mesoporous structure for thermoelectrics

Mesoporous Structure for Thermoelectrics

Hyung-Ho Park*, Sin-Young Jung, Min-Hee Hong and Chang-Sun Park Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Korea

Thermoelectric properties are defined through the figure of merit, Z = σS2/κ, where σ, S, and κ are electrical conductivity, Seebeck coefficient and thermal conductivity, respectively. For increase ZT value, high electrical conductivity, low thermal conductivity, and high Seebeck coefficient are required. However, it is difficult to control these factors individually because thermal conductivity is generally proportional to electrical conductivity. By controlling microstructure on a nano-scale, thermal conductivity and electrical conductivity can be controlled individually. So, in this chapter, it was focused on the synthesis of mesoporous TiO2 film for its application in thermoelectric generation. The mesoporous TiO2 film was synthesized with titanium tetraisopropoxide. The triblock copolymer, Pluronic P-123 (EO20PO70EO20) was used as surfactant in 1-propanol. Furthermore Nb, Ag and Pt were introduced to increase electrical conduction. As a result, an improvement in the electrical conductivity and a reduction in the thermal conductivity through the control of pores distributions were found to be effective to enhance the thermoelectric property.

Keywords: thermoelectric; mesoporous thin film; TiO2; thermal conductivity; electrical conductivity; Seebeck coefficient

1. Introduction

1.1 Thermoelectric background

Global warming is accelerated by the use of fossil fuels, alters the amount and pattern of precipitation, leading to expansion of tropical and desert areas and a rise in sea levels. So, a global movement toward the generation of renewable energy is therefore under way to help meet increased energy needs. Novel energy conversion techniques are urgently required to decrease reliance on fossil fuels. Thermoelectric generation refers to a direct conversion of waste heat into useful electricity. Because this energy conversion process does not generate pollution and renewable technique, it is a suitable candidate for future energy conversion.[1-3] However, despite recent advances, thermoelectric generation is not as efficient as steam engines, thermoelectric remain limited for applications that are currently poorly served. The efficiency of thermoelectric device depends on materials properties through the figure of merit, Z = σS2/κ, where σ, S, and κ are electrical conductivity, Seebeck coefficient and thermal conductivity, respectively. To obtain high ZT value, high electrical conductivity, low thermal conductivity, and high Seebeck coefficient are required. However, it is very difficult to control these factors individually because electrical conductivity and thermal conductivity has an inverse relationship. To overcome this problem, nano-scale control technique is introduced.[4] In this work, thermoelectric properties are analysed with various porosity. Transparent conducting ceramic thin films have good properties to be used in a wide range of applications including coatings and gas sensors. And, pore structure has low thermal conductivity, thermoelectric property can be increased by adapting mesoporous structure. Thermal conductivity can be controlled by porosity. Meanwhile, electrical conductivity is decreased by pore structure then a reduced heating process becomes essential to create oxygen vacancy.

Figure 1. Optimization of the thermoelectric property of oxide by adaption of mesoporous structure.[5] Phonon-glass electron-crystal (PGEC) for new design concept of low thermal conductivity like amorphous and high electrical conductivity like crystal has been recently reported. By adopting this concept, Because electron mean free path is higher than phonon mean free path in oxide semiconductor materials, electrical conductivity and thermal

Materials and processes for energy: communicating current research and technological developments (A. Méndez-Vilas, Ed.)____________________________________________________________________________________________________

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conductivity can be controlled individually. Thus if nanoporous structure is adopted in an oxide thermoelectric materials, low thermal conductivity is decreased because phonon is scattered by existence of ordered pore structure. Moreover, impurity doping and reduced annealing process could enhance the electrical conductivity. In other words, oxide based thermoelectric material has a limit to increase the thermoelectric property, but nanoporous structure induces a change in the electrical conductivity and Seebeck coefficient. So, optimized thermoelectric property can be obtained as shown in Figure 1.[5]

1.2 Mesoporous thin films

As shown in Figure 2, mesoporous thin film has a structure containing nanosized pores.[6] In synthesis process, pore size, pore distribution (regular/irregular, open/close) and pore shape of mesoporous thin film can be controlled easily.[7] Mesoporous structure has distinctive properties such as large surface area and a decrease in phonon conduction. So, mesoporous ceramic thin film can be used in many applications such as sensors, thermoelectric, thermal insulators, and so on. Among many mesoporous structure, closed-pore structure has very high thermal insulation property. To obtain low thermal conductivity and high ZT value, closed-pore structure can be adopted using a self-assembly process. Also, in thermal insulation field, the nanoporous ceramic thin film is a good candidate due to its extremely low thermal conductivity. Through the studies on the various syntheses of nanoporous ceramic thin films and their characteristics, it has been revealed that nanoporous ceramic thin films are very useful in many applications.

