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Porous-structured Cu2O/TiO2 nanojunction material toward efficient CO2 photoreduction
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Nanotechnology Nanotechnology 25 (2014) 165402 (8pp)
doi:10.1088/0957-4484/25/16/165402
Porous-structured Cu2O/TiO2 nanojunction material toward efficient CO2 photoreduction Hua Xu1,2,3 , Shuxin Ouyang1,2,3 , Lequan Liu3 , Defa Wang1,2 , Tetsuya Kako3 and Jinhua Ye1,2,3,4 1
TU-NIMS Joint Research Center, and Tianjin Key Laboratory of Composite and Functional Materials, School of Materials Science and Engineering, Tianjin University, 92 Weijin Road, Nankai District, Tianjin 300072, People’s Republic of China 2 Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, People’s Republic of China 3 Environmental Remediation Materials Unit, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan 4 International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan E-mail: [email protected] and [email protected] Received 4 December 2013, revised 18 February 2014 Accepted for publication 24 February 2014 Published 26 March 2014
Abstract
Porous-structured Cu2 O/TiO2 nanojunction material is successfully fabricated by a facile method via loading Cu2 O nanoparticles on the network of a porous TiO2 substrate. The developed Cu2 O/TiO2 nanojunction material has a size of several nanometers, in which the p-type Cu2 O and n-type TiO2 nanoparticles are closely contacted with each other. The well designed nanojunction structure is beneficial for the charge separation in the photocatalytic reaction. Meanwhile, the porous structure of the Cu2 O/TiO2 nanojunction can facilitate the CO2 adsorption and offer more reaction active sites. Most importantly, the gas-phase CO2 photoreduction tests reveal that our developed porous-structured Cu2 O/TiO2 nanojunction material exhibits marked photocatalytic activity in the CH4 evolution, about 12, 9, and 7.5 times higher than the pure TiO2 , Pt–TiO2 , and commercial Degussa P25 TiO2 powders, respectively. The greatly enhanced activity can be attributed to the well designed nanojunction structure combined with the porous structure, which can simultaneously enhance the charge separation efficiency and facilitate the CO2 adsorption. Keywords: Cu2 O/TiO2 junction, nanocomposite, charge separation, CO2 adsorption, CO2 photoreduction S Online supplementary data available from stacks.iop.org/Nano/25/165402/mmedia (Some figures may appear in colour only in the online journal)
1. Introduction
on the surface of a photocatalyst; then, the photogenerated electrons migrate from the bulk to the surface to reduce CO2 to CH4 or other hydrocarbons [7–10]. Nevertheless, in the second step, a large proportion of photogenerated electrons recombine with the holes in the bulk and cannot migrate to the surface to take part in the photocatalytic reaction, leading to a poor photocatalytic activity [11, 12]. Therefore, to achieve a higher
The reduction of carbon dioxide (CO2 ) into hydrocarbon fuels (e.g. CH4 ) is a promising solution to both global warming and energy shortage [1–6]. Generally, the gas-phase CO2 photoreduction comprises two steps: CO2 adsorption and CO2 -to-hydrocarbon conversion. First, CO2 molecules adsorb 0957-4484/14/165402+08$33.00
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considering both the effect of photogenerated carriers and gas adsorption properties, we suggest that porous-structured Cu2 O/TiO2 nanojunction materials will be a good candidate for CO2 photoreduction. There have been some studies carried out on the Cu2 O/TiO2 heterojunction before, which have confirmed that the Cu2 O/TiO2 heterojunction could enhance the charge separation efficiency and lead to a high photocatalytic activity. For instance, Lalitha et al reported that Cu2 O/TiO2 nanocomposites with size about 20–40 nm exhibited higher activity than pure TiO2 in the H2 evolution [31]; Chu et al designed a Cu2 O@TiO2 core–shell structure, and this material showed an excellent photocatalytic activity in 4-nitrophenol degradation [24]; Yang et al deposited a Cu2 O nanowire on a TiO2 nanotube, and the Cu2 O/TiO2 composites exhibited twice the activity of the pure TiO2 nanotube [32]. Nevertheless, to date, no study has been focused on the porousstructured Cu2 O/TiO2 material, especially in CO2 photoreduction. Herein, we demonstrate a facile method to synthesize porous-structured Cu2 O/TiO2 nanojunction material with size less than 10 nm via loading Cu2 O nanoparticles on the a network of TiO2 porous materials. As a promising material, the Cu2 O/TiO2 nanojunction exhibits excellent performance in the CO2 photoreduction test. The simultaneously enhanced charge separation efficiency and CO2 adsorption make great contributions to the high photocatalytic activity.
Scheme 1. Schematic illustration of the charge transfer process
occurring in the Cu2 O/TiO2 nanojunction under UV–visible light irradiation.
CO2 conversion efficiency requires that the photocatalyst not only have a strong ability to capture CO2 on its surface but also it can motivate more electrons to migrate to the surface reactive sites to reduce CO2 . To effectively suppress the photogenerated electron–hole recombination probability, one of the most effective methods is to fabricate photocatalysts with heterojunction structure, which can facilitate charge rectification and induce faster carrier migration [13–17]. Herein, we will focus on the Cu2 O/TiO2 p–n heterojunction, in which Cu2 O is a typical p-type semiconductor with band gap 2.0–2.2 eV [18–20] and anatase TiO2 is a popular n-type semiconductor with band gap 3.2 eV [21, 22]. As shown in scheme 1, the band structures of the Cu2 O and TiO2 match well with each other, in which the conduction band edge of Cu2 O is higher than that of TiO2 , while the valence band edge of TiO2 is lower than that of Cu2 O [1, 23, 24]. Under UV–visible light irradiation, the electrons on the conduction band of Cu2 O will quickly move to the conduction band of TiO2 , whereas the holes on the valence band of TiO2 will transfer to the valence band of Cu2 O, effectively realizing the charge separation process [24]. More interestingly, a p–n junction effect is supposed to exist in the interface region between the p-type Cu2 O and n-type TiO2 in the Cu2 O/TiO2 heterojunction, which can further facilitate the charge transfer between Cu2 O and TiO2 [25, 26]. In parallel, if we make the particle size of the material smaller by several nanometers, the transfer distance of the electrons and holes from the bulk to the surface reactive sites can be greatly reduced [27]. Therefore, it is expected that the Cu2 O/TiO2 nanojunction material will exhibit excellent charge separation properties. Moreover, due to the outstanding structural characteristics of porous-structured materials, which not only facilitate the reactant adsorption but also can offer more reaction sites in the photocatalytic reaction, they have been widely adopted in the field of photocatalysis [2, 28]. We recently demonstrated that mesoporous structure could promote CO2 adsorption on the surface of a photocatalyst, thus significantly enhancing the photocatalytic activity [29, 30]. Inspired by the above studies,
2. Experimental details 2.1. Synthesis of porous-structured TiO2
In a typical synthetic procedure, 0.5 ml of titanium tetrachloride (TiCl4 ) was first dissolved into 30 ml of benzyl alcohol containing 2 ml of ethanol to obtain a yellowish solution as the Ti precursor. After this, the above solution was transferred to a Teflon vessel and heated with microwave assistance at 200 ◦ C for 40 min. After reaction, the resulting product was collected, thoroughly washed with ethanol and deionized water, and finally dried in the oven at 70 ◦ C for 6 h. 2.2. Synthesis of porous-structured Cu2 O/TiO2 heterojunction
A schematic illustration of the reaction process is shown in scheme 2. Typically, 2 mmol of the above-prepared porous TiO2 powders were first dispersed into 2 ml of ethanol via ultrasonic treatment for 15 min. Then, the TiO2 suspension was transferred into a Teflon vessel containing 30 ml of benzyl alcohol. After this, 0.4 mmol of copper (II) acetylacetonate (Cu(acac)2 ) was added to the vessel, and kept stirring for another 15 min. Finally, the reaction vessel was fixed in the microwave oven; after microwave–solvothermal treatment at 200 ◦ C for 40 min, the final Cu2 O/TiO2 heterojunction with a mass ratio between Cu2 O and TiO2 equal to 1:5 can be obtained. In addition, pure Cu2 O can also be synthesized via a method similar to that mentioned above. 0.4 mmol of Cu(acac)2 was dispersed into 30 ml of benzyl alcohol containing 2 ml of ethanol. After microwave–solvothermal treatment at 200 ◦ C for 40 min, we can get the final Cu2 O. The related 2
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Scheme 2. Schematic illustration of the formation process of the porous-structured Cu2 O/TiO2 nanojunction.
