Cristina V.
Manzano
*,
Cristina
Llorente del Olmo
,
Olga
Caballero-Calero
and
Marisol
Martín-González
*
Instituto de Micro y Nanotecnología, IMN-CNM, CSIC (CEI UAM+CSIC), Isaac Newton, 8, E-28760, Tres Cantos, Madrid, Spain. E-mail: cristina.vicente@csic.es; marisol.martin@csic.es
First published on 30th July 2021
In the last few years, the exploration of new thermoelectric materials with low-toxicity, earth-abundance, and high-efficiency has become essential. Following this trend, sustainable, easily scalable, and cost-effective fabrication methods, such as electrochemical deposition, are also desirable. In this work, the Pourbaix diagram of silver–selenium–water was developed to find an adequate pH and reduction potential for the electrodeposition of stable silver selenide. Based on this diagram, a solution without the incorporation of additives was developed. Silver selenide films were electrodeposited at different reduction potentials, and after the deposition, the compositional, morphological, and structural characterizations of the silver selenide thin films were analyzed. The thermoelectric properties of the electrodeposited silver selenide films were measured at room temperature. The maximum power factor was found for the films grown at 0.071 V with a value of 3421 ± 705 μW m−1 K−2 and a thermal conductivity of 0.56 ± 0.06 W m−1 K−1. Even better, when it can be done by employing a technique that is easily scalable to an industrial level and allows large areas to be obtained, such as electrodeposition. Finally, films with similar properties were deposited on a flexible Kapton substrate. A unileg thermoelectric power generator was produced with maximum output powers of 14.7, 29.4, and 37 μW under temperature differences of 10, 15, and 19 K, respectively; and maximum power densities of 55.1, 110.1, and 138.6 mW m−2 under temperature differences of 10, 15, and 19 K, respectively.
In the last two decades, different strategies have been applied to improve the efficiency of thermoelectric materials, which is related to the figure of merit (zT). This is defined as zT = (σS2/k)T, where σ is the electrical conductivity, S is the Seebeck coefficient, k is the thermal conductivity, and T is the absolute temperature.6 These strategies consist either of enhancing the thermoelectric properties of conventional materials or in the development of new materials. Intensive work was undertaken in the development of novel techniques and approaches to uncoupling the electrical conductivity, Seebeck coefficient, and thermal conductivity, to increase the figure of merit. One of the most studied approaches, which has given good results, is nanostructuration.7–9 Moreover, great efforts have also been applied to understanding the thermal and electrical transport mechanisms to enhance the thermoelectric properties of conventional thermoelectric materials, such as Bi2Te310,11 and PbTe.12 However, these materials, which are widely used in industry, must be replaced with new materials due to their scarcity and high toxicity. In recent years, the development of new materials that can substitute telluride compounds with high-efficiency, low-toxicity, and earth-abundant elements has become fundamental.13,14 In this sense, selenides are perfect candidates for such a purpose, due to their high-power factor and low thermal conductivity. In addition, selenides are cheaper and more abundant on earth than tellurides.15 In the last few years, world-record thermoelectric figures of merit were reported for SnSe,16,17 Cu2Se,18 and Ag2Se,19 and these values were 2.6 at 923 K along the b-axis, 1.5 at 1000 K, and 1.2 at room temperature, respectively.
In the last decade, different studies on silver selenide bulk materials20–23 have been reported. However, two years ago, silver selenide thin films were developed.19,24,25 These materials were obtained using spark plasma sintering (SPS),22,26 a ball-milling process,27 microwave-assisted solution,28 hot pressing,21 pulsed hybrid reactive magnetron sputtering (PHRMS),19 or vacuum-assisted infiltration.24,25 The thermoelectric properties of silver selenide grown by these techniques were studied. Silver selenide has the highest electrical conductivity in bulk, with values of approximately 2000–3000 S cm−1,20,21,23 while in films the values are between 750 and 1000 S cm−1.19,24,25 The Seebeck coefficient of silver selenide is slightly higher for films, with values of between −180 and −120 μV K−1 for thin films19 and bulk materials,21 respectively. In addition, the thermal conductivity is very low in both cases, with values of between 1.521 and 0.64 W m−1 K−1.19 These thermoelectric properties result in a figure of merit of between 0.626 and 1.219) at room temperature. These high values of zT at room temperature make this material of interest for room-temperature applications. Table 1 shows the growth method and the thermoelectric properties of silver selenide in studies where zT was estimated.
