Halide perovskites are exciting candidates for broad-spectrum photocatalysts but have the problem of ambient stability. Protective shells of oxides and polymers around halide perovskite nano- and micro-crystals provide a measure of chemical and photochemical resilience but the photocatalytic performance of perovskites is compromised due to low electron mobility in amorphous oxide or polymer shells and rapid charge carrier recombination on the surface. Herein an in situ surface passivation and stabilization of CsPbBr3 nanocrystals was achieved using monolayered graphenic carbon nitride (CNM). Extensive characterization of carbon nitride protected CsPbBr3 nanocrystals (CNMBr) indicated spherical CsPbBr3 nanoparticles encased in a few nm thick g-C3N4 sheets facilitating better charge separation via percolation/tunneling of charges on conductive 2D nanosheets. The CNMBr core-shell nanocrystals demonstrated enhanced photoelectrochemical water splitting performance and photocurrent reaching up to 1.55 mA cm−2. The CNMBr catalyst was successfully deployed for CO2 photoreduction giving carbon monoxide and methane as the reaction products.

1. S1
SUPPORTING INFORMATION
for
Air- and Water-Stable Halide Perovskite Nanocrystals
Protected with Nearly-Monolayer Carbon Nitride for CO2
Photoreduction and Water Splitting
Devika Laishram,1,2
Sheng Zeng,1
Kazi M. Alam,1, 4
Aarat P. Kalra,1,3
Kai Cui,4
Pawan Kumar,1,5
*
Rakesh K. Sharma,2
* and Karthik Shankar1
*
1
Department of Electrical and Computer Engineering, University of Alberta, 9211 - 116 St,
Edmonton, Alberta, Canada T6G 1H9
2
Department of Chemistry, Indian Institute of Technology Jodhpur, Jodhpur, Rajasthan, India
34201
3
Department of Physics, Faculty of Science, 4-181 CCIS, University of Alberta, Edmonton, T6G
2E1, Canada
4
Nanotechnology Research Centre, National Research Council of Canada, Edmonton, Canada
T6G 2M9
5
Department of Chemical and Petroleum Engineering, University of Calgary, 2500 University Dr.
NW, Calgary, Alberta, Canada T2N 1N4
Emails: 1,5
Pawan Kumar (pawan.kumar@ucalgary.ca); 1
Karthik Shankar (kshankar@ualberta.ca);
2
Rakesh K. Sharma (rks@iitj.ac.in)

2. S2
Contents
1.0 Additional experimental details .......................................................................................................... S2
Physicochemical characterization.......................................................................................................... S2
2.0 A brief review of approaches towards achieving single layer carbon nitride.................................... S6
3.0 X-ray diffraction………………………………………………………………………………………………..…………………………..S14
4.0 Raman spectroscopy ………………………………………………………………………………………………………………..…..S15
5.0 Fourier Transform Infrared spectroscopy…………………………………………………………………………………..…S16
6.0 Fluorescence lifetime imaging microscopy (FLIM)..........................................................................S199
7.0 Electrochemical impedance spectroscopy ......................................................................................S221
8.0 Kelvin Probe Force Microscopy (KPFM)............................................................................................. S23
9.0 Additional photoelectrochemical data.............................................................................................. S24
10.0 Results of isotope labeled mass spectrometry ............................................................................... S28
11.0 Interfacial energy level alignment in core-shell CNBr nanocrystals............................................... S29
12.0 References........................................................................................................................................ S33
1.0 Additional experimental details
Physicochemical characterization
The morphological attributes of materials were determined using Field emission scanning electron
microscopy (FESEM). The SEM images of samples deposited on FTO were acquired using a Zeiss
Sigma FESEM operating at an accelerating voltage of 5 keV. Energy dispersive X-ray (EDS)
spectrum and elemental mapping using FE-SEM were acquired by Zeiss Sigma FESEM
w/EDX&EBSD integrated with Oxford AZtecSynergy system with an acquisition time of 300 s.
The FE-SEM images and EDX elemental mapping of the samples deposited on lacy carbon-coated
copper TEM grids were performed on a Hitachi S5500 SEM/STEM working at 30 keV
accelerating voltage. High-resolution transmission electron microscopy (HR-TEM), to investigate
ultrafine morphological attributes of CNM sample were performed using a JEOL JEM-
ARM200CF S/TEM operating at an acceleration voltage of 200 keV. For preparing the CNM

