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This chapter provides recent progress in developments of BiVO4- and Bi2Te3-based materials for high efficiency photoelectrodes and thermoelectric applications. The self-assembling nanostructured BiVO4-based materials and their heterostructures (e.g., WO3/BiVO4) are developed and studied toward high efficiency photoelectrochemical (PEC) water splitting via engineering the crystal and band structures and charge transfer processes across the heteroconjunctions. In addition, crystal and electronic structures, optical properties, and strategies to enhance photoelectrochemical properties of BiVO4 are presented. The nanocrystalline, nanostructured Bi2Te3-based thin films with controlled structure, and morphology for enhanced thermoelectric properties are also reported and discussed in details. We demonstrate that BiVO4-based materials and Bi2Te3-based thin films play significant roles for the developing renewable energy.
Faculty of Basic Sciences, Can Tho University of Medicine and Pharmacy, Vietnam
Nguyen Trung Kien
Faculty of Medicine, Can Tho University of Medicine and Pharmacy, Vietnam
Chien Nguyen Van
Department of Materials Science and Engineering, National Chiao Tung University, Taiwan
Institute of Materials Science, Vietnam Academy of Science and Technology, Vietnam
*Address all correspondence to: lhuuphuoc@ctump.edu.vn
1. Bismuth vanadate (BiVO4) material: a highly promising photoanode for use in solar water oxidation
The photoelectrochemical (PEC) water splitting, which uses semiconductors to directly harvest and convert the abundant solar energy into storable and friendly environmental energy in the form of hydrogen and oxygen from water, has been regarded as a promising approach to solve our current energy challenges [1, 2, 3, 4, 5, 6]. The PEC cell for energy conversion on a global scale requires the development of devices that are highly efficient, stable, and simple in design. The effective and economic energy transfer technologies are still under developing stage for more than 40 years. The PEC water splitting becomes more competitive as the cost continues to decrease with the development of nanotechnology, which offers the use of new nano-heterostructure photocatalysts. The current direction along this research field focuses on the development of high efficiency photoelectrodes based on metal oxides due to their high chemical and photostability, low cost, and ease of fabrication. Among these interesting and important photoelectrode materials for PEC, BiVO4 is demonstrated as a promising photocatalyst for PEC water splitting. Originally, the BiVO4was intensively studied as a ferroelasticity, acoustic-optical, and ionic conductivity materials [7]; however, recently most studies focus on its photocatalytic activity owing to its direct bandgap of ˜2.5 eV together with the desirable band edges and good stability [8, 9, 10, 11]. Additionally, the theoretical solar-to-hydrogen conversion efficiency of the monoclinic BiVO4 could reach 9.2% with a maximum photocurrent of 7.5 mA cm−2 under standard AM 1.5 solar light irradiation, which is significantly higher than those of the common metal oxides, and thus the monoclinic BiVO4 has been recognized as one of the most promising photoanode materials for PEC water splitting [1]. However, its photoactivity performance is limited by its poor electrical conductivity, slow hole transfer kinetics for water oxidation, and poor charge separation [12]. A concise overview of BiVO4 electronic and optical properties as well as the alternative strategies to improve its photocatalytic activities is discussed in this section.
