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Applied Surface Science 317 (2014) 818–827

Contents lists available at ScienceDirect

Applied Surface Science

journa l h om epa ge: www.elsev ier .com/ locate /apsusc

olyamide–thallium selenide composite materials via temperaturend pH controlled adsorption–diffusion method

emigijus Ivanauskasa, Linas Samardokasa, Marius Mikolajunasb, Darius Virzonisb,onas Baltrusaitis c,∗

Department of Physical and Inorganic Chemistry, Kaunas University of Technology, Radvilenu str. 19, Kaunas LT-50254, LithuaniaDepartment of Technology, Kaunas University of Technology, Panevezys Faculty, Daukanto 12, 35212 Panevezys, LithuaniaDepartment of Chemical and Biomolecular Engineering, Lehigh University, B336 Iacocca Hall, 111 Research Drive, Bethlehem, PA 18015, USA

r t i c l e i n f o

rticle history:eceived 27 June 2014eceived in revised form 29 August 2014ccepted 29 August 2014vailable online 6 September 2014

eywords:hallium selenideurface propertiesunctional materials

a b s t r a c t

Composite materials based on III–VI elements are promising in designing efficient photoelectronicdevices, such as thin film organic–inorganic solar cells. In this work, TlSe composite materials weresynthesized on a model polymer polyamide using temperature and pH controlled adsorption–diffusionmethod via (a) selenization followed by (b) the exposure to the group III metal (Tl) salt solu-tion and their surface morphological, chemical and crystalline phase information was determinedwith particular focus on their corresponding structure–optical property relationship. XRD analy-sis yielded a complex crystalline phase distribution which correlated well with the optical andsurface morphological properties measured. pH 11.3 and 80 ◦C yielded well defined, low struc-tural disorder composite material surface. After annealing in N2 at 100 ◦C, polycrystalline PA-TlxSey

PSFM

composite materials yielded a single TlSe phase due to the enhanced diffusion and reaction ofthallium ions into the polymer. The method described here can be used to synthesize variety ofbinary III–VI compounds diffused into the polymer at relatively low temperatures and low over-all cost, thus providing for a flexible synthesis route for novel composite solar energy harvestingmaterials.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

Inorganic–organic composites are a quickly developing groupf materials with a tremendous range of highly tunable structural,hysical and chemical properties. Those include but are not limitedo energy transfer and storage [1–3] and electromagnetic proper-ies [4–8]. High compositional variability of these materials allowsncorporating variety of inorganic compounds. Of particular inter-st are composite materials with group III metal – Ga, In and Tl –inary or ternary compounds, which, in combination with group VIonmetals – S, Se and Te – possess unique physical properties ofremendous importance to modern science and technology [9–13].or example, CuInSe2 is widely used in solar energy harvesting

11], GaSe is used as far-infrared conversion material [13], In2Se3

in optoelectronic devices [14] since, depending on the growthonditions and doping, it exhibits either n- or p-type conduction

∗ Corresponding author. Tel.: +1 6107586836.E-mail address: job314@lehigh.edu (J. Baltrusaitis).

ttp://dx.doi.org/10.1016/j.apsusc.2014.08.192169-4332/© 2014 Elsevier B.V. All rights reserved.

[15]. Tl5Te3 is a superconductor at low temperatures [16,17] whileTlS and Tl2S are photoconductors [18,19]. Tl2S also exhibits pho-tocurrent behavior [19]. In general, thallium selenides can be usedfor the production of photocells [20] and photoconductors [21], aswell in the applications in modern microelectronics [22]. Particu-larly established, yet still rapidly progressing area of III–VI materialdevelopment and use is solar energy harvesting [23]. For example,compounds, such as InSe, are particularly suitable for photovoltaicsdue to their bandgap of ∼1.3 eV, as well as their appropriate opti-cal and electronic properties. Furthermore, solar cells based onCIS/CIGS (CuInSe2/Cu(In,Ga)Se2) are at the level of maturity [24]with 19.9% efficiency demonstrated for CIGS devices [25]. Unprece-dented interest in these so-called 2nd generation photovoltaic (PV)materials is due to their high absorbance with very thin films ofmaterials needed to harvest solar energy. The main issue of CISmaterial thin films is associated with the complexity of the CIS

material layer multicomponent systems, which provide challengesin forming well defined films with uniform properties across thelarge-area substrates using high-throughput equipment [26]. Inaddition, polytypism and variety of stoichiometries available for

urface Science 317 (2014) 818–827 819

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R. Ivanauskas et al. / Applied S

II–VI materials present direct problems since many physical prop-rties, including bandgap, are in turn affected [23].

An opportunity of improving thin III–VI material surface mor-hology, as well as their electronic properties is in combining themith conducting polymers. Composite materials comprised of poly-ers with thin film inorganic semiconductor deposited on their

urface exhibit robust structural properties due to the polymerlasticity and resistance to harsh environments, while also func-ioning as effective semiconductors. Organic–inorganic solar cellsave been the area of rapid development since they potentiallyould adopt the advantages of inorganic materials, such as sta-ility, high carrier mobility and compatible fabricating process,nd utilize the advantages of organic conducting polymers, suchs enhanced light absorption at a wide range of wavelengths,djustable molecular structures for energy band alignment, facileolution based manufacturing [27,28]. Three general approachesollowed in organic–inorganic hybrid materials synthesis are (a)olloidal synthesis followed by mixing with conjugated polymer,b) inorganic materials structured on an electrode and filled witholymer and (c) ligand free inorganic particles prepared directly

n polymer matrix [29]. For example, flexible PET/CuInS2 solarells were recently synthesized using metal xanthate route for then situ preparation of ligand-free inorganic nanocrystals directlyn the conjugated polymer matrix [29]. However, this in situpproach requires rather high temperatures of ∼200 ◦C to per-orm thermal conversion to the corresponding metal sulfides [30].dsorption–diffusion can be viewed as an emerging alternativeethod in obtaining these binary III–VI compounds incorporated

nto polymers. This method does not require high temperatures,acuum or inert atmosphere. As in chemical bath depositionethod, deposition of thin film occurs due to substrate in contactith dilute chemical bath containing corresponding ions resulting

n an excellent adhesion between the thin film and the polymer dueo the partial film incorporation [31–36]. However, since synthe-is proceeds at relatively low temperatures (<100 ◦C), multitude ofrystalline phases are typically obtained thus limiting the applica-ility of the method [34].

