Highly Enhanced Photocatalytic Water-splitting Activity of Gallium Zinc

Subscriber access provided by Kaohsiung Medical University

Kinetics, Catalysis, and Reaction Engineering

Highly Enhanced Photocatalytic Water-splitting Activity of Gallium Zinc Oxynitride Derived from Flux-assisted Zn/Ga Layered Double Hydroxides Jae-Hun Yang, YI-RONG PEI, Seung-Joo Kim, Goeun Choi, Ajayan Vinu, and Jin-Ho Choy Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03908 • Publication Date (Web): 07 Nov 2018 Downloaded from http://pubs.acs.org on November 8, 2018

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 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 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.

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 32 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

Industrial & Engineering Chemistry Research

Revised manuscript submitted to Ind. Eng. Chem. Res.

Highly Enhanced Photocatalytic Water-splitting Activity of Gallium Zinc Oxynitride Derived from Flux-assisted Zn/Ga Layered Double Hydroxides Jae-Hun Yang,†,‡,⊥ Yi-Rong Pei,†,⊥ Seung-Joo Kim,§ Goeun Choi,† Ajayan Vinu ‡ and Jin-Ho Choy*,† † Center

for Intelligent Nano-Bio Materials (CINBM), Department of Chemistry and Nanoscience, Ewha Womans University, Seoul 03760,

Republic of Korea. ‡ Global

Innovative Centre for Advanced Nanomaterials, School of Engineering, Faculty of Engineering and Built Environment, The

University of Newcastle, Callaghan, New South Wales, 2308, Australia. § Department

of Energy Systems Research, Ajou University, Suwon 16499, Republic of Korea.

KEYWORDS : Overall water splitting • Layered double hydroxides • Gallium Zinc Oxynitride • Photocatalyst • Flux

1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 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 2 of 32

ABSTRACT

The Ga/Zn-oxynitride solid solution [(GaN)1-x(ZnO)x] is one of the promising visible-light harvesting photocatalysts for overall watersplitting. A series of (GaN)1-x(ZnO)x (0.11 ≤ x ≤ 0.33) are synthesized by calcining the carbonate-type Zn/Ga-LDH precursor with and without sodium carbonate flux at 850 oC for 8 - 14 h under a NH3 gas-flow. The solid solutions without flux are determined to be low in crystallinity but plate-like in morphology with preferred orientation could be observed. On the other hand, those with flux turn out to be better in crystallinity, and eventually exhibit significantly higher photocatalytic activity for overall water splitting under visible-light irradiation than those without flux. In addition, the bandgap energies can also be engineered from 2.57 eV to 2.72 eV by changing the synthetic parameter such as nitridation time. It is, therefore, suggested that the present new approach can offer new opportunities for designing the next generation photocatalytic systems.


Page 3 of 32 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 Recently, human society is encountered with tremendous environmental problems such as contaminated ground waters, global warming caused by greenhouse gases, and tiny dust particles.1,2 The burning of fossil fuel is considered as the major cause of these environmental problems. In order to reduce air pollution caused by fossil fuels, and remediate polluted environment, it is highly required to develop clean energy resources alternating the fossil fuel. Energy derived from sun is thought to be one of the best clean energy sources of alternative energy available to modern humans on the planet Earth. For example, hydrogen fuel can be produced by solar-driven photocatalytic water splitting reaction without any extra energy consumption but with the help of semiconducting photocatalysts. Hydrogen, as a chemical energy, can be storable and renewable, and convertible to mechanical energy in a combustion engine or to electrical one in a fuel cell as a clean fuel.3,4 For that reason, various photo-catalysts with the activity even under visible light irradiation, such as metal-doped TiO2 and SrTiO3, CdS, BiVO4, Ta3N4, graphitic carbon nitride, etc., have been steadily developed for water-splitting reactions.4-12 Gallium zinc oxynitride [(GaN)1-x(ZnO)x] with the wurtzite structure, the solid solution of ZnO and GaN, is considered as one of the most promising photocatalysts for overall water splitting under visible light whereas each of ZnO and GaN shows only the UV-light harvesting photocatalytic activity.13-20 Its absorption edge can be finely tuned by controlling the content of ZnO during the synthesis by simply adjusting the reaction temperature, time, and the initial content of reactants. In general, the (GaN)1-x(ZnO)x catalysts can be synthesized by the conventional solid state reaction at ~ 850 oC for more than 10 h under ammonia gas flow by using the precursors such as ZnO and Ga2O3 or ZnO and ZnGa2O4. In the present study, an attempt has been made to develop a new precursor route for the preparation of (GaN)1-x(ZnO)x solid solution under milder synthesis condition compared to the conventional solid state reaction method using 2D materials such as layered double hydroxides (LDHs).