2. Crystal and electronic structures of bismuth vanadate
The synthesis BiVO4 crystallizes in three main crystal structures of tetragonal scheelite, monoclinic phase, and tetragonal zircon structure. The photocatalytic activities of BiVO4 strongly depend on the optical properties of each crystal structure. Among these structures, the monoclinic scheelite BiVO4 was demonstrated to exhibit the highest photocatalytic water oxidation under visible light illumination [13]. The higher photocatalytic activity of the monoclinic scheelite was attributed to its smaller energy band gap and higher crystal distortion, which enhance the optical absorption and the electron-hole separation [14]. The conduction-band minimum of BiVO4 is formed mainly by V 3d, which splits into triplet bands V 3d x2−y2/z2, V 3d z2/zx, and V 3d zy/xy [15]. The valence band (VB) of tetragonal zircon structure BiVO4 is formed by O 2p orbital, but the top region of VB of monoclinic structure is formed by both Bi 6 s and O 2p orbitals. It was demonstrated that Bi 6 s is located above O 2p and the smaller bandgap of monoclinic BiVO4 attributes to the transition from the 6 s electrons of Bi to the 3d orbital of V [14, 16]. Recently, Zou et al. demonstrated that the crystal distortion in monoclinic BiVO4 improves the lone-pair impact of Bi 6 s states which raises up the O 2p states and reduces the bandgap [17]. In addition, the monoclinic scheelite structure possesses the better electron-hole separation owing to its higher degree of structural distortion than that in the tetragonal scheelite structure to result in enhanced photocatalytic activity for the monoclinic structure. In addition, the conduction-band edge of BiVO4 is only slightly below the H+/H2 redox potential, while its valence band is much below the oxidation potential of water resulting in low required external bias [15].
In order to investigate the experimental crystal and electronic structure of monoclinic BiVO4, the single crystal BiVO4, which possesses good crystallinity and eliminates the structural and electronic defects as well as impurity phases, is required. The vacuum deposition such as sputtering, molecular beam epitaxy, and pulsed laser deposition for the single crystal monoclinic BiVO4 has been developed recently [18, 19, 20]. A good lattice match (mismatch less than 1%) of cubic yttrium-stabilized zirconia (YSZ, a = 5.145 Å) was used as substrate for epitaxial growth of high-quality epitaxial monoclinic BiVO4 (a = 5.1935 Å, b = 5.0898 Å, c = 11.6972, γ = 90.3871) films. The X-ray diffraction θ–2θ scan (Figure 1a) shows only high intensity and sharp (00 l) reflections of monoclinic BiVO4 phase indicating that growth was c-axis oriented: BiVO4 (001) || YSZ (001). The high-resolution TEM image (the inset of Figure 1) reveals the in-plane and out-of-plane lattice parameters of the BiVO4 with d200 = 2.53 Å and d002 = 5.81 Å which are in good agreement with the lattice constants of the bulk monoclinic BiVO4. The orientation and growth direction of BiVO4 crystal were identified and constructed in the corresponding atomic model as shown in Figure 1b.
Figure 1.
(a) XRD 2θ/θ scans of BiVO4 film grown on YSZ substrate, and the inset shows HR-TEM image of BiVO4; (b) atomic model of BiVO4/ YSZ heteroconjunction.
The electronic structural properties of single crystal BiVO4 were investigated by the photoluminescence (PL) spectroscopy and UV-Visible spectroscopy. Figure 2 presents the PL and UV-Vis absorption spectra of a single crystal BiVO4 film. As presented in the UV-Vis absorption spectrum, the observed absorption in the range of 440–480 nm corresponding to the bandgap of ˜ 2.67 eV could attribute to the transition from the 6 s electrons of Bi (or the hybrid orbital of Bi 6 s and O 2p) to the empty 3d x2-y2/z2 orbital of V, which is consistent with emission peak of 2.65 eV in the PL spectrum. The second absorption band edge of approximately 3.15 eV and the second PL emission peak at 3.1 eV could be ascribed to the electron transition from O 2p valence band to V 3d z2/zx and/ or V 3d zy/xy orbitals.
Figure 2.
Photoluminescence taken under the irradiation of 325 nm laser at room temperature (green curve) and UV-Vis absorption spectrum (red curve) of BiVO4 epitaxial film.
