesized by a fast combustion method using urea (CO(NH2)2) as the fuel. TiO2 (anatase) powder (ST-01, particle size ~7 nm) was mixed with NbCl5 (99.99%) powder in the molar ratio [NbCl5]/{[TiO2]+[NbCl5]} = x:1, where x was varied; the amount of urea added was based on the mass ratio m(urea) : m(TiO2+NbCl5) = 3 : 1. NbCl5 hydrolyzes readily in humid air, so to avoid formation of Nb2O5, the relative humidity was kept below 60% (see Figure S1 in Supporting Information) when mixing the 7



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raw materials. The mixed reagents were transferred rapidly to a muffle furnace and heated in air atmosphere at 600 oC isothermally for 45 minutes. After natural cooling to room temperature, yellow powders were obtained. Next, powders were pressed into either rectangular bars or disks by a cold pressing process at a pressure of 1×104 kgf/cm2. In some cases, Ti metal particles were added to the TiO2 powders (4 g) in the molar ratio [Ti metal]/[Ti1-xNbx(O,N)2±δ] =Y, where Y was varied, by milling in 20 ml absolute ethanol with a planetary ball mill at 600 rpm for 24 hours. The dark blue precipitates were washed with 600 ml absolute ethanol and dried at 120 oC for 2 hours. Finally, the pressed pieces were placed in a tube furnace and sintered in a pure N2 flow with 0.2 L/min at 1100 oC for 10 hours using a constant heating rate of 5 oC/min. During sintering the samples were placed on a graphite boat, with another boat containing carbon powder placed in front of it. A small amount of CO gas is expected to be generated in the furnace because the N2 atmosphere still contains small quantities of O2 and H2O gas. After cooling down naturally to room temperature, the blue-black and black sintered samples were incised and polished ready for thermoelectric characterization. The compositions of the resulting N and Nb co-doped anatase TiO2 nanoparticles can be written Ti1-xNbx(O,N)2±δ, where x is the molar ratio of the starting reagents, [NbCl5]/{[TiO2]+[NbCl5]}. In this work, materials sintered with addition of Ti powder are denoted by TNOX-Y, where X is the Nb content x expressed as a percentage, and Y is the molar ratio [Timetal]/[Ti1-xNbx(O,N)2±δ] of 8


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the mixture before sintering expressed as a percentage; sintered samples without added Ti powder are denoted simply by TNOX. Characterization X-ray diffraction (XRD) patterns were collected using a powder diffractometer equipped with a heating stage (PANalytical X’pert Pro MPD) and operated at 40 kV and 40 mA, with Cu Kα radiation (λ = 0.154 nm). The heating rate was 5 o

C/min and data collection began once the temperature reached the preset values.

Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) were performed using a Hitachi S-4800 FESEM microscope with an Oxford energy dispersive X-ray detector. Transmission electron microscopy (TEM) observations were carried out using a Tecnai G220 microscope at 200 kV. Focused ion beam (FIB) technique was used to prepare TEM samples of sintered compacts. X-ray photoelectron spectroscopy (XPS) was performed using an AXIS Ultra DLD spectrometer using Al Kα radiation (hv = 1486.6 eV). Electrical resistivities and Seebeck coefficients were measured by the static direct-current (DC) method (ULVAC-RIKO, ZEM-3) under low argon (99.999 %) atmosphere with temperature gradients of 20 oC, 30 oC and 40 oC. The thermal conductivity was calculated using the formula κ = λρcp, where λ, ρ and cp represent thermal diffusivity, density and specific heat capacity, respectively. The heat capacity was measured using a differential scanning calorimeter (TA Instruments, DSC-2910), and thermal diffusivity using the laser-flash method (ULVAC-RIKO, TC-9000V). Densities were estimated using 9



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Archimedes’ method. Uncertainties in the measured electrical conductivity, Seebeck coefficient and thermal conductivity were estimated to be within ± 10%, ± 7% and ±6 %, respectively.

