Materials Science

Mild Solvothermal Synthesis and Characterisation of Sulfur Rich Cu2-xS Nanocrystals with Varied Sulfur Precursors for Thermoelectrics

Chong Ka Shing1, Tam Teck Lip Dexter2

1Student, River Valley High School

2Institute of Materials Research and Engineering, A*STAR

ABSTRACT

Recent studies have proven that several Cu2-xS compounds displayed satisfactory figure of merit (ZT values, which evaluates the conversion efficiency of a thermoelectric material. , where α is the Seebeck coefficient, is the electrical conductivity, T is the absolute temperature and κ is the thermal conductivity. In this study, sulfur rich Cu2-xS hexagonal flakes nanocrystals were synthesised with dithiooxamide or thiourea as sulfur precursors using a mild solvothermal method. Apart from investigating the two synthetic methods of Cu2-xS, this paper optimises the power factor (PF), where PF = α2σ for Cu2-xS by uncovering the optimum parameters between electrical conductivity and Seebeck coefficient. X-Ray Diffraction affirmed the identity of the sample as covellite (CuS) while Energy Dispersive X-Ray Spectroscopy showed that dithiooxamide samples contained a considerable amount of impurities. Due to high purity, thiourea samples showed high electrical conductivity of 579-1m-1 at room temperature. However, these samples have moderate Seebeck coefficient and higher thermal conductivity due to their elevated hole concentration. Despite TU2, which is made from thiourea and copper bromide in a 2:1 ratio, having the lowest Seebeck coefficient of 12.3μVK−1, this value is nearly 3 times greater than that of the single crystalline CuS with α = 5μVK−1, emphasising the significance of nanostructuring to increase seebeck coefficient. Nevertheless, the increment in figure of merit owing to electrical conductivity is far greater than the deduction in figure of merit by Seebeck coefficient and thermal conductivity, especially at lower temperatures. CuS nanocrystals synthesised from thiourea precursor shows great potential as a room temperature thermoelectric device due to its unique temperature dependence of increasing power factor at lower temperatures. TU2 has the highest power factor of 51.4μWm-1K-2 at room temperature and was substantially higher than previously reported CuS by Narjis et al with a power factor of 4.5μWm-1K-2 due to solvothermal synthesis method adopted. This resulted in a highest ZT value of 0.00220 at room temperature, 20% higher than that of CuS synthesized by Tarachand et al with ZT of 0.00183. Regardless of a relatively lower maximum figure of merit, chemically stable CuS nanocrystals fill the research gap of room temperature thermoelectric by offering satisfactory thermoelectric performance with a low synthetic cost coupled with its non-toxicity nature.

Keywords: Cu2-xS, Nanocrystals, Solvothermal, Power factor, Electrical conductivity, Seebeck coefficient, Thermal conductivity, Figure of merit, Hole concentration, Dithiooxamide, Thiourea

Background and Purpose of Research

A temperature gradient induces the flow of charge carriers from the hotter to the cooler end, generating an electric field and creates a voltage difference directly proportional to the temperature difference (Ge 2016). Utilizing low grade heat from industrial process or waste heat from electrical appliances is vital to reducing the growing carbon emissions of our society (Chen 2003). The above mentioned Seebeck effect directly recovers electricity from thermal energy, thus receiving much attention as a promising green technology (Dresselhaus 2007). One notable thermoelectric material is Cu2-xS due to its low cost and non-toxicity (Han 2014). (Refer to Annex F) Being a p-type semiconductor, Cu vacancies act as charge carriers, making Cu2-xS an excellent electrical conductor (Rabinal 2018). Moreover, Cu2-xS has variable stoichiometric compositions, ranging from sulfur rich CuS to copper rich Cu2S (Grozdanov 1995). As a result, it has a Cu/S ratio dependent hole concentration and various band gap values ranging from 1.2 to 2.0 eV suitable for a wide range of applications (Zhang 2014). The crystalline structure of Cu2-xS can be subdivided into groups based on the sulfur packing in the lattice. (Refer to Annex A) Reported Cu2-xS can be further classified into covellite (CuS), yarrowite (Cu1.12S), spionkopite (Cu1.40S), anilite (Cu1.75S), digenite (Cu1.80S), djurleite (Cu1.97S) and chalcocite (Cu2S) (Sun 2017).

