Controlling the Assembly of Nanocrystalline ZnO Films by a Transient

J. Phys. Chem. C 2008, 112, 5373-5383

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Controlling the Assembly of Nanocrystalline ZnO Films by a Transient Amorphous Phase in Solution Peter Lipowsky,*,† Niklas Hedin,‡ Joachim Bill,*,† Rudolf C. Hoffmann,† Anwar Ahniyaz,‡,§ Fritz Aldinger,† and Lennart Bergstro1 m*,‡ Max-Planck-Institut fu¨r Metallforschung und Institut fu¨r Nichtmetallische Anorganische Materialien der UniVersita¨t Stuttgart, PulVermetallurgisches Laboratorium, Heisenbergstrasse 3, D-70569 Stuttgart, Germany, Materials Chemistry Research Group, Department of Physical, Inorganic, and Structural Chemistry, Arrhenius Laboratory, Stockholm UniVersity, SE 106 91 Stockholm, Sweden, and YKI, Institute for Surface Chemistry, Box 5607, SE 114 86 Stockholm, Sweden ReceiVed: September 7, 2007; In Final Form: NoVember 30, 2007

The chronological sequence of the precipitation and crystallization of Zn species in methanol solutions containing poly(vinyl pyrrolidone) (PVP) was studied with dynamic light scattering, electron microscopy, and chemical analysis and correlated with the film formation on substrates. It was found that the polymer inhibited uncontrolled precipitation of ZnO and promoted the formation of smooth ZnO films on substrates. The reaction solution was quenched from 60 °C to room temperature after different periods of time. It could be shown that amorphous needle-like particles formed in the first place. This intermediate and metastable phase dissolved, and aggregated ZnO nanocrystals formed concurrently, whereby the transformation rate was determined by the PVP concentration. The formation of high-quality ZnO films, with respect to uniformity and low surface roughness, on substrate surfaces modified by self-assembled monolayers was strongly related to the presence of the intermediate amorphous phase in solution. Uncontrolled crystallization at low PVP concentrations resulted in defective and rough deposits. Very high PVP concentrations resulted in rapid aggregation of the zinc oxide nanocrystals in solution and a strongly decreased deposition rate. Control of the reaction conditions through intermediate amorphous phases commonly occurs in biomineralization processes. The employment of this mechanism could facilitate the development of novel chemical bath deposition processes of artificial nanostructured materials.

Introduction Controlled deposition of inorganic and crystalline films from solution has attracted interest because of its versatility for applications and its similarity to biomineralization.1,2 In particular, the fabrication of ZnO films has been extensively studied3-5 owing to the favorable properties and applications of this material.6 Recent work has shown that the conditions for successful deposition of films with tailored properties rely on the ability to promote the deposition and suppress the competing (bulk) processes (e.g., crystallization and particle aggregation).7 Commonly, the deposited films display a granular structure, which has been related to models where the build-up of the film is assumed to occur through attachment and assembly of nanoparticles formed in the bulk of the solution.8 Attempts have been made to model the deposition process based on colloidal stability.9,10 Materials mineralized in solutions, regardless of whether in biological or in artificial environments, frequently exhibit a hierarchical internal structure, which cannot be explained using traditional concepts of ion-by-ion growth and non-directional * Corresponding authors. (P.L.) Tel.: +49 (0)711 689 3202; fax: +49 (0)711 689 3255; e-mail: [email protected]. (J.B.) Tel.: +49 (0)711 689 3202; fax: +49 (0)711 689 3255; e-mail: [email protected]. (L.B.) Tel.: +46 (0)8 162368; e-mail: [email protected]. † Max-Planck-Institut fu ¨ r Metallforschung und Institut fu¨r Nichtmetallische Anorganische Materialien der Universita¨t Stuttgart. ‡ Stockholm University. § Institute for Surface Chemistry.

aggregation processes.11 Recently, crystalline nanoparticles formed in solution have been shown to attach to each other with a congruent crystallographic orientation12 and finally fuse together in a single-crystal-like material of oriented and selfassembled nanocrystals.13 Indeed, oriented attachment appears to be a rather general phenomenon,14,15 which is responsible for the highly oriented superstructures of nanocrystalline building blocks often called mesocrystals.16 A similar mechanism has been suggested for the formation of oriented domains in zinc oxide thin films deposited from solutions.17 Amorphous phases are common intermediates in biomineralization.18,19 Such phases occur, for example, during the formation of the spines of larval20 and adult21 sea urchins, mollusc larval shells,22,23 the carapace of crabs,24 and the teeth of chitons.25,26 It has also been proposed that hybrid nanoparticles, containing amorphous inorganics in an organic matrix, can serve as precursors for crystalline materials.11 In recent studies, it has been demonstrated that nanostructured zinc oxide thin films can be deposited on various surfacecharged organic templates from methanol solutions.5,27,28 Control of the reaction conditions permitted the fabrication of smooth and textured polycrystalline ZnO films.5 In particular, the concentration of the additive poly(vinyl pyrrolidone) (PVP) polymer was found to be crucial. The deposited nanocrystalline films, produced in a solution with a volume fraction φ ≈ 0.1 of PVP, were virtually free of included polymer and exhibited a pronounced crystallographic texture with the [001] axis being perpendicular to that of the substrate plane. The formation of

