Ag-Nanoinclusion-Induced Enhanced Thermoelectric Properties of Ag2S

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Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Ag-Nanoinclusion-Induced Enhanced Thermoelectric Properties of Ag2S Tarachand,† Bodhoday Mukherjee,† Monika Saxena,† Yung-Kang Kuo,‡ Gunadhor Singh Okram,*,† Siddhartha Dam,§ Shamima Hussain,§ Archana Lakhani,† Uday Deshpande,† and Thoudinja Shripathi† †

UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452001, India Department of Physics, National Dong-Hwa University, Hualien 974, Taiwan § UGC-DAE Consortium for Scientific Research, Kalpakkam Node, Kokilamedu 603104, India Downloaded via 5.189.205.210 on August 21, 2019 at 05:52:40 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: The effects of Ag nanoinclusions on thermoelectric properties of Ag2S semiconducting nanostructures, synthesized by a novel one-pot facile polyol method, have been investigated. The resulting products are characterized by powder XRD, EDAX, XPS, and UV−vis techniques. FESEM images reveal the formation of disc-shaped Ag2S nanoparticles with an average thickness of 52 nm and diameters ranging from 50 nm to a few hundreds of nm. All samples show a systematic reduction in electrical resistivity with increasing Ag content in the composites. The Seebeck coefficient (α) values for the Ag nanoparticle-incorporated Ag2S nanocomposites are notably high near 300 K because of the low-energy charge-carrier filtering effect, which is due to preferential scattering of low-energy electrons at the barrier potentials set up at metal−semiconductor interfaces. The theoretical fitting of α data reveals a systematic shift of the Fermi level toward the conduction band edge with increasing Ag content in the composites. A significantly improved thermoelectric power factor at 325 K is observed for a wide range of Ag nanoinclusions with the highest ZT of 0.0029 at 325 K in the Ag2S−Ag nanocomposite with 20.1% Ag. KEYWORDS: polyol method, thermoelectricity, Ag nanoinclusion, ductile Ag2S, Ag2S−Ag, nanocomposites, band bending, semiconductor to metal transition

1. INTRODUCTION

emerging materials are semiconductors in which one can reduce the value of κ by several ways such as nanostructuring, point defect introduction, and selecting highly anisotropic layered materials that contain constituent heavy elements in large unit cells. Among these, nanostructuring is known as a promising way that can be done in three ways: (i) reduction of the grain size down to nanoscale,1,3,4 (ii) introduction of nanocrystals into nanomaterial hosts,5−8 and (iii) introduction of nanoparticles (NPs) into bulk hosts.2,9,10 A good ZT value (∼300 K) can be achieved by nanostructuring alone,1,3,4 but the formation of nanocomposites by introducing nanocrystals in nanomaterials/bulk host is

Production of energy through alternative sources with less waste heat, harmful gases, or noise is of utmost importance for sustainability. Given the huge power consumption demand, thermoelectric (TE) devices have attracted much attention from researchers because TE devices convert waste heat into electricity silently without producing harmful byproducts and have long durability with low maintenance. To achieve the goal of their compatibility with conventional sources of energy, however, highly efficient TE materials are needed. The performance of thermoelectric devices is determined by a dimensionless figure of merit, ZT = α2T/(ρκ), where ρ, κ, and T are electrical resistivity, total thermal conductivity, and absolute temperature, respectively.1,2 The main challenge is the construction of the solid materials such that they should possess high α and σ (1/ρ) but low κ. For this, the best © XXXX American Chemical Society

Received: May 22, 2019 Accepted: August 5, 2019

A

DOI: 10.1021/acsaem.9b01016 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials a unique strategy for simultaneous reduction of κ through combined grain boundary scattering of heat carriers, together with the enhancement of a power factor (α2/ρ) induced by low-energy charge-carrier filtering.2,5−11 Mehta et al.4 demonstrated a readily scalable bottom-up approach to create bulk TE nanomaterials of both n- and p-type pnictogen chalcogenides with ZT > 1 at 300 K. Li et al.8 demonstrated the synthesis of Bi2Te2.7Se0.3/SnS2 nanocomposites through the polyol method and reported an enhanced ZT ∼ 0.93 at 450 K for this nanocomposite with 1.47 vol % of SnS2, which is 2.3 times higher than that of pristine Bi2Te2.7Se0.3. They attributed the enhancement to the precise control of SnS2 layer thickness of atomically thin interface in Bi2Te2.7Se0.3/SnS2 nanocomposites which leads to better phonon scattering and lowenergy charge-carrier filtering.8 Biswas et al.2 achieved a ZT value of 2.2 at 915 K in powder-processed and spark plasmasintered mesostructured p-type PbTe incorporated with endotaxially nanostructured 4 mol % SrTe. The enhancement was attributed to the multiscale scattering of phonons by point defects, nanoscale endotaxial precipitates, and mesoscale grain boundaries. Unfortunately, the most promising TE devices thus available commercially contain rare and toxic elements (Pb, Te, Se, Bi, Sb, etc.) which are harmful to the environment. In addition, these materials are also rigid and hard with negligible ductility, increasing the risk of mechanical damage, device failure, and finally, creating environmental problems.12 This tends to restrict their uses in several places including automobiles. This issue can be resolved if such devices are made with naturally abundant nontoxic ductile materials. To this end, semiconducting Ag2S, with good ductility at room temperature,13 has emerged as a potential candidate for replacing them. It crystallizes in a monoclinic crystal structure (space group P21/ c) below ∼450 K known as α-phase (acanthite), above which it transforms into β-phase (argentite) with cubic structure.14 It is a direct, narrow band gap (∼1.1 eV) layered semiconductor with relatively large absorption coefficients (∼106 cm−1),13,15,16 high chemical stability, natural abundance, and negligible toxicity,17 attracting great interest for its potential applications in resistance-switches and nonvolatile memory devices,18 photocatalysts,15 good antibacterial properties,19 solar cells,20 and thermoelectrics.21,22 The electronegativity difference (Δχ) among constituent elements of Ag2S is 0.65 close to the required value of ≤0.5 for higher TE power factor (higher carrier mobility)23 indicating its potential as TE material. Xiao et al.21 show 12 nm monodisperse Ag2S nanocrystals with the highest ZT values of 0.12 at its semiconducting (α-Ag2S) to superionic conductor (β-Ag2S) phase transition temperature of 454 K. Furthermore, a high Seebeck coefficient of −198 μV/K was achieved in nanocrystalline Ag2S thin film at 400 K with an Ag:S = 0.33 at. % concentration in spray solution.22 This value of α is significantly enhanced for single-phase Ag2S bulk to nearly α400 K = −625 μV/K.13 Even though it has relatively high α and low κ,13,21,22 less attention has been paid because of its high electrical resistance (>106 Ω). They suggest that the thermoelectric performance of Ag2S can be improved at low temperature by reducing ρ through crystallite size variation and tuning the atomic ratio of Ag:S.13,21,22 For instance, Faleev et al.11 reported theoretically an enhancement of power factor (α2/ρ) for an optimum volume fraction of metal nanoinclusions, wherein the electrical resistivity was decreased without a simultaneous decrease in the Seebeck coefficient. Lee et al.7 showed a 67% ZT

