Catal Lett (2009) 132:75–80 DOI 10.1007/s10562-009-0063-3
Experimental Study on Photocatalytic Activity of Cu2O/Cu Nanocomposites Under Visible Light Bo Zhou Æ Zhiguo Liu Æ Hongxia Wang Æ Yanqiang Yang Æ Wenhui Su
Received: 16 January 2009 / Accepted: 5 June 2009 / Published online: 18 June 2009 Ó Springer Science+Business Media, LLC 2009
Abstract Cu2O/Cu nanocomposites (NCs) are synthesized using a two-step hydrothermal method, their different phase compositions are obtained by adjusting the reaction time, and then, they are used as photocatalysts to degrade dye Procion Red MX-5B (PR), methylene blue (MB) and methyl orange (MO) under visible-light. Experimental results indicate Cu2O/Cu NCs exhibit a much higher photocatalytic activity than pure Cu2O, they remain almost unchanged in their phase compositions in the long photocatalytic reaction process, except for partial oxidation of particle surface. They still exhibit a high photocatalytic activity even at the end of four photocatalytic reaction cycles. It can therefore be concluded that Cu2O/Cu nanocomposites are good candidates for processing of pollutant water. Keywords Cu2O/Cu Nanocomposites Hydrothermal method Photocatalyst
B. Zhou Z. Liu (&) Y. Yang W. Su Department of Physics, Center for Condensed Matter Science and Technology, Harbin Institute of Technology, 150080 Harbin, China e-mail:
[email protected] B. Zhou e-mail:
[email protected] H. Wang College of Chemistry and Chemical Engineering, Harbin Normal University, 150025 Harbin, China W. Su International Center for Materials Physics, Academia Sinica, 110015 Shenyang, China
1 Introduction It is increasingly important nowadays to find possible solutions to environmental pollution problems. Photo-degradation is one of the ways to treat polluted water and air. Semiconductor-based photocatalysts have attracted much attention from the research community because of their low cost and availability, TiO2 and ZnO are the most commonly used ones. However, the broad band gap (3.0–3.2 eV) of TiO2 or ZnO-based photocatalysts limits their applications because they can be activated by ultraviolet (UV) light only [1–4]. The maximum irradiation of sunlight lies in the visible range, and so, only photocatalysts with corresponding band gap can make full use of the solar energy. Cuprous oxide (Cu2O) has a direct band gap of ca. 2.2 eV, and it is therefore widely used for solar energy conversion [5–7], photocatalytic degradation of organic pollutants [8, 9] and decomposition of water into O2 and H2 [10, 11]. Another important quality of photocatalysts is quantum efficiency. Due to the easy recombination between photoelectrons and holes, pure semiconductors exhibit a very low quantum efficiency. The quantum efficiency can be improved by adding noble metal (e.g., Ag, Au, Cu, Pt) into photocatalysts to quickly transfer photogenerated electrons, and prevent the recombination, so that the metal/semiconductor heterostructure exhibits an excellent photocatalytic activity [11–17]. However, to the best of our knowledge, not much work has been done so far on the synthesis and photocatalytic property of Cu2O/Cu nanocomposites. Therefore, we synthesized Cu2O/Cu NCs using a stepwised hydrothermal route, controlled their phase compositions by adjusting the reaction time, and found Cu2O/Cu NCs are photocatalysts with high quantum efficiency and high activity under visible light irradiation.
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2 Experimental
3 Results and Discussion
2.1 Preparation of Cu2O/Cu NCs
3.1 Phase Composition
In a way similar to what is reported in Ref. [19], 0.073 g of Cu(NO3)23H2O was dissolved in 30 mL of N,N-dimethylformamide (DMF) and ethanol at a ratio of 1:2 by volume. The solution was sealed in a Teflon-lined stainless steel autoclave with a capacity of 50 mL, first heated at 180 °C for 5 h in an electric oven, and then heated at 200 °C for 0.5–3.0 h, and finally cooled to room temperature in the oven. The precipitation was washed with ethanol for several times, and dried at 100 °C for 3 h in a vacuum oven.
