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rectorite composite
Journal of Colloid and Interface Science 376 (2012) 217–223
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Facile synthesis and photocatalytic properties of AgAAgClATiO2/rectorite composite Yanqing Yang a, Gaoke Zhang a,⇑, Wei Xu b a b
School of Resources and Environmental Engineering, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, PR China Wuhan Second Ship Design and Research Institute, Wuhan 430064, PR China
a r t i c l e
i n f o
Article history: Received 21 December 2011 Accepted 1 March 2012 Available online 10 March 2012 Keywords: Photocatalyst Deposition–photoreduction method Visible light ARG Surface plasmon resonance
a b s t r a c t In this study, we prepared a new visible light induced plasmonic photocatalyst AgAAgClATiO2/rectorite using a facile deposition–photoreduction method. The catalysts were characterized using X-ray diffraction (XRD), UV–visible diffused reflectance spectra (UV–vis DRS), Raman spectra, high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS). The as-prepared AgAAgClATiO2/rectorite powders exhibited an efficient photocatalytic activity for the degradation of acid orange (ARG) and 4-nitrophenol (4-NP) under visible light irradiation (k > 400 nm). Moreover, the mechanism suggested that the high photocatalytic activity is due to the charge separation and the surface plasmon resonance of metallic Ag particles in the region of visible light. The active species measurements suggested that HO is not the dominant photooxidant. Direct hole transfers and O 2 were involved as the active species in the photocatalytic reaction. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction TiO2, as a cheap, nontoxic, efficient photocatalyst for the degradation of various pollutants, has received much research attentions during the past two decades [1–3]. However, it is activated only under UV light irradiation because of its large band gap and hence hindered its practical application. Therefore, enhancing the photocatalytic efficiency and visible light utilization of TiO2 is the most urgent issue in this topic. The works including metal deposition, nonmetal doping or ion-implanting methods have been explored to improve the properties of TiO2 [4–8]. Among them, noble metal nanoparticles (NPs) have attracted considerable interest due to their surface plasmon resonance (SPR), which accelerates the separation process of the photo-generated electrons and holes in the semiconductor catalyst [9,10]. And many researchers have demonstrated that the Ag NPs deposited on semiconductor show efficient plasmon resonance in the visible region [11–16]. On the other hand, silver halide has been supposed to be a new visible light photocatalytic material for its good sensitivity to light. Ag/silver halide composites as an excellent charge-separation promoter and builtin acceptor have been widely investigated in recent years. Recently, Huang and co-workers [17] fabricated the Ag@AgCl photocatalyst by first treating Ag2MoO4 with HCl to form AgCl powder and then reducing some Ag ions to Ag0 species, which is efficient for MO degradation under visible light irradiation. Yu et al. [18] introduced the Ag/AgCl structure into nanotube arrays and ⇑ Corresponding author. Fax: +86 27 87887445. E-mail address: [email protected] (G. Zhang). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.03.003
indicated the photocatalyst shows high visible light photocatalytic activity and stability during the photocatalytic degradation of methylorange in water. Clays, such as montmorillonite (MMT), rectorite, and kaolinite, have attracted much attention in recent years. These natural materials possess layered structures, large surface areas, and a high cation exchange capacity and can adsorb organic substances either on their external surfaces or within their interlaminar spaces by interaction or substitution [19]. Rectorite is a regular clay minerals consisting of alternate regular (1:1) stacking of dioctahedral mica-like layer and dioctahedral smectite-like layer [20]. As the support for the AgAAgClATiO2 composite, rectorite may facilitate the catalyst separation, recovery, and recycling after the photocatalytic reaction. In this study, we prepared AgAAgClATiO2 loaded rectorite composite photocatalyst. The visible light-driven photocatalytic activity of this composite material was investigated by the photocatalytic decomposition of azo dyes and nitrophenols. The relationship between the photocatalytic activity and the structural features of the prepared catalysts was investigated through a systematic characterization analysis, and the photocatalytic mechanism of the reaction was also discussed.
2. Experimental section 2.1. Materials The rectorite clay was obtained from Zhongxiang Hubei, PR. China. Titania P25 (TiO2, ca. 80% anatase, 20% rutile) was purchased
Y. Yang et al. / Journal of Colloid and Interface Science 376 (2012) 217–223
2.2. Preparation of photocatalyst The TiO2/rectorite was prepared according to our previous work [21], the difference from the previous work was that the final solution pH was adjusted by NaOH solution but not by distilled water. At room temperature, TiO2/rectorite was dispersed in 10 mL ethanol, and then the mixture of silver nitrate solution (50 mL, 0.05 M) and ethanol (40 mL) was dropwise added to the dispersion with vigorous stirring for 2 h. The white AgCl appeared because the TiO2 sol was prepared by addition of TTIP into HCl solution and the TiO2/rectorite containing chlorine element. The wet cakes were separated and dried at 70 °C. Finally, the product was mixed with distilled water and was irradiated by visible light (k > 400 nm) for 3 h to reduce partly absorbed Ag+ to Ag via AgCl photocatalysis. 2.3. Characterization Powder X-ray diffraction patterns of the as-synthesized powders were obtained on a D/MAX-RB X-ray diffractometer (Rigaku, Japan) equipped with Cu Ka radiation (k = 1.5406 Å) and recorded with 2h ranging from 2.5° to 70°, while the accelerating voltage and the applied current were held at 40 kV and 50 mA, respectively. Scanning electron microscopy (SEM, JSM-5610LV) was used to characterize the morphologies of the products. Morphologies and microstructures of the prepared samples were further examined with transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images by a JEM 2100F electron microscope operated at an accelerating voltage of 200 kV. Chemical analysis of the photocatalyst was performed by X-ray analysis (EDX) joined a JSM-5610LV scanning electron microscopy (SEM). X-ray photoelectron spectroscopy (XPS) analysis was carried out using a Thermo VG Multilab 2000 spectrometer (the degree of vacuum in analysis room excelled 3 108 Pa). All binding energies were referred to the C 1s peak at 284.63 eV of the surface adventitious carbon and were revised. The Raman spectrum was recorded using a Renishaw invia spectrometer equipped with notch filter and a CCD detector. The diffuse reflectance spectra (DRS) were measured by a UV–vis spectrometer (UV2550, Shimadzu, Japan) in the range of 190–800 nm. BaSO4 was used as the reflectance standard material. The HO trapping fluorescence spectra were taken on a fluorescence spectrophotometer (Shimadzu RF-5300PC).
