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Cu2O decorated carbon-incorporated TiO2 microspheres with enhanced visible light photocatalytic activity
Materials Research Bulletin 74 (2016) 265–270
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Cu2O decorated carbon-incorporated TiO2 microspheres with enhanced visible light photocatalytic activity Yalin Fang, Mingxuan Sun* , Ying Wang, Shanfu Sun, Jia He School of Materials Engineering, Shanghai University of Engineering Science, Shanghai 201620, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 May 2015 Received in revised form 3 September 2015 Accepted 29 October 2015 Available online 1 November 2015
Carbon-incorporated TiO2 microspheres (CTMs), produced by a flame assisted hydrolysis method, were successfully decorated with Cu2O nanoparticles via a chemical bath deposition process. A red shift of absorption edge and enhanced visible light absorption were observed for the resulted Cu2O/CTMs composites compared with bare TiO2 and CTMs. Their effective visible-light-driven photocatalytic activity and the effect of deposition amount of Cu2O on the photocatalytic properties were determined by the degradation of methylene blue. The highest degradation rate was achieved for Cu2O/CTMs composite (29.2%) compared to CTMs (18.5%) and bare TiO2 (7.3%) under visible light illumination for 80 min. The improved photoactivity can be ascribed to the integrative synergistic effect of carbon and Cu2O on TiO2. This work develops a rapid and facile approach for the fabrication of Cu2O decorated carbon-incorporated TiO2 microspheres composites and demonstrates it is an efficient route to improve the photocatalytic activity of TiO2. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: A. Composites A. Oxides A. Surfaces B. Chemical synthesis B. Microstructure D. Catalytic properties
1. Introduction Energy crisis and environmental pollution have become two major problems for human society. Photocatalytic technique of semiconductor has great potential for the utilization of solar energy to solve environmental challenges. Among the reported semiconductors, TiO2 has attracted much attention due to its low cost, non-toxicity and high stability [1–4]. However, the fast recombination of photogenerated electron-holes and activation only under ultraviolet light irradiation of TiO2 greatly reduce its utilization efficiency of solar light and hamper its full potential application. To date, numerous strategies have been explored to extend its optical response into the visible region and restrain the recombination of electron-hole pairs, such as doping with metal [5,6] or non-metal [7,8] and coupling with narrow bandgap semiconductors [9,10]. Among these, modification of TiO2 with other semiconductors (such as SnO2 [11,12], ZnO [13,14], CdS [15], a-Fe2O3 [16,17], Nb2O5 [18,19], SrTiO3 [20,21], etc.) to form a heterojunction is a smart means to enhance its photocatalytic performance. Cubic cuprous oxide (Cu2O), a p-type semiconductor with a direct forbidden band gap of about 2.1 eV, has been regarded as a promising photocatalytic material for the conversion of solar
* Corresponding author. E-mail addresses: [email protected], [email protected] (M. Sun). http://dx.doi.org/10.1016/j.materresbull.2015.10.053 0025-5408/ ã 2015 Elsevier Ltd. All rights reserved.
energy into electrical or chemical energy, due to its nontoxicity, environment friendly and natural abundance [22,23]. It has been proved that the modification of TiO2 with Cu2O nanoparticles is an effective way to extend the photosensitivity of TiO2 into the visible light. In addition, the matching of the band structure between Cu2O and TiO2 also favors the effective separation of photoinduced electron-hole pairs [24–28]. Zhang et al. [29] decorated Cu2O nanoparticles onto the inner surfaces and interfaces of TiO2 nanotube arrays via electrochemical deposition technique. The Cu2O-TiO2 composites exhibited considerably enhanced photocurrent response and higher photocatalytic or photoelectrocatalytic degradation rate of methyl orange under visible light irradiation. Carbon species incorporated into TiO2 can be also a good choice to modified TiO2 [30–33], which has been reported to extend light absorption range into visible region and improve the separation of photoinduced electrons and holes. Significant studies inclined to combine TiO2 with carbon species, such as graphene [34], carbon nanotube [35], carbon nanodots [36], and so on. Zhang et al. [37] reported carbon-incorporated TiO2 microspheres exhibited remarkable photoresponse and excellent photocatalytic activity. In an earlier paper, we demonstrated that graphene-TiO2 nanorod hybrid composites showed high performance in photocatalytic degradation of methylene blue under visible light illumination [38]. Furthermore, we also revealed the sensitizing effect of carbon quantum dots on the photoelectrochemical and photocatalytic performance of TiO2 nanotube arrays under visible light [39].
