Synthesis of Reduced Grapheme Oxide as A Platform for loading β-NaYF4:Ho3+@TiO2Based on An Advanced Visible Light-Driven Photocatalyst

In this paper a novel visible light-driven ternary compound photocatalyst (β-NaYF4:Ho3+@TiO2-rGO) was synthesized using a three-step approach. This photocatalyst was characterized using X-ray diffraction, Raman scattering spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, Transmission electron microscopy, X-ray photoelectron spectroscopy, fluorescence spectrometries, ultraviolet-visible diffuse reflectance spectroscopy, Brunauer–Emmett–Teller surface area measurement, electron spin resonance, three-dimensional fluorescence spectroscopy, and photoelectrochemical properties. Such proposed photocatalyst can absorb 450 nm visible light while emit 290 nm ultraviolet light, so as to realize the visible light-driven photocatalysis of TiO2. In addition, as this tenary compound photocatalyst enjoys effecitve capacity of charge separation, superior durability, and sound adsorb ability of RhB, it can lead to the red shift of wavelength of absorbed light. This novel tenary photocatalyst can reach decomposition rate of RhB as high as 92% after 10 h of irradiation by visible-light Xe lamp. Compared with the blank experiment, the efficiency was significantly improved. Recycle experiments showed that theβ-NaYF4:Ho3+@TiO2-rGOcomposites still presented significant photocatalytic activity after four successive cycles. Finally, we investigated visible-light-responsive photocatalytic mechanism of the β-NaYF4:Ho3+@TiO2-rGO composites. It is of great significance to design an effective solar light-driven photocatalysis in promoting environmental protection.

be enhanced from UV to Vis. In addition to the microcrystals catalyst β-NaYF 4 :Ho 3+ @TiO 2 -rGO which can improve the utilization of sunlight, rGO can inhibit the recombination of photogenerated electron-hole pairs and enhance the adsorption capacity of the photocatalyst. In addition, we conducted detailed discussion on the Vis responsive photocatalytic mechanism of β-NaYF 4 :Ho 3+ @TiO 2 -rGO.

Results and Discussion
Structure and morphology characterizations. Figure 1 shows the XRD patterns of β-NaYF 4 :Ho 3+ , β-NaYF 4 :Ho 3+ @TiO 2 and β-NaYF 4 :Ho 3+ @TiO 2 -rGO, respectively. From the XRD images of the three samples in Figure. 1, we can see that diffraction peaks are very sharp. This indicates that the samples resulted from hydrothermal synthesis are of high purity and excellent crystallinity, and such excellent crystallinity plays a vital role in determmining the photocatalysis efficiencies 54 . In addition, we can see that the diffraction peaks of UC material at 17.2°, 30.1°, 30.8°, 43.5°, and 53.7° can perfectly match with the standard card of β-NaYF 4 (JCPDS no. , without the occurrence of impurity-contained diffraction peak. This indicates that the high purity UC material β-NaYF 4 prepared using hydrothermal method is the current well-recognized substrate of the highest luminous efficiency 53 . Through comparing with standard card, it can find the compound material β-NaYF 4 :Ho 3+ @TiO 2 not only involved all diffraction peaks of β-NaYF 4 , but also showed the characteristic diffraction peak of TiO 2 when 2 θ = 25.4°, which was consistent with anatase standard card (JCPDS no. . Therefore, it proved that the TiO 2 included in photocatalyst prepared by sol-gel method was anatase-typed TiO 2 , which enjoyed higher photocatalysis effect than rutile type TiO 2 and was more helpful in increasing the photocatalysis activity of the compound material 55 . Moreover, β-NaYF 4 :Ho 3+ @TiO 2 prepared by such method will not change the crystal form of β-NaYF 4 , therefore it can be known such method has sound stability and reproducibility. Through comparing the XRD pattern between β-NaYF 4 :Ho 3+ @TiO 2 -rGO and the compound material, it can observe no occurrence of new peak value, and all peak values are either hexagonal hexagonal or anatase-typed TiO 2 . It can also be noted that the diffraction peak of rGO is not reflected in such XRD pattern, which we think may be due to its low proportion in compound material and its low concentration that is under the LOD of XRD. Therefore, other characterization methods are still needed to verify the existence of rGO.
