materials Article
The Synthesis of a Core-Shell Photocatalyst Material YF3:Ho3+@TiO2 and Investigation of Its Photocatalytic Properties Xuan Xu 1,2 , Shiyu Zhou 1,2 , Jun Long 1,2 , Tianhu Wu 1,2 and Zihong Fan 3, * 1
2 3
*
Key Laboratory of Three Gorges Reservoir Region’s Eco-Environment, Ministry of Education, Chongqing University, Chongqing 400045, China;
[email protected] (X.X.);
[email protected] (S.Z.);
[email protected] (J.L.);
[email protected] (T.W.) National Centre for International Research of Low-carbon and Green Buildings, Chongqing University, Chongqing 400045, China College of Environmental and Resources, Chongqing Technology and Business University, Chongqing 400067, China Correspondence:
[email protected]; Tel.: +86-139-9622-1283
Academic Editor: Dirk Poelman Received: 30 December 2016; Accepted: 21 February 2017; Published: 16 March 2017
Abstract: In this paper, YF3 :Ho3+ @TiO2 core-shell nanomaterials were prepared by hydrolysis of tetra-n-butyl titanate (TBOT) using polyvinylpyrrolidone K-30 (PVP) as the coupling agent. Characterization methods including X-ray diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS) under TEM, X-ray photoelectron spectroscopy (XPS), fluorescence spectrometry, ultraviolet-visible diffuse reflectance spectroscopy, and electron spin resonance (ESR) were used to characterize the properties and working mechanism of the prepared photocatalyst material. They indicated that the core phase YF3 nanoparticles were successfully coated with a TiO2 shell and the length of the composite was roughly 100 nm. The Ho3+ single-doped YF3 :Ho3+ @TiO2 displayed strong visible absorption peaks with wavelengths of 450, 537, and 644 nm, respectively. By selecting these three peaks as excitation wavelengths, we could observe 288 nm (5 D4 →5 I8 ) ultraviolet emission, which confirmed that there was indeed an energy transfer from YF3 :Ho3+ to anatase TiO2 . In addition, this paper investigated the influences of different TBOT dosages on photocatalysis performance of the as-prepared photocatalyst material. Results showed that the YF3 :Ho3+ @TiO2 core-shell nanomaterial was an advanced visible-light-driven catalyst, which decomposed approximately 67% of rhodamine b (RhB) and 34.6% of phenol after 10 h of photocatalysis reaction. Compared with the blank experiment, the photocatalysis efficiency was significantly improved. Finally, the visible-light-responsive photocatalytic mechanism of YF3 :Ho3+ @TiO2 core-shell materials and the influencing factors of photocatalytic degradation were investigated to study the apparent kinetics, which provides a theoretical basis for improving the structural design and functions of this new type of catalytic material. Keywords: upconversion; visible light photocatalysis; core-shell structure; Ho3+ -single-doped
1. Introduction With the development of industry, organic matter such as drugs, pesticides, surfactants, and raw chemical materials cause an increasing amount of pollutants in surface water, groundwater, sewage, and drinking water. It is even worse that most of these contaminants are complex and non-biodegradable, and therefore traditional water treatment methods cannot completely remove them. In recent years, photocatalysis has gained increasing attention due to the discovery of water splitting on a semiconductor electrode [1–4]. Over the past 20 years, photocatalysis has become a Materials 2017, 10, 302; doi:10.3390/ma10030302
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research topic of interest because of its practical applications in air and water remediation, self-cleaning, and self-sterilizing surfaces [5,6]. TiO2 , a well-known semiconductor, has been intensively investigated during this time. Some studies showed that TiO2 represents one of the most promising materials for Surface Enhanced Raman Scattering (SERS), because of its high refractive index, versatile surface functionalization, synergistic coupling to plasmonic nanoparticles, cocatalysts, and so on. Thus it could be used to analyze and monitor the process of photocatalytic degradation [7–9]. Meanwhile, TiO2 with its extraordinary chemical stability, environmentally friendly, and biocompatible characteristics, has been intensively investigated as a benchmark material for many photocatalytic reactions [10–14]. However, the application range of TiO2 is still limited due to its wide band gap (3.2 eV) which requires ultraviolet irradiation for photocatalysts reaction [10,15,16]. As we all know, ultraviolet (UV) light only accounts for 5% of the total solar energy, while visible light and near-infrared (NIR) account for 45% and 48%, respectively. Many approaches have been adopted to extend the absorption range of TiO2 towards the visible light region. Some researchers have made attempts to shift the band gap of TiO2 towards visible light by doping metals [17], rare earth metal [18], or non-metals [19–21], and by cationic substitutions [22]. Also, it have been reported that TiO2 coupling with nanoantennas could stimulate the photon-driven process and enhance the photodegradation rate because of the nanoantenna Surface Plasmon Resonance [23]. However, these methods may introduce defects such as increased recombination of photogenerated electrons and holes, and result in decreased stability, service life, and photocatalyst efficiency of TiO2 , which is an even more serious problem [24,25]. Recently, rare-earth-doped upconversion (UC) nanophosphors have attracted a number of interests for their capability of extending the absorption range of TiO2 . UC luminescence can switch the long-wave radiation into short-wave radiation through multiphoton mechanisms, so that low-energy light can be changed into high-energy light. UC material could convert visible light into UV light. Rare-earth doped fluorides with low photon energies and high quantum efficiency can be used as luminescent upconversion materials [26]. As a host material, fluorides such as NaYF4 [27–29], YF3 [30,31], and LiYF4 [32] have become the hotspot in the research of upconversion luminescence transformation. Fluoride has relatively lower phonon energy and high UC efficiency, which can reduce the loss of non-radiation [33,34]. Moreover, fluoride has advantages such as high chemical stabilization, high mechanical strength, and a simple preparation process. Studies have reported that the YF3 doped with rare-earth ions can emit UV light under visible light excitation [31,35]. With a favorable energy level structure and abundant transitions from UV to NIR region at various wavelengths, the rare-earth ion Ho3+ is one of the most important active ions in upconversion luminescence (UCL) applications [36,37]. Therefore in this paper, YF3 :Ho3+ was selected as an intermediate matrix to absorb visible light and emit UV light which was then transferred to TiO2, so that high photocatalytic reaction efficiency was realized. The core-shell structure as a coupling model has the potential to increase luminous efficiency. A series of studies have shown that such methods can be used to improve the use ratio of solar energy. For example, NaYF4 :Yb3+ , Tm3+ @TiO2 core-shell nanoparticles have been reported to emit UV and visible light under 980 nm excitation and perform with higher efficiencies [38,39]. It is reported that NaYF4 is better than YF3 as an upconversion nanocrystal host matrix, but it remains difficult to obtain these kinds of hierarchical nanostructures and achieve a uniform TiO2 coating. There were already some studies that have proven that YF3 could be coated with TiO2 successfully and stably [16,40,41]. This paper aims to prepare a YF3 :Ho3+ @TiO2 core/shell structure which is conducive for a TiO2 shell to absorb the UV light from UCL [42]. It has been previously reported that the YF3 :Ho3+ nanoparticles can be synthesized by a facile hydrothermal method. Such YF3 :Ho3+ nanoparticles exhibit good upconversion properties, which are conducive to emitting upconversion fluorescence around 288 nm under excitation at 450 nm [43]. In this work, to realize the good UC properties of YF3 :Ho3+ , a highly efficient photocatalyst was prepared by coating YF3 :Ho3+ nanoparticles with TiO2 . Therefore, the TiO2 could use visible light to improve
