Biphasic Sol–Gel Synthesis of Microstructured/Nanostructured YVO4:Eu3+ Materials and Their H2O2 Sensing Ability

Microstructured/nanostructured YVO4:Eu3+ powders and films were synthesized through a biphasic sol–gel method, aiming at their application as H2O2 sensing materials based on the turn-off luminescence of Eu3+ ions. The synthesis was typically carried out at temperatures of 80 °C or lower by using organic solutions to dissolve vanadium alkoxide and aqueous solutions to dissolve yttrium and europium salts together with sodium carboxylates. The resultant crystalline YVO4:Eu3+ powders and films were characterized as containing micrometer-sized particles comprising primary nanoparticles with high specific surface areas. A comparative study was performed on the H2O2-responsive turn-off luminescence properties for the above samples and those synthesized by a single-phase sol–gel or a conventional solid-state reaction method. The results indicated that the microstructural feature of the samples from the biphasic sol–gel method was effective for detecting H2O2 through its adsorption on the particle surface and quenching of the Eu3+ luminescence. The film samples showed repeatable and quantitative turn-off luminescence, thereby demonstrating their suitability as solid-state H2O2 sensors.


INTRODUCTION
Fluorescence can be used for detecting certain kinds of chemical species. This is based on changes of the fluorescence intensity or wavelength caused by the interaction between target chemical species and fluorescent materials. Much effort has been devoted so far to designing fluorescent materials aiming at biosensing and bioimaging. Besides traditional organic fluorescent dyes, quantum dots (QDs) have been intensively studied due to their higher stability and tunable fluorescence properties. They include semiconductor QDs such as CdS and CdSe, carbon dots, graphene QDs, and so on. 1−3 There have also been inorganic phosphors that can be used as solid-state luminescence sensors. Due to their higher mechanical strength, thermal stability, and chemical durability, inorganic phosphors are suitable for application under severe circumstances such as high mechanical loads, 4 high temperatures, 5 or highly reactive conditions. 6 A key technique for realizing solid-state luminescence sensors is the precise control of the microstructure of inorganic phosphors in accordance with the respective sensing principles: dense bulks for stress sensing, thin films for high-temperature sensing, and nanoparticles for chemical sensing. 4−6 In our previous studies, we focused on the luminescence sensing of redox species using rare-earth-activated phosphors (Ce-PO 4 :Tb 3+ , CeO 2 :Sm 3+ , and CaWO 4 :Eu 3+ ) synthesized as nanorods, 7 porous microspheres, 8−10 or nanostructured films. 11 In the present study, motivated by the increasing demand for the detection of hydrogen peroxide (H 2 O 2 ), 12,13 we attempted to synthesize microstructure-controlled YVO 4 :Eu 3+ phosphors. Among Eu 3+ -activated, red-emitting phosphors, YVO 4 :Eu 3+ is known to show highly efficient emissions because of the presence of the strongly lightabsorbing VO 4 3− units that can easily transfer excitation energy to the doped Eu 3+ ions. 14,15 The highly efficient phosphors are favorable for designing sensors like turn-off type fluorescent probes. 16,17 A difficulty arises, however, when applying the highly efficient phosphors to chemical sensors; they need to be synthesized as nanometer-scale particles having large surface areas. Generally, nanometer-scale phosphors are inferior to their bulk counterparts due to the much increased amount of surface defects that quench the luminescence considerably. 18 This is the specific reason for the necessity of the precise microstructural control of inorganic phosphors for chemical sensing applications.