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2. Experimental All chemical reagents were of analytical grade and used as received. Deionized water was used to prepare all the solution. The synthesis procedures of Cu2O/CTMs are illustrated in Scheme 1. Carbon-incorporated TiO2 microspheres (CTMs) were prepared by a flame-assisted hydrolysis of tetrabutyl orthotitanate (TBOT) process [37]. In a typical procedure, 5 mL TBOT was added into a 100 mL beaker containing 35 mL ethanol. Subsequently, stirring was applied for a period of time to obtain clear and stable solution. Finally, the as-prepared solution was ignited and yellow flame immediately spread over the liquid surface. CTMs were obtained after the burning out of the ethanol solution. A chemical bath deposition process was developed to couple Cu2O nanoparticles onto the surface of the CTMs. Typically, CTMs (0.2 g) were added into ethanol solution of Cu(CH3COO)2H2O with a series concentration (0.005 M, 0.03 M and 0.06 M), followed by ultrasonic treatment for 1 h to allow the adsorption of Cu ions onto the surface of CTMs. Subsequently, the powders were collected by centrifugation and added into (10 mL, 0.1 M) NaOH solution. Ultrasound irradiation was again applied to disperse the powders. The suspension solution was maintained at 60 C in a water bath and an aqueous solution of (10 mL, 0.1 M) glucose was slowly dropped into the above suspension with continuous stirring for 10 min. After the addition of glucose, Cu2+ ions were reduced to Cu+ ions. Thus, Cu2O nanoparticles were deposited onto the surface of CTMs as follows: HOCH2(CHOH)4CHO + 2Cu(OH)2!HOCH2(CHOH)4COOH + Cu2O + 2 H2O Finally, the obtained precipitate was rinsed with deionized water and ethanol for several times, and dried at 60 C for 10 h. The as-prepared samples were labeled as Cu2O/CTMs (x), where x represents the concentration of Cu(CH3COO)2H2O ethanol solution. The phase composition and crystal structure were characterized by X-ray diffraction (XRD) (PANalytical X’Pert, Holland) using Cu Ka radiation operated at 40 kV and 40 mA. Field emission scanning electron microscope (FE-SEM) (JEOL JSM-7000F, Japan) and transmission electron microscope (TEM) (FEI Tecnai F20, USA) were used to investigate the morphologies and microstructures of the samples. The UV–vis diffuse reflectance spectra of the samples were recorded by UV–vis spectrophotometer (Shimadzu UV 3600, Japan). X-ray photoelectron spectroscopy (XPS) was measured
using thermo ESCALAB 250XI system with Al Ka radiation as exciting X-ray source. The specific surface area of the as-prepared samples were detected by N2 adsorption-desorption analysis conducted at 77K (Micromeritics ASAP 2020 V4.01, USA). The photocatalytic activity of the photocatalysts was evaluated by the degradation of MB solution under visible light illumination. Typically, 7.5 mg of the photocatalysts was added to 15 mL of 5 mg/ L MB solution, followed by stirring for 2 h in the dark to establish adsorption/desorption equilibrium of MB. Then, the mixture solution was irradiated under 500 W Xenon lamp (CHF-XM35, Beijing) light source. An optical filter was used to cut off wavelength below 420 nm to provide visible light with intensity 38.0 mW cm2. The samples were tested under continuous stirring at 20 min intervals for 80 min. UV–vis spectrophotometer (Shimadzu UV 1601-PC, Japan) was applied to detect the adsorption spectrum of MB solution in the wavelength range of 400-800 nm. The relative concentration of MB in the solution was determined by the absorbance at 664 nm. In addition, the photocatalytic measurement of bare TiO2 (P25) was performed under the same condition for comparison. 3. Results and discussion 3.1. Characterizations Fig. 1 shows the XRD patterns of the as-prepared CTMs and Cu2O/CTMs (0.005, 0.03 and 0.06). The typical diffraction peaks at 25.3 , 37.9 , 48.1, 53.9 , 55.1, and 62.7 for CTMs and Cu2O/CTMs are assigned to (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 11), and (2 0 4) crystal planes of anatase TiO2, respectively. The result indicated that anatase TiO2 can be prepared by flame assisted hydrolysis of TBOT alcohol solution without further annealing treatment. The burning flame of ethanol can provide a high temperature above 400 C. Thus, TiO2 is gradually crystallized with the assistance of the ethanol burning. Compared with the XRD pattern of CTMs, no obvious peaks for Cu2O were observed for Cu2O/CTMs (0.005 and 0.03), which is ascribed to the small amount and weak intensity of Cu2O. A slight additional diffraction peak at 36.4 can be observed for Cu2O/CTMs (0.06), which was assigned to (111) planes of Cu2O (PDF Card No.00-003-0892). However, the peak is so weak that we cannot directly confirm that Cu2O is deposited onto the surface of CTMs by XRD. The presence of Cu2O in Cu2O/CTMs composites is further confirmed by SEM, TEM and XPS results, as described later. Fig. 2 presents the FE-SEM images of CTMs (A and B) and Cu2O/ CTMs (C and D). The morphology of bare TiO2 was investigated in our earlier paper [38], which was not shown here. As shown in Fig. 2A and 2B, the profile of single microsphere can be clearly distinguished with diameter around 1.0–2.0 mm, which indicate microsphere structure can be obtained without template. The constituent nanocrystals of CTMs are strongly combined together and cannot be broken apart even under grinding or ultra-
d
Intensity (a.u.)
Microsphere structure could bring multiple reflection of incident light, leading to outstanding photoabsorption associated with light scattering [40]. Thus, TiO2 microsphere has attracted much attention owing to its large specific surface area and excellent light absorption performance. In this paper, Cu2O decorated carbon-incorporated TiO2 microspheres were synthesized and their enhanced photodegradation of methylene blue under visible light was confirmed. To the best of our knowledge, Cu2O decorated carbon-incorporated TiO2 microspheres ternary composites have not been reported before. In addition, the synthesis of carbon-incorporated TiO2 microspheres and the loading of Cu2O both possessed rapid, facile and clean properties.
c b a
Cu2O
20
Scheme 1. Schematic illustration for preparation of Cu2O/CTMs.
30
40 50 60 2Theta (degree)
TiO2
70
80
Fig. 1. XRD patterns of bare CTMs (a) and Cu2O/CTMs ((b) 0.005, (c) 0.03, (d) 0.06).
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Fig. 2. The SEM images of CTMs (A) and (B), Cu2O/CTMs (0.06) composites (C) and (D).