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the degradation efficiency of the catalyst. The YF3 :Ho3+ @TiO2 photocatalyst was prepared in this work by the hydrolysis of tetrabutyltitanate (TBOT) using PVP as the coupling agent. The influences of different dosages of TBOT on the materials’ morphology, size, and photocatalysis efficiency were investigated. In addition, the photocatalysis mechanism of YF3 :Ho3+ @TiO2 and the apparent kinetics of RhB degradation are discussed in details. 2. Experimental and Methods 2.1. Materials All chemicals were used as received without further purification. Y2 O3 (99.999%, Chengdu Kelong Chemical Co., Ltd., Chengdu, China), Ho2 O3 (>99.9%, Shanghai TongNai Environmental Protection Co., Ltd., Shanghai, China), NaF (Chengdu Kelong Chemical Co., Ltd., Chengdu, China), ethanol (Chongqing Chuandong Chemical Group Co., Ltd., Chongqing, China), ethylenediamine tetraacetic acid (EDTA) (Chongqing Boyi Chemical Co., Ltd., Chongqing, China), acetic acid (CH3 COOH) (Chongqing Chuandong Chemical Group Co., Ltd., Chongqing, China), and tetrabutyltitanate (TBOT) (Chengdu Kelong Chemical Co., Ltd., Chengdu, China) were of analytical grade. 2.2. Preparation of YF3 :Ho3+ @TiO2 Photocatalyst In this paper, YF3 nanoparticles were first prepared based on Jun’s study [42,43], and then the YF3 :Ho3+ @TiO2 photocatalyst was prepared by hydrolysis of TBOT using PVP as the coupling agent based on Qin’s work. In preparation processes, TBOT (6.0 mL) was first dissolved in ethanol (30.0 mL) and CH3 COOH (2.0 mL), and then the solution was vigorously stirred for 30 min to form precursor A; YF3 :Ho3+ nanoparticles (0.02 g) were dispersed in ethanol (20.0 mL) and H2 O (4.0 mL) to form precursor B. After that, precursor B was added dropwise into precursor A at a rate of 1 mL/min while stirring for 1 h. After standing for 24 h, the resulting nanoparticles were dried at 105 ◦ C, and then calcined by a heating rate of 2 ◦ C/min to 400 ◦ C for 2 h. The experimental parameters of the TBOT dosage and the hydrolysis reaction time are shown in Table 1 below. Table 1. Synthesis condition of all samples. TBOT: tetrabutyltitanate. Number
Dosage of TBOT/mL
Hydrolysis Reaction Time/min
A B C D E F G
0.1 0.5 1 2 4 6 8
60 60 60 60 60 60 60
2.3. Photocatalytic Activity Measurements In this research, RhB and phenol were used to test the photocatalytic activity of YF3 :Ho3+ @TiO2 . The photocatalytic activity of YF3 :Ho3+ @TiO2 was evaluated via degradation of RhB and phenol under the irradiation of a 500 W Long arc xenon lamp with a UV cutoff filter (λ > 420 nm) under laboratory conditions using a Hitachi U-3010 UV-Vis spectrophotometer (Hitachi Corp., Tokyo, Japan). For specific test procedures, first add 0.15 g of photocatalyst material in 500 mL of 5 mg/L solution of rhodamine B (RhB) and 500 mL of 5 mg/L solution of phenol respectively for a dark-reaction for half an hour, so as to achieve adsorption—desorption equilibrium between the pollutants and photocatalyst. Then, place the reaction system 30 cm away from the light source to be irradiated for 10 h. Take out 8 mL of the samples once every 2 h. Finally, perform centrifugation treatment for the samples and then test the absorbance of RhB at 552 nm using UV-Vis. Test the absorbance of phenol at 510 nm using 4-amino antipyrine as the chromogenic reagent under UV-Vis.
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2.4. Characterization The crystal structures of all prepared samples were characterized by X-ray diffraction (XRD) using a Rigaku D/Max-2500pc diffractometer (JEOL Ltd., Tokyo, Japan) with Cu Kα radiation, where ◦ ◦ the 2θ scanning angle Materials 2017, 10, 302 ranged from 10 to 80 . The surface chemical environments were 4 ofanalyzed 14 by X-ray photoelectron spectra (XPS) on a PHI5000 Versa Probe system (JEOL Ltd., Tokyo, Japan) 2.4. Characterization with monochromatic Al Kα X-rays. Scanning electron microscopy (SEM) images were acquired with The crystal structures of all prepared samples were characterized by X-ray diffraction (XRD)Energy a JSM-7800F JEOL emission scanning electron microscope (Zeiss, Oberkochen, Germany). using a Rigaku D/Max-2500pc diffractometer (JEOL Ltd., Tokyo, Japan) with Cu Kα radiation, where dispersive X-ray (EDS) images were acquired with an EDX-100A-4 (Zeiss, Oberkochen, Germany). the 2θ scanning angle ranged from 10° to 80°. The surface chemical environments were analyzed by Transmission electron (TEM) microscopy was carried out on a FEI Tecnai G20 (JEOL Ltd., Tokyo, X-ray photoelectron spectra (XPS) on a PHI5000 Versa Probe system (JEOL Ltd., Tokyo, Japan) with Japan) operated at an acceleration voltage of 200 kV. UV-Vis diffuse-reflectance spectroscopy (UV-Vis monochromatic Al Kα X-rays. Scanning electron microscopy (SEM) images were acquired with a DRS) and UV-Vis absorption were conducted with a Hitachi U-3010 UV-Vis Energy spectrometer JSM-7800F JEOL emissionspectra scanning electron microscope (Zeiss, Oberkochen, Germany). (Hitachidispersive Ltd., Tokyo, Japan). The sample for electron resonance(Zeiss, (ESR)Oberkochen, measurement was prepared X-ray (EDS) images were acquired with spin an EDX-100A-4 Germany). Transmission (TEM) microscopy out on asolution FEI Tecnai G20(aqueous (JEOL Ltd.,dispersion Tokyo, by mixing β-NaYF4electron :Ho3+ @TiO in was a 50carried mM DMPO tank for 2 samples operated at an acceleration voltage of 200 kV. diffuse-reflectance spectroscopy (UV-Visspectra − ). Upconversion DMPO-Japan) OH and methanol dispersion for DMPO·OUV-Vis photoluminescence 2 DRS) and UV-Vis absorption spectra were conducted with a Hitachi U-3010 UV-Vis spectrometer were recorded using a Horiba Jobin Yvon fluorescence spectrophotometer (Fluorolog-3; excitation (Hitachi Ltd., Tokyo, Japan). The sample for electron spin resonance (ESR) measurement was source power, 0–450 W, Horiba Ltd., Tokyo, Japan). prepared by mixing β-NaYFScientific 4:Ho3+@TiO2 samples in a 50 mM DMPO solution tank (aqueous dispersion for DMPO- OH and methanol dispersion for DMPO-·O2−). Upconversion photoluminescence spectra were
3. Results and Discussion recorded using a Horiba Jobin Yvon fluorescence spectrophotometer (Fluorolog-3; excitation source power, 0–450 W, Horiba Scientific Ltd., Tokyo, Japan).
