Hybrid Cu_xO–TiO₂ porous hollow nanospheres: preparation, characterization and photocatalytic properties

Abstract

A series of Cu_xO–TiO₂ hollow nanospherical composites with different mass percents of copper were prepared by an impregnation method, and the TiO₂ hollow nanospheres were prepared by hydrolysis of tetraethoxy titanium on the surface of spherical SiO₂ nanoparticles and corrosion of SiO₂ in NaOH solution. Cu_xO–TiO₂ hollow nanospheres displayed higher photocatalytic activities towards acetaldehyde degradation to CO₂ than TiO₂@SiO₂ nanospheres and TiO₂ hollow nanospheres under UV and visible-light irradiation due to the narrowed band gap of TiO₂ and enhanced charge separation after copper was introduced. In particular, the Cu_xO–TiO₂ hollow nanospheres show high catalytic activities in degradation under visible-light irradiation.

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1. Introduction

In the past decades, titanium dioxide (TiO₂) has attracted increasing attention due to its extensive application such as in sensor technology,^1,2 pigments,³ solar cells,⁴ self-cleaning coatings⁵ and photocatalysis.^6,7 Particularly, study on the photocatalytic properties of TiO₂ is the most common because of its several advantages including low cost, innocuity, chemical stability and high refractive index.^8–11 However, there are some disadvantages in using TiO₂, such as small surface area, phase transition, crystal growth and low quantum efficiency, which limit the photocatalytic efficiency of the catalysts.^12–14

In order to overcome these problems, many investigations have been reported.^15–17 Morphology control and preparation of titania-based materials are two very effectual strategies.¹⁸ One of the most efficient ways is to coat TiO₂ in form of a layer on surface of core materials with high surface area, such as ZrO₂, MoO₃, Fe₂O₃ and SiO₂.^19,20 Silica is one of the best core materials for its well-known surface chemistry, absorption capacity, controllable preparation and it can be easily eliminated.²¹ Another is to synthesize TiO₂ with different morphologies and microstructures to enhance the surface area of the semiconductor. Many new morphologies of TiO₂ were synthesized in the past years, such as nanotubes, nanorods, and hollow spheres, etc.²² The synthesis of TiO₂ hollow nanosphere is especially concerned for its large surface area, low density, and high light-harvesting efficiency.^23,24

Volatile organic compound is the sources of indoor air pollution, and it is always the central issues in the environmental protection field. Among various photocatalytic oxidations over semiconductors, TiO₂ photocatalyst possesses the function of air purification. Despite of good photocatalytic activities, TiO₂ is only effective to degrade some photosensitive organic compounds under ultraviolet (UV) irradiation with strong intensity. However, the intensity of UV-light from common filament lamp is below several μW cm⁻³, which is one thousand times low to the sunlight.²⁵ So the catalyst cannot reach the desired effect to get rid of the indoor contaminated gases. Therefore, development of visible-light-sensitive photocatalysts based on TiO₂ is necessary.

Another, due to high prices and scarce of the noble metal, attention has been given to search an alternative catalytic component to replace the noble metal. Such as, copper or copper oxide is better substitute component because of their low cost and wide use. In 2009, TiO₂ powder grafted by Cu⁺ ions was found to be sensitive to visible light by Irie and his co-workers,²⁶ which lighted up the hope to TiO₂ on its applications in daily life. They studied the photocatalytic activity of Cu/TiO₂ under visible light irradiation. Huang et al. prepared the CuO/TiO₂ by the deposition–precipitation (DP) and investigated its catalytic behavior.²⁷ Also, Gnanasekaran et al. did many research to expand the absorption of TiO₂ to the visible light range for improving the photocatalytic efficiency.^28,29 Although photocatalytic efficiency of TiO₂ is improved markedly, catalytic efficiency of the catalyst is still unsatisfying. Based on these works, we determine to use high specific surface area porous TiO₂ hollow spheres as the carrier of loading copper, and study their catalytic activities by degradation of acetaldehyde gas.

