Interplay of Internal Structure and Interfaces on the Emitting Properties

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Interplay of internal structure and interfaces on the emitting properties of hybrid ZnO hierarchical particles Monica Distaso, Giovanni Bertoni, Stefano Todisco, Sergio Marras, Vito Gallo, Liberato Manna, and Wolfgang Peukert ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00777 • Publication Date (Web): 12 Apr 2017 Downloaded from http://pubs.acs.org on April 14, 2017

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ACS Applied Materials & Interfaces

Interplay of internal structure and interfaces on the emitting properties of hybrid ZnO hierarchical particles Monica Distaso,1,* Giovanni Bertoni,2 Stefano Todisco,3 Sergio Marras,4 Vito Gallo,3 Liberato Manna,5 Wolfgang Peukert1 Corresponding author: [email protected] 1.

Institute of Particle Technology, FAU Erlangen-Nuremberg; Cauerstraße, 4 – 91058 Erlangen, Germany; 2. IMEM-CNR, Parco Area delle Scienze 37/A, Parma, Italy; 3. Dipartimento di Ingegneria Civile, Ambientale, del Territorio, Edile e di Chimica (DICATECh), Politecnico di Bari, via Orabona 4 (CAMPUS), I-70125 Bari, Italy; 4. Materials Characterization Facility, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy; 5 Nanochemistry Department, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy. Supporting Information. 13 Solid state C NMR characterization of Poly-N-vinyl-pyrrolidone; Additional characterization of the emitting properties of ZnO/PVP particles; Details on the statistical evaluation of Scanning Electron Microscopy images

Keywords:

mesocrystals;

structure-function

relationship;

nanocomposites;

nanoreactors; post-processing; coordination Abstract The design of hybrid organic/inorganic nanostructures with controlled assembly drives the development of materials with new or improved properties and superior performances. In this paper the surface and internal structure of hybrid ZnO Poly-Nvinyl-pyrrolidone (ZnO/PVP) mesocrystals is investigated in details and correlated with their emitting properties. A photoluminescence study at room temperature reveals that the as-synthesized particles show a remarkable Ultraviolet (UV) emission, whereas the emission from defects in the visible is not observed. On the other hand, the visible emission is achieved upon calcination of the hybrid ZnO/PVP particles in air, whereas its intensity is found to increase with the calcination temperature and in some cases, to overwhelm the emission in the UV. A molecular description is proposed for the absence of visible emission from defects in the assynthesized ZnO/PVP mesocrystals based on Fourier Transform Infrared (FTIR) and 1 ACS Paragon Plus Environment


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solid state

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C-NMR (SSNMR) spectroscopy. An in depth electron microscopy study

sheds light on the internal organization of mesocrystals and reveals the formation of nanoreactors, i.e. particles with enclosed porosity, upon thermal treatment. 1. Introduction The assembly of organic and inorganic moieties by bottom-up approach allows the design of complex systems with various morphologies, compositions, and functionalities, with the advantage of a great precision even at the nanometer level. The resulting hybrid organic/inorganic nanostructures show new or improved properties and superior performances, which imply that a particular attribute in the hybrid can exceed the sum of the individual phases.1,2 In the last years, our group has shown that the use of polymers as templating agents during the synthesis enables the engineering of hybrid materials by tuning their morphology, size, and effective refractive indices.3,4,5 Furthermore, the polymer on the particle surface provides colloidal stability and may offer additional anchoring groups for further surface functionalization.6,7,8 Among the inorganic materials investigated, ZnO is a technologically relevant semiconductor that has received significant attention for opto-electronic, catalytic, and biological applications.9-14 In general, for optoelectronic applications based on ZnO such as UV lasers9,15,16 or UV Light Emitting Diodes (UV-LED),11 the co-presence of UV and visible emission affects the performance of the final device. The non-radiative recombinations at defects sites are detrimental for the emission efficiency of the active material and, therefore, of the device.9,10,11 To boost the UV emission efficiency, it is necessary to implement strategies such as annealing and plasma curing17 to control and monitor the defects distributions.17,18 Rarely, ZnO structures that do not present any visible emission could be obtained.11,19 However, understanding these properties at

