rGO ternary composites with synergistic enhanced photocatalytic activity

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Ultrasonic impregnation assisted in-situ photoreduction deposition synthesis of Ag/TiO2 /rGO ternary composites with synergistic enhanced photocatalytic activity Yaqi Hou a, Shengyan Pu a,b,∗, Qingqing Shi a, Sandip Mandal a, Hui Ma a,c, Shengyang Xue a, Guojun Cai a, Yingchen Bai b a

State Key Laboratory of Geohazard Prevention and Geoenvironment Protection (Chengdu University of Technology), No.1, Dongsanlu, Erxianqiao, Chengdu 610059, Sichuan, China b State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China c Department of Plant and Environmental Sciences, University of Copenhagen, Thorvaldsensvej 401871 Frederiksberg, Denmark

a r t i c l e

i n f o

Article history: Received 18 May 2019 Revised 21 August 2019 Accepted 31 August 2019 Available online xxx Keywords: Graphene oxide Titanium dioxide Photocatalyst Ternary composites

a b s t r a c t New approaches and practices to improve photocatalytic efficiency of materials for remediation of organic and inorganic pollutant has become a top priority. We have developed a graphene-oxide based ternary composite Ag/TiO2 /rGO as a green catalyst, and the photocatalytic oxidation and reduction efficiency of the catalyst was thoroughly evaluated for Cr(VI) and tetracycline (TC) remediation in water as potential contaminants. The results suggested that the electronic coupleing system exhibits excellent photocatalytic activity, mainly due to affecting light absorption and higher e− /h+ separation efficiency. The photocatalytic activity of the silver-deposited graphene-supported silver and titanium dioxide was 4.3 and 2.5 times for Cr (VI) and TC remediation, respectively. Furthermore, the service life of the photocatalyst was also evaluated within five recycling studies, indicating that the efficiency of the obtained photocatalyst was maintained above 80%. Electron paramagnetic resonance spectroscopy and the addition of free radical scavengers confirmed that e− act as reducing agents and may be the key (O2 ·− ) to the degradation process. The photocatalytic mechanism was proposed based on the studies obtained. Green catalysts based on graphene-based ternary composites developed in this work can provide effective and economic remediation at higher scale and offer practical application of photo catalysis technology. © 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction The photocatalyst based on semiconductor materials is considered as a new type of green photocatalytic material, which are widely used in catalytic research [1,2]. These kind of materials provided an economic technology for water treatment [3] because of their operative performance under mild reaction conditions, superior decontamination ability, improved physiochemical stability, lower energy consumption and strong oxidizing properties in broad spectrum [4]. The photocatalytic properties and use of titanium dioxide in catalysis is being studied for nearly 40 years, however a standard methodology is still missing. This insufficiency led to urgent needs in the studies of environmental ∗ Corresponding author at: State Key Laboratory of Geohazard Prevention and Geoenvironment Protection (Chengdu University of Technology), Chengdu 610059, China. E-mail addresses: [email protected], [email protected] (S. Pu).

pollution control, energy conversion, and synthesis of new materials [5]. Adopting various methods to improve the performance of the catalyst has been one of the significant goals in the catalyst technology development [6]. Hence, in this direction, carbon forms has been exploited for its exceptional electron transport properties [7]. It is used and studied to establish the catalytic properties in different allotropic forms of carbon such as carbon microspheres [8], carbon fibers [9], fullerenes [10], carbon nanotubes [11], and graphene’s [12]. Furthermore, it has been also used as a co-catalyst or catalyst carrier in photoelectric conversion and photocatalytic systems, and has become an important component of many composite catalysts, and has received widespread attention as heterogeneous catalyst [13]. It is established that, the introduction of carbon material to composite materials such as carbon nanotube has significantly improved the photocatalytic performance in terms of higher specific surface area, promotes the dispersion of the active components and increases the effective reactive sites [14]. Furthermore, the increase of electron transport rate by the carbon material

https://doi.org/10.1016/j.jtice.2019.08.023 1876-1070/© 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Please cite this article as: Y. Hou, S. Pu and Q. Shi et al., Ultrasonic impregnation assisted in-situ photoreduction deposition synthesis of Ag/TiO2/rGO ternary composites with synergistic enhanced photocatalytic activity, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.08.023

