Cu–TiO2 nanorods with enhanced ultraviolet- and visible-light photoactivity for bisphenol A degradation

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Cu–TiO2 nanorods with enhanced ultraviolet- and visible-light photoactivity for bisphenol A degradation Li-Fen Chiang, Ruey-an Doong ∗ Department of Biomedical Engineering and Environmental Sciences, National Tsing-Hua University, 101, Sec. 2, Kuang-Fu Road, Hsinchu 30013, Taiwan

h i g h l i g h t s • • • •

TiO2 nanorods have been successfully fabricated by microwave-assisted sol–gel method. Cu ions were reduced to Cu0 /Cu2 O mixture and well distributed onto TiO2 nanorods. The Cu–TiO2 composites enhance the BPA photodegradation efficiency by inhibiting e–h recombination. The rate constant for BPA photodegradation by Cu–TiO2 increases 6.6 times higher than that of P25 in visible light.

a r t i c l e

i n f o

Article history: Received 31 August 2013 Received in revised form 21 January 2014 Accepted 27 January 2014 Available online xxx Keywords: Microwave-assisted Sol–gel Copper ions TiO2 nanorods Bisphenol A Visible-light-driven photodegradation

a b s t r a c t In this study, the microwave-assisted sol–gel method and chemical reduction were used to synthesize Cu–TiO2 nanorod composites for enhanced photocatalytic degradation of bisphenol A (BPA) in the presence of UV and visible lights. The electron microscopic images showed that the Cu nanoparticles at 4.5 ± 0.1 nm were well-deposited onto the surface of TiO2 nanorods after chemical reduction of Cu ions by NaBH4 . The X-ray diffractometry patterns and X-ray photoelectron spectroscopic results indicated that Cu species on the Cu–TiO2 nanorods were mainly the mixture of Cu2 O and Cu0 . The Cu-TiO2 nanorods showed excellent photocatalytic activity toward BPA photodegradation under the irradiation of UV and visible lights. The pseudo-first-order rate constant (kobs ) for BPA photodegradation by 7 wt% Cu–TiO2 nanorods were 18.4 and 3.8 times higher than those of as-synthesized TiO2 nanorods and Degussa P25 TiO2 , respectively, under the UV light irradiation. In addition, the kobs for BPA photodegradation by 7 wt% Cu–TiO2 nanorods increased by a factor of 5.8 when compared with that of Degussa P25 TiO2 under the irradiation of 460 ± 40 nm visible light. Results obtained in this study clearly demonstrate the feasibility of using one-dimensional Cu–TiO2 nanorods for photocatalytic degradation of BPA and other pharmaceutical and personal care products in water and wastewater treatment plants. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Endocrine disrupting chemicals (EDCs) such as bisphenol A (BPA), estradiol, and estrone are typical emerging pollutants commonly found in water resources and effluents from wastewater treatment plants [1,2]. BPA has been widely used in the plastic industry for the production of polycarbonate plastics and epoxy resin [3]. Because of the easy release into the environment through the domestic sewages and industrial wastewaters, BPA may cause endocrine-disruptive effects on human beings and aquatic biota [4]. Therefore, the development of an effective strategy to rapidly remove BPA from water and wastewater is thus needed. Various technologies including adsorption [5,6], chemical oxidation [7], advanced oxidation processes [8,9], and photocatalysis

∗ Corresponding author. Tel.: +886 3 5726785; fax: +886 3 5718649. E-mail address: [email protected] (R.-a. Doong).

[10,11] have been developed to effectively remove BPA from water and wastewater. Photocatalytic degradation has been reported to be a promising method for the degradation of BPA in aqueous solutions. Nanostructured TiO2 is one of the most effective materials used in photocatalytic and photovoltaic technologies [12–15]. Nanoparticles with small size are usually preferred photocatalysts due to their high specific surface area and more reactive sites. However, the electron–hole recombination rate would be increased in nano-sized photocatalysts due to the confined space in sphere-shaped nanoparticles [16]. More recently, one-dimensional (1-D) nanomaterials have been demonstrated to be the effective photocatalysts for pollutant removal, carbon dioxide reduction, and water splitting because of their large specific surface areas and unique morphology for electron transport [17–20]. Nanorods have several advantages including high surface-to-volume ratios, high active sites for surface reactions, and high interfacial charge carrier transfer rates. In addition, the delocalization of carriers can move freely throughout the length of nanorods [20], resulting in

