Electrodeposition of tellurium has been investigated in both acidic and alkaline media. Qiu et al. (1989) electrodeposited Te thin films with a thickness up to 4 µm on monocrystalline tellurium substrate from a TeO₂-saturated aqueous solution. The thickness was relatively uniform. The needle-like surface morphology with random crystal orientation was observed when deposited on (10 1 ¯ 0) surfaces. At high current densities, polycrystalline films consisting of 1 µm blades with random crystal orientation were produced (Qiu and Shih, 1989).

Suggs et al. (1991) investigate the electrochemical nucleation and growth of Te on gold (Au) (100) surface in acidic sulfate baths (i.e., 0.4 mM TeO₃ ^2- in X M H₂SO₄). Under potentiodynamic deposition, Te initially electrodeposited under underpotential deposition (UPD). As the deposition potential becomes more cathodic, Te electrodeposits under overpotential deposition (OPD) to from three dimensional nuclei. (Suggs and Stickney, 1991).

Ikemiya et al. (1996) electrodeposited Te films on Au (100) and Au (111) from acidic sulfate solutions (0.1 mM HTeO₂ ⁺ + 0.05 M H₂SO₄). The atomic structures and growth morphologies of the films were investigated by in situ atomic force microscopy. Accordingly, the atomic structure of the Te deposits was independent to the substrate crystal orientation, this support the conclusion that the surface diffusion process of Te adsorbed atoms is rate-limiting steps (Ikemiya et al., 1996).

Yagi et al. (1996) electrodeposited Te in acidic perchlorate solutions with 0.1 M HClO₄ and 0.5 mM TeO₂ using polycrystalline gold as substrate. AuAdditionally, in situ optical second harmonic (SH) generation at two different excitation wavelengths was utilized. On 1,064 nm excitation, the SH signal varied with the surface coverage of Te (Yagi et al., 1996).

Sorenson et al. synthesized tellurium atomic layers on Au (110) by electrodeposition in the acidic bath (i.e., 0.25 mM TeO₂ + 20 mM H₂SO₄). Additionally, the phase transitions associated with those layers was investigated. The voltammetry indicates two sub-monolayer deposition features and one for bulk. The result of the slow deposition kinetics is that surfaces composed of a single atomic layer structure are not observed. (Sorenson et al., 1999; Sorenson et al., 2001)

Jiang et al. electrodeposited Te film on polyaniline-coated macroporous phenolic foam in the solution with 1 M HNO₃ and 10 mM HTeO₂ ⁺. The deposited film was composed of columnar structures and had a growth direction along c-axis direction (Jiang et al., 2011). The highest Seebeck coefficient achieved for the macroporous Te film is 342 μV/K at 473 K (Jiang et al., 2012).

Abad et al. (2015) electrodeposited Te films from acidic nitrate baths (e.g., 10 mM HTeO₂ ⁺ and 1 M HNO₃) with sodium lignosulfonate (SLS) as additives. The presence of SLS reduced the average grain size resulted in higher electrical resistivity (∼798 µΩ m) compared to Te electrodeposits (∼229 µΩ m) in the absence of SLS. The Seebeck coefficient values were about 285 µV/K for both samples which resulted in the power factor of 280 µW/(mK²) and 82 µW/(mK²) without SLS and with SLS, respectively, at room temperature (Abad et al., 2015).

Ha et al. (2000) reported the electrochemical behavior of tellurium in alkaline baths (e.g., 10 mM TeO₃ ^2- in2.5 M NaOH). In this bath, Te was able to electrodeposit between -0.8 V and -0.95 V vs. Hg/HgO, but the Te morphology was porous with needle-like radial growth (Ha et al., 2000).

Sadeghi et al. (2008) electrodeposited Te using a nickel-coated copper as substrate in alkaline plating baths. The influence of current density, temperature, and pH were systematically studied. They found that the optimum conditions to electrodeposit Te was: 6 g/L (37.6 mM) TeO₂, pH of 10, and DC current density of 8.55 mA/cm² at room temperature (Sadeghi et al., 2008).

Our group also demonstrated the ability to electrodeposit thick Te films from alkaline baths (Wu et al., 2017a) where the applied potentials were optimized to electrochemically reduced TeO₃ ⁻² (aq) to Te_(s) without further reduction of Te to Te₂ ^2- (aq). The XRD data revealed that the preferred orientation of thick Te films altered from (001) to (101) as the applied potential varied from −0.9 V to −1.0 V. The optimum pH ranges to deposit compact thick films was between 11.3 and 12.5. Additionally, sufficient magnetic agitation is also essential to deposit compact films. The average grain size ranged from 66 to 135 nm where larger grain size resulted in lower carrier concentration (e.g., n = 7.1 × 10¹⁸ cm⁻³) which might be due to lower defect density. The Highest deposition rate (upto 130 µm/h) with high current efficiency (upto 85%) was achieved by adjusting deposition conditions. Additionally, galvanic displacement reaction which is another facile method to synthesize various nanostructured Te was investigated (Chang et al., 2010; Chen et al., 2010; Hangarter et al., 2010; Lee et al., 2011; Jung et al., 2012; Elazem et al., 2013; Park et al., 2013; Suh et al., 2014; Suh et al., 2017).

PbTe is also a narrow band-gap semiconductor with E_g of 0.31 eV measured at room temperature and a rock-salt crystal structure. PbTe can be n-type or p-type as a result of departures from stoichiometry (n-type for Pb-rich PbTe, while p-type for Te-rich PbTe). (Dughaish, 2002). The state-of-the-art commercially available PbTe based thermoelectric materials have the highest ZT of ∼0.8 at ∼ 600 K, which makes the materials a good candidate for thermoelectric application in the middle-high temperature range.

The Electrodeposition of PbTe was investigated by several groups. Saloniemi et al. reported electrodeposition of Te-rich PbTe thin films in alkaline electrolytes containing TeO₂, disodium salt of ethylenediaminetetraacetic acid (EDTA), and Pb(CH₃COO)₂ethylenediaminetetraacetic. They utilized various electrochemical analysis methods including cyclic voltammetry and quartz crystal microbalance to investigate the electrodeposition of PbTe. They observed that Te-rich PbTe deposition through UPD of Pb on Te via six electron reduction (Saloniemi et al., 1998). The reduction of the PbEDTA^2- complex to Pb₍₀₎ was a two-electron reaction whereas Te deposits via a four-electron reaction. As the potential becomes more negative, the film becomes powdery and Te₍₀₎ further reduced to Te₂ ^2- as the deposition potential becomes more negative (Saloniemi et al., 2000).

Miranda et al. electrodeposited polycrystalline PbTe thin films on porous silicon from alkaline solutions with EDTA as a complexing agent for Pb. They were able to deposit PbTe thin films with the average grain size of 100 nm (Miranda et al., 2004).

Qiu et al. (2005) synthesized uniform and single-crystalline PbTe nanorods with a diameter in the sub-10-nm regime at ambient conditions using sonoelectrochemical method. In the experiment, the Pb²⁺ and TeO₃ ^2- ions concentration were fixed at 10 mM, and the solution pH was kept at approximately 8. Nitrilotriacetic acid (NTA) was used as a complex reagent. The composition of PbTe can be controlled by ratio of precusor ion/ligand concentration. When the [Pb²⁺]/[NTA] changed from 0.20:1 to 0.10:1 to 0.05:1, the composition of deposits changed from pure PbTe to a mixture of PbTe/Te to pure Te (Qiu et al., 2005).

Yang et al. (2008) electrodeposited PbTe nanowire arrays using template which is patterned by lithographic method. The cross-section of the synthesized PbTe nanowires is rectangular, and the width and height of the nanowires can be tuned from 60 to 400 nm and 20–100 nm, respectively. Polycrystalline PbTe with face centered cubic crystal structure and was produced by a cyclic electrodeposition-stripping method, which have grain size ranged from 10 to 20 nm. The nanowires have a length over 1 mm (Yang et al., 2008).

Erdogan et al. (2009) electrodeposited stoichiometric PbTe thin films on Au (111) substrates from alkaline baths containing EDTA, Pb²⁺, and TeO₃ ^2- ions. They observed two dimensional nucleation and growth with the preferred orientation of (200) (Erdoğan et al., 2009).

Li et al. (2008a) electrodeposited symmetrical PbTe dendritic structures in the solution containing 10 mM Na₂TeO₃, 5 mM Pb(NO₃)₂ and 0.1 M tartaric acid. The formation of the PbTe dendritic structure is affected by the potential oscillation. The morphology of particle with dendritic structures were star-like or trigonal, and the size of the particles were varied from 100 to 500 nm. The deposited PbTe structures had a band gap energy of about 0.272 eV (Li et al., 2008a).

Additionally, many other groups reported the results of characterization of PbTe electrodeposits based on various experimental conditions which are summarized on the Table 1. (Banga et al., 2008; Diliberto et al., 2008; Jung et al., 2011; Ni et al., 2011; Frantz et al., 2015; Wu et al., 2016a; Frantz et al., 2016; Bae et al., 2017).

Bi₂Te₃ with a bandgap of 0.16 eV is an excellent candidate for TE application near room temperature range (Figure 4). Electrodeposition of Bi₂Te₃ was investigated by various groups.

