Composite nanostructures with enhanced physical and chemical properties have gained scientific and technological interest due to their applications in photocatalysis [1-4], optoelectronic devices [5, 6], solar cells [7-10], and drug delivery [11-13]. In the past two decades, much effort has been made to develop techniques for fabricating nanocomposites with well controlled sizes and morphologies. These have included calcination [14], co-precipitation [15-17], epitaxial growth [18-20], self-assembly [21-28], electrospinning [29-31], sol-gel [32] and hydrothermal [33, 34] methods. In particular, electrospinning has been recognized as an efficient, low cost and versatile method for synthesizing composite nanofibers including Au/PVP [35], TiO₂/polyvinyl pyrrolidone (PVP) [36], graphene oxide/PVA [37], Ag/multi-wall nanotubes/polyacrylonitrile (PAN) [38], upconversion nanoparticles (UCNPs)@SiO₂ [32], and SnO₂@TiO₂ [39]. Lanthanide ion-doped upconversion phosphors have been widely used as efficient nanotransducers. They can transform near infrared (NIR) photons to high energy photons with ultraviolet-visible (UV-Vis) wavelengths. This has led to their application in solid state lasers, solar cells, flat-panel displays, bioimaging, chemotherapy, and photodynamic therapy [40-48]. Nanocomposites incorporated with UCNPs including UCNPs/CdSe [49, 50], UCNPs/CdTe [51, 52], UCNPs/CdS [53], UCNPs/TiO₂ [54-61], and UCNPs/reduced graphene oxide [62] have been prepared and used for optimizing solar absorption. We recently demonstrated a facile process to fabricate UCNPs/CdS/TiO₂ nanofibers, which enabled full solar spectrum absorption for enhanced photocatalysis [63]. However, the distance between the UCNP and CdS chromophores was difficult to control, which reduced the energy transfer efficiency. CdS can be excited by photons from the excited state levels including ¹I₆→³F₄, ¹D₂→³F₄, ¹D₂→³H₆, and ¹G₄→³H₆ of Tm³⁺ for NaYF₄:Yb/Tm nanoparticles [53, 63]. The excited state levels (²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2) of Er³⁺ can also excite CdS, according to the proposed energy transfer process in Fig. 1. The fluorescence energy transfer efficiency will also be enhanced if NaYF₄:Yb/Tm or NaYF₄:Yb/Tm@NaYF₄ nanoparticles are replaced by NaYF₄:Yb/Tm@NaYF₄:Yb/Er nanoparticles. The distance between the UCNP and CdS or TiO₂ chromophores can significantly affect the fluorescence energy transfer efficiency [64].

In the current study, we used an electrospinning process to combine NaYF₄:Yb/Tm@NaYF₄:Yb/Er and CdS nanoparticles in close proximity in a SiO₂ matrix. We first prepared UCNPs/ethyl silicate (TEOS)/CdS/PVP nanofibers and UCNPs/SiO₂/CdS/PVP nanofibers by electrospinning, according to our modified protocol [63, 65]. A sol-gel process was then used to coat a layer of TiO₂ on the UCNPs/SiO₂/CdS/PVP nanofibers, and subsequent calcination at 500 ℃ for 2 h yielded TiO₂ composite nanotubes embedded with UCNPs and CdS nanoparticles. NaYF₄:Yb/Tm@NaYF₄:Yb/Er core-shell nanocrystals (i.e. UCNPs) with enhanced fluorescence emission were prepared as chromophores and nanotransducers. Their fluorescence properties and photocatalytic performance are discussed.

All chemicals were of analytical grade and used as received without further purification. CdS nanoparticles with an average diameter of 100 nm were synthesized via a hydrothermal method [66]. β-NaYF₄:Yb(30%), Tm(0.5%)@NaYF₄:Yb(20%), Er(2%) core-shell nanoparticles with an average diameter of 40 nm were prepared using a sequential growth process [67].

The UCNPs/TEOS/CdS/PVP composite nanofibers were fabricated via a modified electrospinning process [63]. In a typical procedure, 0.284 g of PVP was dissolved in 4 mL of absolute ethanol to form a clear solution with vigorous stirring. 0.22 g of as-prepared hydrophilic UCNPs and 0.58 g of CdS nanoparticles were added to the previous solution, which was then ultrasonicated for 10 min. 1.5 mL of TEOS was added and the resulting mixture was stirred vigorously. The mixture was then poured into a 10 mL plastic syringe for electrospinning under a flow rate of 1.1 mL h^-1 and a voltage of 7 kV. The nanofibers acquired from the collector were UCNPs/TEOS/CdS/PVP nanofibers.