sonication. The progress of the formation of TiO2 microsphere can be concluded to be the hydrolysis reaction of TBOT with water generated from alcohol combustion, in which the titania nuclei, formed in the initial state, gradually grow bigger to form TiO2 sphere [41,42]. As shown in Fig. 2C and 2D, cubic Cu2O nanoparticles are observed with average size of about 60 nm on the surface of CTMs, which imply that Cu2O/CTMs are successfully obtained. The morphology of the as-prepared Cu2O/CTMs composites is further studied by TEM. As shown in Fig. 3, CTMs with average diameter about 2 mm and the cubic Cu2O nanoparticles (Fig. 3B) are further clearly presented. The results are in accordance with the SEM results. The surface chemical composition and valence states of Cu2O/ CTMs composites were further determined by XPS analysis (Fig. 4). Fig. 4A illustrates the XPS spectra of Cu2O/CTMs composites, which
exhibits the presence of C, Ti, O and Cu elements. The peaks with binding energy of 458.5 eV and 464.2 eV are assigned to Ti 2p3/2 and Ti 2p1/2 for Ti (IV) of the titania, respectively, which confirm the present of TiO2 (Fig. 4B). Fig. 4C displays two characteristic peaks with binding energies at 952.2 eV and 932.2 eV, which can be identified for Cu 2p1/2 and Cu 2p3/2, respectively. The binding energies can be ascribed to Cu (I) of Cu2O, which can further confirm that Cu2O is successfully assembled onto the surface of the CTMs via the chemical bath deposition process. The results are consistent with the XRD, SEM and TEM analyses. Fig. 4D exhibits the deconvolution of C 1s peak which has four fitting curves centered at 284.6 eV, 285.7 eV, 286.7 eV, and 288.3 eV. The strong peak at 284.6 eV is assigned to elemental carbon, arising from the incomplete burning of organic compounds during the flame assisted hydrolysis [37]. The curves at 285.7 eV, 286.7 eV, and
Fig. 3. The TEM (A) and HR-TEM (B) images of Cu2O/CTMs (0.06).
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(A)
a
a
Ti 2p Cu 2p
(eV)1/2
b e
1/2
b d e c
(α hv)
Absorbance (a.u.)
Intensity
O 1s
d c 2.0 2.5 3.0 3.5 4.0 4.5 hv (eV)
C1s
1000 900 800 700 600 500 400 300 200 Binding Energy (eV)
Intensity
(B)
Ti 2p1/2
468 466 464 462 460 458 456 454 452 Binding Energy (eV)
(C)
Intensity
Cu 2p3/2
Cu 2p1/2
960 955 950 945 940 935 930 Binding Energy (eV)
Intensity
285.7
290
286.7
288
286
800
284
absorption properties of Cu2O/CTMs were investigated. The optical absorption edges for Cu2O/CTMs (0.005, 0.03 and 0.06) are 458 nm, 481 nm and 469 nm, respectively. As shown in the insert, a plot of the square root of absorbance coefficient versus the energy of light affords the band gap. The optical band gap energy is determined to be 3.10 eV and 2.80 eV for bare TiO2 and CTMs, while that of Cu2O/ CTMs (0.005), (0.03) and (0.06) is estimated to be 2.75 eV, 2.55 eV, and 2.65 eV, respectively. Obviously, the absorption edges firstly shift to longer wavelength and then shift to shorter wavelength with the increase of the Cu2O contents. The longest absorption edge is the resulted Cu2O/CTMs (0.03) sample. Furthermore, it is obvious that all of the absorption edges for Cu2O/CTMs are longer than that of CTMs. The absorption range of light played an important role in the photocatalysis, especially for the visiblelight-driven photodegradation of contaminants. Thus, the enhanced visible light absorption and narrow band gap promised that Cu2O/CTMs were more active than CTMs under visible light illumination. 3.2. Photocatalytic measurements
284.6
288.3
400 500 600 700 Wavelength (nm)
Fig. 5. The UV–vis diffuse reflectance absorption spectra of bare TiO2 (a), CTMs (b) and Cu2O/CTMs ((c) 0.005, (d) 0.03, (e) 0.06). The inset shows the corresponding hv vs. (ahv)1/2 curves of as-prepared samples.
Ti 2p3/2
(D)
300
282
Binding Energy (eV) Fig. 4. XPS spectra for Cu2O/CTMs (0.06): survey spectrum (A), Ti 2p (B), Cu 2p (C), C 1s (D).
288.3 eV were corresponded to C O bonds, caused by the insufficient hydrolysis of TBOT [43]. The peak ascribed to Ti C bonds at around 281.0 eV has not been detected, indicating that the carbon species incorporated in TiO2 matrix is present as elemental state [44]. The UV–vis absorption spectra of the prepared samples are shown in Fig 5. As shown in Fig. 5, compared to that of bare TiO2 (387 nm), the absorption edges of the CTMs (402 nm) apparently shifted to the visible region with enhanced absorption intensity. This result indicates that the narrowing of the band gap of TiO2 occurs with the carbon incorporation, which can be act as sensitizer like dyes [34].The effect of Cu2O contents on the optical
The photodegradation of MB was employed to detect the photocatalytic activity of the as-prepared samples. The adsorption capacity of as-prepared pure TiO2, CTMs and Cu2O/CTMs was evaluated by monitoring the residual concentration fraction of MB after equilibrium in the dark for 120 min, as shown in Fig. 6A. The adsorption rates of MB are 5.5% and 40.0% for bare TiO2 and CTMs, respectively. It can be observed that as-prepared CTMs showed higher adsorption capacity than that of bare TiO2 (approximately 8 times). The improvement of the as-prepared samples in adsorption capacity should be attributed to the enhanced specific surface area for microsphere structure [45,46]. However, adsorption capacity decreased with the introduction of the Cu2O nanoparticles on the surface of CTMs. The specific surface areas of the as-prepared samples were further investigated with N2 adsorption-desorption analysis. The values of the BET surface areas were 51 m2 g1, 103 m2 g1, and 90 m2 g1 for bare TiO2, CTMs, and Cu2O/CTMs (0.005). Therefore, there was an appreciable increase in the surface area of CTMs. Upon Cu2O decoration, the specific surface areas of CTMs decreased, which was consistent with the literature [47]. Compared with CTMs, the decreased adsorption capacity of Cu2O/CTMs may be attributed to the smaller specific surface areas. Fig. 6B depicted the TiO2, CTMs, and Cu2O/CTMs induced photocatalytic degradation of MB under visible light illumination. After 80 min illumination, the degradation rate for CTMs was 18.5%, indicating the photocatalytic activity of CTMs is higher than that of pure TiO2 (7.3%). Cu2O/CTMs presents even higher degradation rate compared to that of CTMs. The influence of
Y. Fang et al. / Materials Research Bulletin 74 (2016) 265–270
(A) Remaing Concentration Fraction of MB
1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3
Initial
a
b
c
e
d
(B)
(Scheme 2). Efficient separation of photogenerated electron-hole pairs and the use of visible light can also be achieved by Cu2O in Cu2O/CTMs. Once Cu2O/CTMs is irradiated by visible light, photogenerated electrons can transfer from the conduction band (CB) of Cu2O to the CB of TiO2, while holes remain on the valance band (VB) of Cu2O. On the surface of the CTMs, the accumulated electrons can be transferred to the adsorbed oxygen and change it to O or O2 [29]. Furthermore, the photogeneraged holes of CTMs can migrate to the VB of Cu2O. These holes can react with water molecules or hydroxide ions (OH) to produce hydroxyl radicals (OH) [48]. Both of the O/O2 and OH have strong oxidation ability and can decompose the MB into CO2 and H2O. In a word, both Cu2O and carbon afford the effective utilization of solar energy for Cu2O/ CTMs composites. 4. Conclusions
1.00 a
0.95 0.90
Ct/C0
269
0.85
b
e c d
0.80 0.75 0.70 0
20
40 60 Time (min)
80
Fig. 6. (A) Bar plot showing the remaining MB in solution: initial and equilibrated with bare TiO2 (a), CTMs (b) and Cu2O/CTMs ((c) 0.005, (d) 0.03, (e) 0.06) in the dark after stirring. (B) Photodegradation of MB under visible light: bare TiO2 (a), CTMs (b) and Cu2O/CTMs ((c) 0.005, (d) 0.03, (e) 0.06).
