band-gap engineering of zinc oxide colloids via lattice ... · abstract: the advanced application...

Band-Gap Engineering of Zinc Oxide Colloids via Lattice Substitutionwith Sulfur Leading to Materials with Advanced Properties forOptical Applications Like Full Inorganic UV ProtectionDaniela Lehr,† Martin Luka,† Markus R. Wagner,‡ Max Bugler,‡ Axel Hoffmann,‡ and Sebastian Polarz*,†

†Department of Chemistry, University of Konstanz, D-78457 Konstanz, Germany.‡Institute of Solid State Physics, Technical University of Berlin, Hardenbergstrasse 36, D-10623 Berlin, Germany

*S Supporting Information

ABSTRACT: The advanced application of wide-band gap semiconductors in areas likephotovoltaics, optoelectronics, or photocatalysis requires a precise control over electronicproperties. Zinc oxide is favorable for large-scale technological applications now and in thefuture because of the large, natural abundance of the involved, chemical elements. Often it isimportant that the band gap can be controlled precisely. While a blue-shift of the band gap canbe reached quite easily using the quantum-size effect, it is still very difficult to achieve a red-shift. We present a powerful method for the band gap engineering of ZnO via the incorporationof sulfur as a solid solutions. The reduction of the energy gap is controlled by ZnO1−xSxcomposition, whereas the latter is adjusted via special organometallic precursor molecules. Thematerial can be supplied in a continuous fashion and in a more refined morphology, forinstance spherical ZnO1−xSx colloids with sizes below λvis/2 (≈ 200 nm). As a concrete application of contemporary importancefirst steps toward the full inorganic UV protection are made.

KEYWORDS: metal oxides, semiconductors, band gap engineering, precursor chemistry, UV protection, aerosol synthesis

■ INTRODUCTIONThe exploitation of semiconductors is of utmost importance forexisting and forthcoming technologies. Some have even arguedthat in analogy to the “Copper Age” following the “Stone Age”,we are currently living in the “Semiconductor Age”.1 For sure,silicon represents the semiconductor, which is used the most intechnological context (e.g., microelectronics). However, thereare various applications for which silicon is not suitable eitherbecause of its relatively narrow band gap or its indirect bandstructure. Therefore, there is large interest in wide, direct gapsemiconductors like III/V compounds, for instance, galliumnitride (GaN),2 or II/VI compounds such as zinc oxide(ZnO).3,4

At first sight, ZnO is a simple material. Unlike typicaltransition metals, zinc does not exhibit a broad redox chemistry.ZnO is diamagnetic and occurs almost exclusively in oneallotrope,5 the Wurtzite structure. Because the Wurtzite (spacegroup P63mc) belongs to the so-called polar crystal classes, it ispyroelectric and piezoelectric which represents the basis forapplications in electromechanical or thermoelectrical couplingdevices.6 Furthermore, ZnO is a semiconductor with a large,direct bandgap of 3.37 eV at room temperature.7 Thus, one ofits most elemental functions is the absorption of lightcorresponding to the energy gap between highest state of thevalence band and the lowest state of the conduction band.8

Although one major advantage of ZnO is its low price and a lowtoxicity, many advanced, optical applications would benefitfrom a smaller band gap as soon as the sun represents therelevant source of radiation.9−11

The electronic properties of binary semiconductors can becontrolled by intentional contamination, as when otherelements are substituted in the anion or cation sublattice.12 Ifan element is introduced that possesses more electrons in itsvalence shell than the substituted constituent, n-doping of thesemiconductor is achieved. Important examples are Al- or F-containing ZnO materials, which are promising candidates forindium tin oxide (ITO) substitutes.13 P-doped ZnO is also ofmajor interest and can be obtained either by Zn2+ substitutionwith Li+,14 or by the exchange of oxygen with nitrogen.9,15 Theengineering of the band gap energy is in principle possiblewhen a solid solution is prepared from two isomorphicsemiconductor compounds with two distinct band gap energiesin their pure form.4,16 For instance, Egap can be adjustedprecisely between Egap(GaN) = 3.4 eV and Egap(GaAs) = 1.4 eVin dependence of composition of GaN1−xAsx solids.