10.1021/jp077201a CCC: $40.75 © 2008 American Chemical Society Published on Web 03/18/2008

5374 J. Phys. Chem. C, Vol. 112, No. 14, 2008 the texture was attributed, on the one hand, to the anisotropic interaction of polar ZnO nanocrystals,29 which were preformed in solution, with the charged functional groups on the substrate surface and, on the other hand, to oriented attachment of the ZnO nanocrystals to each other. This study elucidated the role of the polymer in detail and examined the relationships between the processes active in the solution and on the substrate. The time evolution of the particle size was studied using dynamic light scattering (DLS), and their morphology, structure, and composition were investigated by electron microscopy, X-ray diffraction (XRD), and chemical analysis. The microstructure and surface roughness of the deposited films were analyzed over a wide range of PVP concentrations. The kinetics of the deposition process on the substrates was determined using a quartz crystal microbalance (QCM). We will show that the deposition is directly connected to the processes occurring in solution and that the formation of an intermediate amorphous phase in solution is the most important parameter controlling the quality of the inorganic films. Experimental Procedures Chemicals, Solution Preparation, Substrate Preparation, and Deposition. All reactions were performed in methanol (VLSI selectipur, BASF) solutions. Zn(CH3COO)2‚2H2O (puriss. p.a., Fluka) was dissolved in methanol to a concentration of 40 mmol L-1. PVP of two different molecular weights (SigmaAldrich or Fluka, PVP10/K15 with Mw ≈ 10 000 g mol-1 or K25 with Mw ≈ 40 000 g mol-1) also was dissolved in methanol and mixed with the zinc acetate solution. A solution of tetraethylammonium hydroxide (TEAOH, purum, Fluka, 100 mmol L-1 in methanol) or NaOH (Fluka, 100 mmol L-1 in methanol) was slowly added to the PVP/zinc acetate mixture under gentle stirring over a period of 25 min using a peristaltic pump. Unless otherwise indicated, all measurements were performed with PVP10/K15 and TEAOH. A typical composition was [Zn2+] ) 10 mmol L-1, [TEAOH] ≈ 25 mmol L-1, and [PVP10] ≈ 10 mmol L-1 (i.e., a volume fraction of approximately φ ≈ 0.10). Both sequences of solutions and sequences of film samples were prepared by varying the concentration and molecular weight of PVP, keeping the zinc acetate and TEAOH concentrations constant. For comparison, deposition solutions were prepared using NaOH of the same concentration instead of TEAOH. The reactions were initiated by heating the solutions to 60 °C using an oil bath or an oven. After various reaction times, the reactions were halted by quick quenching to room temperature by immersion in a water bath. As supports for the self-assembled monolayer (SAM) substrates, polished, cleaned, and surface-oxidized p-type borondoped single-crystal Si (100) wafers were used. Sulfo-SAMs were created by binding thioacetato-16-trichlorosilyl hexadecane30,31 onto such silicon wafers or onto glass slides from a toluene-based silane solution at 4 °C for 5 h in an argon atmosphere. The excess of silanes was removed by rinsing in chloroform and methanol. The thioacetate functionality was oxidized to a sulfonic acid moiety by placing the substrates in a saturated aqueous solution of potassium hydrogen monopersulfate for at least 12 h. Subsequently, the substrates were washed abundantly with distilled water, dried in a stream of argon, and stored in the dark. Amino-SAMs were produced from 1-cyano-16-trichlorosilyl hexadecane30 bound to the substrate as described previously. The cyano functionalities were reduced to amino groups by placing the substrates in a saturated solution of LiAlH4 in absolute diethyl ether for at least 12 h. The amino-