Figure 2. Transmission electron microscopy (TEM) image of mesoporous structure.[6]

1.3 Application of mesoporous TiO2 film to thermoelectric

Semiconductive TiO2 in particular enjoys broad applicability as a gas sensors, membranes, photocatalysts, and optical applications.[8, 9] Ordered mesoporous material has good thermal insulation properties because of the regular distribution of nano-sized pore structures. So, mesoporous TiO2 structure could reduce its thermal conductivity and electrical conductivity. However, conductivity ratio can be increased as a decrease in electrical conductivity is expected to be smaller than a decrease in thermal conductivity because of the inelastic mean free path difference between electrons and phonons due to the highly ordered pore structure. If these properties are exploited, the thermoelectric properties of TiO2 can be potentially improved. Moreover, impurity atoms can be incorporated in mesoporous TiO2 structure to increase electrical conductivity. Thus, both doping process and nanoparticle incorporation process could potentially enhance the thermoelectric properties of TiO2 films. Research in this field has been focused on the synthesis of mesoporous TiO2 films with an emphasis on the effects of ordered pore structure and doping.

2. Structure and electrical property of ordered mesoporous TiO2 film

2.1 Preparation of mesoporous TiO2 thin films

TiO2 thin films were largely prepared by sol synthesis, spin coating and heat treatment. Titanium tetraisopropoxide (Ti(OPri)4, TTIP, Aldrich, 97%) was used as a titania precursor. 1-propanol was used as a solvent, Pluronic P-123 (EO20PO70EO20, Aldrich, MW 5800) was used as pore forming agent for mesoporous structure. HCl (Duksan, 35%) was used as catalyst. The mixed solution was coated on substrate and aged to control a speed of hydrolysis and condensation reaction.[10] And the as-prepared thin films were aged. Mesoporous TiO2 thin film could be synthesized after anneal at high temperature.

2.2 Analysis of structure and electrical property of mesoporous TiO2 thin films

X-ray diffraction (XRD) with Cu Kα radiation (λ = 1.5418 Å) was performed to investigate the structural ordering and crystal structure. Wide-angle pattern was recorded in the 2θ range from 1o to 60o with 0.01o interval. The refractive


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indices of synthesized mesoporous TiO2 films were obtained using an ellipsometer. From these refractive indices, porosities were calculated by using the Lorentz-Lorenz equation.[11] The cross-sectional image and thickness of mesoporous TiO2 film were observed using scanning electron microscopy (SEM). A detailed micro structural analysis was performed by using TEM. The electrical conductivity of mesoporous TiO2 thin film was measured by Hall effect measurement system, which uses van der Pauw method at room temperature. And the electrical parameters were measured by the direct current (Idc = 10 mA) four probe method in the magnetic field up to 0.57 T. Indium metal soldering was carried out to the sample surface coated on the substrate for a good ohmic contact..

2.3 Structural characterization of ordered mesoporous TiO2 thin films

In the synthesis process of mesoporous material, especially evaporation induced self-assembly process, surfactant/precursor molar ratio is the most important factor to influence on porosity. With increasing surfactant concentration, the number of micelles that formation of pore skeleton also increased. These increased micelle structure induced a result of porosity increase. After calcination process, porosity of mesoporous TiO2 film can be controlled depends on the surfactant/precursor molar ratio. In this work, the 0.015 sample of surfactant/precursor molar ratio had a highest porosity which has 32.4%. However, when surfactant molar ratio is increased the 0.020, porosity is decreased rather than increased, it was caused by an existence of critical surfactant/precursor molar ratio.[12] Because of this, in this work, the surfactant/precursor molar ratio was fixed as 0.015. Mesoporous TiO2 thin films should have high electrical conductivity/thermal conductivity ratio for the application of thermoelectric materials. Also, formation of reduced TiO2 is essential to enhance the electrical conductivity that is decreased by pore structure. A stoichiometric TiO2 has insulation property. However, if oxygen vacancy is created by reduction anneal on TiO2, the electrical conductivity will be highly increased. An oxygen vacancy in TiO2 could be formed by vacuum or reduced gas atmosphere annealing process. Among many reduction gases such as H2, N2 and NH3, vacuum and H2 effectively reduce TiO2. Then in this experiment, vacuum-anneal treatment was used to reduce mesoporous TiO2 film.

Figure 3. Color of mesoporous TiO2 films with different annealing atmosphere on the transparent quartz substrate.[13]

Figure 4. XRD spectrum of mesoporous TiO2 film with surfactant/precursor molar ratio of 0.015 after vacuum-anneal at 900oC (R represents rutile structure).[13]