Growth method | Electrical conductivity (S cm−1), measurement technique | Seebeck coefficient (μV K−1), measurement technique | Power factor (μW m−1 K−2), measurement technique | Thermal conductivity (W m−1 K−1), measurement technique | zT | References |
---|---|---|---|---|---|---|
Quartz tubes (melt cooled), bulk | 1988, Van der Pauw (in-plane) | −133, differential steady state (in-plane) | 3520 (in-plane) | 1.1, Laser flash (out-of-plane) | 0.96 | 20 |
Ball-milling process, bulk | 1000, Van der Pauw (in-plane) | −150, Van der Pauw (in-plane) | 186–1693 (in-plane) | 0.65–1.1, Laser flash (out-of-plane) | 2.016 × 10−3 K−1 | 27 |
Hydrothermal reaction. SPS, bulk | 1000, four-probe (in-plane) | −120, PMMS, Phys. Prop. Meas. system (out-of-plane) | — | 1, PMMS, Phys. Prop. Meas. system (out-of-plane) | 0.6 (at RT) | 26 |
Ball-milling process, bulk | 3333, ULVAC ZEM-2 (in-plane) | −120, ULVAC ZEM-2(in-plane) | 4900ULVAC ZEM-2(in-plane) | 1.5, Laser flash (out-of-plane) | 1 (at RT) | 21 |
Spark plasma sintering, bulk | 1290, modified thermal expansion equipment (Netzsch, DIL402 C) (in-plane) | −153, modified thermal expansion equipment (Netzsch, DIL402 C) (in-plane) | 3010, modified thermal expansion equipment (Netzsch, DIL402 C) (in-plane) | 1.08, Laser flash (out-of-plane) | 0.84 | 22 |
Sputtering, films | 750, Linseis, LSR-3 (in-plane) | −180, Linseis, LSR-3(in-plane) | 2440 at RT, Linseis, LSR-3 (in-plane) | 0.643, ω-SThM (out-of-plane) | 1.2 at 100 °C | 19 |
For such a purpose, it is essential to produce high-quality silver selenide by inexpensive and industrially scalable techniques. In this sense, electrodeposition is a good candidate. It is also performed at room temperature, so it is compatible with polymeric substrates, it does not require vacuum conditions, and it allows perfect control over the composition, morphology, and crystallographic structure. To date, the thermoelectric properties of electrodeposited silver selenide have not been reported. The few studies found in the literature are focused on the growth and crystallographic structure, and morphological and optical characterization of thin films29–32 or nanostructures,33,34 but not on their thermoelectric properties. In the mentioned studies, to avoid precipitation of the elements, either aqueous solutions with a complexing agent such as EDTA,30,32 SCN−,29 TEA, or TSC34 or organic solvents such as DMSO33 were employed. These additives affect the grain size, purity, and, therefore, the thermoelectric properties.
This study is focused on the electrodeposition of silver selenide films and their thermoelectric properties from aqueous solutions without the use of complexing agents. To find an adequate pH and reduction potential for the electrodeposition, the Pourbaix diagram of silver and selenium was obtained. Then, the compositional, morphological, and structural characterizations of the silver selenide films were analyzed. The thermoelectric properties, electrical conductivity, Seebeck coefficient, and thermal conductivity, of electrodeposited silver selenide films, have been studied for the first time. Moreover, a flexible thermoelectric power generator composed of five single legs of silver selenide was developed, obtaining its maximum output power and maximum power density.
