Article pubs.acs.org/IECR
Photocatalytic Application of Au−TiO2 Immobilized in Polycarbonate Film Duck Kun Hwang,† Yong-Gun Shul,‡ and Kyeongseok Oh*,§ †
Department of Corporate Diagnosis, Small & Medium Business Corporation, Yeouido-dong 24, Gukjaegeumyung-ro, Yeongdeungpo-gu, Seoul 150-718, Republic of Korea ‡ Department of Chemical and Biomolecular Engineering, Yonsei University 134 Shinchon-dong, Seodaemun-gu, Seoul 120-749, Republic of Korea § Department of Chemical and Environmental Technology, Inha Technical College, 100 Inha-Ro, Incheon 402-752, Republic of Korea ABSTRACT: Nanosize Au−TiO2 is becoming popular to photocatalytic application in visible light. A new fabrication process of photocatalyst is introduced for the purpose of its potential utilization under sunlight. A mixture of photocatalyst and polycarbonate (PC) was cast on a glass substrate to form a film. The photocatalytic experiment was performed in the UV region and visible region separately with powdery samples first, followed by composite films. When powdery Au−TiO2 was applied to decompose of acetaldehyde, activity inferior to Degussa P-25 was observed in UV light, but superior activity was observed in visible light. In the case of Au−TiO2 in PC composite film, evidence of acetaldehyde decomposition was observed, unlike the case of P-25 in PC composite, which showed negligible traces of acetaldehyde decomposition.
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INTRODUCTION TiO2 is a well-known photocatalyst that has the multiple advantages of high chemical stability, nontoxicity, and operability at room temperature.1 When it is illuminated by UV light, TiO2 catalyzes the decomposition of many organic compounds. Meanwhile, relatively poor efficiency occurs in the presence of visible light because TiO2 has wide band gap energy.1−3 In order to improve photocatalytic performance in visible light, nanometals embedded TiO2 have been explored.3,4 Among metals-doped TiO2, nano Au−TiO2 is becoming popular to catalyze in the presence of visible light.2−7 Various application of Au−TiO2 is summarized in Table 1. This study is motivated by the questions below: • Can we fabricate a powdery photocatalyst (TiO2, Au− TiO2) in various shapes instead of placing it in a batch reactor? • How to overcome the mechanical cracks of photocatalyst film if it is coated on the surface of polymers12 or other substrates, which may be caused by different thermal expansions? To solve the first question above, we decided to employ a polymer to immobilize photocatlysts. Composites of photocatalyst/polymer are much easier to fabricate than metal-matrix or ceramic composites. In order to cast polymer composite on a substrate, an appropriate solvent can be used to mix photocatalysts and a polymer. When we consider future utilization of photocatalyst/polymer composite, the transparent property of the polymer materials may be important. For example, either direct exposure of composite to light or secondary transmittance of irradiating light may be possible. Table 2 shows the typical properties of polymer materials. Polycarbonate (PC) has a superior value in impact strength compared to those of any other transparent polymers presented. PC is also satisfactory in its thermal property © 2013 American Chemical Society
(glass temperature) and clearness (refractive index). There has been a great deal of interest in the utilization of PC because of its competitive set of mechanical, thermal, and electrical properties as an engineering plastic. PC has dimensional stability, good adhesion, thermal stability, transparency, and is relatively less expensive than TiO2. Large amounts of PC are produced globally and consumed for various applications such as automotive interior and exterior materials, safety helmets, construction exterior walls, and so on. In addition, gas permeability data of PC is presented in Table 3. Specifically, the higher value in water permeability may be advantageous to apply to aqueous phase phtocatalytic reactions as well as gas phase reactions. Nevertheless, it should be noted that PC is sensitive to UV irradiation. With the help of various anti-UV agents,15 it will be possible to use for the long-term when photocatalyst/PC composite is placed outdoors. In this study, photocatalytic decomposition was attempted by using Au−TiO2 