We report herein our strategy and results for synthesizing micro-/nano-structured YVO 4 :Eu 3+ materials for H 2 O 2 sensing. Contrary to the previous methods for the synthesis of YVO 4 :Eu 3+ nanomaterials, 15,18−23 a biphasic sol−gel method has been employed because it is beneficial to obtaining microstructure-controlled materials ranging from nano-to micro-meter scales. 24 4 :Eu 3+ powder samples by the sol−gel method with the above-mentioned biphasic system and a simple single-phase liquid system. A powder precipitated in the biphasic system (named BP) was initially tinged with yellow, as shown in Figure 1. The BP powder was then washed with a 1.0 M aqueous NaOH solution under stirring for 5 h as a base treatment. The color of the resultant powder (BP-BT) was changed to white during washing. Although the yellow color was lighter for a powder precipitated in the single-phase system (SP), the same base treatment was conducted to obtain SP-BT powder ( Figure 1). Figure 2 shows the X-ray diffraction (XRD) patterns of the BP, BP-BT, SP, and SP-BT powder samples. All the peaks appearing in the patterns can be indexed to the tetragonal YVO 4 phase (ICDD 17-0341). Comparing their peak intensity, the powders obtained from the biphasic system exhibit higher intensities and are crystallized better than those obtained from the single-phase system. The crystallite size along the a-axis of (Y,Eu)VO 4 is calculated to be 25.0 and 11.6 nm for BP-BT and SP-BT, respectively, from the (200) peak using Scherrer's equation.
X-ray fluorescence (XRF) analysis was carried out to examine the origin of the yellow color of the as-precipitated BP and SP samples. The V/(Y + Eu) ratio was determined to be 2.18 and 1.58 for BT and ST, respectively. This result indicates that the BP and SP samples have excess amounts of vanadium against a stoichiometric V/(Y + Eu) ratio of 1. Because of the lack of any XRD peaks due to a secondary phase in Figure 2, the excess vanadium is thought to be present in the samples as a vanadium oxide gel. On the other hand, the base-treated samples, BP-BT and SP-BT, had V/(Y + Eu) ratios of 1.16 and 1.06, respectively. The color of the BP-BT and SP-BT samples was also changed to white by the base treatment ( Figure 1). The yellow colorization is therefore because of the vanadium oxide gel, which can be removed by washing in an aqueous NaOH solution. A further experiment was carried out to confirm the presence of the vanadium oxide gel before the base treatment. That is, the sample BP was annealed at 800°C for 4 h in air in expectation of the crystallization of the amorphous gel. Figure S1 (see Supporting Information) shows a field-emission transmission electron microscopy (FETEM) image and a selected area electron diffraction (SAED) pattern of BP after annealing. The SAED pattern can be indexed to the orthorhombic V 2 O 5 phase (ICDD 41-1426). This confirms that the excess vanadium exists separately from the (Y,Eu)VO 4 phase in the sample BP.
The precipitation of the vanadium oxide gel in the samples BP and SP can be explained as follows. As reported in the literature, 27 a large variety of chemical species containing V 5+ ions exist in aqueous solutions depending on their pH values as well as V 5+ concentrations ([V 5+ ]). Any species stable at the respective conditions is defined in the log[V 5+ ] − pH diagram. 27 Among these, V 2 O 5 is the most stable phase at both a lower pH and a higher log [V 5+ ]. We observed in the biphasic system that the pH value of the aqueous solution decreased from 4.5 before the reaction to 3.6 after the reaction for 24 h. This pH decrease is because of the progress of the hydrolysis of VO(OC 2 H 5 ) 3 and the subsequent formation of (Y,Eu)VO 4 through a series of chemical reactions as described below.
This reaction occurs when VO(OC 2 H 5 ) 3 in the organic phase comes into contact with water molecules at the interface of the biphasic system. According to the diagram, VO 2 (OH) 2 − is stable at a low log [V 5+ ] around pH = 4.5 and, therefore, H + is released through the hydrolysis.
