Nitrogen-Doping in ZnO via Combustion Synthesis? - Chemistry of

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Nitrogen-Doping in ZnO via Combustion Synthesis? Stefan Söllradl, Magnus Greiwe, Vanessa J. Bukas, Magnus R. Buchner, Marc Widenmeyer, Timur Kandemir, Tobias Zweifel, Anatoliy Senyshyn, Sebastian Günther, Tom Nilges, Andreas Tuerler, and Rainer Niewa Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm504200q • Publication Date (Web): 20 May 2015 Downloaded from http://pubs.acs.org on May 26, 2015

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Chemistry of Materials

Nitrogen-Doping in ZnO via Combustion Synthesis? Stefan Söllradl*†1,2,3, Magnus Greiwe4, Vanessa J. Bukas4, Magnus R. Buchner4, Marc Widenmeyer5, Timur Kandemir6, Tobias Zweifel3, Anatoliy Senyshyn3, Sebastian Günther4, Tom Nilges4, Andreas Türler1,2, Rainer Niewa5 1

Paul Scherrer Institute, Laboratory for Radiochemistry and Environmental Chemistry, 5232 Villigen-PSI, Switzerland 2 Universität Bern, Department of Chemistry & Biochemistry, Freiestraße 3, 3012 Bern, Switzerland 3 Heinz Maier-Leibnitz Zentrum (MLZ), Technische Universität München, Lichtenbergstraße 1, 85748 Garching, Germany 4 Technische Universität München, Department Chemie, Lichtenbergstraße 4, 85747 Garching, Germany 5 Universität Stuttgart, Institut für Anorganische Chemie, Pfaffenwaldring 55, 70569 Stuttgart, Germany 6 Fritz-Haber-Institut der Max-Planck-Gesellschaft, Abteilung Anorganische Chemie, Faradayweg 4-6, 14195 Berlin, Germany KEYWORDS ZnO, nitrogen doping, p-type doping, solution combustion method, prompt gamma activation analysis

ABSTRACT: We report the synthesis and characterization of colored ZnO-based powders via solution combustion reaction of urea and zinc nitrate hexahydrate in varying molar ratios between 1 : 1 and 10 : 1. Among other techniques, we employ X-ray diffraction, nuclear magnetic resonance and Raman spectroscopy to characterize the products. Within a narrow range of reactant ratios, we reproducibly find an unidentified, crystalline precursor phase related to isocyanuric acid next to ZnO. Finally, we complement our investigations by performing Prompt Gamma Activation Analysis (PGAA) on selected products in order to directly determine elemental bulk compositions and compare these with X-ray photoelectron spectroscopy (XPS) measurements. Our data show traces of nitrogen mainly on the surface of the particles and thus we question solution combustion method as a reliable synthesis towards N-doped ZnO. Furthermore, we exclude nitrogen as being responsible for the appearance of the four controversially discussed Raman bands superimposed onto the spectrum of pure ZnO (at 275, 510, 582, and 643 cm-1) and show, that the combination of PGAA and XPS is an excellent and complementary method to obtain information about the distribution of the elements in question.

Introduction Zinc oxide is a IIb-VI semiconductor compound that crystallizes preferentially in the hexagonal wurtzite-type structure and is experiencing a veritable research boom1,2,3 in recent years due to its unique properties. It is characterized by a direct wide band gap of approximately 3.4 eV and a high excitation binding energy of ca. 60 meV at ambient temperature3, thus rendering it a transparent, clear and colorless material. Due to these properties, ZnO-based materials find their way in a wide variety of applications exploiting, e.g., the potential to act as catalyst4-6, application as gas sensor7 or as antibacterial material8. In the field of optoelectronics ZnO holds great promises towards applications in the blue/UV range of the light spectrum3,9-11 as alternative to GaN (e.g. for LEDs or laser diodes), as a radiation hard material for electronic devices, as material transparent in the visible part of the light spectrum for the use in electronic circuits, and as a

cheap, transparent, conducting oxide to replace indium tin oxide layers. In particular for its use towards high-end applications, a stable form of p-doped ZnO is essential. As-grown, however, zinc oxide typically exhibits n-type conductivity12 and, depending on the production process, hydrogen can be observed as an impurity acting as a shallow donor when located on interstitial sites12-15. The production of p-type ZnO requires these n-type donors to be overcompensated and shallow acceptor levels to be stabilized instead16. Neighboring group V elements such as nitrogen, phosphorous, and arsenic offer promising prospects of acting as stable acceptors within ZnO and have therefore been the target of considerable recent research17. Among these, nitrogen has probably been the most extensively discussed candidate to act as an acceptor on oxygen sites18 due to its low ionization energy and the similarity of the two species in terms of ionic radii and electronic structure16. However, recent experiments19,20 and refined theoretical models21 report nitro-