immobilized in PC film in visible light. Serial processes were conducted to prepare a composite film. At first, we carefully prepared nano Au−TiO2 in sol-gel method in advance. The size distribution of Au−TiO2 nanoparticles were verified by a light-scattering apparatus. As a comparison study, commercial TiO2 (Degussa, P-25) and synthesized TiO2 without gold (pure-TiO2) was also used instead of Au−TiO2. Photocatalytic activity of powdery catalysts (P-25, pure-TiO2, and Au−TiO2) was examined by acetaldehyde decomposition under UV and visible lights separately. Second, a mixture of a certain amount of photocatalyst/PC/solvent was cast to form a film on a glass substrate. Then, Ar plasma etching was employed to remove PC to expose photocatalyst particles. The Received: Revised: Accepted: Published: 17907
August 26, 2013 November 12, 2013 November 28, 2013 November 28, 2013 dx.doi.org/10.1021/ie402800f | Ind. Eng. Chem. Res. 2013, 52, 17907−17912
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Table 1. Applications of Au−TiO2 type
decomposition material
preparation
powder
film
polymer composite
Au−TiO2 nanocrystals synthesized through sol-gel reactions gold nanoparticles supported on P25 titania Au−TiO2 films obtained by electron beam evaporation Au−TiO2 coated on CaF2 disk Au-SrTiO3 films synthesized through sol-gel reactions Au−TiO2/ Cellulose composite Au−TiO2/ Polycarbonate composite
wavelength
reference
methanol
xenon lamp (>320 nm)
2
water methyl orange acetic acid CuSO4 methylene blue acetaldehyde
xenon lamp (>400 nm) blacklight (max. @ 350 nm) Hg lamp (>400 nm) xenon lamp (560 nm) xenon arc lamp (ASTM standard spectrum) fluorescent lamp (>400 nm)
4 8 9 10 11 this work
Table 2. Typical Properties of Optical Polymers13,14 polymer
refractive index
glass transition temperature (°C)
impact strength (kg·cm/cm)
polycarbonate (PC) poly(methylmethacrylate) (PMMA) polystyrene (PS) poly(styrene-co-acrylonitrile) (SAN) poly(vinylchloride) (PVC)
1.586 1.491 1.590 1.571 1.540
150 105 100 109 87
80 1.6 1.0 1.0
dissolved in chloroform (100 mass unit) at room temperature with intense stirring until a clear solution was obtained. In the other side, chloroform (100 mass unit) and photocatalyst (0.8 mass unit) were mixed in vigorous stirring at room temperature. The mixture of photocatalyst/chloroform was slowly dropped into the mixture of PC/chloroform with agitation. Then, the mixture of photocatalyst/PC/chloroform was cast to make a film on a glass substitute. Film was dried and placed in an oven at 90 °C for 6 h. Plasma Etching of Composite Films. Argon (Ar) plasma etching was processed on dried photocatalyst/PC composite films at room temperature. A plasma source (Wavemate MPDR 610i) was operated at 2.45 GHz with permanent magnets for electron cyclotron resonance. Typical operating parameters were 100 W of microwave power (maximum 300W), 7 × 10−6 mbar of Ar pressure in the preparation chamber, and a treatment time of 30−300 s. The etched surfaces of films were characterized by scanning electron microscopic (SEM, Hitachi S350N). Acetaldehyde Decomposition of Photocatalyst/PC Composite Films. Photocatalytic decomposition of gaseous acetaldehyde was examined by measuring the concentration of acetaldehyde as a function of irradiation time under light illumination with UV (Sankyo Denki G6T5, 6W × 2) and visible lights (Phillips F6T5/CW, 6W × 2), separately. Fixed amount (0.03 g) of each of the powdery photocatalysts was attached on a glass substrate. And photocatalyst/PC composites also contained 0.03 g of photocatalyst. The gas phase reaction was carried out in a quartz reactor that was equipped with a gas recirculation system. The inlet concentration of acetaldehyde was set to 500 ppm in an excess oxygen environment. Irradiation was conducted at room temperature after the equilibrium of reactants was fully achieved and verified by gas chromatography. Finally, the concentration of acetaldehyde was analyzed by gas chromatography (Shimadzu GC-8A, Porapak T column) with a flame ionization detector (FID). Additionally, a blank test was also carried out before the decomposition started. It should be noted that the concentration of acetaldehyde was only affected by the decomposition because sampling volume contains the exact sample concentration of acetaldehyde no matter how much GC injection was performed.