3.1. X-ray Diffraction (XRD) Pattern Analysis 3. Results and Discussion
The phase structures of the materials were characterized by XRD measurements. The XRD 3+ @TiO with different TBOT dosages are shown in Figure 1. patterns3.1. of X-ray pure Diffraction TiO2 , YF3(XRD) , andPattern YF3 :Ho 2 Analysis 3+ All the diffractions of the YF :Ho @TiO could be assignedbytoXRD themeasurements. anatase TiO2The (JCPDS No. 21-1272). 2 were characterized The phase structures3 of the materials XRD patterns As we all among all the crystal types of TiO2 , the anatase TiO theinhighest 2 has of know, pure TiO 2, YF3, and YF3:Ho3+@TiO2 with different TBOT dosages are shown Figure 1.photocatalysis All the efficiency. Hence, the prepared materials excellent photocatalytic capacity. diffractions of the YF3:Ho3+@TiO 2 couldhave be assigned to the anatase TiO2 (JCPDS No. 21-1272). As we all know, the among allshows the crystal typesYF of 3TiO , the anatase TiO2with has 0.1 the mL highest photocatalysis In addition, XRD that only :Ho23+ @TiO TBOT coincides weakly 2 doped efficiency. Hence, the prepared materials have excellent photocatalytic capacity. with the YF3 standard (JCPDS No. 74-0911), while the others only show the diffraction peak of anatase In addition, the XRD shows that only YF3:Ho3+@TiO2 doped with 0.1 mL TBOT coincides weakly TiO2 . This is mainly because when the dosage of TBOT was too high, the content of TiO2 in the material with the YF3 standard (JCPDS No. 74-0911), while the others only show the diffraction peak of anatase 3+ would be relatively high, making the YF3 :Ho lower than its detection may also be TiO 2. This is mainly because when the dosagecontent of TBOT was too high, the contentlimit. of TiOIt 2 in the 3+ was covered by TiO 3+ due to the fact that when YF :Ho , the strong diffraction peak of TiO 3 2 2 would material would be relatively high, making the YF3:Ho content lower than its detection limit. It may 3+ was hide thealso diffraction ofthat YF3when :Ho3+YF , so that thecovered diffraction YF3diffraction :Ho3+ would only be due to peak the fact 3:Ho by TiOpeak 2, the of strong peakshow of TiOup 2 3+, so that the diffraction peak of YF3:Ho3+ would show up would hide the diffraction peak of YF 3 :Ho when the dosage of TBOT was reduced to a certain degree. only when theresults, dosage of TBOT a certain degree. From the XRD we can was see reduced that thetophotocatalyst material YF3 :Ho3+ @TiO2 is synthesized From the XRD results, we can see that the photocatalyst material YF3:Ho3+@TiO2 is synthesized by the hydrolysis of TBOT. Changing the dosage of the TBOT cannot change the phase and crystal by the hydrolysis of TBOT. Changing the dosage of the TBOT cannot change the phase and crystal of of the UCL material, butbut cancan only affect proportionbetween between anatase and YF :Ho3+ , the UCL material, only affectthe therelative relative proportion anatase TiO2TiO and2 YF 3:Ho3+, 3 thereby thereby affecting the degradation efficiency. affecting the degradation efficiency.
Figure 1. X-ray diffraction (XRD) patterns of the photocatalyst YF3:Ho3+@TiO2 with different
Figure 1. X-ray diffraction (XRD) patterns of the photocatalyst YF3 :Ho3+ @TiO2 with different tetrabutyltitanate (TBOT) dosages. tetrabutyltitanate (TBOT) dosages.
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3.2. Morphology and Composition Analysis by Transmission Electron Microscopy (TEM)
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3.2. Morphology and Composition Analysis by Transmission Electron Microscopy (TEM) 3.2.1. TEM 3.2.1. TheTEM TEM images of YF3 :Ho3+ @TiO2 with different dosages of TBOT are shown in Figure 2a–g, from which we can see that nanoparticles with particle sizes of about 10 nm in stick to the UCL The TEM images of YFTiO 3:Ho @TiO2 with different dosages of TBOT are shown Figure 2a–g, 2 3+ 3+ @TiO is overall homogeneous, but in an agglomerated state. The amount of TiO material. YF :Ho from which 3 we can see2 that TiO2 nanoparticles with particle sizes of about 10 nm stick to the UCL 2 3+ increases with the increase of the TBOT dosage. When the dosage of TBOT is 3+@TiO doped on the material. YFYF 3:Ho 2 is overall homogeneous, but in an agglomerated state. The amount of TiO2 3 :Ho 3+ , making 3+ increases doped the YF 3 :Ho of the TBOT of TBOT small, TiOon cannot evenly coat thewith YF3the :Hoincrease a part ofdosage. the YF3When :Ho3+ the stilldosage exposed. Withisthe 2 3+ 3+, making a part of the YF3:Ho3+ still exposed. With the small, TiO 2 cannot evenly coat the YF 3 :Ho increase of the TBOT dosage, YF3 :Ho is coated by the TiO2 particles gradually. increase of the TBOT dosage, YF3:Ho3+ is coated by the TiO2 particles gradually. 3.2.2. Energy Dispersive X-ray (EDS) Line Scan under High Resolution TEM (HRTEM) 3.2.2. Energy Dispersive X-ray (EDS) Line Scan under High Resolution TEM (HRTEM) Figure 2h shows the high resolution image of TiO2 shell coated on the surface of the YF3 . Figure 2j Figure shows the high resolution image of TiO2 shell coated on theline surface of the YF3.inFigure 2j 2i. shows the EDS2hline scanning profiles which are recorded along the white as presented Figure shows the EDS line scanning profiles which are recorded along the white line as presented in Figure 2i. The EDX elementary line scanning was used to further determine the composition of the composite, elementary linesynthesized scanning wasmaterials used to further determine the composition the As composite, andThe to EDX prove whether the were of a core-shell structure orofnot. shown in and to prove whether the synthesized materials were of a core-shell structure or not. As shown in 3+ Figure 2j, there are signal detections of both YF3 :Ho and TiO2 at point A where the scanning starts. Figure 2j, there are signal detections of both YF 3:Ho3+ and TiO2 at point A where the scanning starts. As the scanning goes outside and comes close to point B, the signal of Ti drops while Y increases. As the scanning goes outside and comes close to point B, the signal of Ti drops while Y increases. At point C, Ti gradually reduces to the minimum level while Y grows to the maximum level. When the At point C, Ti gradually reduces to the minimum level while Y grows to the maximum level. When scanning reaches the other end of YF3 :Ho3+ 3+ at point D, Y begins to decrease while Ti gradually the scanning reaches the other end of YF3:Ho at point D, Y begins to decrease while Ti gradually increases. This is obvious evidence to prove that the synthesized materials have a core-shell structure increases. This is obvious evidence to prove that the synthesized materials have a core-shell structure and the TiO2 is strongly coupled on YF3 :Ho3+ . and the TiO2 is strongly coupled on YF3:Ho3+.
