Size-Tunable Solvothermal Synthesis of Zn2GeO4:Mn2+

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DOI: 10.1021/cg100753g

Size-Tunable Solvothermal Synthesis of Zn2GeO4:Mn2þ Nanophosphor in Water/Diethylene Glycol System

2010, Vol. 10 4494–4500

Satoru Takeshita,† Joji Honda,† Tetsuhiko Isobe,*,† Tomohiro Sawayama,‡ and Seiji Niikura‡ †

Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan, and ‡SINLOIHI Company, Limited, 2-19-12 Dai, Kamakura 247-8550, Japan Received June 6, 2010; Revised Manuscript Received July 22, 2010

ABSTRACT: Zn2GeO4:Mn2þ nanophosphors are synthesized from germanium(IV) oxide and acetates of zinc and manganese(II) in a mixed solvent of water and diethylene glycol (DEG) by the solvothermal reaction at 200 °C for 2 h. Phase pure Zn2GeO4:Mn2þ is obtained for the samples prepared at 0 e xDEG e 91.7, where xDEG is the volume percentage of DEG in the mixed solvent. The particle and crystallite sizes decrease with the increase in xDEG. The sample prepared at xDEG = 91.7 comprises the nanorods with 30.2 and 12.2 nm in mean length and width, respectively. The actual Mn concentration measured by X-ray fluorescence analysis increases with the increase in xDEG and approaches to the nominal concentration, 2.0 at %. The samples show green luminescence corresponding to the d-d transition of Mn2þ under the irradiation of UV and near-UV light. The photoluminescence intensity reaches a maximum around xDEG ∼ 80. This is attributed to the competitive factors, the Mn concentration and the sizes of crystallites and particles.

Introduction Luminescent inorganic nanoparticles have been extensively studied in the recent decades due to their tremendous potential applications as well as fundamental science researches in many fields such as optics, optoelectronics, and biolabeling.1 In recent years, these inorganic nanophosphors have attracted much attention for use as a transparent wavelength convertor because of lower light scattering from nanoparticles in comparison to micrometer-sized particles.2,3 Especially, near-UV to visible wavelength conversion by nanophosphors is one of the current challenges required in the field of photovoltaics.4 Several researchers have focused on the sulfide- and selenide-based luminescent quantum dots (QDs), e.g., semiconductor CdS, CdSe, and CdSe/ZnS core/shell nanoparticles and discussed the potential of these luminescent QDs as the transparent wavelength convertors from near-UV to visible light.5-7 However, practical applications of luminescent QDs are limited to the field of biolabeling so far because QDs are not appropriate for the large-scale industry from their several aspects: a high toxicity of cadmium, high costs for the synthesis processes, and an extreme particle-size-sensitivity of emission wavelength.4,8,9 Large spectral overlap between absorption and emission spectra of QDs would also be disadvantage with respect to the wavelength conversion where a part of the converted photons from one QD is reabsorbed by other QDs.4 Therefore, we have focused on the impurity-doped oxide nanophosphors from the aspects of low toxicity, low costs for the materials, the synthesis processes, negligible spectral overlap, and relatively higher stability compared to sulfide- and selenide-based QDs. We recently reported the wet chemical synthesis of YVO4:Bi3þ, Eu3þ nanoparticles which emits red under the excitation of UV and near-UV light.10,11 This YVO4:Bi3þ,Eu3þ is one of the promising materials for wavelength converting applications from near-UV to red light because it has a broad absorption *To whom correspondence should be addressed. Phone: þ81 45 566 1554. Fax: þ81 45 566 1551. E-mail: [email protected]. Web site: http:// www.applc.keio.ac.jp/∼isobe/. pubs.acs.org/crystal