The thermal conductivity was measured out-of-plane using the photoacoustic (PA) method in the silver selenide films deposited on a conductive substrate. To perform the measurements, 80 nm titanium film was deposited as a transductor. The photoacoustic method consists of incident radiation from a modulated fiber-coupled laser (Alphalas) of 980 nm wavelength with an optical power of 260 mW periodically, which heats the Ti film and then the silver selenide alloy. The air in contact with the film's surface expands and contracts in response to the periodic heating, and acts as a thermal piston that generates acoustic waves. The acoustic waves are detected by a microphone (40 BL 1/4′′ CCP pressure type, with a 26 CB, 1/4′′ preamplifier, both from G.R.A.S. Sound & Vibration) and they are compared to the incident modulated signal by a lock-in amplifier. From the phase shift between the signals, the thermal properties can be delineated by applying the multilayer model developed by Hu et al.37 The error of the thermal conductivity measurements from the PA technique is ∼10%.10,38 This technique has previously been used to obtain the thermal conductivity of other films and reference materials.7,38–43
The maximum output power and the maximum power density obtained for the fabricated device have been measured from the open circuit voltage and the short circuit current (Pmax = VOIO/4) using a Keithley 2000 multimeter under temperature differences of 10, 15, and 19 K. The temperature difference was established using a controlled heating source applied to one side of the device, and a container with iced water on the other, and then the same thermal gradient was applied along the five legs of the device.
As shown in Fig. 1, Ag2Se (the Ag–Se region) is stable between pH = 0 and pH = 14, and when an electrode potential of around 0 V vs. NHE is applied. It is important to note that 0 V vs. NHE corresponds to −0.21 V at room temperature for Ag/AgCl. According to the Pourbaix diagram and the selenium stability diagram,47 the general reaction that will occur at low pH is:
2Ag+ + H2SeO3 + 4H3O+ + 6e− → Ag2Se + 7H2O | (1) |
To determine whether Ag2Se can be experimentally obtained without any additive added to the solution, as seems to be indicated by the Pourbaix diagram, cyclic voltammetry was performed in an aqueous solution of 5 Mm H2SeO3 + 5 mM AgNO3 + 0.2 M H2SO4 over the OCP (open circuit potential) = 0.468 V to −0.8 V, 0.8 V, to OCP vs. Ag/AgCl (see Fig. 2a). The scan rate was 10 mV s−1 and the electrochemical experiments were carried out at room temperature.
As Fig. 2a shows, there are three reduction peaks and two oxidation peaks in the cyclic voltammetry. The reduction potential located at 0.4 V corresponds to the reduction of Ag+ to Ag. This was confirmed by depositing at that potential and analyzing the resulting deposit. The reduction peak at around 0.1 V is due to the reduction of Ag+ and H2SeO3 to Ag2Se according to reaction (1). And the last reduction peak around −0.6 V corresponds to the reduction of Ag2Se to elemental silver and H2Se anions. This was confirmed by depositing at that potential, where no deposit was obtained after 2 h. Fig. 2b shows the silver selenide reduction peak in detail. Since the purpose of this study is to obtain silver selenide films, the electrodeposition was performed at different reduction potentials located around the silver selenide reduction peak, as shown in Fig. 2b. The electrodeposition time was 2 h for all the grown films.
Films | Atomic% Se | Atomic% Ag | Formula | Thickness |
---|---|---|---|---|
0.084 V | 35.1 ± 2 | 63.0 ± 2 | Ag1.8Se | 1043 nm |
0.078 V | 32.2 ± 2 | 60.7 ± 2 | Ag1.9Se | 964 nm |
0.071 V | 31.5 ± 2 | 68.2 ± 2 | Ag2.2Se | 860 nm |
0.044 V | 32.8 ± 2 | 67.0 ± 2 | Ag2.1Se | 1035 nm |
−0.015 V | 34.7 ± 2 | 64.1 ± 2 | Ag1.9Se | 1054 nm |
The compositions are Ag1.8Se, Ag1.9Se, Ag2.2Se, Ag2.1Se, and Ag1.9Se for 0.084, 0.078, 0.071, 0.044, and −0.015 V, respectively. All the films presented in this study exhibit a composition close to Ag2Se. The thickness of the films is collected in Table 1, showing values between 860 and 1043 nm.