Lipowsky et al. SAMs were washed abundantly with pure ether and immersed in an aqueous 10 vol % HCl solution to protonate the amino groups prior to being dried in a stream of argon. The ZnO depositions were conducted on 10 mm × 10 mm SAM-coated Si wafers placed in 1 mL aliquots of precursor solutions in a closed container heated to 60 °C. The deposition process was conducted at various extents of time by replacing the methanol-based solution every 1.5 h. Up to 20 such cycles were conducted before the substrate was removed. The formed films were abundantly washed with distilled water. Characterization. The particle size in solution was measured by DLS (Malvern Zetasizer Nano ZS). The size was obtained by fitting the correlation function to a bimodal exponential decay function that yields the diffusion constants D. The hydrodynamic diameter was calculated using d ) kBT/(3πηD). Here, kB is the Boltzmann constant, T is the absolute temperature, and η is the viscosity. For the in situ monitoring of the reaction and aggregation processes, a sample cell containing the solution was heated to 60 °C inside the DLS device, and photon correlation measurements were conducted. The data typically showed a bimodal size distribution with a contribution from very small particles (less than 10 nm) corresponding to the polymer molecules. This contribution is not reported in the Results and Discussion section. The electrophoretic mobility of the precipitating particles and the conductivity of the solutions were determined using the Zetasizer device. These experiments were evaluated using the automated routines provided by the manufacturer. A viscosity of 2 mPa s was determined for a methanol solution of 10 mmol L-1 PVP at 60 °C using a Thermo Haake RheoStress1 rheometer. The value for pure methanol at 60 °C and an ambient atmospheric pressure of 0.1 MPa (0.32 mPa s) was obtained by linear extrapolation of data from the literature.32 Values of viscosities for other polymer concentrations were calculated assuming a linear dependence between these parameters. The film growth rate was determined by QCM (RQCM, Maxtek Inc.) experiments. A sulfo-SAM was deposited onto the silica surface of the probe according to the procedures outlined previously. The QCM measurements were performed at a controlled temperature of 60 °C in a closed reaction chamber. For scanning electron microscopy (SEM), a JEOL JSM-6300 F instrument with an accelerating voltage of 3 kV and a working distance of 15 mm was used. Cross-sectional specimens of the deposited films were obtained by cracking the brittle Si or glass substrate bearing the film. Using a pair of pliers, a small notch was made on the substrate edge to control the position of the crack. Transmission electron microscopy (TEM) was performed using JEOL JEM-4000 FX (400 kV) and JEOL JEM-3010 (300 kV) instruments. The images from the JEM-4000 FX instrument were captured on an electron image film (Kodak) and from the JEM-3010 instrument by a CCD camera (Keen View, SIS Images). Nanoparticle samples were prepared by applying a few drops of the particle-containing solution onto a copper TEM grid with a lacey carbon film (Plano or Okenshoji) and allowing the solvent to evaporate at ambient conditions. Atomic force microscopy (AFM) images were recorded with a Digital Instruments Nanoscope III instrument operating in the tapping mode with silicon cantilevers. X-ray diffraction (XRD) experiments were executed on a Philips X’Pert diffractometer in a Bragg-Brentano geometry using monochromatic Cu- KR1 radiation. Scherrer analysis was performed by fitting three Lorentzian maxima with interdepen-

Controlling Assembly of Nanocrystalline ZnO Films

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Figure 2. TEM images of crystalline ZnO particles prepared from a methanolic Zn2+/TEAOH solution at room temperature without the addition of PVP. (a) Bright field TEM image and (b) inverted dark field TEM image (i.e., negative). Note the domains in the dark field image, which are typical of polycrystalline particles.

Figure 1. Temporal evolution of the particle size in a methanol solution of Zn acetate and TEAOH at different PVP concentrations at 60 °C.

dent full widths at half maximum γ to the (100), (002), and (101) peaks of the ZnO diffraction diagram. The volumeweighted domain size in the direction parallel to the diffraction vector (d) was calculated using the Scherrer formula d ≈ λ/(γ cos θ), where λ is the wavelength, and 2θ is the peak position. d was taken as a measure of the average crystallite size. Instrumental peak broadening was neglected since it is insignificant as compared to the peak broadening because of grain size effects. For XRD analysis, samples were prepared by centrifuging the dispersions at 1000g for 45 min. The procedure was repeated with pure methanol before the sediment was retrieved and dried. The nuclear magnetic resonance (13C NMR) spectra were obtained using a Bruker Avance 250 spectrometer operating at

62.895 MHz. A broadband proton decoupling technique was applied. The measurements were performed in a mixture of methanol and deuterated methanol. Elemental analysis was performed using a range of apparatuses (ELEMENTAR Vario EL C/H/N-Determinator and LECO TC-436-DR N/O Determinator) including atomic emission spectrometry (ISA Jobin Yvon JY70 Plus). The thermal properties were studied by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) using a Netzsch STA Jupiter 449C instrument. The thermal measurements were carried out in air, by heating the samples from 25 to 1000 °C with a heating rate of 10 °C min-1 in alumina crucibles. A quadrupole mass spectrometer (GAM 200, In Process Instruments) was used for the detection of the gaseous species. The samples were prepared by centrifugation and being washed in methanol, followed by being ground in a mortar. Investigation of Particulate Species in Solution We studied how the precipitation, assembly, and crystallization of Zn species proceed with time in a methanol solution.

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Figure 3. X-ray diffractogram of the particles prepared from methanolic Zn2+ solutions (no PVP or heating), with the addition of NaOH and TEAOH, respectively. The individual graphs are shifted along the y-axis. The data were corrected by subtraction of the background intensity caused by the substrate. The zincite peaks are indexed.

