Allylic selective oxidation of tert-butyl alcohol to methacrolein: Cooperative catalysis of two different active sites

Subscriber access provided by UNIV OF GEORGIA

Energy, Environmental, and Catalysis Applications

Enhanced Photoelectrocatalytic Reduction and Removal of Atrazine: Effect of Co-Catalyst and Cathode Potential Haoying Wang, Jie Li, Huijie Shi, Siqi Xie, Chao-Jie Zhang, and Guohua Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b12139 • Publication Date (Web): 25 Sep 2019 Downloaded from pubs.acs.org on October 5, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Enhanced

Photoelectrocatalytic

Reduction

and

Removal of Atrazine: Effect of Co-Catalyst and Cathode Potential Haoying Wang†, Jie Li†, Huijie Shi*†, Siqi Xie†, Chaojie Zhang‡ and Guohua Zhao*†

†School

of Chemical Science and Engineering, and Shanghai Key Lab of Chemical

Assessment and Sustainability, Tongji University, 1239 Siping Road, Shanghai 200092, China ‡State

Key Laboratory of Pollution Control and Resources Reuse, College of

Environmental Science and Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Photoelectrocatalytic (PEC) reduction and removal of atrazine, one typical endocrine disruptor chemical, was achieved on Pd quantum dots modified TiO2 nanotubes (PdQDs@TiO2NTs) under regulating potentials. Compared with that on TiO2NTs, the PEC reduction efficiency of atrazine on PdQDs@TiO2NTs significantly increased, mainly attributed to the reduced electron transfer resistance, longer lifetime of the photo-generated electrons and the faster electron injection from the catalyst to atrazine in the solution. Meanwhile, PdQDs could also function as co-catalyst so that the electrocatalytic activity of PdQDs@TiO2NTs was evidently improved. Moreover, the investigation indicated that the applied potential not only played important role in accelerating the separation of photo-generated electrons and holes, but also with the increment of the cathodic potential, the PEC reduction mechanism of atrazine underwent the variation of electro-assisted photo-catalysis, synergetic photo-electrocatalysis, and photo-assisted electro-catalysis. A highest atrazine PEC reduction efficiency was achieved as 99.5% on PdQDs@TiO2NTs in about 5 hours under the potential of -1.3 V vs. SCE, whereas the highest synergetic effect of photo- and electrocatalysis was achieved at a lower potential of -0.9 V vs. SCE. KEYWORDS: photoelectrocatalytic reduction, atrazine, co-catalyst, negative potential, PdQDs@TiO2NTs

ACS Paragon Plus Environment

Page 2 of 41

Page 3 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

INTRODUCTION Atrazine, a commonly used herbicide in agriculture, has attracted much attention in the field of environmental pollution control because of its widely distribution in the polluted waters and threat to the environment and public health1. It is a possible carcinogen and endocrine disrupting chemical identified by the U.S. EPA2. Although the concentration of atrazine in water is very low, once it enters into the living organisms, it will be bio-accumulated and pass from one to another through the food chain, which may cause cancer or problems to the reproductive system, endocrine system and so on3. Despite the self-purification ability of waters can alleviate environmental pollution problems to some degree, it will still exceed the selfpurification capacity with the continuous accumulation of atrazine. Moreover, the stable chemical structure of atrazine leads to its long half-life and low biodegradability4. As a result, it is particularly necessary to effectively remove atrazine from waters. So far, there have been many techniques developed for treating polluted waters containing atrazine, including adsorption5, microbial degradation6, electro-catalysis7, photo-catalysis8, and photo-electro-catalytic (PEC) oxidation method9 etc. Among them, PEC technology has been given much attention owing to its high efficiency compared to the photo-catalysis, environmental friendliness and utilization of solar energy10. Especially the PEC oxidation method has been extensively investigated in the treatment of atrazine9, alky phenols11, bisphenol A12 and etc. It is noted that in the PEC system, upon the radiation of light, both holes with oxidizing activity and electrons with reductive activity are generated. However, most published research works discussed

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the utilization of the holes and hydroxyl radicals, produced by the reaction of holes with water molecules, for the oxidation and degradation of the environmental pollutants. Few works pay attention to the use of photo-generated electrons for reduction, especially for treating organic pollutants. Considering the chemical structure of atrazine, the substituted chlorine atom makes the triazine-ring electron deficiency attributed to the electron-withdrawing effect, so that the hydroxyl radicals always attack only the side chains of atrazine to generate de-alkylation products13. It is speculated that the electron-deficient chemical structure of atrazine may prefer to react with the photogenerated electrons and to be reduced14. As a result, we propose that a negative potential is applied on the working electrode in the PEC system, and atrazine would be reduced and removed by the photo-generated electrons with high efficiency on the cathode. To obtain the high PEC reduction efficiency of atrazine, it is highly desirable to design and fabricate an electrode with good photoelectrocatalytic properties and reductive activity. TiO2 nanotubes (TiO2NTs) prepared by three-step anodization15 on metal titanium substrate was a kind of well reported electrode with good photocatalytic properties. However, on account of its poor conductivity and electro-catalytic performance, modification of suitable co-catalyst with good electro-catalytic activity is necessary. Numerous reports have shown that electro-catalytic de-chlorination of organic halides become easier in the presence of co-catalysts such as Pd, Ru, and Ni16, which were endowed with excellent electro-catalytic activity. Among them, Pd exhibits superior property for the adsorption and storage of H atom, which can promote the cleavage of the C-Cl bond17, 18. By the modification of Pd on TiO2NTs as co-catalyst,

ACS Paragon Plus Environment

Page 4 of 41

Page 5 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

not only the electro-catalytic activity of TiO2NTs will be improved by introduction of catalytic reaction sites which facilitate the subsequent chemical reactions19, 20, but also Schottky junction will be formed between Pd and TiO2NTs for accelerated charge transfer21, 22. Upon light irradiation, the photo-electron on the conduction band of TiO2 will transfer to Pd. The Fermi energy level of Pd then moves to a more negative potential, which leads to the secondary electron transfer from Pd to the electron acceptor in the electrolyte solution23, 24, such as atrazine in this work. However, in most cases, Pd nanoparticles modified on TiO2NTs are larger and not evenly distributed. It leads to the insufficient contact between Pd nanoparticles and TiO2NTs, which can impede the efficient separation of photo-generated electrons and holes25, 26. Based on this consideration, Pd quantum dots (PdQDs) with small size effect and high electrocatalytic property are selected here for modification of TiO2NTs to construct PdQDs@TiO2NTs electrode with high PEC reduction activity. Herein, PdQDs@TiO2NTs electrode was fabricated by a three-step anodization method followed by growth of PdQDs via hydrothermal method. PEC reduction and removal of atrazine was conducted on PdQDs@TiO2NTs under negative potential and the performance was compared with that on TiO2NTs. The detailed functions of PdQDs in improving the atrazine removal was investigated via various PEC methods, timeresolved fluorescence emission decay spectra and open circuit photo-voltage decay (OCPD) to elucidate the improved electro-catalytic performance, the charge carrier dynamics and enhanced charge transfer across the interface and etc. Then another factor, the applied potential, influencing the atrazine reduction efficiency was investigated, and