Wang et al. (2008) synthesized high-density thermoelectric Bi₂Te₃/Sb heterostructure nanowire arrays with diameter of tens using AAO template-directed pulsed electrodeposition. The electrolyte included 12 mM TeO₂, 4 mM Bi(NO₃)₃, 0.1 M Sb₂O₃, 0.5 M K₂C₆H₅O₇, 1 M C₆H₈O₇, and 2 M HNO₃. Additionally, was used as template (Wang et al., 2008).

Li et al. synthesized the hierarchical Bi₂Te₃ nanostructures by electrodeposition in the solution with 10 mM Na₂TeO₃, 5 mM Bi(NO₃)₃, 10 mM tartaric acid and 1 M HNO₃ at room temperature (Li et al., 2008b; Li et al., 2008c).

Liu and Li (2008) reported that electrodeposited Bi₂Te₃ films had a preferential orientation of (110) and platelet grain morphology. The grain morphology changed from single-to multi-order platelets, and the texture decreased when the deposition potential became more negative, which was explained by considering geometrical selection growth and (1ī010) (ī 105) twinning of Bi₂Te₃ crystals (Liu and Li, 2008).

Glatz et al. (2008) electrodeposited Bi_2+xTe_3−x by combining potential controlled deposition pulses with galvanostatic-controlled resting pulses. The deposited had a uniform stoichiometry composition along the entire thickness. A deposition rates of 50 µm/h was achieved, and Layers thickness of 800 μm was obtained. The composition of Bi_2+xTe_3−x can be controlled by varying Bi ion concentration in the electrolyte with 80 mM HTeO₂ ⁺ and 2 M HNO₃. Bath n-type and p-type Bi_2+xTe_3−x, which is determined by Seebeck coefficients, was deposited (Glatz et al., 2008).

Lee et al. (2008) electrodeposited Bi₂Te₃ nanowires arrays using AAO as template by potentiostatic, galvanostatic, and pulsed method in aqueous solution at room temperature. Uniform Bi₂Te₃ nanowire arrays with highly oriented crystalline structure was synthesized, The bandgap of the deposited can be controlled from 0.21 to 0.29 eV by different relaxation times in the pulsed electrodeposition. The electrical resistances increased slightly with increasing temperatures, which was owing to enhanced carrier-phonon scattering. All samples showed a positive Seebeck coefficient (12–33 µV/K). (Lee et al., 2008).

Li et al. electrodeposited Bi_xSb_2-xTe_y in nitric acid and hydrochloric acid solutions. A composition of Bi_0.5Sb_1.5Te₃ was gained in both acid solutions with significantly different morphology. The Bi_0.47Sb_1.36Te_3.17 thin film prepared in the nitric acid solution has the highest Seebeck coefficient of 213 µV/K. The Bi_0.22Sb_1.48Te_3.30 film prepared in the hydrochloric acid solution has the highest power factor of 111.5 µW/(mK²), which had an electrical resistivity of 1.27 × 10^–4 Ω m and Seebeck coefficient of 119 µV/K (Li and Wang, 2009).

Diliberto et al. (2008) synthesized Bi₂Te₃ thin films using pulsed electrodeposition from electrolytes of 20 mM Te(IV) ion and 1 M HNO₃. The Bi ion concentration was varied, where increasing Bi concentration in the electrolyte would lead to higher Bi composition. The results also indicated that pulsed electrodeposition would improve the morphology and the electrical conductivity of films compared to direct electrodeposition. The film near stoichiometry (Bi_1.93Te_3.07) have a Seebeck coefficient of -65 µV/K (Diliberto et al., 2008).

Zhu et al. (2008) synthesized Bi₂Te₃ thin sheets on Au by electrochemical atomic layer epitaxy method using Bi solution with 0.25 mM Bi(NO₃)₃ and 0.1 M HClO₄, and Te solutions with 0.25 mM TeO₂ and 0.1 M HClO₄. The bandgap of the Bi₂Te₃ film was 0.33 eV measured by Fourier transform infrared spectroscopy. Compared to the bulk Bi₂Te₃ single crystal, the bandgap is blue shifted (Zhu et al., 2008).

Mavrokefalos et al. (2009) reported electrodeposition of n-type Bi₂Te₃ nanowires (NW). The results showed that monocrystalline NWs have higher electrical conductivity and thermal conductivity than polycrystalline NWs. Additionally, the carrier mobility of the monocrystalline NW is about 2.5 times higher than that of the polycrystalline NW, but it about 19% lower than that of bulk materials. The electron mean-free path was decreased from 61 nm for bulk materials to 40 nm for the 52 nm nanowires, which is owing to electron scattering specularity parameter by nanowire surface is 0.7. Furthermore, the thermal conductivity of the polycrystalline nanowires is lower. The ZT is about 0.1 at 400 K for both monocrystalline and polycrystalline NWs (Mavrokefalos et al., 2009).

Li et al. electrodeposited Bi_0.5Sb_1.5Te₃ thin film from nitric acid baths. The results show that electrodeposition mechanism varied with applied potential, where at low applied potential, Te was deposited because of electrochemical reduction of HTeO₂ ⁺, while at more negative applied potential the reduction reaction of Bi³⁺ with Te occurred with formation of Bi₂Te₃. Additionally, when the applied potential is negative enough, formation of Bi_0.5Sb_1.5Te₃ compound took place (Köse et al., 2009).

Li et al. examined the electrodeposition of Bi₂Te₃ in a solution containing TeCl₄, Bi(NO₃)₃ and dimethyl sulfoxide (DMSO) by combining cyclic voltammetry with electrochemical quartz crystal microbalance. The results indicated Te⁴⁺ concentrations in and applied potential had an effect on Bi₂Te₃ composition. Bi₂Te₃ was electrodeposited in applied potential between −0.2 and −0.8 V vs. Ag/AgCl with 10 mM Te⁴⁺ and 7.5 mM Bi³⁺. However, Te-rich Bi₂Te₃ were electrodeposited at applied potential between -0.2 and -0.8 V vs. Ag/AgCl in the solution with 50 mM Te⁴⁺ and 37.5 mM Bi³⁺ (Li, 2009).

Suresh et al. (2009) electrodeposited Bi₂Te₃ thin films at various pH values in HNO₃ solution of Bi (NO₃)₃ and TeO₂. The increase in pH resulted in a decrease in grain size and the film morphology transformed from dispersed nanoparticles to connected chain-like nanostructures as pH was increased. At the temperature between 300 and 425 K, the data showed a four-times increase in Seebeck coefficient between its maximum and minimum value as the solution pH changes from 1 to 3.5, which is attributed to the improved connectivity of the nanostructures at higher pH (Suresh et al., 2009).

Kim and Oh (2009) electrodeposited n-type Bi₂Te₃ and p-type Sb₂Te₃ films. The n-type Bi₂Te₃ had a power factor of 7.1 × 10^–4 W/(K²·m) with a Seebeck coefficient of −51.6 μV/K, which was electrodeposited at applied potential of −0.05 V with 25 mM Bi ion and 25 mM Te ion. Additionally, The p-type Sb₂Te₃ film had a power factor of 1.7 × 10^–4 W/(K²·m) with a Seebeck coefficient of 52.1 μV/K, which is deposited at applied potential of 0.02 V in the solution containing 63 mM Sb ion and 7 mM Te ion. (Kim and Oh, 2009).

Mannam et al. (2009) electrodeposited Bi_xTe_y nanowires from aqueous acidic solutions containing different [Bi³⁺]/[HTeO₂ ⁺] (20/20 and 20/10 mM) with 2.5 M HNO₃. The nanowires deposited at low applied potentials had a dominant orientation of (110) according to the XRD pattern. In both electrolytes, n-type nanowires were deposited. However, p-type nanowires can be deposited only in the [Bi³⁺]/[HTeO₂ ⁺] = 20/10 mM solution. Nanowires formed in the 20/10 mM electrolyte showed at transition from intrinsic to extrinsic. The Seebeck coefficient of -318.7 and 117 μV/K were achieved for n-type and p-type Bi_xTe_y nanowires, respectively (Mannam et al., 2009).

Kuleshova et al. (2010) electrodeposited BiSbTe films in nitric acid baths. In the electrolyte, sodium ligninsulfonate was added as surfactant, which would improve uniformity of the films as well as the thermoelectric properties. Additionally, the surfactant would also affect the composition of films, where Bi_0.32Sb_1.33Te₃ was deposited in the solution with surfactant and Bi_0.35Sb_1.33Te₃ was deposited without surfactant in the solution with 10 mM HTeO₂ ⁺, 1 mM Bi³⁺, 20 mM Sb³⁺, 1 M HNO₃, 0.1 M H₃Cit and 50 mM Na₃Cit (Kuleshova et al., 2010).

Ma et al. (2010) electrodeposited Bi₂Te₃ on stainless steel, in which the reaction mechanism and the effect of deposition parameters on composition and morphology were investigated. The CV results showed that onset potential for Bi₂Te₃ is more positive than Bi and Te deposition. Furthermore, the Te reduction reaction is kinetically hindered with the presence of Bi ions. (Ma et al., 2010).

Kim and Oh (2010a) synthesized p-type Sb_xTe_y and n-type Bi_xTe_y films by electrodeposition. The Bi_xTe_y film with a thickness of 5.3 µm was electrodeposited in 1 M HNO₃ solution at -0.05 V, which contained 50 mM Bi and Te ion. Moreover, the Bi/(Bi + Te) mole ratio is 0.5. The Sb_xTe_y film with a thickness of 5.2 µm was electrodeposited at 0.02 V in the electrolyte, where the total concentration of Sb and Te ion is 70 mM and Sb/(Sb + Te) mole ratio is 0.9. The Bi_xTe_y and Sb_xTe_y films have an electrical conductivity of -67 and 63 µV/K(Kim and Oh, 2010a).