3. S3
sample for HR-TEM, a very dilute dispersion of samples in methanol was sonicated for 30 min
and deposited on a 300-mesh lacy carbon-coated copper TEM grid followed by drying under a
solar simulator for 2h. While the fine structure of the CNMBr sample, EDX elemental line scan
and Electron energy-loss spectroscopy (EELS) were performed on a JEOL 2200 FS TEM/STEM
with EDX operating at an acceleration voltage of 200 keV. As the CNMBr sample was changing
shape under high-energy electron beam a cryo-stage that maintains the sample at low temperature
during measurement was used to slow the rate of morphological change. A very dilute suspension
of CNMBr in hexane was directly deposited on a TEM grid for TEM analysis. The acquired
electronic TEM images in .dm3 format were processed with Gatan micrograph software to analyze
morphology, d spacing FFT and iFFT of the image. Electron energy-loss spectroscopy (EELS)
was performed on CN, CNM and CNMBr samples to gain information about the structural,
coordination and electronic state of samples. EELS spectra that measure inner shell ionization edge
(core loss) of electrons transmitted through the sample was determined in C K-edges and N K-
edges energy loss region. The obtained EELS data was processed and corrected for energy edge
in Gatan micrograph and exported .csv files were plotted using Origin®
2018. Dynamic light
scattering (DLS) to measure the average particle size distribution of samples was performed in
hexane by using Malvern Zetasizer. The surface/subsurface (~10 nm) chemical composition of
samples and binding energy (BE) of elements were determined using X-ray photoelectron
spectroscopy (XPS) using Axis-Ultra, Kratos Analytical instrument and a monochromatic Al-Kα
source (15 kV, 10 mA, 50 W, Rowland circle monochromator) and 1486.7 eV photon energy under
ultrahigh vacuum (∼10−9
Torr). The instrument work function was calibrated with respect to the
binding energy (BE) of the metallic Au 4f7/2 line at 83.96 eV. The spot size was kept at 300
microns. For survey scan the Pass Energy was kept 80 eV, hybrid lens, energy region 1-1100 eV,

4. S4
step size 1 eV and 100 ms dwell time while for HRXPS the pass energy was 10 eV hybrid lens,
energy width 12-16 eV, step size 0.025 eV and 100 ms dwell time. The source energy resolution
for monochromatic Al K(alpha) X-rays was > 760 kcps @ 0.48 eV FWHM (Ag 3d 5/2) at 10 eV
pass energy. The binding energy of the C1s peak of adventitious carbons (BE ≈ 284.8 eV) was
used for the charging correction and calibrating BE of all other elements (Carbon correction). The
deconvolution of acquired raw data in .vms into various peak components was performed using
CasaXPS (2.3.23PR1.0) software and exported data in .csv file was plotted in Origin®
2018. To
XPS fitting was performed by following previously reported literature [1, 2] and obtained fitting
parameters were reported in table S3. For the peak fitting Shirley background function was used
to create a region, while the peak shape was fitted using Lorentzian asymmetric function, LA
(1.53,243) and Gaussian-Lorentzian, GL(30) function. The measurement was done at a magic
angle (~54.7º) and the relative sensitivity factor was acquired from existing data in Casa XPS
software. For the measurement, the sample dispersed in ethanol was deposited on an FTO glass
and dried to make a thin film. The XPS valence band spectra to determine the band structure of
CN and CNM were collected over an 18-eV binding range. The ultraviolet photoemission
spectroscopy (UPS) was used to determine the work function and valence band spectra of samples,
carried out using a 21.21 eV He lamp as an excitation source. The nature of chemical functional
groups using IR active vibrational modes was determined using Fourier transform infrared (FT-
IR) spectroscopy on a Digilab (Varian) FTS 7000 FT-Infrared Spectrophotometer with UMA 600
Microscope equipped with a ZnSe ATR accessory. The measurement was carried out by depositing
samples on a ZnSe crystal and maintaining nitrogen flow using ATR assembly. The spectra were
accumulated by averaging 32 scans in the frequency range of 400–4000 cm-1
. The phase structure
and crystallinity of materials were determined using X-ray powder diffraction (XRD) spectra