RESULTS AND DISCUSSION

Structure, Morphologies and Compositions of the N and Nb co-doped TiO2 Nanoparticles All combusted powders were found to consist of the anatase phase, as shown by the X-ray diffraction (XRD) patterns in Figure 1. Comparison of the peak positions with those for the reference (undoped) TiO2, as highlighted in the insets in Figure 1, reveals that no obvious structural change occurs if urea alone is used; in contrast, the peaks are shifted to lower angles as the amount of NbCl5 is increased. This is strong evidence that Nb ions have been successfully substituted onto Ti sites, because Nb5+ ions (ionic radius = 0.064 nm) are larger than Ti4+ ions (ionic radius = 0.0605 nm), causing the lattice to expand upon substitution.33 Scanning electron microscopy (SEM) observations were carried out to study the difference in morphologies of the different powders. Typical micrographs are shown in Figure 2. Although all powders consist of uniform nanoparticles, their particle sizes differ with composition. Particle size distributions estimated from representative SEM micrographs are shown in the insets of Figure 2. The 10


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particle diameter corresponding to the maximal ratio in sample Ti(O,N)2±δ (Figure 2B) increases to 35 nm compared with about 30 nm for pure TiO2 (Figure 2A), while those also containing Nb are smaller at about 20 nm for Ti0.91Nb0.09(O,N)2±δ (Figure 2C) and 14 nm for Ti0.83Nb0.17(O,N)2±δ (Figure 2D). The greater particle growth in the N-doped powders may be caused by a higher transient temperature reached during combustion when no Nb has been added.34 Also, the particle growth of the latter two samples may have been suppressed by the lower oxygen vacancy concentrations when higher valence Nb ions (+5) are present.35 Transmission electron microscopy (TEM) images of the reference TiO2 and N and Nb co-doped TiO2 are compared in Figure 3. When NbCl5 is added, the diameter of the nanoparticles decreases noticeably, consistent with the findings from SEM. In addition, the morphologies of co-doped nanoparticles (Figures 3C and 3E) are more irregular than those of the reference sample (Figure 3A), suggesting that particle growth is suppressed locally by the addition of Nb. However, the crystal structures identified from the corresponding selected area electron diffraction (SAED) patterns are that of the anatase phase and are nearly identical in all cases, as shown in the insets of Figures 3A and 3E. This is consistent with the XRD results which showed that single-phase anatase had been formed. High-resolution TEM images, shown in Figures 3B, 3D and 3F, confirm the suppression of crystallite growth caused by Nb doping. The particle sizes observed are consistent with the crystallite sizes (see Table SI in 11



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Supporting

Information)

estimated

from

the

XRD

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data

using

the

Debye-Scherrer equation. Although the above results strongly suggest that substitution of Nb was successful,36,37 in situ high-temperature XRD in air was also carried out as further confirmation, since it is known that Nb doping retards the phase transition from anatase to rutile.36 In the remaining discussion, we focus mainly on materials with x = 0.17 (Ti0.83Nb0.17(O,N)2±δ, TNO17 and TNO17-12), because the sample TNO17-12 showed the highest thermoelectric performance. The high temperature XRD measurements indeed showed that the transition temperature was increased by Nb doping from about 850 oC (see Figure S2A in Supporting Information) to 900 oC when x = 0.17 (see Figure S2B in Supporting Information). At the same time, the secondary phase TiNb2O7 is generated because Nb is less soluble in the rutile lattice than the anatase lattice,35 indicating that the solubility limit for Nb incorporation into rutile lattice has been reached; TiNb2O7 is typically observed in air when the Nb concentration exceeds 6.0%.36,37 Quantitative EDX analysis of the sintered material TNO17, which has the same x value, namely Ti0.83Nb0.17(O,N)2±δ, also showed that the actual Nb content is close to the nominal value of 17 at % (see section 3.2), suggesting that the effective Nb concentration in Ti0.83Nb0.17(O,N)2±δ is close to 17 at %. It is worth noting that TiNb2O7 forms at lower temperature than the anatase-rutile phase transition. This is also likely related to the fact that Nb incorporation into the anatase lattice delays the onset of the phase transition. 12


