Published: August 04, 2011 r2011 American Chemical Society 16904 dx.doi.org/10.1021/jp2053579 | J. Phys. Chem. C 2011, 115, 16904–16909 ARTICLE pubs.acs.org/JPCC Understanding the Decomposition Pathways of Mixed Sulfur/Selenium Lead Phosphinato Complexes Explaining the Formation of Lead Selenide Javeed Akhtar, †,‡ Mohammad Afzaal, § Mark A. Vincent, || Neil A. Burton, || James Raftery, || Ian H. Hillier,* ,|| and Paul O’Brien* ,† † School of Chemistry and School of Materials, The University of Manchester, Oxford Road, Manchester M13 9PL, U.K. ‡ NanoScience & Materials Synthesis Lab, Department of Physics, COMSATS, Institute of Information Technology, Islamabad Campus, Islamabad, Pakistan § Center of Research Excellence in Renewable Energy, King Fahd University of Petroleum and Minerals, P.O. Box 1292, Dhahran, 31261 Saudi Arabia ) School of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9PL, U.K. b S Supporting Information ’ INTRODUCTION The synthesis of novel metal-molecular precursors to deposit binary 13 or ternary phases 46 of inorganic materials with critical dimensions of the order of nanometers have received consider- able attention in the last three decades. To date, a large number of molecular precursors have been synthesized and used to grow metal sulfide, selenide, and telluride materials. 7,8 One of the driving factors is to avoid using pyrophoric and especially noxious chemicals in preparing chalcogenide thin films and nanoparticles. 9 Moreover, carefully designed metal precursors that provide the relevant elemental entities can suppress impu- rities associated with multiple source materials. 10 The presence of only one precursor molecule in the supply stream potentially reduces the likelihood and extent of prereaction in CVD and can help to control intrinsic stoichiometry. Metal dithiocarbamates and xanthates are the most commonly used single-source precursors for thin films and nanoparticles. 11 A number of studies have appeared reporting the decomposition products of the compounds in the presence of primary amines which promote nanoparticle formation at low temperatures. 12,13 In attempts to understand the formation of metal sulfide nanoparticles in the presence of alkylamines, a recent study has led to suggestions of a new mechanism, on the basis of the identification of isolated intermediates and the side products. 14 Studies of the side products from different alkylamines at various concentrations have revealed that steric hindrance at the alkyla- mine is critical to the rate of decomposition of the dithiocarba- mates and the competitive reaction pathways involving the intermediates. Information on the thermal properties of related mixed chalcogenide compounds are scant. Our group has been recently investigating the use of mixed-chalcogenide compounds in CVD experiments e.g., [Ni{ i Pr 2 P(E1)NP(E2) i Pr 2 } 2 ] (E1 = S, E2 = Se (1); E1 = E2 = S (2), and E1 = E2 = Se (3)), which lead to the deposition of nickel sulfide, selenide, or phosphide; the material deposited depending on both the temperature and method used for the deposition. 15 In continuation of such efforts, we here analyze the decomposition behavior of the lead thioseleno- phosphinate, [Pb{(C 6 H 5 ) 2 PSSe} 2 ] 16,17 in both the vapor and solution. The precursor decomposes in aerosol-assisted (AA)CVD Received: June 8, 2011 Revised: July 23, 2011 ABSTRACT: Lead selenide (PbSe), as micro- and nanocrystals, has been produced from a mixed lead thioseleno-phosphinato compound, [Pb{(C 6 H 5 ) 2 PSSe} 2 ] by chemical vapor deposition. The formation of PbSe nanocrystals in the solution has also been demonstrated at room temperatrure in a mixture of oleylamine and dodecanethiol. Density functional theory calculations of the decomposition of the complex are consistent with a dominant role for thermodynamic factors rather than kinetic ones in controlling the material formed.
