Photocatalyst synthesis. A rutile TiO2 nanowire film was hydrothermally grown on a FTO substrate16,17. First, 15 mL of hydrochloric acid (35%, Samchun Chemical Co., Ltd., Republic of Korea) was diluted with 15 mL of deionised (DI) water and mixed with 0.5 mL of titanium (IV) butoxide (97%, MilliporeSigma, USA). This clear solution was transferred to a Teflon-lined stainless-steel autoclave (125 mL) and the FTO substrate was submerged in the solution. The sealed autoclave was kept in an oven at 150°C for 5 h and then cooled to room temperature (RT) slowly. After rinsing and drying, the sample was annealed at 550°C for 5 h to increase crystallinity.
A nanoporous BiVO4 film was prepared according to previously reported methods27. Briefly, three solutions for synthesising BiVO4 were prepared. First, a bismuth nitrate solution was prepared by dissolving 0.04 M of bismuth nitrate pentahydrate (≥99.99%, MilliporeSigma, USA) in 0.4 M (50 mL) of a potassium iodide solution (99%, FujiFilm Wako Chemicals USA Corp., USA). The pH of this solution was then reduced to 1.7 by adding HNO3 (60%, Junsei Chemical Co., Ltd., Japan). The second solution consisted of 0.23 M of p-benzoquinone (≥98%, MilliporeSigma, USA) in absolute ethanol (20 mL). Later, both solutions were mixed to obtain a dark brown BiOI precursor solution. A dark orange BiOI layer was electrodeposited on the FTO by applying a constant potential of -0.1 V against Ag/AgCl (RE-1B, ALS) for 420 s. Next, 0.4 M of vanadyl acetylacetonate (98%, MilliporeSigma, USA) in dimethyl sulfoxide (99%, Kanto Chemical Co., Inc., Japan) was prepared for converting the BiOI to BiVO4. This solution was uniformly drop-cast on the BiOI layer and then the film was annealed at 450°C for 2 h. Finally, a brownish crust was removed from the samples by washing with 1 M of NaOH (98%, Alfa Aesar, USA) for 30 min and CoPi was applied onto the BiVO4 photoanode via photo-assisted electrodeposition28. A three-electrode cell was used with the BiVO4 working electrode (one sun position), Ag/AgCl reference electrode, and a Pt mesh counter electrode and CoPi was deposited for 6 min at ~0.2−0.3 V (vs. RHE) with photocurrent densities of ~1−4 µA cm−2 in a 0.1 M potassium phosphate buffer at pH 6.7 and containing 0.5 mM of cobalt nitrate (98%, Alfa Aesar, USA).
An m-BiVO4 powder was synthesised under ambient conditions using a novel solid–liquid reaction31, in which equimolar concentrations of bismuth (III) oxide (99.9%, MilliporeSigma, USA) and vanadium oxide (≥99.6%, MilliporeSigma, USA) were dissolved in 250 mL of 0.5 M HNO3 with high-speed stirring. Yellow m-BiVO4 particles were collected via suction filtration after four days of mixing, followed by repeated washings with DI water to remove the unreacted dissolved species and HNO3 before drying at 110°C.
Photoanode characterisation. The XRD patterns of photoanode films were recorded using an X’Pert PRO diffractometer (Malvern Pananalytical, UK) with Cu Kα radiation (λ = 1.5406 Å) from 10–70° in 2θ at a scanning speed of 1° min−1. The HRTEM and SEM images were obtained using a JEM-2100F (JEOL, Ltd., Japan) microscope and a SU8220 (Hitachi High-Technologies Corp., Japan), respectively. Optical properties were determined by UV–vis diffuse reflectance spectroscopy using a UV-3600 (Shimadzu Corp., Japan).
Electrocatalyst synthesis. Synthesis of Co–N/CNT, employing CoTMPP as the Co and N precursors, was conducted following a silica coating-mediated process with some modifications23. We ground 0.28 g of CoTMPP in a mortar for 5 min and the resulting fine powder was mixed with 0.2 g of acid-treated CNT for 10 min. The CoTMPPCl–CNT mixture was heated to 400°C at a ramping rate of 2°C min−1 and maintained at 400°C for 3 h under N2 gas (99.999%) and a flow rate of 1 L min−1. The resulting composite was mixed with 1.0 mL of TEOS in a mortar, followed by mixing with 1.0 mL of formic acid. The paste-like mixture was kept at RT for 12 h in a fume hood, then heated to 900°C at a ramping rate of 2°C min−1, and maintained at that temperature for 3 h. To etch the silica, the pyrolyzed composite was mixed with 1:1 (v/v) of ethanol–10% aqueous HF solution. The slurry was stirred for 30 min, filtered, and washed with ethanol several times. Finally, the HF etching process was repeated in the same manner and the product was oven-dried at 60°C.