Table 3. Permeability Coefficient of Polycarbonate permeate
He
H2
Ne
Ar
O2
CO2
H2O
N2
P × 10
7.5
9.0
100
0.6
1.05
6.0
1050
0.225
a
a
13
The permeability coefficient is defined as
P=
quantity of permeate × film thickness area × time × pressure drop across the film
photocatalytic activity of photocatalyst/PC films was investigated by acetaldehyde decomposition as a function of irradiation time under light illumination with UV and visible lights. It should be noted that a batch reactor for powdery photocatalysts and a flow-type reactor for Au−TiO2/PC was used separately to evaluate the acetaldehyde decomposition.
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EXPERIMENTS Preparation of Au−TiO2. Au−TiO2 was prepared by a precipitation method. Titanium tetrachloride (TiCl4, Aldrich Co.) and tetrachloroauric acid trihydrate (HAuCl4·3H2O, Aldrich Co.) were dissolved in distilled water individually and kept below 1 °C. Aqueous TiCl4 solution was maintained at pH = 7 using 0.1 N NaOH solution and then added slowly to the aqueous gold ion solution with care to suppress an active hydrolysis reaction. Suspensions obtained were kept in a cooling bath for 1 h with continuous stirring. The solution was then dialyzed for 3 h using Spectra/Por (MWCO: 6-8000) membrane. After the solvent was removed by a rotary vacuum evaporator, the filtered solid was placed at 30 °C. In other case, pure-TiO2 was synthesized from TiCl4 in the aqueous phase without gold precursor. Particles synthesized were characterized by a dynamic light scattering (Brookhaven, 90Pus) and UV−vis spectrophotometer (Shimazu, UV1601). Film Formation of Photocatalyst Immobilized in PC. Polycarbonate (PC, 200-10 grade) was supplied by MitsubishiDow Chemical Co. with a melt flow index of 10 g/10 min (at 300 °C, 2.16 kg). The average molecular weight was about 41 000 with polydispersity index of 3.0 (Mw/Mn) measured by gel permeation chromatography. As photocatalysts, commercial TiO2 (Degussa, P-25), pure-TiO2, and Au−TiO2 were used in this study. Chloroform (Jusei Co., first grade) was used as a solvent without further purification. PC (9.2 mass unit) was 17908
dx.doi.org/10.1021/ie402800f | Ind. Eng. Chem. Res. 2013, 52, 17907−17912
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RESULTS AND DISCUSSION Characterization of Powdery Photocatalyst. Au−TiO2 nanoparticles were synthesized by the sol-gel method. After hydrolysis, the aqueous remnant was dialyzed to remove ions. Figure 1 presents the size distribution of Au−TiO2 up to 40 nm
The absorbance shifts approaching the visible region will provide the potential capability of Au−TiO2 when irradiated in visible light. Photocatalytic activity was evaluated by acetaldehyde decomposition and the result is shown in Figure 3. P-25
Figure 1. Particle size distribution of Au−TiO2 in aqueous phase after dialysis. Figure 3. Photocatalytic decomposition of acetaldehyde under UV irradiation (254 nm).