3. Strategies to enhance photoelectrochemical properties of BiVO4
The development of suitable techniques to fabricate high efficiency BiVO4 photoelectrode for PEC water spitting is crucial. To date, there are many methods, which have been developed to prepare BiVO4 photoelectrodes for use in solar water oxidation. The representative synthesis methods are (1) metal organic decomposition (MOD) combining with the spin coating or spray pyrolysis deposition [21, 22, 23], (2) electrophoretic deposition (EPD) and chemical bath deposition (CBD) [24, 25, 26, 27], and (3) vacuum deposition methods such as sputtering deposition and pulsed laser deposition (PLD) [18, 19, 20, 28, 29]. The MOD is a facile method to synthesize BiVO4 materials for photoelectrodes. The main advantages of the MOD method are the easy composition tuning and morphology engineering. However, the existence of crystal and surface defect and low adhesion between BiVO4 and conducting substrate could limit its performance. The EPD and CBD methods are simple, have low cost, and are easy to scale up. The vacuum deposition is a powerful method to synthesize good crystalline BiVO4 photoelectrodes. The samples prepared by this method could easily control the doped composition; could facilely tune the crystalline in form of amorphous, polycrystalline, and single crystalline; and possess a good adhesion.
As described above, BiVO4 has many advantages features, however, the actual PEC performance of undoped BiVO4 is still far below its theoretical value, indicating that its advantages have not been fully developed. Many research groups have improved photoelectrochemical water oxidation by the introduction of (1) controlled morphologies, (2) forming nanocomposite structure, (3) doping, and (4) decorating with cocatalysts [30]. A summary of advantages and alternative strategies to enhance photoelectrochemical properties of BiVO4 was depicted in Figure 3. The best PEC performance of BiVO4 photoelectrodes that has been demonstrated to date is a combination of several above strategies to take advantages of high surface area, high electron mobility, high water oxidation kinetics, and low charge carrier recombination. Therefore, understanding the benefits and disadvantages of each strategy could provide an effective way to enhance the PEC properties.
Figure 3.
The advantages and strategies to enhance photoelectrochemical properties of BiVO4.
First, the morphologies of BiVO4 photoelectrode such as shape, size, and particle contact strongly effect on their interfacial energetics, kinetics, and charge transport properties as well as reactive sites. Because the electrochemical reactions only occur at the electrode-electrolyte interfacial area, the photogenerated electron-hole needs to be transferred to the surface for the reactions. Thus, the development of high surface area photoelectrodes is necessary. The porous structure and one-dimensional nanostructure such as nanorod/nanowire arrays reveal high surface areas per electrode volume, which directly improves the PEC efficiency. The high efficient PEC devices based on the BiVO4 porous structure and the one-dimensional nanostructures have been reported by many research groups [31, 32, 33, 34]. However, creating more porosity or smaller nanorod/nanowire structures to improve surface area could increase the defect site, increase grain boundaries, and reduce crystallinity leading reduction photocatalytic activity [35]. In addition, with hole diffusion length of ∼100 nm and carrier mobility of 0.044 cm2 V−2 s−1 [36], the length and thickness of nanostructure should be finely optimized to maximum light absorption and photogenerated charge carrier transport. Furthermore, researchers have recently demonstrated that the charge separation and photoactivity properties are closely related to the exposed crystal facet of BiVO4 photocatalysts [8, 9, 37, 38, 39]. Therefore, an optimized design and morphological control of crystal facets could improve the PEC performance.
Second, nanocomposites provide a powerful route to overcome limitations in the current studies of single material systems for water splitting, where the photoelectrochemical performance of photoelectrode can be significantly improved by the choice of proper interactions of constituents. The construction of nanocomposite consisting of BiVO4 and addition semiconductor/or conductor can improve the optical absorption, charge carrier separation, and charge transport processes. Combination of BiVO4 with additional metal oxide semiconductors could enhance overall photon absorption and improve electron-hole separation by the rapid photogenerated electron and hole injections at their heterojunctions and extension the optical absorption range. Many BiVO4-based heterostructures have been fabricated successfully such as TiO2/BiVO4, Ag3PO4/BiVO4, Bi2S3/BiVO4, g-C3N4/BiVO4, BiVO4/SnO2, WO3/BiVO4, BiOCl/BiVO4, Fe2O3/BiVO4, Cu2O/BiVO4, and ZnO/BiVO4 [40, 41, 42, 43]. Among them, the WO3 is a commonly coupled semiconductor with BiVO4 because it is a stable n-type semiconductor with better electron transport properties than the BiVO4 and a suitable conduction-band edge for facile electron injection from conduction band of BiVO4 [29, 44, 45, 46].