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molecular level remained an unaddressed issue. In 2005 and later, Cölfen introduced the concept of mesocrystals to describe superstructures comprising ordered arrangements of nano- or micro- building blocks.20,21 When mesocrystals are grown in the presence of polymers or proteins acting as templating agents, a nanocomposite material with hierarchical organization can be fabricated.22,23 In the present paper, we describe mesocrystal nanocomposites based on ZnO and Poly-Nvinyl-pyrrolidone (ZnO/PVP) and we demonstrate that the close interaction between the organic moiety and the inorganic framework in a mesocrystalline organization allows achieving remarkably high UV emission at room temperature with no emission in the visible region. The results open the possibility to design materials with enhanced properties for blue optoelectronic applications.9,15,16 The strategy described might be generalized to a wide range of other direct semiconductor materials, such as CdS, CdSe, PbS, PbSe, GaAs, GaN, to mention only few.24,25,26,27 The growth of these materials in a mesocrystalline habit might boost the exciton emission by reducing the emission from defects. 2. Experimental part All chemicals were of high purity grade and have been used without any further purification. Zn(NO3)2⋅6H2O (99 %), absolute ethanol and N,N-Dimethylformamide (DMF, 99.5 %) were acquired from VWR International GmbH. PVP (K30, MW 55000 g/mol) and ZnO (99.9 %) were Sigma-Aldrich products. 2.1 Synthesis of ZnO mesocrystals under solvothermal conditions. The synthesis of ZnO/PVP mesocrystals have been widely described previously.3,4 In the current work, the synthesis was performed using a Hastelloy C22 reactor with a volume of 1.5 l (Büchi AG). The reactor is equipped with a mechanical rotor for the stirring of the reaction mixture with a three-blades-propeller geometry. In a typical



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synthesis, 500 ml of a solution of Zn(NO3)2⋅6H2O and PVP in DMF with a concentration of 0.025 M and 0.01 g/ml, respectively, was heated for 3 h at 120 °C with or without stirring (Re = 0, 12800 or 25500, respectively) applying a heating profile of 7 K/min and cooling down to room temperature naturally over 5.5 h. The particles without polymer were prepared by solubilizing Zn(NO3)2⋅6H2O (1 mmol) in DMF (40 ml). The solution was transferred in a 250 ml Teflon liner of a stain less steel autoclave (DAB 3, Berghof Products + Instruments GmbH, Germany). The reactor was placed in a dedicated metal jacket on a hotplate and heated at 120 °C for 3 h. Upon cooling to room temperature, the suspensions were centrifuged in order to separate the particles from the mother solution. The particles were then washed with DMF and absolute ethanol via re-dispersion and centrifugation cycles (three cycles for each solvent). The final ethanol suspension of ZnO/PVP mesocrystals was stable for several months, whereas the ethanol suspension of ZnO particles without polymer was stable for about 30 minutes, before sedimentation occurred. The obtained powders were dried overnight in an oven at 60 °C. 2.2 Transmission Electron Microscopy (TEM). High resolution TEM images (HRTEM), selective area electron diffractions (SAED) and scanning TEM images (STEM) acquired in high angle annular mode (HAADF) were obtained on a JEOL JEM-2200FS microscope working at 200 kV accelerating voltage. The particles were deposited by drop casting on ultrathin carbon film copper grids. 2.3 Scanning Electron Microscopy (SEM). The SEM was a Zeiss ULTRATM 55. The image analysis was carried out by a threshold/watershed method using the freeware ImageJ package.28 The particle size distribution was calculated as described in details elsewhere.4,5 2.4 Attenuated Total Reflectance – Fourier Transfer Infrared (ATR-FTIR) spectroscopy. The FT-IR instrument was a Varian Excalibur HE Series. The 4 ACS Paragon Plus Environment

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powdered ZnO/PVP sample and the commercial PVP were deposited onto an ATR diamond crystal and slightly pressed. The spectral resolution was 4 cm-1 at 2000 cm-1, and 64 transients were acquired for each spectrum, as well as for the background. 2.5 Solid State NMR Spectroscopy (SSNMR).