Then, the VO 2 (OH) 2 − ions react with Y 3+ and Eu 3+ in the aqueous phase to form Y 1−x Eu x VO 4 . This reaction also releases H + and the pH value would be decreased further. However,

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Article HCOO − derived from HCOONa, which was added to the aqueous phase, works as a base and consumes H + to achieve the following equilibrium reaction. Thus, the pH value steadies down to 3.6 after the 24 h reaction. With the progress of reactions 1 and 2, the stable V 5+ species changes from VO 2 (OH) 2 − to less-soluble V 2 O 5 , which has a great tendency to form an amorphous gel. 28 Figure 3 shows the photoluminescence (PL) excitation and emission spectra of the samples BP, BP-BT, SP, and SP-BT.
Several sharp peaks are observed in both the excitation and the emission spectra. They are characteristic of the 4f−4f electronic transitions of Eu 3+ doped in the YVO 4 phase, 29 thereby confirming that reaction 2 involves Eu 3+ as the reactant in the present liquid-phase synthesis. Strong, broad excitation bands ranging from 220 to 360 nm are also seen in the PL excitation spectra of all the samples. They are assigned to the overlapping VO 4 3− absorption and O 2− → Eu 3+ charge transfer (CT). 30,31 Because of the presence of the amorphous V 2 O 5 gel, the PL intensity of BP and SP is much lower than that of BP-BT and SP-BT, respectively. According to the literature, 32 the V 2 O 5 gel has two absorption bands centered at 270 and 380 nm. The latter band is the reason for the yellow colorization and would have less influence on the Eu 3+ emissions. In contrast, the former band overlaps with the CT band and a large part of the excitation light is absorbed by the V 2 O 5 gel in the samples BP and SP. Therefore, the base treatment of these samples in the 1.0 M aqueous NaOH solution under stirring for 5 h effectively removes the V 2 O 5 gel and enhances the Eu 3+ emissions, as seen for the samples BP-BT and SP-BT in Figure 3.
It is also obvious in Figure 3 that the samples from the biphasic system exhibit a higher PL intensity than those from the single-phase system both before and after the base treatment. This is one of the major reasons why we employ the biphasic sol−gel method to produce phosphor materials. Formerly, we synthesized two kinds of self-activated luminescent materials, namely CaNb 2 O 6 and Ba 2 V 2 O 7 , by a similar biphasic method. 25,26 In the case of CaNb 2 O 6 , particles obtained from the biphasic method had a rough and dimpled surface which was more suitable for the extraction of the emitted light. Ba 2 V 2 O 7 from the biphasic method also had better crystallinity and morphology for emission. A fundamental difference between the biphasic and the single-phase system lies in the reaction rate and the morphological evolution. The hydrolysis of the metal alkoxides proceeds much more slowly and mildly due to a very limited reaction space as well as a low amount of water at the organic−aqueous interface. The nucleation and crystal growth of Y 1−x Eu x VO 4 , according to reaction 2, take much more time in the biphasic system and the precipitates are matured well. This is why the crystallinity is better ( Figure 2) and hence the PL intensity is higher ( Figure 3) for the samples from the biphasic system.
