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Use of biomass for a development of nanocellulose-based biodegradable flexible thin film thermoelectric material
Solar Energy 201 (2020) 21–27
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Use of biomass for a development of nanocellulose-based biodegradable flexible thin film thermoelectric material
T
⁎
N.P. Klochkoa, V.A. Barbashb, K.S. Klepikovaa, V.R. Kopacha, I.I. Tyukhovc, , O.V. Yashchenkob, D.O. Zhadana, S.I. Petrushenkod, S.V. Dukarovd, V.M. Lyubova, A.L. Khrypunovaa National Technical University “Kharkiv Polytechnic Institute”, 2, Kirpichov St., 61002 Kharkiv, Ukraine National Technical University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute”, 37, Prospect Peremohy, 03056 Kyiv, Ukraine c San Jose State University, Department of Mechanical Engineering, One Washington Square, San Jose, CA 95192-0087, USA d Kharkiv National University named after V. N. Karazin, 4, Svobody Square, 61022 Kharkiv, Ukraine a
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Biological solar energy conversion Nanocellulose Biodegradable electronics, copper iodide Thermoelectric material
In this work, we used solar energy converted via photosynthesis into chemical energy of the biomass of the fastgrowing perennial herb Miscanthus × giganteus for the manufacture of nanocellulose (NC) films, which are biodegradable alternative to common petroleum-based polymer substrates used in flexible electronics. To create the NC substrates, we applied an environmentally friendly method of organosolv delignification of plant raw materials carried out at a low temperature and in a relatively short time. Then by means of the low-temperature cheap and scalable method Successive Ionic Layer Adsorption and Reaction (SILAR) we deposited copper iodide (CuI) film of 0.72 µm thickness on both sides of the 12 µm thick NC substrate, and thus obtained light-weight and flexible biodegradable nontoxic thermoelectric material CuI/NC. Crystal structure, morphology, chemical composition, and optical, electrical and thermoelectric properties of the CuI/NC have been researched. Studies have shown that nanostructured p-type semiconductor CuI film in the CuI/NC TE material is quite dense and completely covers the NC surface. It has typical optical direct band gap ≈ 3.0 eV, is single-phase γ-CuI with crystallite sizes in the 19–25 nm range, with moderate dislocation density of (1.6–2.8) × 1015 lines/m2, and tolerable microstrains ε of (4–9) × 10−3 a.u. The determined value of the Seebeck coefficient S is ~228 μV K−1, at that, S is constant in the temperature range 290–335 K. Together with the thermoelectric power factor ≈ 36 μW·m−1·K−2it is favorable for the use of CuI/NC as new thermoelectric material for an in-plane design of biodegradable flexible thin film thermoelectric generator (TEG). At temperature gradient of 50 K, the single p-CuI thermoelectric leg made from CuI/NC strip of 3 cm long and 0.5 cm wide generates open circuit voltage 8.4 mV, short circuit current 0.7 µA and maximum output power 1.5 nW. It corresponds to the output power density 10 µW/m2, and thus confirms the suitability of CuI/NC to obtain electricity by the harvesting the waste environmental heat.
1. Introduction Conversion of solar energy into useful chemical energy of biomass via the process of photosynthesis provides the ability to create diverse cellulose-based materials, which are alternatives to petroleum-based polymers, and therefore they help to alleviate the depletion of nonrenewable natural resources (LaVan and Cha, 2006; Barber, 2007; Zhu et al., 2016; Shak et al., 2018; Fang et al., 2019). According to Barber (2007), photosynthesis produces more than 100 billion tons of dry biomass annually. This was in 2007 equivalent to a hundred times the weight of the total human population and amounted to a mean annual rate of energy storage of approximately 100 TW. It is estimated (LaVan ⁎
and Cha, 2006; Fang et al., 2019) that the harnessing solar power via biological systems may have high benefits in the case of multidisciplinary efforts, especially through nanotechnology approaches, due to the earth abundance of biomass, its renewability, biodegradability, biocompatibility, broad modification capacity, adaptability, and versatile morphology (Jung et al., 2017;Vicente et al., 2018). Today, a re-engineering of cellulose fibers at the nanoscale giving nanocellulose (NC) allows this renewable material to be applied to advanced energy storage systems, biosensors, and in the different electronic and optoelectronic devices. Among them, transparent transistors, light emitting diodes, solar cells, antennas and radiofrequency identification devices, high-performance loudspeakers, and lightweight actuators (LaVan and
Corresponding author. E-mail address: [email protected] (I.I. Tyukhov).
https://doi.org/10.1016/j.solener.2020.02.091 Received 23 January 2020; Received in revised form 23 February 2020; Accepted 26 February 2020 0038-092X/ © 2020 Published by Elsevier Ltd on behalf of International Solar Energy Society.
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solution of NaOH:15-crown-5ether (1:1) or in polyethyleneimine. The all above listed leads to an increase in the cost of production. The presented by Abol-Fotouh et al. (2019) simple TE module composed of six pairs of legs having average Seebeck coefficient 30 mV K−1 and power factor ~20 µW m−1 K−2 each, generated power of 14.5 nW at temperature gradient of 20 K. The thermoelectric-based temperature sensor in (Jung et al., 2017) combines various materials such as poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT:PSS), silver nanoparticles (AgNP) and CNTs to form a thermocouple on the nanocellulose substrate, and it generates a thermoelectric voltage output of 1.7 mV for a temperature difference of 125 K. However, the NC substrates in (Jung et al., 2017) were made from quite expensive material cotton. In this work, we use low-cost plant-derived nanocellulose substrates for a creation of new biodegradable eco-friendly flexible thin film thermoelectric material to convert low-grade waste heat into electricity at near-room temperatures. For this, solar energy converted via photosynthesis into chemical energy of Miscanthus (Miscanthus × giganteus), the fast-growing perennial herb widely cultivated in Europe and the United States for use in electricity and heat generation through combustion, and as feedstock for biofuels. According to Barbash et al. (2019), Miscanthus is an unpretentious, inexpensive fast growing non-wood plant with a high content of cellulose (up to 49.7%). It is quite frost resistant and resistive to diseases. The yield of Miscanthus can reach 10–35 dry tons per hectare annually during 15–20 years. Compared to wood, which requires about 10–12 years to fully mature, Miscanthus requires a maximum of 3 years after planting to reach its peak dry biomass production which further showcases its potential as a viable and cheap source of biomass. For the manufacture of nanocellulose substrates, we use an environmentally friendly method of organosolv delignification of plant raw materials carried out at a low temperature and in a relatively short time, described in (Barbash et al., 2017, Barbash et al., 2018). Since the biodegradable electronic devices, as well as the flexible TE appliances, in particular, thermoelectric generators (TEGs), combine both inorganic and organic functional materials, inorganic dissolvable thin-film materials, e.g. metals and their compounds, must all be necessary elements for the human body and not hazardous to the environment. The non-biodegradable part of the e-waste should be at least non-toxic, and environmentally friendly (Zhou et al., 2013; Li et al., 2018). Accordingly, we deposited copper iodide (CuI) film of 0.72 µm thickness on the NC substrate by means of the low-temperature cheap and scalable method Successive Ionic Layer Adsorption and Reaction (SILAR). SILAR is an alternative to vapour-phase and chemical-precursor techniques, which successfully used to produce a large variety of semiconductor materials (Shishiyanu et al., 2005; Lupan et al., 2008). As indicated by Nicolau (1985), this method is suitable for a growing of polycrystalline thin films of water-insoluble ionic or ionocovalent compounds of the CmAn type by heterogeneous chemical reaction at the solid-solution interface between adsorbed Cn+ cations and Am− anions. This is intended to allow ion-by-ion growth of the compound film via sequential addition of individual atomic layers as it involves an alternate immersion of the substrate into solution containing a soluble salt of the cation of the compound to be grown and then into solution containing a soluble salt of the anion. The substrate supporting the growing film rinsed in high-purity water after each immersion (Niesen, 2002). Thus we obtained light-weight and flexible biodegradable thermoelectric material CuI/NC. Notably, that this thermoelectric material is exclusively nontoxic, because in addition to macrobiotic elements of biomass it consists of Cu, I, and of S traces, which all are essential for life (Zoroddu et al., 2019). Here we analyze crystal structure, morphology, chemical composition, and optical, electrical and thermoelectric properties of CuI/NC. With the aim to demonstrate the potential of CuI/NC as a new thermoelectric material for an in-plane design of biodegradable flexible thin film TEG, we explore its possibility to obtain electricity under conditions of temperature gradients from 5
