Generalized Water-Processed Metal Chalcogenide Complexes:Synthesis and ApplicationsZhe Xia,† Jie Zhong,† Meiying Leng,† Long Hu,† Ding-Jiang Xue,*,† Bo Yang,† Ying Zhou,†

Xinsheng Liu,† Sikai Qin,† Yi-Bing Cheng,†,‡ and Jiang Tang*,†

†Wuhan National Laboratory for Optoelectronics (WNLO) and School of Optical and Electronic Information, Huazhong Universityof Science and Technology (HUST), Wuhan 430074, China‡Department of Materials Engineering, Monash University, VIC 3800, Australia

*S Supporting Information

ABSTRACT: Metal chalcogenide complexes (MCCs) have attracted considerableattention recently due to their high solubility in polar solvents, low-temperaturedecomposition to metal chalcogenide films, and service as ligands for colloidalnanocrystals (NCs). However, most of the MCCs were typically synthesized in thehighly toxic and explosive hydrazine (denoted as hydrazine-MCCs), severely restrictingthe wide applications of MCCs. Here we present a versatile and environmentally benignwater-based solution method for the preparation of various MCCs (denoted as water-MCCs) through directly dissolving a series of bulk V2VI3 chalcogenides (V = Sb, As andVI = S, Se, Te) in water with the presence of (NH4)2S at room temperature and ambientatmosphere. We further show that such water-MCCs can be readily processed intocorresponding semiconducting thin films upon mild thermal treatment and then beextended to fabricate compositionally controlled semiconductor alloys with tunable bandgaps (i.e., Sb2(S1−x,Sex)3, 0 ≤ x ≤ 1) through simple control of substrate temperature.Furthermore, we present that our water-MCCs, especially for Sb4S7

2−, can be utilized toserve as ligands for in-situ-synthesized water-based PbS quantum dots (QDs), achieving a homogeneous and stable aqueous QDssolution without needing further conventional secondary ligand exchange. Our study provides a general strategy for the synthesisof various MCCs using water as safer and more environmentally friendly solvent alternative to hydrazine, thus greatly enhancingthe wide applications of MCCs in solution-processed inorganic semiconductors.

1. INTRODUCTION

Solution processing of metal chalcogenide thin films holds greatpotential for wide applications in solar cells, transistors, andthermoelectrics due to its low manufacturing cost and goodcompatibility with high-throughput deposition techniques suchas spray and printing.1−9 However, the covalent nature of metalchalcogenides, despite providing their outstanding electronicproperties, makes it challenging for direct dissolution of thesematerials. Fortunately, the utility of hydrazine as solvent opensthe door for dissolution of most metal chalcogenides throughthe formation of highly soluble precursors: metal chalcogenidecomplexes (MCCs), denoted as hydrazine-MCCs, thusproviding true solutions down to the molecular level.10,11

More importantly, MCCs can be readily decomposed to thestarting metal chalcogenides upon mild thermal treatment dueto the weak incorporation of hydrazinium cations.1,10−13

Therefore, the feature of high solubility combined with low-temperature decomposition makes MCCs ideal for solution-processed thin-film semiconductors. The most successfulexample of this MCCs-based approach is Cu(In,Ga)(S,Se)2(CIGS) and Cu2ZnSn(S,Se)4 (CZTSSe) thin-film solar cellsdeveloped by Mitzi et al., demonstrating remarkable efficienciesof 15.2% and 12.7%, respectively,14,15 thereby leading to a

breakthrough in molecular MCCs precursors for high-qualitythin semiconducting films.Another attractive feature of MCCs is serving as ligands for

colloidal nanocrystals (NCs) due to their high affinity ofterminal chalcogen atoms toward binding undercoordinatedmetal atoms on NC surface based on Pearson’s hard and softacids and bases (HSAB) principle as well as their small sizesuch as thiostannate ligands (Sn2S7

6−, SnS44−) formed in

different solvents of less than 1 nm, much smaller thantraditional long-chain organic ligands with a length of ∼2nm.16−19 The above characteristics enable MCCs to directlyreplace organic ligands and attach firmly to the surface of NCs,thus not only providing colloidal stabilization in solution butalso effectively minimizing interparticle spacing and facilitatingthe charge transport between individual NCs when processedinto films. This concept of MCCs-ligands was first introducedby Talapin et al. in 2009,16 demonstrating the dramatic impactof various MCCs on coupled Au and CdSe NCs in thin films toincrease conductivity by several orders of magnitude,respectively. Very recently, Kovalenko et al. reported a detailed

Received: September 15, 2015Revised: November 5, 2015Published: November 6, 2015

Article

pubs.acs.org/cm

© 2015 American Chemical Society 8048 DOI: 10.1021/acs.chemmater.5b03614Chem. Mater. 2015, 27, 8048−8057

experimental and theoretical study of the inorganic surfacefunctionalization of CdSe NCs by MCCs (Sn2S6

4−, Sn2S76−,

and SnS44−),18 illustrating the atomistic details of the organic-

to-inorganic ligand exchange and binding motifs at the NCsurface, greatly promoting the full understanding of MCCs. Inaddition, MCCs can also be used as “solders” for semi-conductors widely used in photovoltaics and thermoelectrics,thus opening new prospects for semiconductor technologies.20