Recently, we reported a self-assembled nanocomposite photoanode composed of epitaxial BiVO4 matrix embedded with WO3 mesocrystal for photoelectrochemical application in the visible light regime using PLD [29]. By taking the advantage of the structural feature of this heterostructure, the well-defined crystal facet and interface between WO3 and BiVO4 phases provide a template for the fundamental understanding of photoactivity in the nanocomposite. The BiVO4-WO3 crystal structure details were investigated by XRD and TEM as shown in Figure 4. The XRD 2θ-θ scans, in-plane Φ-scans, and reciprocal space map results clearly show that BiVO4 and WO3 spontaneously separated into two single phases during the deposition with an in-plane orientation relationship as [100]BiVO4 //[110]WO3 //[001]YSZ. Both monoclinic BiVO4 and orthorhombic WO3 phases exhibit two sets of structural domains, which are separated by a 90° rotation around c-axis. The microstructure and the epitaxial relationship of BiVO4-WO3 heterostructure were further investigated as displayed in Figure 4(b–e), which are in an excellent agreement with the XRD analyses. The self-assembled mesocrystal-embedded heterostructure that composed WO3 and BiVO4 phases is illustrated in Figure 4g. The energy band alignment of BiVO4-WO3 heterojunction illustrated using X-ray photoelectron spectroscopy technique shows that the valence band maxima of WO3 lies 0.55 eV below that of BiVO4 and the conduction-band minimum of WO3 lies 0.25 eV below that of BiVO4. This energy band structure could favor the enhancement of photogenerated charge carrier separation, which was further confirmed by photoluminescence and ultrafast transient absorption spectroscopies. Figure 5 shows Nyquist electrochemical impedance spectroscopy and Mott-Schottky analysis for the BiVO4-WO3 photoelectrodes. The results demonstrated a much higher electron conductivity of WO3 with carrier densities of 9.68 × 1018 cm−3 under light illumination than BiVO4. Therefore, the BiVO4 matrix serves as the light absorber due to its narrow bandgap, and the WO3 mesocrystal acts as an electron conductor owing to its high electron conductivity resulting in a significantly enhanced photoelectrochemical performance.
Figure 4.
Structure characterization of BiVO4-WO3 nanocomposite. (a) X-ray diffraction 2θ-θ scans of pure WO3, BiVO4, and the BiVO4-WO3 nanocomposite showing only (00 l) type peaks of BiVO4 and WO3. Inset shows XRD ϕ-scans of YSZ (220), BiVO4 {013}, and WO3 {022} reflections for BiVO4-WO3 heterostructure. (b-e) Cross-sectional TEM images of BiVO4-WO3 (BVO-WO) nanocomposite taken along [010]YSZ zone axis. The enlarged images taken from the marked areas of (d) BiVO4 and (e) WO3 showing their out-of-plane and in-plane d-spacing parameters. (f) X-ray reciprocal space maps (RSMs) of the composite grown on ITO/YSZ. (g) Illustration of self-assembled BiVO4-WO3 nanocomposite [29].
Figure 5.
(a) The Nyquist plots and (b) the Mott-Schottky plots for the WO3, BiVO4, and BiVO4-WO3 photoelectrodes in 0.5 M Na2SO4 solution under AM 1.5 illumination [29].
