1 Electronic Supplementary Information (ESI) for2

3 Hydrophobic three-dimensionally networked boron-doped

4 diamond electrode towards electrochemical oxidation 5 Yapeng He,a Haibo Lin,*ab Xue Wang,a Weimin Huang,*a Rongling Chen,a and 6 Hongdong Lic

78 a. College of Chemistry, Jilin University, Changchun 130012, PR China.9 b. Key Laboratory of Physics and Technology for Advanced Batteries of Ministry of

10 Education, Jilin University, Changchun, 130012, PR China.11 c. State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, 12 PR China.13 Email: lhb910@jlu.edu.cn; huangwm@jlu.edu.cn14 Tel (Fax): +86 431 85155189;1516171819202122232425262728293031323334353637383940414243

Electronic Supplementary Material (ESI) for ChemComm.This journal is © The Royal Society of Chemistry 2016

44 1. Experimental section4546 1.1 Preparation and characterization of BDD film on mesh Ti substrate4748 The BDD film electrodes were prepared by HFCVD method on flat and mesh titanium 49 substrates. Before the deposition, the substrates were pre-treated under 70 0C for 20 min in the 18% 50 HCl solution to clean the surfaces firstly and construct coarse surface. Afterwards, they were 51 cleaned in deionized water for 5 min. Then, they were pre-treated in a suspension of diamond 52 powder for 30 min. At last, ultrasonically clean was conducted in acetone, ethanol, and deionized 53 water in sequence for 5 min and the substrates were dried in nitrogen for use. Tantalum wire with 54 diameter of 0.75 mm was severed a filament. The distance between the filament and the substrate 55 was 8-10 mm. The mixture of CH4 and H2 was used as the gas source, while B(OCH3)3 liquid 56 taken out by bubbling H2 gas was as the boron source and the amount of boron doping level was 57 adjusted by B(OCH3)3/H2 flow rate. The total gas flux was kept at 300 sccm for all experiments 58 with the 1.0 vol% CH4 in H2 while the substrate temperature was kept at 750 ± 20 0C.59 The film crystanllinity and phase composition of Ti/BDD films were analyzed by X-ray 60 diffraction (XRD) using a Rigaku D/Max 2550 diffractometer (Japan) with Cu-Kα radiation 61 (λ=1.5418 Å). Scanning electron microscopy (SEM, SU8020, HITACHI, Japan) was employed to 62 examine the surface morphology the BDD films. The quality of BDD film was characterized by 63 Raman spectroscopy (Renishaw inVia Raman microscope, argon laser 514 nm, UK).6465 1.2 Electrochemical measurements6667 Electrochemical properties were measured on PARSTAT 2273 electrochemical work station 68 (Princeton Applied Research, USA) using a conventional three-electrode cell system. Flat and 69 3DN-Ti/BDD electrodes were employed as working electrodes, Pt electrode as the counter 70 electrode, and KCl-saturated calomel electrode (SCE) as the reference electrode. All the potentials 71 were referred to SCE.72 The electrochemical effective area is also calculated by the slope of the plot of charge 73 amount (Q) versus t1/2 from chronocoulometry using 0.1 mmol L-1 K3[Fe(CN)6] as a model based 74 on the equation:1, 2

75

1/ 2 1/ 2

1/ 2

2ads dl

nFACD tQ Q Q

76 where is Qads the Faradic charge, Qdl is double layer charge, A is the electrode surface area, C is 77 the substrate concentration, D is the diffusion coefficient. Here, D of K3[Fe(CN)6] is 7.6×10-6 cm2 78 s-1. Based on the parameters, the slope of the line is 2nFACD1/2/π1/2, and the electrochemical 79 effective surface area of flat and 3DN-BDD electrode could be calculated respectively.80 The mass transfer coefficient3 was obtained employing ferro/ferricyanide redox couple (5 81 mmol L-1 K4[Fe3(CN)6] and 5 mmol L-1 K3[Fe3(CN)6]) in alkaline media (NaOH 0.5 mol L-1). 82 When the potential of the working electrode was controlled at -0.1 V vs. SCE, in the plateau zone 83 of the ferricyanide reduction, the resulting reduction current was diffusion controlled and related 84 to:

85 ,ox SL ox

Ci nFSD

86 where iL is the diffusion current (in A), n is the number of exchanged electrons per anion (n=1), F 87 is the Faraday constant (96487 C mol-1), S is the electrode surface area (1×10-4 m2), DOX is the 88 diffusion coefficient of ferricyanide (0.9×10-9 m2 s-1), COX, S is the bulk solution concentration of 89 ferricyanide (5 mol m-3), and I is the diffusion layer thickness (in m).90 Then, the mass transfer conefficient (kd) and the diffusion layer thickness (δ) can be 91 calculated by：

92,

ox Ld

ox S

D ik

nFSC

93 1.3 Electrochemical degradation 9495 Terephthalic acid was used as a spin trap for hydroxyl radicals. The fluorescence property of 96 the solution was measured using a fluorescence spectroscopy (FluoroMax-4, Horiba, France). 97 Electrolysis experiment was performed in a 100 mL of 0.25 mol L-1 Na2SO4 solution with 1 mmol 98 L-1 terephthalic acid and 4 mmol L-1 NaOH at a constant current density (30 mA cm-2). The 99 anodes were flat and 3DN-BDD electrodes, while the cathode was a Ti/RuO2-TiO2-SnO2 net with