Whereas MCCs hold great promise for solution processing ofthin-film semiconductors and ligands for NCs, it is unfortunatethat most of the MCCs were typically synthesized in the highlytoxic and explosive hydrazine, such that the synthesis processmust be performed with great caution under inert environment,thus severely restricting the wide applications of MCCs.Consequently, it is highly desirable to develop a green, general,and low-cost approach free of hydrazine to synthesize MCCs.In the search for alternative solvent, Brutchey et al. recentlymade impressive progress through using a simple thiol−aminesolvent mixture.4,21−23 We take the view that water canundoubtedly be considered the cleanest and cheapest solventavailable. Inspired by the synthesis of (NH4)4Sn2S6 reported byTalapin et al.,24 we propose a water-based strategy to synthesizeAs- and Sb-based MCCs in the presence of (NH4)2S, denotedas water-MCCs, based on the following design principle:Aqueous (NH4)2S solution can offer a combination of twoessential functional groups for the formation of MCCs via thewell-known dimensional reduction process: S2− and NH4

+

(serving as counterion), analogous to the excess reducedchalcogen and N2H5

+ in hydrazine.10 More importantly,especially for the following thin-film deposition, water as theonly solvent can be easily evaporated upon moderate thermaltreatment, while (NH4)2S can be readily eliminated completelyvia H2S and NH3 gas. Combined with no carbon source in ouraqueous system, O-, N-, and C-free samples would be obtainedthrough our proposed water-MCCs method.In this work, we present a general approach to synthesize

various MCCs using water as a safer and more environmentallyfriendly alternative to hydrazine. Raman spectroscopy wasapplied to thoroughly study the synthesis mechanism of water-MCCs. As shown in Figure 1, the resulting water-MCCs weredirectly solution processed into corresponding metal chalcoge-nides or alloys films by spray pyrolysis, and a completeSb2(S1−x,Sex)3 thin-film solar cell was built achieving 1.43%solar energy conversion efficiency. In addition to thin-film solarcells, such water-MCCs solutions may be further extended tofabricate chalcogenides-sensitized solar cells, another importantstructure of photovoltaic devices, due to their true solutioncharacteristic. Furthermore, our water-MCCs were applied forcapping in-situ-synthesized PbS quantum dots (QDs),achieving a homogeneous and stable aqueous QDs solution.Compared with conventional hydrazine-based approach, ourwater-based synthesis of MCCs greatly enhance the potentialapplication of MCCs in a wide range of photovoltaic,electronic, and thermoelectric devices with high throughput,low cost, and improved safety.

2. EXPERIMENTAL SECTION2.1. Chemicals. Antimony sulfide (Sb2S3, powder, 99.999%),

antimony selenide (Sb2Se3, powder, 99.999%), antimony telluride(Sb2Te3, powder, 99.96%), arsenic(III) sulfide (As2S3, powder, 99.9%),arsenic(III) selenide (As2Se3, powder, 99.999%), arsenic(III) telluride(As2Te3, powder, 99%), and selenium (Se, amorphous, 99.999%) wereall purchased from Alfa Assar. Ammonium sulfide (40−48 wt % in

water) and tellurium (Te, 99.99%) were purchased from Aladdin.Lead(II) nitrate (Pb(NO3)2, analytical reagent grade) and sodiumhydroxide (NaOH, analytical reagent grade) were purchased fromSinopharm Chemical Reagent Co., Ltd. NOTE: X-ray diffractiondemonstrated that elemental Se and Te were amorphous andhexagonal tellurium, respectively. All chemicals were used as receivedwithout any further purification.

2.2. Synthesis of Metal Chalcogenide Complexes Solutions.All syntheses were performed at room temperature in a well-ventilatedfume hood. To synthesize a series of V2VI3 chalcogenides (V = Sb, Asand VI = S, Se, Te) based water-MCCs, Sb2S3 (1 mmol), Sb2Se3 (1mmol), Sb2Te3 (0.1 mmol), As2S3 (1 mmol), As2Se3 (1 mmol), andAs2Te3 (0.1 mmol) were mixed with 10 mL of deionized water and 10mL of ammonium sulfide solution (40−48 wt % in water) in a 25 mLconical flask at room temperature. The mixtures were fully dissolvedwithin 7 days of magnetic stirring at room temperature.

2.3. Synthesis of PbS Quantum Dots with in-Situ Water-MCCs Capping. A 2 mL amount of Pb(NO3)2 aqueous solution (0.1mol/L) and 14 mL of NaOH aqueous solution (0.1 mol/L) weremixed in a 25 mL conical flask with magnetic stirring until atransparent colorless solution was formed. Next, once adding theabove solution drop by drop into the vigorously stirred (NH4)2Sb4S7aqueous solution, a dark brown colloidal solution was formedimmediately, indicating the formation of PbS QDs. Note that the(NH4)2Sb4S7 aqueous solution used for the synthesis of PbS QDs waspurified in the following steps: (i) The resulting Sb2S3-MCCs solutionwas heated at 65 °C in a N2-filled glovebox for several hours to removewater and (NH4)2S. (ii) The obtained yellow solid was redissolved inwater only upon addition of small quantities of (NH4)2S and thenfiltered to remove excessive sulfur as a precipitate. Finally, the resultantdark brown PbS QDs were separated out via centrifugation andredispersed in water for the following measurement. All syntheses wereperformed at room temperature in a fume hood.