Figure 2. Transmission electron microscopy (TEM) images (a–g) of the YF3:Ho3+@TiO2 with different Figure 2. Transmission electron microscopy (TEM) images (a–g) of the YF3 :Ho3+ @TiO2 with different TBOT dosages at different magnifications; (h) is the corresponding high-resolution TEM image. TBOT dosages at different magnifications; (h) and is the corresponding TEM image. Energy dispersive X-ray (EDS) line scan profiles a TEM image of thehigh-resolution YF3:Ho3+@TiO2 composite is 3+ @TiO composite is Energy dispersive X-ray (EDS) line scan profiles and a TEM image of the YF :Ho 3 2 shown in i and j. Points A–D in (j) correspond to the same point shown in (i). shown in i and j. Points A–D in (j) correspond to the same point shown in (i).
3.3. Chemical States Investigation by X-ray Photoelectron Spectroscopy (XPS) 3.3. Chemical States Investigation by X-ray Photoelectron Spectroscopy (XPS) X-ray photoelectron spectroscopy (XPS) was used to examine the chemical states of the elements 3:Ho3+@TiO2 core-shell Figure 3athe shows the full survey spectrum onX-ray the surface of the YFspectroscopy photoelectron (XPS) wasmaterials. used to examine chemical states of the elements reveals of Ti,2 core-shell O, Y, F, and Ho. In Figure 3b,3a theshows binding of 462.98 and on which the surface ofthe theco-presence YF3 :Ho3+ @TiO materials. Figure theenergy full survey spectrum 457.28 eV, which are respectively labeled as Ti 2p 1/2 and Ti 2p 3/2 , are consistent with the typical values which reveals the co-presence of Ti, O, Y, F, and Ho. In Figure 3b, the binding energy of 462.98 and reported for TiO2 [44]. According to the asymmetric profile of O 1s shown in Figure 3c, it can be seen
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457.28 eV, which are respectively labeled as Ti 2p1/2 and Ti 2p3/2 , are consistent with the typical values Materialsfor 2017, 10, 302 6 of 14 reported TiO 2 [44]. According to the asymmetric profile of O 1s shown in Figure 3c, it can be seen that more than one kind of oxygen species exists. It was reported that when the binding energy was that more than one kind of oxygen species exists. It was reported that when the binding energy was around 528.58 eV, the peak corresponded to the characteristic peak of Ti–O–Ti; while if the binding around 528.58 eV, the peak corresponded to the characteristic peak of Ti–O–Ti; while if the binding energy was around 530.08 eV, the peak was attributed to H–O. According to Figure 3d, the three peaks energy was around 530.08 eV, the peak was attributed to H–O. According to Figure 3d, the three peaks at 156.38, 158.88, and 161.08 eV all corresponded to Y 3d. Element F shows two peaks at 682.78 and at 156.38, 158.88, and 161.08 eV all corresponded to Y 3d. Element F shows two peaks at 682.78 and 685.28 eV,eV, respectively, which correspond In addition additionto tothe themain mainelements elements 685.28 respectively, which correspondtotoFF1s1s(see (seeFigure Figure 3e). 3e). In inin thethe YF3YF nanoparticles, the doping elements in these nanoparticles were also detected. The peaks at 3 nanoparticles, the doping elements in these nanoparticles were also detected. The peaks at 3+ ions. XPS results show that rare 156.28, 158.78, and 160.88 eVeV (see Figure 156.28, 158.78, and 160.88 (see Figure3f) 3f)are areattributed attributedto tothe the Ho Ho3+ ions. XPS results show that rare earth ions have been successfully host matrix. matrix. earth ions have been successfullyincorporated incorporatedinto intothe the YF YF33 host
Figure 3. High-resolution X-ray photoelectron spectroscopy (XPS) analysis of the YF3:Ho3+@TiO2: Figure 3. High-resolution X-ray photoelectron spectroscopy (XPS) analysis of the YF3 :Ho3+ @TiO2 : (a) Wide spectrum; (b) Ti 2p; (c) O 1s; (d) Y 3d; (e) F 1s; (f) Ho 4d. (a) Wide spectrum; (b) Ti 2p; (c) O 1s; (d) Y 3d; (e) F 1s; (f) Ho 4d.
3.4. Optical Spectra Investigation 3.4. Optical Spectra Investigation 3.4.1. UV-Vis Diffuse Reflection Spectroscopy 3.4.1. UV-Vis Diffuse Reflection Spectroscopy To analyze the optimal absorption wavelength of the synthesized YF3:Ho3+@TiO2 material, the 3+ @TiO material, the To analyze the optimalspectroscopy absorption wavelength of the Figure synthesized YFthe 3 :Ho 2 UV-Vis diffuse reflection was investigated. 3 shows representative spectra 3+ 3+ 3+ UV-Vis diffuse reflection spectroscopy was investigated. Figure 3 shows the representative of YF3:Ho @TiO2 and YF3:Ho . From the spectrum of YF3:Ho @TiO2, we can observe a spectra light 3+ 3+ . From the spectrum of YF :Ho3+ @TiO we can observe a light of YF and YF absorption edge2 before 400 nm, which is overlapped with that of it confirms that the 3 :Ho @TiO 3 :Ho 3 TiO2. Moreover, 2, absorption edge 2before 400can nm, which overlapped with that of TiO2 . Moreover, it which confirms that the YF3:Ho3+@TiO material absorb theislight with wavelengths between 300–700 nm, is shown 3+ @TiO 3+absorb YF3in :Ho material can the light with wavelengths between 300 and 700 nm, which the spectrum of YF 3 :Ho . There are three absorption peaks in the visible light region (450 nm, is 2 3+ 538 nm, nm), where intensity ofare thethree 450 nm peak is peaks relatively higher. we region can see(450 from shown in the644 spectrum of YFthe . There absorption in the visibleAslight nm, 3 :Ho 4, for onewhere wavelength, the stronger absorption light absorption ability more 538Figure nm, 644 nm), the intensity of thethe 450 nm peakpeak’s is relatively higher. As we is, canthe see from suitable is for Hence, 450 nm peak’s was selected as the excitation Figure 4, forthat onewavelength wavelength, theexcitation. stronger the absorption light absorption abilitywavelength is, the more 3+@TiO2, which is consistent with the goal of utilizing visible light as the excitation source of YF 3 :Ho suitable that wavelength is for excitation. Hence, 450 nm was selected as the excitation wavelength of for UCL materials.