Recent studies have proven that several Cu2-xS compounds displayed satisfactory figure of merit (ZT values (Zhao 2015), which evaluates the conversion efficiency of a thermoelectric material. , where α is the Seebeck coefficient, is the electrical conductivity, T is the absolute temperature and κ is the thermal conductivity (Goldsmid 2017). Most notably, Cu1.97S can display a maximum ZT value of 1.5 at 900K which represents the highest reported value for p-type copper semiconductors (He 2014). However, there exists limited reports regarding thermoelectric properties of sulfur rich Cu2-xS due to their inferior ZT merit value, with Tarachand et al reporting CuS of highest ZT of 0.0187 (Tarachand 2018). Nevertheless, recent nanocrystal engineering demonstrated increased Seebeck coefficient and decreased thermal conductivity due to the reduced dimensionality of its quantum dot structures (Martin 2009). Hence, this paper study methods to raise the Seebeck coefficient and electrical conductivity and uncover the potential CuS through synthesis of nanocrystals with strong interface phonon-scattering and charge carrier filtering ability. Despite the current extensive study into Cu2-xS crystals, prior research has failed to compare the trends and relationship between amount of sulfur source, types of sulfur precursors, and their corresponding ZT at different temperatures using solvothermal synthesis. Investigating trends of different sulfur rich Cu2-xS nanocrystals using different synthetic conditions explores its potential and lay the foundation for future studies.

Different methods for the synthesis of Cu2-xS with varying oxidation states of Cu have been reported. These includes the melting-quenching, solvothermal methods and microwave assisted synthesis (Liao 2003). However, this paper focuses solely on the economically efficient solvothermal synthesis of Cu2-xS using CuBr2 with two different sulfur-containing precursors, thiourea (TU), SC(NH₂)₂ (Rahmani 2017) and dithiooxamide (DTO), NH2(CS)2NH2 (Roy 2006) under different conditions. Several studies have generally confirmed that thiourea provides a high reducing power to form an intermediate Cu[(NH2)2CS]2+ complex (Annex B) and is likely to contain less copper (II) oxide impurities in its final product (Madarász 2001). On the other hand, dithiooxamide forms a stable and easily synthesisable Cu-DTO complex intermediate, hence lowering the activation energy in forming CuS. Both synthetic conditions are predicted to form sulfur rich Cu2-xS nanocrystals (Zhao 2012). The two reactions consist of sulfur precursors which form stable intermediate, eliminating the need of a capping ligand. The synthetic media used will be N, N-dimethylformamide (DMF), an inexpensive and common solvent for inorganic synthesis (Adler 1970). Apart from investigating the two synthetic methods of Cu2-xS, this paper optimises the power factor (PF), PF = for Cu2-xS by uncovering the optimum parameters between electrical conductivity and Seebeck effect through evaluating trends related to the choice between different Cu : S stoichiometric ratios and type of sulphur precursor. One significant synthetic challenge is controlling the stoichiometric ratio of copper and sulfur due to the absence of a detailed mechanism to describe their formation (Kumar 2011).

Methodology

Different stoichiometric ratios (Refer to Annex D) of copper (II) bromide, thiourea and dithiooxamide were dissolved in DMF and subjected to oil bath of 160 in an inert atmosphere of nitrogen for 24 hours. Stoichiometric ratios were adjusted to produce different phase compositions and morphologies of sulfur rich Cu2-xS (Kumar 2011). This solvothermal process allows for the creation of grain boundaries between Cu2-xS nanocrystals which had been proven to be effective in phonon scattering and enriching defect structures, hence potential to increase both Seebeck coefficient and electrical conductivity (Zhu 2009). Samples were purified through washing it with DMF twice and methanol once after centrifuging at 4000 rpm for 5 minutes. Sample were dried in a vacuum oven of 60 overnight and grinded into fine powders before pressing into a pellet. Samples are named with their sulfur precursor as prefix and the copper to precursor concentration ratio as suffix. (Refer to Annex D)

Pellets are tested for electrical conductivity, σ and Seebeck coefficient, α using ZEM-3 while hall measurements are taken using BioRad HL5500. Energy-dispersive X-ray spectroscopy (EDX) and X-Ray Diffraction (XRD) are carried out using FESEM 7600F and Bruker D8 Advance respectively.

