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Selective Electrocatalytic Degradation of Odorous Mercaptans Derived from S-Au Bond Recognition on a Dendritic Gold/Boron-doped Diamond Composite Electrode Shouning Chai, Yujing Wang, Ya-nan Zhang, Meichuan Liu, Yanbin Wang, and Guohua Zhao Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b00393 • Publication Date (Web): 19 Jun 2017 Downloaded from http://pubs.acs.org on July 4, 2017
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Selective
Electrocatalytic
Degradation
of
Odorous
2
Mercaptans Derived from S-Au Bond Recongnition on a
3
Dendritic Gold/Boron-doped Diamond Composite Electrode
4 5
Shouning Chai,†,‡ Yujing Wang,§ Ya-nan Zhang,† Meichuan Liu,† Yanbin Wang, †
6
and Guohua Zhao*†
7 8
†
9
Assessment and Sustainability, Tongji University, Shanghai 200092, China.
School of Chemical Science and Engineering, Shanghai Key Lab of Chemical
10
‡
11
Shaanxi 710055, China
12
§
13
Xi’an, Shaanxi 710032, China
College of Science, Xi’an University of Architecture and Technology, Xi’an,
School of Materials and Chemical Engineering, Xi’an Technological University,
14 15 16 17 18 19 20 21 22 1
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ABSTRACT:
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To improve selectivity of electrocatalytic degrading to toxic, odorous mercaptans,
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the fractal-structured dendritic Au/BDD (boron doped diamond) anode with
26
molecular recognition is fabricated through facile replacement method. The
27
characterizations of SEM and TEM show that the gold dendrites compose of
28
single crystalline and have high population of the Au (111) facet. The distinctive
29
structure endows the electrode with advantages of low resistivity, high active
30
surface area, and prominent electrocatalytic activity. To evaluate selectivity, the
31
dendritic Au/BDD is applied in degrading two groups of synthetic wastewater
32
containing thiophenol/2-mercaptobenzimidazole (targets) and phenol/2-Hydroxyb
33
-enzimidazole (interferences), respectively. Results show that targets removals
34
reach 91%/94%, while interferences removals are only 58%/48% in a short time.
35
The corresponding degradation kinetic constants of targets are 3.25 times and 4.1
36
times that of interferences in the same group, demonstrating modification of
37
dendritic
38
target-selectivity.
39
electrocatalytic degradation derives from preferential recognition and fast
40
adsorption to thiophenol depending on strong Au-S bond. The efficient, selective
41
degradation is attributed to the synergetic effects between accumulative behaviour
42
and outstanding electrochemical performances. This work provides a new strategy
43
for selective electrochemical degradation of contaminants for actual wastewater
44
treatment.
gold
on XPS
BDD and
could
effectively
EXAFS
further
enhance reveal
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electrocatalytic the
selective
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Table of Contents (TOC)
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INTRODUCTION
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Mercaptans, as raw materials for synthesis of pharmaceuticals, pesticides and fine
66
chemicals, are a kind of highly toxic, offensively odorous and corrosive pollutants.1
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Thiophenol and 2-mercaptobenzimidazole (2-MBI) are two representative members
68
that more toxic than common aliphatic mercaptans.2,3 It is reported that, thiophenols
69
could target central nervous system, liver and kidney, which prolonged exposure in
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water can cause a series of serious health problems in living body, for example, their
71
median lethal dose (LC50) values is at a low range of 0.01−0.4 mM in fish.4 Previous
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studies also reported that the use of 2-MBI may induce tumors, cause allergic
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reactions and be toxic to aquatic organisms.5 Considering their serious harmfulness
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and the continuing environmental concerns, the disposal of wastewater containing
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mercaptans has become an urgent problem at present. In the past decades, several
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techniques have been studied for removing organosulfur pollutants such as
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adsorption, photocatalysis, biodegradation, and so on.1,6-9 Nevertheless, their practical
78
applications are also limited by some defects including hard reutilization, low
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efficiency, and easy sulfur-poisoning. Besides that, electrocatalytic oxidation with
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anode is known as a promising approach for recalcitrant organic pollutants
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decomposing since its strong oxidation performance, mild treatment condition and
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environmental compatibility,10-12 which should also be suitable for degrading
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mercaptans efficiently. However, toxic mercaptans with low level are always
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coexisting with a great deal of less toxic, biodegradable contaminants in many effluent
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streams. Regarding to these wastewaters, the application of electrocatalytic 4
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degradation is not optimal if it wants to achieve complete detoxification and
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purification due to poor reaction selectivity to target contaminants and high electric
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energy consumption. Previous studies have indicated that the poor selectivity is
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resulted from some oxidative free radical with a strong redox potential.13 The poor
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selectivity may be ascribed to nonselective hydroxyl radicals dominated reaction and
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approximate oxidation potential of various pollutants with similar chemical
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structure.14 To overcome these drawbacks, the combination electrocatalytic oxidation
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with conventional biological treatment would be a more effective and energy-saving
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process for actual multi-pollutants complex wastewater, i.e. preferentially degrade
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toxic, odorous mercaptans to harmfulless, inodorous substances highly selective
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electrocatalytic oxidation firstly, and then biological treatment is used to decompose
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residuals.15,16 Thus, the problem how to realize selective electrocatalytic degradation
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to target mercaptans becomes a great challenge and arduous task in the environmental
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catalysis field.