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3+ Materials 2017, 10, 302 7 of 14 YF 3 :Ho @TiO2 , which is consistent with the goal of utilizing visible light as the excitation source for UCL materials. Materials 2017, 10, 302 7 of 14
Figure 4. UV-Vis absorption spectra of upconversion nanoparticles (the inset shows the enlarged
Figure 4. 4. UV-Vis UV-Visabsorption absorption spectra of upconversion nanoparticles the enlarged spectra of upconversion nanoparticles (the (the inset inset showsshows the enlarged figure figure of UV-Vis absorption spectra from 400 to 800 nm). figure of UV-Vis absorption spectra from 400 to 800 nm). of UV-Vis absorption spectra from 400 to 800 nm).
3.4.2. Fluorescence Spectrum Analysis
3.4.2. Fluorescence Spectrum Analysis
Figure 5 shows the fluorescence emission spectra of YF3:Ho3+@TiO2 under the visible light 3+@TiO excitation at 450 the nm, from which we can seespectra that all of theYF prepared samples sharethe thevisible similarlight shows the fluorescence fluorescence emission spectra of YF :Ho3+ 22 under Figure 5 shows emission under 3 3:Ho 3+ upconversion properties YF3see :Ho that . There a strong emission peak at share 288 nm,the which at 450 luminescence we as can excitation nm, from which allisthe prepared samples similar 3+ ion from 5D4→5I8. When YF3:Ho3+ is doped with TiO2, the resulted from the transition of the Ho 3+. 3+ upconversion luminescence properties as YF 3 :Ho There is a strong emission peak at 288 nm, which luminescence properties as YF3 :Ho . There is a strong emission peak at 288 nm, upconversion luminescence capacity becomes weaker. This may be due to the fact that the UV light 3+ 5 5 3+ 3+ 5 5 3+ resulted from from the transition of the ion ion fromfrom D4→DI48→ . When YF3:Ho is doped withwith TiO2TiO , the which resulted the transition of Ho the Ho I8 . When YF3 :Ho is doped 2, emitted by the UCL after absorbing visible light would be absorbed by TiO2, and moreover, with the upconversion luminescence capacity becomes weaker. This may be due to the fact that the UV light the upconversion luminescence capacity becomes weaker. This may be due to the fact that the UV increase of the TBOT dosage, the TiO2 doped on the YF3:Ho3+ would hinder the excitation light from UCL after absorbing visible light would be absorbed by TiO22, and moreover, with the emitted by the 3+ [45]. In addition, arriving at the YF3:Ho3+, resulting in reduced excitation light energy for YF3:Ho a 3+ would hinder the excitation light from 3+ TBOT dosage, the TiO 2 doped doped on the YF 3:Ho :Ho increase of the TBOT dosage, the TiO on the YF would hinder the 3 large number of TiO2 loaded on the2YF3 surface would also absorb most of the UV light and decrease 3+3+ arriving the YF33intensity :Ho InIn addition, a 3+ :Ho3+3+, ,resulting resulting in.reduced reducedexcitation excitationlight lightenergy energyfor forYF YF3:Ho [45]. addition, the at excitation of YF3:Hoin 3 :Ho [45].
alarge largenumber numberofofTiO TiO2 2loaded loadedon onthe theYF YF3 3surface surfacewould wouldalso alsoabsorb absorbmost mostof ofthe theUV UV light light and and decrease 3+. . the excitation intensity of YF33:Ho3+
Figure 5. Upconversion luminescence spectra of YF3:Ho3+@TiO2 photocatalysts with different TBOT dosages.
Figure 5. Upconversion luminescence spectra of YF3:Ho3+@TiO2 photocatalysts with different Figure 5. Upconversion luminescence spectra of YF3 :Ho3+ @TiO2 photocatalysts with different TBOT dosages. TBOT dosages.
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3.5. Photocatalysis Mechanism of RhB Degradation 3.5. Photocatalysis Mechanism of RhB Degradation According to Jun’s study [43], there are two upconversion mechanisms for YF3:Ho3+, of which 3+ , of which According to Jun’s study [43], there are two upconversion mechanisms one is a two-photon upconversion fluorescence mechanism and the other for oneYFis3 :Ho a three-photon upconversion fluorescence mechanism. one is a two-photon upconversion fluorescence mechanism and the other one is a three-photon To furtherfluorescence determine the photocatalysis mechanism of YF3:Ho3+@TiO2, this paper detected the upconversion mechanism. 3+ @TiO photogenerated radicals during the photocatalysis processofby ESR technique To further determine the photocatalysis mechanism YFthe paper detected the 3 :Ho 2 , this[46–48]. For the capacity of generating such as the DMPO-hydroxyl radical[46–48]. (·OH) and DMPOphotogenerated radicals during the radicals, photocatalysis process by the ESR technique − superoxide (·O2of), generating 5,5-dimethyl-1-pyrroline-N-oxide has been radical generally used and for For theradical capacity radicals, such as the(DMPO) DMPO-hydroxyl (·OH) −the trapping radicals. According results shown in Figure 6, we can see that the signals of ·OH and DMPO-superoxide radical (·Oto ), 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) has been generally used 2 ·O are obvious and clear. The intensities of these two considerably for2−trapping radicals. According to the results shown in radicals’ Figure 6,signals we canincrease see that the signals of after ·OH − − 3+ irradiation forobvious 6 min. and Hence, theThe ·OH and ·O2 of are twotwo main oxidative species for the YF3:Ho @TiO and ·O2 are clear. intensities these radicals’ signals increase considerably after2 3+ @TiO − are system. Moreover, is speculated with these two radicals during its photocatalytic irradiation for 6 min.RhB Hence, the ·OH andto·Oreact two main oxidative species for the YF :Ho 2 3 2 degradation, and theRhB response equation is as follows. system. Moreover, is speculated toshown react with these two radicals during its photocatalytic degradation, and the response equation is shown as follows. (R1) YF3 :Ho3 + + visible light → YF3 :Ho3 + + UV YF3 :Ho3 + + visible light → YF+3 :Ho3 + + UV TiO2 + UV → TiO2 + (h + e- ) TiO2 + UV → TiO2 + (h + + e− ) H2 O + TiO2 h + → ·OH + H + H2 O + TiO2 h + → ·OH + H + H +++TiO TiO2 ee−- → ·O2− ·O +2H+ + →·O·2O+2−H+2 O H22 O2 2
(R1) (R2)
·O2−·O+-2 H O22O→ OH- − +2O2 + 2H ·OH + + OH +O 2 →·OH
(R5) (R5)
·OH (or·O2− )-2 ++RhB → degradation degradation++ products ·OH(or·O RhB → products
(R6) (R6)
(R2) (R3) (R3) (R4) (R4)
based on the response equation, this paper also theorizes a possible reaction process Besides, based 3+ emits UV light after absorbing visible light 3+ emits shown in inFigure Figure7.7.Firstly, Firstly,the theUCL UCL material 3 :Ho 3:Ho UV light after absorbing visible light (R1). material YFYF (R1). Then, TiO2 trapped the surface UCL surface will generate electron-hole through UV excitation Then, TiO2 trapped on theon UCL will generate electron-hole pairs pairs through UV excitation (R2). − (R2).excited The excited electrons on the react with oxygentotoform form ·O ·O22− and andHO HO2,2 ,and andthen then the 2 surface The electrons on the TiOTiO 2 surface react with oxygen + − resulted HO 2O 2 ;2;R4). HO22 combines combines with with H H+ to to form form hydrogen hydrogenperoxide peroxidelater later(H (H 2O R4).When When·O ·O2 2− meets H22 O22,, OH (R5); meanwhile, the photogenerated photogenerated holes holes at (R2) will react with H2O to form it will generate ··OH ·OH (R3). For For the the conduction conduction band band of of TiO TiO22 located located above above the the RhB RhB redox redox potential, potential, the oxygen species ·OH ((·OH, ·OH, ·O ·O22−− , and H O ) could oxidize the RhB to realize the purpose of RhB’s degradation. , and H2O2 2) 2could oxidize the RhB to realize the purpose of RhB’s degradation.