In addition, it is worth noting that it can observe no diffraction peaks of Ho 3+ from UC material, compound material, or ternary complex in above figure, which is either because Ho is not successfully doped in, or because it is exsited in other forms. The existence form of Ho is of vital significance to UC luminousmechanism, therefore it is needed to seek other characterization methods.
To determine the micromorphologies and the presence of rGO these samples, we conducted SEM and TEM characterization for β-NaYF 4 :Ho 3+ , β-NaYF 4 :Ho 3+ @TiO 2 , andβ-NaYF 4 :Ho 3+ @TiO 2 -rGO, respectively, and the results are shown in Figure. 2. From Figure. 2a,b, it can be seen that the prepared UC materials are hexagonal prisms in uniform sizes, regular shapes, and with smooth surfaces. These hexagonal microcrystals have lengths of around 8 um and diameters of nearly 2.4 um, which is consistent with XRD characterization result. There is a close correlation between UC luminous efficiency and host material size. The larger the host material size is, the higher the luminous efficiency will be 56 . This indicates that the UC host material prepared by hydrothermal method is of sound luminous efficiency. After being compounded with TiO 2 shell by sol-gel method, the UC material had not changed very much on its morphology, but still remain in shape of hexagonal prism. After coating, the surfaces become coarser ( Figure. 2c), showing that the microcrystals are successfully coated with a TiO 2 layer. The β-NaYF 4 :Ho 3+ microcrystals are equably coated by the TiO 2 shell. For further detailed structure analysis, the characterization of the β-NaYF 4 :Ho 3+ @TiO 2 core-shell microcrystals was carried out by TEM. Figure 2d show enlarged image in which the β-NaYF 4 :Ho 3+ @TiO 2 core-shell structure is clearly seen; the core of β-NaYF 4 :Ho 3+ exhibits a dark color. These images confirm that the UC microcrystals are uniformly coated by a TiO 2 layer. The average thickness of the TiO 2 shells is about 50 nm. Core-shell structure model is beneficial to realizing high efficient energy transfer between UC material and photocatalyst. The SEM image of ternary Figure 1. The XRD patterns of β-NaYF 4 :Ho 3+ , β-NaYF 4 :Ho 3+ @TiO 2 , and β-NaYF 4 :Ho 3+ @TiO 2 -rGO.
complex β-NaYF 4 :Ho 3+ @TiO 2 -rGO in Figure. 2e,f shows that there are massive amounts of rGO lamellas deposited on the surface of compound material. There are small amount of deciduous TiO 2 existing on the surfaces of some rGO lamellas; some rGO serve as substrates, on which compound materials are loaded; some rGO serve as carriers, which wrap the compound materials; The coupling between rGO and β-NaYF 4 :Ho 3+ @TiO 2 is positive to the high efficient transferring of charge, and thus increasing the separation efficiency of photongenerated carriers.
Composition and chemical states. Energy dispersive X-ray spectroscopy (EDS) was conducted to determine the element composition of β-NaYF 4 :Ho 3+ @TiO 2 -rGO and to further confirm the successful doping of Ho element. Results are shown in Figure. 3. According to the scan results of EDS surface of ternary compound material in Figure. 3b-h, it can observe the existences of Y, F, Na, Ho, Ti, O and C in ternary compound material β-NaYF 4 :Ho 3+ @TiO 2 -rGO, wherein C element is in homogeneous distribution, which indicates a sound dispersion effect of rGO. Figure 3i presents the mass percent of each element. However, EDS can only characterize the existence of in prepared sample, rather than charaterizing the existence form of Ho.