Published on Web 09/09/2010

in the near-UV region due to the charge transfer transition from Bi3þ to V5þ, followed by converting to the red emission of Eu3þ at a high efficiency. We also reported that the wavelength conversion film containing YVO4:Bi3þ,Eu3þ nanoparticles has an advantage of the low light scattering loss in comparison to the micrometer-sized particles and a sufficient potential for the use as the spectral shifter for multicrystalline silicon solar cells.12,13 Mn2þ-doped Zn2GeO4 is an efficient green-emitting phosphor for the UV and near-UV excitation.14 In recent years, Zn2GeO4 host crystal has also attracted much attention as a water decomposition photocatalyst.15 Typical synthesis methods for micrometer- and submicrometer-sized particles of Zn2GeO4 and Zn2GeO4:Mn2þ are conventional solid state reaction,15-19 hydrothermal synthesis,20,21 coprecipitation,22 and hydrolysis of alkoxides.23 Zn2GeO4 and Zn2GeO4:Mn2þ thin films were prepared for the use as cathodoluminescence and electroluminescence devices by radio frequency magnetron sputtering,24-26 pulsed laser deposition,27 and sol-gel method.28 One-dimensional Zn2GeO4 nanostructures were also prepared by vapor growth techniques29-31 and aqueous process using Zn-containing Ge nanoparticles.32 Recently, Huang et al. reported the surfactant-assisted hydrothermal synthesis of Zn2GeO4 nanorod of ∼300 and ∼20 nm in length and width, respectively.33,34 In this work, we report the facile solvothermal route without any surfactants to prepare Zn2GeO4:Mn2þ nanoparticles of less than tens of nanometers in size. Mixed solvents of ultrapure water and diethylene glycol (DEG) with various DEG concentrations are used as the solvents to control the size of the nanoparticles. We also discuss the influences of the mixed solvent on the properties of the final products such as their phases, sizes, morphologies, compositions, and photoluminescence (PL) properties. We focus on DEG by the following reasons. The polarity of water/DEG mixed solvent is tunable by changing its ratio because DEG is totally miscible with water at any ratio, and the static dielectric constant ranges from ∼80 (pure water) to ∼30 (pure DEG) r 2010 American Chemical Society

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at room temperature.35 DEG is also known as a typical coordinating solvent and hence is often used for the wet chemical syntheses of various kinds of nanoscale inorganic materials.36 Experimental Section Preparation of Samples. Germanium(IV) oxide (2.50 mmol, Kanto, 99.99%) and sodium hydroxide (5.00 mmol, Wako, 97.0%) were put in 2.5 mL of the solvent in the Teflon-lined stainless steel autoclave (Berghof, DAB-2). Zinc acetate dihydrate (4.90 mmol, Wako, 99.9%) and manganese(II) acetate tetrahydrate (0.10 mmol, Soekawa, 99%) were put in 5.0 mL of another solvent. This solution was added to the above-mentioned autoclave with stirring. Next, 22.5 mL of the additional solvent was put to this autoclave with stirring and the resulting mixture was purged with

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argon gas for 10 min at the flow rate of 300 mL min-1. Then this autoclave was sealed and heated at 200 °C for 2 h. After cooling to room temperature, the resulting suspension was washed with ethanol by centrifugation at 12000 rpm for 30 min once and at 10000 rpm for 10 min twice to obtain the precipitate. Finally, this precipitate was dried at 50 °C for 1 day to obtain the powdered sample. A mixed solvent (total volume 30.0 mL) of ultrapure water and DEG (Kanto, 99.5%) was used as the solvent. The volume percentage of DEG, xDEG, ranged from 0 to 100 vol %. In addition, we also prepared the sample without Mn2þ, i.e., nondoped Zn2GeO4, at xDEG = 91.7 to clarify the luminescence property of Mn2þ. Characterizations. The powder X-ray diffraction (XRD) profile and the crystallite size were measured using an X-ray diffractometer (Rigaku, Rint 2200) with a Cu KR radiation source. The particle size and morphology were observed using field emission transmission electron microscopes (TEM, FEI, Tecnai F20 and Tecnai 12), where the powdered sample dispersed in ethanol was dropped on a copper microgrid and dried at 30 °C. The Fourier transform infrared (FT-IR) reflectance spectrum was measured using a spectrometer (JASCO, FT/IR-4200) by means of diffuse reflection method, where the powdered sample was diluted with KBr powder. The atomic percentage of Mn/(Zn þ Mn) was determined using an X-ray fluorescent analyzer (XRF, Rigaku, ZSXmini II) by means of the fundamental parameter method. The PL spectrum, its excitation (photoluminescence excitation: PLE) spectrum, and the absolute quantum efficiency were measured using a fluorescence spectrometer (JASCO, FP-6500) with a 150 W Xe lamp. The spectral response was calibrated using an ethylene glycol solution of Rhodamine B (5.5 g L-1) and a standard light source (JASCO, ESC-333). The integrating sphere (JASCO, ISF-513) was used to measure the absolute quantum efficiency.

Results

Figure 1. XRD profiles of Mn2þ-doped samples prepared at different volume percentages of DEG. xDEG (vol %): (a) 0, (b) 50, (c) 75, (d) 91.7, and (e) 100. The Miller indices of rhombohedral Zn2GeO4 (ICDD 11-687) are shown. b: hexagonal GeO2 (ICDD 36-1463). 0: hexagonal ZnO (ICDD 36-1451).