Further example could be observed in the double-deck inverse opal WO3/BiVO4 structure that could reach a photocurrent density of 3.3 mA/cm2 at 1.23 V versus RHE and the core-shell WO3/BiVO4 helix nanostructure, in which BiVO4 was doped with Mo and naturally doped with W from the WO3 core, exhibits a high photocurrent density of 3.6 mA/cm2 at 1.23 V versus RHE [34, 47]. More recently, the highest photocurrent density of 6.72 mA/cm2 at 1.23 V versus RHE for BiVO4-based photoelectrode was obtained in the CoPi-coated WO3/BiVO4 core-shell nanorods [21]. The high efficiency photoelectrode was attributed to the superior charge carrier separation and efficient light scattering. In addition, by coupling with a conductor such as reduced graphene oxide and carbon nanotubes, the photocurrent density of the composite electrodes was improved significantly compared to those of the corresponding BiVO4 electrodes which could attribute to the enhancement of charge separation and transport properties [49, 50, 51, 52, 53, 54, 55, 56, 57]. However, due to the low crystallinity of BiVO4 as well as the poor BiVO4-conductor and conductor-substrate contact in these nanocomposites, it needs to be further developed to improve the PEC performance.
Third, doping with the metal (such as W, Fe, B, Cu, Zn, Ti, Nb, Sn, Co, Pd, Rb, Ru, Ag, Ga, Sr, and Ir) or nonmetal (such as P and N) materials can change the electrical and optical properties of BiVO4. Among them, only Mo- and W-doped BiVO4 show a significantly enhanced photocurrent and IPCE [21]. The intentional introduction of n-type conductivity W and/or Mo atoms does not change the bandgap energy and band edge positions of doped-BiVO4; however, it can increase the charge carrier density and the electrical conductivity. A suitable doping concentration of Mo and/or W results in significant improvement PEC performance of BiVO4 [54, 55, 56]. On the other hand, P doping forms an internal electric field which may improve the photogenerated electron–hole separation. However, it is noted that the doping could induce trap carrier which can enhance charge carrier recombination and reduce the charge transport [35]. Therefore, the optimum doping concentration is a key factor to improve the overall PEC performance of the photoelectrode.
Finally, decoration of BiVO4 with various oxygen evolution catalysts (OECs) such as Co–Pi, Co3O4, RhO2, Pt, CoO, and FeOOH improves the kinetic for oxygen evolution and provides unique active sites for catalytic reactions, thereby strongly enhanced the photocurrent density [3, 27, 40, 57, 58]. In addition, it decreases the bias potential and improves the stability of BiVO4 photoanodes. Perhaps the most successful route in the development of high efficient BiVO4 photoelectrode for water oxidation is the modification of the BiVO4-semiconductor nanocomposite surface with OECs. To date, the highest photocurrent density 6.72 mA/cm2 at 1.23 V versus RHE was obtained for the CoPi-coated WO3/BiVO4 core-shell nanorods [48].
4. Introduction to thermoelectric bismuth-based materials and applications
Thermoelectric (TE) materials are used in appliances such as heat pumps and power generators [59, 60, 61, 62, 63]. TE devices offer unique features of low environmental impact, no moving parts, quiet operation, and high reliability. The performance of TE materials is determined by a dimensionless figure of merit, ZT = α2σT/κ, in which α, σ, T, and κ are the Seebeck coefficient, the electrical conductivity, absolute temperature, and the thermal conductivity, respectively. A TE material must exhibit a high power factor (PF = α2σ) and low thermal conductivity (κ) to achieve a high ZT value. However, it is challenging for enhancing the ZT value due to the coupling among the TE parameters [62]: the relationship between α and the carrier concentration n (expressed by |α|˜ n−2/3 approximately [60]) limits the increase of the PF (=α2σ), while the proportional relationship between electrical conductivity and electronic thermal conductivity (the Wiedemann-Franz law) restricts the enhancement of the σ/κ ratio.
