APPLIED PHYSICS LETTERS 96, 183112 共2010兲
Synthesis and optical properties of GaN/ZnO solid solution nanocrystals Wei-Qiang Han,1,a兲 Zhenxian Liu,2 and Hua-Gen Yu3 1
Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, USA Geophysical Laboratory, Carnegie Institution of Washington, Washington DC 20015, USA 3 Department of Chemistry, Brookhaven National Laboratory, Upton, New York 11973, USA 2
共Received 24 March 2010; accepted 15 April 2010; published online 6 May 2010兲 We devised a synthesis route to prepare narrow band gap GaN/ZnO solid solution nanocrystals via nitriding a homogeneous Ga-Zn-O nanoprecursor. The nanocrystals were characterized by several following methods: x-ray diffractometer, transmission electron microscopy, ultraviolet-visible diffuse reflection, and Raman spectroscopy. Here, we can control the composition of nanocrystals by the nitridation temperature. From 550 to 850 ° C, the corresponding crystalline size varies from 6.1 to 27 nm. It has been demonstrated that the sample prepared at 650 ° C had the narrowest band gap of 2.21 eV. Microstructural investigations show that the 共101兲 surface is the predominantly exposed one for the GaN/ZnO solid solution nanocrystals. We also discuss the influence of chemical disorder based on the Raman spectra acquired. © 2010 American Institute of Physics. 关doi:10.1063/1.3428393兴 A solid-solution is a crystalline solid formed by a mixture of two or more crystalline solids, and has many important applications in catalysis, optoelectronic-, and electronicdevices applications.1–4 Recently, 共Ga1−xZnx兲共N1−xOx兲 solid solutions have emerged as the most promising photocatalyst for visible-light-driven water splitting.5,6 Although both GaN and ZnO have a wurtzite structure with similar lattice constants and band gaps 共about 3.3 eV兲, the band gap of 共Ga1−xZnx兲共N1−xOx兲 depends on the compositional ratio of Ga/Zn, values down to 2.43 eV were achieved experimentally.5 The 共Ga1−xZnx兲共N1−xOx兲 solid solutions are commonly synthesized by nitriding a mixture of Ga2O3 and ZnO or ZnGa2O4 共submicrometers兲 and/or its mixture with ZnO.6 Since these precursors are larger than submicrometers, the 共Ga1−xZnx兲共N1−xOx兲 particles obtained often range from submicrometers to several micrometers. In photocatalytic applications, nanosemiconductors have better quantum-conversion efficiencies than bulk semiconductors do. This advantage of the nanomaterials reflects their high efficiency of electron-hole pair separation, short electron-hole diffusion lengths to the interface, and large interfacial surface areas.7 So far, it is still a challenge to prepare nanostructured 共Ga1−xZnx兲共N1−xOx兲 solid solutions, in particular, those with even narrower band gaps. In this work, we present a novel route of synthesizing 共Ga1−xZnx兲 ⫻共N1−xOx兲 nanocrystals via nitriding a Ga-Zn-O 共GZO兲 nanoprecursor. Besides its attractive application in photocatalysis, this solid solution can also serve as a good model system for fundamental explorations of the disorder effect on physical properties, and consequently, on optical properties. We prepared GZO precursor by mixing gallium nitrate hydrate 共12.8 g兲 and zinc acetate dehydrate 共11.1 g兲 in solution of ethanolamine 共6 ml兲. After stirring at 65 ° C for 2 h, the GZO solution was aged for 1 week at 0 ° C. Thereafter, the resulting gel-like precursor was sintered at 400 ° C for 1 h. The GZO dry-precursor was reacted with ammonia for 10 h at different nitridation temperatures from 550 to 850 ° C. a兲
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Figure 1 shows x-ray diffraction 共XRD兲 spectra of a sequence of samples A–G 共see Table I兲 prepared in a series of nitridation temperatures. The XRD patterns of the samples 共C–G兲, where the temperature was above 650 ° C, are characterized by a single hexagonal wurtzite phase. The existence of the single-phase is also verified in the enlarged panel in Fig. 1共b兲 by the strong 共101兲 peak. This peak correlates linearly with the a-axis lattice constant of a wurtzite crystal. The length of a-axis is set by the atomic ratio of Ga/Zn because the ionic radius of Zn2+ is larger than that of Ga3+. The smaller the a-axis parameter 共or the Zn/Ga ratio兲, the larger becomes the 2 angle of the 共101兲 peak. Here, we have used the fact that the influence of the c-axis parameter is small according to the very small shifting in the 共002兲 peak. Figure 1共b兲 clearly demonstrate this correlation; thus, the 共101兲 peak shifts to larger 2 toward the direction of pure GaN with the rise in nitridation temperature. The XRD results confirm that the materials obtained are 共Ga1−xZnx兲
FIG. 1. 共Color online兲 共a兲 XRD spectra of the temperature-series of 共Ga1−xZnx兲共N1−xOx兲 crystals and 共b兲 the enlarged section of the region of the strongest 共101兲 peak. Open circle denotes ZnGa2O4.