Cu2O content in Cu2O/CTMs on the photocatalytic performance was also investigated. The degradation rate is 25.5% for the Cu2O/ CTMs (0.005). The degradation rates enhanced was observed with the increasing of Cu2O content and the highest degradation rate was 29.2% by Cu2O/CTMs (0.03). Further increasing the Cu2O content, the degradation rate decreased to 20.8% for Cu2O/CTMs (0.06). All the above observation indicates that the introduction of Cu2O plays an active part in the photocatalytic degradation process of MB. The enhanced photocatalytic activity of Cu2O/CTMs composites in the visible light irradiation can be attributed to the synergistic effect of carbon elements and Cu2O nanoparticles. The incorporated carbon elements act as graphite-like carbon, preventing the recombination of photogenerated electron-hole pairs and extending the visible light response range [33,34]. Meanwhile, the band structure of Cu2O matches well with TiO2 in CTMs composites
Scheme 2. The schematic mechanism of photocatalytic activity for Cu2O/CTMs.
We have successfully fabricated carbon-incorporated TiO2 microspheres by flame-assisted hydrolysis method, and decorated cubic Cu2O nanoparticles on the surface of CTMs through a chemical bath deposition process. The combination of Cu2O leads to the improved visible light absorbance of CTMs. The Cu2O/CTMs composites exhibits enhancement of photocatalytic degradation rate of MB under visible light irradiation compared with bare TiO2 and CTMs. In particular, with loading of the appropriate amount of Cu2O nanoparticles, Cu2O/CTMs (0.03) composite achieves the highest degradation rate of 29.2%, as 1.6 times larger than that of CTMs. Both Cu2O and carbon are responsible for the improvement of the photocatalytic activity of TiO2. This study provides a new technique to improve the photocatalytic activity of TiO2 and paves an alternative way for novel insight to seeking new and desirable photocatalysts. Acknowledgments This work was financially supported by Innovation Program of Shanghai Municipal Education Commission (15ZZ092), Training Program for Young Teachers in Shanghai Colleges and Universities (ZZgcd14010), Shanghai University of Engineering Science Innovation Fund (14KY0511, 15KY0516), and Startup Foundation of Shanghai University of Engineering Science (NO. 2014-22). References [1] A.L. Linsebigler, G. Lu, J.T. Yates, Photocatalysis on TiO2 surface, Chem. Rev. 95 (1995) 735. [2] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Light-induced amphiphilic surfaces, Nature 388 (1997) 431. [3] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Visible-light photocatalysis in nitrogen-doped titanium oxides, Science 293 (2001) 269. [4] D. Li, H. Haneda, S. Hishita, N. Ohashi, Visible-light-driven N-F-codoped TiO2 photocatalysts. 2. Optical characterization, photocatalysis, and potential application to air purification, Chem. Mater. 17 (2005) 2596–2602. [5] S.S.R. Putluru, L. Schill, A.D. Jensen, B. Siret, F. Tabaries, R. Fehrmann, Mn/TiO2 and Mn-Fe/TiO2 catalysts synthesized by deposition precipitation-promising for selective catalytic reduction of NO with NH3 at low temperatures, Appl. Catal. B-Environ. 165 (2015) 628–635. [6] S.N.R. Inturi, T. Boningari, M. Suidan, P.G. Smirniotisa, Visible-light-induced photodegradation of gas phase acetonitrile using aerosol-made transition metal (V, Cr, Fe, Co, Mn, Mo, Ni, Cu, Y, Ce, and Zr) doped TiO2, Appl. Catal. BEnviron. 144 (2014) 333–342. [7] M.X. Sun, P. Song, J. Li, X.L. Cui, Preparation, characterization and application of novel carbon and nitrogen codoped TiO2 nanoparticles from annealing TiN under CO atmosphere, Mater. Res. Bull. 48 (2013) 4271–4276. [8] W. Yu, X.J. Liu, L.K. Pan, J.L. Li, J.Y. Liu, J. Zhang, P. Li, C. Chen, Z. Sun, Enhanced visible light photocatalytic degradation of methylene blue by F-doped TiO2, Appl. Surf. Sci. 319 (2014) 107–112. [9] J.F. Zhang, W. Yan, C.P. Yu, X. Shu, L. Jiang, J.W. Cui, Z. Chen, T. Xie, Y.C. Wu, Enhanced visible-light photoelectrochemical behaviour of heterojunction composite with Cu2O nanoparticles-decorated TiO2 nanotube arrays, New J. Chem. 38 (2014) 4975–4984.