In this work, model III–VI binary compound, thalliumelenide, with varying ratio of thallium-to-selenium was depositednto the model polymer–polyamide (PA) – substrates usingdsorption–diffusion method. Even though use of thallium com-ounds has almost been eliminated in recent years due to theiroxicity, thallium may still become attractive as group IIIA elementue to its relatively low price, when compared to that of In, asell as its greater abundance in the Earth’s crust (0.6 ppm Tl vs

.1 ppm In) [37]. Physical and electrical properties of the resultingomposite materials were analyzed using X-ray diffraction (XRD),-ray photoelectron spectroscopy (XPS), scanning electron micro-cope (SEM) to obtain the broader picture of structural thalliumelenide transformations during the low temperature synthesis.

single phase thallium selenide was then attempted to obtainerforming annealing in inert environment. Thallium selenides onubstrates are commonly manufactured using vacuum evapora-ion (references within [38]), by directly coalescing the elementsnder inert argon environment [22,39,40] or by direct combina-ion of the elements at room temperature in liquid ammonia andrganic solvents [41]. Additionally, layers of thallium selenidesre formed using chemical bath deposition [38,42]. In our case,A as a semi-hydrophilic polymer was used since it is capablef adsorbing ions from aqueous solution of various electrolytes43]. This fact has previously enabled incorporation of anions fromqueous solutions of the simplest selenopolythionate homologue –

elenotrithionate (SeS2O6

2−) – into the PA surface as thin films [31].fter treating these anion containing polymer films with Cu(II/I)r silver(I) salt aqueous solutions, CuxSe and AgxSe thin films werebtained [33–35,44]. Here we utilize thallium (I) salts while varying

Fig. 1. Schematics of PA-TlxSey composite materials synthesis procedure.

solution temperature and pH to understand the resulting thal-lium selenide material structure and physical properties withinthe hybrid material. We then obtain a single phase TlSe heatingat moderate temperatures in inert environment.

2. Experimental methods

2.1. Synthesis of PA-TlxSey composite materials

PA films (Tecamid 6, 500 �m thickness, Germany) of15 mm × 70 mm were used as substrates. PA was non-porouswith the pore size less than 1.5 nm [45], as measured using a�-method and Quantasorb (Japan). PA was boiled in distilledwater for 120 min to remove the remainder of the monomerand dried using the filter paper, followed by desiccation withCaCl2 for 24 h. Schematics of the two step PA-TlxSey compositematerial preparation is shown in Fig. 1. First, PA films were initiallyselenized for 1.5 h in 0.05 M K2SeS2O6 at pH 2.15, adjusted using0.1 M HCl at 60 ◦C. Potassium selenotrithionate salt, K2SeS2O6,was prepared according to procedures reported previously [46].Potassium selenotrithionate synthesis was performed using potas-sium disulphite (K2S2O5) (≥98.0% from Sigma–Aldrich), selenousacid (H2SeO3), selenium (Se) pellets (100 mesh powder, 99.99%trace metals basis from Aldrich) and hydrochloric acid (HCl) 0.1 M(0.1 N) from Fluka were used.

Next, to form TlxSey composites, selenized samples of PA 6 weretreated in thallium (I) salt solution for 10 min at various pH andtemperature. These experimental conditions are summarized in

Table 1. Thallium (I) sulfate salt solution was prepared using crys-talline Tl2SO4. Concentration of this solution was 0.1 M (pH ∼2.8).pH of Tl2SO4 solution was adjusted using KOH granules. After thetreatment in thallium (I) sulfate solution, samples were rinsed with

820 R. Ivanauskas et al. / Applied Surface

Table 1Experimental conditions of TlxSey thin film deposition on PA.a

Sample Deposition conditions

pH Temperature (◦C)

PA-TlxSey-1 9.2 60PA-TlxSey-2 11.3 60PA-TlxSey-3 13.0 60PA-TlxSey-4 11.3 50PA-TlxSey-5 11.3 70PA-TlxSey-6 11.3 80

s

d((

2

TemKo0ts

2

mai

2

atwsaetoetotpttttqe

2

iQl

a Samples were reacted with 0.05 M K2SeS2O6 at 60 ◦C followed by 0.1 M Tl2SO4

olution. See Section 2 for further details.

istilled water, dried over CaCl2 for 24 h. Thallium sulphate (Tl2SO4)≥99.9% trace metals basis from Aldrich), potassium hydroxideKOH) (reagent grade, 90%, flakes from Sigma–Aldrich) were used.

.2. XRD characterization

X-ray diffraction analysis of the as deposited and heated PA-lxSey materials was performed using a DRON-6 diffractometerquipped with a special device for beam limitation at low andedium diffraction angles. It uses graphite-monochromatized Cu-

� radiation source (� = 1.54178 A) operating at 30 kV and currentf 30 mA. The XRD patterns were recorded with a step size of.05◦ from 25 to 70◦. Obtained XRD patterns were processed usinghe software packages Search Match, ConvX and Xfit for baselinemoothing and peak identification.

.3. SEM characterization

SEM imaging was performed using Raith GMBH ELine instru-ent equipped with a field emission gun operating at 1 kV

ccelerating voltage. No sample coating was performed beforemaging and secondary electron signal was used for imaging.

.4. XPS characterization

Surface elemental analysis of the PA-TlxSey was performed using custom-designed Kratos Axis Ultra X-ray photoelectron spec-roscopy system [47]. The surface analysis chamber is equippedith aluminum K� X-ray gun and 500 mm Rowland circle silicon

ingle crystal monochromator. The X-ray gun was operated using 15 mA emission current at an accelerating voltage of 15 kV. Low-nergy electrons were used for charge compensation to neutralizehe sample. High-resolution spectra were acquired in the regionf interest using the following experimental parameters: 20–40 eVnergy window; pass energy of 20 eV; step size of 0.1 eV, and dwellime of 1000 ms. One sweep was used to acquire a survey spectrumf all binding regions. The absolute energy scale was calibrated tohe Cu 2p2/3 peak binding energy of 932.6 eV using an etched copperlate. A Shirley-type background was subtracted from each spec-rum to account for inelastically scattered electrons that contributeo the broad background. CasaXPS software was used to processhe XPS data [48]. Transmission corrected relative sensitivity fac-or (RSF) values from the Kratos library were used for elementaluantification. An error of ±0.2 eV is reported for all peak bindingnergies.

.5. AFM characterization

Surface of PA6-TlxSey composite materials was character-zed using atomic force microscope (AFM) measurements usingScope-250 (Quesant Corp.) instrument. Dry samples were ana-

yzed in contact mode using Si cantilevers (CSG10 series, Nano

Science 317 (2014) 818–827

Technology Instruments – Europe BV) with a force constant0.2 Nm−1 and tip curvature of 10 nm. Images were analyzed usingSPIP (Image Metrology) and Quesant Corp. software.

2.6. Optical property measurements

Optical properties of PA-TlxSey composite materials were mea-sure using PerkinElmer Lambda 35 UV/VIS spectrometer.

Bandgap and Urbach energy calculation details are provided inSupplemental Information. Briefly, bandgap was calculated usingEq. (1) which relates absorption coefficient with the photonenergy in crystalline semiconductors [49,50]:

˛h� = B(h� − Eg)m, (1)

where Eg – bandgap (eV), B – constant and h� – photon energy (eV).According to (4), dependence between the bandgap and �g can bederived

Eg = 1239.83�g

(2)

�g was determined by extrapolating the linear portion of thecurve to the abscissa(

A

�

)1/m

= f(

1�

). (3)

Since absorption (A) and transmission (T) are related viaA = 2 − lgT, bandgap can be determined using transmission spec-trum. m value of 0.5 was used.