sample (the TiO2 content in the composite was obtained by ICP measurements). 3. Results and discussion 3.1. XRD analysis XRD was used to determine the phase structure of the samples. Fig. 1 shows the XRD patterns of the rectorite, TiO2/rectorite and AgAAgClATiO2/rectorite composite. From Fig. 1a, the rectorite exhibits basal (0 0 1) and (0 0 2) diffraction peaks, which are essential for rectorite with a highly ordered and oriented silicate layer structure [22]. As seen in Fig. 1b, the (0 0 1) and (0 0 2) peaks in the XRD patterns of the TiO2/rectorite composite disappear, suggesting that the layered structure of rectorite was destroyed to some extent. The TiO2 sample existed as a mixed phase (brookite, anatase, and rutile). The peaks of brookite phase appear because the chloride ions (an appropriate ratio of [Cl]:[Ti]) seem to be essential for the formation of brookite in highly acidic solution (pH = 3.0 in this study) [23]. The peaks of brookite in Fig. 1c disappeared probably due to the pH value changes in the preparation process. The diffraction peaks attributed to AgCl (JCPDS No. 31– 1238) and Ag (JCPDS No. 04–0783) were observed in Fig. 1c. This suggests that the deposited AgCl has a crystalline cubic phase and the metallic silver produced was due to the photoreduction process under visible light irradiation. Moreover, the diffraction peak assigned to metal Ag is broad and weak, which may result from its low content and small particle size on the surface of the as-prepared composite. 3.2. TEM images Fig. 2 displays the SEM and TEM images of the rectorite and the AgAAgClATiO2/rectorite composite. Some particles presented on the surface of the AgAAgClATiO2/rectorite composite (Fig. 2b) as compared to the pristine rectorite (Fig. 2a). As shown in Fig. 2c, the intercalation of the AgAAgClATiO2 with the rectorite particles destroyed the ordered structure of rectorite to some extent, resulting in some exfoliated rectorite particles [24]. The corresponding HRTEM image (Fig. 2d) of the sample states the lattice spacing of
001
from the Degussa Co. All other reagents, including AgNO3, ethanol, titanium tetraisopropoxide (TTIP), acid orange (ARG) and 4-nitrophenols (4-NP), were analytical grade and used as received without purification. All experiments were carried out using distilled water.
(a)
A: anatase B: brookite R: rutile C: AgCl S: silver
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2.4. Evaluation of photocatalytic activity
C(200)
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B(132) A(200) C(220)
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Rectorite A(101) R(110) B(121) R(101) A(004)
The photocatalytic activity of the photocatalyst was evaluated by the degradation of ARG and 4-NP in aqueous solution under visible light irradiation. A 300 W Dy lamp was used as the light source with a 400 nm cutoff filter to ensure complete removal of radiation below 400 nm. In a typical experiment, aqueous suspensions of ARG (100 mL, 50 mg/L) or 4-NP (100 mL, 5 mg/L) with 0.15 g of the photocatalyst were placed in the beaker. Prior to irradiation, the suspensions were magnetically stirred in the dark for 20 min to ensure that an adsorption/desorption equilibrium was established between the dye and the photocatalyst. At given irradiation time intervals, about 6 mL dispersions were collected and centrifuged to remove the particles. The filtrates were analyzed by an UV–visible spectrophotometer (Unico UV-2102PC). For comparison, the photocatalytic degradation of ARG by P25 was performed using the same procedure as above. The dosage of P25 was equivalent to the TiO2 content in the 0.15 g AgAAgClATiO2/rectorite
TiO2/rectorite
Ag-AgCl-TiO2/rectorite
Ag-AgCl 0
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2 Theta (degree) Fig. 1. XRD patterns of (a) rectorite, (b) the TiO2/rectorite, (c) the AgAAgClATiO2/ rectorite composite.
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Fig. 2. SEM images of (a) rectorite, (b) the AgAAgClATiO2/rectorite composite; TEM image (c) and HRTEM image (d) of the AgAAgClATiO2/rectorite composite.
the as-prepared photocatalyst. The determined lattice spacing of 0.35 nm and 0.32 nm are in good agreement with the values for anatase (1 0 1) and rutile (1 1 0) phase, respectively. The crystal lattice stripe of 0.197 nm belongs to the (2 2 0) phase of cubic AgCl (JCPDS 31–1238), suggesting the AgCl particles have deposited on the surface of TiO2/rectorite. 3.3. EDX and XPS analysis Energy dispersive X-ray spectroscopy (EDX) is an analytical technique used for the element analysis or chemical characterization of a sample. XPS analysis is useful for detecting valency states
of elements on the surface [25]. Thus, the compositions and the chemical state of its constituent elements of the AgAAgClATiO2/ rectorite composite were investigated by EDX and XPS analysis (Fig. 3 and Fig. 4). The energy dispersive X-ray spectroscopy (EDX) in the Fig. 3 shows the presence of Ag, Ti, Cl, and O elements on the as-prepared photocatalyst. Ti and O peaks result from the TiO2 particles, Ag and Cl elements indicate the presence AgCl and/or Ag in the as-prepared photocatalyst. Fig. 4 shows the high-resolution XPS spectra of the Cl 2p and Ag 3d regions. As shown in Fig. 4a, the Cl 2p peak is deconvoluted into two peaks (197.8 and 199.3 eV), which are assigned to the Cl 2p3/2 and Cl 2p1/2, respectively. This result suggests that the Cl elements
Fig. 3. EDX pattern of the AgAAgClATiO2/rectorite composite.