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After confirming that Nb had been successfully incorporated into the anatase lattice, its distribution was examined by scanning transmission electron microscopy (STEM) elemental mapping and line-scan measurements. In Figure 4, the STEM image of Ti0.83Nb0.17(O,N)2±δ (Figure 4A) and the corresponding elemental maps (Figures 4B and 4C) reveal that Nb is distributed homogeneously as expected, confirming that no niobium oxide phases are present. On the other hand, line-scans across a single nanoparticle, as shown in Figures 4D and 4E, reveal that the concentration of Nb at the surfaces of a nanoparticle is higher than that in the particle interior. The chemical states of Nb and Ti were determined by XPS (the survey scan is provided in Figure S3A in Supporting Information). The peak positions at 206.9 and 209.7 eV for Nb 3d, and 464.4 and 458.7 eV for Ti 2p as shown in Figures 5A and 5B, respectively, confirm that both Nb and Ti are in their highest valence states in the Nb-doped nanoparticles (x = 0.17).38 The extra charge introduced by doping with pentavalent Nb is likely compensated by the creation of Ti vacancies.36 Because urea (CO(NH2)2) was used as the fuel, it is possible that C is incorporated into the crystal lattice in addition to N. XPS was carried out to determine whether this was the case or not. Figure 5C shows the C 1s spectrum of Ti0.83Nb0.17(O,N)2±δ. The peaks at 284.8, 286.3 and 288.9 eV can be assigned to adventitious carbon species in the XPS apparatus.34 A C 1s peak would be observed at about 281.8 eV if C had substituted on the O site in the anatase 13



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lattice;39,40 this is not evident in our spectrum. In the N 1s spectrum shown in Figure 5D, the peak at 395.6 eV may be associated with Ti-N bonds (Ns),40 indicating that N substitution on O sites occurs during the combustion process. The peak at 401.9 eV has been assigned to chemisorbed nitrogen (Ni),34,41 which usually suggests interstitial N doping.42 The chemical composition of atomic Ni/Ns is about 1.076 based on the fitted areas. By comparing with the peak for O 1s (Figure S3B in Supporting Information), the surface atomic N/O was estimated to be 0.047. Similarly, the surface Nb/Ti ratio atomic was calculated to be 1.195, corresponding to 54 at % Nb doping at the surface of the nanoparticles, suggesting substantial Nb segregation just as observed in the above EDX line-scan. Recently Breault and Bartlett43 proposed that N and Nb co-doped anatase TiO2 nanoparticles synthesized by sol-gel processing techniques are effective catalysts for the degradation of methylene blue dye. Our synthesized nanoparticles may also be suitable for this application. For use in thermoelectric generators, however, the crystal structures, microstructures and their effect on thermal and electrical conductivities of sintered N and Nb co-doped anatase TiO2 nanoparticles need to be determined. Sintered Compacts Stoichiometric TiO2 is a semiconductor with wide band gap, 3.2 eV for anatase and 3.0 eV for rutile,44 and thus exhibits relatively low electrical conductivity. To improve the electrical conductivity, co-doped TiO2 pellets and bars were 14


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sintered in an N2 atmosphere. During sintering, the materials were reduced by the slight CO gas generated from the boats of carbon powders in the tube furnace. For comparison, non-doped TiO2 was fabricated under the same conditions. XRD patterns of the sintered samples are shown in Figure 6. These reveal that all samples consist of the rutile phase without any secondary phase. The disappearance of TiNb2O7 which formed during sintering in air is consistent with the observation that a reducing atmosphere increases the solubility limit of Nb in the rutile lattice.45 In Figure 6A, the diffraction peaks shift to lower angles with increasing Nb concentration, confirming that Nb has been successfully substituted onto Ti sites. Microstructures of each sample were observed by SEM. As seen in Figure 7, the grain sizes apparently decrease upon Nb doping from several microns for the reference (non-doped) TiO2 (Figure 7A) to hundreds of nanometers for samples with x = 0.17 (Figure 7B). The mechanism by which the grain growth is constrained is likely analogous to that described previously for the N and Nb co-doped anatase powders. EDX spectra were obtained to calculate the atomic ratio Nb/(Nb+Ti) in each sample, with relatively large areas (about 400 × 400 µm2) used to collect the data (Figure S4 in Supporting Information). The calculated Nb content in the sample TNO17 was 0.172, while that of TNO23 was 0.235, which are both extremely close to the nominal values. To investigate the possible incorporation 15



