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Two Photoenergy Conversion Modes of YVO4:Eu3+ Nanoparticles: Photoluminescence and Photocatalytic Activity Yusuke Shiraishi, Satoru Takeshita, and Tetsuhiko Isobe J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 01 Jun 2015 Downloaded from http://pubs.acs.org on June 3, 2015

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

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The Journal of Physical Chemistry

Two Photoenergy Conversion Modes of YVO4:Eu3+ Nanoparticles: Photoluminescence and Photocatalytic Activity

Yusuke Shiraishi, Satoru Takeshita*, and Tetsuhiko Isobe*

Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan

ABSTRACT In this work we propose Eu3+-doped YVO4 as a model compound of a multimodal photoenergy converter that shows photoluminescence and photocatalytic activity, and discuss relation between these two photoenergy conversion modes. YVO4:Eu3+ nanoparticles with different Eu3+ concentrations and distributions were prepared by coprecipitation and ion exchange methods. The obtained samples consisted of elliptical nanoparticles of 20–25 nm in mean size, irrespective of the Eu3+ concentration and distribution. The nanoparticles showed two photoenergy converting functions under UV irradiation: (i) Red photoluminescence corresponding to a 4f → 4f transition of Eu3+ and (ii) photocatalytic decomposition of methyl orange. Increase in the Eu3+ concentration increased the photoluminescence quantum yield, but decreased the photocatalytic activity. Localizing Eu3+ ions at the particle surface increased the photocatalytic activity, but decreased the photoluminescence quantum yield. These results imply a competitive relation between photoluminescence and photocatalytic activity for the compositional range used in this work.

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INTRODUCTION Photoenergy converting materials are today one of the most important categories of functional materials. Photocatalysts absorb excitation light and accelerate chemical reactions, including thermodynamically nonspontaneous ones. In other words, photocatalysts are light-to-chemical photoenergy converters. Phosphors absorb excitation light and radiate emission light of different wavelengths. In other words, phosphors are light-to-light photoenergy converters. Although photocatalysts and phosphors have been extensively studied in the past several decades, they have been focused on only as single-modal photoenergy converters. In this study, we focus on the possibility of multimodal photoenergy converters that show both photocatalytic activity and photoluminescence (PL) at the same time. Several semiconductors have been reported as not only photocatalysts but also as phosphor host crystals. TiO2 is one of the most well-studied photocatalysts,1,2 but it has also been studied as a host crystal for rare-earth-doped phosphors.3,4 Zn2GeO4 is known as not only a photocatalyst for CO2 reduction,5,6 but also as a host crystal for the green-emitting phosphor Zn2GeO4:Mn2+.7,8 YVO4 is a typical host crystal for rare-earth-doped phosphors,9,10 but it is also known as a UV-active photocatalyst for organic dye decomposition.11,12 While our group has focused on Bi3+- and Eu3+codoped YVO4 nanoparticles as potential red-emitting phosphors,13–16 we have also found that these nanoparticles show activity for photoredox reactions similar to photocatalytic reactions.17–19 Furthermore, a number of semiconductor quantum dots, such as CuInS2–AgInS2 and carbon dots,20– 22

have been studied from the aspects of their multimodal photofunctions, including PL,

photocatalysis, and photovoltaic effects. These previous works guarantee the possibility of multimodal photoenergy conversion and stress its diverse potential. However, to our knowledge, no research has focused on the relation between PL and photocatalysis as different photoenergy conversion modes in one material. The fundamental physicochemical properties of multimodal