2.4. Film Deposition and Device Fabrication. Sb2S3 films weredeposited on FTO glass at a temperature of 350 °C by spray pyrolysisusing the Sb2S3-MCCs solution. The Sb2(S1−x,Sex)3 alloy thin filmswere obtained using the Sb2Se3-MCCs solution through a similar spraypyrolysis technique except using different substrate temperatures.Sb2Se3 films were prepared by an additional selenization ofSb2(S0.44,Se0.56)3 films in a covered hot plate under Se atmospheresat 380 °C. All of the above experiments were done in a N2-filledglovebox. Solar cells were fabricated with a conventional planar devicestructure (glass/FTO/TiO2/Sb2(S0.44,Se0.56)3/Au). First, TiO2 electro-

Figure 1. Schematic illustration for the synthesis and applications ofwater-MCCs.

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des were fabricated by depositing Ti sols on FTO substrates using spincoating with following heat treatment. The preparation of Ti sols wascarried out in air ambient at room temperature. Titanium(IV) n-butoxide (4.25 mL) was mixed with triethanolamine (2 mL) andanhydrous ethyl alcohol (25 mL) in a flask under continuous magneticfor 2 h. Acetic acid (5 mL) and deionized water (5 mL) were thenadded into the mixture with continuous magnetic agitation for 24 h.The mixture was then stored in a beaker and placed inside a fumehoodto allow condensation reactions until the volume reached 15 mL. Tisols were deposited on the FTO substrates by spin coating at 2500rpm for 15 s. One edge of the as-deposited film was then wiped free ofthe sols with a swab soaked in anhydrous ethyl alcohol to expose aregion of clean FTO for electrical contacting. This wipe wasimmediately followed by heat treatment in air on a hot plate at 520°C for 1 h. Sb2(S0.44,Se0.56)3 absorber layers were then deposited ontothe TiO2 electrode at a temperature of 380 °C by spray pyrolysis usingthe Sb2Se3-MCCs solution. Finally, gold contacts (50 nm thickness)were deposited by thermal evaporation. Each device had a total area ofapproximately 0.09 cm2 (3 mm × 3 mm) fixed by the mask pattern.2.5. Materials and Device Characterization. Raman analysis

(Horiba JobinYvon, LabRAM HR800, 532 nm excitation) wasperformed on the V2VI3 chalcogenides (V = Sb, As and VI = S, Se,Te) solutions in a backscattering confocal configuration at roomtemperature using 2 mL glass bottles as the solution containers andfocusing the laser beam in the liquid. Thermogravimetric analysis(TGA, PerkinElmer Instruments, Diamond TG/DTA6300) wasperformed in a flowing N2 atmosphere at 10 °C/min. The crystalstructures of the products were characterized by X-ray diffraction(XRD, Philips, X pert pro MRD, with Cu Kα radiation, λ = 1.54178Å). The absorption was recorded by a UV−vis−IR spectrophotometer(PerkinElmer Instruments, Lambda 950 using integrating sphere). Themorphology of V2VI3 chalcogenides films was tested by scanningelectron microscopy (SEM, FEI Nova NanoSEM450, without Ptcoating). The compositions of thin films were obtained throughenergy-dispersive spectroscopy (FEI Quanta 600 scanning electronmicroscope, 20 kV). Transmission electron microscopy (TEM, TecnaiG2 20U-TWIN) was used to characterize the as-synthesized PbS QDs.The photoluminescence of the sample was excited by a Ti:sapphire(Mira 900) laser with an excitation wavelength of 488 nm andrecorded by an liquid N2-cooled InGaAs CCD spectrometer(Princeton, OMV5). The dynamic light scanning (DLS) and zeta-potential results were obtained by a Malvern NanoZS ZEN3600analyzer. Fourier transform infrared spectroscopy (FTIR) tests werecarried out using a Bruker VERTEX 70 instrument. Current density−voltage characteristics in the dark and under light were measured by aKeithley 2420 source meter under ambient conditions. The lightsource is a standard AM1.5G (100 mW/cm2) made by the NewportSol3A Class AAA solar simulator (Oriel, model 94023 A). Nointentional temperature control or aperture was used for the efficiencymeasurement.