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

different reaction mechanism was proposed for PEC atrazine reduction under different applied potential. This work provided fundamental experimental data for applying PEC reduction technique in removing organic environmental pollutants in the waters. EXPERIMENTAL SECTION Materials and Apparatus. Ti plate was provided by the Sinopharm chemical reagent co. LTD. Atrazine, sodium iodide and palladium chloride were all obtained from Aladdin Industrial Corporation. Polyvinylpyrrolidone (PVP) was purchased from TCI Shanghai. The morphologies and elemental distribution on the surface of PdQDs@TiO2NTs electrode was characterized by a Hitachi S-4800 field emission scanning electron microscope (FE-SEM, Japan) equipped with an X-ray energy dispersive spectrometer. Detailed microscopic structure of the samples were recorded on a JEM2100 transmission electron microscope (TEM, Japan). The crystal structure of the electrodes was monitored by a Bruker D8 advance X-ray diffractometer (XRD, Germany). The elements and oxidation states of the electrode was characterized by an AXIS Ultra DLD X-ray photoelectron spectrometer (XPS, British) using focused monochromatic Al Kα X-ray radiation. The fluorescence lifetime was determined on the Fluorolog-3-11 transient fluorescence spectrometer (Horiba Scientific, USA). The excitation wavelength was set as 340 nm, and the decay curve was monitored at the wavelength of 390 nm. The UV–vis diffuse reflectance spectra (UV-vis DRS) was obtained by Agilent Carry 5000 UV/VIS/NIR spectrophotometer (USA). Preparation of PdQDs@TiO2NTs. Highly ordered TiO2NTs were prepared by a

ACS Paragon Plus Environment

Page 6 of 41

Page 7 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

three-step electrochemical anodization method27. Subsequently, PdQDs growth on the TiO2NTs was conducted by hydrothermal method28. Typically, PVP (800 mg), PdCl2 (8 mg) and NaI (300 mg) were dissolved in 13 mL deionized water by ultrasonic treatment for 30 min. Then the mixture was transferred into a Teflon-sealed autocalave with the volume of 25 mL, where TiO2NTs electrode was placed against the wall at an angle with the TiO2NTs-grown side facing down. After reaction at 200 C for 1.5 h, the autocalave was cooled to the ambient temperature, and the electrode was taken out and rinsed with deionized water repeatedly to produce the PdQDs@TiO2NTs electrode. Electrochemical Measurements. The PEC experiments were carried out on the CHI6043E electrochemical workstation. TiO2NTs or PdQDs@TiO2 NTs was used as the working electrode, a saturated calomel electrode (SCE) was employed as the reference electrode, and platinum electrode was used as the counter electrode. The working area of the working electrodes was confined as 0.25 cm2. The excitation light source was provided by a LA-410UV UV lamp (HAYASHI, Japan) with a light power of 20 mWcm-2. During the experiments, the entire device was fixed so that the position of the electrode immersed into the electrolyte remained unchanged and the light intensity was constant. The supporting electrolyte was 0.1 molL-1 Na2SO4 solution, in which nitrogen was pumped before characterization to avoid the interference of oxygen. PEC Reduction of Atrazine in Water. PEC reduction and removal of atrazine was performed using PdQDs@TiO2NTs or TiO2NTs as working cathode in 40 mL aqueous solution containing 2 ppm atrazine and 0.1 molL-1 Na2SO4. The working area of the electrodes were controlled as about 4 cm2. Pt sheet was used as the anode, and

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

SCE was used as the reference electrode. The experiments were carried out in a singlecompartment vessel, and the temperature (such as 25 ± 2 C) was kept constant with cooling water. The light irradiation was provided by a 300 W xenon lamp with the wavelength of 320-800 nm, and the light intensity was about 100 mWcm-2. To investigate the influence of applied negative potential on atrazine reduction efficiency, the potential was set as -0.5, -0.9, -1.3 and -1.5 V vs. SCE respectively. The changes of atrazine concentration was monitored by high-pressure liquid chromatography (HPLC), and the separation of the intermediates were performed on an Agilent TC-C18 column (150 mm4.6 mm, 5 m). Acetonitrile and water were used as mobile phase with the volume ratio of 60/40, and the flow rate was set as 1 mLmin-1. The detection wavelength was 220 nm. RESULTS AND DISCUSSION Characterization of PdQDs@TiO2 NTs Electrode. The typical SEM images of TiO2NTs and PdQDs@TiO2NTs were shown in Figure 1. Ordered, densely packed, and one-dimensional nanotube arrays were uniformly grown on Ti substrate, on the top of which was a periodically porous layer. As shown, the surface of the nanotubes was very smooth. The average tube diameter was about 40 nm, the tube length was approximately 0.55 µm, and the wall thickness was nearly 24 nm (Figure 1B). After hydrothermal reaction, PdQDs of about 4.0-8.0 nm were homogeneously grown and distributed on the surface of the nanotubes (Figure 1 and S1). The HR-TEM images shown in Figure S2 further supported the results. The lattice fringes with spacing of 0.35 nm and 0.22 nm were ascribed to the (101) and (111) planes of anatase TiO2 and metallic Pd