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of C and N, XPS data were taken in an attempt to detect any C 1s and N 1s peaks in the TNO17 sample (a typical scan is shown in Figure S5A in Supporting Information). The C 1s peak (Figure S5B in Supporting Information) is similar to that for the Ti0.83Nb0.17(O,N)2 ± δ powder, indicating no Ti-C bonds were formed. Interestingly, the N 1s peak observed in the case of the powder is no longer visible. This suggests that N is removed at high temperature even under an N2 atmosphere. In fact, N is very easy to be removed in air atmosphere because the color of N and Nb co-doped anatase nanoparticles were transformed from yellow into white after they had been further thermally heated at 650 oC in air atmosphere for 30 minutes (see Figure S6 in Supporting Information). The change of color can be attributed to the physical origin that N doping induces formation of localized occupied states in the gap, accounting for the visible light activity,41 while Nb doping shows optical transparency in the visible region because of the missing of impurity states in the gap.7,46 Scans around the Ti 2p peak revealed two new peaks at 457.2 and 461.9 eV, as shown in Figure 8A, indicating that some Ti4+ was reduced to Ti3+ during the sintering process.47 At the same time, the weak new peaks (at 205.5 eV and 208.2 eV) in Figure 8B indicate that a slight amount of Nb5+ was reduced to Nb4+.41 Based on these XPS data, the estimated atomic ratios Ti3+/ Ti4+ and Nb4+/ Nb5+ are 0.083 and 0.032, respectively. Additionally, the Nb/Ti atomic ratio is estimated to be 0.748, namely 43 at % Nb incorporation, which may indicate that Nb segregation remains substantial, since XPS analysis is surface sensitive. 16


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The degree of reduction could be further tailored by ball milling the TiO2 powders together with metallic Ti powder before sintering. The XRD patterns in Figure 9 reveal that the pressed and sintered N and Nb co-doped samples (x = 0.17 and 0.23) mixed with Ti powder have been successfully transformed into the rutile phase. Furthermore, the shift in the XRD peaks to lower angles with increasing Nb concentration is still observed, as shown in the inset of Figure 9. It is also worth noting that mixtures with lower Nb concentrations (x = 0.00 and 0.09) underwent phase separation to form multiphase materials with poor thermoelectric properties. The influence of N doping of the initial powders on the electronic properties of the sintered samples (from which N had ostensibly been removed) was also investigated. Samples synthesized by sintering Nb doped powders (without N doping) shows poorer electrical conductivities (power factors) than those prepared from N and Nb co-doped powders. This improvement in properties of the sintered samples may be attributed to an increase in defect concentration and distribution brought about by N doping and N burn-off, since it is known that N doping introduces large number of O vacancies, especially near particle surfaces.42 TEM micrographs such as those shown in Figure 10 allow the grain boundaries in polycrystalline TNO17-12 to be characterized directly. Bright-field (BF) (Figure 10A) image reveals that the sintered material consists of nanosized grains, with particle diameters on the order of hundreds of 17



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nanometers. In addition, the black areas in the BF image hint at high concentrations of point defects formed during the sintering process. Analysis of the crystal structure observed in the HRTEM image in Figure 10B also confirmed that the material had transformed to the rutile phase. EDX line-scans were taken to determine the Nb distribution across a single grain boundary; the results are shown in Figure 10D. The higher concentration of Nb at the grain boundary compared to the crystal interior indicates segregation of the dopant ions has occurred. XPS measurements taken of the reduced samples typically failed to detect any C or N doping. They did, however, reveal that some of the cations exist in their reduced states, namely Ti3+ and Nb4+ (see Figure S7 in Supporting Information). The Ti3+/Ti4+ and Nb4+/Nb5+ ratios for TNO17-12 are about 0.108 and 0.042, respectively, which are higher than the corresponding values for TNO17, as expected. Thermoelectric Properties The different effect of Nb doping and Ti reduction on thermoelectric properties as well as their relationship is theoretically modeled, which will be discussed systematically in our next article. Here we report the results for the TNO17-12 sample, which exhibits the best thermoelectric performance. Figure 11 shows the thermoelectric properties for the different samples as a function of temperature. From Figure 11A, it can be seen that the electrical conductivity of TNO17-12 is significantly higher than that of undoped TiO2, as 18