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photoenergy converters, e.g., the factors determining the predominant photoenergy conversion mode, whether it is a competitive relation between different photoenergy conversion modes or not, and the effects of the surrounding environment on the photoenergy conversion modes, have not yet been clarified. Investigation into these topics is necessary not only for potential practical applications of multimodal photoenergy converters but also for deeper understanding of photocatalysis and PL. As a matter of fact, in some semiconductors photocatalysis and PL occur through the same excitation process. We suggest that Eu3+-doped YVO4 may be a model compound for multimodal photoenergy conversion. YVO4 has a valence band that mainly consists of O2− 2p, and its conduction band mainly consists of V5+ 3d.23,24 As illustrated in Figure 1, YVO4 absorbs 250–350 nm UV light through interband transition, namely, charge transfer transition from O2− to V5+ of VO43− anions. If the photogenerated electrons and holes reach the particle surface, they can be consumed in photocatalytic reactions. In the field of phosphor research, the migration of excited electrons and holes is also described as a thermally activated hopping of excitons between VO43− anions.25,26 If the electrons and holes are trapped at a VO43− anion near a Eu3+ ion during the migration process, they recombine to excite the 4f electrons of Eu3+, which is usually described as “energy transfer from VO43− to Eu3+”.25,26 In this case, the excitation energy is converted into red emission corresponding to 4f → 4f transitions of Eu3+.

Figure 1. Illustration of relaxation pathways for photogenerated electrons and holes in YVO4:Eu3+.

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In this work, we focus on Eu3+-doped YVO4 nanoparticles as a model compound of a multimodal photoenergy converter and discuss the relationship between the two photoenergy conversion modes. To achieve this purpose, we prepared YVO4:Eu3+ nanoparticles with different Eu3+ concentrations and distributions and investigated their effects on the PL and photocatalytic activity. Various methods have been reported for synthesizing YVO4:Eu3+, including solid-state reactions,8 coprecipitation,27,28 sol–gel methods,29 microemulsion methods,30 polyol and ionic liquid methods,31,32 combustion methods,33 spray pyrolysis,34 and solvothermal methods.35–38 We chose a coprecipitation method combined with an annealing treatment for the following reasons: (i) Nanoparticles of ~20 nm in size, which is suitable for photocatalytic reactions, can be easily formed by this method. (ii) The method does not use any organic ligands that strongly bond to the nanoparticle surface. We also focused on a cation exchange approach to prepare Eu3+-surfacelocalized YVO4, because this approach does not require any additional high-energy treatments that would change the structural and particulate properties of the nanoparticles.39

EXPERIMENTAL SECTION Materials. Yttrium nitrate hexahydrate (99.99%), europium(III) nitrate hexahydrate (99.95%), lanthanum nitrate hexahydrate (99.0%), and methyl orange (MO) were purchased from Kanto Chemical. Sodium orthovanadate(V) heptahydrate (99%) was purchased from Mitsuwa Chemical. Sodium hydroxide (97.0%) was purchased from Wako Pure Chemical Industries. All reagents were of analytical grade and used without further purification.

Synthesis of Uniformly Eu3+-Doped YVO4 Nanoparticles. Uniformly Eu3+-doped YVO4 nanoparticles were prepared by a coprecipitation method combined with an annealing treatment. A

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total of 4.00 mmol of yttrium nitrate hexahydrate and europium(III) nitrate hexahydrate were dissolved in 40.00 mL of ultrapure water. Sodium orthovanadate(V) heptahydrate (4.00 mmol) was dissolved in 40.00 mL of ultrapure water and the pH value was adjusted to 12.9 by adding 1.20 mL of aqueous sodium hydroxide solution (1.00 mol L−1). The (Y,Eu)(NO3)3 solution was added to the sodium orthovanadate(V) solution and aged at 85 °C for 60 min under constant stirring at 300 rpm. The obtained suspension was centrifuged at 13000 rpm for 10 min to remove the unreacted ions and the supernatant was carefully removed. The as-prepared YVO4:Eu3+ nanoparticles were obtained by washing the precipitate with ethanol and centrifuging at 13000 rpm for 10 min. Finally, the nanoparticles were dried at 50 °C overnight and then annealed at 500 °C for 120 min in a tubular furnace under a constant air flow (300 mL min−1) to yield a powdered sample of uniformly Eu3+doped YVO4 nanoparticles. The actual Eu/(Y + Eu) content measured by X-ray fluorescence analysis (XRF) was controlled from 0 at% (undoped) to 12.5 at% by changing the nominal content, as shown in Figure S1. For comparison, we also prepared La3+-doped, non-luminescent YVO4 nanoparticles using lanthanum nitrate hexahydrate instead of europium nitrate hexahydrate.

