Supplementary data

Construction of Bi2WO6/RGO/g-C3N4 2D/2D/2D hybrid Z-scheme heterojunctions with

large interfacial contact area for efficient charge separation and high-performance

photoreduction of CO2 and H2O into solar fuels

Wan-Kuen Jo, a Santosh Kumar, b Salvador Eslava, b Surendar Tonda a,*

a Department of Environmental Engineering, Kyungpook National University, Daegu 702

701, South Korea.

b Department of Chemical Engineering, University of Bath, Claverton Down, Bath, BA2

7AY, United Kingdom.

.⃰ Corresponding Author:

S. Tonda, E-mail: surendar.t86@gmail.com; surendart@knu.ac.kr

Contact No.: +82 53 950 6584.

Experimental section

Material characterization

The crystal structures of the prepared samples were examined by an X-ray diffractometer

(XRD: Rigaku (D/Max-2500)) with Cu Kα radiation (λ = 1.5406 Å). The surface morphology

and composition of the samples were characterized using a Hitachi (SU8220) field-emission

scanning electron microscope (FESEM). The detailed microstructure of catalysts was

investigated by a transmission electron microscope (TEM, Hitachi, HT 7700) and a field-

emission TEM (FETEM, Titan G2 ChemiSTEM Cs Probe (FEI Company, Netherlands))

equipped with an energy-dispersive X-ray spectrometer (EDS). Fourier transform infrared

1

(FT-IR) patterns were obtained on a PerkinElmer (Frontier) FT-IR/NIR spectrometer. The

ultraviolet-visible diffuse reflectance spectra (UV-vis DRS) were recorded on a Shimadzu

UV-2600 UV-vis spectrophotometer, using BaSO4 as a reflectance standard.

Thermogravimetric analysis (TGA) was performed on a TA Instruments Q500

thermogravimetric analyzer. Nitrogen adsorption and desorption isotherm measurements

were carried out using a BELSORP-max (Japan) equipment at liquid N2 temperature. CO2

adsorption isotherms were recorded on a BELSORP-max (Japan) equipment at 298 K.

Steady-state photoluminescence (PL) measurements were conducted on a Shimadzu RF-6000

spectrofluorophotometer with an excitation wavelength of 365 nm. PL lifetime data were

obtained with an Edinburgh Photonics FLS 980 spectrometer using a picosecond pulsed light-

emitting diode (LED) with an excitation wavelength of 380 nm. A Thermo Scientific K-

Alpha X-ray photoelectron spectrometer (XPS) was applied to analyze the surface electronic

states of samples. The transient photocurrent responses of the prepared samples were

monitored on an IVIUM Technologies electrochemical workstation in a standard three-

electrode cell. A Pt foil and Ag/AgCl electrode (in saturated KCl) served as the counter and

reference electrodes, respectively, and indium tin oxide (ITO) deposited with catalyst was

used as the working electrode. An aqueous Na2SO4 (0.5 M) solution was used as supporting

electrolyte, and a 300 W Xe lamp was chosen as the light source. The working electrode was

prepared as follows: approximately 15 mg of as-prepared catalyst was suspended in 20 µL of

5 wt% Nafion solution and 0.5 mL of ethanol. Then, the mixture was ground to make a

slurry, which was then evenly spread as a thin film onto an ITO glass substrate with an active

area of 1.0 cm2. The resultant coated ITO substrate was then dried at 80 °C. Mott-Schottky

measurements were carried out on a CHI 7081C electrochemical workstation in a

conventional three-electrode glass cell. Glassy carbon electrode coated with catalyst was used

as a working electrode. Ag/AgCl and Pt foil served as the reference and counter electrodes,

2

respectively, and aqueous KNO3 (0.5 M) was used as the electrolyte solution. The Mott-

Schottky plots of the samples were recorded in the dark.

Photocatalytic activity tests

Photocatalytic CO2 reduction experiments were carried out in a gas-closed circulation system

by using a stainless steel reactor (250 mL) with a quartz window. A 300-W Xe arc lamp with

a UV cut-off filter of 420 nm served as the light source to trigger the CO2 reduction reaction.

In a typical process, 50 mg of the catalyst powder was uniformly distributed in the

photoreactor. Prior to light illumination, the reactor was vacuum-treated and purged with high

purity CO2 gas (5 mL/min) for 1 h to ensure that air was eliminated from the reactor. The CO2

gas was passed through a water bubbler to generate a mixture of CO2 and water vapor.

During the irradiation, 1 mL gas was periodically withdrawn from the reactor for quantitative

analysis of products on a Shimadzu Tracera GC-2010 Plus gas chromatograph fitted with

Barrier Ionization Detector and He as a carrier gas. The products were calibrated with a

standard gas mixture and determined by the retention time.

For reusability experiment, the optimum catalyst was collected after each photocatalytic

run and refreshed by washing with deionized water and heat treatment at 100 °C. Then, its

CO2 reduction performance was re-evaluated under the similar conditions mentioned above.

The selectivity toward the formation of CO, CH4, and H2 were simply deduced according to

the following equations.

CO selectivity (% )=2NCO

8NCH4+2NCO+2N H 2

×100

C H 4 selectivity (% )=8NCH4

8NCH4+2NCO+2N H 2

×100

H 2 selectivity ( %)=2N H 2

8NCH 4+2NCO+2N H 2

×100

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In which NCO , NCH4 and N H 2 stand for yields of the different products.