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Cu2O decorated carbon-incorporated TiO2 microspheres with enhanced visible light photocatalytic activity Yalin Fang, Mingxuan Sun* , Ying Wang, Shanfu Sun, Jia He School of Materials Engineering, Shanghai University of Engineering Science, Shanghai 201620, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 May 2015 Received in revised form 3 September 2015 Accepted 29 October 2015 Available online 1 November 2015
Carbon-incorporated TiO2 microspheres (CTMs), produced by a flame assisted hydrolysis method, were successfully decorated with Cu2O nanoparticles via a chemical bath deposition process. A red shift of absorption edge and enhanced visible light absorption were observed for the resulted Cu2O/CTMs composites compared with bare TiO2 and CTMs. Their effective visible-light-driven photocatalytic activity and the effect of deposition amount of Cu2O on the photocatalytic properties were determined by the degradation of methylene blue. The highest degradation rate was achieved for Cu2O/CTMs composite (29.2%) compared to CTMs (18.5%) and bare TiO2 (7.3%) under visible light illumination for 80 min. The improved photoactivity can be ascribed to the integrative synergistic effect of carbon and Cu2O on TiO2. This work develops a rapid and facile approach for the fabrication of Cu2O decorated carbon-incorporated TiO2 microspheres composites and demonstrates it is an efficient route to improve the photocatalytic activity of TiO2. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: A. Composites A. Oxides A. Surfaces B. Chemical synthesis B. Microstructure D. Catalytic properties
1. Introduction Energy crisis and environmental pollution have become two major problems for human society. Photocatalytic technique of semiconductor has great potential for the utilization of solar energy to solve environmental challenges. Among the reported semiconductors, TiO2 has attracted much attention due to its low cost, non-toxicity and high stability [1–4]. However, the fast recombination of photogenerated electron-holes and activation only under ultraviolet light irradiation of TiO2 greatly reduce its utilization efficiency of solar light and hamper its full potential application. To date, numerous strategies have been explored to extend its optical response into the visible region and restrain the recombination of electron-hole pairs, such as doping with metal [5,6] or non-metal [7,8] and coupling with narrow bandgap semiconductors [9,10]. Among these, modification of TiO2 with other semiconductors (such as SnO2 [11,12], ZnO [13,14], CdS [15], a-Fe2O3 [16,17], Nb2O5 [18,19], SrTiO3 [20,21], etc.) to form a heterojunction is a smart means to enhance its photocatalytic performance. Cubic cuprous oxide (Cu2O), a p-type semiconductor with a direct forbidden band gap of about 2.1 eV, has been regarded as a promising photocatalytic material for the conversion of solar
* Corresponding author. E-mail addresses: [email protected], [email protected] (M. Sun). http://dx.doi.org/10.1016/j.materresbull.2015.10.053 0025-5408/ ã 2015 Elsevier Ltd. All rights reserved.
energy into electrical or chemical energy, due to its nontoxicity, environment friendly and natural abundance [22,23]. It has been proved that the modification of TiO2 with Cu2O nanoparticles is an effective way to extend the photosensitivity of TiO2 into the visible light. In addition, the matching of the band structure between Cu2O and TiO2 also favors the effective separation of photoinduced electron-hole pairs [24–28]. Zhang et al. [29] decorated Cu2O nanoparticles onto the inner surfaces and interfaces of TiO2 nanotube arrays via electrochemical deposition technique. The Cu2O-TiO2 composites exhibited considerably enhanced photocurrent response and higher photocatalytic or photoelectrocatalytic degradation rate of methyl orange under visible light irradiation. Carbon species incorporated into TiO2 can be also a good choice to modified TiO2 [30–33], which has been reported to extend light absorption range into visible region and improve the separation of photoinduced electrons and holes. Significant studies inclined to combine TiO2 with carbon species, such as graphene [34], carbon nanotube [35], carbon nanodots [36], and so on. Zhang et al. [37] reported carbon-incorporated TiO2 microspheres exhibited remarkable photoresponse and excellent photocatalytic activity. In an earlier paper, we demonstrated that graphene-TiO2 nanorod hybrid composites showed high performance in photocatalytic degradation of methylene blue under visible light illumination [38]. Furthermore, we also revealed the sensitizing effect of carbon quantum dots on the photoelectrochemical and photocatalytic performance of TiO2 nanotube arrays under visible light [39].