Bismuth (Bi)-based chalcogenide narrow-bandgap semiconductors such as Bi2Te3, Bi2Se3, Sb2Te3, Bi2(TexSe1-x)3, and (BixSb1-x)2Te3 are of high interest [64, 65, 66, 67, 68, 69]. They have been widely exploited for Peltier coolers and thermoelectric generators at low temperature regime (≤150°C) [70, 71]. Thanks to the extensive phonon scattering at grain boundaries, nanocrystalline and nanostructured Bi-based chalcogenide thin films have achieved reduced thermal conductivity [72, 73, 74, 75, 76, 77]. However, the lattice imperfections and grain-boundary defects impair the electrical transport properties of the films [72], which call for further investigations to improve PF or the electronic part of ZT. Currently, enhancing the PF of Bi-based thin films is challenging due to not only the coupling among TE material properties [60] but also the tendency of forming nonstoichiometric films at elevated substrate temperatures (Ts) [78]. Numerous charge carriers arising from vacancy defects of volatile elements can constrain the enhancement of ׀α׀; however, low carrier concentrations can suppress electrical conductivity if carrier mobility (μ) is poor.
The crystal structures of Bi2Se3 and Bi2Te3 are usually described by a hexagonal cell that consists of 15 layers of atoms stacking along the c-axis with a sequence shown below [60], as shown in Figure 6. ··· Se(1)–Bi–Se(2)–Bi–Se(1) ··· Se(1)–Bi–Se(2)–Bi–Se(1) ··· Se(1)–Bi–Se(2)–Bi–Se(1) ··· and ···Te(1)–Bi–Te(2)–Bi–Te(1) ··· Te(1)–Bi–Te(2)–Bi–Te(1)···Te(1)–Bi–Te(2)–Bi–Te(1) ···. The superscripts refer to two different types of bonding for Se or Te atoms. The five-atomic-layer thick lamellae of –(Se(1)–Bi–Se(2)–Bi–Se(1))– or –(Te(1)–Bi–Te(2)–Bi–Te(1))– is called quintuple layers, QLs. The Se(1)…Se(1) or Te(1)…Te(1) refers to van der Waals force between Se and Te atoms, whereas the Se(1)–Bi and Bi–Se(2) or Te(1)–Bi and Bi–Te(2) are ionic-covalent bonds. This weak binding between the Se(1) – Se(1) and Te(1)…Te(1) accounts for the easy cleavage of these materials perpendicular to the c-axis and the anisotropic thermal and electrical transport properties. For example, the thermal conductivity along the c-axis direction is ˜0.7 Wm−1K −1, while it is ˜1.5 Wm−1K −1 for the plane perpendicular to the c-axis [79].
Figure 6.
The hexagonal structures of one unit cell of Bi2Se3 and Bi2Te3.
Thin-film technology is advantageous for obtaining nanocrystalline and nanostructured materials by adjusting deposition conditions and subsequent thermal treatments. The extensive phonon scattering at grain boundaries in the nanostructures causes a large reduction in thermal conductivity while maintaining reasonable electrical conductivity, leading to enhanced ZT. Among physical vapor deposition techniques, PLD offers a great versatility in the fabrication of films with multielement stoichiometry and with a variety of structures, from amorphous or nanostructured to polycrystalline or even epitaxial [66, 67, 68, 76, 77, 80, 81]. Thin-film TE devices offer some distinctive advantages.
First, a thin-film device has the natural advantage of a small volume (thickness of ≤10 μm, length and width of ˜100 μm) as compared to size of millimeters for a bulk TE couple. As a result, thin-film TE cooling can be integrated into microelectronic systems (Figure 7a). In principle, the bulk device can be scaled down to micro sizes; however, fabrication processes to do so are difficult.
Figure 7.
(a) An example for optoelectronics of the continuing reduction in package size. (b) A schematic of grain-boundary scattering for thin-film materials with (i) disoriented-small grains and (ii) highly oriented large grains. (c) The comparison of thermal/cooling time response of thin-film (˜5 μm) superlattice device and a bulk device [59]. (d) Thermoelectric module performance chart which presents the temperature drop ΔT versus pumping power [83]. (e) A commercial thin-film TE module. (f) Size comparison between a thin-film TE device and bulk TE device [83].