ACS Paragon Plus Environment

Page 8 of 41

Page 9 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

respectively.27 The results showed that the integrality of the nano-tubular structure of TiO2NTs was well-retained and not influenced by the following hydrothermal treatment. The uniform distribution of PdQDs on the electrode surface was further confirmed by the EDS analysis, suggesting that the deposited amount of Pd was about 2.3 wt % (Figure S3). The elemental composition and the oxidation state of PdQDs@TiO2NTs was also analyzed by XPS method. It showed that the main elements on PdQDs@TiO2NTs were Ti, O and Pd (Figure 2A). From the XPS Ti 2p spectrum in Figure 2B, we could find the doublet peaks at 464.2 eV and 458.4 eV, which could be ascribed to Ti 2p1/2 and Ti 2p3/2, indicating the presence of Ti4+ on the electrode surface. It was basically the same as that of TiO2NTs, indicating that there was no specific interaction between PdQDs and TiO2NTs. There were also two peaks of Pd 3d5/2 and Pd 3d3/2 at 334.6 and 339.9 eV in the XPS Pd 3d spectrum shown in Figure2C. The peak interval of 5.3 eV together with the peak intensity ratio (3/2) of Pd 3d5/2 to 3d3/2 indicated that Pd0 was the main chemical state of Pd on the surface of PdQDs@TiO2NTs. In addition, the XRD patterns of PdQDs@TiO2NTs (Figure 2D) were also collected. Both rutile and anatase phase of TiO2 were present in PdQDs@TiO2NTs, which was beneficial for the charge separation in PEC process by forming homo-junction29. The diffraction peak at about 47.95° could be ascribed to the (200) plane of metallic Pd, further confirming the successful loading of PdQDs on TiO2NTs surface30. The optical absorption properties of PdQDs@TiO2NTs was also investigated and compared with that of TiO2NTs. As shown in Figure S4, enhanced absorption in the

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

visible light region was observed for PdQDs@TiO2NTs, owing to the surface plasma resonance effect of PdQDs. The band gap of 2.81 eV was calculated based on the Tauc plots for PdQDs@TiO2NTs31, 32, which was narrower than that (3.20 eV) of TiO2NTs. It was beneficial for harvesting more light irradiation, leading to improved PEC performance. Enhanced PEC Reduction of Atrazine on PdQDs@TiO2NTs. PEC reduction and removal of atrazine was then performed on PdQDs@TiO2NTs, and the performance was compared with that on TiO2NTs. As shown in Figure 3A and 3B, the PEC removal of atrazine was achieved as 99.5 % on PdQDs@TiO2NTs cathode under the potential of -1.3 V with a reaction rate constant of 1.036 h-1, whereas those on TiO2NTs were about 73.1 % and 0.265 h-1. It indicated that by modification of PdQDs, not only the PEC removal rate of atrazine was improved, but also the reaction rate constant was significantly enlarged to be about 3.9 times. It was presumed that except for the enhanced visible light absorption (Figure S4), the excellent electro-catalytic activity PdQDs@TiO2NTs compared with TiO2NTs could be one reason for the enhanced PEC performance in atrazine reduction, owing to the modification of PdQDs as co-catalyst. So the electrochemical reduction and removal of atrazine on PdQDs@TiO2NTs was investigated, and compared with that on TiO2NTs. As indicated in Figure 3C and 3D, without light irradiation, the electrochemical reduction efficiency of atrazine on PdQDs@TiO2NTs was approximately 96.0 % with a reaction rate constant of 0.604 h-1, which were about 1.63 and 3.56 times higher than that on TiO2NTs. It confirmed that by modification of

ACS Paragon Plus Environment

Page 10 of 41

Page 11 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

PdQDs, the electro-catalytic activity of TiO2NTs was significantly improved. Meanwhile, it could be observed that although the PEC removal rate of atrazine on PdQDs@TiO2NTs was only slightly increased (from 96.0 % to 99.5 %), the reaction rate constant increased for about 72.4 %. Meanwhile, it was known that the separation efficiency of photo-generated holes and electrons was very important in determining the PEC activity. Electrochemical impedance spectroscopy (EIS) is a commonly used technique for investigating the properties of charge separation and transfer at the electrode-electrolyte interface33. Figure S5A showed the Nyquist plots of TiO2NTs and PdQDs@TiO2NTs electrode before and after illuminating. After PdQDs modification, the diameter of the semicircle sharply decreased, suggesting that the Schottky junction formed between

PdQDs and

TiO2NTs could significantly facilitate the charge transfer directionally by reducing the charge carrier recombination and decreasing the electron transfer resistance of TiO2NTs34. Meanwhile, the charge carrier density (ND) was determined from the slope of the Mott-Schottky plots as shown in Figure S5B. PdQDs@TiO2NTs exhibited the lower slope and the higher carrier density of 3.12×1017 cm-3, which was about 3 times that

of

TiO2NTs,

indicating

the

improved

electrical

conductivity

of

[email protected] The flat band potentials (EFB) were also obtained to be -0.23 V and -0.085 V for TiO2NTs and PdQDs@TiO2NTs, respectively. Obviously, compared with TiO2NTs electrode, a positive shift of EFB in PdQDs@TiO2NTs exhibited a decrease in bending of the band edge, and accordingly facilitated the separation and transfer of photo-generated electrons36, leading to the enhanced PEC performance.

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 41

Furthermore, time-resolved fluorescence emission decay spectra were used for monitoring the electron transfer dynamics in PdQDs@TiO2NTs and TiO2NTs. The results shown in Figure 4 indicated that the fluorescence decays were well fitted with a three-exponential decay model37, and the lifetimes of the charge carriers were listed in Table 1. The significantly increased carriers' lifetimes on PdQDs@TiO2NTs further confirmed that the modification of PdQDs on the surface of TiO2NTs could effectively reduce the recombination of photo-generated holes and electrons, which would play significant role in improving the PEC performance. The enhanced lifetimes of the photo-generated electrons was further investigated by measuring the accumulated electron lifetime, which was analyzed by the OCPD technique38, 39. It could be calculated by the following equation 40: 𝜏=

𝑘𝐵𝑇 𝑒

𝑑Voc ―1

∙ ( 𝑑𝑡 )

Where, 𝜏 was the lifetime of photo-generated electrons dependent on the electrical potential, kB was the boltzmann's constant, T was the temperature, e was the charge of a single electron, and Voc was the open circuit potential at time t. It could be found that PdQDs@TiO2NTs exhibited longer accumulated photoelectron lifetime than TiO2NTs and slow charge recombination, owing to capacity of PdQDs in storing photo-generated electrons (Figure S5C and S5D)41,

42.