As shown in Fig. 1, the relative intensity of Cu(111) increases with the increase in the reaction time, which indicates a higher content of Cu in the Cu2O/Cu NCs. The mass fraction of Cu in the composites can be calculated using the relative ratio reported in Ref. [20] as shown below:
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B: Cu
B(220)
B(220)
B(111)
S5
A: Cu2O
S4 S3
20
30
40
A(220)
S1
50
60
2 θ (°)
Fig. 1 XRD patterns of S1, S2, S3, S4 and S5
A(311)
S2 A(200)
The adsorption behavior and photocatalytic activity of the samples were evaluated by the degradation of the aqueous solution of Procion Red MX-5B (PR-10), methylene blue (MB) and methyl orange (MO) at the concentration of 10 mg/L under visible-light. 0.030 g of Cu2O/Cu NCs powder was dispersed in the 50 mL of dye solution. Before irradiation, the suspension was stirred in dark for 120 min to achieve an adsorption/desorption equilibrium. The photocatalytic reaction of Cu2O/Cu NCs was carried out at room temperature with a 40-W tungsten lamp as the visible-light source. 5 mL of suspension was sampled at different intervals for analysis. After being centrifugated at 11,000 rpm for 10 min, the supernatant was measured on a UV-visible spectrophotometer (752 PC, Shanghai spectrum instruments CO. Ltd.) for absorbance (A). The decolorization efficiency of dyes was calculated using the following equation, D ¼ ðA0 AÞ=A0 100% ¼ ðC0 CÞ= C0 100%, where A is the absorbance of and C is the concentration of dye solutions at different intervals. The degradation of PR was evaluated by Fourier Transformation Infrared Spectroscope (FT-IR, Perkin Elmer 1600). The surface properties of as-prepared samples and samples after photocatalysis were characterized using PHI5700ESCA with monochromatic Al Ka X-ray radiation (1486.6 eV).
It can be seen from Fig. 2 that all the Cu2O/Cu NCs are similar in grain size and morphology. As shown in Fig. 2a, Cu2O/Cu NCs consists of octahedron particles with edge length in the range of 120–170 nm. As shown in Fig. 2b, a hollow structure is indicated by the lightened part of particle. As a result, without any change in morphology, the phase compositions of Cu2O/Cu NCs is changed by adjusting the reaction time, which is very important for the design of the heterostructure of photocatalysts.
A (111)
2.3 Adsorption and Photocatalytic Activity
3.2 Morphology
A(110)
The phase compositions of the samples was characterized using a X-ray diffractometer (Rigaku 12 kW) with Cu Ka radiation (k = 0.15418 nm). Their morphology was studied using a JEM-S4800 field-emission scanning electron microscope (FESEM) operated at 15 kV and a JEOL-2010 transmission electron microscope (TEM) working at 200 kV. N2 adsorption–desorption experiments were made using a Quantachrome Autosorb-1 apparatus.
ð1Þ
where ICuð111Þ and ICu2 Oð111Þ are the heights of the characteristic diffraction peaks in Cu (111) and Cu2O (111) planes, respectively. As shown in Table 1, the content of Cu increases from 16 to 87 wt% when the reaction time is increased from 0.5 to 3.0 h. Formic acid HCOOH generated from the hydrolysis of DMF acts as a weak reducing agent in this reaction [19]. It can therefore be concluded that the reaction time has a significant effect on the component ratio of Cu2O/Cu NCs.
Intensity (arb.u.)
2.2 Characterization
ICuð111Þ ICu2 Oð111Þ þ ICuð111Þ
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Experimental Study on Photocatalytic Activity
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WCu2 O ðwt%Þ
SBET (m2/g)
0
100
12.4
0.5
16
84
13.3
S3 S4
1.0 2.0
36 52
64 48
13.5 14.2
S5
3.0
87
13
12.2
Samples
Secondary time (h)
S1
0
S2
WCu (wt%)
(a)
0.14
S4
0.12
Absorbance (A)
Table 1 Experimental conditions, phase composition and characterized parameters of different samples
0.10
0 min
0.08
20 min
0.06
40 min
0.04
60 min
0.02 0.00
3.3 Adsorption and Photocatalytic Activity
Figure 3a shows the time-dependent adsorption spectra of PR-10 with the presence of sample S4 under visible light irradiation. The peak lies at about 512 nm is selected to monitor the adsorption and photocatalytic degradation process. As shown in Fig. 3a, the hight of the characteristic peak reduces with the irradiation time. After being irradiated for 60 min, the peak vanishes, no new adsorption peaks appears, and no obvious hypsochromic shift is observed, which means PR-10 is degraded. As shown in Fig. 3b, the concentration of PR-10 decreases by more than 67% when an adsorption/desorption equilibrium is achieved, which indicates the samples have a strong adsorption. It can be concluded from Fig. 2 and Table 1 that all the samples have similar adsorptions possibly because of their similarities in morphology and specific surface area SBET. As shown in Fig. 3b, with the presence of pure Cu2O (S1), there is little decrease the concentration of PR-10 under visible light irradiation. In comparison, with the presence of Cu2O/Cu NCs (S2–S5), the concentration of PR-10 almost decreases to zero after it is irradiated for 60 min. As shown in Fig. 4, the peaks of absorption at 627 and 1,394 cm-1 are attributed to the vibration of Cu–O bond in
400
500
600
700
Wavelength (nm)
(b) 0.30 in dark 120 min.