Synthesis of Eu3+-Surface-Localized YVO4 Nanoparticles. Eu3+-surface-localized YVO4 nanoparticles were prepared through cation exchange by aging undoped YVO4 nanoparticles in aqueous Eu(NO3)3 solution. The as-prepared YVO4 nanoparticles were redispersed in 40.0 mL of Eu(NO3)3 solution (1.38 or 9.20 mmol L−1) and aged at 85 °C for 60 min under constant stirring at 300 rpm. After the cation exchange, the suspension was centrifuged at 13000 rpm for 10 min to remove the unreacted ions and the precipitate was washed with ethanol. The obtained precipitate was dried at 50 °C overnight and then annealed at 500 °C for 120 min to yield a powdered sample of Eu3+-surface-localized YVO4 nanoparticles. The actual Eu3+ concentrations of the final products were 1.8 and 7.0 at% for the Eu(NO3)3 concentrations of 1.38 and 9.20 mmol L−1, respectively.

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Characterization. The atomic compositions of the samples were determined with the fundamental parameter method using an XRF analyzer (Rigaku, ZSX mini II). Powder X-ray diffraction (XRD) profiles were obtained using an X-ray diffractometer (Rigaku, RINT-2200) with a monochromator and a Cu Kα radiation source. The crystallite size perpendicular to the (200) plane, D200, was calculated from the XRD peak width using Scherrer’s equation. The plane spacing of (200), d200, was calculated from the XRD peak position. The particle size and morphology were examined using field-emission transmission electron microscopy (TEM; FEI, Tecnai G2). The samples for TEM observations were prepared by dropping an aqueous suspension onto a copper microgrid and drying at room temperature. PL and corresponding excitation (PLE) spectra were measured using a fluorescence spectrometer (Jasco, FP-6500) with a Xe lamp. The PL quantum yield, Φ, of the powdered samples was measured on the same spectrometer equipped with an integrating sphere (Jasco, ISF-513) based on the following equation (1),

Φ=

I em I ex − I ref

(1)

where Iem is the integrated emission intensity of the sample, Iex is the integrated intensity of the incident excitation light, and Iref is the integrated intensity of the excitation light reflected by the sample. A reflectance standard (Labsphere, Spectralon SRS-99) was used to determine Iex. We also measured the apparent PL quantum yield for an aqueous YVO4:Eu3+ suspension using the same apparatus, but its absolute value was not reliable because the scattering loss was not negligible. The photocatalytic activity of the YVO4:Eu3+ nanoparticle suspension was evaluated based on the rate of photocatalytic decomposition of MO under UV irradiation. The YVO4:Eu3+ nanoparticle powder (60.0 mg) was redispersed in ultrapure water (29.25 mL) under ultrasonic irradiation. An aqueous MO solution (0.75 mL, 100 mg L−1) was added to the YVO4:Eu3+ suspension and stirred for 120 min in the dark to achieve adsorption equilibrium. A portion of the suspension (3.80 mL) was 6


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then poured into a quartz vessel and continuously irradiated with UV light (λex = 315.1 nm) at 25 °C under constant stirring using a Xe lamp in the fluorescence spectrometer (Jasco, FP-6500) as a UV light source. After UV irradiation for 0–300 min, the suspension was centrifuged at 15000 rpm for 10 min to remove the nanoparticles, and the MO concentration was determined using a UV-vis-NIR spectrometer (Jasco, V-570) and a calibration line (Figure S2).

