GaN: Powder Synthesis, Characterization, and Potential

Subscriber access provided by READING UNIV

Article 2

Quaternary Wurtzitic Nitrides in the System ZnGeN-GaN: Powder Synthesis, Characterization, and Potentiality as a Photocatalyst Takayuki Suehiro, Masataka Tansho, and Tadashi Shimizu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09135 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 19, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Quaternary Wurtzitic Nitrides in the System ZnGeN2–GaN: Powder Synthesis, Characterization, and Potentiality as a Photocatalyst Takayuki Suehiro,∗,† Masataka Tansho,‡ and Tadashi Shimizu‡ †

SiAlON Group, Research Center for Functional Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan ‡

High Field NMR Group, Research Center for Advanced Measurement and Characterization, National Institute for Materials Science, 3-13 Sakura, Tsukuba 305-0003, Japan *To whom correspondence should be addressed. E-mail: [email protected].

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: We developed a new quaternary wurtzitic nitride system by formation of the solid solution between ZnGeN2 and GaN. Near stoichiometric and monophasic powder samples in the composition Zn1`x Ge1`x Ga2x N2 (x ≤ 0.50) were obtained by the reduction–nitridation synthesis conducted at 900‹ C. The results of crystal structure refinement clearly revealed that the cation ordering in the structure of ZnGeN2 (P na21 ) tends to disappear by introducing Ga into the lattice, and the structure transforms to a simple wurtzite phase (P 63 mc) with the composition of x ≥ 0.33. The observed structural evolution was further confirmed by the results of

71

Ga solid-state nuclear magnetic resonance

(NMR) spectroscopy, showing an unsplit single peak observed for x ≥ 0.33. The dissolution of GaN into ZnGeN2 also resulted in a marked narrowing of the band gap, from the ultraviolet region of 3.42 eV to the visible-light range of 3.02–3.05 eV, depending scarcely on the value of x. The results of photocatalytic test reactions for water splitting showed that the synthesized Zn1`x Ge1`x Ga2x N2 solid solution possessed the H2 evolution rate of 2.8–3.6 µmol/h and the relatively high O2 evolution rate of 100.4–126.6 µmol/h, as well as the capability for overall water splitting under the visible-light irradiation of λ > 400 nm.


Page 2 of 35

Page 3 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

Introduction

The band-gap engineering of wurtzitic semiconductors covering the ultraviolet (UV) to near-infrared region for optoelectronic, photovoltaic, and photocatalytic applications has attracted considerable interest in recent years.1−10 One strategy for the development is to make existing binary and ternary systems into a further multinary system, which is expected to possess higher compositional flexibility and thus, more tunable physical properties. This methodology has been already adopted for the oxide semiconductors, e.g., LiGaO2 –ZnO4 and β-AgGaO2 –ZnO5 quaternary systems, which enabled the band-gap tuning of ZnO in the UV (up to 5.6 eV) and visible (down to 2.2 eV) regions, respectively. For the nitridic systems, visible-light sensitization of GaN and ZnGeN2 has been attained by formation of solid solutions with ZnO,1,2 and photocatalytic overall splitting of water by the developed Ga1−x Znx N1−x Ox and Zn1+x GeN2 Ox ternary oxynitride systems has been successfully demonstrated. Recently, Cai et al.10 proposed a series of new wurtzite-derived quaternary nitride systems, I–III–Ge2 N4 (I = Cu, Ag, Li, Na, K; III = Al, Ga, In), through the “cation mutation” in GaN, and predicted their crystal structure and thermodynamic stability based on the first-principles calculations. Their results showed that most of the designed nitrides are not stable with respect to the phase separation into binary or ternary nitrides, whereas there are two exceptions, LiAlGe2 N4 and LiGaGe2 N4 , which is expected to be synthesized without secondary phases. They are both lattice-matched to GaN and their calculated band gaps are direct and larger than GaN, 4.63 and 3.74 eV, respectively, which may be potentially suitable for UV optoelectronic applications. It should be noted that formation of quaternary wurtzitic nitrides has been scarcely reported as claimed in their study, with 3 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

only a few examples such as CaAlSiN3 11 and CaGaSiN3 ,12 as well as alloyed thin films of Alx Ga1−x−y Iny N13,14 and ZnSnx Ge1−x N2 .8,9 ZnGeN2 can be regarded as a II–IV–N2 derivative of GaN generated by the aforementioned cation-mutation concept, and possesses a wurtzite-derived β-NaFeO2 structure consisting of an ordered arrangement of Zn and Ge.15,16 Despite the minimal lattice mismatch (∼1.0% in the a − b plane and ∼0.2% along the c axis15,17 ) and the similar processing conditions as compared to the parent GaN, the solid-solution formation between ZnGeN2 and GaN has not been attempted experimentally or predicted theoretically, as was missed in the previous computational exploration.10 In this work, we successfully synthesized fine powder samples of a new quaternary wurtzitic nitride system, Zn1−x Ge1−x Ga2x N2 (x ≤ 0.50), via a facile reduction–nitridation process. The formation of the solid solution in the system ZnGeN2 –GaN was established and the structural evolution was examined based on the results of crystal structure refinement and

71

Ga solid-state NMR spectroscopy. The optical absorption properties of

Zn1−x Ge1−x Ga2x N2 in the visible range were revealed by diffuse-reflectance and photoluminescence measurements, and their potentiality for photocatalytic water splitting under visible-light irradiation was demonstrated.