Cha, 2006; Zheng et al., 2013; Huang et al., 2013; Zhou et al., 2013; Gomez and Steckl, 2015; Hoeng et al., 2016; Zhu et al., 2016; Tan et al., 2016; Feig et al., 2018; Li et al., 2018; Fang et al., 2019; Abol-Fotouh et al., 2019; Zhang and Park, 2019). In the last few years, the cellulosic biopolymer-based green electronics is supplemented by lightweight, portable, flexible NC-based power generators to provide energy to wearable electronics through harvesting mechanical energy in triboelectric and piezoelectric appliances (Zhu et al., 2016; Jung et al., 2017; Fang et al., 2019) or by using of waste heat via thermoelectric (TE) devices (Jung et al., 2017; Zhang and Park 2019; Zhao et al., 2019; Abol-Fotouh et al., 2019). Huge ecological advantage of the NC-based electronic and thermoelectric devices is their inherent biodegradability. As these devices have become ubiquitous in modern society, and are prevalent in every facet of human activities, and the lifetime of electronics get shorter and shorter, the pressure on electronic waste (e-waste) management systems is mounting with no abate in sight. This poses a growing ecological problem, and an alternative to traditional electronics is biodegradable electronics as the most viable replacement to address the issue of uncontrollable e-waste to reduce the environmental footprint of devices (Irimia-Vladu et al., 2012; Tan et al., 2016; Zhu et al., 2016; Feig et al., 2018; Abol-Fotouh et al., 2019). In accordance with Abol-Fotouh et al. (2019), Feig et al. (2018), biodegradable electronic and thermoelectric devices can be broken down into smaller non-toxic constituent pieces at biologically benign or physiological conditions, and thereafter they must dissolve, resorb, or physically disappear into physiological or environmental solutions, partially or completely, at controlled rates after the expecting working period without leaving a permanent mark. More precisely, the quality of biodegradability has been defined by the EU harmonized standard EN 13432:2000 “Packaging: requirements for packaging recoverable through composting and biodegradation” as the property where a minimum of 90% of the mass can be converted into water, carbon dioxide, and biomass under defined temperature, humidity, and oxygen conditions within six months, in the presence of fungi or microorganisms. As indicated by Tan et al. (2016), the substrate in electronic device serves as a foundation upon which functional layers such as semiconductor and metal thin films are deposited. As the volume of substrate is larger than that of any layer, it generates more ewaste in general, compared to other layers. Hence, replacing conventional substrates with biodegradable ones tremendously reduces the ewaste problem. As nanocellulose is able to degrade slowly in the presence of naturally occurring fungi (Irimia-Vladu et al., 2012; Tan et al., 2016; Zhu et al., 2016; Feig et al., 2018; Abol-Fotouh et al., 2019), it is suggested that NC substrates can be used in the multiple consumer electronic and thermoelectric devices. The most common type of plant fibers used for the nanocellulose fabrication is a wood pulp (Zhu et al., 2016; Shak et al., 2018). So, in spite of the obvious advantages of nanocellulose, the rather high cost of the wood-derived NC substrates compared with classical petroleumbased polymer substrate of flexible electronics polyethylene terephthalate (PET) (Hoeng et al., 2016) serves as a limiting factor for the wide distribution of the NC-based electronic devices, especially for countries that do not have large stocks of free wood. Authors AbolFotouh et al. (2019) report on the fabrication of a thermoelectric module from farmed thermoelectric paper on the base of bacterial nanocellulose films. Highly dispersed single-wall carbon nanotubes (CNTs) were embedded as percolating networks into produced by microorganisms nanocellulose polymer matrixes representing intertwined networks of fibres with fine porosity composed of fibrils with diameters of tens of nanometers to obtain composites CNTs/NC. The generation of composites CNTs/NC regulated directly in situ during the biosynthesis, however, the farming of this TE material carried out in artificial conditions, in the dark at 30 °C in a special incubator. Then, the grown CNTs/NC composites were purified and doped chemically. For example, for the n-doping of CNTs/NC composites authors Abol-Fotouh et al. (2019) used solution processing in trimethylammonium hydroxide, in 22
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the NC surface with I− ions to obtain CuI. After this immersion, the final stage of one SILAR cycle was a rinsing in the distilled water for 10 s. Such SILAR cycles for the CuI deposition onto NC repeated 40 times. The thickness of the CuI film was determined gravimetrically, taking for a calculation the bulk CuI density 5.67 g/cm3. Surface morphology of NC substrate and CuI/NC thermoelectric material was observed by scanning electron microscopy (SEM) in a secondary electron mode. The SEM instrument “Tescan Vega 3 LMH” operated at an accelerating voltage 30 kV without the use of additional conductive coatings when scanning conductive CuI/NC thermoelectric material. However, for morphology studies of the pure NC substrate by SEM we used thin Cr film (~10–15 nm thick) as conductive coating, which evaporated in vacuum at 10−6 Torr residual gas pressure immediately before SEM research. Chemical analysis of the pure NC and of the CuI/NC thermoelectric material was carried out by X-ray fluorescence (XRF) microanalysis using an energy dispersive spectrometry (EDS) system “Bruker XFlash 5010”. Energy dispersion spectra were taken from the 50 × 50 μm areas. Quantification of the spectra was carried out in the self-calibrating detector mode. Optical properties of the thermoelectric material CuI/NC in UV–visible-near IR range (at wavelengths (λ) 300–1100 nm) studied with an “SF-2000” spectrophotometer equipped with “SFO-2000” specular and diffuse reflection attachment. We recorded optical transmission To(λ) spectrum to obtain total optical absorption A(λ) = −lgTo (λ) of the optical medium consisted of 0.72 µm CuI, interface, 12 µm NC, interface and 0.72 µm CuI. Optical spectra of specular reflectance Rs(λ) and diffuse reflectance Rd(λ) for the CuI coating were studied using a reflection attachment “SFO-2000” at light incidence angle ϑ = 8° relative to normal to the surface. The light scattering in the CuI film, haze factor (Hf), was calculated in accordance with Klochko et al. (2019a) as the ratio of the diffuse reflectance Rd to the total reflectance R, where R is sum of Rs and Rd. That is, Hf = Rd/R. Optical band gap Eg for direct allowed transitions in the CuI film was determined as described in Klochko et al. (2017) from the Kubelka-Munk function:
to 50 °C at near-room temperatures, for example, by the harvesting the waste environmental heat. 2. Experimental procedures The manufacture of nanocellulose substrates was carried out in accordance with Barbash et al. (2019). As feedstock we used the biomass from the second-year miscanthus herb planted in a field in the Kyiv's region. Miscanthus stalks were exempted from leaves and nodes, crushed into particles of 5–7 mm in size and placed in a desiccator to maintain constant moisture and chemical composition. Cooking of Miscanthus pulp carried out in three stages. At the first stage, a delignification of the feedstock carried out. With this, the shredded Miscanthus stalks were treated for 30–240 min in a hot (95 ± 2 °C) solution, contained glacial acetic acid and 35% hydrogen peroxide at volume ratio of 7:3, when the liquid to solid ratio was 10:1. Thus, we obtained an organosolv Miscanthus pulp (OMP). At the second stage, we carried out an alkaline treatment of OMP by hot (95 ± 2 °C) aqueous solution contained 7% NaOH at the liquid to solid ratio 12:1 for 15–240 min. The OMP after alkaline treatment was washed with hot distilled water. Then, at the third stage, the OMP was hydrolyzed by means of 43 and 50% sulfuric acids at the liquid to solid ratio 10:1, at temperatures of 40 and 60 °C for 30–90 min to obtain a suspension of hydrolyzed nanocellulose in accordance with Barbash et al. (2019). Then this suspension was rinsed with distilled water three times by means of centrifugation at 4000 rev/min and subsequent dialysis, until it reached pH 7. Ultrasound treatment of 0.6% nanocellulose suspension was performed in an ice bath using an ultrasound disintegrator UZDN-A (SELMI, Ukraine) at 22 kHz during 30–60 min to prevent overheating during treatment. Eventually, the suspension turned into a homogenous gel-like dispersion with nanoparticle diameters in the 5–20 nm range. The required amount of the prepared dispersion was poured into Petri dishes and dried at room temperature in air to obtain a 12-μm thick nanocellulose substrate. According to Barbash et al. (2019), the obtained nanocellulose substrates have density up to 1.6 g/cm3, transparency up to 82%, and crystallinity up to 76.5%. (Barbash et al., 2019). Fig. 1(on the left) shows photo of the NC substrate. To create biodegradable thermoelectric material CuI/NC with NC substrate coated at both sides by 0.72 µm thick CuI films presented in Fig. 1(on the right), we employed CuI deposition from aqueous solutions at room temperature through SILAR method described earlier in Klochko et al. (2018). We used aqueous solution containing 0.1 M CuSO4 and 0.1 M Na2S2O3 as a cationic precursor, into which NC substrate immersed for 20 s. Then, the substrate rinsed in distilled water for 10 s. After that, the nanocellulose substrate was immersed for 20 s into aqueous NaI solution (anionic precursor), which concentration was 0.075 M to carry out the reaction of strongly adsorbed Cu+ ions on
F (R) =
(1 − Rd )2 2Rd
(1) 2
As shown in Klochko et al. (2017), a plot of (F(R)·hν) vs hν yields a direct band gap value Eg of the CuI by extrapolating of (F(R)·hν)2 linear part on hν. To analyze crystal structure of CuI thin films in the thermoelectric material CuI/NC, we recorded X-ray diffraction (XRD) patterns by a “DRON-4” diffractometer with Bragg–Brentano focusing (θ–2θ). The crystalline phases were identified by comparing the experimental diffraction patterns with the reference database JCPDS by using PCPDFWIN v.1.30 software. The experimental interplanar d-spacing were calculated using the Bragg’s equation (Palatnik,1983; Tsybulya and Cherepanova, 2008):
nλ = 2d sin θ .
