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TiO2 composite for enhancing visible light photocatalytic performance
Applied Surface Science 498 (2019) 143855
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Au nanoparticle modified three-dimensional network PVA/RGO/TiO2 composite for enhancing visible light photocatalytic performance
T
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Liang Zhanga,b,c, , Haojie Qib, Yuan Zhaod, Lvling Zhonga, Yujuan Zhanga, Yao Wanga, Juanqin Xuea, Yue Lie a
School of Chemistry and Chemical Engineering, Xi'an University of Architecture and Technology, Xi'an, Shaanxi, 710055, China School of Environmental and Municipal Engineering, Xi'an University of Architecture and Technology, Xi'an, Shaanxi, 710055, China c Shaanxi Provincial Key Laboratory of Gold and Resource, Xi'an University of Architecture and Technology, Xi'an, Shaanxi, 710055, China d Reactor Engineering and Safety Research Center, China Nuclear Power Technology Research Institute Co., Ltd, Shenzhen, 518031, China e College of Materials Science and Engineering, Xi'an University of Architecture and Technology, Xi'an, Shaanxi, 710055, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Photocatalyst TiO2 Au nanoparticle Visible light Rhodamine B
To improve photocatalytic performance, a composite of gold nanoparticle-modified titanium dioxide/reduced graphene oxide/polyvinyl alcohol (PVA/RGO/TiO2/Au) was successfully synthesized via a mild, low-temperature wet chemical method and characterized by FTIR, XRD, SEM, TEM, XPS, UV–vis DRS, PL, BET, and BJH. The PVA/RGO/TiO2/Au composite had a uniform, hierarchically network structure with many pores, which showed great absorbance in the visible light range and effectively inhibited the recombination of photogenerated electron-hole pairs. The synthesized composite showed a high photocatalytic degradation activity for rhodamine B (RhB) under high pressure sodium lamp irradiation. The photocatalytic degradation activity was closely related to the catalyst concentration, the initial RhB concentration, the pH, and the H2O2 concentration. The composite remained at nearly 94% of the initial photocatalytic degradation activity after five cycles.
1. Introduction Acceleration of global industrialization has highlighted the dye wastewater discharged from textile and dye factories. The complex structure of the organic pollutants in dye wastewater makes their degradation extremely difficult. Organic pollution has serious implications for aquatic organisms and human beings [1]. Efforts have been made to develop more effective treatment processes [2]. Photocatalysts, semiconductors capable of effective catalytic degradation of a wide range of organic substances under the excitation of photons, have been extensively studied, due to their low cost, non-toxicity, high efficiency and stability, and no secondary environmental pollution. Many semiconductor materials can be used as photocatalysts, including titanium dioxide (TiO2) [3–9] and other oxide or sulfide semiconductors [10–14]. Titanium dioxide is the most popular nano-photocatalyst material due to its strong oxidizing ability, chemical stability, and nontoxicity. Drawbacks of titanium dioxide include the high recombination rate of photogenerated electron-hole pairs, the low utilization of solar energy, and poor recyclability for reuse [15]. Research addressing photon quantum efficiency and solar energy utilization include combining non-metal substances [16–21] or doping
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metals [22–24]. Within the combined non-metal materials, carbonaceous materials and their derivatives possess excellent absorptivity, excellent electron conduction, and large surface area, which help to degrade organic pollutants in the photocatalytic reaction [21]. An example is graphene, which exhibits excellent electrical and optical properties, such as high electron mobility and high absorption properties in the visible range [16–20]. Graphene-modified titanium dioxide adsorbs visible light and enlarges the specific surface area. Wang et al. [18] synthesized a core-structured TiO2 NCs/RGO composite that exhibits high optical absorption in a wide spectral scope. Zhao et al. [19] synthesized three-dimensional graphene/TiO2 nanotube nanocomposite, effectively inhibiting charge recombination of TiO2. Yang et al. [20] demonstrated graphene can be photoexcited under visible light irradiation. Modifying titanium dioxide with graphene can improve the photocatalytic activity. However, the prepared materials are nanoparticles in wastewater and are prone to agglomeration, which decreases their specific surface area due to van der Waals forces. The nanocatalysts are difficult to recover and purify from the reaction mixture, resulting in great challenges to a wide practical application, due to high cost and limited resources [23]. The modified semiconductor composite as a
Corresponding author at: School of Chemistry and Chemical Engineering, Xi'an University of Architecture and Technology, Xi'an, Shaanxi 710055, China. E-mail address: [email protected] (L. Zhang).
https://doi.org/10.1016/j.apsusc.2019.143855 Received 20 May 2019; Received in revised form 24 August 2019; Accepted 1 September 2019 Available online 02 September 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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cooled to room temperature, the resulting precipitate was washed and dried via the same approach. The prepared composite particle was PVA/RGO/TiO2. The PVA/TiO2 composite was prepared by the same solvothermal method without RGO.
carrier requires further study [25]. Polymers have become popular carriers in recycling applications of photocatalysts, due to their mechanical properties, biocompatibility, and nontoxicity [26–29]. Polyvinyl alcohol (PVA) is non-toxic, biocompatible, and its mechanical properties make it a popular suitable polymeric matrix to support the catalyst. However, PVA has a high water solubility in the treatment of wastewater such as high water solubility. Glutaraldehyde (GA) was therefore crosslinked with PVA to form an acetal compound, while graphene further improved the water resistance and mechanical strength of the PVA polymeric material [28]. The graphene/TiO2 composite combined with PVA was successfully recycled by Jung et al. [29]. The photocatalytic activity of catalyst after combining with the carrier was reduced, because the contact between the carrier and the catalyst reduces the surface area of the catalyst, which decreases the numbers of active sites. Doping noble metal nanoparticles (NPs) can circumvent this, due to their electrical conductivity properties. Gold NPs are widely applied to the surface of the photocatalyst [22–24]. In this study, we successfully synthesized the TiO2/PVA/RGO/Au with a hierarchical network structure via a wet chemical method. The synthesized composite was applied to the photodegradation of rhodamine (RhB). The degradation process indicated that the catalyst improved the light response range and photodegradation efficiency, but also reduced the agglomeration effect and was more convenient for recycling. This composite is an excellent prospect for extensive applications in wastewater treatment.
2.3. Synthesis of PVA/RGO/TiO2/Au composite 0.1 g of the synthesized PVA /RGO/TiO2 composite and 5.8 ml of chloroauric acid (4.36 × 10−3 mol/l) were added to the 100 ml beaker with 50 ml of deionized water. After stirring for 2 h, 5.8 ml of sodium borohydride (1.09 × 10−2 mol/l) was added. The solution was continually stirred for an additional 24 h. The sample was filtered and washed with deionized water, then dried at 80 °C for 12 h.
2.4. Characterization of composite The chemical bond and the functional group were tested with Fourier transform infrared spectroscopy (FTIR, FEI Nicolet iS 50ATR, US). The crystal phase structure was obtained on a power X-ray diffraction (XRD, Bruker D8, Germany) using Cu kα radiation. The surface morphologies and nanoparticle size were observed on a scanning electron microscopy (SEM, FEI verous 460, US) and transmission electron microscopy (TEM, FEI Tecnai G2 F20, US). The chemical composition and state were determined via X-ray photoelectron spectroscopy (XPS, ThermoFisher ESCALAB 250Xi, USK) with Al kα radiation (hv = 1486.6 eV). The UV–vis diffuse reflectance spectroscopy (DRS, UV2600, Shimadzu, Japan) was used to determine the optical properties of the samples. Photoluminescence (PL) spectrum obtained under excitation at 397 nm with a Gangdong F-320 spectrophotometer fitted with a 200 W xenon lamp as the excitation source was used to determine the recombination rate of photogenerated electron-hole pairs. The Brunauer–Emmett–Teller (BET) surface areas, pore volume, and the pore size distributions of the samples were obtained on an automatic specific surface and porosity analyzer (Micromeritics, ASAP 2460, USA). After the pretreatment at 130 °C for 6 h, the absorption and desorption of nitrogen (N2) was carried out successively. The BET surface area was calculated by the multipoint BET method with nitrogen adsorption data. The total pore volume was calculated with single point method using the adsorption data at the relative pressure (p/p0) of 0.99. The pore size distribution and the average pore size were calculated by the Barrett-Joyner-Halenda (BJH) method with nitrogen desorption data.