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the reduction of e− /h+ recombination rate and the increase in photocatalytic activity toward organic decomposition. In addition to the morphological effect on photocatalysis, several studies have used dopants including metal ions [21–24], metal oxide [25], and anions [26–28] to effectively decrease the electron–hole recombination rate and to improve the photocatalytic activity of TiO2 -based nanostructures. The use of copper ions as dopants has recently attracted much attention because copper species including Cu2 O and CuO can serve as the electron mediator and can extend the absorption to the long wavelength region [29–31]. However, the preparation of metaldoped nanorods is usually time tedious and energy consuming. Therefore, the development of a facile method to prepare 1-D TiO2 based nanomaterials for effectively photodegradation of BPA in a wide wavelength range is important. Microwave-assisted sol–gel method is a promising technique for preparation of 1-D nanomaterials because of the advantages of short reaction time and environmental friendliness. However, the fabrication of Cu-doped TiO2 nanorods using microwave sol–gel method in nonaqueous solution has rarely reported and the photocatalytic degradation of BPA by Cu–TiO2 nanorods remains unclear. In this study, the photocatalytic degradation of BPA by Cu–TiO2 nanorods was investigated in the presence of UV and visible lights. The Cu–TiO2 nanorod composites were synthesized via microwaveassisted sol–gel method followed by chemical reduction method using NaBH4 as the reducing agent. The dimension and morphology of Cu-modified TiO2 nanorod composites were examined by transmission electron microscopy (TEM). The X-ray powder diffractometry (XRD) and X-ray photoelectron spectroscopy (XPS) were used to characterize the crystallinity and chemical species of Cu ions, respectively. Effect of Cu loading and humic acid on the photodegradation efficiency and rate of BPA was evaluated. In addition, the reaction kinetics for BPA photodegradation by Cu-modified TiO2 nanorods was examined and compared with those by Degussa P25 TiO2 . 2. Materials and methods 2.1. Chemicals Degussa P25 TiO2 powder was obtained from Degussa Co. Titanium(IV) isopropoxide (TTIP, 98 + %) was purchased from ACROS. Benzyl alcohol (ACS reagent, ≥99.0%), oleic acid (90%) and hexane (anhydrous, 95%) were purchased from Sigma–Aldrich. Tetramethylammonium hydroxide pentahydrate (TMAOH, ≥97%) was purchased from Sigma. Sodium borohydride (95%), copper(II) nitrate pentahemihydrate (Cu(NO3 )2 ·2.5H2 O, 98%), and absolute ethanol (99.8%) were purchased from Riedel-de Haën. Hydrochloric acid (36.5–38.0%) was purchased from J.T. Baker. Bisphenol A (BPA, 99 + % purity) and humic acid were purchased from Aldrich. All chemicals were used as received without further treatment. All solutions were prepared with high-purity bidistilled deionized water (Millipore Co., 18.3 M cm) unless otherwise mentioned. 2.2. Synthesis of TiO2 nanorods The TiO2 nanorods were prepared using the microwave-assisted sol–gel method in non-aqueous solutions. 0.2–0.8 mL of TTIP, 0.4 mL of bidistilled de-ionized water, and 8 mL of oleic acid was dissolved in 20 mL benzyl alcohol. The mixture was stirred for 10 min at room temperature and followed by heating up to 180 ◦ C for 3 h. The obtained TiO2 nanorods were harvested by addition of absolute ethanol, centrifugation at 11,000 rpm for 10 min and re-dispersion into hexane.