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well as that previously reported for reduced pure rutile TiO2.48 For example, the doped sample has a conductivity of 2.4×104 S/m at 700 oC while the undoped sample has 2.0 ×103 S/m. The higher conductivity can be attributed to enhanced carrier concentrations resulting from Nb doping and Ti reduction. Furthermore, the electrical conductivities of the two samples increase only slowly with temperature, suggesting that they are degenerate semiconductors. The negative values of the Seebeck coefficients (Figure 11B) further reveal that they are n-type semiconductors. Although the Seebeck coefficient of TNO17-12 decreases somewhere in the range of about -195 to -230 µV/K, which is lower than that of undoped TiO2 (-250 to -285 µV/K), the power factor of the doped sample is seven times greater, up to about 9.8×104 W/mK2 at 700 oC. This phenomenon can be ascribed to the fact that the decrease in Seebeck coefficient is outweighed by the increase in electrical conductivity, which should be mainly contributed by the large number of intrinsic defects. Actually, intrinsic defect tuning is very powerful to enhance the thermoelectric properties correlated with electronic structure, Seebeck coefficient and electrical conductivity, of thermoelectric materials such as Bi2Te3.49 In Figure 11C, the total (lattice) thermal conductivity of TNO17-12 is seen to be much lower than that of undoped TiO2, especially in the low temperature region. At 700 oC, the total (lattice) thermal conductivity of TNO17-12 is 2.6 (2.1) W/mK, which is 1.4 (1.9) W/mK less than that of the reference sample. According to the Callaway model,50 the phonon heat current is usually limited 19



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by the following scattering mechanisms: (a) grain boundary scattering; (b) point defect

scattering;

(c)

phonon-phonon

Umklapp

scattering;

and

(d)

electron-phonon scattering. Based on the results of SEM and TEM observation, Nb doping increases the number and concentration of grain boundaries, which according to the above model may be one of the major factors causing the decreased thermal conductivity. Also, Ti reduction during the sintering process (and possibly also N removal) introduces more point defects, which increases the occurrence of the second scattering mechanism. In the case of rutile TiO2, the last two mechanisms are negligible. In other words, the thermal conductivity is mainly suppressed by increased grain boundary scattering and point defect scattering. As a result of the improved power factor and simultaneous decrease in thermal conductivity, the value of ZT increases about ten times to 0.35 at 700 o

C for TNO17-12 (Figure 11D). To the best of our knowledge, this is the highest

value reported for rutile TiO2, and also one of the highest values reported for n-type thermoelectric oxides. Even higher values of ZT may be possible if the grain size is decreased further without causing a marked deterioration in the electrical conductivity. We also tested the thermal stability of TNO17-12 under pure N2 atmosphere (see Figure S8 in Supporting Information). The electrical conductivity, Seebeck coefficient and power factor remain relatively unchanged after successive heat treatments at high temperatures (800, 600 and 400 oC) for long times. This improved stability may be ascribed to the higher concentration of Nb at particle 20


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surfaces, which decreases the concentration of O vacancies at the surfaces of doped rutile TiO2,51 suppressing any electrochemical interactions with the atmosphere that could lead to breakdown of the crystal structure. The excellent thermoelectric performance of our sintered sample demonstrates that chemical tuning of low-cost semiconducting oxides is a promising means of designing superior materials for use in thermoelectric generators at high temperature (> 400 oC).

CONCLUSIONS In this work, we developed a novel and fast single-step combustion method for synthesizing anatase TiO2 nanoparticles co-doped with nitrogen and niobium and an average grain size of about 14 nm. Nb ions occupy Ti sites while N ions occupy O sites and apparently also interstitial sites. In addition, N doping increases the grain size (crystallite size) on account of a transient higher temperature during combustion; conversely Nb doping decreases the grain size owing to a smaller concentration of O vacancies. The Nb distribution investigated by EDX line-scans confirms that the concentration of Nb at the surface of the nanoparticle is higher than that in the crystal interior. This combustion method may be extended to conveniently synthesize other foreign-atom-doped metal oxide nanoparticles. Co-doped TiO2 nanopowders sintered at 1100 oC in an N2 atmosphere transformed into an Nb-doped rutile phase, with nitrogen being removed. Nb 21