Figure 4. TEM micrograph of spicular particles formed after 5 min at 60 °C in a methanol-based solution with [PVP]/[Zn2+] ) 1:1.

Three distinct stages were identified for the processes in the solution: an initial stage, after the reactants were mixed in the methanol solution at room temperature; an intermediate stage, immediately after being heated to 60 °C; and a final stage incipient after a few hours and lasting longer than the experimental time scale. Time Evolution of Particle Size. Apart from PVP-free solutions, the evolution of the particle size was studied at four different levels of polymer concentration: low (φ ≈ 0.04-0.06), intermediate (φ ≈ 0.08-0.12), high (φ ≈ 0.14-0.18), and very high (φ ≈ 0.20 and more). At all of these four concentrations, the polymer chains can be assumed to show interpenetration.33 The time evolution of the particle size (Figure 1) depends strongly on the PVP concentration. At a low PVP concentration (φ ≈ 0.06; Figure 1a), relatively large particles form immediately after the solution is heated to 60 °C. During this intermediate stage, the particle size does not change significantly. However, after a few hours, the size decreases rapidly to approximately 10 nm. We identify this drastic change in the particle size as the transition between the intermediate stage

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Figure 5. Time evolution of the electrical conductivity in a methanolbased solution with [PVP]/[Zn2+] ) 1:1 at 60 °C.

and the final stage. Systems with an intermediate PVP concentration (Figure 1b) show a similar but less pronounced timedependent behavior. The particles initially formed were smaller, ∼25 nm, and the transition to the final stage was less drastic. At a high PVP concentration (Figure 1c), the transition point was only characterized by the change from a constant to an increasing particle size because of the similar sizes of the particles in the intermediate and in the beginning of the final stage. The evolution of the particle size with respect to the reaction time suggests a transformation reaction. Thereby, the particulate phase, which is formed first, dissolves and thus contributes to the formation of the final product. In the PVP concentration range of φ ≈ 0.06-0.14, the measured average particle size was 20-50 nm during the intermediate stage. These particles are present in the reaction solution for a period between 1 and 3 h before they eventually disappear. The time scale for the disappearance depends on the PVP concentration; at higher PVP concentrations, the intermediate stage lasts a shorter time. In the final stage, the particles grow with an approximately linear increase of the particle diameter. The particle growth is faster at higher PVP concentrations; at φ ≈ 0.14, the growth rate is 10 nm h-1. Note that at very high PVP concentrations, no particle formation was detected by DLS. Initial Stage at Room Temperature. In PVP-free solutions, nanoparticles formed at room temperature immediately after the components were mixed (Figure 2). The morphology of the crystallites depended on the cation of the added base. TEM investigations revealed that rod-like polycrystalline particles formed in tetraethylammonium solutions; in NaOH solutions, approximately spherical nanocrystals were formed, similar to the particles that Spanhel and Anderson observed when performing a similar experiment in ethanol.34 The XRD diffractograms (Figure 3) matched those of nanocrystalline ZnO (zincite) with an additional small contribution of Zn(OH)2 (wu¨lfingite). In the absence of a polymer, these particles dissolved within a few hours when the solution was heated to 60 °C. At PVP concentrations above φ ≈ 0.06, the particle formation was inhibited at room temperature. Indeed, it is only in these solutions that smooth ZnO films can be formed on SAM-coated substrates. Intermediate Stage after Heating to 60 °C. Heating the methanol-based solutions of zinc acetate, TEAOH, and PVP to 60 °C resulted in rapid particle formation. The TEM micrograph in Figure 4 shows spicular (i.e., needle-like) particles with a length of ∼30 nm. These particles formed within 5 min at



Figure 6. TEM micrographs of the time-dependent morphological evolution of the ZnO nanoparticles in methanol-based solutions at 60 °C at different PVP concentrations. (a) Coexistence of spicular particles and compact aggregates after 1.5 h at an intermediate PVP concentration. (b) Fractal aggregates of nanosized particles after 3.5 h at a low PVP concentration. (c) Compact aggregates of nanosized ZnO particles after 3.5 h at a high PVP concentration. (d) High-resolution TEM image of a particle from panel c. The angular distribution of the lattice fringes demonstrates the polycrystallinity of the particles.