after dialysis. Average particle size was 13.6 nm after 3 h of dialysis. The procedure establishment for nanosize TiO2 synthesis was also presented in the previous work.16 Figure 2
shows the best activity until UV irradiation reached 80 min. After then, pure-TiO2 contributed the most to decompose the acetaldehyde until UV irradiation reached 220 min. However, the activity difference between P-25 and pure-TiO2 was not large after 200 min. Meanwhile, Au−TiO2 did not show the superior activity to TiO2 samples in UV light. Inferior activity of Au−TiO2 to TiO2 during UV irradiation was reported elsewhere.2 It is well known that the photocatalytic activity of TiO2 is dependent upon how many electrons transfer from the valence band to the conduction band located in TiO2. If nano Au particles are deposited on the surface of TiO2, electrons excited in the conduction band of TiO2 may migrate to Au particles when Au−TiO2 is irradiated by UV light (refer to Scheme 1 in ref 4). If this is true, it will result in less activity of acetaldehyde decomposition (Figure 3). It is suspected that the photocatalytic decomposition of acetaldehyde may depend upon the number of electrons excited and located in the conduction band of TiO2. In other words, it is also predictable that decomposition activity under UV light may be worse when the amount of doped gold increases on TiO2.2 When visible light irradiates either TiO2 or Au−TiO2, the mechanism of electron migration will be different. First, we can easily imagine that no significant electron excitation will occur during visible light irradiation to no-doped TiO2. The reason is that the bad gap energy of TiO2 requires high energy at the wavelength of the UV region. The band gap energy can be represented in the simplified equation17
Figure 2. UV−vis absorption spectra of various Au−TiO2 in aqueous phase: (a) 0.5 mol %, (b) 1 mol %, (c) 5 mol %, and (d) 10 mol % of Au in Au−TiO2.
shows the absorbance data when Au−TiO2 was irradiated by UV−vis light. Au concentration in Au−TiO2 was set to 0.5, 1, 5, and 10 mol %. With increasing the gold amount, UV−vis absorbance shifted gradually to higher wavelength region. It should be noted that 5 and 10 mol % Au−TiO2 responded with relatively higher values in absorbance at the wavelength range of 400−500 nm (visible region) as shown in Figure 2c and 2d.
E(eV) =
hC 1, 236 = λ (nm) λ (nm)
(1)
Here, E, h, C, and λ represent the band gap energy, the Plank constant, light velocity, and wavelength, respectively. The band 17909
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controlled by lower irradiation power. What was obtained was that more photocatalyst particles were becoming exposed with increasing plasma-etching time. Figure 5 showed the surface
gap energy will be decreased with increasing wavelength. In the presence of Au on TiO2, we can see the reduction of band gap energy in Figure 2. A higher concentration of Au in Au−TiO2, responding absorbance shift to longer wavelength, results in the reduction of band gap energy. Seh and co-workers18 reported the synergic effect of Au and TiO2 in Au−TiO2 during watersplit-induced hydrogen generation in visible light. They interpreted that plasmon placed in the interface between Au and TiO2 played an important role in exciting electrons by visible light. Nanosized gold caused generation of surface plasmons when it is irradiated by visible light. Seh and coworkers18 reported the noticeable result that P-25 did not show any evidence to generate hydrogen from the water split reaction in the presence of visible light. In this study, photocatalytic activities of TiO2 and Au−TiO2 are also compared in the visible region and shown in Figure 4.
Figure 5. SEM images of the surfaces of Au−TiO2/PC composite film after a plasma etching time of (a) 0 s, (b) 0 s (cross section), (c) 60 s, (d) 120 s, (e) 300 s, and (f) 300 s.
Figure 4. Photocatalytic decomposition of acetaldehyde under fluorescent light.