characterizations of Cu2 O are analyzed in the supporting information (available at stacks.iop.org/Nano/25/165402/mmedia). In comparison, the Pt-loaded TiO2 was prepared via photodeposition of Pt (1 wt%) on porous TiO2 in a mixed solution (50 ml of methanol and 220 ml of water) under UV–visible light irradiation for 2 h. H2 PtCl6 was used as the Pt precursor, and the light source is a 300 W xenon arc lamp. To remove the surface-adsorbed organics, the Pt-loaded TiO2 was finally calcined at 400 ◦ C for 2 h with a temperature ramping rate of 1 ◦ C min−1 . This sample was denoted as Pt–TiO2 . 2.3. Characterization
X-ray diffraction patterns were recorded on a Rigaku Multiflex diffractometer (RINT 2000; Rigaku, Japan) with monochromatized Cu K α radiation (λ = 1.541 78 Å). X-ray photoelectron spectroscopy (XPS) were performed on Thermo ESCALAB250 using monochromatized Al Kα at hν = 1486.6 eV. The binding energies were calibrated to the C 1s peak at 284.6 eV. The size and morphology of the samples were observed with a transmission electron microscope (TEM, JEM-200 CX, JEOL) operating at 200 kV. The Brunauer– Emmett–Teller (BET) surface areas were characterized by a surface area analyzer (Autosorb-1C, Quantachrome, USA) with nitrogen adsorption at 77 K. UV–visible absorption spectra were measured on a UV–visible spectrophotometer (UV-2500 PC, Shimadzu, Japan). The CO2 pulse chemisorption tests were carried out on a Micromeritics AutoChem II chemisorption analyzer at 35 ◦ C with a fixed volume loop of 0.05 ml.
Figure 1. XRD patterns of the standard anatase TiO2 (JCPDS no
71-1167) (a), porous TiO2 (b), porous Cu2 O/TiO2 nanojunction material (c), and the standard cubic Cu2 O (JCPDS no 78-2076) (d).
Shimadzu) equipped with a flame ionized detector (FID) and methanizer. 3. Results and discussion
To characterize the crystal structure of the samples, XRD measurements were carried out on the as-prepared TiO2 and Cu2 O/TiO2 heterojunction as shown in figure 1. The diffraction peaks at 25.26◦ , 37.83◦ , 47.90◦ , 53.91◦ , and 54.99◦ can be assigned to (101), (004), (200), (105), and (211) planes of anatase TiO2 (JCPDS no 71-1167) (figure 1(a)), respectively. Thus, as shown in figure 1(b), our prepared TiO2 is well indexed to the pure anatase TiO2 . For the Cu2 O/TiO2 heterojunction (figure 1(c)), besides the peaks matching well with the anatase TiO2 , the relative peak around at 36.22◦ is indexed to the diffraction of the (111) plane of the cubic Cu2 O (JCPDS no 78-2076) (figure 1(d)), indicating that TiO2 and Cu2 O coexist in the Cu2 O/TiO2 heterojunction. Meanwhile, the Cu 2p XPS spectrum was also recorded to demonstrate the valence state of Cu in the Cu2 O/TiO2 composite. As shown in figure S1 (available at stacks.iop.org/Nano/25/ 165402/mmedia), the XPS spectrum revealed the Cu 2p3/2 and Cu 2p1/2 peaks located at 932.3 and 952.2 eV, respectively,
2.4. CO2 photoreduction
In the photocatalytic reduction of CO2 , 40 mg of the photocatalyst was uniformly dispersed on a glass reactor with a base area of 8.1 cm2 . A 300 W xenon arc lamp was used as the light source for the photocatalytic reaction. The volume of the reaction system was around 390 ml. The reaction setup was vacuum treated several times, and then high-purity CO2 gas was introduced into the reaction system to achieve a pressure of 80 kPa. Deionized water (3 ml) was injected into the reaction system. During the irradiation, about 0.5 ml of gas was taken from the reaction cell at given intervals for subsequent CH4 concentration analysis using a gas chromatograph (GC-14B, 3
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Figure 2. TEM image of pure TiO2 (a), TEM ((b), (c)), SAED
pattern (inset of (c)), and HRTEM (d) images of the porous-structured Cu2 O/TiO2 nanojunction material. Figure 3. EDS elemental mapping analysis of the Cu2 O/TiO2
nanojunction material: (a) survey, (b) O element, (c) Ti element, and (d) Cu element.
which are in good accordance with the characteristic peak of Cu (I) [33]. The size and morphology of the TiO2 and Cu2 O/TiO2 heterojunction were observed by TEM. As shown in figure 2(a), TiO2 was composed of large numbers of ultrafine nanoparticles with sizes of several nanometers, and these particles aggregated together to from the porous structure. The Cu2 O/TiO2 heterojunction material exhibited morphology similar to that of TiO2 (figure 2(b)), which also consisted of ultrafine nanoparticles, and these nanoparticles agglomerated together to form the porous structure (figure 2(c)). The corresponding selected area electron diffraction (SAED) pattern of the agglomerated nanoparticles is shown in the inset of figure 2(c), in which the clear diffraction rings indicate that the Cu2 O/TiO2 nanojunction material is well crystallized. The well designed interface between Cu2 O and TiO2 nanoparticles can be observed by the HRTEM image as shown in figure 2(d). The lattice spacings of the two nanoparticles were measured to be 0.246 and 0.352 nm, which correspond to the (111) crystal plane of cubic Cu2 O and (101) crystal plane of anatase TiO2 , respectively. It is clear that the Cu2 O nanoparticle (∼5 nm) and TiO2 nanoparticle (∼6 nm) are closely connected with each other to build the good nanojunction interface. Furthermore, the energy-dispersive x-ray spectroscopy (EDS) elemental mapping analyses (figure 3) reveal that the distribution of both Ti and Cu is homogeneous. Therefore, we can conclude that the as-prepared Cu2 O/TiO2 nanojunction has a good nanojunction structure, in which TiO2 nanoparticles and Cu2 O nanoparticles are closely contacted with each other. Such a well designed interface is beneficial for the charge separation between Cu2 O and TiO2 . Figure 4(a) shows the nitrogen adsorption–desorption isotherms of the prepared TiO2 and Cu2 O/TiO2 nanojunction. The adsorption isotherms of both TiO2 and the Cu2 O/TiO2 nanojunction can be classified as type IV with a hysteresis loop of type H1, which is associated with porous materials [34]. The
Table 1. Total pore volumes, BET surface areas, as well as the
amounts of adsorbed CO2 of the prepared TiO2 , Cu2 O/TiO2 nanojunction, Pt–TiO2 , and commercial P25 TiO2 powder.