5. S5
acquired on a Bruker D8 advance diffractometer radiation source was a Cu X-ray tube (Cu-Kα,
IμSμ, λ = 0.15418 nm) operating at 50 W. This tool is equipped with a 2D detector (VANTEC-
500) and performs Bragg diffraction with a 2θ resolution of ±0.007°. Diffrac.eva software was
used to acquire spectra with a scan size of 0.02° in the 5°–85°range of 2θ values. The UV-Vis
absorption spectra of materials were obtained using a Perkin Elmer Lambda-1050 UV–Vis-NIR
spectrophotometer equipped with an integrating sphere accessory. The UV-Vis spectra for solid
sample deposited on a glass slide in the form of films was acquired in diffuse reflectance mode
while for liquid samples a quartz cuvette was used followed by measuring the absorbance. The
charge carrier’s recombination dynamic was determined using steady-state photoluminescence
(PL) spectra collected a Varian Cary Eclipse fluorimeter with a xenon lamp excitation source and
maintaining a slit width of 5 nm. Raman spectra of the samples were acquired on a Thermo
Scientific DXR2 Raman Microscope with 632 nm laser excitation and an incident power of 20
mW cm−2
. For collecting the spectra a 50 µm confocal pinhole apertures slit, a 2 cm-1
/CCD pixel
element spectral dispersion grating was used and spectra were averaged for 60 s. The mechanism
of charge carrier generation/transport dynamics on the surface of the sample under dark and
irradiation conditions was determined by surface potential (SP)/contact potential difference (CPD)
measured using a peak force KPFM (Kelvin probe force microscopy) Dimension Fast Scan Atomic
Force Microscope (Bruker Nanoscience Division, Santa Barbara, CA, USA). For the
measurement, samples were prepared on conductive FTO glass by depositing a dilute solution of
material by spin-casting followed by drying. For the measurement of the SP map and SP
distribution samples were illuminated perpendicularly with 450 and 520 nm diode laser (Thorlabs)
using a custom-made optical setup. For comparison measurements were also performed under
dark conditions. The surface potential of the sample was acquired using an SCM-PIT cantilever

6. S6
(4.4 N/m stiffness) at 75 nm lift height, 2 kHz lockin bandwidth, and a scan speed of 1 Hz. The
samples were grounded with the AFM chuck using conducting copper tape and surface potential
was mapped by sample routing at zero tip bias. Before the measurement samples were maintained
under dark and light conditions for 5 min to attain charge equilibrium of charge carrier transport.
The work function of Pt-Ir tip was calibrated by measuring the contact potential difference (CPD)
of HOPG and the Pt tip. Fluorescence imaging of samples was performed on a Zeiss Axio
Examiner epifluorescence microscope, using 350, 480 and 535 nm excitations and measuring the
emission at 460, 530 and 610 nm emission. A Zeiss 63x Plan-Apochromat objective was used for
imaging. The average lifetime of the sample was calculated using Fluorescence lifetime imaging
microscopy (FLIM) using 750 nm femtosecond Ti:sapphire laser excitation, equipped with a Zeiss
LSM 510 NLO multiphoton microscope, a FLIM module consisting of a Hamamatsu RS-39
multichannel plate detector, a filter wheel, and a Becker Hickl Q5 SPC730 photon counting board.
2.0 A brief review of approaches towards achieving single layer carbon
nitride
Breaking the intersheets hydrogen bonding by the transformation of bulk g-C3N4 into few to
single-layered sheets is envisaged to improve the performance of CN.[3-5] In this regard,
solvent exfoliation using sonication of bulk CN in cheap solvents such as isopropanol, butanol
and even water have been employed to obtain few-layered thick CN sheets.[6-8] Another
approach to achieve few-layered CN sheets consists of the use of precursors such as NH4Cl
which pyrolyze at high temperatures and produce gas bubbles that work to separate CN into
sheets.[9-11] Although these approaches reduce the number of stacked sheets and improve
performance, the vast majority of charge carriers still experience inter-sheet recombination.
Chemical exfoliation using harsh chemicals such as H2SO4, KMNO4, H3PO4 etc which
protonates strand NH/-NH2 forming ammonium substructure and oxidation/intercalation with

