Band-Gap Engineering of Zinc Oxide Colloids via Lattice Substitution

May 2, 2012 - As a concrete application of contemporary importance first steps toward the full inorganic UV protection are made. ... Citation data is ...
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Band-Gap Engineering of Zinc Oxide Colloids via Lattice Substitution with Sulfur Leading to Materials with Advanced Properties for Optical Applications Like Full Inorganic UV Protection Daniela Lehr,† Martin Luka,† Markus R. Wagner,‡ Max Bügler,‡ 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 like photovoltaics, optoelectronics, or photocatalysis requires a precise control over electronic properties. Zinc oxide is favorable for large-scale technological applications now and in the future because of the large, natural abundance of the involved, chemical elements. Often it is important that the band gap can be controlled precisely. While a blue-shift of the band gap can be reached quite easily using the quantum-size effect, it is still very difficult to achieve a redshift. We present a powerful method for the band gap engineering of ZnO via the incorporation of sulfur as a solid solutions. The reduction of the energy gap is controlled by ZnO1−xSx composition, whereas the latter is adjusted via special organometallic precursor molecules. The material can be supplied in a continuous fashion and in a more refined morphology, for instance spherical ZnO1−xSx colloids with sizes below λvis/2 (≈ 200 nm). As a concrete application of contemporary importance first steps toward the full inorganic UV protection are made. KEYWORDS: metal oxides, semiconductors, band gap engineering, precursor chemistry, UV protection, aerosol synthesis

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INTRODUCTION The exploitation of semiconductors is of utmost importance for existing and forthcoming technologies. Some have even argued that 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 in technological context (e.g., microelectronics). However, there are various applications for which silicon is not suitable either because of its relatively narrow band gap or its indirect band structure. Therefore, there is large interest in wide, direct gap semiconductors like III/V compounds, for instance, gallium nitride (GaN),2 or II/VI compounds such as zinc oxide (ZnO).3,4 At first sight, ZnO is a simple material. Unlike typical transition metals, zinc does not exhibit a broad redox chemistry. ZnO is diamagnetic and occurs almost exclusively in one allotrope,5 the Wurtzite structure. Because the Wurtzite (space group P63mc) belongs to the so-called polar crystal classes, it is pyroelectric and piezoelectric which represents the basis for applications in electromechanical or thermoelectrical coupling devices.6 Furthermore, ZnO is a semiconductor with a large, direct bandgap of 3.37 eV at room temperature.7 Thus, one of its most elemental functions is the absorption of light corresponding to the energy gap between highest state of the valence band and the lowest state of the conduction band.8 Although one major advantage of ZnO is its low price and a low toxicity, many advanced, optical applications would benefit from a smaller band gap as soon as the sun represents the relevant source of radiation.9−11 © 2012 American Chemical Society

The electronic properties of binary semiconductors can be controlled by intentional contamination, as when other elements are substituted in the anion or cation sublattice.12 If an element is introduced that possesses more electrons in its valence shell than the substituted constituent, n-doping of the semiconductor is achieved. Important examples are Al- or Fcontaining ZnO materials, which are promising candidates for indium tin oxide (ITO) substitutes.13 P-doped ZnO is also of major interest and can be obtained either by Zn2+ substitution with Li+,14 or by the exchange of oxygen with nitrogen.9,15 The engineering of the band gap energy is in principle possible when a solid solution is prepared from two isomorphic semiconductor compounds with two distinct band gap energies in their pure form.4,16 For instance, Egap can be adjusted precisely between Egap(GaN) = 3.4 eV and Egap(GaAs) = 1.4 eV in dependence of composition of GaN1−xAsx solids.17 A similar case is well-known for alloys of AlN and InN.18 Adopting this principle for ZnO one has to look for II/VI compounds preferentially with Wurtzite structure, or at least binary solids containing elements capable of tetrahedral coordination (see Table 1). In fact ZnO1−xMgxO materials exhibit a blue shift with higher Mg content.19 To achieve a redshift is more demanding. Zn1−xCdxO and ZnO1−xSex could be realized and the change of Egap was proven,20 but because of the significant toxicity of Cd2+ and Se2− their use is problematic. In Received: January 21, 2012 Revised: April 25, 2012 Published: May 2, 2012 1771