improvement in Au-nanodot decorated Bi2Te3 composite over the pure Bi2Te3 because of electron energy filtering and phonon scattering effects. Zhang et al.24 also reported a maximum ZT ∼ 0.71 at 475 K, 4.43 times that of its pure counterpart (ZT ∼ 0.16) in a bottom-up synthesis of 1.5 vol % Ag nanowire dispersed in Bi2Te3 nanoparticles. This better performance is again due to the high density of interfaces plus increased electrical conductivity. Ibáñez et al.25 introduced 4.4 mol % Ag nanodomains in nanostructured PbS by a bottom-up approach to obtain ZT ∼ 1.7 at 850 K, which is over 3 times that of pure PbS NPs. They revealed the assistance of Ag NPs in the scattering of heat-carrier phonons, in addition to injecting electrons and facilitating charge transport between PbS nanocrystals. This leads to partial decoupling of α and σ in bottom-up-processed PbS-Ag nanocomposites and hence exhibits improved σ/κ. Thus, introduction of a small fraction of resistive nanocrystals into a relatively less resistive/conducting host5,6,8−10 or conducting nanocrystals into a resistive/semiconducting host7,11,24,25 normally provides a well-stabilized strategy for optimization of charge concentration and hence enhancement of ZT. Moreover, inhomogeneous distribution and/agglomeration of incorporated nanoparticles in the TE matrix is of significant importance and a challenge to get an enhancement in the TE performance.26 With this motivation, we have incorporated Ag NPs in nanostructured Ag2S by a simple bottom-up approach using a novel polyol method which produces homogeneous Ag2S−Ag composites. Their enhanced thermoelectric properties have thus been investigated systematically.

2. RESULTS AND DISCUSSION 2.1. X-ray Diffraction and Energy-Dispersive Analysis of X-rays. Control of the phase fractions of insulating Ag2S and metallic Ag NPs in the reaction conditions during their synthesis will decide the electrical and thermal conductivity and the thermoelectric properties of the final composite. To enable this, a series of samples with different Ag:S ratios have been prepared by varying the quantity of thiourea (S-source) systematically (Table S1). The resulting products are first characterized by powder X-ray diffraction (XRD) to get information about phase purity and crystallinity (Figure 1). In this analysis, the XRD peaks of sample (ASA0) prepared with 2.5 times excess quantity of thiourea of nominal composition (i.e., 2.5 × 3 mmol) match well with JCPDS card no. 240715 and 652356 of monoclinic phase of pure Ag2S without any impurity peaks. The Rietveld refinement of ASA0, acanthite δAg2S, confirmed its single-phase nature with space group P21/c (Figure 2a). On the other hand, the samples viz. ASA1, ASA2, ASA3, ASA4, ASA5, and ASA7, synthesized with thiourea ≤3 mmol exhibit extra peaks corresponding to the face-centered cubic (FCC) phase of Ag metal (JCPDS 893722) in addition to the monoclinic Ag2S (Figure 1). They well-fit in doublephase Rietveld refinement (Figure S1). Initially, the required parameters were taken from refs 27,28; the calculated phase fractions along with other obtained parameters are tabulated in Tables 1 and S2. The mass density of monoclinic phase of Ag2S in composites is slightly increased as compared with the single-phase Ag2S (ASA0). The lattice parameter a of FCC phase of Ag is slightly larger in composites (Table S2), which is associated with slightly lower mass densities than its bulk counterpart.28 The measured mass density of Ag2S−Ag nanocomposites increases with increasing Ag content and is B


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some of the Ag atoms may lie at grain boundaries in the amorphous state or go to Ag2S lattice and that a fraction of this may be converted to crystalline Ag with FCC phase. Typical arrangements of atoms in monoclinic crystal structures (P21/c) of δ-Ag2S are presented in Figure 2b−d. These structures are generated using the CIF file obtained after Rietveld refinement of XRD of ASA0 through VESTA software. Figure 2b shows the unit cell of α-Ag2S in which Ag atoms occupy two different 4e Wyckoff sites, viz., Ag1 (0.0715, 0.0151, 0.3093) and Ag2 (0.7264, 0.324, 0.4375), whereas sulfur atoms occupy one 4e site S (0.492, 0.2339, 0.1321). Clear slipping planes (100) stacked along direction are visualized in Figure 2c where zigzag atomic layers of (100) planes can slip along the direction for α -Ag2S. The Ag−S framework comprises a distorted, 4 × Ag−S, octagon (Figure 2d). The minimum distance between Ag1−S along the direction is 2.46676 Å, whereas the Ag2−S bond length along the direction is 2.53358 Å, respectively (Figure 2d). These lengths are smaller than those of nonstoichiometric silver sulfide Ag1.93S,27 confirming the formation of dense structured stoichiometric Ag2S nanocrystals. It is interesting to note that proper stirring of the solution during the reaction of the precursors and careful washing steps are essential for the systematic formation of Ag2S−Ag composites at 160 °C. Otherwise, impurity peaks corresponding to the unreacted precursors can appear in XRD (Figure S2). Furthermore, for compositional study, the energy dispersive analysis of X-ray (EDAX) measurements are performed on pellets of ASA0, ASA1, ASA3, and ASA5, and spectra are shown in Figure S3a−d. This gives the atomic percentage of constituent elements in composites (Table 2). Here, ASA0 shows exactly 2:1 elemental ratio of Ag:S. However, in the case of ASA1, the Ag component is greater than the nominal value, which reveals the presence of excess