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For the electrocatalytic degradation process, the selection of anode is crucial since
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the removal efficiency and energy consumption is directly determined by anode’s
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properties.13 Boron-doped diamond film (BDD), as a new-style carbonaceous material,
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has been widely considered as an outstanding electrode for applications in wastewater
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treatment, in virtue of its remarkable chemical inertness, electrochemical stability,
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low background current, and wide window potential.17,18 In operation, abundant
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weakly physisorbed •OH are generated on the BDD surface when a certain positive
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potential applied, which presents strong oxidation ability and plays a decisive role in 5
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decomposing the organic pollutants.19,20 Compared with Pt, CeO2, SnO2, and PbO2
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etc. anodes, BDD as a non-active anode has a weak interaction between the anode
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surface and electrogenerated •OH, leading to a larger overpotential for O2
111
evolution.12,13,21 Thus, BDD exhibits outstanding mineralization ability and great
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current efficiency for recalcitrant organic pollutants decomposing. In spite of these
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advantages, as-grown BDD also shows the disadvantage of lack of selectivity to target
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contaminants in
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target-selective oxidation on BDD can be achieved via in situ ingenious modification.
heterogeneous catalysis.11,22,23 Herein,
our concern
is if
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It is noteworthy that many approaches have been proposed to improve selectivity in
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photocatalysis field recently, including control of electrostatic attraction or repulsion
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between pollutants and catalyst surface,24 construction of shape/size-selective
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titanosilicate molecular sieves,25,26 graft a layer of precognition functional polymer or
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specific organic molecules that might physisorb target molecules on photocatalyst,27-29
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and even design of all inorganic molecular imprinted photocatalyst using target
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template.14,30 Obviously, it could be concluded from previous studies that directed
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diffusion of target pollutants and even preferentially enriched on the surface of
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catalyst is prerequisite to achieve selective degradation through increasing their
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chance of catalytic reaction. Nevertheless, aforementioned several approaches are not
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suitable for enhancing electrocatalytic selectivity of BDD because of its distinctive
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inertness, rigid microstructure, and high oxidative decomposing ability to modified
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polymeric host matrix in molecular imprinted technology.
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Besides, others specific adsorption and recognition of catalysts to target molecules
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could also use to improving electrocatalytic target-selectivity and efficiency. It is
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well-known that the strong covalent S-Au bond (bond energy about 30~40 kJ mol-1)
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between surfur and gold atoms could be easily formed, which has been widely
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application in self-assembly and molecule detection.31-34 Therefore, we assume that
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the selective electrocatalytic oxidation to low-level mercaptans wastewater would be
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achieved by virtue of strong affinity of the −SH group toward gold surface. In view of
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this, we can devote to design a novel anode via constructing nanostructured gold on
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BDD to reach our goal. In general, the catalytic activity of nanostructured gold
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depends on its particular architecture and configuration show different generally.35,36
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Recently, various fractal nano-dendritic gold catalysts were prepared in a solution of
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HAuCl4 in the ionic liquid [BMIM][PF6] or by means of dodecyl trimethyl
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ammonium bromide, cyclodextrin or cysteine as assistant additions in aqueous
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solution, and which was found possessing unusual performances due to its unique
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nano architecture and high surface energy.29,37 Moreover, their dispersive,
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unconsolidated, hyperbranched construction and exposed Au (111) facet are
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appropriate to selectively adsorb sulfur-containing contaminants. In this work, a
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dendritic Au/BDD composite electrode was constructed by a displacement grown
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method with electrodeposited Zn as template in aqueous solution without any
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structure-director and special medium. The physicochemical and electrochemical
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characterizations of dendritic Au/BDD are investigated systematically. To determine
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the catalytic selectivity of sample, thiophenol and phenol mixture solution, and 2-MBI 7
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and 2-hydroxybenzimidazole (2-HBI) mixture solution are used in degradation
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experiment. This work would provide a distinctive technique for selective removal of
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sulfur-containing organic pollutants in wastewater treatment.
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EXPERIMENTAL
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Materials. BDD (10 × 60 mm) was purchased from Centre Suisse d'Electronique et
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de Microtechnique SA (CSEM, Switzerland) and synthesized on a single crystal p-Si
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(100) wafer by microwave-assisted chemical vapor deposition (MP-CVD) technique.