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Ag 3d5/2 Ag 3d3/2
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Cl 2p 1/ 2
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2 3/ 2p Cl
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Binding energy (eV)
Binding energy (eV)
Fig. 4. Cl 2P (a) and Ag 3d (b) XPS spectra of the AgAAgClATiO2/rectorite composite.
are mainly in the form of Cl [26,27]. Fig. 4b shows the XPS spectra of Ag of the AgAAgClATiO2/rectorite composite. The Ag 3d3/2 peak is divided into two different peaks at 373.6 and 373.0 eV, and the peak of Ag 3d5/2 is divided into 367.58 and 366.8 eV. The peaks at 373.6 and 367.58 eV are attributed to metallic silver, respectively. And the two peaks with a spin energy separation of 6.02 eV further indicate that the zerovalent silver exists in the AgAAgClATiO2/rectorite composite [28]. The Ag 3d peak appears at the binding energy at 373.0 and 366.8 eV belongs to the monovalent Ag, indicating the formation of AgCl on the as-prepared photocatalyst [29].
3.4. Raman analysis Fig. 5 shows the scatting Raman spectra for (a) the TiO2/rectorite and (b) the AgAAgClATiO2/rectorite composite. The spectrum intensity of the AgAAgClATiO2/rectorite composite increased as compared to the TiO2/rectorite composite. The SERS (surface enhancement of Raman signals) phenomenon was attributed to electromagnetic factors or chemical effects, and this conclusion has been theoretically debated and experimentally proven [30– 34]. Liu et al. [35] suggested the SERS was related to the ‘‘induced resonance effect’’ in the AgATiO2 system, which was due to the interaction of silver particles with semiconductor or the adsorption of molecules or ions on the surface of silver particles. Honma et al. [36] recognized the SERS effects were induced by the plasmon excitation of the silver particle in AgACdS hybrid particles.
Considering these, we suggest that the changes in intensity are probably due to the AgAAgCl particles in the composite. 3.5. UV–vis DRS analysis The photoabsorption ability of the prepared samples were detected by the UV–vis diffuse reflectance spectrum, as shown in the Fig. 6, the TiO2/rectorite composite and AgAAgClATiO2/rectorite photocatalyst have absorption in the visible region in contrast to P25 and TiO2. Importantly, aside from the photoabsorption from TiO2/rectorite, the AgAAgClATiO2/rectorite composite displays another absorption band around 450–750 nm. This can be attributed to the plasmon resonance of Ag particles in AgAAgClATiO2/rectorite composite. As the results of UV–vis DRS, the as-prepared photocatalyst is expected to have excellent photocatalytic activity for the degradation of organic contaminants. 3.6. Photodegradation of ARG and 4-NP under visible light irradiation ARG belongs to the commercial dye and widely used in textiles, soaps, and cosmetic products. The dye often occurs in wastewater and causes serious environment problems because the dye is chemically stable and biologically less active. Thus, the photocatalytic degradation of ARG was chosen to investigate the photocatalytic activity of the as-prepared photocatalyst. When dissolved in distilled water, ARG displays a major absorption band centered at 505 nm that is used to monitor the photocatalytic degradation. Fig. 7 illustrates the UV–vis absorption spectra of ARG aqueous solution under different reaction times. As shown in Fig. 7, the
1.5
1.2
Absorbtance (a.u.)
Intensity (a.u.)
(a) Ag-AgCl-TiO2 /rectorite (b) TiO2 /rectorite
(a) (b)
(a) rectorite (b) Ag-AgCl-TiO2 /rectorite (c) TiO2 /rectorite
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(d) TiO2 (e) P25
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Wavelength (nm) Fig. 5. Raman spectrum of (a) the AgAAgClATiO2/rectorite composite and (b) the TiO2/rectorite composite.
Fig. 6. UV–vis diffuse reflectance spectra of different samples.
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Fig. 7. UV–vis spectra changes of ARG solution during the photocatalytic degradation by the as-prepared photocatalyst under visible light illumination.
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C/C0
absorption spectrum of the original solution shows three distinctive peaks at 215, 331, and 505 nm, which correspond to the structure of the benzene ring, the naphthalene ring, and the nitrogen to nitrogen double bond (AN@NA), respectively. It is found that the ARG absorption peak at 505 nm decreased rapidly and disappeared after 20 min, and the color of the azo dye changed from red to colorless as shown in the inset of Fig. 7, this confirms the photodegradation of ARG (i.e., the breakup of the chromophore responsible for the characteristic color of the azo dyes, rather than its discoloration or bleaching) [37]. The characteristic peaks of naphthalene ring and benzene ring also became more smoothness after visible light irradiation for 20 min, and no new absorption spectra appeared in either the visible or ultraviolet regions, indicating that the sample exhibited excellent photocatalytic activity. Fig. 8 shows the adsorption and photodegradation of ARG by different catalysts under visible light irradiation (k > 400 nm). The blank test in the absence of photocatalyst was performed and the results show that the photolysis of ARG was negligible under visible light irradiation. Meanwhile, the adsorption of ARG by the photocatalyst in the dark was also checked. In dark within 20 min, the adsorption reached equilibrium, and about 16% of ARG was adsorbed onto AgAAgClATiO2/rectorite. As a comparison, the photodegradation of ARG with P25 was also performed, and the results demonstrate that ARG was not significant degraded under visible light irradiation. However, the removing efficiency of the contaminant by AgAAgClATiO2/rectorite composite could reach nearly 100% within 20 min under visible light irradiation, indicating the excellent photocatalytic activity of the as-prepared photocatalyst. To further investigate the photocatalytic properties of the asprepared photocatalyst, 4-NP was selected to evaluate the photocatalytic activity as it is produced in the highest quantities worldwide has the relatively high toxicity and persistence in aqueous media [38]. In order to determine the nature of 4-NP in different systems, the spectra of 4-NP in photocatalytic reaction system and in distilled water were compared. As shown in Fig. 9, after adding the as-prepared photocatalyst, the peak at 400 nm disappeared and the intensity of peak at 317 nm increased as compared to the original 4-NP aqueous solution. This is so because that 4-NP is easily deprotonated to yield 4-NP– and develops a typical yellow color in basic media. After adding the photocatalyst, the pH values changed from 6.6 to 5.0, and thus, the yellow color disappeared as depicted in the Fig. 9. Fig. 10 displays the UV–vis spectra changes of 4-NP (5 mg/L) solution during the photocatalytic degradation
Ag-AgCl-TiO2 /rectorite adsoption in the dark blank P25
dark
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Time (min) Fig. 8. The photodegradation efficiencies of ARG solution as a function of time under different conditions.