2 2.1

Experimental Section Powder Synthesis by GRN

The powder samples of the system ZnGeN2 –GaN were synthesized by using the gasreduction–nitridation (GRN) method.18−25 The oxide starting materials were prepared by simple wet mixing of ZnO (Sigma-Aldrich Co., no. 544906), GeO2 (Sigma-Aldrich Co.,


Page 4 of 35

Page 5 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


99.99%), and Ga2 O3 (Sigma-Aldrich Co., 99.99%) powders, with the target composition of Zn1−x Ge1−x Ga2x N2 , where x = 0, 0.10, 0.20, 0.33 (1/3), and 0.50. The content of ZnO in the starting composition was optimized to be 3 times the stoichiometric quantity, by considering the volatilization of Zn during the reduction–nitridation reaction.2,26 The raw powder mixture of ∼0.4 g was loaded in an alumina boat and set in a horizontal alumina tube furnace (inner diameter of 24 mm). The furnace was subsequently heated to the optimized reaction temperature of 900◦ C in an NH3 –1.5 vol% CH4 gas mixture, introduced at a constant flow rate of 0.13 L/min. After the reaction time of 4 h (x = 0) or 5 h (0.10 ≤ x ≤ 0.50), the sample was furnace cooled in an NH3 atmosphere.

2.2

Characterization

The phase composition of the synthesized powders was analyzed by X-ray diffraction (XRD) using Cu Kα1 radiation operated at 45 kV and 200 mA (SmartLab, Rigaku). Crystal structure was analyzed by the Rietveld refinement of the XRD patterns using the program RIETAN-FP.27 A high-purity Si powder (SRM640c, NIST) was used as an external standard for refining the lattice constants with a fixed zero-shift parameter. The cationic compositions of the synthesized powders were analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES; IRIS Advantage, Jarrell Ash). The nitrogen and oxygen contents were analyzed by the selective hot-gas extraction method (TC436, LECO Co.). The specific surface area and the equivalent particle size of the powders were measured by the single-point Brunauer–Emmett–Teller (BET) method (FlowSorb III, Micromeritics). The

71

Ga magic-angle spinning (MAS) NMR measurement was con-

ducted at 11.7 T using a JEOL ECA 500 spectrometer equipped with a 4 mm CP-MAS



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

probe spinning at ca. 13–15 kHz. The Larmor frequency at 11.7 T was 152.55 MHz for 71

Ga. The spectra were acquired by 3200–11000 scans using the single pulse sequence

with a π/6 pulse of 1.3 µs and a relaxation delay of 20 s, and were referenced to 0 ppm for a 1.0 M aqueous Ga(NO3 )3 solution. Diffuse reflectance spectrum (DRS) of the samples was measured with a spectrophotometer (V-650, Jasco), by referring to the Spectralon reflectance standard. Photoluminescence (PL) spectra were measured at room temperature using a spectrofluorometer (FP-8500, Jasco).

2.3

Photocatalytic Reactions

The photocatalytic test reactions were carried out by using a top irradiation-type cell connected to a closed gas circulation system, as described in our previous work.25 The RuO2 cocatalyst was loaded to the powder samples by impregnation from a RuCl3 (Wako Pure Chemical Industries Ltd., 99.9%) ethanol solution, followed by the calcination in air at 400◦ C for 4 h. Photocatalytic H2 evolution was examined with ∼0.22 g of the RuO2 loaded samples dispersed in 200 mL of deionized water containing 10 vol% of methanol as an electron donor. O2 evolution was evaluated in an aqueous silver nitrate solution (0.01 M) without loading the cocatalyst. The reaction system was initially evacuated to remove air completely, and then the cell was irradiated by the visible light of λ > 400 nm using a 300-W Xe lamp (PE300BF, Excelitas Technologies Co.) equipped with a cutoff filter (LU0400, Asahi Spectra Co.). The light irradiation for 20 h followed by reevacuation was adopted prior to the H2 evolution measurements, to maintain possible N2 evolution from the samples to an undetectable level. The amounts of H2 and O2 evolved were measured using on-line gas chromatography (GC-8A, Shimadzu).


Page 6 of 35

Page 7 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3

Results and Discussion

3.1

Zn1−x Ge1−x Ga2x N2 powders synthesized by GRN

Table 1 lists the main physicochemical properties of the samples synthesized by the GRN reaction at 900◦ C. Almost ideal cation stoichiometry was attained by optimizing the ZnO contents in the starting material and the reaction time, and the compositional deviations were well within 5% relative to the target values for each element. The analyzed nitrogen contents (CN ) for x ≥ 0.10 were in the range of 16.0–15.4 wt% (96–94% of the nominal contents), while the oxygen contents (CO ) were found to be ∼2 wt%, which may be ascribed mainly to the surface oxidation,18 due to their relatively high specific surface areas (SBET ) of ∼13–14 m2 /g. The corresponding particle sizes (DBET ) were as small as 71–77 nm. The impurity carbon contents of the synthesized powders were negligibly low levels of 0.03–0.04(1) wt%. Figure 1 shows the XRD patterns of the synthesized Zn1−x Ge1−x Ga2x N2 powder samples. For the composition of x = 0 (ZnGeN2 ), the diffraction pattern could be indexed with the β-NaFeO2 structure (space group: P na21 ), in which the strongest two peaks of the wurtzite lattice, i.e., 100 and 101 split into 120/200 and 121/201 of the orthorhombic lattice, resulting in higher relative intensity of the 002 diffraction peak. While this tendency was faintly observed for the composition of x = 0.10, the Ga-containing quaternary samples exhibited apparently the diffraction patterns of a single-phase wurtzite structure (space group: P 63 mc) with the lattice constants close to GaN (a = 3.1926 and c = 5.1858 ˚ A, for x = 0.50). These results indicated that the atomic ordering of Zn and Ge in the ZnGeN2 structure tends to disappear by the dissolution of Ga into the lattice, as will be revealed by the crystal structure refinement. 7 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3.2