Second, thin-film devices have a much shorter response time than bulk devices, as shown in Figure 7c [59]. The thin-film device achieves the steady state in 15 μs, while the bulk device requires 0.35 s [59]. This is a result of the response time associated with the transport of heat through the thin film (micrometers) rather than through the millimeters associated with bulk devices.
Third, the thin-film device has the ability to handle much larger density of heat pumping power than does the bulk device. Typically, bulk devices are working with pumping power density lower than 10 W/cm2; however, the pumping power density in a thin-film device is on the order of hundreds of W/cm2 [82]. Figure 7d illustrates a comparison performance chart between thin-film and bulk TE modules, in which the load line represents the temperature difference (ΔT) between the top and bottom substrate and possible pumping power density (Q) for a given drive current of a TE module [83]. At the maximum drive current for a TE module, the load line is determined by the maximum power density (Qmax) and the ΔTmax. For a commercial example, under a given drive current, an eTEC™ Series thin-film TE module (Laird technology) possesses a larger pumping power up to ten times (Figure 7e) and a comparable maximum temperature difference ΔTmax than the conventional bulk TE modules [83].
Fourth, the granular structure-morphology of thin films increases the grain-boundary scattering which is helpful for suppressing κ and enhanced ZT values. Figure 7b represents the roles of grain orientation and grain size in carrier mobility. The grains with the same colors illustrate that they have the same/similar orientations. The grain-boundary scattering in Figure 7b(i) will be greater than that of the Figure 7b(ii) case because of its smaller grain size and the greater grain disorientation. The effective mobility is given by [84]
µg=Lq2πm∗kTexp−φbkTE1
where q is the carrier charge, m* is the effective mass, k is the Boltzmann constant, T is the temperature, and φb is the grain-boundary potential barrier in the depletion region. In polycrystalline silicon, the potential barrier height is approximately twice as high at random boundaries as at low-energy coincidence boundaries.
5. Thermoelectric properties of nanocrystalline and nanostructured Bi2Te3-based thin films
Thermoelectric Bi2Te3-based materials have played a dominant role in the field of thermoelectrics. The traditional cooling materials are alloys of Bi2Te3 with Sb2Te3 (such as Bi0.5Sb1.5Te3; p-type) and of Bi2Te3 with Bi2Se3 (such as Bi2Te2.7Se0.3; n-type), with a ZT at room temperature approximately equal to one [63]. Recently, nanocrystalline and nanostructured Bi2Te3-based thin films have recently attracted great interests because of their superior TE performance [72, 73, 74, 75, 76, 77, 80, 81, 85, 86]. The PF of Bi2Te3 films was 8.8 μWcm−1 K−2 for an (015)-oriented film with layered-hexagonal morphology [85] and 33.7 μWcm−1 K−2 for a highly (00l)-oriented layered film [86]. Furthermore, the hexagonal-Bi2Te3 film grown using MBE obtained a PF of 27 μWcm−1 K−2 [87], and the smooth-epitaxial- Bi2Te3 film prepared by co-evaporation achieved a PF of 39.9 μWcm−1 K−2 [88].
Despite fewer studies performed on thermoelectric Bi2Se3 as compared to Bi2Te3, a considerable amount of recent efforts to enhance TE performance has been devoted to the synthesis of Bi2Se3 nanostructures such as hexagonal flakes (PF ≈ 0.28 μWcm−1 K−2) [89] and nanoflakes (PF ≈ 0.97 μWcm−1 K−2) [90], using the solvo-thermal method and chemical bath deposition, respectively. Moreover, the Bi2Se3 film grown by metal organic-chemical vapor deposition has reached a PF = 5.8 μWcm−1 K−2 [91]. In PLD, tightly controlling substrate temperatures (Ts) and ambient pressures enable the morphologies and compositions of films to be manipulated extensively, which offer a new method for enhancing the TE properties of films. For example, self-assembled Bi2Te3 films featuring well-aligned zero- to three-dimensional nanoblocks have been fabricated (Figure 8), but the room temperature PFs of these films remain low (≤ 1.9 μWcm−1 K−2) due to the low electrical conductivity of the voided structures [77].