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C SSNMR experiments were

performed on a Bruker Avance I 400 spectrometer (13C, 100.6 MHz) using a 4.0 mm HX MAS probe at 298 K. Samples were packed in zirconia rotors. Chemical shifts are referenced to adamantane (δ 38.48). A two-pulse phase-modulation (TPPM) 1

decoupling scheme was used for the

H- decoupling.

1

H-13C CP/MAS NMR

experiments were performed using 3.25 µs as proton π/2 pulse length, νCP of 55.0 kHz, contact time of 1.0 ms, νdec of 76.9 kHz, a recycle delay of 4 s and a MAS rate of 13.5 KHz. 1162 transients were acquired for the

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C NMR spectrum of PVP and

11652 for the 13C NMR spectrum of ZnO/PVP mesocrystals. 2.6 Film preparation. Nanoparticulate films were deposited by spin coating (Novocontrol technologies GmbH & Co KG) of ZnO/PVP suspensions in ethanol on squared silicon wafers with a lateral size of 2 cm. In all cases, the substrates were previously washed with acetone and ethanol under ultrasonication (15 minutes) and finally dried under a gentle flux of nitrogen. For reproducibility purposes, 3 films were prepared for non-treated particles and for each calcination temperature namely a total number of 12 films were deposited. Additionally, bare silicon wafers of the same size were washed as described above and treated at the named temperature. The latter set of substrates was analyzed at photoluminescence spectrometer to exclude any possible contribution of the silicon wafers surface to the luminescence under the experimental conditions of the analysis. 2.7 Thermal treatment. The experiments were carried out on two sets of samples, namely powders and particles deposited onto silicon substrates. For the investigation 5 ACS Paragon Plus Environment


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of the effect of thermal treatment on the internal structural organization of mesocrystals, the dry powders were heated under air atmosphere in a porcelain crucible using a Nabertherm Controller P320, at a rate of 5 K/min up to the desired temperature, maintained 1 hour at that temperature, and cooled down inside the furnace. For the thermal treatment of the films, the coated silicon wafers were calcined for one hour at different temperatures, namely 400 °C, 500 °C and 600 °C on a hotplate (Praezitherm, Harry Gestigkeith GmbH). For statistical evaluation and reference, respectively, there were always three coated plates and one uncoated silicon wafer at each temperature, the latter was used as reference in photoluminescence measurements. 2.8 Statistical Evaluation of the number of particles per unit area. In order to verify that the number of particles was constant from one silicon wafer to another, the following statistical analysis was carried out by SEM on non-calcined films and on films calcined at 400 °C, 500 °C and 600 °C. For this set of experiments the magnification was set to 2000 (2 k) and for every film three non-overlapping areas were selected at the edge, in the middle and on the opposite edge of the substrate, respectively. In each micrograph, the number of particles was counted by threshold/watershed analysis using ImageJ and a mean value of 1950 particles with a standard deviation 82 was obtained. This analysis already demonstrated that the coverage of the substrates was reproducible and not affected by the thermal treatment. In order to improve the standard deviation, the analysis was repeated on non-calcined film and on the film calcined at 600 °C at higher magnification of 5000 (5 k) and 10000 (10 k), respectively. Again, three non-overlapping areas at the opposite edges and in the middle of the films, respectively, were imaged. The mean values obtained for non-calcined films and for films calcined at 600 °C were in