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The final boron content was of the order of 8000 ppm, and the average resistivity was
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~0.1 Ω cm.
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Methanol was of HPLC grade, and ZnCl2, KCl, H3BO3, HAuCl4, thiophenol,
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phenol, 2-MBI, 2-HBI and other chemicals used in this study were analytical grade,
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purchased from Sigma-Aldrich Company. All these chemicals were used as received
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without further purification. For solution preparation and chromatographic purposes,
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ultrapure water (Milli-Q water, Millipore) was used.
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Fabrication and Characterization of Dendritic Au/BDD. BDD was first immersed
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in boiling aqua regia for 30 min to completely remove the contaminants on its surface,
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then for -OH to terminate the sample surface, BDD underwent an anodic treatment at
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+3 V in 2 M H2SO4, and finally washed ultrasonically by water, ethanol and water in
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turn.38,39 The electro-deposition on the oxygen-terminated BDD was carried out in the
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static 0.05 M ZnCl2/0.1 M KCl/0.05 M H3BO3 mixed solution with a constant applied
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voltage -3.5 V for 120 s. And then, it was placed in the aqueous solution of 1 mM
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HAuCl4 for a certain period and ensured that the replacement reaction of Zn particles 8
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was finished completely. Finally, the treated BDD electrode was kept in 0.1 M H2SO4
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to remove residual Zn before the calcined treatment at 450 °C for 1 h and the
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dendritic Au/BDD obtained.
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The morphology of electrode samples was characterized by a scanning electron
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microscopy (EFEG-SEM, Model Quanta 200 FEG, FEI) and a transmission electron
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microscopy (TEM, JEM-2100, JEOL) with an accelerating voltage of 200kV. X-ray
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diffraction (XRD, Model D/max2550VB3+/PC, Rigaku) analysis was performed
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using a diffractometer with Cu K⍺ radiation, with an accelerating voltage of 40 kV
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and current of 30mA. The static state contact angle of pure water on the electrode
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surface was determined by drop shape analysis system DSA100 (Krüss, Germany) at
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room temperature. The XPS spectra were determined on a X-ray photoelectron
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spectrometer (AXIS-ULTRA DLD, Shimadzu, Japan) using monochromatized Al Kα
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radiation (1486.6 eV). The binding energy measurements were corrected for charging
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effects with reference to the C1s peak of the adventitious carbon (284.6 eV). X-ray
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absorption measurements were performed at the beam line BL14W1 of the Shanghai
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Synchrotron Radiation Facility (SSRF) of China. X-ray absorption near-edge structure
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(XANES) spectra and Au L3-edge extended X-ray absorption fine structure (EXAFS)
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were recorded in a transmission mode by using ion chambers to measure the radiation
191
intensity. The station was operated with a Si (111) double-crystal monochromator.
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All electrochemical measurements were carried out in a three-electrode cell system
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of CHI 660c (CHI Co., USA) electrochemical workstation. A saturated calomel
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electrode (SCE) served as the reference electrode and Pt foil as the counter electrode. 9
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The electrochemical impedance spectroscopy (EIS) was used to determine the
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conductivity of catalysts at the open circuit potential, with the frequency range from
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1×105 to 1×10−3 Hz and amplitude 5 mV, and the electrolyte was 0.5 mM [Fe(CN)6]3-/
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[Fe(CN)6]4- solution.
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Selective Elecrocatalytic Oxidation Experiment and Analysis. The elecrocatalytic
200
oxidation was carried out in a cylindrical single compartment cell equipped with a
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magnetic stirrer and a jacketed cooler to maintain a constant temperature (25 ± 2 °C).
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The dendritic Au/BDD and BDD electrodes with working area of 4 cm2 worked as
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anode, respectively, and a Pt foil with the same area was used as cathode with the
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electrode gap of 1 cm. Two groups of 50 mL mixed binary pollutants solution with
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0.05 M Na2SO4 electrolyte were degraded in the cell. One was composed of 50 mg
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L−1 thiophenol and 50 mg L−1 phenol, the other was composed of 50 mg L−1 2-MBI
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and 50 mg L−1 2-HBI. The current density was controlled to be constant at 20 mA
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cm-2 by a direct current potentiostat. The stirring rate was about 600 rpm.
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The concentration of four pollutants during the degradation was measured by
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high-performance liquid chromatography (HPLC, Agilent HP1100) with AQ-C18
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column (4.6 mm × 250 mm, particles size 5 µm) and UV detector, the detective
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wavelength were at λ = 270 nm, 236 nm, 300 nm, and 278 nm for thiophenol and
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phenol, 2-MBI, and 2-HBI, respectively. Methanol/1 wt% acetum mixtures (70:30,
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V/V) were employed as the mobile phase at the flow rate of 1 mL min−1. The injection
215
volume was 20 µL.