XPS characterization method was used to determine the chemical states of elements on the surface of β-NaYF 4 :Ho 3+ @TiO 2 -rGO as shown in Figure. 4. According to the full spectrum image in Figure. 4a, it shows that such sample contains Ti, O, Na, Y, F, Ho and C elements. C 1 s peak (284.1 eV) is the spectrum internal reference. Figure 4b shows that the Ti2p photoemission peak of compound material consists of two sub-peaks, with binding energies of 458.3 eV and 464.1 eV, corresponds to Ti 2p 3/2 and Ti 2p 1/2 , respectively, which is consistent with the XPS spectrum of TiO 2 as described 57 . As shown by the O1s in Figure. 4c, there is at least one type of oxygen in the compound material. The binding energies of two peaks are 529.9 eV and 532.3 eV, which are corresponded to the characteristics of Ti-O-Ti and H-O, respectively. The element F displays one characteristic peak at 684.3 eV because of the core level of F1s (see Figure. 4d). The C1s XPS spectrum shows two characteristic peaks, corresponding to oxygenated ring C bonds (284.7 eV for C-C, C=C and C-H,286.2 eV for C-O, and 288.6 eV for the C=O bond). These results indicate that there exist abundant oxygen-containing functional groups onrGO surface. However, in the C 1 s XPS spectra of β-NaYF 4 :Ho 3+ @TiO 2 -rGOas shown in Fig. 4e, the relative intensities of the three components associated with C-O/C=O bonds decrease significantly, indicating that some of the oxygen functional groups were reduced during the chemical reduction process 58,59 . Doped elements can also be detected by XPS. Figure 4f represents the characteristic peak of Ho 3+ at binding energy of 159.5 eV and 161.1 eV, respectively. XPS characterization results show that the Ho element in the sample exists in the form of Ho 3+ and has been successfully doped into the crystal lattice of the host material β-NaYF 4 .
To further confirm the reduction of GO, we conducted FTIR analysis for the samples, and the results are shown in Figure. 5. It shows that GO contains abundant oxygen containing functional groups. The strong peak occurred at 1731 cm −1 is mainly caused by the tensile vibration of C=O in carboxyl functional groups, while the strong peak at around 1623 cm −1 is mainly caused due to the lack of oxidized C=C structure in graphite structure. In addition, the occurrence of peak at 1401 cm −1 is due to the oxidized C-OH on the surface. The occurrences of peaks at 3405 cm −1 and 1048 cm −1 are due to the vibrations of OH and C-O 60 , respectively. Through comparing β-NaYF 4 :Ho 3+ @TiO 2 -rGO and GO, it can find that C-O and C=O, at 1084 cm −1 and 1731 cm −1 respectively, are almost disappeared, which indicates that some carboxy groups are differentially reduced according to different conditions. The functional groups at 1623 cm −1 and 1401 cm −1 are also significantly reduced. Results show that the GO can be effectively reduced into rGO using hydrothermal method, which is consistent with XPS result. β-NaYF 4 :Ho 3+ @TiO 2 -rGO displays a strong and wide absorption peak, which is proved to be a combination peak caused by the stretching vibration of Ti-O-Ti and Ti-O-C 61 . Moreover the existence of Ti-O-C bond indicates existence of chemical bond force between rGO and TiO 2 , and such chemical bond is beneficial to the red shift of absorbed wavelength 49 , so as to better take the advantage of solar photocatalysis.