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gen to act as a deep acceptor. The suitability of nitrogen to form stable p-ZnO therefore remains an unresolved issue, and establishing a reliable and controllable process for its production lies at the forefront of ongoing research worldwide8,16,22-29. Towards the production of nitrogen-doped ZnO, a number of different synthesis routes have been suggested and tested including molecular beam epitaxy22,23,27,29, reactive sputtering25, chemical vapor 24,25,28, and ammonolysis of ZnO2 at low temperatures16. A rather straightforward yet nevertheless promising approach towards nitrogen incorporation into an oxide is to use an appropriate nitrogen source and promote nitrogen intake to the oxide via the solution combustion method (SCM). Along these lines, Mapa and Gopinath heated aqueous solutions of zinc nitrate and urea and reported the formation of orange-colored ZnO nano-crystals with bulk nitrogen concentrations of up to 15 %8. In the present work, we adopt the process described by Mapa and Gopinath8 and prepare a range of ZnO samples by altering the initial reactant ratios. The products are subsequently characterized by a series of techniques including (powder and single crystal) X-ray diffraction, nuclear magnetic resonance (NMR) and Raman spectroscopy. We complement the investigation by performing Prompt Gamma Activation Analysis (PGAA), a robust and, compared to X-ray photoelectron spectroscopy, much more bulk-sensitive characterization technique which, to the best of our knowledge, has not been applied in the context of ZnO:N before. Selected samples were analyzed in addition to PGAA with X-ray photoelectron spectroscopy (XPS) and the results compared with the PGAA data. We are thus able to simultaneously determine the bulk and surface nitrogen incorporation and relate it to the appearance of characteristic Raman bands, reported in earlier works as an indication for the presence of nitrogen.

Experimental section Synthesis All samples were synthesized via the solution combustion method (SCM), as described by Mapa and Gopinath8. Zinc nitrate hexahydrate (Zn(NO3)2 6 H2O, Chempur 98+%) and urea (CO(NH2)2 Chempur 98+%) of analytical grade were used without further purification. The reactants were mixed in specific molar ratios (their mass sum typically in the range of 15 g) and filled into glass beakers. After dissolving the reactants in 10 ml of deionized water, the beakers were placed in a Nabertherm muffle furnace, which was maintained at 500 °C. Due to the exothermal combustion reaction, extreme care regarding fire hazard should be taken. A video of three different combustion ratios and their reactions with an open furnace are available free of charge in the supplementary material. After the combustion reaction, the beakers were removed from the furnace and left in air at ambient temperature for cooling. Upon reaching room temperature, the products were collected, ground and stored in closed glass tubes. A ZnO reference sample was produced by annealing 15 g of pure zinc nitrate hexahydrate in a glass beaker at

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400 °C in air for 20 minutes. The container was subsequently left to cool in air at ambient temperature, to yield a white/pale yellow powder. In the following, all samples will be denoted according to the employed urea : zinc nitrate hexahydrate molar ratios (i.e. a ratio of 2 : 1 corresponds to two molar parts of urea and one part of zinc nitrate hexahydrate). Based on the described synthesis route, several samples were prepared with reactant ratios ranging from 0.5 : 1 to 10 : 1. The colors of the products ranged from dark red for samples with a molar ratio of 1 : 1, over pale orange/white for samples with ratios in the range of 6 : 1, and towards pale brownish/white for samples prepared with molar ratios in the range of 9 : 1. An overview of the different sample colors is depicted in Fig. S1 of the supplementary material. To extract the additional organic phase detected in certain samples (vide infra), the powders were completely dissolved in 20 % hydrochloric acid maintained at 70 °C. Upon cooling small white crystals were found to form on the surface of the liquid at temperatures of ca. 50 °C. These crystals were subsequently removed by filtration and re-dissolved in 20 % hydrochloric acid at 100 °C. After complete dissolution, the acid was cooled to room temperature over a period of 8 hours. During this cooling process the organic phase crystalized in form of white crystals at the surface of the acid. Once ambient temperature was reached, these crystals were collected and analyzed.