In this paper, TiO₂@SiO₂ core–shell nanosphere was prepared by using silica gel nanosphere as hard template, and TiO₂ hollow nanosphere was obtained after SiO₂ was etched by NaOH solution with a certain concentration. Cu²⁺ ions were adsorbed and Cu_xO was formed on the surface of the TiO₂ hollow nanosphere to enhance separation of the photogenerated charge carriers. Four kinds of photocatalysts, TiO₂@SiO₂ nanosphere, TiO₂ hollow nanosphere, Cu_xO–TiO₂@SiO₂ nanosphere and Cu_xO–TiO₂ hollow nanosphere, were synthesized. The catalytic activities of these catalysts were studied by degradation of acetaldehyde under UV and visible-light irradiations. The results indicate that all catalysts have good catalytic activities in degradation of acetaldehyde under UV-light irradiation, and the Cu_xO–TiO₂ hollow nanosphere shows high catalytic activities in the degradation under visible-light irradiation. As shown in Scheme 1, the visible-light is considered to initiate interfacial charge transfer. If the photon energy of light is greater than the semiconductor band gap, the electron (e⁻) on the valence band will be excited to the conduction band and the hole (h⁺) will be generated at the valence band. When Cu_xO–TiO₂ is used as photocatalyst, electrons in the valence band (VB) of TiO₂ are stimulated and directly transferred to Cu(II) to form Cu(I), as well as holes (h⁺) in the VB of TiO₂. Also the adsorbed O₂ on the surface can capture the electrons in VB of TiO₂, but it is harder than that of Cu(II). Thus, the energy to separate electrons and holes becomes low, which narrows the band gap of TiO₂ and prolongs the absorption edge from UV-light to visible-light region. The holes produced in VB of TiO₂ decompose organic substances.

Scheme 1 Possible mechanism on degradation of acetaldehyde under the existence of Cu_xO@TiO₂ catalysts.

2. Experimental

2.1. Chemicals and instruments

All chemicals used are of analytical reagent grade available from a commercial supplier without further purification. Tetraethoxysilane (TEOS, ≥99%) and copper chloride dihydrate (CuCl₂·2H₂O, AR) were purchased from Sinopharm Chemical Reagent Company (in Shanghai, PR China). Tetraethoxy titanium (C₈H₂₀O₄Ti, ≥98%) was purchased from Aladdin Industrial Corporation (in Shanghai, PR China). L-Lysine (BR) was purchased from Beijing Biological Technology Company (in Beijing, PR China). Hydrochloric acid (38 wt%), acetic acid (38 wt%), sodium hydroxide (AR) and ethanol (AR, 99%) were all purchased from Beijing Chemical Reagent Company (in Beijing, PR China).

Fourier transform infrared (FT-IR) spectra of the samples were recorded in range of 4000–400 cm⁻¹ with 2 cm⁻¹ resolution on a Bruker Vector-22 Fourier transform spectrometer (made in Germany). Electronic micrographs (including energy-dispersive X-ray spectroscopy, EDS) of the samples were observed using a S-4700 scanning electron microscope (SEM, made in Japan) operated at 15 kV, a J-2010 high resolution transmission electron microscope (HRTEM, made in Japan) and a Tecnai G² 20S-TWIN transmission electron microscope (TEM, made in USA) operated at 200 kV, respectively. Crystal structures of samples were determined by powder X-ray diffraction (PXRD), using a Rigaku D/MAX diffractometer (made in Japan) with Cu Kα radiation (λ = 0.15406 nm, scanning speed = 10° min⁻¹). N₂ sorption isotherms of the samples were recorded on a Quantachrome NOVA 2000e sorption analyzer (made in USA) at the temperature of liquid nitrogen (77 K). Samples were degassed at 200 °C overnight prior to the measurement. Surface areas and pore size distributions of the samples were obtained by Brunauer–Emmett–Teller (BET) method and calculated using Barrett–Joyner–Halenda (BJH) model, respectively. UV-Vis absorption of the samples were obtained by Perkin Elmer Lambda 900 UV-Vis spectrophotometer (made in Germany) in range of 220–800 cm⁻¹. X-ray photoelectron spectroscopic (XPS) analysis was measured on Shimadzu ESCA-250 and ESCA-1000 spectrometers (made in Japan) with Mg K_α X-ray sources.