(2)
Average crystallite size D of CuI was determined from the X-ray line broadening method using the Scherer’s formula as in Klochko et al. (2019a); Klochko et al. (2019b):
D = (0. 9·λ )/(β ·cosθ),
(3)
Calculation of the CuI lattice parameter a was performed from the positions of the X-ray diffractions by the Nelson–Reilly graphical extrapolation method and refined using the least-squares method by UnitCell software in accordance with Palatnik (1983); Tsybulya and Cherepanova (2008). Crystal lattice microstrains we obtained from the relation ɛ = Δd/d (where d is the crystal interplanar spacing according to JCPDS, and Δd is the difference between the corresponding experimental and reference interplanar spacing), and dislocation density evaluated according to Dongol et al. (2015) through 1/D2 as in Klochko et al. (2019a). The resistivity ρ of CuI film in the TE material CuI/NC was measured
Fig. 1. Images of the pure nanocellulose substrate NC on the background of National Technical University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute” logos (on the left) and of the thermoelectric material CuI/NC with nanocellulose substrate coated at both sides by 0.72 µm thick CuI films on the background of National Technical University “Kharkiv Polytechnic Institute” logo (on the right). 23
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at near-room temperatures in the T range of 293–327 K in accordance with Chen et al. (2018) by using a four-point collinear probe resistivity method. The resistivity calculated as follows:
ρ = (πtδU23)/(I14 ln(2)),
(4)
where U23 is the voltage between the second and third probe; I14 is the current between the first and fourth probes; δ is a correction factor for the accounting the ratio of the distance between the probes and the size of the substrate; πδ/ln(2) ≈ 4.45. To assess the room-temperature thermoelectric performances of the obtained CuI films, the in-plane Seebeck coefficients S at the 293–330 K temperature range measured as thermoelectric voltages ΔU induced in response to the temperature gradients ΔT along the TE material CuI/ NC. The distance between hot and cold probes in the form of gold rings was 2.3 cm. Then, the thermoelectric power factor p for the thermoelectric material CuI/NC was calculated as S2/ρ, pursuant to Faustino et al. (2018). In accordance with Faustino et al. (2018), output voltage Vout and output power Pout of the thermoelectric thin film leg in the form of CuI/NC strip of 3 cm long and 0.5 cm wide obtained as function of output currents Iout measured at different load resistances Rload for several temperature differences in the ΔT = 5–50 K range. From Vout plotted against Iout we obtained values of open circuit voltage Voc at load resistance Rload = ∞, and short circuit current Isc, when Rload = 0. According to Yang et al. (2017); Faustino et al. (2018), the open circuit voltage Voc for such single-leg thin film thermoelectric module was described by the equation Voc = SΔT, hence, the output power Pout can be calculated using the equation (Yang et al., 2017; Faustino et al.,2018):
Pout = (S ΔT )2Rload /(Rload + Rint )2
Fig. 2. (a) – SEM images of thermoelectric material CuI/NC and nanocellulose substrate NC (insets) obtained at two different magnifications; (b) – XRF spectra of nanocellulose substrate NC and thermoelectric material CuI/NC. Table 1 X-ray fluorescence spectroscopy data for nanocellulose substrate NC and thermoelectric material CuI/NC.
where Rint is the internal resistance of the single-leg thermoelectric module. As the maximum output power Pmax is obtained when Rload = Rint (Faustino et al.,2018), thus, Rint can be described as:
Rint = Voc2 /4Pmax .
Element
NC at. (%)
CuI/NC at. (%)
C O Cu S I
42 58 – <1 –
52 22 14 <1 12
(5)
(6)
Accordingly, maximum output power per unit area, i.e. output power density P*max (in nW/m2) of the single-leg thermoelectric module was evaluated for the obtained p-type thermoleg CuI/NC as ∗ Pmax = Pmax /(0.5 × 3)·10−4 . 3. Results and discussion Fig. 2(a) shows top view SEM images of the CuI surface in the thermoelectric material CuI/NC and of the pure nanocellulose substrate NC in insets. It is seen, that CuI film is quite dense, but it is nanostructured and rough compared to the smooth NC substrate. Both XRF spectra in Fig. 2(b) contain C, O and traces of sulfur. On the one hand, NC contains eSO3− functional groups, as it obtained through sulphuric acid hydrolysis (Shak et al., 2018). On the other hand, CuI film contains sulfur from the chemically unstable compound sodium thiosulfate Na2S2O3 in the cationic precursor that is typical for CuI films fabricated on different substrates by the SILAR method using such solutions (Klochko et al., 2018; Klochko et al., 2019a; Klochko et al., 2019b). Note that Cr in the XRF spectrum of NC belongs to the electrically conductive chromium film and is not a part of nanocellulose substrate. Chemical compositions of NC and TE material CuI/NC obtained by the EDS microanalysis presented in Table 1. The rather high content of copper and iodine in the CuI/NC against the background of C and O from the NC substrate is another confirmation of the density of the thin (0.72 µm) copper iodide film, and a full NC surface coating with CuI film in this TE material. Probably, it originates owing to the good wettability of NC in the aqueous solutions, in which CuI film was obtained by the SILAR method, due to hydrophilic functional groups in the nanocellulose (Shak et al., 2018). In this case, the hydrophilicity of the nanocellulose is a significant advantage of this material as
Fig. 3. (a) – Photo and XRD pattern of the TE material CuI/NC; (b) – its optical properties: transmittance and absorbance spectra, To(λ) and A(λ) respectively, total reflectance spectrum R(λ), haze factor spectrum Hf(λ), and plot for the determining of optical band gap Eg of the CuI film in the TE material CuI/NC by using the Kubelka-Munk function.
compared to organic polymers derived from oil. XRD pattern in Fig. 3(a) demonstrates eight X-ray diffraction peaks. Their analysis has revealed that CuI film in TE material CuI/NC is 24
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PEDOT:PSS, AgNP and CNTs (Jung et al.,2017; Abol-Fotouh et al., 2019; Zhao et al., 2019), and even exceeds the S values for the different thin-film composites of such well-known inorganic TE material as Bi2Te3 (Zhao et al., 2019). The positive sign of S in Fig. 4(a) indicates the p-type conductivity of the copper iodide film. The temperature dependence of the CuI resistivity ρ(T) in Fig. 4(b) corresponds to the transport of the charge carriers (holes) via the localized states according to the nearest neighboring hopping model for the nanostructured copper iodide thin films described by Kaushik et al. (2017). The ρ values are in the range of (1.5–1.7)·10−1 Ω·cm. Fig. 4(c) shows temperature dependence of the thermoelectric power factor p(T) = S2/ρ(T) for TE material CuI/NC. As it seen, p grows with increasing temperature and reaches value 36 μW·m−1·K−2 at T = 330 K. Thus, the thermoelectric power factors of the TE material CuI/NC are higher than that of the best examples bacterial nanocellulose films with embedded highly dispersed CNT networks (p ≈ 20 μW·m−1·K−2) in Abol-Fotouh et al. (2019). Moreover, p values of CuI/NC exceed p = 25.5 μW·m−1·K−2 of the Bi2Te3/bacterial cellulose nanofiber in Zhao et al. (2019). Plots of the output voltages and output powers versus output currents obtained for the strip with (0.5 × 3) cm2 area of the TE material CuI/NC as single p-CuI thermoelectric leg in Fig. 4(d) show output thermoelectric characteristics of CuI/NC. According to the graphs of Vout vs. Iout and Pout vs. Iout in Fig. 4(d), the output parameters of the single p-CuI thermoelectric leg naturally increase with the increase of the temperature difference between the cold and warm edges of the CuI/NC strip. At temperature gradient of 50 K, the single p-CuI thermoelectric leg made from CuI/NC generates Voc = 8.4 mV, Isc = 0.7 µA and Pmax = 1.5 nW at Rload = Rint = 11.8 kΩ. It corresponds to the power density P*max = 10 µW/m2. These values are even better than we obtained (Klochko et al., 2018) for the same CuI films deposited via SILAR on glass substrates. It must be borne in mind that Vout generated by a single p-type TE leg is always very low, therefore, to achieve high output voltage and power, TEGs are typically made of dozens, or even hundreds, of TE couples (Du et al., 2018). The output thermoelectric data obtained in this work have the prospect of being improved by designing TEG on the nanocellulose substrate using a large number of pn thermocouples by connecting CuI/NC strips with wires of n-type thermoelectric material.