Photoluminescence properties. Figure 6a shows the UV-Vis diffuse reflection spectrum of ternary compound material β-NaYF 4 :Ho 3+ @TiO 2 -rGO. Results show that the UV-Vis diffuse reflection spectrum of β-NaYF 4 :Ho 3+ @TiO 2 -rGO has two distinguished features as compared with β-NaYF 4 :Ho 3+ @TiO 2 , β-NaYF 4 :Ho 3+ , β-NaYF 4 , and P25, wherein one is the overall improvement of Vis light absorption property, and the other one is the red shift of absorption cross section. Both features are positive to increasing the photocatalytic activity 7 . In addition, we observed that β-NaYF 4 :Ho 3+ displayed three weak absorption peaks at 450 nm, 537 nm, and 642 nm, respectively, while β-NaYF 4 did not show absorption peak. This indicates the UC material excited ion Ho 3+ is of vital significance to luminescence. On the whole, the absorption peak at 450 nm is relatively stronger. As we know that the stronger the light-absorbing capacity of the absorption peak is, the more suitable the absorption peak will be serving as excitation wavelength. Therefore the absorption peak at 450 nm was finally selected as the excitation wavelength for the luminescence spectrum of UC material β-NaYF 4 :Ho 3+ .In the absorption spectrum of GO, it shows the absorption peaks of GO at 230 nm (π-π* transitions of C=C bonds) 62 referred to pure GO films shown in Figure. 6a. rGO has a characteristic absorption peak at 270 nm, compared with GO 40 nm red shift of absorption spectrum, the overall absorption intensity increases, indicating that GO in hydrothermal conditions reduction. The carbon atoms of sp 3 hybridized into sp 2 hybrid structure, improved GR was largeπelectron conjugated structure 63 . sUV-Vis characterization results show that the rGO was successfully prepared by hydrothermal synthesis.
The bandgaps of P25, β-NaYF 4 :Ho 3+ @TiO 2 , and β-NaYF 4 :Ho 3+ @TiO 2 -rGO can be calculated using Tauc's formula, and the calculation results are shown in Figure. 6b. The bandgap of P25 is 3.20 eV (387.5 nm), the bandgap of β-NaYF 4 :Ho 3+ @TiO 2 is 3.18 eV (389.9 nm), and the bandgap of β-NaYF 4 :Ho 3+ @TiO 2 -rGO is 3.09 eV (401.3 nm). From the results, it can conclude that the UC materialβ-NaYF 4 :Ho 3+ microcrystal is of limited influence to the bandgap of TiO 2 , while the introduced rGO is of larger influence to its absorption cross section, such as reducing its bandgap and leading to the red shift of absorbed wavelength. From Figure. 6c, it can be seen that the VB edges ofβ-NaYF 4 :Ho 3+ @TiO 2 are estimated to be 0.32 eV. According to the bandgap and the valence band (VB) position of the samples, we can draw the bandgap structures as displayed in Figure. 6c inset, from which it can be clearly seen that the conduction band (CB) and VB position of β-NaYF 4 :Ho 3+ @TiO 2 . Figure 6d shows the UC Luminescence spectra of β-NaYF 4 :Ho 3+ , β-NaYF 4 :Ho 3+ @TiO 2 , and β-NaYF 4 :Ho 3+ @ TiO 2 -rGO respectively under 450 nm excitation. It can be seen that under 450 nm excitation, there is a emission peak at 290 nm in ultraviolet region, which corresponds to the radiative transition of Ho 3+ from 5 D 4 to 5 I 8 . After being doped with TiO 2 , the emission peak wavelength location of composite material of β-NaYF 4 :Ho 3+ has not been changed, which indicates that the UC luminance properties remain unchanged after compounded with TiO 2 . In addition, we can observe that β-NaYF4:Ho 3+ @TiO 2 shows a fairly weak luminous intensity at 290 nm under the 450 nm excitation, wherein the emission peak is almost disappeared. This may be because the UV-light emitted from β-NaYF 4 :Ho 3+ was absorbed by the TiO 2 wrapped on the surface, so that the phenomenon of photocatalysis is resulted. As it is well known that rGO is an effective material which is of high ability in absorbing light 64 . For rGO-assisted β-NaYF 4 :Ho 3+ @TiO 2 , there were less light emitted from the system, while more irradiated lights were absorbed in. It is reasonable that stronger irradiated light will induce higher intensity of the Figure 5. FTIR spectra of GO, TiO 2 , β-NaYF 4 :Ho 3+ , β-NaYF 4 :Ho 3+ @TiO 2 , and β-NaYF 4 :Ho 3+ @TiO 2 -rGO, respectively. Figure 6. (a) The UV-Vis absorbance spectra of the P25, β-NaYF 4 , β-NaYF 4 :Ho 3+ , β-NaYF 4 :Ho 3+ @TiO, β-NaYF 4 :Ho 3+ @TiO 2 -rGO, GO, and the RGO prepared by hydrothermal method under the same conditions asβ-NaYF 4 :Ho 3+ @TiO 2 -rGO; inset: the enlarged spectra ranging from 400 nm to 500 nm. converted light. Hence, the low emission intensity detected here can be ascribed to the fact that most of the converted lights were absorbed by the composite with the help of rGO.