STEM analysis was conducted on a JEOL JEM 2200FS equipped with a CEOS aberration corrector (CEOS GmbH, Heidelberg, Ger). Images and EDS maps were acquired in analytical mode (AMAG), with a probe size of 0.8 nm and a nominal beam current of 450 pA. The EDS signal was collected with a Bruker X-Flash silicon-drift detector and was processed using the Bruker Esprit software. EDS maps were collected for >10 min. The final resolution of the EDS maps was estimated to be ∼0.8 nm.

2.2. Synthesis of TiO₂@SiO₂ nanosphere

Silica nanosphere was used as template to synthesize TiO₂ hollow nanosphere, and its synthesis was according to the method in ref. 30.

TiO₂@SiO₂ with core–shell structure was composited via sol–gel method. Primarily, the as-prepared SiO₂ nanosphere with diameter about 25 nm was dispersed in an ethanol solution (ethanol in water) and oscillated in ultrasonic oscillator for several minutes. A solution of tetraethoxy titanium in ethanol was then added dropwise to 35 mL of the above dispersion system with continuous stirring. And a mixture solution of 35 mL of absolute ethanol, 2.0 mL acetic acid and two drops of hydrochloric acid was added to the mixture rapidly. After stirred for 30 min, the mixture was heated at 40 °C and stirred vigorously for 1.0 h. The final product TiO₂@SiO₂ nanosphere was obtained after the mixture was filtered, washed with water and ethanol for several times, dried at 70 °C for about 4.0 h and calcined at 550 °C for 4.0 h.

2.3. Synthesis of TiO₂ hollow nanosphere

The silica template was removed by immersing TiO₂@SiO₂ nanosphere in a NaOH solution (3.0 M) at 60 °C for 8.0 h under continuous stirring, and the hollow nanosphere was obtained after the mixture was centrifuged, washed with ethanol and water for several times and dried at 60 °C for 4.0 h.

2.4. Synthesis of Cu_xO–TiO₂ hollow nanosphere and Cu_xO–Ti–TiO₂@SiO₂ nanosphere

The Cu_xO–TiO₂ hollow nanosphere and Cu_xO–TiO₂@SiO₂ nanosphere were prepared by an impregnation method using CuCl₂·2H₂O as source of Cu²⁺ ions. 1.0 g of TiO₂ hollow nanosphere (or TiO₂@SiO₂ nanosphere) was added into 10 mL distilled water in a vial reactor. Different amounts of CuCl₂·2H₂O (mass percents of Cu²⁺ relative to TiO₂ are 0.25%, 0.50% and 1.0%, respectively) were added to the above reactor, and the system was heated at 90 °C for 1.0 h under continuous stirring. The products were obtained by centrifuged, washed with water for several times and dried at 110 °C overnight. The products are named as 0.25%-Cu_xO–TiO₂ hollow nanosphere, 0.5%-Cu_xO–TiO₂ hollow nanosphere and 1.0%-Cu_xO–TiO₂ hollow nanosphere, respectively; or 0.25%-Cu_xO–TiO₂@SiO₂ nanosphere, 0.5%-Cu_xO–TiO₂@SiO₂ nanosphere and 1.0%-Cu_xO–TiO₂@SiO₂ nanosphere, respectively.