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2. Experimental All chemical reagents were of analytical grade and used as received. Deionized water was used to prepare all the solution. The synthesis procedures of Cu2O/CTMs are illustrated in Scheme 1. Carbon-incorporated TiO2 microspheres (CTMs) were prepared by a flame-assisted hydrolysis of tetrabutyl orthotitanate (TBOT) process [37]. In a typical procedure, 5 mL TBOT was added into a 100 mL beaker containing 35 mL ethanol. Subsequently, stirring was applied for a period of time to obtain clear and stable solution. Finally, the as-prepared solution was ignited and yellow flame immediately spread over the liquid surface. CTMs were obtained after the burning out of the ethanol solution. A chemical bath deposition process was developed to couple Cu2O nanoparticles onto the surface of the CTMs. Typically, CTMs (0.2 g) were added into ethanol solution of Cu(CH3COO)2H2O with a series concentration (0.005 M, 0.03 M and 0.06 M), followed by ultrasonic treatment for 1 h to allow the adsorption of Cu ions onto the surface of CTMs. Subsequently, the powders were collected by centrifugation and added into (10 mL, 0.1 M) NaOH solution. Ultrasound irradiation was again applied to disperse the powders. The suspension solution was maintained at 60 C in a water bath and an aqueous solution of (10 mL, 0.1 M) glucose was slowly dropped into the above suspension with continuous stirring for 10 min. After the addition of glucose, Cu2+ ions were reduced to Cu+ ions. Thus, Cu2O nanoparticles were deposited onto the surface of CTMs as follows: HOCH2(CHOH)4CHO + 2Cu(OH)2!HOCH2(CHOH)4COOH + Cu2O + 2 H2O Finally, the obtained precipitate was rinsed with deionized water and ethanol for several times, and dried at 60 C for 10 h. The as-prepared samples were labeled as Cu2O/CTMs (x), where x represents the concentration of Cu(CH3COO)2H2O ethanol solution. The phase composition and crystal structure were characterized by X-ray diffraction (XRD) (PANalytical X’Pert, Holland) using Cu Ka radiation operated at 40 kV and 40 mA. Field emission scanning electron microscope (FE-SEM) (JEOL JSM-7000F, Japan) and transmission electron microscope (TEM) (FEI Tecnai F20, USA) were used to investigate the morphologies and microstructures of the samples. The UV–vis diffuse reflectance spectra of the samples were recorded by UV–vis spectrophotometer (Shimadzu UV 3600, Japan). X-ray photoelectron spectroscopy (XPS) was measured
using thermo ESCALAB 250XI system with Al Ka radiation as exciting X-ray source. The specific surface area of the as-prepared samples were detected by N2 adsorption-desorption analysis conducted at 77K (Micromeritics ASAP 2020 V4.01, USA). The photocatalytic activity of the photocatalysts was evaluated by the degradation of MB solution under visible light illumination. Typically, 7.5 mg of the photocatalysts was added to 15 mL of 5 mg/ L MB solution, followed by stirring for 2 h in the dark to establish adsorption/desorption equilibrium of MB. Then, the mixture solution was irradiated under 500 W Xenon lamp (CHF-XM35, Beijing) light source. An optical filter was used to cut off wavelength below 420 nm to provide visible light with intensity 38.0 mW cm2. The samples were tested under continuous stirring at 20 min intervals for 80 min. UV–vis spectrophotometer (Shimadzu UV 1601-PC, Japan) was applied to detect the adsorption spectrum of MB solution in the wavelength range of 400-800 nm. The relative concentration of MB in the solution was determined by the absorbance at 664 nm. In addition, the photocatalytic measurement of bare TiO2 (P25) was performed under the same condition for comparison. 3. Results and discussion 3.1. Characterizations Fig. 1 shows the XRD patterns of the as-prepared CTMs and Cu2O/CTMs (0.005, 0.03 and 0.06). The typical diffraction peaks at 25.3 , 37.9 , 48.1, 53.9 , 55.1, and 62.7 for CTMs and Cu2O/CTMs are assigned to (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 11), and (2 0 4) crystal planes of anatase TiO2, respectively. The result indicated that anatase TiO2 can be prepared by flame assisted hydrolysis of TBOT alcohol solution without further annealing treatment. The burning flame of ethanol can provide a high temperature above 400 C. Thus, TiO2 is gradually crystallized with the assistance of the ethanol burning. Compared with the XRD pattern of CTMs, no obvious peaks for Cu2O were observed for Cu2O/CTMs (0.005 and 0.03), which is ascribed to the small amount and weak intensity of Cu2O. A slight additional diffraction peak at 36.4 can be observed for Cu2O/CTMs (0.06), which was assigned to (111) planes of Cu2O (PDF Card No.00-003-0892). However, the peak is so weak that we cannot directly confirm that Cu2O is deposited onto the surface of CTMs by XRD. The presence of Cu2O in Cu2O/CTMs composites is further confirmed by SEM, TEM and XPS results, as described later. Fig. 2 presents the FE-SEM images of CTMs (A and B) and Cu2O/ CTMs (C and D). The morphology of bare TiO2 was investigated in our earlier paper [38], which was not shown here. As shown in Fig. 2A and 2B, the profile of single microsphere can be clearly distinguished with diameter around 1.0–2.0 mm, which indicate microsphere structure can be obtained without template. The constituent nanocrystals of CTMs are strongly combined together and cannot be broken apart even under grinding or ultra-
d
Intensity (a.u.)
Microsphere structure could bring multiple reflection of incident light, leading to outstanding photoabsorption associated with light scattering [40]. Thus, TiO2 microsphere has attracted much attention owing to its large specific surface area and excellent light absorption performance. In this paper, Cu2O decorated carbon-incorporated TiO2 microspheres were synthesized and their enhanced photodegradation of methylene blue under visible light was confirmed. To the best of our knowledge, Cu2O decorated carbon-incorporated TiO2 microspheres ternary composites have not been reported before. In addition, the synthesis of carbon-incorporated TiO2 microspheres and the loading of Cu2O both possessed rapid, facile and clean properties.
c b a
Cu2O
20
Scheme 1. Schematic illustration for preparation of Cu2O/CTMs.
30
40 50 60 2Theta (degree)
TiO2
70
80
Fig. 1. XRD patterns of bare CTMs (a) and Cu2O/CTMs ((b) 0.005, (c) 0.03, (d) 0.06).
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Fig. 2. The SEM images of CTMs (A) and (B), Cu2O/CTMs (0.06) composites (C) and (D).