The apparent quantum yield (AQY) of the photocatalyst was calculated using the

following equation according to previously reported works [S1-S3].

AQY (% )= thenumber of reacted electronsthe number of incident photons

×100

AQY (% )= thenumber of evolvedCOmolecules×2the number of incident photons

×100

Photocatalytic H2 generation experiments were carried out in a quartz reactor (volume ~80

mL) at room temperature and atmospheric pressure, and the outlet of the reactor was sealed

with a silicone rubber septum. A 300-W Xe lamp was used as the light source, and the UV

part of the output light was removed by a 420 nm cut-off filter. In a typical experiment, 5 mg

of catalyst powder was suspended in 40 mL of an aqueous solution containing 10 vol%

triethanolamine as a sacrificial donor. Before light illumination, the catalyst suspension was

dispersed through ultrasonic treatment for 10 min, and subsequently, argon was bubbled

through the reactor for 20 min to completely remove the dissolved oxygen and ensure an

anaerobic condition. Throughout the experiment, continuous magnetic stirring was applied at

the bottom of the reactor to maintain homogeneity of the suspension and avoid the

sedimentation of the catalyst particles. During the irradiation, gas samples of 500 µL were

taken out intermittently through the septum to analyze the amount of generated H2 using a

Shimadzu GC-2010 Plus gas chromatograph equipped with a thermal conductivity detector

and He as a carrier gas.

The terephthalic acid (TA) PL probing technique was used to explore the generation of

hydroxyl radicals (•OH) by the visible-light irradiated catalyst. As is well known, TA readily

reacts with •OH to form a highly fluorescent material, 2-hydroxyterephthalic acid (TAOH). In

brief, 50 mg of catalyst was dispersed in 100 mL of a mixture of 5 × 10−4 M TA and 2 × 10−3

4

M NaOH aqueous solutions. The obtained suspension was stirred and then exposed to visible

light. At regular irradiation time intervals, an aliquot of the suspension was collected and

filtered to measure the variation in the maximum PL emission intensity of TAOH at 425 nm

under excitation at 315 nm.

Figures

Fig. S1. Magnified XRD patterns of the prepared CN, BWO, and BWO/RGO/CN catalysts.

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Fig. S2. Tauc plots to determine the optical band gaps of the prepared CN, BWO, RGO/CN,

BWO/CN, and BRC-15 catalysts.

Fig. S3. SEM image of RGO.

Fig. S4. SEM image and corresponding EDS profile of BRC-15 hybrid heterojunction.

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Fig. S5. TEM images of (a) CN, (b) RGO, and (c) BWO samples.

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Fig. S6. TGA patterns of the prepared CN, BWO, and BWO/RGO/CN catalysts.

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Fig. S7. N2 adsorption/desorption isotherms for the CN, BWO, and BWO/RGO/CN

composite samples.

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Fig. S8. CO2 adsorption isotherms for the CN, BWO, and BWO/RGO/CN composite

samples.

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Fig. S9. Time-dependent H2 amounts generated over all the synthesized photocatalysts

(conditions: 300 W xenon lamp with a UV cut-off filter (λ ≥ 420 nm) as light source and 5

mg catalyst).

Fig. S10 (a) XRD and (b) FT-IR patterns for BRC-15 hybrid heterojunction before and after

photocatalytic experiments.

Fig. S11. Mott-Schottky plots of CN and BWO catalysts.

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Fig. S12. •OH trapping PL patterns of (a) CN and (b) BRC-15 in TA solution under visible-

light irradiation.

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Table S1. Elemental composition of CN, BWO, and BRC-15 catalysts from XPS and EDS

measurements.

Analysis Sample Composition in at. % C:N

(0.75)

Bi: W: O

(2:1:6)

C N Bi W O

XPS(surface)

CN 42.68 57.32 - - - 0.74 -

BWO - - 21.82 11.04 67.14 - 1.98:1:6.08

BRC-15 43.54 50.03 0.95 0.46 5.02 0.87 2.06:1:10.91

EDS (bulk)

BRC-15 36.54 41.8 4.51 2.13 15.02 0.87 2.11:1:7.05

Table S2. Fitted parameters from time-resolved PL spectroscopy of CN, RGO/CN,

BWO/CN, and BRC-15 photocatalysts for a 380 nm excitation wavelength.

Photocatalyst τ1/ns τ1/ns B1 B2 τ/ns χ2

CN 1.43 4.1 202.51 48.81 2.52 1.09

RGO/CN 1.97 7.15 310.52 103.44 4.80 1.13

BWO/CN 2.18 8.05 284.67 136.8 5.93 1.24

BRC-15 2.73 11.54 705.14 832.69 10.07 1.05

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References

[S1] J. Liu, Y. Liu, N. Liu, Y. Han, X. Zhang, H. Huang, Y. Lifshitz, S.T. Lee, J. Zhong, Z.

Kang, Science 347 (2015) 970−974.

[S2] S. Tonda, S. Kumar, M. Bhardwaj, P. Yadav, S.B. Ogale, ACS Appl. Mater. Interfaces

10 (2018) 2667–2678.

[S3] S. Tonda, S. Kumar, Y. Gawli, M. Bhardwaj, S.B. Ogale, Int. J. Hydrogen Energy 42

(2017) 5971–5984.

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