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excellent agreement with each other, with a standard deviation of 3 in both cases (Supporting Information, Table S1, Figure S4 and S5). 2.9 Functionalization of commercial bulk ZnO with PVP. Control experiments were carried out using commercial bulk ZnO powder and the same polymer used for the synthesis, PVP K30. PVP (0.1044 g, 0.939 mmol of monomer units) was dissolved in 10 ml of absolute ethanol leading to a clear solution. Commercial ZnO powder (0.1046 g, 1.285 mmol) was added under stirring in a molar ratio ZnO to PVP of 1.4 mol/mol. The stable suspension was kept under stirring and then dried by rotary evaporation (Rotavapor R-100, Büchi Labortechnik AG) over 30 minutes at room temperature until a dry powder was obtained. To exclude the influence of ethanol, the commercial ZnO powder was treated similarly without addition of the polymer. Suspensions with suitable concentrations were prepared and deposited on silicon wafers by drop casting aliquots of the suspensions with the same ZnO concentration. Silicon wafers with same size were employed. 2.10 Photoluminescence (PL) Spectroscopy. The emission spectra were recorded at room temperature using a FluoroLog-3 (Horiba Jobin Yvon GmbH) equipped with a double-grating on the excitation as well as on the emission side. The instrument was calibrated with deionized water (Raman peak at 397 nm). The measurements were carried out in liquid phase or on non-calcined and calcined films, respectively, as specified throughout the paper. In the former case, ethanol suspensions of the assynthesized ZnO/PVP mesocrystals and ZnO particles were prepared with optical density lower than 0.1 and a quartz cuvette with 1 cm path length was employed. The PL measurements on films comprising the same number of particles were carried out at an angle of 45 °.



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3. Results and discussion The implementation of process parameters is a useful tool to control the morphology and size of hierarchical structures. Under the experimental conditions used in the current paper (Experimental part), it was found that the width of the particle size distribution (PSD) was narrowed at higher Reynolds number from 50 nm to 27 nm (Table 1), whereas the mean particles size (x50,0) was almost constant. Table 1. Influence of stirring on the PSD of ZnO/PVP particles synthesized under solvothermal conditions using DMF as solvent

Stirring frequency / Re 0 12800 25500

x50,0 / nm x5,0 / nm 160 110 153 113 150 123

x95,0 / nm 215 190 177

Mean width of PSD ± 53 ± 39 ± 27

Figure 1a reports representative transmission electron microscopy (TEM) images of the as-synthesized ZnO/PVP nanoparticles, with their typical morphology and corresponding narrow size distribution. The structures were highly porous due to the presence of many channels and holes. Electron diffraction patterns (SAED) carried out on a group of particles present the expected rings, due to different orientations of the particles with respect to the electron beam (Figure 1b), whereas a diffraction with spots in hexagonal pattern was obtained when the analysis was performed on a single particle (Figure 1 c). This indicates that the single particles are mainly crystalline, and consisting of quasi-oriented (between few degrees) smaller building blocks of ZnO in the wurtzite phase (hexagonal). A slight misalignment of the primary particles for about 6 ° (Figure 1 c) confirms the presence of strain inside each particle. Scanning transmission electron microscopy (STEM) images confirm that the particles are formed by the aggregation of smaller units with diameters between 5 and 10 nm (Figure 1d).


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a)

group of particles

b)

one particle

c)

d)

top view side view

(2-10) (100) ∼6º

Figure 1. a) Bright field TEM image of ZnO/PVP particles; b) SAED pattern from a group of particles, with the characteristic rings as expected from differently oriented crystals; c) SAED pattern from a single particle. The pattern is similar to that of a single crystal, with small arcs formed by the diffraction spots due to the slightly different orientation of few degrees in the (a,b) plane of the ZnO domains constituting the particles. d) HAADF-STEM indeed shows the small ZnO domains forming the particles, and the peculiar shape compressed in z direction, as is visible by a comparison of top view and side view.

The single particles orient preferentially with the c axis orthogonal to the support film, whereas no preferential orientation was observed among particles in a and b directions (indeed the diffraction from groups of particles always shows the rings). The reason for the preferential orientation of the particles with the c axis perpendicular to the substrate can be ascribed to their squashed morphology, as can 9 ACS Paragon Plus Environment


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be easily observed in the insert of Figure 1d, comparing top and side views. By filtering the TEM images around the elastically transmitted electrons, the visibility of the polymer layer (PVP) with variable thickness around the particles is enhanced (Figure 2 a,b). In order to shed light on a possible coordination of PVP on the surface of the particles, ATR-FTIR and