2. Experimental section 2.1. Materials Polyvinyl alcohol-124 (PVA-124) was purchased from Guangdong Guanghua Sci-Tech Co., Ltd. Tetrabutyl titanate was purchased from Shanghai Sanpu Chemical Co., Ltd. Glutaraldehyde (GA), rhodamine B (RhB), sodium hydroxide and hydrogen peroxide (30%) were purchased from Tianjin Kermel Chemical Reagent Co., Ltd. Crystalline flake graphite was purchased from Qingdao Tengshengda Carbon Machinery Co., Ltd. Sodium nitrate, sulfuric acid (98%) and hydrochloric acid (HCl) were purchased from Xi'an Chemical Reagent Factory. Potassium permanganate was purchased from Henan Jiaozuo Three Chemical Plant. Absolute ethanol (99.5%) was purchased from Tianjin Tianli Chemical Reagent Co., Ltd. These reagents were used as received from the manufacturer. Deionized (DI) water was used throughout the experiments. 2.2. Synthesis of PVA/RGO/TiO2 and PVA/TiO2 composites
2.5. Photodegradation of RhB solution PVA/RGO/TiO2 composite was prepared as follows: Graphene oxide (GO) was synthesized via an improved Hummers method [30]. The prepared GO was reduced via hydrazine hydrate [31] and the reduced GO (RGO) composite was made. 0.0385 g of RGO was completely dissolved in 30 ml of deionized water for 2 h via ultrasound. 2.4 g of polyvinyl alcohol-124 (PVA-124) was transferred to the graphene solution and treated at 90 °C. When the PVA was completely dissolved, the HCl was slowly added to adjust the pH to 4. The mixture was transferred to the Hydrothermal reactor, and 10 ml of GA (diluted to 25%) was slowly added into the solution under continuous stirring. The systemic temperature was 90 °C. After 3 h, the system was slowly cooled to room temperature. The precipitate was filtered, washed with ethanol, and washed deionized water three times, then dried in vacuum at 80 °C for 12 h. A gray PVA/RGO composite was obtained. To coat TiO2 on the PVA/RGO composite, 0.25 ml of tetrabutyl titanate was dissolved in 20 ml of ethanol, and 0.25 ml of DI water was dropped into the solution under continuous stirring. When the sol was generated, the mixture and 0.5 g of the prepared PVA were added into the Hydrothermal reactor and heated for 18 h at 190 °C. After the system slowly
Photodegradation experiments were performed in batch systems under visible light irradiation. A high-pressure sodium lamp (150 W, Philips) employed as the visible light source provided a power density of approximately 278 W/m2. To avoid lamp heating, the high-pressure sodium lamp (HPSL) was jacketed by a water-cooled condenser [32]. Photocatalyst was added to the RhB standard solution. To ensure the adsorption-desorption equilibrium between organic molecules and the photocatalyst, the suspensions were stirred for 30 min continuously in the dark. After centrifugal separation, the concentration of the solution was measured at 552 nm with a 722 G UV–visible spectrophotometer (China). Then the solution was irradiated under visible light. During the photodegradation process, the concentration of the solution was measured at definite time intervals. The residual concentration Ct of the RhB in the solution was calculated by using standard curve method, and the degradation efficiency of the photodegradation of RhB in the solution was calculated as follows: D % = [(C0 − Ct)/C0] × 100% [26], where C0 and Ct are the initial and time t concentration of RhB solution.
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Fig. 1. Infrared spectroscopy of (a) PVA/RGO, (b) PVA/RGO/TiO2, (c) PVA/ RGO/TiO2/Au, (d) PVA/RGO, and (e) PVA/RGO/TiO2 in 550–450 cm−1.
Fig. 2. XRD patterns of (a) PVA/RGO/TiO2, and (b) PVA/RGO/TiO2/Au.
are shown in Fig. 3d with a diameter about 100 nm. The network structure of PVA/RGO/TiO2 appeared cluster of TiO2 NPs in Fig. 3b. The TiO2 NPs aggregated on the PVA/RGO with a size < 50 nm, see Fig. 3e, h. There was little change on the surface morphology of the PVA/RGO/TiO2 with the Au doped, as shown in Fig. 3c. The TEM image of the PVA/RGO/TiO2/Au (Fig. 3h) revealed the porous structure with a range between 10–100 nm. The surface of network structure was relatively smooth in the high-magnification SEM image (Fig. 3f), but the Au NP was not clearly distinguished. The magnified TEM image in Fig. 3i shows the single atomic Au sites highlighted by red circles, which were decided by the lattice distance. The Au and Ti lattices are clearly presented in the high-resolution TEM (HRTEM) image in Fig. 3k. The lattice spacing was about 0.23 nm in Fig. 3j, which indicated the (111) facets of the Au nanocrystal. The lattice spacing of the magnified TiO2 nanocrystal at about 0.35 nm corresponded to the (101) facets, see Fig. 3l [18,22,37]. The results agreed with the XRD analysis. The energy-dispersive X-ray (EDX) spectrum also demonstrated that the Au NPs were distributed on the PVA/RGO/TiO2, see Fig. 3g.
3. Results and discussion 3.1. Characterization analysis 3.1.1. Chemical bonds obtained by infrared spectroscopy analysis To confirm that PVA/RGO/TiO2/Au was successfully prepared, infrared spectra of PVA/RGO, PVA/RGO/TiO2 and PVA/RGO/TiO2/Au composites were obtained (Fig. 1a–c). The characteristic absorption peaks of PVA/RGO were observed (Fig. 1a). The strong and wide absorption peak between 3700 and 3000 cm−1 was attributed to the –OH group stretching vibration. The peaks at 2930 cm−1 and 2864 cm−1 corresponded to the –CH2 group anti-symmetric and symmetric stretching vibration. The peaks at 1560 cm−1 and around 816 cm−1 were attributed to the plane bending vibration of CeH. The peak at 1643 cm−1 was attributed to the C]O of the alkyl aldehyde unit, and the peak at 1124 cm−1 was attributed to the O-C-O stretching vibration, which demonstrated the acetal reaction between PVA and GA [26,33,34]. The characteristic absorption peaks of PVA/RGO are seen in Fig. 1b. The peaks within 550–450 cm−1 corresponded to the Ti-O-Ti stretching vibration, which was shown in the comparison between Fig. 1d and e [35,36]. The characteristic absorption peaks of PVA/RGO/TiO2/Au were similar to the PVA/RGO/TiO2 in Fig. 1b, because Au atoms slightly affected the material structure. These characteristic peaks proved a successful combination between TiO2 and PVA/RGO in the composites.
3.1.4. Chemical composition and state obtained by XPS analysis The chemical composition and the state of PVA/RGO/TiO2/Au were studied by X-ray photoelectron spectroscopy (XPS), as shown in Fig. 4. The O 1s, Ti 2p, C 1s, and Au 4f peaks are shown in the XPS survey spectrum in Fig. 4a. The C 1s spectrum was displayed with three binding energy peaks presented in Fig. 4b. The three peaks at 284.47 eV, 286.37 eV, and 288.77 eV were attributed to CeC, CeO, and C]O bonds [19,26]. The O 1s spectrum was shown in Fig. 4c. The two peaks at 529.63 eV and 530.70 eV were attributed to O-Ti3+ and OTi4+ species. The last peak at 532.33 eV was attributed to OeC bonds [19]. The Ti 2p spectrum in Fig. 4d presented two typical peaks, indicating Ti 2p3/2 and Ti 2p1/2 were located at the binding energy of 458.48 eV and 464.16 eV [18,19,32]. The Au 4f spectra in Fig. 4e showed the peaks, indicating Au 4f7/2 and Au 4f5/2 were located at the binding energy of 83.36 eV and 87.26 eV from the metallic state Au [22].
3.1.2. Crystal phase structures determined by XRD analysis The crystal phase structures of the PVA/RGO/TiO2 and the PVA/ RGO/TiO2/Au were further characterized by XRD. The peak at 2θ values of 19.14° corresponded to the (101) faces of the PVA [28]. The main peaks at 2θ values of 25.14°, 37.77°, 48.01°, 53.77°, 54.95°, and 74.88° in Fig. 2a corresponded to the (101), (004), (200), (105), (211), and (204) faces of anatase TiO2 [26], which matched well with the JCPDS powder diffraction pattern 21-1272. The (111), (200), (220), and (311) faces presented in Fig. 2b were ascribed to the diffraction peak of Au [23], confirming the coexistence of metal Au.
3.1.5. UV–vis DRS and PL spectra analysis Optical absorption properties of PVA, PVA/RGO, PVA/RGO/TiO2, and PVA/RGO/TiO2/Au were revealed by the UV–vis diffuse reflectance spectra (UV–vis DRS) (Fig. 5a). Pure PVA had almost no absorbance in the visible wavelength range. The PVA modified with RGO had a conspicuous visible-light absorbance, which helped to improve the TiO2 optical properties. The combined composite of the PVA/RGO
3.1.3. Morphological observation by SEM and TEM The morphologies of PVA/RGO, PVA/RGO/TiO2, and PVA/RGO/ TiO2/Au were observed by field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) (Fig. 3). The panoramic image of PVA/RGO in Fig. 3a shows a uniform, hierarchically network structure with many pores. Some macropores 3
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Fig. 3. SEM image of (a, d) PVA/RGO, (b, e) PVA/RGO/TiO2, and (c, f) PVA/RGO/TiO2/Au. (g) EDX spectrum of PVA/RGO/TiO2/Au. TEM image of PVA/RGO/ TiO2/Au: (h) TEM image. (i) TEM image of the single atomic Au sites highlighted by red circles. (k) HRTEM image. (j) HRTEM image showing clear lattice fringes of Au. (l) HRTEM image showing clear lattice fringes of anatase TiO2. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
the photogenerated carriers separation efficiency. The lowest PL intensity for PVA/RGO/TiO2/Au indicated the lower recombination rate, which showed that the Au NPs further improved the separation efficiency of the photogenerated carriers. This was because both the gold nanoparticles and the graphene have excellent electrical conductivity for the transfer of photoexcited electrons, which meant that they acted as electron acceptors [20,22].