RESULTS AND DISCUSSION Structural and Particulate Properties. Figure 2(a–e) show the XRD profiles of samples prepared with different Eu3+ concentrations. All the XRD peaks belonged to YVO4 with the tetragonal zircon structure. Table S1 summarizes the D200 and d200 values for these samples. The crystallite size was ~18 nm, irrespective of the Eu3+ concentration. No systematic changes in D200 and d200 were observed with Eu3+ concentration. This is attributed to the small amount of Eu3+ present and the similar ionic radii of Y3+ (1.159 Å) and Eu3+ (1.206 Å).40 As the XRD profiles indicate the absence of impurity phases, we conclude that the samples are phase-pure YVO4:Eu3+. Figure 2(f and g) show the XRD profiles of the Eu3+-surface-localized samples. The profiles indicated the presence of phase-pure YVO4, showing that the Eu3+ ions were incorporated into the YVO4 host crystal without forming any detectable impurity phases. No systematic change in the XRD peak width and position were observed compared with the uniformly Eu3+-doped samples. We also confirmed the phase-pure nature of the La3+-doped YVO4 samples, as shown in Figure S3.

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Surface localized (g) (f)

Intensity (a.u.)

Uniformly doped

(e) (d) (c) (b) (a)

10

20

30

40

50

(420)

(321) (312) (400)

(103)

(200)

(112) (220) (202) (301)

Tetragonal YVO4 (ICDD No. 17-341) (101)

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60

2θ (deg) Figure 2. XRD profiles of (a–e) uniformly Eu3+-doped and (f,g) Eu3+-surface-localized YVO4 samples at different actual Eu3+ concentrations (at%): (a) 0.0, (b) 0.8, (c) 1.6, (d) 6.8, (e) 12.5, (f) 1.8, and (g) 7.0. The ICDD card data for tetragonal YVO4 is also shown.

Figure 3(a–f) show the TEM images of uniformly Eu3+-doped samples prepared with different Eu3+ concentrations. All the samples were comprised of elliptical nanoparticles, irrespective of the Eu3+ concentration. High-resolution TEM images [Figure 3(d)] showed a lattice fringe with a spacing of 0.355 nm, which corresponds to the (200) spacing of YVO4. The mean particle size and its standard deviation were measured from the TEM images and are plotted as a function of the actual Eu3+ concentration in Figure 4. The mean particle size was 20–25 nm, and no significant change with the Eu3+ concentration was observed. According to the comparison with the crystallite size, each nanoparticle consisted of a single crystallite domain.

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Figure 3(g–i) show TEM images of Eu3+-surface-localized samples prepared with different Eu3+ concentrations. The size and shape of these samples did not differ significantly from those of the uniformly Eu3+-doped samples. High-resolution TEM images [Figure 3(h)] showed no impurity phase or change in the surface morphology. From these results, we conclude that the Eu3+ concentration and distribution had no significant influence on the structural and particulate properties of the material. Energy dispersive X-ray (EDX) analysis confirmed the presence of Eu3+ in each nanoparticle (see Figure S4). We also measured an EDX line profile across a nanoparticle (see Figure S5), but could not obtain clear evidence for surface localization of Eu3+ ions owing to the low resolution and low Eu3+ concentration.

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Figure 3. TEM images of (a–f) uniformly Eu3+-doped and (g–i) Eu3+-surface-localized YVO4 samples at different actual Eu3+ concentrations (at%): (a) 0.0, (b) 0.8, (c,d) 1.6, (e) 6.8, (f) 12.5, (g,h) 1.8, and (i) 7.0.

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Mean particle size (nm)

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30

20

10

0

0

5 3+

Actual Eu

10

concentration (at%)

Figure 4. Change in mean particle size and its standard deviation with actual Eu3+ concentration. Circles: uniformly Eu3+-doped YVO4. Triangles: Eu3+-surface-localized YVO4.