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C SSNMR experiments were carried out on the pure

polymer and on the as-synthesized particles. As a cyclic amide (lactam), the structure of N-vinyl-pyrrolidone is best described as a resonance hybrid (III) of the primary structures (I) and (II) reported in Figure 2c. Consequently, this moiety of the polymer has two possible centers for the coordination, either through the oxygen (structure IV, Figure 2 f) or through the nitrogen atom (structure V, Figure 2 f), respectively. Based on the widely accepted description of the influence of inductive effects on chemical bonds,29 the coordination modus (IV) implies a weakening of the C=O bond, and an increase of the order of the N-C bond, hence, the contribution of (II) to the ground state of the structure increases. On the contrary, if the coordination occurs through the nitrogen atom, the structure (I) is a better description of the lactam moiety. In FTIR spectroscopy, the coordination through the oxygen atom (IV) should lead to a red shift (lower wavenumbers) of the stretching vibration of the N-C=O bond (νN-C=O), whereas the coordination through the nitrogen should produce a blue-shift (towards higher wavenumbers).


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Figure 2. a) Low magnification and b) high magnification filtered TEM images of ZnO/PVP mesocrystals in which the shell of PVP polymer surrounding the ZnO core is visible; c) resonance structures of the N-vinyl-pyrrolidone moiety of PVP; d) ATR-FTIR spectra of PVP (blue curve) and of the as-synthesized ZnO/PVP particles (red curve); e) 1H-13C CP/MAS spectra of PVP (blue curve) and of the as-synthesized ZnO/PVP particles (red curve); f) exemplified monodentate coordination of N-vinyl-pyrrolidone moiety of PVP to a generic metal center.

The coordination mode (V) in Figure 2 f implies an appreciable steric hindrance and, consequently, in the liquid phase and on ionic metal centers, the coordination of



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lactam groups has been found to occur almost exclusively through the oxygen atom, especially for a coordination number equal to six.3,30,31,32,33 The situation might be completely different when the coordination of PVP occurs on solid surfaces, where movements of the atoms are severely restricted if not prevented. Under these conditions, different conformations are possible for the polymer, according to the surface coverage.34 Figure 2d shows that the stretching of the amide group (νN-C=O) in pure PVP is observed at 1647 cm-1, whereas in the same spectral region the spectrum of the ZnO/PVP mesocrystals shows two vibrations at around 1662 cm-1 and 1600 cm-1, respectively. The former is blue-shifted by 15 cm-1, whereas the latter is red-shifted by 42 cm-1 with respect to the νN-C=O of PVP. Thus, FTIR spectroscopy suggests that the coordination of PVP onto the ZnO surface can occur either through the oxygen or through the nitrogen atom of the N-C=O bond. The magnitude of the shifts observed by infrared spectroscopy can be directly correlated with the relative strength of the interaction.31 The red-shift is larger than the blue-shift, therefore the coordination through the oxygen atom seems to be favored, in agreement with the above steric considerations. Concerning the NMR spectroscopy, the resonance of the amide carbon in pure PVP was found at δ 176, whereas the spectrum of the ZnO/PVP mesocrystals showed two broad amide carbon signals centered at δ 177 and δ 170 (Figure 2e). The presence of two differently coordinated lactam groups is in agreement with FTIR spectroscopy findings. However no conclusive signal attributions can be drawn based solely on the chemical shift of the carbon nuclei, as in the 13C NMR spectroscopy the resonances are mainly dominated by orbital symmetry features (paramagnetic contributions), rather than only by the electron density.35