Powder X-ray diffraction X-ray powder diffraction data were collected on a STOE StadiP diffractometer, using Cu Kα1 radiation (λ = 1.54051 Å) and a flat-bed sample holder in transmission setup. The patterns were recorded with a 130° image plate detector at room temperature and ambient pressure. Samples prepared with molar ratios of 1 : 1 and 2.3 : 1 were additionally measured at the high-resolution powder diffractometer P02 at the German electron synchrotron DESY. These samples were placed on a quartz glass capillary with a diameter of 0.3 mm and exposed to radiation of λ = 0.53845 Å at ambient pressure and room temperature. Single crystal X-ray diffraction Single crystal diffraction measurements were performed on a IPDS 2T STOE diffractometer using Mo Kα radiation (λ = 0.71073 Å) at ambient temperature. The program suite Jana 2006 was used for the refinement and structure solution. The acquired data and resulting structural parameters may be found in the supplementary material (Tables S1 and S2). Nuclear magnetic resonance spectroscopy 1 H NMR (400 MHz), 13C NMR (100 MHz), and 15N NMR (41 MHz) measurements were performed on a Bruker Avance III 400 spectrometer. The chemical shifts of 1H and 13C NMR are given relative to the solvent signal for dmso-d6 (2.50 ppm and 39.5 ppm)29,30, while


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90 % CH3NO2 in CDCl3 was used as an external standard for the measurement of 15N NMR.

Raman Spectroscopy Raman spectra were recorded on a Jobin Yvon – Horiba HR800 UV Raman microscope with an attached Jobin Yvon Symphony CCD detector of 1,024 x 359 pixel2. A 633-nm laser beam was used for excitation, which was dispersed by a grating of 600 lines/mm. The fine powder samples were pressed by hand on a glass flat-bed carrier. Spectra in the range of 150 cm-1 to 1,300 cm-1 were collected in three measurement cycles and with an accumulation time of 15 s per scan. X-ray Photoelectron Spectroscopy X-ray photoelectron spectra were recorded for the quantification of surface nitrogen concentrations using a Leybold Heraeus LHS-10, which was operated in constant-pass energy mode32. As excitation source nonmonochromatic Al Kα radiation (λ = 1,486.6 eV) was used. The binding energy was normalized on the Zn 2p3/2 peak with a BE of 1,021.6 eV. Scanning Electron Microscopy and Energy Dispersive X-Ray Spectroscopy Scanning Electron Microscopy (SEM) and Energydispersive X-Ray Spectroscopy (EDX) measurements were performed on a Zeiss EVO microscope equipped with a tungsten cathode and an ultra-dry EDX detector. The sample powders were dispersed on a sticky carbon pad. To improve the image quality, a thin layer of Ag was deposited on the sample surface. Prompt Gamma Activation Analysis Prompt Gamma Activation Analysis (PGAA) was performed at the FRM II in Garching. Powder samples with masses of ca. 200 mg were sealed in Teflon bags and irradiated in an evacuated chamber at 0.3 mbar in a cold neutron beam with a thermal equivalent flux of 4x1010 cm-2 s-1. The standard PGAA setup33 was modified by inserting a 10 mm lead attenuator in front of the high-purity germanium (HPGe) detector as described in previous work34. The latter was found necessary in order to allow the use of a higher neutron flux and therefore higher count rates to reduce the statistical uncertainties in the high-energy range of the spectrum and thus improve the detection limit for nitrogen. A 60 % Ortec (ntype) PopTop HPGe detector was used, surrounded by a bismuth germanate (BGO) Compton suppressor and attached to an Ortec DSpec-50 digital spectrometer. The gamma spectra were acquired with Ortec Maestro-3235 software and evaluated using Hypermet-PC36. It is noted at this point that the detection limit of a specific element existing within a matrix of another depends on three primary factors: the element’s neutron-capture cross section, its molar concentration in the sample matrix, and the efficiency of the detection system. For determining the elemental composition within the ZnO matrix, PGAA was directly applied to quantitatively eval-