The morphology of silver selenide films was analyzed using FE-SEM images (see Fig. 3). The morphology of the silver selenide films presents large grains for all cases, but depending on the reduction potential applied, the grain size is different. The grain size was in the ranges 80–650 nm, 110–900 nm, 110–640 nm, 140–500 nm, and 140–525 nm for 0.084, 0.078, 0.071, 0.044, and −0.015 V, respectively. Moreover, the small and bright grains correspond to a higher Ag content, as corroborated by EDX measurements. The films which present these grains were grown at 0.084 and −0.015 V.
Fig. 3 FE-SEM images of silver selenide films grown at different reduction potentials: (a) 0.084 V, (b) 0.078, (c) 0.071, (d) 0.044, and (e) −0.015 V. |
In the studies reported in the literature for electrodeposited Ag2Se films,29–32 the morphology observed is very different from that observed in this work, probably due to the electrodeposition solution. In this study, a solution without any additives was applied. The additives normally change the grain size and then the electrical and thermal conductivities. However, silver selenide thin films grown by sputtering exhibit a similar morphology, but with larger grains of 1–2 μm.19
To study the crystallographic structure of the silver selenide films, XRD measurements were performed. Fig. 4 shows the X-ray diffractograms of the films grown at different reduction potentials. Different peaks can be seen, which can be associated with the components of the substrate: Au (JCPDS 040784) and Si (JCPDS 27-1402), along with others that correspond to orthorhombic β-Ag2Se (JCPDS 01-071-2410) with a space group P212121.
Fig. 4 X-ray diffractograms of silver selenide films grown at different reduction potentials. The orthorhombic β-Ag2Se phase seems to be present in all the films (JCPDS: 01-071-2410). |
Harris texture analysis48 was performed to obtain the degree of preferred orientation quantitatively. The texture coefficient (TC) and the standard deviation (σ), which indicates the deviation intensity of the experimental XRD from published values of JCPDS, equations can be written as:
(2) |
(3) |
Films | Formula | Carrier concentration (cm−3) | Mobility (cm2 V−1 s−1) | Electrical conductivity (S cm−1) | Seebeck coefficient (μV K−1) | Power factor (μW m−1 K−2) |
---|---|---|---|---|---|---|
0.084 V | Ag1.8Se | −1.1 × 1019 ± 0.5 × 1018 | 8.1 × 102 | 1380 ± 69 | −136 ± 14 | 2537 ± 523 |
0.078 V | Ag1.9Se | −1.1 × 1019 ± 0.5 × 1018 | 8.9 × 102 | 1550 ± 78 | −94 ± 9 | 1376 ± 76 |
0.071 V | Ag2.2Se | −2.5 × 1019 ± 0.1 × 1019 | 7.5 × 102 | 3010 ± 15 | −107 ± 11 | 3421 ± 705 |
0.044 V | Ag2.1Se | −1.9 × 1019 ± 0.1 × 1019 | 4.4 × 102 | 1280 ± 69 | −112 ± 11 | 1612 ± 332 |
−0.015 V | Ag1.9Se | −8.7 × 1018 ± 0.4 × 1018 | 9.7 × 102 | 1380 ± 69 | −118 ± 12 | 1916 ± 395 |
The thermoelectric properties, electrical conductivity, Seebeck coefficient, and power factor of selenide films, are shown in Fig. 5 as a function of the reduction potential.