The photostability of the samples BP and BP-BT was evaluated because the nano-sized phosphors synthesized from the chemical solutions containing organic ligands often suffer from degradation during irradiation with the excitation light. 33,34 Figure S2 shows the PL intensity at 619.5 nm plotted against time of the ultraviolet (UV) irradiation at 320 nm for the samples BP and BP-BT. While the PL intensity of BP, normalized to its initial intensity, decreases rapidly to 0.24 within 10 min, BP-BT maintains its intensity as high as 0.85 after 10 min. We confirmed from the Fourier-transform infrared (FT-IR) spectroscopy analysis that the HCOO − ions were present in the sample BP and initially questioned whether they acted as the luminescence quencher. However, we came to know later that a film sample also had low photostability even after annealing at 400°C for 3 h in air: the photostability was finally improved after the base treatment. Because the HCOO − ions were no longer present in the annealed film, we consider that the low photostability of the sample BP arises from the amorphous V 2 O 5 gel. Figure 4a shows the field-emission scanning electron microscopy (FESEM) images of the samples BP-BT and SP-BT. Both the samples are defined as micrometer-sized spherical particles, which are constructed of primary nanoparticles. The apparent sizes of these micro-/nano-structured particles are 0.5−3.0 and 0.5−1.5 μm for BP-BT and SP-BT, respectively. Figure 4b shows the FETEM images of the samples BP-BT and SP-BT. It is observed that both the samples are formed by the aggregation of non-spherical nanoparticles. The sample BP-BT exhibits a larger size of the

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Article primary nanoparticles than the sample SP-BT, indicating that the crystal growth was promoted in the biphasic system. This result also supports the higher PL intensity of the BP-BT sample as mentioned above. Figure 5 shows the N 2 adsorption−desorption isotherms and pore size distribution curves of BP-BT and SP-BT. The isotherm of both BP-BT and SP-BT corresponds to that of type IV with a type H2 hysteresis loop in the IUPAC classification. 35 This suggests that the present YVO 4 :Eu 3+ particles have mesopores with an ink bottle shape. The fitting analysis based on the Brunauer−Emmett−Teller (BET) equation revealed that the specific surface areas of BP-BT and SP-BT were 157 and 67.4 m 2 g −1 , respectively. The pore size distribution was examined using the MP and the Barrett− Joyner−Halenda (BJH) method. It is seen from Figure 5b,c that both BP-BT and SP-BT have micropores and mesopores with diameter ranges of 0.7−2 and 2−10 nm, respectively. The pore volume is larger in BP-BT than in SP-BT. These results demonstrate that the micro-/nano-structured particles from the biphasic system are promising as porous sensing materials.
To examine the H 2 O 2 -sensing ability of our YVO 4 :Eu 3+ micro-/nano-structured particles, PL spectra were measured before and after immersing them in a 100 ppm H 2 O 2 solution for 10 min. Figure 6 shows the PL excitation and emission spectra of the samples BP-BT and SP-BT, together with those of another powder synthesized by a conventional solid-state reaction method (the sample SS) before and after H 2 O 2 immersion. Note that the sample SS was composed of micrometer-sized particles and its specific surface area was as low as 0.8 m 2 g −1 . It is seen from Figure 6 that BP-BT and SP-BT undergo a decrease of 33 and 18%, respectively, in their PL intensity after immersion in the 100 ppm H 2 O 2 solution. On the other hand, the sample SS does not show any remarkable change in the PL intensity after immersion. This observation indicates that the micro-/nano-structured particles from the biphasic system have higher H 2 O 2 -sensing ability based on the PL quenching because of their higher specific surface areas and larger pore volumes.
For BP-BT, the PL intensity was also measured using different amounts of the powders immersed in the 100 ppm H 2 O 2 solution. As shown in Figure S3, there seems to be no systematic trend between the PL intensity and the amount of the powders. We also confirmed that the PL intensity no longer decreased with a much larger amount of the powders, basically indicating that the number of H 2 O 2 molecules was excessive under the experimental conditions for Figure S3. Considering that the total surface area increases proportionally with the amount of the powders, the result in Figure S3 indicates that the effective surface area for H 2 O 2 sensing is not constant among the samples. This can be deduced from the particle morphology shown in Figure 4a; the BP-BT powder is composed of particles with different sizes from 0.5 to 3.0 μm. The larger and the smaller particles would have a different H 2 O 2 sensing ability, taking account of the penetration depth of the solution inside the particles. The regulation of the particle size and the penetration depth is necessary to evaluate the H 2 O 2 sensitivity more exactly, which is our next challenge.