17 A similarcase is well-known for alloys of AlN and InN.18

Adopting this principle for ZnO one has to look for II/VIcompounds preferentially with Wurtzite structure, or at leastbinary solids containing elements capable of tetrahedralcoordination (see Table 1). In fact ZnO1−xMgxO materialsexhibit a blue shift with higher Mg content.19 To achieve a red-shift is more demanding. Zn1−xCdxO and ZnO1−xSex could berealized and the change of Egap was proven,

20 but because of thesignificant toxicity of Cd2+ and Se2− their use is problematic. In

Received: January 21, 2012Revised: April 25, 2012Published: May 2, 2012

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© 2012 American Chemical Society 1771 dx.doi.org/10.1021/cm300239q | Chem. Mater. 2012, 24, 1771−1778

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analogy, for solid solutions of the type ZnO1−xSx one wouldexpect gap energies between 3.3 and 3.6 eV (Table 1).Interestingly, some researchers have made an oppositeobservation. Meyer and co-workers prepared thin films ofZnO1−xSx over a broad range of compositions using radiofrequency reactive sputtering.21 They found that Egap goesthrough a minimum for x = 0.45 (Egap(ZnO0.55S0.45) = 2.6 eV).The latter results were confirmed by Locmelis et al. who haveprepared ZnO1−xSx with x < 5% from a chemical transportreaction at 900 °C using ZnO and ZnS as starting materials.22

In the meantime theoretical studies have shed some light ontothe unusual electronic situation.23 It was reported that thevalence band and the conduction band are affected differentlyby sulfur incorporation. The energy of the valence bandincreases strongly for small sulfur ratios while the energy of theconduction band remains almost constant. The latterphenomenon is responsible for the observed decrease of Egap.

23

ZnO and also TiO2 are already used for UV protection in sunlotions and other products.24 While both materials (ZnO andTiO2) cover the UV−B region (λ = 280−315 nm; Ephoton = 4.4- 3.9 eV) additional measures need to be implemented for fullprotection in the UV-A (λ = 315−380 nm; Ephoton = 3.9−3.2eV) region. The full UV protection is currently achieved viaadditional organic dyes. Such organic compounds are currentlyunder intense discussion because they are not absolutelyphotostable and the resulting degradation products couldpotentially cause skin cancer. Therefore, there is a profounddemand for the all inorganic UV protection. Obviously, aneffective UV rotection requires a small but stable shift of theband gap energy Egap to lower values. Therefore, it seems thatthe preparation of ZnO1−xSx represents a promising route to gofor the all-inorganic UV protection. However, the methodsmentioned by Meyer and Locmelis are hardly suitable for amass-production of the materials. Besides sputtering techni-ques, suitable methods for producing ZnO1−xSx materials over abroad range of compositions do not exist. Consequently,kinetically controlled routes to ZnO1−xSx are highly desired.Without the use of high reaction temperatures, phaseseparation into ZnO and ZnS can be avoided.It is well-known in the meantime that kinetically controlled

pathways to functional inorganic materials can be pursued usingmolecular precursors.25,26 A nice summary about the potentialof molecular precursor routes to functional, inorganic materialswas published recently.27 A special class of precursors are thosethat contain all elements necessary for the formation of thedesired materials. This special class of precursors is calledsingle-source precursors.28 Thus, of particular relevance for thework here are molecular and in particular single-sourceapproaches toward materials related to ZnS and ZnO. Themost common way to prepare ZnS that can be found in theliterature is via simple salts.29 Several researchers could show

that the use of molecular compounds as precursors has someinherent advantages for the preparation of ZnS nanostruc-tures.30,31 It is worth mentioning the work published by Lieberand co-workers in 2003. Nanocrystalline ZnS wire-networkscould be prepared using Zn(S2CNEt2)2 as a single-sourceprecursor.30 Very recently, Dossing et al. have used thisprecursor for the preparation of a ZnS shell around a CdSequantum dot.32 Alkylzinc alkylsulfides represent promising ZnSprecursors as well but their potential has been explored onlyrarely.33,34 There has also been significant activity in thepreparation of ZnO materials by molecular precursors andsingle-source compounds.25,35 For instance Chaudret et al.could prepare a range of interesting ZnO materials via thecontrolled oxidation of dialkylzinc compounds. Our group hasgained significant experience in the preparation of differentZnO nanomaterials starting from organometallic alkylzincalkoxide precursors [MeZnOR]4 and several papers havebeen published lately.5,11,36−38