under visible light irradiation (k > 400 nm). As shown in Fig. 10, an apparent decrease in 4-NP at the wavelength of 317 nm was observed, which indicated the photocatalyst also showed an efficient photocatalytic activity for the degradation of 4-NP. Fig. 11 shows the photocatalytic rate of 4-NP solution under different conditions. The absorbance values obtained at 317 nm were used to calculate the decomposition ratio of 4-NP after 120 min. It can be seen that the characteristic peak at 317 nm dramatically decrease with increasing photocatalytic reaction time and the degradation rate of 4-NP reach 84% after visible light irradiation. The photolysis and the adsorption of 4-NP on the as-prepared photocatalyst in the dark were also given in Fig. 11. After 120 min, the concentration of 4-NP is nearly unaltered, suggesting the decrease in 4-NP is mainly caused by photodegradation but not adsorption and photolysis. 3.7. Visible light photodegradation mechanism The photo-induced active species such as trapped holes (h+), hydroxyl radical (HO ), and superoxide radical anions ðO 2 Þ are potentially important substance for the mineralization of toxic organic contaminants. Therefore, to investigate the mechanism of the process, the corresponding experiments were employed to determine the specific reactive species that played important roles in the photodegradation process. The formation of hydroxyl radicals at the photocatalytic system was detected by PL technique using
Fig. 9. UV–vis spectra changes of 4-NP solution in different reaction systems.
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Fig. 10. UV–vis spectra changes of 4-NP solution during the photocatalytic degradation by the as-prepared photocatalyst under visible light illumination.
Fig. 12. HO trapping PL spectra of the AgAAgClATiO2/rectorite composite on TA solution under visible light irradiation.
BQ þ O þ O2 2 ! BQ
1.0
blank test adsorption in the dark photocatalysis
0.6 0.4 0.2 0.0
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Time (min) Fig. 11. The degradation efficiencies of 4-NP solution as a function of irradiation time under different conditions.
terephthalic acid as a probe molecule. 2-Hydroxyl-terephthalic acid (TAOH), which is generated when terephthalic acid captures the HO radicals (as shown in Eq. (1)), performs a strong fluorescence at around 426 nm on the excitation of its own 312 nm absorption band [39,40].
HO þ TA ! TAOH
As shown in Fig. 13, the removing efficiency of ARG was about 10% after adding AO to the reaction system. The addition of AO seriously inhibited the photocatalytic process, which indicated the photo-generated holes and the active species generated by positive holes were the major responsibility in the degradation of ARG. Meanwhile, the photodegradation efficiency was also decreased to a certain degree after 2 mM BQ was added to the reaction system. The experimental result suggests that the superoxide may also another important intermediate species to the oxidative degradation of ARG. Therefore, a possible mechanism for the contaminant degradation over the as-prepared photocatalyst could be proposed on the basis of the above experimental results (as shown in Fig. 14). Silver particles can be excited by visible light and generated electronhole pairs due to surface plasmon resonance. The electrons can inject into the CB (conduction band) because the formed Schottky barrier at the Ag/TiO2 interface [44]. The conduction band gap of TiO2 was deduced about 0.7 V [38], which was more negative than the standard redox potential of O2 =O 2 (0.33 V vs NHE) [45]. As a result, the photo-generated electrons are expected to be trapped by absorbed O2 to produce O 2 . On the other hand, holes were scavenged by Cl– to form active oxidation production Cl0 which was consistence with Huang and Yu’s reports [18,46]. These
ð1Þ
Therefore, the photoluminescence technique by TA–PL to detect the formation of the HO radicals was conducted. Experimental procedures are similar to the measurement of the photocatalytic activity except that ARG aqueous solution was replaced by the TA aqueous solution. As shown in Fig. 12, the photoluminescence emission intensity at 426 nm barely increased after visible light irradiation, which indicates the HO radicals are not the main active species in the photocatalytic system. To further investigate the effect of other active species such as positive holes and superoxides on the photocatalytic degradation process, the comparison experiments for ARG degradation in the presence of the quenchers were performed. Ammonium oxalate (AO) is an effective scavenger of holes [41] and 1,4-Benzoquinone (BQ) is a quencher of the superoxide radical anion. O 2 via fast electron transfer according to Eq. (2) generating benzoquinone radicals [42,43]. and the BQ radical formation would inhibit the participation of O 2 in the contaminant decomposition.
1.0 0.8
C/C0
C/C0
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ð2Þ
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no quencher add AO add BQ
dark
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0
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Time (min) Fig. 13. Photocatalytic degradation of ARG over the AgAAgClATiO2/rectorite particles under different conditions with exposure to visible light (k > 400 nm).
Y. Yang et al. / Journal of Colloid and Interface Science 376 (2012) 217–223
Fig. 14. Schematic illustration for the charge separation in the visible light irradiated AgAAgClATiO2/rectorite system [18].