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could promote the interface heterojunction formation between the carbon-metal and inhibit the e− /h+ pairs and considerably improve the photocatalytic efficiency [15]. It is also observed that, with incorporation of carbon material as a photosensitizer in semiconductors, shifts the Fermi level of the composite in a more affirmative direction, thereby increasing the absorption range and intensity of visible light to improve the utilization of light energy [16]. Graphene as a photocatalyst carrier, in comparison to other conventional carbon materials presents superior surface area with proximity around, 2600 m2 /g [17,18], which is twice than that of single-walled carbon nanotubes. This particular feature makes it an ideal carrier for the catalysts [19]. Therefore, many researchers have explored the behavior and symmetry of graphene as an auxiliary agent, as carrier in photocatalysis and photoelectric conversion process. It worthy to explore further until a standard material obtained [20]. Kamat et al. [21] systematically studied the electron transport mechanism between graphene and TiO2 , and confirmed the role of graphene as an electron acceptor in storing electrons and as a wire to transfer electrons. In recent years, several graphene-based photocatalysts were well reported for binary systems. The graphene is only shared with one component, which limits the structural benefits of graphene in significant way [22,23]. To overcome the drawback of binary system, it is important to study the preparation of ternary or even multi-graphene-based photocatalytic nanomaterials, which provide a new progressive direction for graphene-based photocatalytic nanomaterials in the future [24]. In this aspect, the ternary hybrid nanomaterials fashioned with two-dimensional graphene/SnO2 /Pt presented excellent degradation performance to 4-nitrophenol (4-NP) was reported [25]. Similarly, the use of TiO2 /carbon nanotubes/reduced graphene oxide composites for degradation of Rhodamine B (RhB) and the excellent degradation because of the reduction by recombination rate of holes and increase of hydroxyl radical on the surface of catalyst by addition of rGO and CNTs respectively was investigated [26]. In another study, the Au@Fe3 O4 -G material presented admirable catalytic property in the reduction of 4-NP by NaBH4 , and the Pt@Fe3 O4 -G showed remarkable electro catalytic performance for hydrazine oxidation [27]. The above studies suggested that, the graphene-based photocatalytic nanomaterials could provide superior photocatalytic activity comparable to traditional materials; therefore, further study is required to understand the ternary or even multi-component composite systems. Hence, we developed the highly efficient Ag/TiO2 /rGO ternary photocatalyst by ultrasonic impregnation assisted in-situ photoreduction strategy. The effect of various influencing factors on photoreduction of Cr(VI) and photodegradation of tetracycline (TC) was investigated. This study provided a proper methodology for ternary system photocatalyst with high regeneration and removal efficiency at large scale water treatment contaminated with organic and inorganic comtanminants. 2. Materials and methods 2.1. Materials Potassium dichromate (K2 Cr2 O7 ), potassium hypermanganate (KMnO4 ), hydrogen peroxide (H2 O2 , 30 wt%), sulfuric acid (H2 SO4 , 98 wt%), hydrochloric acid (HCl, 36.0–38.0%), sodium hydroxide (NaOH), ammonium hydroxide (NH3 ·H2 O, 25–28%) were all of analytical grade and used as received without further purification. Absolute ethanol (C2 H5 OH) was premium grade pure. All of them were purchased from Kelong Chemical Reagent Company (Sichuan, China). TiO2 (P25) nanoparticles powder was supplied by Degussa Company (Germany). Graphite powder (purity > 99.95%), and silver nitrate (AgNO3 ) were obtained from Aladdin Industrial Corporation (Shanghai, China). Tetracycline (TC) was obtained from

Sigma-Aldrich. Deionized (D.I.) water used for all experiments was produced by Ultrapure Milli-Q water purification system.

2.2. Synthesis of graphene oxide (GO) The GO is synthesized by modified Hummers method [28], as shown in Fig. S1a. Specifically, 2.0 g graphite powder and 1.2 g NaNO3 were added to a three necked flask, and concentrated H2 SO4 (60 ml) was added drop wise under ultra-sonication to obtain a dark blue mixture. Then, the three-necked flask was put in an ice bath and mixed vigorously using a magnetic stirrer for approx. 15 min. Potassium permanganate (8.8 g) was slowly added to the reaction system and continuously stirred for 12 h while using an ice bath to avoid temperatures exceeding 20 °C. The mixture was continuously stirred for 6 h at 50 °C and 35 °C, respectively. Next, H2 O2 (22 mL) was slowly added to the vessel with a large amount of bubbles, and the resultant mixture was stirred for another 3 h. Lastly, the obtained mixture was washed successively with HCl solution (5%) and absolute ethanol until the pH of the supernatant became neutral, and the initial product was dispersed in absolute ethanol by sonication. Finally, the obtained product was uniformly dispersed by ultrasonication in absolute ethanol.

2.3. Preparation of Ag/TiO2 /RGO ternary composites The Ag/TiO2 /rGO photocatalyst is prepared by ultrasonic impregnation assisted photoreduction strategy (Fig. S1b). Briefly, trace silver and GO were deposited on TiO2 by photoreduction. Firstly, 0.8 g of titanium dioxide nanoparticles were uniformly mixed with 80 ml of an absolute ethanol solution (as an electron donor). Then, adding 0.74 mM AgNO3 solution (precious metal precursor) and ultrasonically dispersing for 30 min under light-shielding conditions (avoid silver ion oxidation) to completely disperse the suspension. Next, a 300 W mercury lamp was used to activate the titanium dioxide in a nitrogen atmosphere about 30 min to obtain an Ag/TiO2 suspension. Thereafter, a desired volume of GO ethanol solution (0.5 mg/ml) was added to the above suspension and ultrasonic dispersion was carried out for another 30 min followed by UV irradiation for 60 min for sufficient photoreduction. Finally, the obtained Ag/TiO2 /rGO composite was washed with ultrapure water and freeze-dried for 32 h to obtain a dry gray-black powder.

2.4. Batch experiments All simulated contaminant solutions were diluted from a 500 mg/L stock solution prior to the experiment. The data presented in this paper are all removal experiments in a single system (contaminants). The experimental results for the mixed system are shown in Fig. S7. The obtained gray-black powder was suspended in a Cr(VI) and TC solution (50 mL, 20 mg/L) by sonication for about 2 min, and then sufficiently stirred under light-shielding conditions for 30 min to carry out equilibrium adsorption. Next, turn on the light source (300 W mercury lamp, <400 nm) and take a sample aliquot at the specified time (0, 2, 5, 10, 20, 30, 40, 50 and 60 min). The suspension was centrifuged and filtered using a 0.25 μm aqueous or organic filter to analyze TC and Cr(VI). The pH was monitored by a pH meter (pHS-320) and the different pH values (3, 5, 7, 9, 11) were adjusted using 1.0 M HCl or NaOH to study the effect of pH. The absorbance of TC and Cr(VI) (Diphenylcarbazide method [29]) were measured at 365 nm and 540 nm using a TU-1901 UV–vis spectrophotometer (Beijing Purkinje General Instrument Co., Ltd, China). All data used for analysis are averaged from three sets of parallel.

