NANOCRYSTALLINE ZINC OXIDE POWDER FOR X-RAY DIFFRACTION METROLOGY

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1 61 NANOCRYSTALLINE ZINC OXIDE POWDER FOR X-RAY DIFFRACTION METROLOGY D. Black, J.J. Ritter, J. E. Bonevich, A. Henins and J.P. Cline National Institute of Standards and Technology, Gaithersburg, MD ABSTRACT Nano scale zinc oxide powder has been produced using a precise thermal decomposition process from a zinc oxalate precursor powder. The size of the crystallites is determined by the specifics of the thermal processing which were chosen to yield crystallites in two size ranges, one with a distribution centered at approximately 15 nm and another centered at 70 nm. The resulting material is composed of essentially strain-free crystallites that are aggregated to a certain extent, but can nonetheless be examined with a TEM to augment X-ray diffraction analyses. The X-ray analysis shows domains to be in the form of discs of a fairly small aspect ratio, containing stacking faults in the 15 nm powder. This material will be used for a National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) 1979, a crystallite size standard. INTRODUCTION The ability to analyze peak broadening as a way to characterize the crystallite size of a wide range of complex nano scale materials is one of the many attributes of modern x-ray diffraction analysis methods. Successful realization of accurate size determination is dependent on an accurate characterization of the instrument broadening, the instrument profile function (IPF), as well as an understanding of the capabilities and limitations of the method by which the specimen function is analyzed. One way to assess performance of a complete measurement method is to acquire and analyze data from a standard sample with known properties. Toward this end NIST is developing SRM 1979, a crystallite size standard. It is based on a nano scale powder where the crystallites are single diffracting domains and to the largest extent possible, the particles are single grains. The artifact will be composed of two separate powders. One, with a size distribution centered at about 15 nm, the other with a size distribution centered at about 70 nm. A satisfactory material for a crystallite size standard for x-ray diffraction must satisfy a variety of conditions. The desired characteristics include: 1) environmentally stable and non-toxic, 2) exhibiting a strong diffraction signal with non-overlapped lines, 3) consist of crystallites of a specific, controllable size range with a narrow size distribution, 4) embody a minimum of crystallographic defects, 5) be dispersible for analysis by complimentary techniques such as transmission electron microscopy (TEM) and 6) be producible in kilogram quantities for SRM production. Based on an extensive body of earlier work by Louër and Langford, see for example: Louër et al. (1983), Langford et al. (1993) and Guillou et al. (1995), CeO 2 and ZnO were identified as candidate materials. In order to prepare bulk quantities of materials prepared by the precipitation method, the services of a chemical manufacturer were secured with the understanding that the vendor set up and operate a fixed element flow reactor. This device was pioneered by one of the authors, J.J Ritter,

2 This document was presented at the Denver X-ray Conference (DXC) on Applications of X-ray Analysis. Sponsored by the International Centre for Diffraction Data (ICDD). This document is provided by ICDD in cooperation with the authors and presenters of the DXC for the express purpose of educating the scientific community. All copyrights for the document are retained by ICDD. Usage is restricted for the purposes of education and scientific research. DXC Website ICDD Website -