0.25
Visible irradiation 120 min. 0.20
C/C0
3.3.1 Adsorption and Photocatalytic Performance on Organic Dyes
0.15 0.10
S1
0.05
S2
S3
S5
S4
0.00 0
20
40
60
80
100
Mass fraction of Cu (wt%)
Fig. 3 a Time-dependent absorption spectra of PR-10 with presence of S4 under visible light irradiation, b relative concentration (C/C0) of PR-10 versus mass fraction of Cu in Cu2O/Cu NCs. m: in dark for 120 min; : under irradiation for 120 min
Cu2O [21, 22]. The weak peak at about 1,000 cm-1 corresponds to the peak of PR in S4 after adsorption, the characteristic peak of PR totally disappears in S4 at the end of the first photocatalytic reaction cycle (S4–R1). Combing the results of Figs. 3a and 4, it can therefore be concluded
Fig. 2 a FESEM image of S4, b TEM image of S4
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(a)
50 mg/L
S4
40 mg/L 30 mg/L
0.6
(b)
C/C 0
Transmittence (%)
0.8
20 mg/L 10 mg/L
0.4
(c) 0.2
(d) 0.0
1394 3000
2500
2000
1500
627 1000
500
-1
Wavenumber (cm )
0
20
40
60
80
100
120
140
160
Irradiation time (min)
Fig. 6 Initial PR concentration-dependent photocatalytic performance with presence of S4
Fig. 4 FT-IR spectra of: a pure PR, b as-prepared S4, c residual S4 after adsorption PR-10 and d residual S4 after first recycling photocatalytic reaction
0.8
S1 MB
S4
0.4
C/C0
heavily on the concentration of interfaces or defects, which can increase the separation efficiency of photogenerated electron-hole pairs [23–27]. The metal nanoparticles in the metal/semiconductor hererostructures act as the sinks for electrons, which promote the interfacial charge-transfer kinetics between the metal and the semiconductor, and improve the separation of photogenerated electron-hole pairs, thereby improving the photocatalytic activity of semiconductors [28]. The interfaces between Cu and Cu2O in the Cu2O/Cu NCs are the sites where the photogenerated electrons can be quickly transferred. This is the reason why Cu2O/Cu NCs exhibit a better photocatalytic performance than pure Cu2O.
0.0 0.8
MO
0.4 0.0 0
20
40
60
80
100
120
Irradiation time (min.)
Fig. 5 Degradation of aqueous solution of MB (10 mg/L) and MO (10 mg/L) with presence of S1 (j) and S4 ( )
that PR is completely decomposed with the presence of S4 under visible light irradiation. The photocatalytic performance of Cu2O/Cu NCs is also evaluated by the photo-degradation of methylene blue (MB) and methyl orange (MO). As shown in Fig. 5, both S1 and S4 have good photocatalytic performance indicated by the photo-degradation of MB and MO. Compared with S1, the concentration of MB and MO decrease faster with the presence of S4, which indicates Cu2O/Cu NCs have a higher photocatalytic activity. As a result, the coexistence of Cu2O and Cu in the NCs is good for the high photocatalytic activity, which is attributed to their heterojunction effect. According to the theory of heterostructural catalysis, the photocatalytic activity of semiconductor-based heterostructures depend
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3.3.2 Effect of Initial PR Concentration on Photo-Degradation Efficiency The effect of initial concentration of dyes on photo-degradation is evaluated by increasing the concentration of PR from 10 to 50 mg/L with the presence of S4. As shown in Fig. 6, the degradation rate decreases with the concentration of PR. With the increasing concentration of PR, some positions of adsorbed OH- radicals are replaced by dye ions (dye-), thereby reducing the number of active sites for OH radicals [28], which is an extremely strong oxidant for the mineralization of organic chemicals [16]. Since the irradiation time and the amount of catalyst are constant, the number of OH species attacking the molecules of PR decreases. On the other hand, as the concentration of PR increases, the penetration of light in the solution is retarded, and so, the number of photons participating in the photo-oxidation reactions decreases. It can therefore be concluded that the degradation rate of PR decreases with the dye concentration.
