Article pubs.acs.org/JPCC
Thermal Conductivity of TiO2 Nanotubes Tao Gao*,† and Bjørn Petter Jelle‡,§ †
Department of Architectural Design, History and Technology, and ‡Department of Civil and Transport Engineering, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway § Department of Materials and Structures, SINTEF Building and Infrastructure, NO-7465 Trondheim, Norway ABSTRACT: Thermal conductivity of TiO2 nanotubes prepared from a NaOH treatment of TiO2 particles with subsequent acid washing and annealing has been investigated. The obtained TiO2 nanotubes have a tetragonal anatase structure, and have a typical inner diameter of about 4−5 nm, wall thickness of about 2−3 nm, and length up to several hundred nanometers. TiO2 nanotubes show a significantly reduced thermal conductivity of about 0.40−0.84 W/(m·K) (average 0.62 W/(m.K)) at room temperature, as compared to about 8.5 W/ (m·K) for the bulk TiO2 materials. The great suppression in thermal conductivity can be understood by means of increased phonon-boundary scattering and enhanced phonon confinement in TiO2 nanotubes with unique nanotubular morphology, small featured sizes, and large surface area (∼258 m2/ g). A theoretical analysis including the surface scattering and size confinement effects of phonon transport in TiO2 nanotubes is also reported, which results in an intrinsic thermal conductivity of 0.30−0.77 W/(m·K) (average 0.54 W/(m.K)) for individual TiO2 nanotubes with wall thickness of 2−3 nm, in harmony with the experimental values.
1. INTRODUCTION In recent years, understanding the heat transport at nanometer scale has attracted considerable attention due to its significance in various scientific and technical fields. For example, with the continuous miniaturization of electronic and micromechanical devices, there is an increasing interest in materials that can dissipate heat efficiently, thus preventing the deterioration of the devices’ performance.1,2 Carbon-related nanomaterials,3−6 in particular carbon nanotubes,3,4 have been extensively studied due to their novel electrical, mechanical, and thermal properties. Carbon nanotubes have an unusually high thermal conductivity, about 6600 W/(m·K) at room temperature,4 which can be attributed to, among others, the strong sp2 bonds that hold a single graphene nanosheet into a seamless and atomically perfect cylinder with diameter of a few nanometers.3,4 In contrast to the ultrahigh thermal conductivity observed in carbon nanotubes, both theoretical and experimental studies have revealed that,7,8 when crystalline solids are confined to nanometer range, thermal transport within them can be significantly reduced. Si nanowires, for example, exhibit a 100-fold reduction in thermal conductivity as compared to the bulk material.8−11 Si nanowires have therefore been of great interest in thermoelectric (TE) devices,10,11 where materials with low thermal conductivity and high electrical conductivity are highly desirable to achieve high TE efficiency. Inspired probably by Si nanowires, thermal properties of other onedimensional (1D) semiconducting nanomaterials, such as Ge and Si−Ge core−shell nanowires,12−16 GaAs nanowires,14,17 GaN nanowires and nanotubes,18−21 SnO2 nanobelts,22 perovskite oxide nanowires,23 TiO2 nanowires and nanotubes,24−26 and ZnO nanowires,27 have also been reported in the literature. © 2013 American Chemical Society
In general, a significantly suppressed thermal conductivity of 1D semiconducting materials has been observed, as compared to the corresponding bulk values, which can be attributed to the increased phonon-boundary scattering and phonon confinement that are typical for 1D nanomaterials with small diameters and large surface-to-volume ratios.1,7 As a matter of fact, thermal conductivity of nanowires decreases with their diameters, which implies that nanowires with smaller diameters, although difficult to fabricate, are preferred to achieve lower thermal conductivities.8−11 Recently, Chen et al. have proposed a Si nanotube structure to reduce further the thermal conductivity of Si nanowires;28 their calculation shows that only a 1% reduction in cross-section area (i.e., by removing the central atoms of nanowires along the axial direction) can bring about a 35% decrease in thermal conductivity. Apparently, the hollow nanotubes, which have an additional inner surface and consequently more phononboundary or phonon-defect scattering than those of the solid nanowire counterparts with similar dimensions,16,20,21,28 represent a promising system to achieve materials with low thermal conductivities. However, in contrast with the extensive studies on the synthesis and electrical and optical property of nanotubes with different chemical compositions,29 investigations on their thermal properties are scarce. Although theoretical approaches have shown promising features of Si nanotubes16,28 and GaN nanotubes,20,21 no experimental work has so far been performed to verify these calculations. It is Received: November 3, 2012 Revised: December 27, 2012 Published: January 3, 2013 1401
dx.doi.org/10.1021/jp3108655 | J. Phys. Chem. C 2013, 117, 1401−1408
The Journal of Physical Chemistry C
Article
Thermal Conductivity Measurement. Thermal properties of the TiO2 nanotubes were investigated with a Hotdisk Thermal Constants Analyzer (TPS 2500S), and a transient plane source technique was applied.42 A disk-type Kapton Sensor 5465 with radius 3.189 mm was used; the sensor, which acts both as heat source and temperature recorder, was sandwiched between two TiO2 nanotube samples. The temperature increase of the samples as a function of time was recorded to compute their thermal conductivity. The final thermal conductivity value reported here was the arithmetic mean of four individual measurements under different conditions (heating power, 0.02−0.2 W; measurement time, 1−160 s). TiO2 nanotube samples for thermal conductivity measurements were nanotube pellets, which were prepared by hydrostatically pressing TiO2 nanotube powders into small disks with diameters of about 19 mm and thicknesses of 1.5−4 mm; the density of the samples was dependent on the pressure applied and the amount of nanotube powders used.