Transcript
Published: August 04, 2011
r 2011 American Chemical Society 16904 dx.doi.org/10.1021/jp2053579 | J. Phys. Chem. C 2011, 115, 16904–16909
ARTICLE
pubs.acs.org/JPCC
Understanding the Decomposition Pathways of MixedSulfur/Selenium Lead Phosphinato Complexes Explaining theFormation of Lead SelenideJaveed Akhtar,†,‡ Mohammad Afzaal,§ Mark A. Vincent,|| Neil A. Burton,|| James Raftery,|| Ian H. Hillier,*,||
and Paul O’Brien*,†
†School of Chemistry and School of Materials, The University of Manchester, Oxford Road, Manchester M13 9PL, U.K.‡NanoScience &Materials Synthesis Lab, Department of Physics, COMSATS, Institute of Information Technology, Islamabad Campus,Islamabad, Pakistan§Center of Research Excellence in Renewable Energy, King Fahd University of Petroleum and Minerals, P.O. Box 1292, Dhahran,31261 Saudi Arabia
)School of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9PL, U.K.
bS Supporting Information
’ INTRODUCTION
The synthesis of novel metal-molecular precursors to depositbinary1�3 or ternary phases4�6 of inorganic materials with criticaldimensions of the order of nanometers have received consider-able attention in the last three decades. To date, a large number ofmolecular precursors have been synthesized and used to growmetal sulfide, selenide, and telluride materials.7,8 One of thedriving factors is to avoid using pyrophoric and especiallynoxious chemicals in preparing chalcogenide thin films andnanoparticles.9 Moreover, carefully designed metal precursorsthat provide the relevant elemental entities can suppress impu-rities associated with multiple source materials.10 The presenceof only one precursor molecule in the supply stream potentiallyreduces the likelihood and extent of prereaction in CVD and canhelp to control intrinsic stoichiometry.
Metal dithiocarbamates and xanthates are the most commonlyused single-source precursors for thin films and nanoparticles.11
A number of studies have appeared reporting the decompositionproducts of the compounds in the presence of primary amineswhich promote nanoparticle formation at low temperatures.12,13
In attempts to understand the formation of metal sulfidenanoparticles in the presence of alkylamines, a recent study has
led to suggestions of a new mechanism, on the basis of theidentification of isolated intermediates and the side products.14
Studies of the side products from different alkylamines at variousconcentrations have revealed that steric hindrance at the alkyla-mine is critical to the rate of decomposition of the dithiocarba-mates and the competitive reaction pathways involving theintermediates.
Information on the thermal properties of related mixedchalcogenide compounds are scant. Our group has been recentlyinvestigating the use of mixed-chalcogenide compounds in CVDexperiments e.g., [Ni{iPr2P(E1)NP(E2)
iPr2}2] (E1 = S, E2 = Se(1); E1 = E2 = S (2), and E1 = E2 = Se (3)), which lead to thedeposition of nickel sulfide, selenide, or phosphide; the materialdeposited depending on both the temperature and method usedfor the deposition.15 In continuation of such efforts, we hereanalyze the decomposition behavior of the lead thioseleno-phosphinate, [Pb{(C6H5)2PSSe}2]
16,17 in both the vapor andsolution. The precursor decomposes in aerosol-assisted (AA)CVD
Received: June 8, 2011Revised: July 23, 2011
ABSTRACT: Lead selenide (PbSe), as micro- and nanocrystals, has been producedfrom a mixed lead thioseleno-phosphinato compound, [Pb{(C6H5)2PSSe}2] bychemical vapor deposition. The formation of PbSe nanocrystals in the solution hasalso been demonstrated at room temperatrure in a mixture of oleylamine anddodecanethiol. Density functional theory calculations of the decomposition of thecomplex are consistent with a dominant role for thermodynamic factors rather thankinetic ones in controlling the material formed.
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experiments to deposit only scattered PbSe microcrystals asopposed to homogeneous thin films, whereas, in solution,[Pb{(C6H5)2PSSe)}2] leads to crystalline PbSe nanoparticlesat room temperature in a mixture of oleylamine and dodeca-nethiol. The possible mechanisms involved in the formation ofonly PbSe materials and room temperature decomposition ofthe precursor are explored and discussed with the aid densityfunctional theory calculations.