The maximum value of the electrical conductivity, 3010 ± 15 S cm−1, was observed for the film grown at 0.071 V, which has a composition of Ag2.2Se and a thickness of 860 nm, that is, the thinnest film in this study. Moreover, the morphology of this film is the smoothest. The electrical conductivity values are of the same order as those for bulk Ag2Se found in the literature.20–23,26,27 However, for films, the electrical conductivity was found to be lower with values of 750,19 1000,24 and 920 S cm−125 for films grown by sputtering and vacuum-assisted infiltration on nylon substrate. The value of the electrical conductivity obtained in this study is 3 times higher than the highest values measured in films grown by other techniques.
The Seebeck coefficients (see red triangles in Fig. 5) were found to be −136 ± 14, −94 ± 9, −107 ± 11, −112 ± 11, and −118 ± 12 μV K−1 for 0.084, 0.078, 0.071, 0.044, and −0.015 V, respectively. The maximum Seebeck coefficient of −136 ± 14 was observed for the film grown at 0.084 V. The Seebeck coefficient values obtained in this study are similar to the values found in the literature for bulk silver selenide20–23,26 and films.24,25 The maximum value at this magnitude (−180 μV K−1 at RT) found in the literature was obtained in thin films grown by sputtering.19
The power factor is shown by blue stars, presenting a value of 2537 ± 523, 1376 ± 76, 3421 ± 705, 1612 ± 332, and 1916 ± 395 μW m−1 K−2 for 0.084, 0.078, 0.071, 0.044, and −0.015 V, respectively. The maximum value, 3421 ± 705 μW m−1 K−2, is observed for the film grown at 0.071 V, which is again the thinnest film (860 nm) with the composition Ag2.2Se. The maximum power factor reported in the literature was 3520 μW m−1 K−2 at RT for bulk silver selenide bulk material grown using quartz tubes followed by melting,20 this value being similar to the best value obtained in this study. And it was 1.5 times higher than the values for silver selenide films.19,24,25
Regarding the thermal conductivity of the film grown at 0.071 V, it is 0.56 ± 0.06 W m−1 K−1. This thermal conductivity value is a bit lower than the values found in the literature: 0.64 and 1.5 W m−1 K−1, for thin films19 and bulk material,21 respectively. The values of thermal conductivity are similar to the values obtained in other films, taking into account the measurement error, but are much lower than in bulk. This can be explained by the small crystallite size observed in our film, which is in the range of 80–190 nm according to the XRD calculations.
It is important to note that this material can only be used for thermoelectric devices working at room temperature. The presence of silver mobile ions has been observed previously at temperatures higher than 125 °C.19,50
Finally, to study the performance of the electrodeposited Ag2.2Se film as a flexible thermoelectric power generator, Ag2.2Se films were grown on a Kapton substrate to serve as n-type legs. For such a purpose, the electrodeposition conditions were optimized at the reduction potential which presents the highest thermoelectric properties and compact films were obtained. This thermoelectric power generator is composed of five single legs of silver selenide film (see Fig. 6). The legs were interconnected by a gold film deposited by e-beam evaporation using a shadow mask.
Fig. 6 Photograph of the flexible unileg thermoelectric power generator device composed of five electrodeposited silver selenide films connected by e-beam evaporated gold films. |
The maximum powers for the flexible thermoelectric power generator formed of five silver selenide unilegs were approximately 14.7, 29.4, and 37 μW under temperature differences of 10, 15, and 19 K, respectively. Therefore, maximum power densities of 55.1, 110.1, and 138.6 mW m−2 under temperature differences of 10, 15, and 19 K, respectively, were achieved. In our case, a temperature difference of 10–20 K was considered because it is a realistic value for wearable applications. By comparing these results with previous thermoelectric devices based on Ag2Se film fabricated by a screen printing technique in which 13 thermocouples were measured at 30 K of temperature difference,51 it can be seen that our results are very promising. To compare that value with our results, we divided the maximum power density reported by the total number of pairs of legs, obtaining ∼5 mW m−251versus ∼138.6 mW m−2, in our case. This is a very interesting result, even more so, taking into account that a third of the temperature gradient was used in our study.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1se01061a |
This journal is © The Royal Society of Chemistry 2021 |