The aqueous TiO2 nanorods were prepared by washing the organic-capped nanorods with a mixture of hexane and ethanol (1:2) several times to remove excess capping agent on the surface of TiO2 nanorods. The washed TiO2 nanorods were then added into the aqueous solution containing 0.2 M tetramethylammonium hydroxide and heated to 70 ◦ C for 60 min. The final products were separated by centrifugation at 11,000 rpm for 20 min and redisperse into bidistilled deionized water. 2.3. Synthesis of Cu–TiO2 nanorod composites The Cu–TiO2 nanorod composites were synthesized by chemical reduction method. Typically, 10 mg TiO2 nanorods were dispersed into de-ionized water under vigorous stirring, and various amounts of Cu(NO3 )2 solutions were added into the TiO2 nanorod suspensions to get final concentrations of 0.1–6 mM. The suspension was equilibrated for 60 min, and then a fresh prepared 20 mM NaBH4 solution was added and stirred for another 60 min in ice bath. The obtained Cu–TiO2 nanorods were harvested by centrifugation at 11,000 rpm for 20 min and washed with ethanol for 3 times. The residues were dried at 110 ◦ C for overnight, and subsequently ground into fine powders using an agate mortar. The energy dispersive X-ray spectroscopy analysis showed that the mass loadings of Cu on the TiO2 nanorods were in the range 0.4–20 wt%. 2.4. Photodegradation of BPA by Cu–TiO2 nanorod composites The photocatalytic degradation of BPA by TiO2 -based nanomaterials was carried out in a glass tube surrounded by eight 8 W UV or visible lamps. The BPA solution was dissolved in bidistilled deionized water to obtain the final concentration of 10 mg/L. In addition, 0.1 wt% ethanol was added to the solution for the enhancement of water solubility of BPA. The 1 g/L titanium-based nanorods and commercial P25 TiO2 nanoparticles were added into 20 mL of BPA solutions. Prior to the irradiation, the suspension was magnetically stirred in the dark for 60 min to ensure the adsorption equilibrium of BPA onto the photocatalysts. As shown in Fig. S1, a decrease of 2–4% in BPA concentration was observed in solutions containing 0.4–20 wt% Cu–TiO2 nanorods within 60 min, indicating the little adsorption of BPA onto Cu–TiO2 nanorods. After the equilibrium, the UV light at 365 nm or the visible light at 460 ± 40 nm was turned on and aliquots (0.5 mL) were withdrawn from the solution at various time intervals for analysis after removal of catalysts by centrifugation. The aqueous concentrations of BPA were determined by a high-performance liquid chromatograph (HPLC) equipped with C-18 column (LUNA 5u 100A, 4.6 mm × 250 mm, Phenomenex) and a diode array detector (HPLC-DAD, Agilent Technologies, series 1200). The isocratic methanol/acetonitrile/water mixture (50:30:20, v/v) at a flow rate of 0.5 mL/min was used as the eluent. The absorbance wavelength at 235 nm was used to determine the BPA concentration. In addition, HPLC with quadrapole tandem mass spectrometer (Quattro Micro, Waters, HPLC/MS/MS) equipped with C-18 column was used to identify the photodegradation intermediates of BPA. The mass spectrometer was operated in the m/z range 50–800 and the isocratic methanol/water mixture (60:40, v/v) at a flow rate of 0.2 mL/min was used as the eluent. The mass accuracy of HPLC/MS/MS is in the range of ±0.2 Da. 2.5. Characterization The TEM images were obtained on a JEOL 2011 microscope operated at 120 kV. High-resolution transmission electron microscopy (HR-TEM) was carried out on a JEOL JEM-2010 microscope at 200 kV. The Brunauer–Emmett–Teller (BET) specific surface area and pore size distribution were carried out by nitrogen adsorption and desorption at 77 K using a surface area and porosimetry

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system (ASAP 2020, Micromeritics). The diffuse reflection spectra were obtained in the wavelength range 250–550 nm using a Hitachi U-3010 UV–vis spectrometer equipped with the integrating sphere accessory for diffuse reflectance spectra. BaSO4 was used as a reference. The XPS measurements were performed by an ESCA PHI 1600 photoelectron spectrometer (Physical Electronics, Eden Prairie, MN) using Al K␣ X-ray source (1486.6 ± 0.2 eV photon energy). The crystalline structures of Cu–TiO2 were identified by XRD using X-ray diffractometer (Bruker NEW D8 ADVANCE, Germany) with a Lynx eye high-speed strip detector and Ni-filtered ˚ operating at a generator voltCu K␣-radiation source ( = 1.5406 A) age and an emission current of 40 kV and 40 mA, respectively. Samples were mounted on sample holder, and the scan range for all samples was between 5◦ and 70◦ 2 at a scan rate of 30◦ /min. 2.6. Reaction kinetics It is known that the rate of photocatalytic degradation of organic pollutants at liquid–solid interface can be described by the Langmuir–Hinshelwood kinetic model [28]: r0 =

KF St C dC = kapp 1 + KF C dt

(1)

where r0 is the reaction rate of BPA photodegradation, C is the aqueous concentration of BPA, kapp is the limiting-step rate constant of reaction at maximum coverage under the given conditions, St is the total reaction sites of Cu–TiO2 nanorods, and KF is the adsorption coefficient of BPA. When the BPA concentration (C) is low, Eq. (1) can be simplified to the pseudo-first-order kinetics: ln

C C0

= −kobs t

(2)