Figure 3 shows a color change of mesoporous TiO2 films with different annealing atmosphere on the transparent quartz substrate. When vacuum annealing process, the color of mesoporous TiO2 thin films are changed from transparent to dark yellow. This result could be explained that commonly known TiO2 thin film has transparent color because energy bandgap (3.2 eV) is higher than the wavelength of visible light region. However, when energy bandgap of TiO2 is decreased, TiO2 absorbs the visible light. So, a color of the thin film is changed to dark. This result affects on the electrical conductivity. For thermoelectric application, a high temperature anneal of mesoporous TiO2 film was required to promote a reduction of TiO2. Moreover, densification of TiO2 that surrounded pores was required high temperature annealing to increase electrical conductivity. To increase thermoelectric property of mesoporous TiO2 film, porosity and annealing temperature must be optimized. Then in this experiment, 900oC of annealing temperature was chosen. The sample with H2/Ar atmosphere annealing showed 10-1 to 1 Ωcm of electrical resistivity, but the sample after vacuum atmosphere anneal showed 10-2 to 10-3 Ωcm of electrical resistivity. So, a reduction of mesoporous TiO2 under vacuum atmosphere was found to be the most effective. Figure 4 shows XRD result of mesoporous TiO2 film with surfactant/precursor molar ratio of 0.015 after vacuum-anneal at 900oC. The peak indexed as (110), (101), (111) and (211) indicated that phase of TiO2 film was changed to rutile structure because of high temperature annealing process. Figure 5 corresponds to a cross-sectional SEM image of mesoporous TiO2 film after anneal at 900oC under vacuum atmosphere. The thickness of the film was measured as around 260 nm. When compare to 450 oC annealing sample, thickness of mesoporous TiO2 film is decreased. Then it can be said that through the high temperature anneal,


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the thickness loss, i.e., porosity loss was only less than 13% because of grain growth. Actually the porosity of the film was 32.4% before anneal and changed to 28.2% after anneal at 900oC under vacuum atmosphere.

Figure 5. Cross-sectional SEM image of mesoporous TiO2 film after anneal at 900oC under vacuum atmosphere.[13]

Figure 6. TEM image of mesoporous TiO2 film after anneal at 900oC under vacuum atmosphere.[14]

However through the SEM observation, a particle growth of TiO2 was observed because normally the size of primary particle was around several tens nanometers. During the high temperature anneal, crystal structure is changed with rutile structure and the grain growth occurred, but particle growth is inhibited because presence of the interior pores. Then with the sample after anneal at 900oC, a disordered and enlarged mesoporous structure was obtained. A detailed micro structural analysis was performed by using TEM and the result was given in Figure 6 [14]. As we can expect from the SEM observation as given in Figure 5, well crystallized and randomly distributed particles were clearly observed and the connected particles during the particle growth seemed to form cavities as micro pores. From this observation, we can say that grain growth and crystallization was limited by porous structure, i.e., controlled mass transfer even after high temperature annealing at 900oC from the presence of interior pores. That is to say, disordered micro-porous structure with agglomerated and enlarged pores could be synthesized because mesopores act as template for the inter-pores between enlarged TiO2 grains.

3. Effect of metal dopant on the thermoelectric property of mesoporous thin film

3.1 Preparation of Nb-doped mesoporous TiO2 thin films

When synthesize Nb added mesoporous TiO2 films, sol concentration and catalyst is changed slightly. When Nb added in TiO2 structure, niobium chloride (NbCl5, Aldrich) is used as Nb precursor. NbCl5 was used as the precursor for Nb, and the concentration of Nb ions was varied between x = 0 and x = 6 at an interval of 2, where x is the cation atomic percent of Nb and is defined as x = [Nb/(Ti + Nb)] X 100.

3.2 Analysis of structure and thermoelectric property

To investigate the crystalline state of the TiO2 thin films, XRD with Cu Kα radiation (λ = 1.5418 Å) was performed. Small-angle and wide-angle pattern were recorded in the 2θ range from 0.8 o to 5 o and from 1o to 60o with 0.01o interval, respectively. For porosity measurement, the refractive indices of the mesoporous TiO2 film was measured using an ellipsometer. From these refractive indices, porosities were calculated using the Lorentz-Lorenz equation.[11] Microstructure of mesoporous thin film was obtained using a TEM at 300 kV. Cross-sectional sample preparation for observation was produced in the form of a thin disk by using glue and Ar ion milling. The thickness and cross-sectional image of mesoporous TiO2 film were observed using SEM. The electrical property of mesoporous TiO2 thin film was measured by van der Pauw method at room temperature using a Hall effect measurement system, and the electrical parameters were measured by the direct current (Idc = 10 mA) four probe method in the magnetic field up to 0.57 T. The thermal conductivities of the films were measured by the 3ω method using a 50-μm-wide gold electrode. The 3ω method is a one-dimensional, heat conduction-based thermal conductivity measurement technique that is widely applied to nano-scaled thin films.[15] All the measurements were performed at room temperature and under ambient atmosphere.