Structural and Particulate Properties. Parts a-e of Figure 1 show the typical XRD profiles of the Mn2þ-doped samples prepared at xDEG = 0, 50, 75, 91.7, and 100, respectively. All the XRD peaks belong to Zn2GeO4 with rhombohedral phenacitetype structure for the samples prepared at 0 e xDEG e 91.7 (Figure 1a-d). On the other hand, the sample prepared at xDEG = 100 (Figure 1e) comprises the mixture of ZnO and GeO2. This result reveals that a small amount of water is

Figure 2. TEM images of Mn2þ-doped samples prepared at different volume percentages of DEG. xDEG (vol %): (a) 0, (b) 25, (c) 50, (d) 75, (e) 87.5, and (f) 91.7. Scale bars: 100 nm.

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Figure 3. Change in mean (a) length and (b) width of the particles with volume percentage of DEG. The sketches of average particles are also shown together with the scale bar.

Figure 5. High resolution TEM images of the Mn2þ-doped sample prepared at xDEG = 87.5. Scale bars: 10 nm. The insets show the corresponding SAED patterns.

Figure 4. Change in mean aspect ratio of the particles with volume percentage of DEG.

required to obtain phase pure Zn2GeO4 under this solvothermal condition. Parts a-f of Figure 2 show the TEM images of the Mn2þdoped samples prepared at xDEG = 0, 25, 50, 75, 87.5, and 91.7, respectively. These samples comprise polygonal or rodlike particles, and their sizes obviously depend on xDEG. The values of mean length and width calculated from 150 particles of each sample are plotted as a function of xDEG, as shown in Figure 3. Sketches of average particles are also shown in Figure 3, together with the mean length and width. The sample prepared at xDEG = 0 (Figure 2a) comprises polygonal particles of 124 and 73 nm in mean length and width, respectively. The size of the particles gradually decreases with the increase in xDEG. The sample prepared at xDEG =91.7 (Figure 2f) comprises rod-like particles of 30.2 and 12.2 nm in mean length and width, respectively. The mean aspect ratio, i.e., the mean ratio of length by width, is plotted as a function of xDEG, as shown in Figure 4. Judging from this result, the mean aspect ratio shows a maximum value of 3.7 for the sample prepared at xDEG = 50.

Figure 5 shows typical high resolution TEM images of the Mn2þ-doped sample prepared at xDEG = 87.5. A uniform lattice fringe and a spot pattern of the selected area electron diffraction (SAED) indicate that each particle comprises a single crystal domain. The lattice fringe of (110) with an interplanar spacing of 7.1 A˚ is observed parallel to the rod direction (see top image of Figure 5). Another lattice fringe of (113) with an interplanar spacing of 2.9 A˚ is observed at an angle of 66° to the rod direction (see bottom image of Figure 5). Therefore, the nanorods grow in the direction of the c-axis of the rhombohedral phenacite-type structure. Figure 6 shows the change in the crystallite sizes estimated from the corresponding XRD peaks of (113) and (410) of the Mn2þ-doped samples using Scherrer’s equation with xDEG. When xDEG increases from 0 to 91.7, both crystallites decrease from ∼40 to ∼20 nm. According to the comparison between the particle sizes (Figure 3) and the crystallite sizes (Figure 6), each particle comprises a single crystal domain for the samples prepared at xDEG g 75. In addition, the crystallites estimated from (113) (Figure 6a) are 5-10 nm larger than those from (410) (Figure 6b) irrespective of any values of xDEG. This is consistent with the anisotropic crystal growth in the direction of the c-axis, as already shown in Figure 5. Surface Organic Species. Parts a and b of Figure 7 show the typical FT-IR diffuse reflectance spectra of the Mn2þdoped samples prepared at xDEG = 0 and 87.5, respectively. Strong absorption bands at 450-650 cm-1 (peak 1) and 700-900 cm-1 (peak 2) are assigned to vibration modes of

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Figure 6. Change in crystallite sizes estimated from (a) (113) and (b) (410) of Mn2þ-doped samples with volume percentage of DEG.

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Figure 8. Change in actual Mn concentration of Mn2þ-doped samples with volume percentage of DEG.

Figure 7. FT-IR diffuse reflectance spectra of the Mn2þ-doped samples prepared at (a) xDEG = 0 and (b) xDEG = 87.5.