Figure 1. Powder XRD spectra of Ag2S−Ag nanocomposites synthesized by varying the quantity of thiourea from 2.55−7.5 mmol (written in parentheses) with a fixed Ag source. Standard reference cards of face-centered cubic (FCC) structured Ag (JCPDS893722) and monoclinic structured Ag2S (JCPDS652356) are inserted at the bottom of spectra.

very close to the values calculated through Rietveld fitting as tabulated in Table S1. Interestingly, the phase fractions found for ASA1 (Table 1) prepared using nominally required quantities of initial precursors; viz., 6:3 of Ag2S are however 94.64% monoclinic Ag2S and 5.36% fcc Ag, against the nominal 100% monoclinic phase of Ag2S. This confirms the volatile nature of sulfur that comes out from the reaction in the form of gases (H2S). These deficiencies of sulfur lead to the formation of Ag2S−Ag nanocomposites. This suggests the controlled increase of the metallic phase fraction of Ag can be possible by reducing the quantity of the sulfur-source. This is enabled by reducing thiourea from 3 to 2.55 mmol. For higher Ag-containing composites like ASA3, ASA4, ASA5, and ASA7, the obtained phase fraction of Ag is less than nominal values, indicating that

Figure 2. (a) Rietveld refinement of powder XRD of ASA0 (pure Ag2S). The crystal structures of α-Ag2S in (b) 3D unit cell, (c) a−b plane, and (d) b−c plane views. In panels b−d, the rectangular box made of gray lines is the unit cell of Ag2S. C


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Table 1. Obtained Parameters from Rietveld Refinement of All Samples Prepared with Varying the Quantity of S-Source for Both Monoclinic (M) Ag2S and FCC Ag Phasesa parameters a (Å) monoclinic FCC b (Å) c (Å) V (Å3) M FCC ρv(g/cm3) M FCC β (deg) phase fraction (%) M [nominal] FCC [nominal]

ASA0

ASA1

ASA2

ASA3

ASA4

ASA5

ASA7

(6:7.5)

(6:3)

(6:2.85)

(6:2.75)

(6:2.7)

(6:2.65)

(6:2.55)

4.227(1)

4.226(1) 4.087(1) 6.926(1) 9.534(1) 227.028 68.254 7.27 10.284 125.549(1) 93.45 [100] 6.55 [0]

4.227(1) 4.087(1) 6.928(1) 9.534(1) 227.14 68.276 7.274 10.278 125.559(2) 88.85 [90.48] 11.15 [9.52]

4.227(1) 4.086(1) 6.927(1) 9.533(1) 227.037 68.239 7.272 10.29 125.566(2) 87.69 [84.62] 12.31 [15.38]

4.228(1) 4.087(1) 6.928(1) 9.534(1) 227.1 68.25 7.261 10.284 125.578(2) 85.77 [84.82] 14.23 [18.18]

4.226(1) 4.086(1) 6.926(1) 9.531(1) 226.941 68.211 7.297 10.29 125.570(2) 85.19 [79.11] 14.81 [20.09]

4.228(1) 4.087(1) 6.928(1) 9.535(1) 227.165 68.266 7.24 10.283 125.567(2) 80.99 [73.91] 19.01 [26.09]

6.927(1) 9.533(1) 227.061 7.23 125.576(1) 100 [100] 0 [0]

a

The errors are written in parentheses.

Table 2. Elemental Compositions of Ag2S−Ag Nanocomposites Obtained from EDAX along with Nominal Values of Both Monoclinic (M) Ag2S and Face-Centered Cubic (FCC) Ag Phases elements

ASA0

ASA1

ASA3

ASA5

observed Ag (at. %) [nominal M + FCC] observed S (at. %) [nominal M]

66.5 [66.7] 33.5 (33.3) [33.3]

67.6 [66.7] 32.4 [33.3]

69.7 [62.9 + 5.7 = 68.6] 30.3 [31.4]

70.4 [61.3 + 8.1 = 69.4] 29.4 [30.6]

Figure 3. FESEM images of (a and b) Ag2S NPs (ASA0) and (c and d) Ag2S−Ag nanocomposites (ASA6) collected with different magnifications.

images show that particles are in random sizes and shapes. Most of the particles have circular, cubic, rectangular, or striplike disk shapes with thicknesses varying from 25 to 65 nm with an average of 52 ± 1 nm and have wide variation in length or width in the range of 50 nm to few hundreds of nm (Figure 3a,b). On other hand, the FESEM images of ASA6 show a slightly thicker and clear nanodisks (NDs) along with some additional spherical NPs attached on them. These spherical NPs have diameters in the range of 50 to 100 nm, which may be Ag-NPs formed due to the presence of excess quantity of

metallic Ag as a second phase. This is formed due to the volatile nature of sulfur which is also evident from XRD data (Figure 1). In ASA3 and ASA5 also, the sulfur component is less than the expected values (Table 2). 2.2. Field Emission Scanning Electron Microscope (FESEM). For additional information on size and shape of NPs, FESEM measurements are performed for single-phase Ag2S (ASA0) and heterostructure Ag2S−Ag (ASA6) by spreading the fraction of as-synthesized powder on a conducting carbon tape followed by 10 Å gold coating (Figure 3a−d). These D


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Figure 4. EDAX spectra of Ag2S−Ag nanocomposite (ASA6) collected by focusing electron beam size at very small regions of a (a) bigger nanodisk and (b) smaller spherical nanoparticle. Insets are FESEM images of corresponding samples, and spots of interest are indicated by “::” (four points).

