Greatly enhanced photocatalytic activity of semiconductor CeO₂ by integrating with upconversion nanocrystals and graphene

Abstract

CeO₂ is an important semiconductor photocatalyst that has been extensively utilized in photocatalysis. However, its photocatalytic efficiency is still not desirable for practical applications with solar light due to the low light absorption efficiency and high recombination rate of photogenerated electron–hole pairs. Up to now, many approaches have been developed to improve the photocatalytic activity of CeO₂ and much progress has been made. However, the NIR light that accounts for 46% of the total solar light is unutilized for photocatalysis. In the present study, greatly enhanced photocatalytic activity of CeO₂ by integrating upconversion nanocrystals and graphene into CeO₂ was achieved for the first time. A novel nanocomposite photocatalyst of NaLuF₄:Gd,Yb,Tm@SiO₂@CeO₂:Tm/graphene was developed, which took advantage of a synergetic effect to enhance the photocatalytic activity of CeO₂. The upconversion nanocrystals (UCNCs) absorb the NIR light and transfer energy to CeO₂, thus extending the light-responsive range of CeO₂ to the NIR region and making CeO₂ produce highly energetic electron–hole pairs. Tm-doping narrows the band-gap of CeO₂, resulting in a red-shift of the absorption edge for CeO₂. Graphene enhances adsorption of pollutants and serves as an effective electron acceptor and transporter. This work provides new insights into the greatest improvement of catalytic activity of CeO₂ through effective integration of upconversion material, CeO₂, and graphene into a hetero-nanocomposite structure. This strategy can be widely used to fabricate semiconductor-based nanocomposite photocatalysts with high photocatalytic activities and facilitate their applications in environmental protection issues using solar light.

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1. Introduction

CeO₂ is an n-type semiconductor, which has a band gap (3.2 eV) similar to TiO₂. It has some properties like titania such as chemical inertness, low cost, non-toxicity, and stability against photoirradiation.^1–6 Moreover, CeO₂ can absorb a larger fraction of the solar spectrum than TiO₂ (ref. 7 and 8) and has an easy redox nature of Ce⁴⁺/Ce³⁺ couple transformation that may support the charge carrier transfer to the catalyst surface and give rise to oxygen vacancies.^9,10 Owing to its unique properties, CeO₂ is a superior semiconductor photocatalyst.¹¹ To date, CeO₂ has attracted much attention and has been extensively utilized in photocatalysis.^12–19 However, the photocatalytic efficiency of CeO₂ is still not desirable for practical applications due to its low light absorption efficiency and high recombination rate of photogenerated electron–hole pairs.²⁰ So far, many approaches have been developed to improve the photocatalytic activity of CeO₂, such as doping with transition metal ions, coupling with narrow bandgap semiconductors, combining with carbon supporting materials, etc.^21–27 Although much progress has been made to improve the catalytic activity of CeO₂, efficient NIR absorption for photocatalysis is still unreached with previously reported methods and organic pollutants are hard to degrade completely with the catalyst under irradiation of visible light. Therefore, a substantial improvement of photocatalytic activity of CeO₂ and large-scale applications of CeO₂-based photocatalysts to solve environmental and energy problems need further innovational works.

In the present study, we designed a novel CeO₂-based nanocomposite photocatalyst. The composite includes core/shell structure upconversion nanocrystals (UCNCs) of NaLuF₄:Gd,Yb,Tm@SiO₂ and nanoparticles of Tm³⁺-doped CeO₂ dispersing around the UCNCs, which are loaded on the supporter of graphene (GN). Based on the designed structure, the upconversion nanocrystals, Tm-doping, and graphene have a synergistic effect that can improve catalytic activity of CeO₂ tremendously. The NaLuF₄:Gd,Yb,Tm absorbs the NIR light and transfers energy to CeO₂, which extends the light-responsive range of CeO₂ to the NIR region and makes CeO₂ produce highly energetic electron–hole pairs.^28,29 Tm-doping can narrow the band-gap of CeO₂, resulting in a red-shift of the absorption edge for CeO₂. Graphene can enhance adsorption of pollutants and serve as an effective electron acceptor and transporter owing to its two-dimensional π-conjugation structure.^30–33