Lee et al. (2010) electrodeposited Bi₂Te₃ nanowires in AAO templates. They claimed the electrical conductivity can be improved from 0.053 to 0.169 × 10⁶ S/m by tailoring the structural properties. Meanwhile, the Seebeck coefficient can be enhanced from 46.6 μV/K to 55 μV/K. As a result, a power factor of 476.3 μW/(K²·m) was achieved (Lee et al., 2010).

Richoux et al. (2010) synthesized p-type (Bi_1-xSb_x)₂Te₃ thermoelectric compounds by pulsed electrodeposition in the electrolyte with 1 M HClO₄ and 0.1 M tartaric acid. The deposited film had a Seebeck coefficient of 150 μV/K. Additionally, pulsed electrodeposition method can be used to reduce resistivity of the films, where 200 μΩ m was achieved by pulsed electrodeposition method, compared to 5,000 μΩ m by direct-current electrodeposition method (Richoux et al., 2010).

Li et al. electrodeposited Bi_xSb_2−xTe_y film by potentiodynamic electrodeposition technique from mixed dimethyl sulfoxide solution containing Bi(NO₃)₃·5H₂O, TeCl₄ and SbCl₃. Their results showed that electrodeposition of Bi_xSb_2−xTe_y can be realized in a wide range of applied potential. However, the films deposited at applied potential of -0.2 to -0.4 V achieved the highest S of 185 μV/K and the lowest electrical resistivity of 3.34 × 10^–5 Ω m after annealing. Additionally, the deposited nano-crystalline Bi_0.49Sb_1.53Te_2.98 film had a preferred orientation of (015) (Li and Wang, 2010).

Chen et al. (2010) fabricated Te-rich n-type Bi_xTe_y films and nanowires array with rhombohedral structure (Figure 5) by potentiostatically electrodeposition from nitric baths. The Seebeck coefficient was about -70 µV/K at 300 K and decreased monotonically with temperature. Additionally, thermal conductivity of 0.75 W/(mK) was obtained at 300 K. Aa a result, The ZT Bi₂Te₃ nanowire was 0.45 at 300 K and 0.9 at 350 K for (Chen et al., 2010).

Frantz et al. (2010) also synthesized polycrystalline Bi₂Te₃ nanowires with rhombohedral phase by electrodeposition using porous polycarbonate as template. Their results showed that dimethyl sulfoxide would help to increase the filling ratio to 80%. Moreover, DMSO in the electrolyte can help to improve the electrical conductivity of the nanowires (Frantz et al., 2010).

Gan et al. (2010) investigated Nanoscale Bi-Te particles with thermoelectric properties electrodeposited on copper substrate in 2.0 M HNO₃. The atomic ratio 1:1 for Bi:Te in the alloy, which is equivalent to the weight percentage of Bi:Te = 62%:38% was confirmed from the EDS data (Gan et al., 2010).

Li et al. (2010a) investigated the electrochemical behavior Bi_xSb_2-xTe_y in the solution consisting of 20 mM TeCl₄, 20 mM Bi(NO₃)₃, 20 mM SbCl₃, DMSO, and 0.1 mM KNO₃. A smooth morphology was obtained for Bi_xSb_2-xTe_y films deposited at different applied potential. The resistances reduced to about 0.04 Ω by post-annealing process. Seebeck coefficient of 85 μV/K was obtained for Bi_0.49Sb_1.53Te_2.98 film (Li et al., 2010a).

Li et al. (2010b) electrodeposited Bi₂Te_2.7Se_0.3 nanowire arrays using AAO as template in the electrolyte composing of 2 mM TeO₂, 2.5 mM Bi(NO₃)₃, 0.3 mM SeO₂ and 0.1 M HNO₃. The post-annealing process was carried out at 300°C under an argon atmosphere. The single crystalline nanowires with diameter of about 14 nm were obtained (Li et al., 2010b).

Golgovici et al. (2010) synthesized BiSbTe films by electrodeposition in choline chloride (ChCl) and malonic acid based ionic liquids with a molar ratio of 1:1. The reaction temperature was controlled between 80 and 85°C. The concentration of Bi, Sb and Te ions ranged from 1.5 to 50 mM. The CV data showed that the Te reduction reaction happened first, followed by formation of binary or ternary compounds by codeposition. Furthermore, pulsed electrodeposition technique was also used to synthesize BiSbTe films (Golgovici et al., 2010).

Rostek et al. (2011) synthesized n-type Bi₂Te₃ films by electrochemical deposition. The films with composition near stoichiometric was deposited in the solution containing 20 mM Te ions and 30 mM Bi ions at a current density of 3.75 mA/cm². The Seebeck coefficient of as-deposited Bi₂Te₃ films is about -55 μV/K. However, after annealed at 250°C for 60 h, the Seebeck coefficient increased to -130 μV/K(Rostek et al., 2011).

Ma et al. (2011) electrodeposited thin Bi₂Te₃ film onto stainless steel from acidic nitrate baths. The carrier concentration of the deposited films was ten times higher than the bulk Bi₂Te₃, while the Seebeck coefficient and Hall mobility is lower than bulk Bi₂Te₃ (Ma et al., 2011).

Erdogan et al. (2009) synthesized Bi₂Te₃ nanofilm and nanowire by electrodeposition. The acidic electrolyte containing 1 mM TeO₂ and 1 mM Bi(NO₃)₃ with a pH of 1.5, in which Bi₂Te₃ nanofilm was deposited with a preferential orientation of (015). Additionally, the alkaline electrolyte containing 2 mM Bi(NO₃)₃, 1 mM TeO₂, and 10 mM EDTA with a pH of 9.0, in which nanowire was deposited with (110) as preferential orientation. They claimed that the EDTA in the basic solution leading to the 2D growth mechanism. Furthermore, the band gap energy of Bi₂Te₃ nanostructures can be tuned by size and morphology of the nanostructures, as shown in the reflection absorption Fourier transform infrared spectroscopy (Erdoan and Demir, 2011).

Li et al. (2011a) electrodeposited polycrystalline Bi₂Te₃ nanowire arrays using AAO templates by a pulse electrodeposition method from a electrolyte containing DMSO. The results showed that the applied potential can be used to tune the composition of the nanowires. The Bi₂Te₃ nanowire array have a preferential orientation of (110). Additionally, Bi₂Te₃/Te multilayered nanowires were electrodeposited by the same method (Li et al., 2011a).

Kose et al. (2009) electrodeposited thin Bi₂Te_3−ySe_y films in the solution containing 2 mM TeO₂, 2.5 mM Bi(NO₃)₃, 0.3 mM SeO₂ and 0.1 M HNO₃ on Au (111) at room temperature. Bi₂Te_2.7Se_0.3 films was obtained at applied potential of −0.02 V vs. Ag/AgCl (3 M NaCl), which has micron-sized granular crystallites (Köse et al., 2009).

Lim et al. (2009) synthesized BiSbTe films via electrodeposition in the electrolyte containing 0.5 mM Bi³⁺, 32 mM SbO⁺, 2 mM HTeO₂ ⁺, 0.2 M citric acid, 30 mM EDTA and 1 M HNO₃. A Seebeck coefficient of 71 µV/K and a power factor 1.2 × 10^–4 W/(K²·m) was achieved for BiSbTe films. Additionally, the amorphous Sb₂Te₃ films was electrodeposited at 0.01–0.03 V in the electrolyte containing 70 mM Bi³⁺, 70 mM SbO⁺, 3.5 M perchloric acid and 0.35 M tartaric acid. A Seebeck coefficient of 250 µV/K and a power factor 57 × 10^–4 W/(K²·m) was achieved for Sb₂Te₃ films (Lim et al., 2009).

Kim et al. (2018a) electrodeposited Bi_xSb_2-xTe_y films in the solution with 2.4 mM TeO₂, 3.6 mM Sb₂O₃, 400–1,000 μM Bi(NO₃)₃5H₂O, 33 mM L-tartaric acid, and 1 M HNO₃ at fixed applied potential of −0.1 V (vs. SCE). The composition of the films were controlled by [Sb]/[Bi] ratio. The results showed that the substitution of Bi with Sb would improve the mobility, while suppress the carrier concentration. The deposited Bi₁₀Sb₃₀Te₆₀ film has a high Seebeck coefficient, which results in a power factor (PF) of ∼490 μW/m K² (Kim et al., 2018a).

Ma et al. (2010) electrodeposited Bi_1-xSb_x and Bi_2-xSb_xTe₃ thin films at 25°C with different morphologies, such as thin sheets, rods, dendrites, and spherical particles. The Bi_1-xSb_x film was deposited in the electrolyte containing 2 mM Bi(NO₃)₃, 1 mM SbCl₃, 0.2 M C₄H₆O₆, and 0.1 M HNO₃. Additionally, The Bi_2-xSb_xTe₃ film was deposited in the electrolyte containing 0.3 mM TeO₂, 0.2 mM Bi(NO₃)₃, 1 mM SbCl₃, 0.2 M C₄H₆O₆, and 0.1 M HNO₃. Furthermore, the results indicated that the underpotential deposition mechanism would lead to the formation of (Bi_0.5Sb_0.5)₂Te₃, however the overpotential deposition would result in the formation of Bi_0.5Sb_1.5Te₃. Meanwhile different deposition mechanism can be triggered by applied potential (Ma et al., 2010).

