Article pubs.acs.org/IC
Average and Local Crystal Structures of (Ga1−xZnx)(N1−xOx) Solid Solution Nanoparticles Mikhail Feygenson,*,† Joerg C. Neuefeind,† Trevor A. Tyson,‡ Natalie Schieber,§ and Wei-Qiang Han⊥ †
Chemical and Engineering Materials Division, Spallation Neutron Source (SNS), Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ‡ Department of Physics, New Jersey Institute of Technology, Newark, New Jersey 07102, United States § Vanderbilt University, Nashville, Tennessee 37235, United States ⊥ Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China ABSTRACT: We report a comprehensive study of the crystal structure of (Ga1−xZnx)(N1−xOx) solid solution nanoparticles by means of neutron and synchrotron X-ray scattering. In our study, we used four different types of (Ga1−xZnx)(N1−xOx) nanoparticles, with diameters of 10−27 nm and x = 0.075−0.51, which show energy band gaps from 2.21 to 2.61 eV. Rietveld analysis of the neutron diffraction data revealed that the average crystal structure is hexagonal wurtzite (space group P63mc) for the larger nanoparticles, while the crystal structure of smaller nanoparticles is disordered hexagonal. Pair-distribution-function analysis found that the intermediate crystal structure retains a “motif” of the average one; however, the local structure is more disordered. The implications of disorder on the reduced energy band gap are discussed.
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INTRODUCTION The search for clean and recyclable energy sources is an ongoing effort of a variety of scientific communities. One of the approaches is hydrogen generation using solar water-splitting photocatalysis, which will provide a cheap and renewable source of hydrogen fuel. The most promising photocatalyst is the (Ga1−xZnx)(N1−xOx) solid solution of GaN and ZnO. It is stable and capable of solar water splitting because of its narrow energy band gap Eg. The narrowest gap estimated for the bulk is Eg = 2.4 eV, limited by the highest Zn concentration x = 0.33 achievable during the synthesis.1 The efficiency of (Ga1−xZnx)(N1−xOx) can be further improved when its size is reduced to nanometers because of more efficient electron−hole pair separation, short electron−hole diffusion lengths to the interface, and larger surface area.2−4 However, the most striking advantage of (Ga1−xZnx)(N1−xOx) nanoparticles is that Eg can be reduced below the 2.4 eV limit predicted for the bulk. For example, in this work we study nanoparticles with the lowest Eg = 2.21 eV for x = 0.51.4 The narrowing of Eg in (Ga1−xZnx)(N1−xOx) bulk from the value of about 3.3 eV for both GaN and ZnO end members is well documented in the literature, despite its origin still being under debate.1,2,5−15 In general, the energy band gap is narrower for samples with higher concentrations of Zn. Moreover, the energy gap seems to depend on the zinc impurity levels, structural relaxation, and clustering of Zn atoms.16 The crystal structure distortion, lattice defects, and variation of the surface-to-bulk atomic composition were also suggested as alternative explanations.1,7,15 All of these © 2015 American Chemical Society
explanations might apply to the (Ga1−xZnx)(N1−xOx) nanoparticles as well, which are prone to impurities and generally more disordered than the bulk counterparts because of a larger number of surface atoms with broken bonds.17 Experimental studies of the local crystal structure would be the first step in understanding the visible-light absorption mechanism and its dependence on the Zn concentration in (Ga1−xZnx)(N1−xOx) nanoparticles. Previous attempts to measure the local structure include extended X-ray absorption fine structure (XAFS) studies of the bulk (Ga1−xZnx)(N1−xOx) samples with x > 0.3.5 They indicated that the local environment of Ga atoms is similar to that of the GaN precursor, although distortions in the second and third atomic shells were found for the samples with higher Zn concentration. The Ga−N and Zn−O bond lengths were found to be distorted compared to single-phase end members GaN and ZnO. In this work, we used pair-distribution-function (PDF) analysis of neutron and synchrotron X-ray diffraction (XRD) data to study the local crystal structure of (Ga1−xZnx)(N1−xOx) nanoparticles. A high penetration depth of neutrons enables probing of all interatomic distances within nanoparticles. Neutrons are sensitive to light elements; thus, the positions of the O and N atoms can be accurately determined. PDF analysis provides structural information on the local and intermediate length scales, complementing the average structure information obtained from the Rietveld refinements of the same data. Received: July 21, 2015 Published: November 6, 2015 11226