3

where kobs is the pseudo-first-order rate constant for BPA photodegradation (min−1 ). 3. Results and discussion 3.1. Characterization of Cu–TiO2 nanorod composites Fig. 1 shows the TEM images of TiO2 nanorods synthesized by addition of various amounts of TTIP precursors ranging from 0.2 to 0.8 mL. It is clear that the TiO2 nanorods prepared at various amounts of TTIP showed a similar diameter of around 2 nm. However, the length of as-synthesized TiO2 nanorods was highly dependent on the added amounts of TTIP. As shown in Fig. 1, the length of TiO2 nanorods increased from 10 nm at 0.2 mL of TTIP to 15 nm at 0.4 mL of TTIP, and then to 21 nm at 0.8 mL of TTIP. The aspect ratios of as-synthesized TiO2 nanorods were 5, 7.5, and 10.5, respectively. The highest aspect ratio of as-synthesized TiO2 nanorods was further used for synthesis of Cu–TiO2 nanorod composites by chemical reduction method using NaBH4 as the reducing agent. Fig. 2 shows the TEM and HRTEM images of Cu–TiO2 nanorod composites. The TEM image clearly showed that copper ions were well dispersed onto the surface TiO2 nanorods and the nanoparticles on the surface increased with the increase in mass loading of Cu from 0.4 to 20 wt% (Fig. 2a–d). In addition, the distribution of particle size of Cu nanoparticles was homogeneous and the average diameter was 4.5 ± 0.1 nm. From the HRTEM image of 7 wt% Cu–TiO2 nanorods (Fig. 2e), a lattice fringe with average d-spacing of 0.35 nm, which is in good agreement with the (1 0 1) reflection of anatase TiO2 was clearly observed. Another nanocrystals with a lattice fringe of 0.36 nm, which corresponds to the (1 1 1) reflection of zerovalent

Fig. 1. TEM images of TiO2 nanorods synthesized by various amounts of TTIP as the precursor. (a) 0.2 mL, (b) 0.4 mL, and (c) 0.8 mL.

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Fig. 2. The TEM images of Cu–TiO2 nanorod composites synthesized by using 0.8 mL TTIP as the precursor followed by reduction of various Cu loading of (a) 0.4 wt%, (b) 2 wt%, (c) 7 wt%, and (d) 20 wt% by NaBH4 . Figures (e) and (f) are the HR-TEM image and EDS spectrum of Cu–TiO2 nanorod composites at 7 wt% Cu.

Cu, was also observed [32]. The EDS analysis indicated that the nanocomposites contained 52.69 wt% Ti, 40.44 wt% O and a small amount of Cu (6.87 wt%) (Fig. 2f), which means that the Cu ions have been successfully deposited onto the surface of TiO2 nanorods. Table S1 (see supplementary data) shows the specific surface areas of as-synthesized TiO2 and Cu–TiO2 nanorods. The specific surface area of as-synthesized TiO2 nanorods was 206.6 m2 /g and increased to 207.5–242.1 m2 /g when 0.4–20 wt% Cu ions were deposited onto the surface of TiO2 nanorods, showing that addition of Cu ions slightly increases the specific surface area of Cu–TiO2 nanorods. Fig. 3a shows the XRD patterns of as-synthesized TiO2

nanorods and Cu–TiO2 nanorod composites prepared by using 0.8 mL of TTIP as the precursor. The XRD patterns of as-synthesized TiO2 nanorods clearly showed peaks centered at 25.42◦ , 37.99◦ , 48.17◦ , 54,11◦ , 55.2◦ , and 62.88◦ 2, which can be assigned as (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1), and (2 0 4) orientations of the anatase phase of TiO2 (JCPDS 21-1272). In addition, two sets of additional peaks of Cu species were obtained. Peaks centered at 36.4◦ and 61.6◦ 2 were characteristic peaks of Cu2 O, while another set of peaks at 43.4◦ , 50.4◦ and 74.2◦ 2, which corresponded to the (1 1 1), (2 0 0), and (2 2 0) reflections of Cu0 , were observed, clearly indicating the formation of mixture of Cu2 O and Cu0 (Cu0 –Cux O). This means that

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100

(a)

0 wt% Cu

Intensity (a.u.)

1-Reflectance (%)

80

0.4 wt% Cu 2 wt% Cu

60

7 wt% Cu 20 wt% Cu

40 20

20

30

40

50

60

70

80

2θ (degree)

0 250

300

350

400

450

500

550

Wavelength (nm)

(b)

Intensity (a.u.)