Results and Discussion

Synthesis of CuS nanocrystals results in the formation of a dark precipitate upon mixing copper (II) bromide and all concentrations of thiourea and dithiooxamide.

Figure 1: XRD diffraction patterns of (a) DTO1 (b) DTO2 (c) TU1 (d) TU1.5 (e) TU2 (f) TU4

XRD patterns of the products were recorded using Cu Kα radiation (λ=1.5418Å) in the 2θ range of 10°-85° and plotted in Figure 1. Above patterns are in good agreement with CuS of hexagonal structure with cell parameters of a = 3.792Å and c = 16.34Å (JCPDS: 06-0464). The (006) diffraction peak has a much stronger intensity than the (102) and (103) peaks, suggesting a preferential orientation of the crystals along the (006) plane (Chang 2011). This reveals the possibility to obtain anisotropic conductivity to be advantageous for thermoelectrics (Liang 1993).

TABLE 1: Average crystallite size calculated from XRD data pattern
Average crystallite Size/nm DTO1 DTO2 TU1 TU1.5 TU2 TU4
35.86 29.05 28.09 19.24 29.42 22.82

Average crystallite sizes of each sample were then calculated and recorded in Table 1 using the Scherrer equation, D = Kλ/Bcosθ where D is crystallite size, K is shape factor (0.9), λ is wavelength of Cu Kα emission, θ is Braggs angle corresponding to the maximum of the diffraction peak and β is full width at half maximum (FWHM) of diffraction peaks (Holzwarth 2011). DTO samples have many outliers with massive crystallite size due to the presence of impurities.

Figure 2: SEM image of copper sulphide nanoparticles in (a) TU1 (b) TU1.5 (c) TU2 (d) TU4

TABLE 2: EDX analysis of different samples
S.N. Sample Atomic % Total %
Cu S N O Br
1 TU1 33.9 34.7 11.7 13.8 5.9 100
2 TU1.5 48.6 38.5 0 12.9 0 100
3 TU2 54.4 45.6 0 0 0 100
4 TU4 53.4 46.6 0 0 0 100
5 DTO1 34.8 34.1 22.9 7.5 0.7 100
6 DTO2 29.8 31.8 31.8 6.6 0 100

Table 2 revealed that all samples give Cu : S stoichiometric ratio of about 1:1 which reaffirms the identity of covellite. Powders of DTO1 and DTO2 contain elevated amounts of O, N due to presence of unreacted dithiooxamide, thus restricting flow of electrons and making it highly non-conducting. Addition of thiourea shifts the equilibrium of the reaction from CuO to CuS, decreasing the atomic percentage of O due to its reducing power. SEM images in Figure 2[1] illustrates that samples of thiourea precursors contain hexagonal flakes, which exhibits strong thermoelectric properties (Du 2007). Clusters of nanoparticles are present in TU1 and TU1.5, providing the possibility that there exists another phase of CuS in them. The chemical compositions of all samples were evaluated by energy dispersive X-ray.