Page 8 of 35

Crystal Structure Refinement

Figure 2 shows the crystal structure of the end member, ZnGeN2 , viewed along the c axis. The structure of ZnGeN2 with the space group P na21 (no. 33) is regarded as a 2 × 2 superlattice of wurtzite with relationships a ≈

√

3aw and b = 2aw , in which two

inequivalent N sites (each, located above Zn and Ge) are surrounded tetrahedrally by two Zn and two Ge.15,16 The orthorhombic cell is slightly distorted from the ideal cation arrangement of wurtzite due to the tilting of larger ZnN4 and smaller GeN4 tetrahedra, with the ratio of a/aw being 1.694,15 appreciably lower than

√

3. Although neutron

diffraction experiments should be required to directly determine the site occupancy factors of Zn and Ge possessing the close electron density, we could examine the existence of cation ordering in the synthesized Zn1−x Ge1−x Ga2x N2 system by estimating the a/aw ratio, as well as the Zn–N and Ge–N bond distances by the Rietveld refinement of the XRD data. Table 2 lists the structure parameters for the synthesized Zn1−x Ge1−x Ga2x N2 samples obtained by the Rietveld refinement based on the orthorhombic P na21 model. The refinement was conducted with the assumption that Ga is distributed to Zn and Ge sites with an identical occupancy factor corresponding to x. The bond valence sum (BVS) values at the (Zn,Ga) and (Ge,Ga) mixed occupancy sites were calculated according to the weighted long-range BVS,28 and all the bond valence parameters were taken from the work by Brese and O’Keeffe.29 In the end member of ZnGeN2 , the refined lattice constants indicated a distorted cation arrangement with a/aw of 1.697, while the average bond distances for Zn–N (dZn−N ) and Ge–N (dGe−N ) were clearly distinctive by considering the estimated standard deviations, and the corresponding BVS values for Zn and Ge agreed well with the formal valency of 2 and 4, suggesting the almost completely ordered 8 ACS Paragon Plus Environment

Page 9 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


cation sublattice. On the other hand, the introduction of 10% of Ga (x = 0.10) readily decreased the distortion of the orthorhombic cell, and a/aw approached nearly

√

3 with

x ≥ 0.33, indicative of the disappearance of the cation ordering. The values of dZn−N and dGe−N were no longer distinguishable for the quaternary compositions with the BVS values for both the sites being around the averaged valency of 3, further indicating the transition to the cation-disordered structure. The observed continuous transition from the cation-ordered P na21 structure to the wurtzite phase contrasted with the case for the LiGaO2 –ZnO system,4 in which selective dissolution of Zn into the Li site (tetracoordinated ionic radius of Li+ : 0.590 ˚ A30 ) is expected, resulting in formation of an intermediate compound and an immiscible region. The length of a tended to increase and b decreased with the increase of x, as expected from the increase of the a/aw ratio, whereas the variation of the lattice constants were very small, ca. 0.26% for a, 0.23% for b, and 0.05% for c, among the relatively broad compositional range of x = 0.10–0.50. This can be explained by the fact that the arithmetic average of the tetracoordinated ionic radii of Zn2+ and Ge4+ (0.60 and 0.390 ˚ A), viz, 0.495 ˚ A is close to the ionic radius of Ga3+ (0.47 ˚ A),30 suggesting minimal long-range structural changes upon the substitution. These results conclusively showed that the synthesized Zn1−x Ge1−x Ga2x N2 with the compositions of at least, x ≥ 0.33 should have a completely cation-disordered simple wurtzite structure, as will be further confirmed by the 71 Ga MAS NMR measurements. Figure 3 shows the XRD Rietveld refinement pattern of the synthesized Zn1−x Ge1−x Ga2x N2 sample with the composition of x = 0.33. The relevant structure parameters and the estimated (Zn,Ge,Ga)–N bond distances are listed in Tables 3 and 4, respectively. The refinement was conducted on the basis of the wurtzite structure with the space group P 63 mc (no. 186) and converged



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

with Rwp = 9.27%, goodness of fit, S = 1.17, RB = 2.37%, and RF = 1.34%, which were better than Rwp = 9.84%, S = 1.24, RB = 2.65%, and RF = 1.82% for the P na21 model, indicating reliably the disordered cation distribution expected for x = 0.33. The obtained (Zn,Ge,Ga)–N bond distances indicated a reasonable BVS value of 2.91.

3.3

71

Ga MAS NMR Spectroscopy

The one-dimensional 71 Ga MAS NMR spectra of the synthesized Zn1−x Ge1−x Ga2x N2 powder samples are shown in Figure 4. The spectrum for the composition of x = 0.10 showed a resonance splitting into ∼320 and ∼270 ppm, implying that Ga dissolved into two slightly different coordination environments, i.e., local ordering of the Zn and Ge sites still remained, which was hardly discernible from the average bond distances obtained by the crystal structure refinement. This tendency of peak splitting was also remained in the asymmetrically broadened peak observed for x = 0.20, while the compositions of x ≥ 0.33 exhibited an essentially single peak with the chemical shift of ∼303 ppm, indicating the single coordination environment of Ga in the wurtzite structure, consistent with the structure refinement results. The chemical shift was close to ∼320–330 ppm observed for wurtzite GaN.31−36 These results further supported the view that the cation ordering in ZnGeN2 remains partly for x ≤ 0.20, while the samples with x ≥ 0.33 are considered to possess a wurtzite structure without the cation ordering, as was indicated by the XRD results.