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that the use of molecular compounds as precursors has some inherent advantages for the preparation of ZnS nanostructures.30,31 It is worth mentioning the work published by Lieber and co-workers in 2003. Nanocrystalline ZnS wire-networks could be prepared using Zn(S2CNEt2)2 as a single-source precursor.30 Very recently, Dossing et al. have used this precursor for the preparation of a ZnS shell around a CdSe quantum dot.32 Alkylzinc alkylsulfides represent promising ZnS precursors as well but their potential has been explored only rarely.33,34 There has also been significant activity in the preparation of ZnO materials by molecular precursors and single-source compounds.25,35 For instance Chaudret et al. could prepare a range of interesting ZnO materials via the controlled oxidation of dialkylzinc compounds. Our group has gained significant experience in the preparation of different ZnO nanomaterials starting from organometallic alkylzinc alkoxide precursors [MeZnOR]4 and several papers have been published lately.5,11,36−38

Table 1. Band-Gap Energies for Important II/VI Semiconductors semiconductor

band gap (eV)

ZnO ZnS ZnSe ZnTe CdO BeO MgO

3.37 (WZ) 3.54 (cubic)/3.91 (WZ) 2.71 (cubic) 2.39 (cubic) 2.22 (cubic) 10.58 (WZ) 7.83 (cubic)

analogy, for solid solutions of the type ZnO1−xSx one would expect gap energies between 3.3 and 3.6 eV (Table 1). Interestingly, some researchers have made an opposite observation. Meyer and co-workers prepared thin films of ZnO1−xSx over a broad range of compositions using radio frequency reactive sputtering.21 They found that Egap goes through a minimum for x = 0.45 (Egap(ZnO0.55S0.45) = 2.6 eV). The latter results were confirmed by Locmelis et al. who have prepared ZnO1−xSx with x < 5% from a chemical transport reaction at 900 °C using ZnO and ZnS as starting materials.22 In the meantime theoretical studies have shed some light onto the unusual electronic situation.23 It was reported that the valence band and the conduction band are affected differently by sulfur incorporation. The energy of the valence band increases strongly for small sulfur ratios while the energy of the conduction band remains almost constant. The latter phenomenon is responsible for the observed decrease of Egap.23 ZnO and also TiO2 are already used for UV protection in sun lotions and other products.24 While both materials (ZnO and TiO2) cover the UV−B region (λ = 280−315 nm; Ephoton = 4.4 - 3.9 eV) additional measures need to be implemented for full protection in the UV-A (λ = 315−380 nm; Ephoton = 3.9−3.2 eV) region. The full UV protection is currently achieved via additional organic dyes. Such organic compounds are currently under intense discussion because they are not absolutely photostable and the resulting degradation products could potentially cause skin cancer. Therefore, there is a profound demand for the all inorganic UV protection. Obviously, an effective UV rotection requires a small but stable shift of the band gap energy Egap to lower values. Therefore, it seems that the preparation of ZnO1−xSx represents a promising route to go for the all-inorganic UV protection. However, the methods mentioned by Meyer and Locmelis are hardly suitable for a mass-production of the materials. Besides sputtering techniques, suitable methods for producing ZnO1−xSx materials over a broad range of compositions do not exist. Consequently, kinetically controlled routes to ZnO1−xSx are highly desired. Without the use of high reaction temperatures, phase separation 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 using molecular precursors.25,26 A nice summary about the potential of molecular precursor routes to functional, inorganic materials was published recently.27 A special class of precursors are those that contain all elements necessary for the formation of the desired materials. This special class of precursors is called single-source precursors.28 Thus, of particular relevance for the work here are molecular and in particular single-source approaches toward materials related to ZnS and ZnO. The most common way to prepare ZnS that can be found in the literature is via simple salts.29 Several researchers could show

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RESULTS AND DISCUSSION The strategy toward the desired ZnO1−xSx materials is shown in chart 1 and will be summarized briefly in the current paragraph: Chart 1. Formation of Different Materials in the Zn/S/O System from Organometallic Precursors via Thermal Elimination Reactions (Δ)