Electrochemical characterisation (RRDE). Electrochemical measurements were conducted using an electrochemical workstation (CHI760E, CH Instruments, Inc., USA). The three-electrode system was constructed for measurement using a graphite rod counter electrode, an Ag/AgCl reference electrode, and a catalyst-loaded RRDE, AFE7R9GCPT, Pine Research Instrumentation, Inc., USA). Before each use, the RRDE was polished with 1.0 and 0.3 μm of aqueous alumina suspensions and a microcloth. The Co–N/CNT catalyst ink was prepared by mixing 5 mg of catalyst, 50 μL of DI water, 12.5 μL of Nafion dispersion (5 wt.%, D521, Dupont de Nemours, Inc., USA), and 537 μL of anhydrous ethanol. The catalyst ink was homogenised using an ultrasonic bath. Then, 6 μL of the ink was deposited onto the RRDE disk. The resulting catalyst loading was 0.2 mg cm−2. After soaking the catalyst-loaded RRDE into a N2-saturated electrolyte, cyclic voltammetry (CV) was performed between 0.05 and 1.20 V (vs. RHE) at a scan rate of 100 mV s−1. Steady CV responses were observed within 20 cycles. The Pt ring of the RRDE was electrochemically cleaned in the same potential range with a scan rate of 500 mV s−1 for 50 cycles. Electrochemical impedance spectroscopy was performed at 0.68 V (vs. RHE) from 100,000–1 Hz at an electrode rotation speed of 1600 rpm in an O2-saturated electrolyte. The series resistance was determined at the high-frequency tail of the Nyquist plot for iR-compensation. The ORR polarization curve was obtained by LSV from 1.2–0.2 V (vs. RHE) at a scan rate of 5 mV s−1 and at electrode rotation speeds of 2025, 1600, 1225, and 900 rpm for Koutecky–Levich analysis (Equation (1)). To correct the non-Faradaic current (capacitive current) from the LSV curve, the same measurements were made in an N2-saturated electrolyte.
![](https://myfiles.space/user_files/58653_1b1c6aeb34a62c68/58653_custom_files/img1610571265.jpg)
In equation (1), i, ik, n, F, A, DO, v, CO, and ω represent the measured current, kinetic current, electron transfer number, Faraday constant (96,485 C mol−1), diffusion coefficient of O2, kinematic viscosity, O2 concentration, and the electrode rotation speed, respectively. The plot of i−1 as a function of ω−1/2 yields a line with a slope of B−1, which was used to calculate n. The DO, v, and CO values vary with the electrolyte chosen; here, DO = 8.90 × 10−6 cm2 s−1, v = 0.0111 cm2 s−1, and CO = 1.21 × 10−6 mol cm−3 for a 0.1 M NaPi electrolyte32-34.
To measure H2O2 yield, the Pt ring potential was held at 1.3 V (vs. RHE) during the LSV measurements and the H2O2 selectivity was then calculated according to the following equation:
![](https://myfiles.space/user_files/58653_1b1c6aeb34a62c68/58653_custom_files/img1610571326.jpg)
where id, ir, and N indicate the disk current, ring current, and the collection efficiency (37%, provided by the manufacturer), respectively. To assess the durability of the catalysts, potential cycling tests were performed between 0.6 and 1.0 V (vs. RHE) with an electrode rotation of 1600 rpm and a scan rate of 50 mV s−1 for 1000 cycles in the O2-saturated electrolyte. After cycling, the ORR activity was measured in a fresh electrolyte.
Photo-electrochemical measurements of photoanode and cathode. The LSV curves of the photoanodes were measured using a digital multimeter (Ivium-n-Stat Multichannel potentiostat, Ivium Technologies, The Netherlands) with an Ag/AgCl reference electrode and a Pt wire counter electrode in an Ar-saturated 0.1 M NaPi solution from 0–2 V (vs. RHE) and at a scan rate of 10 mV s−1. The photoanode (3.4 cm2) was illuminated using a 300 W Xe arc lamp (66902 Newport Corp., USA) with an AM 1.5 G filter at 100 mW cm−2 under one sun of simulated illumination. For visible light illumination, the intensity was adjusted to 160 mW cm−2 with a 420 nm cutoff filter. The light intensity was adjusted using a standard Si cell (PEC-SI01, Peccell Technologies, Inc., Japan). The LSV curves of the Co–N/CNT cathode (geometric area = 3.74 cm2) were also measured from 1.2–0.05 V (vs. RHE) at a scan rate of 5 mV s−1 in an O2-saturated 0.1 M NaPi solution at pH 6.