uate zinc (characterized by a neutron-capture cross section of 1.30 b37), hydrogen (332.6 mb37) and nitrogen (79.5 mb37) species. In contrast, the detection of oxygen (0.19 mb37) and carbon (3.51 mb37) proved not feasible due to their low neutron-capture cross section. The bulk concentrations of zinc, hydrogen, and nitrogen were determined based on selected, elementspecific gamma lines37: For zinc all characteristic gamma lines above 5,000 keV were taken into account, while hydrogen is known to provide only a single gamma line at 2,223 keV37. The presence of nitrogen was examined by checking for its characteristic line at 10,829 keV37 in the normally background-free region of the PGAA spectrum. If detected, and depending on the sample's nitrogen concentration, other weaker gamma lines were additionally taken into account and fitted.

Results & Discussion A number of different samples were prepared via the SCM by varying the ratio of urea to zinc nitrate. The ratio of reactants was found to have a pronounced effect on the combustion reaction that takes place upon heating: For reactant ratios of 4 : 1 and less, the first steps of the process involve water starting to evaporate, boil and subsequently a change in color to dark orange-brown. The dark color can be attributed to the presence of nitrogen oxides that evolve from zinc nitrate. Once the solution is saturated, the emission of brownish nitrogen oxides starts and is followed by an intense flame striking out of the beaker. In particular for low relative amounts of urea (between 0.5 : 1 and 3 : 1), the flame is extremely bright and of a white-yellowish color as depicted in Fig. 1. With increasing ratios of urea however, the combustion becomes less intense and the emission of nitrous gases decreases. For reactant ratios of 5 : 1 and higher, black carbon is observed on the upper part of the beaker indicating the deficiency of oxygen for a complete combustion of urea. For ratios higher than 6 : 1, white smoky gases are emitted from the solution due to the decomposition of urea which takes place after the reactants in the solution have been saturated. This starts with the formation of bubbles and foam, which solidifies over time to form an amorphous material. A short movie of the reaction with different molar preparation ratios (1:1, 6:1, 9:1) in an open furnace can be viewed in the supplementary material.



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ducibility of the results exclude the possibility of sample contamination, but clearly suggest the formation of an additional, crystalline phase within the aforementioned samples. An identification of this phase, based on the PDF-2 database38 was not possible and a structure solution based on the obtained powder data (neutron and Xray) not successful.

Figure 1. Combustion reaction of a sample with a molar ratio of 1 : 1 of urea and Zn(NO3)2 6 H2O at 500 °C.

The powder XRD pattern of all prepared samples are depicted in Fig. 2, along with that of the pure ZnO reference (black line, bottom). A clear decrease in the intensity of the diffraction features is observed as a function of the relative amount of urea used in synthesizing the samples. This is associated with the corresponding increase in the amount of urea’s decomposition products and therefore, decreases in the absolute mass of the crystalline products of interest. The main diffraction features of the as-synthesized powders can be indexed to hexagonal wurtzite-type ZnO. The unit cell dimensions of the products are essentially unaffected by the varying synthesis conditions within error of determination (see below).

Figure 2. Powder X-ray diffraction patterns of ZnO-samples prepared with molar ratios of urea to zinc nitrate from 1 : 1 to 10 : 1.

A number of additional reflections however are observed in the low angle region (< 40° in two-theta) for samples prepared with a reactant molar ratio of ca. 2 : 1. These appear consistently within a narrow range of employed reactant ratios (ca. 2 : 1 – 2.6 : 1) and are most intense for the 2.3 : 1 sample, as indicated by the powder XRD patterns of Fig. 3. The appearance of additional reflections in consecutive products along with the repro-

Figure 3. Powder X-ray diffraction of samples prepared with molar ratios of 1.6 : 1 to 3 : 1 of urea and Zn(NO3)2 6 H2O showing the appearance of an additional phase. Main reflections of the additional phase are indicated with arrows.