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Article 2.2. YVO 4 :Eu 3+ Films. For the possible sensing applications, it is favorable that the micro-/nano-structured particles are fixed on substrates to facilitate their handling. Films of YVO 4 :Eu 3+ were then deposited on silica glass substrates from the biphasic system with the addition of HCOONa (film F) or CH 3 COONa (film A) in the aqueous phase. Figure 7 shows XRD patterns of the films F and A after annealing at 400°C for 3 h and base treatment for 1 h. All the peaks appearing in the patterns can be indexed to the tetragonal YVO 4 phase (ICDD 17-0341). The pattern of film A exhibits a prominent (200) peak as compared to the other weakened peaks, thereby indicative of a preferential a-axis orientation of the particles grown on the substrate. Figure 8 shows FESEM and FETEM images of the films F and A. From the surface FESEM images at low magnification, both film F and A consist of micrometer-sized spherical particles. These particles originate from the initial heterogeneous nucleation at the substrate surface, the subsequent homogeneous nucleation in the aqueous phase, and the final growth together on the substrate. The size of the spherical particles is observed to be 1.0−4.0 and 1.0−2.0 μm for film F and A, respectively. As a whole, the spherical particles of film A are smaller than those of film F, and the accumulation of the particles is denser in film A than in film F. From the surface FESEM images at high magnification in Figure 8a and FETEM images in Figure 8b, it can be seen that the micrometer-sized spherical particles are actually constructed of primary nanoparticles, similar to the YVO 4 :Eu 3+ powder obtained from the biphasic system. The primary nanoparticles in film F are spherical in shape, whereas those in film A have a sheet-like structure. From the cross-sectional FESEM images, the thickness of film A is observed to be larger than that of film F. This difference would cause changes in the optical properties between the films. For example, the diffuse reflectance of film A in the visible light region is twice or many times as high as that of film F, as shown in Figure S4.
We found that film A was attached more tightly to the substrate than film F. This was confirmed experimentally through the repetition of immersing/heating treatments of the films as described below. As seen from Figure 8, the surface of the substrate is fully covered by the densely accumulated smaller particles in film A, whereas film F exhibits an uncovered substrate surface due to the inhomogeneous distribution of the particles. Such a difference may originate from the nucleation stage of the film deposition. An experiment was then performed to compare the number of the particles formed on the substrate through the initial heterogeneous nucleation. That is, the substrate was made to stand against the wall of a glass centrifuge tube when the films (called film F′ and A′ in this experiment) were deposited using the biphasic system under the same reaction conditions. Figure  S5 shows the FESEM images of the substrate surface after the reaction. A clear difference is observed between film F′ and A′ in the number and the size of the particles formed on the substrate. The larger number and the smaller size of the particles in film A′ coincide with those observed for film A. This is why the connection between the particles and the substrate, as well as that among the particles, is sufficiently strong in film A, which is important for its application as the H 2 O 2 sensor.
The enhanced heterogeneous nucleation in film A can be explained as follows. HCOOH and CH 3 COOH have pK a values of 3.75 and 4.76, respectively. Because the pH value of the aqueous phase before the reaction was measured to be 4.50, the concentrations of HCOO − and CH 3 COO − can be calculated to be 0.085 and 0.024 M, respectively, according to the formula "pH = pK a + log 10 ([A − ]/[HA])" (A = HCOO or CH 3 COO). In the aqueous phase, the HCOO − and the CH 3 COO − anions can react with the Y 3+ (0.095 M) and the Eu 3+ cations (0.005 M) to form complexes, which decreases the concentration of the free Y 3+ and Eu 3+ cations. Due to the higher HCOO − concentration, reaction 2 is retarded and the formation of Y 1−x Eu x VO 4 is suppressed in the deposition of film F. In contrast, the heterogeneous nucleation becomes

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Article more frequent on the substrate in the case of film A because of the lower CH 3 COO − concentration. Figure 9 shows PL excitation and emission spectra of film F and A, those immersed in the 100 ppm aqueous H 2 O 2 solution (film F-I and A-I), and those subsequently heated at 200°C in air (film F-H and A-H). Both film F and A exhibit a remarkable decrease in the PL intensity after immersing in the H 2 O 2 solution (F-I and A-I). This H 2 O 2 sensitivity corresponds with that observed for the powder sample ( Figure 6). We confirmed here that the PL intensity did not change when the films were immersed in water. The decreased PL intensity of the H 2 O 2treated films is then recovered by heating in air at 200°C (F-H and A-H).