2.5. Measurement of photocatalytic activities

Photocatalytic activities of the synthesized materials were evaluated by degradation of acetaldehyde in gas phase under ultraviolet and visible-light irradiations. 0.05 g microsphere powder was uniformly spread in a sample holder, which was placed on the bottom of a 500 mL cylinder-type Pyrex glass vessel. The glass vessel was flushed with O₂ (20%)/N₂ gas to remove carbon dioxide from the system, and the relative humidity of atmosphere inside the vessel was controlled to 45% by passing the O₂/N₂ gas through chilled water formerly. 5.2 mL acetaldehyde was introduced into the reaction vessel using a Pressure-Lock syringe to reach a concentration of 200 ppmv. After kept for 15 min in dark environment, the glass vessel was irradiated from top by a 150 W xenon lamp (Hayashi UV410) which emits light of wavelength range of 350–400 nm at an intensity of 3.0 mW cm⁻². The degradation of acetaldehyde and the generation of carbon dioxide were monitored using a gas chromatograph (SP-2100A, BFRL Co.), equipped with a 2 m Porapak-Q column and a flame ionization detector.

The same method was used in visible-light irradiation (420 nm in wavelength and 119 mW cm⁻² in intensity) to the degradation of acetaldehyde to check the photocatalytic activities of the samples.

3. Results and discussion

3.1. Characterizations of the catalysts synthesized

SEM images of silica nanosphere (used as template) and TiO₂@SiO₂ nanosphere, TEM image of TiO₂ hollow nanosphere and HRTEM image of Cu_xO–TiO₂ hollow nanosphere are shown in Fig. 1. It is found that the silica has regular ball-shape morphology with an average diameter about 25 nm (Fig. 1A). The average diameter of the microsphere increases to about 35 nm after TiO₂ framework formed on the surface (Fig. 1B). After immersed in NaOH solution (3.0 M) at 60 °C for 8.0 h, Si–O structure in TiO₂@SiO₂ nanosphere was damaged and the shell of TiO₂ remained. TEM image (Fig. 1C) indicates that final product has regular microspherical morphology with hollow structure, and its average diameter is about 35 nm. The shell thickness of these hollow microspheres is about several nanometers. HRTEM image (Fig. 1D) shows that there are Cu_xO clusters attached on the highly crystallized TiO₂ surfaces and formed clear lattice fringes. Particle size distributions of unmodified SiO₂ nanosphere, TiO₂@SiO₂ nanosphere and TiO₂ hollow nanosphere are also shown in Fig. 1.

Fig. 1 SEM images and particle size distributions of unmodified SiO₂ (A) nanosphere and TiO₂@SiO₂ nanosphere (B); TEM image and particle size distribution of TiO₂ hollow nanosphere (C) and HRTEM image of Cu_xO–TiO₂ hollow nanosphere (D).

The FT-IR spectra of SiO₂ nanosphere, TiO₂@SiO₂ nanosphere and TiO₂ hollow nanosphere are shown in Fig. 2. All the samples have stretching and bending vibrations absorptions of hydroxyl groups from surface of the samples and the adsorbed water, which are at 3400 and 1600 cm⁻¹ respectively.³¹ The antisymmetric stretching vibration absorption of Si–O–Si in SiO₂ (Fig. 2a) and TiO₂@SiO₂ nanospheres (Fig. 2b) are found at 1100 cm⁻¹, but it cannot be observed in Fig. 2c. Disappearance of Si–O–Si absorption in Fig. 2c indicates that there isn't any SiO₂ in the TiO₂ hollow nanosphere. Vibration absorption of Ti–O–Si in TiO₂@SiO₂ (Fig. 2b) is found at 960 cm⁻¹,³² indicating that the framework of (Ti–O)_n was connected on the surface of silica gel after tetraethoxy titanium was adsorbed and hydrolyzed. At the same time, the absorptions of Ti–O in TiO₂@SiO₂ and TiO₂ hollow nanospheres are found at about 520 cm⁻¹.

Fig. 2 FT-IR spectra of SiO₂ nanosphere (a), TiO₂@SiO₂ nanosphere (b) and TiO₂ hollow nanosphere (c).