7. S7
counter oxidizing anions can afford one to two-layer thick carbon nitride sheets.[12-14]
Unfortunately, the formed monolayer CN loses crystallinity due to intra-sheet hydrogen
bonding within the same sheets.[3] Recently, approaches to form crystalline carbon nitride in
bulk, few-layered sheets and atomically thick sheets have been introduced.[15-18] HNO3
assisted exfoliation which can protonate stand NH/-NH2 more efficiently and produce strong
oxidizing NO3
-
ions that facilitate the separation of sheets is most appealing.[19] Due to the
reversibility of H-bonding, the formed sheet fragments can re-polymerize to constitute the
long-range π-conjugation systems (gelling process) after removal of HNO3, resulting in
increased periodicity and crystallinity. Inspired by these observations, we coupled CsPbBr3
nanocrystals with monolayer sheets (CNM) to stabilize the materials in harsh polar solvents
and boost photocatalytic performance.

8. S8
Figure S1. SEM images of CNM showing agglomerates and edges of sheets.
Figure S2. FE-SEM images of (a-c) CNMBr on FTO substrate, (d) Electron image of mapped area for SEM
EDX elemental mapping, Elemental mapping of selected area showing distribution of (e) O, (f) Cs (g) Pb (h)
Br (h) Br (i) C (j) N (k) Sn (l) composite image showing distribution of Cs, Pb, Br, C, N and Sn, (m) EDX
spectra of CNMBr showing various elements present.

9. S9
Figure S3. SEM EDX elemental mapping of CNMBr showing (a) Composite image for Cs, Pb, Br, C and N
elements, (b) Cs, (c) Pb, (d) Br, (e) C, (f) N (g) SEM EDX spectra of CNMBr.

10. S10
Figure S4. SEM STEM elemental mapping of CNMBr (a) area of scan and distribution of elements (b) Cs (c)
Pb (d) Br (e) C (f) N

11. S11
Figure S5. HR-TEM images of CNMBr at (a) 200 nm scale bar (b) 50 nm scale bar (c) 5 nm scale bar showing
non-uniform distribution of CsPbBr3 in carbon nitride matrix (d) and (e) at 50 nm scale bar showing CNM
wrapped around CsPbBr3 nanocrystal (f) at 10 nm scale bar showing CNM coated on CsPbBr3 and lattice fringe.

12. S12
Figure S6. Fluorescence microscope images of (a-b) CsPbBr3 nanocrystals under 480 nm excitation and 530
nm emission (c-d) CN under 350 nm excitation and 460 nm emission (e-f) CN under 535 nm excitation and
610 emission.

13. S13
Figure S7. XPS elemental survey scan of CN (black), CNM (blue) and CNMBr (red)
Figure S8. Average particle size distribution of CsPbBr3 (yellow) and CNMBr (red) in hexane determined with
dynamic light scattering (DLS).

14. S14
3.0 X-Ray diffraction (XRD)
The crystalline structure and stacking features of the CN, CNM and CNMBr were determined
by X-ray diffraction (XRD) (Fig. S9a in Supporting Information). Two distinct peaks located at
27.3° and 13.7° in XRD patterns of CN were assigned to (002) and (100) planes of stacked
graphitic carbon nitride framework [20, 21]. The interplanar stacking of carbon nitride sheets gives
broad (002) peak with a 0.32 nm interplanar d spacing while another relatively weak XRD signal
centered at 13.7° with 0.68 nm separation originated from in-plane packing of heptazine units in
finite CN network. CNM also displayed these signature peaks. Interestingly, the (002) peak for
CNM was intense and narrow suggesting improved crystallinity and ordered structure [15, 22]. It
is worthwhile to mention that the transformation of bulk CN sheets into monolayered sheets breaks
inter-layer hydrogen bonding which promotes more ordered re-stacking when the solvent is
removed. The improved periodicity of crystalline CN sheets promotes charge delocalization and
reduces intralayer carrier recombination. The XRD patterns of CNMBr displayed various peaks at
14.8°, 21.4°, 30.4°, 34.3°, 37.7° and 45.6° for the perovskite structure. However, the CNM signals
were hard to identify because a very thin crust of CNM was present on CsPbBr3 nanocrystals. The
additional peaks were observed due to various phases of CsPbBr3 and residual Cs salt.
4.0 Raman spectroscopy
Raman spectra of CN, CNM and CNMBr samples were acquired under 632 nm laser excitation
(Fig. S10b in Supporting Information). Raman spectra of CN displayed a broad Raman signal at
1237 cm-1
due to a combinational D+G band of graphitic structure [23, 24]. The out-of-plane
vibration of sp3
hybridized C/N in graphitic CN structure generates D band while in-plane
vibration of sp2
hybridized C/N atoms gives rise to G band. Unlike graphene oxide/N-doped