sonication. The progress of the formation of TiO2 microsphere can be concluded to be the hydrolysis reaction of TBOT with water generated from alcohol combustion, in which the titania nuclei, formed in the initial state, gradually grow bigger to form TiO2 sphere [41,42]. As shown in Fig. 2C and 2D, cubic Cu2O nanoparticles are observed with average size of about 60 nm on the surface of CTMs, which imply that Cu2O/CTMs are successfully obtained. The morphology of the as-prepared Cu2O/CTMs composites is further studied by TEM. As shown in Fig. 3, CTMs with average diameter about 2 mm and the cubic Cu2O nanoparticles (Fig. 3B) are further clearly presented. The results are in accordance with the SEM results. The surface chemical composition and valence states of Cu2O/ CTMs composites were further determined by XPS analysis (Fig. 4). Fig. 4A illustrates the XPS spectra of Cu2O/CTMs composites, which
exhibits the presence of C, Ti, O and Cu elements. The peaks with binding energy of 458.5 eV and 464.2 eV are assigned to Ti 2p3/2 and Ti 2p1/2 for Ti (IV) of the titania, respectively, which confirm the present of TiO2 (Fig. 4B). Fig. 4C displays two characteristic peaks with binding energies at 952.2 eV and 932.2 eV, which can be identified for Cu 2p1/2 and Cu 2p3/2, respectively. The binding energies can be ascribed to Cu (I) of Cu2O, which can further confirm that Cu2O is successfully assembled onto the surface of the CTMs via the chemical bath deposition process. The results are consistent with the XRD, SEM and TEM analyses. Fig. 4D exhibits the deconvolution of C 1s peak which has four fitting curves centered at 284.6 eV, 285.7 eV, 286.7 eV, and 288.3 eV. The strong peak at 284.6 eV is assigned to elemental carbon, arising from the incomplete burning of organic compounds during the flame assisted hydrolysis [37]. The curves at 285.7 eV, 286.7 eV, and
Fig. 3. The TEM (A) and HR-TEM (B) images of Cu2O/CTMs (0.06).
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(A)
a
a
Ti 2p Cu 2p
(eV)1/2
b e
1/2
b d e c
(α hv)
Absorbance (a.u.)
Intensity
O 1s
d c 2.0 2.5 3.0 3.5 4.0 4.5 hv (eV)
C1s
1000 900 800 700 600 500 400 300 200 Binding Energy (eV)
Intensity
(B)
Ti 2p1/2
468 466 464 462 460 458 456 454 452 Binding Energy (eV)
(C)
Intensity
Cu 2p3/2
Cu 2p1/2
960 955 950 945 940 935 930 Binding Energy (eV)
Intensity
285.7
290
286.7
288
286
800
284
absorption properties of Cu2O/CTMs were investigated. The optical absorption edges for Cu2O/CTMs (0.005, 0.03 and 0.06) are 458 nm, 481 nm and 469 nm, respectively. As shown in the insert, a plot of the square root of absorbance coefficient versus the energy of light affords the band gap. The optical band gap energy is determined to be 3.10 eV and 2.80 eV for bare TiO2 and CTMs, while that of Cu2O/ CTMs (0.005), (0.03) and (0.06) is estimated to be 2.75 eV, 2.55 eV, and 2.65 eV, respectively. Obviously, the absorption edges firstly shift to longer wavelength and then shift to shorter wavelength with the increase of the Cu2O contents. The longest absorption edge is the resulted Cu2O/CTMs (0.03) sample. Furthermore, it is obvious that all of the absorption edges for Cu2O/CTMs are longer than that of CTMs. The absorption range of light played an important role in the photocatalysis, especially for the visiblelight-driven photodegradation of contaminants. Thus, the enhanced visible light absorption and narrow band gap promised that Cu2O/CTMs were more active than CTMs under visible light illumination. 3.2. Photocatalytic measurements
284.6
288.3
400 500 600 700 Wavelength (nm)
Fig. 5. The UV–vis diffuse reflectance absorption spectra of bare TiO2 (a), CTMs (b) and Cu2O/CTMs ((c) 0.005, (d) 0.03, (e) 0.06). The inset shows the corresponding hv vs. (ahv)1/2 curves of as-prepared samples.
Ti 2p3/2
(D)
300
282
Binding Energy (eV) Fig. 4. XPS spectra for Cu2O/CTMs (0.06): survey spectrum (A), Ti 2p (B), Cu 2p (C), C 1s (D).
288.3 eV were corresponded to C O bonds, caused by the insufficient hydrolysis of TBOT [43]. The peak ascribed to Ti C bonds at around 281.0 eV has not been detected, indicating that the carbon species incorporated in TiO2 matrix is present as elemental state [44]. The UV–vis absorption spectra of the prepared samples are shown in Fig 5. As shown in Fig. 5, compared to that of bare TiO2 (387 nm), the absorption edges of the CTMs (402 nm) apparently shifted to the visible region with enhanced absorption intensity. This result indicates that the narrowing of the band gap of TiO2 occurs with the carbon incorporation, which can be act as sensitizer like dyes [34].The effect of Cu2O contents on the optical
The photodegradation of MB was employed to detect the photocatalytic activity of the as-prepared samples. The adsorption capacity of as-prepared pure TiO2, CTMs and Cu2O/CTMs was evaluated by monitoring the residual concentration fraction of MB after equilibrium in the dark for 120 min, as shown in Fig. 6A. The adsorption rates of MB are 5.5% and 40.0% for bare TiO2 and CTMs, respectively. It can be observed that as-prepared CTMs showed higher adsorption capacity than that of bare TiO2 (approximately 8 times). The improvement of the as-prepared samples in adsorption capacity should be attributed to the enhanced specific surface area for microsphere structure [45,46]. However, adsorption capacity decreased with the introduction of the Cu2O nanoparticles on the surface of CTMs. The specific surface areas of the as-prepared samples were further investigated with N2 adsorption-desorption analysis. The values of the BET surface areas were 51 m2 g1, 103 m2 g1, and 90 m2 g1 for bare TiO2, CTMs, and Cu2O/CTMs (0.005). Therefore, there was an appreciable increase in the surface area of CTMs. Upon Cu2O decoration, the specific surface areas of CTMs decreased, which was consistent with the literature [47]. Compared with CTMs, the decreased adsorption capacity of Cu2O/CTMs may be attributed to the smaller specific surface areas. Fig. 6B depicted the TiO2, CTMs, and Cu2O/CTMs induced photocatalytic degradation of MB under visible light illumination. After 80 min illumination, the degradation rate for CTMs was 18.5%, indicating the photocatalytic activity of CTMs is higher than that of pure TiO2 (7.3%). Cu2O/CTMs presents even higher degradation rate compared to that of CTMs. The influence of
Y. Fang et al. / Materials Research Bulletin 74 (2016) 265–270
(A) Remaing Concentration Fraction of MB
1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3
Initial
a
b
c
e
d
(B)
(Scheme 2). Efficient separation of photogenerated electron-hole pairs and the use of visible light can also be achieved by Cu2O in Cu2O/CTMs. Once Cu2O/CTMs is irradiated by visible light, photogenerated electrons can transfer from the conduction band (CB) of Cu2O to the CB of TiO2, while holes remain on the valance band (VB) of Cu2O. On the surface of the CTMs, the accumulated electrons can be transferred to the adsorbed oxygen and change it to O or O2 [29]. Furthermore, the photogeneraged holes of CTMs can migrate to the VB of Cu2O. These holes can react with water molecules or hydroxide ions (OH) to produce hydroxyl radicals (OH) [48]. Both of the O/O2 and OH have strong oxidation ability and can decompose the MB into CO2 and H2O. In a word, both Cu2O and carbon afford the effective utilization of solar energy for Cu2O/ CTMs composites. 4. Conclusions
1.00 a
0.95 0.90
Ct/C0
269
0.85
b
e c d
0.80 0.75 0.70 0
20
40 60 Time (min)
80
Fig. 6. (A) Bar plot showing the remaining MB in solution: initial and equilibrated with bare TiO2 (a), CTMs (b) and Cu2O/CTMs ((c) 0.005, (d) 0.03, (e) 0.06) in the dark after stirring. (B) Photodegradation of MB under visible light: bare TiO2 (a), CTMs (b) and Cu2O/CTMs ((c) 0.005, (d) 0.03, (e) 0.06).