■ RESULTS AND DISCUSSIONThe strategy toward the desired ZnO1−xSx materials is shown inchart 1 and will be summarized briefly in the current paragraph:

First, a suitable organometallic precursor for zinc sulfide isidentified, and the formation of ZnS is discussed in brief. Anelegant route to the desired ZnO1−xSx would be the directintroduction of oxygen during treatment of the precursor in thepresence of O2. Alternatively, in the so-called ’two sourceapproach’, a mixture of two precursors (one for ZnS and onefor ZnO) will be tested for the synthesis of the ternaryZnO1−xSx phase. The optical properties of the materials will bestudied. Finally, it is necessary to obtain ZnO1−xSx in a refinedmorphology which makes the material more suitable forpotential applications.

Molecular, Organometallic Precursor System for ZincSulfide (ZnS). Three types of [RZn(SR′)]n compounds aredescribed in the literature with R = Me, Et; R′ = Et, isoPr, tertBu;and n = 5, 8, 10.33 We have selected the octameric compound[MeZnSisoPr]8 reported by Shearer in 1969 (see scheme 1) andheated it under nitrogen atmosphere at T = 250, 450, and 650°C. The resulting samples were analyzed using powder X-raydiffraction (PXRD; see Figure 1). It is seen that even at lowtemperatures (250 °C), the entire precursor has converted tonanocrystalline ZnS. The evaluation of the PXRD peakbroadening via Scherrer equation shows that the averagecrystallite size is 8 nm.39 The crystallinity of the materials canbe enhanced by using higher synthesis temperatures (DP (450°C) = 31 nm; DP (650 °C) = 44 nm). The majority of thesamples (∼75%) consist of the Sphalerite modification, but

Table 1. Band-Gap Energies for Important II/VISemiconductors

semiconductor band gap (eV)

ZnO 3.37 (WZ)ZnS 3.54 (cubic)/3.91 (WZ)ZnSe 2.71 (cubic)ZnTe 2.39 (cubic)CdO 2.22 (cubic)BeO 10.58 (WZ)MgO 7.83 (cubic)

Chart 1. Formation of Different Materials in the Zn/S/OSystem from Organometallic Precursors via ThermalElimination Reactions (Δ)

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some ZnS in Wurtzite modification (∼25%) is present as well.Because the latter crystal structure represents the high-temperature modification, it can be concluded that at least toa certain degree kinetically controlled conditions can beaccessed using the mentioned precursor route. If the trans-formation of the precursor into ZnS is performed at a muchhigher reaction rate, up to ∼40% of ZnS in the Wurtzitemodification can be obtained. It is interesting to note that thereis a certain similarity between the structure of the precursor andthe metastable ZnS product. The precursor contains acharacteristic element formed by two fused [ZnS]3 ringswhich is highlighted in Scheme 1. A very similar structuralelement can also be identified in the lattice of crystalline ZnS inWurtzite modification. Consequently, one can interpret the

emergence of Wurtzite as the result of the transformation of theprecursor under at least partial preservation of its internalstructure. Such a strong relation between the precursor and theresulting product has also been characterized elsewhere as atopological synthesis.5,25,27

Unfortunately, it is not possible to obtain ZnO1−xSx by anoxidative treatment directly. The products obtained from thetreatment of [MeZnSisoPr]8 at 450 °C under different O2/N2

Scheme 1. Structure of the Organometallic Zinc SulfidePrecursor [MeZnSisoPr]8

a

aZn ≅ blue; S ≅ green; C ≅ grey; hydrogen atoms are omitted forbetter visibility. Sections from the structures of ZnS in the Wurtzitemodification and the Sphalerite modification are also shown on theright-hand side. The similarity between the molecular precursor andWurtzite is highlighted by the polyhedron colored in dark grey.

Figure 1. PXRD patterns of the ZnS materials prepared at differenttemperatures. The reference patterns of ZnS in Wurtzite modification(blue) and Sphalerite modification (blue) are also shown.