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TABLE I. List of crystalline size, atomic ratio of Zn/ 共Zn+ Ga兲, and band gap Eg of 共Ga1−xZnx兲共N1−xOx兲 nanocrystals at a series of temperatures.
Sample No. A B C D E F G
Nitridation temperature Crystalline size Atomic ratio Eg 共°C兲 共nm兲 Zn/ 共Ga+ Zn兲 共eV兲 550 600 650 700 750 800 850
6.1 6.7 10.4 11.7 17.0 19.9 27.0
¯ ¯ 0.482 0.432 0.379 0.144 0.088
2.36 2.30 2.21 2.32 2.37 2.61 2.65
⫻共N1−xOx兲 solid solutions, rather than mixtures of GaN and ZnO phases. This result is consistent with the atomic ratio of nanocrystals given in Table I 共discussed below兲 as well as with previous reports.6 However, at low nitridation temperatures 共550 and 600 ° C兲, both samples A and B comprise two following phases: the wurtzite 共Ga1−xZnx兲共N1−xOx兲 phase and the cubic ZnGa2O4 phases. The latter labeled with open circles in Fig. 1, formed by the nonreacted precursors. We calculated the sizes of the 共Ga1−xZnx兲共N1−xOx兲 nanocrystals from the XRD spectra using Scherrer’s formula. Table I lists the results. Their size, ranging from 6.1 to 27 nm, is affected strongly by the nitridation-temperature. All nanocrystals are larger than their precursors with a size of about 4.4 nm. Figures 2共a兲 and 2共b兲 are high-resolution TEM images of samples C and G. Each single particle is typically a single crystal. Electron diffraction patterns show that the 共101兲 crystal plane is the preferred exposed surface of all the nanocrystals generated at different temperatures. The atomic ratios in Table I were measured from the energy-dispersive x-ray spectrometry 共EDS兲 spectra. Figure 2共c兲 displays a typical EDS spectrum with the sample C
FIG. 2. 关共a兲 and 共b兲兴 High-resolution TEM images of the samples C 共650 ° C兲 and G 共850 ° C兲. The inset panel in 共b兲 is the selected area diffraction pattern obtained by fast Fourier transform techniques; 共c兲 the EDS spectrum of the sample C 共650 ° C兲; and 共d兲 the EELS spectrum of sample C.
FIG. 3. 共Color兲 The UV-visible diffuse reflectance spectra of the temperature-series of 共Ga1−xZnx兲共N1−xOx兲 crystals.
共650 ° C兲 that has the determined Zn/ 共Zn+ Ga兲 atomic and GaN ratios being, respectively, 0.482 and close to 1. Figure 2共d兲 is the electron energy loss spectrometry 共EELS兲 spectrum taken from the same nanocrystal at which the O / 共N + O兲 atomic ratio is recorded as 0.48. No other element has been detected. Combining the EDS and EELS results, we conclude that the ratios of Ga/N and Zn/O are roughly unity, within the errors of our experimental methods. That is, the atomic ratios are in accord with the XRD findings of a single-phase nanocrystal. As Table I shows, the Zn/ 共Zn + Ga兲 atomic ratio of the samples declines as temperature rises. Figure 3 shows the UV-visible diffuse reflectance spectra of the series of 共Ga1−xZnx兲共N1−xOx兲 samples. We estimated the band gap energy 共Eg兲 of each sample according to the onset of its spectrum. As expected, the band gap of all samples is much narrower than that of the pure GaN 共3.4 eV兲 and ZnO 共3.2 eV兲 materials. Table I summarizes the results. Sample C 共650 ° C兲 has the narrowest band gap 共Eg兲 ⬃ 2.21 eV with a Zn/ 共Zn+ Ga兲 atomic ratio of 0.482. Both numbers are smaller than theoretical prediction of Jensen et al.8 共2.29 eV and 0.525 eV, respectively兲. For pure wurtzite-phase 共Ga1−xZnx兲共N1−xOx兲 nanocrystals, owing to previous theoretical investigations,8,9 it is most likely that the narrow band gap originates from a p-d 共N 2p and Zn 3d兲 repulsion in the upper valence band. This repulsion elevates the maximum of the valence-band without affecting the conduction-band minimum, which results in a narrowing of the band gap.9 In other words, increasing the Zn concentration should maximize the p-d interactions for reducing the band gap of desired nanocrystals. In this work, we demonstrated that this goal is met by preparing solidsolution samples at relative low-temperatures. This important outcome mainly benefited from using our specially designed nanosized GZO-precursors that allowed us to lower the time and temperature of nitridation for the formation of the 共Ga1−xZnx兲共N1−xOx兲 samples. High temperature and/or a long nitridation time often leads to less ZnO content in products because ZnO is easily reduced and volatilized when exposed to the reductive atmosphere during the nitridation process. Preparing a 共Ga1−xZnx兲共N1−xOx兲 solid-solution at lowtemperature decreases nitrogen vacancies that are mainly responsible for the low-activity of H2 evolution on 共oxy兲ni-
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FIG. 4. 共Color online兲 Raman spectra of the temperature-series of 共Ga1−xZnx兲共N1−xOx兲 crystals and also ZnO bulk and GaN nanocrystals.