Please cite this article as: Y. Hou, S. Pu and Q. Shi et al., Ultrasonic impregnation assisted in-situ photoreduction deposition synthesis of Ag/TiO2/rGO ternary composites with synergistic enhanced photocatalytic activity, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.08.023

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Fig. 1. TEM images of (a) pure GO, (b) TiO2 , and (c) Ag/TiO2 /rGO composite, inset: distribution of AgTiO2 /rGO NPs size (obtained by counting 132 particles); (d) HR-TEM image of Ag/TiO2 /rGO composite.

2.5. Photoelectrochemical measurements All photocurrent and electrochemical measurements were performed using the Gamry 30 0 0 electrochemical workstation. This test uses a standard three-electrode system in which a platinum (Pt) wire is used as a counter electrode and an Ag/AgCl electrode is used as a reference electrode. The catalyst is applied to the FTO glass as a working electrode by a coating method, and the electrolyte used for the electrochemical reaction is Na2 SO4 (0.5 M). A 500 W Xenon lamp equipped with a Newport Solar Simulator AM 1.5 G filter was used as the solar simulator. 2.6. Characterization All the studies were carried out by using sophisticated instruments. The crystallography of Ag/TiO2 /rGO was studied by XRD (Ultima IV, Japan) employing Cu Kα radiation (λ = 0.154 nm, 40 KV) in the range of 5° to 80° The structural morphology was observed by TEM (JEM2100, Japan) operated at 200 kV. XPS measurements were performed by a K-Alpha Probe (Thermo Scientific, USA) with a monochromatic Al-K radiation (300 W). UV–vis spectra study was carried out by using a spectrophotometer (Shanghai United Instrument Co., Ltd, China). Electron paramagnetic resonance spectra (EPR) was performed on an EMX-8 spectrometer (Bruker BioSpin Corp., Germany). Zeta Potential (ZP) of the nanoparticles was determined by Malvern ZETA SIZER Nano series 3600 (Malvern instruments, UK). X-Ray Fluorescence data obtained from DSDP Site 80-550. The BET surface area was measured at 77 K using F-Sorb 2400 N2 adsorption analysis (Gold APP Instruments, China). 3. Results and discussion 3.1. Preparation and characterization of Ag/TiO2 /rGO ternary composites The morphology and structure of Ag/TiO2 /rGO composite were characterized by HR-TEM, as displayed in Fig. 1. The

semi-translucent graphene nanostructures can be observed remaining to its ultrathin sheet like nature (Fig. 1a). Fig. 1b is a TEM image of commercial titanium dioxide (P25), and the average diameter obtained by counting 122 TiO2 particles was about ca. 24.5 nm, as shown in Fig. S2. The TEM images (Fig. 1c) evidently show the sphere-like morphology of the developed product with a diameter about ca. 24±0.3 nm. Fig. 1d presented the HR-TEM observation, revealing that TiO2 nanoparticles formed well with a spacing of 0.35 nm between nearby lattice fringes, which well matches the d-spacing of (111) plane of TiO2 (d = 0.35 nm). The TiO2 precursor crystal structure of the obtained Ag/TiO2 /rGO is well retained due to the mild experimental conditions, showing a similar XRD pattern to that of pure TiO2 (Fig. S3), and the XRD patterns of obtained catalysts with different GO loadings are not significantly different, as shown in Fig. S4. However, no noticeable Ag nanoparticles is observed because of trace silver dosage of 0.679 wt%. (Table S1). The UV–vis diffuse reflectance spectroscopy was used to investigate the optical absorption property of TiO2 , Ag/TiO2 , TiO2 /rGO and Ag/TiO2 /rGO ternary composites respectively and as presented in Fig. 2a. It can be seen that the addition of reduced graphene oxide and silver nanoparticles can extend the absorption range to visible light, and the absorption edge of the all composite material has a clear red shift compared to the bare titanium oxide. This may be due to plasma effect in the metal surface enhancing the light absorption range of the semiconductor photocatalyst, and the incorporation of GO can be used as a photosensitizer for semiconductors, which shifts the Fermi level of the composite in a more favorable direction, thereby enhancing the absorption of visible light by the material. Obviously, a wide strong absorption band appears in the range of 40 0∼60 0 nm of Ag/TiO2 /rGO compared to the spectrum of TiO2 /rGO sample and bare TiO2 (a significant absorption shoulder at 380 nm), which was similar to the previous study [30]. On the other hand, as the amount of addition of graphene oxide increases, the composite exhibits an incessantly enhanced visible light absorption in the range of 40 0–80 0 nm, which is consistent with a color variation from white to black of the material. The color change of the composite material from white to yellow (gray)