3.4

Optical Properties

Figure 5(a) shows the absorbance spectra of the synthesized Zn1−x Ge1−x Ga2x N2 samples, obtained through the Kubelka-Munk transformation of the DRS data. The synthesized 10 ACS Paragon Plus Environment

Page 10 of 35

Page 11 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ga-containing quaternary samples exhibited obvious visible-light absorption up to ∼450 nm, in contrast with the end member of ZnGeN2 , resulting in their light yellow coloration under daylight. The plots of (αhν)2 vs hν obtained from the absorbance data is shown in Figure 5(b), considering the direct band gaps reported for both the end members.37,38 The band gap energy (Eg ) for ZnGeN2 obtained by the extrapolation was 3.42 eV (363 nm), in close agreement with the reported values of 3.3–3.5 eV.26,39,40 The Eg values for the synthesized Zn1−x Ge1−x Ga2x N2 samples estimated from the linear plots were in the visible range of 3.02–3.05 eV (411–407 nm) and depended scarcely on the concentration of Ga (Table 1), reflecting the aforementioned small variation of the lattice constants. In the previously reported Ga1−x Znx N1−x Ox system, more pronounced and successive variation of the band gap upon the compositional change has been observed and predicted,41,42 due to the larger lattice mismatch between the end members (∼2.0% for a and ∼1.0% for c). The marked reduction of the band gap attained in the current system can be rationalized by the theoretically predicted staggered (type-II) band offset between ZnGeN2 and GaN,6 in which the introduction of Ga into ZnGeN2 will lead to a downward shift of the conduction-band minimum, thereby reducing the band gap, as has been also expected for the incorporation of ZnO in GaN.43 Figure 6 shows the PL excitation and emission spectra of the synthesized Zn1−x Ge1−x Ga2x N2 samples, observed at room temperature. ZnGeN2 is known to exhibit a yellow emission under UV excitation without any activators, which is attributable to the electronhole pair recombination via defect-related levels.26 The synthesized ZnGeN2 exhibited a yellow broadband emission peaking at ∼620 nm, in conformity with the results reported earlier,26,40 while the Ga-containing quaternary samples showed significantly redshifted



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

emission bands peaking at ∼680 nm. The excitation peak wavelengths, corresponding to the inter-band transition, agreed well with the Eg values of ∼360 nm for ZnGeN2 and ∼410 nm for the quaternary compositions, further demonstrating the narrowing of the band gap attained by the formation of the solid solution.

3.5

Photocatalytic Activity

Table 5 lists the results of the photocatalytic test reactions for H2 and O2 evolution from water under the presence of sacrificial reagents, conducted with the visible-light responsive quaternary samples under the light irradiation of λ > 400 nm. The synthesized Zn1−x Ge1−x Ga2x N2 possessed the nearly constant H2 evolution rate of 2.8–3.6 µmol/h, reflecting the almost invariable powder characteristics and optical properties among the quaternary compositions. The O2 evolution rate increased slightly from 100.4 µmol/h for x = 0.10 to 126.6 µmol/h for x = 0.50, which were appreciably high compared to ∼60 µmol/h attained by Ga1−x Znx N1−x Ox .44 Figure 7 shows the time course of overall splitting of pure water up to the reaction time of 25 h, performed with the composition of x = 0.10 under the visiblelight irradiation of λ > 400 nm. The total amounts of H2 and O2 evolved were 21.5 and 10.2 µmol, respectively, and the resulting H2 evolution rate of 0.9 µmol/h was comparable to ∼0.6 µmol/h reported for Zn1+x GeN2 Ox 2 and ∼1.5 µmol/h for Ga1−x Znx N1−x Ox ,41 measured under the similar reaction conditions (RuO2 cocatalyst, light irradiation by 300-W Xe lamp, λ > 420 nm). The photocatalytic activity of the developed Zn1−x Ge1−x Ga2x N2 will be further improved by loading more complicated cocatalysts,45 as well as by optimizing the particle size or crystallinity,41 as have been indicated by the earlier studies.


Page 12 of 35

Page 13 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4

Conclusions

We have developed a new quaternary wurtzitic nitride system, Zn1−x Ge1−x Ga2x N2 . Fairly stoichiometric and monophasic powder samples with fine particle sizes of 71–77 nm were synthesized in the quaternary compositions of 0.1 ≤ x ≤ 0.5, via the facile reduction– nitridation process conducted at 900◦ C. The results of the crystal structure refinement revealed that the cation ordering observed in the orthorhombic P na21 structure of ZnGeN2 tends to disappear by the incorporation of Ga into the lattice, and the refinement conducted on the basis of the space group P 63 mc for x = 0.33 converged with the sufficient reliability factors of RB = 2.37% and RF = 1.34%, conclusively indicating the transformation to the completely cation-disordered wurtzite structure. This structural evolution was further confirmed by the

71

Ga NMR results showing a single peak at ∼303 ppm for

the compositions of x ≥ 0.33. A marked reduction of the band gap from 3.42 eV for x = 0 to 3.02–3.05 eV for the quaternary compositions was attained by the formation of the solid solution, which was also confirmed by the PL excitation peaks redshifting to the visible region of ∼410 nm. The synthesized Zn1−x Ge1−x Ga2x N2 exhibited photocatalytic activity for water splitting with the H2 evolution rate of 2.8–3.6 µmol/h and the O2 evolution rate of 100.4–126.6 µmol/h under the presence of sacrificial reagents, and also possessed promising capability for overall splitting of pure water, under the visible-light irradiation of λ > 400 nm.