3.3 Effect of metal dopant on the structure and thermoelectric property

The anatase phase of TiO2 can only be crystallized when the annealing temperature is within the range of 400oC to 700oC. Therefore, Nb-doped mesoporous TiO2 films were annealed at 450oC; the XRD spectra of the Nb-doped mesoporous TiO2 films after annealing at 450oC are shown in Figure 7.[16] The intense diffraction peak at 25.3o could be indexed as (101) and indicated that the TiO2 films crystallized with an anatase structure. Meanwhile, no diffraction peaks from metallic Nb, even at a Nb concentration of 6%, were observed in the XRD patterns. Based on these results,


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we concluded that an addition of Nb dopant up to 6% had no effect on the formation of the TiO2 matrix with anatase crystal structure. In addition, to investigate the effects of Nb dopant on the mesoporous pore structure, small-angle XRD was performed at angles from 0.8o to 5o. The existence of a diffraction peak at low angles implied a regular distribution of pores in the mesoporous TiO2 film. The intense diffraction peak at 1.40o to 1.44o could be indexed as a (100) diffraction and corresponded to the interpore distance. The intensity of this diffraction peak for the undoped TiO2 film (Figure 8(a)) was slightly higher than that of Nb-doped TiO2 films, indicating that the introduction of Nb induced a slight disordering of pores in the TiO2 mesoporous films.[16, 17] Furthermore, the diffraction peak position was somewhat shifted to a greater angle in the Nb-doped films, i.e., there was a decrease in the interpore distance.[18] The interpore distance varied between 6.31 nm and 6.13 nm according to the concentration of Nb. Based on the similar ionic radii of Nb5+ (0.064 nm) and Ti4+ (0.061 nm) at different valence states, we suggest that Nb5+ is a substitutional n-type dopant that replaces the Ti4+ cation in TiO2.[19]

Figure 7. Wide-angle XRD patterns of the Nb-doped mesoporous TiO2 films with Nb concentrations of (a) 0%, (b) 2%, (c) 4% and (d) 6%.[16]

Figure 8. Small-angle XRD patterns of Nb-doped mesoporous TiO2 films with Nb concentrations of (a) 0%, (b) 2%, (c) 4% and (d) 6%.[16]

To investigate the effect of Nb-doping on interpore distance, porosity was calculated from the refractive index measured by an ellipsometer. Figure 9 shows the porosity of Nb-doped mesoporous TiO2 films according to the concentration of Nb.[16] The films had porosity values ranging between 27.1% and 31.3% according to the Nb concentration. Undoped samples had a porosity of 28.2%, while the porosity of samples doped with 4% and 6% Nb increased to 30.7% and 31.3%, respectively. This small increase in porosity was likely due to changes in the interpore distance, as shown in Figure 9. An exception was observed for the case with 2% Nb doping, possibly due to a decrease in the ordering of the pore structures, as shown in Figure 8; a decrease in the ordering of mesoporous structures can cause measurement errors when using ellipsometry and thus errors in the calculated porosity.[20, 21] To maximize the thermoelectric properties of a material, the material should have high electrical conductivity and low thermal conductivity. However, these parameters are proportional to each other and are therefore difficult to manipulate independently. Thermal conductivity can be represented by an electron component and a phonon component, κ = κ el + κ ph. To decrease thermal conductivity, it is more efficient to control the phonon component than the electrical component, because the electrical component is related to electrical conductivity according to the Wiedemann–Franz law (κ /σ = LT).[22] By incorporating a mesoporous structure into thermoelectric materials, thermal conductivity can be lowered due to the presence of pores. To determine the thermal conductivity of ordered mesoporous TiO2 films according to the existence of pores, we used the 3-omega method. As shown in Figure 10, the thermal conductivity of Nb-doped mesoporous TiO2 films decreased as the concentration of Nb-dopant increased because of the decrease in phonon contribution. The 6% Nb-doped samples showed the lowest thermal conductivity, 0.53 W/mK (Figure 10), which is very low compared with that of a TiO2 thin film, ~10 W/mK.[16, 23]

Figure 9. Porosity variation in mesoporous TiO2 films containing various concentrations of Nb.[16]

Figure 10. Thermal conductivity in mesoporous TiO2 films containing various concentrations of Nb.[16]


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This lowered thermal conductivity due to the ordered distribution of pores decreased the electrical conductivity, and Nb dopants were therefore added to the mesoporous TiO2 to overcome the loss in electrical conductivity. The electrical resistivity values of mesoporous TiO2 films with various Nb-dopant concentrations are shown in Figure 11.[16] Resistivity was calculated from the sheet resistance measured using a four-point probe; an undoped mesoporous TiO2 film had resistivity of 0.216 Ωcm. Generally, TiO2 has electrical insulating properties, in contrast to reduced TiO2 that has improved electrical conductivity due to the presence of oxygen vacancies.[23] It is therefore necessary to increase the number of oxygen vacancies in TiO2 for downstream thermoelectrical applications. Annealing in a vacuum (1 X 10-6 torr) atmosphere is an efficient way to produce reduced TiO2, which has better electrical conductivity than does nonreduced TiO2.[23] The 2% and 4% Nb-doped samples had resistivity values of 0.107 Ωcm and 0.058 Ωcm, respectively, and the resistivity decreased gradually by approximately 50% for every 2% increase in the Nb concentration. When 6% Nb was introduced, the electrical resistivity decreased to 0.026 Ωcm, 12% of the resistivity of the undoped TiO2 mesoporous film. The above results indicate that, although the introduction of a pore structure reduced the thermal conductivity and decreased the electrical conductivity, annealing under a reducing atmosphere and the introduction of dopant increased the electrical conductivity of the resultant films.[24] This can clearly be observed in Figure 12, which shows a plot of the ratio of electrical conductivity to thermal conductivity according to dopant concentration.[16] The undoped sample had an electrical to thermal conductivity ratio of 0.055 X 104 K/V2; however, as the Nb concentration increased, this ratio also increased gradually, and the sample with 6% Nb had the highest conductivity ratio value of 0.734 X 104 K/V2.