3. RESULTS AND DISCUSSION3.1. Synthesis and Characterization of Water-MCCs.

As shown in the top of Figure 2, a series of six bulk V2VI3chalcogenides (V = As, Sb; VI = S, Se, Te) can be readilydissolved in aqueous (NH4)2S solution under magnetic stirringat room temperature and ambient pressure. The resultingsolutions were free of visible scattering, indicating a completedissolution to form true solutions rather than nanoparticulatedispersions. Moreover, the prepared solutions were stable whenstored in ambient conditions for over 6 months, and noprecipitation was observed. More importantly, this dissolutionprocess could be easily scalable, which was demonstrated onthe scale-up dissolution of As2Se3 and Sb2S3 (bottom of Figure2), highlighting the great potential of this water-baseddissolution process for mass production.We applied Raman spectroscopy to characterize our

chalcogenides solutions to investigate the dissolution mecha-

nism. Figure 3 showed typical Raman spectra of all six V2VI3chalcogenides solutions. In the case of As2S3 and Sb2S3, thedistinct peak located at 2560 cm−1 can be assigned to the S−Hvibration in HS− species formed upon dissolution of (NH4)2Sin water and subsequent hydrolysis of S2−,25 while the peakslocated at 394 (Figure 3a) and 363 cm−1 (Figure 3d) couldpresumably be attributed to the As−S and Sb−S stretchingmodes of As and Sb chalcogenide complexes: AsS3

3− andSb4S7

2−, respectively.26−28 Therefore, we proposed that thedissolution mechanism of As2S3 and Sb2S3 in aqueous (NH4)2Ssolution may be processed by forming highly soluble MCCsthrough the following reactions

+ → ++ −As S 3(NH ) S 6NH 2AsS2 3 4 2 4 33

(1)

+ → ++ −2Sb S (NH ) S 2NH Sb S2 3 4 2 4 4 72

(2)

With regard to Sb2Se3, besides the predictable Sb−Se peaklocated at 216 cm−1, a Sb−S peak at 363 cm−1 was observed, asshown in Figure 3e. Meanwhile, the Se chain (235 cm−1) andSe8 ring (260 cm−1) peaks were also detected, despite noelemental Se added to the solution. Moreover, as shown inFigure S1 in the Supporting Information, there was a certainamount of (NH4)2Sx present in aqueous (NH4)2S solution,which could be easily decomposed into elemental S.29 On thebasis of the above information, we therefore proposed that thedissolution process of Sb2Se3 could be attributed to substitutionof Se with S, thus forming Sb4(S1−x,Sex)7

2− while leaving Se insolution through the following overall chemical equation

+ − +

→ + + −+−

−

x

x

2Sb Se (6 7 )S (NH ) S

2NH Sb (S , Se ) (6 7 )Sex x

2 3 4 2

4 4 1 72

(3)

Such substitution reaction was expected considering the verysimilar chemistry between S and Se and the unchangedcomplex configuration after the replacement. This result was

Figure 2. (Top) Photograph of a series of solutions by dissolving sixV2VI3 chalcogenides in aqueous (NH4)2S solution. (Bottom) Photo-graph of scale-up dissolution of As2Se3 and Sb2S3 in aqueous (NH4)2Ssolution.

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further strengthened by the control experiment in directlydissolving the elemental Se in aqueous (NH4)2S solution(Figure S2), similar to the dissolution of Se and Te in a thiol−amine solvent mixture.30 Furthermore, the analogous sub-stitution reaction was also observed in the dissolution processof As2Se3 due to the presence of Se chain, Se8 ring, and As−Speaks (Figure 3b). However, the frequency of the As−Sestretching mode was not available in the literature. Here wecalculated the As−Se vibration frequency based on the knownAs−S vibration frequency through the harmonic oscillatorapproximation for isolated binary vibrational modes31

− = −++

v vM M MM M M

(As Se) (As S)( )( )

S As Se

Se As S (4)

The v(As−Se) calculated from this expression was approx-imately 297 cm−1, agreeing well with our experimentallyobserved Raman peak located at 291 cm−1. We thereforeproposed that this new Raman peak resulted from an As−Sevibrational mode, thus confirming the formation of As-(S1−x,Sex)3

3−. Analogous to As2Se3 and Sb2Se3, As2Te3 andSb2Te3 were dissolved in aqueous (NH4)2S solution through asimilar substitution reaction of S exchanging Te to formcorresponding As(S1−x,Tex)3

3− and Sb4(S1−x,Tex)72−, as shown

in Figure 3c and 3f. In brief, on the basis of the design principle,we successfully synthesized various MCCs through directlydissolving a series of bulk V2VI3 chalcogenides (V = Sb, As andVI = S, Se, Te) in water with the presence of (NH4)2S.3.2. Thermal Analysis of As-Prepared Water-MCCs.

With the formation of water-MCCs solutions, we are nowexploring their application for solution deposition of corre-sponding chalcogenides. The decomposition temperature ofMCCs is the most fundamental parameter for the deposition;therefore, thermogravimetric analysis (TGA) of dried water-MCCs solutions was first measured to determine the

temperature at which water-MCCs decomposed. As shown inFigure 4a (blue curve), decomposition of (NH4)2Sb4S7(denoted as Sb2S3-MCCs) occurred in essentially a singlestep and was completed at ∼200 °C. The observed weight lossmay correspond to the dissociation and loss of volatiledecomposition products through the following reaction

= + ↑ + ↑(NH ) Sb S 2Sb S 2NH H S4 2 4 7 2 3 3 2 (5)