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RESULTS AND DISCUSSION 10
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The BDD film is cleaned after pre-treatment, which is a polycrystalline thin film
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grown on the Si wafer. Figure 1A shows that the continuous BDD surface is smooth,
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crack free, and dense. The average crystallite grain size is in micrometers. Zn was
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chosen as a bridge to construct dendritic Au on the BDD surface since Zn was one of
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the active metals with higher hydrogen over potential compared with BDD. After
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electro-deposition, the Zn nanoparticles dispersed grow on hollow position among
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several grains preferentially in virtue of innumerable micro hydrogen bubbles in-situ
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generated as template (Figure 1B). Then, the Zn/BDD was immersed in the HAuCl4
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solution, the galvanic displacement reaction (4Zn + 2HAuCl4→2Au + 4ZnCl2 + H2↑)
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occurred immediately, and it was observed that the irregular stellate Zn/Au alloy
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nanoparticles demonstrating anisotropic crystal growth after treated 15 min, as shown
228
in Figure 1C. After 180 min, the displacement reaction was complete sufficiently,
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Figure 1D displays the morphology of dendritic Au/BDD, the well-defined fractal
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structured dendritic gold with micron stem length grow outward on the BDD film,
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which performs three-fold symmetry (inset) and every stems grow out the similar
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secondary structured branches and tertiary leaves. Meanwhile, partial BDD surface is
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still exposed avoiding totally covered. A possible mechanism is proposed for the
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formation of dendritic Au/BDD in Figure 1E. As the elemental Au generated initially,
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innumerable primary cell are also formed, which consist of Zn cathode, Au anode,
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BDD as the conducting line, and [AuCl4]- solution as electrolyte. The half-cell
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reaction as follows: 1/2Zn - e-=1/2Zn2+ (cathode); 1/3[AuCl4]- + e-=1/3Au + 4/3Cl-.
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This cell reaction take place on the Au crystal nucleus, instead of the direct surface 11
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replacement reaction, owing to relative high activation energy for the latter.40 The
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peculiar hierarchical Au dendrites are seldom obtained via common directly
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electrodeposition method because of the same deposition rate on different gold crystal
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facets.
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Figure 2 displays the XRD patterns of the BDD (curve a), Zn/BDD (curve b),
244
Zn@Au/BDD (curve c), and dendritic Au/BDD (curve d), which indicates that the Au
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dendrites have face-centered cubic (fcc) crystal structure, and the characteristic peaks
246
of Au are stronger while those peaks assigned to Zn become weaker and disappear
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eventually after 180 min along with replacement reaction proceeding. The sharp
248
diffraction peak at 38° corresponding to (111) crystal facet also demonstrates that the
249
crystallinity of Au dendrites is better. The ratios of the peak intensities assigned to
250
(111) facet relative to those (200) and (220) facet could effectively reflect preferential
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growth tendency of different Au facets. Those values are 3.7 and 8.8 respectively,
252
which are notably greater than common poly-Au (1.9 and 3.1, respectively).36 The
253
results confirm the higher population of the Au (111) facet, and the Au dendrites grow
254
along the direction preferentially. 41,42
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Further TEM observations were carried out to study the morphology and
256
crystallographic orientation of the Au dendrites. Figure 3A shows a typical TEM
257
image with low resolution of two main stems on an Au dendrite. The planar
258
projection angels between the branches groups and the stems ac. 70°, approximately
259
equal to the theoretical angle 67.8° between two direction of cubic structure in
260
the (112) projection plane, which is consistent with the observations reported in
—
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previous literatures.37 The corresponding selected area electron-diffraction (SAED)
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pattern (Figure 3B) shows clear diffraction spots, confirming that the dendrites are
263
composed of single crystalline Au. In addition, the high-resolution TEM images
264
(Figure 3C and 3D) display the equivalent lattice fringe spacing of 0.24 nm at the tips
265
of the branch and the trunk, which are all in good agreement with the d-spacing value
266
between the (111) planes of fcc structure Au. 41,43