EDS spectrum of TiO₂@SiO₂ nanosphere (Fig. 3A) shows that the nanosphere is consisted by Si (20.49%), Ti (7.86%) and O (71.65%) elements, proving the existence of both SiO₂ and TiO₂ frameworks in the sample. Calculation indicates that the proportion of TiO₂ and SiO₂ in TiO₂@SiO₂ nanosphere is about 1 : 3. However, only Ti and O elements are found in Fig. 3B, and the proportion of Ti and O is about 1 : 2, which confirms the removal of template and generation of TiO₂ hollow nanosphere.

Fig. 3 EDS spectra of TiO₂@SiO₂ nanosphere (A) and TiO₂ hollow nanosphere (B).

The crystal phases of TiO₂ largely determines their suitabilities for photocatalytic activities.^33,34 The powder X-ray diffraction (PXRD) patterns of TiO₂@SiO₂ nanosphere and TiO₂ hollow nanospheres deposited by different concentrations of Cu²⁺ ions are shown in Fig. 4. Six characteristic peaks of anatase TiO₂ are observed at 25.2° (101), 37.8° (004), 48.1° (200), 53.9° (105), 56.1° (211) and 62.7° (204) in Cu_xO–TiO₂@SiO₂ nanospheres (Fig. 4A), which are indexed to diffractions of TiO₂ (ref. 35) except for the diffractions of SiO₂ at 21.79°. However, when the Cu content in the nanocomposite, diffraction peaks of two weak diffractions of Cu_xO (CuO and Cu₂O) were observed in 27.5° and 42.3°.³⁶ As to Cu_xO–TiO₂ hollow nanosphere (Fig. 4B), seven characteristic diffractions from anatase TiO₂ are observed, and all the diffractions are stronger than the corresponding diffractions in Cu_xO–TiO₂@SiO₂ nanosphere. Also the diffractions associated with Cu₂O at 27.5° and 42.3° on the surface of TiO₂ hollow nanosphere are found (Fig. 4B). All the results indicate that Cu_xO was attached to the surface of TiO₂, and the diffraction intensity from Cu₂O increases with its content. Elemental mapping analysis of the sample Cu_xO–TiO₂ hollow nanosphere (shown in Fig. S1 in the ESI†) indicates that the Cu element distributed uniformly though its content on TiO₂ surface was little.

Fig. 4 (A) XRD patterns of TiO₂@SiO₂ nanospherical ((a) TiO₂@SiO₂; (b) 0.25%-Cu_xO–TiO₂@SiO₂; (c) 0.5%-Cu_xO–TiO₂@SiO₂; (d) 1.0%-Cu_xO–TiO₂@SiO₂); (B) XRD patterns of TiO₂ hollow nanospherical materials ((a) TiO₂ hollow sphere; (b) 0.25%-Cu_xO–TiO₂ hollow sphere; (c) 0.5%-Cu_xO–TiO₂ hollow sphere; (d) 1.0%-Cu_xO–TiO₂ hollow sphere).

Nitrogen adsorption–desorption isotherms of TiO₂@SiO₂ and TiO₂ hollow nanospheres are shown in Fig. 5. Isotherms exhibiting behavior between those of types I and IV are characteristic of mesopores, according to IUPAC classifications. In addition, the prepared TiO₂@SiO₂ nanosphere and TiO₂ hollow nanosphere own surface areas of 251 m² g⁻¹ and 402 m² g⁻¹, pore volumes of 0.613 cm³ g⁻¹ and 0.983 cm³ g⁻¹, respectively. These parameters are far more than those of P25 (49 m² g⁻¹ of surface area and 0.090 cm³ g⁻¹ of pore volume), a kind of commercial TiO₂ microsphere.

Fig. 5 The N₂ adsorption–desorption isotherms of TiO₂@SiO₂ nanosphere (a) and TiO₂ hollow nanosphere (b).