TiO2 Cu2 O/TiO2 Pt/TiO2 P25
Pore volume (cm3 g−1 )
Surface area (m2 g−1 )
Amounts of CO2 adsorption (ml g−1 )
0.21 0.29 0.22 0.10
198.2 206.3 199.8 51.1
0.12 0.27 0.13 0.05
pores result from the agglomeration of nanoparticles, which is in good accordance with the TEM images shown in figure 2. In parallel, the pore size distributions of TiO2 and Cu2 O/TiO2 nanojunction are also listed, in which the micropores below 2 nm were analyzed by the Horvath–Kawazoe (HK) method (figure 4(b)), and the mesopores above 2 nm were studied by the Barrett–Joyner–Halenda (BJH) method (figure 4(c)). It is obvious that our developed Cu2 O/TiO2 nanojunction material exhibits a sharp pore size distribution around 4–8 Å (figure 4(b)), indicating that the Cu2 O/TiO2 nanojunction material is mainly in micropores, and the size distribution of the Cu2 O/TiO2 nanojunction is sharper than the pure TiO2 below 2 nm (figure 4(b)) and similar to TiO2 in the range larger than 2 nm (figure 4(c)). As shown in scheme 2, the Cu2 O/TiO2 nanojunction was fabricated via loading Cu2 O nanoparticles on a network of porous TiO2 ; after loading ultrafine Cu2 O nanoparticles on the TiO2 , some of the pores in TiO2 might be partially filled, leading to a smaller pore size. Meanwhile, as shown in table 1, the total pore volumes and BET surface areas were recorded to be (0.21 cm3 g−1 , 198.2 m2 g−1 ) and (0.29 cm3 g−1 , 206.3 m2 g−1 ) for TiO2 and 4
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Figure 4. N2 adsorption–desorption isotherms (a) and pore size distributions analyzed by the HK method (b) and BJH method (c) of the prepared TiO2 and Cu2 O/TiO2 nanojunction material.
porous structure can entrap the CO2 in the pores and enhance the CO2 adsorption over the surface of nanoparticles, thus offering more opportunities for the electrons transferred from the surface to CO2 to reduce it into CH4 or other hydrocarbons, leading to a higher photocatalytic activity. The UV–visible absorption spectra of the prepared TiO2 and Cu2 O/TiO2 heterojunction are shown in figure 5. It can be clearly seen that the absorption edge of TiO2 is around 380 nm; as for the Cu2 O/TiO2 heterojunction, apart from the absorption edge (nearly 388 nm) of pure TiO2 , another absorption band edge around 510 nm can also be detected, which results from the existence of Cu2 O in the hybrid Cu2 O/TiO2 material. To experimentally investigate whether the band structures of Cu2 O and TiO2 match well with each other in the Cu2 O/TiO2 composite, valence-band XPS spectra in addition to the UV–visible absorption spectra were adopted to study the electronic band structures of Cu2 O and TiO2 (figure 6). The Cu2 O was prepared via the same synthetic method as the Cu2 O/TiO2 nanojunction, and the detailed characterizations are shown in the supporting information (available at stacks. iop.org/Nano/25/165402/mmedia). As shown in the insets of figures 6(a) and (b), the UV–visible absorption spectra demonstrate that the absorption edges of TiO2 and Cu2 O are nearly 371 and 644 nm, respectively. TiO2 is known as an indirect semiconductor, while Cu2 O is a direct semiconductor. As a semiconductor, the band gaps can be calculated by the equation of αhν = A(hν − E g )n/2 , in which α is the absorption coefficient, hν is the incident photon energy, A is the proportionality constant, E g is the band gap, and n is equal to 1 and 4 for direct and indirect semiconductors, respectively [12, 35]. According to this Butler equation, plotting (αhν)1/2 or (αhν)2 versus hν from the absorption spectra, and extrapolating the linear part to zero on an abscissa axis, from the intersections of the straight lines obtained we can deduce the bandgaps of 3.2 and 2.1 eV for TiO2 and Cu2 O, respectively (figures 6(a) and (b)). Then, from the valence-band XPS spectra, the valenceband edges were deduced to be 2.8 and 1.0 eV for TiO2 (figure 6(c)) and Cu2 O (figure 6(d)), respectively. Finally, on the basis of the above experimental results, we can draw a schematic illustration of the band structures of Cu2 O and TiO2 as shown in figure 6(e). As shown in figure 6(e), the conduction band edge of Cu2 O (−1.1 eV) is much higher than that of TiO2 (−0.4 eV), while the valence band edge of TiO2 (2.8 eV) is lower than that of Cu2 O (1.0 eV); the band
Figure 5. UV–visible absorption spectra of the prepared TiO2 and Cu2 O/TiO2 nanojunction.
the Cu2 O/TiO2 nanojunction, respectively. Both the large total pore volume and surface area strongly support the fact that our developed Cu2 O/TiO2 nanojunction has the porous structure, which is beneficial for the CO2 adsorption in the photocatalytic reaction. In addition, to investigate the CO2 adsorption ability on the porous TiO2 and Cu2 O/TiO2 nanojunction, CO2 pulse chemisorption tests were carried out. As shown in figure S1 and table S1 (available at stacks.iop.org/Nano/25/165402/ mmedia), the amounts of adsorbed molecular CO2 were calculated to be 0.12 and 0.27 ml g−1 for our developed porous TiO2 and Cu2 O/TiO2 nanojunction, respectively. The better CO2 adsorption ability of the Cu2 O/TiO2 nanojunction is attributed to its much sharper pore size distribution in the micropore region and its larger total pore volume (0.29 cm3 g−1 ) than TiO2 (0.21 cm3 g−1 ). In comparison, the Pt-loaded porous TiO2 and the commercial P25 TiO2 powder were also tested. Pt–TiO2 shows similar BET surface area and CO2 adsorption ability to TiO2 , while the P25 TiO2 powder with a lower surface area (51.1 m2 g−1 ) and smaller total pore volume (0.10 cm3 g−1 ) exhibits a relatively limited CO2 adsorption, about 0.05 ml g−1 . These results further experimentally confirm that the porous structure with a larger total pore volume is beneficial for the CO2 adsorption, especially in the case of smaller pore size. In the CO2 photoreduction test, the 5
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Figure 6. UV–visible absorption spectra (inset) as well as their related estimation of band gaps of TiO2 (a) and Cu2 O (b). Valence-band XPS spectra of TiO2 (c) and Cu2 O (d). Schematic illustration of the determined valence band and conduction band edges of TiO2 and Cu2 O (e).