267
Nyquist diagram of the electrochemical impedance spectroscopy is always
268
employed to estimate the electron transfer resistance (Ret). The size of arc diameter
269
on the Nyquist plot is closely relevant to the resistance of electron transfer on the
270
electrode surface. Nyquist plots at the dendritic Au/BDD and BDD are shown in
271
Figure 4A. At the same time, the equivalent circuit, often used to depict the
272
electrochemical behaviour of electrode, was employed for fitting the impedance
273
spectra. The fitted equivalent circuit for the dendritic Au/BDD agrees with Randles
274
model. Rs, Rct, Cdl, and Zw represent the solution resistance, charge transfer
275
resistance, double layer capacitance, and Warburg resistance, respectively. The fitted
276
values are given in Table S1. The Rct value of dendritic Au/BDD is 62 ohm, much
277
less than that of BDD (3850 ohm). The results indicate that the growth of gold
278
dendrites with remarkable electrical conductivity is beneficial to the charge transfer
279
directionally and decrease the electrical resistance of BDD. Dendritic Au growth
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facilitates mass and electron transports, accordingly improves the elecctocatalytic
281
ability and reduces the corresponding energy consumption. Furthermore, the
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probability of lattice distortion at interfacial regions and the number of crystal 13
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boundaries greatly increase in dendritic gold comparing with common gold
284
nanoparticles. Consequently, abundant formed surface defect sites would be the active
285
centres responsible for the high catalytic activity.44 To investigate electrocatalytic
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behaviour of dendritic Au/BDD, the cyclic voltammograms (CVs) in the
287
[Fe(CN)6]3-/[Fe(CN)6]4- solution are performed. As illustrated in Figure 4B, the redox
288
peak current density on BDD is small, which equal to 0.79 mA cm-2 and -0.50 mA
289
cm-2, respectively, due to its high electrochemical impedance and poor electrocatalytic
290
activity. However, the corresponding peak values increased to 2.05 mA cm-2 and
291
-1.73 mA cm-2 on the dendritic Au/BDD. Compared with BDD, the potential
292
difference of the dendritic Au/BDD between the anodic and the cathodic peaks, ∆Ep,
293
reduces from 1.70 to 0.57 V, which means that the reversibility of the electrode is
294
better. The negative shift of the anodic peak potential and positive shift of the
295
cathodic peak potential of the CVs at dendritic Au/BDD indicates that the redox
296
reaction of the [Fe(CN)6]3-/[Fe(CN)6]4- redox couple needs less energy. Therefore, it
297
can be expected that the modification of dendritic Au is beneficial to BDD
298
electrochemical performances including enhanced conductivity and electrocatalytic
299
activity. Figure 4C displays CVs recorded in deoxygenated 0.5 M H2SO4 for dendritic
300
Au/BDD. Distinct three oxidation peaks of Au surface monolayer corresponding to
301
different crystal facets were observed in the potential range of 1.0 to 1.4 V,
302
respectively, while the background current is very low in the same condition. The
303
construction of dendritic Au could significantly enhancee the conductivity and
304
electrocatalytic activity. The sharp peak at ca. 1.4 V is often indicative of Au (111) 14
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facet, which further demonstrating the (111) facet is predominant crystallographic
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plane on the dendritic Au/BDD.45 Moreover, the electrochemical active surface area
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of dendritic Au is calculated to be 0.014 cm2 (SA=Qe/390 µC cm2, Qe is equal to 5.4
308
µC, which is integral electrical quantity of reduction peak) from the charge consumed
309
during the reduction of AuOx corresponding to the peak at 0.87 V (in Figure 4C)
310
using the reported value of 390 µC cm2 for a clean Au electrode.44 The active surface
311
area is much less than the real surface area of BDD substrate, indicating that only a
312
small part of BDD surface is covered by dendritic Au and most is exposed.
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The selectivity of dendritic Au/BDD was evaluated in two groups of synthetic
314
wastewater composed of thiophenol/2-MBI (target pollutant) and phenol/2-HBI
315
(interference pollutant) with equal mass concentration due to their similar chemical
316
structure (Supporting Information (SI) Figure S1). Inset of Figure 5 shows the
317
removal of two pollutants. After 100 min, 91% thiophenol and 58% phenol are
318
removed on the dendritic Au/BDD, while the corresponding removal rate is 62% and
319
75% on bare BDD, respectively. The phenol is more easily oxidized than thiophenol
320
on nonselective BDD, which maybe attribute to the stronger electron donating
321
conjugation effect of –OH than that of –SH, leading to higher electron density of
322
benzene ring in phenol and easily be attacked by •OH. Meanwhile, it is easily noted
323
that the efficiency of electrocatalytic oxidation to thiophenol increase significantly
324
after dendritic Au assembled on the BDD. With regard to mixed solution of 2-MBI
325
and 2-HBI, the time dependence of pollutants removal is presented in inset of Figure
326
S2. The removal rates of 2-MBI and 2-HBI reach to 94% and 48% on the dendritic 15
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Au/BDD, respectively at 75 min, while the corresponding values are 54% and 82% at
328
bare BDD. The above results not only reveal that the dendritic Au/BDD has excellent
329
electrocatalytic performance, but also exhibit high selectivity to target pollutants.