Urbach energy relates absorption coefficient and photon energy[51]

= ˛0ehv/EU (4)

where ˛0 – constant, EU – Urbach energy. It was calculated usingtangent of the angle of the linear portion of the curve lnA = �−1

according to EU·tg� = 1239.83.

2.7. Other experimental methods

Total amount of selenium and thallium in PA-TlxSey compositematerials was determined using Perkin-Elmer 503 atomic absorp-tion spectrometer (using � = 196 nm and � = 276.8 nm), equippedwith electrode-less discharge lamp and air-acetylene flame. Typicalsensitivity toward Se and Tl was about 0.5 �g/ml for 1% absorption.

3. Results and discussion

3.1. PA-TlxSey composite material formation mechanism andbulk chemical composition

Series of PA-TlxSey composites were synthesized on Tecamid6 polyamide sheet using adsorption/diffusion method via main-taining the selenization parameters constant, while systematicallyaltering Tl2SO4 solution pH and temperature. These experimentalconditions are shown in Table 1. A two-step mechanism of TlxSey

formation on PA samples can be proposed. In the first step SeS2O62−

anions adsorb and diffuse into the subsurface of polyamide. Forthe first 1.5 h both adsorption and diffusion of SeS2O6

2− anionsproceeds while afterwards chemical reactions of SeS2O6

2− anionstakes place both in solution and on the polymer according to reac-tions (5) and (6)

SeS2O62− + H2O → SeSO3

2− + 2H+ + SO42− (5)

SeS2O62− + SeSO3

2− → Se2S2O62− + SO3

2− (6)

Divalent selenium containing compounds remain adsorbedwithin the polymer, while hexavalent sulfur compounds dissolve

urface Science 317 (2014) 818–827 821

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S

S

S

iFgitT

sTmoatt

3

mwTmcisttprIpti1tfiTfbai0sTtrpa((Et

Fig. 2. (Top) X-ray diffraction patterns of PA 6 sheets of (a) PA-TlxSey-1, (b) PA-TlxSey-2 and (c) PA-TlxSey-3. Peaks were identified and assigned as follows: (�) –Tl2Se1.2 (75–1007) tetragonal thallium selenide; (©) – Tl2Se2 (75–880) tetragonalthallium selenide; (�) – Se8 (71–528) monoclinic selenium; (�) – Se8 (76–1865)monoclinic selenium. PA 6 was selenized 1.5 h with 0.05 M K2SeS2O6 solution in0.1 M HCl at 60 ◦C and in Tl2SO4 0.1 M salt solution with different pH. The pH ofTl2SO4 solution: (a) – 9.22, (b) – 11.3 and (c) – 13.0. (Bottom) X-ray diffractionpatterns of PA 6 of (a) PA-TlxSey-4, (b) PA-TlxSey-2, (c) PA-TlxSey-5 and (d) PA-TlxSey-6. Peaks were identified and assigned as follows: (�) – Tl2Se1.2 (75–1007) tetragonalthallium selenide; (©) – Tl2Se2 (75–880) tetragonal thallium selenide; (�) – Tl5Se3

(41–1295) tetragonal thallium selenide; (�) – Se8 (71–528) monoclinic selenium; (�)– Se8 (76–1865) monoclinic selenium. PA 6 was selenized 1.5 h with 0.05 M K2SeS2O6

R. Ivanauskas et al. / Applied S

ack into the solution. During the second step, selenized polymers reacted with Tl(I) sulfate solution and TlxSey formation proceedsia Tl+ ion reactions with the adsorbed SeS2O6

2−, Se2S2O62− and

eSO32− ions according to reactions (7)–(9)

eS2O62− + 2Tl+ + 4OH− → Tl2Se + 2SO4

2− + 2H2O (7)

e2S2O62− + 2Tl+ + 4OH− → Tl2Se + Se + 2SO4

2− + 2H2O (8)

eSO32− + 2Tl+ + 2OH− → Tl2Se + SO4

2− + H2O (9)

Formation of TlxSey during the reactions (7) through (9) resultsn change of the PA color from transparent to dark gray, as shown inig. 1. In particular, increasing reaction pH or temperature for theiven pH value, resulted in the loss of PA film transparency signify-ng deposition of TlxSey. Since the reactions (7) through (9) involvewo Tl ions per one Se and proceed at basic pH values, resultinglxSey films would be Tl rich.

Bulk chemical analysis performed of the selected PA-TlxSey

amples using atomic absorption spectroscopy (AAS) is shown inable 2. It can be seen that molar Tl concentration, as well as Tl/Seolar ratio increases with the increased temperature and pH value

f Tl2SO4 solution. Notably, our deposition experiments performedt lower pH values of 4.7 resulted in the absence of thallium withinhe sample, as inferred from AAS analysis. This is due to the fact thathallium selenides are unstable at lower pH values and decompose.

.2. PA-TlxSey composite material XRD characterization

Structural characterization of the obtained TlxSey compositeaterials was performed using XRD. Complex diffraction patternsere obtained due to the polycrystalline nature of the resulting

lxSey composite material due to the simultaneous existence ofany thallium selenide and selenium phases of varying chemical

omposition and high crystallinity degree of the PA 6 itself. Thentensity of PA 6 XRD peak maximum at 2� < 25◦ exceeded inten-ity of the thallium selenide films several times. Therefore, onlyhe spectral region of 2� ≥ 25.0◦ was used in detail in combina-ion with the available literature data [52–58] and JCPDS referenceatterns. X-ray diffraction analysis presented in Fig. 2 with the cor-esponding peak values tabulated and assigned in Supplementalnformation Table S1 revealed the existence of multiple crystallinehases summarized in Table 2, presence of which was dependent onhe deposition conditions. Selected XRD spectra of TlxSey compos-te materials on PA sheets with the different pH values (9.2, 11.3 and3.0) of thallium sulfate salt solution is shown in Fig. 2 top whereashose for varying temperature (50, 60, 70 and 80 ◦C) of thallium sul-ate at constant reaction time of 10 min and pH of 11.3 are shownn Fig. 2 bottom. It can be seen that with the increasing pH values ofl2SO4 solution there is an increase in the number of TlxSey phasesormed. This can be observed in Fig. 2 with the increase in the num-er of peaks associated with different phases, concurrent with themount of thallium detected in composites using AAS, as shownn Table 2. In particular, thallium amount increases from 0.076 to.334 �mol/cm2 from PA-TlxSey-1 to PA-TlxSey-3 with the corre-ponding two and four crystalline phases detected, respectively.he increase in thallium amount in the composites obtained withhe increasing pH can be attributed to the formation of thalliumich phase Tl2Se1.2 (75–1007) at high pH values. This phase is veryronounced in XRD patterns for pH 11.2 and 13.0 (Fig. 2, top, bnd c), whereas at pH 9.22 only one TlxSey phase is formed Tl2Se2

75–880) (Fig. 2, top, a). Additionally, two monoclinic Se phases Se871–528) ir Se8 (76–1865) are present, especially at high pH values.lemental Se formation is due to the Se2S2O6

2− anion decomposi-ion reaction (8), as well as due to the low stability of the polymer

solution in 0.1 M HCl at 60 ◦C and in Tl2SO4 0.1 M solution with pH 11.3 at differenttemperatures. The temperatures of Tl2SO4 solution were (a) 50, (b) 60, (c) 70 and(d) 80 oC.

adsorbed SeS2O62− ions in alkaline medium, which decomposes

according to the reaction (10)

SeS2O62− + 2OH− → Se + SO3

2− + SO42− + H2O (10)

Hence, with the increasing pH value of the solution reactions (7)through (10) proceed faster and elemental Se is formed via (8) and(10).