Jin and Wang (2010) electrodeposited n-Type thin Bi₂Te_3-ySe_y films using Au, Bi, and Bi₂Te_3-ySe_y as substrates. The electrolyte contained 8 mM HTeO₂ ⁺, 8 mM Bi³⁺, 1 mM H₂SeO₃, and 1 M HNO₃. The substrates have significant effect on the morphology of films, as well as the crystal orientation. The preferred orientation of (015) with rhombohedral structure was obtained when using Bi₂Te_3-ySe_y as substrate. Additionally, the films deposited on the Bi₂Te_3-ySe_y substrate showed the highest power factor after annealing (Jin and Wang, 2010).

Golgovici et al. (2011) investigated electrodeposition of Bi₂Te₃, Sb₂Te₃, BiSb, and BiSbTe films in an aqueous solution containing 5 M NaCl and 1 M HCl or an ionic liquid with choline chloride and malonic acid mixture. The concentrations of Bi, Sb and Te ion were controlled between 10 and 90 mM. Morphology and composition of BiSbTe was modified by increasing the current pulses (Golgovici et al., 2011).

Liu et al. electrodeposited Bi₂Te₃ pillars using multi-channel glass molds as template. The results showed that pulsed electrodeposition method is helpful to achieve high aspect ratio filling. The n-type Bi₂Te₃ arrays with aspect ratio exceeding ten was obtained at a pulse circle of −0.2 V for 4 s, +0.5 V for 1 s, and 0 mV for 3 s (vs. SCE). The precursor concentration in the electrolyte includes 7.5 mM Bi³⁺ and 10 mM HTeO₂ ⁺. Furthermore, the electrical conductivity of as-deposited Bi₂Te₃ pillars is the same magnitude as bulk Bi₂Te₃ (Liu and Li, 2011).

Li et al. (2011b) synthesized heterogeneous thermoelectric nanowire arrays of multilayer Bi₂Te₂Se/Te using template direction electrodeposition. The thickness of the Te section can be modulated by tailoring Te ion concentration. The diameter of the heterogeneous nanowires is from 60 to 85 nm. Additionally, the Bi₂Te₂Se segment can change to Bi₂Se₂Te by lowing the Te ion concentration to a certain level (Li et al., 2011b).

Pinisetty et al. (2011a) fabricated polycrystalline Bi₂Te₃ nanowires and nanotubes arrays by electrodeposition. The applied potential had effect on the composition, where both Bi-rich (p-type)and Te-rich (n-type) nanowires or nanotubes can be deposited. The lamellar thickness of bath morphologies were about 17–24 nm. The nanowires and nanotubes had a Seebeck coefficient of 11.5 and 17 μV/K, respectively, which were deposited at −0.4 V. However, when applied potential was −0.065 V, Seebeck coefficient of −48 and −63 μV/K were obtained for the nanowires and nanotubes, respectively (Pinisetty et al., 2011a).

Lim et al. (2012a) synthesized Bi_xSb_2-xTe₃ films by electrodeposition in an electrolyte containing 0.8 mM TeO₂, 0.2 mM Bi(NO₃)₃, 0.8 mM Sb₂O₃, 33 mM tartaric acid, and 1 M HNO₃. The composition of the thin films can be controlled by applied potential, where stoichiometry can be achieved from −0.10 to −0.15 V vs. SCE. Additionally, at more negative applied potential, the thermoelectric property of Bi_xSb_2-xTe₃ films was degraded, which might owing to higher defect density. The electrical and thermoelectric properties can be enhanced by annealing in reducing environment (Lim et al., 2012a).

Peranio et al. (2012) synthesized Bi₂Te₃ nanowires by a potential-pulsed electrodeposition using AAO as template in a solution with 15 mM HTeO₂ ⁺, 10 mM Bi³⁺ and 1 M HNO₃. The nanowires had a stoichiometric composition with diameters of 50–80 nm and a length of 56 μm. The nanowires are single-crystalline with no grain boundaries. The XRD pattern revealed that growth direction of the nanowires were (110) and (210). Additionally, the c axis of the Bi₂Te₃ crystal was perpendicular to nanowire axis (Peranio et al., 2012).

Frantz et al. (2012) electrodeposited bismuth telluride nanowires from an electrolyte with1.5 mM Bi³⁺, 15 mM HTeO₂ ⁺ and DMSO using polycarbonate as template. The DMSO would shift the reduction potential to more negative side and inhibit the cation diffusion. The nanowires deposited -0.1 V vs Ag/AgCl at have a diameter of 60 nm diameter with stoichiometric composition. The crystal structure of the nanowires was polycrystalline with a preferential orientation perpendicular to the (015) planes (Frantz et al., 2012).

Ma et al. (2012a) synthesized thin Sb₂Te₃ and Bi₂Te₃ films using goldthe Au-coated silicon as substrate in an acidic bath with Bi(NO₃)₃·5H₂O, TeO₂, Sb₂O₃, 1 M HNO₃ and 0.5 M tartaric acid at room temperature by electrochemical deposition. The as-deposited Bi₂Te₃ films were polycrystalline, but the Sb₂Te₃ films were amorphous. Additionally, the Sb₂Te₃ films showed both Sb₂Te₃ and Te phase after annealing (Ma et al., 2012a).

Zhu et al. (2008) electrodeposited p-type quaternary thin BiSbTeSe films using Au as substrate in a acidic solution with 0.5 mM Se(IV), 12 mM Te(IV), 2.5 mM Bi(III), 10 mM Sb(III), 0.67 M tartaric acid at room temperature. The thickness of the films was controlled to 8 μm. The applied potential can be used to tailoring the composition of the films. The as-deposited films were amorphous, however they changed to polycrystalline after annealing based on the XRD patterns. A maximum power factor of 620 µW/(K²·m) was achieved for the thin BiSbTeSe films after post-annealing with Seebeck coefficients of 116–133 μV/K (Zhu and Wang, 2012).

Banga et al. (2012) fabricated Bi₂Te₃/Bi_2−xSb_xTe₃ heterostructure by pulsed potentiostatic electrodeposition method. The solution consisted Na₂TeO₃, Bi(NO₃)₃, Sb(III), 2 M nitric acid, and 0.3 M tartaric acid. The heterostructure had a layer periodicity in the range of 10–30 nm. The XRD data showed that the multilayer films possessed a (015) texture (Banga et al., 2012).

Zhu et al. (2008) synthesized n-type Bi₂Te_3-ySe_y films using ITO-coated glass as substrates in the acidic solution containing 10.0 mM HTeO₂ ⁺, 7.5 mM Bi³⁺, 1.1 mM SeO₃ ^2- and 0.5 M HNO₃ at room temperature by pulsed electrodeposition. The smooth and compact Bi₂Te_3-ySe_y films were obtained. Increasing the cathodic current density would decrease the grain size of the films. The Bi₂Te_3-ySe_y films had a Seebeck coefficient of about -92 μV/K and electrical resistivity of about 109.4 μΩ m (Zou et al., 2012).

Naylor et al. (2012) synthesized Bi₂Te₃ films with stoichiometric composition Bi₂Te₃ in the electrolyte consisting of 10 mM Te(IV), 7.5 mM Bi(III), sodium lignosulfonate (SL) and 1 M HNO₃. The sodium lignosulfonate acted as a surfactant, which would improve morphology and roughness of the Bi₂Te₃ films and achieve better alignment in the (110) plane. The optimal concentration of SL is from 60 to 80 mg/L at a deposition potential of -0.1 V vs SCE (Naylor et al., 2012).

Limmer et al. (2012) reported the electrodeposition of 75 nm diameter nanowire arrays with a composition of Bi₂(Te_0.95Se_0.05)₃ onto Si substrates using AAO as template in the electrolyte containing 80 mM Bi(NO₃)₃•5H₂O, 40–80 mM TeCl₄, 0.8–1.2 mM SeO₂ and 0.1 M KClO₄ in dimethyl sulfoxide. The nanowires are polycrystalline with grain size of 5–10 nm (Limmer et al., 2012).

Ma et al. (2012b) synthesized ternary compounds (Bi_xSb_1-x)₂Te₃ and Bi₂(Te_1-ySe_y)₃ by electrodeposition using gold-coated silicon as substrates in the electrolyte with TeO₂, Bi(NO₃)₃·5H₂O, SbCl₃ and Na₂SeO₃, 1 M HNO₃ and 0.67 M tartaric acid at room temperature. The p-type (Bi_xSb_1-x)₂Te₃ films had the highest power factor obtained with composition close to Bi_0.5Sb_1.5Te₃ deposited at a relatively large negative potential. In addition, Bi₂(Te_1-ySe_y)₃ thin films showed n-type behaviors with composition close to Bi₂Te_2.7Se_0.3 (Ma et al., 2012b).

Fu et al. (2013) fabricated Ag/Bi₂Te₃ multilayer nanowires by pulse electrochemical deposition using AAO as the template in the electrolyte consisted of 0.1 M HTeO₂ ⁺, 75 mM Bi(NO₃)₃, 10 mM AgNO₃, and 1 M HNO₃. The deposited the Bi₂Te₃ had a rhombohedral lattice phase and Ag had a cubic lattice phase. The length of each layer ranged from 25 to 45 nm (Fu et al., 2013).