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Previous studies have revealed that there exists some correlation among
331
hydrophobicity, oxygen evolution potential (OEP), and mineralization capability for
332
BDD. As-grown BDD via CVD is largely hydrogen terminated and hydrophobic in
333
nature.46 The wettability test of water at BDD and dendritic Au/BDD were determined,
334
and the contact angle is about 95º at BDD. After chemically cleaned with acid and
335
modified with hydrophilic dendritic Au, a distinct decrease of hydrophobicity was
336
observed (Figure S3). Although the hydrophobic property was change, the good
337
situation is that a relatively high OEP of ~2.1 V still maintained for dendritic
338
Au/BDD. The OEP is also much higher than that of common DSA anode (Figure S4),
339
ensuring that it still exhibits strong mineralization ability to recalcitrant organic
340
pollutants. Furthermore, the decrease of OEP is maybe related to the wettability of
341
electrode surface.
342
The selectivity of dendritic Au/BDD maybe attribute to the preferential adsorption
343
of thiophenol molecules on the exposed surfaces with single-crystalline gold dendrites
344
crystal planes as a result of formation of Au-S bonds. This hypothesis could be proved
345
by equilibrium adsorption test, the adsorptive capability of thiophenol on dendritic
346
Au/BDD is much higher than BDD, and the equilibrium adsorption removal to phenol
347
is only one-third to thiophenol on the dendritic Au/BDD after 60 min. Meanwhile,
348
XPS with high sensitivity was used to study fast adsorption of thiophenol on the 16
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electrode and the combination of Au and S atoms. As shown in the survey spectra
350
(Figure S5), the peaks of Au 4f and S 2p indicate that the grown dendritic Au and
351
adsorbed thiophenol. The Au 4f spectra (Figure 6A) display a doublet characteristic of
352
Au 4f7/2 and Au 4f5/2 centered at 84.0 eV and 87.7 eV, respectively, and peak
353
separation equal to 3.7 eV, which is in agreement with the spectral values for single
354
crystal Au.47 Figure 6B shows that the S 2p peak is broad and complicated because of
355
the overlapping contribution of sulfur with different chemical state, which would be
356
fitted by four peaks. The domain peak of S 2p3/2 at 162.1 eV is characteristic of
357
sulfur in Au-S bond, illustrating that plenty of thiophenol molecules assembled on the
358
single crystal Au surface. The peak of S 2p3/2 at 163.9 eV is assigned to the
359
sulfhydryl groups for noncovalently conjugated thiophenol enriched on the electrode
360
surface.47,48 In order to investigate the interaction between thiophenol and the
361
dendritic Au/BDD during degradation, the sample used for degrading mixed solution
362
of thiophenol and phenol for 30 min was selected to analyse. Figure 6C shows Au
363
L3-edge XANES spectra for the dendritic Au/BDD sample together with the Au foil as
364
a reference. The characteristic three-peak pattern following the edge jump at ~11919
365
eV is an indication of the existence of an (fcc) gold structure. The first resonance at
366
the edge is often known as the white line, arising from electronic transition from the
367
2p3/2 to the 5d5/2, 3/2 states.49,50 Although the 5d orbitals in Au atoms are nominally full
368
because of s-p-d hybridization, a small white line still could be detected in the
369
XANES of bulk Au. The white-line intensity for the Au foil is rather low, while it is
370
intensified for the dendritic Au/BDD, suggesting a growing number of charges 17
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371
transferred from Au to S atoms due to the preferential adsorption of thiophenol and
372
formation of Au-S bond during degradation.51 In the post-edge region, the resonance
373
peak of the dendritic Au/BDD is significantly broadened and attenuated relative to
374
that of the Au foil, which is attributed to the nanosize effect of the former. The
375
nanosize effect can be understood as that, namely, when the nanoparticle size
376
decreases, the increase in the percentage of surface atoms results in a decrease in the
377
average number of neighboring Au atoms. More explicit information for the
378
thiophenol adsorption behavior could be observed from the Fourier transforms (FT) of
379
k2χ(k) curves of EXAFS spectra (Figure 6D). In terms of dendritic Au/BDD, the FTs
380
curve demonstrate an apparent peak at 1.9 Å ascribed to the Au–S shell, while the
381
intensity of the peak in the range of 2.0–3.0 Å assigned to the first Au–Au shell shows
382
weaker comparing with that of gold foil, which is imply the lower coordination
383
number of Au surface atoms and increasing disorder due to the emergence of the
384
surface Au-S contribution near 2 Å. These results reveal that the strong thiophenol
385
adsorption via Au-S bond truly modulate the dendritic Au surface states, which are
386
consistent
387
chronocoulometric method (Text in SI, Figure S6),14,53
388
of target thiophenol and nontarget phenol on dendritic Au/BDD surface are calculated
389
as 1.1×10-10 and 4.1×10-11 mol cm-2, respectively, which is in accordance with the sole
390
equilibrium adsorption result.
with
the
previous
reports.51,52
Additionally,
according
to
a
the adsorption capacity (Γ0)
391
To further study the selectivity of electrocatalytic oxidation, the kinetics of
392
thiophenol and phenol decay is analysed in detail. Figure 5 shows the relationships 18
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393
between ln (C0/C) and time on two electrodes, in which all processes follow
394
pseudo-first-order kinetic. The values of apparent rate constant k are listed in Table 1.