Figure 5. High-resolution XPS spectra of (a) Carbon 1s, (b) Ag 3d, (c) S 2p, and (d) Oxygen 1s. Experimental data are represented by black open circles, and the red curve is fitting line which is the summation of all peaks.

penetration depth of the electron beam used is larger than the size of targeted particles. In addition, the electron beam can also penetrate inside the Ag2S ND. This confirms our expectation that larger disk-shaped particles are Ag2S covered or attached with excess Ag atoms perhaps in amorphous form and smaller spherical Ag NPs. These FESEM images and EDAX analysis of Ag2S−Ag nanocomposites (ASA0 and ASA6) suggest that Ag content increases in composites wherein excess Ag atoms start nucleating and form smaller nanoparticles of the Ag metal on the surface of larger Ag2S nanodisks. Therefore, FESEM in combination with EDAX analysis of ASA6 shows the existence of two types NPs (Ag2S and Ag) together in the composite. These results are well-corroborated with the Rietveld refinements of XRD described above. 2.3. X-ray Photoelectron Spectroscopy (XPS). Surface information on Ag2S−Ag nanocomposites is further explored using the XPS technique which can probe down to a few nanometers only. The survey scan spectrum of ASA1, synthesized with nominally required quantity of initial

AgNO3 in the reaction (Figure 3c,d). Some of the bigger particles are also observed, which may be due to aggregation of smaller NPs (Figure 3a−d). In order to reveal the formation of Ag2S and Ag heterostructures in ASA6 nanocomposites, we collected location-specific EDAX of ASA6 by focusing the electron beam on individual ND of Ag2S (Figure 4a) and spherical NPs of Ag (Figure 4b). For confirmation of this, EDAX data were collected from three different spherical NPs, and their obtained atomic ratios along with one scan for disk-shaped Ag2S NPs are tabulated in Table S3. When EDAX was collected on a compressed pellet of ASA0 (Ag2S), the atomic ratio of Ag:S was almost exactly 2:1 (Table 2), but on disk-shaped Ag2S (Figure 4a), the ratio is 73.7:26.3, which is much greater than 2:1, indicating the presence of excess Ag lying on Ag2S nanodisks (Figure 3c,d). In all three runs on spherical particles, the atomic ratio of Ag:S has an average of 96.5:3.5 (Table S3), indicating well that these spherical NPs could be mostly Ag NPs. The presence of ∼3.5 at. % of sulfur is expected to come from larger Ag2S NPs below Ag NPs because spot size and E


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Figure 6. (a) UV−visible absorption spectra of Ag2S−Ag nanocomposites with Ag nanoinclusions of 0−23.5% in Ag2S host. (b) Photoluminescence emission spectra of Ag2S (ASA0) collected using an excitation wavelength of 476 nm.

Figure 7. Schematics of (a) band diagram of separate Ag and Ag2S NPs, (b) band bending in Ag2S−Ag nanocomposites, and (c) possible mechanism for observance of photoluminescence in Ag2S NPs.

with Ag−S bonds.19,33 This result also confirms the absence of unreacted AgNO3 in the products for which the absorption peak is expected at 300 nm.34 The second absorption peak edge at ∼600 (600−700) nm is due to its blue-shifted optical band gap,13,15,16 which is consistent with observed mode for Ag2S NPs.13,15,16,19 However, in all other samples, the optical absorption edges of Ag2S nanoparticles have shifted toward the longer-wavelength region with an increase in the Ag content. The expected additional broad absorption edge peaks in the visible region (320−500 nm) due to emergence of surface plasmon resonance (SPR) of metallic Ag NPs in Ag2S−Ag nanocomposites are absent and are attributed to the highly sensitive nature of the aspect ratio of NPs35 and the influence of the surroundings.29,35,36 This may serve as an additional evidence for good attachments of Ag NPs on Ag2S nanodisks corroborating well that found in EDAX. Now, the Fermi level of metallic Ag and the bottom of the conduction band of Ag2S are located from vacuum level at −4.26 and −4.42 eV, respectively (Figure 7a).29 When Ag NPs come into contact with Ag2S NPs, electron transfer will be facilitated when the conduction electrons near the Fermi level of metallic Ag fill the empty, low-energy states available in Ag2S (Figure 7b). Because of this charge transfer from Ag NPs to Ag2S NDs, a broad SPR mode that is expected at 320−500 nm is missing (Figure 6a), and hence, band bending of the semiconducting side at the interface is expected to take place (Figure 7b). This also leads to the formation of a contact potential barrier, which prevents the further charge transfer. This may suggest that a large number of Ag NPs are not able to separate completely from the host Ag2S NPs, and only a fraction of Ag NPs come into solution (water) freely when they are dispersed in water, for example. The zeta potential (ζ) study of these composites with very low ζ values in deionized water (Figure S5) may support this (see the section Dynamic

precursors, is recorded in the range of 0−1300 eV (Figure S4), which confirms the absence of impurity elements. The highresolution spectra of C, Ag, S, and O are recorded for their detailed valence state analysis (Figure 5). The C 1s peak can be deconvoluted into three peaks centered at 284.6, 286.5, and 288.12 eV and are assigned as C 1s peaks for C−C/CC, −C−OH, and −CO,6 respectively (Figure 5a). The C 1s peak (284.6 eV) is taken as the internal reference for the whole spectra. In Figure 5b, the Ag 3d spectrum is deconvoluted into two pairs of peaks. The first pair of peaks occurring at 367.51 and 373.5 eV should be assigned to the binding energies of Ag 3d5/2 and Ag 3d3/2 of the Ag+ ions in Ag2S, respectively.29−32 On the higher-energy side, the smaller components at 368.25 and 374.21 eV could be attributed to Ag 3d5/2 and Ag 3d3/2 of metal Ag0.29−32 The relative spectral intensities between Ag0 and Ag+ components (Figure 5b) confirmed the presence of a fraction of Ag0 atoms in the composite. The deconvolution of S 2p peak gives two peaks located at around 160.8 and 162.02 eV, which can be assigned to S 2p3/2,31 and S 2p1/2,29,30 respectively (Figure 5c). They are excellently consistent with the S2− in the lattice of Ag2S. Figure 5d shows the spectrum of O 1s located at 532.24 eV which correspond to adsorbed byproduct containing >CO group or H2O molecules.6,31 These results further confirm the coexistence of two separate phases of Ag and Ag2S in composites and corroborated well with the findings from XRD, EDAX, and FESEM above. 2.4. UV−visible Absorption and Photoluminescence Spectroscopy. To validate further the existence of Ag2S and Ag together in heterostructured nanocomposites, we performed UV−vis absorption spectroscopy for samples synthesized with varying quantities of thiourea (Figure 6a). Ag2S NPs (ASA0) dispersed in deionized water show two obvious absorption edges. The first is at 260 nm, which is due to the n →; σ* transition of nonbonding electrons of sulfur in Ag2S F


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Figure 8. Temperature-dependent (a) electrical resistivity and (b) Seebeck coefficient (α) and (c) thermal conductivity data of Ag2S−Ag nanocomposites synthesized with different ratios of initial precursors. Inset in (b): expanded portion of α in smaller scale.