single-phase and polycrystalline with cubic copper iodide crystal structure (zincblende, γ-CuI, JCPDS #06-0246). Calculations of the CuI film structural parameters show, that the average crystallite sizes D are in the 19–25 nm range with moderate dislocation density of (1.6–2.8) × 1015 lines/m2 and tolerable microstrains ε of (4–9) × 10−3 a.u. The CuI lattice parameter a = 6.058 ± 0.005 Å according to the Nelson–Reilly graphical extrapolation, and a = 6.067 ± 0.005 Å using the least-squares method, which is slightly higher than a = 6.051 Å in JCPDS #06-0246. Fig. 3(b) shows spectra of the optical transmittance To and absorbance A of the TE material CuI/NC in the UV–Vis-NIR range, which are typical for CuI films deposited via SILAR method onto different transparent substrates (Klochko et al., 2017; Klochko et al., 2018; Klochko et al., 2019a; Klochko et al., 2019b). Thermoelectric material CuI/NC is translucent in the visible range, its maximum transparency To equals 7%. The To(λ) spectrum in Fig. 3(b) does not contain any interference extremes, which indicates that the level of irregularities on the CuI surface is commensurable with the visible wavelengths. The reflection spectrum in Fig. 3(b) confirms the significant reflectance in the entire visible range. According to the Haze factor spectrum in Fig. 3(b), the reflection is predominantly diffuse. The band gap Eg for direct optical transitions in the SILAR deposited CuI films Eg ≈ 3.0 eV is near the value 2.95–3.1 eV measured for bulk CuI at near-room temperature (Grundmann et al., 2013; Kneiß et al., 2018). Electrical and thermoelectric properties of the obtained flexible thin-film TE material were studied using an experimental sample CuI/ NC in the form of a thin-film strip 3 cm long and 0.5 cm wide, consisted of 12 µm thick NC substrate coated by 0.72 µm thick CuI films at both sides. It is seen in Fig. 4(a) a plot of the thermoelectric voltages induced in response to the temperature gradients ΔT along the CuI/NC thermoelectric strip. The determined value of the Seebeck coefficient S is ~228 μV K−1, at that, S is constant in the temperature range 290–335 K, which is favorable for the use of this material CuI/NC in TEG. Note, that this S value more than an order of magnitude higher than the Seebeck coefficients that were measured earlier for the nanocellulose-derived organic and composite TE materials contained
4. Conclusions In this work, we used conversion of solar energy into chemical energy of biomass of fast-growing perennial herb Miscanthus to fabricate via an environmentally friendly method of organosolv delignification low-cost nanocellulose films used as substrates for the creation of new biodegradable thin film thermoelectric material. For this, we deposited CuI film of 0.72 µm thickness on the 12 µm thick NC substrate by means of the low-temperature cheap and scalable SILAR method and thus obtained light-weight and flexible biodegradable thermoelectric material CuI/NC. Notably, that this thermoelectric material is exclusively nontoxic, because in addition to macrobiotic elements of biomass it consists of Cu, I, and of S traces, which all are essential for life. By means studying the surface morphology, crystal structure, chemical composition, optical, electrical and thermoelectric properties of CuI/NC we found out that nanostructured p-type semiconductor CuI film in the CuI/NC TE material is quite dense and completely covers the NC surface. It has typical optical direct band gap ≈ 3.0 eV, is single-phase γCuI with crystallite sizes in the 19–25 nm range, with moderate dislocation density of (1.6–2.8) × 1015 lines/m2, and tolerable microstrains ε of (4–9) × 10−3 a.u. The determined value of the Seebeck coefficient S is ~228 μV K−1 more than an order of magnitude higher than the Seebeck coefficients that were measured earlier for the nanocellulose-derived organic and composite TE materials contained PEDOT:PSS, AgNP and CNTs (Jung et al.,2017; Abol-Fotouh et al., 2019; Zhao et al., 2019), and even exceeds the S values for the different
Fig. 4. (a) – Thermoelectric voltage induced in response to the temperature gradient ΔT along the CuI/NC thermoelectric strip; (b) – temperature dependence of the CuI resistivity ρ; (c) – temperature dependence of the thermoelectric power factor p = S2/ρ for TE material CuI/NC; (d, e) – output voltage Vout (solid symbols) and output power Pout (opened symbols) for some temperature gradients ΔT versus output current Iout obtained for (0.5 × 3) cm2 strip of the TE material CuI/NC. 25
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thin-film composites of such well-known inorganic TE material as Bi2Te3 (Zhao et al., 2019). At that, S is constant in the temperature range 290–335 K, which is favorable for the use of CuI/NC as new thermoelectric material for an in-plane design of biodegradable flexible thin film TEG. The thermoelectric power factor of CuI/NC p ≈ 36 μW·m−1·K−2 is higher than that of the best examples bacterial nanocellulose films with embedded highly dispersed CNT networks (p ≈ 20 μW·m−1·K−2) in Abol-Fotouh et al. (2019). Moreover, p values of CuI/NC exceed p = 25.5 μW·m−1·K−2 of the Bi2Te3/bacterial cellulose nanofiber in Zhao et al. (2019). At temperature gradient of 50 K, the single p-CuI thermoelectric leg made from CuI/NC strip of 3 cm long and 0.5 cm wide generates Voc = 8.4 mV, Isc = 0.7 µA and Pmax = 1.5 nW. It corresponds to the power density P*max = 10 µW/m2. So, by virtue of the complex of physical and chemical properties, the developed eco-friendly biodegradable flexible thin film thermoelectric material can be effectively used for the conversion of low-grade waste heat into electricity at near-room temperatures. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors gratefully acknowledge the financial support of Ministry of Education and Science of Ukraine under project number М 5487. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solener.2020.02.091. References Abol-Fotouh, D., Dörling, B., Zapata-Arteaga, O., Rodríguez-Martínez, X., Gómez, A., Reparaz, J.S., Laromaine, A., Roig, A., Campoy-Quiles, M., 2019. Farming thermoelectric paper. Energy Environ. Sci. 12, 716–725. https://doi.org/10.1039/ c8ee03112f. Barbash, V.A., Yashchenko, O.V., Kedrovska, A., 2017. Preparation and properties of nanocellulose from peracetic flax pulp. J. Sci. Res. Rep. 16 (1), 1–10. https://doi.org/ 10.9734/JSRR/2017/36571. Barbash, V.A., Yashchenko, O.V., Opolsky, V.O., 2018. Effect of hydrolysis conditions of organosolv pulp from kenaf fibers on the physicochemical properties of the obtained nanocellulose. Theor. Exp. Chem. 54 (3), 193–198 0040-5760/18/5403-0193. Barbash, V.A., Yashchenko, O.V., Vasylieva, O.A., 2019. Preparation and properties of nanocellulose from miscanthus x giganteus. ID 3241968-1–ID 3241968-8. J. Nanomater. 2019. https://doi.org/10.1155/2019/3241968. Barber, J., 2007. Biological solar energy. Phil. Trans. R. Soc. A. 365 (1853), 1007–1023. https://doi.org/10.1098/rsta.2006.1962. Chen, X., Dai, W., Wu, T., Luo, W., Yang, J., Jiang, W., Wang, L., 2018. Thin film thermoelectric materials: classification, characterization, and potential for wearable applications. 244-1–244-16. Coating 8 (7). https://doi.org/10.3390/coatings8070244. Dongol, M., El-Denglawey, A., Abd El Sadek, M.S., Yahia, I.S., 2015. Thermal annealing effect on the structural and the optical properties of nano CdTe films. Optik 126, 1352–1357. https://doi.org/10.1016/j.ijleo.2015.04.048. Du, Y., Xu, J., Paul, B., Eklund, P., 2018. Flexible thermoelectric materials and devices. Appl. Mater. Today 12, 366–388. https://doi.org/10.1016/j.apmt.2018.07.004. Fang, Z., Hou, G., Chen, C., Hu, L., 2019. Nanocellulose-based films and their emerging applications. 100764-1–100764-11. Curr. Opin. Solid State Mater. Sci. 23. https:// doi.org/10.1016/j.cossms.2019.07.003. Faustino, B.M.M., Gomes, D., Faria, J., Juntunen, T., Gaspar, G., Bianchi, C., Almeida, A., Marques, A., Tittonen, I., Ferreira, I., 2018. CuI p-type thin films for highly transparent thermoelectric p-n modules. 6867-1−6867-10. Sci. Rep. 8. https://doi.org/ 10.1038/s41598-018-25106-3. Feig, V.R., Tran, H., Bao, Z., 2018. Biodegradable polymeric materials in degradable electronic devices. ACS Cent. Sci. 4 (3), 337–348. https://doi.org/10.1021/ acscentsci.7b00595. Gomez, E.F., Steckl, A.J., 2015. Improved performance of OLEDs on cellulose/epoxy substrate using adenine as a hole injection layer. ACS Photonics 2 (3), 439–445. https://doi.org/10.1021/ph500481c. Grundmann, M., Schein, F.-L., Lorenz, M., Böntgen, T., Lenzner, J., Wenckstern, H., 2013.