Nguyen et al. (2012) investigated the electrodeposition of Bi₂Te₃ film in the electrolyte consisting of 50 mM of 50 mM TeCl₄, Bi(NO₃)₃, 0.5 M lithium nitrate, and ethylene glycol. The results showed that the electrochemical reduction reaction of both Bi³⁺ and Te⁴⁺ ions were carried out at applied potential more negative than 0.2 and 0.55 V vs. SHE, and the reaction is one step without the formation of intermediates. The Bi and Te ions had the similar diffusion coefficients and the reaction rate constants. Bi₂Te₃ films stoichiometric composition were deposited at current densities up to 5 A/dm² (Nguyen et al., 2012).

Agapescu et al. (2013) electrodeposited of Bi, Te, and Bi₂Te₃ films in an ionic liquids consisting of 10 mM BiCl₃ and TeO₂, choline chloride, and oxalic acid (ChCl–OxA) at 60°C.

Kim and Oh (2013) fabricated a thermoelectric device using n-type Bi₂Te₃ and p-type Sb₂Te₃ thin films as basic element legs. The device has a cross-plane configuration with 242 pairs of legs by flip-chip bonding of top electrodes. The thickness of both Bi₂Te₃ and Sb₂Te₃ films were about 20 μm. Additionally, the n-type Bi₂Te₃ and p-type Sb₂Te₃ films showed Seebeck coefficients of -59 μV/K and 485 μV/K, respectively. Furthermore, an open-circuit voltage of 0.294 V and a maximum output power of 5.9 μW were achieved at a temperature difference of 22.3 K (Kim and Oh, 2013).

Manzano et al. (2013) electrodeposited Bi₂Te₃ films with preferentially oriented of (110) direction in the electrolyte containing 10 mM HTeO₂ ⁺, 7.5 mM Bi³⁺ and 1 M HNO₃ at applied potential of 0.02 V vs. Ag/AgCl on a Pt substrate. When using pulsed electrodeposition method, the results indicated that at a pulse of on-time = off-time = 0.1 s the films achieved a Seebeck coefficient of −72 μV/K and power factor of 440 μW/(K²·m), which is measured at 107°C. Additionally, when using potentiostatic method, a Seebeck coefficient of -73 μV/K at 107°C and power factor of 600 μW/(K²·m) was obtained at 107°C (Manzano et al., 2013).

Zhou et al. (2013) electrodeposited n-type phosphorus-doped Bi₂Te₃ films on a stainless-steel electrode in the solution containing 10 mM TeO₂, 8 mM Bi(NO₃)₃, 4 mM H₃PO₄ and 1 M HNO₃. The as-prepared films had the thermal conductivity of 0.47 W/(mK) and the electrical conductivity of 280 S/cm (Zhou et al., 2013).

Rashid et al. (2013) synthesized Bi₂Te₃ films by galvanostatic electrodeposition in a solution containing 8 mM HTeO₂ ⁺, 8 mM Bi³⁺ and 1 M nitric acid. The results indicated that annealing process would enhance the carrier mobility while suppressing the carrier concentration. Additionally, the Seebeck coefficient can be enhanced from -57 to -169.49 µV/K and the power factor can be enhanced from 2.74 to 1737 µW/(K²·m) by post annealing process for p-type Bi₂Te₃ film. Moreover, the Seebeck coefficient can be improved from 28 to 112.3 µV/K and the power factor can be improved from 2.57 to 443 µW/(K²·m) by post-annealing process (Rashid et al., 2013).

Cao et al. (2013) fabricated thin Bi₂Te₃ films by electrodeposition in the solution with 10 mM HTeO₂ ⁺, 8 mM Bi³⁺ and 1 M HNO₃ at room temperature. The substrates used during the deposition had an epitaxial seed layer, which would help to reduce the lattice mismatch between Bi₂Te₃ and Silicon. Moreover, more uniform structure and better crystallinity can be achieved. Both doped and intrinsic silicon were used as substrate, while the results showed that a more compact thin Bi₂Te₃ film with preferential orientation of (001) was obtained for intrinsic silicon, which also showed better thermoelectric performance and smoother surface morphology. Compared to the thin film with preferential orientation of (110), the electrical conductivity is about 72% higher and the power factors is about 45% higher. Additionally, the electrical conductivity and Seebeck coefficient was suppressed by reducing the seed layer thickness from 40 to 20 nm, which can be attributed to the insufficient charge transfer during electrodeposition (Cao et al., 2013).

Wu et al. (2013) investigated the effect of chloride on the electrodeposition of Bi₂Te₃ films in the solution containing TeCl₄, Bi(NO₃)₃·5H₂O and ethylene glycol. The results indicated that the presence of chloride could enhance the reduction reaction rate of Te significantly, where the reaction rate with chloride in the solution is three orders of magnitude higher than the rate without chloride. Additionally, Bi₂Te₃ films with stoichiometric composition and smooth morphology were electrodeposited in certain potential window. A Seebeck coefficient of -120 µV/K was achieved for the Bi₂Te₃ films (Wu et al., 2013).

Yoo et al. (2013a) electrodeposited Bi_xTe_y thin films from nitric acid baths with 2.5–10 mM Bi(NO₃)₃, 10 mM TeO₂, and 1.5 M HNO₃ using Au/Ni/Si as substrates. The films with surface morphologies of granular and needle-like structures were deposited at different Te content. Higher of Bi ions concentration in electrolytes would result in higher power factor. Additionally, the power factor was not improved significantly owing to the interdependence of the electrical conductivity and the Seebeck coefficient (Yoo et al., 2013a).

Wang et al. (2013) synthesized Bi₂Te_2.7Se_0.3 and Bi_0.5Sb_1.5Te₃ by electrodeposition combined with post annealing. The solution to electrodeposit n-type Bi₂Te_2.7Se_0.3 contained 8 mM HTeO₂ ⁺, 8 mM Bi³⁺, 1 mM H₂SeO₃ and 1 M HNO₃, while the electrolyte to electrodeposit p-type Bi_0.5Sb_1.5Te₃ contained 2 mM Bi³⁺, 10 mM HTeO₂ ⁺, 100 mM Sb(III) and 1 M HNO₃. The as-deposited films possess amorphous structure and can be transferred to nanocrystalline after annealing. The annealed films show a preferred orientation of (015). The maximum power output of 77 μW was achieved with open-circuit voltage of 660 mV with a temperature difference of 20 K at 25°C. Additionally, a power density of 770 μW/cm³ was obtained (Wang et al., 2013).

Rashid et al. (2013) synthesized n-type Bi₂Te₃ films with a prominent orientation of (110) in the acidic solution with TeO₂ and Bi(NO₃)₃ on gold electrode. The Bi₂Te₃ films are nanocrystalline with grain size ranged from 21 to 45 nm. The results showed that the electrodes distance could be used to tune electrical and thermoelectric properties of the films, thus improving carrier charge mobility without varying of the Seebeck coefficient and carrier concentration. The highest power factor of 820 μW/K²·m was achieved with an electrical conductivity of 2.13 × 10³ S/cm and Seebeck coefficient of -61.2 μV/K (Rashid and Chung, 2013).

Yoo et al. (2013b) synthesized Bi_xSb_2-xTe₃ films use potentiostatic electrodeposition method at room temperature in an acidic electrolyte containing 0.8 mM TeO₂, 0.2 mM Bi(NO₃)₃, 0.8 mM Sb₂O₃, 1 M HNO₃, and 33 mM tartaric acid. When the applied potential was controlled between -0.10 and -0.15 V versus SCE, thin films with composition near stoichiometric were deposited. Additionally, reducing the applied potentials would result in suppressing the electrical and thermoelectric properties, probably owing to higher defect density (Yoo et al., 2013b).

Ng et al. (2014) fabricated the binary Bi₂Te₃ and ternary BiSbTe nanowires using template (AAO) directed electrodeposition method in a solution compose of 10 mM TeO₂, 20 mM Bi(NO₃)₃·5H₂O and 1 M HNO₃,. The results showed that reducing the applied potentials can increase the Sb composition, while increasing the applied potentials would facilitate the formation of Bi₂Te₃ (Ng et al., 2014).

Zou et al. (2014) investigated electrodeposition of n-type Bi₂Te_3-ySe_y film in the solution containing 10.0 mM HTeO₂ ⁺, 1.1 mM SeO₃ ^2-, 7.5 mM Bi³⁺, and 1 M HNO₃ at room temperature. The nucleation and growth mechanism were examined. The electrochemical reaction rate was controlled by diffusion and irreversible with the limiting current density of 1.78 mA/cm². A flocculent film was deposited when the applied potential was larger than limiting current without agitation. However, Bi₂Te_3-ySe_y film with smooth morphology was deposited at 4 mA/cm² with agitation. Bi₂Te_3-ySe_y film deposited at 1 mA/cm² have relatively high power factor and electrical conductivity (Zou et al., 2014).

Maas et al. (2014) electrodeposited Bi₂Te₃ in acidic solution with 20 mM HTeO₂ ⁺ and 20 mM Bi³⁺. The anode is Bi₂Te₃ as a sacrificial the source of cations. A homogeneous Bi₂Te₃ film with a thickness of 300 µm was deposited using Bi₂Te₃ as anode, while without Bi₂Te₃ as anode the thickness can be obtained is 10 times thinner. A power factor of 500 µW/(K²·m) was achieved (Maas et al., 2014).

