Unique Zinc Germanium Oxynitride Hyperbranched Nanostructures with Enhanced Visible‐Light Photocatalytic Activity for CO 2 Reduction

Unique (Zn1+xGe)(N2Ox) hyperbranched nanostructures with rough surfaces were prepared by nitriding Zn2GeO4 bundles at 700 °C. In this process, the constituent smooth nanobelts of the Zn2GeO4 bundles are transformed into chains composed of nanoparticles; therefore, the (Zn1+xGe)(N2Ox) hyperbranched nanostructures have a lager specific surface area, which is twice that of the Zn2GeO4 precursor. Compared to reference (Zn1+xGe)(N2Ox) particles synthesized by a solid‐state reaction approach as well as (Zn1+xGe)(N2Ox) prepared by nitriding Zn2GeO4 nanorods at 700 °C for 6 h, the optimal hyperbranched (Zn1+xGe)(N2Ox) particles exhibit enhanced activity for the photoreduction of CO2 to CH4 under visible light due to their unique 3D hyperbranched nanostructure. A GaN–ZnO solid solution with a 3D microsphere structure can be obtained by a similar process. This work provides valuable information for the preparation of oxynitrides with high specific surface areas.

Unique (Zn1+xGe)(N2Ox) hyperbranched nanostructures with rough surfaces, tailored by simply nitriding Zn2GeO4 bundles at 700 °C, exhibit enhanced activity toward the reduction of CO2 to CH4 under visible light due to their 3D hyperbranched nanostructure.

Zinc; Germanium; Oxynitrides; Nanostructures; Nitridation; Photocatalysis

The photocatalytic reduction of CO2 to valuable hydrocarbon fuels in the presence of water through the help of photocatalysts would be like killing two birds with one stone in terms of saving our environment.[1] Fujishima and Honda first introduced the possibility of reducing CO2 photocatalytically by semiconductor photocatalysts in aqueous solution.[2] TiO2 is a widely applied material in photocatalysis.[3] Nevertheless, the large band gap of TiO2 (3.2 eV for anatase) undermines the possible use of visible light. Some visible‐light photocatalysts are unstable under illumination (e.g., CdS and CdSe). Doping is a popular way to expand the absorption wavelength edge of TiO2 into the visible region. Owing to the considerable bulk defects and localized d states in the electronic structure that suppress the migration of carriers, the doping of TiO2 with 3d transition elements has a restricted applicability.[4] Anion‐doped (N, C, B, and S mostly) TiO2 visible‐light photocatalysts have attracted considerable attention.[5] However, only a limited enhancement in visible‐light absorption has been obtained.[4] , [6]

Zinc germanium oxynitride, (Zn1+xGe)(N2Ox), which is a ZnO and ZnGeN2 solid solution, is well known as a stable and active visible‐light‐responsive photocatalyst for overall water splitting[7] and the photodegradation of organic pollutants in water.[8] In the normal preparation process of (Zn1+xGe)(N2Ox), a mixture of GeO2 and ZnO powders is calcined under flowing NH3 at high temperature.[7] , [8] Lee and co‐workers successfully prepared (Zn1+xGe)(N2Ox) through the reaction between ZnO and GeO2 at 1123 K under NH3 flow (20 mL min–1) for the first time and demonstrated that (Zn1+xGe)(N2Ox) (x = 0.44) serves as an efficient photocatalyst for total water splitting under UV/Vis light.[[7] ], [[7] ] If (Zn1+xGe)(N2Ox) is loaded with RuO2 and Rh2–yCryO3 as cocatalysts, its photocatalytic activity in water splitting for hydrogen production under visible light is greatly enhanced.