Page 4 of 32

LDH, also known as a hydrotalcite-like compound, is a type of solid solution between divalent metal hydroxide and trivalent metal hydroxide, which have been widely studied as catalysts, absorbents, drug delivery carriers, nanofillers in polymer nanocomposites or ceramic precursors.21-31 LDHs composed of Zn and Ga hydroxides have a potential as good precursors for (GaN)1-x(ZnO)x solid solution since they are solid solution of Zn(OH)2 and Ga(OH)2+ with layered structure and its layered structure is effective for nitridation, where the molar ratio of Zn/Ga in LDHs can easily be controlled from 1.5 to 4.29,30 The (GaN)1-x(ZnO)x solid solutions synthesized from LDH precursors with various Zn/Ga ratios were prepared through a simple calcination of these precursors under NH3 gas flow at 800 oC for less than 5 h, and their photo-reduction activity of Cr6+ (toxic species) to Cr3+ (nontoxic species) was demonstrated in an aqueous solution.29 Recently, nano-structured (GaN)1-x(ZnO)x solid solutions were synthesized via nitridation by calcining the ball-milled mixture of Zn/Ga LDH, Zn metal and urea at 900 oC for 15 min under N2 flow and further calcination at 600 oC for 1 h.31 These nanostructured solid solution loaded with 1 wt% Rh cocatalyst demonstrated moderate overall water-splitting activity, where the photocatalyst with Zn/(Ga + Zn) = 0.2 showed the highest photocatalytic activity. But, these synthetic methods required the additional Zn species such as Zn or ZnO except Zn/Ga LDH, which can increase the synthetic cost. However, there is no report on the overall water-splitting efficiency of (GaN)1- x(ZnO)x synthesized directly from Zn/Ga LDH precursors without any additional Zn source. In this report, we suggest a flux-assisted direct route to the preparation of (GaN)1-x(ZnO)x solid solution with a high crystallinity from Zn/Ga LDH precursor, and demonstrate how their photocatalytic activity can be affected by their crystallinity. Crystallinity is the key issue as the amorphous sites (defect sites) can play a role as recombination sites of photo-excited electrons and holes which may decrease the photocurrent and further reduce the photocatalytic activity. Therefore, much efforts have been devoted on the synthesis of well crystallized metal nitride or metal oxynitride using the flux-assisted nitridation method where Na2CO3, NaCl, K2CO3, KCl, etc. were used as the flux.32,33 However, there is no report on the flux-assisted method for the highly crystalline (GaN)1-x(ZnO)x solid solution. Here, we demonstrate the synthesis of highly crystalline (GaN)1-x(ZnO)x solid solution with a high overall water splitting activity by simply mixing a 4 ACS Paragon Plus Environment

Page 5 of 32 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


flux with Zn/Ga LDH followed by nitridation under NH3 flow without the additional Zn species such as ZnO or Zn metal. Crystallographic and optical properties of these solid solutions synthesized by flux-assisted method from LDH precursors have been examined in detail together with the photocatalytic overall water splitting activity.

2. EXPERIMENTAL SECTION 2.1. Materials Zinc nitrate hexahydrate (Zn(NO3)26H2O, >98.0%), sodium carbonate anhydrous (Na2CO3, >99.0%) and sodium hydroxide (98.0%) were purchased from DaeJung Chemical & Metals Co., LTD. Gallium nitrate solution (Ga(NO3)3, 10wt% based on Ga metal) was purchased from Molycorp, Inc. NH3 gas (5N-grade) was obtained from Daesung Industrial Gases Co. Ltd. Cr(NO3)39H2O and Na3RhCl62H2O (97.0% as Rh) were purchased from Sigma-Aldrich and Kanto Chemicals, respectively. All the chemicals were used without further purification. 2.2. Synthesis of Zn2Ga-LDH Zinc gallium LDH (Zn/Ga = 2) was prepared by coprecipitation method based on the solubility diagram of Zn2+ and Ga3+ hydroxides (Figure S1).30,34 Briefly, 250 mL of the mixed aqueous solution of Zn(NO3)26H2O (0.4 M) and Ga(NO3)3 (0.2 M) was drop-wisely titrated with the mixed aqueous solution of Na2CO3 (0.15 M) and NaOH (0.5 M) upto pH at 9.0 (± 0.2) at 30 oC to form white precipitate, where the molar ratio of Zn/Ga was fixed to 2. The resulting white precipitate in suspension was aged for 12 h, then collected by centrifugation, further washed with deionized water in order to remove possibly adsorbed or unreacted trace ions. The sample was finally freeze-dried for 24 hours, and further dried at 100 oC for 12 h. The prepared sample was denoted as Zn2Ga-LDH. 2.3. Synthesis of (GaN)1-x(ZnO)x solid solutions 5 ACS Paragon Plus Environment


Page 6 of 32

The (GaN)1-x(ZnO)x solid solutions were prepared via nitridation of Zn2Ga-LDH with or without a flux under dry ammonia flow. Briefly, 4.0 g of Zn2Ga-LDH powder was mixed with the flux, Na2CO3, corresponding to the molar ratio of 0.1, and mechanically grinded in an agate mortar. Subsequently, the prepared mixtures were calcined at 850 oC in vertical quartz tube-furnace under NH3 flow with a rate of 250 mL/min. The furnace was cooled down to room temperature under NH3 flow after the nitridation reaction for 8, 10, 12 and 14 h, finally washed with distilled water and dried at 100 oC for 12 h, where the final samples were denoted as GZLDH-F8h, GZLDH-F10h, GZLDHF12h and GZLDH-F14h depending on the nitridation time, respectively. For the comparison, the (GaN)1-x(ZnO)x solid solution was also prepared directly from Zn2Ga-LDH using the above procedure but without the addition of flux and the sample was denoted as GZLDH10h. The weights for finally obtained GZLDH-F8h, GZLDH-F10h, GZLDH-F12h, GZLDH-F14h and GZLDH-10h were 1.20 g, 1.06 g, 1.01 g, 0.97 g and 1.08 g, respectively. 2.4. Characterization Powder X-ray diffraction (PXRD) patterns of all the samples such as Zn2Ga-LDH and the (GaN)1-x(ZnO)x solid solutions were recorded with Rigaku D/MAX RINT 2200-Ultima+ diffractometer equipped with Cu-Kα radiation (λ = 1.5418 Å) at 40 kV and 30 mA. A step scan mode was employed in a 2θ range of 5 – 75° with a step size of 0.02° and the counting time of 2 seconds for each step. Fourier transform infrared (FT-IR) spectra were obtained with a JASCO FT/IR-6100 spectrometer by the KBr disk method. The UV-Vis diffuse reflectance spectra were recorded with JASCO V-550. Scanning electron microscopic images (SEM) were obtained with JEOL JSM-6700F field emission scanning electron microscope (FE-SEM) after Pt-coating. The Zn and Ga contents for all the samples were determined with the inductively coupled plasma - optical emission spectrometry (ICP-OES, Agilent, 5100). The specific surface areas of all the solid solutions prepared were studied by N2 adsorption-desorption measurements at liquid nitrogen temperature by using ASAP 2040. The samples were degassed under vacuum (10-5 torr) at 200 °C for 12 h before the measurement. 6 ACS Paragon Plus Environment