Figure 7. (a) CH4 evolution over the prepared TiO2 , Cu2 O/TiO2 nanojunction, Pt–TiO2 , and P25 under UV–visible light irradiation (λ > 300 nm) in 10 h. (b) Average CH4 evolution rates of the prepared TiO2 , Cu2 O/TiO2 nanojunction, Pt–TiO2 , and P25.
structures of Cu2 O and TiO2 match well with each other. In this situation, under light illumination, the electrons will transfer from Cu2 O to TiO2 , while the holes will move from TiO2 to Cu2 O, effectively realizing the charge separation. The CO2 photoreduction tests of the as-prepared TiO2 and Cu2 O/TiO2 nanojunction material were evaluated under UV– visible light irradiation (figure 7(a)). For purposes of compari-
son, the Pt-loaded porous TiO2 (Pt–TiO2 ) and commercial P25 were also tested under the same experimental conditions. As shown in figure 7(b), the CH4 evolution rates were calculated to be 2.4, 28.4, 3.2, and 3.7 ppm g−1 h−1 for the TiO2 , Cu2 O/TiO2 nanojunction, Pt–TiO2 , and P25, respectively. Among these samples, the Cu2 O/TiO2 nanojunction exhibits the highest photocatalytic activity, about 12, 9, and 7.5 times higher than 6
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Scheme 3. Schematic illustration of the charge transfer and CO2 reduction processes occurring on the Cu2 O/TiO2 nanojunction (a), porous TiO2 (b), Pt-loaded porous TiO2 (c), and commercial P25 powder (d) under UV–visible light irradiation.
TiO2 , Pt–TiO2 , and P25, respectively. Besides CH4 , trace amounts of CO can also be detected. In addition, controlled experiments show that no CH4 was evolved when the photocatalytic reaction was carried out in the dark or irradiated in the absence of photocatalyst, revealing that the reduction of CO2 is driven by light irradiation over the photocatalyst. The isotopic tracer experiments were also processed to investigate the reaction mechanism of CO2 photoreduction (figure S3 available at stacks.iop.org/Nano/25/165402/mmedia). The results showed that the C and H in the product CH4 are mainly from CO2 (figure S3(a) available at stacks.iop.org/Nano/25/165402/ mmedia) and H2 O (figure S3(b) available at stacks.iop.org/ Nano/25/165402/mmedia), respectively. The well designed porous structure combined with the closely contacted interface of the Cu2 O/TiO2 nanojunction plays important roles in enhancing the photocatalytic activity. First, in contrast to pure TiO2 , the superior activity of the Cu2 O/TiO2 material can be attributed to its well designed nanojunction structure. As shown in scheme 3(a), once excited, the electrons will transfer from Cu2 O to, and accumulate on, the TiO2 , while the holes will move from TiO2 to Cu2 O, to realize the charge separation, while for pure TiO2 (scheme 3(b)), the electrons and holes easily recombine on the surface, leading to a poor charge separation efficiency and lower photocatalytic activity. Then, after Pt loading, the photocatalytic activity of Pt–TiO2 is slightly enhanced compared with pure TiO2 , but still lower than that of the Cu2 O/TiO2 nanojunction. As shown in scheme 3(c), Pt has a lower Fermi energy than TiO2 , so Pt–TiO2 can enhance the charge separation efficiency by pushing the photogenerated electrons transfer from the conduction band of TiO2 to the surface Pt sites; then, the accumulated electrons on the Pt sites reduce CO2 to CH4 . However, most of the molecular CO2 is adsorbed on the porous TiO2 , and to finish the whole CO2 photoreduction process the molecular CO2 on the TiO2 surface needs to migrate dozens of nanometers to the Pt sites; during this migration, the charges (electrons and holes) are easily recombined. In the case of our developed Cu2 O/TiO2 nanojunction, as shown in scheme 3(a), the CO2 adsorption and electron accumulation occur simultaneously on the TiO2 surface, thus the distance for electron migration becomes shorter and the ratio for recombination on the surface becomes lower, leading to higher activity.
Furthermore, compared with commercial P25 TiO2 powder, which is composed of 80% anatase and 20% rutile (scheme 3(d)), our developed Cu2 O/TiO2 nanojunction exhibits 7.5 times higher activity in the CH4 evolution (figure 7). The higher activity of the Cu2 O/TiO2 nanojunction in contrast to P25 might be ascribed to the porous structure and larger surface area, as shown in table 1, which facilitate the CO2 adsorption (0.27 ml g−1 for Cu2 O/TiO2 nanojunction versus 0.05 ml g−1 for P25) and can offer more reaction active sites. On the other hand, our designed Cu2 O/TiO2 nanojunction has a smaller particle size (about 5–8 nanometer) than that of P25 TiO2 (around 25 nm). With a smaller particle size, the migration distance for the electrons and holes from the bulk to surface can be greatly reduced, which can somewhat inhibit the charge recombination in the bulk. For Cu2 O/TiO2 composite material, the content of Cu2 O in the composites might affect their photocatalytic properties. Accordingly, Cu2 O/TiO2 composites in different mass ratios (such as 1:1, 1:3, and 1:7) were prepared to investigate their photocatalytic properties. As shown in figure S4(a) (available at stacks.iop.org/Nano/25/165402/mmedia), with more Cu2 O contained in the composite material, the intensity of the relative peak around 36.22◦ (the diffraction of the (111) plane of Cu2 O) increased; in parallel, the light absorption edge of the composites was also red-shifted to the longer wavelength region (figure S4(b) available at stacks.iop.org/ Nano/25/165402/mmedia). When testing the photocatalytic activities in CO2 photoreduction, the CH4 evolution rates were calculated to be 2.4, 4.0, 28.4, 9.4, 8.5, and 5.9 ppm g−1 h−1 for TiO2 , CT1:7 , CT1:5 , CT1:3 , CT1:1 , and Cu2 O, respectively (figures S4(c)–(d) available at stacks.iop.org/Nano/25/165402/ mmedia). Obviously, all the Cu2 O/TiO2 composites exhibit better photocatalytic activity than TiO2 , Pt–TiO2 , or P25. With the optimized concentration, the Cu2 O/TiO2 nanojunction with the mass ratio of 1:5 exhibits the highest CH4 evolution rates. 4. Conclusions
We have successfully prepared porous-structured Cu2 O/ TiO2 nanojunction material with a size of several nanometers. The well designed nanojunction structure facilitates the charge separation between Cu2 O and TiO2 ; while the porous structure is beneficial for the CO2 adsorption. These combined 7
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properties endow the Cu2 O/TiO2 nanojunction with marked photocatalytic performance in CO2 photoreduction, about 12, 9, and 7.5 times higher than that of TiO2 , Pt–TiO2 , and the commercial P25 TiO2 , respectively. This study demonstrates that construction of porous structure coupled with a nanojunction is an interesting strategy to enhance the photocatalytic activity of CO2 photoreduction. We anticipate that this concept can provide a versatile route to fabricate other porous-structured heterojunction materials for higher solar-to-energy conversion efficiency.