The activity of a photocatalyst is not only heavily dependent on its inherent physical properties but also correlated to the morphology of the crystals (such as particle size, nanostructure, and surface arrangement).[9] The fabrication of morphology‐controlled catalysts is a highly efficient way to enhance their properties, as heterogeneous catalysis is more sensitive to the surface and interface of the catalyst. However, these (Zn1+xGe)(N2Ox) materials typically have a nanoparticle morphology, which restricts the systematic study of the effect of microstructure in (Zn1+xGe)(N2Ox) on its photocatalytic activity. Ye and co‐workers adopted a different approach and prepared mesoporous zinc germanium oxynitride, and the mesoporous structure proved effective in enhancing the activity of the catalyst in the photoreduction of CO2 under visible light.[10]

Herein, a simple and facile approach for the preparation of (Zn1+xGe)(N2Ox) solid solution with three‐dimensional (3D) morphologies is reported. Zn2GeO4 bundles were synthesized solvothermally in a mixed solvent of ethylenediamine and distilled water at 180 °C. By nitriding Zn2GeO4 under NH3 flow (200 mL min–1), a yellow (Zn1+xGe)(N2Ox) solid solution with a hyperbranched nanostructure can be obtained. The prepared (Zn1+xGe)(N2Ox) solid solution was applied to reduce CO2 to CH4 under visible light. In comparisons with the activities of reference (Zn1+xGe)(N2Ox) particles synthesized through a solid‐state reaction as well as (Zn1+xGe)(N2Ox) prepared by nitriding Zn2GeO4 nanorods at 700 °C for 6 h, the optimal hyperbranched (Zn1+xGe)(N2Ox) exhibited enhanced CO2 photoreduction activity under visible‐light irradiation owing to their unique 3D hyperbranched nanostructures.

A (Zn1+xGe)(N2Ox) solid solution was prepared by nitriding Zn2GeO4 under NH3 flow. The nitridation temperature and time play very important roles in controlling the crystal structure, chemical composition, and surface morphology of the final product. The XRD patterns of the Zn2GeO4 precursor and the samples nitrided under different temperature for 6 h are shown in Figure [NaN] . For the as‐synthesized Zn2GeO4 precursor, all of the diffraction peaks can be indexed to a Zn2GeO4 phase with the rhombohedral crystal structure (JCPDS card 11‐0687). No additional signals corresponding to impurities were identified. After the samples was nitrided at 600 °C for 6 h (Figure [NaN] b), no new diffraction peaks were observed; therefore, no phase transition occurs. In addition, the intensity and sharpness of the XRD peaks increase slightly, which implies that the Zn2GeO4 crystals have improved crystallinity owing to the growth of crystals at high temperature. The XRD peaks of these two samples corresponding to the (113) and (410) planes of Zn2GeO4 were analyzed. The peaks of the sample nitrided at 600 °C are broader and shifted toward higher 2θ values. These results indicate that the crystals nitrided at 600 °C are distorted and internal strain exists within the crystals after N doping. The number and intensities of the diffraction peaks began to change when the reaction temperature was increased to 650 °C (Figure [NaN] c). The XRD peaks of the Zn2GeO4 crystals disappear or become weaker, and (Zn1+xGe)(N2Ox) becomes the dominant phase, as indicated by the intense peaks ascribed to (Zn1+xGe)(N2Ox). All of the XRD diffraction peaks of the Zn2GeO4 crystals disappeared as the annealing temperature was increased further to 700 °C (Figure [NaN] d). The remaining XRD peaks can be ascribed to (Zn1+xGe)(N2Ox) (JCPDS card 24‐1443) with lattice constants of a = b = 3.2 Å and c = 5.19 Å. The diffraction peaks of the hexagonal (Zn1+xGe)(N2Ox) phase are similar to those of ZnGeN2 (JCPDS card 47‐1426) and ZnO (JCPDS card 36‐1451), and the diffraction angles are between those of ZnGeN2 and ZnO. These results show that the obtained material is a solid solution of ZnO and ZnGeN2. However, some weak peaks in the JCPDS standard card do not appear in our XRD pattern owing to the broadening of the peaks.