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Moreover, the presence of the quadrupolar nuclei nitrogen and zinc introduces other relaxation contributions, thus making the assignments of the respective coordination mode not straightforward.36 The broad signal at δ 177 includes also the non-coordinated PVP units. This finds a correspondence with the TEM images in Figure 2a and 2b where a shell of polymer with variable thickness on ZnO surface is visible. The resonances of methylene carbons of the pyrrolidone ring, as well as the -CH and -CH2 groups of the polymer backbone, do not shift upon coordination (Figure 2e and Figure S1, Supporting Information). Figure 3 shows the optical properties of the as-synthesized ZnO/PVP mesocrystals in ethanol. The extinction spectrum shows the typical UV-absorption and visible scattering, which we have described in detail in our previous work (Figure 3a).3,4 The PL spectrum shows that the particles comprise a remarkable near band UV emission at 372 nm,21 whereas the characteristic green emission usually observed for ZnO centered at around 500 nm is not present.37,38,39,40,41 Upon suitable selection of the excitation wavelength (Figure S2), the PL spectra of ZnO/PVP mesocrystals excited between 280 and 320 nm, reported in the Supporting Information (Figure S3), confirmed that no emission was observed in the green region of the visible spectrum. This latter feature is highly interesting and deserves a more careful analysis. In particular, we were interested in understanding the relation between the hierarchical structure organization of the polymer and the inorganic framework and the absence of green emission in the visible region of the PL spectrum. Thus, the polymer was removed by calcination in air at 400 °C, 500 °C and 600 °C and the particles were analyzed by electron microscopy. Removing the polymer from the particles by calcination has a dramatic effect on the hierarchical internal organization of the particles. 13 ACS Paragon Plus Environment


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Figure 3. Optical properties of the as-synthesized ZnO/PVP mesocrystals dispersed in absolute ethanol. a) extinction; b) PL spectra at different excitation wavelength. PVP in ethanol shows no emission under these experimental conditions (Figure S2). Size of the excitation and emission slits: 5 nm.

Figure 4 shows that, upon calcination at 400 °C a remarkable coarsening of the structures occurs, leading to the formation of bulk crystalline particles, with multiple voids at their interior. By rising the calcination temperature to 500 °C and further increasing the calcination time to 1h, the number of voids per particle was reduced and simultaneously their nominal size increased. At 600 °C, the inner distribution of the voids is very similar to the particles calcined at 500 °C for 1h; however, the surface of the particles appears coarser and more faceted. Furthermore, the particles undergo a partial sintering and aggregation process once the polymer is removed and this effect is stronger at higher temperature or for longer calcination time. These observations suggest that the internal structure of the particles collapses once the polymer is removed, which is followed at higher temperature by an oriented attachment among the primary particles. The collapse of the hierarchical structure upon removal of the polymer component of the nanocomposite is similar to previous observations on the effect of thermal treatment on hematite/PVP mesocrystals.42


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Figure 4. HAADF-STEM images of the ZnO particles after thermal treatment at different temperatures and different calcination time.

The formation of voids inside the structures upon calcination also suggests that the polymer is present not only at the surface of the particles, but also at their interior, between the oriented building blocks. The next step was to verify the modifications in the emitting properties of the ZnO mesocrystals, once the polymer was removed and the hierarchical organization of the particles suppressed. To carry out this study the particles were immobilized on a silicon substrate by spin coating. This is an important step, as the removal of the 15 ACS Paragon Plus Environment


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polymer promotes sintering and aggregation of the particles (Figure 4) with consequent loss of stability of the corresponding suspensions. This would prevent a correct measurement of the optical properties in the liquid phase due to sedimentation during the analysis. Figure 5a shows a typical coverage of the silicon substrate coated with a diluted ethanol suspension of ZnO/PVP mesocrystals. A statistical analysis was carried out on SEM images taken at low magnification (2k) before calcination and after calcination at different temperatures, confirming that the number of emitting species was constant around a mean value of 1950 particles, with a standard deviation of 82 (Figure 5b and Experimental part). A closer look at higher magnification (10k) SEM images from the samples calcined at 600 °C for 1h confirms the results obtained at lower magnification with an even higher accuracy (Table S1, Figure S4 and S5, Supporting Information). Figure 5c compares the PL spectra of ZnO mesocrystals deposited on silicon substrates by spin-coating before and after the thermal treatment. The spectra show that the as-synthesized ZnO/PVP particles are characterized by a strong UV emission without any emission in the visible region up to 625 nm (Figure 5c), in agreement with the results obtained in the liquid phase (Figure 3b). The situation changes dramatically upon thermal treatment. The calcination at progressively higher temperatures of 400 °C, 500 °C and 600 °C leads to a reduction of the UV emission together with a corresponding increase of the emission from defects. In order to better visualize the consequences of the thermal treatment in the visible region of the spectrum, normalization at the maximum was carried out. This normalization is possible because the number of particles is kept constant on the substrate throughout the experiment, as demonstrated by our statistical analysis (Figure 5b). The insert in Figure 5c clearly shows that the visible green emission increases progressively as soon as the polymer is removed by calcination at high temperature. A small shift of the visible emission towards the blue 16 ACS Paragon Plus Environment