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Acknowledgments
This work was partially supported by the National Basic Research Program of China (No 2014CB239301); Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Ministry of Education of China; and the World Premier International Research Center Initiative (WPI), MEXT, Japan. References [1] Roy S C, Varghese O K, Paulose M and Grimes C A 2010 ACS Nano 4 1259–78 [2] Yan S C et al 2010 Angew. Chem. Int. Edn 49 6400–4 [3] Tu W G, Zhou Y, Liu Q, Tian Z P, Gao J, Chen X Y, Zhang H T, Liu J G and Zou Z G 2012 Adv. Funct. Mater. 22 1215–21 [4] Xu H, Ouyang S X, Li P, Kako T and Ye J H 2013 ACS Appl. Mater. Interfaces 5 1348–54 [5] Chen X Y, Zhou Y, Liu Q, Li Z D, Liu J G and Zou Z G 2012 ACS Appl. Mater. Interfaces 4 3372–7 [6] Zhou Y, Tian Z P, Zhao Z Y, Liu Q, Kou J H, Chen X Y, Gao J, Yan S C and Zou Z G 2011 ACS Appl. Mater. Interfaces 3 3594–601 [7] Li P, Ouyang S X, Kako T and Ye J H 2013 J. Mater. Chem. A 1 1185–91 [8] Li P, Ouyang S X, Xi G C, Kako T and Ye J H 2012 J. Phys. Chem. C 116 7621–8 [9] Liu Q, Zhou Y, Kou J H, Chen X Y, Tian Z P, Gao J, Yan S C and Zou Z G 2010 J. Am. Chem. Soc. 132 14385–7 [10] Liu Q, Zhou Y, Tian Z P, Chen X Y, Gao J and Zou Z G 2012 J. Mater. Chem. 22 2033–8 [11] Tong H, Ouyang S X, Bi Y P, Umezawa N, Oshikiri M and Ye J H 2012 Adv. Mater. 24 229–51
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Porous-structured Cu2O/TiO2 nanojunction material toward efficient CO2 photoreduction
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 165402 (http://iopscience.iop.org/0957-4484/25/16/165402) View the table of contents for this issue, or go to the journal homepage for more
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Nanotechnology Nanotechnology 25 (2014) 165402 (8pp)
doi:10.1088/0957-4484/25/16/165402
Porous-structured Cu2O/TiO2 nanojunction material toward efficient CO2 photoreduction Hua Xu1,2,3 , Shuxin Ouyang1,2,3 , Lequan Liu3 , Defa Wang1,2 , Tetsuya Kako3 and Jinhua Ye1,2,3,4 1
TU-NIMS Joint Research Center, and Tianjin Key Laboratory of Composite and Functional Materials, School of Materials Science and Engineering, Tianjin University, 92 Weijin Road, Nankai District, Tianjin 300072, People’s Republic of China 2 Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, People’s Republic of China 3 Environmental Remediation Materials Unit, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan 4 International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan E-mail: [email protected] and [email protected] Received 4 December 2013, revised 18 February 2014 Accepted for publication 24 February 2014 Published 26 March 2014
Abstract
Porous-structured Cu2 O/TiO2 nanojunction material is successfully fabricated by a facile method via loading Cu2 O nanoparticles on the network of a porous TiO2 substrate. The developed Cu2 O/TiO2 nanojunction material has a size of several nanometers, in which the p-type Cu2 O and n-type TiO2 nanoparticles are closely contacted with each other. The well designed nanojunction structure is beneficial for the charge separation in the photocatalytic reaction. Meanwhile, the porous structure of the Cu2 O/TiO2 nanojunction can facilitate the CO2 adsorption and offer more reaction active sites. Most importantly, the gas-phase CO2 photoreduction tests reveal that our developed porous-structured Cu2 O/TiO2 nanojunction material exhibits marked photocatalytic activity in the CH4 evolution, about 12, 9, and 7.5 times higher than the pure TiO2 , Pt–TiO2 , and commercial Degussa P25 TiO2 powders, respectively. The greatly enhanced activity can be attributed to the well designed nanojunction structure combined with the porous structure, which can simultaneously enhance the charge separation efficiency and facilitate the CO2 adsorption. Keywords: Cu2 O/TiO2 junction, nanocomposite, charge separation, CO2 adsorption, CO2 photoreduction S Online supplementary data available from stacks.iop.org/Nano/25/165402/mmedia (Some figures may appear in colour only in the online journal)
1. Introduction
on the surface of a photocatalyst; then, the photogenerated electrons migrate from the bulk to the surface to reduce CO2 to CH4 or other hydrocarbons [7–10]. Nevertheless, in the second step, a large proportion of photogenerated electrons recombine with the holes in the bulk and cannot migrate to the surface to take part in the photocatalytic reaction, leading to a poor photocatalytic activity [11, 12]. Therefore, to achieve a higher
The reduction of carbon dioxide (CO2 ) into hydrocarbon fuels (e.g. CH4 ) is a promising solution to both global warming and energy shortage [1–6]. Generally, the gas-phase CO2 photoreduction comprises two steps: CO2 adsorption and CO2 -to-hydrocarbon conversion. First, CO2 molecules adsorb 0957-4484/14/165402+08$33.00
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considering both the effect of photogenerated carriers and gas adsorption properties, we suggest that porous-structured Cu2 O/TiO2 nanojunction materials will be a good candidate for CO2 photoreduction. There have been some studies carried out on the Cu2 O/TiO2 heterojunction before, which have confirmed that the Cu2 O/TiO2 heterojunction could enhance the charge separation efficiency and lead to a high photocatalytic activity. For instance, Lalitha et al reported that Cu2 O/TiO2 nanocomposites with size about 20–40 nm exhibited higher activity than pure TiO2 in the H2 evolution [31]; Chu et al designed a Cu2 O@TiO2 core–shell structure, and this material showed an excellent photocatalytic activity in 4-nitrophenol degradation [24]; Yang et al deposited a Cu2 O nanowire on a TiO2 nanotube, and the Cu2 O/TiO2 composites exhibited twice the activity of the pure TiO2 nanotube [32]. Nevertheless, to date, no study has been focused on the porousstructured Cu2 O/TiO2 material, especially in CO2 photoreduction. Herein, we demonstrate a facile method to synthesize porous-structured Cu2 O/TiO2 nanojunction material with size less than 10 nm via loading Cu2 O nanoparticles on the a network of TiO2 porous materials. As a promising material, the Cu2 O/TiO2 nanojunction exhibits excellent performance in the CO2 photoreduction test. The simultaneously enhanced charge separation efficiency and CO2 adsorption make great contributions to the high photocatalytic activity.
Scheme 1. Schematic illustration of the charge transfer process
occurring in the Cu2 O/TiO2 nanojunction under UV–visible light irradiation.