3+@TiO2 composite Figure electron spin spin resonance resonance(ESR) (ESR)spectra spectraof ofthe theYF YF3:Ho :Ho3+ Figure 6. 6. DMPO DMPO spin-trapping spin-trapping electron @TiO2 composite 3 − in for ··OH dispersion for for ··O in the the methanol methanol dispersion dispersion for OH (a) (a) and and in in the the aqueous aqueous dispersion O2 −(b). (b).
2
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Figure 7. YF3:Ho3+@TiO2 photocatalysis mechanism for RhB degradation (the insets show the transition
3+ @TiO photocatalysis mechanism for RhB degradation (the insets show the 3+) Ps: Figure of 7. Ho YF [1] Xu, X. 2et al. Synthesis and intense ultraviolet to visible upconversion luminescence of 3 :Ho 3+ 3 3+ GSA: state absorption; ESA: excitation state UV: YF :Ho transition of Honanoparticles. ) Ps: [1] Xu, X. ground et al. Synthesis and intense ultraviolet to absorption; visible upconversion ultraviolet; CB: conduction band. ground state absorption; ESA: excitation state luminescence of VB: YF3valence :Ho3+ band; nanoparticles. GSA: absorption; UV: ultraviolet; VB: valence band; CB: conduction band.
3.6. Photocatalysis Application
Figure 8a,b 7. YF3show :Ho3+@TiO photocatalysis mechanism for 2RhB degradation insetsof show the transition Figure the 2influences of YF 3:Ho3+@TiO with different (the dosages TBOT on RhB and 3.6. Photocatalysis Application 3+
X. et al. Synthesis andsamples intense ultraviolet to visible luminescence of for Ho ) Ps: [1] Xu, phenolof degradation, respectively. All the have almost zero upconversion adsorption efficiency both 3:Ho3+ nanoparticles. GSA: ground state3+absorption; ESA: excitation state absorption; UV: YF RhB and wherein the most is 2less than 1%. For dosages the RhB and phenol,on the Figure 8a,bphenol, show the influences of significant YF3 :Ho one @TiO with different of TBOT RhB and ultraviolet; VB: valence band; CB: conduction band. then decreases with the increase of the TBOT efficiency firstsamples increases phenol photocatalytic degradation,degradation respectively. All the have almost zero adsorption efficiency both for dosage. When the dosages of TBOT are 6 mL, it can obtain the highest degradation efficiency (67%) RhB and phenol, wherein the most significant one is less than 1%. For the RhB and phenol, the Photocatalysis Application of3.6. RhB as well as the highest degradation efficiency (34.6%) of phenol. According to the analyses photocatalytic degradation efficiency first increases then decreases with the2 increase of the TBOT above,Figure this could duethe to the fact that of when the 3+ TBOT increased, more TiO would doped 8a,b be show influences YF3:Ho @TiOdosage 2 with different dosages of TBOT onbeRhB and dosage.on When the dosages of TBOT are 6 mL, it can obtain the highest degradation efficiency (67%) of the YFdegradation, 3:Ho3+, and thus more UV light would be absorbed, resulting in more excited electron-hole phenol respectively. All the samples have almost zero adsorption efficiency both for RhB as well the highest degradation ofphenol. phenol. According toand thephenol, analyses pairs and an improved degradation efficiency RhBisand However, when the dosage of above, RhB as and phenol, wherein the mostefficiency significantof(34.6%) one less than 1%. For the RhB the TBOT to fact increase 8 mL, thefirst degradation efficiency wouldmore be decreased for both RhB and photocatalytic degradation efficiency increases then decreases with the of the TBOT this could be continued due to the thattowhen TBOT dosage increased, TiOincrease would be doped on the 2 3+ in Figure 3, when the dosage reaches 8 mL, the YF3:Ho3+ is 3+ phenol. According to the TEM of YF 3:Ho dosage. When the dosages of TBOT are 6 mL, it can obtain the highest degradation efficiency (67%) YF3 :Ho , and thus more UV light would be absorbed, resulting in more excited electron-hole pairs covered which will affectefficiency the absorption of visible lightAccording and the conversion from of RhB by as TiO well2 completely, as the highest degradation (34.6%) of phenol. to the analyses and an improved degradation efficiency of RhB and phenol. However, when the dosage of TBOT 3+@TiO2. visible to UV be light, reducing photocatalytic activity of YF3:Homore above,light this could duethus to the fact thatthe when the TBOT dosage increased, TiO2 would be doped continued toThis increase to 8tested mL, the degradation efficiency be decreased foraboth RhB and phenol. paper photocatalytic efficiency ofwould P25, TiO 2 (prepared using similar method on the YF 3:Ho3+also , and thus the more UV light would be absorbed, resulting in more excited electron-hole 3+ 3+ 3+@TiO 3+),Figure According to3:Ho the TEM YF3 :Ho 3, when the dosage reaches 8 mL,when the YF isofcovered 3 :Ho 2 of without YF3:Hoin andefficiency BiVO 4, respectively. RhB degradation, the results showed aspairs YF and an improved degradation of RhB andFor phenol. However, the dosage thecontinued efficiency of increase P25will wasaffect 12.2%, the of TiO 2visible was 17.8%, and the efficiency BiVO 4 was by TiO2that completely, which the absorption of light and the conversion from TBOT to to 8 mL, theefficiency degradation efficiency would be decreased forof both RhB andvisible 3+and 3+ For phenol degradation, the showed the efficiency of P25 TiO 2 was nearly zero, phenol. According to the TEM YFresults 3:Ho3+ in Figure 3, when light to 22.6%. UV light, thus reducing theof photocatalytic activity ofthe YFdosage @TiO 3 :Ho reaches 28. mL, the YF3:Ho is 3+@TiO2. Therefore, it and the efficiency of BiVO 4 was 9.7%, all of which were less than that of YF 3:Ho covered by TiO 2 completely, which will affect the absorption of visible light and the conversion from This paper also tested the photocatalytic efficiency of P25, TiO2 (prepared using a similar method 3+@TiO2 material can make up for the defects 3+ can be concluded that the YF 3:Ho TiO22.(which is unable visible light to UV light, thus reducing the photocatalytic activity of YF3:Ho of@TiO as YF3 :Ho3+ @TiO2 without YF3 :Ho3+ ), and BiVO4 , respectively. For RhB degradation, the results to have a paper photocatalytic under visible light irritation), has a higher This also testedreaction the photocatalytic efficiency of P25, TiOand 2 (prepared using a photocatalytic similar method showedefficiency that efficiency of P25 waslight 12.2%, the efficiency of TiO was 17.8%,theand theshowed efficiency of 3+ the common visible BiVO4.For :Hothan @TiO 2 without YF 3:Ho3+), andphotocatalyst BiVO 4, respectively. RhB2degradation, results as YF3the BiVO4 was For phenol degradation, the results theand efficiency of P25 and4 was TiO2 was that 22.6%. the efficiency of P25 was 12.2%, the efficiency of TiOshowed 2 was 17.8%, the efficiency of BiVO 3+ @TiO . nearly zero, and efficiency of BiVO was 9.7%, all the of which were lessand than ofnearly YF3 :Ho 22.6%. Forthe phenol degradation, the4 results showed efficiency of P25 TiOthat 2 was zero, 2 3+ @TiO anditthe of BiVO4that was the 9.7%, all3 :Ho of which were less than that of YF3:Ho3+@TiO2. Therefore, it Therefore, canefficiency be concluded YF 2 material can make up for the defects of TiO2 be concluded thatathe YF3:Ho3+@TiO2 material make up for thelight defects of TiO2 (which unable (which iscan unable to have photocatalytic reactioncan under visible irritation), andis has a higher to have a photocatalytic reaction under visible light irritation), and has a higher photocatalytic photocatalytic efficiency than the common visible light photocatalyst BiVO4 . efficiency than the common visible light photocatalyst BiVO4.