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Use of biomass for a development of nanocellulose-based biodegradable flexible thin film thermoelectric material
T
⁎
N.P. Klochkoa, V.A. Barbashb, K.S. Klepikovaa, V.R. Kopacha, I.I. Tyukhovc, , O.V. Yashchenkob, D.O. Zhadana, S.I. Petrushenkod, S.V. Dukarovd, V.M. Lyubova, A.L. Khrypunovaa National Technical University “Kharkiv Polytechnic Institute”, 2, Kirpichov St., 61002 Kharkiv, Ukraine National Technical University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute”, 37, Prospect Peremohy, 03056 Kyiv, Ukraine c San Jose State University, Department of Mechanical Engineering, One Washington Square, San Jose, CA 95192-0087, USA d Kharkiv National University named after V. N. Karazin, 4, Svobody Square, 61022 Kharkiv, Ukraine a
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Biological solar energy conversion Nanocellulose Biodegradable electronics, copper iodide Thermoelectric material
In this work, we used solar energy converted via photosynthesis into chemical energy of the biomass of the fastgrowing perennial herb Miscanthus × giganteus for the manufacture of nanocellulose (NC) films, which are biodegradable alternative to common petroleum-based polymer substrates used in flexible electronics. To create the NC substrates, we applied an environmentally friendly method of organosolv delignification of plant raw materials carried out at a low temperature and in a relatively short time. Then by means of the low-temperature cheap and scalable method Successive Ionic Layer Adsorption and Reaction (SILAR) we deposited copper iodide (CuI) film of 0.72 µm thickness on both sides of the 12 µm thick NC substrate, and thus obtained light-weight and flexible biodegradable nontoxic thermoelectric material CuI/NC. Crystal structure, morphology, chemical composition, and optical, electrical and thermoelectric properties of the CuI/NC have been researched. Studies have shown that nanostructured p-type semiconductor CuI film in the CuI/NC TE material is quite dense and completely covers the NC surface. It has typical optical direct band gap ≈ 3.0 eV, is single-phase γ-CuI with crystallite sizes in the 19–25 nm range, with moderate dislocation density of (1.6–2.8) × 1015 lines/m2, and tolerable microstrains ε of (4–9) × 10−3 a.u. The determined value of the Seebeck coefficient S is ~228 μV K−1, at that, S is constant in the temperature range 290–335 K. Together with the thermoelectric power factor ≈ 36 μW·m−1·K−2it is favorable for the use of CuI/NC as new thermoelectric material for an in-plane design of biodegradable flexible thin film thermoelectric generator (TEG). At temperature gradient of 50 K, the single p-CuI thermoelectric leg made from CuI/NC strip of 3 cm long and 0.5 cm wide generates open circuit voltage 8.4 mV, short circuit current 0.7 µA and maximum output power 1.5 nW. It corresponds to the output power density 10 µW/m2, and thus confirms the suitability of CuI/NC to obtain electricity by the harvesting the waste environmental heat.
1. Introduction Conversion of solar energy into useful chemical energy of biomass via the process of photosynthesis provides the ability to create diverse cellulose-based materials, which are alternatives to petroleum-based polymers, and therefore they help to alleviate the depletion of nonrenewable natural resources (LaVan and Cha, 2006; Barber, 2007; Zhu et al., 2016; Shak et al., 2018; Fang et al., 2019). According to Barber (2007), photosynthesis produces more than 100 billion tons of dry biomass annually. This was in 2007 equivalent to a hundred times the weight of the total human population and amounted to a mean annual rate of energy storage of approximately 100 TW. It is estimated (LaVan ⁎
and Cha, 2006; Fang et al., 2019) that the harnessing solar power via biological systems may have high benefits in the case of multidisciplinary efforts, especially through nanotechnology approaches, due to the earth abundance of biomass, its renewability, biodegradability, biocompatibility, broad modification capacity, adaptability, and versatile morphology (Jung et al., 2017;Vicente et al., 2018). Today, a re-engineering of cellulose fibers at the nanoscale giving nanocellulose (NC) allows this renewable material to be applied to advanced energy storage systems, biosensors, and in the different electronic and optoelectronic devices. Among them, transparent transistors, light emitting diodes, solar cells, antennas and radiofrequency identification devices, high-performance loudspeakers, and lightweight actuators (LaVan and
Corresponding author. E-mail address: [email protected] (I.I. Tyukhov).
https://doi.org/10.1016/j.solener.2020.02.091 Received 23 January 2020; Received in revised form 23 February 2020; Accepted 26 February 2020 0038-092X/ © 2020 Published by Elsevier Ltd on behalf of International Solar Energy Society.
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solution of NaOH:15-crown-5ether (1:1) or in polyethyleneimine. The all above listed leads to an increase in the cost of production. The presented by Abol-Fotouh et al. (2019) simple TE module composed of six pairs of legs having average Seebeck coefficient 30 mV K−1 and power factor ~20 µW m−1 K−2 each, generated power of 14.5 nW at temperature gradient of 20 K. The thermoelectric-based temperature sensor in (Jung et al., 2017) combines various materials such as poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT:PSS), silver nanoparticles (AgNP) and CNTs to form a thermocouple on the nanocellulose substrate, and it generates a thermoelectric voltage output of 1.7 mV for a temperature difference of 125 K. However, the NC substrates in (Jung et al., 2017) were made from quite expensive material cotton. In this work, we use low-cost plant-derived nanocellulose substrates for a creation of new biodegradable eco-friendly flexible thin film thermoelectric material to convert low-grade waste heat into electricity at near-room temperatures. For this, solar energy converted via photosynthesis into chemical energy of Miscanthus (Miscanthus × giganteus), the fast-growing perennial herb widely cultivated in Europe and the United States for use in electricity and heat generation through combustion, and as feedstock for biofuels. According to Barbash et al. (2019), Miscanthus is an unpretentious, inexpensive fast growing non-wood plant with a high content of cellulose (up to 49.7%). It is quite frost resistant and resistive to diseases. The yield of Miscanthus can reach 10–35 dry tons per hectare annually during 15–20 years. Compared to wood, which requires about 10–12 years to fully mature, Miscanthus requires a maximum of 3 years after planting to reach its peak dry biomass production which further showcases its potential as a viable and cheap source of biomass. For the manufacture of nanocellulose substrates, we use an environmentally friendly method of organosolv delignification of plant raw materials carried out at a low temperature and in a relatively short time, described in (Barbash et al., 2017, Barbash et al., 2018). Since the biodegradable electronic devices, as well as the flexible TE appliances, in particular, thermoelectric generators (TEGs), combine both inorganic and organic functional materials, inorganic dissolvable thin-film materials, e.g. metals and their compounds, must all be necessary elements for the human body and not hazardous to the environment. The non-biodegradable part of the e-waste should be at least non-toxic, and environmentally friendly (Zhou et al., 2013; Li et al., 2018). Accordingly, we deposited copper iodide (CuI) film of 0.72 µm thickness on the NC substrate by means of the low-temperature cheap and scalable method Successive Ionic Layer Adsorption and Reaction (SILAR). SILAR is an alternative to vapour-phase and chemical-precursor techniques, which successfully used to produce a large variety of semiconductor materials (Shishiyanu et al., 2005; Lupan et al., 2008). As indicated by Nicolau (1985), this method is suitable for a growing of polycrystalline thin films of water-insoluble ionic or ionocovalent compounds of the CmAn type by heterogeneous chemical reaction at the solid-solution interface between adsorbed Cn+ cations and Am− anions. This is intended to allow ion-by-ion growth of the compound film via sequential addition of individual atomic layers as it involves an alternate immersion of the substrate into solution containing a soluble salt of the cation of the compound to be grown and then into solution containing a soluble salt of the anion. The substrate supporting the growing film rinsed in high-purity water after each immersion (Niesen, 2002). Thus we obtained light-weight and flexible biodegradable thermoelectric material CuI/NC. Notably, that this thermoelectric material is exclusively nontoxic, because in addition to macrobiotic elements of biomass it consists of Cu, I, and of S traces, which all are essential for life (Zoroddu et al., 2019). Here we analyze crystal structure, morphology, chemical composition, and optical, electrical and thermoelectric properties of CuI/NC. With the aim to demonstrate the potential of CuI/NC as a new thermoelectric material for an in-plane design of biodegradable flexible thin film TEG, we explore its possibility to obtain electricity under conditions of temperature gradients from 5