and the TiO2 showed a great absorbance in the visible light range (Fig. 5A, b, c). The PVA/RGO/TiO2/Au showed the broad range of the visible light response, which was beneficial to enhance utilization of light for the photocatalytic reaction [26]. The recombination rate of photogenerated electron-hole pairs was investigated; see Fig. 5B for the photoluminescence (PL) spectra of PVA/TiO2, PVA/RGO/TiO2, and PVA/RGO/TiO2/Au. The PL intensity of the PVA/RGO/TiO2 was lower than the PVA/TiO2, which implied a low recombination rate [26,38]. This proved that the RGO improved
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Fig. 4. XPS of the PVA/RGO/TiO2/Au (a) survey spectrum, (b) C 1s, (c) O 1s, (d) Ti 2p, and (e) Au 4f.
to hundreds of nanometers; the number of pores in PVA/RGO/TiO2, and PVA/RGO/TiO2/Au was higher than that of PVA/TiO2. The BET surface area, pore volume, and pore size values of the prepared samples are presented in Table 1. The BET surface area and pore volume of PVA/RGO/TiO2 were 14.936 m2/g and 0.02788 cm3/g, respectively, which were higher than that of PVA/TiO2 (2.250 m2/g and 0.00624 cm3/g). The surface area and pore volume increased, which was attributed to the adhesion and aggregation of TiO2 NPs on the
3.1.6. Brunauer–Emmett–Teller (BET) surface area and pore size distribution The N2 absorption-desorption type IV isotherm (Fig. 6a) and the pore size distribution (Fig. 6b) of PVA/TiO2, PVA/RGO/TiO2, and PVA/ RGO/TiO2/Au displayed distinct hysteresis loops close to H3 type at a relative pressure range of 0.45–1.0, which were a typical characteristic of mesopores (2–50 nm) and macropores (> 50 nm) features [26]. The pore size distribution showed the pore diameters ranging from dozens 5
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Fig. 5. (A) The UV–vis DRS of PVA, PVA/RGO/TiO2, PVA/RGO/TiO2/Au, and PVA/RGO. (B) PL spectra of PVA/TiO2, PVA/RGO/TiO2, and PVA/RGO/TiO2/Au.
surface of PVA/RGO, resulting in the formation of some new mesopores, macropores, and micro-protrude (observed in SEM images). The surface area and pore volume of PVA/RGO/TiO2 modified with Au NPs (10.536 m2/g and 0.02027 cm3/g) were slightly lower than that of PVA/RGO/TiO2, which indicated the Au NPs are distributed in it's pores, causing some pores to disappear, as seen in the inset of Fig. 6. The high specific surface area can provide more surface active sites, and the mesopores and macropores can facilitate the injection of dye and photons. These help to improve photocatalytic performance [48,49].
Table 1 BET surface area, pore volume, and pore size. Samples
BET surface area (m2/g)
Pore volume (cm3/ g)
Pore size (nm)
PVA/RGO/TiO2/Au PVA/RGO/TiO2 PVA/RGO
10.536 14.936 2.250
0.02027 0.02788 0.00624
72.971 62.246 108.893
prepared PVA/RGO/TiO2/Au showed considerable photocatalytic efficiency, which resulted from the Au NPs improved separation of electron-hole pairs via their high conductivity [22–24]. As the same time, the photodegradation kinetics of RhB was also investigated using the Langmuir-Hinshelwood pseudo-first-order reaction: ln (C0/Ct ) = kapp × t, where C0 and Ct are the adsorption-desorption equilibrium and irradiation time t concentration of RhB solution, kapp is an apparent rate constant (Fig. 7b) [18,26]. The corresponding initial reaction rates of RhB degradation were obtained using the first equation: r = kapp × C0, where r is the initial reaction rate of the dye [44]. The initial reaction rates of No catalyst, PVA/RGO, PVA/RGO/TiO2, and PVA/RGO/TiO2/Au were 0.00841, 0.01775, 0.04959, and 0.08178 mg/ l·min−1, respectively. The initial reaction rate of RhB degradation with PVA/RGO/TiO2/Au was 9.7-fold higher than that with PVA/RGO.
3.2. Photocatalytic degradation experiments 3.2.1. Photocatalytic degradation performance The degradation performance of PVA/RGO/TiO2/Au was confirmed by comparing the degradation properties on the RhB in the presence of PVA/RGO, PVA/RGO/TiO2, and PVA/RGO/TiO2/Au under HPSL irradiation. The experimental results are shown in Fig. 7. The PVA/RGO/TiO2/Au composite was the most efficient for degradation of the RhB. The PVA/RGO, compared with the solid blank, showed a slight degradation performance. The PVA/RGO/TiO2 composite showed some improvement on the photodegradation of RhB. The results revealed that the TiO2-containing composite with PVA/RGO could adsorb visible light to generate the electron-hole pairs and decompose the organic dye [29,50]. When the Au NPs was deposited, the
Fig. 6. (a) N2 absorption-desorption isotherm, (b) pore size distribution curves of the prepared samples. 6
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Fig. 7. (a) Comparison of degradation performance of the different catalysts on the photodegradation of RhB under HPSL irradiation. (b) The photodegradation kinetics of RhB with different photocatalysts (Experimental conditions: 50 ml of 10 mg/l RHB, pH 7, and 0.05 g catalyst.)
solution showed an increasing trend, as the catalyst concentration (CPVA/RGO/TiO2/Au) increased. As the same time, the corresponding photodegradation kinetics of RhB over PVA/RGO/TiO2/Au with different concentrations fit the pseudo-first-order model (Fig. 8b). The initial reaction rates (Table 2) were in the following order: 0.03878 mg/
3.2.2. Effect of catalyst and initial RhB concentration A conspicuous difference of the degradation performance was found in the RhB with the various amounts of catalysts. The degradation performance of the RhB over PVA/RGO/TiO2/Au with different concentrations is shown in Fig. 8a. The removal efficiency of the RhB in the
Fig. 8. (a) and (b) The degradation performance of the RhB over PVA/RGO/TiO2/Au with different concentrations, and the corresponding photodegradation kinetics (Experimental conditions: 50 ml of 10 mg/l RHB, pH 7.); (c) and (d) The degradation performance of the RhB with different concentrations over PVA/RGO/TiO2/Au, and the corresponding photodegradation kinetics. (Experimental conditions: pH 7, 0.05 g catalyst.) 7
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competition for the adsorption site [40]. The degradation capability basically exhibited similar behavior as the adsorption quantity. At pH 5, the perfect catalytic effect occurred because the acidic environment could support the mass generation of the reactive intermediates. Moreover, the corresponding photodegradation kinetics of RhB roughly followed pseudo-first-order reaction (Fig. 9b), and the initial reaction rates for photodegradation of RhB with PVA/RGO/TiO2/Au at different pH values were shown in the Table 3. The PVA/RGO/TiO2/Au exhibited excellent photocatalytic activity at pH 7, with the initial reaction rate being 0.07730 mg/l·min−1. Hydrogen peroxide (H2O2) that was added to the initial solution promoted the catalytic process. In order to evaluate the effect that the performance of the catalyst had under HPSL irradiation [43], the test was performed in RhB solution with added H2O2. The photodegradation performance of the RhB at different H2O2 concentrations (CH2O2) (Fig. 9c), and the corresponding photodegradation kinetics of RhB (Fig. 9d) were shown. The degradation efficiency of the RhB without H2O2 was 78% after 180 min of irradiation, but that with 3 mM H2O2 reached 76% after 30 min of irradiation, and 92% after 60 min of irradiation. The degradation efficiency of the RhB with 6 mM H2O2 hardly improved. The H2O2 added to the initial solution trapped many photoelectrons, resulting in effective inhibition of the recombination process of the photogenerated electron-hole pairs. The ionization of H2O2 generated hydroxyl radicals (%OH). The excess H2O2 would scavenge hydroxyl radicals and generate relatively weaker peroxyhydroxyl radicals (HO2%) or further produce oxygen and water [43,44]. Moreover, the corresponding photodegradation kinetics of RhB with H2O2 for PVA/RGO/TiO2/Au was roughly in agreement with the pseudo-firstorder reaction, and the initial reaction rates of RhB with different H2O2 contents degradation were shown in the Table 3. The initial reaction rate at 3 mM H2O2 was 4-fold higher than that without H2O2, which meant the appropriate amount of H2O2 was beneficial for photocatalytic degradation of the dyes.