Figure 9. Schematic of (a) sulfur vacancy (VS) and Ag antisite defect (AgS) creation in Ag2S and formation of nucleation centers for Ag NPs at the Ag-rich grain boundary regions of two neighboring grains (region inside the pink solid line) and (b) the possible scattering of low-energy carriers at the semiconductor to metal interfaces in Ag2S−Ag nanocomposites. In (b), very small gray patches are indicative of clusters of Ag atoms.

mmol) and ASA1 (TU = 3 mmol) samples are not possible to record by four-probe ρ measurements because they had high ρ. This high ρ of the nanocomposites can be suppressed by increasing the Ag content on it.22 Figures 8a and S7a show the ρ data of Ag2S−Ag nanocomposites made with varying the quantity of sulfur source. Herein, ASA2 (TU = 2.85 mmol) shows a drastic reduction in ρ values as compared with ASA1, and its sharp exponential increase with lowering of temperature (Figure S7a) is attributed to semiconducting behavior. Furthermore, nanocomposites with 2.75 mmol TU (ASA3) shows initially a semiconducting nature down to 225 K, below which it exhibits metallic behavior down to 5 K (Figure 8a). Similar trends are followed by other samples made with a further reduction of TU wherein the semiconducting to metal (SM) transition temperature (TSM) shifts toward the higher temperature side, and for the last sample, ASA7 (2.55 mmol TU), the SM transition disappeared and exhibits fully metallic nature in the entire range of 5−325 K (Figure 8a). This behavior of ρ can be understood in terms of formation of Ag2S−Ag nanocomposites which are already evident in Rietveld refinement of powder XRD, EDAX, XPS, and UV− vis. Here metallic Ag NPs are introduced in the semiconducting Ag2S via a bottom-up approach. Single-phase Ag2S is highly resistive because of its wide band gap and is reduced by formation of sulfur vacancies (see Figure 9a). In these nanocomposites, the presence of the excess Ag source increases sulfur defects, particularly near the grain boundaries because of the incomplete atomic bonding and atomic disorder near the grain boundary region. This leads to an accumulation of excess Ag atoms on the grain boundaries of Ag2S NPs. These Agatom-dominated regions will turn into nucleation centers,

light scattering in Supporting Information), wherein most of the bigger crystalline particles get settled at the bottom of the cuvette and absorption comes from very small or amorphous NPs in the solution. Additionally, the optical band gap of Ag2S NPs is estimated from UV−visible absorption data using the Tauc or Bardeen equation16 (Figure S6). As Ag2S is a direct band gap semiconductor, the straight line fit to a plot between (ahν)2 versus hν gives Eg, where a is the absorption coefficient and hν is the photon energy. The obtained band gap for the pure Ag2S (ASA0) is 1.7 eV, in good agreement with previously reported values of NPs.15,16,19 To justify the above assumption, the photoluminescence (PL) emission spectra are collected for ASA0 (Ag2S) using a laser exciting source with a wavelength of 476 nm (2.6047 eV) (Figure 6b). The main emission peak of ASA0 (pure Ag2S) is centered at 717 nm (1.729 eV), which can be ascribed to the band gap recombination of electron−hole pairs.13,15,16,19 The schematic of the PL phenomenon is illustrated in Figure 7c. 2.5. Transport Properties. 2.5.1. Resistivity Studies. Adsorbed EG or moisture on the obtained powder samples as seen from XPS data has been removed before the measurements of transport properties. For this, pellets of all samples are made by applying a nominal pressure and placing them into a tubular furnace at 200 °C for 5 h under a continuous flow of Ar gas at the rate of 160 cc/min. After calcination, all pellets were crushed into fine powder, which was then consolidated into pellets by applying ∼1 GPa pressure at 25 °C and held for 1 min. Thermopower (α) data are recorded for the resulting pellets, and four-probe electrical resistivity (ρ) measurements are performed in the bar shape piece of the pellets. The ρ data for ASA0 (pure Ag2S, TU = 7.5 G