It was

beneficial for improving the reduction activity of the electrons accumulated on PdQDs, which then would transfer to atrazine in the electrolyte for reduction reaction. To investigate the electron transfer efficiency from the catalyst to atrazine across the interface, the lifetime of accumulated electrons on PdQDs@TiO2NTs and TiO2NTs in

ACS Paragon Plus Environment

Page 13 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

the presence of atrazine were further determined by the OCPD technique, and the results were shown in Figure 5. The data of the accumulated photoelectron lifetime were summarized in Table 2. As shown, under the potential of -0.6 V, the accumulated photoelectron lifetime on PdQDs@TiO2NTs was calculated to be about 25.6 s in the absence of atrazine, which was about 3 times that on TiO2NTs. In the present of atrazine, the accumulated photoelectron lifetime on PdQDs@TiO2NTs decreased significantly to 1.2 s, about 95.3 % decrement, indicating the fast charge transfer from PdQDs@TiO2NTs to atrazine across the interface41. However, the lifetime decrement on TiO2NTs in the present of atrazine was about 49.4 %, much smaller than that on PdQDs@TiO2NTs.

The

efficient

surface

charge

transfer

efficiency

of

PdQDs@TiO2NTs was also confirmed by measuring the photocurrent responses using H2O2 as electron scavenger, which could increase the surface charge transfer efficiency to almost 100% (Text S3)43, 44 . As shown in Figure 5C and 5D, by comparison of the photocurrent response in the absence and presence of H2O2, the surface charge transfer efficiency of PdQDs@TiO2NTs was calculated to be 45.5%, which was much higher than that (35.7%) of TiO2NTs. All the above results confirmed the faster electron transfer across the interface, i.e. the faster electron injection from the catalyst into the electron acceptors (atrazine or H2O2 in this work) in the PEC process, resulted in the enhanced PEC removal efficiency of atrazine on PdQDs@TiO2NTs compared with that on TiO2NTs. Based on the discussion and reported literatures27, 45, a possible charge carriers transfer pathway was proposed. In the PEC reduction process, a negative potential was

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

applied to PdQDs@TiO2NTs. Under light irradiation, electrons were readily excited to the conduction band of TiO2NTs to produce the photo-generated electrons, leaving the holes on the valence band. The electrons from the external circuit could effectively suppress the recombination of the photo-generated electrons and holes by scavenging the holes on the valence band. Schottky junction formed between PdQDs and TiO2NTs was also beneficial for the fast separation of the photo-generated electrons and holes. Because that the electron affinity of TiO2 NTs was about 4.2 eV46, which was lower than the work function of PdQDs (about 5.12 eV)47, the photo-generated electrons would be injected and stored into the Femi level of PdQDs, which would further transfer to atrazine in the solution to make the reduction reaction occur. PEC Reduction of Atrazine under Regulating Potentials. Except for the modification of co-catalyst, the applied potential was another factor influencing the PEC removal efficiency of atrazine on PdQDs@TiO2NTs, as well as the catalytic mechanism. The influence of the negative potential on the PEC performance of PdQDs@TiO2NTs in reducing atrazine was firstly investigated by measuring the photocurrent response under different potentials. Figure 6A showed the photocurrent response on PdQDs@TiO2NTs before and after adding atrazine under regulating potentials. It could be observed that when there was no atrazine in the electrolyte, the cathode photocurrent increased with the negative increment of potentials. After addition of atrazine, a bigger photocurrent signal could be observed under each potential, owing to the reduction of atrazine on PdQDs@TiO2NTs. It was worth noting that the biggest photocurrent response originated from atrazine was observed under the

ACS Paragon Plus Environment

Page 14 of 41

Page 15 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

potential around -1.4 V. When the potential reached -1.6 V, the atrazine-originated photocurrent response became smaller. This phenomenon may be attributed to the competition of hydrogen evolution reaction (HER) under this potential. As shown in Figure 6B, the onset potential of HER was determined to be about -1.3 V in PEC condition. It may indicate under the potential of -1.6 V, fast hydrogen evolution reaction competed successfully with atrazine reduction reaction, leading to decreased atrazine reduction efficiency. PEC reduction and removal of atrazine on PdQDs@TiO2NTs were further performed under different negative potentials. As shown in Figure 6C and 6D, all the atrazine reduction reactions kept well with a pseudo-first-order reaction kinetics. The PEC removal ratio of atrazine increased accordingly with the increment of the cathode potential, and reached the maximum under the potential of -1.3 V with the removal rate of 99.5 % and reaction rate constant of 1.036 h-1. But it decreased when the potential was -1.5 V, which was consistent with the tendency of the photocurrent response, owing to the co-existent side reaction of HER. In order to further elucidate the catalytic mechanism of the PEC reduction of atrazine on PdQDs@TiO2NTs under different negative potentials and the synergetic effect between electro-catalysis and photo-catalysis in PEC process, electrochemical (EC) reduction of atrazine on PdQDs@TiO2NTs was also carried out under different potentials. The results were shown in Figure 7 and summarized in Table 3. It could be observed that with the increasing of the cathode potential, the EC removal rate of atrazine increased accordingly from 2.3 % to 97.7 % in the potential range from -0.5 to

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

-1.5 V. As we know, in the PEC process, the electro-catalysis and photo-catalysis would promote each other to achieve a synergetic effect. We plotted the simple addition of photocatalytic removal rates and the EC removal rates against the applied potential, and compared the results with the PEC removal rates. As observed in Figure 7, under the potentials of -0.5 V and -0.9 V, the PEC removal rate was higher than the simple sum up, and the biggest synergetic effect was obtained under the potential of -0.9 V. When the potential was -1.3 V, although the highest PEC removal of atrazine was obtained to be 99.5 %, the proportion of EC removal was quite high to be 96.0 %. When the potential was -1.5 V, the PEC removal was a little lower than the EC removal. It may be because that in the EC process, the onset potential of HER was about -1.5 V (inset of Figure 6B), atrazine removal reaction dominated against HER under this condition in EC process. However, in PEC process, the onset potential of HER decreased to -1.3 V, HER dominated against atrazine reduction instead, leading to the lower removal rate of atrazine in PEC process under the potential of -1.5 V compared with that in EC process. Furthermore, the reaction activation energy in atrazine PEC reduction on PdQDs@TiO2NTs under different potentials was also determined48, and the data were presented in Figure S6 and Table 3. The results were consistent with the PEC reduction efficiency of atrazine, that the highest removal rate was obtained under -1.3 V with a smallest activation energy of 26.33 kJmol-1. Based on the above discussion, we could summarize that with the negative potential increased, atrazine reduction underwent a catalytic mechanism transition from electro-assisted photo-catalysis (-0.5 V), synergetic PEC process (-0.9 V) and photo-assisted electro-catalysis (-1.3 V). When