Cu2O content in Cu2O/CTMs on the photocatalytic performance was also investigated. The degradation rate is 25.5% for the Cu2O/ CTMs (0.005). The degradation rates enhanced was observed with the increasing of Cu2O content and the highest degradation rate was 29.2% by Cu2O/CTMs (0.03). Further increasing the Cu2O content, the degradation rate decreased to 20.8% for Cu2O/CTMs (0.06). All the above observation indicates that the introduction of Cu2O plays an active part in the photocatalytic degradation process of MB. The enhanced photocatalytic activity of Cu2O/CTMs composites in the visible light irradiation can be attributed to the synergistic effect of carbon elements and Cu2O nanoparticles. The incorporated carbon elements act as graphite-like carbon, preventing the recombination of photogenerated electron-hole pairs and extending the visible light response range [33,34]. Meanwhile, the band structure of Cu2O matches well with TiO2 in CTMs composites
Scheme 2. The schematic mechanism of photocatalytic activity for Cu2O/CTMs.
We have successfully fabricated carbon-incorporated TiO2 microspheres by flame-assisted hydrolysis method, and decorated cubic Cu2O nanoparticles on the surface of CTMs through a chemical bath deposition process. The combination of Cu2O leads to the improved visible light absorbance of CTMs. The Cu2O/CTMs composites exhibits enhancement of photocatalytic degradation rate of MB under visible light irradiation compared with bare TiO2 and CTMs. In particular, with loading of the appropriate amount of Cu2O nanoparticles, Cu2O/CTMs (0.03) composite achieves the highest degradation rate of 29.2%, as 1.6 times larger than that of CTMs. Both Cu2O and carbon are responsible for the improvement of the photocatalytic activity of TiO2. This study provides a new technique to improve the photocatalytic activity of TiO2 and paves an alternative way for novel insight to seeking new and desirable photocatalysts. Acknowledgments This work was financially supported by Innovation Program of Shanghai Municipal Education Commission (15ZZ092), Training Program for Young Teachers in Shanghai Colleges and Universities (ZZgcd14010), Shanghai University of Engineering Science Innovation Fund (14KY0511, 15KY0516), and Startup Foundation of Shanghai University of Engineering Science (NO. 2014-22). References [1] A.L. Linsebigler, G. Lu, J.T. Yates, Photocatalysis on TiO2 surface, Chem. Rev. 95 (1995) 735. [2] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Light-induced amphiphilic surfaces, Nature 388 (1997) 431. [3] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Visible-light photocatalysis in nitrogen-doped titanium oxides, Science 293 (2001) 269. [4] D. Li, H. Haneda, S. Hishita, N. Ohashi, Visible-light-driven N-F-codoped TiO2 photocatalysts. 2. Optical characterization, photocatalysis, and potential application to air purification, Chem. Mater. 17 (2005) 2596–2602. [5] S.S.R. Putluru, L. Schill, A.D. Jensen, B. Siret, F. Tabaries, R. Fehrmann, Mn/TiO2 and Mn-Fe/TiO2 catalysts synthesized by deposition precipitation-promising for selective catalytic reduction of NO with NH3 at low temperatures, Appl. Catal. B-Environ. 165 (2015) 628–635. [6] S.N.R. Inturi, T. Boningari, M. Suidan, P.G. Smirniotisa, Visible-light-induced photodegradation of gas phase acetonitrile using aerosol-made transition metal (V, Cr, Fe, Co, Mn, Mo, Ni, Cu, Y, Ce, and Zr) doped TiO2, Appl. Catal. BEnviron. 144 (2014) 333–342. [7] M.X. Sun, P. Song, J. Li, X.L. Cui, Preparation, characterization and application of novel carbon and nitrogen codoped TiO2 nanoparticles from annealing TiN under CO atmosphere, Mater. Res. Bull. 48 (2013) 4271–4276. [8] W. Yu, X.J. Liu, L.K. Pan, J.L. Li, J.Y. Liu, J. Zhang, P. Li, C. Chen, Z. Sun, Enhanced visible light photocatalytic degradation of methylene blue by F-doped TiO2, Appl. Surf. Sci. 319 (2014) 107–112. [9] J.F. Zhang, W. Yan, C.P. Yu, X. Shu, L. Jiang, J.W. Cui, Z. Chen, T. Xie, Y.C. Wu, Enhanced visible-light photoelectrochemical behaviour of heterojunction composite with Cu2O nanoparticles-decorated TiO2 nanotube arrays, New J. Chem. 38 (2014) 4975–4984.
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