From a technological perspective, PbSe is a promisingmaterialin many applications, including in laser materials,18 thermo-electric devices,19,20 near-infrared (near-IR) luminescence,21
and IR detectors.22�24 The recently discovered phenomenonof the multiple exciton generation (MEG) effect in PbE (where E =S, Se, or Te) materials could lead to an entirely new paradigm forhigh-efficiency and low-cost solar cell technology.20,21
’EXPERIMENTAL SECTION
a. Synthesis of [Pb{(C6H5)2PSSe)}2]. The preparation of[Pb{(C6H5)2PSSe}2] was prepared according to previouslypublished reports.16,17 Yield: 1.86 g, 69%. Mp: 212 �C. Anal.Found: C, 35.2; H, 2.2; P, 7.3; S, 6.0, Pb, 27.3. Calcd: C, 36.0; H,2.5; P, 7.7; S, 6.8; Pb, 26.0. IR: 2928 υ(C�H), 1472, υ(CdC),720, υ(C�S) cm�1. 1H NMR (δ, CDCl3, 400 MHz): 7.89 (8H,m, Ar�H), 7.49 (8H, m, Ar�H), 7.40 (4H, m, Ar�H). NegativeESI-MS, [(C6H5)2PSSe]
+: m/z 296 (100%).b. Deposition of Films by AACVD. Thin films composed of
PbSe crystallities were deposited using a home-built AACVDKit.25 Deposition was carried out on glass (450 and 500 �C)substrates by dissolving 0.02 M of the precursor in tetrahydro-furan (THF). Prior to deposition, the precursor was welldissolved in THF by sonication for 10 min. This precursorsolution was placed in a two-necked 100 mL round-bottom flask
with a gas inlet that allowed the carrier gas (argon) to pass intothe solution to effect transport of the aerosol. This flask wasconnected to the reactor tube by a piece of reinforced tubing. Theargon flow rate was controlled by a Platon flow gauge. Eight glasssubstrates (approximately 1 � 3 cm) were placed inside thereactor tube and inserted in a Carbolite furnace. The precursorsolution in a round-bottom flask was kept in a water bath abovethe piezoelectric modulator of a PIFCO ultrasonic humidifier(Model No. 1077). The aerosol droplets of the precursor thusgenerated were transferred into the hot-wall zone of the reactorby carrier gas. Both the solvent and the precursor underwentthermolysis as the precursor vapors reached the heated substratesurface where thermally induced reactions and film depositiontook place.c. Synthesis of PbSeNanocrystals.The compound (20mM)
was dissolved in a 1:1 molar mixture of oleylamine (10 mL) anddodecanethiol (7 mL) under nitrogen and stirred at roomtemperature. After 15 min, dry methanol was added to thebrown/black solution, resulting in the flocculation of the parti-cles. The deposit was washed several times with methanol, dried,and stored at room temperature.d. Material Characterization. All synthetic reactions were
performed under an inert atmosphere of dry nitrogen usingstandard Schlenk line techniques. All reagents were purchasedfrom Sigma-Aldrich Ltd. and used as received. Solvents weredistilled and dried prior to use when necessary. Electrospraymassspectra were recorded on a Micromass Platform II. 1H NMRspectra were obtained using a Bruker AC400 FT-NMR spectro-meter. Infrared spectra were obtained on a Specac singlereflectance ATR instrument (4000�400 cm�1). Elementalanalysis was performed by the University of Manchester