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Fig. 2. (a) UV–vis diffuse spectra of TiO2 , Ag/TiO2 , TiO2 /rGO, and Ag/TiO2 /rGO in different doping ratio; (b) curve of [F(R∞)hv]1/2 versus photon energy of pure TiO2 and the prepared Ag/TiO2 , TiO2 /rGO and Ag/TiO2 /rGO composites. Table 1 Relative content of elements in the XPS spectrum of Ag/TiO2 /rGO. Name

Area (N) TPP-2M

Atomic%

Ti 2p O 1s C 1s Ag 3d

0.20 0.58 0.54 0

14.82 39.93 45.12 0.13

and further to black, corresponding to pure TiO2 , Ag/TiO2 , TiO2 /rGO and Ag/TiO2 /rGO, respectively, as shown in the inset of Fig. 2b. The band gap of the obtained photo catalyst could be calculated from the Kubelka-Munk equations [31]:

F (R∞ ) = (1 − R∞ )2 /2R∞ F (R∞ )hv = C1 (hv − Eg )

2

(1) (2)

where R = 10−A and A is the optical adsorption, hv is the photon energy and C1 is the proportionality constant. The relationship between [F(R∞ )hv] 1/2 and photon energy is shown in Fig. 2b, The band gap of pure TiO2 is 3.10 eV, and the forbidden band width of the prepared composite is efficiently reduced to 3.06 eV (TiO2 /rGO), 2.94 eV (Ag/TiO2 ) and 2.75 eV (Ag/TiO2 /rGO), respectively. This change can be explained as follows: The surface functional groups on the GO (for example, -OH, -COOH, etc.) are reduced by photogenerated electrons generated during ultraviolet light irradiation. The π electrons of a carbon atom are not absolutely bonded to another atoms to form a delocalized large bond and some unpaired π electrons associate with free electrons on the surface of TiO2 to form a Ti-O-C structure (Fig. 3b), causing the valence band edge to move upward and reduce the band gap. Thus, a composite material can be thought of as a “ternary” structure created by the beneficial interaction of the three. Ag/TiO2 /rGO form an electronic connection system that effectively enhances light absorption and improves detachment efficiency of electrons and holes. In order to explore possible photocatalytic mechanism, the XPS spectra analysis was studied and the surface elemental and its valences of the resultant photocatalytic materials before and after modification is observed. Fig. 3a represents full XPS spectrum of GO, TiO2 , Ag/TiO2 and Ag/TiO2 /rGO nanopowder in XPS survey spectrum, and the chemical binding energies at 529.98, 458.68, 367.62, and 284.74 eV were assigned to O 1 s, Ti 2p3/2 , Ag 3d5/2 , and C 1 s, respectively (Table 1). It clearly showed that the peak of C 1 s of Ag/TiO2 /rGO, relative to Ti 2p3/2 , is higher than that of pure TiO2 and Ag/TiO2 , which may be due to the increase of C content caused by the addition of GO. Specifically, Fig. 3b highlights three characteristic components of diverse carbon-containing functional groups, the binding energies of 284.74 eV attributed to

sp2 hybridized carbon atoms of graphene; one of the weak peak at 286.49 were allocated to the oxygenated carbon species of C–OH; and the peak at 288.52 eV indicates the presence of C–O bonds, assigned to Ti-O-C, revealing that the C atoms may substituted some of the TiO2 lattice in the preparation process and formed a Ti-O-C structure after the in-situ approach. As shown in Fig. 3c, the Ti-OTi bond and the Ti-OH bond in lattice oxygen were corresponded to the spectrum at 529.98 eV and 531.81 eV, respectively, and the bond of C–OH or C–O–C species (533.15 eV) was mainly because of adsorbed oxygen on the Ag/TiO2 /rGO surface [32]. FT-IR analysis for GO, TiO2 , TiO2 /rGO, and Ag/TiO2 /rGO are illustrated in Fig. S5 to further confirm the decrease of GO to rGO and the interactions between TiO2 and rGO [33,34]. It can be seen from the XPS spectrum of the Ti 2p orbital (Fig. 3d) that the binding energy of pure TiO2 exhibits characteristic peaks of Ti 2p3/2 and Ti 2p1/2 at 458.68 eV and 464.48 eV, proving the existence of Ti4+ . Related with pure TiO2 , the shift of Ti 2p orbital of Ag/TiO2 /rGO at 458.99 eV and 464.76 eV was reduced by about 0.28 eV and 0.33 eV, respectively, due to the effect of Ag element on the charge density around the Ti and O atoms adjacent to the TiO2 surface. The peaks observed at 368.29 and 374.27 eV were attributed to Ag 3d5/2 and Ag 3d3/2 of the metallic silver, respectively (Fig. 3e). The difference of binding energy between Ag 3d3/2 and Ag 3d5/2 is 6.0 eV, which indicated the metallic nature of the silver in the Ag/TiO2 /rGO [35,36]. The XPS data of each element of the obtained ternary composite material prepared is shown in Table S2, wherein the quantized value of Ag is 0.681 wt% (mass ratio of Ag to TiO2 ), which matches well with the ICP-AES data (0.679 wt%). However, the XPS spectrum of Ag undergoes a certain degree of movement after the photocatalytic reaction (Fig. 3e, After). The peaks at Ag 3d5/2 were peaked by the Gauss Lorentz method to obtain two peaks, and the peaks at 367.66 eV and 368.28 eV were designated as Ag2 O and Ag, respectively, indicating the formation of Ag2 O after photocatalytic reaction. This is probably because some of the silver adsorbed on the surface of the titanium dioxide are oxidized to form Ag2O during the photocatalytic reaction. But, the presence of silver oxide does not have significant effect on the photocatalytic performance because of its self-stabilizing process. On the one hand, a very small amount of silver oxide silver oxide can reduce Ag+ to monotonic enthalpy under illumination, providing an effective trap for electrons. On the other hand, the adsorption of oxygen on the silver element will inhibit the reduction of silver oxide [37]. 3.2. Catalytic performance for Cr (VI) and TC removal 3.2.1. Effect of different catalysts The photocatalytic reaction, generally considered to be an interfacial reaction, consists of two processes in which contaminants