Ramna spectra. The Raman spectra of NaYF 4 :Ho 3+ , NaYF 4 :Ho 3+ @TiO 2 , and NaYF 4 :Ho 3+ @TiO 2 -rGO composite are presented in Figure. Figure. 6, the I D /I G ratios for NaYF 4 :Ho 3+ @TiO 2 -rGO were1.04, further confirmation theNaY-F 4 :Ho 3+ @TiO 2 -rGO composite further confirmed the formation of rGO sheets 63 . Raman spectroscopy is one of the most common, rapid, non-destructive and high-resolution techniques for characterizing carbon materials 67 . Previous studies have shown that certain Raman peaks change sensitively with the number of GR layers (n) 68 . This discovery that exhibits Raman fingerprints for single-layer, bilayer, and few-layer GR can be used as identification. The position of the G (1582.2 cm −1 ) peak of the Raman spectrum was deduced from previous studies that the number of rGO layers was within six layers 67 .
Photoelectrochemistry measurements. Photoelectrochemical measurements were performed to investigate the excitation, separation, transfer, and recombination of photoinduced charge carriers 69 . Figure 8 shows the characterization results of photocurrents of electrode TiO 2 , β-NaYF 4 :Ho 3+ @TiO 2 , and β-NaYF 4 :Ho 3+ @ TiO 2 -rGO, based on which it can deduce the electron inteaction between rGO and β-NaYF 4 :Ho 3+ @TiO 2 . From the Figure. 8 we can see that steady and transient photocurrents can be obtain during the process of cyclically opening and closing Vis light irradiation. Enjoying a large bandgap, the TiO 2 nanoparticle is of no influence to  Vis light, and nearly no photocurrents were resulted. However regarding composite material β-NaYF 4 :Ho 3+ @ TiO 2 , the UC material within first absorbed Vis light and converts it into UV light, and then excited TiO 2 to produce electron-hole pairs, so that photocurrents were produced. It is worth noting that the photocurrent resulted from electrode β-NaYF 4 :Ho 3+ @TiO 2 -rGO has stronger intensity as compared with photocurrent resulted from β-NaYF 4 :Ho 3+ @TiO 2 . This is owe to rGO's excellent conductivity that led to the effective separation of electron-hole pairs. The effective separation of electron-hole pairs is a key factor for the enhancement of photocatalysis activity of β-NaYF 4 :Ho 3+ @TiO 2 -rGO. The results above are consistent with PL analysis results.