15. S15
graphene where D and G bands are well separated, the D and G bands were merged together in the
Raman spectra of CN that might be due to cessation of some vibrational mode in extensively
hydrogen-bonded stacked sheets. Additionally, a broad Raman band at 1403 cm-1
originated due
to cumulative vibrations of residual -CN and C=O. The simulated Raman spectra of CN sheets
also validated the presence of these bands (Fig. S11a in Supporting Information). Interestingly, the
D+G band of CNM was shifted toward higher wavenumbers and the contribution of the D band
was decreased which might be due to the formation of a more ordered crystalline structure during
film formation after removal of solvent. Raman spectra of CNMBr also exhibited signature peaks
of CNM demonstrating a well-preserved carbon nitride structure on the surface of CNMBr.
Figure S9. (a) XRD patterns of CN, CNM and CNMBr (b) Raman spectra of CN, CNM and CNMBr Color: CN
(black), CNM (cyan) and CNMBr (red)

16. S16
5.0 Fourier transform infrared (FTIR) spectroscopy
The infrared active vibrational features due to various chemical functional groups were
investigated using Fourier transform infrared (FTIR) spectroscopy (Fig. S10 in Supporting
Information). The FTIR spectra of bulk carbon nitride (CN) samples contain a signature peak at
792 cm-1
due to triazine (C3N3) ring bending vibration of fused heptazine (C6N7) unit while peaks
in between 1080-1398 cm-1
are due to triazine ring stretching vibrations [25]. The IR absorption
bands between 1440-1700 cm-1
were assigned to bending vibrations of surface adsorbed H2O and
C=O (δH2O, νO–H) stretch [26]. A broad absorption band centered at 3174 cm-1
was due to
terminal/strand –NH2/NH and –OH (νN–H, νO–H) stretch. In order to definitively assign FTIR peaks
to stretching and bending vibrations of specific functional groups/units, we generated a simulated
IR spectrum using GAMESS-US (Mark Gordon, Iowa State University) software. After drawing
the basic heptazine based structure of C3N4 in ChemDraw (PerkinElmer, Waltham, MA), a .sdf
file was imported into MOE (Molecular Operating Environment; Chemical Computing Group,
Montreal, Quebec, Canada), where the overall potential energy of the chemical structure was
minimized in vacuum using defaults. Subsequently, SCF (self-consistent field) calculations were
employed within the RHF/RM1 level of theory in GAMESS-US for geometry optimization and
Hessian calculations [27]. The simulation, which was performed in a vacuum, was used as a
starting point for the final simulation, using water used as a solvent using the polarizable
continuum model (PCM) framework. Raman spectra were predicted in a vacuum, using input CN
IR vacuum spectra, within the RHF/6311G level of theory. A correction factor of 0.96 was used
for plotting all IR and Raman spectra. The Gaussian convolution of 6 cm-1
was applied on all
spectral peaks. All vibrational features of the CN scaffold in the experimental FTIR spectra were
well matched with simulated FTIR spectra further verifying the presence of heptazine constituted