Corroboration of the decomposition reaction was obtained byX-ray diffraction (XRD) using Sb2S3-MCCs solutions annealedat 300 °C. As shown in Figure 4c, all of the diffraction peaksmatched well with orthorhombic Sb2S3 (JCPDS 06-0474),indicating the decomposition product was phase-pure Sb2S3without any impurities or secondary phase. Fourier transforminfrared spectroscopy (FTIR) further verified the thermaldecomposition of (NH4)2Sb4S7. Figure 4b showed strong ν(N−H) and ν (O−H) stretching bands at 2500−3500 cm−1 forthe dried Sb2S3-MCCs solution corresponding to NH4

+ andwater, which disappeared after annealing at 300 °C for 10 min,in accordance with TGA result. Similarly, Sb2Se3-MCCs, As2S3-MCCs, and As2Se3-MCCs could also be decomposed intocorresponding chalcogenides, as shown in Figures S5 and S6.X-ray photoelectron spectroscopy (XPS) was further

performed to investigate the chemistry state of as-preparedSb2S3 and check the presence of oxygen impurity. As shown inFigure S4, magnified XPS spectra of Sb (3d), S (2p)demonstrated that the Sb and S were in the expected valencestates (Sb2

3+S32−), and no detectable oxygen (<0.1%, the

detection limits) was present in the sample.32,33 This oxygenconcentration was consistent with our previously reportedwater-based solution-processed CZTSSe film where high-quality CZTSSe film was obtained with carbon and oxygencontamination as low as hydrazine-MCCs-derived CZTSSefilms,10,34 confirming that our designing principle for water-MCCs was effective at excluding oxygen impurity for the

Figure 3. Raman spectra of (a) As2S3, (b) As2Se3, (c) As2Te3, (d) Sb2S3, (e) Sb2Se3, and (f) Sb2Te3 precursor solutions.

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following reasons: (i) The only oxygen source in our system,water, can be easily evaporated upon moderate thermaltreatment. (ii) Thermodynamic calculation indicated thatthere was a spontaneous reaction between Sb2O3 and H2S toform Sb2S3 and H2O (g) at 600 K, as shown in Table S1.Therefore, even if some Sb2O3 were formed during the processof thermal decomposition, the surrounding excess H2S formedthrough the decomposition of (NH4)2S would convert theseoxides into sulfides efficiently.Decomposition of the analogous As2Te3-MCCs occurred in

several steps (Figure 4a, pink curve), which was morecomplicated than that of (NH4)2Sb4S7 by the fact that the as-prepared bulk precursor was not a single phase but, rather, amixture of (NH4)3As(S1−x,Tex)3 and Te similar to Sb2Se3-basedprecursor, as described in the Raman spectrum characterization.To understand the decomposition of As2Te3-MCCs better, wefirst calculated the temperature-dependent saturated vaporpressure of elemental Te and As2Te3 as well as elemental S andSb2S3, according to the following equations35

= −° +

pB

t Clog (mm Hg) A

( C) (6)

= −p AB

T Klog (kPa)

( ) (7)

Constants involved for calculation are shown in Table 1,36,37

and the temperature-dependent equilibrium pressure is shownin Figure 4e and 4f. As shown in Figure 4a (pink curve), thelow-temperature transition (200−300 °C) indicated the loss ofvolatile decomposition products such as NH3 and H2S. On the

basis of the calculated temperature-dependent vapor pressure ofTe and As2Te3, the transition (300−380 °C) probablyrepresented the loss of As2Te3 due to the higher vaporpressure of As2Te3 versus Te; the high-temperature transition(>380 °C) may correspond to the evaporation of both As2Te3and Te. As shown in Figure 4d, XRD study of As2Te3 precursorsolutions annealed at 300 °C further confirmed the formationof As2Te3 and elemental Te, analogous to the recovery ofdissolved bulk As2Te3 in a thiol−amine solvent mixture via low-temperature annealing.4 Note that the excess Te, inevitablyremaining in the sample after the low-temperature stage ofdecomposition due to the lower volatility of Te versus S, provesmuch more difficult to be removed from the sample than forSb2S3. With regard to Sb2Te3-MCCs, the decompositionproducts were analogous to that of As2Te3-MCCs, as shownin Figure S6d. Briefly, the above characterizations gatheredfrom TGA, XRD, and FTIR showed that our water-MCCs werewell suited for the solution deposition of correspondingchalcogenides upon mild thermal treatment.

3.3. Fabrication of Sb2(S1−x,Sex)3 Alloy Thin Films andTheir Application in Solar Cells. To further demonstrate theutility of our water-MCCs for solution processing of thin-filmchalcogenides, we sought to use the Sb2S3-MCCs and Sb2Se3-MCCs solutions as an example to prepare high-qualitySb2(S1−x,Sex)3 alloy thin films across the entire compositionalrange from x = 0 to 1 through simply controlling the annealingconditions. Very recently, our group as well as others havedemonstrated the great potential of this Sb2(S1−x,Sex)3 alloy asabsorber material for photovoltaic devices due to their suitableand continuously tunable band gaps between 1.08 and 1.62 eV,high absorption coefficient (>104 cm−1), and nontoxic, low-cost, and earth-abundant nature.26,38−41 Importantly, the truesolution characteristic of our water-MCCs solution allowed usto choose the simple and inexpensive spray pyrolysis techniqueto deposit Sb2(S1−x,Sex)3 alloy thin films, which can also beadapted easily for large-scale manufacturing.Figure 1 shows a schematic illustration of the deposition