Fig. 4. The UV–visible diffuse reflectance spectra of as-synthesized TiO2 nanorods and Cu–TiO2 nanorod composites at various mass loadings of Cu(II) ions ranging from 0.4 to 20 wt%.

960

950

940

930

Binding energy (eV)

(c)

Intensity (a.u.)

0

Cu

3.2. Photodegradation of BPA by Cu–TiO2 under UV light irradiation conditions

CuO

938

936

934

XPS spectrum may be mainly attributed to the conversion of Cu2 O, which is in good agreement with the results obtained from XRD patterns. The optical property of Cu–TiO2 nanorod composites was determined by UV–visible spectroscopy. Fig. 4 shows the diffuse reflectance spectra of the as-synthesized TiO2 and Cu–TiO2 nanorod composites. The adsorption edge of as-synthesized TiO2 nanorods started to increase at around 404 nm, which corresponded to the band gap of 3.1 eV. This adsorption is mainly attributed to the interband transition of TiO2 [34]. The spectra showed a slight red-shift to 410–416 nm (2.98–3.0 eV) when 0.4–20 wt% Cu(II) ions were added. The absorption at 410 nm of Cu–TiO2 is the direct interfacial charge transfer from the valence band of TiO2 to Cu [35,36]. This result clearly indicates that Cu was successfully deposited onto the surface of TiO2 to extend the absorption to a long wavelength range.

932

930

Binding energy (eV) Fig. 3. (a) XRD patterns of as-synthesized TiO2 nanorods and Cu–TiO2 nanorods composite, (b) Cu 2p XPS spectra and (c) deconvolution of Cu 2p3/2 XPS spectra of 7 wt% Cu–TiO2 nanorod composites.

addition of NaBH4 can convert Cu2+ ions into the reduced species of Cu2 O and Cu0 . Fig. 3b shows the Cu 2p XPS spectra of 7 wt% Cu–TiO2 nanorod composites. Two peaks centered at 932.8 and 953 eV were observed, indicating the formation of a mixture containing Cu(I) and Cu(0). It is noteworthy that the differentiation of Cu(I) and Cu(0) peaks in this study was difficult because the resolution of XPS used in this study was around 0.2 eV. After peak deconvolution of Cu 2p3/2 XPS spectrum shown in Fig. 3c, peaks centered at 933.6 and 932.7 eV, which can be assigned as CuO and Cu0 , respectively, were obtained. Chang and Hsu [33] investigated the phosphine adsorption by the sol–gel-derived Cu/TiO2 adsorbents and found that the Cu0 and Cu+ would be oxidized to CuO and Cu(OH)2 after exposure to ambient air. Since Cu2 O is an unstable species and can be easy to oxidize to CuO, the formation of CuO shown in the

Fig. 5 shows the effect of Cu loading on the photocatalytic efficiency of BPA by Cu–TiO2 nanorod composites under the 365 nm UV light irradiation. No obvious photodegradation of BPA was observed after 120 min of irradiation in the absence of photocatalyst (direct photolysis). Addition of photocatalysts including as-synthesized TiO2 nanorods and Cu–TiO2 nanorod composites increased the photodegradation efficiency of BPA and 52% of BPA was photodegraded by as-synthesized TiO2 nanorods after 120 min of UV light irradiation. Addition of Cu ions significantly enhanced the photodegradation efficiency and rate of BPA by TiO2 nanorods. 53% of the original BPA was photodegraded by Cu–TiO2 nanorod composites at 0.4 wt% Cu after 120 min of irradiation, and a nearly complete photodegradation of BPA was observed when the mass loading of Cu were in the range of 2–20 wt%, clearly indicating that the addition of copper species significantly enhanced the photocatalytic activity of TiO2 nanorods toward BPA photodegradation. The photodegradation of BPA by Cu–TiO2 nanorod composites followed the pseudo-first-order kinetics, and the kobs for BPA photodegradation increased upon increasing concentration of Cu. As shown in Fig. 5b, the kobs for BPA photodegradation by as-synthesized TiO2 nanorods was 0.0063 min−1 . After addition of Cu ions, the kobs for BPA photodegradation increased from 0.0083 ± 0.0015 min−1 at 0.4 wt% Cu to 0.0373 ± 0.0045 min−1 at 2 wt% Cu, and then reached the maximum value of 0.116 ± 0.022 min−1 at 7 wt% Cu. Further increasing the Cu concentration to 20 wt% decreased the kobs for BPA photodegradation to 0.023 ± 0.005 min−1 , clearly showing that 7 wt% is the

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(a)