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doping also increased the grain boundary density compared with undoped TiO2 sintered under the same conditions. Modification of the charge states of Ti cations was demonstrated to be a powerful tool for tuning the defect concentration, and thereby enhancing the thermoelectric properties. By sintering under reducing conditions, the power factor was increased by seven times up to 9.87×104 W/mK2 at 700 oC; this is attributed to the large increase in electrical conductivity without a significant decrease in the Seebeck coefficient. The decrease in thermal conductivity from 4.0 W/mK to 2.6 W/mK at 700 oC is attributed to increased grain boundary scattering and point defect scattering. As a result, the figure of merit, ZT, increased by an order of magnitude to 0.35 (700 oC), which is the highest value reported for rutile TiO2, and also one of the best results for any n-type thermoelectric oxide. The excellent thermal stability at high temperature makes the materials prepared by the combustion synthesis technique and optimized sintering process promising candidates for use in thermoelectric generators at high temperature (> 400 oC).

SUPPORTING INFORMATION Room- and high-temperature XRD patterns of nanoparticles, XPS data of nanoparticles and sintered compacts, EDX spectra of the sintered compacts, a photograph of the powders, thermal stability results, and crystallite sizes calculated using the Debye-Scherrer equation. This material is available free of 22


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charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Mailing Address: Key Laboratory of Renewable Energy and Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou,

510640,

P.

R.

China;

Tel:

+86-20-87035351;

Fax:

+86-20-87035351; E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (Grant No. 51172234), JSPS-CAS Joint Research Project： Enhancement of thermoelectric power of nano-titanate p-type material by metallic cation intercalation.

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Figure 1. XRD patterns of N and Nb co-doped TiO2 powders Ti1-xNbx(O,N)2±δ. The inset shows a magnified view of the peaks at 25o.

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Figure 2. SEM images of N and Nb co-doped TiO2 nanoparticles (scale bars = 100 nm): (A) TiO2, (B) Ti(O,N)2±δ, (C) Ti0.91Nb0.09(O,N)2±δ, and (D) Ti0.83Nb0.17(O,N)2±δ. The insets show the corresponding estimated particle size distributions.

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Figure 3. TEM and HRTEM images of N and Nb co-doped TiO2 nanoparticles: (A), (B) TiO2, (C), (D) Ti0.91Nb0.09(O,N)2±δ and (E), (F) Ti0.83Nb0.17(O,N)2±δ; Scale bars = 100 nm for A and E, and 10 nm for B, C, D and F. The insets in A and E are the corresponding SAED patterns.

33



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Figure 4. Distribution of Nb in Ti0.83Nb0.17(O,N)2±δ: (A) HAADF-STEM image of nanoparticles, with corresponding elemental maps of (B) Ti, and (C) Nb; and (D) a HAADF-STEM image of a single nanoparticle, with (E) of the same. Scale bars = 50 nm.

34


EDX line-scan

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Figure 5. XPS data for Ti0.83Nb0.17(O,N)2±δ: (A) Nb 3d, (B) Ti 2p, (C) C 1s and (D) N 1s peaks.

35



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Figure 6. XRD patterns of sintered TiO2 bulk materials without Ti reduction.

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Figure 7. SEM images of the sintered TiO2 bulk materials without Ti reduction (scale bars = 5 µm): (A) TiO2±δ and (B) TNO17.

Figure 8. XPS data of the sintered TiO2 bulk materials TNO17: (A) Ti 2p and (B) Nb 3d 37



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Figure 9. XRD patterns of the sintered TiO2 bulk materials TNO17-12 and TNO23-12. The inset shows a magnified view of the peaks near 27.5o.

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Figure 10. TEM micrographs of TNO17-12: (A) BF image (scale bar = 500 nm), (B) HRTEM image (scale bar = 5 nm), (C) high-magnification HAADF-STEM image (scale bar = 50 nm), and (D) an EDX line-scan taken across the region in (C).

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Figure 11. Thermoelectric properties of sintered TiO2 bulk materials: (A) electrical conductivity, (B) Seebeck coefficient, (C) thermal conductivity, and (D) ZT. The lattice thermal conductivity in C is estimated by excluding the contribution of electrons, which can be calculated using the Wiedemann-Franz law, κe=LσT, where L is the Lorentz constant, approximately equal to 2.44×10-8 V2K-2, and the other symbols have their usual meanings.

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Nanoparticles and Sintered Compacts for Enhanced Thermoelectric

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