Please cite this article as: Y. Hou, S. Pu and Q. Shi et al., Ultrasonic impregnation assisted in-situ photoreduction deposition synthesis of Ag/TiO2/rGO ternary composites with synergistic enhanced photocatalytic activity, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.08.023

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Fig. 3. The survey spectrum (a); C 1 s (b), O1s (c), Ti 2p (d), and Ag 3d (e) XPS spectra of the Ag/TiO2 /rGO composites (3 wt%, 60 min irradiation).

are first adsorbed to the surface of the catalyst and then reacted with photogenerated holes, photogenerated electrons or free radicals (OH· and O2 ·− ) generated by the catalyst. Therefore, the adsorption efficiency of pollutants on the surface of the photocatalyst is critical to the following photocatalytic efficiency. It can be evidently seen from Figs. 4a" and b” that the doping of silver particles and GO greatly improves the adsorption efficiency of the photocatalyst for TC and Cr(VI) in the darkness, which was due to the large specific surface area of GO and the role of unreduced functional groups providing a good premise for the photocatalysis. And then, in terms of photocatalytic removal, the obtained ternary composite Ag/TiO2 /rGO shows excellent photoreduction properties and oxidizing power, seen from Figs. 4a and b. It can be observed that more than 80% of TC and Cr(VI) can be removed in 10 min and 25 min, respectively, and had better efficiency within 60 min (catalytic, Figs. 4a"and b”). Langmuir-Hinshelwood first order kinetic model has been applied to evaluate the reaction kinetics between ternary composite system nanocomposites and contamination solution:

ln

C  t

C0

= −kapt + b

(3)

Where, Ct is the concentration in the solution at t min; C0 is the initial concentration; b represents a constant; kap is the apparent first-order rate constant (min−1 ). All the fitted images are shown in Fig. S6, and the relevant data are shown in Table S2. Compared with pure TiO2 , all modified photocatalytic composites exhibited higher photocatalytic activity for the removal of TC and Cr(VI) (K = 0.0602 min−1 and K = 0.0208 min−1 , respectively), moreover, the ternary composite system showed the highest catalytic activity, which were 2.5 (K = 0.1578 min−1 ) and 4.3 (K = 0.0895 min−1 ) times as much as it, respectively (Figs. 4a and b ). This is attributed to the fact that the loading of silver nanoparticles and GO provides the critical position for the storage and transfer of photogenerated electrons, which means that the recombination probability of photogenerated electrons and holes is reduced to improve the photocatalytic efficiency. It is worth noting that binary composites (Ag/TiO2 and TiO2 /rGO) exhibit different results when removing two simulated contaminants. Specifically, TiO2 /rGO presented better activity than Ag/TiO2 during photocatalytic oxidation of TC, as opposed to photocatalytic reduction of hexavalent chromium. The reason may be that the incomplete reduction of GO in TiO2 /rGO during the preparation process leads to the photogenerated electrons in the catalytic process being used for the reduction of GO to rGO, which

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Fig. 4. Effect of different photocatalytic materials on TC (a) and Cr(VI) (b) removal; The apparent first-order rate constant (k) of as-prepared catalysts in degradation of TC (á) and reduction of Cr(VI) (b́); Contribution of adsorption and photocatalytic removal of TC (a") and Cr(VI) (b"). The experimental conditions were [Cat] = 1 g/L, [Cr(VI)]0 = 20 mg/L, [TC]0 = 20 mg/L, 60 min irradiation.

results in two outcomes: first is to improve the separation of electrons and holes (conducive to photodegradation), and the second is to compete with the main active species (photogenerated electronics, proved in the mechanism part) of hexavalent chromium reduction. For Ag/TiO2 , as an excellent conductor and storage medium, Ag nanoparticles can effectively separate photogenerated electrons from holes and store some electrons for direct reduction of hexavalent chromium. This is also consistent with the photocurrent test results (Fig. 9). While, more active species participate in the process of photodegradation of TC, hence, the ability of photogenerated holes to be directly utilized is lower. 3.2.2. Effect of GO doping ratio Graphene content is the main catalytic factor for obtaining Ag/TiO2/ rGO ternary composites. Figs. 5a" and b" clearly shows that the photocatalytic material has enhanced adsorption capacity for both contaminants with more GO loading, while the photocatalytic activity change is different (Figs. 5a and b). There is no doubt that the Ag/TiO2 /rGO nanocomposites with different doping ratios presented higher photocatalytic activity than bare TiO2 and Ag/TiO2 . The degradation of TC by catalysts with different GO