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semiconducting Ag2S NPs and inject electrons to the latter. These electrons may modify donor states (ED) available just below the conduction band (Ec) (Figure 7a) that might be formed because of S defects present in Ag2S; in fact, each S defect provides two donor states. This leads to the creation of midgap levels (ED, partially filled d levels of Ag atoms). As temperature increases, the charge carriers get thermally activated from ED to Ec as the activation energy Eac = Ec − ED is low. This hopping from ED to Ec may lead to an exponential decrease in ρ with increasing T. In the highertemperature region (∼300 K), the conductivity of crystalline Ag2S−Ag nanocomposites follows Arrhenius type behavior. The activation energies (Eac) are obtained from the fitting of the relation σ(T) = σo exp[−(Eac/kBT)] to the dc electrical conductivity data (Figure S8), where σ0, kB, and T are preexponential factor, Boltzmann constant, and absolute temperature, respectively. The Eac calculated from the slope of ln(σ) versus 1000/T are 252.9, 223.9, and 168.93 meV for ASA2, ASA3, and ASA4, respectively (Table S4). It suggests that the conduction of charge carriers from one grain to another is due to the thermal excitation of charge carriers. The Eacs are much less than the actual band gap (Eg ∼ 1.7 eV) of pure Ag2S, as evident from UV−vis and photoluminescence (Figures 6, S6, and 7), confirm the presence of midgap levels (ED) of defects and occurrence of thermally activated transition of electrons from ED to the conduction band (Ec). 2.5.2. Study of Seebeck Coefficient and Hall Coefficient. Seebeck coefficient (α) measurements are also not possible for highly resistive samples, viz., pure Ag2S (ASA0) and first ASA1 nanocomposite. As discussed above, ρ of composites decreases with increasing Ag content because of the simultaneous increase in the number of sulfur defects in Ag2S matrix and formation of Ag NPs (crystalline or amorphous) at the grain boundaries (Figures 8a and 9a). Consequently, the overall n increases with Ag in the composites, which make them possible to produce a measurable voltage by applying a small temperature difference (∼2 K). It is well-known that a very small change in n can be reflected in the Seebeck coefficient α and have an inverse relation, α ∝ 1/n.1,6,37 The temperature dependence of α for ASA2 and all other samples with higher Ag content are shown in Figure S7b and Figure 8b, respectively. ASA2 (9.5% Ag NPs) is a more resistive sample with low n and exhibits very high α values (−574 μV/K at 325 K) with a negative sign. The magnitude of α decreases with decreasing T and goes beyond the recordable range of instruments below 234 K (Figure S7b). These results demonstrate that the majority of charge carriers are electrons. Taking into account its ρ analysis, it is concluded that ASA2 is an n-type nondegenerate semiconductor. The α values of other samples with higher Ag content (except last ASA7), above their SM transition, also increase with T. They attain reasonably higher values near 300 K (Figure 8b) because of low-energy charge-carrier filtering at grain boundaries and SM interfaces (Figure 9b). Since the Seebeck coefficient is nothing but the average energy per charge carrier with respect to Fermi level,23,26 the optimum SM interface potential barriers are seemingly capable of enhancing α by increasing average energy per charge carrier. The negative sign of α for these samples and its decrease with lowering T may be due to diffusion of thermally excited electrons to lower energy levels.23,38 These α data, combined with the ρ analysis above, with SM transition temperature confirm that all samples are n-type nondegenerate semiconductors, except ASA7.

indicated by the red line in Figure 9a, for crystalline Ag NPs or formed amorphous clusters of Ag atoms. This probable atomic contact between the Ag2S and Ag particles in the nanocomposites will contribute to a more favorable carrier transport for its application in TE devices. Thus, a systematic decrease in the quantity of TU with fixed AgNO3 in a reaction at 160 °C (Table S1) leads to the formation of metallic Ag phase and increases the number of sulfur vacancies in the nanocomposites (Figures 8 and 9). In fact, there are two possible ways for current flow: the first way is the percolation via metallic Ag atoms (amorphous or crystalline channels) at grain boundaries of Ag2S NDs, and the second way is across the semiconducting Ag2S NDs. However, the ρ of sulfur-deficient semiconducting Ag2S NDs (ρAg2S) will increase exponentially with decreasing temperature, whereas that of the metallic path through Ag NPs (ρAg) at grain boundaries will go down linearly with decreasing temperature. These two competing phenomena operate together in these materials, and domination of one over the other at different temperature regions is thus evident (Figure 8). All samples (with Ag nanoinclusions of 0−23.5%), except ASA7, showed semiconducting ρ below 325 K down to about 220 K maximally but with varying ranges because ρAg< ρAg2S here since electric current follows the least resistive path; thus, below TSM, all samples have to perhaps switch to metallic ρ. Below a particular temperature (TSM), ρAg will exhibit lower values as compared with ρAg2S, because ρAg goes down linearly and ρAg2S increases exponentially with decreasing T and exhibits semiconductor to metal (SM) transition temperature. When TU reduces from 3 mmol (ASA1) to 2.85 mmol (ASA2), nearly insulating ρ (not shown as this was unmeasurable) to purely semiconducting behavior is observed (Figure S7 a) due perhaps to the formation of sulfur vacancies. On further reduction of TU as in ASA3 (2.75 mmol), the number of Ag atoms (crystalline and amorphous) increases significantly enough to make current percolation possible through it, and therefore, the metallic behavior region appears below TSM (5−220 K). In Figure 8a, the value of ρ and its exponentially varying region for samples with 9.5−23.5% Ag nanoinclusions decreases with increasing Ag content, which is attributed to creation of a large number of sulfur vacancies and interactions with crystalline or amorphous Ag NPs, as discussed further later. Thus, overall charge-carrier density (n) and Ag NP concentration increases with increasing Ag content in the composites. The SM transition temperature depends on the ratio of Ag:S in nanocomposites, which increases with reducing quantity of the sulfur source in composites (Table S1). Consequently, the SM transition shifts toward higher temperature, occurring near 225, 260, 275, and 300 K for ASA3, ASA4, ASA5, and ASA6, respectively. Thus, there is systematic evolution of resistivity from insulating to semiconducting and finally to metallic behavior. It is thus seen that an excess quantity of TU in the reaction is required in the synthesis of single-phase Ag2S, and Ag2S−Ag composites are formed when nominal or less quantity of TU is taken (Figure 1 and Table 1). This indicates that an excess of sulfur does not create Ag vacancies (VAg), but a lack of sulfurprecursor may create sulfur vacancies (VS), which leads to the formation of Ag NPs as the second phase. Moreover, the Ag content that is less than the nominal value (Table 1) appears to suggest that Ag could be in nanocrystalline as well as amorphous phases. These (metallic) nanocrystalline and/or amorphous Ag phases would, most probably, interlink the H


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Figure 10. (a) Linear fitting of α versus 1000/T curves of Ag2S−Ag nanocomposites with different% of Ag nanoinclusions (15.4−23.5%), (b) Hall coefficient and charge concentration of ASA3 at different T, and (c) thermoelectric figure of merit of ASA2, ASA3, and ASA5.