Page 7 of 32 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


2.5. Photocatalytic Reaction For the photocatalytic overall water-splitting reaction, the solid solutions were modified with Rh-Cr mixed-oxide nanoparticles as cocatalysts via the previously reported impregnation method.17-20 Briefly, 0.3 g of the (GaN)1-x(ZnO)x powder was dispersed in 3 mL of aqueous solution containing Cr(NO3)39H2O and Na3RhCl62H2O in the evaporating dish over the water bath, where the contents of Cr and Rh were 1.5 wt% and 1.0 wt%, respectively. The suspension was evaporated by stirring with a glass rod to form a powder, which was further calcined at 350 oC for 1 h. The photocatalytic reactions were carried out in an inner-irradiation Pyrex reaction vessel connected to a closed gas circulation and evacuation system. The (GaN)1-x(ZnO)x solid solution loaded with Rh-Cr mixed oxides powder (0.2 g) was dispersed by a magnetic stirrer in aqueous solution (400 mL) in an inner-irradiation type Pyrex vessel. The reaction vessel was first evacuated several times to completely remove air, and Ar gas was introduced to the vessel upto ~40 torr. Then visible light (λ > 400 nm) was irradiated on the suspension by using a 450 W high-pressure Hg lamp (Ushio Inc. UM-452) via a Pyrex glass tube filled with 2M of NaNO2 aqueous solution to filter the light with a wavelength less than 400 nm. The evolved gas was periodically analyzed with an in-situ gas chromatograph (GC-2014, Shimadzu) equipped with TCD detector and stainless steel column packed with Molecular Sieve 5A, where Ar gas was used as a carrier gas.

3. RESULTS AND DISCUSSION 3.1. Zn2Ga-LDH



Page 8 of 32

According to the solubility diagram (Figure S1), the lowest solubility for both the Zn2+ and Ga3+ hydroxides with respect to pH calculated was expected to be around 9.0, resulting in the coprecipitation.33 Based on the above diagram, LDH sample was successfully prepared by coprecipitating the aqueous mixed solution containing Zn2+ and Ga3+ species at pH = 9.0. The PXRD patterns of Zn2Ga-LDHs showed well developed (00l) diffraction peaks which correspond to the basal spacing of 7.60 Å (Figure 1) and consistent with those of the carbonate phase of Zn2Ga-LDH previously reported.29,30 All the peaks could be indexed on the basis of the rhombohedral crystal system (R3m) as shown in Figure 1. The unit cell parameters were determined to be a = 3.113 Å and c = 22.820 Å. According to the FT-IR spectrum (Figure S2), the peaks at ~3500 cm-1, 1630 cm-1 and 1370 cm-1 could be attributed to the (OH) symmetric and asymmetric, the bending vibrations of water, and the O-C-O symmetric one of CO32- with D3h symmetry, respectively. Those FT-IR peaks are known to be characteristic for the carbonate intercalated LDHs. The morphology of Zn2Ga-LDH was also monitored by SEM measurement. As shown in the inset of Figure 1, the prepared LDH has the plate-like morphology with ~ 100 nm along the basal plane direction with a thickness of several nm. Based on the ICP-OES analysis and TGA analysis (Figure S3), the chemical formula of LDH was determined as Zn0.69Ga0.31(OH)2(CO3)0.1550.6H2O. 3.2. (GaN)1-x(ZnO)x solid solutions The Zn2Ga-LDH precursor was transformed to the (GaN)1-x(ZnO)x solid solution through calcination at 850 °C via ammonolysis with and without Na2CO3 flux. PXRD patterns of (GaN)1-x(ZnO)x solid solutions synthesized from LDH with and without Na2CO3 flux together with the reference ZnO (PDF no. 36-1451) and GaN (PDF no. 50-792) are shown in Figure 2. As clearly observed in Figure 2, all the peaks corresponding to Zn2Ga-LDH disappeared after ammonolysis. However, new peaks corresponding to the Wurzite structure (hexagonal crystal system with P63mc space group) appeared without any impurities which are assigned to ZnO, GaN and ZnGa2O4, indicating that the (GaN)1-x(ZnO)x solid solutions were successfully synthesized from the LDH precursor. All the peaks could be observed between the peaks of ZnO and GaN, indicating the formation of solid solution, as in the previous report but with different intensity.20 For example, the 8 ACS Paragon Plus Environment

Page 9 of 32 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


difference between GZLDH-10h and GZLDH-F10h was the relative intensity of (100) and (002) peaks which is again depending upon crystal growth with or without flux. The (002) peak intensity of GZLDH-10h was determined to be more pronounced than that of GZLDHF10h, which could be attributed to the in-packing arrangement of platelet crystals grown along the direction of the specific crystal plane. To understand the detailed structures of those samples, the Rietveld refinements were performed with the Fullprof program.35 For structural refinement, the structural parameters of GaN (PDF no. 50-791) were adopted as an initial model and assumed as if the Ga and N atoms were substituted by Zn and O atoms, respectively. Since the Ga and Zn atoms were randomly distributed on the same crystallographic sites (2b Wyckoff positions) and N and O atoms were considered to occupy the other 2b Wyckoff positions, the occupation factors of Ga, Zn, N, and O atoms were fixed to the values evaluated from ICP-OES analysis. The observed, calculated and difference patterns from the Rietveld refinements of the X-ray diffraction data are shown in Figure 3. The preferred orientation in crystal growth can be examined by means of the March-Dollase algorithm in Rietveld refinements, where a parameter G1, is refined.34 If there is no preferred orientation, G1 should be one (G1 = 1), whereas if the material is plate-like crystal, G1 is smaller than one (G1 < 1).36 The G1 parameters for GZLDH-10h and GZLDH-F10h were determined to be 0.88 and 1.0, respectively, indicating that GZLDH-10h is plate-like crystal while GZLDH-F10h does not have any preferred orientation. The atomic coordinates and overall temperature factors are summarized in Table S1 and the bond distances are also listed in the Table S2. The position of N(O) in the solid solution is slightly different depending on the reaction condition, but the average bond length between Ga(Zn) and N(O) for all the samples are almost same as 1.96 Å. The morphology for the present solid solutions with and without flux was monitored with FE-SEM measurements as shown in Figure 4. When Zn2Ga-LDH was calcined under NH3 at a high temperature, it was transformed into the (GaN)1-x(ZnO)x solid solution (GZLDH-10h) which is highly dispersed and topochemically retained its plate-like morphology with the size of 100 nm along the plane direction. However, the morphology of GZLDH-F10h is different from that of GZLDH-10h since the former was at first fused in the flux and 9 ACS Paragon Plus Environment