2. Experimental section

2.1 Materials

Rare earth oxides Lu₂O₃ (99.999%), Gd₂O₃ (99.999%), Yb₂O₃ (99.999%), and Tm₂O₃ (99.999%) were purchased from Shanghai Yuelong New Materials Co. Ltd. Oleic acid (OA) (>90%), 1-octadecene (>90%), (3-aminopropyl)triethoxysilane (APTES) (>98%), titanium(IV) ethoxide (>98%), sodium carbonate, ethanol, octanol, Triton X-100, tetraethoxysilane (TEOS), NH₃·H₂O (28 wt%), Ce(NO)₃, trifluoroacetic acid, sodium citrate, and cyclohexane were supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai). Rhodamine B (RhB) and cerium nitrate hexahydrate (Ce(NO₃)₃·6H₂O) were purchased from Sigma-Aldrich. Ln(CF₃COO)₃ (Ln: Lu, Gd, Yb, Tm) precursors were prepared by dissolving the corresponding metal oxides in trifluoroacetic acid at elevated temperatures.

2.2 Synthesis of NaLuF₄:Gd,Yb,Tm@SiO₂

The core–shell upconversion nanocrystal of NaLuF₄:Gd,Yb,Tm@SiO₂ was prepared by using our previous method.²⁸ To synthesize NaLuF₄:Gd,Yb,Tm, 10 mL of the coordinating oleic acid (90%) ligand and 10 mL of the non-coordinating solvent 1-octadecene (90%) were added to the reaction vessel (solution A). Sodium trifluoroacetate (98%, 2 mmol) and lanthanide trifluoroacetate precursors (1 mmol) were added to the solution containing 5 mL of oleic acid and 5 mL of 1-octadecene (solution B). Both solutions A and B were heated to 120 °C under vacuum with magnetic stirring and kept for 30 min. Then solution A was heated to 310 °C under nitrogen and solution B was added to the reaction vessel dropwise at a rate of 1.5 mL min⁻¹. The mixture was maintained at this temperature and stirred vigorously for 60 min, then cooled down to 80 °C. The formed nanocrystals were precipitated from the solution by addition of excess of ethanol and collected by centrifugation after washing with hexane/ethanol (1 : 1, v/v) three times.

To synthesize NaLuF₄:Gd,Yb,Tm@SiO₂, 6.7 mL of octanol and 12 mL of Triton X-100 were dispersed in 50 mL of cyclohexane with sonication and stirring. Then, 5 mL of cyclohexane suspension containing UCNCs (0.1 mmol mL⁻¹) was added. The mixture was magnetically stirred for 30 min, and then 0.4 mL of (28 wt%) ammonium hydroxide solution was added to obtain a reverse microemulsion solution. After stirring for 1 h, 0.3 mL of TEOS was added and the resulted reaction mixture was aged for 14 h with stirring. The final product of NaLuF₄:Gd,Yb,Tm@SiO₂ was collected by centrifugation and washed several times with water and ethanol.

2.3 Preparation of CeO₂

CeO₂ was prepared by homogeneous precipitation and subsequent calcination process. First, cerium nitrate hexahydrate (Ce(NO₃)₃·6H₂O) was dissolved in 80% ethylene glycol (C₂H₆O₂) and kept under stirring at 50 °C until a homogeneous solution was obtained. Then, 3.0 mol L⁻¹ ammonia solution (NH₄OH) was added. The color of solution changed from purple to yellow. Afterwards, the mixture was aged at 50 °C for 12 h. The purple suspension became turbid and yellow. The suspension was washed with deionized water/ethanol, and then dried in a vacuum oven at 70 °C for 24 h. Finally, the single-phase CeO₂ product was calcined at 500 °C for 1 h.

2.4 Preparation of UCNCs@SiO₂@CeO₂

The NaLuF₄:Gd,Yb,Tm@SiO₂@CeO₂ core–shell nanoparticles were synthesized by using a method similar to single-phase CeO₂ preparation but with a minor modification, which was that the as-prepared NaLuF₄:Gd,Yb,Tm@SiO₂ core–shell particle was added to the mixed solution of cerium nitrate hexahydrate and 80% ethylene glycol solution before ammonia solution (3.0 mol L⁻¹) was introduced into the mixture.