active species will decompose the contaminants to the final carbon dioxide or other intermediate products. 4. Conclusions Visible light induced AgAAgClATiO2/rectorite composite was prepared using a facile deposition–photoreduction method. The as-obtained composite showed a high photocatalytic activity for the degradation of ARG and 4-NP solutions. The HO trapping PL studies suggested that HO was not the dominant photooxidant in the photocatalytic reaction system. The free radical experiments 0 suggested that O 2 and Cl were formed in the ARG degradation process, which were generated by charge separation and transfer during the surface plasmon resonance of silver particles. A possible mechanism of dye photodegradation over AgAAgClATiO2/rectorite composite was proposed. The study would be helpful to understand the theoretical and practical application of visible light induced plasmon photocatalyst. Acknowledgments This work was supported by the National Natural Science Foundation of China (50872103), the Program of Wuhan Subject Chief Scientist (201150530147), the Wuhan Technologies R and D Program, National Basic Research Program of China (973 Program) 2007CB613302 and the Project-sponsored by SRF for ROCS, SEM. References [1] [2] [3] [4]
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Contents lists available at SciVerse ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Facile synthesis and photocatalytic properties of AgAAgClATiO2/rectorite composite Yanqing Yang a, Gaoke Zhang a,⇑, Wei Xu b a b
School of Resources and Environmental Engineering, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, PR China Wuhan Second Ship Design and Research Institute, Wuhan 430064, PR China
a r t i c l e
i n f o
Article history: Received 21 December 2011 Accepted 1 March 2012 Available online 10 March 2012 Keywords: Photocatalyst Deposition–photoreduction method Visible light ARG Surface plasmon resonance
a b s t r a c t In this study, we prepared a new visible light induced plasmonic photocatalyst AgAAgClATiO2/rectorite using a facile deposition–photoreduction method. The catalysts were characterized using X-ray diffraction (XRD), UV–visible diffused reflectance spectra (UV–vis DRS), Raman spectra, high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS). The as-prepared AgAAgClATiO2/rectorite powders exhibited an efficient photocatalytic activity for the degradation of acid orange (ARG) and 4-nitrophenol (4-NP) under visible light irradiation (k > 400 nm). Moreover, the mechanism suggested that the high photocatalytic activity is due to the charge separation and the surface plasmon resonance of metallic Ag particles in the region of visible light. The active species measurements suggested that HO is not the dominant photooxidant. Direct hole transfers and O 2 were involved as the active species in the photocatalytic reaction. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction TiO2, as a cheap, nontoxic, efficient photocatalyst for the degradation of various pollutants, has received much research attentions during the past two decades [1–3]. However, it is activated only under UV light irradiation because of its large band gap and hence hindered its practical application. Therefore, enhancing the photocatalytic efficiency and visible light utilization of TiO2 is the most urgent issue in this topic. The works including metal deposition, nonmetal doping or ion-implanting methods have been explored to improve the properties of TiO2 [4–8]. Among them, noble metal nanoparticles (NPs) have attracted considerable interest due to their surface plasmon resonance (SPR), which accelerates the separation process of the photo-generated electrons and holes in the semiconductor catalyst [9,10]. And many researchers have demonstrated that the Ag NPs deposited on semiconductor show efficient plasmon resonance in the visible region [11–16]. On the other hand, silver halide has been supposed to be a new visible light photocatalytic material for its good sensitivity to light. Ag/silver halide composites as an excellent charge-separation promoter and builtin acceptor have been widely investigated in recent years. Recently, Huang and co-workers [17] fabricated the Ag@AgCl photocatalyst by first treating Ag2MoO4 with HCl to form AgCl powder and then reducing some Ag ions to Ag0 species, which is efficient for MO degradation under visible light irradiation. Yu et al. [18] introduced the Ag/AgCl structure into nanotube arrays and ⇑ Corresponding author. Fax: +86 27 87887445. E-mail address: [email protected] (G. Zhang). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.03.003
indicated the photocatalyst shows high visible light photocatalytic activity and stability during the photocatalytic degradation of methylorange in water. Clays, such as montmorillonite (MMT), rectorite, and kaolinite, have attracted much attention in recent years. These natural materials possess layered structures, large surface areas, and a high cation exchange capacity and can adsorb organic substances either on their external surfaces or within their interlaminar spaces by interaction or substitution [19]. Rectorite is a regular clay minerals consisting of alternate regular (1:1) stacking of dioctahedral mica-like layer and dioctahedral smectite-like layer [20]. As the support for the AgAAgClATiO2 composite, rectorite may facilitate the catalyst separation, recovery, and recycling after the photocatalytic reaction. In this study, we prepared AgAAgClATiO2 loaded rectorite composite photocatalyst. The visible light-driven photocatalytic activity of this composite material was investigated by the photocatalytic decomposition of azo dyes and nitrophenols. The relationship between the photocatalytic activity and the structural features of the prepared catalysts was investigated through a systematic characterization analysis, and the photocatalytic mechanism of the reaction was also discussed.
2. Experimental section 2.1. Materials The rectorite clay was obtained from Zhongxiang Hubei, PR. China. Titania P25 (TiO2, ca. 80% anatase, 20% rutile) was purchased
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2.2. Preparation of photocatalyst The TiO2/rectorite was prepared according to our previous work [21], the difference from the previous work was that the final solution pH was adjusted by NaOH solution but not by distilled water. At room temperature, TiO2/rectorite was dispersed in 10 mL ethanol, and then the mixture of silver nitrate solution (50 mL, 0.05 M) and ethanol (40 mL) was dropwise added to the dispersion with vigorous stirring for 2 h. The white AgCl appeared because the TiO2 sol was prepared by addition of TTIP into HCl solution and the TiO2/rectorite containing chlorine element. The wet cakes were separated and dried at 70 °C. Finally, the product was mixed with distilled water and was irradiated by visible light (k > 400 nm) for 3 h to reduce partly absorbed Ag+ to Ag via AgCl photocatalysis. 2.3. Characterization Powder X-ray diffraction patterns of the as-synthesized powders were obtained on a D/MAX-RB X-ray diffractometer (Rigaku, Japan) equipped with Cu Ka radiation (k = 1.5406 Å) and recorded with 2h ranging from 2.5° to 70°, while the accelerating voltage and the applied current were held at 40 kV and 50 mA, respectively. Scanning electron microscopy (SEM, JSM-5610LV) was used to characterize the morphologies of the products. Morphologies and microstructures of the prepared samples were further examined with transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images by a JEM 2100F electron microscope operated at an accelerating voltage of 200 kV. Chemical analysis of the photocatalyst was performed by X-ray analysis (EDX) joined a JSM-5610LV scanning electron microscopy (SEM). X-ray photoelectron spectroscopy (XPS) analysis was carried out using a Thermo VG Multilab 2000 spectrometer (the degree of vacuum in analysis room excelled 3 108 Pa). All binding energies were referred to the C 1s peak at 284.63 eV of the surface adventitious carbon and were revised. The Raman spectrum was recorded using a Renishaw invia spectrometer equipped with notch filter and a CCD detector. The diffuse reflectance spectra (DRS) were measured by a UV–vis spectrometer (UV2550, Shimadzu, Japan) in the range of 190–800 nm. BaSO4 was used as the reflectance standard material. The HO trapping fluorescence spectra were taken on a fluorescence spectrophotometer (Shimadzu RF-5300PC).