Page 10 of 32

aggregated, resulting in a crystal growth with the particle size greater than 200 nm. This is mainly due to the effect of the flux Na2CO3, which played a role of well mixing and melting of the calcined LDH, and the crystal growth of the solid solution during ammonolysis. This is in good agreement with the XRD result. During the nitridation process, Zn(OH)2 in the precursor LDH was transformed to ZnO, which could be reduced to the Zn metal under a reductive atmosphere. Thus, the formed Zn metal could be volatilized at 850 oC under NH3 flow because the boiling point (907 oC) of Zn is rather low and the vapor pressure of Zn at 850 oC is more than 60 kPa.37 This process makes a drastic reduction of ZnO content during the nitridation process. The content of ZnO in (GaN)1-x(ZnO)x for GZLDH-10h was determined to be x = 0.23, which is slightly higher than that of GZLDH-F10h (x = 0.19). The UV-vis diffuse reflectance spectra for these (GaN)1-x(ZnO)x samples (GZLDH-10h and GZLDH-F10h) together with ZnO and GaN as the reference samples are displayed in Figure 5. Both samples absorb the light of visible region while ZnO and GaN absorb the UV light only, and Zn2Ga-LDH does not absorb the light longer than 300 nm (Figure S4). The absorption edge of GZLDH-F10h is about 470 nm, which is slightly shorter than that of GZLDH-10h (479 nm). It was found, however, that GZLDH-F10h absorbs the visible light between 400 nm and 470 nm more effectively than GZLDH-10h, surely due to the high crystallinity of the former. The bandgap energy (Eg) was also calculated by using the expression of (h)2 = A(h - Eg)n/2, where  is absorption coefficient, h is Plank’s constant,  is the light frequency, Eg is the band gap, and A is constant.38,39 The bandgap energies of GZLDH-F10h and GZLDH-10h were calculated to be 2.64 eV and 2.59 eV, respectively, which are smaller than that of ZnO (3.2 eV). We also investigated the effect of nitridation time on the crystal grown of the (GaN)1-x(ZnO)x solid solutions from the Zn2Ga-LDH precursors with the Na2CO3 flux. PXRD profiles for the solid solutions prepared at 850 oC for 8 h to 14 h were well indexed with a single phasic hexagonal wurtzite structure such as ZnO and GaN (Figure 6). The (110) peak position at 2θ = ~ 32.2o and the (101) one at ~36.7o were up-shifted to higher angle as the nitridation time was increased from 8 h to 14 h, indicating that the synthesized samples were not 10 ACS Paragon Plus Environment

Page 11 of 32 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 simple mixture of ZnO and GaN crystals, but their solid solution. The peak shift to higher angle could be attributed to the decrease of ZnO concentration in the solid solution as the sum of ionic radii of tetrahedral Zn2+ (0.74 Å) and O2- (1.24 Å) was estimated to be larger than that of tetrahedral Ga3+ (0.61 Å) and N3- (1.32 Å).40-42 The (002) peak was also up-shifted to higher angle with the increase of nitridation time, but the degree of shift was found to be smaller than that of (100) and (101) peaks, indicating a smaller change along the c-axis compared to the a-axis. An evolution in the unit cell parameters in the solid solutions could be more precisely determined by the Rietveld refinement considering the preferred orientation parameter (G1) as displayed in Figure S5, Table 1 and Table S1. With increasing the nitridation time from 8 h to 14 h, the cell parameters were proportionally reduced from a = 3.2105 Å and c = 5.1997 Å to a = 3.1961 Å and c = 5.1901 Å, respectively. The G1 parameter for all the solid solution from the LDH precursors with flux was found to be almost 1.0, indicating no preferred orientation in crystal growth. This is also one of the reasons for the absence of plate-like or needle-like crystals in the solid solution. According to the SEM analyses (Figure S6), the morphology for all the samples were found to be fused and aggregated. The particle size of the solid solution synthesized from the LDH precursor and flux for 8h (GZLDH-F8h) was around 150 nm, which was determined to be a little smaller than those of others (~ 200 nm). The chemical compositions for the solid solutions determined by ICP-OES analysis, and the unit cell parameters are summarized in Table 1. With increasing the nitridation time from 8 h to 14 h, the ZnO content (x) in (GaN)1x(ZnO)x

was decreased from 0.24 to 0.09, indicating that ZnO was reduced to Zn and subsequently volatilized, resulting in the lower

concentration in the solid solution, which is in good agreement with the XRD results. The UV-Vis diffuse reflectance spectra for the (GaN)1-x(ZnO)x solid solutions with respect to the nitridation time are shown in Figure 7. It can be seen in Figure 7 that the absorption edges were significantly red-shifted to the longer wavelength from 456 nm to 482 nm with decreasing the nitridation time from 14 h to 8 h, indicating that the visible light could be more efficiently harvested by the GZLDHF8h. The bandgap energies (Eg) for GZLDH-F8h, GZLDH-F10h, GZLDH-F12h and GZLDH-F14 were calculated to be 2.57 eV, 2.64 eV, 2.69 11 ACS Paragon Plus Environment