Figure 3.1: Electrical Conductivity dependence on temperature
Figure 3.2: Seebeck Coefficient dependence on temperature

Measurements of electrical conductivity, Seebeck coefficient, power factor, thermal conductivity and figure of merit at different temperatures are recorded in Table 5 (Refer to Annex G) and plotted in Figure 3.1, 3.2, 3.3, 3.4 and 3.5 respectively. The measured electrical conductivity of different samples are plotted in Figure 3.1 with values ranging from 7.94-1m-1 to 595.2-1m-1. Exceptional purity and hole concentrations in the order of 1019 cm-3 are believed to contribute to the remarkable electrical conducting properties of samples synthesised from thiourea. (Refer to Annex E) This hole concentration is attained mainly due to the low to minimal band gap value of CuS, around 1.2 eV (Itoh 2006) as well as the presence of grain boundaries which are shown to create abundant defect states (Feng 2017). Since electrical conductiviy is given by:

where n, e and u are the carrier concentration, charge and charge carrier mobility respectively, the trends among the samples can be evaluated using their grain size and hole concentration (Nolas 2013). Small grain sizes induces a considerable number of grain boundaries which greatly reduces hole mobility by hole scattering (Yamada 2010). Hence electrical conductivity generally rises along with increased grain size with the anomaly of TU1 due to its exceptionally poor hole concentration. Moreover, the presence of a sizeable amount of impurites reduces the electrical conductivity of TU1 further, to a 97.1 -1m-1 low at room temperature. Among all samples, TU2 has the highest ever reported Cu2-xS electrical conductivity of 595.2 -1m-1 at room temperature, a factor of 102 higher than TU1, labelling it as a suitable thermoelectric material to recover electricity even at room temperature (Narjis 2018).

Seebeck coefficients are shown in Figure 3.2 in the order of μVK−1. The values of Seebeck coefficient are optimistic for CuS nanocrystals owing to the existence of grain boundaries. Despite increasing interface phonon-scattering(Lan 2010) and suppression of the phonon drag effect, preferential scattering of low-energy electrons increases the number of high energy electrons in CuS and hence make the system more conducive for charge to move into a lower energy state, increasing Seebeck coefficient. This effect is apparent in TU1.5 with the most grain boundaries. Seebeck coefficient can be determined using the Mott formula (Mott 1968):

where T is absolute temperature, n is the carrier concentration and m* is the effective mass of the carrier. Positive values of Seebeck coefficient indicates the charge carriers as holes (Snyder 2011). Generally, p-type materials demonstrates poor Seebeck coefficient due to lower m* of holes relative to electrons (Hosono 2006). However, due to low hole concentrations, DTO2 reached a Seebeck coefficient of 21.3 μVK−1 high at 573K while TU2 has the lowest Seebeck coefficient of 12.3 μVK−1 . However, this lowest value is nearly 3 times greater than single crystalline CuS with α = 5 μVK−1,which emphasises the significance of nanostructuring in improving α. Nevertheless, the α2 of samples increases less than proportionately than their decrease in in temperatures > 300K, shifting the focus on optimising power factor at room temperature to synthesising samples with higher electrical conductivity in the range where Seebeck coefficient remains similar.

Power factor of all samples is determined using . Despite having the lowest Seebeck coefficient, TU2 has the highest power factor of 51.4 μWm-1K-2 at room temperature and reaches a peak of 55.7 μWm-1K-2 at 523K. This value is substantially higher than previously reported CuS with a power factor of 4.5 μWm-1K-2 and Cu1.8S at 1.93 μWm-1K-2 at room temperature. (Narjis 2018).

As shown in Figure 3.3, CuS is a potential candidate for room temperature thermoelectric as its synthetic cost is relatively inexpensive and has a unique temperature dependence of power factor. Owing to its metallic behavior of decreasing electrical conductivity and increasing Seebeck coefficient as temperature rises, there exists a reasonably high power factor of 51.4 μWm-1K-2 for CuS at low temperatures as shown by sample TU2. DTO1 and DTO2 has comparatively lower power factors of 1.33 μWm-1K-2 and 2.82 μWm-1K-2 despite their higher Seebeck coefficient. Hence, having pure samples is an indispensable requirement to obtaining the optimum power factor.