CO2 conversion efficiency requires that the photocatalyst not only have a strong ability to capture CO2 on its surface but also it can motivate more electrons to migrate to the surface reactive sites to reduce CO2 . To effectively suppress the photogenerated electron–hole recombination probability, one of the most effective methods is to fabricate photocatalysts with heterojunction structure, which can facilitate charge rectification and induce faster carrier migration [13–17]. Herein, we will focus on the Cu2 O/TiO2 p–n heterojunction, in which Cu2 O is a typical p-type semiconductor with band gap 2.0–2.2 eV [18–20] and anatase TiO2 is a popular n-type semiconductor with band gap 3.2 eV [21, 22]. As shown in scheme 1, the band structures of the Cu2 O and TiO2 match well with each other, in which the conduction band edge of Cu2 O is higher than that of TiO2 , while the valence band edge of TiO2 is lower than that of Cu2 O [1, 23, 24]. Under UV–visible light irradiation, the electrons on the conduction band of Cu2 O will quickly move to the conduction band of TiO2 , whereas the holes on the valence band of TiO2 will transfer to the valence band of Cu2 O, effectively realizing the charge separation process [24]. More interestingly, a p–n junction effect is supposed to exist in the interface region between the p-type Cu2 O and n-type TiO2 in the Cu2 O/TiO2 heterojunction, which can further facilitate the charge transfer between Cu2 O and TiO2 [25, 26]. In parallel, if we make the particle size of the material smaller by several nanometers, the transfer distance of the electrons and holes from the bulk to the surface reactive sites can be greatly reduced [27]. Therefore, it is expected that the Cu2 O/TiO2 nanojunction material will exhibit excellent charge separation properties. Moreover, due to the outstanding structural characteristics of porous-structured materials, which not only facilitate the reactant adsorption but also can offer more reaction sites in the photocatalytic reaction, they have been widely adopted in the field of photocatalysis [2, 28]. We recently demonstrated that mesoporous structure could promote CO2 adsorption on the surface of a photocatalyst, thus significantly enhancing the photocatalytic activity [29, 30]. Inspired by the above studies,
2. Experimental details 2.1. Synthesis of porous-structured TiO2
In a typical synthetic procedure, 0.5 ml of titanium tetrachloride (TiCl4 ) was first dissolved into 30 ml of benzyl alcohol containing 2 ml of ethanol to obtain a yellowish solution as the Ti precursor. After this, the above solution was transferred to a Teflon vessel and heated with microwave assistance at 200 ◦ C for 40 min. After reaction, the resulting product was collected, thoroughly washed with ethanol and deionized water, and finally dried in the oven at 70 ◦ C for 6 h. 2.2. Synthesis of porous-structured Cu2 O/TiO2 heterojunction
A schematic illustration of the reaction process is shown in scheme 2. Typically, 2 mmol of the above-prepared porous TiO2 powders were first dispersed into 2 ml of ethanol via ultrasonic treatment for 15 min. Then, the TiO2 suspension was transferred into a Teflon vessel containing 30 ml of benzyl alcohol. After this, 0.4 mmol of copper (II) acetylacetonate (Cu(acac)2 ) was added to the vessel, and kept stirring for another 15 min. Finally, the reaction vessel was fixed in the microwave oven; after microwave–solvothermal treatment at 200 ◦ C for 40 min, the final Cu2 O/TiO2 heterojunction with a mass ratio between Cu2 O and TiO2 equal to 1:5 can be obtained. In addition, pure Cu2 O can also be synthesized via a method similar to that mentioned above. 0.4 mmol of Cu(acac)2 was dispersed into 30 ml of benzyl alcohol containing 2 ml of ethanol. After microwave–solvothermal treatment at 200 ◦ C for 40 min, we can get the final Cu2 O. The related 2
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Scheme 2. Schematic illustration of the formation process of the porous-structured Cu2 O/TiO2 nanojunction.
characterizations of Cu2 O are analyzed in the supporting information (available at stacks.iop.org/Nano/25/165402/mmedia). In comparison, the Pt-loaded TiO2 was prepared via photodeposition of Pt (1 wt%) on porous TiO2 in a mixed solution (50 ml of methanol and 220 ml of water) under UV–visible light irradiation for 2 h. H2 PtCl6 was used as the Pt precursor, and the light source is a 300 W xenon arc lamp. To remove the surface-adsorbed organics, the Pt-loaded TiO2 was finally calcined at 400 ◦ C for 2 h with a temperature ramping rate of 1 ◦ C min−1 . This sample was denoted as Pt–TiO2 . 2.3. Characterization
X-ray diffraction patterns were recorded on a Rigaku Multiflex diffractometer (RINT 2000; Rigaku, Japan) with monochromatized Cu K α radiation (λ = 1.541 78 Å). X-ray photoelectron spectroscopy (XPS) were performed on Thermo ESCALAB250 using monochromatized Al Kα at hν = 1486.6 eV. The binding energies were calibrated to the C 1s peak at 284.6 eV. The size and morphology of the samples were observed with a transmission electron microscope (TEM, JEM-200 CX, JEOL) operating at 200 kV. The Brunauer– Emmett–Teller (BET) surface areas were characterized by a surface area analyzer (Autosorb-1C, Quantachrome, USA) with nitrogen adsorption at 77 K. UV–visible absorption spectra were measured on a UV–visible spectrophotometer (UV-2500 PC, Shimadzu, Japan). The CO2 pulse chemisorption tests were carried out on a Micromeritics AutoChem II chemisorption analyzer at 35 ◦ C with a fixed volume loop of 0.05 ml.
Figure 1. XRD patterns of the standard anatase TiO2 (JCPDS no
71-1167) (a), porous TiO2 (b), porous Cu2 O/TiO2 nanojunction material (c), and the standard cubic Cu2 O (JCPDS no 78-2076) (d).
Shimadzu) equipped with a flame ionized detector (FID) and methanizer. 3. Results and discussion
To characterize the crystal structure of the samples, XRD measurements were carried out on the as-prepared TiO2 and Cu2 O/TiO2 heterojunction as shown in figure 1. The diffraction peaks at 25.26◦ , 37.83◦ , 47.90◦ , 53.91◦ , and 54.99◦ can be assigned to (101), (004), (200), (105), and (211) planes of anatase TiO2 (JCPDS no 71-1167) (figure 1(a)), respectively. Thus, as shown in figure 1(b), our prepared TiO2 is well indexed to the pure anatase TiO2 . For the Cu2 O/TiO2 heterojunction (figure 1(c)), besides the peaks matching well with the anatase TiO2 , the relative peak around at 36.22◦ is indexed to the diffraction of the (111) plane of the cubic Cu2 O (JCPDS no 78-2076) (figure 1(d)), indicating that TiO2 and Cu2 O coexist in the Cu2 O/TiO2 heterojunction. Meanwhile, the Cu 2p XPS spectrum was also recorded to demonstrate the valence state of Cu in the Cu2 O/TiO2 composite. As shown in figure S1 (available at stacks.iop.org/Nano/25/ 165402/mmedia), the XPS spectrum revealed the Cu 2p3/2 and Cu 2p1/2 peaks located at 932.3 and 952.2 eV, respectively,
2.4. CO2 photoreduction
In the photocatalytic reduction of CO2 , 40 mg of the photocatalyst was uniformly dispersed on a glass reactor with a base area of 8.1 cm2 . A 300 W xenon arc lamp was used as the light source for the photocatalytic reaction. The volume of the reaction system was around 390 ml. The reaction setup was vacuum treated several times, and then high-purity CO2 gas was introduced into the reaction system to achieve a pressure of 80 kPa. Deionized water (3 ml) was injected into the reaction system. During the irradiation, about 0.5 ml of gas was taken from the reaction cell at given intervals for subsequent CH4 concentration analysis using a gas chromatograph (GC-14B, 3
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Figure 2. TEM image of pure TiO2 (a), TEM ((b), (c)), SAED
pattern (inset of (c)), and HRTEM (d) images of the porous-structured Cu2 O/TiO2 nanojunction material. Figure 3. EDS elemental mapping analysis of the Cu2 O/TiO2
nanojunction material: (a) survey, (b) O element, (c) Ti element, and (d) Cu element.