(a)
(b)
Figure 8. Effect of TBOT dosage on the removal rate of RhB (a) and phenol (b).
(a)
(b)
Figure 8. Effect of TBOT dosage on the removal rate of RhB (a) and phenol (b).
Figure 8. Effect of TBOT dosage on the removal rate of RhB (a) and phenol (b).
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3.7. Influencing Factors of the Photocatalytic Degradation Reaction
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The influencing factors of photocatalytic reaction include photocatalyst dosage (mcata ), substrate 3.7. Influencing(C Factors of the Photocatalytic Degradation Reaction concentration 0 ), and irradiation intensity (E), which are also the three major factors that affect the ka coefficient of the photocatalytic kinetics equation, according the Langmuir-Hinshelwood equation The influencing factors of photocatalytic reaction includetophotocatalyst dosage (mcata), substrate (other factors are considered). paper(E), haswhich studied seriesthe of three experiments to figure out howthe the concentration (C0),not and irradiationThis intensity area also major factors that affect apparent photocatalytic degradation kinetics change with these three factors. ka coefficient of the photocatalytic equation, according to the Langmuir-Hinshelwood
equation (other factors are not considered). This paper has studied a series of experiments to figure 3.7.1. The effect of m out how the apparentcata photocatalytic degradation kinetics change with these three factors. Figure 9 shows how different dosages of YF3 :Ho3+ @TiO2 affect the photocatalytic efficiency. 3.7.1. The effect of mcata For the experiment, this paper changes the dosages of the photocatalyst from 0.05 to 0.25 g. And the 3+ @TiO dosage, the degradation efficiency of RhB first result shows that with thedifferent increasing YF3 :Ho Figure 9 shows how dosages of YF 3:Ho23+@TiO2 affect the photocatalytic efficiency. For increases and then decreases. This may be due to thephotocatalyst fact that the increased not the experiment, this paper changes the dosages of the from 0.05photocatalyst to 0.25 g. Anddosage the result only improved photon efficiency to generate more photogenerated radicals, but also increased 3+@TiO2 dosage, shows that withthe the increasing YF3:Ho the degradation efficiency of RhB first effluent turbidity, causing light scattering to decrease photon efficiency. After fitting, an equation can increases and then decreases. This may be due to the fact that the increased photocatalyst dosage not only be obtained as shown below. improved the photon efficiency to generate more photogenerated radicals, but also increased effluent turbidity, causing light scattering to decrease photon efficiency. After fitting, an equation can be obtained ka = −10.643 m2 + 3.2156 m + 0.161 (R2 = 0.9717) as shown below. 2 If ka = k1 mb , i.e., ln kkaa= lnk1 +m2blnm, there be(Rtwo conditions, m = 0.05 g–0.15 g and = -10.643 + 3.2156 m +will 0.161 = 0.9717) m = 0.15 g–0.25b g. Through the lnk-lnm graph, we can obtain k1 = 0.76638, b = 0.31835 (R2 = 0.93077) If k = k1 m , i.e., ln ka = ln k12 + b ln m , there will be two conditions, m = 0.05 g–0.15 g and and k1 =a 0.14273, b = −0.56254 (R = 0.92079). Then there is: m = 0.15 g–0.25 g. Through the lnk-lnm graph, we can obtain k1 = 0.76638, b = 0.31835 (R2 = 0.93077) and k1 = 0.14273, b = −0.56254 k(R2 = = 0.92079). Then there is: ≤ m ≤ 0.15) 0.31835 0.76638 m (0.05
a
0.76638 . 0.05 0.15 ka = 0.14273 m−0.56254 (0.15 ≤ m ≤ 0.25) ka = 0.14273 m-0.56254 (0.15 ≤ m ≤ 0.25)
Figure Figure9.9.Degradation Degradationrate rateof ofRhB RhBwith withdifferent differentdosages dosagesof ofthe thephotocatalyst. photocatalyst.
