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Scripta Materialia 64 (2011) 510–512

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Modified structures and improved thermoelectric property inAg-added polycrystalline In2Se3

J.L. Cui,a,⇑ X.J. Zhang,b Y. Deng,c H. Fu,b Y.M. Yan,b Y.L. Gaob and Y.Y. Lid

aInstitute of Materials Engineering, Ningbo University of Technology, Ningbo 315016, People’s Republic of ChinabMaterials Science and Engineering College, China University of Mining and Technology, Xuzhou 221116, People’s Republic of China

cSchool of Chemistry and Environment, Beihang University, Beijing 100191, People’s Republic of ChinadMaterials Science and Engineering College, Taiyaun University of Technology, Taiyuan 030024, People’s Republic of China

Received 15 October 2010; accepted 14 November 2010Available online 18 November 2010

a-In2Se3 has a two-layer structure with 1/3 of the cation sites vacant. After addition of Ag to In2Se3, we identified the main phaseas In5AgSe8 with nanoinclusions of InSe forming in situ. Ag incorporation favors the formation of Ag2Se slab, which is largelyresponsible for the decrease in band gap Eg, accounting for much of the increase (decrease) in electrical conductivity (Seebeck coef-ficient). This effect, combined with a reduction in lattice thermal conductivity, results in a big improvement in thermoelectric prop-erty over a-In2Se3.� 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Thermoelectric properties; Band gap; Ag-added In2Se3; Structural modifications

For the past decade there has been a renewedfocus on the development of thermoelectric (TE) materi-als. In order to achieve high TE performance, which ischaracterized by the dimensionless figure of meritZT = a2 rT/(Ke + KL), where a, r, T, Ke and KL arethe Seebeck coefficient, electrical conductivity, absolutetemperature, and the electronic and lattice contributionto the total thermal conductivity K, respectively, manynew materials have been developed, such as bulkPbTe-based alloys (ZT = 1.45 at 627 K) [1], siliconnanowires (ZT = 0.6 and one at room temperatureand 200 K, respectively) [2,3], disordered layered WSe2

with ultralow thermal conductivity [4] and In4Se3-basedcrystals (ZT = 0.6 and 1.48 at around 700 K) [5,6].

Concerning the In–Se-based alloys, many theoreticalcalculations have indicated that the structure of a-In2Se3

is based on two-layer hexagonally packed arrays of Seatoms with 1/3 of the In atom sites being empty [7,8].This structure allows [7] incorporated impurities or for-eign elements to take the most stable valence state, sincethe intermolecular bond lowers the barrier to atomicrearrangement. Liang et al. [9] proposed that the intro-

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duction of Ag into As2Se3 (which has a structure similarto that of In2Se3) would cause the appearance of ex-tended states into the midgap region, resulting in im-proved electrical conduction. Mott [10] suggested,however, that the impurities in a-In2Se3 do not formany active levels in the gap, and therefore no remarkablechange in electrical property would be observed. In thispaper, we present modified structures and demonstratean improved TE property of In2Se3 through additionof Ag, and also report on the in situ formation of nano-structured InSe and the effect of this phase.

Ag-free In2Se3 (hereafter referred to as a-In2Se3) andAg-added In2Se3 (Ag–In2Se3) were prepared by meltingthe mixtures of In, Ag and Se single elements in vacuumsilica tubes at 1273 K for 24 h, slow cooling to 860 K,followed by quenching in water. This process enables astable a-In2Se3 to be obtained [11,12], which providesmuch higher electrical conduction than b- [11] andc-In2Se3 [13,14]. Powder X-ray diffraction (XRD) pat-terns (Fig. 1) confirm that, as expected, a-In2Se3 wascrystallized in a phase (JCDPS: 34–1279), but Ag–In2Se3

reveals a ternary alloy In5AgSe8 (JCDPS: 76–1439) withprecipitation of a minor phase InSe (JCDPS: 34–1431).

Detailed experimental procedures have been reportedelsewhere [15]. The thermal conductivities were calcu-lated as the products of measured densities, specificheats and thermal diffusivities.