ACS Paragon Plus Environment

Page 16 of 41

Page 17 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

the potential exceeded the onset potential of HER, the PEC reduction rate of pollutants tended to be decreased. Furthermore, the influence of the negative potential on atrazine PEC reduction pathway on PdQDs@TiO2NTs was also investigated, and the intermediates were qualitatively identified. As shown in Figure S7, three main intermediates were detected in all the PEC process under the potential of -0.9, -1.3, and -1.5 V, respectively, including the dechlorination products HA and DEHA, as well as the deethylation products DEA. The results indicated the applied potentials had no influence on the types of the intermediates, but the evolution rate was different under different negative potential. Under -0.9 V, DEHA was the main intermediate, and the concentration of DEHA increased gradually in 5 hours (Figure S7A). Under -1.3 V and -1.5 V, both HA and DEHA were the main intermediates (Figure S7B and S7C). Under both potentials, HA reached the maximum concentration in about 3 hours, and then decreased gradually. However, DEA reached the maximum at about 2 hours under the potential of -1.3 V, whereas it was 1 hour under the potential of -1.5 V. And in about 5 hours, DEA diminished under both -1.3 V and -1.5 V, while it remained about 0.2 ppm under -0.9 V. The results indicated that more negative potential was beneficial for generating the dechlorination products. Since that the concentration of atrazine in the water was low, it was hard to determine the concentration of chloridion. But from the quantification of the intermediates, the dechlorination rate could be calculated indirectly to be nearly 100 % when the potential was negative enough. It indicated that PEC reduction applying PdQDs@TiO2NTs as photocathode was a promising method for dechlorination of

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 41

organic pollutants, in order to reduce the toxicity caused by halogen substitution49. And this high dechlorination performance could be mainly ascribed to the high dechlorination activity of PdQDs as co-catalyst. Stability of PdQDs@TiO2NTs in PEC Reduction of Atrazine. The stability of catalyst is pretty important for its application in real water treatment, and it is also an important factor to evaluate the performance of catalyst. Hence, we repeated the PEC reduction of atrazine 5 times using the same PdQDs@TiO2NTs electrode and tested the removal rate of atrazine. Figure 8A showed the degradation curve of atrazine for 5 cycles with an external bias of -1.3 V. It could be observed that the PEC removal rate of atrazine at the second cycle was slightly higher than that of the first, and then it remained basically unchanged after continuous use. Besides, the SEM image, and XRD pattern of PdQDs@TiO2NTs after usage were recorded and shown in Figure 8. It could be observed that after use in PEC reduction of atrazine, the morphology and the crystalline structure of PdQDs@TiO2NTs remained well. Meanwhile, the surface Pd content was also determined by EDS to be about 2.2 wt % (Figure S8), which remained nearly

unchanged

after

5-cycle

usage,

indicating

the

good

stability

of

PdQDs@TiO2NTs. CONCLUSIONS In this work, PdQDs@TiO2NTs photo-cathode with good PEC reduction activity were prepared by three-step electrochemical anodization and hydrothermal method. It presented enhanced PEC reduction activity toward atrazine compared to TiO2NTs electrode. The investigation showed that deposition of PdQDs was beneficial to the

ACS Paragon Plus Environment

Page 19 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

accumulation of photo-generated electrons, thereby improving the lifetime of the charge carriers and the reduction activity. The results also testified the faster electron transfer from PdQDs to atrazine across the interface, leading to the high reduction efficiency in PEC atrazine removal. In addition, the influence of the applied negative potential on PEC reduction efficiency and the reaction mechanism was investigated. It suggested that the external negative bias played a vital role in governing the separation of the photo-generated carriers on PdQDs@TiO2NTs. The PEC reduction removal efficiency of atrazine exceeded 99 % within 5 h at -1.3 V, which indicated the satisfactory reduction activity toward atrazine. Meanwhile, with the negative potential increased, atrazine reduction underwent a catalytic mechanism transition from electro-assisted photo-catalysis, synergetic PEC process and photo-assisted electro-catalysis. However, the negative potential did not change the reduction intermediates, but altering the evolution trends instead. ASSOCIATED CONTENT Supporting information The supporting information including the calculation methods of charge carrier density, lifetime of accumulated electrons and surface charge transfer efficiency; Particle size distribution of PdQDs on the surface of PdQDs@TiO2NTs; TEM images of PdQDs@TiO2NTs; EDX spectra of PdQDs@TiO2NTs before and after usage; DRS, Nyquist and Mott-schottky plots of TiO2NTs and PdQDs@TiO2NTs, open circuit potential and time curve of PdQDs@TiO2NTs and TiO2NTs electrodes under radiation,

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

potential-dependent life-time of accumulated photo-electrons, measurement of the activation energy of PEC reduction of atrazine on PdQDs@TiO2NTs, and the evolution of intermediates in PEC reduction of atrazine on PdQDs@TiO2NTs under different conditions was available on the website. AUTHOR INFORMATION Corresponding Authors: Huijie Shi: Fax: (86)-21-65982287; E-mail: [email protected] Guohua Zhao: Fax: (86)-21-65982287; E-mail: [email protected] Author Contributions This manuscript was written through contributions from all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was financially supported by the National Natural Science Foundation of China (NSFC, No. 21677110 and 21537003), the Science & Technology Commission of Shanghai Municipality (14DZ2261100), the Fundamental Research Funds for the Central Universities (22120180118) and the State Key Laboratory of Pollution Control and Resource Reuse Foundation (NO. PCRRF17013). REFERENCES (1) Mkhalid, I. A., Photocatalytic Remediation of Atrazine under Visible Light Radiation Using Pd-Gd2O3 Nanospheres. J. Alloy. Compd. 2016, 682, 766-772.