60 °C and φ ≈ 0.1. Studies at different PVP concentrations showed that analogous spicular particles immediately were formed after heating in most of the studied systems, except at zero or very high PVP concentrations. The featureless X-ray diffraction pattern and the absence of lattice fringes in highresolution TEM micrographs suggest that the spicular particles formed during the intermediate stage are amorphous. At low PVP concentrations (about 0 < φ < 0.08), these elongated amorphous particles form even at room temperature, without heating. Very high concentrations of PVP (φ > 0.18) inhibit the particle formation, even at 60 °C. Further proof of the rapid formation of a particulate phase is given by the conductivity measurements shown in Figure 5. Heating the solution from room temperature to 60 °C resulted in a slight increase of the conductivity during the first few minutes, which is probably related to an increased ionic mobility during the temperature rise. After this initial period, the conductivity fell very rapidly from a level of around 2-3 mS

cm-1 to a conductivity of about 0.05 mS cm-1. The Zn2+ ions provided the largest contribution to the conductivity since Zn2+ has the highest ion mobility among the ions in the solution. Relating the change in conductivity to the zinc ion concentration suggests that less than 2% of the added zinc remains in solution when the intermediate amorphous phase has formed. Final Stage. As the reaction proceeds, the amorphous spicular particles gradually disappear and are displaced by a more equiaxed species of particles, as shown in Figure 6. Figure 6a illustrates that the spicular particles coexist with polycrystalline aggregates, consisting of spheroidal ZnO crystallites with diameters of only few nanometers, at reaction times on the order of 1.5 h. The electron microscopy studies show that the spicular particles have completely disappeared after 3.5 h. At that point, only nanocrystalline aggregates were detected. These observations correspond well with the time scales for the change in particle size (Figure 1). Indeed, the coexistence of spicular particles and nanoparticle aggregates at 1.5 h, at an intermediate


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TABLE 1: Results from Elemental Analysis of Three Uncalcined Samples (wt %)

Zn O N C H

rod-like particles, zero PVP, no heating (%)

needle-like particles, [PVP]/[Zn2+] ) 0.4:1, 20 min at 60 °C (%)

compact aggregates, [PVP]/[Zn2+] ) 1.4:1, 24 h at 60 °C (%)

51 34 1 9 3

44 31 3 18 4

41 30 4 20 4

PVP concentration, match very well with the transition region in Figure 1b. The electron microscopy studies also show that the morphology of the nanoparticle aggregates strongly depends on the PVP concentration, whereas the size and shape of the nanocrystals themselves remained nearly unaffected. At low PVP concentrations, the aggregates have a fractal shape (Figure 6b), whereas at intermediate and high PVP concentrations, they are compact and approximately spherical (Figure 6c). Composition of the Particles in Solution. The compositions of the particulate species formed during the initial, intermediate, and final stage were investigated by thermal and elemental analyses. Initial Stage. The rod-like particles, prepared without PVP at room temperature, displayed an initial mass loss around 150200 °C, which continued up to about 400 °C, resulting in a total mass loss of about 25%. Mass spectrometry analysis revealed the expulsion of H2O, CO, CO2, and fragments with Mr ) 17, indicating a decomposition of methanol or residual acetate moieties. A weakly endothermic process occurred concurrently with a slight increase in mass in the temperature range of 700-800 °C. Intermediate Stage. The spicular particles, prepared at low PVP concentrations during 20 min at 60 °C, experienced a slight TABLE 2: Overview of the Observed Processes in Solution

mass loss between 150 and 200 °C (about 5%). Mass spectrometry indicated that H2O and fragments with Mr ) 17 were released. This suggests that methanol and/or acetate species were expelled. A strong mass loss occurred around 400-500 °C (about 30%), together with an endothermic reaction. Mass spectrometry showed the expulsion of CO, H2O, fragments with Mr ) 17 and 30, and a very strong signal of CO2. This suggests the presence of an organic component, probably PVP. Indeed, PVP decomposes by endothermic reactions in that temperature range.35 Final Stage. The compact aggregates, prepared at high PVP concentrations during 24 h at 60 °C, lost ∼40% mass at 300500 °C. This mass loss was accompanied by endothermic reactions. Mass spectroscopy showed a release of H2O, fragments with Mr ) 17 and 30, and plenty of CO2, suggesting the presence of PVP. The results of the elemental analysis of particles at the three different stages are summarized in Table 1. The thermal and elemental analyses indicated that the precipitates contained little water but a fair share of either methanol or residual acetate (in particular, the rod-like particles in the PVP-free solutions). Considering their nitrogen contents and their mass losses between 400 and 500 °C, it is clear that the particles produced in the PVP-containing solutions contain a relatively high fraction of PVP: around 20-35% by weight. A proportion of this high amount of PVP can probably be ascribed to imperfect purification of the samples, especially in the case of the sample from the final stage, which was obtained from a highly concentrated PVP solution. In an earlier section of this paper, it was described that during the final stage of the process, polycrystalline ZnO aggregates are present as indicated by TEM. The main inorganic constituent of the needle-like particles produced during the intermediate stage was probably Zn(OH)2 and/or ZnO, which



Figure 8. Film thickness and roughness depend on the polymer concentration in solution. The thickness was measured with the help of SEM images, and the roughness was determined using AFM. See also Lipowsky et al.5