Photocatalytic activity and recyclability. Based on RhB as target degradation product, we researched the photocatalytic activity of β-NaYF 4 :Ho 3+ @TiO 2 -rGO to Vis light. We used 500 Xe lamp coupled with 420 nm cut-off filter as the light source. The time-dependent absorption spectrogram of RhB is shown in Figure. 9a, from which we can see that RhB shows a gradual decreasing absorbance at 554 nm with the extension of time. This indicates β-NaYF 4 :Ho 3+ @TiO 2 -rGO is of sound degradation effect to RhB under Vis light irradiation. Figure 9b shows the variation curve of C/C 0 with the increase of optical radiation time, wherein C 0 and C respectively represent the initial concentration of RhB solution and the concentration of RhB after certain period of irradiation time. Results show that photocatalyst β-NaYF 4 :Ho 3+ @TiO 2 -rGO can reach decomposition rate of RhB as 92% after 10 h of Xe lamp irradiation, which is increased by 25% as compared to the decomposition rate of RhB using β-NaYF 4 :Ho 3+ @TiO 2 after 10 h of Xe lamp irradiation. Theβ-NaYF 4 :Ho 3+ @TiO 2 -rGO exhibited the best photocatalytic efficiency, which was because its larger BET surface area and pore volume ( Figure. 10 and Table 1) are beneficial for theβ-NaYF 4 :Ho 3+ @TiO 2 -rGO composite contacting with organic contaminants, and thus the photocatalytic performance can be enhanced after loading of the rGO sheets. In addition, the rGO facilitated the transport of electrons photogenerated in the TiO 2 , and therefore led to efficient separation of photogenerated carriers in the coupledβ-NaYF 4 :Ho 3+ @TiO 2 -rGO system. Moreover, the photocatalytic degradation of RhB was closely dependent on the Vis light adsorption ability of TiO 2 , and the loading of the rGO shortened the bandgap of TiO 2 . This explains why there was an increased absorptionof Vis light on TiO 2 , and thus why an increased decomposition rate of RhB was resulted in. To eliminate external factors to the degradation effect of phototacatlysis, we conducted a series of comparative tests. We found that the self-decomposition rate of RhB after 10 h of Xe lamp irradiation was only 6%, which indicates the thermal radiation of xenon lamp is of limited influence. By using UV catalyst P25, the decomposition rate of RhB after 10 h of Xe lamp irradiation was only 12%, wherein half of the decomposition rate was ascribed to adsorption degradation. To our surprise, the UC materialβ-NaYF 4 :Ho 3+ reached the decomposition rate of RhB as high as 37% after 10 h of Xe lamp irradiation, which was higher than those of β-NaYF 4 , P25, and β-NaYF 4 @TiO 2 . This is mainly because the non-radiative transition of activator Ho 3+ in UC material resulted in the UV light of short wavelength (290 nm) which is of higher thermal energy and photosensitization effect, so that RhB can be decomposed more effectively 70 . In addition, due to the absence of doping with Ho 3+ , the β-NaYF 4 and β-NaYF 4 @TiO 2 after 10 h of Xe lamp irradiation can achieve decomposition rate of RhB as 7% and 11%, respectively, which were far below the levels after doping with Ho 3+ . This indicates that suitable activator must be selected to induce photon transition before realizing effective UC luminescence.
Photocatalyst should remain unchanged photochemical stability and durability after repeated irradiation, which is of vital significance in practical application. Figure 9c shows the photocatalystic activity of β-NaYF 4 :Ho 3+ @TiO 2 -rGO after four repeated irradiations. From this figure we can see that after four repeated irradiations, the photocatalytic activity of the sample is insignificantly decreased from 92% to 81% and then tends to be stable. This reveals that the composite photocatalyst is of relatively better stability and reproducibility.