Table 2 Apparent constants kapp, initial reaction concentration C0, and initial reaction rate r over different catalyst and initial RhB concentration. CPVA/ RGO/
kapp (min−1)
C0 (mg/l)
r (mg/ l·min−1)
CRhB (mg/l)
kapp (min−1)
C0 (mg/l)
r (mg/ l·min−1)
0.00392 0.00507 0.00804 0.01541 0.01987
9.894 9.435 9.052 8.055 7.477
0.03878 0.04783 0.07278 0.12413 0.14856
10 20 50 70 100
0.01624 0.00882 0.00694 0.00431 0.00387
8.855 18.270 46.245 66.157 96.940
0.14381 0.16114 0.32094 0.28514 0.37516
TiO2/Au
(g/l) 0.5 1.0 1.2 1.6 2.0
l·min−1 (0.5 g/l) < 0.04783 mg/l·min−1 (1.0 g/l) < 0.07278 mg/ l·min−1 (1.2 g/l) < 0.12413 mg/l·min−1 (1.6 g/l) < 0.14856 mg/ l·min−1 (2.0 g/l). The photodegradation date increased rapidly, when the quantity of the catalyst ranged from 0.5 to 1.6 g/l. The photodegradation date beyond 1.6 g/l increased slowly. The increase of the catalyst concentrations could promote the degradation rate of the organic substance by providing additional active sides for the generation of oxidizing species and the greater photoactive surface area. When excess catalyst concentration is present, the catalytic efficiency is slightly relevant to it, as seen in the current study. The higher concentration of catalysts even caused agglomeration and subsequently reduced the available surface area, while blocking the incident light that further decreased the light transmittance through the solution [39]. Initial RhB concentration (CRhB) was used to assess its effect on photocatalytic performance, in the presence of PVA/RGO/TiO2/Au under HPSL irradiation. Fig. 8c shows the values of removal efficiency for the RhB that decreased, as the initial RhB concentration increased from 10 to 100 mg/l. Fig. 8d shows the typical first-order photodegradation feature over the different initial RhB concentration. The initial reaction rates (Table 2) showed the weak increased trend, compared to the rapid increase in the initial RhB concentration. The probability of formation of reactive species on the catalyst surface and reactive species accepted by RhB determine the degradation rate. As the initial dye concentration increases, more dyes molecules are available for excitation and energy transfer [42]. However, with an excessive concentration of dyes, the relative residual amount of dye molecules increases after the adsorption capacity of the catalyst reaches equilibrium. The photons reaching the catalyst surface are reduced, which resulted from the interception of excess organic substrate concentration. [26,36].
3.2.4. Reusability The reusability of the photocatalyst repeatedly tested the photocatalytic performance of the composite catalyst degrading rhodamine B under HPSL irradiation. The reusability testing was performed for five times in the same test conditions. After the degradation was finished, the catalyst was filtered and washed with ethanol and deionized water for three times for each trial. The catalyst was dried in a vacuum oven for 12 h at 80 °C and applied to the next run. The five runs were carried out with PVA/RGO/TiO2/Au under the HPSL irradiation for photocatalytic experiments (Fig. 10). The percentage degradation efficiency of the RhB decreased from 85% to 80% after five cycles, and the degradation capacity of PVA/ RGO/TiO2/Au composite did not decrease the removal of RhB. According to the reusability tests, the composite catalyst had a longterm usage for the photodegradation under the HPSL irradiation. The decrease was a result of some catalyst surface being covered by the thin layer of intermediate dye after the first run, so that the adsorption capacity of the composite was lower in following runs. The surface of the covered catalyst lost some active sites trapping photons after each run, so that the generated reactive intermediate for the reaction with the dye molecules under the HPSL irradiation was reduced [45]. The degradation capability decreased, as the number of runs increased.
3.2.3. Effect of pH and H2O2 concentration The pH value was a vital factor in the photocatalytic reaction, because it was closely related to the surface charge of the catalyst, the adsorption of organic substances on the catalyst surface [40], and the amount of charged free radicals produced during the photocatalysis process [26]. The RhB-solution degradation performance was tested with different pH values under HPSL irradiation. The experimental results (Fig. 9a) showed that the pH values affected both the adsorption capacity and degradation performance [41,42]. The percent adsorption capacity of the RhB on the catalyst surface increased from 3% to 36%, with a pH value from 3 to 9, and then decreased for pH values higher than 9. The optimum adsorption capacity was obtained at pH 9. The surface of the composite material was positively charged at lower pH, because of protonation of the active functional groups on the catalyst surface. This positive surface charge resulted in competition against the cationic RhB for adsorption sites. As the pH increased, the catalyst surface could be negatively charged, enhancing the electrostatic attraction between the catalyst and dyes. When the pH was higher than 9, the decreased adsorption efficiency could be attributed to the enhanced hydroxyl ionic strength of the solution, which resulted in more
3.2.5. Possible photocatalytic mechanism The possible mechanism is shown in Fig. 11, which illustrates the charge migration and the photocatalytic process under Vis-light irradiation. Both the Au nanoparticle and the RGO exhibited excellent electrical conductivity, and the RGO could be band gap-photoexcited under Vis-light irradiation [20,22,46]. The TiO2 alone cannot absorb Vis-light, due to the 3.2 eV band gap energy [22,26]. The RGO can absorb photons under the HPLS irradiation and then become excited to generate photoelectron-hole pairs. The hot electron transferred to the 8
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Fig. 9. (a) and (b) The degradation performance of the RhB over PVA/RGO/TiO2/Au at different pH values, and the corresponding photodegradation kinetics (Experimental conditions: 50 ml of 10 mg/l RHB, 0.05 g catalyst.); (c) and (d) The degradation performance of the RhB over PVA/RGO/TiO2/Au at different H2O2 concentrations, and the corresponding photodegradation kinetics. (Experimental conditions: 50 ml of 10 mg/l RHB, pH 7, and 0.05 g catalyst.) Table 3 Apparent constants kapp, initial reaction concentration C0, and initial reaction rate r at different pH and H2O2 concentration. pH value
kapp (min−1)
C0 (mg/l)
r (mg/ l·min−1)
CH2O2 (mM)
kapp (min−1)
C0 (mg/l)
r (mg/ l·min−1)
3 5 7 9 11
0.00390 0.01001 0.00889 0.01067 0.00536
9.568 9.278 8.695 6.345 9.703
0.03732 0.09287 0.07730 0.06770 0.05201
0 1 2 3 6
0.00928 0.01052 0.02005 0.03851 0.03973
9.435 9.362 9.244 9.144 9.052
0.08756 0.09845 0.18534 0.35214 0.35964
conduction band the TiO2 or the surface of the Au nanoparticles. In addition, the RhB molecules with photosensitization can also absorb Vis-light for excitation [49,50]. The hot electron from the excited RhB molecules (RhB*) absorbed to the surface of the catalyst transferred to the conduction band of TiO2 and RGO semiconductor and then to the RGO or the Au nanoparticles. The successfully transferred hot electrons were trapped by the oxygen in the surface of the TiO2, RGO, and the Au nanoparticles to generate superoxide anion radicals (%O2−). The powerful oxidizing free radicals %O2– in the reaction system were accepted by RhB absorbed to the surface of the catalyst in the reaction system, where the degradation of RhB was obtained. The photogenerated holes reacted with the RhB molecules to produce oxidation RhB [26,49,50].
Fig. 10. Five cycles of the photocatalyst reusability on the photodegradation of the RhB under the HPSL irradiation. (Experimental conditions: 50 ml of 10 mg/l RHB, pH 7, and 0.05 g catalyst.)
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Fig. 11. Possible photocatalytic mechanism for the electron-hole migration in the PVA/RGO/TiO2/Au catalyst under Vis-light irradiaition.
4. Conclusions PVA/RGO/TiO2/Au was successfully synthesized via a mild, lowtemperature wet chemical method. The SEM results revealed that the PVA/RGO/TiO2/Au had a uniform, hierarchically network structure with many pores. The TEM revealed (111) facets of the Au nanocrystal and (101) facets of TiO2 nanocrystal. The results agreed with the XRD analysis. The UV–vis DRS and PL indicated that the composite had great absorbance in the visible light region and effectively inhibited the recombination of photogenerated electron-hole pairs. As a result, the high photocatalytic performance of PVA/RGO/TiO2/Au was confirmed on the photodegradation of RhB in the photocatalytic degradation experiments. The photocatalytic performance was closely related to the catalyst concentration, the initial RhB concentration, the pH value, and the H2O2 concentration. These factors affected the amount of active sides, the adsorption efficiency of RhB, and the formation of reactive species. The high reusability of PVA/RGO/TiO2/Au was determined in the five cycles of testing. The composite remained nearly 94% of the initial degradation activity after five cycles. This study revealed that the prepared composite had great potential for dye wastewater treatment. Acknowledgements The authors acknowledge partial support from the National Natural Science Foundation of China (51874223), the Natural Science Major Research Plan in Shaanxi Province, China (2017ZDJC-25), Shaanxi Key Laboratory of Environmental Engineering of Xi'an University of Architecture and Technology, and Key Laboratory of Northwest Water Resource, Environment and Ecology, MOE. References [1] L. Thao, T. Dang, W. Khanitchaidecha, D. Channei, A. Nakaruk, Photocatalytic degradation of organic dye under UV-A irradiation using TiO2-vetiver multifunctional nano particles, Materials 10 (2017) 122. [2] D. Deng, N. Aryal, A. Ofori-Boadu, M.K. Jha, Textiles wastewater treatment, Water Environment Research 90 (2018) 1648–1662. [3] C.W. Lai, J.C. Juan, W.B. Ko, S. Bee Abd Hamid, An overview: recent development of titanium oxide nanotubes as photocatalyst for dye degradation, International Journal of Photoenergy 2014 (2014). [4] N. Tzikalos, V. Belessi, D. Lambropoulou, Photocatalytic degradation of Reactive Red 195 using anatase/brookite TiO2 mesoporous nanoparticles: optimization using response surface methodology (RSM) and kinetics studies, Environ. Sci. Pollut. Res. 20 (2013) 2305–2320.