First, a suitable organometallic precursor for zinc sulfide is identified, and the formation of ZnS is discussed in brief. An elegant route to the desired ZnO1−xSx would be the direct introduction of oxygen during treatment of the precursor in the presence of O2. Alternatively, in the so-called ’two source approach’, a mixture of two precursors (one for ZnS and one for ZnO) will be tested for the synthesis of the ternary ZnO1−xSx phase. The optical properties of the materials will be studied. Finally, it is necessary to obtain ZnO1−xSx in a refined morphology which makes the material more suitable for potential applications. Molecular, Organometallic Precursor System for Zinc Sulfide (ZnS). Three types of [RZn(SR′)]n compounds are described 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) and heated it under nitrogen atmosphere at T = 250, 450, and 650 °C. The resulting samples were analyzed using powder X-ray diffraction (PXRD; see Figure 1). It is seen that even at low temperatures (250 °C), the entire precursor has converted to nanocrystalline ZnS. The evaluation of the PXRD peak broadening via Scherrer equation shows that the average crystallite size is 8 nm.39 The crystallinity of the materials can be enhanced by using higher synthesis temperatures (DP (450 °C) = 31 nm; DP (650 °C) = 44 nm). The majority of the samples (∼75%) consist of the Sphalerite modification, but 1772

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Scheme 1. Structure of the Organometallic Zinc Sulfide Precursor [MeZnSisoPr]8a

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

a

Figure 2. (a) PXRD patterns of ZnO1−xSx materials prepared with different 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 pure ZnO as a reference are shown as gray bars. The data plotted over the full 2θ range and a zoom of the region for the [110] diffraction are given in SI-3 in the Supporting Information. (b) Deviation of the ZnO lattice constants as a function of the amount of sulfur in the ZnO lattice x (crossed circle; black ≅ lattice parameter a; gray ≅ lattice parameter c). As a comparison, the prediction according to Vegard’s law is also shown (straight lines with points at x = 0, 1), taking into consideration the reference lattice parameters of pure ZnO and pure ZnS from single-crystal data.

Table 2. Composition of ZnO1−xSx Materials Figure 1. PXRD patterns of the ZnS materials prepared at different temperatures. The reference patterns of ZnS in Wurtzite modification (blue) and Sphalerite modification (blue) are also shown.

some ZnS in Wurtzite modification (∼25%) is present as well. Because the latter crystal structure represents the hightemperature modification, it can be concluded that at least to a certain degree kinetically controlled conditions can be accessed using the mentioned precursor route. If the transformation of the precursor into ZnS is performed at a much higher reaction rate, up to ∼40% of ZnS in the Wurtzite modification can be obtained. It is interesting to note that there is a certain similarity between the structure of the precursor and the metastable ZnS product. The precursor contains a characteristic element formed by two fused [ZnS]3 rings which is highlighted in Scheme 1. A very similar structural element can also be identified in the lattice of crystalline ZnS in Wurtzite modification. Consequently, one can interpret the

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

ZnO1−xSx (theoretical)

ZnO1−xSx (EDX)

ZnO1−xSx (elemental analysis)

0.5 1.0 2.5 5.0 15 25

ZnO0.99S0.01 ZnO0.98S0.02 ZnO0.95S0.05 ZnO0.9S0.1 ZnO0.7S0.3 ZnO0.5S0.5

ZnO0.980S0.020 ZnO0.960S0.040 ZnO0.933S0.067 ZnO0.891S0.109

ZnO0.983S0.017 ZnO0.974S0.026 ZnO0.939S0.061 ZnO0.892S0.108

emergence of Wurtzite as the result of the transformation of the precursor under at least partial preservation of its internal structure. Such a strong relation between the precursor and the resulting product has also been characterized elsewhere as a topological synthesis.5,25,27 Unfortunately, it is not possible to obtain ZnO1−xSx by an oxidative treatment directly. The products obtained from the treatment of [MeZnSisoPr]8 at 450 °C under different O2/N2 1773

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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 lines indicate the Raman modes in the pure ZnO sample. (b) Raman shift of the LO mode as function of the sulfur content.