Sample
Mean pore size(nm) Pore volume(cm 3 g −1 ) Surface area(m 2 g −1 ) which implies that the recombination of photogenerated electrons and holes is much less in the β-NaYF 4 :Ho 3+ @ TiO 2 -rGO. The results above are consistent with Photoelectrochemical measurements analysis results. A possible reaction process is proposed in Figure. 13, which can be summarized in the equations below. The β-NaYF 4 :Ho 3+ UC absorbs visible light and emits UV light (R1). The detailed process can be described as below. First Ho 3+ jumps from ground state 5 I 8 energy level to excited state 5 F 1 ( 5 I 8 → 5 F 1 ) energy level via Ground State Absorption (GSA) under the excitation of 450 nm light source, meanwhile it jumps via nonradiative cross relaxation back to excited state 5 I 4 energy level ( 5 F 1 → 5 I 4 ), rather than back to excited state 5 I 6 energy level. In the second stage, the Ho 3+ , which locates on excited state 5 I 4 energy level, absorbs the photon of the same energy, and then directly jumps via ESA onto excited state 5 D 4 energy level ( 5 I 4 → 5 D 4 ). Finally, Ho 3+ jumps from highly excited level 5 D 4 back to ground state 5 I 8 ( 5 D 4 → 5 I 8 ), while emitting 288 nm UV light, so that the two-photon UC luminescence mechanism is completed. Through UV excitation, electron-hole pairs are generated on the TiO 2 surface (R2), which is followed by rapid transfer of photogenerated electrons to rGO sheets via percolation mechanism (R4). The excited electrons on the TiO 2 surface react with the absorbed oxygen, resulting in the formation of ·O 2 − or HO 2 ·. After that, it can form hydrogen peroxide (H 2 O 2 ; R5) through combining H + with ·O 2 − or HO 2 ·. H 2 O 2 reacts with the superoxide radical anion (·O 2 − ), and then reduces it into a hydroxyl radical (·OH; R7). The photogenerated holes react with H 2 O, resulting in the formation of hydroxyl radicals (·OH; R3). The conduction band of TiO 2 is located above the RhB redox potential, which allows TiO 2 to be catalytically active. Therefore, these reactive oxygen species (i.e., ·OH, ·O 2 − , and H 2 O 2 ), especially ·OH, can oxidize the organic molecules and perform photocatalysis (R7). The entire sequence is summarized as below:  Figure 11. DMPO spin-trapping ESR spectra of β-NaYF4:Ho 3+ @TiO2-rGO in methanol dispersion for OH (a) and in aqueous dispersion for·O2 − (b); (c) superoxide radical, (d) hydroxyl radicals of TiO2, β-NaYF4:Ho 3+ , and β-NaYF4:Ho 3+ @TiO2-rGO after four min of Vis light irradiation.  Figure 12. 3D fluorescence spectra of NaYF4:Ho 3+ (a), NaYF4:Ho 3+ @TiO2 (b), and NaYF4:Ho 3+ @TiO2-rGO composite (c). Figure 13. Photocatalytic reaction mechanism of β-NaYF4:Ho 3+ @TiO2-rGO.

Conclusions
In summary, we have successfully prepared β-NaYF 4 :Ho 3+ @TiO 2 -rGO composites as an advanced Vis-driven photocatalyst by using a simple hydrothermal method. We demonstrated a new strategy by integrating the Vis-to-UV UC property of β-NaYF 4 :Ho 3+ with the excellent electrical properties of GR to enhance the photocatalytic efficiency of TiO 2 . Photocatalytic properties of the β-NaYF 4 :Ho 3+ @TiO 2 -rGO composites were evaluated by the degradation of an RhB solution. The enhanced photocatalytic activity was associated with the large extended photoresponsive range, great adsorptivity of dyes and high electron-hole separation efficiency due to the synergetic interactions among TiO 2 , GR and UC material. This work is expected to promote practical applications of photocatalysts under solar irradiation in the hope of addressing various environmental issues.

Experimental Section
Materials and reagents. All  Synthesis of NaYF 4 :Ho 3+ @TiO 2 -rGOternarycomposites. The NaYF 4 :Ho 3+ @TiO 2 core-shell microcrystals were synthesized by referring preliminary work 35 . UV-irradiation of theNaYF 4 :Ho 3+ @TiO 2 -graphene oxide samples was performed using a hydrothermal method. Firstly, 2 mg/mL GO (2.5 mL) was ultrasonicated in 100 mL of anhydrous ethanol solution till being well dispersed; after that, 0.1 g of NaYF 4 :Ho 3+ @TiO 2 was added into the above GO solution and then keep vigorous stirring for 1 h. The resulting mixture was transferred into a 100 mL stainless Teflon-lined autoclave filled with deionized water up to 80% of its capacity. The autoclave was tightly sealed and heated at 130 °C for 4 h, after that the system was allowed to cool to room temperature naturally. The precipitate was centrifuged and washed with deionized water for two times and then dried in the vacuum freeze drier at −60 °C for 24 h, before resulting in NaYF 4 :Ho 3+ @TiO 2 -rGO composites.