In order to explore the chemical states of some relative elements in the nanocomposite samples, X-ray photoelectron spectroscopy (XPS) was carried out. In Fig. 6A, two peaks at 463.9 eV and 458.2 eV were observed from the Ti 2p spectrum, which are assigned to Ti 2p_1/2 and Ti 2p_3/2. No obvious differences were observed between the two spectra of TiO₂ and 0.5%-Cu_xO–TiO₂ hollow nanospheres, which indicate that Cu_xO was deposited on the surface of TiO₂ hollow nanospheres rather than the lattice. As shown in Fig. 6B, the appearance of CuO can be confirmed by the binding energies of 934.6 and 954.5 eV, respectively. There are another two characteristic peaks located at binding energies of 932.5 and 952.5 eV, which can be assigned to Cu(I) in Cu₂O.³⁷ Mole ratio of Cu⁺ to Cu²⁺ is about 1.6 : 1, which calculated from the peak area in Fig. 6B. In process of the reaction, the hydrated Cu²⁺ ions were firstly adsorbed on the surface of TiO₂ hollow nanospheres. Some of Cu²⁺ ions were hydrolyzed to Cu(OH)₂ in this system, and later they were changed into CuO under a certain temperature (110 °C). Due to the semiconductive property, the electron–holes in TiO₂ would be separated under illumination of UV-light in the sunshine, and the electrons separated would be transferred to Cu(II) to form Cu₂O.

Fig. 6 XPS analysis of Ti 2p in TiO₂ hollow nanosphere (A) and Cu 2p in 0.5%-Cu_xO–TiO₂ hollow nanosphere (B).

UV-Vis absorption spectra (shown in Fig. 7) indicate that all the samples, including TiO₂@SiO₂ nanosphere, TiO₂ hollow nanosphere, Cu_xO–TiO₂@SiO₂ nanosphere and Cu_xO–TiO₂ hollow nanosphere, have the intrinsic interband absorptions of TiO₂ at about 400 nm. The weak absorptions in range of 400–500 nm are assigned to the interfacial charge transfer in Cu_xO–TiO₂@SiO₂ nanosphere and Cu_xO–TiO₂ hollow nanosphere. In addition, the absorptions from 600 to 800 nm in the spectra of Cu_xO–TiO₂@SiO₂ nanosphere and Cu_xO–TiO₂ hollow nanosphere are assigned to the intrinsic excitation band of Cu_xO and the d–d transition of Cu(II).³³ Sequence of absorption intensities in range of 500–800 nm of the relative materials is TiO₂@SiO₂ nanosphere ≅ TiO₂ hollow nanosphere < Cu_xO–TiO₂@SiO₂ nanosphere < Cu_xO–TiO₂ hollow nanosphere, indicating that the Cu²⁺ ions on the surface of TiO₂ strengthen the absorption of the host in visible-light region, and the intensified degree on the absorptions increases with the increasing contents of Cu element in the composite materials, especially to TiO₂ hollow nanosphere.

Fig. 7 UV-Vis absorption spectra of TiO₂@SiO₂, TiO₂ hollow nanosphere and Cu_xO–TiO₂@SiO₂ and Cu_xO–TiO₂ hollow nanospheres.