sample (the TiO2 content in the composite was obtained by ICP measurements). 3. Results and discussion 3.1. XRD analysis XRD was used to determine the phase structure of the samples. Fig. 1 shows the XRD patterns of the rectorite, TiO2/rectorite and AgAAgClATiO2/rectorite composite. From Fig. 1a, the rectorite exhibits basal (0 0 1) and (0 0 2) diffraction peaks, which are essential for rectorite with a highly ordered and oriented silicate layer structure [22]. As seen in Fig. 1b, the (0 0 1) and (0 0 2) peaks in the XRD patterns of the TiO2/rectorite composite disappear, suggesting that the layered structure of rectorite was destroyed to some extent. The TiO2 sample existed as a mixed phase (brookite, anatase, and rutile). The peaks of brookite phase appear because the chloride ions (an appropriate ratio of [Cl]:[Ti]) seem to be essential for the formation of brookite in highly acidic solution (pH = 3.0 in this study) [23]. The peaks of brookite in Fig. 1c disappeared probably due to the pH value changes in the preparation process. The diffraction peaks attributed to AgCl (JCPDS No. 31– 1238) and Ag (JCPDS No. 04–0783) were observed in Fig. 1c. This suggests that the deposited AgCl has a crystalline cubic phase and the metallic silver produced was due to the photoreduction process under visible light irradiation. Moreover, the diffraction peak assigned to metal Ag is broad and weak, which may result from its low content and small particle size on the surface of the as-prepared composite. 3.2. TEM images Fig. 2 displays the SEM and TEM images of the rectorite and the AgAAgClATiO2/rectorite composite. Some particles presented on the surface of the AgAAgClATiO2/rectorite composite (Fig. 2b) as compared to the pristine rectorite (Fig. 2a). As shown in Fig. 2c, the intercalation of the AgAAgClATiO2 with the rectorite particles destroyed the ordered structure of rectorite to some extent, resulting in some exfoliated rectorite particles [24]. The corresponding HRTEM image (Fig. 2d) of the sample states the lattice spacing of
001
from the Degussa Co. All other reagents, including AgNO3, ethanol, titanium tetraisopropoxide (TTIP), acid orange (ARG) and 4-nitrophenols (4-NP), were analytical grade and used as received without purification. All experiments were carried out using distilled water.
(a)
A: anatase B: brookite R: rutile C: AgCl S: silver
002
218
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2.4. Evaluation of photocatalytic activity
C(200)
S(111)
(c)
A(105) R(220)
B(132) A(200) C(220)
(b)
Rectorite A(101) R(110) B(121) R(101) A(004)
The photocatalytic activity of the photocatalyst was evaluated by the degradation of ARG and 4-NP in aqueous solution under visible light irradiation. A 300 W Dy lamp was used as the light source with a 400 nm cutoff filter to ensure complete removal of radiation below 400 nm. In a typical experiment, aqueous suspensions of ARG (100 mL, 50 mg/L) or 4-NP (100 mL, 5 mg/L) with 0.15 g of the photocatalyst were placed in the beaker. Prior to irradiation, the suspensions were magnetically stirred in the dark for 20 min to ensure that an adsorption/desorption equilibrium was established between the dye and the photocatalyst. At given irradiation time intervals, about 6 mL dispersions were collected and centrifuged to remove the particles. The filtrates were analyzed by an UV–visible spectrophotometer (Unico UV-2102PC). For comparison, the photocatalytic degradation of ARG by P25 was performed using the same procedure as above. The dosage of P25 was equivalent to the TiO2 content in the 0.15 g AgAAgClATiO2/rectorite
TiO2/rectorite
Ag-AgCl-TiO2/rectorite
Ag-AgCl 0
10
20
30
40
50
60
70
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2 Theta (degree) Fig. 1. XRD patterns of (a) rectorite, (b) the TiO2/rectorite, (c) the AgAAgClATiO2/ rectorite composite.
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Fig. 2. SEM images of (a) rectorite, (b) the AgAAgClATiO2/rectorite composite; TEM image (c) and HRTEM image (d) of the AgAAgClATiO2/rectorite composite.
the as-prepared photocatalyst. The determined lattice spacing of 0.35 nm and 0.32 nm are in good agreement with the values for anatase (1 0 1) and rutile (1 1 0) phase, respectively. The crystal lattice stripe of 0.197 nm belongs to the (2 2 0) phase of cubic AgCl (JCPDS 31–1238), suggesting the AgCl particles have deposited on the surface of TiO2/rectorite. 3.3. EDX and XPS analysis Energy dispersive X-ray spectroscopy (EDX) is an analytical technique used for the element analysis or chemical characterization of a sample. XPS analysis is useful for detecting valency states
of elements on the surface [25]. Thus, the compositions and the chemical state of its constituent elements of the AgAAgClATiO2/ rectorite composite were investigated by EDX and XPS analysis (Fig. 3 and Fig. 4). The energy dispersive X-ray spectroscopy (EDX) in the Fig. 3 shows the presence of Ag, Ti, Cl, and O elements on the as-prepared photocatalyst. Ti and O peaks result from the TiO2 particles, Ag and Cl elements indicate the presence AgCl and/or Ag in the as-prepared photocatalyst. Fig. 4 shows the high-resolution XPS spectra of the Cl 2p and Ag 3d regions. As shown in Fig. 4a, the Cl 2p peak is deconvoluted into two peaks (197.8 and 199.3 eV), which are assigned to the Cl 2p3/2 and Cl 2p1/2, respectively. This result suggests that the Cl elements
Fig. 3. EDX pattern of the AgAAgClATiO2/rectorite composite.
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202
Ag 3d5/2 Ag 3d3/2
Intensity (a.u.)
Cl 2p 1/ 2
Intensity (a.u.)
(b)
2 3/ 2p Cl
(a)
200
198
196
194
6.02 eV
378 376 374 372 370 368 366 364 362
Binding energy (eV)
Binding energy (eV)
Fig. 4. Cl 2P (a) and Ag 3d (b) XPS spectra of the AgAAgClATiO2/rectorite composite.
are mainly in the form of Cl [26,27]. Fig. 4b shows the XPS spectra of Ag of the AgAAgClATiO2/rectorite composite. The Ag 3d3/2 peak is divided into two different peaks at 373.6 and 373.0 eV, and the peak of Ag 3d5/2 is divided into 367.58 and 366.8 eV. The peaks at 373.6 and 367.58 eV are attributed to metallic silver, respectively. And the two peaks with a spin energy separation of 6.02 eV further indicate that the zerovalent silver exists in the AgAAgClATiO2/rectorite composite [28]. The Ag 3d peak appears at the binding energy at 373.0 and 366.8 eV belongs to the monovalent Ag, indicating the formation of AgCl on the as-prepared photocatalyst [29].