Photocatalytic activity measurement. The photocatalytic activity of the NaYF 4 :Ho 3+ @TiO 2 -rGO composites was measured via comparing the concentration of rhodamine B (RhB) after irradiation to the original concentration of RhB using a Hitachi U-3010 UV-Vis spectrophotometer (Hitachi Corp., Tokyo, Japan). The percentage of degradation is indicated as C/C 0 , where C is the concentration of RhB at the irradiation time t and C 0 is the concentration at adsorption equilibrium with the photocatalyst before irradiation. Typically, 50 mg of photocatalyst was suspended in 250 mL RhB aqueous solution (5 mg/L) by sonication. Prior to irradiation, the suspension was stirred in the dark for 0.5 h to establish the adsorption-desorption equilibrium. Then the solution was exposed to the irradiation of a500 W Xenon arc lamp with a UV cutoff filter (λ > 400 nm). Every 2 hour, 8 mL of the transparent, aqueous solution was collected and then centrifuged (10,000 r/min) prior to analysis with the Hitachi U-3010 UV-Vis spectrophotometer. As reference experiments, t the photodegradation of RhB with P25 (Degussa P25), with β-NaYF 4 :Ho 3+ , and without any catalyst were tested, respectively. All the experiments were at the same conditions for comparison.
Characterization. The crystal structures of all prepared samples were characterized by X-ray diffraction (XRD) using a Rigaku D/Max2500pc diffractometer with Cu Kα radiation. Scanning electron microscopy (SEM) images were obtained with a Zeiss AURIGA FE microscope (EHT = 5 kV, WD = 8.8 nm; Zeiss, Oberkochen, Germany). An energy-dispersive X-ray analysis (EDS) of the samples was also performed during the SEM measurements. Transmission electron microscopy (TEM) measurements were carried out on a FEI Tecnai G20 operated at an acceleration voltage of 200 kV. The surface chemical environments were analyzed by X-ray photoelectron spectra (XPS) on a PHI5000 VersaProbe system with monochromatic Al Kα X-rays. The composite was applied with Fourier transform infrared spectroscopy analysis (FT-IR, IRPrestige-21, Shimadzu, Japan) using FT-IR spectrophotometer (KBr as the reference sample). UV-Vis diffuse-reflectance spectroscopy (UV-Vis DRS) was performed using the Hitachi U-3010 UV-Vis spectrophotometer. Raman spectra were recorded on an HR Evolution instrument with an Ar + laser source of 488 nm. The Brunauer-Emmett-Teller (BET) surface areas measurements and evaluation of porosity of the samples were conducted on the basis of nitrogen adsorption isotherms measured at 400 °C using a gas adsorption apparatus (Gemini VII 2390, Micromeritics Instrument Corp, Norcross, GA, USA). The sample for electron spin resonance (ESR) measurement was prepared by mixing NaYF 4 :Ho 3+ @TO-rGO samples in a 50 mM DMPO solution tank (aqueous dispersion for DMPO-·OH and methanol dispersion for DMPO-·O 2 − ). Photoelectrochemical properties were evaluated using CHI Electrochemical Workstation (CHI 760E, Shanghai Chenhua, China). All the photoelectrochemical measurements were performed under Vis light of a 300 W Xe lamp coupled with 420 nmcutoff filters. Three-dimensional (3D) fluorescence spectra were obtained with a Hitachi F-7000 fluorescence spectrophotometer with a 150 W Xe lamp as excitation source. All experiments were performed at room temperature.