process for Sb2(S1−x,Sex)3 alloy thin films. Binary Sb2S3 filmswere deposited on FTO substrates at a temperature of 300 °Cby spray pyrolysis using the Sb2S3-MCCs solution, while theSb2(S1−x,Sex)3 alloy thin films were prepared using the Sb2Se3-MCCs solution through spray pyrolysis at different substratetemperatures. XRD measurements were first employed tocharacterize the crystalline structure of as-prepared films. Asshown in Figure 5a, all of the films crystallized intoorthorhombic structure with no additional and split diffractionpeaks observed, indicating the formation of phase-pureSb2(S1−x,Sex)3 alloys without any impurities or secondaryphase. As expected with the smaller atom radius of S, a gradualshift to larger 2θ angle was clearly visible in the magnified viewof (020) and (120) peaks shown in Figure 5b with increasing Sconcentration. Figure 5c showed the lattice constants a, b, and cderived from the position of (200), (020), and (221) diffractionpeaks, respectively. The Se concentration x was then obtained

Figure 4. (a) TGA of Sb2S3-MCCs (blue curve) and As2Te3-MCCs(pink curve; run at 10 °C min−1 in a N2 flowing environment). (b)FTIR spectra of dried (black curve) and annealed (300 °C; red curve)Sb2S3-MCCs. (c) XRD pattern of Sb2S3-MCCs annealed at 300 °C.(d) XRD pattern of As2Te3-MCCs annealed at 300 °C. Temperature-dependent saturated vapor pressure of (e) Sb2S3 and S and (f) As2Te3and Te in the temperature range from 300 to 500 °C.

Table 1. Constants Involved for Calculation

material eq A B C

S 6 6.84 2500.12 186.30Te 6 7.301 5370.6 221Sb2S3 7 13.96 10 490As2Te3 7 10.45 8185

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from Vegard’s law using the calculated lattice parametersdeduced from the XRD data, which can be written as26,40

= × + − ×m x x m x m( ) (Sb Se ) (1 ) (Sb S )2 3 2 3 (8)

where x, m(x), m(Sb2Se3), and m(Sb2S3) are the respective Seconcentration and lattice parameters a, b, and c of theseorthorhombic structured samples. To further verify thecomposition more precisely, we applied energy-dispersivespectroscopy (EDS) to determine the Se concentration. Asshown in Table S2, the Se concentration measured directlyfrom EDS is very close to the results obtained from XRDpatterns, further confirming the composition of the as-preparedfilms as Sb2(S1−x,Sex)3. The linear dependence of latticeparameters on alloy composition x further confirmed thecompositional homogeneity of our Sb2(S1−x,Sex)3 alloy films.On the basis of XRD and EDS results, we therefore concludedthat as the substrate temperature was increased, sulfur could bemore prone to volatilize than selenium due to the higher vaporpressure of sulfur versus selenium (Figure S7) and thensubstituted by excess selenium from the solution, thus leadingto the gradually increased selenium content from x = 0.44 to0.76 in the final Sb2(S1−x,Sex)3 alloy films. This chalcogenexchange during annealing has already been widely used in thefabrication of CIGS and CZTSSe films.10 It should also benoted that even at a high substrate temperature of 400 °C, westill failed to get the pure Sb2Se3 film. Therefore, we applied apostselenization step for our alloy films to obtain the pureSb2Se3 films; please read more details in the ExperimentalSection.Transmission spectroscopy was then applied to investigate

the optical properties of our Sb2(S1−x,Sex)3 alloy films. Asshown in Figure 5d, there was an obvious systematic red shift intransmission spectra of Sb2(S1−x,Sex)3 alloy films with an

increase of Se composition, due to a narrower band gap ofSb2Se3 than that of Sb2S3.

39 Furthermore, band gaps of our sixsamples were estimated by plotting (αhν)1/2 versus (hν) andfound to be 1.61, 1.33, 1.24, 1.20, 1.16, and 1.10 eV for x = 0,0.44, 0.48, 0.56, 0.76, and 1, respectively, covering the entirerange of bulk band gaps previously reported for Sb2S3 andSb2Se3.

33,42

Scanning electron microscopy (SEM) was utilized tocharacterize the morphologies of Sb2(S1−x,Sex)3 alloy filmswith increasing Se composition from 0 to 1. As shown in Figure6, almost all of the alloy films showed a smooth and compact

surface morphology except the Sb2(S0.24,Se0.76)3 film (Figure6e) with apparent pinholes and cracks, which may arise fromthe high annealing temperature of 400 °C, thus leading toeasier volatilization and larger volume shrinkage. It was notablethat the grain size of Sb2Se3 was quite large, close to 1 μm,which could be desirable for high-performance optoelectronicdevices. In brief, the above material and optical characterizationdemonstrated that our water-MCCs solution could be readilyutilized to deposit chalcogenide thin films via spray pyrolysisand even precisely control the composition of Sb2(S1−xSex)3alloy films and then conveniently tune the resulting band gapsthrough simply regulating the substrate temperatures.To explore the suitability of the resulting alloy films for solar