Effect of Eu3+ Concentration on Photoluminescence Properties. Figure 5 shows the PL and PLE spectra of the powdered samples of uniformly Eu3+-doped YVO4 with different Eu3+ concentrations. The PLE spectra consisted of a broad peak at ~300 nm corresponding to the interband transition of YVO4 followed by energy transfer to Eu3+. The emission spectra consisted of four sharp peaks corresponding to the intrinsic 4f → 4f transitions of Eu3+, 5D0 → 7F1 (594 nm), 5D0 → 7F2 (619 nm), 5

D0 → 7F3 (651 nm), and 5D0 → 7F4 (699 nm). The intensity ratios between the four emission peaks

agree with those given in previous reports.27,35 This confirms that the Eu3+ ions substituted for Y3+ ions in the host YVO4 crystal. The PL quantum yields of the powdered samples are plotted as a function of the actual Eu3+ concentration in Figure 6 (squares). The PL quantum yield increased simply with Eu3+ concentration in this concentration range. This is attributed to the increase in the 11



probability of energy transfer from VO43− to Eu3+ as a result of the increased number of Eu3+ ions. The saturation of the PL quantum yield at Eu3+ ~12.5 at% is attributed to concentration quenching among Eu3+ ions. This is consistent with Huignard et al.’s report in which the optimum Eu3+ concentration was ~16 at% for YVO4:Eu3+ nanoparticles prepared by the coprecipitation method.27

300 EX. (d)

PL intensity (a.u.)

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200

EM.

(c)

(b) 100 (a)

0

300

400

500

600

700

Wavelength (nm) Figure 5. PLE (λem = 619.4 nm) and PL (λex = 315.1 nm) spectra of powdered samples of uniformly Eu3+-doped YVO4 at different actual Eu3+ concentrations (at%): (a) 0.8, (b) 1.6, (c) 6.8, and (d) 12.5.

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50

10

40

8 30 6 20 4

PL quantum yield (%)

12

Rate constant of -3 -1 MO decomposition (10 min )

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10

2 0 0

5

10 3+

Actual Eu

0 15

concentration (at%)

Figure 6. Effect of the actual Eu3+ concentration on the PL quantum yield (squares) and rate constant of MO decomposition (circles) for uniformly Eu3+-doped YVO4 samples.

Effect of Eu3+ Concentration on Photocatalytic Activity. Figure 7 shows the change in the MO concentration with UV irradiation time for the uniformly Eu3+-doped samples. The MO decomposition rate increased in the presence of YVO4:Eu3+ nanoparticles. This result verifies the photocatalytic activity of the nanoparticles. Assuming a first order reaction, the MO concentration, [MO] (mg L−1), at irradiation time t (min), is given by equation (2) as follows, ln[MO] = ln[MO]0 – kt

(2)

where k (min−1) is the rate constant and [MO]0 (mg L−1) is the initial MO concentration at t = 0. As shown in Figure S6, the MO decomposition curves agreed well with a first order reaction. The k value calculated from Figure S6 is plotted as a function of the actual Eu3+ concentration in Figure 6 (circles). Increasing the Eu3+ concentration decreased the photocatalytic activity but increased the PL 13



quantum yield. Such a decrease in photocatalytic activity was not observed for La3+-doped samples, as shown in Figures S7 and S8. We therefore conclude that the local lattice distortion caused by the large dopant ions, La3+ (1.3 Å) and Eu3+ (1.206 Å),39 did not affect the photocatalytic activity of the samples. As the Eu3+ concentration increased, more electrons and holes recombined to excite Eu3+ ions during their migration process, which lead to the higher PL quantum yield. As a result, fewer electrons and holes reached the particle surface, where photocatalytic reactions occur. These results reflect the competitive relationship between the PL and photocatalytic activity.

3.0 (f) MO concentration (mg L-1)

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2.0

(e)

(d)

1.0 (b)

(c)

(a) 0.0 0

100 200 Irradiation time (min)

300

Figure 7. Change in MO concentration of aqueous MO solutions containing dispersed YVO4:Eu3+ with UV irradiation time for uniformly Eu3+-doped YVO4 samples. Actual Eu3+ concentrations (at%): (a) 0.0, (b) 0.8, (c) 1.6, (d) 6.8, (e) 12.5, and (f) blank sample without YVO4:Eu3+.