ZnO4 and GeO4 tetrahedra, respectively, in Zn2GeO4.23 Two peaks at 1438 and 1561 cm-1 (peaks 6 and 7) are assigned to the symmetric and asymmetric stretching vibration modes of carboxyl groups, respectively, derived from acetate ions.37,38 These are characteristic of carboxyl groups coordinating to metal ions at the surface of the particles. A small peak at 1022 cm-1 (peak 3) is assigned to the stretching vibration modes of C-C and C-O bonds in acetate ions.38 A broad absorption band around ∼3250 cm-1 (peak 10) and a small peak at 1661 cm-1 (peak 8) are assigned to the stretching and deformation vibration modes of O-H groups, respectively, derived from water and DEG.23,39 Small peaks are observed at 1050-1150 cm-1 (peaks 4), corresponding to the stretching vibration mode of C-O bonds, at 1200-1400 cm-1 (peaks 5), corresponding to the deformation vibration mode of C-H bonds, and at 2850-3000 cm-1 (peaks 9), corresponding to the stretching vibration mode of C-H bonds for the sample prepared at xDEG= 87.5 (Figure 7b).39 These peaks 4, 5, and 9 are not observed for xDEG = 0 (Figure 7a). These results verify the presence of DEG molecules adsorbed on the surface of particles. Mn Concentration. Figure 8 shows the change in the actual Mn concentration, i.e., the atomic percentage of Mn/(Zn þ Mn)

Figure 9. Photographs of DEG colloidal solution of the Mn2þdoped sample prepared at xDEG = 87.5 under the irradiation of (a) white light and (b) 302 nm light.

measured by XRF analysis, of the Mn2þ-doped samples with xDEG. The Mn concentration for the sample prepared at xDEG = 0 is 0.1 at %, which is 1/20 of the nominal concentration, 2.0 at %. The Mn concentration increases with the increase in xDEG and approaches to the nominal concentration. The Mn concentrations for the samples prepared at xDEG = 87.5 and 91.7 are 2.2 and 2.1 at %, respectively. Photoluminescence Properties. Figure 9 shows the photographs of the colloidal solution of the Mn2þ-doped sample prepared at xDEG = 87.5 under the irradiation of white light and 302 nm light. The Mn2þ-doped samples show green luminescence under the excitation of UV ∼ near UV light, as shown in Figure 9. Parts a and b of Figure 10 show the PL and PLE spectra of the powdered samples prepared at xDEG. = 91.7 with and without Mn2þ doping, respectively. A broad emission peaking at 470 nm is observed in the PL spectrum (Figure 10a) of the nondoped sample. This emission might be attributed to the luminescent defects such as

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Figure 10. PLE (top) and PL (bottom) spectra of the samples prepared at xDEG = 91.7. (a) Nondoped Zn2GeO4 (λem = 470 nm, λex = 264 nm) and (b) Mn2þ-doped Zn2GeO4 (λem = 538 nm, λex = 304 nm).

oxygen vacancy, Zn interstitial, Zn vacancy, and Ge vacancy.40 This defect luminescence disappears and an strong emission peaking at 538 nm corresponding to the d-d transition (4T1 f 6A1) of Mn2þ is observed in the PL spectrum (Figure 10b) of the Mn2þ-doped sample.19 Two broad excitation bands are observed at 230-270 and 270-370 nm in the PLE spectra (Figure 10) of both samples. The valence band of Zn2GeO4 is mainly composed of the 2p orbital of O2-, while the conduction band is mainly composed of the 4s and 4p orbitals of Ge4þ.34 The excitation band at 230-270 nm is assigned to the transition from the valence band to the conduction band for Zn2GeO4 host crystal, followed by the energy transfer to Mn2þ or luminescent defects.19,40 On the other hand, the assignment of the excitation band at 270370 nm is not yet clear. In the case of the Mn2þ-doped sample, several groups proposed that this excitation band is possibly assigned to the transition between valence band and the impurity level of oxygen vacancy, followed by the energy transfer to Mn2þ.19,25 Another possible assignment of this excitation band is the charge transfer transition from O2- to Mn2þ.41,42 Such charge transfer transition is observed for Zn2SiO4:Mn2þ, which is isostructural to Zn2GeO4:Mn2þ. Figure 11 shows the PL and PLE spectra of the Mn2þdoped samples prepared at various values of xDEG. The peak

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Figure 11. PLE (top) and PL (bottom) spectra of Mn2þ-doped samples prepared at different volume percentages of DEG. xDEG (vol%): (a) 0, (b) 25, (c) 50, (d) 75, (e) 87.5, and (f) 91.7. Each spectrum was measured using its optimum wavelength of emission or excitation.