TS-1 synthesis. We synthesised TS-1 as previously reported in the literature35; 2 g of Tween 20 (MilliporeSigma, USA) was added to 21 ml of DI water and this solution was added to 30.3 g of a tetrapropylammonium hydroxide solution (1 M in water, MilliporeSigma, USA) dropwise under stirring. To this mixed solution, 36 g of tetraethyl orthosilicate (≥99.0%, MilliporeSigma, USA) was added dropwise under stirring. After 1 h of stirring, 0.61 g of titanium (IV) butoxide (97%, MilliporeSigma, USA) in 9.12 g of isopropyl alcohol (IPA, ≥99.5%, Daejung Chemicals and Metals Co., Ltd., Republic of Korea) was added dropwise. The molar ratio of the synthesis gel was 1 SiO2:0.01 TiO2:0.17 TPAOH:14.5 H2O:0.009 Tween20:0.88 IPA. After stirring for another 1 h, the resulting solution was transferred into a Teflon-lined stainless autoclave and heated at 150°C for 20 h. The precipitates were separated by centrifugation, washed with water several times, and dried at 110°C overnight. The synthesised sample was calcined in a muffle furnace at 550°C for 15 h.
TS-1 characterisation. The XRD pattern of TS-1 was collected on a D2 phaser (Bruker Corp., USA) using Cu Kα radiation from 5–50° in 2θ at a scanning speed of 0.1° s−1. The N2 sorption isotherm was performed using Belsorp-Max II (MicrotracBEL, Japan). Scanning electron microscopy was performed using an SU-8220 (Hitachi High-Technologies Corp., Japan) at 5 kV. Ultraviolet–visible diffuse reflectance and infrared spectra were recorded on a Cary 5000 (Agilent Technologies, Inc., USA) and Nicolet iS 50 FT-IR spectrometer (Thermo Fisher Scientific, USA), respectively.
Hydrogen peroxide detection. The amount of H2O2 was estimated using the DPD method22. First, DPD and peroxidase (POD) solutions were prepared by dissolving 0.05 g of DPD (≥98.0%, MilliporeSigma, USA) in 5 mL of 0.1 N H2SO4 and 5 mg of POD (horseradish, MilliporeSigma, USA) in 5 mL of DI water, respectively. These solutions were kept at 5°C. Next, 2.7 mL of a 0.1 M NaPi buffer (pH 6.0), 0.05 mL of DPD solution, 0.05 mL of POD solution, and 0.2 mL of sample were mixed. The absorbance of the mixed solution at λ = 551 nm was measured using a UV-2600 (Shimadzu Corp., Japan). The amount of H2O2 was calculated from an external standard curve (R2 > 0.99).
Propylene epoxidation in photo-electro-heterogeneous catalytic system and product analysis. The overall reaction was performed in a single reactor composed of an anode cell and a cathode cell. The anode cell was separated from the cathode cell through a 0.18 mm-thick Nafion membrane (Nafion® 117, MilliporeSigma, USA) and a 0.1 M NaPi solution (pH 2−8) was used as the electrolyte. The anode and cathode cells consisted of a photoanode (3.4 cm2) with 28 mL electrolyte and Co-N/CNT cathode (geometric area = 3.74 cm2) with TS-1 (5 mg mL−1) in a 9 mL electrolyte, respectively. The photoanode and cathode were connected with alligator clips and copper wire as an external circuit.
In the photocatalytic powder system, BiVO4 powder (2.5 mg mL−1) and TS-1 (5 mg mL−1) were added together in a 9 mL electrolyte in a single cell. The photoanode (or BiVO4 powder) was illuminated using a 300 W Xe arc lamp (66902 Newport Corp., USA) with an AM 1.5 G filter at 100 mW cm−2 under one sun of illumination. For visible light illumination, the intensity was adjusted to 160 mW cm−2 with a 420 nm cutoff filter. Argon gas was continuously bubbled into the anode electrolyte and O2 and C3H6 gas were continuously bubbled into the cathode electrolyte from 20 min before the reaction until the reaction was completed. During the reactions, the outlet gases were periodically analysed via gas chromatography (GC, 7820A Agilent Technologies, Inc., USA) equipped with an HP–FFAP column (30 m × 530 µm × 1.00 µm) and flame-ionization detector. For liquid analysis, the sample was filtered using a 0.45-μm hydrophilic syringe filter (13HP045AN, Advantec MFS, Inc., USA). Then, it was injected into the inlet of the GC equipped with an HP-5 (30 m × 320 µm × 0.25 µm) column. The products were quantified based on the external standard curves (R2 > 0.99) of the respective compounds. Propylene oxide selectivity and H2O2 utilisation were calculated according to the following equations:
![](https://myfiles.space/user_files/58653_1b1c6aeb34a62c68/58653_custom_files/img1610571394.jpg)