Page 12 of 32

eV and 2.72 eV, respectively, as included in Table 1. The bandgap energy of the (GaN)1-x(ZnO)x solid solution was found to be closely related to the concentration of ZnO in this solid solution.17,18 Density functional theory (DFT) calculations on the (GaN)1-x(ZnO)x solid solutions reported by Domen et al. demonstrated that the lowest energy level of conduction band were mostly consisted of 4s and 4p atomic orbitals of Ga whereas the highest energy levels of valence band were built up with N2p atomic orbital interacting with Zn3d and O2p ones.15,18 According to this calculation, the p-d repulsion lifts the valance band position upward without affecting the minimum conduction band level. Therefore, the minimum energy level of conduction band for this solid solution could be determined by the conduction energy level of Ga3d orbital of GaN, and the maximum level of valence band could also be changed by the degree of p-d repulsion interaction of Zn3d and O2p orbitals of ZnO and N2p orbital of GaN. As summarized in Table 1, Eg of (GaN)1-x(ZnO)x is decreased with increasing the concentration of ZnO, which is well consistent with the result of the DFT calculations. For example, a high concentration of ZnO in this solid solution results in the higher p-d repulsion, which give rise to the upward shift of valance band position and the decrease of Eg. 3.3. Photocatalytic water-splitting activity The photocatalytic activity of the (GaN)1-x(ZnO)x solid solutions was evaluated for overall water-splitting reaction under visible light irradiation in the distilled water medium without any electron or hole scavengers and pH adjustment, where the pH was fixed with ~ 6.8. Rhodium and chromium mixed oxide (Rh2-yCryO3) nanoparticles (1.0 wt% Rh and 1.5 wt% Cr) were loaded as cocatalysts on the (GaN)1x(ZnO)x

solid solutions via the impregnation method for the efficient photocatalytic overall water-splitting reaction.17,19 A typical time

course of photocatalytic overall water splitting for the (GaN)1-x(ZnO)x solid solutions from Zn2Ga-LDH modified with Rh2-yCryO3 nanoparticles under visible light irradiation (λ > 400 nm) is demonstrated in Figure 8 and the calculated rates of evolved H2 and O2 are summarized together with their specific surface areas (Figure S7) in Table 2. H2 and O2 gases were sustainably evolved by photocatalytic 12 ACS Paragon Plus Environment

Page 13 of 32 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


electrolysis of water under visible light with the evolution ratio of H2 to O2 = 2.2, which was slightly larger than that expected from the stoichiometry. The solid solution prepared from the Zn2Ga-LDH precursor with the Na2CO3 flux for 10h (GZLDH-F10h) resulted in the production of H2 and O2 gases with the evolution rates of 78 mol/h and 34 mol/h, respectively, while that prepared from intact Zn2GaLDH (GZLDH-10h) led to the generation of H2 and O2 gases with the evolution rates of 11 mol/h and 5 mol/h, respectively. Though the photocatalytic activity of the (GaN)1-x(ZnO)x solid solutions is thought to be mainly affected by the crystallinity, specific surface area, atomic composition, and bandgap energy, the present sample of GZLDH-F10h showed 7 times higher photocatalytic activity than GZLDH10h. It should be noted that Eg of GZLDH-F10h (2.64 eV) was slightly higher than that of GZLDH-10h (2.59 eV) but their ZnO contents were almost similar with x = 0.19 for the former and x = 0.23 for the latter. Furthermore, the specific surface area of GZLDH-F10h (9.4 m2g1)

was estimated to be significantly smaller than that of GZLDH-10h (14.6 m2g-1) as summarized in Table 2. Although the morphology of

GZLDH-F10h is quite different from GZLDH-10h with the plate-like morphology as shown in Figure 2, a high photocatalytic activity was observed for GZLDH-F10h, due to its high crystallinity without any preferred orientation, resulting in better light absorption as shown in UV-vis spectra (Figure 3). Furthermore, GZLDH-F10h showed the sustainable water-splitting activity in the repeated cycles for 10 h without a noticeable degradation of the activity (Figure S8), which is similar to the previous reports.13,43 As described above, the present flux assisted route could be considered as an useful way of preparing the (GaN)1-x(ZnO)x solid solutions with a high water-splitting activity. In addition, the relation between nitridation time and photocatalytic activity was investigated for the (GaN)1-x(ZnO)x solid solutions. As shown in Figure 8, the longer the nitridation time was, the larger bandgap energy became with lowered visible light absorption, and as a consequence, the photocatalytic activity was decreased, even though the specific surface area (~ 8.7  0.7 m2g-1) was almost the same. In particular, the activity was drastically dropped down after 14 h nitridation, due to a strong reduction of ZnO concentration x=0.09) in the solid solution together with the large band gap (Figure S9). GZLDH-F8h and GZLDH-F10h were also similar in terms of photocatalytic



Page 14 of 32

activity under visible light with the gas evolution rates of 81 mol/h (H2) and 37 mol/h (O2), and 78 mol/h (H2) and 34 mol/h (O2), respectively.

4. CONCLUSIONS Single phasic (GaN)1-x(ZnO)x solid solutions with a high crystallinity showing efficient photocatalytic overall water- splitting activity were successfully synthesized from Zn2Ga-LDH precursor via Na2CO3-flux-assisted nitridation method. According to the XRD and SEM analyses, the (GaN)1-x(ZnO)x solid solutions prepared without flux was determined to be low in crystallinity but plate-like in morphology with preferred orientation toward (00l) direction. When the Na2CO3 flux was used, however, all the solid solution phases were turned out to be highly crystalline without any preferred orientation. With increasing the nitridation time from 8 h to 14 h via the flux- assisted method, the concentration of ZnO in (GaN)1-x(ZnO)x was decreased, giving rise to a reduction of unit cell parameters (a and c parameters) with the blue-shift of absorption edges. In this study, we were able to precisely control the concentration of ZnO in the range from x = 0.09 to 0.24, and the bandgap energy from 2.54 eV to 2.72 eV. The GZLDH-F10h showed 7 times higher gas evolution rate with 78 mol/h (H2) and 34 mol/h (O2) than that prepared without flux (GZLDH-10h) [11 mol/h (H2) and 5 mol/h (O2)] for the photocatalytic overall water splitting even though the specific surface area of GZLDH-F10h (9.4 m2g-1) was significantly smaller than that of GZLDH-10h (14.6 m2g-1). The highest photocatalytic activity with gas evolution rate of 81 mol/h (H2) and 37 mol/h (O2) under visible light irradiation was observed for the sample of GZLDH- F8h with the bandgap energy of 2.54 eV and the ZnO concentration x = 0.24, suggesting that the Na2CO3-flux-assisted nitridation reaction could be an effective route to the zinc gallium oxynitride photocatalyst from Zn/Ga-LDHs precursor for overall water-splitting applications.