17. S17
CN framework (Fig. S11b in Supporting Information). The FTIR spectra of CNM also displayed
almost identical vibrational features demonstrating that the tertiary N-linked tris-s-triazine network
remains intact during HNO3 mediated exfoliation [28]. The protonation of stand NH2/NH group is
a reversible process and the chemical structure of CN is restored once HNO3 is removed;
subsequently, no change in the IR spectra was observed. However, the –OH peak intensity was
slightly increased which might be due to the surface adsorbed oxygen functionalities and fractional
oxidation of the C/N structure. As evident from previous reports, long-chain alkyl ligands control
and stabilize the CsPbBr3 crystal during synthesis due to grafting on the crystal surface. The
pristine CsPbBr3 nanocrystal displayed an intense IR absorption band at 2850 and 2925 cm–1
due
to symmetric and asymmetric CH2 stretches of CsPbBr3 grafted long alkyl chains [29].
Additionally, several signatures bending vibration peaks of alkyl chains were present in the 927-
1740 cm–1
wavenumber range verifying the presence of dense alkyl chain network on CsPbBr3
crystals [30]. For the in situ synthesized CsPbBr3 embedded CNM (CNMBr), all signature peaks
of CsPbBr3 grafted alkyl chains and heptazine motif of CN structure remain preserved suggesting
successful coupling of CNM with CsPbBr3.

18. S18
Figure S10. FTIR spectra of CN (black), CNM (cyan), CsPbBr3 (yellow)and CNMBr (red)
Figure S11. Simulated (a) Raman and (b) FTIR spectra of CN.

19. S19
6.0 Fluorescence lifetime imaging microscopy (FLIM)
To understand the charge separation mechanism, the lifetime of excited species was measured
from the fluorescence lifetime imaging decay curve (Figure S13). The fluorescence decay curve
was fitted tri-exponentially by the following expression:
I(t) = A1e−t/τ1
+ A2e−t/τ2
+ A3e−t/τ3
(6)
where, A1, A2 and A3 are the percentage contribution of each decay component and τ1, τ2 and τ3 are
values of lifetime components, respectively. The existence of three decay components in the FL
decay curve was assigned to band-to-band recombination, trap assisted recombination and surface-
mediated recombination. The system generated lifetime value of each decay component and their
percentage contribution is given in Table S1. The coherent measure of charge carrier separation
efficiency is an average lifetime (τave) which was calculated from the three tri-exponential
components by the following expression.
τave = (A1τ1
2
+ A2τ2
2
+ A3τ3
2
)/ (A1τ1 + A2τ2 + A3τ3) (7)
The calculated average lifetime for CN and CNM samples was found to be 1.14 and 1.72 ns,
respectively. It is evident from the increased average lifetime value that transformation for bulk
sheets into monolayer sheets increases charge transport on the conjugated CNM sheets leading to
better charge separation as reported in earlier literature.

20. S20
Table S1. The fluorescence decay lifetime and their relative percentage contribution for CN and
CNM
Sample τ1(ns) - A1(%) τ1(ns) - A1(%) τ1(ns) - A1 (%) Average lifetime (τave) ns
CN
CNM
0.471 – 74.34
0.738 – 70.95
1.621 – 16.48
3.606 – 21.21
5.415 – 9.18
5.481 – 7.84
1.14
1.72
Figure S12. Fluorescence lifetime imaging (FLIM) lifetime decay curve of CN (black), CNM
(blue).

21. S21
7.0 Electrochemical impedance spectroscopy
Nyquist plot obtained from Electrochemical impedance spectroscopy (EIS) was used for
determining the semiconductor-electrolyte interfacial (SEI) behaviors of photocatalysts.[31, 32]
The EIS impedance spectra were collected under dark and AM1.5 G irradiation in a frequency
range of 0.1 to 100,000 Hz by applying an external bias of -0.4 V vs Ag/AgCl (Figure S8). The
experimentally obtained EIS Nyquist plot and their best fitted equivalent circuit are shown in Fig.
14d and Inset. The symbol, Rs, Csc, RT, CH, Rs, Q and n are electrolyte resistance, charge transfer
resistance, charge transport resistance, space-charge capacitance, constant phase element,
coefficient. Table S1 demonstrates the values of the fitting parameter obtained from the equivalent
circuit. It can be seen from the graph that the diameter of semicircles arc in the Nyquist plot which
represents to charge transfer resistance of materials was largest for CN. This shows that CN
displayed the highest semiconductor electrolyte resistance for charge transfer. However, when
bulk CN was transformed to monolayer CNM the diameter of the semicircle was decreased
significantly demonstrating reduced inter-sheets charge recombination due to better charge
transport on monolayer sheets. For CNMBr a negligible increase in diameter was observed which
might be due to interfacial charge recombination between CN sheets and CsPbBr3 core. As
expected under irradiation conditions the arc diameter becomes smaller for all the materials due to
better transfer of photogenerated charge between material and electrolyte. The recombination
lifetime of materials under dark and light conditions was also calculated. Evidently, the
recombination lifetime of CN was decreased after transformation into monolayer CNM sheets
which complies with improved charge transport on the π conjugated monolayer sheets.
Interestingly, the CNMBr displayed a manifold increase in lifetime value inferred due to improved
charge separation in between the CsPbBr3 core and CNM shell.