Figure 7. 13C NMR spectra recorded of two solutions containing only PVP and PVP plus zinc acetate plus TEAOH, respectively. The carbon atoms in the PVP monomer were assigned numbers or letters, and the corresponding numbers or letters in the spectra indicate which peak can be attributed to which carbon atom. Gray italic numbers indicate signals from free monomeric vinylpyrrolidone.

is amorphous according to the TEM and XRD investigations. In contrast, the rod-like particles formed during the initial stage at room temperature have a high content of crystalline ZnO and contain some residual solvent. Effect of PVP on Particle Formation and Aggregation Table 2 gives an overview of the results of the solution experiments and the film deposition studies, which will be discussed in more detail next. The intermediate amorphous spicular particles were formed in a certain PVP concentration range, which coincides with the PVP concentration range where smooth ZnO films can be deposited. Hence, it appears that the formation, and slow dissolution, of the amorphous phase in solution is essential for the deposition of smooth inorganic films. Organic additives can prevent nucleation, disperse small particles, or inhibit crystal growth.36 These effects can be exploited for the preferential fabrication of certain crystal structures. Often unexpected crystal morphologies are ascribed to the specific adsorption of molecules to certain faces of the growing crystals.37,38 To investigate this, 13C NMR spectroscopy was applied. No significant line broadening of the signals or change in the chemical shift of the PVP polymer was found in the presence of the amorphous spicular particles formed in solutions containing PVP plus zinc acetate and TEAOH (Figure 7).39 Polymers that are adsorbed to small and freely tumbling nanoparticles experience a slower reorientation dynamics and have broader NMR lines. If the PVP immediately undergoes a

rapid exchange between surface and solution, broader lines appear as well.40 If the PVP, on the other hand, adsorbs on large particles, and has a slow exchange, narrow NMR lines are expected.41 The signal from the adsorbed polymer is lost in liquid-state NMR experimentation. Our results suggest the conclusion that there is no or only a weak direct interaction of the PVP with the nanoparticles. However, additional solid-state NMR experiments would have to be undertaken to firmly state if there is a strong molecular interaction between the ZnO nanoparticles and the PVP polymer, but this is beyond the scope of this study. During the later stage of the process, PVP controls the growth of the polycrystalline ZnO aggregates. A linear increase of the particle diameter was found (Figure 1) and confirmed by longtime experiments. This implies a growth that is much faster than the linear increase in particle volume with time, which one would expect in the case of Ostwald ripening. The growth rate was higher at higher PVP concentrations. The fractal aggregates, observed at low PVP concentrations, suggest a colloidal instability and rapid aggregation.42 In contrast, at high PVP concentrations, the fast growth and compact morphology of the aggregates cannot be explained merely by one of the prevalent models of crystal growth or aggregation, such as reaction-limited aggregation43,44 or diffusion-limited aggregation,45 without consideration of the effects of the dissolved, nonadsorbing polymers. For entropic reasons, the exclusion (depletion) of a nonadsorbing polymer from the gap between two colloidal particles, or between a colloidal particle and a surface, at sufficiently short separation distances results in an attractive force, a depletion force, between the particles,46 whose magnitude increases with increasing polymer concentration in solution. Theoretical studies,47,48 simulations,49 and experimental investigations50,51 on dispersions containing dissolved polymers at relatively high concentrations support the conjecture that such polymer-mediated interactions promote the formation of compact aggregates: When the difference in size between the nanoparticles and the polymer coils is relatively small, like in the system described here, the dispersion will eventually phaseseparate into a virtually nanoparticle-free solution and a condensed phase that contains the nanoparticles (e.g., in the form of agglomerates or as a polycrystalline film on a substrate). Indeed, support for this conjecture is given by the absence of PVP in the deposited ZnO films (for the composition of the films, see ref 5) and the very low concentration of individual ZnO nanocrystals in the solution (as observed in the TEM


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Figure 9. X-ray diffractograms of ZnO films deposited from basic methanol solutions at a range of [PVP]/[Zn2+] ratios. The individual graphs are shifted along the y-axis.

micrographs, Figure 6). The concentration dependence of the magnitude of the depletion forces explains the faster aggregation rate and film deposition rate at higher PVP concentrations. Furthermore, the shift of the equilibrium by the rapid removal of the crystalline ZnO nanoparticles from the solution may also be the cause of the faster dissolution rate of the intermediate amorphous phase at higher PVP concentrations. The surface charge and associated zeta potential of particles in a solution are important parameters that can influence both the colloidal stability and the deposition on a substrate. The electrophoretic mobility (and thus also the associated zeta potential) of the particles in methanolic solutions containing zinc acetate, PVP, and TEAOH is very low and fluctuates around zero. Hence, the particles have a very low surface charge, suggesting that long-range electrostatic interactions have no significant influence on the aggregation and deposition processes. In summary, the PVP-free films,5 the compact aggregates, and the NMR results suggest that the interaction between PVP and surfaces of the formed particles is very weak, and it is unlikely that adsorption of PVP controls either the growth of the ZnO crystallites or the particle interactions governing their assembly. However, PVP is clearly involved in the formation of the amorphous spicular particles and is an important constituent of these inorganic-organic hybrid particles. The elongated shape of these amorphous particles may be related to a preferred precipitation direction induced by the polymer. Possibly the polymer strands, or fragments thereof, act as templates during the formation of the intermediate particles. It is interesting to note that elongated amorphous particles have indeed been observed previously and reported to act as intermediates in the biomineralization of magnesium-containing calcites.52 The transition from the metastable amorphous spicular