Experimental Study on Photocatalytic Activity
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3.3.3 Potential Recycling Use B (111)
Intensity (arb.u.)
B: Cu
(a)
(b)
20
30
40
50
2 θ (°)
60
70
80
Fig. 9 XRD patterns of S4: a as-prepared S4, b residual S4 photocatalytic reaction at dye concentration of 10 mg/L at room temperature at the end of 4th recycle (S4–R4)
1.0
first run
S4
second run
0.8
third run C/C0
A (111)
A: Cu2O
As shown in Fig. 7, even at the end of the fourth cycle, the degradation rate of PR-10 under visible light irradiation is as high as 91%. Compared with the as-prepared S4, the degradation efficiency of PR-10 decreases just a little at the end of the fourth run, due to the deactivation of active centers in the catalysts resulting from the oxidation of Cu2O and Cu. As shown in Fig. 8, the fitting on Cu 2p3/2 peak, Cu0 (932.7 eV), Cu? (932.5 eV) and Cu2? (935.2, 943.8 eV) are present in as-prepared S4 [29–31]. At the end of the fourth cycle of photocatalytic reaction (S4–R4), only Cu? (932.5 eV), Cu2? (933.3, 935.2, 943.8 eV) can be
0.6
fourth run
0.4
0.2
0.0 0
20
40
60
80
100
120
Irradiation time (min)
Fig. 7 Photocatalytic degradation of PR-10 with recycled S4
2+
1 943.81 (Cu )
observed. It is generally accepted that a small fraction of some Cu and Cu2O surfaces are oxidized to CuO during the sample drying and handling under normal ambient condition [32]. However, no peaks corresponding to CuO can be seen from the XRD patterns of S4 and S4–R4 (Fig. 9). As reported in Ref. [33], CuO is present only on the surface of Cu2O nanocrystals and they form a thin amorphous shell. That is the reason why CuO phase can not be detected using XRD technique, because XRD can detect the crystalline parts of the nanoparticles only. On the other hand, XPS is very sensitive to surfaces only. The content of Cu2O in the S4–R4 goes up to 51 wt%, possibly because of the partial oxidation of Cu nanocrystals. Although the partial oxidation of Cu element occurs on the surface, S4 still performs well in the long photocatalytic reaction process.
2+
2 935.15 (Cu ) 0
3 932.73 (Cu )
3 2
1+
Intensity (arb. u.)
4 932.50 (Cu )
4 Conclusions
S4 4
1
Cu2O/Cu photocatalysts of heterostructure are synthesized using a facile two-step hydrothermal route. The mass content of Cu increases with the increase in reaction time. As photocatalysts, Cu2O/Cu NCs exhibit excellent properties in degrading organic dyes PR, MB and MO. They perform well in the long photocatalytic reaction process, which is essential for wide applications. It can therefore be concluded that a way has been found for the design of photocatalysts composites for a variety of applications.
2+
1 943.81 (Cu ) 2+
2 935.15 (Cu ) 2+
3 933.33 (Cu )
3
1+
4 932.50 (Cu )
S4-R4 2
1
950
945
940
935
4
930
925
Binding Energy (eV)
Fig. 8 XPS patterns of a as-prepared S4, b residual S4 photocatalytic reaction at dye concentration of 10 mg/L at room temperature at the end of the 4th recycle (S4–R4)
Acknowledgments This research work is funded by the National Natural Science Foundation of China (no. 10504005 and no. 10674034), and Development Program of outstanding Young Teachers in Harbin Institute of Technology (Grant no. HITQNJS. 2006.059).
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