3.4. Raman analysis Fig. 5 shows the scatting Raman spectra for (a) the TiO2/rectorite and (b) the AgAAgClATiO2/rectorite composite. The spectrum intensity of the AgAAgClATiO2/rectorite composite increased as compared to the TiO2/rectorite composite. The SERS (surface enhancement of Raman signals) phenomenon was attributed to electromagnetic factors or chemical effects, and this conclusion has been theoretically debated and experimentally proven [30– 34]. Liu et al. [35] suggested the SERS was related to the ‘‘induced resonance effect’’ in the AgATiO2 system, which was due to the interaction of silver particles with semiconductor or the adsorption of molecules or ions on the surface of silver particles. Honma et al. [36] recognized the SERS effects were induced by the plasmon excitation of the silver particle in AgACdS hybrid particles.
Considering these, we suggest that the changes in intensity are probably due to the AgAAgCl particles in the composite. 3.5. UV–vis DRS analysis The photoabsorption ability of the prepared samples were detected by the UV–vis diffuse reflectance spectrum, as shown in the Fig. 6, the TiO2/rectorite composite and AgAAgClATiO2/rectorite photocatalyst have absorption in the visible region in contrast to P25 and TiO2. Importantly, aside from the photoabsorption from TiO2/rectorite, the AgAAgClATiO2/rectorite composite displays another absorption band around 450–750 nm. This can be attributed to the plasmon resonance of Ag particles in AgAAgClATiO2/rectorite composite. As the results of UV–vis DRS, the as-prepared photocatalyst is expected to have excellent photocatalytic activity for the degradation of organic contaminants. 3.6. Photodegradation of ARG and 4-NP under visible light irradiation ARG belongs to the commercial dye and widely used in textiles, soaps, and cosmetic products. The dye often occurs in wastewater and causes serious environment problems because the dye is chemically stable and biologically less active. Thus, the photocatalytic degradation of ARG was chosen to investigate the photocatalytic activity of the as-prepared photocatalyst. When dissolved in distilled water, ARG displays a major absorption band centered at 505 nm that is used to monitor the photocatalytic degradation. Fig. 7 illustrates the UV–vis absorption spectra of ARG aqueous solution under different reaction times. As shown in Fig. 7, the
1.5
1.2
Absorbtance (a.u.)
Intensity (a.u.)
(a) Ag-AgCl-TiO2 /rectorite (b) TiO2 /rectorite
(a) (b)
(a) rectorite (b) Ag-AgCl-TiO2 /rectorite (c) TiO2 /rectorite
0.9
(d) TiO2 (e) P25
(a)
0.6
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0
300
600
900
1200
1500
1800
Wavenumber (cm-1)
(d) (e)
0.0 200
300
400
500
600
700
Wavelength (nm) Fig. 5. Raman spectrum of (a) the AgAAgClATiO2/rectorite composite and (b) the TiO2/rectorite composite.
Fig. 6. UV–vis diffuse reflectance spectra of different samples.
800
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Fig. 7. UV–vis spectra changes of ARG solution during the photocatalytic degradation by the as-prepared photocatalyst under visible light illumination.
1.0 0.8
C/C0
absorption spectrum of the original solution shows three distinctive peaks at 215, 331, and 505 nm, which correspond to the structure of the benzene ring, the naphthalene ring, and the nitrogen to nitrogen double bond (AN@NA), respectively. It is found that the ARG absorption peak at 505 nm decreased rapidly and disappeared after 20 min, and the color of the azo dye changed from red to colorless as shown in the inset of Fig. 7, this confirms the photodegradation of ARG (i.e., the breakup of the chromophore responsible for the characteristic color of the azo dyes, rather than its discoloration or bleaching) [37]. The characteristic peaks of naphthalene ring and benzene ring also became more smoothness after visible light irradiation for 20 min, and no new absorption spectra appeared in either the visible or ultraviolet regions, indicating that the sample exhibited excellent photocatalytic activity. Fig. 8 shows the adsorption and photodegradation of ARG by different catalysts under visible light irradiation (k > 400 nm). The blank test in the absence of photocatalyst was performed and the results show that the photolysis of ARG was negligible under visible light irradiation. Meanwhile, the adsorption of ARG by the photocatalyst in the dark was also checked. In dark within 20 min, the adsorption reached equilibrium, and about 16% of ARG was adsorbed onto AgAAgClATiO2/rectorite. As a comparison, the photodegradation of ARG with P25 was also performed, and the results demonstrate that ARG was not significant degraded under visible light irradiation. However, the removing efficiency of the contaminant by AgAAgClATiO2/rectorite composite could reach nearly 100% within 20 min under visible light irradiation, indicating the excellent photocatalytic activity of the as-prepared photocatalyst. To further investigate the photocatalytic properties of the asprepared photocatalyst, 4-NP was selected to evaluate the photocatalytic activity as it is produced in the highest quantities worldwide has the relatively high toxicity and persistence in aqueous media [38]. In order to determine the nature of 4-NP in different systems, the spectra of 4-NP in photocatalytic reaction system and in distilled water were compared. As shown in Fig. 9, after adding the as-prepared photocatalyst, the peak at 400 nm disappeared and the intensity of peak at 317 nm increased as compared to the original 4-NP aqueous solution. This is so because that 4-NP is easily deprotonated to yield 4-NP– and develops a typical yellow color in basic media. After adding the photocatalyst, the pH values changed from 6.6 to 5.0, and thus, the yellow color disappeared as depicted in the Fig. 9. Fig. 10 displays the UV–vis spectra changes of 4-NP (5 mg/L) solution during the photocatalytic degradation
Ag-AgCl-TiO2 /rectorite adsoption in the dark blank P25
dark
0.6 0.4 0.2 0.0 -20
-15
-10
-5
0
5
10
15
20
25
Time (min) Fig. 8. The photodegradation efficiencies of ARG solution as a function of time under different conditions.