ACS Paragon Plus Environment

Page 20 of 41

Page 21 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

(2) De la Casa-Resino, I.; Valdehita, A.; Soler, F.; Navas, J. M.; Perez-Lopez, M., Endocrine Disruption Caused by Oral Administration of Atrazine in European Quail. Comp. Biochem. Physiol. C-Toxicol. 2012, 156, 159-165. (3) Schnoor, B.; Elhendawy, A.; Joseph, S.; Putman, M.; Chacon-Cerdas, R.; FloresMora, D.; Bravo-Moraga, F.; Gonzalez-Nilo, F.; Salvador-Morales, C., Engineering Atrazine Loaded Poly (Lactic-Co-Glycolic Acid) Nanoparticles to Ameliorate Environmental Challenges. J. Agric. Food. Chem. 2018, 66, 7889-7898. (4) Zhang, J. J.; Lu, Y. C.; Yang, H., Chemical Modification and Degradation of Atrazine in Medicago Sativa through Multiple Pathways. J. Agric. Food. Chem. 2014, 62, 9657-9668. (5) Botta, F.; Fauchon, N.; Blanchoud, H.; Chevreuil, M.; Guery, B., Phyt'Eaux Cites: Application and Validation of a Programme to Reduce Surface Water Contamination with Urban Pesticides. Chemosphere 2012, 86, 166-176. (6) Dominguez-Garay, A.; Boltes, K.; Esteve-Nunez, A., Cleaning-up ArazinePolluted Soil by Using Microbial Electroremediating Cells. Chemosphere 2016, 161, 365-371. (7) Komtchou, S.; Dirany, A.; Drogui, P.; Robert, D.; Lafrance, P., Removal of Atrazine and Its By-Products from Water Using Electrochemical Advanced Oxidation Processes. Water Res. 2017, 125, 91-103. (8) Wang, W. K.; Chen, J. J.; Gao, M.; Huang, Y. X.; Zhang, X.; Yu, H. Q., Photocatalytic Degradation of Atrazine by Boron-Doped TiO2 with a Tunable Rutile/Anatase

Ratio. Appl. Catal. B 2016, 195, 69-76.

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(9) Shi, H. J.; Wang, Y. L.; Tang, C. J.; Wang, W. K.; Liu, M. C.; Zhao, G. H., Mechanism Investigation on the Enhanced and Selective Photoelectrochemical Oxidation of Atrazine on Molecular Imprinted Mesoporous TiO2. Appl. Catal. B 2019, 246, 50-60. (10) Feng, N. D.; Wang, Q.; Zheng, A. M; Zhang, Z. F; Fan, J.; Liu, S. B.; Amoureux, J. P.; Deng, F., Understanding the High Photocatalytic Activity of (B, Ag)-Codoped TiO2 under Solar-light Irradiation with XPS, Solid-state NMR, and DFT Calculations. J. Am. Chem. Soc. 2013, 135, 1607-1616. (11) Fan, Z. Y.; Wei, T.; Shi, H. J.; Tang, B.; Zhao, G. H., Adsorption Driven Preferential Degradation of Alkyl Phenols on Hydrophobic Perfluoroalkyl Modified {001}-TiO2. Chem. Eng. J. 2019, 357, 689-697. (12) Zhang, Y. N.; Qin, N.; Li, J. Y.; Han, S. N.; Li, P.; Zhao, G. H., Facet ExposureDependent Photoelectrocatalytic Oxidation Kinetics of Bisphenol A on Nanocrystalline {001} TiO2 /Carbon Aerogel Electrode. Appl. Catal. B 2017, 216, 30-40. (13) Chen, X.; Hu, X. M.; An, L.; Zhang, N. L.; Xia, D. G.; Zuo, X.; Wang, X. Y., Electrocatalytic Dechlorination of Atrazine Using Binuclear Iron Phthalocyanine as Electrocatalysts. Electrocatalysis 2013, 5, 68-74. (14) Chen, Y. L.; Xiong, L.; Song, X. N.; Wang, W. K.; Huang, Y. X.; Yu, H. Q., Electrocatalytic Hydrodehalogenation of Atrazine in Aqueous Solution by Cu@Pd/Ti Catalyst. Chemosphere 2015, 125, 57-63. (15) Gong, J. J.; Lai, Y. K.; Lin, C. J., Electrochemically Multi-anodized TiO2 Nanotube Arrays for Enhancing Hydrogen Generation by Photoelectrocatalytic Water

ACS Paragon Plus Environment

Page 22 of 41

Page 23 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Splitting. Electrochim. Acta 2010, 55, 4776-4782. (16) Liu, H. Y.; Zhang, L. Y.; Wang, N.; Su, D. S., Palladium Nanoparticles Embedded In The Inner Surfaces of Carbon Nanotubes: Synthesis, Catalytic Activity, and Sinter Resistance. Angew. Chem. Int. Ed. 2014, 53, 12634-12638. (17) Chaplin, B. P.; Reinhard, M.; Schneider, W. F.; Schuth, C.; Shapley, J. R.; Strathmann, T. J.; Werth, C. J., Critical Review of Pd-Based Catalytic Treatment of Priority Contaminants in Water. Environ. Sci. Technol. 2012, 46, 3655-3670. (18) Shao, M. H., Palladium-Based Electrocatalysts for Hydrogen Oxidation and Oxygen Reduction Reactions. J. Power Sources 2011, 196, 2433-2444. (19) Tan, C.L.; Cao, X.H.; Wu, X. J.; He, Q.Y.; Yang, J.; Zhang, X.; Chen, J.Z.; Zhao, W.; Han, S.K.; Nam, G. H., M, S.; Zhang, H. Recent Advances in Ultrathin TwoDimensional Nanomaterials. Chem. Rev. 2017, 117, 6225-6331. (20) Fan, Z.X.; Huang, X.; Tan, C.L.; Zhang, H., Thin mMetal Nanostructures: Synthesis, Properties and Applications. Chem. Sci., 2015, 6, 95–111. (21) Dhar, S.; Chakraborty, P.; Majumder, T.; Mondal, S. P., CdS-Decorated Al-Doped ZnO Nanorod/Polymer Schottky Junction Ultraviolet–Visible Dual-Wavelength Photodetector. ACS Appl. Nano Mater. 2018, 1, 3339-3345. (22) Xu, Y. F.; Yang, M. Z.; Chen, H. Y.; Liao, J. F.; Wang, X. D.; Kuang, D. B., Enhanced Solar-Driven Gaseous CO2 Conversion by CsPbBr3 Nanocrystal/Pd Nanosheet Schottky-Junction Photocatalyst. ACS Appl. Energy Mater. 2018, 1, 50835089. (23) Dong-Kyun, K.; Murray, C. B., Probing the Fermi Energy Level and the Density