395
The k value of thiophenol decay on BDD is 0.010 min−1, approximately equal to that
396
of phenol, 0.014 min−1, demonstrating that no selective electrocatalytic oxidation
397
occurred. In contrast, the k value of thiophenol on dendritic Au/BDD increases to
398
0.026 min−1, which is 3.25 (αDendritic
399
revealing its distinguished electrocatalytic selectivity to target thiophenol. For the
400
other mixed solution of 2-MBI and 2-HBI degradation, the similar selectivity to
401
2-MBI was demonstrated, and the α2-MBI/2-HBI value reached to 4.10 (Figure S2 and
402
Table 1). Moreover, high selectivity factor R (defined as the ratio αDendritic
403
to αBDD) of 4.58 and 8.20 further confirms that the modification of single crystal
404
dendritic Au with dominant (111) facet on BDD enhances the electrocatalytic
405
selectivity toward the sulfur-containing target contaminant.14,29
Au/BDD=kthiophenol/kphenol)
times for phenol,
Au/BDD
406
The selective electrocatalytic degradation mechanism of mercaptans on the
407
dendritic Au/BDD is investigated. Cyclic voltammetry was performed in 0.05 M
408
Na2SO4 blank solution and 0.05 M Na2SO4 + 50 mg L-1 thiophenol solution (Figure
409
S7), respectively. The cyclic voltammograms in the two solutions almost completely
410
overlap each other in the potential range of -0.2 to 2.0 V. There is no obvious
411
oxidative peak of thiophenol appearing in the latter solution, indicating that
412
electrocatalytic oxidation of thiophenol mainly triggered by•OH-induced indirect
413
electrocatalysis rather than direct electrocatalysis on dendritic Au/BDD.19,20 At the
414
same time, the electrocatalytic oxidation behavior of binary mixed pollutants (2-MBI 19
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415
and 2-HBI) on dendritic Au/BDD were also determined as presented in Figure S8.
416
There are three oxidation peaks appearance at the potential range of 0~1.5 V. With
417
respect to the strongest oxidation peak, the corresponding potential of pollutants
418
oxidation at dendritic Au/BDD (about 1.05 V) (vs. SCE) is lower than that on BDD
419
(about 1.2 V) (vs. SCE). The enhanced electrocatalytic activity should be attributed to
420
construction of dendritic Au on BDD. The peak current density of pollutants
421
electro-oxidation in a forward scan of dendritic Au/BDD is higher that of BDD due to
422
a good deal of pollutant molecules participate in the electrochemical reaction at
423
dendritic Au surface. The results reveal that, as a highly reactive electrocatalyst, gold
424
also plays a significant contribution to pollutants degradation of via direct elecatalytic
425
oxidation way simultaneously. Herein, it is believed that the electrocatalytic
426
selectivity of mercaptans is achieved depending on specific adsorption and
427
accumulation effect. When the analogous substances coexisting, mercaptans
428
molecules are able to surpass interference pollutants molecules in the adsorption
429
competition and occupy mostly active adsorption sites on Au crystal, and adsorbed
430
mercaptans degradation occurs directly over the anode trough the adsorbed •OH. On
431
the other side, most of micron-sized dendritic Au grows outward from the BDD plane
432
rather than tiling horizontal as shown in Figure 1D, which would form an
433
“adsorption-controlled layer” with tens of micron thickness in the vicinity of BDD
434
surface during degradation. The micron magnitude of this layer just is just the order
435
that the distribution distance of free •OH on BDD.54 Under the driving effect of strong
436
Au-S interaction, the continuous directed diffusion and accumulation of mercaptans 20
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437
toward gold interface results in a relative higher concentration than interferences in
438
this layer. In this case, more mercaptans also would be rapidly oxidized by abundant
439
of free •OH electro-generated on BDD before •OH quenching.18,55
440
Thus, dendritic Au/BDD, as a novel fractal nanostructured electrode, not only has
441
prominent electrochemical performance, but also presents highly selective capability.