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is also observed when the calcination temperature is increased from 400 °C to 500 and 600 °C. When the calcination is carried out at 600 °C, the intensity of the green emission even overcomes that of the UV emission. As already mentioned, the green emission has been attributed by many authors to oxygen vacancies, although in a non-conclusive way.37,38,39,40,41 The emergence of green emission upon polymer removal, taken together with the electron microscopy, ATR-FTIR and

13

C SSNMR

results (Figure 2), suggests that the polymer has a fundamental role in quenching the emission from the defects of the ZnO/PVP mesocrystals.

Figure 5. Photoluminescence spectra of ZnO/PVP mesocrystals before and after calcination: a) SEM image showing a typical coverage of Si/SiO2 substrate with as-synthesized ZnO/PVP particles; b) results of the statistical analysis carried out on films before and after thermal treatment; c) PL spectra of ZnO particles deposited on substrate before and after calcination. The insert shows the same spectra normalized at the maximum; d) PL spectra of commercial bulk ZnO and commercial bulk ZnO functionalized with PVP. Excitation wavelength 330 nm; slits size 5 nm.



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In order to verify this hypothesis, commercial bulk ZnO powder was functionalized with PVP and the PL properties of the corresponding films were analyzed and compared before and after adsorption of PVP (Figure 5 d). The results show that the visible emission of ZnO commercial powder comprises a very broad and intense band centered at 510 nm and a shoulder at 450 nm. Interestingly, it was observed that upon adsorption of PVP on the surface of the commercial powder the intensity of the band at 510 nm was reduced, supporting the model proposed of the interaction of PVP with the surface of ZnO superstructures. It might be questioned whether the loss of emission in the calcined particles is due to the removal of the polymer, or to the coarsening of the internal nanostructures inside the ZnO/PVP mesocrystals. In order to shed light on this aspect, particles without polymer were synthesized and characterized by both electron microscopy and PL spectroscopy. Figure 6a shows that the monodispersity of the sample is lost when the polymer is not employed as templating agent during the synthesis, and that this translates into a broad distribution of particles, from 20 to 100 nm in diameter. Crystalline particles were obtained, as demonstrated by the fast Fourier transform (FFT) in the inset of Figure 6b. However, a lack of internal organization of the primary crystallites inside each hierarchical structure was observed by HRTEM and SAED (Figure 6 b and c). The degree of misalignment can be estimated by the streaking of the diffraction spots in Figure 6c, and it is around 30 degrees and above. The pattern indeed evidences polycrystallinity, with contributions from [001] and [011] or [111] orientations. Being the alignment of the primary crystallites much lower in this case, the obtained structures cannot be classified as mesocrystals.20,21,23


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Figure 6. ZnO particles synthesized under solvothermal conditions without the polymer. a) TEM; b) HRTEM and the fast Fourier transform (FFT) from the highlighted area; c) electron diffraction analysis carried out on the single particle reported in b); d) STEM analysis on an ensemble of particles; e) PL spectra at different excitation wavelength. Size of the excitation and emission slits: 5 nm. A close comparison between the diffraction patterns from the particles with and without polymer is presented in Figure S6 of the Supporting Information. The size of the primary crystallites was around 5-6 nm, thus, relatively smaller in comparison with the ZnO/PVP mesocrystals shown in Figure 2, leading to a nanostructured material. The STEM analysis confirmed the porosity of the as-obtained particles (Figure 6 d). Finally, the particles exhibited very low emission intensity both in the UV and in the visible region up to 540 nm. These evidences further support the role of PVP in boosting the UV emission of ZnO/PVP nanocomposites. The results obtained in this work show that ZnO/PVP nanocomposite mesocrystals show interesting emitting properties, with a remarkable UV emission and negligible 19 ACS Paragon Plus Environment