rest mass of electrons calculated using NC = 2(2πme*kBT/ h2)3/2 (Table S5), confirms the nondegenerate semiconducting nature of ASA3 at 300 K. Since noble metal Ag and sulfur-deficient Ag2S both have electrons as majority charge carriers,22,38 all nanocomposites should exhibit negative α values only in whole temperature range of 5−325 K. However, below SM transitions, α values are suppressed to very small values for all samples and switched to positive values at low temperature because of the likely dominance of positive α of metallic Ag NPs as in bulk which have α values from α5K = 0.5 to α 300 K = 1.5 μV/K but with negative Hall coefficient.38 This can be understood in terms of its complex Fermi surface having connected necks. In FCC silver, the Fermi surface curvatures along each neck axis (111) are positive, sharp, and just touching the Brillouin zone boundary. The density of hole-like states, nh, associated with the necks is much greater than that of electron-like states, ne, associated with the belly, and hence, the Fermi surface generates holes.38 As a result, the density of thermally excited holes is much higher than that of electron density yielding a positive α. When two types of charge carriers exist in a system, the resulting thermopower will be the sum of αe (−ve) and αh (+ve). Therefore, the magnitude and sign of total α is decided by the dominance of electrons or holes over the other at a particular temperature range (Figure 8b). For further confirmation of domination of different types of charge carriers at different T, Hall coefficient (RH) measurements for ASA3 are performed, and n values are calculated (Figure 10 b). RH would be the slope of linear fit of Hall resistivity (ρxy) versus applied magnetic field (Bz) graphs at different Ts (Figure S10). In the high-temperature region (200 to 300 K), the obtained slopes are negative, which indicates electrons as the majority charge carriers, and give negative RH values (Table S5). Obtained n values at 250−300 K lie in the range of 1−2 × 1017 cm−3. However, below 200 K, a variation in the positive slopes with Bz is observed, reflecting the existence of two types of charge carriers with holes in majority (Figure S10). Different slopes found from low and high Bz regions give different positive RH values (Figure 10 and Table S5). These positive RH values below 200 K are due to the complex Fermi surface in Ag2S−Ag nanocomposites and well consistent with α data in which very low or positive α values are attained (Figure 8b). 2.5.3. Study of Power Factor, Thermal Conductivity, and ZT. The thermoelectric power factor (PF) values for all the samples have been calculated (Figure S7c). Even though the values of α of ASA2 are the highest (α325 K = −574 μV/K), it shows very poor PF of 0.91 μWm−1 K−2 at 325 K due to poor

Since higher temperature electrical resistivity is governed by thermally activated electrons and is well-fitted with the Arrhenius equation, we have tried to fit high-temperature α data with Mott’s equation of thermoelectric power due to electron diffusion in a nondegenerate semiconductor,39,40 α=−

kB e

(A + ), where E Es kBT

s

is the difference between the

bottom of the conduction band to Fermi energy level (Ef), A is the constant which depends on scattering mechanism of charge carriers in nondegenerate semiconductors. From the slope of the α versus 1/T curve (Figures 10a and S9), the estimated values of energy difference Es are 178.61, 164.67, 162.69, 158.42, and 55.57 meV for ASA2, ASA3, ASA4, ASA5, and ASA6, respectively (Table S4). These values, smaller than half of optical band gap (Eg),13,15,16 indicate the existence of mid gap levels in the band gap region. Moreover, activation energy Eac obtained from resistivity curves greater than Es appear to confirm the charge transport is mostly dominated by thermally activated hopping of carriers. The magnitude of α at 300 K systematically decreases with increasing Ag content in nanocomposites (αASA3 > αASA4 > αASA5 > αASA6 at ∼300 K). It is due to the increase in carrier density (n) with Ag content that is consistent with α ∝ 1/n relation.1,6,37 As Ag content increases, the number of nucleation centers of Ag NPs (amorphous clusters) at the grain boundary region increases, and the combination of nearby centers creates growth of fully grown crystallites of Ag NPs. Consequently, composites with higher Ag content have less affective SM interfaces and more chance for percolation of conduction electrons through Ag grains; therefore, α values get suppressed to smaller values.11 For ASA2, Ag content is low; thus, the size of Ag NPs is small, and they can distribute uniformly. This creates more SM interfaces and hence exhibits high α values (α325 K = −575 μV/K). For others, the α value at 325 K systematically reduces to −229.7, −162.2, −134.5, −52.8, and −0.3 μV/K for ASA3, ASA4, ASA5, ASA6, and ASA7 nanocomposites, respectively. These results are excellently in accordance with theory11,41 as well as experiment.7,24,25 They strongly suggest that a significant improvement in α is possible through introduction of metal NPs in the semiconducting host which leads the band bending at metal NPs to semiconductor interfaces (Figure 7b). This can strongly block the low-energy charge carriers through an optimum potential barrier height at SM interfaces and transmits high-energy carriers only (Figure 9b). The calculated density of defects states (NC) found (1.38 × 1020 cm−3) using n = NC exp(−(EC − EF)/kBT), greater than n ∼ 1.9 × 1017 cm−3 (discussed below) with an effective mass 3.22 times the I


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mmol of thiourea (98%, Merck) was separately prepared in another 20 mL of EG. These were mixed together in a three-neck roundbottom flask using 10 mL of extra EG. This milky solution containing initial precursors was heated up at the rate of 6 °C/min under a continuous flow of Ar gas. At ∼155 °C, black precipitates (ppt) of Ag2S started forming, and the temperature was then maintained at 160−165 °C for 2 h to get a complete product. The resulting black ppt was washed using steps described in ref 6. The resulting ppt was dried at 60 °C for 3 h. This dried product, named as ASA1, was directly used for all characterizations. To incorporate Ag NPs into Ag2S nanomaterials, we simply reduced the quantity of thiourea in the above synthesis procedures, which leads to the formation of Ag2S−Ag nanocomposites. Samples with different quantities of thiourea, such as 7.5, 3, 2.85, 2.75, 2.7, 2.65, 2.6, and 2.55 mmol with a fixed 6 mmol of Ag source, were prepared while keeping all other reaction parameters/steps the same. This produces the series of nanocomposites (Ag2S)1−xAgx with expected x = 0, 0, 9.52, 15.38, 18.18, 20.09, 23.53, and 26.09% coded as ASA0, ASA1, ASA2, ASA3, ASA4, ASA5, ASA6, and ASA7, respectively. Characterization Techniques. Powder X-ray diffraction (XRD) patterns were collected for structural analysis of resulting nanocomposites using a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (1.54 Å) in θ−2θ geometry from 20° to 80°. Energydispersive analysis of X-rays (EDAX) was carried out using a Model JEOL JSM 5600 scanning electron microscope equipped with EDAX. For morphological study, field emission scanning electron microscopy (FESEM) images were obtained using a Carl Zeiss AURIGA FIBSEM in secondary emissions mode. X-ray photoelectron spectroscopy (XPS) measurements were performed in X-ray photoelectron spectroscope (SPECS, Germany) using Al Kα radiation with applied an anode voltage of 13 kV and an emission current of 22.35 mA; a survey scan and high-resolution spectra were recorded with an energy of 40 and 30 eV, respectively. UV−visible spectra were measured with a UV−visible spectrophotometer (Shimadzu 3600). Photoluminescence measurements were performed by using a Shimadzu model RF 510 spectrophotofluorimeter equipped with a 150-W xenon lamp as an excitation source. Thermoelectric power and resistivity measurements of the consolidated pellets of nanocomposites were performed in a specially designed commercially available Dewar using load-based differential direct current42 and four-point probe homemade setup,43 respectively. Thermal conductivity (κ) as a function of temperature was measured using a dc pulse laser technique in the temperature range of 5−300 K.6 Hall measurements were carried out in a 9 T AC Transport PPMS (Quantum Design) system using a five probe method.