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

of States Distribution in PbTe Nanocrystal (Quantum Dot) Solids by TemperatureDependent Thermopower Measurements. Acs Nano 2011, 5, 4810-4817. (24) Naya, S.; Teranishi, M.; Aoki, R.; Tada, H., Fermi Level Control of Gold Nanoparticle by the Support: Activation of the Catalysis for Selective Aerobic Oxidation of Alcohols. J. Phys. Chem. C 2016, 120, 12440-12445. (25) Chen, M.; Goodman, D. W., Catalytically Active Gold: From Nanoparticles to Ultrathin Films. Acc. Chem. Res. 2006, 39, 739-746. (26) Corma, A.; Serna, P.; García, H., Gold Catalysts Open a New General Chemoselective Route to Synthesize Oximes by Hydrogenation of α, β-Unsaturated Nitrocompounds with H2. J. Am. Chem. Soc. 2007, 129, 6358-6359. (27) Zhang, Y. J.; Zhao, G. H.; Shi, H. J.; Zhang, Y. N.; Huang, W. N.; Huang, X. F.; Wu, Z. Y., Photoelectrocatalytic Glucose Oxidation to Promote Hydrogen Production over Periodically Ordered TiO2 Nanotube Arrays Assembled of Pd Quantum Dots. Electrochim. Acta 2015, 174, 93-101. (28) Shi, H. J.; Wang, Y. L.; Zhao, J. Z.; Huang, X. R.; Zhao, G. H., Cathodic Photoelectrochemical Detection of PCB101 in Environmental Samples with High Sensitivity and Selectivity. J. Hazard. Mater. 2018, 342, 131-138. (29) Shi, W. W.; Yu, W.; Li, D. D.; Zhang, D.; Fan, W. J.; Shi, J. Y.; Li, C., PTB7: PC61BM Bulk Heterojunction-Based Photocathodes for Efficient Hydrogen Production in Aqueous Solution. Chem. Mater. 2019, 31, 1928-1935 (30) Ye, M. D.; Gong, J. J.; Lai, Y. K.; Lin, C. J.; Lin, Z. Q., High-efficiency Photoelectrocatalytic Hydrogen Generation Enabled by Palladium Quantum Dots-

ACS Paragon Plus Environment

Page 24 of 41

Page 25 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Sensitized TiO2 Nanotube Arrays. J. Am. Chem. Soc. 2012, 134, 15720-15723. (31) Tang, L.; Feng, C.Y.; Deng, Y.C.; Zeng, G.M.; Wang, J.J.; Liu, Y.N.; Feng, H.P.; Wang, J.J., Enhanced Photocatalytic Activity of Ternary Ag/g-C3N4/NaTaO3 Photocatalysts under Wide Spectrum Light Radiation: The High Potential Band Protection Mechanism. Appl. Catal. B 2018, 230, 102-114. (32) Wu, Z.B; Sheng, Z.Y.; Liu, Y.; Wang, H.Q.; Tang, N.; Wang, J., Characterization and Activity of Pd-Modified TiO2 Catalysts for Photocatalytic Oxidation of NO in Gas Phase. J. Hazard. Mater. 2009, 164, 542-548. (33) Li, G. H.; Zhu, M.; Ma, L.; Yan, J. R.; Lu, X. L.; Shen, Y. F.; Wan, Y. K., Generation of Small Single Domain Nanobody Binders for Sensitive Detection of Testosterone by Electrochemical Impedance Spectroscopy. ACS Appl. Mater. Interfaces 2016, 8, 13830-13839. (34) Chai, S. N.; Wang, Y. J.; Zhang, Y. N.; Liu, M.C.; Wang, Y. B.; Zhao, G. H., Selective Electrocatalytic Degradation of Odorous Mercaptans Derived from S-Au Bond Recongnition on a Dendritic Gold/Boron-Doped Diamond Composite Electrode. Environ. Sci. Technol. 2017, 51, 8067-8076. (35) Bera, B.; Chakraborty, A.; Kar, T.; Leuaa, P.; Neergat, M., Density of States, Carrier Concentration, and Flat Band Potential Derived from Electrochemical Impedance Measurements of N-Doped Carbon and Their Influence on Electrocatalysis of Oxygen Reduction Reaction. J. Phys. Chem. C 2017, 121, 20850-20856. (36) Matsumoto, Y.; Miura, Y.; Takata, S., Thickness-Dependent Flat Band Potential of Anatase TiO2 (001) Epitaxial Films on Nb: SrTiO3 (001) Investigated by UHV-

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Electrochemistry Approach. J. Phys. Chem. C 2016, 120, 1472-1477. (37) Liu, X. Y.; Wang, S.; Liu, X. S.; Zhao, X. T.; Gu, Y. Z., Competition of Carrier Separation and Recombination for an Optimized Electrode Configuration for Flexible Thin-Film Solar Cells. ACS Appl. Mater. Interfaces 2018, 10, 32067-32077. (38) Liu, X. Y.; Chen, Z.; Li, W. X.; Cao, M. S., Distinctly Improved Photocurrent and Stability in TiO2 Nanotube Arrays by Ladder Band Structure. J. Phys. Chem. C 2017, 121, 20605-20612. (39) Li, J.; Pei, Q.; Wang, R. Y.; Zhou, Y.; Zhang, Z. M.; Cao, Q. Q.; Wang, D. H.; Mi, W. B.; Du, Y. W., Enhanced Photocatalytic Performance through Magnetic Field Boosting Carrier Transport. ACS Nano 2018, 12, 3351-3359. (40) Wang, H.; Zhang, B. H.; Zhao, F. Q.; Zeng, B. Z., One-Pot Synthesis of NGraphene Quantum Dot-Functionalized I-BiOCl Z-Scheme Cathodic Materials for "Signal-Off" Photoelectrochemical Sensing of Chlorpyrifos. ACS Appl. Mater. Interfaces 2018, 10, 35281-35288. (41) Zhu, Q.; Wang, Y. Y.; Zhang, H. N.; Duan, R.; Chen, C. C.; Song, W. J.; Zhao, J. C., Localized TiIII Mediated Dissociative Electron Transfer for Carbon Halogen Bond Activation on TiO2. Appl. Catal. B 2017, 219, 322-328. (42) Xu, H. M.; Lin, Y. H.; Harumoto, T.; Shi, J.; Nan, C. W., Highly (001)-Textured Tetragonal BiFeO3 Film and Its Photoelectrochemical Behaviors Tuned by Magnetic Field. ACS Appl. Mater. Interfaces 2017, 9, 30127-30132. (43) Feng, C.Y.; Deng, Y.C.; Tang, L.; Zeng, G.M.; Wang, J.J.; Yu, J.F.; Liu, Y.N.; Peng, B.; Feng, H.P.; Wang, J.J., Core-Shell Ag2CrO4/N-GQDs@g-C3N4 Composites