Cha, 2006; Zheng et al., 2013; Huang et al., 2013; Zhou et al., 2013; Gomez and Steckl, 2015; Hoeng et al., 2016; Zhu et al., 2016; Tan et al., 2016; Feig et al., 2018; Li et al., 2018; Fang et al., 2019; Abol-Fotouh et al., 2019; Zhang and Park, 2019). In the last few years, the cellulosic biopolymer-based green electronics is supplemented by lightweight, portable, flexible NC-based power generators to provide energy to wearable electronics through harvesting mechanical energy in triboelectric and piezoelectric appliances (Zhu et al., 2016; Jung et al., 2017; Fang et al., 2019) or by using of waste heat via thermoelectric (TE) devices (Jung et al., 2017; Zhang and Park 2019; Zhao et al., 2019; Abol-Fotouh et al., 2019). Huge ecological advantage of the NC-based electronic and thermoelectric devices is their inherent biodegradability. As these devices have become ubiquitous in modern society, and are prevalent in every facet of human activities, and the lifetime of electronics get shorter and shorter, the pressure on electronic waste (e-waste) management systems is mounting with no abate in sight. This poses a growing ecological problem, and an alternative to traditional electronics is biodegradable electronics as the most viable replacement to address the issue of uncontrollable e-waste to reduce the environmental footprint of devices (Irimia-Vladu et al., 2012; Tan et al., 2016; Zhu et al., 2016; Feig et al., 2018; Abol-Fotouh et al., 2019). In accordance with Abol-Fotouh et al. (2019), Feig et al. (2018), biodegradable electronic and thermoelectric devices can be broken down into smaller non-toxic constituent pieces at biologically benign or physiological conditions, and thereafter they must dissolve, resorb, or physically disappear into physiological or environmental solutions, partially or completely, at controlled rates after the expecting working period without leaving a permanent mark. More precisely, the quality of biodegradability has been defined by the EU harmonized standard EN 13432:2000 “Packaging: requirements for packaging recoverable through composting and biodegradation” as the property where a minimum of 90% of the mass can be converted into water, carbon dioxide, and biomass under defined temperature, humidity, and oxygen conditions within six months, in the presence of fungi or microorganisms. As indicated by Tan et al. (2016), the substrate in electronic device serves as a foundation upon which functional layers such as semiconductor and metal thin films are deposited. As the volume of substrate is larger than that of any layer, it generates more ewaste in general, compared to other layers. Hence, replacing conventional substrates with biodegradable ones tremendously reduces the ewaste problem. As nanocellulose is able to degrade slowly in the presence of naturally occurring fungi (Irimia-Vladu et al., 2012; Tan et al., 2016; Zhu et al., 2016; Feig et al., 2018; Abol-Fotouh et al., 2019), it is suggested that NC substrates can be used in the multiple consumer electronic and thermoelectric devices. The most common type of plant fibers used for the nanocellulose fabrication is a wood pulp (Zhu et al., 2016; Shak et al., 2018). So, in spite of the obvious advantages of nanocellulose, the rather high cost of the wood-derived NC substrates compared with classical petroleumbased polymer substrate of flexible electronics polyethylene terephthalate (PET) (Hoeng et al., 2016) serves as a limiting factor for the wide distribution of the NC-based electronic devices, especially for countries that do not have large stocks of free wood. Authors AbolFotouh et al. (2019) report on the fabrication of a thermoelectric module from farmed thermoelectric paper on the base of bacterial nanocellulose films. Highly dispersed single-wall carbon nanotubes (CNTs) were embedded as percolating networks into produced by microorganisms nanocellulose polymer matrixes representing intertwined networks of fibres with fine porosity composed of fibrils with diameters of tens of nanometers to obtain composites CNTs/NC. The generation of composites CNTs/NC regulated directly in situ during the biosynthesis, however, the farming of this TE material carried out in artificial conditions, in the dark at 30 °C in a special incubator. Then, the grown CNTs/NC composites were purified and doped chemically. For example, for the n-doping of CNTs/NC composites authors Abol-Fotouh et al. (2019) used solution processing in trimethylammonium hydroxide, in 22
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the NC surface with I− ions to obtain CuI. After this immersion, the final stage of one SILAR cycle was a rinsing in the distilled water for 10 s. Such SILAR cycles for the CuI deposition onto NC repeated 40 times. The thickness of the CuI film was determined gravimetrically, taking for a calculation the bulk CuI density 5.67 g/cm3. Surface morphology of NC substrate and CuI/NC thermoelectric material was observed by scanning electron microscopy (SEM) in a secondary electron mode. The SEM instrument “Tescan Vega 3 LMH” operated at an accelerating voltage 30 kV without the use of additional conductive coatings when scanning conductive CuI/NC thermoelectric material. However, for morphology studies of the pure NC substrate by SEM we used thin Cr film (~10–15 nm thick) as conductive coating, which evaporated in vacuum at 10−6 Torr residual gas pressure immediately before SEM research. Chemical analysis of the pure NC and of the CuI/NC thermoelectric material was carried out by X-ray fluorescence (XRF) microanalysis using an energy dispersive spectrometry (EDS) system “Bruker XFlash 5010”. Energy dispersion spectra were taken from the 50 × 50 μm areas. Quantification of the spectra was carried out in the self-calibrating detector mode. Optical properties of the thermoelectric material CuI/NC in UV–visible-near IR range (at wavelengths (λ) 300–1100 nm) studied with an “SF-2000” spectrophotometer equipped with “SFO-2000” specular and diffuse reflection attachment. We recorded optical transmission To(λ) spectrum to obtain total optical absorption A(λ) = −lgTo (λ) of the optical medium consisted of 0.72 µm CuI, interface, 12 µm NC, interface and 0.72 µm CuI. Optical spectra of specular reflectance Rs(λ) and diffuse reflectance Rd(λ) for the CuI coating were studied using a reflection attachment “SFO-2000” at light incidence angle ϑ = 8° relative to normal to the surface. The light scattering in the CuI film, haze factor (Hf), was calculated in accordance with Klochko et al. (2019a) as the ratio of the diffuse reflectance Rd to the total reflectance R, where R is sum of Rs and Rd. That is, Hf = Rd/R. Optical band gap Eg for direct allowed transitions in the CuI film was determined as described in Klochko et al. (2017) from the Kubelka-Munk function:
to 50 °C at near-room temperatures, for example, by the harvesting the waste environmental heat. 2. Experimental procedures The manufacture of nanocellulose substrates was carried out in accordance with Barbash et al. (2019). As feedstock we used the biomass from the second-year miscanthus herb planted in a field in the Kyiv's region. Miscanthus stalks were exempted from leaves and nodes, crushed into particles of 5–7 mm in size and placed in a desiccator to maintain constant moisture and chemical composition. Cooking of Miscanthus pulp carried out in three stages. At the first stage, a delignification of the feedstock carried out. With this, the shredded Miscanthus stalks were treated for 30–240 min in a hot (95 ± 2 °C) solution, contained glacial acetic acid and 35% hydrogen peroxide at volume ratio of 7:3, when the liquid to solid ratio was 10:1. Thus, we obtained an organosolv Miscanthus pulp (OMP). At the second stage, we carried out an alkaline treatment of OMP by hot (95 ± 2 °C) aqueous solution contained 7% NaOH at the liquid to solid ratio 12:1 for 15–240 min. The OMP after alkaline treatment was washed with hot distilled water. Then, at the third stage, the OMP was hydrolyzed by means of 43 and 50% sulfuric acids at the liquid to solid ratio 10:1, at temperatures of 40 and 60 °C for 30–90 min to obtain a suspension of hydrolyzed nanocellulose in accordance with Barbash et al. (2019). Then this suspension was rinsed with distilled water three times by means of centrifugation at 4000 rev/min and subsequent dialysis, until it reached pH 7. Ultrasound treatment of 0.6% nanocellulose suspension was performed in an ice bath using an ultrasound disintegrator UZDN-A (SELMI, Ukraine) at 22 kHz during 30–60 min to prevent overheating during treatment. Eventually, the suspension turned into a homogenous gel-like dispersion with nanoparticle diameters in the 5–20 nm range. The required amount of the prepared dispersion was poured into Petri dishes and dried at room temperature in air to obtain a 12-μm thick nanocellulose substrate. According to Barbash et al. (2019), the obtained nanocellulose substrates have density up to 1.6 g/cm3, transparency up to 82%, and crystallinity up to 76.5%. (Barbash et al., 2019). Fig. 1(on the left) shows photo of the NC substrate. To create biodegradable thermoelectric material CuI/NC with NC substrate coated at both sides by 0.72 µm thick CuI films presented in Fig. 1(on the right), we employed CuI deposition from aqueous solutions at room temperature through SILAR method described earlier in Klochko et al. (2018). We used aqueous solution containing 0.1 M CuSO4 and 0.1 M Na2S2O3 as a cationic precursor, into which NC substrate immersed for 20 s. Then, the substrate rinsed in distilled water for 10 s. After that, the nanocellulose substrate was immersed for 20 s into aqueous NaI solution (anionic precursor), which concentration was 0.075 M to carry out the reaction of strongly adsorbed Cu+ ions on
F (R) =
(1 − Rd )2 2Rd
(1) 2
As shown in Klochko et al. (2017), a plot of (F(R)·hν) vs hν yields a direct band gap value Eg of the CuI by extrapolating of (F(R)·hν)2 linear part on hν. To analyze crystal structure of CuI thin films in the thermoelectric material CuI/NC, we recorded X-ray diffraction (XRD) patterns by a “DRON-4” diffractometer with Bragg–Brentano focusing (θ–2θ). The crystalline phases were identified by comparing the experimental diffraction patterns with the reference database JCPDS by using PCPDFWIN v.1.30 software. The experimental interplanar d-spacing were calculated using the Bragg’s equation (Palatnik,1983; Tsybulya and Cherepanova, 2008):
nλ = 2d sin θ .