After the complete dissolution of a 2.3 : 1 powder sample (including the additional, unidentified phase) in hydrochloric acid, followed by the recrystallization of the organic species, responsible for the additional reflections, at the surface of the acid upon cooling, crystals of isocyanuric acid were isolated and identified via single crystal XRD. The obtained crystal structure data agrees well with values reported by Verschoor et al.39 and Coppens et al.40. Further details on crystallographic data and atomic coordinates can be found in the supplementary material (Tab. S1 and Tab. S2). The presence of the precursor phase, forming crystals of isocyanuric acid after a complete dissolution of the sample was additionally confirmed by liquid state NMR. The measured chemical shifts for 1H, 13C, and 15N are listed in Tab. S3 in the supplementary material and show perfect agreement with values reported for isocyanuric acid by Yamada et al.41 using liquid state NMR and by Damodaran et al.42 with solid state NMR. The NMR spectra obtained within this work can be found in the supplementary material as well (Figs. S2 – S4). The identification of isocyanuric acid strongly suggests that the additional crystalline phase, coexisting within the 2.3 : 1 powder sample forms within a narrow range of reactant ratios and is a crystalline precursor for the later formation of isocyanuric acid in HCl. Isocyanuric acid is a main decomposition product during the pyrolysis of urea.43,44 A good performance of urea as NH3 source was also observed45 via a selective catalytic reduction, however, in this case an additional CuZSM5 catalyst was required to maximize the yield of NH3 and reduce HNCO successfully. A thermal decomposition of urea without catalyst in contrast, yielded always in residual HNCO species in the reaction products.


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With that reason, the observed precursor phase indicates an incomplete combustion during the synthesis of samples with a molar preparation range of 1.6 : 1 to 3 : 1 in our experiments. Based on the above, the focus is directed towards samples with a high crystalline character, prepared with low relative amounts of urea and in particular those with 1 : 1 and 2.3 : 1. While interest in the 2.3 : 1 sample lies in the presence of a crystalline precursor phase related to isocyanuric acid, the motivation for investigating the 1 : 1 sample derives from the report by Mapa and Gopinath to expect a high nitrogen bulk concentration for a similarly prepared sample8. Tab.1 lists the lattice parameters (α and c) as determined by Rietveld analysis: for the 1 : 1 and 2.3 : 1 samples in the present work, for the aforementioned 1 : 1 sample reported by Mapa and Gopinath and named ZU18, for ZU1 after annealing at 950 °C (ZU-950)8 and for two literature values of pure ZnO17,46. Mapa and Gopinath attributed the overall lattice contraction observed for ZU1 to a nitrogen incorporation of 15 at.%. Upon high-temperature annealing, the authors suggest nitrogen release, as the lattice parameters reflect those of pure ZnO and the sample color changes from orange to white. In the present work however, despite the similar orange coloration of the samples no deviation of the unit cell parameters from those expected for pure ZnO is observed for the 1 : 1 or the 2.3 : 1 product.

nal at 384, 414, 441, and 586 cm -1)47 the synthesized powder samples give rise to four additional Raman bands at 275, 510, 582, and 643 cm-1 (marked by stars in the figure). These bands have in fact been detected in numerous past studies. However, no unambiguous interpretation regarding their origin was presented and, thus, they are a matter of a long-lasting controversy. These bands have repeatedly been assigned to local vibrational modes of nitrogen8,28,48-50 and used as evidence to support nitrogen incorporation into the ZnO lattice8. Raman bands at these wave numbers however have also been reported when intentionally excluding nitrogen species from the growth process and doping ZnO thin films with different elements (such as Fe, Sb, Ga, or Al)51,52. Accordingly, in their relevant overview Manjón et al.53 attributed the presence of these bands to disorder-activated Raman scattering, which occurs due to induced defects and the consequent breaking of symmetry. A feature, to the best of our knowledge so far unreported, is the appearance of yet two additional Raman bands (both marked by crosses in Fig. 4) in the spectra of the 2 : 1, 2.3 : 1, and 2.6 : 1 samples with wavenumber values of 748 cm-1 (accompanied by a weaker band at ca. 735 cm-1) and 1043 cm-1 (accompanied by a broad shoulder in a slightly higher frequency region). These bands could not be directly related to existing data54,55 for ZnO nor for substances related to isocyanuric acid.