The XRD patterns of the as‐synthesized crystalline Zn2GeO4 and the samples nitrided at 700 °C for different times were also investigated, as shown in Figure [NaN] . For the sample treated with NH3 for 2 h, rhombohedral Zn2GeO4 was dominant, and some additional XRD peaks corresponding to (Zn1+xGe)(N2Ox) appeared. As the nitridation time increased from 2 to 4 h, the peaks of the Zn2GeO4 phase were apparent, and the peaks of the (Zn1+xGe)(N2Ox) phase became prominent. The treatment of the sample with NH3 for 6 h resulted in pure (Zn1+xGe)(N2Ox) phase without residues or impurity phases (Figure [NaN] d). The XRD patterns of the samples nitrided for 6 and 8 h are similar, and only the peak intensity increases slightly.

The optical absorption spectra of the samples nitrided at 700 °C for different times and the Zn2GeO4 precursor are illustrated in Figure [NaN] a. The absorption edges of the nitrided products exhibit remarkable redshifts compared with that of Zn2GeO4 precursor. There is a gradual redshift of the absorption edge toward λ = 550 nm with the increase of nitridation reaction time. For a crystalline semiconductor, the optical absorption near the band edge follows Equation (1).[11]

(αhν)n=A(hν−Eg)

Here, α is the absorption coefficient, and h is the Plank constant; ν, Eg, and A are the frequency (1/λ), the band gap, and a constant, respectively; n is either 2 for a direct interband transition or 1/2 for an indirect interband transition. Both (Zn1+xGe)(N2Ox) and Zn2GeO4 exhibit direct interband transitions, and the value of n equals to 2.[[7] ], [[9] ]Through the extrapolation of the straight portion of the plot of (αhν)2 versus (hν) to the α = 0 point, the Eg values of the prepared samples can be estimated (Figure S1). The band gap of the as‐synthesized Zn2GeO4 precursor is about 4.53 eV and it has an optical absorption edge at λ ≈ 274 nm. The absorption onsets of the samples treated with NH3 for 2, 4, 6, and 8 h are at λ ≈ 490, 498, 512, and 519 nm, respectively, corresponding to band gaps of 2.53, 2.49, 2.42, and 2.39 eV, respectively; therefore, the band gap becomes narrower as the reaction time increases. As expected from the band‐gap changes, the color of the samples changed from gray to yellow. The X‐ray photoelectron spectroscopy (XPS) studies show that there is no clear N 1s peak for the Zn2GeO4 precursor. After NH3‐treatment for 2 h, a weak N 1s peak at a binding energy (BE) of 397–399 eV indicated the low nitrogen content of this sample. The intensity of the N 1s peak increased with extended nitridation time because more N atoms were introduced into the Zn2GeO4 precursor. The N 1s peaks for the (Zn1+xGe)(N2Ox) solid solution were observed at BE = 397.5–396.7 eV (NH3‐treated for 6 and 8 h). The positions of theses N 1s peaks differ from that for Zn3N2 (BE = 395.8 eV), and this suggests that the Zn valence state in the (Zn1+xGe)(N2Ox) powder is different from that of Zn3N2.[[7] ] The XPS elemental analysis results (see Table [NaN] ) show that the nitrogen concentration in the nitrided samples increased remarkably from 10.7 to 42.1 atom‐% with the increase of the nitridation reaction time from 2 to 8 h, whereas the oxygen content decreased (from 35.1 to 11.3 atom‐%). This is because more O was replaced by N as NH3 treatment time under high temperature increased. The zinc content decreased only slightly from 28 to 25.7 atom‐% as the nitridation time was increased from 2 to 8 h. For the sample prepared for 6 h, the chemical composition is Zn1.35GeN2.08O0.67. Interestingly, the Brunauer–Emmett–Teller (BET) surface areas of these samples increased with the increasing nitridation time from 2 to 6 h, and this result is inconsistent with previous reports.[10] The sample prepared for 6 h shows the highest specific surface area of about 40.47 m2 g–1, which is approximately two times that of the Zn2GeO4 precursor (21.14 m2 g–1).

Chemical compositions of the prepared samples

Typical field‐emission SEM (FE‐SEM) images of Zn2GeO4 and (Zn1+xGe)(N2Ox) solid solution nitrided at 700 °C for 6 h are shown in Figure [NaN] . The FE‐SEM images with different magnifications show that Zn2GeO4 is mostly composed of symmetric bundles. These bundles are self‐assembled from nanobelts. The individual bundle are 2.5–5 µm long and 500–700 nm wide. Frequently, these bundles are intergrown to generate branched star‐shaped configurations or highly hierarchical structures. Structures made up of several bundles with flowerlike architectures can be observed frequently (Figure [NaN] b). Each petal is composed of numerous uniform and smooth nanobelts, which have an average diameter of 20–100 nm. After nitridation, the resulting product inherits the bundle shape of the Zn2GeO4 precursor with a length of 3–8 µm and a diameter of 0.5–2 µm (Figure [NaN] c). This high‐temperature nitridation process causes the constituent smooth nanobelts to be transformed into rough chains composed of nanoparticles. The rough surface gives the (Zn1+xGe)(N2Ox) solid solution a high specific surface area.

The TEM image of a typical (Zn1+xGe)(N2Ox) architecture is shown in Figure [NaN] , in which the flowerlike structure composed of several bundles is illustrated. The rough surface formed by the nitridation of the particles is clearly shown. The clear diffraction rings (inset of Figure [NaN] a) show that the (Zn1+xGe)(N2Ox) material is highly crystalline. As is clearly shown in Figure [NaN] b, the constituent nanobelts are fused slightly into rough nanoparticle chains after high‐temperature treatment, and the typical chains have sizes of about 100–200 nm. The representative HRTEM image shows an interplanar d spacing of about 0.274 nm, which corresponds to the 100 lattice planes of hexagonal (Zn1+xGe)(N2Ox) (see inset of Figure [NaN] b).

To measure the photocatalytic activities of the prepared (Zn1+xGe)(N2Ox) solid solution samples (the products treated with NH3 for 6 and 8 h are referred to as ZGON‐6 and ZGON‐8, respectively), the photocatalytic CO2 conversion with water vapor under visible‐light irradiation (λ > 420 nm) was investigated. For CO2 reduction with water vapor, the transfer of a single electron to CO2 is highly improbable as it is very unfavorable energetically. The situation is better for a proton‐assisted transfer of multiple electrons.[9] Different products such as HCOOH, CO, HCHO, CH3OH, and CH4 can been obtained by CO2 reduction. The band‐gap structure of a semiconductor is an important factor for the selectivity of the CO2 reduction product. The possible reactions related to the photocatalytic reduction of CO2 with H2O and the corresponding redox potentials versus the normal hydrogen electrode (NHE) are listed in Equations (2), (3), (4), (5), (6), (7), and (8).[13] The formation of CH4 and CH3OH, which needs eight and six electrons, respectively, for reduction, is more feasible owing to their less‐negative redox potentials of CH4 and CH3OH, and CH3OH is a much better hole scavenger than H2O. Therefore, significant amounts of CH3OH cannot be obtained if the reduction and oxidation occur with the same catalyst. Therefore, CH4 is a pretty normal product of CO2 photoreduction with a semiconductor photocatalyst.

CO2+2H++2e−→HCOOH; Eredox0=−0.61V

CO2+2H++2e−→CO+H2O; Eredox0=−0.53V

CO2+4H++4e−→HCHO+H2O; Eredox0=−0.48V

CO2+6H++6e−→CH3OH+H2O; Eredox0=−0.38V

CO2+8H++8e−→CH4+2H2O; Eredox0=−0.24V

2H++2e−→H2; Eredox0=−0.41V

H2O→1/2O2+2H++2e−; Eredox0=0.82V

On the other hand, the adsorption strengths of the intermediates on the catalyst surface play a crucial role in the product selectivity. If CO desorbs readily, the reaction goes only to CO and not CH4.[12] For (Zn1+xGe)(N2Ox) solid solution as the photocatalyst, only CH4 was obtained. H2O is oxidized by the photogenerated holes to generate H+ ions and O2 according to Reaction (8). The CO2 is reduced by the photogenerated electrons to CH4 according to Reaction (6). For reference, (Zn1+xGe)(N2Ox) prepared trough a solid‐state reaction (ZGON‐SSR; SEM images, XRD pattern, and UV/Vis spectrum in the Support Information, Figure S2) and (Zn1+xGe)(N2Ox) prepared by nitriding Zn2GeO4 nanorods at 700 °C for 6 h (ZGON‐ROD; Figures S3–S5) were also tested for the CO2 photoreduction under the same conditions. No CH4 or any other hydrocarbon species were detected for the Zn2GeO4 because of its large band gap (Eg ≈ 4.53 eV, see Figure S1). The evolutions of the CH4 yields over these four samples under visible‐light irradiation are shown in Figure [NaN] a. The ratio for the generation rate of O2 to CH4 over ZGON photocatalysts is about 2:1 (the theoretical O2/CH4 molar ratio is 2:1 for CO2 + 2H2O → CH4 + 2O2). On the basis of the data shown in Figure [NaN] b, no other reduction products besides CH4 formed. The CH4 yield over ZGON‐SSR is about 0.031 µmol over a 12 h period, whereas the CH4 evolution over ZGON‐ROD is about 0.654 µmol, and these values are much lower than those for ZGON‐6 (ca. 1.613 µmol) and ZGON‐8 (ca. 1.178 µmol). The low photoactivity for CH4 production over ZGON‐SSR is mainly attributed to its small specific surface area (3.75 m2 g–1). The CO2 adsorption–desorption isotherms of these four samples are shown in Figure [NaN] c. The highest CO2 uptakes for ZGON‐6, ZGON‐8, ZGON‐ROD, and ZGON‐SSR are 14.3,12.4, 6.2, and 3.5 cm3 g–1, respectively, at 1 atm and 273 K. ZGON‐6 has the highest adsorption capability for CO2 mainly because of its high BET surface area. The lower CH4 generation rate over ZGON‐ROD than those over ZGON‐6 and ZGON‐8 is attributed to the following three factors: (1) The longitudinal dimensions of the constituent rough chains of ZGON‐6 and ZnGeON‐8 provide abundant transport channels for charge separation. (2) The specific surface areas of the ZGON‐6 and ZGON‐8 (40.47 and 35.78 m2 g–1, respectively) hyperbranched nanostructures are much higher than that of ZGON‐ROD (22.05 m2 g–1); therefore, they offer more CO2 adsorption (Figure [NaN] b) and photocatalytic reaction centers. (3) The unique 3D hierarchical architectures of ZGON‐6 and ZGON‐8 increase light scattering, which favors enhanced light absorption. Furthermore, to determine the reusabilities of the ZGON photocatalysts, photocatalytic CO2 reduction experiments were performed for three cycles with ZGON‐6 as the photocatalyst. After each reaction, the used photocatalyst can be regenerated through only exposure to air for 24 h without any other particular treatment. As shown in Figure [NaN] d, no apparent loss of the CH4 evolution rate was observed; therefore, the ZGON‐6 solid solution is structurally stable during the photocatalytic reaction.

The CH4 generation rate over ZGON‐6 is 0.135 µmol h–1, which is 38 % higher than that over ZGON‐8 (0.098 µmol h–1). The ZGON‐8 particles (a length of ca. 3–10 µm, see Figure [NaN] ) are larger than those of ZGON‐6 owing to the high‐temperature crystal growth, and this is consistent with the BET areas of these two samples. There are some clear cracks in the ZGON‐8 hierarchical structures, and some 3D hyperbranched structures collapse to form particles. The lower activity over ZGON‐8 was ascribed to the lower number of reaction sites arising from the coarse grain size, small specific surface area, and collapse of the 3D hyperbranched architectures. The stability and reusability of a photocatalyst are very important for its photocatalytic application. To evaluate further the long‐term performance of the ZGON‐6 photocatalysts, the photocatalytic reduction of CO2 was conducted by irradiation using the recycled photocatalysts. The used photocatalyst can be regenerated by exposed to air for 24 h without any other particular treatment. As shown in Figure [NaN] d, the photocatalyst ZGON‐6 can retain its good photocatalytic activity even after three reaction cycles. Identical CO2 reduction experiments operated in the absence of light or photocatalyst produced no detectable product; the CO2 reduction reaction occurs under light over the photocatalysts. Control experiments under visible‐light irradiation were also performed over the photocatalysts in an Ar atmosphere. In the absence of CO2, no CO, CH4 or other carbon compounds were detected, and this implies that there was no residual carbon in the system. The apparent quantum yield of CH4 evolution over ZGON‐8 photocatalyst at λ = 420 ± 15 nm was measured to be 0.016 % according to Equation (9).[14]

EQ=[N(CH4)×8]/N(photons)×100%

The photoreduction activity of the CO2 over the solid solution can be enhanced further by the loading of cocatalysts (such as Pt and RuO2) to improve the separation of the photogenerated electron–hole pairs and, hence, enhance the multielectron reaction.

A GaN–ZnO solid solution with a 3D microsphere structure can be obtained by a similar process (see Figures S6–S8). The unique 3D hierarchical nanostructures of the ZnGa2O4 precursor possess great CO2 photocatalytic performance.[[9] ] After nitridation, the resulting GaN–ZnO solid solution inherits the 3D hierarchical nanostructures of the ZnGa2O4 precursor with a high specific surface area (65.82 m2 g–1), and this will help discussions of the effect of the GaN–ZnO microstructure on the activities of photocatalytic reactions.

Unique (Zn1+xGe)(N2Ox) hyperbranched nanostructures with rough surfaces were successfully synthesized through the simple nitridation of Zn2GeO4. The prepared (Zn1+xGe)(N2Ox) solid solution inherit and retain the bundle morphology of the Zn2GeO4 precursor. The constituent smooth nanobelts of the Zn2GeO4 bundles change into rough chains composed of nanoparticles after nitridation, and this provides the (Zn1+xGe)(N2Ox) solid solution with hyperbranched nanostructures and a high specific surface area, approximately two times of that of Zn2GeO4 precursor. As the nitridation time increased from 2 to 6 h, the CH4 evolution rates under visible light increased, because the quantity of (Zn1+xGe)(N2Ox) hyperbranched nanostructures in the sample increased. However, the CH4 generation rate over the sample treated with NH3 for 8 h decreased due to its lower BET surface and collapsed morphology. This work provides useful information for the preparation of oxynitride solid solutions with high specific surface areas and some new insights into the effect of the microstructures in materials on their photocatalytic activities.

Preparation of the Zn2GeO4 Particles: All chemicals were of analytical grade or higher quality. The Zn2GeO4 particles were synthesized by a solvothermal method with ethylenediamine/water as the solvent according to our previously reported work.[15] Typically, GeO2 (5 mmol) and Zn(CH3COO)2·2H2O (1.10 g) were added to ethylenediamine/water (15 mL). The mixture was stirred for 40 min and then transferred to a Teflon‐lined autoclave with an inner volume of 25 mL. Subsequently, the solvothermal synthesis was performed at 180 °C under an autogenerated pressure for 24 h, and then the mixture was cooled naturally to room temperature. After the reaction, the product was obtained through centrifugation, washed with alcohol and deionized water repeatedly, and then dried at 60 °C for 12 h. Zn2GeO4 was collected as a white powder.

Preparation of the Solid Solution: (Zn1+xGe)(N2Ox) solid solutions were prepared by nitriding Zn2GeO4 powder at the given temperature (600–700 °C) under NH3 flow. After the nitridation reaction, the sample was cooled naturally to room temperature under NH3 flow.

Characterization of Materials: The phase structures of the samples were characterized with an X‐ray diffractometer (XRD, Rigaku Co. Ltd, Japan) with Cu‐Kα radiation (λ = 0.154178 nm) at an operation voltage of 40 kV and a current of 40 mA. The surface morphologies were examined by FE‐SEM (FEI Nova NanoSEM 230). The TEM images and high‐resolution images were recorded with a transmission electron microscope (JEM‐2100, JEOL) with a LaB6 filament and operated at 200 kV. The chemical compositions were analyzed with a ThermoScientific K‐Alpha XPS instrument. The UV/Visible diffuse reflection spectra were measured with a UV/Vis spectrophotometer with an integrating sphere (UV/Vis, Shimadzu UV‐2550, Japan). The specific surface areas were measured with a surface‐area analyzer (Micromeritics TriStar‐3000 USA) at 77 K and calculated by the BET method.

Measurement of Photocatalytic Activities: The photocatalytic activities of the samples were evaluated for the photocatalytic reduction of CO2 under irradiation with a 300 W Xe arc lamp with a UV cutoff filter (λ > 420 nm) and a cooling water filter. The reactions were performed in a gas‐closed circulation system with a volume of about 230 mL (see Figure S9). The photocatalyst powder (0.1 g) was spread uniformly on the bottom of a Pyrex glass cell. Then, the whole system was evacuated several times and filled with high‐purity CO2 gas. Next, distilled water (0.4 mL) was added into the gas‐closed reaction system to generate water vapor. After the adsorption of CO2 reached equilibrium in the dark, light irradiation was performed. During the reaction, gas samples (ca. 0.5 mL) were taken from the reaction system at given time intervals. The products were measured by gas chromatography (Shimadzu, GC‐14B with FID detector or GC8A, Japan) against a standard curve.

This work was supported by the International Science & Technology Cooperation Program of Anhui, China (no. 1704e1002212), the Scientific Research Project of Anhui Provincial Department of Education (no. KJ2017A102), the National Natural Science Foundation of China (no. 51302001), the Scientific Research Foundation for the Returned Overseas Chinese Scholars (2016), and the Funds for Distinguished Young Scientists of Anhui Polytechnic University (no. 2015JQ02).

Graph: XRD patterns of (a) as‐synthesized Zn 2 GeO 4 and samples prepared under different temperatures for 6 h: (b) 600, (c) 650, and (d) 700 °C. (f) The peaks for the (113) and (410) faces from (a) and (b).

Graph: XRD patterns of (a) as‐synthesized Zn 2 GeO 4 and samples prepared at different nitridation times of (b) 2, (c) 4, and (d) 8 h.

Graph: (a) UV/Vis diffuse reflectance spectra of Zn 2 GeO 4 and samples prepared at 700 °C for different nitridation times (inset: photographs of the samples); (b) N 1s XPS spectra of these samples.

Graph: Typical FE‐SEM images of (a and b) the Zn 2 GeO 4 precursor and (c and d) (Zn 1+x Ge)(N 2 O x ) solid solution.

Graph: (a) TEM image and selected‐area electron diffraction (SAED) pattern (inset) and (b) magnified TEM image and HRTEM image (inset) of the as‐synthesized (Zn 1+x Ge)(N 2 O x ) solid solution.

Graph: (a) CH 4 evolution activities, (b) CH 4 and O 2 generation rates, and (c) CO 2 adsorption–desorption isotherms for the prepared samples; (d) recycling test on ZGON‐6 for the photocatalytic reduction of CO 2.

Graph: Typical FE‐SEM images of ZGON‐8 (prepared at 700 °C by nitridation for 8 h).

Graph: Supporting Information