DOI: 10.1021/acs.inorgchem.5b01605 Inorg. Chem. 2015, 54, 11226−11235
Article
Inorganic Chemistry
mg of each sample was measured in a 6-mm-diameter vanadium can for 60 min at room temperature. NOMAD detectors were calibrated using scattering from diamond powder, and standard silicon powder was measured to obtain the instrument parameter file for the Rietveld refinements. The highest q-resolution backscattering bank was used for the Rietveld refinements of the diffraction data. In order to obtain the structural factor S(q), the scattering intensity was normalized to the scattering from a solid vanadium rod and the background was subtracted. The PDF was obtained by the Fourier transform of S(q) at qmin = 0.08 Å−1 and qmax = 26 Å−1:
Hence, the structural behavior of nanoparticles on largely different length scales can be directly probed in a single powder diffraction experiment. Such an approach to modeling was demonstrated in the crystal structure studies of ZnSe1−xTex and InxGa1−xAs bulk ternary alloys, where a combination of Rietveld and PDF analysis revealed strain-induced local disorder in both samples.18,19 We report here the crystal structure measurements of (Ga1−xZnx)(N1−xOx) nanoparticles of different dimensions and Zn concentrations. We used the combination of neutron diffraction, synchrotron XRD, and XAFS measurements to accurately determine the average and local crystal structures of our nanoparticles. We argue that the local crystal structure of our nanoparticles deviates significantly from a simple hexagonal wurtzite structure reported by the neutron diffraction experiments on the bulk (Ga1−xZnx)(N1−xOx) sample.20 Such disorder was predicated by the density functional theory (DFT) calculations of Jensen et al.21 to explain the narrowing of the band gap.
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G(r ) =
EXPERIMENTAL SECTION
Table 1. Dimensions of the Nanoparticle Powders Used in This Worka sample
TN (°C)
d (nm)
x
Eg (eV)
650 700 750 850
10.4(0.9) 11.7(0.6) 17.0(0.8) 27.0(0.6)
0.51 0.48 0.44 0.075
2.21 2.32 2.37 2.65
∫q
qmax
min
q[S(q) − 1] sin(qr ) dq
(1)
The synchrotron X-ray PDF measurements were carried out on beamline 11-ID-B at the Advanced Photon Source (APS), Argonne National Laboratory. The nanoparticles were measured in 0.5 mm Kapton capillaries, at room temperature with X-ray wavelength λ = 0.2127 Å. Standard nickel bulk was measured for calibration. The scattering structure factor S(q), with corrections for background scattering, X-ray transmission, and Compton scattering, was obtained from the diffraction data using the PDFgetX2 software package.23 The Rietveld and PDF refinements of XRD and neutron diffraction data were completed using the GSAS and PDFgui software packages, respectively.24,25 For the XAFS measurements in transmission mode, samples were prepared by grinding and sieving the materials ( 0.3 were not reported in the work of Jensen et al.; therefore, our measurements provide further experimental validation of his model. 11234
DOI: 10.1021/acs.inorgchem.5b01605 Inorg. Chem. 2015, 54, 11226−11235
Article
Inorganic Chemistry
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nanoparticles. More studies are needed to fully explore the possibilities of reducing Eg by doping.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank J. Carruth for his help with neutron scattering experiments at SNS. We are grateful to K. Beyer and K. Freeman for helping with the synchrotron X-ray measurements at APS. We also acknowledge Th. Proffen for stimulating discussions. We thank Pamela Whitfield for reading the manuscript and providing her comments. A portion of this research at Oak Ridge National Laboratory’s SNS was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. This research was conducted at the Center for Functional Nanomaterials, which is sponsored at Brookhaven National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. This research used resources of the APS, a U.S. Department of Energy (DOE), Office of Science User Facility, operated for the DOE Office of Science by Argonne National Laboratory under Contract DE-AC02-06CH11357. Support for T.A.T. was provided by DOE Grant DE-FG02-07ER46402. X-ray absorption data acquisition was performed at Brookhaven National Laboratory’s NSLS, which is funded by the U.S. Department of Energy.
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DOI: 10.1021/acs.inorgchem.5b01605 Inorg. Chem. 2015, 54, 11226−11235