Page 15 of 32 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


ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXXXXXX. The atomic coordinates, the overall temperature parameters (Boverall), the preferred orientation parameters (G1), and the Ga(Zn)-N(O) bond distances of (GaN)1-x(ZnO)x refined from the Rietveld analysis. FT-IR spectrum and TG-DTA curves of Zn2Ga-LDH. Rietveld refinements of the powder XRD profiles and SEM images for prepared (GaN)1-x(ZnO)x solid solutions. Relationship between photocatalytic water-splitting activity and the nitridation time for (GaN)1x(ZnO)x

solid solutions. (PDF)

AUTHOR INFORMATION Corresponding Author * [email protected] Author Contributions ⊥These authors contributed equally. Notes The authors declare no competing financial interest.



Page 16 of 32

ACKNOWLEDGMENT This work was supported by grants from the Ministry of Knowledge Economy (1004 1239) and the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (2005-0049412 and 2013R1A1A2062239). Dr. Jae-Hun Yang was supported by RP-Grant 2016 of Ewha Womans University

REFERENCES (1) Boonen, E.; Beeldens, A. Recent Photocatalytic Applications for Air Purification in Belgium. Coatings 2014, 4, 553-573. (2) Gielen, D. ; Boshell, F.; Saygin, D. Climate and energy challenges for materials science. Nat. Mater. 2016, 15, 117–120. (3) Rodriguez, C. A.; Modestino, M. A.; Psaltis, D.; Moser, C. Design and cost considerations for practical solar-hydrogen generators. Energy Environ. Sci. 2014, 7, 3828-3835. (4) Hisatomi, T.; Kubota, J.; Domen, K. Recent advances in semiconductors for photocatalytic and photoelectrochemical water splitting. Chem. Soc. Rev. 2014, 43, 7520-7535. (5) Wenderich, K.; Mul, G. Methods, Mechanism, and Applications of Photodeposition in Photocatalysis: A Review. Chem. Rev. 2016, 116, 14587-14619. (6) Wang, X.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.; Domen, K.; Antonietti, M. A metal-free polymeric photocatalyst for hydrogen production from water under visible light. Nat. Mater. 2009, 8, 76-80.


Page 17 of 32 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


(7) Ong, W. J.; Tan, L. L.; Ng, Y. H.; Yong, S. T.; Chai, S. P. Graphitic Carbon Nitride (g-C3N4)-Based Photocatalysts for Artificial Photosynthesis and Environmental Remediation: Are We a Step Closer To Achieving Sustainability? Chem. Rev. 2016, 116, 7159-7329. (8) Kim, H. N.; Kim, T. W.; Kim, I. Y.; Hwang, S. J. Cocatalyst‐Free Photocatalysts for Efficient Visible‐Light‐Induced H2 Production: Porous Assemblies of CdS Quantum Dots and Layered Titanate Nanosheets. Adv. Funct. Mater. 2011, 21, 3111-3118. (9) Tee, Y.; Win, K. Y.; Teo, W. S.; Koh, L.D.; Liu, S.; Teng, C. P.; Han, M. Y. Recent Progress in Energy-Driven Water Splitting. Adv. Sci. 2017, 4, 1600337 (10) Yuliati, L.; Yang, J. H.; Wang, X.; Maeda, K.; Takata, T.; Antonietti, M.; Domen, K. Highly active tantalum(V) nitride nanoparticles prepared from a mesoporous carbon nitride template for photocatalytic hydrogen evolution under visible light irradiation. J. Mater. Chem. 2010, 20, 4295-4299. (11) Lee, J. M.; Yang, J. H.; Kwon, N. H.; Jo, Y. K.; Choy, J. H., Hwang, S. J. Intercalative hybridization of layered double hydroxide nanocrystals with mesoporous g-C3N4 for enhancing visible light-induced H2 production efficiency..Dalton Trans. 2018, 47, 2949-2955. (12) Shakeel, M.; Li, B.; Yasin, G.; Arif, M.; Rehman, W.; Khan, H. D. In Situ Fabrication of Foamed Titania Carbon Nitride Nanocomposite and Its Synergetic Visible-Light Photocatalytic Performance. Ind. Eng. Chem. Res. 2018, 57, 8152−8159. (13) Maeda, K.; Teramura, K.; Lu, D. L.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K. Photocatalyst releasing hydrogen from water. Nature 2006, 440, 295. (14) Maeda, K.; Takata, T.; Hara, M.; Saito, N.; Inoue, Y.; Kobayashi, H.; Domen, K. GaN:ZnO solid solution as a photocatalyst for visiblelight-driven overall water splitting. J. Am. Chem. Soc. 2005, 127, 8286–8287.