contents is not significantly improved (the photocatalyst with the best removal efficiency of 5 wt% is only 1.6 times that of Ag/TiO2 ), but the reduction of Cr(VI) by Ag/TiO2 /rGO (3 wt%) is 2.4 times higher than that of the original (Fig. 5b ). The possible cause of this phenomenon is that more GO loading lead to stronger adsorption of Cr(VI), and the Cr(III) produced by reduction is also adsorbed on the surface of the material, which will enhance the shielding effect of light and reduce the utilization of light. In addition, since the active site is occupied, the unreduced Cr(VI) is difficult to contact the surface of the catalyst, thus it is difficult to participate in the interfacial reaction. In summary, an appropriate amount of GO loading enhances the adsorption activity while increasing the activity of the photocatalytic reaction as an excellent conductor of photogenerated electrons. 3.2.3. Effect of initial pH The initial pH of the solution not only affects surface adsorption, but also affects the reaction activity of oxygen-containing free radicals and the redox potential of the catalyst or metal conduction band and valence band, which also plays an important role in the entire photocatalytic process. The effects of different pH values

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Fig. 5. Effect of different GO loading on TC (a) and Cr(VI) (b) removal; The apparent first-order rate constant (k) of as-prepared catalysts in degradation of TC (á) and reduction of Cr(VI) (b́); Contribution of adsorption and photocatalytic removal of TC (a") and Cr(VI) (b"). [Cat] = 1 g/L, [Cr(VI)]0 = 20 mg/L, [TC]0 = 20 mg/L, 60 min irradiation.

on the adsorption efficiency and photodegradation efficiency of TC are presented in Fig. 6a-a". It is observed that the degradation efficiency of tetracycline can reach over 97% within 40 min at pH = 5, 7, and 9, but it is slightly lower at the pH is 3 and 11. This is mainly because the pH of the solution will change the surface charge of the catalyst and the protonation state of the organic matter [38]. On the one hand, TiO2 is an amphoteric oxide, and the pH of the solution will affect the charge properties and density of the surface of the TiO2 particles (Fig. S8) [39]. On the other hand, tetracycline is also an amphoteric compound containing an acidic phenolic hydroxyl group, an enol hydroxyl group and a basic dimethylamino group, and has three pKa values (3.3, 7.7, 9.7) [40]. When the solution is lower (pH < 3), the main form of the charged tetracycline (TCH3 + ) and the surface of Ag/TiO2 /rGO are positively repulsive, resulting in a decrease in adsorption efficiency; Similarly, when the pH of the solution is higher than 9, the surface of the TiO2 becomes negatively charged and the TC dissociated anions (TCH− and TC2− ) repel each other to reduce the catalytic removal efficiency [41]; And, the appropriate H+ concentration in the weakly acidic solution can promote the photocatalyst to produce sufficiently strong oxidizing OH. The stability of the free radical is better, and the oxidation performance can be fully exerted. Under

acidic and basic conditions, free radicals are easily decomposed, which is not conducive to the photocatalytic reaction [42]. This indicates that the pH adaptation range of tetracycline degradation by this catalyst is weak acid, weak base and neutral and strong acid, strong base will inhibit the degradation efficiency of tetracycline. The effect of pH on Cr(VI) reduction and removal efficiency pH value from 3 to 11 is presented in Fig. 6b-b". First, the adsorption process of Cr(VI) is a primary step for photocatalytic reduction, and the effect of initial pH on adsorption and the removal rate produced are shown in Fig. 6b". It can be seen that the adsorption of Cr(VI) at the catalyst interface increases rapidly as the pH decreases, according to the results of the study, the maximum adsorption rate of 31.6% (pH = 3), which was 13 higher than the lowest 2.3% (pH = 11). Furthermore, the zeta potential, pHpzc (point charge with a pH of zero) of Ag/TiO2 /rGO was found to be in between 5–6. Therefore, when the pH < 5, the positive charge on the surface of the catalyst attracts the negatively charged Cr2 O7 2− through electrostatic attraction. Conversely, when the initial pH higher than 5, the surface of the catalyst behaves negatively charged and deprotonation takes place resulting in a decrease of Cr(VI) in the adsorption capacity [43].

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Fig. 6. Effect of different pH on TC (a) and Cr(VI) (b) removal; The apparent first-order rate constant (k) of as-prepared catalysts in degradation of TC (á) and reduction of Cr(VI) (b́); Contribution of adsorption and photocatalytic removal of TC (a") and Cr(VI) (b"). [Cat] = 1 g/L, [Cr(VI)]0 = 20 mg/L, [TC]0 = 20 mg/L, 60 min irradiation.

The metal interface is important for the redox potential, which determines the oxidation or reduction ability of the metal ion. As shown in the Fig. 6á, the reduction rate of Cr(VI) decreases slowly with the increase of pH. In addition, the reaction can reach equilibrium within 40 min at pH=3, but Cr(VI) has no obvious reduction at pH 11, indicating the oxidation of Cr(VI). The ability is strongly influenced by the H+ concentration. It is also observed that when the initial pH is high, the redox potential of Cr(VI) is lowered, resulting in a decrease in its oxidizing ability (Eqs. (4)–(6)). Therefore, the reduction of Cr(VI) should be easier to carry out under acidic conditions apart from neutral or alkaline conditions. Equation of reduction mechanism of Cr(VI) in acidic conditions:

Cr2 O27− + 14H + + 6e− → 2Cr 3+ + 7H2 O

(4)

E = 0.98 eV (SHE ) 0

Equation of reduction reaction mechanism of Cr(VI) in neutral conditions:

C r2 O27− + 8H + + 3e− → 2C r 3+ + 4H2 O

(5)

E 0 = 0.56 eV (SHE ) Equation of reduction mechanism of Cr(VI) in alkaline conditions:

CrO24− + 4H2 O + 3e− → Cr (OH )3 + 5OH −

(6)

E 0 = 0.24 eV (SHE ) Some reports indicated that the relationship between the oxidation–reduction potential of the catalyst conduction band and the valence band and pH is as follows [44]:

ECB (V ) = −0.050 − 0.059 pH (25 ◦C )

(7)

EV B (V ) = 3.159 − 0.059 pH (25 ◦C )

(8)

It can be obtained that the potential of CB and the VB decreases by 59 mV for each unit increase of the pH of the solution. The larger the VB and CB reduction potential of TiO2 , the stronger the reduction ability of photogenerated electrons. In addition, the difference between the conduction band of TiO2 and the redox point

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Fig. 7. (a) The reusability of ternary composite Ag/TiO2 /rGO by 5 recycle on TC (a) and Cr (VI) (b). [Cr(VI)]0 = 20 mg/L, [TC]0 = 20 mg/L, [Cat]=1 g/L.

Fig. 8. EPR spectra of radical adducts: (a) DMPO–O2 ·− and (b) DMPO–·OH; Temporal evolution curves of the DMPO–O2 ·− adduct (c) and DMPO−·OH adduct (d) obtained from the second peak intensity. Effect of different quenchers on the Ag/TiO2 /rGO photocatalytic removal rate of TC (e) and Cr(VI) (f).

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of heavy metal ions can accelerate the photocatalytic reduction of heavy metal ions. 3.2.4. Reusability of the Ag/TiO2 /rGO catalysts The regeneration and reusability were the key properties in evaluating the potential application of material. The reusable properties of the ternary composite Ag/TiO2 /rGO are shown in Fig. 7. Obviously, with the recycling process, the removal effect of both kinds of pollution decreased to some extent, and the reduction of Cr(VI) was more obvious. The reason for this difference may because that the small molecules generated during the degradation of tetracycline do not easily adhere to the surface of the catalyst, and the Cr(III) produced by the reduction of Cr(VI) can easily adsorb to the active sites [45], resulting in the remaining Cr(VI) not reaching the reaction interface. Moreover, the invalid adsorption also reduces the efficiency of light utilization, which greatly reduces the catalytic performance. In this experiment, after 5 cycles, the reduction efficiency is still more than 80%, indicating that the catalyst presented greater regeneration capability and stability towards aqueous phase and account as an excellent catalyst for contaminant removal from water. 3.3. Photocatalytic mechanism The EPR spectra of O2 ·− and ·OH were captured by DMPO in methanol dispersion and aqueous medium system to study the active radical species of different reaction systems (Reaction conditions:catalysts = 1 mg/mL, [DMPO] = 100 mM), shown in Fig. 8. Six characteristic peaks of O2 ·− adducts (Fig. 8a) and four characteristic peaks (1:2:2:1) of ·OH adducts (Fig. 8b) were detected on three different reaction systems under UV light irradiation. To evaluate the relative cumulative amount of oxygenated free radicals further, the time evolution of the EPR signal (intensity of the second peak) is compared under the same conditions as in Fig. 8c and 8d (EPR spectra strength of O2 ·− and ·OH of the as obtained TiO2 /rGO, Ag/TiO2 /rGO and pure TiO2 powder under different illumination times was shown in Fig. S9 and S10). The results show that the rate of two free radical formation increases in the order of Ag/TiO2 /rGO > TiO2 /rGO > TiO2 , and the trend is in good agreement with the experimental results (Fig. 4). This in turn explains reasonably why a higher photocatalytic activity of Ag/TiO2 /rGO than TiO2 /rGO and TiO2 toward the removal of contaminants. The free radical trap experiments were performed to evaluate the contribution of each free radical and photogenerated holes and electrons to contaminant removal. EDTA-2Na is an inhibitor of photogenerated h+ ; Isopropanol and p-benzoquinone are often used to identify the role of ·OH and O2 ·− in oxidation reactions, respectively [46]. Therefore, EDTA-2Na, isopropanol and p-benzoquinone were used to demonstrate whether h+ , ·OH and O2 ·− are involved in photo degradation of Cr(VI) and tetracycline. From Fig. 8e, the order of inhibition of photodegradation of tetracycline by several quenchers is: p-benzoquinone > EDTA-2Na > isopropanol, that is, the order of efficiency of each radical in the reaction process is: O2 ·− >h+ >OH·. In general, dissolved oxygen is an essential factor in photodegradation reaction, mainly because dissolved oxygen can capture conduction band electrons, thereby inhibiting photoelectron-hole pair recombination, prolonging the lifetime of holes, increasing the amount of active free radicals such as OH· [47]. After the nitrogen gas was introduced into the reaction solution, the degradation efficiency of TC was suppressed by about 30%, indicating that adsorption of oxygen plays a significant role in this study. It is also noticed that when p-benzoquinone O2 ·− is added to the reaction solution, the degradation efficiency of TC was suppressed up to 90%, which is much higher than the inhibition rate after N2 . First, the reaction solution is provided with N2 , and it does not completely remove the dissolved oxygen in the

Fig. 9. Photocurrent responses of TiO2 , TiO2 /rGO, Ag/TiO2 and Ag/TiO2 /rGO.