22. S22
Figure S13. Mott-Schottky plots showing flat and potential of (a) CN (b) CNM (c) CNMBr. Experimental
(points) and fitted (line) EIS Nyquist plots of (d) CN (black), CNM (blue) and CNMBr (red) under dark and CN
(purple), CNM (yellow) and CNMBr (cyan) under AM1.5 G solar simulated light (100 mW cm-2
). Insets show
an Equivalent circuit of EIS data under dark and illumination conditions respectively.

23. S23
Table S2. Various equivalent circuit elements obtained from EIS Nyquist plot fitting shown in Figure S14.
8.0 Kelvin Probe Force Microscopy (KPFM)
The surface potential change of CNMBr under dark and irradiation conditions was determined
using Kelvin Probe Force Microscopy (KPFM) to evaluate the mechanism of charge carrier's
dynamics (Figure S15). The surface potential map of CNMBr under dark, 450 and 520 nm laser is
shown in Figure S15a. Under dark conditions, the sample displayed an equal charge distribution
over the surface of the sample. However, when the sample was irradiated under 450 nm laser a
significant contrast change was attributed to increased charge carrier density on the surface of
samples due to improved charge generation/separation. Contrarily, under 520 nm laser, an explicit
small change was observed demonstrating the poor charge generation/separation. The measured
SP under dark, 450 and 520 nm light was found to be 55, 85 and 57 mV Figure S9b. A large surface
shift in average SP difference (+30 mV) was observed at 450 nm suggesting excellent charge
generation under relatively more energetic photons.
Sample RS (Ω) CSC
(F.s-1+n)
RT
( Ω )
CH
( F.s-1+n )
RC
(Ω)
Q
(F.s(-1 + n)
n τ
(µs)
CNBr
light
15.83 8.864×10-7 248.1 1.108×10-8 50.5 2.429×10-4 0.434 220
CNBr
dark
15.69 7.579×10-7
246.7 1.179×10-8
54.73 1.609×10-4
0.5 187
CN light 38.47 7.057×10-9
136.5 3.647×10-5
214.3 4.264×10-4
0.5 0.96
CN dark 41.99 8.191×10-9 146.7 5.225×10-6 0 2.317×10-4 0.5 1.2
CNFM
light
17.52 9.454×10-9
56.41 4.945×10-5
28.1 9.522×10-4
0.5 0.5
CNFM
dark
17.05 1.068×10-8
58.41 5.223×10-5
23.45 1.007×10-3
0.5 0.63

24. S24
9.0 Additional photoelectrochemical data
Figure S14. Photocurrent vs time (i-t) plot showing photoresponse during light On-Off cycle at +0.6 V applied
bias for of (a) CN under solar simulated AM1.5G light irradiation without filter (100 mW cm−2
) (black) and
AM1.5G light irradiation with 420 nm cut-off filter (purple), (b) CNM under solar simulated AM1.5G light
irradiation without filter (100 mW cm−2
) (blue) and AM1.5G light irradiation with 420 nm cut-off filter (yellow),
(c) CNMBr under solar simulated AM1.5G light irradiation without filter (100 mW cm−2
) (red) and AM1.5G
light irradiation with 420 nm cut-off filter (cyan). All the measurement was performed in 0.1 M Na2SO4 solution
at a scan rate of 0.1 mV/sec.