Acknowledgment.

This work was supported by JSPS KAKENHI, grant no.

26420689 (T. Suehiro). The authors are grateful to Mr. Y. Yajima of National Institute



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

for Materials Science (NIMS) for the chemical analyses, and Mr. K. Deguchi and Mr. S. Ohki of NIMS for the NMR measurements.

Supporting Information.

Crystallographic data for the sample with x = 0.33

(CIF), supplied as Supporting Information.


Page 14 of 35

Page 15 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


References (1) Maeda, K.; Takata, T.; Hara, M.; Saito, N.; Inoue, Y.; Kobayashi, H.; Domen, K. GaN:ZnO Solid Solution as a Photocatalyst for Visible-Light-Driven Overall Water Splitting. J. Am. Chem. Soc. 2005, 127, 8286–8287. (2) Lee, Y.; Terashima, H.; Shimodaira, Y.; Teramura, K.; Hara, M.; Kobayashi, H.; Domen, K.; Yashima, M. Zinc Germanium Oxynitride as a Photocatalyst for Overall Water Splitting under Visible Light. J. Phys. Chem. C 2007, 111, 1042–1048. (3) Ouyang, S.; Ye, J. β-AgAl1−x Gax O2 Solid-Solution Photocatalysts: Continuous Modulation of Electronic Structure toward High-Performance Visible-Light Photoactivity. J. Am. Chem. Soc. 2011, 133, 7757–7763. (4) Omata, T.; Kita, M.; Tachibana, K.; Otsuka-Yao-Matsuo, S. Structural Variation and Optical Properties of ZnO–LiGaO2 Pseudo-Binary System. J. Solid State Chem. 2012, 188, 92–99. (5) Suzuki, I.; Nagatani, H.; Arima, Y.; Kita, M.; Omata, T. Pseudo-Binary Alloying System of ZnO–AgGaO2 Reducing the Energy Band Gap of Zinc Oxide. Appl. Phys. Lett. 2013, 103, 222107. (6) Punya, A.; Lambrecht, W. R. L. Band Offsets between ZnGeN2 , GaN, ZnO, and ZnSnN2 and Their Potential Impact for Solar Cells. Phys. Rev. B 2013, 88, 075302. (7) Omata, T.; Nagatani, H.; Suzuki, I.; Kita, M.; Yanagi, H.; Ohashi, N. Wurtzite CuGaO2 : A New Direct and Narrow Band Gap Oxide Semiconductor Applicable as a Solar Cell Absorber. J. Am. Chem. Soc. 2014, 136, 3378–3381. 15 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(8) Narang, P.; Chen, S.; Coronel N. C.; Gul, S.; Yano, J.; Wang, L.-W.; Lewis, N. S.; Atwater, H. A. Bandgap Tunability in Zn(Sn,Ge)N2 Semiconductor Alloys. Adv. Mater. 2014, 26, 1235–1241. (9) Shing, A. M.; Coronel, N. C.; Lewis, N. S.; Atwater, H. A. Semiconducting ZnSnx Ge1−x N2 Alloys Prepared by Reactive Radio-Frequency Sputtering. APL Mater. 2015, 3, 076104. (10) Cai, Z.-H.; Narang, P.; Atwater, H. A.; Chen, S.; Duan, C.-G.; Zhu, Z.-Q.; Chu, J.H. Cation-Mutation Design of Quaternary Nitride Semiconductors Lattice-Matched to GaN. Chem. Mater. 2015, 27, 7757–7764. (11) Ottinger F. Synthese, Struktur, und Analytische Detailstudien Neuer Stickstoffhaltiger Silicate und Aluminosilicate. Ph.D. Thesis, ETH Z¨ urich, Z¨ urich, Switzerland, 2004. (12) Häusler, J.; Neudert, L.; Mallmann, M.; Niklaus, R.; Kimmel, A. L.; Alt, N. S. A.; Schl¨ ucker, E.; Oeckler, O.; Schnick, W. Ammonothermal Synthesis of Novel Nitrides: Case Study on CaGaSiN3 . Chem. Eur. J. 2017, 23, 2583–2590. (13) Morales, F. M.; Mánuel, J. M.; Garc´ıa, R.; Reuters, B.; Kalisch, H.; Vescan, A. Evaluation of Interpolations of InN, AlN and GaN Lattice and Elastic Constants for Their Ternary and Quaternary Alloys. J. Phys. D: Appl. Phys. 2013, 46, 245502. ¨ ol, V. B.; Sigle, W.; van Aken, P. A.; Garc´ıa, (14) Mánuel, J. M.; Koch, C. T.; Ozd¨ R.; Morales, F. M. Inline Electron Holography and VEELS for the Measurement of