Table 1. Crystal Data and Structural Refinement for[Pb{(C6H5)2PSSe}2]
compound C24H20P2PbS2Se2formula weight 799.57
temperature 100(2) K
crystal system, space group triclinic, P1
wavelength 0.710 73 Å
unit cell dimensions a = 9.006 Å, R = 99.36�b = 10.928 Å, β = 90.01�c = 12.770 Å, γ = 98.07�
volume 1227.4 Å3
Z, calculated density 2, 2.163 Mg/m�3
absorption coefficient 10.150 mm�1
F(000) 752
crystal size 0.15 � 0.15 � 0.05 mm
θ range for data collection 1.62�26.55�limiting indices �11 e h e 11, �13 e k e 8, �16 e l e 15
final R indices [I > 2σ(I)] R1 = 0.0450, wR2 = 0.0991
R indices (all data) R1 = 0.0653, wR2 = 0.1073
largest diff peak and hole +1.927 and �1.863 e �3
Table 2. Selected Bond Lengths (Å) and Bond Angles (deg)for [Pb{(C6H5)2PSSe}2]
bonds length (Å) bonds angles (deg)
Pb(1)�S(4) 2.812(8) S(4)�Pb(1)�S(2) 94.1(7)
Pb(1)�S(2) 2.863(9) S(2)�Pb(1)�S(3) 94.4(5)
Pb(1)�S(3) 2.876(6) S(4)�Pb(1)�Se(4) 3.00(3)
Pb(1)�Se(4) 2.890(3) S(2)�Pb(1)�Se(4) 92.60(5)
Pb(1)�S(1) 2.903(6) S(3)�Pb(1)�Se(4) 75.62(19)
Pb(1)�Se(2) 2.978(2) S(3)�Pb(1)�S(1) 154.40(2)
Pb(1)�Se(1) 3.017(5) S(4)�Pb(1)�Se(2) 94.40(2)
Pb(1)�Se(3) 3.044(2) P(1)�Se(1)�Pb(1) 84.00(18)
Se(2)�P(1) 2.151(6) P(2)�S(4)�Pb(1) 89.10(3)
S(2)�P(1) 2.19(3) S(4)�Pb(1)�S(2) 94.10(7)
Figure 1. X-ray single crystal structure of [Pb{(C6H5)2PSSe}2].
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Microanalytical facility. Thermogravimetric measurements werecarried out using a Seiko SSC/S200 thermal analyzer witha heating rate of 10 �C min�1 under nitrogen at a flow rate10 mL/min.X-ray powder diffraction patterns were obtained using a
Bruker D8 AXE diffractometer (Cu KR). The samples werescanned between 20 and 80� in a step size of 0.05 with a countrate of 9 s. Thin films were carbon coated using Edwards E-306Acoating system before carrying out scanning electron microscope(SEM) and energy-dispersive X-ray spectroscopy (EDX). SEMwas performed using a Philips XL 30FEG and EDX analysiswere carried out using a DX4 instrument. Transmissionelectron microscope (TEM) samples were prepared by eva-porating a dilute toluene solution of the nanoparticles on lacycarbon coated copper grids (S166-3, Agar Scientific). PhilipsCM20 and technai 30 TEM were used to obtain TEM imagesof the nanoparticles.e. X-ray Single Crystallography. Single-crystal X-ray crystal-
lography study for the compound was carried out on a BrukerAPEX diffractometer using a graphite monochromated Mo KRradiation (λ = 0.710 73 Å). The structures were solved by directmethods and refined by full-matrix least-squares on F2.26 Allnon-H atoms were refined anisotropically. The crystal refine-ment parameters are given Table 1. H atoms were placed incalculated positions by assigned isotropic thermal parametersand allowed to ride on their parent carbon atoms. All calculationswere carried out by using the SHELXTL package.27 CCDC685544 contains the supplementary crystallographic data forthis paper.