2.5 Preparation of UCNCs@SiO₂@CeO₂:Tm

The as-obtained NaLuF₄:Gd,Yb,Tm@SiO₂@CeO₂ nanoparticle were mixed with 17 mL of deionized water containing 0.17 g of Tm(NO₃)₃ with sonication and magnetic stirring for 30 min. The resulted suspension was transferred to a dry Teflon-lined stainless steel autoclave and heated at 180 °C for 24 h in an electric oven. The final product was collected by centrifugation, washed with deionized water and ethanol three times, and then dried at 60 °C in a vacuum oven overnight.

2.6 Preparation of UCNCs@SiO₂@CeO₂:Tm/GN

The graphene oxide (GO) nanosheets were produced as described previously.²⁴ To synthesize UCNCs@SiO₂@CeO₂:Tm/GN composites, 2.5 mg of GO was dispersed in 10 mL of deionized water with sonication for 2 h to form a stable GO colloidal, and then a specific amount of UCNCs@SiO₂@CeO₂:Tm was added and the mixture was stirred for 10 min. Subsequently, 20 mL of ethylene glycol was added and the mixture was stirred for 30 min. The resulted suspension was transferred to a round bottom flask and heated at 100 °C for 6 h in an oil bath. The final product UCNCs@SiO₂@CeO₂:Tm/GN was collected by centrifugation, washed with deionized water three times, and then dried at 60 °C in a vacuum oven overnight.

2.7 Characterization

The sizes and morphologies of the prepared products were characterized using a JEOL JEM-2010F transmission electron microscope (TEM) operating at 200 kV. The crystal phase structures of the as-prepared samples were examined by powder X-ray diffraction (XRD) measurements performed on a Rigaku D/max-2500 X-ray diffractometer using Cu-Kα radiation. The scan was performed in the 2θ range from 10° to 80° with a scanning rate of 8° min⁻¹. The upconversion luminescence spectra were recorded with an Edinburgh LS-920 fluorescence spectrophotometer using an external 0–2 W adjustable laser (980 nm, Beijing Hi-Tech Optoelectronic Co, China) as the excitation source instead of the xenon lamp source in the spectrophotometer. The UV-Vis absorption spectra were obtained on a Hitachi 3010 UV-Vis spectrophotometer (Hitachi, Japan).

2.8 Photocatalytic experiments

Photocatalysis was performed via monitoring RhB degradation by measuring the variation of optical absorption of RhB with a Hitachi U-3010 spectrophotometer, using SGY-IB multifunction of a photochemical reactor (Nanjing Sidongke Electric Co. Ltd) as a photocatalytic reaction device. In a typical experiment, 20 mg of sample (catalyst) was dispersed into a quartz cuvette containing 50 mL of RhB aqueous solution (20 mg L⁻¹). The suspension was magnetically stirred in the dark for 30 min to attain an adsorption–desorption equilibrium between the dye and catalyst. Then the photoreaction vessels were exposed to simulated solar light irradiation produced by a 500 W Xe lamp (PL-X500D) (wavelength range: 300–2500 nm). At given time intervals, the photoreacted solutions were analyzed by recording variations of the absorption band maximum (554 nm) in the UV-visible spectra of RhB. The degradation ratio η (%) of the dye was calculated by using the following equation:

η (%) = 100 × [(C₀ − C)]/C₀

= 100 × [(A₀ − A)]/A₀

where C₀ and C are the initial and residual concentrations of RhB in the solution, respectively, and A₀ and A are the absorbance of RhB at 554 nm before and after exposure under simulated solar light, respectively.