obviously interesting to conduct both theoretical and experimental research in nanotubular materials, where an improved understanding on heat transport at nanometer scale may lead to a better material design and tailor-making of advanced TE devices and also for other applications.30 We report here the thermal properties of TiO2 nanotubes prepared via an alkaline treatment of crystalline TiO2 particles with subsequent acid-washing and annealing treatment.31−33 As an important oxide nanomaterial, TiO2 nanotubes have been extensively studied due to their extraordinary structural and physical properties,26,31−38 and promising applications in, for example, photocatalysis,39 lithium batteries,40 and sensors.41 In this work, it is found that TiO2 nanotubes have a significantly reduced thermal conductivity of about 0.40−0.84 W/(m·K), as compared to about 8.5 W/(m·K) for the bulk TiO2 materials.25 The significant reduction in thermal conductivity of TiO2 nanotubes can be explained by the increased phonon-boundary scattering and phonon confinement effects due to their novel nanotubular structure and small featured sizes, that is, with inner diameters of about 4−5 nm and wall thicknesses of about 2−3 nm. A theoretical analysis including the surface scattering and size confinement effects of phonon transport in TiO2 nanotubes results in an intrinsic thermal conductivity of 0.30− 0.77 W/(m·K) for individual TiO2 nanotubes, in harmony with the experimental values. Details of both theoretical and experimental studies will be reported in the following sections.
3. RESULTS AND DISCUSSION Structural Features of TiO2 Nanotubes. It is known that the nanotube product from the alkaline treatment of crystalline TiO2 particles and followed by acid washing is actually protonic titanate, of which the detailed crystal structure is still under debate.33 As shown in Figure 1a, there are only a few broad
2. EXPERIMENTAL SECTION Materials and Chemicals. Reagent-grade sodium hydroxide (NaOH, ≥98%), titania nanoparticle (anatase TiO2, 95%), and titanate nanotubes with high purity can be readily achieved via the alkaline treatment of crystalline TiO2 particles,31−37 which represent an important and promising feature for their practical applications. Energy dispersive X-ray spectroscopic (EDX) analysis revealed that the as-prepared titanate nanotubes are composed of only two elements, titanium and oxygen (note that hydrogen is not detectable by EDX), indicating that titanate nanotubes were obtained after the acid washing.33−37 After the annealing treatment, no obvious morphological changes were observed for the obtained TiO2 materials (Figure 2c and d),43 implying that the nanotubular morphology of titanate nanotubes is probably preserved; that is, the transformation from protonic titanate H0.7Ti1.825□0.175O4·H2O to anatase TiO2 was topotactic.31 This assumption is confirmed by the subsequent TEM analysis (Figure 3). As reported in Figure 3, the corresponding TEM data reveal that the as-prepared titanate nanotubes have typically inner diameters of about 5 nm, outer diameters of about 11 nm, and lengths of up to several hundred nanometers. Moreover, the asprepared titanate nanotubes are poorly crystallized, as demonstrated by the corresponding electron diffraction (ED) analyses. The individual nanotube shows only diffused diffraction features. A typical ED pattern taken from an area that contains many nanotubes is shown in the inset of Figure 3a. Diffraction rings are observed as a result of the polycrystalline nature of the sample, with many nanotubes oriented along all directions. The strong diffraction rings with d-spacings of 0.366 and 0.187 nm may correspond, respectively, to the (110) and (200) crystallographic planes of the orthorhombic H0.7Ti1.825□0.175O4·H2O. After the annealing treatment, the obtained TiO2 nanotubes show improved crystallinity, as demonstrated by the corresponding XRD
Figure 4. Isotherms of nitrogen adsorption and desorption of (a) asprepared titanate nanotubes and (b) TiO2 nanotubes. 1403
dx.doi.org/10.1021/jp3108655 | J. Phys. Chem. C 2013, 117, 1401−1408
The Journal of Physical Chemistry C
Article
Teller (BET) surface area (SBET) of the TiO2 nanotubes is about 258 m2/g, which is smaller than that of the titanate nanotube precursors, about 317 m2/g. Moreover, an increase in mean pore diameter (rp) and a decrease in pore volume (Vtotal) are also noticeable after the annealing treatment, Table 1.
t ·k (2) α where t is the time measured from the start of the transient recording, and k is the thermal diffusivity of the as-prepared TiO2 nanotube samples. By knowing the heating power P0 and the radius of the disk spiral α, thermal conductivity λ of the samples can be calculated by plotting the recorded temperature increase ΔT(τ) versus D(τ). As reported in Table 2, the τ=
Table 1. Surface Area and Pore Structure of the Nanotube Samplesa
a
nanotube sample
SBET (m2/g)
Vtotal (cm3/g)
rp (nm)
titanate TiO2
317 258
1.72 0.85
1.5 2.1
Table 2. Measured Thermal Properties of TiO2 Nanotube Samples
Measurement/calculation error:
