Amorphous Semiconductor Nanowires Created by Site-Specific

Aug 5, 2016 - Semiconductor nanowires that have been extensively studied are typically in a crystalline phase. Much less studied are amorphous semicon...
4 downloads 13 Views 6MB Size
Download PDF
Amorphous Semiconductor Nanowires Created by Site-Specific Heteroatom Substitution with Significantly Enhanced Photoelectrochemical Performance Ting He,‡,§,⊥ Lianhai Zu,†,⊥ Yan Zhang,† Chengliang Mao,¶ Xiaoxiang Xu,† Jinhu Yang,*,†,§ and Shihe Yang*,∥ †

School of Chemical Science and Engineering, Tongji University, Shanghai 200092, P. R. China School of Materials Science and Engineering, Tongji University, Shanghai 201804, P. R. China § Research Center for Translational Medicine & Key Laboratory of Arrhythmias of the Ministry of Education of China, East Hospital, Tongji University School of Medicine, No. 150 Jimo Road, Shanghai 200120, P. R. China ∥ Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong P. R. China ¶ Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, Institute of Environmental Chemistry, College of Chemistry, Central China Normal University, Wuhan 430079, P. R. China ‡

S Supporting Information *

ABSTRACT: Semiconductor nanowires that have been extensively studied are typically in a crystalline phase. Much less studied are amorphous semiconductor nanowires due to the difficulty for their synthesis, despite a set of characteristics desirable for photoelectric devices, such as higher surface area, higher surface activity, and higher light harvesting. In this work of combined experiment and computation, taking Zn2GeO4 (ZGO) as an example, we propose a site-specific heteroatom substitution strategy through a solution-phase ions−alternativedeposition route to prepare amorphous/crystalline Si-incorporated ZGO nanowires with tunable band structures. The substitution of Si atoms for the Zn or Ge atoms distorts the bonding network to a different extent, leading to the formation of amorphous Zn1.7Si0.3GeO4 (ZSGO) or crystalline Zn2(GeO4)0.88(SiO4)0.12 (ZGSO) nanowires, respectively, with different bandgaps. The amorphous ZSGO nanowire arrays exhibit significantly enhanced performance in photoelectrochemical water splitting, such as higher and more stable photocurrent, and faster photoresponse and recovery, relative to crystalline ZGSO and ZGO nanowires in this work, as well as ZGO photocatalysts reported previously. The remarkable performance highlights the advantages of the ZSGO amorphous nanowires for photoelectric devices, such as higher light harvesting capability, faster charge separation, lower charge recombination, and higher surface catalytic activity. KEYWORDS: site-specific heteroatom substitution, bonding distortion, amorphous nanowires, semiconductor, photoelectrochemical water splitting

S

Zn2GeO4 are wide-bandgap semiconductors (3.2−4.5 eV) and are generally in a crystalline state.15,19−23 However, the widebandgap semiconductors can only absorb UV light, which accounts for only ZGO nanowire powder. The ZSGO nanowire array photoanode shows the highest photoresponse with the ratio of the photocurrent to the dark current (Ion/Ioff) of ∼9.4. In addition, the ZSGO nanowire array photoanode also exhibits higher photoresponse stability and shorter response time, in contrast with the other two. For example, the photoresponse of the ZSGO nanowire array photoanode remains steady during five on/off cycles with a timespan over 550 s (Figure 4c), and its response and recovery times are both less than 0.2 s, observed from an enlarged single on/off cycle shown in Figure 4d. It is

noteworthy that the ZSGO nanowire array photoanode shows remarkably improved performance, such as higher photocurrent and better stability, compared with that of ZGO nanowires as described in previous reports (Table S4). In addition, the ZSGO nanowire arrays could preserve the original morphology even after PEC and photoresponse measurements over 550 s (Figure S10), showing the high structural stability. Moreover, the amorphous ZSGO nanowire arrays also display a significantly enhanced photocurrent that is almost twice that of its crystalline counterpart of the ZGSO nanowire arrays (Figure S11), though the PEC performance of the ZGSO nanowire arrays may be underestimated because of the coexistence of GeO2 and Si nanoparticles. These results demonstrate that the ZSGO nanowire arrays can serve as a promising photocatalyst for highly efficient PEC devices. The intrinsic mechanism for the best PEC water-splitting performance of the ZSGO nanowire arrays is ascribed to their structural advantage, high light harvesting, and low charge recombination. First, ZSGO nanowire arrays with oriented architecture can avoid the aggregation of nanowires, which offers high specific surface area and abundant active sites, resulting in the higher photocurrent densities than the disordered nanowires. Second, the ZSGO nanowire arrays also demonstrate a better light-harvesting capability. As shown in Figure 5a, UV−vis absorption spectra of the ZSGO nanowire arrays show stronger and broader peaks in the region from ∼300 to ∼500 nm, compared with that of ZGSO and ZGO samples. This may be attributed to two factors. One is the narrowed bandgap due to Si heteroatom substitution as calculated and discussed above, which extends the absorption band to the visible light region of ZSGO nanowire arrays. The other is the presence of plenty of dangling bonds or crystal defects in the amorphous phase of ZSGO nanowires, resulting in a “band tail” adjacent to the intrinsic bandgap of 7887

DOI: 10.1021/acsnano.6b03801 ACS Nano 2016, 10, 7882−7891

Article

ACS Nano

Figure 5. (a) UV−vis absorption spectra of the three samples. Inset in (a) is a Tauc plot of F(R∞)2 vs hv (eV) obtained according to UV−vis absorption spectra. Direct bandgaps were determined by the intercept of a linear fit to the absorption edge. (b) Photoluminescence spectra of the three samples. (c) Schematic illustration shows the comparison of a photoelectrochemical sequence occurred over ZSGO and ZGO phase.

Zn2GeO4.24,61 It is also found that there is a stronger absorption in the ultraviolet waveband for the ZSGO nanowire arrays than the ZGSO/ZGO nanowire powders, possibly owing to the more open architecture of the former which is more accessible for light entrance and harvesting. Last, but not the least, the ZSGO nanowire arrays display a lower recombination rate of photoinduced charges compared with the other two. Figure 5b shows the photoluminescence (PL) spectra of three nanowire samples. Three broad emission peaks centered at ∼440 nm appear in the PL spectra for three samples, which can be assigned to the triplet to singlet (T1−S0) transitions of the Ge-related oxygen-deficient centers. According to the principle of fluorescence spectroscopy, the characteristic luminescence occurs when the excited state of electrons decays radioactively to the ground state, accompanying with recombination of exited electron from conduction band and the holes in valence band.62 Accordingly, the stronger peak intensity in the PL spectrum means a higher charge recombination. It is noted the PL measurement was conducted in the same Na2SO4 solution as the PEC tests, which can simulate the real working situation for the three photocatalysts. The PL peak intensity of the ZSGO nanowire arrays is the lowest among the three (Figure 5b), suggesting the lowest charge recombination. This agrees with the finding that the luminescence intensity of a luminescence center in an amorphous phase is much weaker than that in a crystal phase.62 The existence of abundant

dangling bonds and defects in amorphous nanowires may serve as effective electron traps, preventing recombination of electrons and holes. Accordingly, for amorphous ZSGO nanowire arrays with a narrower bandgap and weaker fluorescence, more effective photoinduced charge generation with less recombination occurs under light irradiation, giving rise to more active holes and electrons compared with crystalline ZGO or ZGSO nanowire powders (scheme in Figure 5c). Subsequently, the electrons will transfer directly to the counter electrode from the arrayed ZSGO nanowires, leaving a quantity of holes on the surface of ZSGO nanowires on the working electrode. These electrons and holes separated at two electrodes serve as catalytic reduction and oxidation sites to facilitate efficiently generation of H2 and O2, respectively. The amorphous ZSGO nanowire arrays with structural and photoelectric advantage promise many important applications related to energy and environmental areas.

CONCLUSION In summary, a site-specific substitution strategy has been proposed for the preparation of amorphous/crystalline Siincorporated ZSGO/ZGSO nanowires with tunable band structure and optimized PEC performance. The substitution of Si heteroatoms for Zn atoms leads to the amorphous nanowires with a narrowed bandgap, while for Ge atoms it 7888

DOI: 10.1021/acsnano.6b03801 ACS Nano 2016, 10, 7882−7891

Article

ACS Nano

were employed for all the atoms with a kinetic energy cutoff of 340 eV. A unit cell (a = b = 14.284 Å, c = 9.547 Å, α = β = 90°, γ = 120°) with trigonal symmetry was constructed for Zn2GeO4 simulations (total atom number = 138). ZSGO and ZGSO were considered by assuming one Zn atom in a unit cell was substituted by one Si atom and one Ge atom in unit cell was substituted by one Si atom, respectively. During the geometrical optimization, all structures were fully relaxed to an energy convergence of 10−5 eV/atom and a force convergence of 0.03 eV/Å. Static calculations were done with a 2 × 2 × 3 Monkhorst−Pack k-point grid. Evaluation of Photoelectrochemical Performance. Electrochemical measurements were performed in a standard three-electrode setup using a CHI660E electrochemical analyzer (CH Instruments, Inc., Shanghai). A Pt electrode and a saturated calomel electrode (SCE) were used as the counter and the reference electrode, respectively. The Si substrate (1 × 1 cm2) covered with thin film of ZSGO nanowire arrays was used as the working electrode. For comparison, 6 mg of ZGSO nanowire powder and ZGO nanowire powder were dispersed in 0.5 mL of ethanol and sonicated for 30 min to form a slurry, respectively. The slurry was spread onto the Si substrate (1 × 1 cm2), dried in air, and employed as a working electrode. A 300 W Xe arc lamp was utilized as the light source, and a 0.5 M Na2SO4 aqueous solution was used as the electrolyte. Prior to the reaction, the whole system was evacuated by a mechanical pump and then filled with 101 kPa high-purity N2 (>99.99%). Linear sweep voltammograms were measured with a scan rate of 5 mV s−1, and amperometric I−t curves were tested at a bias voltage of +1.6 V (vs SCE).

results in crystalline nanowires with a widened bandgap, which are investigated in depth by substantial characterizations and discussed in term of structure distortion comparisons and DFT calculations. PEC tests demonstrate that the ZSGO nanowire arrays as a photoanode exhibit better PEC performance than ZGSO and ZGO nanowire powders. The performance improvement is ascribed to the advantages in structural and photoelectric properties of the ZSGO amorphous nanowire arrays, such as higher light-harvesting capability, faster charge separation, lower charge recombination, and higher specific catalytic activity. The ZSGO amorphous nanowire arrays can be employed as an efficient, stable, and fast-response photocatalyst for photoelectric applications. The concept of site-specific heteroatom substitution may be applicable to other semiconductor materials for precise phase engineering as well as band structure tailoring. This study may offer an opportunity for the development of amorphous materials with desirable structure and property for promising applications.

METHODS Raw Materials. All chemicals were of analytical grade and used without further purification. Zn(CH3COO)2·2H2O, GeO2, ethylenediamine (En), and Na2SO4 were purchased from Adamas Reagent Ltd. Si (100) foils were purchased from SuZhou Technology Co., Ltd. Before use, the Si foils were washed with acetone, 2-propanol, ethanol, and deionized water several times to ensure the surfaces of the Si substrates were thoroughly cleaned. Preparation of Amorphous Zn1.7Si0.3GeO4 (ZSGO) Nanowire Arrays. In a typical synthesis, 1.10 g of Zn(CH3COO)2·2H2O and 0.26 g of GeO2 were added into a mixture of ethylenediamine (En, 10 mL) and deionized water (5 mL) under stirring. The resulting homogeneous solution was transferred into a 25 mL Teflon-lined stainless steel autoclave. Subsequently, the pretreated Si substrate (1 × 1 cm2) was immersed vertically into the above solution. The autoclave containing Si substrate and the solution was maintained at 180 °C for 20 h, leading to the formation of ZSGO nanowire arrays on Si substrate. After being cooled to room temperature, the product of ZSGO nanowire arrays on Si substrate was washed with absolute alcohol and distilled water at least three times and then dried at 60 °C for 12 h. The loading mass of ZSGO nanowire arrays on Si substrate was about 6 mg·cm−2. Zn2(GeO4)0.88(SiO4)0.12 nanowire in the form of dispersive powder formed in solution was also collected and washed similarly according to the above treatment steps. The Zn2GeO4 nanowire powder was also prepared by the same method except for the use of Si substrate. Structure Characterization. The crystal structure of the samples was characterized by XRD using a D/max2550VB3+/PC X-ray diffractometer with Cu Kα radiation (λ = 0.15418 nm). The morphology was observed using a scanning electron microscope (SEM, Hitachi S4800, 3 kV) equipped with an X-ray EDS analysis system and a high-resolution transmission electron microscopy (HRTEM, JEM 2011, 200 kV). Surface electronic states were analyzed by X-ray photoelectron spectroscopy (XPS, PHI-5000C ESCA system (Perkin Elmer) with Mg Kα radiation). All binding energies (BEs) were referred to the C 1s peak (284.6 eV) arising from surface hydrocarbons (or adventitious hydrocarbon). The Fourier transform infrared (FTIR) and UV−vis spectra were recorded on Nicolet 6700 FTIR spectrometric analyzer and Cary-50 UV−vis spectrophotometer, respectively. Raman spectra were recorded by using a spectrophotometer (inVia, Renishaw, Germany) with a 514 nm laser. Photoluminescence measurements were carried out at room temperature using a fluorescence spectrometer (F-7000, Hitachi, Japan) with an excitation wavelength of 270 nm, scanning speed of 1200 nm/min, and width of excitation slit of 10 nm. Theoretical Calculations. All calculations were performed using DFT with the exchange-correlation functions described by GGA-PBE. The calculation was implemented by the CASTEP code in which the plane-wave pseudopotential approach and ultrasoft pseudopotentials

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b03801. EDS pattern of ZSGO nanowire arrays, SEM images of ZSGO short-nanowire arrays, characterizations of ZSGO nanowire arrays after annealing, SEM images and XRD patterns of ZGSO nanowire powder and ZGO powder, Tauc plot of F(R∞)2 vs hv (eV) of ZSGO, ZGO, and ZGSO nanowire samples, PDOS of ZSGO, ZGO, and ZGSO, SEM image of ZSGO nanowire arrays after tests over 550 s, the periodic on/off photocurrent responses of the ZGSO crystalline nanowire arrays with comparison of ZSGO amorphous nanowire arrays, element contents measured by EDS for ZSGO nanowire arrays and ZGSO nanowire powder, comparisons of theoretical structure parameters between Si(II)O4, ZnO4, Si(IV)O4, and GeO4 tetrahedrons, and comparison of photoelectricrelated performances of ZSGO nanowire arrays and ZGO nanowire-based structures (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions ⊥

T.H. and L.H.Z. contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation (21001082 and 21273161), The Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, Shanghai Innovation 7889

DOI: 10.1021/acsnano.6b03801 ACS Nano 2016, 10, 7882−7891

Article

ACS Nano

(17) Yang, X. Y.; Wolcott, A.; Wang, G. M.; Sobo, A.; Fitzmorris, R. C.; Qian, F.; Zhang, J. Z.; Li, Y. Nitrogen-Doped ZnO Nanowire Arrays for Photoelectrochemical Water Splitting. Nano Lett. 2009, 9, 2331−2336. (18) Zong, X.; Yan, H. J.; Wu, G. P.; Ma, G. J.; Wen, F. Y.; Wang, L.; Li, C. Enhancement of Photocatalytic H2 Evolution on CdS by Loading MoS2 as Cocatalyst under Visible Light Irradiation. J. Am. Chem. Soc. 2008, 130, 7176−7177. (19) Zhao, X. W.; Jin, W. Z.; Cai, J. G.; Ye, J. F.; Li, Z. H.; Ma, Y. R.; Xie, J. L.; Qi, L. M. Shape- and Size-Controlled Synthesis of Uniform Anatase TiO2 Nanocuboids Enclosed by Active {100} and {001} Facets. Adv. Funct. Mater. 2011, 21, 3554−3563. (20) Lu, N.; Su, Y.; Li, J. Y.; Yu, H. T.; Quan, X. Fabrication of Quantum-Sized CdS-Coated TiO2 Nanotube Array with Efficient Photoelectrochemical Performance Using Modified Successive Ionic Layer Absorption and Reaction (SILAR) Method. Sci. Bull. 2015, 60, 1281−1286. (21) Lu, F.; Cai, W. P.; Zhang, Y. G. ZnO Hierarchical Micro/ Nanoarchitectures: Solvothermal Synthesis and Structurally Enhanced Photocatalytic Performance. Adv. Funct. Mater. 2008, 18, 1047−1056. (22) Liang, J.; Cao, Y. Q.; Lin, H.; Zhang, Z. Z.; Huang, C. C.; Wang, X. X. A Template-Free Solution Route for the Synthesis of WellFormed One-Dimensional Zn2GeO4 Nanocrystals and Its Photocatalytic Behavior. Inorg. Chem. 2013, 52, 6916−6922. (23) Zou, F.; Hu, X. L.; Qie, L.; Jiang, Y.; Xiong, X. Q.; Qiao, Y.; Huang, Y. H. Facile Synthesis of Sandwiched Zn2GeO4-Graphene Oxide Nanocomposite as A Stable and High-Capacity Anode for Lithium-Ion Batteries. Nanoscale 2014, 6, 924−930. (24) Mott, N. F.; Davis, E. A. Electronic Processes in Non-Crystalline Materials; Clavendon Press: Oxford, 1979. (25) Santra, P. K.; Kamat, P. V. Mn-doped Quantum Dot Sensitized Solar Cells: A Strategy to Boost Efficiency over 5%. J. Am. Chem. Soc. 2012, 134, 2508−2511. (26) Guo, X. L.; Wang, L. L.; Tan, Y. W. Hematite Nanorods CoDoped with Ru Cations with Different Valence States as High Performance Photoanodes for Water Splitting. Nano Energy 2015, 16, 320−328. (27) Yue, B.; Li, Q. Y.; Iwai, H.; Kako, T.; Ye, J. H. Hydrogen Production Using Zinc-Doped Carbon Nitride Catalyst Irradiated with Visible Light. Sci. Technol. Adv. Mater. 2011, 12, 1462−1470. (28) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. VisibleLight Photocatalysis in Nitrogen-Doped Titanium Oxides. Science 2001, 293, 269−271. (29) Khan, S. U.; Al-Shahry, M.; Ingler, W. B. Efficient Photochemical Water Splitting by A Chemically Modified N-TiO2. Science 2002, 297, 2243−2245. (30) Sakthivel, S.; Kisch, H. Daylight Photocatalysis by CarbonModified Titanium Dioxide. Angew. Chem., Int. Ed. 2003, 42, 4908− 4911. (31) In, S.; Orlov, A.; Berg, R.; Garcia, F.; Pedrosa-Jimenez, S.; Tikhov, M. S.; Wright, D. S.; Lambert, R. M. Effective Visible LightActivated B-Doped and B, N-Codoped TiO2 Photocatalysts. J. Am. Chem. Soc. 2007, 129, 13790−13791. (32) Liu, Y. W.; Hua, X. M.; Xiao, C.; Zhou, T. F.; Huang, P. C.; Guo, Z. P.; Pan, B. C.; Xie, Y. Heterogeneous Spin States in Ultrathin Nanosheet Inducing Subtle Lattice Distortion for Efficiently Triggering Hydrogen Evolution. J. Am. Chem. Soc. 2016, 138, 5087− 5092. (33) Yan, S. C.; Wan, L. J.; Li, Z. S.; Zou, Z. G. Facile TemperatureControlled Synthesis of Hexagonal Zn2GeO4 Nanorods with Different Aspect Ratios toward Improved Photocatalytic Activity for Overall Water Splitting and Photoreduction of CO2. Chem. Commun. 2011, 47, 5632−5634. (34) Sato, J.; Kobayashi, H.; Ikarashi, K.; Saito, N.; Nishiyama, H.; Inoue, Y. Photocatalytic Activity for Water Decomposition of RuO2Dispersed Zn2GeO4 with d10 Configuration. J. Phys. Chem. B 2004, 108, 4369−4375. (35) Qian, L.; Chen, J. F.; Li, Y. H.; Wu, L.; Wang, H. F.; Chen, A. P.; Hu, P.; Zheng, L. R.; Yang, H. G. Orange Zinc Germanate with

Program (13ZZ026), Scientific Research Foundation for the Returned Overseas Chinese Scholars of SEM, and the Fundamental Research Funds for the Central Universities.

REFERENCES (1) Kongkanand, A.; Tvrdy, K.; Takechi, K.; Kuno, M.; Kamat, P. V. Quantum Dot Solar Cells. Tuning Photoresponse through Size and Shape Control of CdSe-TiO2 Architecture. J. Am. Chem. Soc. 2008, 130, 4007−4015. (2) Liu, B.; Aydil, E. S. Growth of Oriented Single-Crystalline Rutile TiO2 Nanorods on Transparent Conducting Substrates for Dyesensitized Solar Cells. J. Am. Chem. Soc. 2009, 131, 3985−3990. (3) Giordano, F.; Abate, A.; Correa Baena, J. P.; Saliba, M.; Matsui, T.; Im, S. H.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; Hagfeldt, A.; Graetzel, M. Enhanced Electronic Properties in Mesoporous TiO2 via Lithium Doping for High-Efficiency Perovskite Solar Cells. Nat. Commun. 2016, 7, 10379. (4) Wolcott, A.; Smith, W. A.; Kuykendall, T. R.; Zhao, Y. P.; Zhang, J. Z. Photoelectrochemical Water Splitting Using Dense and Aligned TiO2 Nanorod Arrays. Small 2009, 5, 104−111. (5) Dholam, R.; Patel, N.; Adami, M.; Miotello, A. Hydrogen Production by Photocatalytic Water-Splitting Using Cr-or Fe-Doped TiO2 Composite Thin Films Photocatalyst. Int. J. Hydrogen Energy 2009, 34, 5337−5346. (6) Cheng, C.; Karuturi, S. K.; Liu, L. J.; Liu, J. P.; Li, H. X.; Su, L. T.; Tok, A. I. Y.; Fan, H. J. Quantum-Dot-Sensitized TiO2 Inverse Opals for Photoelectrochemical Hydrogen Generation. Small 2012, 8, 37− 42. (7) Wolcott, A.; Smith, W. A.; Kuykendall, T. R.; Zhao, Y. P.; Zhang, J. Z. Photoelectrochemical Study of Nanostructured ZnO Thin Films for Hydrogen Generation from Water Splitting. Adv. Funct. Mater. 2009, 19, 1849−1856. (8) Shang, L.; Tong, B.; Yu, H. J.; Waterhouse, G. I. N.; Zhou, C.; Zhao, Y. F.; Tahir, M.; Wu, L. Z.; Tung, C. H.; Zhang, T. R. CdS Nanoparticle-Decorated Cd Nanosheets for Efficient Visible LightDriven Photocatalytic Hydrogen Evolution. Adv. Energy Mater. 2016, 6, 1501241. (9) Mohapatra, S. K.; John, S. E.; Banerjee, S.; Misra, M. Water Photooxidation by Smooth and Ultrathin α-Fe2O3 Nanotube Arrays. Chem. Mater. 2009, 21, 3048−3055. (10) Wang, W. N.; An, W. J.; Ramalingam, B.; Mukherjee, S.; Niedzwiedzki, D. M.; Gangopadhyay, S.; Biswas, P. Size and Structure Matter: Enhanced CO2 Photoreduction Efficiency by Size-Resolved Ultrafine Pt Nanoparticles on TiO2 Single Crystals. J. Am. Chem. Soc. 2012, 134, 11276−11281. (11) Tu, W. G.; Zhou, Y.; Liu, Q.; Tian, Z. P.; Gao, J.; Chen, X. Y.; Zhang, H. T.; Liu, J. G.; Zou, Z. G. Robust Hollow Spheres Consisting of Alternating Titania Nanosheets and Graphene Nanosheets with High Photocatalytic Activity for CO2 Conversion into Renewable Fuels. Adv. Funct. Mater. 2012, 22, 1215−1221. (12) Ryu, J.; Lee, S. H.; Nam, D. H.; Park, C. B. Rational Design and Engineering of Quantum-Dot-Sensitized TiO2 Nanotube Arrays for Artificial Photosynthesis. Adv. Mater. 2011, 23, 1883−1888. (13) Xiong, J. Y.; Han, C.; Li, Z.; Dou, S. X. Effects of Nanostructure on Clean Energy: Big Solutions Gained from Small Features. Sci. Bull. 2015, 60, 2083−2090. (14) Habibi, M. H.; Hassanzadeh, A.; Mahdavi, S. The Effect of Operational Parameters on the Photocatalytic Degradation of Three Textile Azo Dyes in Aqueous TiO2 Suspensions. J. Photochem. Photobiol., A 2005, 172, 89−96. (15) Huang, J. H.; Ding, K. N.; Hou, Y. D.; Wang, X. C.; Fu, X. Z. Synthesis and Photocatalytic Activity of Zn2GeO4 Nanorods for the Degradation of Organic Pollutants in Water. ChemSusChem 2008, 1, 1011−1019. (16) Zhang, Z. H.; Hossain, M. F.; Takahashi, T. Photoelectrochemical Water Splitting on Highly Smooth and Ordered TiO2 Nanotube Arrays for Hydrogen Generation. Int. J. Hydrogen Energy 2010, 35, 8528−8535. 7890

DOI: 10.1021/acsnano.6b03801 ACS Nano 2016, 10, 7882−7891

Article

ACS Nano

Germanate Nano-Materials. J. Solid State Chem. 2011, 184, 1054− 1062. (53) Chandra Babu, B.; Buddhudu, S. Analysis of Structural and Electrical Properties of Ni2+: Zn2GeO4 Ceramic Powders by Sol-Gel Method. J. Sol-Gel Sci. Technol. 2014, 70, 405−415. (54) Zhao, Y. X.; Yang, S. W.; Zhu, J.; Ji, G. F.; Peng, F. The Study of Oxygen Ion Motion in Zn2GeO4 by Raman Spectroscopy. Solid State Ionics 2015, 274, 12−16. (55) Sasaki, M.; Ehara, T. Silicon Oxide Thin Films Prepared by Vacuum Evaporation and Sputtering Using Silicon Monoxide. J. Phys.: Conf. Ser. 2013, 417, 012028. (56) Alfonsetti, R.; Lozzi, L.; Passacantando, M.; Picozzi, P.; Santucci, S. XPS Studies on SiOx Thin Films. Appl. Surf. Sci. 1993, 70−71, 222− 225. (57) Nguyen, T.; Lefrant, S. XPS Study of SiO Thin Films and SiOMetal Interfaces. J. Phys.: Condens. Matter 1989, 1, 5197−5204. (58) Sun, C.; Kuan, C.; Kao, F. J.; Wang, Y. M.; Chen, J. C.; Chang, C. C.; Shen, P. On the Nucleation, Growth and Impingement of Platelike α-Zn2SiO4 Spherulites in Glaze Layer: A Confocal and Electron Microscopic Study. Mater. Sci. Eng., A 2004, 379, 327−333. (59) Oh, I.-k.; Kim, M.-K.; Lee, J.-s.; Lee, C.-W.; Lansalot-Matras, C.; Noh, W.; Park, J.; Noori, A.; Thompson, D.; Chu, S. The Effect of La2O3-Incorporation in HfO2 Dielectrics on Ge Substrate by Atomic Layer Deposition. Appl. Surf. Sci. 2013, 287, 349−354. (60) Xiao, H.; Tahir-Kheli, J.; Goddard, W. A., III Accurate Band Gaps for Semiconductors from Density Functional Theory. J. Phys. Chem. Lett. 2011, 2, 212−217. (61) Simmons-Potter, K.; Potter, B. G., Jr; Warren, W. L. Investigation of Defects in Highly Photosensitive Germanosilicate Thin Films. In Photonics West’97 Conference; Andrews, M. P., Ed.; SPIE: San Jose, CA, 1997; pp 93−99. (62) Lin, G.; Dong, G. P.; Tan, D. Z.; Liu, X. F.; Zhang, Q.; Chen, D. P.; Qiu, J. R.; Zhao, Q. Z.; Xu, Z. Z. Long Lasting Phosphorescence in Oxygen-Deficient Zinc-Boron-Germanosilicate Glass-Ceramics. J. Alloys Compd. 2010, 504, 177−180.

Metallic Ge-Ge Bonds as a Chromophore-Like Center for VisibleLight-Driven Water Splitting. Angew. Chem., Int. Ed. 2015, 54, 11467− 11471. (36) Liu, Q.; Zhou, Y.; Kou, J. H.; Chen, X. Y.; Tian, Z. P.; Gao, J.; Yan, S. C.; Zou, Z. G. High-Yield Synthesis of Ultralong and Ultrathin Zn2GeO4 Nanoribbons toward Improved Photocatalytic Reduction of CO2 into Renewable Hydrocarbon Fuel. J. Am. Chem. Soc. 2010, 132, 14385−14387. (37) Liu, Q.; Zhou, Y.; Tian, Z. P.; Chen, X. Y.; Gao, J.; Zou, Z. G. Zn2GeO4 Crystal Splitting toward Sheaf-like, Hyperbranched Nanostructures and Photocatalytic Reduction of CO2 into CH4 under Visible Light after Nitridation. J. Mater. Chem. 2012, 22, 2033−2038. (38) Yan, S. C.; Wang, J. J.; Gao, H. L.; Wang, N. Y.; Yu, H.; Li, Z. S.; Zhou, Y.; Zou, Z. G. Zinc Gallogermanate Solid Solution: A Novel Photocatalyst for Efficiently Converting CO2 into Solar Fuels. Adv. Funct. Mater. 2013, 23, 1839−1845. (39) Yin, L. W.; Li, Q.; Miao, X. G.; Wang, C. X. Three-Dimensional Mn-Doped Zn2GeO4 Nanosheet Array Hierarchical Nanostructures Anchored on Porous Ni Foam as Binder-Free and Carbon-Free Lithium-Ion Battery Anodes with Enhanced Electrochemical Performance. J. Mater. Chem. A 2015, 3, 21328−21336. (40) Li, X. D.; Feng, Y.; Li, M. C.; Li, W.; Wei, H.; Song, D. D. Smart Hybrids of Zn2GeO4 Nanoparticles and Ultrathin g-C3N4 Layers: Synergistic Lithium Storage and Excellent Electrochemical Performance. Adv. Funct. Mater. 2015, 25, 6858−6866. (41) Yan, C. Y.; Singh, N. D.; Lee, P. S. Wide-Bandgap Zn2GeO4 Nanowire Networks as Efficient Ultraviolet Photodetectors with Fast Response and Recovery Time. Appl. Phys. Lett. 2010, 96, 2−5. (42) Wu, S. P.; Wang, Z. L.; Ouyang, X.; Lin, Z. Q. Core-Shell Zn2GeO4 Nanorods and Their Size-Dependent Photoluminescence Properties. Nanoscale 2013, 5, 12335−12341. (43) Liang, J.; Xu, J.; Long, J. L.; Zhang, Z. Z.; Wang, X. X. SelfAssembled Micro/Nano-Structured Zn2GeO4 Hollow Spheres: Direct Synthesis and Enhanced Photocatalytic Activity. J. Mater. Chem. A 2013, 1, 10622. (44) Gu, Z. J.; Liu, F.; Li, X. F.; Pan, Z. W. Luminescent Zn2GeO4 Nanorod Arrays and Nanowires. Phys. Chem. Chem. Phys. 2013, 15, 7488−7493. (45) Li, W. W.; Wang, X. F.; Liu, B.; Xu, J.; Liang, B.; Luo, T.; Luo, S. J.; Chen, D.; Shen, G. Z. Single-Crystalline Metal Germanate Nanowire-Carbon Textiles as Binder-Free, Self-Supported Anodes for High-Performance Lithium Storage. Nanoscale 2013, 5, 10291− 10299. (46) Jiang, G. H.; Tang, B. L.; Chen, H.; Liu, Y. K.; Li, L.; Huang, Q.; Chen, W. X. Controlled Growth of Hexagonal Zn2GeO4 Nanorods on Carbon Fibers for Photocatalytic Oxidation of p-toluidine. RSC Adv. 2015, 5, 25801−25805. (47) Zhou, X.; Zhang, Q.; Gan, L.; Li, X.; Li, H. Q.; Zhang, Y.; Golberg, D.; Zhai, T. Y. High-Performance Solar-Blind Deep Ultraviolet Photodetector Based on Individual Single-Crystalline Zn2GeO4 Nanowire. Adv. Funct. Mater. 2016, 26, 704−712. (48) Chen, W. M.; Lu, L. Y.; Maloney, S.; Yang, Y.; Wang, W. Y. Coaxial Zn2GeO4@Carbon Nanowires Directly Grown on Cu Foils as High-Performance Anodes for Lithium Ion Batteries. Phys. Chem. Chem. Phys. 2015, 17, 5109−5114. (49) Yi, R.; Feng, J. K.; Lv, D. P.; Gordin, M. L.; Chen, S. R.; Choi, D.; Wang, D. H. Amorphous Zn2GeO4 Nanoparticles as Anodes with High Reversible Capacity and Long Cycling Life for Li-Ion Batteries. Nano Energy 2013, 2, 498−504. (50) Yang, J. H.; Liu, G. M.; Lu, J.; Qiu, Y. F.; Yang, S. H. Electrochemical Route to the Synthesis of Ultrathin ZnO Nanorod/ Nanobelt Arrays on Zinc Substrate. Appl. Phys. Lett. 2007, 90, 103109. (51) Lu, C. H.; Qi, L. M.; Yang, J. H.; Tang, L.; Zhang, D. Y.; Ma, J. M. Hydrothermal Growth of Large-Scale Micropatterned Arrays of Ultralong ZnO Nanowires and Nanobelts on Zinc Substrate. Chem. Commun. 2006, 3551−3553. (52) Boppana, V. B. R.; Hould, N. D.; Lobo, R. F. Synthesis, Characterization and Photocatalytic Properties of Novel Zinc 7891

DOI: 10.1021/acsnano.6b03801 ACS Nano 2016, 10, 7882−7891