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Au nanoparticle modified three-dimensional network PVA/RGO/TiO2 composite for enhancing visible light photocatalytic performance
T
⁎
Liang Zhanga,b,c, , Haojie Qib, Yuan Zhaod, Lvling Zhonga, Yujuan Zhanga, Yao Wanga, Juanqin Xuea, Yue Lie a
School of Chemistry and Chemical Engineering, Xi'an University of Architecture and Technology, Xi'an, Shaanxi, 710055, China School of Environmental and Municipal Engineering, Xi'an University of Architecture and Technology, Xi'an, Shaanxi, 710055, China c Shaanxi Provincial Key Laboratory of Gold and Resource, Xi'an University of Architecture and Technology, Xi'an, Shaanxi, 710055, China d Reactor Engineering and Safety Research Center, China Nuclear Power Technology Research Institute Co., Ltd, Shenzhen, 518031, China e College of Materials Science and Engineering, Xi'an University of Architecture and Technology, Xi'an, Shaanxi, 710055, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Photocatalyst TiO2 Au nanoparticle Visible light Rhodamine B
To improve photocatalytic performance, a composite of gold nanoparticle-modified titanium dioxide/reduced graphene oxide/polyvinyl alcohol (PVA/RGO/TiO2/Au) was successfully synthesized via a mild, low-temperature wet chemical method and characterized by FTIR, XRD, SEM, TEM, XPS, UV–vis DRS, PL, BET, and BJH. The PVA/RGO/TiO2/Au composite had a uniform, hierarchically network structure with many pores, which showed great absorbance in the visible light range and effectively inhibited the recombination of photogenerated electron-hole pairs. The synthesized composite showed a high photocatalytic degradation activity for rhodamine B (RhB) under high pressure sodium lamp irradiation. The photocatalytic degradation activity was closely related to the catalyst concentration, the initial RhB concentration, the pH, and the H2O2 concentration. The composite remained at nearly 94% of the initial photocatalytic degradation activity after five cycles.
1. Introduction Acceleration of global industrialization has highlighted the dye wastewater discharged from textile and dye factories. The complex structure of the organic pollutants in dye wastewater makes their degradation extremely difficult. Organic pollution has serious implications for aquatic organisms and human beings [1]. Efforts have been made to develop more effective treatment processes [2]. Photocatalysts, semiconductors capable of effective catalytic degradation of a wide range of organic substances under the excitation of photons, have been extensively studied, due to their low cost, non-toxicity, high efficiency and stability, and no secondary environmental pollution. Many semiconductor materials can be used as photocatalysts, including titanium dioxide (TiO2) [3–9] and other oxide or sulfide semiconductors [10–14]. Titanium dioxide is the most popular nano-photocatalyst material due to its strong oxidizing ability, chemical stability, and nontoxicity. Drawbacks of titanium dioxide include the high recombination rate of photogenerated electron-hole pairs, the low utilization of solar energy, and poor recyclability for reuse [15]. Research addressing photon quantum efficiency and solar energy utilization include combining non-metal substances [16–21] or doping
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metals [22–24]. Within the combined non-metal materials, carbonaceous materials and their derivatives possess excellent absorptivity, excellent electron conduction, and large surface area, which help to degrade organic pollutants in the photocatalytic reaction [21]. An example is graphene, which exhibits excellent electrical and optical properties, such as high electron mobility and high absorption properties in the visible range [16–20]. Graphene-modified titanium dioxide adsorbs visible light and enlarges the specific surface area. Wang et al. [18] synthesized a core-structured TiO2 NCs/RGO composite that exhibits high optical absorption in a wide spectral scope. Zhao et al. [19] synthesized three-dimensional graphene/TiO2 nanotube nanocomposite, effectively inhibiting charge recombination of TiO2. Yang et al. [20] demonstrated graphene can be photoexcited under visible light irradiation. Modifying titanium dioxide with graphene can improve the photocatalytic activity. However, the prepared materials are nanoparticles in wastewater and are prone to agglomeration, which decreases their specific surface area due to van der Waals forces. The nanocatalysts are difficult to recover and purify from the reaction mixture, resulting in great challenges to a wide practical application, due to high cost and limited resources [23]. The modified semiconductor composite as a
Corresponding author at: School of Chemistry and Chemical Engineering, Xi'an University of Architecture and Technology, Xi'an, Shaanxi 710055, China. E-mail address: [email protected] (L. Zhang).
https://doi.org/10.1016/j.apsusc.2019.143855 Received 20 May 2019; Received in revised form 24 August 2019; Accepted 1 September 2019 Available online 02 September 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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cooled to room temperature, the resulting precipitate was washed and dried via the same approach. The prepared composite particle was PVA/RGO/TiO2. The PVA/TiO2 composite was prepared by the same solvothermal method without RGO.
carrier requires further study [25]. Polymers have become popular carriers in recycling applications of photocatalysts, due to their mechanical properties, biocompatibility, and nontoxicity [26–29]. Polyvinyl alcohol (PVA) is non-toxic, biocompatible, and its mechanical properties make it a popular suitable polymeric matrix to support the catalyst. However, PVA has a high water solubility in the treatment of wastewater such as high water solubility. Glutaraldehyde (GA) was therefore crosslinked with PVA to form an acetal compound, while graphene further improved the water resistance and mechanical strength of the PVA polymeric material [28]. The graphene/TiO2 composite combined with PVA was successfully recycled by Jung et al. [29]. The photocatalytic activity of catalyst after combining with the carrier was reduced, because the contact between the carrier and the catalyst reduces the surface area of the catalyst, which decreases the numbers of active sites. Doping noble metal nanoparticles (NPs) can circumvent this, due to their electrical conductivity properties. Gold NPs are widely applied to the surface of the photocatalyst [22–24]. In this study, we successfully synthesized the TiO2/PVA/RGO/Au with a hierarchical network structure via a wet chemical method. The synthesized composite was applied to the photodegradation of rhodamine (RhB). The degradation process indicated that the catalyst improved the light response range and photodegradation efficiency, but also reduced the agglomeration effect and was more convenient for recycling. This composite is an excellent prospect for extensive applications in wastewater treatment.
2.3. Synthesis of PVA/RGO/TiO2/Au composite 0.1 g of the synthesized PVA /RGO/TiO2 composite and 5.8 ml of chloroauric acid (4.36 × 10−3 mol/l) were added to the 100 ml beaker with 50 ml of deionized water. After stirring for 2 h, 5.8 ml of sodium borohydride (1.09 × 10−2 mol/l) was added. The solution was continually stirred for an additional 24 h. The sample was filtered and washed with deionized water, then dried at 80 °C for 12 h.
2.4. Characterization of composite The chemical bond and the functional group were tested with Fourier transform infrared spectroscopy (FTIR, FEI Nicolet iS 50ATR, US). The crystal phase structure was obtained on a power X-ray diffraction (XRD, Bruker D8, Germany) using Cu kα radiation. The surface morphologies and nanoparticle size were observed on a scanning electron microscopy (SEM, FEI verous 460, US) and transmission electron microscopy (TEM, FEI Tecnai G2 F20, US). The chemical composition and state were determined via X-ray photoelectron spectroscopy (XPS, ThermoFisher ESCALAB 250Xi, USK) with Al kα radiation (hv = 1486.6 eV). The UV–vis diffuse reflectance spectroscopy (DRS, UV2600, Shimadzu, Japan) was used to determine the optical properties of the samples. Photoluminescence (PL) spectrum obtained under excitation at 397 nm with a Gangdong F-320 spectrophotometer fitted with a 200 W xenon lamp as the excitation source was used to determine the recombination rate of photogenerated electron-hole pairs. The Brunauer–Emmett–Teller (BET) surface areas, pore volume, and the pore size distributions of the samples were obtained on an automatic specific surface and porosity analyzer (Micromeritics, ASAP 2460, USA). After the pretreatment at 130 °C for 6 h, the absorption and desorption of nitrogen (N2) was carried out successively. The BET surface area was calculated by the multipoint BET method with nitrogen adsorption data. The total pore volume was calculated with single point method using the adsorption data at the relative pressure (p/p0) of 0.99. The pore size distribution and the average pore size were calculated by the Barrett-Joyner-Halenda (BJH) method with nitrogen desorption data.