Figure 11. Electrical resistivity in mesoporous TiO2 films containing various concentrations of Nb.[16]

Figure 12. Conductivity ratios of electrical conductivity to thermal conductivity in mesoporous TiO2 films containing various concentrations of Nb.[16]

4. Effect of incorporated metal nanoparticles on the thermoelectric property of mesoporous thin film

4.1 Preparation and analysis of mesoporous TiO2 thin films incorporated with metal nanoparticles

4.1.1 Preparation of mesoporous TiO2 thin films incorporated with Pt nanoparticles

When synthesize Pt added mesoporous TiO2 thin films, hexachloroplatinic acid hexahydrate (H2PtCl6·6H2O) was used as a Pt precursor. Poly(N-vinyl-2-pyrrolidon) (PVP; MW 55000) was used as a standard protective polymer for the Pt nanoparticles. Ethanol was used as a reduction agent. Solution preparations began with mixing 1-propanol and Pt nanoparticle-dispersed solution. The ethanol in the Pt nanoparticle-dispersed solution acted as mixed solvent with 1-propanol in the solution for the synthesis of ordered mesoporous TiO2 film. P-123 and HCl were then dissolved in the mixed solution. Pt nanoparticle concentrations were 0, 0.02, 0.04, and 0.06 at. %. After aging, the as-prepared films were calcined in a vacuum chamber at 300oC and annealed at 450oC to increase the crosslinking of TiO2.

4.1.2 Preparation of mesoporous TiO2 thin films incorporated with Ag nanoparticles

For Ag added mesoporous TiO2 films, silver nitrate (AgNO3) were used as precursor. And, nitric acid (HNO3, 65%) in water (H2O) is used as catalyst. The chemical solution deposition procedure was the same as the case of Pt nanoparticles.

4.1.3 Analysis of structure and thermoelectric property

To investigate interplanar spacing, structural ordering, and crystal structures, XRD patterns were collected using Cu Kα radiation with a wavelength of 1.5418 Å. The refractive indices of the synthesized films were obtained using an ellipsometer. From these refractive indices, porosities were calculated using the Lorentz-Lorenz equation.[11]


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Resistivity was calculated based on measurements of sheet resistance and thickness. Sheet resistances of films were measured using a four-point probe. The thermal conductivities of the films were measured by the 3ω method. All the measurements were performed at room temperature and under ambient atmosphere.

4.2 Effect of incorporated metal nanoparticles on the thermoelectric property

4.2.1 Effect of incorporated Pt nanoparticles on the thermoelectric property

To determine the crystal structure of the synthesized films, XRD analysis was performed at a range of wide angles, from 20 to 60o. The wide-angle XRD spectra of mesoporous TiO2 films with Pt nanoparticles of 0, 0.02, 0.04, and 0.06 at.% (ratio to the number of Ti atoms) incorporated are shown in Figure 13.[25] Because annealing was performed at 450 oC, all films had an anatase phase, and each diffraction peak was designated (101), (004), or (200). No diffraction peak corresponding to Pt nanoparticles was observed in any of the XRD spectra. The absence of a Pt nanoparticle diffraction peak is likely because the amounts of Pt nanoparticles incorporated were too small to detect with XRD. Addition of Pt nanoparticles did not have any effect on the formation of the anatase phase, based on a comparison of the anatase diffraction peaks from mesoporous films containing different concentrations of Pt nanoparticles. To investigate if the synthesized mesoporous films prepared using various Pt nanoparticle concentrations had an ordered pore structure, structural analysis was performed using small angle XRD; the results are shown in Figure 14.[25] Regardless of the amount of Pt nanoparticles incorporated, all mesoporous TiO2 films had a diffraction peak corresponding to an ordered pore structure at 0.46o of 2Θ; this diffraction angle corresponds to an interpore distance of approximately 19.2 nm. As the concentration of Pt nanoparticles increased, the intensity of the peak became weaker. This is because the incorporated Pt nanoparticles disturbed the formation of a regular pore structure during the evaporation-induced self-assembly process. Generally, when nanoparticles are added to mesoporous materials, the nanoparticles are located inside the mesopores.[26, 27] Addition of Pt nanoparticles to the solution for mesoporous films resulted in location of the Pt nanoparticles between the micelles and titania oligomers. Micelle ordering therefore decreases due to the presence of Pt nanoparticles and mesoporous TiO2 composite films gradually become disordered as the concentration of Pt nanoparticles increases. We conclude that the incorporated Pt nanoparticles had almost no effect on the crystal structure of titania, but slightly degraded the ordering of the pore structures. The porosity behavior of the mesoporous TiO2 films containing various concentrations of Pt nanoparticles (0, 0.02, 0.04 and 0.06 at. %) is shown in Figure 15.[25]