Figure 2. (a) PXRD patterns of ZnO1−xSx materials prepared withdifferent sulfur content (x = 0.02 (black), 0.04 (red), 0.07 (green),0.11 (blue), 0.3 (yellow), 0.5 (orange)). The diffraction signals of pureZnO as a reference are shown as gray bars. The data plotted over thefull 2θ range and a zoom of the region for the [110] diffraction aregiven in SI-3 in the Supporting Information. (b) Deviation of the ZnOlattice constants as a function of the amount of sulfur in the ZnOlattice x (crossed circle; black ≅ lattice parameter a; gray ≅ latticeparameter c). As a comparison, the prediction according to Vegard’slaw is also shown (straight lines with points at x = 0, 1), taking intoconsideration the reference lattice parameters of pure ZnO and pureZnS from single-crystal data.

Table 2. Composition of ZnO1−xSx Materials

relative amount of[MeZnSisoPr]8/(mol%)

ZnO1−xSx(theoretical)

ZnO1−xSx(EDX)

ZnO1−xSx(elementalanalysis)

0.5 ZnO0.99S0.01 ZnO0.980S0.020 ZnO0.983S0.0171.0 ZnO0.98S0.02 ZnO0.960S0.040 ZnO0.974S0.0262.5 ZnO0.95S0.05 ZnO0.933S0.067 ZnO0.939S0.0615.0 ZnO0.9S0.1 ZnO0.891S0.109 ZnO0.892S0.10815 ZnO0.7S0.325 ZnO0.5S0.5



ratios were analyzed by PXRD (see data given in theSupporting Information; SI-1). Only biphasic materials ZnO+ ZnS were observed. The latter result can be explained by thehigh reactivity of the organometallic precursor toward oxygen(see chart 1b) combined with the restricted miscibility of ZnOand ZnS.40

Because still the desired, monophasic ZnO1−xSx could not bereached, we attempted an approach that is related to the so-called coprecipitation used for the preparation of many catalystsystems.41 Two precursors can be mixed in almost any ratio andit can be expected that a broad variety of compositions can beaccessed. The difficulty in using two precursors is that it can bevery difficult to ensure a perfect dispersion. If the precursors doform separate phases, it would be almost impossible to derivethe desired monophasic material. Therefore, one has to accountfor the different chemical characteristics of the two usedcompounds. First a ZnO precursor with similar propertiescompared to the ZnS precursor has to be identified. Then,mixtures of these precursors can be studied regarding theircapability for the formation of ZnO1−xSx materials.

Molecular Precursor System for Zinc Oxide (ZnO).Because of the existing and extensive knowledge aboutorganometallic ZnO precursors of the type [MeZnO]4, thefollowing discussion will be kept very short.5,11,36−38 Ofimportance for the current context is the preparation of ZnOvia a thermal elimination reaction as given in Chart 1c,36,42 andin particular the precursor with R = −CH2CH2OCH3 ≈−OEtOMe needs to be mentioned because it is a liquid atroom temperature.37,43 Thus, the latter is not only a ZnOprecursor, but it can also play the role of a solvent at the sametime.44 Thermogravimetric analysis (TGA) shows that allorganic residues are eliminated from the precursor in one cleanstep (Δmexp = −48%; Δmmeas = −48%) with a maximum at 212°C which can be seen from the differentiated data (DTG). ThePXRD shows that ZnO has been formed. The latter data areshown in SI-2 in the Supporting Information.

Two-Source Approach for the Preparation of SulfurContaining Zinc Oxide Materials (ZnO1−xSx). Now, the

Figure 3. (a) Raman spectra of ZnO1−xSx at room temperature; x = 0 (gray), 0.02 (black), 0.04 (red), 0.07 (green); 0.11 (blue). Vertical drop linesindicate the Raman modes in the pure ZnO sample. (b) Raman shift of the LO mode as function of the sulfur content.