Szymczak et al. (2014) electrodeposited n-type Bi₂Te₃ films in an ionic liquid with 1-ethyl-1-octyl-piperidinium bis(trifluoromethylsulfonyl)imide (EOPipTFSI) and 1-ethyl-1-octyl- piperidinium bromide (EOPipBr). The atomic ratio of EOPipTFSI and EOPipBr is 95:5. According to the result, this ionic liquid is stable at high cathodic applied potential, which provide a larger window to deposited Bi₂Te₃ compound. The morphology of the Bi₂Te₃ film can be tuned by precursor concentration, in which mirror-like films can be deposited with good uniformity. Additionally, an electrical resistivity of 133 µΩ m and Seebeck coefficient of -70 µV/K were achived (Szymczak et al., 2014).

Jiang et al. (2014) fabricated Bi₂Te₃/PEDOT:PSS/Bi₂Te₃ composite film by electrodeposition of Bi₂Te₃ onto poly (3,4-ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT:PSS) film. The solution contained TeO₂, Bi(NO₃)₃, and 1 M HNO₃. A thermal conductivity of 0.169–0.179 W/(mK) was obtained. ZT value of 1.72 × 10^–2 was achieved for Bi2Te3/PEDOT:PSS/Bi₂Te₃ composite film with electrical conductivity of 403.5 S/cm (Jiang et al., 2014).

Caballero-Calero et al. (2014) electrodeposited Bi₂Te₃ films in a solution with 10 mM HTeO₂ ⁺, 7.5 mM Bi³⁺, 1 M HNO₃. The Bi₂Te₃ films have a preferred orientation of (110) with c-axis parallel the substrate. Additionally, the effect of sodium lignosulfonate as surfactant on morphology was examined. Seebeck coefficient was determined to be -80 ± 6 µV/K (Caballero-Calero et al., 2014).

Wu et al. (2014) electrodeposited SbBi, Sb₂Te₃, and BiSbTe alloys in the electrolyte containing TeCl₄, SbCl₃, Bi(NO₃)₃, and ethylene glycol. The electrochemical reaction mechanism of Sb in chloride-free ethylene glycol was investigated. The results showed that the diffusion coefficients of Sb(III), Te(IV) and Bi(III) were comparable in ethylene glycol. Additionally, the onset potential of Sb is more negative than that of Te. During the electrodeposition of BiSbTe alloys, BiTe was deposited first followed by increase of Sb composition at more negative applied potential. (Wu et al., 2014).

Patil et al. (2015) electrodeposited fern shaped Bi₂Te₃ thin film in the solution containing 10 mM Te(IV), 7 mM Bi(NO₃)₃, and 1 M HNO₃.

Matsuoka et al. (2015) electrodeposited Bi₂Te₃/Bi₂Se₃ multiplayer heterostructure in two baths sequentially. The layer thickness was fixed to about 1 µm and the number of layers were varied from 2 to 10. The deposited multilayer structure is n-type with nanocrystalline. The boundaries between different layers were not clear planar. The number of the layers had a dramatic effect on the electrical conductivity, where more layers resulted in higher electrical conductivity, while Seebeck coefficient remained unchanged. The 10-layer Bi₂Te₃/Bi₂Se₃ heterostructure has a power factor of 144 µW/(K²∙m), which is about 3 times higher than that of the 2-layer heterostructure (Matsuoka et al., 2015).

Caballero-Calero et al. (2015) electrodeposited Bi₂Te_3-ySe_y films in a conventional three electrode cell in the solution containing 9 mM HTeO₂ ⁺, 7.5 mM Bi³⁺, 1 mM H₂SeO₃, and 1 M HNO₃. The influence of additives (i.e., sodium signosulfonate (SLS) and EDTA) in morphology, stoichiometry, structure and Seebeck coefficient was studied. The films synthesized with SLS had high crystallographic orientation and better morphology, while films deposited in the presence of EDTA had higher content of bismuth. The combination of both additives would improve the quality of stoichiometric Bi₂Te_2.7Se_0.3 films, namely denser morphology, higher orientation and higher Seebeck coefficients (60% larger) when compared with films deposited without additives. (Caballero-Calero et al., 2015).

Zhou et al. (2015) synthesized Bi₂Te₃ thin films by the pulsed electrodeposition method in the solution consisting of 40 mM HTeO₂ ⁺, 30 mM Bi³⁺ and 1.7 M HNO₃. The effect of deposition parameters on the composition and microstructure was investigated. The results indicated that the stoichiometry and morphology can be improved by a large pulse off-to-on ratio with a pulsed applied potential of 0 mV vs. Ag/AgCl. Additionally, larger pulse off-to-on ratio would enhance the ZT of Bi₂Te₃ films owing to suppressing the thermal conductivity and improving the Seebeck coefficient. The highest ZT value was 0.16 obtained at a pulse off-to-on ratio of 50 (Zhou et al., 2015).

Li et al. (2015) reported the electrodeposition of BiSbTe nanowires in the electrolyte containing 15 mM HTeO₂ ⁺, 40 mM SbO⁺, 2 mM Bi³⁺, 0.3 M tartaric acid and 1 M HNO₃. Their data showed that the pulse electrodeposit method would help to improve the uniformity and crystallinity of Bi_0.5Sb_1.5Te₃ nanowires, which resulted in higher electrical and thermal conductivity, compared to the direct current deposited nanowires. Additionally, the pulse electrodeposit method would also enhance the Seebeck coefficient of nanowires, which was attributed to a more homogeneous distribution of the elements. The highest ZT value was 1.14 at 330 K achieve by pulse-deposited Bi_0.5Sb_1.5Te₃ nanowires (Li et al., 2015).

Song et al. (2015) synthesized thin Bi₂Te₃ films in a acidic bath with 7.5 mM Bi(NO₃)₃, 10 mM TeO₂, cetyltrimethylammonium bromide (CTAB) and 1.5 M HNO₃ at room temperature. CTAB acted as a surfactant. The results indicated that the presence of CTAB would help to improve the surface morphology and mechanical properties Bi₂Te₃ films. However, the electrical and thermoelectric properties were preserved (Song et al., 2015).

Uda et al. (2015) fabricated Bi₂Te₃ thermoelectric micro-device by electrodeposition in an electrolyte composing of Bi(NO₃)₃·5H₂O, TeO₂, and HNO₃. The size effect of electrode was examined. The cross-section of the TE units is 50 × 50 μm² with depth of 20 µm. Additionally, the device had a eight arrays, which composed of 110 TE units. A maximum power output of 0.96 µW was achieved with an open-circuit voltage of 17.6 mV (Uda et al., 2015).

Chang et al. (2015) examined the electrodeposition of individual n-type Bi₂Te₃ nanowires (NWs) using polycarbonate membranes (PCM) as templates in the solution 10 mM HTeO₂ ⁺, 15 mM Bi³⁺, 1 M HNO₃ and 50 v/v % DMSO. The electrodeposition conditions, such as the applied potential can be used to control the composition of Bi₂Te₃. Additionally, increase the Te composition would increase the average grain size of NWs, as well as the electrical conductivity. The maximum power factor of 195.8 µW/(mK²) was achieved at 300 K for the Te-rich NW with diameter of 162 nm (Chang et al., 2015).

Shin and Oh (2015) fabricated a thermoelectric device based on thin film by combining electrodeposition and the flip-chip process. The thermoelectric materials used in the device are the n-type Bi₂Te₃ and p-type Sb₂Te₃ thin film, which is deposited on Ti/Cu/Au substrate in the solutions with 25 mM Bi ion, 25 mM Te ion and 1 M HNO₃ for Bi₂Te₃ and 63 mM Sb ion, 7 mM Te ion. The device with 242 pairs thermoelectric legs have a internal resistance of 21.4 Ω, which have a output voltage of 320 mV and output power of 1.1 mW at 39.7 K temperature difference. Additionally, the calculated power density of 3.84 mW/cm² (Shin and Oh, 2015).

Abellán et al. (2015) synthesized thin Bi₂Te₃ films containing TeCl₄, Bi(NO₃)₃ and dimethyl sulfoxide. Different substrates were used, such as CdTe/FTO and SnO₂:F coated glasses. Additionally, the deposits films were n-type semiconductors with trigonal crystal structure and stoichiometric composition dimethyl sulfoxide (Abellán et al., 2015).

Kulsi et al. (2015) synthesized thin Bi₂Te₃ films with preferred crystal orientation of (018) in the solution consisting of 15 mM TeO₂ and 10 mM Bi (NO₃)₃. The effect of different surfactant on the morphology was examined, including sodium dodecyl sulfate (SDS) and polyvinylpyrrolidone (PVP). The results indicated that improving the surface morphology would help to enhancing the carrier mobility. A ZT value of 0.28 was achieved using SDS as surfactant, which was measured at room temperature (Kulsi et al., 2015).

Şişman and Başoğlu (2016) fabricated thin Bi₂Te_3-ySe_y films by electrodeposition in the solution containing 2 mM TeO₂, 2.5 mM Bi(NO₃)₃, SeO₂ and 0.1 M HNO₃ with Au as substrate. The Se composition was controlled to be 0.3 to 2.5. The results showed that replacement of Te by Se atoms would push the XRD diffraction peaks positions Bi₂Te_3-ySe_y to higher angle, which is attributed to the change of crystal lattice constant (Şişman and Başoğlu, 2016).