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The YVO4:Eu3+ suspensions showed red emission during the photocatalytic decomposition of MO, just as they showed in the absence of MO. To investigate the influence of MO on the PL intensity, the PL spectra of YVO4:Eu3+ suspensions with and without MO were measured under the same conditions as those for the photocatalytic activity measurements. The absorption spectrum of MO and the PLE and PL spectra of YVO4:Eu3+ suspension had little overlap (see Figure S9), so we could roughly evaluate the difference in the PL intensity without considering a reabsorption loss. As shown in Figure 8, the PL intensity was not changed by the addition of MO within the experimental error. We also measured the apparent PL quantum yields of YVO4:Eu3+ suspensions with and without MO (see Figure S10), but did not find any significant difference. These results suggest that the fraction of photoenergy converted into PL was not affected by the presence of MO surrounding the nanoparticles. According to previous studies on TiO2 photocatalysts,1,2 when photogenerated electrons and holes reach the particle surface, the electrons are captured by oxygen molecules and produce oxygen radicals such as O2•−. The holes are captured by surface organic species directly, or by water molecules to produce hydroxyl radicals, OH•. The photocatalytic decomposition of organic species in water originates from oxidative dehydration by the holes and/or OH• radicals. We suggest that migration of electrons and holes to the particle surface occurred at a constant rate, which was exclusively determined by the Eu3+ concentration in the present case, regardless of the presence of MO.

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100

PL intensity (a.u.)

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EX.

80

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EM.

3+

6.8 at% without MO

3+

6.8 at% with MO

3+

1.6 at% without MO

3+

1.6 at% with MO

Eu Eu

60 40 Eu

20

Eu 0

300

400

500

600

700

Wavelength (nm) Figure 8. PLE (λem = 619.4 nm) and PL (λex = 315.1 nm) spectra of uniformly Eu3+-doped YVO4 suspensions with and without MO.

Effect of Eu3+ Distribution on Photoluminescence and Photocatalytic Activity. The PL spectra and MO decomposition curves of the Eu3+-surface-localized samples are summarized in Figures S11 and S12. The ratios between the intensities of the four PL emission peaks of the Eu3+-surfacelocalized samples were almost the same as those of the uniformly Eu3+-doped samples. This confirms that the Eu3+ ions substituted for Y3+ ions in the host crystal. Figure 9 shows the relationship between the PL quantum yield and the rate constant of MO decomposition for samples with different Eu3+ concentrations and distributions. As shown by the triangles in Figure 9, the PL quantum yields of the Eu3+-surface-localized samples were 3–4 times lower than those of the uniformly Eu3+-doped samples at similar Eu3+ concentrations (e.g., compare the triangle at Eu3+ 1.8 at% and the circle at Eu3+ 1.6 at%). This result shows that concentration quenching among surfacelocalized Eu3+ ions could not be negligible for the Eu3+-surface-localized samples. This could be

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indirect evidence for localization of the Eu3+ ions. On the other hand, the rate constants of MO decomposition for the Eu3+-surface-localized samples were 1.6–1.7 times larger than those measured for uniformly Eu3+-doped samples of similar Eu3+ concentrations, indicating that the photocatalytic activity was increased by localizing Eu3+ ions at the particle surface. Although the reason for this increase has not yet been clarified, we suggest the following possible mechanisms: (i) The surface-localized Eu3+ ions changed the electronic structure of YVO4 near the surface, which improved the MO decomposition rate in some way. (ii) Energy migration among surface-localized Eu3+ ions increased the probability of energy transfer to surface chemical species related to the photocatalytic reactions. (iii) The surface-localized Eu3+ ions changed the colloidal properties of the suspension, such as the dispersion stability and MO adsorption equilibrium. However, we think that (iii) is not highly probable because the amount of dopant Eu3+ ions was small. Figure 9 also shows a negative correlation between the photocatalytic activity and PL quantum yield. This result implies a competitive relationship between PL and photocatalytic activity regarding the Eu3+ concentration and distribution. We note that this result does not verify a competitive relationship for YVO4:Eu3+ samples with different crystallographic and particulate properties. There might be three relaxation pathways for photogenerated electrons and holes: (i) PL, (ii) photocatalysis and related surface reactions, and (iii) nonradiative recombination, which turns into heat, in the host crystal. Measuring the rate of pathway (iii) would be quite difficult, and we could not do it in the present work. Further work is needed to investigate other factors that affect the probability of these pathways, such as particle size and defect concentration.