cell applications, we chose the Sb2(S0.44,Se0.56)3 film as anexample to fabricate heterojunction solar cells with thestructure of glass/FTO/TiO2/Sb2(S0.44,Se0.56)3/Au, as shownin Figure 7a. The thickness of the alloy film was directlymeasured to be around 500 nm from Figure 7b, which wasthick enough for absorbing much of the incident sunlight. Wefirst investigated the photoresponse of our Sb2(S0.44,Se0.56)3 thinfilm through evaporating Au electrodes onto this alloy film tobuild a photoconductive photodetector. Current−time (I−t)characteristic of the device was recorded with a light source(650 nm wavelength, 430 μW cm−2) generated by a functionalgenerator-controlled light-emitting diode. As shown in Figure7c, under an external bias of 40 V, the dark and photocurrentwere about 15 and 45 nA respectively, corresponding to aphoto−dark current ratio of 3. The observed strong andreversible photoresponse suggested the good optoelectronicproperties of our alloy film. Figure 7d shows the current densityversus voltage (J−V) characteristics of the device measured inthe dark and under 100 mW cm−2 simulated AM1.5Girradiation. The best device exhibited an open-circuit voltage(Voc) of 0.49 V, a short-circuit current density (Jsc) of 6.6 mA/

Figure 5. (a) XRD patterns of Sb2(S1−xSex)3 alloy films deposited byspray pyrolysis at different substrate temperatures. (b) Magnified viewof (020) and (120) diffraction peaks in the region indicated by thelight blue. (c) Lattice constants a (red), b (blue), and c (green),derived from XRD diffraction peaks, plotted as a function of Seconcentration x in the Sb2(S1−xSex)3 alloy films. (d) Transmittancespectra of Sb2(S1−xSex)3 alloy films.

Figure 6. SEM images of Sb2(S1−xSex)3 alloy films with Secomposition of x = (a) 0, (b) 0.44, (c) 0.48, (d) 0.56, (e) 0.76, and(f) 1.

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cm2, and a fill factor (FF) of 44.2%, corresponding to a powerconversion efficiency of 1.43%. As shown in Table S3, ourdevice efficiency was quite low, mainly limited by the highdefect density at the heterojunction interface as evidenced bythe high value of saturation current density (J0 = 2.0 × 10−2 mAcm−2) and diode ideality factor (A = 2.46) calculated accordingto Sites’s method,43−45 which should be further improved bypassivating interfacial defects. However, this preliminary 1.43%device efficiency is still encouraging and fully demonstrates thegreat potential of our water-MCCs for thin-film solar cellapplications, considering the simple aqueous solution processas well as the very limited optimization work done.3.4. Aqueous Synthesis of PbS Quantum Dots with In-

situ Water-MCCs Passivation. PbS colloidal quantum dots(CQDs) are attractive materials for application in photovoltaicdevices due to their convenient solution processing andquantum size effect band-gap tunability.46−50 To date, thetraditional synthesis method for PbS CQDs is always based onthe use of long hydrocarbon ligands (e.g., oleic acid) to ensuretheir solution processability, subsequently replacing them withshort thiols or halide anions during the layer-by-layer spin-coating process, thus providing high carrier transport and lowdefect density to reduce recombination loss. We took the viewthat introducing our water-MCCs during the growth of the PbSQDs could stabilize in-situ-formed PbS QDs during one-potmixing of Pb2+ and S2− due to the strong binding affinity of ourMCCs to the QDs, thus providing an effective surfacepassivation while achieving a simplified device fabricationprocedure without further solid state ligand exchange.Our aqueous synthesis of PbS QDs with in-situ water-MCCs

passivation was depicted in Figure 1. The PbS QDs wereprepared in water at room temperature using ionic startingmaterials Pb(NO3)2 and (NH4)2S in the presence of(NH4)2Sb4S7. Moreover, NaOH as the chelating agent wasalso added to control the hydrolysis of Pb2+ through theformation of Pb(OH)3

−, considering the high stability constantof Pb(OH)3

− complex β3 = 3.8 × 1014. Once Pb(OH)3−

aqueous solution was directly added to the previously prepared(NH4)2Sb4S7 solution, the dark PbS QDs would be

immediately generated with negligible formation of Pb(OH)2due to the very low Ksp value of PbS (1.0 × 10−28) comparedwith that of Pb(OH)2 (1.2 × 10−15), and Sb4S7

2− could nearlysimultaneously stabilize in-situ-formed PbS QDs to preventthem from growing into large chunks and further increase inkstability, thus leading to a self-stabilized aqueous PbS QDssolution.XRD was applied to investigate the crystal structure of as-

synthesized QDs. As shown in Figure 8a, all of the diffraction

peaks were in good agreement with cubic PbS (JCPDS 05-0592), indicating the as-synthesized QDs were phase-pure PbS.Moreover, the broadness of the diffraction peaks was attributedto the small size of PbS QDs. The average crystal grain sizeestimated from Rietveld refinement of XRD data by theScherrer equation was about 5 nm. Figure 8b showed a typicaltransmission electron microscopy (TEM) image of PbS QDs,which clearly indicated the size of PbS QDs to be 5 ± 0.5 nmwith no agglomeration, consistent with the calculated size fromXRD characterization. Dynamic light scattering (DLS)measurements also confirmed single-particle populations insolutions with similar average diameter (Figure 8c).We applied absorption and photoluminescence spectroscopy