Figure 10. Graphs of mass uptake experiments with a QCM show ZnO film growth on a sulfo-SAM during (a) a single deposition cycle and (b) four consecutive deposition cycles, respectively. The PVP concentration in these experiments was φ ≈ 0.1. The initial vigorous amplitudes relate to temperature differences during the assembling and filling of the reaction chamber. Note that in panel b, the agitation and the temperature changes during the exchange of the solutions may shift the value of the film mass. Therefore, only statements about the film growth during a single deposition cycle can be deduced but not about the total film mass deposited applying several cycles.

particles to the stable polycrystalline aggregates can be seen as a manifestation of Ostwald’s rule of stages where the least dense phase is formed first and then transforms into denser and denser phases with time. Recently, Co¨lfen and Mann11 presented a generalized scheme of the crystallization pathways, which also accommodate a sequential path that commonly involves an amorphous phase. The PVP molecules trigger and boost the growth of the polycrystalline aggregates; this indicates that depletion forces are important for aggregation kinetics. Consequently, the transition between the intermediate and the final stage of the reaction occurs sooner at higher PVP concentrations. Film Deposition on Substrates Figure 8 and Table 2 summarize how the PVP concentration affects the film thickness and surface roughness of the deposited ZnO films.53 The maximum thickness and minimum roughness occur at intermediate PVP concentrations, around φ ≈ 0.1. At zero and very low PVP concentrations (φ e 0.04), large disordered clusters precipitate from the solution onto the substrate (i.e., no continuous film is formed). At φ g 0.04, the thickness of the films increases with an increasing PVP concentration and reaches a plateau. The smoothness and texture are gradually lost when the PVP content is increased even further. At very high PVP concentrations, hardly any particles are formed, and no film is deposited.



Figure 11. Schematic pictorial view of the processes in solution and at the substrate interface during the deposition of a ZnO film. The formation and dissolution of the intermediate spicular particles act as a sink that controls the concentration of Zn species and thus the balance between deposition and precipitation in solution. The palisades symbolize the SAM, the gray block is the silicon carrier, the red dots are Zn2+ ions, the dark purple lines are the amorphous spicular particles, the blue circles are ZnO crystallites, and the green clews are the polymers.

The X-ray diffractograms give more information as to the structure of the deposited ZnO films. All films exhibited three peaks at scattering angles 2θ of about 31.9, 34.6, and 36.4°, indexed as the (100), (002), and (101) peaks from ZnO with the zincite structure (Figure 9). At intermediate PVP concentrations, φ ≈ 0.1-0.12, the (002) signal was much stronger than the (100) and (101) signals, indicating textured films. However, at higher or lower PVP concentrations, the relative intensities of the peaks resemble the pattern obtained from a powder more and more, until at φ ) 0 or φ ≈ 0.18 no indication of a texture was observed. In contrast, the grain size showed no significant dependence on the PVP concentration in a wide range of volume fractions (φ ≈ 0.04-0.16). High PVP concentrations result in the formation of thinner, rougher, and less textured films as compared to the films formed at intermediate PVP concentrations. The combined effect of an increased precipitation rate and the competition between aggregation in solution and film formation explains the poor film quality. At high PVP concentrations, the dissolution of the intermediate spicular amorphous phase occurs at an early stage, which results in a rapid increase of the concentration of Zn species in solution and eventually precipitation. Hence, the time window for film formation becomes small. Uncontrolled crystallization at low PVP concentrations results in defective and rough deposits. Hence, the formation of the amorphous phase in solution at intermediate concentrations of PVP is necessary for the formation of high-quality films; the intermediate phase acts as a sink for zinc. It is interesting to note that an earlier study by Co¨lfen et al.54 has also pointed out the importance of intermediates in solution for successful deposition of inorganic films. In their case, sulfate-containing complexes acted as a reservoir of zirconium ions during the deposition of zirconia films from aqueous solutions. The time-dependent deposition of the inorganic films was studied by QCM on SAM-coated probes. This technique detects very small mass changes from the change in resonance frequency. The reactions were performed in a reaction chamber