ratios were analyzed by PXRD (see data given in the Supporting Information; SI-1). Only biphasic materials ZnO + ZnS were observed. The latter result can be explained by the high reactivity of the organometallic precursor toward oxygen (see chart 1b) combined with the restricted miscibility of ZnO and ZnS.40 Because still the desired, monophasic ZnO1−xSx could not be reached, we attempted an approach that is related to the socalled coprecipitation used for the preparation of many catalyst systems.41 Two precursors can be mixed in almost any ratio and it can be expected that a broad variety of compositions can be accessed. The difficulty in using two precursors is that it can be very difficult to ensure a perfect dispersion. If the precursors do form separate phases, it would be almost impossible to derive the desired monophasic material. Therefore, one has to account for the different chemical characteristics of the two used compounds. First a ZnO precursor with similar properties compared to the ZnS precursor has to be identified. Then, mixtures of these precursors can be studied regarding their capability for the formation of ZnO1−xSx materials. Molecular Precursor System for Zinc Oxide (ZnO). Because of the existing and extensive knowledge about organometallic ZnO precursors of the type [MeZnO]4, the following discussion will be kept very short.5,11,36−38 Of importance for the current context is the preparation of ZnO via a thermal elimination reaction as given in Chart 1c,36,42 and in particular the precursor with R = −CH2CH2OCH3 ≈ −OEtOMe needs to be mentioned because it is a liquid at room temperature.37,43 Thus, the latter is not only a ZnO precursor, but it can also play the role of a solvent at the same time.44 Thermogravimetric analysis (TGA) shows that all organic residues are eliminated from the precursor in one clean step (Δmexp = −48%; Δmmeas = −48%) with a maximum at 212 °C which can be seen from the differentiated data (DTG). The PXRD shows that ZnO has been formed. The latter data are shown in SI-2 in the Supporting Information. Two-Source Approach for the Preparation of Sulfur Containing Zinc Oxide Materials (ZnO1−xSx). Now, the

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

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

Considering all analytical data together, one can conclude that sulfur was successfully incorporated into the lattice of the ZnO matrix. The effect of sulfur substituting oxygen in the ZnO lattice on optical properties can already be seen by the bare eye (see Figure 4a−c). Optical absorption spectra were recorded in diffuse reflectance modus and the data were evaluated using the modified Kubelka−Munk function.48 [F(R∞)hν]1/2 is plotted versus the incident photon energy as required for a direct crystalline semiconductor (Figure 4d). In comparison to pure ZnO, it can be seen that the absorption edge is shifted significantly to lower energies (larger wavelength), and that there is a systematic correlation to the amount of sulfur present in the ZnO lattice as a substituent (Figure 4e). The band-edge is blurred significantly indicating that also the density of states function of the semiconductor is affected, eventually by defect states. For the samples containing the highest amount of sulfur (30%, 50%; see Table 1), one cannot anymore give a reliable value for the correct band gap because of the extension of the absorption over the whole VIS region and the beginning phase separation (see above). ZnO1−xSx Materials with Refined Morphology. The materials obtained so far have the form of powders containing highly agglomerated nanoparticles (TEM data shown in SI-4 in the Supporting Information). This morphology hampers the applicability of the materials because it is almost impossible to disperse the material in a liquid medium, for instance a sunlotion. Therefore, it is also an important step to prepare the material in the form of isolated particles which can be redispersed after their preparation. Spherical nanoparticles of ZnO1−xSx were obtained by the spray aerosol process depicted in scheme 2. A spray with droplets of a solution containing a mixture of [MeZnSisoPr]8 and [MeZnOEtOMe]4 in toluene is created. The solvent evaporates quickly leaving behind smaller liquid aerosol droplets comprising the solution of [MeZnSisoPr]8 in [MeZnOEtOMe]4. The size of the droplets and the total precursor concentration in toluene are parameters for controlling the average size of the final, solid particles. The subsequent treatment of the aerosol at high temperature induces the conversion to ZnO1−xSx. The investigation of the materials using scanning electron microscopy (SEM) and transmission electron microscopy