’RESULTS AND DISCUSSION
The synthesis of [Pb{(C6H5)2PSSe}2] was easily carried outby reaction between (Pb(CH3COO)2) and (NHEt3)(Ph2PSSe)at room temperature.16,17 The compound is perfectly air stableand can be stored for a long period of time. Suitable crystals forsingle X-ray analysis were obtained from a concentrated THFsolution at room temperature. The molecular structure of thecompound is shown in Figure 1, and selected bond lengths andangles are given in Table 2. The geometry at the lead atom is adistorted square pyramidal with two sulfur and two selenium atoms,forming the base of the pyramid and the lone pair occupying the axialposition. The average bond lengths of Pb�S (2.86 Å) are smallerthan the bond lengths of Pb�Se (2.98 Å) as expected. The averagebond angle around S/Se�Pb�S/Se is 95.9� and is similar to thevalue in related compounds such as lead dithiocarbamates.28,29
The decomposition behavior of the compound was studied bythermogravimetric analysis (N2, 10 �C/min), which indicated asingle-step weight loss between 287 and 385 �C. Calculation ofthe precursor efficiency to afford bulk PbSe (35.9%) as the finalproduct, based on the residual material (37.6%) from the TGAexperiments, found the sample to be within 5%. Growth of thinfilms was attempted at 450 and 500 �C with an argon flow rateof 160 SCCM (SCCM denotes cubic centimeter per minute) for1 h at atmospheric pressure. The reactor was purged with argonfor 10 min at the required deposition temperature before carryingout deposition to avoid any in situ oxidation. The deposited blackfilms were found to be nonuniform and could be easily wiped off
Figure 2. XRPD patterns of PbSe crystallities deposited by AACVD at(a) 450 �C and (b) 500 �C. In the insets are shown the SEM images.
Figure 3. (a) XRPD and (b) TEM image of PbSe crystals grown atroom temperature in a mixture of oleylamine and dodecanethiol.
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the surface, indicating that the deposited material is weaklyadsorbed on glass surfaces. This observation may account for
the poor adhesion between the PbSe crystallities and sub-strates.30 X-ray powder diffraction (XRPD) (Figure 2) of depos-ited material corresponds to cubic PbSe (ICDD No. 06�3540)with the rock salt structure. The strong (200) peak suggests a(100) oriented growth. Scanning electron microscope (SEM)images reveal that the deposits consist of disordered cubic PbSecrystallites (∼0.7�2.2 μm in size) randomly orientated on thesubstrates. These results are in marked contrast to our previousstudies on PbSe multifaceted microcrystals grown from leadphosphonodiselenoates in AACVD experiments.30b Energy-dis-persive X-ray spectroscopy (EDS) analysis on PbSe is not fruitfulbecause the commonly observed selenium K line overlaps withthe lead M5 absorption edge. No sulfur peaks were detected.However, phosphorus accumulated during the decomposition ofthe precursor was found. At low deposition temperatures phos-phorus contamination is 6�9% whereas at higher temperatures,lower phosphorus contents 1�2% were seen.PbSe Nanocrystals. Initial attempts were made to prepare
PbSe nanocrystals at room temperature from the mixed S/Secompound. The resulting black precipitate was analyzed byXRPD, which confirmed the formation of crystalline cubic PbSe,without any impurities (Figure 3a). TEM studies indicate thatthe as-deposited material consists of a mixture of randomlyorientated elongated rectangles, cubes, truncated cubes, andtruncated octahedron structures, as shown in Figure 3b. Thoughthey are uniform in shape, they vary significantly by size, which isfound to be highly sensitive toward the reaction conditions.Computational Investigation of the Formation of PbSe
from [Pb{(C6H5)2PSSe}2]. We have carried out density func-tional theory (DFT) calculations to study the formation of PbSefrom [Pb{(C6H5)2PSSe}2], which we have observed during thethermal CVD process and also by reaction in solution witholeylamine and dodecanethiol. The calculations employed a localimplementation of the M06 functional in Gaussian0331 togetherwith the SDD (Pb) and 6-311G** (C, H, S, P, and Se) basis sets.The rigid rotor and harmonic oscillator approximations wereused to obtain corrections to the calculated electronic energies,to yield free energies.We first consider a number of possible mechanisms for the