The H 2 O 2 sensitivity, as observed in the decreasing PL intensity, of the micro-/nano-structured YVO 4 :Eu 3+ particles and films may have a different mechanism than that observed in our previous redox-responsive phosphor materials. 7−11 If the PL quenching is because of the surface redox reaction between H 2 O 2 and YVO 4 :Eu 3+ to promote the V 5+ → V 4+ reduction, the PL intensity can possibly be recovered by the reverse redox reaction using oxidants. However, the PL intensity was not recovered by the typical oxidizing treatment of film F-I by immersing in a 100 ppm KMnO 4 aqueous solution for 10 min, as shown in Figure S6. This implies that the H 2 O 2 -treated film F-I had not been reduced and therefore the PL recovery would need different treatments.
The other plausible cause for the PL quenching is the photocatalytic activity of YVO 4 :Eu 3+ for decomposing H 2 O 2 . It was reported that the YVO 4 :Eu 3+ nanoparticles could decompose methyl orange under UV irradiation. 36 The photoexcited electrons and holes are consumed to decompose the H 2 O 2 molecules that are adsorbed on the surface of the YVO 4 :Eu 3+ particles. The energy transfer to the Eu 3+ ions is then disturbed and their emission intensity is diminished. We found this mechanism possible because the quenched PL in film F-I could be recovered by continuous irradiation with 300 nm UV light, as shown in Figure 10. An almost linear recovery of the PL intensity with the irradiation time corresponds to the progress of the decomposition of H 2 O 2 and the increase in the energy transfer to the Eu 3+ ions. Without the adsorbed H 2 O 2 , YVO 4 :Eu 3+ adversely shows a slight decrease in its PL intensity as seen for film F in Figure 10. The large surface areas as well as the highly porous structure of the particles would be beneficial for preserving the H 2 O 2 molecules inside them during the immersion treatment.
Reproducible and repeatable H 2 O 2 -sensing ability should be achieved to fabricate a reliable sensor using the micro-/nanostructured YVO 4 :Eu 3+ films. Figure 11a shows changes of the relative PL intensity, obtained by integrating emissions between 500 and 750 nm, of film A after repeating five cycles of the immersion in the 100 ppm aqueous H 2 O 2 solution and heat treatment at 200°C. Obviously, film A can show the quenching/recovery switching behavior without losing the initial PL intensity. This result also supports the high mechanical strength of film A which is durable against the above immersion/heating procedure. Actually the diffuse reflectance spectra of the initial film and the final film after

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Article the five-cycle treatments are almost the same (Figure 11b); the particles constructing the film were not detached. In contrast, film F had a lower mechanical strength and was peeled off after the second heat treatment.
Finally, a quantitative H 2 O 2 -sensing ability was evaluated. Four kinds of film A-I were prepared by using aqueous H 2 O 2 solutions of 25, 50, 75, and 100 ppm. The relative PL intensity (integrating emissions between 500 and 750 nm) of the films was normalized with that of film A taken as unity. Figure 12 shows a plot of the relative PL intensity against the H 2 O 2 concentration. The intensity decreases linearly with the increasing H 2 O 2 concentration, thereby indicating the excellent quantitativity in H 2 O 2 sensing. The linearity also supports the PL quenching mechanism, which basically depends on the amount of adsorbed H 2 O 2 molecules, as discussed above.