442
These distinctive properties are ascribed to synergistic effect of two factors. First,
443
BDD is selected as the base electrode, which has higher oxygen evolution potential
444
than common DSA electrodes, so it exhibits strong electrochemical degradation
445
ability to organic pollutants. Second, compared with the bare BDD, the modified
446
dendritic Au single crystal with numerous physisorption and chemisorption sites can
447
preferentially adsorb and accumulate a large number of mercaptans molecules and are
448
decomposed subsequently, which plays dominant role for selective electrocatalytic
449
degradation to the target pollutant in a complex wastewater. From the perspective of
450
actual applications, the stability of electrode is an important issue that must be
451
considered. The reusability of dendritic Au/BDD was successfully carried out by
452
degrading two groups of pollutants over the reused sample (Figure S9). The
453
electrocatalytic degradation efficiencies were nearly maintained at the level of fresh
454
sample after five consecutive runs. To determine the structural stability, the
455
morphology and crystal structure of the used dendritic Au/BDD after five degradation
456
cycles were examined by XRD and SEM. As depicted in Figure S10, the main
457
diffraction peaks corresponding to (111) crystal facet of dendritic Au is still sharp and
458
strong, and no new diffraction peaks appears. Moreover, the dendritic structure of Au 21
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459
was remained the same as fresh sample (Figure S11). All of these results suggested
460
the dendritic Au/BDD shows good stability during utilization. In addition, the
461
passivated one can be recycled after activating with cyclic voltammetry in dilute
462
sulfuric acid solution. Despite two groups of binary pollutant mixtures we selected
463
cannot represent the complex actual wastewater, however, this study provides a new
464
way of thinking and approach for functional electrode design. Meanwhile, the ideas
465
can also be widely applied for removing, detecting or analysing for other
466
sulfur-containing organic contaminants with high toxicity and low concentration.
467
ASSOCIATED CONTENT
468
Supporting Information
469
The Supporting Information is available free of charge via the Internet at
470
http://pubs.acs.org. Figures S1-S6 (PDF)
471
AUTHOR INFORMATION
472
Corresponding Authors
473
*Phone: 86-21-65988570-8244; fax: 86-21-65982287;
474
E-mail:
[email protected] 475
Notes
476
The authors declare no competing financial interest.
477
ACKNOWLEDGMENT
478 479
This
work
Foundation
of
was China
financially supported
by
National
(Project No. 21537003, 21507103).
480 22
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Science
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658 659 660 661 662 663 664 665 666 667 668 669 31
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Figure Captions
671
Figure 1. SEM images of BDD (A), Zn/BDD (B), Zn@Au/BDD of replaced for 15
672
min in HAuCl4 (C), and dendritic Au/BDD (D) with high magnification SEM (inset);
673
Schematic illustration of gold dendrites growth process at BDD (E).
674
Figure 2. XRD patterns of BDD (a), Zn/BDD (b), Zn@Au/BDD of displaced for 15
675
min in HAuCl4 (c), and dendritic Au/BDD (d).
676
Figure 3. (A) TEM image of Au dendrites; (B) SEAD pattern of Au dendrites; (C, D)
677
HRTEM images of the branch tip denoted as C) and stem edge denoted as D) in (A),
678
respectively.
679
Figure 4. (A) Nyquist plot of dendritic Au/BDD in 5 mM [Fe(CN)6]3-/[Fe(CN)6]4-
680
solution, inset: the corresponding equivalent circuit model and Nyquist plot of bare
681
BDD in the same solution; Cyclic voltammograms of dendritic Au/BDD and bare
682
BDD in 5 mM [Fe(CN)6]3-/[Fe(CN)6]4- solution (B) and in 0.5 M H2SO4 solution at
683
scan rate of 50 mV s-1 (C), respectively.
684
Figure 5. The removal kinetics of thiophenol (A) and phenol (B) using dendritic
685
Au/BDD (a, a’) and BDD anode (b, b’), respectively. Inset: the removal of thiophenol
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(A) and phenol (B) with electrocatalytic oxidation time using dendritic Au/BDD (a, a’)
687
and BDD anode (b, b’), respectively; (a, b) obtained from the equilibrium adsorption
688
test; (a’, b’) obtained from electrocatalytic oxidation process.
689
Figure 6. Au 4f (A) and S2p (B) XPS spectra of dendritic Au/BDD performing
690
equilibrium adsorption test in thiophenol and phenol mixed binary solution for 60 min;
691
ex situ Au L3-edge XANES spectra of dendritic Au/BDD used for degrading mixed 32
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solution of thiophenol and phenol for 30 min and a gold foil reference (C);
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Fourier-transformed k2χ(k) EXAFS spectra of the sample and a gold foil reference
694
(D).
695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 33
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714 715
Figure 1
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722 723
Figure 2
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729 730
Figure 3
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Figure 4 37
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749
750 751 752 753 754 755 756 757 758 759 760
Figure 5
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Figure 6
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785
Table 1. Rate constants and selectivity rate for electrocatalytic degradation of target pollutant (thiophenol or 2-MBI) in the presence of interference (phenol or 2-HBI) using two electrodes
k
Dendritic Au/BDD
BDD
kthiophenol (min -1)
0.026
0.010
kphenol (min-1)
0.008
0.014
k2-MBI (min -1)
0.037
0.012
k2-HBI (min -1)
0.009
0.024
αthiophenol/phenol
3.25
0.71
α2-MBI/2-HBI
4.10
0.50
Rthiophenol/phenol
4.58
R2-MBI/2-HBI
8.20
786
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