2. Experimental section 2.1. Materials Polyvinyl alcohol-124 (PVA-124) was purchased from Guangdong Guanghua Sci-Tech Co., Ltd. Tetrabutyl titanate was purchased from Shanghai Sanpu Chemical Co., Ltd. Glutaraldehyde (GA), rhodamine B (RhB), sodium hydroxide and hydrogen peroxide (30%) were purchased from Tianjin Kermel Chemical Reagent Co., Ltd. Crystalline flake graphite was purchased from Qingdao Tengshengda Carbon Machinery Co., Ltd. Sodium nitrate, sulfuric acid (98%) and hydrochloric acid (HCl) were purchased from Xi'an Chemical Reagent Factory. Potassium permanganate was purchased from Henan Jiaozuo Three Chemical Plant. Absolute ethanol (99.5%) was purchased from Tianjin Tianli Chemical Reagent Co., Ltd. These reagents were used as received from the manufacturer. Deionized (DI) water was used throughout the experiments. 2.2. Synthesis of PVA/RGO/TiO2 and PVA/TiO2 composites
2.5. Photodegradation of RhB solution PVA/RGO/TiO2 composite was prepared as follows: Graphene oxide (GO) was synthesized via an improved Hummers method [30]. The prepared GO was reduced via hydrazine hydrate [31] and the reduced GO (RGO) composite was made. 0.0385 g of RGO was completely dissolved in 30 ml of deionized water for 2 h via ultrasound. 2.4 g of polyvinyl alcohol-124 (PVA-124) was transferred to the graphene solution and treated at 90 °C. When the PVA was completely dissolved, the HCl was slowly added to adjust the pH to 4. The mixture was transferred to the Hydrothermal reactor, and 10 ml of GA (diluted to 25%) was slowly added into the solution under continuous stirring. The systemic temperature was 90 °C. After 3 h, the system was slowly cooled to room temperature. The precipitate was filtered, washed with ethanol, and washed deionized water three times, then dried in vacuum at 80 °C for 12 h. A gray PVA/RGO composite was obtained. To coat TiO2 on the PVA/RGO composite, 0.25 ml of tetrabutyl titanate was dissolved in 20 ml of ethanol, and 0.25 ml of DI water was dropped into the solution under continuous stirring. When the sol was generated, the mixture and 0.5 g of the prepared PVA were added into the Hydrothermal reactor and heated for 18 h at 190 °C. After the system slowly
Photodegradation experiments were performed in batch systems under visible light irradiation. A high-pressure sodium lamp (150 W, Philips) employed as the visible light source provided a power density of approximately 278 W/m2. To avoid lamp heating, the high-pressure sodium lamp (HPSL) was jacketed by a water-cooled condenser [32]. Photocatalyst was added to the RhB standard solution. To ensure the adsorption-desorption equilibrium between organic molecules and the photocatalyst, the suspensions were stirred for 30 min continuously in the dark. After centrifugal separation, the concentration of the solution was measured at 552 nm with a 722 G UV–visible spectrophotometer (China). Then the solution was irradiated under visible light. During the photodegradation process, the concentration of the solution was measured at definite time intervals. The residual concentration Ct of the RhB in the solution was calculated by using standard curve method, and the degradation efficiency of the photodegradation of RhB in the solution was calculated as follows: D % = [(C0 − Ct)/C0] × 100% [26], where C0 and Ct are the initial and time t concentration of RhB solution.
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Fig. 1. Infrared spectroscopy of (a) PVA/RGO, (b) PVA/RGO/TiO2, (c) PVA/ RGO/TiO2/Au, (d) PVA/RGO, and (e) PVA/RGO/TiO2 in 550–450 cm−1.
Fig. 2. XRD patterns of (a) PVA/RGO/TiO2, and (b) PVA/RGO/TiO2/Au.
are shown in Fig. 3d with a diameter about 100 nm. The network structure of PVA/RGO/TiO2 appeared cluster of TiO2 NPs in Fig. 3b. The TiO2 NPs aggregated on the PVA/RGO with a size < 50 nm, see Fig. 3e, h. There was little change on the surface morphology of the PVA/RGO/TiO2 with the Au doped, as shown in Fig. 3c. The TEM image of the PVA/RGO/TiO2/Au (Fig. 3h) revealed the porous structure with a range between 10–100 nm. The surface of network structure was relatively smooth in the high-magnification SEM image (Fig. 3f), but the Au NP was not clearly distinguished. The magnified TEM image in Fig. 3i shows the single atomic Au sites highlighted by red circles, which were decided by the lattice distance. The Au and Ti lattices are clearly presented in the high-resolution TEM (HRTEM) image in Fig. 3k. The lattice spacing was about 0.23 nm in Fig. 3j, which indicated the (111) facets of the Au nanocrystal. The lattice spacing of the magnified TiO2 nanocrystal at about 0.35 nm corresponded to the (101) facets, see Fig. 3l [18,22,37]. The results agreed with the XRD analysis. The energy-dispersive X-ray (EDX) spectrum also demonstrated that the Au NPs were distributed on the PVA/RGO/TiO2, see Fig. 3g.
3. Results and discussion 3.1. Characterization analysis 3.1.1. Chemical bonds obtained by infrared spectroscopy analysis To confirm that PVA/RGO/TiO2/Au was successfully prepared, infrared spectra of PVA/RGO, PVA/RGO/TiO2 and PVA/RGO/TiO2/Au composites were obtained (Fig. 1a–c). The characteristic absorption peaks of PVA/RGO were observed (Fig. 1a). The strong and wide absorption peak between 3700 and 3000 cm−1 was attributed to the –OH group stretching vibration. The peaks at 2930 cm−1 and 2864 cm−1 corresponded to the –CH2 group anti-symmetric and symmetric stretching vibration. The peaks at 1560 cm−1 and around 816 cm−1 were attributed to the plane bending vibration of CeH. The peak at 1643 cm−1 was attributed to the C]O of the alkyl aldehyde unit, and the peak at 1124 cm−1 was attributed to the O-C-O stretching vibration, which demonstrated the acetal reaction between PVA and GA [26,33,34]. The characteristic absorption peaks of PVA/RGO are seen in Fig. 1b. The peaks within 550–450 cm−1 corresponded to the Ti-O-Ti stretching vibration, which was shown in the comparison between Fig. 1d and e [35,36]. The characteristic absorption peaks of PVA/RGO/TiO2/Au were similar to the PVA/RGO/TiO2 in Fig. 1b, because Au atoms slightly affected the material structure. These characteristic peaks proved a successful combination between TiO2 and PVA/RGO in the composites.
3.1.4. Chemical composition and state obtained by XPS analysis The chemical composition and the state of PVA/RGO/TiO2/Au were studied by X-ray photoelectron spectroscopy (XPS), as shown in Fig. 4. The O 1s, Ti 2p, C 1s, and Au 4f peaks are shown in the XPS survey spectrum in Fig. 4a. The C 1s spectrum was displayed with three binding energy peaks presented in Fig. 4b. The three peaks at 284.47 eV, 286.37 eV, and 288.77 eV were attributed to CeC, CeO, and C]O bonds [19,26]. The O 1s spectrum was shown in Fig. 4c. The two peaks at 529.63 eV and 530.70 eV were attributed to O-Ti3+ and OTi4+ species. The last peak at 532.33 eV was attributed to OeC bonds [19]. The Ti 2p spectrum in Fig. 4d presented two typical peaks, indicating Ti 2p3/2 and Ti 2p1/2 were located at the binding energy of 458.48 eV and 464.16 eV [18,19,32]. The Au 4f spectra in Fig. 4e showed the peaks, indicating Au 4f7/2 and Au 4f5/2 were located at the binding energy of 83.36 eV and 87.26 eV from the metallic state Au [22].
3.1.2. Crystal phase structures determined by XRD analysis The crystal phase structures of the PVA/RGO/TiO2 and the PVA/ RGO/TiO2/Au were further characterized by XRD. The peak at 2θ values of 19.14° corresponded to the (101) faces of the PVA [28]. The main peaks at 2θ values of 25.14°, 37.77°, 48.01°, 53.77°, 54.95°, and 74.88° in Fig. 2a corresponded to the (101), (004), (200), (105), (211), and (204) faces of anatase TiO2 [26], which matched well with the JCPDS powder diffraction pattern 21-1272. The (111), (200), (220), and (311) faces presented in Fig. 2b were ascribed to the diffraction peak of Au [23], confirming the coexistence of metal Au.
3.1.5. UV–vis DRS and PL spectra analysis Optical absorption properties of PVA, PVA/RGO, PVA/RGO/TiO2, and PVA/RGO/TiO2/Au were revealed by the UV–vis diffuse reflectance spectra (UV–vis DRS) (Fig. 5a). Pure PVA had almost no absorbance in the visible wavelength range. The PVA modified with RGO had a conspicuous visible-light absorbance, which helped to improve the TiO2 optical properties. The combined composite of the PVA/RGO
3.1.3. Morphological observation by SEM and TEM The morphologies of PVA/RGO, PVA/RGO/TiO2, and PVA/RGO/ TiO2/Au were observed by field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) (Fig. 3). The panoramic image of PVA/RGO in Fig. 3a shows a uniform, hierarchically network structure with many pores. Some macropores 3
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Fig. 3. SEM image of (a, d) PVA/RGO, (b, e) PVA/RGO/TiO2, and (c, f) PVA/RGO/TiO2/Au. (g) EDX spectrum of PVA/RGO/TiO2/Au. TEM image of PVA/RGO/ TiO2/Au: (h) TEM image. (i) TEM image of the single atomic Au sites highlighted by red circles. (k) HRTEM image. (j) HRTEM image showing clear lattice fringes of Au. (l) HRTEM image showing clear lattice fringes of anatase TiO2. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
the photogenerated carriers separation efficiency. The lowest PL intensity for PVA/RGO/TiO2/Au indicated the lower recombination rate, which showed that the Au NPs further improved the separation efficiency of the photogenerated carriers. This was because both the gold nanoparticles and the graphene have excellent electrical conductivity for the transfer of photoexcited electrons, which meant that they acted as electron acceptors [20,22].