Lei et al. (2016a) synthesized of 600 μm-thick n-type Bi₂Te₃ films by pulsed and potentiostatic electrodeposition in the electrolyte consisting of 70 mM TeO₂, 52.5 mM Bi³⁺, 2 M nitric acid and polyvinyl alcohol (PVA). The results indicated that compact and uniform Bi₂Te₃ films was electrodeposited which composition near stoichiometric and hexagonal crystal structure. Moreover, the film growth can reach 100 μm/h. Additionally, a Seebeck coefficient of -200 µV/K and an electrical conductivity of 400 S/cm were achieved, resulting in a power factor of 1.6 × 10³ µW/(mK²) (Lei et al., 2016a).

Yang et al. (2016) electrodeposited p-type BiSbTe thin films using ITO glasses as substrate in the electrolyte composing of 2 mM TeO₂, 0.5 mM Bi₂O₃, 3.5 M HClO₄, 1 M HNO₃ and 0.35 M C₄H₆O₆. The Sb³⁺ concentration and current density were the variables during the electrodeposition. Thin BiSbTe films showed different morphologies, such as ball-type, mixed-type and acicular-type. The Seebeck coefficient of 32.89 μV/K was obtained (Yang et al., 2016).

Patil et al. (2016) electrodeposited thin Bi₂Te₃ film in a solution with 10 mM Te(IV), 7 mM Bi(NO₃)₃·5H₂O and 1 M HNO₃. The XRD pattern showed that the Bi₂Te₃ film was nanocrystalline with grain size of 18.08 nm and had a preferred orientation of (015) with rhombohedral crystal structure (Patil et al., 2016).

Na et al. (2016) electrodeposited n-type Bi₂Te₃ films in the electrolyte with 10 mM HTeO₂ ⁺, 8 mM Bi³⁺ and 1 M HNO₃ on a flexible substrate. The effect of applied potential on the crystal structure and thermoelectric properties were systematically studied. The Bi₂Te₃ film with preferred orientation of (110) is deposited. The highest power factor of 1,473 μW/(K²·m) was achieved for the film electrodeposited at applied potential of 0.02 V with electrical conductivity of 691 S/cm. The effect of applied potential and grain size on the electrical and thermoelectric properties were shown in Figure 6. A flexible thermoelectric device was fabricated using Bi₂Te₃ as n-type material and poly (3,4-ethylene dioxythiophene)s as p-type material. The generator achieved a output voltage of 5 mV and output power of 56 nW with temperature difference of 12 K (Na et al., 2016).

Lal et al. (2017) synthesized p-type (Bi_xSb_1-x)₂Te₃ thin films using pulsed electrodeposition in the electrolyte consisting of 15 mM HTeO₂ ⁺, 5 mM Sb₂O₃, 5 mM Bi(NO₃)₃, 0.2 M tartaric acid, sodium dodecyl sulfate (SDS) and dimethyl sulfoxide. The results indicated that the present of SDS would improve the Seebeck coefficient and power factor of the films as shown in Figure 7. (Lal et al., 2017).

Additionally, many other groups reported the results of characterization of BiTe/Se electrodeposits based on various experimental conditions which are summarized on Table 1. (Jagadish et al., 2015; Lei et al., 2016b; Wu et al., 2016b; Hasan et al., 2016; Kulsi et al., 2016; Manzano et al., 2016; Wu et al., 2017b; Kang et al., 2017; Lal et al., 2017; Lei et al., 2017).

Dou et al. (2008) synthesize Bi/BiSb superlattice nanowires by template-directed electrodeposition method, in which AAO was used as template. The electrolyte for electrodeposition contains a mixture of 80 mM SbCl₃, 40 mM BiCl₃, 50 g/L citric acid, 40 g/L tartaric acid, 70 g/L NaCl, 100 g/L glycerol and 1.0 M HCl at pH value of 0.82 (Dou et al., 2008).

Weber et al. (2008) electrodeposited high density nanowire arrays in AAO templates from the electrolyte of 50 mM Bi³⁺ + 50 mM Sb³⁺ in dimethyl sulfoxide.

Dou et al. (2009) synthesized Bi/BiSb multilayer nanowires by pulsed electrodeposition with small bilayer thickness. The electrolyte for the deposition contained a mixture of 80 mM SbCl₃, 40 mM BiCl₃, 0.24 M citric acid, 0.27 M tartaric acid, 1.2 M NaCl, 0.1 M glycerol and 1.0 M HCl. Additionally, the modulating time was used to control the segment length and layer thickness of the nanowires (Dou et al., 2009).

Muller et al. (2012) synthesized Bi_1−xSb_x nanowires with Sb composition in the range from 0.05 to 0.40 and diameter in the range from 20 to 100 nm. The results showed that applied potential and ratio of Bi/Sb ions would influence the composition of Bi_1−xSb_x nanowires (Müller et al., 2012).

Limmer et al. (2015) electrodeposited Bi_xSb_y in the non-aqueous baths consisting of Sb salts, Bi(NO₃)₃·5H₂O, dimethyl sulfoxide and KClO₄. The effect of different Sb salts on the crystalline quality and preferred orientations were investigated. The results showed that nanowire arrays synthesized with SbI₃-based bath were polycrystalline. However, nanowire arrays synthesized with SbCl₃-based bath have a trigonal orientation, and composition of these nanowires remained constant along the nanowires. Additionally, there was a composition gradient along the radius of the nanowires array, where nanowires of Bi_0.75Sb_0.25 were obtained in the center area and nanowires of Bi_0.70Sb_0.30 were obtained in the edge region (Limmer et al., 2015).

Xiao et al. (2009) electrodeposited thin Bi₂Se₃ films using Pt as substrate by atomic layer epitaxy. The electrochemical reaction mechanism of Bi and Se alone were investigated by cyclic voltammetry. The electrodeposition mechanism Bi₂Se₃ is underpotential deposition (UPD). The synthesized Bi₂Se₃ films had an orthorhombic structure with stoichiometric composition. Additionally, the bandgap of Bi₂Se₃ films the was 0.35 eV. (Xiao et al., 2009).

Li et al. (2010c) synthesized Bi₂Se₃ thin films by electrodeposition in the solution containing SeO₂, Bi(NO₃)₃, and HNO₃ using Ti and indium tin oxide-coated glass as substrates at room temperature. The results indicated that the substrate had dramatic effect on the crystal structure of Bi₂Se₃ thin films. Pure rhombohedral crystal structure was obtained on the indium tin oxide-coated glass substrate, while both rhombohedral and orthorhombic crystal structure was obtained on Ti (Li et al., 2010c).

Xue et al. (2014) fabricated Bi₂Se₃/Bi multilayered nanowire arrays by pulsed electrodeposition using AAO as template in the electrolyte with 7.5 mM H₂SeO₃, 25 mM Bi³⁺ and 7 mM HNO₃. Each layer of Bi or Bi₂Se₃ had a thickness of about 100 nm, and the total length of the nanowire was 10 µm with a diameter of 50 nm (Xue et al., 2014).

Li et al. electrodeposited thick Bi₂Se₃ films using ITO-coated glass as substrate in a acidic solution containing 25 mM SeO₂, 25 mM Bi(NO₃)₃ and 1.3 M HNO₃. The results showed that the as-deposited films were p-type Bi₂Se₃ films. The power factors of 52.57 μW/mK² were obtained for the as-deposited films (Xiaolong and Zhen, 2014).

Tumelero et al. (2016) electrodeposited Bi₂Se₃ using Si (100) substrate as substrate in the electrolyte consisting of 1.5 mM SeO₂, 1 mM Bi(NO₃)₃ and 0.5 M nitric acid. The results indicated that Bi₂Se₃ with single orthorhombic phase and stoichiometric composition can be deposited by tuning the applied potential, while the potential window was narrow. Additionally, the deposited Bi₂Se₃ had a bandgap of 1.25 eV (Tumelero et al., 2016).

Souza et al. (2017) synthesized Bi₂Se₃ films by potentiostatic electrodeposition method in the electrolyte consisting of 1.5 mM SeO₂, 0.5 mM Bi₂O₃ and 1.0 M HClO₄ using silicon (100) as substrate. The deposited Bi₂Se₃ films is compact with uniform and smooth morphology. The as-deposited films had a dominant orthorhombic phase with mixture of rhombohedral and amorphous phases. However, pure rhombohedral structure was obtained after annealing (Souza et al., 2017).

Jagadish et al. (2016) synthesized n-type Bi₂S₃ films in the solution consisting of 20.6 mM Bi(NO₃)₃, 0.54 M of lactic acid, 0.78 M of nitric acid and 140.8 mM Na₂SO₄. Virgin carbon fiber and recycled carbon fiber were used as substrates. The deposited Bi₂S₃ had a composition near stoichiometry. The surface morphology and the Seebeck coefficient of Bi₂S₃ films can be tuned by post annealing process. The Bi₂S₃ films had Seebeck coefficient of -16.3 and -12.4 µV/K deposited on virgin carbon fiber and recycled carbon fiber, respectively (Jagadish et al., 2016).

Ueda et al. (2008) synthesized Sb₂Te₃ alloy in the AlCl₃-NaCl-KCl molten salt electrolyte containing 10 mM TeCl₄ and 7 mM SbCl₃, at the temperature of 423 K and applied potential of 0.85 V vs. Al/Al(III). The composition of Sb can be controlled by concentration ratio of the Sb(III) to [Sb(III) + Te(IV)]. The morphology of deposited Sb₂Te₃ alloy is disk-like granule, which had a size of around 10 µm (Ueda et al., 2008).