electrical conductivity. Also, the fully metallic sample ASA7 exhibits a PF less than that of ASA2 because of its very low α values (Table S6). They suggest that neither highly resistive nor highly conductive samples are suitable to achieve good PF and hence ZT values. The highest PF value of 19.53 μWm−1 K−2 at 325 K is achieved for the sample ASA5 with moderate electrical conductivity. This value is significantly large for a nonstate-of-the-art material. It is nearly 3 orders of magnitude higher than the famous tellurium-free good TE material Bi2S3 nanorods.37 This value of PF for Ag2S−Ag nanocomposites, synthesized by the bottom-up approach as a facile scalable and environment-friendly polyol method at low temperature (160 °C), has special importance in terms of its nontoxicity and naturally abundant elements as constituents. These samples are nanomaterials exhibiting a high density of grains boundaries, point defects, and a large number of SM interfaces. They can scatter more phonons, and hence, reduced κ is predicted11,37,41 and experimentally observed for several nanomaterials.1−10,24,25 The thermal conductivity of 2.2045, 2.0429, and 1.6782 Wm−1 K−1 at 325 K (Figure 8c) for ASA5 (Ag = 20.1%), ASA3 (Ag = 15.4%), and ASA2 (Ag = 9.5%) indicates its increase with increasing Ag content in the composites. The disappearance of peak-like features near 40 K with increasing Ag is suggestive of reduction of crystalline Ag2S phase with increasing metallic Ag (amorphous/crystalline) and defects. This value of κ is very low for system containing 20% metallic Ag, which is attributed to the dominant scattering of phonon at the interfaces of Ag2S−Ag nanocomposites and defects. The calculated ZT values for ASA2, ASA3, and ASA5 (Figure 10c and Table S6) show that the sample with 20.1% Ag nanoinclusion exhibits the highest ZT of 0.0029 at 325 K, whereas others showed lower values. Although this value of ZT is quite low, significantly high values are expected for its spark plasma sintered dense pellets and beyond it (especially near structural transition of Ag2S at 450 °C like ref21 due to superionic transition). Spark plasma sintering at optimum temperature can produce a dense pellet with relatively improved σ without affecting α much.1 Notably, these nanocomposites are highly chemically stable and ductile semiconductors,13 which may solve the TE devise failure of crack-prone fragile TE materials. Hence, this report may open a new vista for TE applications of Ag2S.

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3. CONCLUSIONS We have demonstrated a facile method of synthesis of Ag2S− Ag nanocomposites and investigated the effect of Ag nanoinclusions on their thermoelectric properties. The existence of Ag NPs as secondary phase in δ-Ag2S is confirmed by Rietveld refinement of its powder XRD. A significant reduction in electrical resistivity within a measurable range of Ag2S with decreasing sulfur content is observed. A significantly high Seebeck coefficient and power factor are achieved in 15.4 to 23.5% Ag nanoinclusions in semiconducting Ag2S. The Ag2S−Ag nanocomposite with 20.1% Ag nanoinclusion exhibits the highest ZT of 0.0029 at 325 K. This may open up a new strategy for enhancing the ZT value of Ag2S near room temperature and beyond it.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b01016. Figures of Rietveld refinement of powder XRD, EDAX, and XPS survey scan of Ag2S−Ag nanocomposites; brief description of zeta potential and hydrodynamic diameter studies of ASA0 and ASA6; Tauc plot for UV−visible spectroscopy absorption data of Ag2S; figures for temperature-dependent ρ, κ, α and α2/ρ, and Arrhenius equation fitting of σ and Mott’s formula fitting of α for nondegenerate semiconductor; tables of calculated crystallite sizes, obtained parameters from refinements, mass density, atomic ratio of Ag:S obtained from EDAX, calculated Hall coefficient and charge concentration, obtained Es (with Eac), α2/ρ and ZT values of Ag2S−Ag nanocomposites (PDF)

EXPERIMENTAL SECTION

Method. Nominally required quantities of initial precursors in ethylene glycol (EG, ≥ 98.5%, Merck) were used for synthesis of Ag2S NPs. In a typical synthesis, 6 mmol of silver nitrate (99.9999%, SigmaAldrich) was dissolved in 20 mL of EG using magnetic stirrer, and 3 J


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AUTHOR INFORMATION

Corresponding Author

*E-mails for G.S.O.: [email protected], [email protected]. ORCID

Gunadhor Singh Okram: 0000-0002-0060-8556 Author Contributions

All authors have given approval to the final version of the manuscript. Funding

Funding is done by UGC-DAE CSR Indore as a routine research work without any grant number. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Mukul Gupta and Layanta Behera, and D. M. Phase and V. K. Ahire from UGC-DAE Consortium for Scientific Research Indore for the XRD and EDAX data, respectively. We are thankful to G. R. Banjare and D. P. Bisen from Pt. Ravishankar Shukla University Raipur for photoluminescence data, and R. Chatterjee and K. K. Chattopdhyay from Jadavpur University, Kolkata for UV− visible data.

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Ag-Nanoinclusion-Induced Enhanced Thermoelectric Properties of Ag2S

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