3. Results and discussion

3.1 Phase and morphology

Fig. 1 shows the X-ray diffraction (XRD) patterns of the prepared products. As shown in Fig. 1, both prepared CeO₂ and UCNCs@SiO₂@CeO₂:Tm/GN showed characteristic diffraction peaks of cubic fluorite structure CeO₂, which were in good agreement with the standard data (JCPDS no. 34-0394).¹⁸ The peaks originate from (111), (200), (220), (311), (400), and (331) crystal planes, respectively.^18,32 UCNCs@SiO₂@CeO₂:Tm/GN showed a typical hexagonal phase of NaLuF₄ (JCPDS no. 27-0276) and a typical cubic fluorite phase of CeO₂,³⁴ confirming the formation of the composite catalysts. In addition, UCNCs@SiO₂@CeO₂:Tm/GN showed a broad and weak diffraction peak at 2θ of about 24° that was ascribed to graphene.³⁵

Fig. 1 XRD patterns of CeO₂ (a), UCNCs (b), and UCNCs@SiO₂@CeO₂:Tm/GN (c).

TEM images of the prepared products are shown in Fig. 2. As shown in Fig. 2a, NaLuF₄:Gd,Yb,Tm nanocrystals have a hexagonal and plate-like structure with a diameter of about 45 nm, which were well-dispersed and uniformed in morphology and size. The TEM image of UCNCs@SiO₂ (Fig. 2b and c) showed that a light and smooth layer of SiO₂ with a thickness of about 24 nm was coated on the surface of the UCNCs. Fig. 2c showed that a large amount of tiny nanoparticles (<10 nm) of CeO₂ were formed and dispersed around the UCNCs, where the size and morphology of the UCNCs remained unchanged. These small-sized CeO₂ nanoparticles dispersed around the surface of the UCNCs can result in a high specific surface area that enhances the contact with contaminants for efficient photocatalytic degradation. Fig. 2d showed that the nanoparticles of UCNCs@SiO₂@CeO₂:Tm were homogenously loaded on the GN sheets surface. The HRTEM images of NaLuF₄:Gd,Yb,Tm (Fig. 2a, inset) and CeO₂ (Fig. 2c, inset) revealed highly crystalline nature of the as-prepared products.^18,32,34 The interplanar distances between adjacent lattice fringes corresponded to the crystal planes of the nanocrystals, well consistent with the results obtained with XRD. The composition of NaLuF₄:Gd,Yb,Tm@SiO₂@CeO₂:Tm was characterized by EDX analysis. As shown in Fig. 2e, all of the elements, including Na, Lu, F, Gd, Yb, Tm, Ce, Si, and O, were detected, further confirming the formation of the nanocomposite photocatalyst.

Fig. 2 The TEM images of UCNCs (a), UCNCs@SiO₂ (b), UCNCs@SiO₂@CeO₂:Tm (c), and UCNCs@SiO₂@CeO₂:Tm/GN (d). The HRTEM images of UCNCs (a, inset) and CeO₂ (c, inset). The EDX spectra of UCNCs@SiO₂@CeO₂:Tm (e).

3.2 UV-Vis absorption spectroscopy

UV-Vis-NIR absorption spectra of the prepared products are shown in Fig. 3. The light absorption edges before 400 nm for CeO₂ and composites corresponded to the band gap absorptions of CeO₂.²⁰ Based on the spectra, it could be speculated that the UV-visible photos generated by the upconversion process of UCNCs could be absorbed by CeO₂ via energy transfer. In comparison to CeO₂, there was an obvious red shift of the absorption edge and stronger absorption in the visible region for the composites. This could be mainly assigned to the narrowing of the band-gap of CeO₂ caused by the Tm-doping. Tm-doping led to a charge-transfer transition between Tm³⁺ and the CeO₂ conduction or valence band, resulting in a red shift of the absorption edge of CeO₂. It has been reported that metal doping can form a dopant energy level within the band gap of CeO₂, and the electronic transitions from the valence band to the dopant level or from the dopant level to the conduction band can effectively cause a red shift of the band edge absorption threshold.^23,36–38 It was worth noting that UCNCs@SiO₂@CeO₂:Tm/GN presented stronger absorption in the visible region in comparison to UCNCs@SiO₂@CeO₂:Tm. This could be attributed to the presence of GN nanosheets in the composite photocatalyst.^39–41

Fig. 3 The UV-Vis absorption spectra of the prepared samples.