to investigate the optical properties of PbS QDs in aqueoussolution. As seen in Figure 8d (red curve), our as-synthesizedPbS QDs exhibited a pronounced excitonic absorption peakcentered at 1170 nm, corresponding to a quantum confinementinduced band gap E0 of 1.06 eV. The small full width at half-maximum (fwhm) of 70 nm reflected the narrow sizedispersions. We calculated the average diameter of as-synthesized PbS QDs as 4.8 nm through the equation proposedby Hens:51 E0 = 0.41 + (0.0252d2 + 0.283d)−1, in goodagreement with the TEM observation shown in Figure 8b. Inaddition, as shown in Figure 8d (black curve), our PbS QDsshowed band-edge photoluminescence with its peak located at0.95 eV, implying good passivation of nonradiative surfacedefects.To investigate the stabilization mechanism of our PbS QDs,

we carried out zeta-potential measurement on aqueous PbS

Figure 7. (a) Schematic configuration of superstrate TiO2/Sb2(S0.44,Se0.56)3 heterojunction solar cell. (b) Cross-sectional SEMimage of Sb2(S0.44,Se0.56)3 film. (c) Photosensitivity of Sb2(S0.44,Se0.56)3film, illuminated under 650 nm LED, whose power density is 430 μWcm−2. Current was tested using 40 V driving voltage. (d) J−Vcharacteristics of Sb2(S0.44,Se0.56)3 device performance in the dark andunder 100 mW cm−2 simulated AM1.5G irradiation.

Figure 8. (a) XRD pattern of as-prepared PbS QDs. (Bottom)Standard diffraction pattern of PbS (JCPDS 05-0592). (b) TEM imageof as-prepared PbS QDs. (Inset) Magnified TEM image of PbS QDs.(c) Size histogram obtained from dynamic light scattering for aqueousPbS QDs solution: 5 ± 0.8 nm. (d) Absorption and photo-luminescence spectra of aqueous PbS QDs solution.

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QDs solution. As shown in Figure 9a, the measured zetapotential of −35.2 mV indicated the binding of negatively

charged Sb4S72− MCCs to the surface of PbS QDs, thereby

providing the strong electrostatic repulsion between QDsneeded to achieve a stable colloidal dispersion, as widelyobserved in other examples of colloidal NCs in polarsolvents.34,52 We further studied the efficacy of Sb4S7

2−

MCCs ligands through FTIR. Figure 9b showed the FTIRspectra of the precipitates from centrifugation of aqueous PbSQDs solution before and after annealing at 150 °C. Theabsorption bands at 1700−1400 cm−1 indicated Sb−Svibrations of Sb4S7

2− MCCs, while the bands at 3000 cm−1

arise from characteristic N−H stretching which could originatefrom NH4

+ counterions in close proximity to negativelycharged MCC ligands, suggesting the attachment of negativelycharged MCC ligands to the surface of PbS QDs. To conclude,on the basis of our novel in-situ water-MCCs capping strategy,we successfully obtained high-quality and stable aqueous PbSQDs solution readily processed in ambient at room temper-ature, thereby showing great promise of our water-MCCsserving as surface ligands for colloidal NCs.

4. CONCLUSIONWe demonstrated a general and environmentally benign water-based solution approach to synthesize a series of MCCsthrough directly dissolving bulk V2VI3 chalcogenides (V = Sb,As and VI = S, Se, Te) in water with the presence of (NH4)2Sat room temperature and ambient atmosphere. Our synthesizedwater-MCCs were particularly attractive for solution depositionof corresponding chalcogenides thin films as well as serving asin-situ capping ligands for colloidal NCs without further ligandexchange. Overall, the simple and environmentally benignsynthetic method, combined with the versatility, would greatlybroaden the application of MCCs in solution-processedinorganic semiconductors.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.chemma-ter.5b03614.

Raman spectra of (NH4)2S solution; Raman spectra ofsolution of Se in (NH4)2S and solution of Te in(NH4)2S; XRD patterns of as-bought Se and Te; XPSspectra of Sb2S3 prepared through thermal decomposi-tion of Sb2S3-MCCs at 300 °C; TGA of As2S3, As2Se3,As2Te3, Sb2S3, Sb2Se3, and Sb2Te3 precursors; results

summary for the composition of Sb2(S1−xSex)3 filmsmeasured by EDS and XRD; device performanceparameters of the TiO2/Sb2(S0.44Se0.56)3 thin film solarcells (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: djxue@iccas.ac.cn.*E-mail: jtang@email.hust.edu.cn.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was financially supported by the “National 1000Young Talents” project, the National Natural ScienceFoundation of China (91433105, 61322401, 51402115, and21403078), and the director fund of Wuhan NationalLaboratory for Optoelectronics. The authors thank BeijingTechnol Science Co. Ltd. for thermal evaporator technicalassistance and the Analytical and Testing Center of HUST andthe facility support of the Center for Nanoscale Character-ization and Devices, Wuhan National Laboratory for Opto-electronics (WNLO).

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