(2)
Average crystallite size D of CuI was determined from the X-ray line broadening method using the Scherer’s formula as in Klochko et al. (2019a); Klochko et al. (2019b):
D = (0. 9·λ )/(β ·cosθ),
(3)
Calculation of the CuI lattice parameter a was performed from the positions of the X-ray diffractions by the Nelson–Reilly graphical extrapolation method and refined using the least-squares method by UnitCell software in accordance with Palatnik (1983); Tsybulya and Cherepanova (2008). Crystal lattice microstrains we obtained from the relation ɛ = Δd/d (where d is the crystal interplanar spacing according to JCPDS, and Δd is the difference between the corresponding experimental and reference interplanar spacing), and dislocation density evaluated according to Dongol et al. (2015) through 1/D2 as in Klochko et al. (2019a). The resistivity ρ of CuI film in the TE material CuI/NC was measured
Fig. 1. Images of the pure nanocellulose substrate NC on the background of National Technical University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute” logos (on the left) and of the thermoelectric material CuI/NC with nanocellulose substrate coated at both sides by 0.72 µm thick CuI films on the background of National Technical University “Kharkiv Polytechnic Institute” logo (on the right). 23
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at near-room temperatures in the T range of 293–327 K in accordance with Chen et al. (2018) by using a four-point collinear probe resistivity method. The resistivity calculated as follows:
ρ = (πtδU23)/(I14 ln(2)),
(4)
where U23 is the voltage between the second and third probe; I14 is the current between the first and fourth probes; δ is a correction factor for the accounting the ratio of the distance between the probes and the size of the substrate; πδ/ln(2) ≈ 4.45. To assess the room-temperature thermoelectric performances of the obtained CuI films, the in-plane Seebeck coefficients S at the 293–330 K temperature range measured as thermoelectric voltages ΔU induced in response to the temperature gradients ΔT along the TE material CuI/ NC. The distance between hot and cold probes in the form of gold rings was 2.3 cm. Then, the thermoelectric power factor p for the thermoelectric material CuI/NC was calculated as S2/ρ, pursuant to Faustino et al. (2018). In accordance with Faustino et al. (2018), output voltage Vout and output power Pout of the thermoelectric thin film leg in the form of CuI/NC strip of 3 cm long and 0.5 cm wide obtained as function of output currents Iout measured at different load resistances Rload for several temperature differences in the ΔT = 5–50 K range. From Vout plotted against Iout we obtained values of open circuit voltage Voc at load resistance Rload = ∞, and short circuit current Isc, when Rload = 0. According to Yang et al. (2017); Faustino et al. (2018), the open circuit voltage Voc for such single-leg thin film thermoelectric module was described by the equation Voc = SΔT, hence, the output power Pout can be calculated using the equation (Yang et al., 2017; Faustino et al.,2018):
Pout = (S ΔT )2Rload /(Rload + Rint )2
Fig. 2. (a) – SEM images of thermoelectric material CuI/NC and nanocellulose substrate NC (insets) obtained at two different magnifications; (b) – XRF spectra of nanocellulose substrate NC and thermoelectric material CuI/NC. Table 1 X-ray fluorescence spectroscopy data for nanocellulose substrate NC and thermoelectric material CuI/NC.
where Rint is the internal resistance of the single-leg thermoelectric module. As the maximum output power Pmax is obtained when Rload = Rint (Faustino et al.,2018), thus, Rint can be described as:
Rint = Voc2 /4Pmax .
Element
NC at. (%)
CuI/NC at. (%)
C O Cu S I
42 58 – <1 –
52 22 14 <1 12
(5)
(6)
Accordingly, maximum output power per unit area, i.e. output power density P*max (in nW/m2) of the single-leg thermoelectric module was evaluated for the obtained p-type thermoleg CuI/NC as ∗ Pmax = Pmax /(0.5 × 3)·10−4 . 3. Results and discussion Fig. 2(a) shows top view SEM images of the CuI surface in the thermoelectric material CuI/NC and of the pure nanocellulose substrate NC in insets. It is seen, that CuI film is quite dense, but it is nanostructured and rough compared to the smooth NC substrate. Both XRF spectra in Fig. 2(b) contain C, O and traces of sulfur. On the one hand, NC contains eSO3− functional groups, as it obtained through sulphuric acid hydrolysis (Shak et al., 2018). On the other hand, CuI film contains sulfur from the chemically unstable compound sodium thiosulfate Na2S2O3 in the cationic precursor that is typical for CuI films fabricated on different substrates by the SILAR method using such solutions (Klochko et al., 2018; Klochko et al., 2019a; Klochko et al., 2019b). Note that Cr in the XRF spectrum of NC belongs to the electrically conductive chromium film and is not a part of nanocellulose substrate. Chemical compositions of NC and TE material CuI/NC obtained by the EDS microanalysis presented in Table 1. The rather high content of copper and iodine in the CuI/NC against the background of C and O from the NC substrate is another confirmation of the density of the thin (0.72 µm) copper iodide film, and a full NC surface coating with CuI film in this TE material. Probably, it originates owing to the good wettability of NC in the aqueous solutions, in which CuI film was obtained by the SILAR method, due to hydrophilic functional groups in the nanocellulose (Shak et al., 2018). In this case, the hydrophilicity of the nanocellulose is a significant advantage of this material as
Fig. 3. (a) – Photo and XRD pattern of the TE material CuI/NC; (b) – its optical properties: transmittance and absorbance spectra, To(λ) and A(λ) respectively, total reflectance spectrum R(λ), haze factor spectrum Hf(λ), and plot for the determining of optical band gap Eg of the CuI film in the TE material CuI/NC by using the Kubelka-Munk function.
compared to organic polymers derived from oil. XRD pattern in Fig. 3(a) demonstrates eight X-ray diffraction peaks. Their analysis has revealed that CuI film in TE material CuI/NC is 24
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PEDOT:PSS, AgNP and CNTs (Jung et al.,2017; Abol-Fotouh et al., 2019; Zhao et al., 2019), and even exceeds the S values for the different thin-film composites of such well-known inorganic TE material as Bi2Te3 (Zhao et al., 2019). The positive sign of S in Fig. 4(a) indicates the p-type conductivity of the copper iodide film. The temperature dependence of the CuI resistivity ρ(T) in Fig. 4(b) corresponds to the transport of the charge carriers (holes) via the localized states according to the nearest neighboring hopping model for the nanostructured copper iodide thin films described by Kaushik et al. (2017). The ρ values are in the range of (1.5–1.7)·10−1 Ω·cm. Fig. 4(c) shows temperature dependence of the thermoelectric power factor p(T) = S2/ρ(T) for TE material CuI/NC. As it seen, p grows with increasing temperature and reaches value 36 μW·m−1·K−2 at T = 330 K. Thus, the thermoelectric power factors of the TE material CuI/NC are higher than that of the best examples bacterial nanocellulose films with embedded highly dispersed CNT networks (p ≈ 20 μW·m−1·K−2) in Abol-Fotouh et al. (2019). Moreover, p values of CuI/NC exceed p = 25.5 μW·m−1·K−2 of the Bi2Te3/bacterial cellulose nanofiber in Zhao et al. (2019). Plots of the output voltages and output powers versus output currents obtained for the strip with (0.5 × 3) cm2 area of the TE material CuI/NC as single p-CuI thermoelectric leg in Fig. 4(d) show output thermoelectric characteristics of CuI/NC. According to the graphs of Vout vs. Iout and Pout vs. Iout in Fig. 4(d), the output parameters of the single p-CuI thermoelectric leg naturally increase with the increase of the temperature difference between the cold and warm edges of the CuI/NC strip. At temperature gradient of 50 K, the single p-CuI thermoelectric leg made from CuI/NC generates Voc = 8.4 mV, Isc = 0.7 µA and Pmax = 1.5 nW at Rload = Rint = 11.8 kΩ. It corresponds to the power density P*max = 10 µW/m2. These values are even better than we obtained (Klochko et al., 2018) for the same CuI films deposited via SILAR on glass substrates. It must be borne in mind that Vout generated by a single p-type TE leg is always very low, therefore, to achieve high output voltage and power, TEGs are typically made of dozens, or even hundreds, of TE couples (Du et al., 2018). The output thermoelectric data obtained in this work have the prospect of being improved by designing TEG on the nanocellulose substrate using a large number of pn thermocouples by connecting CuI/NC strips with wires of n-type thermoelectric material.