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20 30 40 50 60

(b)

(a)(202

)(109

)(1

14)

(001

0)(1

08)

(110

)(008

)(1

06)

a: Ag−Free In2Se3, o: α−In2Se3b: Ag−added In2Se3, *: In5AgSe8, +: InSe

(312

)

∗ +

(204

)

∗ (105

)

∗ (2

11)

∗ +(103

)

∗ ∗

(112

)

(110

)

∗

(101

)

+

οοοοοο

οοο

(105

)

(103

)

οο(100

)

οο

(101

) (006

)

ο

(102

)

∗

(004

)

ο

Rel

ativ

e In

tens

ity,

I / (a

u)

Diffraction angle, 2θ / (o)

Figure 1. XRD patterns for the two powders: (a) Ag-free In2Se3; (b)Ag-added In2Se3.

J. L. Cui et al. / Scripta Materialia 64 (2011) 510–512 511

The a and r values as a function of temperature arepresented in Figure 2a and b. The a values are negativeover the entire temperature range, indicating n-type semi-conductor behavior. Above 450 K the absolute a valuesof Ag–In2Se3 are much lower than those of a-In2Se3:the highest one is 285.0 lV K�1 at 435 K, whereas the rvalue ranges from 158.7 X�1 m�1 at room temperature(RT) to 7.7 � 103 X�1 m�1 at 884 K, i.e. more than tentimes that of a-In2Se3 at the corresponding temperature.Figure 2c indicates total K values as a function of temper-ature for the two samples. The K value for the Ag–In2Se3

sample is very low when T < 600 K, but above that it ex-ceeds that of a-In2Se3 because of the increased contribu-tion from the electronic component Ke. At 884 K the totalK value is 0.34 W K�1 m�1, about 23% higher than thatof a-In2Se3. The lattice part, KL, shows a decreasing ten-dency, reducing by a factor of 2.6 at RT and of 1.55 at884 K. For the a-In2Se3 sample, the lattice part, KL,reduces with increasing temperature before reaching aminimum value of 0.24 W K�1 m�1 at 708 K, and thenincreases with temperature due to a bipolar effect. TheZT value as function of temperature is depicted inFigure 2d, showing a rapid increase above �550 K forboth samples. The highest ZT value is 0.6 at 884 K, about2.6 times that of a-In2Se3. This value is comparable to thereported values of 0.63 and 0.6 for In4Se3 [6,16].

300 450 600 750 9000

200

400

600

800(a)

−α /

10 − 6

V. K

−1

Temperature, T / K

Ag−free In2Se3 Ag−added In2Se3

1.0 1.5 2.0 2.5 3.0

100

102

104(b) Ag−free In2Se3

Ag−added In2Se3

σ / Ω

− 1.m

− 1

1000 T −1 / K −1

300 450 600 750 900

0.3

0.6

0.9 (c)

ther

mal

con

duct

ivity

/ W. K

− 1. m

− 1

Temperature, T / K

κ for the Ag−free In2Se3κL for the Ag−free In2Se3κ for the Ag−added In2Se3κL for the Ag−added In2Se3

300 450 600 750 9000.0

0.2

0.4

0.6 (d) Ag−free In2Se3 Ag−added In2Se3

Figu

re o

f mer

it, Z

T

Temperature, T / K

Figure 2. Electrical and thermal conductivity plotted as a function oftemperature for Ag-free and Ag-added polycrystalline In2Se3: (a)Seebeck coefficient a; (b) electrical conductivity r; (c) lattice and totalthermal conductivity (KL and K); (d) dimensionless TE figure of merit,ZT.

This improvement in the electrical property may bedue solely to a decrease in the gap, though it has beensuggested [9] that increased hopping paths due to the in-crease in coordination number also play a role. A mea-surement was therefore made on the two samples,showing Eg values of 1.23 eV (Ag–In2Se3) and 1.32 eV(a-In2Se3) (Fig. 3), with the latter a little smaller thanthe experimentally determined value of 1.42 eV [7], butcomparable to reported values [11,13,17,18].