Page 18 of 32

(15) Maeda, K.; Teramura, K.; Takata, T.; Hara, M.; Saito, N.; Toda, K.; Inoue, Y.; Kobayashi, H.; Domen, K. Overall water splitting on (Ga1xZnx)(N1-xOx)

solid solution photocatalyst: relationship between physical properties and photocatalytic activity. J. Phys. Chem. B 2005, 109,

20504–20510. (16) Maeda, K.; Hashiguchi, H.; Masuda, H.; Abe, R.; Domen, K. J. Phys. Chem. C Photocatalytic Activity of (Ga1-xZnx)(N1-xOx) for VisibleLight-Driven H2 and O2 Evolution in the Presence of Sacrificial Reagents. 2008, 112, 3447-3452. (17) Maeda, K.; Teramura, K.; Lu, D.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K. Characterization of Rh−Cr Mixed-Oxide Nanoparticles Dispersed on (Ga1-xZnx)(N1-xOx) as a Cocatalyst for Visible-Light-Driven Overall Water Splitting. J. Phys. Chem. B 2006, 110, 13753-13758. (18) Hisatomi, T.; Maeda, K.; Lu, D.; Domen, K. The Effects of Starting Materials in the Synthesis of (Ga1−xZnx)(N1−xOx) Solid Solution on Its Photocatalytic Activity for Overall Water Splitting under Visible Light. ChemSusChem 2009, 2, 336-343. (19) Maeda, K.; Teramura, K.; Saito, N.; Inoue, Y.; Domen, K. Improvement of photocatalytic activity of (Ga1−xZnx)(N1−xOx) solid solution for overall water splitting by co-loading Cr and another transition metal. J. Catal. 2006, 243, 303–308. (20) Maeda, K.; Teramura, K.; Masuda, H.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K. Efficient Overall Water Splitting under Visible-Light Irradiation on (Ga1−xZnx)(N1−xOx) Dispersed with Rh-Cr Mixed-Oxide Nanoparticles: Effect of Reaction Conditions on Photocatalytic Activity. J. Phys. Chem. B 2006, 110, 13107-13112. (21) Park, D. H.; Hwang, S. J.; Oh, J. M.; Yang J. H.; Choy, J. H. Polymer–inorganic supramolecular nanohybrids for red, white, green, and blue applications. Prog. Polym. Sci. 2013, 38, 1442-1486. (22) Wang, Q.; O’Hare, D. Recent advances in the synthesis and application of layered double hydroxide (LDH) nanosheets. Chem. Rev. 2012, 112, 4124-4155. 18 ACS Paragon Plus Environment

Page 19 of 32 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


(23) Woo, M. A.; Song, M. S.; Kim, T. W.; Kim, I. Y.; Ju, J. Y.; Lee, Y. S.; Kim, S. J.; Choy, J. H.; Hwang, S. J. Mixed valence Zn–Co-layered double hydroxides and their exfoliated nanosheets with electrode functionality. J. Mater. Chem. 2011, 21, 4286-4292. (24) Choi, G.; Jeon, I. R.; Piao, H.; Choy, J. H. Highly Condensed Boron Cage Cluster Anions in 2D Carrier and Its Enhanced Antitumor Efficiency for Boron Neutron Capture Therapy. Adv. Funct. Mater. 2018, 28, 1704470 (DOI: 10.1002/adfm.201704470). (25) Portier, J.; Choy, J. H.; Subramanian, M.A. Inorganic–organic-hybrids as precursors to functional materials. Int. J. Inorg. Mater. 2001, 3, 581-592. (26) Choy, J. H.; Park, J. S.; Kwak, S. Y.; Jeong, Y. J.; Han, Y. S. Layered Double Hydroxide as Gene Reservoir. Mol. Cryst. Liquid Cryst. 2000, 341, 425-429. (27) Choi, S. J.; Choi, G. E.; Oh, J. M.; Oh, Y. J.; Park, M. C., Choy, J. H. Anticancer drug encapsulated in inorganic lattice can overcome drug resistance. J. Mater. Chem. 2010, 20, 9463-9469. (28) Yang, J. H.; Zhang, W.; Ryu, H.; Lee, J. H.; Park, D. H.; Choi, J. Y.; Vinu, A.; Elzatahry, A. A.; Choy, J. H. Influence of anionic surface modifiers on the thermal stability and mechanical properties of layered double hydroxide/polypropylene nanocomposites. J. Mater. Chem. A 2015, 3, 22730-22738. (29) Wang, J.; Huang, B.; Wang, Z.; Wang, P.; Cheng, H.; Zheng, Z.; Qin, X.; Zhang, X.; Dai, Y.; Whangbo, M. W. Facile synthesis of Znrich (GaN)1−x(ZnO)x solid solutions using layered double hydroxides as precursors. J. Mater. Chem., 2011, 21, 4562-4567. (30) Thomas, G. S.; Kamath, P. V. The layered double hydroxide (LDH) of Zn with Ga: Synthesis and reversible thermal behaviour. Soid State Sci. 2006, 8, 1181-1186.



Page 20 of 32

(31) Adeli, B.; Taghipour, F. Facile synthesis of highly efficient nano-structured gallium zinc oxynitride solid solution photocatalyst for visible-light overall water splitting. Appl. Cata. A: General, 2016, 521, 250-258. (32) Takata, T.; Lu, D.; Domen, K. Synthesis of Structurally Defined Ta3N5 Particles by Flux-Assisted Nitridation. Crys. Growth & Design 2011, 11, 33-38. (33) Moon, K. H. ; Kim, J. M.; Sohn, Y.; Cho, D. W.; Kim, Y. I.; Avdeev, M. Crystal structures and color properties of new complex perovskite oxynitrides AMg0.2Ta0.8O2.6N0.4(A = Sr, Ba). Dalton Trans. 2016, 45, 5614-5621. (34) Baes Jr., C. F.; Mesmer, R. E. The Hydrolysis of Cations, Wiley, New York, 1986. (35) Rodriguez-Carvajal, J. Fullprof 2000: A Rietveld Refinement and Pattern Matching Analysis Program, April 2008, Laboratoire Léon Brillouin (CEA-CNRS). (36) Dollase, W. A. Correction of Intensities for Preferred Orientation in Powder Diffractometry: Application of the March Model. J. Appl. Cryst. 1986, 19, 267-272. (37) Haynes, W. M.; Lide, D. R.; Bruno, T. J. CRC Handbook of Chemistry and Physics, 95th Ed., CRC Press, New York, 2014, 6-93. (38) Zeng, J.; Wang, H.; Zhang, Y. C.; Zhu, M. K.; Yan, H. Hydrothermal Synthesis and Photocatalytic Properties of Pyrochlore La2Sn2O7 Nanocubes. J. Phys. Chem. C 2007, 111, 11879-11887. (39) Butler, M. A. Photoelectrolysis and physical properties of the semiconducting electrode WO2. J. Appl. Phys. 1977, 48, 1914–1920. (40) R. D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Cryst. A 1976, A32, 751-767. 20 ACS Paragon Plus Environment