3.3 Upconversion luminescence

The upconversion luminescence properties of the as-prepared products were investigated. The UCL intensities of the samples were measured at the same concentrations of 1.0 mg mL⁻¹ in cyclohexane. As shown in Fig. 4, each of them showed typical characteristic UCL of Tm³⁺ under CW excitation at 980 nm. The UV emission peaks centered at 361 nm was attributed to the transition of Tm³⁺ ion: ¹D₂ → ³H₆.^29,42 The visible emission peaks at 450, 476, 645, and 697 nm were assigned to ¹D₂ → ³H₄, ¹G₄ → ³H₆, ¹G₄ → ³F₄, and ³F₃ → ³H₆ transitions of Tm³⁺ ion, respectively.^42,43 The luminescence intensity of NaLuF₄:Gd,Yb,Tm@SiO₂ decreased to some extent in comparison to that of pure NaLuF₄:Gd,Yb,Tm due to the light scattering effects by the silica layer.⁴⁴ However, this SiO₂ layer could prevent the electron trapping caused by surface defects and ligands of bare NaLuF₄:Gd,Yb,Tm,⁴⁵ and prevent the upconversion nanoparticles from photocatalysis-induced corrosion, thus prolonging its lifetime.⁴⁶ After further CeO₂ coating, the luminescence intensity decreased significantly in comparison to NaLuF₄:Gd,Yb,Tm@SiO₂. The light emitted by UCNCs in the UV and visible regions nearly disappeared, which was attributed to absorbance of the CeO₂ shell caused by an energy transfer process between UCNCs and CeO₂ according to the Förster resonance energy transfer (FRET) mechanism. These results were similar to those obtained in our previous studies on composite photocatalysts of NaLuF₄:Gd,Yb,Tm@SiO₂@TiO₂:Mo and NaLuF₄:Gd,Yb,Tm@SiO₂@Ag@TiO₂.^34,47 The light emitted by UCNCs@SiO₂@CeO₂:Tm/GN in the visible regions was weaker than that in the absence of GN. This may be caused by absorbance of GN.^39–41

Fig. 4 The upconversion luminescence spectra of the prepared products under 980 nm excitation.

3.4 Photocatalysis

Graphene, which consists of a well-defined two-dimensional honeycomb-like network of carbon atoms, has been emerging as a fascinating material due to its superior electronic conductivity, remarkable structural flexibility, prominent thermal stability, and high specific surface area.^{31,34,48–51} The combination of these intriguing properties of graphene with distinctive characteristics of other functional nanomaterials has become a popular pathway for achieving specific applications in various fields.^39,40 Works of ours and others have demonstrated that combination of semiconductor catalyst with graphene could improve the photocatalytic activity of catalyst remarkably.^39–41 In the present study, we coupled the nanocomposite catalyst UCNCs@SiO₂@CeO₂:Tm with graphene to further enhance the catalytic activity of the product.

RhB was used as a model pollutant to investigate the photocatalytic property of composite UCNCs@SiO₂@CeO₂:Tm/GN. A 500 W xenon lamp (wavelength range at 300–2500 nm) was used to simulate the solar light. Upon irradiation for designated periods, 1 mL of RhB aqueous solution was withdrawn and diluted to 4 mL for absorbance measurement. Fig. 5 showed the absorbance spectra of RhB under simulated solar light irradiation as a function of irradiation time with the composite as the photocatalyst. It was clear that the absorption intensity of RhB at 554 nm decreased gradually with the increase in irradiation time, indicating degradation of RhB upon the simulated solar light irradiation.

Fig. 5 The absorbance spectra of RhB catalyzed by the UCNCs@SiO₂@CeO₂:Tm/GN nanocomposite at different irradiation time points under simulated solar light irradiation.