CONCLUSIONS
The micro-/nano-structured YVO 4 :Eu 3+ powders and films were synthesized and their H 2 O 2 sensing ability was evaluated. The PL emission intensity of the Eu 3+ ions was decreased when the YVO 4 :Eu 3+ samples were immersed in a 100 ppm aqueous H 2 O 2 solution for 10 min at room temperature. The intensity could then be recovered by heating the samples at 200°C for 10 min in air. The film sample showed repeatable PL quenching and recovery at least up to five cycles, and also quantitative PL quenching against the H 2 O 2 concentration. The mechanism underlying the PL quenching was suggested to be the consumption of photogenerated electron−hole pairs by the decomposition of the H 2 O 2 molecules adsorbed on the YVO 4  were dissolved together in 45.0 mL of deionized water. The above Eu 3+ dopant level (5 mol %) had been optimized for yielding the strongest Eu 3+ emissions in a preliminary experiment by changing it between 1 and 10 mol %. The pH value of the resultant aqueous solution was adjusted to 4.50 by using HCl (35.0−37.0%, Wako). A 7.0 mL aliquot of the aqueous solution was placed in a glass centrifuge tube, into which 7.0 mL of the organic solution was poured gently. The biphasic organic−aqueous system was thus achieved and maintained at a constant temperature of 80°C for 24 h. The precipitate (BP) formed after 24 h was collected by suction filtration, washed with deionized water, and dried at 60°C for a few hours. The dried powder, tinged with yellow, was further washed with a 1.0 M aqueous NaOH solution under stirring for 5 h as the base treatment. The color of the powder changed to white during the washing. Finally, the white powder (BP-BT) was collected again by suction filtration, washed with deionized water, and dried at 60°C for a few hours.
For the comparative study, another powder was synthesized in the simple single-phase liquid system. The aqueous solution employed for this synthesis was the same as that used in the above biphasic system. A 7.0 mL aliquot of the aqueous solution was placed in a glass centrifuge tube. Then, 0.7 mL of the ethanolic VO(OC 2 H 5 ) 3 solution, which had been used to prepare the organic solution, was poured directly into the aqueous solution. The resultant solution was kept at 80°C for 24 h. The precipitate (SP) was collected by suction filtration, washed with deionized water, and dried at 60°C for a few hours. The base treatment was also carried out in the same way as described above to obtain the SP-BT powder.
4.2. Solid-State Synthesis of Powder. As the reference sample, the stoichiometric YVO 4 :Eu 3+ powder was synthesized by the conventional solid-state reaction method. Commercial powders of Y 2 O 3 (99.99%, Wako), Eu 2 O 3 (99.9%, Wako), and V 2 O 5 (99.0%, Wako) were mixed in a molar ratio of Y/Eu/V = 0.95:0.05:1 using a pestle and mortar with the addition of acetone. The resultant mixture was dried at room temperature and then heated at 1200°C for 6 h in air to obtain the YVO 4 :Eu 3+ powder (SS).
4.3. Liquid-Phase Synthesis of Films. The substrates employed in the present study were silica glass plates, 12.6 mm × 12.6 mm in dimensions and 1 mm in thickness. The solutions used for film deposition were almost the same as those used for powder synthesis in the biphasic system. Additionally, 3.00 mmol of CH 3 COONa (98.5%, Wako) was also selected as the additive, instead of 4.50 mmol of HCOONa, in attempting to modify the nucleation process during the film deposition. The substrate was placed horizontally at the bottom of the glass centrifuge tube so that the film can be deposited on its upper side. A 7.0 mL aliquot of the aqueous solution containing HCOONa or CH 3 COONa was added to the tube so that the substrate could be immersed completely. A 7.0 mL aliquot of the organic VO(OC 2 H 5 ) 3 solution was then poured into the tube to prepare the biphasic system. The system was kept at 50°C for 24 h to promote the film deposition. Subsequently, the substrate was removed from the tube, followed by washing with deionized water and drying at room temperature. The film thus obtained on the substrate was annealed at 400°C for 3 h in air. The annealed film underwent the base treatment for 1 h without stirring, was washed with deionized water, and was finally dried at room temperature.