Next, Tl2SO4 solution temperature effect on the structure andproperties of the resulting TlxSey was investigated and data arepresented in terms of XRD data shown in Fig. 2, bottom. In partic-ular, thallium selenide formation temperature was kept constantat 50, 60, 70 and 80 ◦C for (a) PA-TlxSey-4, (b) PA-TlxSey-2, (c)PA-TlxSey-5 and (d) PA-TlxSey-6, respectively. With the increasingtemperature, thallium amount detected using AAS also graduallyincreased from 0.092 to 0.270 �mol/cm2, as shown in Table 2. Inter-estingly, selenium amount in TlxSey composites does not show any

particular trend and increased thallium amount can be explainedby the increased mobility of Tl(I) ions with temperature, resultingin faster permeation of the polymer and reaction with adsorbedSeS2O6

2− ions, as well as their decomposition products. As a result,

822 R. Ivanauskas et al. / Applied Surface Science 317 (2014) 818–827

Table 2Bulk elemental composition (Tl and Se) and crystalline phase of PA-TlxSey composite materials.a

Sample Amount measured (�mol/cm2) Tl/Se molar ratio Crystalline phase observed in XRDb

Tl Se

PA-TlxSey-1 0.076 0.816 0.093 Tl2Se2; Se8

PA-TlxSey-2 0.126 1.372 0.092 Tl2Se1.2; Tl2Se2; Se8

PA-TlxSey-3 0.334 1.096 0.305 Tl2Se1.2; Tl2Se2; Se8

PA-TlxSey-4 0.092 1.02 0.090 Tl2Se2; Se8

PA-TlxSey-5 0.232 1.12 0.207 Tl2Se1.2; Tl2Se2; Tl5Se3; Se8

PA-TlxSey-6 0.270 1.13 0.240 Tl Se ; Tl Se ; Se

a Elemental composition determined using atomic absorption spectroscopy (AAS).b Full information on crystalline phase assignment in shown in Table S1.

Table 3Surface elemental composition of PA-TlxSey composite materials.a

Sample C 1s % N 1s % O 1s % S 2p % Se 3d % Tl 4f %

PA-TlxSey-1 48.2 14.2 33.0 3.0 1.0 0.6PA-TlxSey-2 48.0 21.1 30.2 0.6 0.0 0.1PA-TlxSey-3 58.7 3.4 33.4 3.5 0.0 1.0PA-TlxSey-4 49.5 20.8 28.7 0.4 0.4 0.2PA-TlxSey-5 39.1 15.0 30.8 4.5 5.8 4.8PA-TlxSey-6 38.2 14.3 35.2 5.1 3.5 3.7

(

Thd(tprieTIsni

3

cinisFdXNtbdttaoaF(sTs

local microstructural disorder [61]. It can be seen from the values

a Elemental composition determined using X-ray photoelectron spectroscopyXPS) and is presented in atomic %.

lxSey crystalline phases formed at higher temperatures are withigher thallium content. This can be seen from the XRD analysisata in Fig. 2 bottom. At 50 ◦C, only one crystalline phase Tl2Se275–880) is detected with Tl/Se ratio of 1 (Fig. 2, bottom a). Withhe increase in Tl2SO4 solution temperature, predominant TlxSey

hases become Tl2Se1.2 (75–1007) and Tl5Se3 (41–1295) with Tl/Seatio of 1.67 (Fig. 2, bottom b–d). Additionally, elemental seleniums formed: Se8 (71–528) and Se8 (76–1865) (Fig. 2, bottom). How-ver, there is fewer number of peaks for these phases than for thelxSey formed using Tl2SO4 at 60 ◦C pH 13.0 solution (Fig. 2, top).ncreased solution temperature and higher pH favored elementalelenium formation, the latter especially due to the highly pro-ounced OH− ion effect on the stability of SeS2O6

2−, Se2S2O62−

r SeSO32− ions.

.3. PA-TlxSey composite material XPS characterization

While AAS represents bulk elemental composition of TlxSey

omposites, X-ray photoelectron spectroscopy provides insightsnto the compositional information of the outermost surface of fewanometers. XPS analysis of the synthesized PA-TlxSey compos-

te sample surfaces was performed and quantification results arehown in Table 3. Several observations can be made from these data.irst, TlxSey composites are truly comprised of thallium selenideiffused into the polymer. Majority of the elements detected viaPS were comprised of C, N and O ranging from 85 to 99% of total./C ratio in all samples except PA-TlxSey-3, was higher (0.3–0.4)

han the theoretical of 0.17, possibly showing depletion of car-on upon TlxSey incorporation. Additionally, up to 33% of O wasetected on the sample surface whereas amide monomer con-ains one structural oxygen atom for every six carbons. This showshat aqueous treatment of the samples resulted in a considerablemount of adsorbed water trapped at the polymer surface. Portionf this oxygen may be originating from the residual selenotrithion-te ion or sulfate ion, as evidenced by the detected sulfur amount.inally, only at lower pH (PA-TlxSey-1) or at higher temperaturesPA-Tl Se -5 and PA-Tl Se -6) detectable amounts of thallium and

x y x y

elenium were obtained with the approximate Tl/Se ratio of unity.his can be explained that at the very surface of the polymer ion dis-olution will be favored or the disproportionation of TlxSey at higher

2 2 5 3 8

pH and could be due to the adsorbed ions, rather than crystallinespecies.

3.4. Optical properties of PA-TlxSey composite materials

Optical properties of PA-TlxSey composite materials were inves-tigated using UV/VIS spectroscopy. Transmittance, Tauc and Urbachenergy plots are shown in Fig. 3 top, middle and bottom, respec-tively, in the 200–800 nm range. Transmittance dependence onboth synthesis pH and temperature was observed. Specifically, redshift in transmittance was observed in both cases with pH 13 sam-ple absorbing into the visible, whereas sample synthesized at 80 ◦Cabsorbs also into the IR region of the spectrum, consistent with theobserved dark color of the material, shown in Fig. 1. Furthermore,there is a more pronounced temperature effect on the transmis-sion of the samples with more intense absorption in the IR region.UV transmission does not exceed 1% for wavelengths < 288 nm(pH 9.2), <355 nm (pH 11.3), <373 nm (pH 13.0). Transmission at800 nm was measured to be 97% (pH 9.2), 88% (pH 11.3), 78% (pH13.0). Furthermore, at for samples synthesized at 80 ◦C (pH 11.3)transmission was measured to be 37% at 800 nm showing a morepronounced synthesis temperature effect. Notably, PA is a trans-parent material and does not absorb in the visible region.