Although only a minor change of the band structurewas observed after addition of Ag in In2Se3, this canalso increase the TE property [19,20]. In order to eluci-date the origin of the band gap decrease in Ag–In2Se3,we analyzed the In5AgSe8 formula, which can be ex-pressed as (In2Se3)2.5(Ag2Se)0.5, where Ag atoms maysubstitute In atoms or occupy In vacancies to form aAg2Se slab. Rhyee et al. [21] indicated that such incorpo-ration is energetically more favorable than interstitial orSe site incorporation for the layered structure. Grootand Moodera [22] pointed out that Ag is more likelyto fill structural vacancies, though it is possible to substi-tute for one of the constituents in In2Se3. The incorpo-rated Ag atom modifies the atomic coordination andhybridizes with Se atoms, forming a Ag2Se slab. How-ever, to determine experimentally the existence of Ag2Sein the present system is difficult, because the bondingenergies of Ag–Se and In–Se are almost indistinguish-able. The compound Ag2Se with a band gap Eg of0.08 eV or less [23,24], interspersed in two layers of thehost structure, generates impurity levels which aremainly responsible for the decrease in the band gap.This result is analogous to those observed by Gridinet al. [25] and Shigetomi et al. [26], who found that highimpurity levels formed by In–Sn complexes or defectcomplexes formed by Zn atoms in the interlayer spaceof the host InSe can shift the energy level downwards.The impurity levels in the midgap facilitate the thermalactivation of electrons from the conduction band, caus-ing an enhancement in charge carrier concentration n(electrons), leading to the decrease (increase) of a (r) val-ues, However, we failed to measure the n values in thepresent work because the electrical conductivities ofthe materials in question are too low, and are lower thanthose of Bi2Te3 and its alloys by orders of magnitude.

By using high-resolution transmission electronmicroscopy (HRTEM) and energy-dispersive spectros-copy (EDS), we confirmed the existence of nanodomainsof InSe (In:Se = 51.7:48.3) about 50 nm in size formed

1 2 3 4 5 6 70

10

20

30

Ag−free In2Se3 Ag−added In2Se3

(A.h

ν)2

photon energy, hν/ eV

1.0 1.2 1.4 1.6 1.8 2.0 2.20

2

4

6

Eg=1.32 (eV)Eg=1.23 (eV)

Ag−free In2Se3 Ag−added In2Se3

(A.h

ν)2

photon energy, hν/ eV

Figure 3. Plots of (A.hm)2 vs. photon energy (hm) for the Ag-free andAg-added polycrystalline In2Se3; the insert shows the expanded plot atlow photon energy.

50 nm

a

InSe

InSe

InSe

InSe

InSe

InSe

InSe

50 nm

InSe

b

InSe

InSe

InSe

InSe

InSe

InSe nanophase

Ag

InIn

In

InIn5AgSe8

InAgSe

Se

25155 2010

Ag,In

Ag

In

Se

Cou

nts

Energy (KeV)

dc

5 nm

InSe

Figure 4. (a,b) Bulk Ag-added polycrystalline In2Se3; the whitearrowheads in (b) show weakly contrasting InSe nanoinclusions�5 nm in size. (c) EDS measurement was performed on selected areas,confirming that the nanoparticles are InSe phase (In:Se = 51.7:48.3),and the remaining area is In5AgSe8. (d) An enlarged image showingInSe nanoinclusions, indicated by two white arrows.

512 J. L. Cui et al. / Scripta Materialia 64 (2011) 510–512

in situ in the Ag–In2Se3 matrix (Fig. 4a–c). Close inspec-tion of the images shows many weakly contrasting nan-oinclusions �5 nm in size, some of which are indicatedby arrowheads in Figure 4b,d; this is possibly indicativeof a nanostructured InSe phase formed in situ at anearly stage of the process. The nanoinclusions areresponsible for the partial reduction in the lattice part,KL [27]. On the other hand, low KL values may partiallybe accounted for by the disordering of the two-layerhexagonally packed arrays [4,5,21] due to Ag incorpora-tion in the In2Se3 structure. Although there is a rapidreduction of KL for a-In2Se3 when T < 708 K, and thena slight increase accompanying the rise in temperature,differential thermal analysis (DTA) confirmed that nophase transformation occurs up to 923 K (data notshown here).

In summary, after addition of Ag to In2Se3, we iden-tified In5AgSe8 and InSe as the major and minor phases,respectively. Incorporation of Ag in the host material fa-vors the formation of Ag2Se slab, accounting for the de-crease in the band gap and much of the increase(decrease) in the electrical conductivity (Seebeck coeffi-cient). Ag incorporation, combined with a significantreduction in the lattice thermal conductivity caused byin situ formation of InSe nanoinclusions or disorderingof the layered structure, results in a large improvementin TE property compared to a-In2Se3.

The work was supported by the NationalNatural Science Foundation of China under GrantNo. 50871056, by the National High TechnologyResearch and Development Program of China underGrant No. 2009AA03Z322 and by the Zhejiang Provin-cial Natural Science Fundation (Y4100182).

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