Figure 13. Wide-angle XRD spectra of mesoporous TiO2 films with incorporated Pt nanoparticles at concentrations of (a) 0, (b) 0.02, (c) 0.04 and (d) 0.06 at.%.[25]

Figure 14. Small-angle XRD spectra of mesoporous TiO2 films with incorporated Pt nanoparticles at concentrations (a) 0, (b) 0.02, (c) 0.04 and (d) 0.06 at.%.[25]

The porosity of ordered mesoporous TiO2 film without Pt nanoparticles was 37.6%; this value decreased gradually down to 29.3% as the concentration of Pt nanoparticles increased. The amount of incorporated Pt nanoparticles was 0.06 at.% at most (approximately 0.29 vol% in the TiO2:Pt nanocomposite film). However, the reduction in porosity was very large, up to 8.3%. There are two possible explanations for the large observed reduction in porosity. The first is that the incorporated Pt nanoparticles occupied the pores. A second possible explanation is the disordering of pore structure caused by the incorporation of Pt nanoparticles, as shown in Figure 14. The porosity of films generally decreases as the pore structure becomes more disordered.[28] This is because pores cannot pack closely when the pore structure is disordered in comparison with an ordered pore structure. Thus, the porosity of mesoporous TiO2 films with incorporated Pt nanoparticles decreased down to 29.3% because of the incorporation of Pt nanoparticles and the resultant disordering of the pore structure. Figure 16 shows the resistivities of the mesoporous TiO2 films with different concentrations of Pt nanoparticles incorporated.[25] Ordered mesoporous TiO2 film without Pt nanoparticles had a resistivity of 0.128 Ωcm, because oxygen deficiencies caused by vacuum annealing provided charge carriers. As the concentration of Pt nanoparticles increased, the resistivity of the film decreased gradually, most likely because the mesopores became filled with Pt nanoparticles. Pt nanoparticles inside mesopores could function as an electrical path


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during charge carrier migration. Furthermore, the introduction of Pt nanoparticles can introduce extra charge carriers into the TiO2 matrix due to the surface plasmon effect, and induce a decrease in resistivity.[29] When the concentration of Pt nanoparticles was 0.06 at. %, the resistivity was reduced to a much greater extent than at the lower Pt nanoparticle concentrations. This indicates that there is a threshold when the effect of the incorporation of Pt nanoparticles becomes noticeable. This type of nanoparticle incorporation effect was also observed in a previous study on ZnO.[30]

Figure 15. Porosity behavior of mesoporous TiO2 films with incorporated Pt nanoparticles at concentrations of 0, 0.02, 0.04 and 0.06 at.%.[25]

Figure 16. Resistivity of mesoporous TiO2 films with incorporated Pt nanoparticles at concentrations of 0, 0.02, 0.04 and 0.06 at.%.[25]

The thermal conductivities of mesoporous TiO2 films with Pt nanoparticles incorporated were measured by the 3ω method. A 300-nm-thick Au electrode layer was prepared on mesoporous TiO2 film/Si substrate using a thermal evaporator. The shape of the Au metal electrode has been reported previously.[31] To calculate thermal conductivity, we used the following equation:

κ= ∆ × 2

Where ΔTh is the temperature oscillation of the heater line, l is the length of the metal line, P is the input power, t is the thickness of the mesoporous TiO2 film, and b is half the width of the metal line.[32] In our system, l and 2b were 1 mm and 50 μm, respectively, and the measured Th/P values were converted to thermal conductivities.

Figure 17. Total thermal conductivity and the thermal conductivity contributed by electron of mesoporous TiO2 films with incorporated Pt nanoparticles at concentrations of 0, 0.02, 0.04 and 0.06 at.%.[25]

Figure 18. Conductivity ratio of mesoporous TiO2 films with incorporated Pt nanoparticles at concentrations of 0, 0.02, 0.04 and 0.06 at.%.[25]

Figure 17 shows the thermal conductivity behaviors of the mesoporous TiO2 films with various concentrations of incorporated Pt nanoparticles (0, 0.02, 0.04, and 0.06 at. %).[25] The thermal conductivity by electron from the electrical conductivity was calculated using Wiedemann–Franz law,[33] κe/σT = CWFL(2.44 X 10-8 WΩK-2). Compared with dense TiO2 (11.7 Wm-1 K-1), the mesoporous TiO2 films had very low thermal conductivities; under 1/10. As the concentration of Pt nanoparticles increased, the thermal conductivity of the films increased slightly. This small increase might be due to the reduction of porosity as shown in Figure 15, and the increase in electrical conductivity as shown in Figure 16. The increase in electrical conductivity could lead to an increase in carrier concentration, that is, the electronic part of thermal conductivity. However, the contribution of the electronic part in thermal conductivity was very low; less than 1/100. Therefore, a slight increase in total thermal conductivity was caused mainly by an increase in the porosity. However, this increase was restricted from an increase in the disordering of the mesoporous TiO2 structure, as shown in