Figure 4. Optical properties demonstrated via (a−c) photographicimages and (d) UV/vis measurements of (a) pure ZnO (blue curve),(b) ZnO0.96S0.04 (purple curve) and (c) ZnO0.9S0.11(red curve). Theradiation spectrum of the sun is also shown in yellow. (e) How thered-shift ΔEgap correlates to the amount of sulfur in ZnO1−xSx.



liquid character of the compound [MeZnOEtOMe]4 becomes amajor advantage. The sulfur-containing precursor [MeZn-SisoPr]8 is very soluble in [MeZnOEtOMe]4. Thus, the resultingsolutions represent ideal molecular dispersions of the sulfurcontaining precursor in a ZnO precursor.Solutions containing different concentration of [MeZn-

SisoPr]8 (from 0.5−25 mol%) in [MeZnOEtOMe]4 wereprepared, thermolyzed at T = 350 °C and analyzed by PXRD(Figure 2a, and SI-3 in the Supporting Information). Thedesired monophasic material could be obtained. The diffractionpatterns are similar to the one of pure ZnO (see also SI-4 in theSupporting Information). A closer inspection reveals that thereis a shift of the diffraction signals [hkl] correlating to adeviation of the lattice constants. Considering Bragg equationone has to expect a shift of the signals to lower angle if sulfurhas substituted oxygen, because the ionic radius of S2−‑ (r = 184pm) is larger than that of O2− (r = 140 pm). Thus, the PXRDdata indicate that S2− has been incorporated into the ZnOlattice like desired.The composition of the prepared materials using different

precursor ratios (see table 2) was determined by twoindependent methods. Energy-dispersive X-ray spectroscopy(EDX) and elemental analysis were performed. The results aresummarized in table 2. Within the errors of the respectivemethods there is a very good agreement between the amount ofsulfur introduced via the precursor and the real composition ofthe final material. Thus, one can conclude that the presentedtwo source method is very suitable for a control of thecomposition of ZnOxS1−x materials.L. Vegard reported in 1921 that there is a linear relationship

for the deviation of lattice constants in solid solutions as afunction of composition, respectively the mole fraction.45

Therefore, the lattice constants were determined and plotted asa function of the relative amount of the sulfur content. It can beseen that there is a linear correlation and an excellent fit to theline indicating a close match to Vegard’s rule (Figure 2b). Theincorporation of sulfur was also investigated by Ramanspectroscopy (Figure 3). With increasing S doping the firstorder Raman modes of ZnO strongly decrease in intensity.46

The observed shift of the longitudinal optical mode to lowervalues is characteristic for the ternary, solid solutionZnO1−xSx.

47 Up to about 4% sulfur, the shift is almost linear.

Considering all analytical data together, one can conclude thatsulfur was successfully incorporated into the lattice of the ZnOmatrix.The effect of sulfur substituting oxygen in the ZnO lattice on

optical properties can already be seen by the bare eye (seeFigure 4a−c). Optical absorption spectra were recorded indiffuse reflectance modus and the data were evaluated using themodified Kubelka−Munk function.48 [F(R∞)hν]

1/2 is plottedversus the incident photon energy as required for a directcrystalline semiconductor (Figure 4d). In comparison to pureZnO, it can be seen that the absorption edge is shiftedsignificantly to lower energies (larger wavelength), and thatthere is a systematic correlation to the amount of sulfur presentin the ZnO lattice as a substituent (Figure 4e). The band-edgeis blurred significantly indicating that also the density of statesfunction of the semiconductor is affected, eventually by defectstates. For the samples containing the highest amount of sulfur(30%, 50%; see Table 1), one cannot anymore give a reliablevalue for the correct band gap because of the extension of theabsorption over the whole VIS region and the beginning phaseseparation (see above).

ZnO1−xSx Materials with Refined Morphology. Thematerials obtained so far have the form of powders containinghighly agglomerated nanoparticles (TEM data shown in SI-4 inthe Supporting Information). This morphology hampers theapplicability of the materials because it is almost impossible todisperse the material in a liquid medium, for instance a sun-lotion. Therefore, it is also an important step to prepare thematerial in the form of isolated particles which can beredispersed after their preparation. Spherical nanoparticles ofZnO1−xSx were obtained by the spray aerosol process depictedin scheme 2. A spray with droplets of a solution containing amixture of [MeZnSisoPr]8 and [MeZnOEtOMe]4 in toluene iscreated. The solvent evaporates quickly leaving behind smallerliquid aerosol droplets comprising the solution of [MeZn-SisoPr]8 in [MeZnOEtOMe]4. The size of the droplets and thetotal precursor concentration in toluene are parameters forcontrolling the average size of the final, solid particles. Thesubsequent treatment of the aerosol at high temperatureinduces the conversion to ZnO1−xSx.The investigation of the materials using scanning electron

microscopy (SEM) and transmission electron microscopy

Scheme 2. Experimental Setup Used for the Aerosol Preparation of Sulfur Containing Zinc Oxide Nanoparticles