with a movable lid holding a pre-heated coil. The deposition of a film from a solution with a PVP content of φ ≈ 0.1 begins immediately when the solution is heated to 60 °C (i.e., at t ) 10 min in Figure 10a, when the heating coil was inserted in the solution). From that moment on, we can relate the frequency change to first-order deposition kinetics with an asymptotically decreasing deposition rate. The mass increase ceased after 3060 min with a specific mass estimated to be 3 µg cm-2. For a uniform and fully dense ZnO film, this mass would correspond to a 5 nm thick film. Taking the porosity of the films and the initial island growth into account, the QCM results are consistent with typical layer thicknesses (Figure 8). Thicker films can be prepared by consecutive deposition cycles. We have also studied the kinetics of such sequential deposition processes with the QCM technique (Figure 10b). It was found that the deposition process is promoted (i.e., the deposition rate is higher) and proceeds for a longer time if the substrate is already coated with ZnO. Indeed, SEM studies suggest that the first two or three deposition cycles produce islands rather than continuous films.35 The slight decrease in mass after t ) 8 h in Figure 10b is possibly related to solvent evaporation. Indeed, the information from the QCM measurements could be exploited to optimize the deposition process. In the standard procedure, we changed solutions every 90 min when thicker ZnO films were to be produced by sequential deposition. This procedure was optimized on the basis of the deposition kinetics data, using an initial deposition cycle of 0.5 h on the bare SAMcoated substrate and subsequent deposition cycles of 3 h. The optimized deposition procedure resulted in a ZnO film that was 46% thicker after 10 deposition cycles as compared to the standard procedure, admittedly with the drawback of an 83% longer total deposition time. Hence, varying the duration of the individual deposition cycles permits balancing an exhaustive use of the deposition solution against a fast increase in film thickness.

5382 J. Phys. Chem. C, Vol. 112, No. 14, 2008 Conclusion The time evolution of the thermally activated mineralization of ZnO nanoparticles in a methanol solution and the relation between the solution processes and the film deposition was studied by a combination of DLS, electron microscopy, X-ray diffraction, and chemical analyses. The studies in solution showed that the polymer PVP determines the precipitation and kinetic polymorph control of zinc-containing species in methanol. By adding PVP to the solution, it was possible to prevent the rapid formation of crystalline rod-like ZnO nanoparticles at room temperature. Increasing the temperature to 60 °C resulted in the rapid formation of amorphous needle-like particles in PVP-containing solutions. PVP is involved in the formation of the amorphous spicular particles, and chemical analysis showed that PVP was incorporated in the particles. The amorphous spicular particles are metastable; they dissolve within a time frame of a couple of hours. Eventually, aggregates of ZnO nanocrystals are formed. It was found that the time scale for the dissolution of the amorphous spicular particles was essentially determined by the PVP concentration. At higher PVP concentrations, the transformation from the amorphous phase to the nanocrystalline ZnO occurs faster than at lower PVP concentrations; the spicular particles dissolve more rapidly, and the ZnO nanocrystals precipitate faster. We suggest that depletion forces are important for the kinetics during the aggregation of the nanocrystals. At low PVP concentrations, tenuous fractal aggregates are observed. At intermediate and high PVP concentrations, the nanocrystals aggregate more rapidly, and the aggregates have a compact morphology. The microstructure and surface roughness of the deposited films were analyzed over a wide range of PVP concentrations, and the deposition kinetics was studied using a QCM. The deposition rate controls the quality of the inorganic films, which is directly coupled to the formation and dissolution of the spicular amorphous particles. Smooth and highly textured ZnO films form only within the PVP concentration range where, on the one hand, the intermediate phase absorbs the zinc ions and releases them again at a convenient rate, and, on the other hand, the final ZnO nanocrystals do not aggregate too quickly. Figure 11 gives a schematic overview of the importance of the intermediate amorphous phase in controlling and fine-tuning the precipitation and aggregation in solution and the deposition of smooth films. Regulating the reaction conditions through intermediate amorphous phases is commonly utilized in biomineralization processes and is probably of significant importance also in many chemical bath deposition processes. Carefully selecting additives that allow the formation of intermediate phases, which can act as a reservoir or a sink from which reactive species can be released at an adjustable rate, may be an important step toward the rational synthesis of materials with a tailored nanostructure. Acknowledgment. The authors thank Dr. Udo Welzel, Maritta Dudek, and Gerd Maier for recording the XRD diffractograms; Sabine Ku¨hnemann for recording the SEM images; Peter Gerstel for the synthesis of the silanes, acquiring the NMR spectra, and for general support in the chemistry laboratory; Dr. Luciana Pitta Bauermann for introduction to the QCM technique; Gerhard Kaiser for chemical analysis; Peter Kopold for supervision during some of the TEM investigations; and Drs. Paul Bellina and Peter van Aken for helpful suggestions

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Controlling the Assembly of Nanocrystalline ZnO Films by a Transient

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