single-phase and polycrystalline with cubic copper iodide crystal structure (zincblende, γ-CuI, JCPDS #06-0246). Calculations of the CuI film structural parameters show, that the average crystallite sizes D are in the 19–25 nm range with moderate dislocation density of (1.6–2.8) × 1015 lines/m2 and tolerable microstrains ε of (4–9) × 10−3 a.u. The CuI lattice parameter a = 6.058 ± 0.005 Å according to the Nelson–Reilly graphical extrapolation, and a = 6.067 ± 0.005 Å using the least-squares method, which is slightly higher than a = 6.051 Å in JCPDS #06-0246. Fig. 3(b) shows spectra of the optical transmittance To and absorbance A of the TE material CuI/NC in the UV–Vis-NIR range, which are typical for CuI films deposited via SILAR method onto different transparent substrates (Klochko et al., 2017; Klochko et al., 2018; Klochko et al., 2019a; Klochko et al., 2019b). Thermoelectric material CuI/NC is translucent in the visible range, its maximum transparency To equals 7%. The To(λ) spectrum in Fig. 3(b) does not contain any interference extremes, which indicates that the level of irregularities on the CuI surface is commensurable with the visible wavelengths. The reflection spectrum in Fig. 3(b) confirms the significant reflectance in the entire visible range. According to the Haze factor spectrum in Fig. 3(b), the reflection is predominantly diffuse. The band gap Eg for direct optical transitions in the SILAR deposited CuI films Eg ≈ 3.0 eV is near the value 2.95–3.1 eV measured for bulk CuI at near-room temperature (Grundmann et al., 2013; Kneiß et al., 2018). Electrical and thermoelectric properties of the obtained flexible thin-film TE material were studied using an experimental sample CuI/ NC in the form of a thin-film strip 3 cm long and 0.5 cm wide, consisted of 12 µm thick NC substrate coated by 0.72 µm thick CuI films at both sides. It is seen in Fig. 4(a) a plot of the thermoelectric voltages induced in response to the temperature gradients ΔT along the CuI/NC thermoelectric strip. The determined value of the Seebeck coefficient S is ~228 μV K−1, at that, S is constant in the temperature range 290–335 K, which is favorable for the use of this material CuI/NC in TEG. Note, that this S value more than an order of magnitude higher than the Seebeck coefficients that were measured earlier for the nanocellulose-derived organic and composite TE materials contained
4. Conclusions In this work, we used conversion of solar energy into chemical energy of biomass of fast-growing perennial herb Miscanthus to fabricate via an environmentally friendly method of organosolv delignification low-cost nanocellulose films used as substrates for the creation of new biodegradable thin film thermoelectric material. For this, we deposited CuI film of 0.72 µm thickness on the 12 µm thick NC substrate by means of the low-temperature cheap and scalable SILAR method and thus obtained light-weight and flexible biodegradable thermoelectric material CuI/NC. Notably, that this thermoelectric material is exclusively nontoxic, because in addition to macrobiotic elements of biomass it consists of Cu, I, and of S traces, which all are essential for life. By means studying the surface morphology, crystal structure, chemical composition, optical, electrical and thermoelectric properties of CuI/NC we found out that nanostructured p-type semiconductor CuI film in the CuI/NC TE material is quite dense and completely covers the NC surface. It has typical optical direct band gap ≈ 3.0 eV, is single-phase γCuI with crystallite sizes in the 19–25 nm range, with moderate dislocation density of (1.6–2.8) × 1015 lines/m2, and tolerable microstrains ε of (4–9) × 10−3 a.u. The determined value of the Seebeck coefficient S is ~228 μV K−1 more than an order of magnitude higher than the Seebeck coefficients that were measured earlier for the nanocellulose-derived organic and composite TE materials contained PEDOT:PSS, AgNP and CNTs (Jung et al.,2017; Abol-Fotouh et al., 2019; Zhao et al., 2019), and even exceeds the S values for the different
Fig. 4. (a) – Thermoelectric voltage induced in response to the temperature gradient ΔT along the CuI/NC thermoelectric strip; (b) – temperature dependence of the CuI resistivity ρ; (c) – temperature dependence of the thermoelectric power factor p = S2/ρ for TE material CuI/NC; (d, e) – output voltage Vout (solid symbols) and output power Pout (opened symbols) for some temperature gradients ΔT versus output current Iout obtained for (0.5 × 3) cm2 strip of the TE material CuI/NC. 25
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thin-film composites of such well-known inorganic TE material as Bi2Te3 (Zhao et al., 2019). At that, S is constant in the temperature range 290–335 K, which is favorable for the use of CuI/NC as new thermoelectric material for an in-plane design of biodegradable flexible thin film TEG. The thermoelectric power factor of CuI/NC p ≈ 36 μW·m−1·K−2 is higher than that of the best examples bacterial nanocellulose films with embedded highly dispersed CNT networks (p ≈ 20 μW·m−1·K−2) in Abol-Fotouh et al. (2019). Moreover, p values of CuI/NC exceed p = 25.5 μW·m−1·K−2 of the Bi2Te3/bacterial cellulose nanofiber in Zhao et al. (2019). At temperature gradient of 50 K, the single p-CuI thermoelectric leg made from CuI/NC strip of 3 cm long and 0.5 cm wide generates Voc = 8.4 mV, Isc = 0.7 µA and Pmax = 1.5 nW. It corresponds to the power density P*max = 10 µW/m2. So, by virtue of the complex of physical and chemical properties, the developed eco-friendly biodegradable flexible thin film thermoelectric material can be effectively used for the conversion of low-grade waste heat into electricity at near-room temperatures. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors gratefully acknowledge the financial support of Ministry of Education and Science of Ukraine under project number М 5487. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solener.2020.02.091. References Abol-Fotouh, D., Dörling, B., Zapata-Arteaga, O., Rodríguez-Martínez, X., Gómez, A., Reparaz, J.S., Laromaine, A., Roig, A., Campoy-Quiles, M., 2019. Farming thermoelectric paper. Energy Environ. Sci. 12, 716–725. https://doi.org/10.1039/ c8ee03112f. Barbash, V.A., Yashchenko, O.V., Kedrovska, A., 2017. Preparation and properties of nanocellulose from peracetic flax pulp. J. Sci. Res. Rep. 16 (1), 1–10. https://doi.org/ 10.9734/JSRR/2017/36571. Barbash, V.A., Yashchenko, O.V., Opolsky, V.O., 2018. Effect of hydrolysis conditions of organosolv pulp from kenaf fibers on the physicochemical properties of the obtained nanocellulose. Theor. Exp. Chem. 54 (3), 193–198 0040-5760/18/5403-0193. Barbash, V.A., Yashchenko, O.V., Vasylieva, O.A., 2019. Preparation and properties of nanocellulose from miscanthus x giganteus. ID 3241968-1–ID 3241968-8. J. Nanomater. 2019. https://doi.org/10.1155/2019/3241968. Barber, J., 2007. Biological solar energy. Phil. Trans. R. Soc. A. 365 (1853), 1007–1023. https://doi.org/10.1098/rsta.2006.1962. Chen, X., Dai, W., Wu, T., Luo, W., Yang, J., Jiang, W., Wang, L., 2018. Thin film thermoelectric materials: classification, characterization, and potential for wearable applications. 244-1–244-16. Coating 8 (7). https://doi.org/10.3390/coatings8070244. Dongol, M., El-Denglawey, A., Abd El Sadek, M.S., Yahia, I.S., 2015. Thermal annealing effect on the structural and the optical properties of nano CdTe films. Optik 126, 1352–1357. https://doi.org/10.1016/j.ijleo.2015.04.048. Du, Y., Xu, J., Paul, B., Eklund, P., 2018. Flexible thermoelectric materials and devices. Appl. Mater. Today 12, 366–388. https://doi.org/10.1016/j.apmt.2018.07.004. Fang, Z., Hou, G., Chen, C., Hu, L., 2019. Nanocellulose-based films and their emerging applications. 100764-1–100764-11. Curr. Opin. Solid State Mater. Sci. 23. https:// doi.org/10.1016/j.cossms.2019.07.003. Faustino, B.M.M., Gomes, D., Faria, J., Juntunen, T., Gaspar, G., Bianchi, C., Almeida, A., Marques, A., Tittonen, I., Ferreira, I., 2018. CuI p-type thin films for highly transparent thermoelectric p-n modules. 6867-1−6867-10. Sci. Rep. 8. https://doi.org/ 10.1038/s41598-018-25106-3. Feig, V.R., Tran, H., Bao, Z., 2018. Biodegradable polymeric materials in degradable electronic devices. ACS Cent. Sci. 4 (3), 337–348. https://doi.org/10.1021/ acscentsci.7b00595. Gomez, E.F., Steckl, A.J., 2015. Improved performance of OLEDs on cellulose/epoxy substrate using adenine as a hole injection layer. ACS Photonics 2 (3), 439–445. https://doi.org/10.1021/ph500481c. Grundmann, M., Schein, F.-L., Lorenz, M., Böntgen, T., Lenzner, J., Wenckstern, H., 2013.
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