trides due to the establishment of large band-bending at the solid-liquid interface and a Schottky-type barrier that hinders the prompt migration of electrons from the bulk to the surface reaction sites.6 Raman scattering affords valuable information on the vibrational states of atoms in materials, which are sensitive to local bonding structures or microscopic disorder.10 Figure 4 shows the Raman spectra for all the solid-state-solution samples synthesized at different temperatures, along with those of ZnO bulk and nanosized GaN crystals 共averaging, 13.4 nm兲 for reference. For wurtzite GaN and ZnO crystals,11,12 one A1, one E1, and two E2 phonon modes are Raman active. For ZnO, the peaks in per centimeter appear at 102关E2共low兲兴, 379关A1共TO兲兴, 409关E1共TO兲兴, 437关E2共high兲兴, 581 关a mixed 574 A1共LO兲 and 591 E1共LO兲 one兴; some are multiphonon modes, such as 330 cm−1. For bulk GaN crystals,12,13 the corresponding fundamental modes appear at 144, 534, 561, 567, 734, and 741. However, we note several features in the Raman spectrum of the nanosized GaN crystal. First, the peaks are broadened, so that the 534 cm−1关A1共TO兲兴 and 561 cm−1关E1共TO兲兴 modes merge to produce a single peak at 555 cm−1. Second, the A1共LO兲 mode appears at 709 cm−1 reflecting a 25 cm−1 redshift relative to that in bulk GaN. In particular, this mode persists in all Raman spectra of 共Ga1−xZnx兲共N1−xOx兲 nanocrystals, and consistently displays a larger redshift as crystalline size decreases. This fact undoubtedly indicates that N atoms are directly connected to Ga atoms in those nanocrystals as they are in pure GaN. The existence of the two peaks at about 270 and 640 cm−1 strongly support our conclusion. The former reflects the recently formed connectivity of Ga-O-Zn. The latter is due to the O-Zn-O local structure that generates the
peak at 658 cm−1 in the Raman spectrum of ZnO 共Fig. 4兲. Clearly, the intensities of both peaks relative to that of the A1共LO兲 mode grow with the increase in the fraction of ZnO in the samples, despite their low resolution at low temperatures. Therefore, those features of the Raman spectra demonstrate that O atoms likely link to Zn whereas N atoms prefer to Ga in the 共Ga1−xZnx兲共N1−xOx兲 nanocrystals. The Raman spectra below 500 cm−1 are more structured than expected in the nanosized GaN, and in both the 850 and 800 ° C 共Ga1−xZnx兲共N1−xOx兲 samples. Some structures are created by the second-order Raman scattering of the 共GaN兲 units.13,14 For instance, we assign the 311 cm−1 peak to the acoustic B1共low兲 mode, while considering the 408 cm−1 one as an overtone due to the A1 acoustic mode. The secondorder Raman processes might be enhanced by thermalinduced disorders of nanocrystals’ surfaces at high temperatures 共⬎800 ° C兲.14 Since the second-order Raman scattering is very sensitive to the long-range 共GaN兲 unit connections, it occurs in samples F and G with low ZnO content. For other samples, ZnO units interrupt such long-range uniform connectivity of GaN. Consequently, the distinct Raman spectra vanish for samples with a higher ZnO content. That is, the 共Ga1−xZnx兲共N1−xOx兲 samples have a good even distribution of ZnO among the 共GaN兲 ones in the solid-solutions; this is a typical chemical-disorder effect of multicompound solidsolutions. In addition, we believe that the strong peaks at 198 and 164 cm−1 represent the surface Ga-O stretching modes15 resulting from the surface oxidation of GaN nanocrystals. This Raman feature may imply that the difficulty of oxidation of the 共Ga1−xZnx兲共N1−xOx兲 nanocrystals rises with high ZnO content; further detailed work is on-going. This work is supported by the U. S. DOE under Contract No. DE-AC02-98CH10886. 1
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