Tauc plots shown in Fig. 3, middle, were obtained using ASFmethod [59]. Similar to UV/VIS data, Tauc plots reveal a system-atic shift toward lower optical bandgap values with the increasingpH, as well as the increase in temperature of the Tl2SO4 solution.Calculated optical bandgap values, in eV, as well as the relevantfitting parameters, are shown in Table S2. Optical bandgap valuesof 2.80 eV for PA-TlxSey-3 and 3.07 eV for PA-TlxSey-1 were esti-mated from the Tauc plots showing rather small pH effect on theoptical bandgap of the material. Temperature, on the other hand,had a more pronounced effect with the estimated optical bandgapof 3.02 and 2.09 eV for PA-TlxSey-4 and PA-TlxSey-6, respectively.Bandgap values reported in the literature for TlSe range from 0.73to 0.84 eV [60], whereas no bandgap values for other stoichiometrythallium selenide materials was found. This is in contrast with theoptical bandgap values determined in this work. This can be due tothe several reasons. Elemental selenium has been detected in all ofthe samples synthesized here and was reported to have measuredbandgap values in the range from 1.56 to 2.4 eV [60]. Low tempera-ture (50 ◦C) and low pH (9.2) samples PA-TlxSey-4 and PA-TlxSey-1,respectively, had very little thallium selenide and their relativelylarge optical bandgap values will be determined by the presence ofthe SeS2O6

2− ion and its decomposition products.Urbach energy plots were constructed and shown in Fig. 3, bot-

tom, with the calculated values shown in Table S2. Urbach energyreflects the density of states in the band tails and therefore the

presented in Table S2 that with the increasing pH, Urbach energyincreases from 0.56 eV to 0.68–0.70 eV. With the increasing synthe-sis temperature, however, Urbach energy initially increases from

R. Ivanauskas et al. / Applied Surface Science 317 (2014) 818–827 823

Fig. 3. PA-TlxSey composite material (top) UV–VIS spectra, (middle) Tauc plots and (bottom) Urbach energy plots of PA-TlxSey-1, PA-TlxSey-2 and PA-TlxSey-3, PA-TlxSey-4,PA-TlxSey-5 and PA-TlxSey-6. For sample labeling refer to Table 1.

824 R. Ivanauskas et al. / Applied Surface Science 317 (2014) 818–827

F xSey-3r

00tcatiiwA

ig. 4. Three dimensional AFM topographical images of (b) PA-TlxSey-1, (c) PA-Tleference in (a).

.64 at 50 ◦C to 0.79 eV for 70 ◦C sample, but then decreases to

.48 eV for 80 ◦C. This can be associated with the increasing struc-ural order of the TlxSey composites at higher temperatures, asonfirmed by the larger amount of thallium detected using AASnalysis. Additionally, thicker GeSe2 films were previously reportedo possess lower structural randomness, since, as the thickness

ncreased, the structural defects were being minimized, thus min-mizing measured Urbach energy [62]. To test this hypothesis,

e performed morphological PA-TlxSey material analysis usingFM.

, (d) PA-TlxSey-5 and (e) PA-TlxSey-6. AFM image of PA (Tecamid 6) is shown for

3.5. PA-TlxSey composite material morphological characterization

Atomic force microscopy (AFM) and scanning electronmicroscopy (SEM) were performed to investigate morpholog-ical changes, resulting in PA surface after TlxSey deposition. AFMallows for a quantitative description of surfaces in terms of their

feature average and maximum height, as well as RMS roughness.In this work, we used AFM to elucidate the mechanism of TlxSey,diffused into the PA, formation and homogeneity. 12 �m × 12 �m3D AFM topography images of selected PA-TlxSey-1, PA-TlxSey-3,

R. Ivanauskas et al. / Applied Surface Science 317 (2014) 818–827 825

Table 4Quantitative parameters obtained via AFM analysis of the selected PA-TlxSey composite material surface.

Sample Max. height, A (nm) Average height, Zmean (nm) Average roughness, Ra (nm) RMS roughness, Rq (nm) Skewness, Rsk (nm)

PA 238.6 92.1 14.2 19.4 0.86PA-TlxSey-1 206.0 59.1 14.7 20.4 1.49

13.9 19.0 0.5735.4 47.5 0.6527.1 35.8 0.76

PsphapsmlIidtPhaRo

stiafciArteppcl

tmtIaouooas[ito

3

Tc

Fig. 5. PA-TlxSey-6 SEM (a) top and (b) side images showing the presence of ∼2 �minterdiffused layer of TlSe on PA surface.

PA-TlxSey-3 231.2 121.7PA-TlxSey-5 450.4 187.0

PA-TlxSey-6 304.6 156.5

A-TlxSey-5 and PA-TlxSey-6, as well as unreacted PA sample arehown in Fig. 4 with the corresponding measured topographicalarameters tabulated in Table 4. It can be seen that the maximumeight, A, of PA-TlxSey-1, PA-TlxSey-3 deposited at 60 ◦C at pH 9.2nd 13.0, respectively, differs very little from the unreacted PA sam-le with A of 238.6 nm. This is concomitant with the low elementurface concentrations measured using XPS. Changes in averageeasured height, Zmean, coincide with those in A with slightly

arger value of 121.7 nm for PA-TlxSey-3 than the unreacted PA.ncrease in thallium concentration from 0.076 to 0.334 �mol/cm2

n Table 2 with the increasing pH from 9.2 to 13.0, as well asecreasing difference between A and Zmean values, in addition tohe decreasing RMS roughness, Rq, (20.4→19.0 nm), shows thatA-TlxSey materials formed at higher pH values contain similareight and size surface features thus being more homogenousnd compact. That is also confirmed by the average roughness,a, (14.7→13.9) and skewness, Rsk, (1.49→0.57) measurementsbtained.

Changes of PA-TlxSey-5 and PA-TlxSey-6 composite materialurfaces from those of unreacted PA are more pronounced dueo the increased reaction temperature, also coinciding with thencreased thallium and selenium amounts detected using XPS. Fewgglomerates of 0.5–1.8 �m can be observed on PA-TlxSey-5 sur-ace, whereas on PA-TlxSey-6 many more similar size agglomeratesan be observed. Data measured and presented in Table 4 show thatncrease in synthesis temperature from 70 to 80 ◦C results in lower

and Zmean values (450.4→304.6 nm and 187.02→156.47 nm,espectively). Rq also decreased from 47.5 to 35.8 nm showing thathe differences between the surface feature height is decreasing,.g. surface roughness is decreasing. Importantly, both higherH and higher temperature resulted in lower surface roughnessarameters, signifying more uniform and cohesive surface. Thisorrelates well with the calculated Urbach energy values that showower structural disorder.