solution, so that O2 ·− is still produced and contributes to the reaction; secondly, p-benzoquinone is used as a quencher for O2 ·− , stopping the annihilation of O2 ·− produced in the aqueous solution accelerating the synthesis rate of O2 ·− , which is superior to the production rate of other active free radicals, and becomes the main free radical of TC degradation. As a result, the addition of pbenzoquinone the degradation efficiency of TC is greatly reduced. It can be seen from Fig. 8f that the removal rate of Cr(VI) is obviously increased after adding oxidizing radical (h+ , OH·) trapping agent, indicating that the addition of the trapping agent can effectively separate the electrons from the holes and improve the reduction efficiency. It is generally believed that superoxide radicals (O2 ·− ) as an amphoteric substance have both strong oxidative and reductive properties. Some studies have suggested that in the presence of oxygen, the adsorbed oxygen competes with Cr(VI) for electrons to produce O2 ·− , which reduces the efficient use of Cr(VI) for electrons [48]; However, some researchers have reported that O2 ·− can also be used as an intermediate transfer of electrons to supply electrons to Cr(VI) by the ability to lose electrons [49]. In this experiment, under the condition of adding superoxide radical scavenger to isopropanol, it was found that the removal efficiency of Cr(VI) was significantly increased, indicating that the production of O2 ·− reduced the efficient use of Cr(VI) for electrons. However, in the case of N2 (Oxygen deficiency), the experimental results show that O2 has no obvious inhibitory effect on the reduction of Cr(VI). This may be due to the fact that the introduction of nitrogen can only remove dissolved oxygen in the solution as much as possible, but the adsorbed oxygen on the surface of the material is difficult to remove by the form of exhaust gas. The adsorption of oxygen on the surface of the material is the key to the formation of superoxide radicals [50]. In order to understand the photoelectrochemical properties of the samples further, the electrochemical properties of the composites and pure TiO2 were determined using a Gamry 30 0 0 electrochemical workstation, and the test results are shown in Fig. 9. Before and after illumination, the photocurrent of the sample is significantly different. In the dark, the photo response current is in a dark current state, and when there is a light source, the system detects a significant current change. It can be observed that different composite materials as well as pure TiO2 powders exhibit stable photocurrent under illumination, indicating that each electrode has an electrophotocatalytic process. Specifically, the photocurrent intensity of the ternary composite Ag/TiO2 /rGO is higher than that of the binary composite (Ag/TiO2 > TiO2 /rGO),

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Fig. 10. The proposed photocatalytic mechanism foe photodegradation of TC and photoreduction of Cr(VI) over Ag/TiO2 /rGO ternary electronic connection system.

which indicates that the effective transfer of electrons successfully inhibits the ineffective composite of photogenerated electrons and photogenerated holes with the addition of Ag and rGO. Based on the results above, a schematic photocatalytic reaction mechanism was shown in Fig. 10. It was confirmed that under light irradiation, photogenerated electrons were gradually stored or transferred from the excited TiO2 through Ag nanoparticles and GO (like a high speed electronic connection system) to the surface-adsorbed species (O2 /Cr(VI)) and a series of reactions were occurred (O2 + e− → O2 ·− , O2 + 2e− + 2H+ → H2 O2 , Cr(VI) + e− → Cr(III)). The photogenerated holes that are effectively separated from them will also react more positively (h+ + Organics → CO2 + H2 O, h+ + H2 O → O2 ·− ). Subsequently, the generated O2 ·− will be used for the degradation reaction of organic matter. This entire process allows the generated photogenerated holes and electrons to undergo exclusive catalytic reactions at different locations. Efficient electron consumption not only inhibits charge recombination to improve interface charge transfer, however, also improves surface catalytic efficiency by providing a reaction center.

tivity. The graphene-based ternary photocatalyst obtained in this work is found to be superior and can be used in large scale water treatment efficiently and economically. Declaration of Competing Interest The authors declare that there is no conflict of interests regarding the publication of this paper. Acknowledgments The authors thank Prof. Anatoly ZINCHENKO and Dr.Sangeeta ADHIKARI for a helpful discussion. This work was supported by the National Natural Science Foundation of China (41772264), the Applied Basic Research Programs of Science and Technology Foundation of Sichuan Province (18YYJC1745) and the Research Fund of State Key Laboratory of Geohazard Prevention and Geoenvironment Protection (SKLGP2018Z001). Supplementary material

4. Conclusion A graphene-based ternary composite catalyst was prepared by a green in-situ photoreduction strategy. The effects of different rGO loading, different reaction systems, and different pH conditions on Cr(VI) reduction and TC degradation were calculated. The photocatalytic activity of graphene-based ternary composite catalyst has been significantly improved compared to commercial available P25, and the removal efficiency is mainly affected by pH value, while the effect of rGO is not obvious. The removal efficiency of Cr(VI) was significantly increased by the addition of oxidizing free radical (h+ , OH·, O2 ·− ) trapping agent, indicating that the addition of the trapping agent can efficiently isolate the electrons from the holes and improve the reduction efficiency. The order of inhibition of tetracycline photodegradation by several quenchers is: p-benzoquinone> EDTA-2Na > isopropanol, the order of efficiency of each free radical in the reaction process is: O2 ·− >h+ >OH·. The electronic coupling in the fabricated system has an effective enhanced light absorption and a higher e− /h+ separation efficiency, therefore presenting excellent photocatalytic ac-

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