3.2. Studies on catalytic activities of the catalysts

Photocatalytic activities of the catalysts were evaluated by monitoring degradation of acetaldehyde under UV-light and visible-light irradiations, and the initial concentration of acetaldehyde gas in all the experiments is about 200 ppmv. The first is to check the activities of the catalysts in a dark environment without any light irradiation. It was found that the concentration of acetaldehyde was almost not changed except for a little amount of acetaldehyde adsorbed (less than 20% of acetaldehyde adsorbed and less than 5% of CO₂ produced, which could be found from Fig. 8), which means that the acetaldehyde was not decomposed in dark under existence of the catalysts. However, the degradation of acetaldehyde happened when the reactor was irradiated by UV-light or visible-light, and large amount of CO₂ generated at the same time. Fig. 8 and 9 display the degradation of acetaldehyde and the generation of CO₂ in the systems under UV-light and visible-light irradiations, respectively. As shown in Fig. 8, the catalytic performance of TiO₂ hollow nanosphere was superior to TiO₂@SiO₂ nanosphere. Also it was found that the content of copper on the surface of TiO₂ affected the activities of catalysts deeply. The catalytic efficiency of 0.5%-Cu_xO–TiO₂ hollow nanosphere is higher than that of 0.25%-Cu_xO–TiO₂ hollow nanosphere. When the copper content is up to 0.5%, increase on catalytic efficiency of the hollow nanosphere is not evident. The above-mentioned regularity can be also found in the Cu_xO–TiO₂@SiO₂ nanospherical system. The decomposed rate of acetaldehyde by different catalysts under UV-light irradiation can be calculated by amount of the residual acetaldehyde and the generated CO₂. In the system of 0.5%-Cu_xO–TiO₂ hollow nanosphere, about 99.72–99.74% acetaldehyde was degraded after 60 minutes. Some data relating to degradation efficiency of acetaldehyde under UV-light and visible-light irradiations catalyzed by the relative catalysts are list in Tables 1 and 2. The clearance ratio is specific value of practical production and theoretical yield of CO₂.

Fig. 8 Degradation of acetaldehyde with the variation of times (A) and amount of CO₂ generated (B) under UV light irradiation and existence of the catalysts (: TiO₂@SiO₂; : 0.25%-Cu_xO–TiO₂@SiO₂; : 0.5%-Cu_xO–TiO₂@SiO₂; : 1.0%-Cu_xO–TiO₂@SiO₂; : TiO₂ hollow sphere; : 0.25%-Cu_xO–TiO₂ hollow sphere; : 0.5%-Cu_xO–TiO₂ hollow sphere; : 1.0%-Cu_xO–TiO₂ hollow sphere).

Fig. 9 Degradation of acetaldehyde with the variation of times (A) and amount of CO₂ generated (B) under visible light irradiation and existence of the catalysts (: TiO₂@SiO₂; : 0.25%-Cu_xO–TiO₂@SiO₂; : 0.5%-Cu_xO–TiO₂@SiO₂; : 1.0%-Cu_xO–TiO₂@SiO₂; : TiO₂ hollow sphere; : 0.25%-Cu_xO–TiO₂ hollow sphere; : 0.5%-Cu_xO–TiO₂ hollow sphere; : 1.0%-Cu_xO–TiO₂ hollow sphere).

Table 1 Initial concentration of acetaldehyde, the amount of CO₂ generated and decomposed rate of acetaldehyde after 60 minutes under UV light irradiation (THN: TiO₂ hollow nanosphere, TS: TiO₂@SiO₂)

Sample	TS	0.25% CuxO–TS	0.5% CuxO–TS	1.0% CuxO–ST	THN	0.25% CuxO–THN	0.50% CuxO–THN	1.0% CuxO–THN
CCH3CHO (ppmv)	193.611	194.139	181.5	193.842	180.361	199.796	193.545	186.103
CCO2 (ppmv)	260.989	342.129	347.763	319.558	282.625	378.431	386.044	361.759
Decomposed rate (%)	67.39–67.41	88.10–88.12	95.78–95.80	82.42–82.44	78.17–78.19	94.69–94.71	99.72–99.74	97.19–97.21

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Table 2 Initial concentration of acetaldehyde, the amount of CO₂ generated and decomposed rate of acetaldehyde after 6 hours under visible light irradiation (THN: TiO₂ hollow nanosphere, TS: TiO₂@SiO₂)

Sample	TS	0.25%-CuxO–TS	0.5%-CuxO–TS	1.0%-CuxO–TS	THN	0.25%-CuxO–THN	0.5%-CuxO–THN	1.0%-CuxO–THN
CCH3CHO (ppmv)	197.620	179.355	180.328	183.991	174.042	178.035	192.159	200.409
CCO2 (ppmv)	50.635	71.583	74.029	62.534	71.488	205.675	241.276	168.007
Decomposed rate (%)	12.80–12.82	19.95–19.97	20.52–20.54	17.69–17.71	20.53–20.55	54.99–55.01	62.77–62.79	41.91–41.93