Page 16 of 35

Page 17 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Strain in Ternary and Quaternary (In,Al,Ga)N Alloyed Thin Films and Its Effect on Bandgap Energy. J. Microsc. 2015, 261, 27–35. (15) Wintenberger, M.; Maunaye, M.; Laurent, Y. Groupe Spatial et Ordre des Atomes de Zinc et de Germanium dans ZnGeN2 . Mater. Res. Bull. 1973, 8, 1049–1054. (16) Lambrecht, W. R. L.; Alldredge, E.; Kim, K. Structure and Phonons of ZnGeN2 . Phys. Rev. B 2005, 72, 155202. (17) Paszkowicz, W.; PodsiadÃlo, S.; Minikayev, R. Rietveld-Refinement Study of Aluminium and Gallium Nitrides. J. Alloys. Compd. 2004, 382, 100–106. (18) Suehiro, T.; Hirosaki, N.; Komeya, K. Synthesis and Sintering Properties of Aluminium Nitride Nanopowder Prepared by the Gas-Reduction–Nitridation Method. Nanotechnology 2003, 14, 487–491. (19) Suehiro, T.; Hirosaki, N.; Xie, R.-J.; Mitomo, M. Powder Synthesis of Ca-α′ -SiAlON as a Host Material for Phosphors. Chem. Mater. 2005, 17, 308–314. (20) Suehiro, T.; Hirosaki, N.; Xie, R.-J.; Sakuma, K.; Mitomo, M.; Ibukiyama, M.; Yamada, S. One-Step Preparation of Ca-α-SiAlON:Eu2+ Fine Powder Phosphors for White Light-Emitting Diodes. Appl. Phys. Lett. 2008, 92, 191904. (21) Suehiro, T.; Hirosaki, N.; Xie, R.-J.; Sato, T. Blue-Emitting LaSi3 N5 :Ce3+ Fine Powder Phosphor for UV-Converting White Light-Emitting Diodes. Appl. Phys. Lett. 2009, 95, 051903.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(22) Suehiro, T.; Onuma, H.; Hirosaki, N.; Xie, R.-J.; Sato, T.; Miyamoto, A. Powder Synthesis of Y-α-SiAlON and Its Potential as a Phosphor Host. J. Phys. Chem. C 2010, 114, 1337–1342. (23) Suehiro, T.; Hirosaki, N.; Xie, R.-J. Synthesis and Photoluminescent Properties of (La,Ca)3 Si6 N11 :Ce3+ Fine Powder Phosphors for Solid-State Lighting. ACS Appl. Mater. Interfaces 2011, 3, 811–816. (24) Suehiro, T.; Xie, R.-J.; Hirosaki, N. Gas-Reduction–Nitridation Synthesis of CaAlSiN3 :Eu2+ Fine Powder Phosphors for Solid-State Lighting. Ind. Eng. Chem. Res. 2014, 53, 2713–2717. (25) Suehiro, T.; Tansho, M.; Shimizu, T. Na-α′ -GeGaON Solid Solution Analogous to α′ -SiAlON: Synthesis, Crystal Structure, and Potentiality as a Photocatalyst. Inorg. Chem. 2016, 55, 2355–2362. (26) Zhang, Q.-H.; Wang, J.; Yeh, C.-W.; Ke, W.-C.; Liu, R.-S.; Tang, J.-K.; Xie, M.-B.; Liang, H.-B.; Su, Q. Structure, Composition, Morphology, Photoluminescence and Cathodoluminescence Properties of ZnGeN2 and ZnGeN2 :Mn2+ for Field Emission Displays. Acta Mater. 2010, 58, 6728–6735. (27) Izumi, F.; Momma, K. Three-Dimensional Visualization in Powder Diffraction. Solid State Phenom. 2007, 130, 15–20. (28) Bosi, F. Bond Valence at Mixed Occupancy Sites. I. Regular Polyhedra. Acta Crystallogr., Sect. B: Struc. Sci. 2014, B70, 864–870.