ACS Paragon Plus Environment

Page 26 of 41

Page 27 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

with

Anti-Photocorrosion

Performance

for

Enhanced

Full-Spectrum-Light

Photocatalytic Activities. Appl. Catal. B 2018, 239, 525-536. (44) Liu, C.Y.; Huang, H.W.; Ye, L.Q.; Yu, S.X.; Tian, N.; Du, X.; Zhang, T.R.; Zhang, Y.H., Intermediate-Mmediated Strategy to Horn-Like Hollow Mesoporous Ultrathin gC3N4 Tube with Spatial Snisotropic Charge Separation for Superior Photocatalytic H2 Evolution. Nano Energy 2017, 41, 738-748. (45) Zhang, Z.H.; Yu, Y.J.; Wang, P., Hierarchical Top-Porous/Bottom-Tubular TiO2 Nanostructures Decorated with Pd Nanoparticles for Efficient Photoelectrocatalytic Decomposition of Synergistic Pollutants. ACS Appl. Mater. Interfaces 2012, 4, 990996. (46) Ratanatawanate, C.; Tao, Y.; Balkus, K. J., Photocatalytic Activity of PbS Quantum Dot/TiO2 Nanotube Composites. J. Phys. Chem. C 2009, 113, 10755-10760. (47) Wang, L.; Nathan, M. I.; Lim, T. H.; Khan, M. A.; Chen, Q., High Barrier Height GaN Schottky Diodes: Pt/GaN and Pd/GaN. Appl. Phys. Lett. 1996, 68, 1267-1269. (48) Chai, S. N.; Zhao, G. H.; Li, P. Q.; Lei, Y. Z.; Zhang, Y. N.; Li, D. M., Novel Sieve-Like SnO2/TiO2 Nanotubes with Integrated Photoelectrocatalysis: Fabrication and Application for Efficient Toxicity Elimination of Nitrophenol Wastewater. J. Phys. Chem. C 2011, 115, 18261-18269. (49) Mu, Y.; Zhan, G.M.; Huang, C.M.; Wang, X.B.; Ai, Z.H.; Zou, J.P.; Luo, S.L.; Zhang, L.Z., Dechlorination-Hydroxylation of Atrazine to Hydroxyatrazine with Thiosulfate: A Detoxification Strategy in Seconds. Environ. Sci. Technol 2019, 53, 3208-3216.

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 41

FIGURE CAPTIONS Figure 1 The SEM images of (A, B) TiO2NTs and (C, D) PdQDs@TiO2NTs. The inset of D showed the elements distribution on PdQDs@TiO2NTs. Figure 2 (A) The survey of the XPS spectra of PdQDs@TiO2NTs; (B) Ti 2p of PdQDs@TiO2NTs and TiO2NTs, (C) Pd 3d of PdQDs@TiO2NTs and (D) the XRD profiles of TiO2NTs and PdQDs@TiO2NTs. Figure 3 (A) The PEC reduction efficiency of atrazine on PdQDs@TiO2NTs and TiO2NTs under -1.3 V and (B) the kinetic curves. (C) The EC removal efficiency of atrazine on PdQDs@TiO2NTs and TiO2NTs under -1.3 V and (D) the kinetic curves. Figure 4 Time-resolved fluorescence decay spectra of PdQDs@TiO2NTs monitored at 390 nm with an excitation wavelength of 340 nm. Figure 5 (A) OCPD curves of PdQDs@TiO2NTs and TiO2NTs; (B) the potential dependent lifetime plots for PdQDs@TiO2 NTs and TiO2NTs derived from the OCPD measurement; Photocurrent density of (C) TiO2NTs and (D) PdQDs@TiO2NTs in 0.1 molL-1 Na2SO4 aqueous solution with and without 0.5 molL-1 H2O2 respectively. Figure 6 (A) Photocurrent response of PdQDs@TiO2NTs before and after adding atrazine at different potentials；(B) LSV curve of PdQDs@TiO2NTs with and without light irradiation; (C) PEC atrazine removal and (D) the first-order reaction kinetic plot on PdQDs@TiO2NTs under different negative potentials.

 

Figure 7 The synergetic effect between the photo-catalysis and electro-catalysis in the PEC reduction of atrazine on PdQDs@TiO2NTs. Figure 8 (A) Stability measurement of PdQDs@TiO2NTs in PEC reduction of atrazine

ACS Paragon Plus Environment

Page 29 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

at under the potential of -1.3 V; (B) SEM images and (C) XRD profiles of PdQDs@TiO2NTs after usage in five PEC reduction cycles.

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 41

Table 1. Charge carrier lifetimes (τ) obtained with the three-exponential fit of the time resolved fluorescence decay curves Samples

τ1 / ns

τ2 / ns

τ3 / ns

PdQDs@TiO2NTs

853.0

93.3

12.5

TiO2NTs

70.1

60.7

11.1

ACS Paragon Plus Environment

Page 31 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Table 2. The accumulated photoelectron lifetimes of PdQDs@TiO2NTs and TiO2NTs in the presence and absence of atrazine Sample

Potential

Accumulated photoelectron lifetime (τ) /s

vs. SCE

Without atrazine With atrazine

PdQDs@TiO2NTs TiO2NTs

-0.6

25.6

1.2

8.5

4.3

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 41

Table 3. The removal rate, reaction rate constants (k) and activation energy (Ea) in atrazine removal on PdQDs@TiO2NTs electrode under various conditions Potential

Method

-0.9V

-1.3V

-1.5V

--

k

%

h-1

EC

2.3

0.006

PEC

24.1

0.060

EC

32.6

0.092

PEC

83.9

0.329

EC

96.0

0.604

PEC

99.5

1.036

EC

97.6

0.791

PEC

93.4

0.541

PC

17.8

0.032

V vs. SCE -0.5V

Removal

ACS Paragon Plus Environment

Ea kJmol-1

62.15

58.57

26.33

45.82

Page 33 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Figure 1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2

ACS Paragon Plus Environment

Page 34 of 41

Page 35 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Figure 3

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4

ACS Paragon Plus Environment

Page 36 of 41

Page 37 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Figure 5

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 6

ACS Paragon Plus Environment

Page 38 of 41

Page 39 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Figure 7

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 8

ACS Paragon Plus Environment

Page 40 of 41

Page 41 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

GRAPHICAL ABSTRACT

ACS Paragon Plus Environment