Table 1. Lattice parameters of samples 1 : 1 and 2.3 : 1 at 298 K and literature values for comparison. a

c

Sample 1:1

3.2509(1) Å

5.2075(5) Å

Sample 1:1 DESY

3.24978(3) Å

5.20583(5) Å

ZnO ref, DESY

3.24908(3) Å

5.20552(5) Å

Sample 2.3:1

3.2510(1) Å

5.2076(2) Å

ZU18

3.2323 Å

5.1784 Å

ZU-9508

3.2511 Å

5.2007 Å

ZnO48

3.2501(1) Å

5.2071(1) Å

ZnO17

3.249 Å

5.206 Å

The Raman spectra of samples prepared with reactant ratios between 1 : 1 and 5 : 1 along with that of pure ZnO (black line, bottom) are depicted in Fig. 4. It is noted that strong fluorescence was observed in the spectra of samples prepared with higher initial urea concentrations, due to the contribution of the organic decomposition products. Therefore, these spectra are not shown. It is clear from Fig. 4 that while retaining the spectral signature of ZnO (through bands of the 1st order Raman sig-

Figure 4. Raman spectra of samples 1 : 1 to 5 : 1 and pure ZnO. Bands, in literature assigned to nitrogen containing zinc oxide are marked with stars. Crosses mark the bands appearing with the additional phase. The spectrum of ZnO is multiplied by a factor of 4 for better comparison.



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Figure 5. SEM images of the investigated samples with PGAA and XPS. Images a) and b) show the hexagonal ZnO pyramids of sample 1 : 1 (1). Images c) and d) illustrate the different sized pyramids of sample 1 : 1 (2).The image e) and f) show the morphology of sample 2.3 : 1.

Figures 5a-d show the morphology of the synthesized sample powders synthesized with a molar ratio of 1 : 1. The dominating particle shape is a hexagonal pyramid of ZnO varying in size between 400 nm and 1.5 µm. In contrast to those, the images of Fig. 5e and 5f illustrate the shape of the sample, synthesized with a molar reaction ratio of 2.3 : 1. In this sample, small hexagonal pyramid shaped particles are agglomerated together with flake like particles. Even due to the fact, that a second phase was found in PXRD and elemental analysis, the organic phase could not be identified during the performed SEM investigation visually. An EDX spectrum of each sample (1 : 1 (1), 1 : 1 (2), 2.3 : 1), recorded in a representative region of agglomerated crystals is supplied in the supplementary material (Fig. S5) but was not used for quantitative conclusions due to the low sensitivity of EDX for light elements in contrast to the other methods used.

reported in the guideline for the evaluation of PGAA data56. Samples

Hydrogen

Nitrogen

Zinc

1 : 1 (1)

0.63% ± 0.04

0.39% ± 0.09

98.9% ± 0.1

1 : 1 (2)

0.51% ± 0.06

< 0.09%

99.5% ± 0.1

2.3 : 1

10.3% ± 0.4

11.0% ± 0.5

78.7% ± 0.9

Based on the peak centered around 2,223 keV (Fig. 6), both 1 : 1 samples were found to contain more than 0.5 at.% of hydrogen. Whether this remains adsorbed on the samples' surface in the form of moisture or is capable of penetrating the surface and stabilizing itself within the host lattice, remains unknown. The possible incorporation of such hydrogen species in the ZnO bulk may in fact be the source of the sample colors, as proposed by Weber et al.12.

The bulk concentrations of zinc, hydrogen, and nitrogen were determined by PGAA measurements for samples with reactant ratios of 1 : 1 and 2.3 : 1. To achieve optimal conditions for the low detection limit of nitrogen, two 1 : 1 samples were irradiated for different durations (21,800 and 11,808 seconds for 1 : 1 (1) and 1 : 1 (2), respectively) and the results are summarized in Tab. 2. Table 2: Results of the PGAA measurements of various samples in atomic percent. Sample 1 : 1 (1) was irradiated for 21,800 s, sample 1 : 1 (2) for 11,808 s, and sample 2.3 : 1 for 7,200 s. The stated confidence interval represents the combined 1s interval based on the statistical (peak area) uncertainty and systematic uncertainties (nuclear data, efficiency) as


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Figure 6. Enlarged region from the prompt gamma spectra around the hydrogen peak from 2,200 to 2,300 keV normalized to count rates. BKG indicates the data of a background spectrum for comparison.