3.7.2. The effect of C0 3.7.2. The effect of C0 This paper tested different concentrations of substrate (RhB) for photodegradation. This paper tested different concentrations of substrate (RhB) for photodegradation. Figure 10 shows how different concentrations of substrate affect the photocatalytic efficiency. Figure 10 shows how different concentrations of substrate affect the photocatalytic efficiency. The degradation efficiency of RhB decreases with the increase of the substrate concentration. When The degradation efficiency of RhB decreases with the increase of the substrate concentration. When the the concentration of the substrate is 4 mg/L, the photocatalytic efficiency reaches the highest value, concentration of the substrate is 4 mg/L, the photocatalytic efficiency reaches the highest value, which which is 76%, and when it is 8 mg/L, the photocatalytic efficiency reaches the lowest value, which is is 76%, and when it is 8 mg/L, the photocatalytic efficiency reaches the lowest value, which is 56%. 56%. This is mainly because when the concentration of the substrate is relatively low, the This is mainly because when the concentration of the substrate is relatively low, the photocatalyst photocatalyst YF3:Ho3+@TiO2 is in excess, and then the photocatalytic efficiency of RhB will be YF3 :Ho3+ @TiO2 is in excess, and then the photocatalytic efficiency of RhB will be relatively high. On the relatively high. On the contrary, when the concentration of the substrate is high, the photocatalyst YF3:Ho3+@TiO2 is no longer sufficient, and all of the photocatalyst needs to take part in the photodegradation. At this time, while the photodegradation rate is at its highest value, the apparent degradation efficiency will decrease with the increase of the substrate concentration, and thus
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contrary, when the concentration of the substrate is high, the photocatalyst YF3 :Ho3+ @TiO2 is no longer sufficient, and all of the photocatalyst needs to take part in the photodegradation. At this time, while the photodegradation rate is at its highest value, the apparent degradation efficiency will decrease with the increase of the substrate concentration, and thus photodegradation efficiency will be worse. Materials 2017, 10, 302 11 of 14 However, when the concentration of the substrate is too high, it will reduce the light transmittance of the solution, thus reducing the photocatalytic activity. photodegradation efficiency will be worse. However, when the concentration of the substrate is too After fitting, an equation can be obtained as shown below. high, it will reduce the light transmittance of the solution, thus reducing the photocatalytic activity. After fitting, an equation can be obtained as shown below. ka = 0.0287C0 + 0.18279 (R22 = 0.99324) ka = 0.0287C0 + 0.18279 (R = 0.99324)
Figure 10. 10. Degradation Degradation rate rate of of RhB RhB with with different different concentrations concentrations of of initial initial glucose. glucose. Figure
3.7.3. The effect of irradiation intensity 3.7.3. The effect of irradiation intensity Figure 11 shows how different irradiation intensities affect the photocatalytic efficiency. When Figure 11 shows how different irradiation intensities affect the photocatalytic efficiency. When the the irradiation intensity reaches 141,500 lx, each factor is at its best level, the RhB is degraded irradiation intensity reaches 141,500 lx, each factor is at its best level, the RhB is degraded completely, completely, and the process is not a zero order reaction. Therefore, in this paper, only the irradiation and the process is not a zero order reaction. Therefore, in this paper, only the irradiation intensities intensities of 87,100 lx, 52,300 lx, 43,500 lx were considered. With the increase of irradiation intensity, of 87,100 lx, 52,300 lx, 43,500 lx were considered. With the increase of irradiation intensity, reactions reactions between the photocatalyst and photon will increase and the rate of the photocatalysis between the photocatalyst and photon will increase and the rate of the photocatalysis reaction will be reaction will be faster, thus leading to an increased efficiency. After fitting, an equation can be faster, thus leading to an increased efficiency. After fitting, an equation can be obtained as shown below. obtained as shown below. = 1.4154 10−-66E (R(2R=2 0.97489) ka =ka1.4154 × ×10 E ++0.2318 0.2318 = 0.97489) The apparent kinetics model of RhB degradation is The apparent kinetics modelCof RhB = C degradation - 2.19967 × 10is-6 m0.31835 C Et A
0
0
(0.05 g ≤ m ≤ C 0.15=g,C4.0 − mg/L ≤ C0 ≤×8.0 −6 0.31835 2.19967 10mg/L, m 43500lx C Et≤ E ≤ 87100lx) A
0
0
CA = C0 - 4.97 × 10-7 m-0.56254 C0 Et (0.05 g ≤gm≤ ≤ mg/L 8.0mg/L, mg/L,43500lx 43500lx≤ ≤ ≤ 87100lx) (0.15 m ≤0.15 0.25g,g,4.04.0 mg/L≤≤ C C00 ≤ ≤ 8.0 E ≤E 87100lx) CA = C0 − 4.97 × 10−7 m−0.56254 C0 Et (0.15 g ≤ m ≤ 0.25 g, 4.0 mg/L ≤ C0 ≤ 8.0 mg/L, 43500lx ≤ E ≤ 87100lx)
The apparent kinetics model of RhB degradation is CA = C0 - 2.19967 × 10-6 m0.31835 C0 Et (0.05 g ≤ m ≤ 0.15 g, 4.0 mg/L ≤ C0 ≤ 8.0 mg/L, 43500lx ≤ E ≤ 87100lx) Materials 2017, 10, 302
CA = C0 - 4.97 × 10-7 m-0.56254 C0 Et
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(0.15 g ≤ m ≤ 0.25 g, 4.0 mg/L ≤ C0 ≤ 8.0 mg/L, 43500lx ≤ E ≤ 87100lx)
Figure 11. Degradation Degradation rate rate of RhB with different irradiation intensities.
4. Conclusions To solve the problem of TiO2 having nearly no photocatalytic efficiency under visible light irradiation, a composite photocatalyst material YF3 :Ho3+ @TiO2 was prepared in this paper using upconversion luminescence technology. Through analyzing the morphology and composition, crystal structure, and optical spectra of YF3 :Ho3+ @TiO2 , it was found that this material had high photocatalytic efficiency under visible light irradiation. In addition, this paper also investigated the influences of different dosages of TBOT on the properties of the photocatalyst. In summary, YF3 :Ho3+ @TiO2 can be successfully prepared using a simple hydrothermal method. By analyzing the XRD images, we found that almost all the samples showed the diffraction peak of anatase TiO2 and that the crystal structure of the material did not change with TBOT dosage. TiO2 was uniformly doped on UCL, and the particle size was about 10 nm. The rice-shaped UCL material had good dispersion, of which the particle size was about 100 nm. The change of TBOT dosage would not cause the change of the material morphology, but would cause the change of the amount of TiO2 doped on UCL, resulting in an impact on the photocatalytic activity of YF3 :Ho3+ @TiO2 . The composite material prepared in this paper shared the same upconversion luminescence property with the UCL material prepared by Jun. It confirmed that the material prepared in this paper could absorb 450 nm visible light and emit UV light, with energy transferred from YF3 :Ho3+ to anatase TiO2 . With the increase of the TBOT dosage, more TiO2 would be doped on the YF3 :Ho3+ , therefore the excitation light was obstructed, resulting in a lower energy of exciting light. The photocatalytic properties of the YF3 :Ho3+ @TiO2 was evaluated by the degradation of RhB and compared with those of traditional photocatalysts such as P25, TiO2 , and the visible light photocatalyst BiVO4 . The results showed that the prepared composite material exhibited better photocatalytic properties as compared with the other three photocatalysts. With the increase of the TBOT dosage, the photocatalytic efficiency of composite YF3 :Ho3+ @TiO2 first increased and then decreased. When the TBOT dosage was 6 mL, the photocatalytic efficiency reached the highest value, which was 67%. The results of this paper indicated that Ho3+ -single-doped hexagonal YF3 could absorb visible light and emit UV light via UC processes. Under UV light irradiation, the composite material YF3 :Ho3+ @TiO2 could exhibit better photocatalytic properties than that of anatase TiO2 , therefore the prepared composite material YF3 :Ho3+ @TiO2 has promising applications in photocatalysis. Acknowledgments: Financial support from the Science and Technology Innovation Special Projects of Social Undertakings and Livelihood Support, Chongqing (cstc2016shmszx20009, cstc2015shmsztzx20003), the Science and Technology Project of Chongqing Education Commission (KJ1500604), the Chongqing Research Program of Basic Research and Frontier Technology (cstc2015jcyjA20013), the Program for Innovative Research Team in University in Chongqing (CXTDX201601003) and the 111 Project (B13041) are gratefully acknowledged.
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Author Contributions: Shiyu Zhou and Xuan Xu conceived and designed the experiments; Shiyu Zhou, Tianhui Wu, and Jun Long performed the experiments; Shiyu Zhou analyzed the data; Zihong Fan contributed reagents/materials/analysis tools; Shiyu Zhou wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.
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