Park et al. (2009) electrodeposited thin Sb_xTe_y films and nanowires at room temperature in an acidic electrolyte. Pt/Si and Au were used as substrate. The applied voltage and film thickness had significant effect on the morphology and grain size of the Sb_xTe_y films. Amorphous Sb_xTe_y films was electrodeposited, while the films became the rhombohedral R3m structure after annealing (Park et al., 2009).

Kim and Oh (2010b) investigated the crystallization behavior of the electrodeposited Sb₂Te₃ film in the electrolyte containing 7 mM Te ion, 63 mM Sb ion, 3.5 M perchloric acid, 0.35 M tartaric acid. The transition crystal structure from amorphous to crystalline would influence the Seebeck coefficient. Moreover, the addition of Cu can improve the thermal stability of the Sb₂Te₃ film, where CuSbTe film had a crystallization temperature of 149.5°C (Kim and Oh, 2010b).

Pinisetty et al. (2011b) electrodeposited polycrystalline Sb₂Te₃ nanowires and nanotubes arrays in the electrolyte consisting of 0.7 mM TeO₂, 1.6 mM Sb₂O₃, 33 or 330 mM tartaric acid, and 3 M HNO₃. The nanowires and nanotube had an average lamellar thickness of 36 and 43 nm, respectively (Pinisetty et al., 2011b).

Lim et al. (2011) electrodeposited p-tyape Sb_xTe_y thin films in an acidic solutions. The effect of TeO₂ concentrations was investigated. Sb₂Te₃ films with composition near stoichiometry was deposited with a rhombohedral structure and preferred orientation of (015). The films had a carrier concentration of 5.8 × 10¹⁸ cm⁻³ and mobility of 54.8 cm²/(Vs). Additionally, more negative applied potential would reduce the carrier concentration and mobility, which is possibly owing to increase in defects. A Seebeck coefficient of 118 μV/K was obtained at room temperature for the as-deposited Sb₂Te₃ film (Lim et al., 2011).

Qiu et al. (2011) synthesized Sb₂Te_x (2 < × < 6) films in the alkaline solution with TeO₃ ^2-, SbO₂ ⁻, diaminourea polymer and triethanolamine. The solution was pretreated by argon gas to fully deaerate, which would enhance the Seebeck coefficient of the films by reducing oxygen contamination in the deposited films. The Sb₂Te_x films were amorphous before annealing. A maximum power factor 1.58 mW/mK² was achieved with a Seebeck coefficient of 532 μV/K after annealing (Qiu et al., 2011).

Schumacher et al. (2012) electrodeposited Sb₂Te₃ films in the electrolyte composing of 10 mM TeO₂, 5.6 mM Sb₂O₃, 0.84 M tartaric acid and 1 M nitric acid with pH of 1. Both Au and stainless steel were used as substrates. The results showed that morphology and composition of the films could be improved by pulsed electrodeposition methods. The p-type Sb₂Te₃ films fabricated by pulsed electrodeposition methods achieved a power factors of about 700 μW/(mK²) at room temperature with the electrical conductivity of 280 S/cm and Seebeck coefficients of 160 μV/K. Additionally, a maximum power factors obtained is 852 μW/(mK²) after annealing (Schumacher et al., 2012).

Li et al. (2012) electrodeposited Sb_xTe_y films in a nonaqueous electrolyte containing 20 mM SbCl₃, 20 mM TeCl₄ and 0.1 M KNO₃. The Sb_xTe_y films had a smooth morphology, which is independent of applied potential. The composition obtained nearest to stoichiometry is Sb_1.87Te_3.13. Additionally, all the films were p-type after annealing (Li et al., 2012).

Lim et al. (2012b) synthesized Sb_xTe_y films in the electrolyte with 2.4 mM HTeO₂ ⁺, 0.8 mM SbO⁺, 33 mM tartaric acid, and 1 M HNO₃ by electrodeposition. The thin Sb₂Te₃ films with composition near stoichiometry were deposited in the applied potential range of -0.15 to -0.30 V vs. SCE. The post-annealing process would reduce the FWHM of the major diffraction peaks and enhance the electrical conductivity. Additionally, the power factor was improved from 44.2 to 372.1 mW/(mK²) by annealing (Lim et al., 2012b).

Lensch-Falk et al. (2012) electrodeposited thin Sb₂Te₃ films in the electrolyte consisting of 7 mM sodium tellurite (IV), 16 mM antimony (III) oxide, 0.3 M tartaric acid, and 2 M nitric acid at room temperature by pulsed electrodeposition method. The results showed that the pulse duration have a significant effect on the texture and microstructure of films, where lamellar microstructure was deposited at short pulse durations, while equiaxed and randomly oriented microstructure was deposited at relative long pulse durations. Additionally, reducing pulse duration would also help to suppress the thermal conductivity of the films, where thermal conductivity of less than 2 W/(Km) was obtained (Lensch-Falk et al., 2012).

Nguyen et al. (2013) fabricated Sb, Te and Sb_xTe_y from molten salts containing acetamide - antimony chloride and tellurium chloride by electrodeposition. The Te composition of Sb_xTe_y alloy films is ranged from 20 to 81 at%, which is obtained in the electrolyte with SbCl₃ up to 0.48 M and TeCl₄ up to 0.12 M (Nguyen et al., 2013).

Yoo et al. (2013c) synthesized Sb₂Te₃ films in the solution consisting of 2.4 mM TeO₂, 0.8 mM Sb₂O₃, 33 mM tartaric acid, and 1 M HNO₃ at room temperature. Additionally, cetyltrimethylammonium bromide (CTAB) was used as surfactant to improve the surface morphology, where the effect of CTAB on the morphology of Sb₂Te₃ films was shown in Figure 8. Moreover, CTAB would also help to enhance the adhesion of Sb₂Te₃ films to substrate. Post-annealing at 200°C can improve electrical conductivity and Seebeck coefficient of the Sb₂Te₃ films, which was attributed to Te nanodots formation within the Sb₂Te₃ crystal structure. A power factor of 716.0 mW/mK² was obtained for Sb₂Te₃ films with 10–20 nm Te nanodots (Yoo et al., 2013c).

Kim et al. (2015) synthesized Te-rich Sb₂Te₃ film in a solution with 3.6 mM Sb₂O₃, 2.4 mM TeO₂, 33 mM L-tartaric acid, 1 M HNO₃. The as deposited films were amorphous, while γ-SbTe embedded nanocrystalline Sb₂Te₃ film was obtained by post annealing process because of solid-state phase transition. The results indicated that γ-SbTe embedded Sb₂Te₃ had higher Seebeck coefficient and P.F. than single phase Sb₂Te₃ film. This was attributed to strong energy-dependent charge scattering, which is confirmed by UPS analysis showing 90 meV valence band difference between Sb₂Te₃ and γ-SbTe nanocrystalline. As a consequence, a Seebeck coefficient of 320 μV/K was obtained for γ-SbTe/Sb₂Te₃ nanocomposite (Kim et al., 2015).

Kim et al. (2018b) also electrodeposited Ag₂Te nanoprecipitates embedded p-type Sb₂Te₃ films (Figure 9). The same electrolyte condition and applied potential (0.1 vs. SCE) was applied to deposit the films except adding 100 μM AgNO₃ as Ag sources. The results indicated that the presence of the β-Ag₂Te phase would improve the Electrical property Sb₂Te₃ films dramatically, which was attributed to energy-dependent charge carrier filtering effect at the β-Ag₂Te/Sb₂Te₃ interface. Additionally, density of states effective mass (m*∼1.8 m₀) increased, leading to a high power factor of 1870 mW/mK² at 300 K without any dramatic suppression of electrical conductivity (Kim et al., 2018b).

Catrangiu et al. (2016) electrodeposited Sb₂Te₃ film in the ionic liquid with 4–10 mM TeO₂, 4–10 mM SbCl₃, choline chloride and oxalic acid. The composition of the films can be controlled by precursor concentration and applied potential. The electrodeposited mechanism is that Te layer was deposited followed by the deposition of Sb_xTe_y compounds at more negative applied potential (Catrangiu et al., 2016).

Hatsuta et al. (2016) synthesized Sb₂Te₃ thin films in the solution consisting of 1.6 mM TeO₂, 1.3 mM SbF₃, and 0.39 M HCl by electrodeposition using stainless steel as substrate. The results indicated that a stoichiometric atomic composition was achieved for the as-deposited thin film. Moreover, the composition of thin film kept at stoichiometry after annealed at the temperature below 250°C. However, when the annealing temperature go up to 300°C, a portion a of alien element, including Fe, Cr, Ni, was detected in the film, which lead to lower Seebeck coefficient and higher electrical conductivity. As a consequence, a maximum power factor of 13.6 µW/(cmK²) was obtained for the Sb₂Te₃ film (Hatsuta et al., 2016).

Kim et al. (2016) electrodeposited thin AgSbTe₂ films. The deposited amorphous Ag₁₅Sb₂₇Te₅₈ film showed a Seebeck coefficient of 1,270 µV/K. The carrier concentration of about 10¹⁶ to 10¹⁹ cm⁻³ was obtained. For deposited nanocrystalline film, The power factors of 90–553 mW/mK² was obtained owing to higher Hall mobility and Seebeck coefficients (Kim et al., 2016).