liquid character of the compound [MeZnOEtOMe]4 becomes a major advantage. The sulfur-containing precursor [MeZnSisoPr]8 is very soluble in [MeZnOEtOMe]4. Thus, the resulting solutions represent ideal molecular dispersions of the sulfur containing precursor in a ZnO precursor. Solutions containing different concentration of [MeZnSisoPr]8 (from 0.5−25 mol%) in [MeZnOEtOMe]4 were prepared, thermolyzed at T = 350 °C and analyzed by PXRD (Figure 2a, and SI-3 in the Supporting Information). The desired monophasic material could be obtained. The diffraction patterns are similar to the one of pure ZnO (see also SI-4 in the Supporting Information). A closer inspection reveals that there is a shift of the diffraction signals [hkl] correlating to a deviation of the lattice constants. Considering Bragg equation one has to expect a shift of the signals to lower angle if sulfur has substituted oxygen, because the ionic radius of S2−‑ (r = 184 pm) is larger than that of O2− (r = 140 pm). Thus, the PXRD data indicate that S2− has been incorporated into the ZnO lattice like desired. The composition of the prepared materials using different precursor ratios (see table 2) was determined by two independent methods. Energy-dispersive X-ray spectroscopy (EDX) and elemental analysis were performed. The results are summarized in table 2. Within the errors of the respective methods there is a very good agreement between the amount of sulfur introduced via the precursor and the real composition of the final material. Thus, one can conclude that the presented two source method is very suitable for a control of the composition 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 a function of composition, respectively the mole fraction.45 Therefore, the lattice constants were determined and plotted as a function of the relative amount of the sulfur content. It can be seen that there is a linear correlation and an excellent fit to the line indicating a close match to Vegard’s rule (Figure 2b). The incorporation of sulfur was also investigated by Raman spectroscopy (Figure 3). With increasing S doping the first order Raman modes of ZnO strongly decrease in intensity.46 The observed shift of the longitudinal optical mode to lower values is characteristic for the ternary, solid solution ZnO1−xSx.47 Up to about 4% sulfur, the shift is almost linear. 1775

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similar effects than before (Figure 5c). Again, the significant shift of the signals to lower diffraction angles is a clear indication for the successful incorporation of sulfur into the ZnO lattice. The enhanced width of the signals shows that the crystallites have remained very small (∼4 nm). EDX and electron diffraction confirm that the particles are composed of ZnO1−xSx.

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CONCLUSION The goal of the current manuscript was to find an effective way to engineer the band gap of ZnO materials and making them more 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 kinetic control. The advantage of the use of the described precursors is that the formation of the targeted materials occurs at relatively low temperature. Most importantly, the temperature is not high enough to overcome the diffusion barriers in the solid state. Thus, it is not only possible to precisely adjust the sulfur content, but one can even incorporate more sulfur than would be allowed considering the thermodynamic solubility limit. It was shown that the incorporation of sulfur effects the optical properties. A significant red-shift of the adsorption edge could be observed. Depending on sulfur contents either the complete UV region or even significant parts of the VIS region can be absorbed. The comparison to the solar spectrum (Figure 4d) shows that the materials presented here will be useful every time it is very important that cost-effective semiconductors like ZnO can be tuned in such a way to absorb more of natural sunlight. Photovoltaics, photocatalysis or sun-protection represent three 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 that the targeted material can be dispersed easily. One further advantage of the described precursor route is that one could obtain ZnO1−xSx in the form of spherical nanoparticle dispersions using an aerosol-spray assisted approach. After purification of the particles a sample could be obtained with particle-sizes below the scattering regime of visible and UV light.

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Figure 5. (a) Particle size distribution function obtained from DLS measurements and (b) TEM micrograph of the dispersed ZnO1−xSx particles. (c) PXRD pattern of a material obtained via the spray-aersol process. Experimental data ≅ black line. Diffraction signals of pure ZnO as a reference ≅ gray bars.

MATERIALS AND METHODS

All starting compounds were received from Aldrich, were purified and carefully dried prior to use. All reactions were performed under strict exclusion 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 and cooled to −7 °C; 4.9 g (64.3 mmol) of 2-propanethiol (in 10 mL of toluene) was added dropwise under intense stirring. After 2 h, the solution was allowed to warm up to RT and stirring was continued for 4 h. The solvent was removed in vacuo and the product obtained as a white 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 five milliliters of (275.5 mmol) ZnMe2 (1.9 M in toluene) was diluted with 60 mL of toluene and cooled to −70 °C; 19.45 g (248 mmol) of 2-methoxyethanol (in 20 mL of toluene) was added dropwise under intense stirring. After the addition, the solution was allowed to warm up to RT overnight under continuous stirring. The solvent was removed in vacuo and the product obtained as a colorless, viscous liquid. Yield: 37.3 g (97%). 1H NMR, 400 MHz, CDCl3: d = 3.85 (t, 2 H, ZnOCH2); 3.49 (t, 2 H, ZnOCH2CH2); 3.34 (s, 3H, OCH3); −0.75 (s, 3 H, ZnCH3).