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Figure 15, because the probability of phonon scattering increases as the disordering of the traveling matrix increases. Figure 18 shows the conductivity ratio (σ/κ) of mesoporous TiO2 films with various concentrations of incorporated Pt nanoparticles.[25] The conductivity ratio was calculated from the resistivity in Figure 16 and the thermal conductivity in Figure 17. The conductivity ratio tended to increase as the concentration of Pt nanoparticles increased. While the extent to which the conductivity ratio increased was small up to 0.04 at. %, it was very large at 0.06 at.%. The small increase was due to the relatively small increase in thermal conductivity when compared with the increase in electrical conductivity according to the concentration of Pt nanoparticles. However, mesoporous TiO2 film with Pt nanoparticles of 0.06 at.% had a much higher conductivity ratio than the other concentrations, up to approximately 1370 KV-2. As discussed previously, when the concentration of Pt nanoparticles reached a certain threshold (see Figure 16), the effect of incorporation became large suddenly, i.e., the conductivity ratio at a Pt nanoparticle concentration of 0.06 at.% increased noticeably. From these results, we conclude that the incorporation of Pt nanoparticles into ordered mesoporous TiO2 films enhances the electrical conductivity of the films, but has little effect on their thermal conductivity. By incorporating Pt nanoparticles into ordered mesoporous TiO2 film, the enhancement in electrical conductivity without the change of thermal conductivity comes into play, thereby improving the thermoelectric properties of mesoporous TiO2 film.

4.2.2 Effect of incorporated Ag nanoparticles on the thermoelectric property

Figure 19 shows the XRD patterns of mesoporous TiO2 films after annealing at 800 °C with various added Ag concentrations.[34] The peaks indexed as (110), (101), (111), and (211) indicated that the TiO2 films crystallized with a rutile structure [26]. The high annealing temperature was chosen for rutile phase formation because the rutile phase shows a 65% lower thermal conductivity than the anatase phase and has the advantage of thermoelectric application among stable TiO2 phases.[35] In addition, as shown in the inset, weak but clearly distinguished diffraction peaks of Ag (111) and (200) were found around 38.1° and 44.6°, respectively with 3 at.% of Ag added to the mesoporous TiO2 film, revealing that the incorporated Ag agglomerated and formed metallic Ag.[36] The diffraction peaks, corresponding to the rutile structure, did not change when the Ag concentration was varied, indicating that variations in Ag concentration had almost no effect on the crystallization of rutile TiO2. A small angle XRD analysis was performed to investigate the effect of Ag incorporation to the formation of the TiO2 mesoporous structure. Figure 20 shows the small angle XRD patterns of the TiO2 mesoporous films with various concentrations of Ag. The intense diffraction peak at 1.4°–1.5° could be indexed as (100) diffraction, and the corresponding interplanar distances, d100, were calculated as 6.31 nm to 6.05 nm, respectively. The undoped TiO2 sample (Figure 20(a)) showed the most intense diffraction peak compared with those of the other doped samples.[34] The existence of a diffraction peak at this extremely low angle in mesoporous samples is an indication of regular ordering in the mesoporous structure.[37]

Figure 19. Wide-angle XRD patterns of the Ag-added mesoporous TiO2 films with Ag concentrations of (a) 0 at.%, (b) 1 at.%, (c) 2 at.% and (d) 3 at.% (inset corresponds to the collection data with longer counter time of sample (d)).[34]

Figure 20. Small-angle XRD patterns of the mesoporous TiO2 films with added Ag concentrations of (a) 0 at.%, (b) 1 at.%, (c) 2 at.% and (d) 3 at.%.[34]

When considering the application of mesoporous TiO2 to thermoelectric materials, a lowering of the thermal conductivity with the mesoporous structure and an enhancement of the electrical conductivity by an elemental addition must be considered. Thus, the optimization of Ag-doping concentration is important for the maintenance of an ordered mesoporous structure. The most well aligned mesoporous materials can be formed by controlling the synthesis conditions and through careful selection of precursor type. Furthermore, an addition of Ag impurity will be considered to efficiently improve the electrical conductivity. However, when a larger amount of Ag was added to the mesoporous TiO2, a lower diffraction intensity was observed. Diffraction intensity decreased notably by about 38.2% with the 3 at.% Ag-doped sample compared with that of the undoped sample. This reduced diffraction intensity likely resulted from degraded crystallinity, i.e., a lowered ordering in the pore structure arrangement of the mesoporous structure. The diffraction peak position shifted slightly to a high angle with the incorporation of Ag, corresponding to a reduction in