SEM top and side images of PA-TlxSey-6 were acquired to assesshe uniformity of the film, as well as its thickness. Fig. 5 shows

icron sized domains, typical of PA structure. Interdispersed onhe surface, bright, electron density rich particles can be observed.nset reveals their size to be of ∼100 nm. These particles are TlxSey

ggregates resulting during the material deposition or dehydrationf the surface. Similar aggregates have previously been observedsing similar solution phase precipitation methods [63]. Side imagef PA-TlxSey-6 revealed the presence of ∼2 �m interdiffused layerf TlxSey on PA surface. Previous studies have shown that thedsorption/diffusion or chemical bath deposition on polymer iself-limiting with the film thickness formed within several microns34,63,64]. This is beneficial for III–VI compound based compos-te materials that have very large absorption coefficient withhin film of material needed and significant material savings canccur.

.6. Single phase PA-TlSe composite materials

As a final step, we attempted to obtain a single crystalline phaselSe containing composite material. PA-TlxSey-5 and PA-TlxSey-3omposite materials, synthesized using the procedure described

Fig. 6. X-ray diffraction patterns of (a) PA-TlxSey-5 and (b) PA-TlxSey-3 compositematerials. Peaks were identified and assigned as (�) – TlSe (22–1476) tetragonalthallium selenide. Samples were heated at 100 ◦C for 12 h in N2 atmosphere.

previously, were subjected to a higher temperature heating in inertatmosphere in order to produce a single phase thallium selenide.Heating was performed for 12 h at 100 ◦C in N2 atmosphere andXRD data obtained are shown in Fig. 6. The only crystalline phase

8 urface

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26 R. Ivanauskas et al. / Applied S

etected in the heated samples was TlSe (22–1476) – tetragonalhallium selenide. This transformation can be explained using Tl-e phase diagrams available in the literature [65]. Only aroundtoichiometrically equivalent amounts of both components doeslSe form, independently of the temperature up until 200 ◦C. Theresence of S8 in the XRD spectra of unheated thallium reactedamples implies that selenium, diffused into the polymer, does notully react to form thallium selenides at the room temperaturend stratifies beneath the thallium rich phases. At elevated tem-erature of 100 ◦C used to heat the sample, residual thallium ionsiffuse faster into the polymer while also binding to the S8. Due tohis diffusion, concentration gradient, otherwise present across theolymer cross-section, becomes smaller and stociohiometric TlSean be formed. This presents a rather simple way of forming a singlehase PA-TlSe composite materials.

. Conclusions

Adsorption–diffusion method was used to obtain ∼2 �m TlxSey

ayers within polyamide (PA) surface thus yielding PA-TlxSey com-osite materials and their morphological, chemical and crystallinehase information was determined. Tl2SO4 solution pH and tem-erature were systematically altered to obtain these materials. Aomplex speciation was inferred from the XRD data that changedith the pH values and temperature of Tl2SO4 solution. pH 11.3

nd 80 ◦C yielded well defined composites with higher pH result-ng in elemental selenium deposited due to the hydroxyl inducedecomposition of SeS2O6

2−, Se2S2O62− and SeSO3

2− ions. Higheremperatures (80 ◦C) yielded larger number of crystalline phasesut more cohesive layers, as inferred from AFM analysis. Calculatedrbach energies correlated with AFM analysis showing decreased

tructural disorder in samples synthesized at 80 ◦C. Annealing theseomposite materials in N2 at 100 ◦C yielded a single TlSe phaseontaining composite material.

The method described here is not limited to thallium selenidesnd can possibly be used to synthesize variety of binary III–VIompounds diffused within the polymer surface. In this work,olyamide was used as a model host polymer. However, a largecale application via two-step process – selenization followed byhe exposure to the group III metal salt solution – on virtually anyolymer can be envisioned. Possibility also remains that ternaryompounds would form during the adsorption–diffusion depo-ition and mild heating at <100 ◦C in inert atmosphere and isurrently under investigation.

cknowledgment

Central Microscopy Research Facility at the University of Iowa,SA, is acknowledged for XPS use.

ppendix A. Supplementary data

Supplementary data associated with this article can beound, in the online version, at http://dx.doi.org/10.1016/j.apsusc.014.08.192.

eferences

[1] H. Wei, X. Yan, S. Wu, Z. Luo, S. Wei, Z. Guo, Electropolymerized polyanilinestabilized tungsten oxide nanocomposite films: electrochromic behavior andelectrochemical energy storage, J. Phys. Chem. C 116 (2012) 25052–25064,

http://dx.doi.org/10.1021/jp3090777.

[2] H. Wei, D. Ding, X. Yan, J. Guo, L. Shao, H. Chen, et al., Tung-sten trioxide/zinc tungstate bilayers: electrochromic behaviors, energystorage and electron transfer, Electrochim. Acta 132 (2014) 58–66,http://dx.doi.org/10.1016/j.electacta.2014.03.056.

[

Science 317 (2014) 818–827

[3] J. Zhu, M. Chen, H. Qu, X. Zhang, H. Wei, Z. Luo, et al., Interfa-cial polymerized polyaniline/graphite oxide nanocomposites towardelectrochemical energy storage, Polymer (Guildford) 53 (2012) 5953–5964,http://dx.doi.org/10.1016/j.polymer.2012.10.002.

[4] P. Saini, M. Arora, G. Gupta, B.K. Gupta, V.N. Singh, V. Choudhary, Highpermittivity polyaniline–barium titanate nanocomposites with excellent elec-tromagnetic interference shielding response, Nanoscale 5 (2013) 4330–4336,http://dx.doi.org/10.1039/C3NR00634D.

[5] X. Zhang, J. Zhu, N. Haldolaarachchige, J. Ryu, D.P. Young, S. Wei, et al.,Synthetic process engineered polyaniline nanostructures with tunable mor-phology and physical properties, Polymer (Guildford) 53 (2012) 2109–2120,http://dx.doi.org/10.1016/j.polymer.2012.02.042.

[6] X. Zhang, S. Wei, N. Haldolaarachchige, H.A. Colorado, Z. Luo, D.P. Young,et al., Magnetoresistive conductive polyaniline–barium titanate nanocompos-ites with negative permittivity, J. Phys. Chem. C 116 (2012) 15731–15740,http://dx.doi.org/10.1021/jp303226u.

[7] X. Yan, M. Li, J. Long, X. Zhang, H. Wei, Q. He, et al., Highly monodispersesub-microspherical poly(glycidyl methacrylate) nanocomposites with highlystabilized gold nanoparticles, Macromol. Chem. Phys. 215 (2014) 1098–1106,http://dx.doi.org/10.1002/macp.201400097.

[8] J. Guo, H. Gu, H. Wei, Q. Zhang, N. Haldolaarachchige, Y. Li, et al.,Magnetite–polypyrrole metacomposites: dielectric properties and mag-netoresistance behavior, J. Phys. Chem. C 117 (2013) 10191–10202,http://dx.doi.org/10.1021/jp402236n.

[9] A.A. Rockett, Current status and opportunities in chalcopyrite solar cells,Curr. Opin. Solid State Mater. Sci. 14 (2010) 143–148, http://dx.doi.org/10.1016/j.cossms.2010.08.001.