and the TiO2 showed a great absorbance in the visible light range (Fig. 5A, b, c). The PVA/RGO/TiO2/Au showed the broad range of the visible light response, which was beneficial to enhance utilization of light for the photocatalytic reaction [26]. The recombination rate of photogenerated electron-hole pairs was investigated; see Fig. 5B for the photoluminescence (PL) spectra of PVA/TiO2, PVA/RGO/TiO2, and PVA/RGO/TiO2/Au. The PL intensity of the PVA/RGO/TiO2 was lower than the PVA/TiO2, which implied a low recombination rate [26,38]. This proved that the RGO improved
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Fig. 4. XPS of the PVA/RGO/TiO2/Au (a) survey spectrum, (b) C 1s, (c) O 1s, (d) Ti 2p, and (e) Au 4f.
to hundreds of nanometers; the number of pores in PVA/RGO/TiO2, and PVA/RGO/TiO2/Au was higher than that of PVA/TiO2. The BET surface area, pore volume, and pore size values of the prepared samples are presented in Table 1. The BET surface area and pore volume of PVA/RGO/TiO2 were 14.936 m2/g and 0.02788 cm3/g, respectively, which were higher than that of PVA/TiO2 (2.250 m2/g and 0.00624 cm3/g). The surface area and pore volume increased, which was attributed to the adhesion and aggregation of TiO2 NPs on the
3.1.6. Brunauer–Emmett–Teller (BET) surface area and pore size distribution The N2 absorption-desorption type IV isotherm (Fig. 6a) and the pore size distribution (Fig. 6b) of PVA/TiO2, PVA/RGO/TiO2, and PVA/ RGO/TiO2/Au displayed distinct hysteresis loops close to H3 type at a relative pressure range of 0.45–1.0, which were a typical characteristic of mesopores (2–50 nm) and macropores (> 50 nm) features [26]. The pore size distribution showed the pore diameters ranging from dozens 5
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Fig. 5. (A) The UV–vis DRS of PVA, PVA/RGO/TiO2, PVA/RGO/TiO2/Au, and PVA/RGO. (B) PL spectra of PVA/TiO2, PVA/RGO/TiO2, and PVA/RGO/TiO2/Au.
surface of PVA/RGO, resulting in the formation of some new mesopores, macropores, and micro-protrude (observed in SEM images). The surface area and pore volume of PVA/RGO/TiO2 modified with Au NPs (10.536 m2/g and 0.02027 cm3/g) were slightly lower than that of PVA/RGO/TiO2, which indicated the Au NPs are distributed in it's pores, causing some pores to disappear, as seen in the inset of Fig. 6. The high specific surface area can provide more surface active sites, and the mesopores and macropores can facilitate the injection of dye and photons. These help to improve photocatalytic performance [48,49].
Table 1 BET surface area, pore volume, and pore size. Samples
BET surface area (m2/g)
Pore volume (cm3/ g)
Pore size (nm)
PVA/RGO/TiO2/Au PVA/RGO/TiO2 PVA/RGO
10.536 14.936 2.250
0.02027 0.02788 0.00624
72.971 62.246 108.893
prepared PVA/RGO/TiO2/Au showed considerable photocatalytic efficiency, which resulted from the Au NPs improved separation of electron-hole pairs via their high conductivity [22–24]. As the same time, the photodegradation kinetics of RhB was also investigated using the Langmuir-Hinshelwood pseudo-first-order reaction: ln (C0/Ct ) = kapp × t, where C0 and Ct are the adsorption-desorption equilibrium and irradiation time t concentration of RhB solution, kapp is an apparent rate constant (Fig. 7b) [18,26]. The corresponding initial reaction rates of RhB degradation were obtained using the first equation: r = kapp × C0, where r is the initial reaction rate of the dye [44]. The initial reaction rates of No catalyst, PVA/RGO, PVA/RGO/TiO2, and PVA/RGO/TiO2/Au were 0.00841, 0.01775, 0.04959, and 0.08178 mg/ l·min−1, respectively. The initial reaction rate of RhB degradation with PVA/RGO/TiO2/Au was 9.7-fold higher than that with PVA/RGO.
3.2. Photocatalytic degradation experiments 3.2.1. Photocatalytic degradation performance The degradation performance of PVA/RGO/TiO2/Au was confirmed by comparing the degradation properties on the RhB in the presence of PVA/RGO, PVA/RGO/TiO2, and PVA/RGO/TiO2/Au under HPSL irradiation. The experimental results are shown in Fig. 7. The PVA/RGO/TiO2/Au composite was the most efficient for degradation of the RhB. The PVA/RGO, compared with the solid blank, showed a slight degradation performance. The PVA/RGO/TiO2 composite showed some improvement on the photodegradation of RhB. The results revealed that the TiO2-containing composite with PVA/RGO could adsorb visible light to generate the electron-hole pairs and decompose the organic dye [29,50]. When the Au NPs was deposited, the
Fig. 6. (a) N2 absorption-desorption isotherm, (b) pore size distribution curves of the prepared samples. 6
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Fig. 7. (a) Comparison of degradation performance of the different catalysts on the photodegradation of RhB under HPSL irradiation. (b) The photodegradation kinetics of RhB with different photocatalysts (Experimental conditions: 50 ml of 10 mg/l RHB, pH 7, and 0.05 g catalyst.)
solution showed an increasing trend, as the catalyst concentration (CPVA/RGO/TiO2/Au) increased. As the same time, the corresponding photodegradation kinetics of RhB over PVA/RGO/TiO2/Au with different concentrations fit the pseudo-first-order model (Fig. 8b). The initial reaction rates (Table 2) were in the following order: 0.03878 mg/
3.2.2. Effect of catalyst and initial RhB concentration A conspicuous difference of the degradation performance was found in the RhB with the various amounts of catalysts. The degradation performance of the RhB over PVA/RGO/TiO2/Au with different concentrations is shown in Fig. 8a. The removal efficiency of the RhB in the
Fig. 8. (a) and (b) The degradation performance of the RhB over PVA/RGO/TiO2/Au with different concentrations, and the corresponding photodegradation kinetics (Experimental conditions: 50 ml of 10 mg/l RHB, pH 7.); (c) and (d) The degradation performance of the RhB with different concentrations over PVA/RGO/TiO2/Au, and the corresponding photodegradation kinetics. (Experimental conditions: pH 7, 0.05 g catalyst.) 7
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competition for the adsorption site [40]. The degradation capability basically exhibited similar behavior as the adsorption quantity. At pH 5, the perfect catalytic effect occurred because the acidic environment could support the mass generation of the reactive intermediates. Moreover, the corresponding photodegradation kinetics of RhB roughly followed pseudo-first-order reaction (Fig. 9b), and the initial reaction rates for photodegradation of RhB with PVA/RGO/TiO2/Au at different pH values were shown in the Table 3. The PVA/RGO/TiO2/Au exhibited excellent photocatalytic activity at pH 7, with the initial reaction rate being 0.07730 mg/l·min−1. Hydrogen peroxide (H2O2) that was added to the initial solution promoted the catalytic process. In order to evaluate the effect that the performance of the catalyst had under HPSL irradiation [43], the test was performed in RhB solution with added H2O2. The photodegradation performance of the RhB at different H2O2 concentrations (CH2O2) (Fig. 9c), and the corresponding photodegradation kinetics of RhB (Fig. 9d) were shown. The degradation efficiency of the RhB without H2O2 was 78% after 180 min of irradiation, but that with 3 mM H2O2 reached 76% after 30 min of irradiation, and 92% after 60 min of irradiation. The degradation efficiency of the RhB with 6 mM H2O2 hardly improved. The H2O2 added to the initial solution trapped many photoelectrons, resulting in effective inhibition of the recombination process of the photogenerated electron-hole pairs. The ionization of H2O2 generated hydroxyl radicals (%OH). The excess H2O2 would scavenge hydroxyl radicals and generate relatively weaker peroxyhydroxyl radicals (HO2%) or further produce oxygen and water [43,44]. Moreover, the corresponding photodegradation kinetics of RhB with H2O2 for PVA/RGO/TiO2/Au was roughly in agreement with the pseudo-firstorder reaction, and the initial reaction rates of RhB with different H2O2 contents degradation were shown in the Table 3. The initial reaction rate at 3 mM H2O2 was 4-fold higher than that without H2O2, which meant the appropriate amount of H2O2 was beneficial for photocatalytic degradation of the dyes.