thermal decomposition of [Pb((C6H5)2PSSe)2] and have calcu-lated the corresponding reaction free energies, both at roomtemperature (298.15 K) and at an elevated temperature (800 K).These are shown in Scheme 1 together with the computedreaction free energies at 298 and 800 K. We have investigated anumber of different fragmentation patterns that can yield theobserved product, PbSe rather than PbS. The first two mechan-isms considered (1, 2) involve the initial loss of a phenyl radical(C6H5) (1a), this process being highly endergonic. The next stepis either the loss of SP(C6H5) (1b in Scheme 1) or of SeP(C6H5)(2b), the former reaction being the somewhat less endergonic ofthe two. In the scheme, we postulate the loss of a second phenylradical (1c), which is only slightly less endergonic than the initialstep. However, the subsequent decomposition of (C6H5)PSSe�Pb-Se (1d) to give the observed product, PbSe, is slightly exergonic.Thus, both reaction sequences 1a�1d and 2a�2b point to thefavorable formation of PbSe.In the second set of possible mechanisms (3�5) that we have
studied, the initial step is the loss of the SeSP(C6H5)2 (3a, 4a)entity to leave (C6H5)2PSSe-Pb, which, like the loss of the phenylradical in steps 1 and 2, is also highly endergonic, but somewhatless so. The subsequent step could be the loss of either PbS (4b)or PbSe (3b) from (C6H5)2PSSe-Pb, the latter being preferred
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on energetic grounds. An alternative route (5) to give PbSeinvolves a two-step decomposition of (C6H5)2PSSe-Pb, invol-ving first the loss of a phenyl radical (5b), and the subsequentdecomposition of (C6H5)PSSe-Pb, giving PbSe (5c). However,this mechanism is considerably more endergonic than the two-step mechanism giving PbSe. Thus, the conclusions of ourcomputations is that the formation of PbSe in CVD may involvemore than one mechanism but the steps that lead to of PbSe aresomewhat more favorable on thermodynamic grounds, thanthose that lead to PbS formation.We now describe our calculations designed to understand the
reaction of the compound in the liquid phase containing theamine and thiol, where the reactive species is likely to be athioalkoxide ion, which we modeled as CH3S
�. An initialcalculation on the isolated complex showed the LUMO to beantibonding between the P and Se atoms of one ligand. Thusaddition of electron density from one of the lone pairs of thesulfide nucleophile into this orbital will weaken the bondbetween the P and Se atoms and hence facilitate the formationof PbSe. Attack on either end of this bond, by CH3S
�, wasstudied, but only when the sulfur nucleophile attacked thephosphorus could a transition state be determined. However,once the initial complex of (I) with CH3S
� is formed (Figure 4)
the decomposition step
ððC6H5Þ2PðSeÞðCH3SÞSÞ 3 Pb 3 ððC6H5Þ2PðSeÞSÞ� f
ððC6H5Þ2PðCH3SÞSÞ 3 PbSe 3 ððC6H5Þ2PðSeÞSÞ�
is quite facile with a barrier of only 13 kJ mol�1 (at 298 K). In thisreaction CH3S
� attacks the phosphorus center such that theselenium and the attacking sulfur are the axial ligands of thephosphorus and lead to the reaction being favorable by �30 kJmol�1 (at 298 K). The product has a well-defined PbSe unit withthe unreacted ligand still coordinated to the lead and the reactedligand loosely coordinated.
’CONCLUSION
In summary, we have analyzed the decomposition behavior ofmixed [Pb{(C6H5)2PSSe}2] in both the vapor and solution,leading to formation of PbSe only. The decomposition of theprecursor was performed at room temperature in the presence ofoleylamine and dodecanethiol. Computational studies indicatethat it is easier to break the Se�P bond than the S�P bond, andthus the formation of PbSe is a result of thermodynamic factorsrather than kinetic ones.
Figure 4. (a) Reactant, (b) transition structure, and (c) product for the formation of PbSe by the decomposition of ((C6H5)2P(Se)-(CH3S)S) 3Pb 3 ((C6H5)2P(Se)S)
�. Distances in Å.
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’ASSOCIATED CONTENT
bS Supporting Information. TGA of [Pb{(C6H5)2PSSe}2].This material is available free of charge via the Internet at http://pubs.acs.org.
Financial support from EPSRC, U.K., is gratefully acknowl-edged. M.A. would like to thank Center of Research Excellence inRenewable Energy, KFUPM for the support. J.A. is thankful to HEC,Pakistan, for financial assistance.
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