100 the electrode gap of 10 mm. The solution was magnetic stirred simultaneously.101 The experiments of anodic oxidation of various contaminants (including phenol, aspirin, 102 paracetamol, xylenol orange, methyl orange and alizarin red S) were carried out in a single 103 compartment cell without diaphragm. The Ti/BDD electrodes were used as the anodes and the 104 cathode was a Ti/RuO2-TiO2-SnO2 net with the electrode gap of 10 mm. The supporting 105 electrolyte was 0.5 mol L-1 Na2SO4. The volume of the solution was 100 mL; the current density 106 was controlled at 30 mA cm-2 all the time using an 8511C potentiostat/galvanostat. During the 107 experiment, the samples were drawn from the solution in a regular interval and the degradation of 108 contaminant molecules were monitored by UV-Vis spectrophotometer (Shimadzu UV-2450, 109 Japan). The concentration of phenols (phenol, aspirin, paracetamol) was determined by high 110 performance liquid chromatography (HPLC) (Shimadzu Prominence LC-20A HPLC, Japan) 111 equipped with Inertsil ODS-SP, C18 column (4.6×150 mm), 5 µm. The concentration change of 112 dyes (xylenol orange, methyl orange and alizarin red S) was determined by the trend of 113 absorbance peak intensity in the visible region of UV-Vis spectrum. The value of chemical 114 oxygen demand (COD) was measured with COD analyzer. All experiments were performed at 115 room temperature. Current efficiency (CE) could be calculated by the following equation:

116 0 100%8000

tCOD CODCE FV

I t

117 Where COD0 and CODΔt are the values of COD (mg L-1) at 0 and Δt, respectively; I the 118 electrolysis current (A); Δt the electrolysis time (s); F the Faraday’s constant, 96500 (C mol-1) and 119 V is the volume of solution (L).120121122

123124125 Fig. S1 SEM image of (a) flat Ti/BDD film; the cross-sectional images of (b) flat Ti/BDD and (c, 126 d) 3DN-BDD film electrode under different magnifications.127128129130131

132133134 Fig. S2 Voltammetry curves of BDD electrodes in 0.5 M H2SO4 solution with scan rate of 50 mV 135 s-1.136

137

138 Fig. S3 Cyclic Voltammetry curves of (a) flat and (b) 3DN-BDD electrodes in 10 mmol L-1 139 K3[Fe(CN)6] + 10 mmol L-1 K4[Fe(CN)6] + 1.0 mol L-1 KCl solution under different scan rates.140141142143144145

146

147 Fig. S4 Plots of (a) Q~t and (b) Q~t1/2 curves of flat and 3DN-BDD electrodes in 0.1 mmol L-1 148 K3Fe(CN)6 + 1 mmol L-1 KCl solution.

149

150 Fig. S5 UV-Vis spectrum of 100 mg L-1 phenolic compounds under current density of 30 mA cm-2 151 on different anodes; (a, c, e) flat BDD, (b, d, f) 3DN-BDD electrode.

152153154 Fig. S6 UV-Vis spectrum of 100 mg L-1 dyes under current density of 30 mA cm-2 on different 155 anodes; (a, c, e) flat BDD, (b, d, f) 3DN-BDD electrode.156157158159160161162163164165166167168169170171172173174

175176 Fig. S7 The change of COD and concentration of different contaminants; hollow: flat Ti/BDD, 177 solid: 3DN-BDD electrode.178179180

181182183 Fig. S8 Fluorescence spectrum of the solution based on (a) flat and (b) 3DN-BDD electrodes at 184 different time.185186187188

189 Although previous research have shown that elimination of organic pollutants could be 190 achieved by electrochemical oxidation, the reaction rates are actually kinetically limited controlled 191 by mass transport, which indicates the movement of reactant from the bulk solution to the 192 electrode surface, as shown in Fig. S9. The reactants are firstly transported to the diffusion layer 193 by bulk diffusion and then electrode surface by interface diffusion. After the electrochemical 194 reaction on electrode surface, the products are transported to the bulk solution in reverse order. 195 Oxidation of contaminants is usually controlled by the rate at which organic molecules are carried 196 from the bulk solution to the electrode surface. Hence, the mass transfer coefficient could reflect 197 the whole driving force towards the mass transfer of contaminants. 198 The diffusion layer, though it has no catalytic reaction sites, is known to play a key role in 199 providing good access of reactants towards electrochemical catalytic sites and effective release of 200 product from the electrode surface. Commonly, there is a negative correlation between the 201 diffusion layer thickness and mass transfer coefficient.202

203204 Fig. S9 Mass transfer model and reaction mechanism on electrode surface. 205206207 Table.1 Comparison of the experimental results on different electrodes208

Electrode Current density(mA cm-2)

Mass transfer coefficient (m s-1)

Diffusion layer thickness (μm)

Flat Ti/BDD 0.373 7.71×10-6 1173DN-BDD 0.636 1.31×10-5 68.7

209210 The meshed structure of 3DN-BDD electrode provides enough channels for fluid volume 211 orthogonally through electrodes as a plunger flow, leading to the intensification of mass transport 212 of the pollutants to electrodes surface.213214215216217

218

219 Fig. S10 The enhancement mechanisms of 3DN-BDD electrode towards electrochemical 220 oxidation of contaminants.221222 1. A. J. Bard and L. R. Faulkner, Electrochemical methods: fundamentals and applications, 223 Wiley New York, 1980.224 2. Y. Li, Y. Gao, Y. Cao and H. Li, Sens. Actuators, B, 2012, 171-172, 726-733.225 3. X. P. Zhu, J. R. Ni, H. N. Li, Y. Jiang, X. A. Xing and A. G. L. Borthwick, Electrochim. Acta, 226 2010, 55, 5569-5575.227