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Meanwhile, degradations of acetaldehyde irradiated under visible-light under the catalysts were studied, which are shown in Fig. 9. Little CO₂ generated from degradation of acetaldehyde under the existence of TiO₂@SiO₂ nanosphere and TiO₂ hollow nanosphere for the wide band gap of TiO₂. However in Cu_xO–TiO₂ hollow nanospherical systems, acetaldehyde could be degraded to CO₂ commendably despite of the efficiencies lower than in UV-light. There is about 62.77–62.79% acetaldehyde was degraded in 0.5%-Cu_xO–TiO₂ hollow nanosphere catalytic system, and the catalyst exhibits the best catalytic performance among all the catalysts. Only 20.52–20.54% of acetaldehyde was degraded in 0.5%-Cu_xO–TiO₂@SiO₂ nanosphere catalytic system. Concentration of CO₂ generated in 0.5%-Cu_xO–TiO₂ hollow nanosphere system reached 241 ppm after 6 h, which is more than three times to the CO₂ generated in Cu_xO–TiO₂@SiO₂ nanosphere system (74 ppm) in the same time. Such high degradation efficiency of acetaldehyde under visible-light irradiation was seldom reported.

Photocatalytic reaction under existence of the semiconductor is a complicated phenomenon. It includes light absorption, stimulation and transference of the carrier and redox reaction on the catalyst surface. Position of energy band determined by the electronic structure in the semiconductor is one of the main controlling factors on photocatalytic activity, which affects range of the light wavelength on catalyst responsibility directly. The visible-light is considered to initiate interfacial charge transfer. Electrons in the valence band (VB) of TiO₂ are directly transferred to Cu(II) to form Cu(I), as well as holes (h⁺) in the VB of TiO₂. Thus, the energy to separate electrons and holes becomes low, which narrows the band gap of TiO₂ and prolongs the absorption edge from UV-light to visible-light region. The holes produced in VB of TiO₂ decompose organic substances. The catalytic degradation mechanism of acetaldehyde is shown in the following equations:³⁸

TiO₂ → h⁺ + e⁻

h⁺ + H₂O → ˙OH + H⁺

O₂ + e⁻ → O²⁻

CH₃CHO + O²⁻ → CH₃CO˙ + HO₂⁻

CH₃CHO + HO₂⁻ → CH₃CO⁻ + H₂O₂

H₂O₂ + CH₃CO˙ → CH₃COOH + ˙OH

CH₃CO⁻ + ˙OH → CH₃COOH + e⁻

CH₃COOH + 2H₂O + 8h⁺ → 2CO₂ + 10H⁺

Due to the larger surface area and more exposed active sites, the catalytic activity of Cu_xO–TiO₂ hollow nanosphere is higher than that of the catalyst Cu_xO–TiO₂@SiO₂ nanosphere. Moreover, the separation of electrons and holes became easier with Cu²⁺ increased on the surface of TiO₂, and superior catalytic activity of the catalyst was exhibited.

4. Conclusions

In summary, four kinds of photocatalysts, TiO₂@SiO₂ nanosphere, TiO₂ hollow nanosphere, Cu_xO–TiO₂@SiO₂ nanosphere and Cu_xO–TiO₂ hollow nanosphere, were synthesized. All the materials were found to have good photocatalytic activities in degradation of acetaldehyde under UV-light irradiations. Compared with TiO₂@SiO₂ nanosphere, TiO₂ hollow nanosphere and Cu_xO–TiO₂@SiO₂ nanosphere, Cu_xO–TiO₂ hollow nanosphere was superior for being used as catalyst in degradation of acetaldehyde under visible-light irradiation. The impregnated Cu²⁺ ions on the surface of TiO₂ hollow nanosphere were possibly to enhance the separation of photogenerated charge carriers.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

We thank the projects of National Natural Science Foundation of China (No. U1362113 and No. 21521005) and the PetroChina Co. Ltd. For financial support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra05281b

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