Page 21 of 32 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


(41) Shannon, R. D.; Prewitt, C. T. Effective ionic radii in oxides and fluorides. Acta Cryst. B 1969, B25, 925-946. (42) Huheey, J. E. ; Keiter, E.A.; Keiter, R. L. Inorganic Chemistry : Principles of Structure and Reactivity 4th Ed., HarperCollins, New York, 1993. (43) Ohno, T.; Bai, L.; Hisatomi, T.; Maeda, K.; Domen, K. Photocatalytic Water Splitting Using Modified GaN:ZnO Solid Solution under Visible Light: Long-Time Operation and Regeneration of Activity. J. Am. Chem. Soc. 2012, 134, 8254−8259.



Page 22 of 32

Table 1. Content of ZnO (x) in (GaN)1-x(ZnO)x solid solutions, their bandgap energy and unit cell parameters. cell parameters (Å)b Samples

xa

Eg a

c

ZnOc

1.00

-

3.2498

5.2066

GZLDH-10h

0.23

2.59

3.2033

5.1960

GZLDH-F8h

0.33

2.57

3.2105

5.1997

GZLDH-F10h

0.19

2.64

3.2005

5.1932

GZLDH-F12h

0.15

2.69

3.1979

5.1917

GZLDH-F14h

0.12

2.72

3.1961

5.1901

GaNd

0.00

-

3.1891

5.1855

a

Calculated by ICP-OES measurement Determined by rietveld refinement c, d Referred from the reference data (PDF no. 50-791 for GaN, and PDF no. 36-1451 for ZnO) b


Page 23 of 32 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. The BET specific surface area and the evolution rate of H2 and O2 in the photocatalytic water-splitting reaction for the (GaN)1-x(ZnO)x solid solutions synthesized from Zn2Ga-LDH.

Samples

a Specific

surface area

b

Activity (mol/h)

(m2/g)

H2

O2

Zn2Ga-LDH

-

ND

ND

GZLDH-10h

14.6

11

5

GZLDH-F8h

9.1

82

37

GZLDH-F10h

9.4

78

34

GZLDH-F12h

8.5

68

30

GZLDH-F14h

7.9

10

5

cGZON-17

7.4

316

162

dGZON-17

8.0

32

16

a

Specific surface area is calculated with BET (Brunauer–Emmett–Teller) theory from the N2 adsorption-desorption isotherm. b(GaN) (ZnO) was loaded with Rh Cr O (1 wt% Rh, 1.5 wt% Cr) in overall water splitting 1-x x 2-y y 3 under visible light ( > 400 nm). Reaction conditions: catalyst 0.2g, distilled water 400 mL, high-pressure mercury lamp (450 W), Pyrex inner-irradiation type vessel with 2M NaNO2 aqueous solution filter. The steady rate of gas evolution was calculated from the linear fitting of the data. c,d GZON- and GZON- are (GaN) (ZnO) solid solutions via the conventional method with 1-x x ZnO and -Ga2O3 and -Ga2O3, respectively.17 * ND indicates that H2 or O2 gases were not detected in this reaction condition.



Page 24 of 32

Figure 1. XRD pattern and SEM image (inset) of Zn2Ga-LDH.


Page 25 of 32 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 2. XRD patterns for (a) ZnO (PDF # 36-1451), (b) GZLDH-10h, (c) GZLDH-F10h and (d) GaN (PDF # 50-792).



(a)

Page 26 of 32

Rp = 6.96 %, Rwp = 10.1 % Rexp = 4.23 %, χ2 = 5.65

(b)

Rp = 7.75 %, Rwp = 11.8 % Rexp = 4.27 %, χ2 = 7.59

Figure 3. Rietveld refinement of the powder XRD profiles of (a) GZLDH-10h, (b) GZLDHF10h, respectively. The observed profiles and fitted results, expected reflection positions, and the difference between the measured and fitted results are expressed as red solid circles, black solid line, green tick markers, and lower blue line, respectively.


Page 27 of 32 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 4. SEM images for (GaN)1-x(ZnO)x solid solutions synthesized from Zn2GaLDH via nitridation without and with flux; (a) GZLDH-10h and (b) GZLDH-F10h, respectively



Page 28 of 32

Figure 5. Diffuse reflectance UV-vis spectra for GZLDH-10h, GZLDH-F10h, ZnO and GaN, respectively.


Page 29 of 32 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 6. XRD patterns for (GaN)1-x(ZnO)x solid solutions synthesized via flux assisted method depending on the nitridation of time; (a) GZLDH-F8h, (b) GZLDH-F10h, (c) GZLDH-F12h, and (d) GZLDH-F14h, respectively.



Page 30 of 32

Figure 7. Diffuse reflectance UV-vis spectra for for (GaN)1-x(ZnO)x solid solutions synthesized via flux assisted method depending on the nitridation of time (8 h, 10 h, 12 h and 14 h).


Page 31 of 32 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 8. Typical time course of photocatalytic overall water splitting on (GaN)1-x(ZnO)x solid solutions from Zn2Ga-LDH modified with Rh2-yCryO3 nanoparticles under visible light irradiation (λ> 400 nm).



Page 32 of 32

Table of Contents. Gallium Zinc oxinitride solid solution from LDH via flux assisted ammonolysis (GZON-LDH-Flux) demonstrated highly enhanced photocatalytic overall water-splitting activity (7 times) compared to that prepared from LDH without flux (GZON-LDH).


Highly Enhanced Photocatalytic Water-splitting Activity of Gallium Zinc

Recommend Documents