10] H. Haeuseler, E. Elitok, A. Memo, A. Osnowsky, Materials with layeredstructures XI: X-ray powder diffraction investigations in the systems CuIn5S8-CuIn5Se8 and AgIn5S8-AgIn5Se8, Mater. Res. Bull. 36 (2001) 737–745,http://dx.doi.org/10.1016/S0025-5408(01)00542-6.

11] Y.-Y. Yu, W.-C. Chien, Y.-H. Ko, S.-H. Chen, Preparation and characterization ofP3HT:CuInSe2:TiO2 thin film for hybrid solar cell applications, Thin Solid Films520 (2011) 1503–1510, http://dx.doi.org/10.1016/j.tsf.2011.08.001.

12] N.B. Singh, G. Kanner, M. Marable, A. Berghmans, D. Knuteson, D. Kahler,et al., Selenides for nonlinear optical applications, Emerg. Mater. Res. 1 (2012)185–200.

13] N.B. Singh, D.R. Suhre, V. Balakrishna, M. Marable, R. Meyer, N. Fernelius,et al., Far-infrared conversion materials: gallium selenide for far-infrared con-version applications, Prog. Cryst. Growth Charact. Mater. 37 (1998) 47–102,http://dx.doi.org/10.1016/S0960-8974(98)00013-8.

14] R. Sreekumar, T.H. Sajeesh, T. Abe, Y. Kashiwaba, C. Sudha Kartha, K.P. Vijayaku-mar, Influence of indium concentration and growth temperature on thestructural and optoelectronic properties of indium selenide thin films, Phys.Stat. Sol. 250 (2013) 95–102, http://dx.doi.org/10.1002/pssb.201248268.

15] H.M. Pathan, S.S. Kulkarni, R.S. Mane, C.D. Lokhande, Preparation and charac-terization of indium selenide thin films from a chemical route, Mater. Chem.Phys. 93 (2005) 16–20, http://dx.doi.org/10.1016/j.matchemphys.2005.01.063.

16] N.N. Greenwood, A. Earnshaw, Chemistry of the Elements, 2nd ed.,Butterworth-Heinemann, Oxford,UK, 1997.

17] M.G. Kanatzidis, Chalcogenides: solid-state chemistry, in: Encyclopediaof Inorganic Chemistry, John Wiley & Sons, Ltd, Hoboken, NJ, 2006,http://dx.doi.org/10.1002/0470862106.ia040m.

18] A. Mondal, P. Pramanik, A chemical method for the prepa-ration of TlS thin films, Thin Solid Films 110 (1983) 65–71,http://dx.doi.org/10.1016/0040-6090(83)90174-8.

19] C.E. Backus, Solar Cells, Wiley, Interscience, New York, 1976.20] P.S. Nayar, W.O. Hamilton, Thallium selenide infrared detector, Appl. Opt. 16

(1977) 2942–2944, http://dx.doi.org/10.1364/ao.16.002942.21] S.A. Hussein, A.T. Nagat, N.M. Mohamed, Characteristic of the photoconduc-

tivity of thallium monoselenide, Cryst. Res. Technol. 24 (1989) 685–692,http://dx.doi.org/10.1002/crat.2170240710.

22] A.M. Panich, M. Shao, C.L. Teske, W. Bensch, Size-dependent properties of Tl2Sestudied by NMR spectroscopy, Phys. Rev. B 74 (2006) 233305.

23] M. Afzaal, P. O’Brien, Recent developments in II–VI and III–VI semiconductorsand their applications in solar cells, J. Mater. Chem. 16 (2006) 1597–1602,http://dx.doi.org/10.1039/B512182E.

24] J.S. Liu, C.-H. Kuan, S.-C. Cha, W.-L. Chuang, G.J. Gau, J.-Y. Jeng, Photovoltaictechnology development: a perspective from patent growth analysis, Sol.Energy Mater. Sol. Cells 95 (2011) 3130–3136, http://dx.doi.org/10.1016/j.solmat.2011.07.002.

25] I. Repins, M.A. Contreras, B. Egaas, C. DeHart, J. Scharf, C.L. Perkins, et al., 19·9%-efficient ZnO/CdS/CuInGaSe2 solar cell with 81·2% fill factor, Prog. Photovolt.Res. Appl. 16 (2008) 235–239, http://dx.doi.org/10.1002/pip.822.

26] A.G. Aberle, Thin-film solar cells, Thin Solid Films 517 (2009) 4706–4710,http://dx.doi.org/10.1016/j.tsf.2009.03.056.

27] M. Wright, A. Uddin, Organic–inorganic hybrid solar cells: a com-parative review, Sol. Energy Mater. Sol. Cells 107 (2012) 87–111,http://dx.doi.org/10.1016/j.solmat.2012.07.006.

28] X. Fan, M. Zhang, X. Wang, F. Yang, X. Meng, Recent progress inorganic–inorganic hybrid solar cells, J. Mater. Chem. A 1 (2013) 8694–8709,

http://dx.doi.org/10.1039/C3TA11200D.

29] C. Fradler, T. Rath, S. Dunst, I. Letofsky-Papst, R. Saf, B. Kunert, et al., Flexiblepolymer/copper indium sulfide hybrid solar cells and modules based on themetal xanthate route and low temperature annealing, Sol. Energy Mater. Sol.Cells 124 (2014) 117–125, http://dx.doi.org/10.1016/j.solmat.2014.01.043.

urface

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[64] V. Krylova, J. Baltrusaitis, Synthesis and properties of polyamide–Ag2S compos-

R. Ivanauskas et al. / Applied S

30] T. Rath, M. Edler, W. Haas, A. Fischereder, S. Moscher, A. Schenk,et al., A direct route towards polymer/copper indium sulfidenanocomposite solar cells, Adv. Energy Mater. 1 (2011) 1046–1050,http://dx.doi.org/10.1002/aenm.201100442.

31] R. Ivanauskas, Studies on application as precursor of selenotrithionate andthe mechanism of seleniumization polyamide 6 films, Mater. Sci. 15 (2009)243–249.

32] R. Ivanauskas, V. Janickis, Formation of copper selenide layers on the surfaceof polyamide films by the use of potassium selenotrithionate, Pol. J. Chem. 82(2008) 2281–2292.

33] R. Ivanauskas, I. Ancutiene, Formation of silver selenide layers on polyamide 6films by the use of potassium selenotrithionate, Mater. Sci. 17 (2011) 396–398,http://dx.doi.org/10.5755/j01.ms.17.4.775.

34] R. Ivanauskas, J. Baltrusaitis, Synthesis and surface properties ofpolyamide–CuxSe composite thin films, Appl. Surf. Sci. 283 (2013) 360–366,http://dx.doi.org/10.1016/j.apsusc.2013.06.116.

35] R. Ivanauskas, V. Janickis, V. Jasulaitiene, Comparison characterization of cop-per selenide thin layers prepared on polyamide 6 films by sorption–diffusionmethod, Cent. Eur. J. Chem. 11 (2013) 636–643, http://dx.doi.org/10.2478/s11532-012-0200-5.