Figure 3.3: Power Factor dependence on temperature

Thermal conductivity results from two contributions. κ = κL + κC where κL is the lattice thermal conductivity and κC is the carrier thermal conductivity. Due to no commercially available device to test for parallel thermal conductivity and the anisotropic properties of all samples as evident by their preferred orientation, κC and κL are obtained from existing literatures and calculations. κC is determined using the equation,, (Snyder 2011) where L is the Lorenz number obtained by the following equation proposed by Kim et al to minimize deviation from experimental thermal conductivity values. (Kim 2015):

where L is in 10−8WΩK−2 and in µVK-1. κL values were taken from Tarachand et al, which synthesised similar CuS nanocrystals, with analogous lattice thermal conductivity (Tritt 2005), and extrapolated to the desired range. Thermal conductivity is then determined through the sum of κC and κL. Samples with a high conductivity allows for the transporting of heat with holes and reveals a greater dependence on κC than κL, with TU2 having the highest thermal conductivity of 7.10 Wm-1K-1 at room temperature. Hence, the key to obtain the optimal power factor is to strive a good balance between thermal conductivity, electrical conductivity and Seebeck coefficient by varying hole concentration (Terasaki 2011). This can be engineered through adjusting concentration of sulphur precursor, which demonstrated a positive relationship with hole concentration in samples.

Figure 3.4: κ dependence on temperature

Thermoelectric figure of merit is calculated for all samples across a range of temperatures. TU2 has the highest figure of merit of 0.00220 at room temperature while TU4 has the maximum ZT of 0.00409 at 564K. While DTO samples has lower thermal conductivity due to limited carrier concentration, it has extremely low electrical conductivity resulted in DTO1 low ZT of 0.000146 at room temperature. Figure 3.5 confirms the hypothesis that power factor is a good comparison of thermoelectric efficiency as it follows the trend of the figure of merit. Thiourea is evident to be a better sulphur precursor for future exploration of Cu2-xS thermoelectric.

Figure 3.5: ZT dependence on temperature

Conclusion, Limitations and Suggestions for Further Research

Both synthesis by thiourea or dithiooxamide sulfur precursor results in the formation of sulfur rich Cu2-xS. Synthesis by dithiooxamide led to the presence of a large amount of impurities while synthesis with high thiourea concentration produces comparatively pure covellite. Due to the presence of impurities, CuS prepared with dithiooxamide has low electrical conductivity and hence a unsatisfactory power factor. Alternatively, despite having the lowest Seebeck coefficient, TU2 has a ZT of 0.00220 and power factor of 51.4 μWm-1K-2 at room temperature which exceeds the CuS reported in Tarachand et al with a ZT of 0.00187 and Najis et al with a power factor of 4.5 μWm-1K-2. Thus, thiourea is a better candidate for the solvothermal synthesis of pure CuS than dithiooxamide, which is also more expensive. Cu:S stoichiometric ratio of 1:2 should be adopted as evident by the superior power factor of TU2.

Regardless of a relatively lower maximum figure of merit, chemically stable CuS nanocrystals fill the research gap of room temperature thermoelectric by offering satisfactory thermoelectric performance with a low synthetic cost coupled with its non-toxicity nature. Despite the lack of thermal conductivity measurements, lattice and carrier thermal conductivity of samples were determined from existing literature and theoretical equations respectively. This predicted value, while not being fully accurate, allows a rough comparison of its ZT with other thermoelectric materials. Furthermore, there also exists a possibility in thiourea samples to induce anisotropic conductivity to be beneficial for thermoelectric device due to the presence of preferred orientation. Moreover, there is an absence of mechanism describing the formation of different grain sizes. Further exploration into these factors will allow the optimising of grain boundaries, hence ZT of CuS. Since α2/κ increases less proportionately than the decrease in electrical conductivity at room temperature, this study suggests that the basis of synthesising room temperature thermoelectric material in future studies is to enhance electrical conductivity further in the range where Seebeck coefficient remains similar. This can be achieved through tuning its grain size to look for the optimal point of power factor at room temperature.

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Annex B

Formation of copper sulfide through the intermediate of a Copper-thiourea complex (Rahmani 2017).

