Heterojunctions of the low bandgap semiconductor bismuth oxyiodide (BiOI) with bulk multilayered graphitic carbon nitride (g-C3N4) and few layered graphitic carbon nitride sheets (g-C3N4-S) are synthesized and investigated as an active photoanode material for sunlight driven water splitting. HR-TEM and elemental mapping reveals formation of a unique heterostructure between BiOI platelets and the carbon nitride (g-C3N4 and g-C3N4-S) network that consisted of dendritic BiOI nanoplates surrounded by g-C3N4 sheets. The presence of BiOI in g-C3N4-S/BiOI and g-C3N4-S/BiOI nanocomposites extends the visible light absorption profile from 500 nm up to 650 nm. Due to excellent charge separation in g-C3N4/BiOI and g-C3N4-S/BiOI, evident from quenching of the carbon nitride photoluminescence (PL) and a decrease in the PL lifetime, a significant increase in photoelectrochemical performance is observed for both types of g-C3N4–BiOI heterojunctions. In comparison to heterojunctions of bulk g-C3N4 with BiOI, the nanocomposite consisting of few layered sheets of g-C3N4 and BiOI exhibits higher photocurrent density due to lower recombination in few layered sheets. A synergistic trap passivation and charge separation is found to occur in the g-C3N4-S/BiOI nanocomposite heterostructure which results in a higher photocurrent and a lower charge transfer resistance.

1. SUPPLEMENTARY INFORMATION
Enhanced Charge Separation in g-C3N4 – BiOI Heterostructures
for Visible Light Driven Photoelectrochemical Water Splitting
Kazi Alam,1†
Pawan Kumar,1†
Piyush Kar,1 Ujwal Thakur,1 Sheng Zeng,1 Kai Cui2 and Karthik
Shankar1
1 Department of Electrical & Computer Engineering, University of Alberta, Edmonton, AB T6G 1H9 Canada
2 Nanotechnology Research Centre, National Research Council of Canada, Edmonton, Canada T6G 2M9
*corresponding author's email address: kshankar@ualberta.ca
†These authors contributed equally
Applied bias photon-to-current efficiency (ABPE):
The ABPE% was calculated by using following folrmula:
ABPE (%) = [J (mA cm–2) x (1.23–Vb)/ P (mW cm–2)] x 100 ……………………… Eqn- (1)
Where, J is the current density, Vb is applied voltage at RHE scale and P is power density of the
incident light.
The applied voltage on Ag/AgCl scale was converted RHE scale by using following expression.
VRHE = VAg/AgCl + 0.059 pH + V0
Ag/AgCl …………………………………………….…Eqn - (2)
Where; V0
Ag/AgCl = 0.197 V.
Electronic Supplementary Material (ESI) for Nanoscale Advances.
This journal is © The Royal Society of Chemistry 2019

2. Figure S1. Equivalent circuit used to model the EIS Nyquist plots for g-C3N4-S, g-C3N4, BiOI, g-
C3N4-S/BiOI, and g-C3N4/BiOI films. EIS data was obtained using AM1.5 G one sun illumination,
in the frequency range of 0.1 to 10000 Hz, and at a potential of -0.2 V vs Ag/AgCl.
Figure S2. Photocurrent response under 425 nm LED light (54.15 W cm –2) during on-off cycle
for (a) g-C3N4-S (black), (b) g-C3N4(blue), (c) BiOI (red), (d) g-C3N4-S/BiOI (wine red) and (e)
g-C3N4/BiOI (green)

3. Figure S3. Mott-Schottky plots of bulk g-C3N4 (blue), g-C3N4-S (black), 40% BiOI/g-C3N4
(green), 60% BiOI/g-C3N4-S (wine red), and BiOI (red).

4. Figure S4. Tauc plots for the determination of the effective optical bandgaps of g-C3N4-S (black),
bulk g-C3N4 (blue), BiOI (red), g-C3N4-S/BiOI (wine) and g-C3N4/BiOI (olive green).

5. Figure S5. UV-Vis absorption spectra collected in diffuse reflectance mode (DR) mode for g-
C3N4-S/BiOI and g-C3N4/BiOI heterostructures before (wine red and green) and after (orange and
light green) several photoelectrochemical cycles respectively.
Figure S6. X-Ray diffractograms of g-C3N4-S/BiOI and g-C3N4/BiOI heterostructures before
(wine red and green) and after (orange and light green) several photoelectrochemical cycles.
Figure S7. Photoelectrochemical re-use data for (a) g-C3N4-S/BiOI (orange) and (b) g-C3N4/BiOI
(light green).

6. Figure S8. ABPE % vs RHE plot under AM1.5G light irradiation (100 mW cm−2) for g-C3N4-
S/BiOI (wine red), g-C3N4/BiOI (green), g-C3N4-S (black), g-C3N4 (blue), BiOI (g-C3N4-S).