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emission from defects. In fact, the ZnO/PVP mesocrystals appear to be built by the hierarchical organization of primary building blocks of wurtzite ZnO with diameter between 5 and 10 nm, aligned with each other (Figure 1a and 1c), embedded in the polymer matrix (Figure 2a and 2b) and having remarkable UV emission and no emission in the visible (Figure 3b and 5c). The particles described in the current study can be used to effectively design materials for optoelectronics such as miniaturized light sources, nano-lasers or diodes with suitable properties, i.e. enhanced emission in the UV region and reduced emission from defects. The design strategy can be applied also to other direct semiconductors, providing that they can be grown in a mesocrystalline habit. Finally, the thermal treatment of mesocrystalline particles can be used as an effective approach to synthesize nanoreactors with internal porosity for materials encapsulation.42,43,44 4. Conclusions In the current paper a correlation between the nanocomposite and hierarchical structure of ZnO/PVP mesocrystals and their emitting properties was investigated. Electron microscopy analysis showed that the mesocrystals are made by primary particles with diameters between 5 and 10 nm embedded in the polymer matrix. FTIR spectroscopy suggests that the polymer coordinates onto the surface of ZnO mainly via the oxygen atom and, to a less extent, through the nitrogen of the –N-C=O group. SS 13C NMR confirms that the polymer interacts with the surface of ZnO particles and supports the presence of two differently coordinated lactam groups. The emitting properties were strictly dependent of the presence of the polymer. Upon thermal treatment, a collapse of the hierarchical organization and the formation of internal voids was observed. By combining the results from PL, electron microscopy, ATRFTIR and

13

C-NMR analysis, we conclude that PVP quenches the green emission


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from defects in the ZnO/PVP mesocrystals via the coordination at the surface of the mesocrystals and at the interfaces between the primary building blocks. This model is supported by the data acquired on commercial bulk ZnO as well as on ZnO particles synthesized without polymer and is in line with the evidences from other authors attributing the green emission in ZnO to oxygen vacancies. This study sheds light on the relationship between the structure and the function of nanocomposite mesocrystalline particles and contributes to the development of knowledge based design strategies of nanomaterials. Acknowledgment MD

and WP would

like

to

acknowledge

the funding

of

the Deutsche

Forschungsgemeinschaft (DFG) through the Cluster of Excellence Engineering of Advanced Materials (EAM) at FAU Erlangen Nurnberg. Dr. Mirko Prato is acknowledged for helpful discussion. References (1) Zheng, H.; Li, Y.; Liu, H.; Yin, X.; Li, Y. Construction of Heterostructure Materials toward Functionality. Chem. Soc. Rev. 2011, 40, 4506–4524. (2) Pignatelli, F.; Carzino, R.; Salerno, M.; Scotto, M.; Canale, C.; Distaso, M.; Rizzi, F.; Caputo, G.; Cozzoli, D. P.; Cingolani, R.; Athanassiou, A. Directional Enhancement of Refractive Index and Tunable Wettability of Polymeric Coatings due to Preferential Dispersion of Colloidal TiO2 Nanorods towards their Surface. Thin Solid Films 2010, 518, 4425–4431. (3) Distaso, M.; Klupp Taylor, R. N.; Taccardi, N.; Wasserscheid, P.; Peukert, W. Influence of Counterion on the Synthesis of ZnO Mesocrystals under Solvothermal Conditions. Chem. − Eur. J. 2011, 17, 2923–2930. (4) Distaso, M.; Segets, D.; Wernet, R.; Klupp Taylor, R.; Peukert, W. Tuning the Size and the Optical Properties of ZnO Mesocrystals Synthesized under Solvothermal Conditions Nanoscale 2012, 4, 864– 873.



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Interplay of Internal Structure and Interfaces on the Emitting Properties

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