which are in good accordance with the characteristic peak of Cu (I) [33]. The size and morphology of the TiO2 and Cu2 O/TiO2 heterojunction were observed by TEM. As shown in figure 2(a), TiO2 was composed of large numbers of ultrafine nanoparticles with sizes of several nanometers, and these particles aggregated together to from the porous structure. The Cu2 O/TiO2 heterojunction material exhibited morphology similar to that of TiO2 (figure 2(b)), which also consisted of ultrafine nanoparticles, and these nanoparticles agglomerated together to form the porous structure (figure 2(c)). The corresponding selected area electron diffraction (SAED) pattern of the agglomerated nanoparticles is shown in the inset of figure 2(c), in which the clear diffraction rings indicate that the Cu2 O/TiO2 nanojunction material is well crystallized. The well designed interface between Cu2 O and TiO2 nanoparticles can be observed by the HRTEM image as shown in figure 2(d). The lattice spacings of the two nanoparticles were measured to be 0.246 and 0.352 nm, which correspond to the (111) crystal plane of cubic Cu2 O and (101) crystal plane of anatase TiO2 , respectively. It is clear that the Cu2 O nanoparticle (∼5 nm) and TiO2 nanoparticle (∼6 nm) are closely connected with each other to build the good nanojunction interface. Furthermore, the energy-dispersive x-ray spectroscopy (EDS) elemental mapping analyses (figure 3) reveal that the distribution of both Ti and Cu is homogeneous. Therefore, we can conclude that the as-prepared Cu2 O/TiO2 nanojunction has a good nanojunction structure, in which TiO2 nanoparticles and Cu2 O nanoparticles are closely contacted with each other. Such a well designed interface is beneficial for the charge separation between Cu2 O and TiO2 . Figure 4(a) shows the nitrogen adsorption–desorption isotherms of the prepared TiO2 and Cu2 O/TiO2 nanojunction. The adsorption isotherms of both TiO2 and the Cu2 O/TiO2 nanojunction can be classified as type IV with a hysteresis loop of type H1, which is associated with porous materials [34]. The
Table 1. Total pore volumes, BET surface areas, as well as the
amounts of adsorbed CO2 of the prepared TiO2 , Cu2 O/TiO2 nanojunction, Pt–TiO2 , and commercial P25 TiO2 powder.
TiO2 Cu2 O/TiO2 Pt/TiO2 P25
Pore volume (cm3 g−1 )
Surface area (m2 g−1 )
Amounts of CO2 adsorption (ml g−1 )
0.21 0.29 0.22 0.10
198.2 206.3 199.8 51.1
0.12 0.27 0.13 0.05
pores result from the agglomeration of nanoparticles, which is in good accordance with the TEM images shown in figure 2. In parallel, the pore size distributions of TiO2 and Cu2 O/TiO2 nanojunction are also listed, in which the micropores below 2 nm were analyzed by the Horvath–Kawazoe (HK) method (figure 4(b)), and the mesopores above 2 nm were studied by the Barrett–Joyner–Halenda (BJH) method (figure 4(c)). It is obvious that our developed Cu2 O/TiO2 nanojunction material exhibits a sharp pore size distribution around 4–8 Å (figure 4(b)), indicating that the Cu2 O/TiO2 nanojunction material is mainly in micropores, and the size distribution of the Cu2 O/TiO2 nanojunction is sharper than the pure TiO2 below 2 nm (figure 4(b)) and similar to TiO2 in the range larger than 2 nm (figure 4(c)). As shown in scheme 2, the Cu2 O/TiO2 nanojunction was fabricated via loading Cu2 O nanoparticles on a network of porous TiO2 ; after loading ultrafine Cu2 O nanoparticles on the TiO2 , some of the pores in TiO2 might be partially filled, leading to a smaller pore size. Meanwhile, as shown in table 1, the total pore volumes and BET surface areas were recorded to be (0.21 cm3 g−1 , 198.2 m2 g−1 ) and (0.29 cm3 g−1 , 206.3 m2 g−1 ) for TiO2 and 4
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Figure 4. N2 adsorption–desorption isotherms (a) and pore size distributions analyzed by the HK method (b) and BJH method (c) of the prepared TiO2 and Cu2 O/TiO2 nanojunction material.
porous structure can entrap the CO2 in the pores and enhance the CO2 adsorption over the surface of nanoparticles, thus offering more opportunities for the electrons transferred from the surface to CO2 to reduce it into CH4 or other hydrocarbons, leading to a higher photocatalytic activity. The UV–visible absorption spectra of the prepared TiO2 and Cu2 O/TiO2 heterojunction are shown in figure 5. It can be clearly seen that the absorption edge of TiO2 is around 380 nm; as for the Cu2 O/TiO2 heterojunction, apart from the absorption edge (nearly 388 nm) of pure TiO2 , another absorption band edge around 510 nm can also be detected, which results from the existence of Cu2 O in the hybrid Cu2 O/TiO2 material. To experimentally investigate whether the band structures of Cu2 O and TiO2 match well with each other in the Cu2 O/TiO2 composite, valence-band XPS spectra in addition to the UV–visible absorption spectra were adopted to study the electronic band structures of Cu2 O and TiO2 (figure 6). The Cu2 O was prepared via the same synthetic method as the Cu2 O/TiO2 nanojunction, and the detailed characterizations are shown in the supporting information (available at stacks. iop.org/Nano/25/165402/mmedia). As shown in the insets of figures 6(a) and (b), the UV–visible absorption spectra demonstrate that the absorption edges of TiO2 and Cu2 O are nearly 371 and 644 nm, respectively. TiO2 is known as an indirect semiconductor, while Cu2 O is a direct semiconductor. As a semiconductor, the band gaps can be calculated by the equation of αhν = A(hν − E g )n/2 , in which α is the absorption coefficient, hν is the incident photon energy, A is the proportionality constant, E g is the band gap, and n is equal to 1 and 4 for direct and indirect semiconductors, respectively [12, 35]. According to this Butler equation, plotting (αhν)1/2 or (αhν)2 versus hν from the absorption spectra, and extrapolating the linear part to zero on an abscissa axis, from the intersections of the straight lines obtained we can deduce the bandgaps of 3.2 and 2.1 eV for TiO2 and Cu2 O, respectively (figures 6(a) and (b)). Then, from the valence-band XPS spectra, the valenceband edges were deduced to be 2.8 and 1.0 eV for TiO2 (figure 6(c)) and Cu2 O (figure 6(d)), respectively. Finally, on the basis of the above experimental results, we can draw a schematic illustration of the band structures of Cu2 O and TiO2 as shown in figure 6(e). As shown in figure 6(e), the conduction band edge of Cu2 O (−1.1 eV) is much higher than that of TiO2 (−0.4 eV), while the valence band edge of TiO2 (2.8 eV) is lower than that of Cu2 O (1.0 eV); the band
Figure 5. UV–visible absorption spectra of the prepared TiO2 and Cu2 O/TiO2 nanojunction.
the Cu2 O/TiO2 nanojunction, respectively. Both the large total pore volume and surface area strongly support the fact that our developed Cu2 O/TiO2 nanojunction has the porous structure, which is beneficial for the CO2 adsorption in the photocatalytic reaction. In addition, to investigate the CO2 adsorption ability on the porous TiO2 and Cu2 O/TiO2 nanojunction, CO2 pulse chemisorption tests were carried out. As shown in figure S1 and table S1 (available at stacks.iop.org/Nano/25/165402/ mmedia), the amounts of adsorbed molecular CO2 were calculated to be 0.12 and 0.27 ml g−1 for our developed porous TiO2 and Cu2 O/TiO2 nanojunction, respectively. The better CO2 adsorption ability of the Cu2 O/TiO2 nanojunction is attributed to its much sharper pore size distribution in the micropore region and its larger total pore volume (0.29 cm3 g−1 ) than TiO2 (0.21 cm3 g−1 ). In comparison, the Pt-loaded porous TiO2 and the commercial P25 TiO2 powder were also tested. Pt–TiO2 shows similar BET surface area and CO2 adsorption ability to TiO2 , while the P25 TiO2 powder with a lower surface area (51.1 m2 g−1 ) and smaller total pore volume (0.10 cm3 g−1 ) exhibits a relatively limited CO2 adsorption, about 0.05 ml g−1 . These results further experimentally confirm that the porous structure with a larger total pore volume is beneficial for the CO2 adsorption, especially in the case of smaller pore size. In the CO2 photoreduction test, the 5
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Figure 6. UV–visible absorption spectra (inset) as well as their related estimation of band gaps of TiO2 (a) and Cu2 O (b). Valence-band XPS spectra of TiO2 (c) and Cu2 O (d). Schematic illustration of the determined valence band and conduction band edges of TiO2 and Cu2 O (e).