top wavelength of the Mn2þ emission (see bottom image of Figure 11) shifts toward the longer wavelength side with the increase in xDEG from 533 nm (xDEG = 0) to 538 nm (xDEG = 91.7). This is attributed to increase in the spin exchange interactions between Mn2þ-Mn2þ pairs with the increase in the actual Mn2þ concentration, as already shown in Figure 8.18,43 The peak top wavelengths of the excitation bands (see top image of Figure 11) also varies with various xDEG, however, the reason for these peak shifts have not been clarified yet. Figure 12 shows change in the PL intensity of the Mn2þ-doped samples with xDEG. The PL intensity of the Mn2þ emission shows a maximum at around xDEG ∼ 80. The internal and external quantum efficiencies are 9.9 and 5.6%, respectively, for the sample prepared at xDEG = 87.5 under the irradiation of 304 nm. Discussion Formation Process of Zn2GeO4 Nanorods. We propose the following reaction mechanism on the formation of Zn2GeO4 in the present study. At the first step of the reaction, water and hydroxide ion are required to dissolve GeO2, as shown in eq 1.44 After mixing germanium- and zinc-containing solutions, the

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Figure 12. Change in PL intensity of Mn2þ-doped samples with volume percentage of DEG.

pH value of the mixture is converged to 6.4 in the case of xDEG = 0. Under such a neutral pH value, most of germanium species would be precipitated as Ge(OH)4, as shown in eq 2.45 GeO2 þ OH - f HGeO3 ð1Þ HGeO3 - þ H3 Oþ a GeðOHÞ4

ð2Þ

On the other hand, zinc acetate is dissolved in water to supply Zn2þ ions, which are under the equilibrium with Zn(OH)2, as shown in eq 3.46 Zn2þ þ 2H2 O a ZnðOHÞ2 þ 2Hþ

ð3Þ

Finally, when the mixture is heated, Zn2GeO4 nuclei form and grow through connecting GeO4 and ZnO4 tetrahedra by the formation of the metal-oxygen-metal bonds, followed by the crystallization into the phenacite structure. This reaction can be described as eqs 4, 40 , and/or 400 . After heating for 2 h, the resulting suspension becomes slightly acidic of pH ∼ 4.9 in the case of xDEG = 0, indicating the formation of excess protons by eqs 3-40 . HGeO3 - þ 2Zn2þ þ H2 O f Zn2 GeO4 þ 3Hþ

ð4Þ

GeðOHÞ4 þ 2Zn2þ f Zn2 GeO4 þ 4Hþ

ð40Þ

GeðOHÞ4 þ 2ZnðOHÞ2 f Zn2 GeO4 þ 4H2 O

ð400Þ

In the case of xDEG = 100, both zinc acetate and GeO2 are not dissolved in the solvent at room temperature because of the absence of water. When the mixture is heated, zinc acetate reacts with DEG molecules to form Zn-OH species, as shown in eq 5,47 while GeO2 remains unchanged and does not take part in any chemical reactions. Subsequently, ZnO is formed by the dehydration and the condensation between Zn-OH species, as shown in eq 6, resulting in the mixture of ZnO and GeO2 as shown in the XRD profile (Figure 1e).

The following three items can explain the dependence of xDEG on crystallite and particle sizes. (i) Polarity of the