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Remaining ratio (C/Co)

(a)

Blank control

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0 wt% Cu 0.4 wt% Cu

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0.2

20 wt% Cu

Blank control Cu-P25 TiO2

0.8

TiO2 nanorod 0.6 0.4

P25 TiO2 Cu-TiO2 nanorod

0.2 0.0

0.0 0

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Illumination time (min)

Illumination time (min) (b)

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-1

kobs (min )

(b) 0.14

-1

kobs (min )

0.10 0.08

0.08 0.06

0.06

0.04

0.04

0.02

0.02

0.00 P25

0.00 0

4

8

12

16

20

Copper loading (wt%) Fig. 5. (a) The photodegradation of 10 mg/L BPA by Cu–TiO2 nanorod composites under UV light irradiation, and (b) kobs for BPA photodegradation as a function of copper loading.

optimal mass loading of Cu ions to enhance the photocatalytic activity of TiO2 nanorods under the irradiation of 365 nm UV light. Several studies have depicted that Cu ions can serve as electron trap center to effectively inhibit the recombination of photoinduced charge carriers and an optimal dosage is usually achieved to improve the photocatalytic activity of TiO2 [30,31,37]. In this study, we have demonstrated that Cu0 –Cux O can significantly enhance the photodegradation efficiency and rate of BPA by 1-D Cu–TiO2 nanorods, and the kobs values for BPA photodegradation by Cu–TiO2 nanorods are 1.3–18.4 times higher than that of assynthesized TiO2 nanorods. It is noteworthy that the decrease in kobs for BPA photodegradation by Cu–TiO2 nanorod composites at high Cu mass loading of 20 wt% is mainly attributed to the production of excessive Cu species on the surface to become the recombination centers for photo-induced electrons and holes, leading to the decrease in photocatalytic activity of Cu–TiO2 nanorod composites. The photocatalytic efficiency and rate of BPA by Cu–TiO2 nanorod composites was further compared with pure and Cumodified P25 TiO2 nanoparticles. As shown in Fig. 6a, 97% of the original BPA was photodegraded by Degussa P25 TiO2 after 120 min of UV light irradiation. However, the total removal efficiency of BPA decreased to 50% when Cu ions were added to Degussa P25 TiO2 and reduced to Cu0 –Cux O by NaBH4 . The photodegradation of BPA by P25 TiO2 also followed the pseudo-first-order kinetics and the

Cu-P25

TiO2 nanorod

Cu-TiO2 nanorod

Fig. 6. (a) The photodegradation of 10 mg/L BPA by various TiO2 -based nanomaterials under the irradiation of 365 nm UV light and (b) the comparison of kobs for BPA photodegradation in the presence of different TiO2 -based materials.

kobs for BPA degradation were 0.03 ± 0.002 and 0.005 ± 0.001 min−1 by pure and Cu-deposited P25 TiO2 , respectively. This means that the kobs for BPA photodegradation by Cu–TiO2 nanorod composites at 7 wt% Cu are 3.8 and 21.8 times higher than those of P25 and Cu–P25 TiO2 , respectively, clearly showing the superior photoactivity of Cu–TiO2 nanorod composites toward BPA photodegradation. It is noteworthy that addition of Cu ions decreases the photodegradation efficiency of BPA by Degussa P25 TiO2 . A previous study showed that addition of 1 wt% Cu(II) to P25 TiO2 lowered the kobs for BPA photodegradation by TiO2 /titanate nanotube composites [30], which is in good agreement with the result obtained in this study. The P25 TiO2 contains 80% anatase and 20% rutile, which may undergo the interparticle electron transfer to enhance the photocatalytic activity of TiO2 . Therefore, the formation of Cu0 –Cux O may block the photo-reactive sites of P25 TiO2 , and results in the decrease in the photodegradation efficiency and rate of BPA. In contrast, TiO2 nanorods contain 100% anatase TiO2 and the formation of Cu0 –Cux O nanoparticles would reduce the electron–hole recombination rate, and subsequently accelerate the photodegradation efficiency and rate of BPA by Cu–TiO2 nanorod composites. Fig. S2 (see supplementary data) shows the HPLC chromatograms for photodegradation of BPA by 7 wt% Cu–TiO2 nanorods under the irradiation of 365 nm UV light. It is clear that some intermediates were produced after the photodegradation. To further confirm the photodegradation of BPA by Cu–TiO2 , HPLC/MS/ MS was used to investigate the photodegradation intermediates of BPA. As shown in Fig. S3 and Table S2 (see supplementary data), several compounds including 4-hydroxybenzaldehyde (m/z