The difference in the height of the zinc peaks centered around 2,110 keV as well as in the baseline of each spectrum can be attributed to the different sample masses. In contrast, the spectral baseline is much weaker in the energy region from 9,000 keV to 11,000 keV, where the characteristic nitrogen peak is located (depicted in Fig. 7). In particular, for samples prepared with molar ratios of 1 : 1, the N-related peak areas could not be fitted using the peak fitting algorithm of HYPERMETPC36 due to the low number of detected nitrogen counts. The detected counts were therefore manually integrated and their statistical uncertainties estimated based on the sum of the peak counts. This explains the high uncertainty for the determined nitrogen concentration in sample 1 : 1 (1), while the detection limit for both measurements with respect to the spectral background equals 0.09 at.%. In any case, the PGAA data indicate very low amounts of nitrogen existing within samples prepared by reactant ratios of 1 : 1.

nitrogen content was found in sample 1 : 1 (1). The N 1s intensity increase is well in agreement with a raised carbon level, both approximately 1 at.%, and the detected binding energy of carbon is in the energy range of the found precursor phase in sample 2.3 : 1. Thus, we attribute the simultaneous increase of the elements carbon and nitrogen in the XPS measurements in contrast to the appearance of nitrogen and hydrogen in the corresponding PGAA measurements to residual decomposition products on the sample surface. Due to the fact, that the penetration depth of XPS is a few nanometers, the obtained elemental concentrations with XPS correspond to their distribution within that penetration depth. In contrast, with PGAA the average bulk signals correspond to the total composition of the sample. Since oxygen cannot be quantified, zinc is the dominating element. As the nitrogen to hydrogen ratio fits well to the later identified decomposition product and lower elemental concentrations of both elements were detected with PGAA within the bulk material, compared to the surface concentrations respectively, we assign these observations to traces of the decomposition product remaining on the surface of that specific sample. Table 3: Results of the XPS measurements in atomic percent for two samples prepared with a molar ratio of 1 : 1 and one with a ratio of 2.3 : 1. The uncertainties represent the 1s interval based on counting statistics only. Samples

Zinc

Oxygen

Nitrogen

Carbon

1.09±0.12

3.56±0.07

1 : 1 (1)

50.30±0.07 45.05±0.17

1 : 1 (2)

49.95±0.06 47.56±0.13 not detected

2.49±0.06

2.3 : 1

48.49±0.05 31.54±0.10

10.63±0.11

9.34±0.08

Figure 7. Comparison of PGAA spectra of different samples in an enlarged region around the characteristic nitrogen peak at 10,829 keV.

In case of the 2.3 : 1 sample, the PGAA measurements indicate hydrogen and nitrogen concentrations of 10.3 at.% and 11.0 at.%, respectively.in the ZnO sample. Within the given error margins, these values agree with the molar ratio of those elements with prospect to the later formation of isocyanuric acid from the precursor phase in HCl. Complementary to the bulk analysis performed with PGAA, surface concentrations of nitrogen, carbon, oxygen, and zinc were determined by XPS. The recorded XPS spectra are shown in Fig. 8 and the derived elemental distribution of the samples are listed in Table 3. For sample 1 : 1 (2) no significant increase of nitrogen was found, supporting the results of the PGAA measurements. In contrast to that, an slight increase of the

Figure 8. XPS spectra for the a) Zn 2p3/2 level and b) N 1s level (if present) for the three samples investigated with PGAA.