Page 18 of 35

Page 19 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(29) Brese, N. E.; O’Keeffe, M. Bond-Valence Parameters for Solids. Acta Crystallogr., Sect. B: Struc. Sci. 1991, B47, 192–197. (30) Shannon, R. D. Revised Effective Ionic-Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1976, 32, 751–767. (31) Han, O. H.; Timken, H. K. C.; Oldfield, E. Solid-State “Magic-Angle” SampleSpinning Nuclear Magnetic Resonance Spectroscopic Study of Group III-V (13–15) Semiconductors. J. Chem. Phys. 1988, 89, 6046–6052. (32) Schwenzer, B.; Hu, J.; Seshadri, R.; Keller, S.; DenBaars, S. P.; Mishra, U. K. Gallium Nitride Powders from Ammonolysis: Influence of Reaction Parameters on Structure and Properties. Chem. Mater. 2004, 16, 5088–5095. (33) Yesinowski, J. P.; Purdy, A. P.; Wu, H.; Spencer, M. G.; Hunting, J.; DiSalvo, F. J. Distributions of Conduction Electrons as Manifested in MAS NMR of Gallium Nitride. J. Am. Chem. Soc. 2006, 128, 4952–4953. (34) Schwenzer, B.; Hu, J.; Morse, D. E. Correlated Compositions, Structures, and Photoluminescence Properties of Gallium Nitride Nanoparticles. Adv. Mater. 2011, 23, 2278–2283. (35) Jung, W.-S. Synthesis and Characterization of GaN Powder by the Cyanonitridation of Gallium Oxide Powder. Ceram. Int. 2012, 38, 5741–5746. (36) Yesinowski, J. P. Solid-State NMR of Inorganic Semiconductors. Topics Curr. Chem. 2012, 306, 229–312. 19 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(37) Stampfl, C; Van de Walle, C. G. Density-Functional Calculations for III-V Nitrides Using the Local-Density Approximation and the Generalized Gradient Approximation. Phys. Rev. B 1999, 59, 5521–5535. (38) Punya, A.; Lambrecht, W. R. L. Quasiparticle Band Structure of Zn-IV-N2 Compounds. Phys. Rev. B 2011, 84, 165204. (39) Du, K.; Bekele, C.; Hayman, C. C.; Angus, J. C.; Pirouz, P.; Kash, K. Synthesis and Characterization of ZnGeN2 Grown from Elemental Zn and Ge Sources. J. Cryst. Growth 2008, 310, 1057–1061. (40) Shang, M.; Wang, J.; Fan, J.; Lian, H.; Zhang, Y.; Lin, J. ZnGeN2 and ZnGeN2 :Mn2+ Phosphors: Hydrothermal-Ammonolysis Synthesis, Structure and Luminescence Properties. J. Mater. Chem. C 2015, 3, 9306–9317. (41) Maeda, K.; Domen, K. Solid Solution of GaN and ZnO as a Stable Photocatalyst for Overall Water Splitting under Visible Light. Chem. Mater. 2010, 22, 612–623. (42) Jensen, L. L.; Muckerman, J. T.; Newton, M. D. First-Principles Studies of the Structural and Electronic Properties of the (Ga1−x Znx )(N1−x Ox ) Solid Solution Photocatalyst. J. Phys. Chem. C 2008, 112, 3439–3446. (43) Huda, M. N.; Yan, Yanfa, Wei, S.-H.; Al-Jassim, M. M. Electronic Structure of ZnO:GaN Compounds: Asymmetric Bandgap Engineering. Phys. Rev. B 2008, 78, 195204.


Page 20 of 35

Page 21 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(44) Maeda, K.; Hashiguchi, H.; Masuda, H.; Abe, R.; Domen, K. Photocatalytic Activity of (Ga1−x Znx )(N1−x Ox ) for Visible-Light-Driven H2 and O2 Evolution in the Presence of Sacrificial Reagents. J. Phys. Chem. C 2008, 112, 3447–3452. (45) Maeda, K.; Teramura, K.; Lu, D.; Saito, N.; Inoue, Y.; Domen, K. NobleMetal/Cr2 O3 Core/Shell Nanoparticles as a Cocatalyst for Photocatalytic Overall Water Splitting. Angew. Chem. Int. Ed. 2006, 45, 7806–7809.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 35

Table 1. Physicochemical Properties of the Synthesized Zn1−x Ge1−x Ga2x N2 Powder Samples

x

a

cationic ratioa Zn Ge Ga -

CN (wt %)

0

0.984(3) 1.016(2)

0.10

0.908(3)

0.886(2)

0.206(2)

16.0(1)

0.20

0.803(8)

0.787(2)

0.410(2)

0.33

0.674(3)

0.646(2)

0.50

0.521(3)

0.476(2)

CO (wt %)

16.7(1) 0.72(1)

SBET DBET (m2 /g) (nm)

Eg (eV)

9.8

101

3.42

1.6(1)

13.5

74

3.04

15.8(1)

2.0(1)

13.9

71

3.05

0.680(2)

15.5(1)

2.3(1)

12.9

77

3.03

1.003(2)

15.4(1)

2.4(1)

13.3

75

3.02

Normalized against the total of 2.


Page 23 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Structure Parameters for the Synthesized Zn1−x Ge1−x Ga2x N2 Samples in the P na21 Model, Estimated by the Rietveld Refinement

BVS at BVS at Zn site Ge site

a/aw

5.18726(12)

1.697

2.04(4)

1.87(4)

1.93

4.19

0.10 5.51851(10) 6.39429(11)

5.18697(6)

1.726

1.97(3)

1.94(3)

2.82

2.98

0.20 5.52055(7)

6.38748(9)

5.18435(4)

1.729

1.95(4)

1.96(3)

2.95

2.85

0.33 5.53184(9)

6.37976(10)

5.18498(4)

1.734

1.94(7)

1.97(7)

3.01

2.81

0.50 5.53309(12) 6.38154(13)

5.18597(5)

1.734

1.94(7)

1.97(6)

3.07

2.80

0

b (˚ A)

average average ˚ dZn−N (A) dGe−N (˚ A)

c (˚ A)

x

a (˚ A)

5.46304(14) 6.43859(17)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 35

Table 3. Refined Structure Parametersa for the x = 0.33 Sample with the Composition Zn1/3 Ge1/3 Ga1/3 N

a

atom site

SOF

x

y

z

B (˚ A2 )

Zn Ge Ga N

1/3 1/3 1/3 1

1/3 1/3 1/3 1/3

2/3 2/3 2/3 2/3

0 0 0 0.3823(6)

0.39(1) 0.39(1) 0.39(1) 0.53(6)

2b 2b 2b 2b

Space group P 63 mc (No. 186), a = 3.19186(2) ˚ A, c = 5.18498(4) ˚ A, V = 45.7472(6) ˚ A3 , Z = 2,

reliability factors: RB = 2.37%, RF = 1.34%.