under visible light irradiation (k > 400 nm). As shown in Fig. 10, an apparent decrease in 4-NP at the wavelength of 317 nm was observed, which indicated the photocatalyst also showed an efficient photocatalytic activity for the degradation of 4-NP. Fig. 11 shows the photocatalytic rate of 4-NP solution under different conditions. The absorbance values obtained at 317 nm were used to calculate the decomposition ratio of 4-NP after 120 min. It can be seen that the characteristic peak at 317 nm dramatically decrease with increasing photocatalytic reaction time and the degradation rate of 4-NP reach 84% after visible light irradiation. The photolysis and the adsorption of 4-NP on the as-prepared photocatalyst in the dark were also given in Fig. 11. After 120 min, the concentration of 4-NP is nearly unaltered, suggesting the decrease in 4-NP is mainly caused by photodegradation but not adsorption and photolysis. 3.7. Visible light photodegradation mechanism The photo-induced active species such as trapped holes (h+), hydroxyl radical (HO ), and superoxide radical anions ðO 2 Þ are potentially important substance for the mineralization of toxic organic contaminants. Therefore, to investigate the mechanism of the process, the corresponding experiments were employed to determine the specific reactive species that played important roles in the photodegradation process. The formation of hydroxyl radicals at the photocatalytic system was detected by PL technique using
Fig. 9. UV–vis spectra changes of 4-NP solution in different reaction systems.
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0.8
Absorbance
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20 min 15 min 10 min 5 min 0 min
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Wavelength (nm)
500
550
600
Wavenumber (nm)
Fig. 10. UV–vis spectra changes of 4-NP solution during the photocatalytic degradation by the as-prepared photocatalyst under visible light illumination.
Fig. 12. HO trapping PL spectra of the AgAAgClATiO2/rectorite composite on TA solution under visible light irradiation.
BQ þ O þ O2 2 ! BQ
1.0
blank test adsorption in the dark photocatalysis
0.6 0.4 0.2 0.0
0
20
40
60
80
100
120
Time (min) Fig. 11. The degradation efficiencies of 4-NP solution as a function of irradiation time under different conditions.
terephthalic acid as a probe molecule. 2-Hydroxyl-terephthalic acid (TAOH), which is generated when terephthalic acid captures the HO radicals (as shown in Eq. (1)), performs a strong fluorescence at around 426 nm on the excitation of its own 312 nm absorption band [39,40].
HO þ TA ! TAOH
As shown in Fig. 13, the removing efficiency of ARG was about 10% after adding AO to the reaction system. The addition of AO seriously inhibited the photocatalytic process, which indicated the photo-generated holes and the active species generated by positive holes were the major responsibility in the degradation of ARG. Meanwhile, the photodegradation efficiency was also decreased to a certain degree after 2 mM BQ was added to the reaction system. The experimental result suggests that the superoxide may also another important intermediate species to the oxidative degradation of ARG. Therefore, a possible mechanism for the contaminant degradation over the as-prepared photocatalyst could be proposed on the basis of the above experimental results (as shown in Fig. 14). Silver particles can be excited by visible light and generated electronhole pairs due to surface plasmon resonance. The electrons can inject into the CB (conduction band) because the formed Schottky barrier at the Ag/TiO2 interface [44]. The conduction band gap of TiO2 was deduced about 0.7 V [38], which was more negative than the standard redox potential of O2 =O 2 (0.33 V vs NHE) [45]. As a result, the photo-generated electrons are expected to be trapped by absorbed O2 to produce O 2 . On the other hand, holes were scavenged by Cl– to form active oxidation production Cl0 which was consistence with Huang and Yu’s reports [18,46]. These
ð1Þ
Therefore, the photoluminescence technique by TA–PL to detect the formation of the HO radicals was conducted. Experimental procedures are similar to the measurement of the photocatalytic activity except that ARG aqueous solution was replaced by the TA aqueous solution. As shown in Fig. 12, the photoluminescence emission intensity at 426 nm barely increased after visible light irradiation, which indicates the HO radicals are not the main active species in the photocatalytic system. To further investigate the effect of other active species such as positive holes and superoxides on the photocatalytic degradation process, the comparison experiments for ARG degradation in the presence of the quenchers were performed. Ammonium oxalate (AO) is an effective scavenger of holes [41] and 1,4-Benzoquinone (BQ) is a quencher of the superoxide radical anion. O 2 via fast electron transfer according to Eq. (2) generating benzoquinone radicals [42,43]. and the BQ radical formation would inhibit the participation of O 2 in the contaminant decomposition.
1.0 0.8
C/C0
C/C0
0.8
ð2Þ
0.6
no quencher add AO add BQ
dark
0.4 0.2 0.0 -20
-15
-10
-5
0
5
10
15
20
25
Time (min) Fig. 13. Photocatalytic degradation of ARG over the AgAAgClATiO2/rectorite particles under different conditions with exposure to visible light (k > 400 nm).
Y. Yang et al. / Journal of Colloid and Interface Science 376 (2012) 217–223
Fig. 14. Schematic illustration for the charge separation in the visible light irradiated AgAAgClATiO2/rectorite system [18].
active species will decompose the contaminants to the final carbon dioxide or other intermediate products. 4. Conclusions Visible light induced AgAAgClATiO2/rectorite composite was prepared using a facile deposition–photoreduction method. The as-obtained composite showed a high photocatalytic activity for the degradation of ARG and 4-NP solutions. The HO trapping PL studies suggested that HO was not the dominant photooxidant in the photocatalytic reaction system. The free radical experiments 0 suggested that O 2 and Cl were formed in the ARG degradation process, which were generated by charge separation and transfer during the surface plasmon resonance of silver particles. A possible mechanism of dye photodegradation over AgAAgClATiO2/rectorite composite was proposed. The study would be helpful to understand the theoretical and practical application of visible light induced plasmon photocatalyst. Acknowledgments This work was supported by the National Natural Science Foundation of China (50872103), the Program of Wuhan Subject Chief Scientist (201150530147), the Wuhan Technologies R and D Program, National Basic Research Program of China (973 Program) 2007CB613302 and the Project-sponsored by SRF for ROCS, SEM. References [1] [2] [3] [4]
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