3 62 now retired from NIST, and consists of a tube with stationary vanes mounted to the tube's interior. These vanes serve to separate and then recombine a fluid passing through the device so that each volume element sees an identical mixing history. Small samples of 20 to 50 grams of the precursor compounds were successfully prepared using this method. These precursor materials were then decomposed to the oxide in a series of experiments to yield 3-5 g quantities of material using a vacuum tube-furnace. The goals were twofold: to duplicate the earlier published results of Louër and Langford and to evaluate the resulting material for its applicability for further development into a standard. It was found that the preparations of CeO 2 yielded unacceptable material: While the diffraction data were promising, the precipitate gelled and upon decomposition yielded large polycrystalline shards of several hundred micrometers in size. The decomposition of an acetate precursor of ZnO however, yielded desirable material. Unfortunately, management changes at the chemical firm led them to lose interest in the project. The decomposition of a commercially available oxalate precursor of ZnO however, yielded acceptable material and duplicated the results of Langford, et al., (1993). The commercial availability of the necessary quantities of the oxalate precursor, coupled with the lack of any immediate commercial means to obtain the acetate precursor, resulted in the adoption of the exoxalate ZnO as being the material of choice for further development for SRM These laboratory scale experiments not only confirmed the results of the earlier work but also established that we could control particle size by appropriate selection of the processing parameters. EXPERIMENTAL The next step in the development of this SRM was to establish the capability to thermally decompose the kilogram quantities of precursor needed for SRM production. This was accomplished by commissioning a large scale vacuum furnace capable of processing material in 125g lots. The furnace itself started out life as a vacuum oven with an internal volume of approximately a cubic foot. Two conventional resistance heating elements, of approximately a square foot in dimension were installed in a parallel configuration approximately an inch apart. Into this gap would reside the boats which contained the precursor to be decomposed. This configuration allowed for uniformity in temperature throughout the powder beds during heating. Temperature control was accomplished with a conventional PID controller. Two Type K thermocouples were installed, one for the temperature controller and a second to permit continuous monitoring of the temperature profile. Vacuum was provided with a roughing pump; 1 inch diameter vacuum lines, including a cold trap, provided for improved conductance to reach a vacuum of 20 Pa (0.15 torr). The furnace is illustrated in Figure 1. Again a series of experiments was performed to duplicate the earlier work using this larger furnace and to establish the specific relationship between particle size and processing parameters to establish our production protocol. Our method used a thermal decomposition performed in vacuum followed by a calcination performed in an air furnace. The precursor zinc oxalate powder, % pure (metals basis), was obtained from Alfa Aesar 1 (Ward Hill, MA). As 1 Certain commercial equipment, instruments, or materials are identified in this report in order to specify the experimental procedure adequately. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the materials or equipment identified are necessarily the best available for the purpose.

4 63 Figure 1. Image of large scale vacuum furnace. expected, these experiments confirmed that the average size and size distribution could be well controlled by adjusting the final temperature of the processing. The production of the ZnO powder was performed in two steps. First, all material was heated in the vacuum furnace, rapidly from room temperature to 70 C, then from 70 C to 110 C at a rate of 2 C/hr followed by another rapid increase to 250 C and then up to 400 C at 2 C/hr and finally a slow return to room temperature. Although a final quench from the high temperature was preferable, this was not possible because of the need to maintain the material in a vacuum while at elevated temperature. This thermal cycle, as measured by the second thermocouple within the furnace, is shown in Figure 2. Next, the material was divided into two approximately equal lots; one for subsequent processing into 15 nm material and the other for processing into the 70 nm material. The material was placed in an air furnace which was rapidly heated to a temperature of 350 C and for the 15 nm particle size, heated at a rate of 2 C/hr to a final temperature of 400 C, while for the 70 nm particle size the same ramp was used but a final temperature of 550 C was needed. To reduce unwanted grain growth, the samples were immediately removed from the furnace once the final temperature was reached; there was no soak at this temperature. Two methods were employed to analyze the X-ray diffraction data (XRD). The first was a Williamson - Hall (WH) analyses used primarily on the small test samples that were decomposed in the tube furnace. The second was Whole Powder Pattern Modeling (WPPM) via PM2K V1.65 (Leoni et al. 2006); this method was used to analyze samples generated with the large scale vacuum furnace. XRD data used for the WH and the early WPPM analyses were collected on a Siemens D500 diffractometer equipped with a focusing Ge incident beam monochromator, sample spinner and a scintillation detector. Copper Ká 1 radiation, ë = nm, was used. The divergence slit was 0.67 while the receiving optics included a slit of 0.05 and 2 Soller slits. The IPF was determined using a split Pearson VII profile shape function (PSF) fit to 24 reflections collected from SRM 660a, LaB 6 (NIST, 2000). Data were collected in discrete