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solvent decreases with the increase in xDEG35 and hence the solubilities of the precursors and solutes such as Ge(OH)4, Zn(OH)2, HGeO3-, and Zn2þ in the solvent decrease. Subsequently, the supersaturation of Zn2GeO4 at the early stage of the reaction of the eqs 4-400 becomes higher, resulting in the larger number of nuclei. This leads to the formation of smaller particles. (ii) Such poor solubilities of the solutes also suppress Ostwald ripening by the dissolution and reprecipitation processes of the particles, resulting in the slower growth rate at the higher xDEG. (iii) DEG molecules coordinate to the surface of the particles and hence prevent their growth. On the other hand, the anisotropic crystal growth in the direction of the c-axis possibly originates from the following two items: (i) the crystallographic feature of phenacite structure and (ii) the preferential coordination of DEG molecules. With respect to the item (i), the phenacite crystal has internal hexagonal hollow tubes and directional polymerization axes of ZnO4 and GeO4 tetrahedra in the direction of the c-axis. According to the previous studies on the willemite-type Zn2SiO4, which is isostructural to the phenacite, this structure has strong bonds in the direction of the c-axis rather than other directions because the so-called periodic bond chain theory predicts that the planes horizontal to the c-axis such as (110) and (100) are energetically more stable than (001).48,49 Therefore, this structure tends to grow in the direction of the c-axis to minimize the area of (001) plane under near equilibrial conditions. Such tendency of anisotropic growth was experimentally confirmed for the willemite-type Zn2SiO4.49 With respect to item (ii), the dependence of xDEG on the aspect ratio (see Figure 4) can be explained by assuming the preferential coordination of DEG molecules to specific planes. In the case of xDEG < 50, DEG molecules might preferentially coordinate to the planes horizontal to the c-axis, e.g., (110) and (100), resulting in the suppression of growth in the directions of the a-and baxes. This leads to the anisotropic growth in the direction of the c-axis. By increasing xDEG over 50 vol %, such preferential coordination might gradually disappear because the number of DEG molecules becomes enough to cover the whole surface of the particles, resulting in the lower aspect ratios. The origin of such preferential coordination is not yet clear, but we note that the Zn2þ and Ge4þ densities of (110) and (100) planes are 1.18 and 1.37 times larger, respectively, than those of (001) plane.50 This indicates that (110) and (100) planes have a larger number of available coordinating sites for OH groups of DEG molecules in comparison to (001) plane. Factors Determining PL Intensity. The dependence of xDEG on the actual Mn concentration for Mn2þ-doped samples can also be explained by the polarity of the solvent. The higher xDEG decreases the solubility of Mn2þ in the solvent because of the lower polarity of the solvent. This results in the higher Mn2þ concentration in the particles. Similar phenomenon was reported for the solvothermal synthesis of ZnGa2O4:Mn2þ nanoparticles in the mixed solvent of water and 1,4-butanediol.51 The PL intensity of the Mn2þ emission shows a maximum around xDEG ∼ 80, as shown in Figure 12. We suggest that the sizes of crystallites and particles and the actual Mn concentration act as predominant parameters for the PL intensity of the Mn2þ-doped samples. Generally, nanosizing the phosphor particles without any surface modification decreases the luminescence efficiency because the increase

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in a surface to volume ratio induces nonradiative processes related to surface defects. In the present study, the particle and crystallite sizes decrease with the increase in xDEG (see Figures 3 and 6). This should contribute to the decrease in the PL intensity of the Mn2þ emission. On the other hand, the optimum concentration of Mn2þ is known to be around 2 at% for bulk Zn2GeO4:Mn2þ.18,41 According to Figure 8, the actual Mn concentration increases with the increase in xDEG and converges to be 2 at % for the sample prepared at xDEG = 87.5. Therefore, the PL intensity of the Mn2þ emission reaches the maximum value around xDEG ∼ 80 by the competition of both factors with opposite effects on the PL intensities. Conclusions Germanium(IV) oxide and acetates of zinc and manganese(II) are treated at 200 °C for 2 h in water/DEG mixed solvents with various volume percentages of DEG using an autoclave. Under this solvothermal condition, a small amount of water, i.e., xDEG = 91.7, is required to dissolve GeO2 to obtain phase pure Zn2GeO4:Mn2þ. Obtained Zn2GeO4:Mn2þ particles comprise the nanorods with the preferential growth in the direction of the c-axis. Their particle and crystallite sizes decrease with the increase in xDEG, and the sample becomes a single crystal domain. This size reaches the minimum at xDEG = 91.7, i.e., 30.2 and 12.2 nm in mean length and width, respectively. Thus, the particle size can be controlled by the solubilities of the precursors and solutes in the mixed solvent and the coordination of DEG molecules. The actual Mn concentration of the Mn2þ-doped samples increases with the increase in xDEG and approaches to the nominal concentration, 2.0 at %. The Mn2þdoped samples show green luminescence corresponding to the d-d transition of Mn2þ under the irradiation of UV and nearUV light. The PL intensity of the Mn2þ emission shows the maximum around xDEG ∼ 80 due to two competitive effects: the decrease in the sizes of crystallites and particles and the increase in the actual Mn concentration with the increase in xDEG. We expect that Zn2GeO4:Mn2þ nanophosphor could be applied to the wavelength convertor from UV ∼ near-UV to green light in the field of photovoltaics, in particular, as a spectral shifter for the amorphous silicon solar cells. Acknowledgment. S.T. thanks the Japan Society for the Promotion of Science (JSPS) for the doctoral fellowship (DC1).

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