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● Uniformly doped ▼ Surface localized

12.5%

40


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6.8% 30

20 1.6%

0.8%

10 7.0% 1.8% 0

0

2

4 6 Rate constant of -3

8

0.0% 10

-1

MO decomposition (10 min ) Figure 9. Relationship between the rate constant of MO degradation and the PL quantum yield of samples with different Eu3+ concentrations and distributions. The numbers show the actual Eu3+ concentrations.

CONCLUDING REMARKS We focused on YVO4:Eu3+ nanoparticles as a model compound of a multimodal photoenergy converter that shows both PL and photocatalytic activity, and investigated the effects of the Eu3+ concentration and distribution on these two properties. When the Eu3+ concentration was below the concentration quenching region, increasing the Eu3+ concentration increased PL quantum yield, but decreased photocatalytic activity. The decrease in the photocatalytic activity was not attributed to local lattice distortion caused by the dopant ions, because the La3+-doped samples did not show a significant decrease in photocatalytic activity. Furthermore, the PL intensity was not affected by the

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presence of MO surrounding the nanoparticles. We suggest that the migration of photogenerated electrons and holes to the particle surface occurred at a constant rate that was exclusively determined by the Eu3+ concentration in this case, regardless of the presence of MO. On the other hand, localizing Eu3+ ions at the particle surface increased the photocatalytic activity, but decreased the PL quantum yield. These results imply a competitive relationship between PL and photocatalytic activity regarding the Eu3+ concentration and distribution in the range of this work’s experiment. Further work will be needed to investigate other factors that affect the probability of photoenergy conversion modes, such as particle size, defect concentration, and high Eu3+ concentration over the quenching concentration.

ASSOCIATED CONTENT Supporting Information Compositional and crystallographic properties of the samples (Table S1), relationship between the nominal and actual Eu3+ concentrations for uniformly Eu3+-doped samples (Figure S1), calibration line for MO concentration (Figure S2), XRD profiles of La3+-doped YVO4 (Figure S3), high-angle annular dark field scanning TEM images and EDX profiles of surface-Eu3+-localized sample (Figures S4 and S5), ln[MO]–t plots for uniformly Eu3+-doped samples (Figure S6), change in the MO concentration with UV irradiation time for YVO4:La3+-dispersed MO aqueous solutions (Figure S7), change in the rate constant of MO decomposition with actual La3+ concentration (Figure S8), absorption spectrum of MO, and PLE and PL spectra of YVO4:Eu3+ suspension (Figure S9), apparent PL quantum yields of YVO4:Eu3+ suspensions with and without MO (Figure S10), PLE and PL spectra of powdered samples of Eu3+-surface-localized YVO4 (Figure S11), and change in the MO concentration with UV irradiation time for Eu3+-surface-localized samples (Figure S12). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]; Tel: +81 45 566 1531; Fax: +81 45 566 1551 (S.T.). *E-mail: [email protected]; Tel: +81 45 566 1554; Fax: +81 45 566 1551 (T.I.).

Author’s Current Affiliation (S.T.) Research Institute for Chemical Process Technology, National Institute of Advanced Industrial Science and Technology (AIST) Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan E-mail: [email protected]

ACKNOWLEDGEMENT This work was supported by JSPS KAKENHI Grant Number 26790017.

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TOC GRAPHIC 50 3+


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Eu 12.5%

40

● Uniformly doped ▼ Surface localized 6.8%

30

20 1.6%

0.8%

10

7.0% 1.8% 0

0

2

4

6

8

0.0%

Rate constant of -3 -1 MO decomposition (10 min )

10

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Two Photoenergy Conversion Modes of YVO4 ... - ACS Publications

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