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122), 4-isopropenylphenol (m/z 134), 4-hydroxyacetophenone (m/z 136), 1,1-bis(4-hydroxyphenyl)ethane (m/z 214), bis(4-hydroxyphenyl)methanol (m/z 216), and 3-hydroxy-2,2bis(4-hydroxyphenyl)propane (m/z 244) were identified as the photodegradation intermediates. Several studies [38,39] have used HPLC/MS to identify the photodegradation byproducts of BPA in UV/TiO2 system, and found that the photocatalytic degradation of BPA was initiated by OH radical attacks at the electron-rich C3 in the phenyl group of BPA. In this study, BPA was attached by the photogenerated hydroxyl radicals (• OH) to produce 1,1bis(4-hydroxyphenyl)ethane and bis(4-hydroxyphenyl)methanol, and then followed by the cleavage of phenyl groups to form 4isopropenylphenol and 4-hydroxyacetophenone, which is in good agreement with the reported data [38,40,41]. In addition, another three new intermediates were obtained after the photocatalytic degradation of BPA by Cu–TiO2 nanorods under the irradiation of UV light. The formation of 4-hydroxybenzaldehyde was generated from the demethylation of 4-hydroxyacetophenone, while the hydroxylation of bisphenol A could produce 3-hydroxy-2,2-bis(4hydroxyphenyl)propane, clearly indicating the occurrence of reaction pathways other than radical reactions. 3.3. Photodegradation of BPA by Cu–TiO2 under visible light irradiation The visible-light-responsive property of Cu–TiO2 nanorod composites was further examined. Fig. 7a shows the photodegradation of BPA by various TiO2 -based nanomaterials under the irradiation

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of visible light at 460 nm. A removal efficiency of 77% was obtained when BPA was irradiated with visible light in the presence of Degussa P25 TiO2 . A nearly complete photodegradation of BPA by as-synthesized TiO2 nanorods was observed, showing the excellent visible-light-responsive characteristics of TiO2 nanorods. After the addition of 7 wt% Cu ions and then converted to Cu0 –Cux O species onto the surface of TiO2 nanorods, the photodegradation efficiency and rate of BPA was significantly enhanced and a nearly complete photodegradation of BPA by Cu–TiO2 nanorods was observed within 60 min. Fig. 7b compares the kobs for BPA photodegradation by TiO2 based nanomaterials under visible light irradiation. The kobs for BPA degradation were 0.009 ± 0.002, 0.011 ± 0.002, 0.019 ± 0.001, and 0.054 ± 0.003 min−1 for Degussa P25 TiO2 , Cu–P25 TiO2 , assynthesized TiO2 nanorods, and Cu–TiO2 nanorod composites, respectively. Several studies have indicated that both Cu2 O and CuO can absorb visible light to enhance the photocatalytic activity of TiO2 . Irie et al. [42] found that the photodegradation of 2-propanol to CO2 by 0.1 wt% Cu(II)/TiO2 was 2.1 times higher than that of Ndoped TiO2 under visible light conditions (>400 nm). In this study, the kobs for BPA photodegradation by as-synthesized TiO2 nanorods and Cu–TiO2 nanorod composites increases by factors of 2.0 and 5.8, respectively, when compared with Degussa P25 TiO2 , clearly indicating that formation of Cu0 –Cux O composites has a significant effect on the enhanced photocatalytic activity of TiO2 nanorods toward BPA photodegradation both in the presence of UV and visible lights. 3.4. Photodegradation of BPA by Cu–TiO2 in the presence of humic acid

Remaining ratio (C/Co)

(a)

1.0 0.8

Blank control P25 TiO2

0.6

Cu-P25 TiO2 TiO2 nanorod Cu-TiO2 nanorod

0.4 0.2 0.0 0

30

60

90

120

150

180

To understand the photocatalytic performance of Cu–TiO2 under environmentally relevant conditions, the photodegradation of BPA by Cu–TiO2 nanorods in the presence of humic acid was further examined. Fig. 8 shows the photodegradation of BPA by 7 wt% Cu–TiO2 nanorods in the presence of 1–25 mg/L humic acid under the irradiation of 365 nm UV light. A nearly complete photodegradation of BPA was observed when the humic acid concentration was lower than 5 mg/L. However, only 93% and 69% of BPA were photodegraded by Cu–TiO2 nanorods when the humic acid concentration increased to 10 and 25 mg/L, respectively. The kobs for BPA photodegradation decreased from 0.113 min−1 at 1 mg/L humic acid to 0.0097 min−1 at 25 mg/L humic acid, clearly showing that addition of humic acid decreased the photodegradation efficiency