The photocatalytic efficiency of the composite could be evaluated through calculating the time-depended degradation ratio of RhB by using CeO₂, CeO₂:Tm, and UCNCs@SiO₂@CeO₂:Tm as controls. Fig. 6 showed the time profile of C/C₀ under simulated solar light irradiation, where C and C₀ are the concentrations of RhB at the indicated time point and before irradiation (after the adsorption equilibrium with the photocatalysts were reached), respectively. It could be seen from Fig. 6, the photocatalytic efficiency of UCNCs@SiO₂@CeO₂:Tm/GN composite was obviously higher than those of controls. RhB almost completely degraded (95%) in the presence of the catalyst after being exposed under Xe lamp for 210 min, whereas the degradation ratios obtained with CeO₂, CeO₂:Tm, and UCNCs@SiO₂@CeO₂:Tm were only 46%, 55%, and 82%, respectively. The reaction kinetics for RhB photodegradation was fitted with a Langmuir–Hinshelwood kinetic model.³⁸ As shown in Fig. 7, the first-order rate equation, −ln(C/C₀) = kt, showed a good fit to the data, where C/C₀ is the value given by Fig. 6 and k is the apparent rate constant. According to the plots, the calculated apparent rate constants were 1.12 × 10⁻², 8.17 × 10⁻³, 3.80 × 10⁻³, and 2.93 × 10⁻³ min⁻¹ for UCNCs@SiO₂@CeO₂:Tm/GN, UCNCs@SiO₂@CeO₂:Tm, CeO₂:Tm, and CeO₂, respectively. Usually, a high apparent rate constant indicates a high catalytic reaction rate and high catalytic activity of the catalyst.

Fig. 6 Photocatalytic degradation of RhB under simulated solar light irradiation.

Fig. 7 Kinetics of RhB degradation under simulated solar light irradiation.

The photocatalytic activity of UCNCs@SiO₂@CeO₂:Tm was higher than that of CeO₂:Tm, which could be attributed to the function of UCNCs. The UCNCs could absorb the NIR light and convert it into UV and visible light, which could be absorbed by CeO₂. By utilizing the UCNCs, NIR irradiation could be absorbed and harvested indirectly by CeO₂ to broaden the absorption region and consequently enhance the photocatalytic activity of CeO₂.⁵² Furthermore, when the highly energetic UV photons created by UCNCs are absorbed by CeO₂, they can drive CeO₂ to generate highly energetic electron and hole pairs.³⁴ These highly energetic electron and hole pairs have strong oxidation–reduction abilities, leading to highly catalytic activity.³⁸ The photocatalytic activities of UCNCs@SiO₂@CeO₂:Tm and CeO₂:Tm were higher than those of UCNCs@SiO₂@CeO₂ and CeO₂, respectively, suggesting that Tm-doping could increase the photocatalytic activity. The introduction of Tm³⁺ into CeO₂ can effectively extend the spectral response from UV to visible area due to the newly-appeared dopant energy band that narrows the band gap of CeO₂.⁵³ Moreover, doping trivalent rare-earth ions in CeO₂ can create oxygen vacancies,³⁴ which increase the density of oxygen vacancies at the surface of the photocatalyst. These oxygen vacancies act as electron traps, preventing the electron–hole recombination.³⁴ Additionally, the photocatalytic activity of UCNCs@SiO₂@CeO₂:Tm/GN was higher than that of UCNCs@SiO₂@CeO₂:Tm, which could be attributed to the function of GN. GN plays two important roles for the photocatalysis. One is to serve as an effective electron acceptor and transporter owing to its two-dimensional π-conjugation structure,⁵¹ which promote the separation of charge carriers and transportation of photogenerated electrons. This is responsible for the enhancement of photocatalytic performance of the catalyst. The other is to increase the specific surface area owing to its giant π-conjugation system and two-dimensional planar structure,⁴¹ which enhances the adsorption for pollutants. Furthermore, the interfacial interaction between semiconductor SeO₂ and graphene causes a small band gap of SeO₂/RN and formation of a heterojunction.³³ The small band gap leads to the absorption in the entire visible spectrum, and even in the infrared region, and thus enhances photocatalytic activity. The heterojunction, charge redistribution takes place for in the whole graphene and several atomic layers in CeO₂, besides the interface due to a strong donor–acceptor interaction, leading to a large amount of charge transferred.³³ In addition, GN has another function that is to prevent the nanoparticles agglomeration.⁴¹

To investigate the durability of the photocatalytic activity of the product UCNCs@SiO₂@CeO₂:Tm/GN under simulated solar irradiation, we repeated the same photocatalytic degradation test for four cycles. As shown in Fig. 8, both the simulated solar and NIR-driven photocatalytic activities of the product changed little, suggesting that the product was very stable.