images of the Au−TiO2/PC composite films etched after 0, 60, 120, and 300 s, respectively. The size of Au−TiO2 was within 90 nm and well dispersed in the PC matrix shown in Figure 5d, e, and f. The thickness of the Au−TiO2/PC composite film was about 1.5 μm as determined by SEM (Figure 5b). The SEM images revealed that the plasma etching led to the sustainable exposure of Au−TiO2 particles, specifically, after 120 and 300 s. In Figure 6, acetaldehyde decomposition was examined by two different photocatalysts’ composite films (P-25/PC and Au−TiO2/PC) as it varied with the UV irradiation time. During the irradiation by UV light over 120 min, both P-25/PC and Au−TiO2/PC films showed a similar trend of acetaldehyde decomposition, with a slightly higher activity in Au−TiO2/PC film (5.6%) than P-25/PC film (5.0%). Compared with the result obtained from powdery photocatalysts (Figure 3), Au− TiO2/PC film showed superior performance to P-25/PC film. Even though the difference in performance is still minimal, the activity trend was opposite to that of the powder loading during UV irradiation. The reason may be because of the inconsistence of particle size of P-25 and Au−TiO2 in photocatalyst/PC film. The average particle size of P-25 particles was about 10 μm. The particle size may not be critical in the powder-loading
As expected, Au−TiO2 showed better acetaldehyde decomposition than pure-TiO2. We interpret that Au contributes to reduce the band gap energy of photocatalyst and that electrons in gold surface plasmons may migrate gradually to the conduction band in TiO2 (refer to scheme 2 in reference 4). After fluorescent irradiation for 240 min, Au−TiO2 showed greater activity than pure-TiO2 (70.4% vs 34.1%). Application of Photocatalysis Immobilized in PC Composite Film. Powdery photocatalysts (P-25, Au−TiO2) were dispersed in chloroform beforehand and mixed with PC/ chloroform mixture. The mixture of TiO2/PC/chloroform was cast to make a film on a glass substrate. After drying the film, it was observed that PC covered most of photocatalyst particles. Ar plasma etching was employed to expose photocatalyst particles by removing thin PC on the surface of photocatalyst/ PC composite film. Plasma etching has often been used in surface modification.19 Plasma etching is applicable to various surfaces, such as glasses, metals, and plastics. In this study, it should be noted that plasma etching was used only to expose photocatalyst particles by removing thin PC. In order to remove thin PC without the loss of photocatalyst, plasma was 17910
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light may be achieved through the increase of Au content, the size reduction of Au or Au−TiO2 in PC composite film, and process development to expose more particles to the surface of PC when casting a film. A long-term experiment would be required to prove a practical installation.
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CONCLUSION Au−TiO2 was synthesized by sol-gel method. The nanosize of Au−TiO2 was verified, with an average size of 13.6 nm. The photocatalytic activity of Au−TiO2 was evaluated in a batch system with powder samples and photocatalyst/PC film samples. The photcatalystic experiment was carried out in the presence of UV light and visible light separately. The performance of Au−TiO2 was compared with P-25 and synthesized TiO2. The reduction of bad gap energy was observed in Au−TiO2. Acetaldehyde decomposition under UV light was conducted with Degussa P-25, synthesized TiO2, and Au−TiO2. Powdery Au−TiO2 showed the least activity in UV light. It was interpreted that the migration of electrons in the conduction band of TiO2 to Au may weaken the activity of TiO2 during UV irradiation. On the contrary, during visible light irradiation, powdery Au−TiO2 showed the best activity in acetaldehyde decomposition. We interpret that the electron migration from gold surface plasmons to the conduction band of TiO2 may activate the decomposition of acetaldehyde. In the case of photocatalyst/PC composite film, unlike in the powdery test, Au−TiO2/PC film was superior to P-25/PC film in UV light, but the difference was marginal. When photocatalyst/PC films were irradiated by visible light, only Au−TiO2/PC film showed evidence of acetaldehyde decomposition. This study explored if future applications of Au−TiO2 can be viable in sunlight. We expect that the shape modification of photocatalyst using polymer composites may be useful to various surfaces.
Figure 6. Acetaldehyde decomposition of plasma-treated photocatalyst/PC composite films under UV irradiation (254 nm).
batch reaction. Well-dispersed nanosize Au−TiO2 in Au− TiO2/PC composite film may become more effective than P-25 in P-25/PC composite film. It should be noted that the quantity of photocatalysts exposed to the surface of composite films are considerably smaller than in the powder-loading experiment. The photocatalytic effect under visible irradiation is presented in Figure 7. Acetaldehyde decomposition was observed to be negligible in P-25/PC film in 120 min. However, the initiation of acetaldehyde decomposition was noticeable in Au−TiO2/PC film (0.63% in 120 min). The result provided the insight that the future capability of Au−TiO2/PC film may be possible where visible light is used under sunlight. We also expect that the better photocatalytic activity in visible
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AUTHOR INFORMATION
Corresponding Author
*K. Oh. Phone: +82-32-870-2271. Fax: +82-32-870-2541. Email:
[email protected]. Notes
The authors declare no competing financial interest.
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Figure 7. Acetaldehyde decomposition of plasma treated photocatalyst/PC composite films under fluorescent irradiation. 17911
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