Figure 8.
The morphology and power factor (PF, unit μWcm−1 K−2) of nanostructured Bi2Te3 thin films grown by PLD at various substrate temperatures and ambient pressures, reported by Chang and Chen [77].
By contrast, the Bi2Te3-based thin films with compact and/or layered structures possess the high electrical conductivities and relatively high Seebeck values if the films obtain stoichiometry. For example, A. Li Bassi et al. reported a high PF of 50.6 μWcm−1 K−2 for the layered-smooth Bi2Te3 films (Figure 9a) [80]. Similarly, the Bi2Te3 films with layered-hexagonal grain structure exhibited a high PF of 33.7 μWcm−1 K−2 (Figure 9b) [86]. P.H. Le et al. found that the optimal PF films also present compact-layered structures, namely, PF = 24.3 μWcm−1 K−2 for the Bi2Te3 film (Figure 9c) and PF = 5.54 μWcm−1 K−2 for the hexagonal Bi2Se3 film (Figure 9d) [66, 67]. Usually, compact or layered structure films obtain high carrier mobility than those with the open/voided nanostructures.
Figure 9.
The morphology and power factor (PF, unit μWcm−1 K−2) of the optimal Bi2Te3 and Bi2Se3 thin films with layered structures grown by PLD, reported in Refs. [66, 67, 80, 86].
Table 1 summarizes thermal transport properties (at room temperature) of nanocrystalline-nanostructured Bi2Te3-based thin films and bulk materials in the literature [62, 72, 73, 74, 75, 76, 87, 92]. Generally, the thermal conductivity κ value for polycrystalline films is expected to be smaller than that of bulk alloys because of the contribution of grain-boundary scattering [62, 72, 74]. Moreover, the κ of nanocrystalline Bi2Te3-based films will further decrease when the grain size of decreases (κ ≤ 0.81 W/mK) [73, 75]. For Bi2Te3/Sb2Te3 superlattice films, the coherent backscattering of phonon waves at the superlattice interfaces is outlined for the reduction of lattice thermal conductivity, resulting in the low κ ≤ 0.4 W/mK [93].
Room temperature thermal transport properties of nanocrystalline-nanostructured Bi2Te3-based thin films and bulk materials in the literature, including sample and fabrication method, average grain size, thermal conductivity κ, electrical conductivity σ, Seebeck coefficient α, power factor PF (= α2σ), and ZT (at 300 K).
For PLD Bi2Te3-based films, Walachova et al. [92] estimated κ starting from direct ZT measurement with the Harman method and found a value of about 0.2–0.3 W/mK for films with a thickness comparable to our films (>100 nm). Recently, Chang et al. [76] reported the κ between 0.93 and 1.16 W/mK for granular BixSb2-xTe3 films. The reported κ (at 300 K) of hexagonal flake Bi2Se3 was 0.75 W/mK.
We report the crystal, electronic structures, optical properties, and PEC activities of BiVO4-based materials. Morphology engineering, cocatalyst loading, and doping with metal and nonmetal composite structures are the approaches to fabricate high efficiency BiVO4 photoelectrode for PEC water spitting. In addition, the Bi2Te3-based thin films with nanocrystalline and nanostructured morphologies obtain the reduced thermal conductivity, meanwhile the close stoichiometric films with layered structure morphologies exhibit high thermoelectric power factors. BiVO4-based materials and Bi2Te3-based thin films play significant roles for the photoelectrode and thermoelectric applications.
Financial support from Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant numbers 103.99–2015.17, 103.99-2016.75 (PHL), and MOST106-2119-M-009-011-MY3, Taiwan (CNV), is gratefully acknowledged.
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Written By
Phuoc Huu Le, Nguyen Trung Kien and Chien Nguyen Van
Submitted: 22 December 2017Reviewed: 19 February 2018Published: 20 June 2018
Open access peer-reviewed chapter