Fig. 8 Repeated photocatalytic activity measurements with the presence of catalysts under the irradiations of simulated solar light.

3.5 Mechanism

Fig. 9 showed the overall processes for photocatalysis of the composite catalyst under solar light irradiation, including the formation of the excited state of Tm³⁺, activation of CeO₂ via energy transfer, the charge migration between the semiconductor and the generation of free radicals. As its intrinsic property, the outer CeO₂ layer can quickly absorb UV from solar light. The high penetrability of NIR light from solar light allows it to enter into the inner to excite the upconversion nanocrystals NaLuF₄:Gd,Yb,Tm, emitting UV and visible light. Then, the upconverted UV light will pass through the transparent SiO₂ layer to excite the outer CeO₂ nanocrystals via an energy transfer process (FRET). After absorbing solar and upconverted light, the excited CeO₂ generates electron–hole pairs. These electron–hole pairs then migrate from the inner region to the surfaces and act as catalytic centres. Meanwhile, as the nanoparticles of UCUCs@SiO₂@CeO₂:Tm are deposited on the graphene sheet surface, the photogenerated electrons easily move to the graphene sheet. This makes the charge separation more efficient and reduces the probability of recombination. These electrons and holes can not only directly decompose pollutants, but also degrade pollutants indirectly through ˙OH and ˙O₂⁻ radicals that are produced from hole-oxidizing H₂O and electron-reducing O₂ molecules, respectively.³⁸ As shown in Fig. 9, the excited electrons arriving on the surface react with the oxygen adsorbed on the surface of CeO₂ to form O²⁻ or O₂²⁻, which combines with H⁺ to form hydrogen peroxide (H₂O₂). H₂O₂ reacts with superoxide radical anion (˙O₂⁻) to generate hydroxyl radical (˙OH). These reactive oxygen species (˙OH, ˙O₂⁻ and H₂O₂), especially ˙OH and ˙O₂⁻, could directly take part in the photocatalytic degradation reaction to degrade RhB. The processes are presented as following equations.

TiO₂ + ħν → e⁻ + h⁺

h⁺ + H₂O₂ → H⁺ + ˙OH

e⁻ + H⁺ + O₂ → H₂O₂

e⁻ + O₂ → ˙O₂⁻

˙O₂⁻ + H₂O₂ → ˙OH + OH⁻ + O₂

Fig. 9 Diagrams of the energy levels of Yb³⁺–Tm³⁺ and the upconversion luminescence process. The energy transfer from the excited Tm³⁺ to CeO₂ through FRET. The main photocatalytic processes.

4. Conclusion

In summary, a novel composite photocatalyst NaLuF₄:Gd,Yb,Tm@SiO₂@CeO₂:Tm/GN was fabricated for the first time. Its morphology, crystalline phase, composition, and spectra were characterized and its photocatalytic properties were investigated. The significantly enhanced photocatalytic activity of the as-prepared nanocomposite in comparison with bare CeO₂ was demonstrated by degradation of RhB under simulated sunlight irradiation, which could be attributed to the extended-light responsive range to the NIR region, the red-shifted absorption edge, improved separation of electron–hole pairs for CeO₂, and enhanced adsorption of pollutants due to the presence of UCNCs, graphene, and Tm-doping. This study presented a new strategy to effectively enhance the photocatalytic activity of CeO₂ under solar light irradiation by artificially integrating CeO₂, UCNCs, and graphene into a hetero-composite nanostructure. It was expected that this convenient assembly approach could be widely utilized to prepare semiconductor-based nanocomposite photocatalysts with highly photocatalytic activity for practical applications in environmental cleaning.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

The authors gratefully acknowledge the National Natural Science Foundation of China (No. 21271126, 11375111, 11428410), Program for Innovative Research Team in University (No. IRT 13078), and National 973 Program (No. 2010CB933901).

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