Before coating the TiO₂ layer on the surface of the UCNPs/CdS/PVP/TEOS nanofibers, the acquired microfibers were calcined in air at 400 ℃ for 1 h at a heating rate of 1 ℃ min^-1, to prepare UCNPs/SiO₂/CdS/PVP nanofibers. In a typical procedure, 0.1 g of UCNPs/SiO₂/CdS/PVP nanofibers were dispersed in ethanol (20 mL) under vigorous stirring. 200 μL of TBT and 90 μL of ammonia solution (28-30 wt.%) were added to the above solution. The resulting mixture was stirred at room temperature for 4 h. The product was collected by centrifugation, washed with ethanol three times, and then dried at 50 ℃ for 8 h. Finally, the as-prepared product was calcined in air at 400 ℃ for 1 h at a heating rate of 1 ℃ min^-1. This improved the crystallization of the formed titania, and removed the PVP matrix, yielding the UCNPs/SiO₂/CdS/TiO₂ composite nanotubes.

The morphologies of the samples were characterized using field-emission scanning electron microscopy (FESEM) with a SU8020 spectrophotometer (Hitachi, Japan). and transmission electron microscopy (TEM) with a JEM-2100F microscope (JEOL, Japan). The crystal phases of the samples were characterized using an X'Pert PRO MPD X-ray diffractometer (PANalytical B.V., Netherlands), using graphite monochromatized Cu K_α radiation at 40 kV and 40 mA. X-ray photoelectron spectra (XPS) were recorded on an ESCALab 250Xi X-ray photocatalysts-electron spectrometer (Thermo-VG Scientific, Massachusetts, USA). Fluorescence spectroscopy (Edinburgh FLS980, UK) was used to obtain steady state and dynamic fluorescence spectra of the samples. Absorption spectra were recorded with a CARY 5000 spectrophotometer (Agilent Technologies Inc, California, USA). Total organic carbon (TOC) analysis was carried out using an elementar Liqui Ⅱ apparatus (Germany).

Fig. 2a and b show FESEM images of the product obtained from electrospinning 0.284 g of PVP, 0.22 g of hydrophilic UCNPs, 0.58 g of CdS nanoparticles, and 1.5 mL of TEOS. The images show that the product consisted of nanofibers of tens of μm in length and 500 nm in diameter. The nanofibers were decorated or embedded with a high population of nanoparticles, as shown by the TEM images in Fig. 2c and d. The chemical composition of the UCNPs/TEOS/CdS/PVP nanofibers was studied using scanning transmission electron microscopy (STEM) and elemental mapping analysis. Fig. 2e-n show that Si, O, Y, Yb, Er, Tm, Si, Cd, S and Na were detected, confirming their co-existence in the as-obtained nanofibers. The chemical composition was also investigated using energy dispersive X-ray (EDX) analysis, as shown in Fig. 3.

Fig. 4 shows SEM and TEM images of the final sample, obtained after calcination of the as-prepared UCNPs/CdS/PVP/SiO₂/TiO₂ derived from 0.1 g of UCNPs/SiO₂/CdS/PVP nanofibers (Fig. 5) and 200 μL of TBT in 90 μL of ammonia solution. Fig. 5 shows that the UCNPs and CdS nanoparticles were combined in close contact by the SiO₂ nanoparticles derived from the UCNPs/TEOS/CdS/PVP nanofibers. Fig. 4a and b show that the as-prepared TiO₂ composites were of several hundred nanometers in diameter and tens of μm in length. This was in accordance with the diameter of the electrospun UCNPs/TEOS/CdS/PVP nanofibers. The TEM image in Fig. 4c demonstrated that the obtained product was tube-like and had a shell thickness of 20 nm. The high resolution TEM image in Fig. 4d showed that the shell layer consisted of octahedrite phase of TiO₂ nanoparticles.

X-ray diffraction (XRD) was used to study the crystalline phases and chemical compositions. Fig. 6 shows the XRD pattern of the as-prepared tube-like composite nanofibers of TiO₂. The diffraction peaks at 25.44, 38.02 and 48.12° 2θ could be indexed to the anatase phase of TiO₂ (JCPDS No. 21-1272), confirming the crystalline TiO₂ shell. Diffraction peaks of the hexagonal phases of CdS (JCPDS No. 41-1049) [68, 69] and NaYF₄ (JCPDS No. 28-1192) [47, 67] were also clearly observed. These results showed that the as-prepared sample consisted of TiO₂, CdS and UCNPs. STEM spectroscopy and XPS were used to characterize the chemical composition and elemental distribution of the as-prepared UCNPs/SiO₂/CdS/TiO₂ composite nanotubes. Fig. 7 shows elemental mapping images for Yb, Y, O, Si, Ti, Tm, S, Na, F, Er and Cd in the composite nanotubes. The images indicated the co-existence of these elements in the UCNPs/SiO₂/CdS/TiO₂ nanotubes. The elemental composition was also investigated by XPS, as shown in Fig. 8. These results collectively indicated that the UCNPs/SiO₂/CdS/TiO₂ nanotubes had been prepared.