Illumination time (min)

Remaining ratio (C/Co)

-1

kobs (min )

0 mg/L 1 mg/L 5 mg/L 10 mg/L 25 mg/L

1.0

(b) 0.06

0.04

0.02

0.8 0.6 0.4 0.2 0.0

0.00

P25

Cu-P25

TiO2 nanorod Cu-TiO2 nanorod

Fig. 7. (a) Photodegradation of 10 mg/L BPA by various TiO2 -based nanomaterials under the irradiation of 460 ± 40 nm visible light and (b) the kobs for BPA photodegradation by different TiO2 materials in the presence of visible light.

0

20

40

60

80

100

120

Illumination time (min) Fig. 8. The photodegradation of 10 mg/L BPA in the presence of 1–25 mg/L humic acid by Cu–TiO2 composites under the irradiation of 365 nm UV light.

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and rate of BPA by Cu–TiO2 nanorods. Humic acid is a naturally occurring organic compound and may compete the active sites on the nanorod surfaces with BPA, resulting in the decrease in photodegradation efficiency and rate of BPA under the irradiation of 365 m UV light. The photodegradation intermediates of BPA in the presence of 10–25 mg/L humic acid were also examined. As shown in Fig. S4 (see supplementary data), several peaks were observed in the HPLC chromatograms and the retention times of peaks were similar to those in the absence of humic acid. After HPLC/MS/MS analysis, 1,1-bis(4-hydroxyphenyl)ethane (m/z 214), bis(4-hydroxyphenyl)methanol (m/z 216), and 3-hydroxy-2,2bis(4-hydroxyphenyl)propane (m/z 244) were identified (Fig. S5, see supplementary data), indicating that the photodegradation of BPA by Cu–TiO2 nanorods in the presence of humic acid may undergo the similar reaction pathways. It is noteworthy that some new peaks appeared in the chromatograms and the distribution patterns of hydrophilic organic compounds at 10–15 min were different, suggesting that other reaction pathways may occur to produce new intermediates in the presence of humic acid. 4. Conclusions In this study, we have synthesized TiO2 nanorods by microwaveassisted sol–gel method and Cu–TiO2 nanorods composites by chemical reduction method for effective photodegradation of BPA in the presence of UV and visible lights. TEM images showed that the diameter of TiO2 nanorods was 2 nm with aspect ratios of 5–10.5. The XRD and XPS results indicated that Cu species were mainly Cu2 O and Cu0 . The Cu–TiO2 nanorods exhibited an enhanced effect on the photocatalytic degradation of BPA under the irradiation of UV and visible lights. The kobs for BPA photodegradation by Cu–TiO2 nanorods at 7 wt% Cu ions were 21.8 and 3.8 times higher than those of as-synthesized TiO2 nanorods and Degussa P25 TiO2 , respectively, under the UV light irradiation. In addition, the kobs for BPA photodegradation by as-synthesized TiO2 nanorods and Cu–TiO2 nanorod composites increased by factors of 2.0 and 5.8, respectively, when compared with Degussa P25 TiO2 in the presence of 460 ± 40 nm visible light. Results obtained in this study clearly demonstrate that Cu–TiO2 nanorod composites are an effective photocatalyst for degradation of BPA under both UV and visible light conditions and open an avenue for a potential application in photocatalytic degradation of pharmaceutical and personal care products in water and wastewater treatment plants. Acknowledgements The authors thank the National Science Council, Taiwan for financial support under Contract No. NSC 101-2221-E-007-084MY3 and NSC 99-2627-M-007-006. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat. 2014.01.047. References [1] S.K. Khanal, B. Xie, M.L. Thompson, S.W. Sung, S.K. Ong, J. Van Leeuwen, Fate, transport, and biodegradation of natural estrogens in the environment and engineered systems, Environ. Sci. Technol. 40 (2006) 6537–6546. [2] C. Ort, M.G. Lawrence, J. Rieckermann, A. Joss, Sampling for pharmaceuticals and personal care products (PPCPs) and illicit drugs in wastewater systems: Are your conclusions valid? A critical review, Environ. Sci. Technol. 44 (2010) 6024–6035. [3] W.T. Tsai, Human health risk on environmental exposure to bisphenol-A: A review, J. Environ. Sci. Health C 24 (2006) 225–255.

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