Conclusions Motivated by reports regarding the formation of stable ZnO with a high concentration of nitrogen doping of up to 15 at.%8, we adopt the similar combustion synthesis process to produce a variety of ZnO-based powder sam-



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ples, including preparation ratios not reported8 yet. The synthesis was found to be highly dependent on the initial molar ratio of the reactants, namely urea : zinc nitrate hexahydrate, in terms of generated heat, gas formation and the color of the collected products, which ranged from dark red to white when increasing the fraction of urea. Powder X-ray diffraction showed no change whatsoever in the dominant ZnO-wurtzite structure for samples prepared with a molar ratio of 1 : 1 to 8 : 1, a fact, which was confirmed by subsequent Rietveld refinements on selected data. Although of a similar orange color, this result disagrees with the earlier observation8 of a minor lattice contraction for similarly prepared 1 : 1 products, denoted to ZnO0.85N0.15. However, our 1 : 1 samples show identical Raman bands as reported8, including the four Raman bands at 275 cm-1, 510 cm-1, 582 cm-1, and 643 cm-1 superimposed onto the spectrum of pure ZnO. The quantification of nitrogen by Prompt Gamma Activation Analysis (PGAA) proved to be below or close to the detection limits, estimated at 0.09 at.%. The additionally performed XPS measurements confirm these data and the lack of nitrogen in 1 : 1 samples. An identification of nitrogen was possible in both samples, however, the attribution to the decomposition products of urea became only possible by a combination of XPS and PGAA. We have shown that the combination of the surface (XPS) and bulk sensitive (PGAA) technique offers a great and versatile potential for the characterization of the origin of different elements of interest. Nevertheless, the performed measurements and their interpretation, especially on the example of the intentionally chosen sample 1 : 1 (1) has also shown, how difficult a reliable interpretation of the localization of the investigated elements is, based on just one of those methods. We therefore refute the significant and stable introduction of nitrogen in the ZnO lattice via the solution combustion method as previously suggested and, based on our results, exclude it from giving rise to the additional Raman bands discussed above. Furthermore, we rule out the association of nitrogen uptake with the sample's orange color. Nevertheless, catalytic activity of ZnO materials, where the four discussed Raman bands were detected, was reported4,7,8 and a detailed knowledge of their origin may also contribute to a better understanding of the catalytic properties of such a material. Finally, within a narrow range of employed reactant ratios an additional co-existing organic phase was detected, which was found to be reproducible and most dominant in 2.3 : 1 samples. Dissolving the complete powder sample (ZnO powder including the observed organic phase) in hot hydrochloric acid and performing single crystal XRD on the grown and extracted crystals achieved the identification. The presence of isocyanuric acid formed from the crystalline precursor phase present in samples of a preparation ratio of 2.3 : 1 was additionally confirmed by liquid state NMR and is in perfect agreement with the PGAA data, which shows significant amounts of both nitrogen and hydrogen.

Corresponding Author * Telephone: +49 (0) 89 289 14768, Fax: +49 (0) 89 289 14911, Email: [email protected]

Present Addresses † Heinz Maier-Leibnitz Zentrum (MLZ), Technische Universität München, Lichtenbergstr. 1, 85748 Garching, Germany

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors would like to thank the Deutsche Forschungsgemeinschaft, the state of Bavaria and the Technische Universität München for the funding of a X-ray powder diffractometer. In addition, the authors would like to thank Dr. Dieter Rau for the videos of the different reactions, Stephanie Ferstl for the support during the synthesis, Fabian Bodensteiner for the support with operating the XPS machine as well as the Elitenetzwerk Bayern, which granted access to all different characterization methods.

ASSOCIATED CONTENT Tables showing the crystallographic data of isocyanuric acid, an overview of the different colors of the samples from 1 : 1 to 10 : 1, the NMR spectra of isocyanuric acid, the EDX spectra, and the XPS spectra of oxygen of the presented data. In addition a movie, showing the combustion reaction in an open furnace with molar preparation ratios of 1 : 1, 3 : 1, 6 : 1, and 9 : 1 is available. This material is available free of charge via the Internet at http://pubs.acs.org.

ABBREVIATIONS PGAA, Prompt Gamma Activation Analysis; SS-NMR, Solid state nuclear magnetic resonance spectroscopy; LSNMR, Liquid state nuclear magnetic resonance spectroscopy; SCM, Solution combustion method; XRD, X-ray diffraction; CCD, Charge Coupled Device; XPS, X-ray photoelectron spectroscopy; SEM, Scanning Electron Microscopy; EDX, Energy-dispersive X-Ray Spectroscopy.

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