36] V. Janickis, R. Maciulevicius, R. Ivanauskas, I. Ancutiene, Study of copper sulfidelayers on a polyamide film formed by the use of higher polythionic acids, Mater.Sci. – Pol. 23 (2005) 715–727.

37] J. Mann, Nature’s Building Blocks: An A–Z Guide to the Elements by John Emsley,Society of Chemical Industry, 2002.

38] M.J. Mangalam, K.N. Rao, N. Ranganajan, M.I.A. Siddiqi, V.C. Suryanarayana,Studies on thin polycrystalline layers of thallium selenide, Jpn. J. Appl. Phys.8 (1969) 1258–1261, http://dx.doi.org/10.1143/JJAP.8.1258.

39] I. Mucha, Z. Sztuba, W. Gaweł, Phase studies on the quasi-binary thallium(I)selenide-thallium(I) telluride system, J. Phase Equilib. 24 (2003) 147–150,http://dx.doi.org/10.1361/105497103770330767.

40] I. Mucha, K. Wiglusz, Phase studies on the quasi-binary thallium(I)selenide–cadmium selenide system, Thermochim. Acta 526 (2011) 107–110,http://dx.doi.org/10.1016/j.tca.2011.09.002.

41] G. Shaw, I.P. Parkin, Room temperature synthesis of thallium chalcogenidesin ammonia and n-butylamine, Main Gr. Met. Chem. 24 (2001) 195–203,http://dx.doi.org/10.1515/mgmc.2001.24.4.195.

42] R.N. Bhattacharya, P. Pramanik, New chemical methods for the deposition ofcopper sulfide (Cu1·8S) and thallium selenide (TlSe) thin film, Bull. Mater. Sci.3 (1981) 403–408, http://dx.doi.org/10.1007/bf02819023.

43] D. Suteu, D. Bilba, F. Dan, Synthesis and characterization of polyamide powdersfor sorption of reactive dyes from aqueous solutions, J. Appl. Polym. Sci. 105(2007) 1833–1843.

44] D. Milasiene, R. Ivanauskas, Silver selenide modification of polyamide fab-ric, Cent. Eur. J. Chem. 11 (2013) 1976–1980, http://dx.doi.org/10.2478/s11532-013-0343-z.

45] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, 2nd ed., Academic

Press, (London) LTD, London NW1 7DX, 1982.

46] B. Rathke, Mittheilungen aus dem Universitäts-Laboratorium zu Königsberg.XV. Beiträge zur Kenntniss der chemischen Aehnlichkeit von Schwefel undSelen. Selendithionige Säure. Selentrithionsäure, J. Für Prakt. Chem. 95 (1865)1–30.

[

Science 317 (2014) 818–827 827

47] J. Baltrusaitis, C.R. Usher, V.H. Grassian, Reactions of sulfur dioxide on calciumcarbonate single crystal and particle surfaces at the adsorbed water carbonateinterface, Phys. Chem. Chem. Phys. 9 (2007) 3011–3024.

48] N. Fairley, CasaXPS Version 2.3.17dev4-8, (n.d.).49] J. Tauc, A. Menth, States in the gap, J. Non-Cryst. Solids 8–10 (1972) 569–585,

http://dx.doi.org/10.1016/0022-3093(72)90194-9.50] N. Mott, E.A. Davis, International Series of Monographs on Physics. Electronic

Processes in Non-Crystalline Materials, Oxford University Press, New York,N. Y., 1978.

51] D. Souri, K. Shomalian, Band gap determination by absorption spectrum fittingmethod (ASF) and structural properties of different compositions of (60 − x)V2O5-40TeO2-xSb2O3 glasses, J. Non-Cryst. Solids 355 (2009) 1597–1601,http://dx.doi.org/10.1016/j.jnoncrysol.2009.06.003.

52] O. Foss, V. Janickis, Crystal structure of [gamma]-monoclinic selenium, J. Chem.Soc. Dalton Trans. (1980) 624–627, http://dx.doi.org/10.1039/dt9800000624.

53] R.E. Marsh, L. Pauling, J.D. McCullough, The crystal structure of [beta]selenium, Acta Crystallogr. 6 (1953) 71–75, http://dx.doi.org/10.1107/S0365110X53000168.

54] P. Cherin, P. Unger, Refinement of the crystal structure of [alpha]-monoclinic Se, Acta Crystallogr. Sect. B 28 (1972) 313–317, http://dx.doi.org/10.1107/S0567740872002249.

55] L.I. Man, V.S. Parmon, R.M. Imamov, A.S. Avilov, Electron diffraction determi-nation of the structure of the tetragonal phase of thallium selenide (Tl5Se3),Kristallografiya 25 (1980), 1070–1072, 1048A.

56] B.K. Vainshtein, B.B. Zvyagin, A.S. Avilov, Electron diffraction structure analysis,Int. Union Crystallogr. Monogr. Crystallogr. 3 (1992) 216–312.

57] J. Weis, H. Schaefer, G. Schoen, New alkali metal tellurides and selenides withgroup IIIA elements, Z. Naturforsch. B Anorg. Chem. Org. Chem. 31B (1976)1336–1340.

58] R.R. Yadav, R.P. Ram, S. Bhan, On the thallium–selenium–tellurium system, Z.Met. 67 (1976) 173–176.

59] N. Ghobadi, Band gap determination using absorption spectrum fitting proce-dure, Int. Nano Lett. 3 (2013) 2, http://dx.doi.org/10.1186/2228-5326-3-2.

60] W.H. Strehlow, E.L. Cook, Compilation of energy band gaps in elemental andbinary compound semiconductors and insulators, J. Phys. Chem. Ref. Data 2(1973) 163, http://dx.doi.org/10.1063/1.3253115.

61] G. Ambrosone, D.K. Basa, U. Coscia, M. Fathallah, Study on the microstructuraland overall disorder in hydrogenated amorphous silicon carbon films, J. Appl.Phys. 104 (2008) 123520, http://dx.doi.org/10.1063/1.3042242.

62] R.T.A. Kumar, P.C. Lekha, B. Sundarakannan, D.P. Padiyan, Influence of thicknesson the optical properties of amorphous GeSe2 thin films: analysis using Ramanspectra, Urbach energy and Tauc parameter, Philos. Mag. 92 (2012) 1422–1434,http://dx.doi.org/10.1080/14786435.2011.647104.

63] V. Krylova, A. Milbrat, A. Embrechts, J. Baltrusaitis, Ag2S deposited on oxidizedpolypropylene as composite material for solar light absorption, Appl. Surf. Sci.301 (2014) 134–141, http://dx.doi.org/10.1016/j.apsusc.201402.014.

ite based solar energy absorber surfaces, Appl. Surf. Sci. 282 (2013) 552–560,http://dx.doi.org/10.1016/j.apsusc.201306.009.

65] H. Okamoto, Se–Tl (selenium–thallium), J. Phase Equilib. Diffus. 32 (2011)570–571, http://dx.doi.org/10.1007/s11669-011-9954-2.