Annex C

Electrical conductivity,

Hence,

(Goldsmid 2017)

Annex D

TABLE 3: Different stoichiometric ratio of precursors added for samples

Sample Copper (II) bromide Dithiooxamide Thiourea Dimethylformamide
DTO1 1 1 0 10ml
DTO2 1 2 0 10ml
TU1 1 0 1 10ml
TU1.5 1 0 1.5 10ml
TU2 1 0 2 10ml
TU4 1 0 4 10ml
Annex E

TABLE 4: Carrier concentrations of samples

Carrier Concentration (cm-3) DTO1 DTO2 TU1 TU1.5 TU2 TU4
1.6×1017 3.7×1017 1.5×1018 1.6×1018 5.7x 1018 9.3×1018
Annex F

Figure 5: Abundance of different thermoelectric elements (Han 2014)

Figure 6: Price of different thermoelectric elements relative to 99.99% Te powder (Han 2014)

Annex G

TABLE 5: Different samples and their electrical conductivity, Seebeck coefficient, power factor, thermal conductivity and figure of merit at different temperatures

Sample Temperature/K Electrical Conductivity/-1m-1 Seebeck Coefficient/ μVK−1 Power Factor/μWm-1K-2 Thermal Conductivity/ Wm-1K-1 Figure of Merit*103
DTO1 305 10.582 11.2 1.33 2.785 0.1459
322 10.384 11.7 1.41 2.818 0.1613
369 9.803 12.9 1.63 2.902 0.2074
418 9.433 14.8 2.08 2.971 0.2927
467 9.090 16.4 2.44 3.029 0.3762
515 8.474 17.5 2.59 3.078 0.4334
564 7.936 20.7 3.42 3.105 0.6213
DTO2 324 16.469 13.1 2.82 2.866 0.3187
370 15.406 14.2 3.12 2.951 0.3912
419 14.487 15.8 3.62 3.021 0.5021
468 13.946 17.6 4.33 3.082 0.6575
517 13.312 19.3 4.96 3.137 0.8176
566 12.673 21.3 5.72 3.167 1.0222
TU1 305 97.087 9.79 9.33 3.422 0.8302
324 95.238 10.6 10.7 3.482 0.9949
369 102.040 10.5 11.2 3.724 1.1099
382 103.305 11.2 13 3.827 1.2975
466 65.359 12.5 10.2 3.658 1.2993
515 134.770 12.4 20.8 4.640 2.3087
563 273.224 11.7 37.4 6.698 3.1436
TU1.5 305 8.620 10.1 0.872 2.771 0.0960
323 8.403 9.98 0.836 2.804 0.0962
370 23.419 11.3 2.97 3.023 0.3633
419 31.055 11.6 4.22 3.190 0.5543
468 19.685 13.6 3.65 3.148 0.5426
516 28.248 16.8 7.95 3.320 1.2357
565 158.730 14.6 33.7 5.136 3.7073
TU2 304 595.238 9.28 51.4 7.095 2.2041
322 571.238 9.31 49.6 7.197 2.2203
369 515.463 9.87 50.1 7.419 2.4941
418 460.829 10.1 47.2 7.532 2.6194
467 413.223 11.4 53.3 7.572 3.2874
516 378.787 12.1 55.7 7.668 3.7483
564 348.432 12.3 52.3 7.715 3.8233
TU4 304 375.939 9.05 30.8 5.477 1.7087
322 359.712 8.91 28.6 5.550 1.6605
369 327.868 9.36 28.8 5.749 1.8504
418 299.401 10.1 30.6 5.901 2.1674
467 272.479 11.5 36.3 5.989 2.8305
515 248.756 12.8 40.6 6.044 3.4595
564 233.644 13.8 44.6 6.147 4.0924
  1. SEM images of samples synthesized from dithiooxamide are in an amorphous state, hence not shown.

About the author

Ka Shing is a Singaporean student from River Valley High School who is interested in Materials Science since 15 years old. He participates in various research attachments to gain insights of how a scientist works.

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