Table 2 Apparent constants kapp, initial reaction concentration C0, and initial reaction rate r over different catalyst and initial RhB concentration. CPVA/ RGO/
kapp (min−1)
C0 (mg/l)
r (mg/ l·min−1)
CRhB (mg/l)
kapp (min−1)
C0 (mg/l)
r (mg/ l·min−1)
0.00392 0.00507 0.00804 0.01541 0.01987
9.894 9.435 9.052 8.055 7.477
0.03878 0.04783 0.07278 0.12413 0.14856
10 20 50 70 100
0.01624 0.00882 0.00694 0.00431 0.00387
8.855 18.270 46.245 66.157 96.940
0.14381 0.16114 0.32094 0.28514 0.37516
TiO2/Au
(g/l) 0.5 1.0 1.2 1.6 2.0
l·min−1 (0.5 g/l) < 0.04783 mg/l·min−1 (1.0 g/l) < 0.07278 mg/ l·min−1 (1.2 g/l) < 0.12413 mg/l·min−1 (1.6 g/l) < 0.14856 mg/ l·min−1 (2.0 g/l). The photodegradation date increased rapidly, when the quantity of the catalyst ranged from 0.5 to 1.6 g/l. The photodegradation date beyond 1.6 g/l increased slowly. The increase of the catalyst concentrations could promote the degradation rate of the organic substance by providing additional active sides for the generation of oxidizing species and the greater photoactive surface area. When excess catalyst concentration is present, the catalytic efficiency is slightly relevant to it, as seen in the current study. The higher concentration of catalysts even caused agglomeration and subsequently reduced the available surface area, while blocking the incident light that further decreased the light transmittance through the solution [39]. Initial RhB concentration (CRhB) was used to assess its effect on photocatalytic performance, in the presence of PVA/RGO/TiO2/Au under HPSL irradiation. Fig. 8c shows the values of removal efficiency for the RhB that decreased, as the initial RhB concentration increased from 10 to 100 mg/l. Fig. 8d shows the typical first-order photodegradation feature over the different initial RhB concentration. The initial reaction rates (Table 2) showed the weak increased trend, compared to the rapid increase in the initial RhB concentration. The probability of formation of reactive species on the catalyst surface and reactive species accepted by RhB determine the degradation rate. As the initial dye concentration increases, more dyes molecules are available for excitation and energy transfer [42]. However, with an excessive concentration of dyes, the relative residual amount of dye molecules increases after the adsorption capacity of the catalyst reaches equilibrium. The photons reaching the catalyst surface are reduced, which resulted from the interception of excess organic substrate concentration. [26,36].
3.2.4. Reusability The reusability of the photocatalyst repeatedly tested the photocatalytic performance of the composite catalyst degrading rhodamine B under HPSL irradiation. The reusability testing was performed for five times in the same test conditions. After the degradation was finished, the catalyst was filtered and washed with ethanol and deionized water for three times for each trial. The catalyst was dried in a vacuum oven for 12 h at 80 °C and applied to the next run. The five runs were carried out with PVA/RGO/TiO2/Au under the HPSL irradiation for photocatalytic experiments (Fig. 10). The percentage degradation efficiency of the RhB decreased from 85% to 80% after five cycles, and the degradation capacity of PVA/ RGO/TiO2/Au composite did not decrease the removal of RhB. According to the reusability tests, the composite catalyst had a longterm usage for the photodegradation under the HPSL irradiation. The decrease was a result of some catalyst surface being covered by the thin layer of intermediate dye after the first run, so that the adsorption capacity of the composite was lower in following runs. The surface of the covered catalyst lost some active sites trapping photons after each run, so that the generated reactive intermediate for the reaction with the dye molecules under the HPSL irradiation was reduced [45]. The degradation capability decreased, as the number of runs increased.
3.2.3. Effect of pH and H2O2 concentration The pH value was a vital factor in the photocatalytic reaction, because it was closely related to the surface charge of the catalyst, the adsorption of organic substances on the catalyst surface [40], and the amount of charged free radicals produced during the photocatalysis process [26]. The RhB-solution degradation performance was tested with different pH values under HPSL irradiation. The experimental results (Fig. 9a) showed that the pH values affected both the adsorption capacity and degradation performance [41,42]. The percent adsorption capacity of the RhB on the catalyst surface increased from 3% to 36%, with a pH value from 3 to 9, and then decreased for pH values higher than 9. The optimum adsorption capacity was obtained at pH 9. The surface of the composite material was positively charged at lower pH, because of protonation of the active functional groups on the catalyst surface. This positive surface charge resulted in competition against the cationic RhB for adsorption sites. As the pH increased, the catalyst surface could be negatively charged, enhancing the electrostatic attraction between the catalyst and dyes. When the pH was higher than 9, the decreased adsorption efficiency could be attributed to the enhanced hydroxyl ionic strength of the solution, which resulted in more
3.2.5. Possible photocatalytic mechanism The possible mechanism is shown in Fig. 11, which illustrates the charge migration and the photocatalytic process under Vis-light irradiation. Both the Au nanoparticle and the RGO exhibited excellent electrical conductivity, and the RGO could be band gap-photoexcited under Vis-light irradiation [20,22,46]. The TiO2 alone cannot absorb Vis-light, due to the 3.2 eV band gap energy [22,26]. The RGO can absorb photons under the HPLS irradiation and then become excited to generate photoelectron-hole pairs. The hot electron transferred to the 8
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Fig. 9. (a) and (b) The degradation performance of the RhB over PVA/RGO/TiO2/Au at different pH values, and the corresponding photodegradation kinetics (Experimental conditions: 50 ml of 10 mg/l RHB, 0.05 g catalyst.); (c) and (d) The degradation performance of the RhB over PVA/RGO/TiO2/Au at different H2O2 concentrations, and the corresponding photodegradation kinetics. (Experimental conditions: 50 ml of 10 mg/l RHB, pH 7, and 0.05 g catalyst.) Table 3 Apparent constants kapp, initial reaction concentration C0, and initial reaction rate r at different pH and H2O2 concentration. pH value
kapp (min−1)
C0 (mg/l)
r (mg/ l·min−1)
CH2O2 (mM)
kapp (min−1)
C0 (mg/l)
r (mg/ l·min−1)
3 5 7 9 11
0.00390 0.01001 0.00889 0.01067 0.00536
9.568 9.278 8.695 6.345 9.703
0.03732 0.09287 0.07730 0.06770 0.05201
0 1 2 3 6
0.00928 0.01052 0.02005 0.03851 0.03973
9.435 9.362 9.244 9.144 9.052
0.08756 0.09845 0.18534 0.35214 0.35964
conduction band the TiO2 or the surface of the Au nanoparticles. In addition, the RhB molecules with photosensitization can also absorb Vis-light for excitation [49,50]. The hot electron from the excited RhB molecules (RhB*) absorbed to the surface of the catalyst transferred to the conduction band of TiO2 and RGO semiconductor and then to the RGO or the Au nanoparticles. The successfully transferred hot electrons were trapped by the oxygen in the surface of the TiO2, RGO, and the Au nanoparticles to generate superoxide anion radicals (%O2−). The powerful oxidizing free radicals %O2– in the reaction system were accepted by RhB absorbed to the surface of the catalyst in the reaction system, where the degradation of RhB was obtained. The photogenerated holes reacted with the RhB molecules to produce oxidation RhB [26,49,50].
Fig. 10. Five cycles of the photocatalyst reusability on the photodegradation of the RhB under the HPSL irradiation. (Experimental conditions: 50 ml of 10 mg/l RHB, pH 7, and 0.05 g catalyst.)
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Fig. 11. Possible photocatalytic mechanism for the electron-hole migration in the PVA/RGO/TiO2/Au catalyst under Vis-light irradiaition.
4. Conclusions PVA/RGO/TiO2/Au was successfully synthesized via a mild, lowtemperature wet chemical method. The SEM results revealed that the PVA/RGO/TiO2/Au had a uniform, hierarchically network structure with many pores. The TEM revealed (111) facets of the Au nanocrystal and (101) facets of TiO2 nanocrystal. The results agreed with the XRD analysis. The UV–vis DRS and PL indicated that the composite had great absorbance in the visible light region and effectively inhibited the recombination of photogenerated electron-hole pairs. As a result, the high photocatalytic performance of PVA/RGO/TiO2/Au was confirmed on the photodegradation of RhB in the photocatalytic degradation experiments. The photocatalytic performance was closely related to the catalyst concentration, the initial RhB concentration, the pH value, and the H2O2 concentration. These factors affected the amount of active sides, the adsorption efficiency of RhB, and the formation of reactive species. The high reusability of PVA/RGO/TiO2/Au was determined in the five cycles of testing. The composite remained nearly 94% of the initial degradation activity after five cycles. This study revealed that the prepared composite had great potential for dye wastewater treatment. Acknowledgements The authors acknowledge partial support from the National Natural Science Foundation of China (51874223), the Natural Science Major Research Plan in Shaanxi Province, China (2017ZDJC-25), Shaanxi Key Laboratory of Environmental Engineering of Xi'an University of Architecture and Technology, and Key Laboratory of Northwest Water Resource, Environment and Ecology, MOE. References [1] L. Thao, T. Dang, W. Khanitchaidecha, D. Channei, A. Nakaruk, Photocatalytic degradation of organic dye under UV-A irradiation using TiO2-vetiver multifunctional nano particles, Materials 10 (2017) 122. [2] D. Deng, N. Aryal, A. Ofori-Boadu, M.K. Jha, Textiles wastewater treatment, Water Environment Research 90 (2018) 1648–1662. [3] C.W. Lai, J.C. Juan, W.B. Ko, S. Bee Abd Hamid, An overview: recent development of titanium oxide nanotubes as photocatalyst for dye degradation, International Journal of Photoenergy 2014 (2014). [4] N. Tzikalos, V. Belessi, D. Lambropoulou, Photocatalytic degradation of Reactive Red 195 using anatase/brookite TiO2 mesoporous nanoparticles: optimization using response surface methodology (RSM) and kinetics studies, Environ. Sci. Pollut. Res. 20 (2013) 2305–2320.
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