The fluorescence spectrum of the as-obtained UCNPs/SiO₂/CdS/TiO₂ composite nanotubes is shown in Fig. 9a. Compared with the spectrum of the UCNPs, the fluorescence emissions of the ¹I₆→³F₄, ¹D₂→³F₄, D₂→³H₆, and ¹G₄→³H₆ transitions of Tm³⁺ and the ²H_11/2→⁴I_15/2 and ⁴S_3/2→⁴I_15/2 transitions of Er³⁺ for the as-prepared TiO₂ composite nanotubes were greatly quenched under excitation by a 980 nm continuous wave (CW) laser. This indicated the enhanced fluorescence energy transfer efficiency of the tube-like UCNPs/SiO₂/CdS/ TiO₂ nanostructures. Most of the transitions for Tm³⁺ or Er³⁺ were greatly quenched, which was ascribed to the close proximity of the UCNPs and CdS nanoparticles in the SiO₂ matrix. The near infrared (NIR) photon energy could be efficiently transferred to the nearby CdS nanoparticles via irradiative energy transfer (IET) and non-irradiative energy transfer (i.e. Förster resonance energy transfer (FRET)) processes, as shown in Fig. 1 [63]. Fig. 9b shows the UV-Vis absorption spectrum of the UCNPs/SiO₂/CdS/TiO₂ composite nanotubes. The as-prepared composite nanotubes exhibited full spectrum absorption of solar light, in contrast to the single component CdS or TiO₂. This would improve the photocatalytic performance under NIR or simulated solar irradiation.

As discussed previously, the composite nanotubes contained UCNPs, CdS and TiO₂ nanoparticles in close proximity. This resulted in the full spectrum response to solar light of the nanotubes, as shown in Fig. 9b. The CdS nanoparticles embedded in the composites could be excited by NIR light via FRET from the UCNPs, or directly by visible light from the solar spectrum [53, 63]. Scheme 1 shows that excited CdS nanoparticles will produce photo-generated electrons and positive holes. This will result in the formation of reactive oxygen species (ROS) which can decompose organic dyes including Rhodamine B (RhB). Terephthalic acid (TPA) was used as a free radical scavenger, to verify the production of ROS under irradiation by a 1500 mW/cm² Xe lamp with or without a UV-Vis filter [70]. Fig. 10 shows fluorescence spectra of aqueous TPA solution in presence of the as-prepared composites after varying irradiation times. The wide fluorescence peak at 421 nm was attributed to the formation of 2-hydroxy-terephthalic acid (TAOH). This indicated that ^*OH was generated in presence of the as-prepared composites under irradiation by UV-Vis-IR and IR light (the latter obtained by using a UV-Vis filter) [70]. More ^*OH was generated with increasing irradiation times. The as-prepared samples should therefore exhibit photocatalytic degradation capability under excitation by IR light or simulated solar light using a Xe lamp.

Photocatalytic experiments were carried out in a beaker using 50 mL of RhB solution with an initial concentration of 4.7 mg·L^-1, in presence of 20 mg of different photocatalysts. Fig. 11a-c shows that more than 90% of RhB was decomposed in 25 min, during excitation by simulated solar light from a 250 W Xe lamp without a filter. More than 50% of RhB was decomposed in 70 min, during IR irradiation from a Xe lamp with an UV-Vis filter. For comparison, different photocatalysts including UCNPs/SiO₂/CdS/PVP and UCNPs were also subjected to photocatalysis experiments. The results are shown in Fig. 11b and c, and demonstrated that the UCNPs/SiO₂/CdS/TiO₂ composite nanotubes had enhanced photocatalytic performance towards the degradation of RhB. Many studies have reported that ROS can decompose RhB to CO₂ [71, 72]. TOC analysis was also carried out to confirm the degradation of RhB. TOC values of 5.2 and 0.5 mg·L^-1 were obtained in RhB solutions before and after the photocatalysis experiment, respectively. This showed that most RhB was oxidized to CO₂. Three consecutive experiments using recycled photocatalyst showed that the TiO₂ composite nanotubes had excellent chemical ability (Fig. 11d).

A colloidal process and electrospinning were used to fabricate UCNPs/SiO₂/CdS/TiO₂ composite nanotubes, which averaged several micrometers in length and 20 nm in thickness. The fluorescence emission of the UCNPs was greatly quenched upon excitation using a 980 nm CW laser, which demonstrated their fluorescence energy transfer efficiency. The composite TiO₂ nanotubes showed excellent photocatalytic performance towards the degradation of RhB dye. This photocatalyst makes efficient use of the full solar spectrum, and provides a means to overcome energy shortages and environmental protection.