Table 4. (Zn,Ge,Ga)–N Bond Distances for Zn1/3 Ge1/3 Ga1/3 N

tetrahedral (Zn,Ge,Ga)–N distances (˚ A) (Zn,Ge,Ga)–N mean

1.941(1) × 3 1.982(3) 1.951


Page 25 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 5. Photocatalytic Activities of the Synthesized Zn1−x Ge1−x Ga2x N2 Powder Samples for H2 or O2 Evolution from Water under the Presence of Sacrificial Reagents and the Visible-Light Irradiation of λ > 400 nm

x 0.10 0.10 0.20 0.20 0.33 0.33 0.50 0.50

activity (µmol/h) H2 a O2 b

cocatalyst 4 wt% none 4 wt% none 4 wt% none 4 wt% none

RuO2

2.8 100.4

RuO2

3.5 100.6

RuO2

3.6 121.5

RuO2

3.0 126.6

a b

Steady-state evolution rate. Initial evolution rate.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure captions Figure 1. XRD patterns of the synthesized Zn1−x Ge1−x Ga2x N2 samples with the various x values. Figure 2. Crystal structure of ZnGeN2 (P na21 ) viewed along the c axis. Zn, Ge, and N atoms are represented by green, blue, and white spheres, respectively. The corresponding wurtzite unit cell (P 63 mc) is also indicated by dashed line. Figure 3. X-ray Rietveld refinement pattern of the synthesized Zn1−x Ge1−x Ga2x N2 sample (x = 0.33). Figure 4.

71

Ga MAS NMR spectra of the synthesized Zn1−x Ge1−x Ga2x N2 samples: (a)

x = 0.10, (b) x = 0.20, (c) x = 0.33, and (d) x = 0.50. Asterisks denote spinning sidebands. Figure 5. (a) Kubelka-Munk transformed absorption spectra and (b) plots of (αhν)2 vs hν showing the Eg values for the synthesized Zn1−x Ge1−x Ga2x N2 powder samples. Figure 6. PL excitation and emission spectra of the synthesized Zn1−x Ge1−x Ga2x N2 powder samples. Figure 7. Time course of overall splitting of pure water by the 4 wt% RuO2 -loaded Zn1−x Ge1−x Ga2x N2 (x = 0.10) powder under the visible-light irradiation of λ > 400 nm.


Page 26 of 35

122 202

040 320

123 203

102

110

103

x = 0 (ZnGeN2)

x = 0.10

x = 0.20

x = 0.50

10

20

30

101

x = 0.33

100 002

Relative intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


120 200 002 121 201

Page 27 of 35

40

50

60

70

80

2θ (degree) Figure 1. XRD patterns of the synthesized Zn1-xGe1-xGa2xN2 samples with the various x values. ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Page 28 of 35

Figure 2. Crystal structure of ZnGeN2 (Pna21) viewed along the c axis. Zn, Ge, and N atoms are represented by green, blue, and white spheres, respectively. The corresponding wurtzite unit cell (P63mc) is also indicated by dashed line. ACS Paragon Plus Environment

Page 29 of 35

Rwp = 9.27% S = 1.17

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


2θ (degree) Figure 3. X-ray Rietveld refinement pattern of the synthesized Zn1-xGe1-xGa2xN2 sample (x = 0.33). ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

(a)

(b)

* *

*

* * 800 600 400 200

0

*

-200 800 600 400 200

(c)

0

-200

0

-200

(d) * *

*

800 600 400 200 71Ga

Page 30 of 35

* * 0

*

-200 800 600 400 200

chemical shift (ppm)

71Ga

chemical shift (ppm)

Figure 4. 71Ga MAS NMR spectra of the synthesized Zn1-xGe1-xGa2xN2 samples: (a) x = 0.10, (b) x = 0.20, (c) x = 0.33, and (d) x = 0.50. Asterisks denote spinning sidebands. ACS Paragon Plus Environment

Page 31 of 35

(a) x=0 x = 0.10 x = 0.20 x = 0.33 x = 0.50

α

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


200

300

400

500

600

700

Wavelength (nm) Figure 5. (a) Kubelka-Munk transformed absorption spectra and (b) plots of (αhν)2 vs hν showing the Eg values for the synthesized Zn1-xGe1-xGa2xN2 powder samples. ACS Paragon Plus Environment


100

(b) 80

[αhν]2 (eV2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

60 40

x = 0.50 Eg = 3.02 eV x = 0.10 Eg = 3.04 eV x=0 Eg = 3.42 eV

20 0 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6

Photon energy (eV) Figure 5. (a) Kubelka-Munk transformed absorption spectra and (b) plots of (αhν)2 vs hν showing the Eg values for the synthesized Zn1-xGe1-xGa2xN2 powder samples. ACS Paragon Plus Environment

Page 32 of 35

Page 33 of 35

x=0

x = 0.10 x = 0.20 x = 0.33 x = 0.50

PL intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


200

300

400

500

600

700

800

Wavelength (nm) Figure 6. PL excitation and emission spectra of the synthesized Zn1-xGe1-xGa2xN2 powder samples. ACS Paragon Plus Environment

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Amount of gases evolved (μmol)


Page 34 of 35

25 20

H2 O2

15 10 5 0 0

5

10

15

20

25

Reaction time (h) Figure 7. Time course of overall splitting of pure water by the 4 wt% RuO2-loaded Zn1-xGe1-xGa2xN2 (x = 0.10) powder under the visible-light irradiation of λ > 400 nm. ACS Paragon Plus Environment

Page 35 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Graphic

N

Ga

N

Zn Ge N

ACS Paragon Plus Environment

GaN: Powder Synthesis, Characterization, and Potential

Recommend Documents