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the pore spacing.[38] Thus, the formation of an ordered TiO2 mesoporous structure and its level of order were affected by Ag-doping. Because of a large difference in ionic radius between Ag+ (rAg+ = 129 pm) and Ti4+ (rTi4+ = 74.5 pm), the Ag+ ion is not able to substitute for the Ti cation in the TiO2 lattice to form a solid solution.[36] Rather, they aggregate and form a metallic Ag state, and this elemental Ag agglomerate was confirmed through the XRD analysis, as shown in Figure 19.[36] Therefore, the formation of the TiO2 crystalline structure and the ordered arrangement in the mesoporous structure were affected by the aggregation of Ag. This was also confirmed through the porosity results, which ranged from 15.6 to 25.8% in the mesoporous TiO2 film according to the amount of Ag-doping, as shown in Figure 21.[34] To investigate the change in thermal conductivity caused by Ag incorporation and varied porosity, the thermal conductivity of the mesoporous film was measured using the 3Ω method. Generally, thermal conductivity is determined by the electron component and phonon component according to κ=κel+κph, where κel is the thermal conductivity of the electron and κph is the thermal conductivity of the phonon. Because κel follows the Wiedemann–Franz law (κ/σ=L·T, where L is the Lorentz constant and T is the absolute temperature), it is beneficial to control κph in the phonon component to simultaneously minimize thermal and electrical conductivity losses [39]. This mesoporous structure can effectively lower the thermal conductivity by minimizing the heat transmission. All samples showed thermal conductivities ranging from 1.38 to 1.69 W/m K with varied Ag concentrations, as shown in Figure 22.[34] These values are much smaller than the 11.7W/m·K of typical TiO2 because pores induce phonon scattering in mesoporous TiO2 thin films. This resulting thermal conductivity is lower than that of non-porous TiO2.

Figure 21. Porosity variation in mesoporous TiO2 films containing various concentrations of Ag.[34]

Figure 22. Thermal conductivity variation in mesoporous TiO2 films containing various concentrations of Ag.[34]

Figure 23 shows the electrical resistivity in the mesoporous TiO2 film according to Ag concentration.[34] The undoped sample had an electrical resistivity of 0.636 Ω cm, but the 1 at.%-Ag doped sample had an electrical resistivity of 0.233 Ω cm, a value that is 63.4% smaller than that of the undoped mesoporous TiO2. In the case of 1 at.%-Ag doping, the porosity was decreased but the pore structure arrangement was maintained so that the electrical resistivity could be decreased from an increased electron conduction area in addition to the intrinsic metal-doping effect. However, with 2 at.% and 3 at.% Ag-doped samples, the electrical resistivity continuously decreased even with an increase in porosity, as shown in Figure 21. These effects may be related to the presence of metallic Ag and its role on electrical conduction, as shown in Figure 19. Therefore, the electrical conductivity of the mesoporous TiO2 thin film was increased due to the incorporation of an Ag dopant with an excellent electrical conductivity. However, according to the Wiedemann–Franz law (κ/σ=L·T), an incremental increase in the electrical conductivity normally causes an incremental increase in the thermal conductivity [39].

Figure 23. Electrical resistivity of mesoporous TiO2 films containing various concentrations of Ag.[34]

Figure 24. Conductivity ratio of electrical conductivity to thermal conductivity in mesoporous TiO2 films containing various concentrations of Ag.[34]


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The ratio of electrical conductivity to thermal conductivity is very important for thermoelectric materials. The electrical conductivity is generally proportional to the thermal conductivity because the thermal conductivity is based on the electrical flow. However, by controlling the nano-structure and through doping, the relation between the electrical conductivity and thermal conductivity can be changed. Figure 24 shows the conductivity ratio of the electrical conductivity to the thermal conductivity in mesoporous TiO2 films containing various concentrations of Ag.[34] The undoped sample showed an electrical to thermal conductivity ratio of 113.14 K/V2. However, as the Ag concentration increased, this ratio also gradually increased, and the sample with 3 at.% Ag had the highest conductivity ratio value of 1122.37 K/V2. This increase in conductivity ratio comes from a decreased electrical resistivity and a similar thermal conductivity, achieved by increasing Ag concentrations. That is to say, an incorporation of Ag into TiO2 mesoporous film induced a substantial reduction in the electrical resistivity, but an ordered pore structure maintained a low thermal conductivity. Moreover, this increase in the conductivity ratio (3 at.% Ag sample) implies that the thermoelectric figure of merit could be increased approximately ten-fold by Ag doping in mesoporous TiO2.

5. Conclusions

Ordered mesoporous TiO2 films with various concentrations of incorporated metal (Pt, Nb, and Ag) were synthesized successfully. As the concentration of metal is increased, the electrical resistivity of the mesoporous TiO2 films rapidly decreased due to a formation of electrical conduction path by metal particles. The porosity of the mesoporous TiO2 films decreased slightly due to an increase in the disordering of pore structures. However, when comparing with a decrease in the resistivity, a decrease of porosity is smaller than that of resistivity. But, thermal conductivity was maintained regardless of metal incorporation. Based on these results, we concluded that incorporation metal into a mesoporous TiO2 film induces an enhancement in electrical conductivity without a change in the thermal conductivity. The observed increase in the conductivity ratio (σ/κ) with metal incorporation conclusively demonstrates that ordered mesoporous TiO2 film is a good candidate for novel thermoelectric application.

Acknowledgements The support by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (No. 2012R1A2A2A01011014) is gratefully acknowledged.

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mesoporous structure for thermoelectrics

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