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Article pH value of the H 2 O 2 solution was measured to be 5.8−6.1, which was almost the same as that of deionized water. The powder samples (30 mg of BP-BT, SP-BT, or SS) were added to the resultant H 2 O 2 solution and kept at room temperature for 10 min under stirring. The powders were collected by suction filtration, washed with deionized water, and dried at 60°C for a few hours. The turn-off luminescence of the powders was examined by measuring the emission spectra before and after the H 2 O 2 treatment.
Separately, the relationship between the turn-off luminescence and the amount of the powder was investigated for the samples obtained from the biphasic system. The BP-BT powder samples, with their amounts varied between 3.4 and 47.3 mg, were added to 10 mL of the 100 ppm H 2 O 2 solution and kept at room temperature for a prolonged time (1 h) under stirring. The turn-off luminescence of the collected powders was then examined similarly.
The film samples were immersed in the 100 ppm H 2 O 2 solution for 10 min at room temperature. The films were removed, washed with deionized water, and dried at room temperature. Additionally, the films were annealed at 200°C for 10 min in air in order to desorb or decompose the H 2 O 2 molecules. The PL of the films was examined similarly before and after the H 2 O 2 treatment and also after annealing.
For the quantitative evaluation of the H 2 O 2 -sensing ability, four kinds of film A-I were prepared by using aqueous H 2 O 2 solutions of 25, 50, 75, and 100 ppm and their PL intensity was measured. The experiment was repeated four times for the respective film samples to deduce the standard deviation.
4.5. Characterization. The crystalline phases of the samples were identified by XRD analysis (D8 ADVANCE diffractometer, Bruker AXS) using Cu Kα radiation. The crystallite size was calculated from the full width at half maximum of the XRD peak using Scherrer's equation with a K value of 0.9. 37 The microstructure of the samples was observed by FETEM (Tecnai F20 microscope, FEI). The specimen for the FETEM observation was prepared by dispersing the powders in ethanol and drying on a carbon coated copper grid. The morphology of the samples was observed by FESEM (Inspect F50, FEI, or S-4700 microscope, Hitachi). Conductive coating was applied to the samples with an osmium plasma coater. The chemical composition of the powder samples was examined by XRF spectroscopy (XGT-2700 analytical microscope, Horiba). The stoichiometric Y 0.95 Eu 0.05 VO 4 powder synthesized by the solid-state reaction method was used as the reference. The pH value of the aqueous solution was measured at room temperature with a pH meter (model F-51, Horiba) and a pH electrode (model 9615S-10D, Horiba). The PL spectra and the photostability of the samples were measured at room temperature with a spectrofluorophotometer (model FP-6500, JASCO) using a xenon lamp (150 W) as the light source. The powder samples were mounted on a silica glass plate having a square well 5 mm × 5 mm in dimensions to regulate their amount in the PL measurement. The film samples were measured when they were mounted directly on the attachment. The diffuse reflectance spectra of the film samples were measured with a UV−visible spectrophotometer (model V-670, JASCO) in the wavelength range between 200 and 800 nm. An integrating sphere (model ISN-723, JASCO) was used together with a Spectralon diffuse reflectance material. The BET surface area and the pore size distribution of the powder samples were evaluated by using nitrogen adsorption−desorption apparatus (model BELSORP-mini II, MicrotracBEL). The presence of organic species in the samples was examined by FT-IR (ALPHA spectrometer, Bruker Optics).

* S Supporting Information
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.9b02915.
The TEM analysis of the annealed powder sample ( Figure S1), the photostability of the powder samples ( Figure S2), the dependence of the H 2 O 2 sensitivity on the amount of the powder samples ( Figure S3), the optical properties of the films (Figure S4), the films on the substrate which was made to stand against the wall ( Figure S5), and the oxidizing treatment with KMnO 4 ( Figure S6)