(TEM) reveals that the product contains spherical particleswith a polydisperse particles size distribution (see SI-5 in theSupporting Information). The sample is still not suitable forUV protection purposes because the large particles (DP > 1/2λvis) will lead to undesired light scattering and a resultingturbidity of the dispersion. The particles could be redispersed invarious organic solvents after surface modification withoctadecyl phosphonic acid. The sample was filtered (pore size0.45 μm) and dynamic light scattering (DLS) data wasacquired. Stable colloidal solutions containing isolated particleswith an average size of Dp ≈ 120 nm have been prepared (seeFigure 5a). The latter information was nicely confirmed byTEM measurements (Figure 5b). PXRD measurements showed

similar effects than before (Figure 5c). Again, the significantshift of the signals to lower diffraction angles is a clearindication for the successful incorporation of sulfur into theZnO lattice. The enhanced width of the signals shows that thecrystallites have remained very small (∼4 nm). EDX andelectron diffraction confirm that the particles are composed ofZnO1−xSx.

■ CONCLUSIONThe goal of the current manuscript was to find an effective wayto engineer the band gap of ZnO materials and making themmore suitable for light absorption in the UV and VIS region.This goal could be reached to full extend.Using special molecular precursors not only ZnO, ZnS but

also solid-solutions ZnO1−xSx could be prepared under kineticcontrol. The advantage of the use of the described precursors isthat the formation of the targeted materials occurs at relativelylow temperature. Most importantly, the temperature is not highenough to overcome the diffusion barriers in the solid state.Thus, it is not only possible to precisely adjust the sulfurcontent, but one can even incorporate more sulfur than wouldbe allowed considering the thermodynamic solubility limit. Itwas shown that the incorporation of sulfur effects the opticalproperties. A significant red-shift of the adsorption edge couldbe observed. Depending on sulfur contents either the completeUV region or even significant parts of the VIS region can beabsorbed. The comparison to the solar spectrum (Figure 4d)shows that the materials presented here will be useful everytime it is very important that cost-effective semiconductors likeZnO can be tuned in such a way to absorb more of natural sun-light. Photovoltaics, photocatalysis or sun-protection representthree important examples.49

The initial materials possessed the form of a nanopowder,respectively highly agglomerated ZnO1−xSx nanocrystals.However, for many potential applications, it is desirable thatthe targeted material can be dispersed easily. One furtheradvantage of the described precursor route is that one couldobtain ZnO1−xSx in the form of spherical nanoparticledispersions using an aerosol-spray assisted approach. Afterpurification of the particles a sample could be obtained withparticle-sizes below the scattering regime of visible and UVlight.

■ MATERIALS AND METHODSAll starting compounds were received from Aldrich, were purified andcarefully dried prior to use. All reactions were performed under strictexclusion of air and humidity using Schlenck technique.

Preparation of [MeZnS-i-Pr]8. Thirty-seven milliliters (70 mmol)of ZnMe2 (1.9 M) in toluene was diluted with 40 mL of toluene andcooled to −7 °C; 4.9 g (64.3 mmol) of 2-propanethiol (in 10 mL oftoluene) was added dropwise under intense stirring. After 2 h, thesolution was allowed to warm up to RT and stirring was continued for4 h. The solvent was removed in vacuo and the product obtained as awhite powder. Yield: 9.8 g (98%). 1H-NMR, 400 MHz, CDCl3: d =3.35 (hept, 1 H, SCH); 1.43 (d, 6 H, CH3); −0.43 (s, 3 H, ZnCH3).