(TEM) reveals that the product contains spherical particles with a polydisperse particles size distribution (see SI-5 in the Supporting Information). The sample is still not suitable for UV protection purposes because the large particles (DP > 1/2 λvis) will lead to undesired light scattering and a resulting turbidity of the dispersion. The particles could be redispersed in various organic solvents after surface modification with octadecyl phosphonic acid. The sample was filtered (pore size 0.45 μm) and dynamic light scattering (DLS) data was acquired. Stable colloidal solutions containing isolated particles with an average size of Dp ≈ 120 nm have been prepared (see Figure 5a). The latter information was nicely confirmed by TEM measurements (Figure 5b). PXRD measurements showed 1776

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Preparation of ZnO1−xSx Materials (Exemplarily). A stock solution of [MeZnS-i-Pr]8 in [MeZnOEtOMe]4 was prepared by dissolving [MeZnS-i-Pr]8 in small amounts of toluene first. Then, the solution was added to, and stirred for 10 min. The solvent was removed in vacuo and a viscous liquid was obtained. Solutions containing different amounts of sulfur were prepared by diluting the stock solution with additional [MeZnOEtOMe]4. Oxide materials were prepared by heating the precursor solution for 3 h at 350 °C under N2- atmosphere in a tube furnace. The resulting materials were calcinated 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 a yellowish powder. Preparation of Dispersions of Spherical ZnO1−xSx Particles. The experimental setup consists of three parts: An atomizer for the aerosol generation (Constant Output Atomizer, model 3076, TSI), two tube furnaces as a heating zone and a filter system for the particle deposition. A 0.1 M precursor solution was prepared by dissolving 1.73 g (2.8 mmol) of [MeZnOEtOMe]4 and 0.17 g of [MeZnS-i-Pr]8 in 29 mL of toluene. The solution was atomized in a nitrogen flow and passed into the tube furnaces (both 500 °C) with a constant flow of 1.5 L/min. At this juncture the solvent evaporates and the precursors decompose to solid particles in the gas phase. The size of the particles is determined by the dimension of the former drop. After the heating zone, the particles are deposited on paper filters. One milligram of the ZnO:S particles was dispersed in 5 mL of a 10 mM solution of Octadecylphosphonic acid in THF and sonicated for 1 h. Prior to DLS measurements, the bigger particles were removed by a 0.45 μm PTFE syringe filter membrane. Analytical Techniques. NMR-spectra were acquired on a Bruker Avance III spectrometer. X-ray diffraction was performed on a Bruker AXS D8 Advance diffractometer using CuKα radiation. Raman measurements were conducted on a Horiba LabRAM HR spectrometer using a 532 nm DPSS laser and a 100× icroscope objective. The UV/vis measurements were done on a Varian Cary 100 scan UV/vis spectrophotometer equipped with an Ulbricht reflecting sphere. The DLS experiment was performed on a Viscotek 802DLS. TEM images were acquired on a Zeiss Libra 120 at 120kv acceleration voltage. SEM images were acquired on a Zeiss Crossbeam IS40XB instrument. TGA was performed on equipment from Netzsch. IR spectra were recorded on a Perkin-Elmer Spectrum 100 equipped with an ATR-unit.

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ASSOCIATED CONTENT

S Supporting Information *

SI-1: PXRD patterns of the materials prepared in the presence of oxygen. SI-2: TGA and PXRD data for the formation of ZnO from [MeZnOEtOMe]4. SI-3: PXRD data for products obtained from the thermolysis of precursors mixtures. SI-4: TEM micrograph and electron diffraction of the ZnO1−xSx materials prepared by thermolysis. SI-5: Electron microscopy data of the as-prepared, polydisperse aerosol.This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Grillo-Werke AG/Grillo Zinkoxid GmbH is most gratefully acknowledged for financial support.

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REFERENCES

(1) Service, R. F. Science 2000, 287, 415. 1777

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Chemistry of Materials

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