5 64 Figure 2. The temperature profile of the initial vacuum processing. regions straddling the maxima of each profile with the step and scan width of each region being varied in correspondence with the FWHM. Count times were varied so as to obtain an approximately constant total number of counts for each scan region. Data from the ZnO were collected in continuous scans from 25 to 125 2è with a step width of 0.02 and a count time of 16 s to yield a total scan time of approximately 24 hours. The specimen broadening function was deconvoluted from observation using TOPAS (Bruker AXS). The IPF was synthesized at the ZnO peak locations by using polynomial fits to the FWHM data and Pearson VII coefficients determined from the profile fitting of SRM 660a. A symmetric Pearson VII PSF was used to fit the specimen induced broadening. The XRD data that were to be used for certification of SRM 1979 were collected on a NIST built diffractometer of similar configuration to the aforementioned Siemens D500. This machine, however, utilizes a highly stiff and fully encoded goniometer assembly for improved accuracy in angle measurement. The Johansson optic is also improved over that of the D500. This improvement allowed for a credible modeling of the emission spectrum using a convolution of Gaussian functions; this, in turn, allows for the data from this machine to be analyzed using the fundamental parameters approach (FPA) (Cheary & Coelho, 1992). The IPF was determined in a manner analogous to that aforementioned, except that SRM 660b (NIST 2010) was used. Data from the ZnO samples were also collected in an analogous manner, except that the step width used for the 15 nm material was increased to 0.25 with a count time of 20 s. An example of these data, with the illustrated IPF profiles being synthesized via the FPA, is shown in Figure 3. With the use of PM2K, the IPF was modeled using the Pearson VII PSF with the Caglioti function (Caglioti et al., 1958) being used to model the FWHM dependence on two-theta and the Finger (Finger et al., 1994) model was used to account for profile asymmetry. The specimen broadening function used was specific to modeling of cylinders with a crystallite size distribution presumed to be log-normal. The Warren (Warren, 1969) model for hexagonal stacking faults was used.

6 65 Figure 3. Three profiles from the ZnO with the IPF. DISCUSSION An initial evaluation of the material was performed using a WH plot. One example from a processing run to a temperature of 400 C is shown in Figure 4. As expected, see for example Auffrédic et al. (1995), the peaks could be segregated into three groups; group 1 containing hk0 peaks and peaks with h k = 3n, group 2 with h k = 3n 1, l odd, and finally group three with h k = 3n 1, l even. This is indicative of the broadening that can occur in hcp materials due to Figure 4. A typical Williamson-Hall plot of material processed to 400 C.

7 66 stacking faults. The peaks in group 1 are unaffected by stacking faults and the slope of a linear fit to these peaks is about 5x10-5, indicating negligible strain, and the intercept of the fit is , giving a particle size of about 80 nm. While the Williamson-Hall plot was sufficient to establish the validity of the processing approach, a quantitative analysis method was needed to extract a more accurate particle size so as to establish the final processing parameters. Following the discussion in Langford et al. (1993), we initially employed their integral breadth analysis method. This method assumed that all peak broadening was due to particle size effects, i.e. no strain broadening, and that the hexagonal ZnO crystallites are well represented by a cylinder. Equations 10 and 17 from Langford and Louër (1982) were then fit to extract the cylinder diameter (D) and height (H). The result of this analysis as applied to our investigation of the effect of final processing temperature on particle size is shown in Figure 5. These results also indicate that the crystallites are more disc-like than rod-like with an average aspect ratio D/H of 1.1 for the lower temperature material and 1.25 for the high temperature material, consistent with the results from Auffrédic et al. (1995). Figure 5. The relationship between the particle size, as expressed as the cylinder diameter and height, and the final processing temperature is shown. This method was sufficient to evaluate the effects of processing by providing a statistical average particle size, but did not characterize the size distribution. To better quantify the particle size distribution we used the analysis package PM2K. This approach assumes a log-normal size distribution and uses whole powder pattern modeling in Fourier space to best fit the data. The temperature dependence on the growth or twin stacking fault parameter â is shown in Figure 6, illustrating that the 70 nm material contains only a very small level of defects. It was observed that the deformation stacking parameter á also dropped off with annealing temperature, but was very small in any case. This indicates that the primary origin of the defects is the growth or twin faults. The size distributions determined from the 20 data sets collected for the certification are shown in Figures 7 and 8. Averaging the median values obtained for the 20 specimens gave

8 67 sizes of 16.0 nm and 69.2 nm. One observes that the range of distributions obtained for the finer material is much narrower than for the coarse material. This is indicative of the increased difficulty of the analysis as the crystallites coarsen and the level of broadening approaches the IPF, this is noted in Figure 3 wherein the 70 nm are surprisingly close to the IPF. Figure 6. Variation in the stacking fault parameter â with annealing temperature Figure 7. The cumulative size distributions obtained from the 20 data sets collected from the 15 nm material.