Figure 7. (a) CH4 evolution over the prepared TiO2 , Cu2 O/TiO2 nanojunction, Pt–TiO2 , and P25 under UV–visible light irradiation (λ > 300 nm) in 10 h. (b) Average CH4 evolution rates of the prepared TiO2 , Cu2 O/TiO2 nanojunction, Pt–TiO2 , and P25.
structures of Cu2 O and TiO2 match well with each other. In this situation, under light illumination, the electrons will transfer from Cu2 O to TiO2 , while the holes will move from TiO2 to Cu2 O, effectively realizing the charge separation. The CO2 photoreduction tests of the as-prepared TiO2 and Cu2 O/TiO2 nanojunction material were evaluated under UV– visible light irradiation (figure 7(a)). For purposes of compari-
son, the Pt-loaded porous TiO2 (Pt–TiO2 ) and commercial P25 were also tested under the same experimental conditions. As shown in figure 7(b), the CH4 evolution rates were calculated to be 2.4, 28.4, 3.2, and 3.7 ppm g−1 h−1 for the TiO2 , Cu2 O/TiO2 nanojunction, Pt–TiO2 , and P25, respectively. Among these samples, the Cu2 O/TiO2 nanojunction exhibits the highest photocatalytic activity, about 12, 9, and 7.5 times higher than 6
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Scheme 3. Schematic illustration of the charge transfer and CO2 reduction processes occurring on the Cu2 O/TiO2 nanojunction (a), porous TiO2 (b), Pt-loaded porous TiO2 (c), and commercial P25 powder (d) under UV–visible light irradiation.
TiO2 , Pt–TiO2 , and P25, respectively. Besides CH4 , trace amounts of CO can also be detected. In addition, controlled experiments show that no CH4 was evolved when the photocatalytic reaction was carried out in the dark or irradiated in the absence of photocatalyst, revealing that the reduction of CO2 is driven by light irradiation over the photocatalyst. The isotopic tracer experiments were also processed to investigate the reaction mechanism of CO2 photoreduction (figure S3 available at stacks.iop.org/Nano/25/165402/mmedia). The results showed that the C and H in the product CH4 are mainly from CO2 (figure S3(a) available at stacks.iop.org/Nano/25/165402/ mmedia) and H2 O (figure S3(b) available at stacks.iop.org/ Nano/25/165402/mmedia), respectively. The well designed porous structure combined with the closely contacted interface of the Cu2 O/TiO2 nanojunction plays important roles in enhancing the photocatalytic activity. First, in contrast to pure TiO2 , the superior activity of the Cu2 O/TiO2 material can be attributed to its well designed nanojunction structure. As shown in scheme 3(a), once excited, the electrons will transfer from Cu2 O to, and accumulate on, the TiO2 , while the holes will move from TiO2 to Cu2 O, to realize the charge separation, while for pure TiO2 (scheme 3(b)), the electrons and holes easily recombine on the surface, leading to a poor charge separation efficiency and lower photocatalytic activity. Then, after Pt loading, the photocatalytic activity of Pt–TiO2 is slightly enhanced compared with pure TiO2 , but still lower than that of the Cu2 O/TiO2 nanojunction. As shown in scheme 3(c), Pt has a lower Fermi energy than TiO2 , so Pt–TiO2 can enhance the charge separation efficiency by pushing the photogenerated electrons transfer from the conduction band of TiO2 to the surface Pt sites; then, the accumulated electrons on the Pt sites reduce CO2 to CH4 . However, most of the molecular CO2 is adsorbed on the porous TiO2 , and to finish the whole CO2 photoreduction process the molecular CO2 on the TiO2 surface needs to migrate dozens of nanometers to the Pt sites; during this migration, the charges (electrons and holes) are easily recombined. In the case of our developed Cu2 O/TiO2 nanojunction, as shown in scheme 3(a), the CO2 adsorption and electron accumulation occur simultaneously on the TiO2 surface, thus the distance for electron migration becomes shorter and the ratio for recombination on the surface becomes lower, leading to higher activity.
Furthermore, compared with commercial P25 TiO2 powder, which is composed of 80% anatase and 20% rutile (scheme 3(d)), our developed Cu2 O/TiO2 nanojunction exhibits 7.5 times higher activity in the CH4 evolution (figure 7). The higher activity of the Cu2 O/TiO2 nanojunction in contrast to P25 might be ascribed to the porous structure and larger surface area, as shown in table 1, which facilitate the CO2 adsorption (0.27 ml g−1 for Cu2 O/TiO2 nanojunction versus 0.05 ml g−1 for P25) and can offer more reaction active sites. On the other hand, our designed Cu2 O/TiO2 nanojunction has a smaller particle size (about 5–8 nanometer) than that of P25 TiO2 (around 25 nm). With a smaller particle size, the migration distance for the electrons and holes from the bulk to surface can be greatly reduced, which can somewhat inhibit the charge recombination in the bulk. For Cu2 O/TiO2 composite material, the content of Cu2 O in the composites might affect their photocatalytic properties. Accordingly, Cu2 O/TiO2 composites in different mass ratios (such as 1:1, 1:3, and 1:7) were prepared to investigate their photocatalytic properties. As shown in figure S4(a) (available at stacks.iop.org/Nano/25/165402/mmedia), with more Cu2 O contained in the composite material, the intensity of the relative peak around 36.22◦ (the diffraction of the (111) plane of Cu2 O) increased; in parallel, the light absorption edge of the composites was also red-shifted to the longer wavelength region (figure S4(b) available at stacks.iop.org/ Nano/25/165402/mmedia). When testing the photocatalytic activities in CO2 photoreduction, the CH4 evolution rates were calculated to be 2.4, 4.0, 28.4, 9.4, 8.5, and 5.9 ppm g−1 h−1 for TiO2 , CT1:7 , CT1:5 , CT1:3 , CT1:1 , and Cu2 O, respectively (figures S4(c)–(d) available at stacks.iop.org/Nano/25/165402/ mmedia). Obviously, all the Cu2 O/TiO2 composites exhibit better photocatalytic activity than TiO2 , Pt–TiO2 , or P25. With the optimized concentration, the Cu2 O/TiO2 nanojunction with the mass ratio of 1:5 exhibits the highest CH4 evolution rates. 4. Conclusions
We have successfully prepared porous-structured Cu2 O/ TiO2 nanojunction material with a size of several nanometers. The well designed nanojunction structure facilitates the charge separation between Cu2 O and TiO2 ; while the porous structure is beneficial for the CO2 adsorption. These combined 7
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properties endow the Cu2 O/TiO2 nanojunction with marked photocatalytic performance in CO2 photoreduction, about 12, 9, and 7.5 times higher than that of TiO2 , Pt–TiO2 , and the commercial P25 TiO2 , respectively. This study demonstrates that construction of porous structure coupled with a nanojunction is an interesting strategy to enhance the photocatalytic activity of CO2 photoreduction. We anticipate that this concept can provide a versatile route to fabricate other porous-structured heterojunction materials for higher solar-to-energy conversion efficiency.
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Acknowledgments
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