Preparation of [MeZnOEtOMe]4. One hundred forty fivemilliliters of (275.5 mmol) ZnMe2 (1.9 M in toluene) was dilutedwith 60 mL of toluene and cooled to −70 °C; 19.45 g (248 mmol) of2-methoxyethanol (in 20 mL of toluene) was added dropwise underintense stirring. After the addition, the solution was allowed to warmup to RT overnight under continuous stirring. The solvent wasremoved in vacuo and the product obtained as a colorless, viscousliquid. Yield: 37.3 g (97%). 1H NMR, 400 MHz, CDCl3: d = 3.85 (t, 2H, ZnOCH2); 3.49 (t, 2 H, ZnOCH2CH2); 3.34 (s, 3H, OCH3);−0.75 (s, 3 H, ZnCH3).

Figure 5. (a) Particle size distribution function obtained from DLSmeasurements and (b) TEM micrograph of the dispersed ZnO1−xSxparticles. (c) PXRD pattern of a material obtained via the spray-aersolprocess. Experimental data ≅ black line. Diffraction signals of pureZnO as a reference ≅ gray bars.



Preparation of ZnO1−xSx Materials (Exemplarily). A stocksolution of [MeZnS-i-Pr]8 in [MeZnOEtOMe]4 was prepared bydissolving [MeZnS-i-Pr]8 in small amounts of toluene first. Then, thesolution was added to, and stirred for 10 min. The solvent wasremoved in vacuo and a viscous liquid was obtained. Solutionscontaining different amounts of sulfur were prepared by diluting thestock solution with additional [MeZnOEtOMe]4. Oxide materialswere prepared by heating the precursor solution for 3 h at 350 °Cunder N2- atmosphere in a tube furnace. The resulting materials werecalcinated for 24 h under “dry air” (0.1 L/min O2, 0.4 L/min N2).Depending on the sulfur content, the product was obtained as ayellowish powder.Preparation of Dispersions of Spherical ZnO1−xSx Particles.

The experimental setup consists of three parts: An atomizer for theaerosol generation (Constant Output Atomizer, model 3076, TSI),two tube furnaces as a heating zone and a filter system for the particledeposition. A 0.1 M precursor solution was prepared by dissolving 1.73g (2.8 mmol) of [MeZnOEtOMe]4 and 0.17 g of [MeZnS-i-Pr]8 in 29mL of toluene. The solution was atomized in a nitrogen flow andpassed into the tube furnaces (both 500 °C) with a constant flow of1.5 L/min. At this juncture the solvent evaporates and the precursorsdecompose to solid particles in the gas phase. The size of the particlesis determined by the dimension of the former drop. After the heatingzone, the particles are deposited on paper filters. One milligram of theZnO:S particles was dispersed in 5 mL of a 10 mM solution ofOctadecylphosphonic acid in THF and sonicated for 1 h. Prior to DLSmeasurements, the bigger particles were removed by a 0.45 μm PTFEsyringe filter membrane.Analytical Techniques. NMR-spectra were acquired on a Bruker

Avance III spectrometer. X-ray diffraction was performed on a BrukerAXS D8 Advance diffractometer using CuKα radiation. Ramanmeasurements were conducted on a Horiba LabRAM HRspectrometer using a 532 nm DPSS laser and a 100× icroscopeobjective. The UV/vis measurements were done on a Varian Cary 100scan UV/vis spectrophotometer equipped with an Ulbricht reflectingsphere. The DLS experiment was performed on a Viscotek 802DLS.TEM images were acquired on a Zeiss Libra 120 at 120kv accelerationvoltage. SEM images were acquired on a Zeiss Crossbeam IS40XBinstrument. TGA was performed on equipment from Netzsch. IRspectra were recorded on a Perkin-Elmer Spectrum 100 equipped withan ATR-unit.

■ ASSOCIATED CONTENT

*S Supporting InformationSI-1: PXRD patterns of the materials prepared in the presenceof oxygen. SI-2: TGA and PXRD data for the formation of ZnOfrom [MeZnOEtOMe]4. SI-3: PXRD data for productsobtained from the thermolysis of precursors mixtures. SI-4:TEM micrograph and electron diffraction of the ZnO1−xSxmaterials prepared by thermolysis. SI-5: Electron microscopydata of the as-prepared, polydisperse aerosol.This material isavailable free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected].

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSGrillo-Werke AG/Grillo Zinkoxid GmbH is most gratefullyacknowledged for financial support.

■ REFERENCES(1) Service, R. F. Science 2000, 287, 415.

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