9 68 The 50% points in the cumulative distributions are at about 16 nm and 70 nm. As a complementary evaluation of the size and shape of the particles, transmission electron microscopy was used. Typical TEM images are shown in Figure 9. As can be seen, the particles are prismatic as expected and, where the electron beam could be isolated to a single particle, the diffraction pattern is clearly that from a single-crystal. The smaller material remains aggregated while the larger material is somewhat irregular in shape. Figure 8. The cumulative size distributions obtained from the 20 data sets collected from the 70 nm material. Figure 9. TEM images of 15 nm material on the left and 70 nm material on the right. The micron marker in both images designates 100 nm.

10 69 CONCLUSIONS Nano scale particles of zinc oxide have been produced by the thermal decomposition of a zinc oxalate precursor. The particle size is well controlled by proper selection of the processing parameters and the resulting material is composed of essentially strain-free single crystal grains. This material will be used for a NIST particle size standard. ACKNOWLEDGMENT We would like to thank Dr. Katherine Mullen for her assistance in developing the software used to implement the integral breadth analysis method for determining D and H. REFERENCES Auffrédic, J. P., Boultif, A., Langford, J. I., Louër, D. (1995). Early Stages of Crystallite Growth of ZnO Obtained from an Oxalate Precursor, J. Am. Ceram. Soc. 78, Caglioti, G., Paoletti, A., Ricci, F.P. (1958), "Choice of Collimators for a Crystal Spectrometer for Neutron Diffraction; Nuclear Instruments," Vol 3,4 pp Cheary, R.W.; Coelho, A.A. (1992), "A Fundamental Parameters Approach to X-ray Line- Profile Fitting," J. Appl. Cryst., Vol. 25, pp Guillou, N., Auffrédic, J. P., Louër, D., (1995). The early stages of crystallite growth of CeO 2 obtained from a cerium oxide nitrate, Powder Diffraction 10, Finger, L.W.; Cox, D.E.; Jephcoat, A.P.(1994), "A Correction for Powder Diffraction Peak Asymmetry due to Axial Divergence," J. Appl. Crystallogr., Vol. 27, pp Langford and Louër, (1982). Diffraction Line Profiles and Scherrer Constants for Materials with Cylindrical Crystallites, J. Appl. Cryst. 15, Langford, J. I., Boultif, A., Auffrédic, J. P., Louër, D., (1993). The use of pattern decomposition to study the combined X-ray diffraction effects of crystallite size and stacking faults in exoxalate zinc oxide, J. Appl. Cryst. 26, Louër, D., Auffrédic, J. P., Langford, J. I., Ciosmak, D., Niepce, J. C., (1983). A precise determination of the shape, size and distribution of size of crystallites in zinc oxide by X-ray diffraction line-broadening analysis, J. Appl. Cryst. 16, Leoni, M., Confente, T., Scardi, P., (2006). PM2K: a flexible program implementing Whole Powder Pattern Modelling, Zeitschrift für Kristallographie Supplements: 2006, Issue suppl 23, pp NIST SRM 660a (2000), "Lanthanum Hexaboride Powder Line Position and Line Shape Standard for Powder Diffraction," National Institute of Standards and Technology; U.S. Department of Commerce: Gaithersburg, MD.

11 70 NIST SRM 660b, (2010), "Lanthanum Hexaboride Powder Line Position and Line Shape Standard for Powder Diffraction," National Institute of Standards and Technology; U.S. Department of Commerce: Gaithersburg, MD. TOPAS, General Profile and Structure Analysis Software for Powder Diffraction Data; V4.2, Bruker AXS GmbH, Karlsruhe, Germany. Warren, B.E., (1969) X-ray Diffraction, Dover Publications, Inc., N.Y.