25. S25
Figure S15. (a) Linear sweep voltammogram of CN showing photocurrent density vs applied potential (J-V),
photoresponse (b) J-V curve during light On-Off cycle Color: under dark (navy blue), solar simulated AM1.5G
light irradiation without filter (100 mW cm−2
) (black) and AM1.5G light irradiation with 420 nm cut-off filter
(purple). Photocurrent response vs time of CN during light On-Off cycle at +0.6 V applied bias, under 365, 420,
460, 520, 580, and 640 nm LEDs (100 mW cm−2
) (d) Enlarged photocurrent vs time graph showing photoresponse
of CN under 520, 580 and 640 nm LEDs (21.1 mW cm−2
) All the measurement were performed in 0.1 M Na2SO4
solution at a scan rate of 0.1 mV/sec. Color: 365 (red), 420 (blue), 460 (yellow), 520 (green), 580 (violet), and
640 (navy blue) nm.

26. S26
Figure S16. (a) Linear sweep voltammogram of CNM showing photocurrent density vs applied potential (J-V),
photoresponse (b) J-V curve during light On-Off cycle Color: under dark (navy blue), solar simulated AM1.5G
light irradiation without filter (100 mW cm−2) (blue) and AM1.5G light irradiation with 420 nm cut-off filter
(yellow). Photocurrent response vs time of CNM during light On-Off cycle at +0.6 V applied bias, under 365,
420, 460, 520, 580, and 640 nm LEDs (100 mW cm−2) (d) Enlarged photocurrent vs time graph showing
photoresponse of CNM under 520, 580 and 640 nm LEDs (21.1 mW cm−2) All the measurement were performed
in 0.1 M Na2SO4 solution at a scan rate of 0.1 mV/sec. Color: 365 (yellow), 420 (dark cyan), 460 (red), 520
(magenta), 580 (purple), and 640 (gray) nm.

27. S27
Figure S17. (a) Linear sweep voltammogram of CNMBr showing photocurrent density vs applied potential (J-
V), photoresponse (b) J-V curve during light On-Off cycle Color: under dark (navy blue), solar simulated
AM1.5G light irradiation without filter (100 mW cm−2
) (red) and AM1.5G light irradiation with 420 nm cut-off
filter (cyan). Photocurrent response vs time of CNMBr during light On-Off cycle at +0.6 V applied bias, under
365, 420, 460, 520, 580, and 640 nm LEDs (100 mW cm−2
) (d) Enlarged photocurrent vs time graph showing
photoresponse of CNMBr under 520, 580 and 640 nm LEDs (21.1 mW cm−2
) All the measurement were performed
in 0.1 M Na2SO4 solution at a scan rate of 0.1 mV/sec. Color: 365 (black), 420 (pink), 460 (blue), 520 (green),
580 (orange), and 640 (red) nm.

28. S28
10.0 Results of isotope labeled mass spectrometry
Figure S18. GC chromatogram of dry air and 13
CO2 reduction product using CNMBr as catalyst
Figure S19. GC-Mass spectra of air and 13
CO2 labeled reduction products showing contribution of (a) ion-17
(m/z-17) and (b) ion 29 (m/z-29).

29. S29
11.0 Interfacial energy level alignment in core-shell CNBr nanocrystals
The bright green fluorescence of CNMBr core-shell nanocrystals (see inset of Fig. 4b) provides an
indication that interfacial quenching of the excited state is inhibited. This provides the first indication
that a Type-I heterojunction is present wherein the lowest unoccupied molecular orbital (LUMO) and
highest occupied molecular orbital (HOMO) of the wider bandgap carbon nitride shell straddle the
energy bands of the CsPbBr3 core. The electronic band structures of CN and CNM were determined
using XPS valence band spectra (Fig. S20a, b). The valence band maxima for CN and CNM were found
to be 2.24 and 2.52 eV below the Fermi level. The optical absorption edge of CN (bulk g-C3N4) occurred
at 480 nm (black curve in Fig. 4a) corresponding to a bandgap of 2.58 eV. CNM nanosheets exhibit a
pronounced quantum confinement effect with a hypsochromic shift of the absorption edge to ~415 nm
(cyan curve in Fig. 4a) corresponding to a bandgap of ~3 eV, very similar to values of 2.89-2.98 eV
obtained for mechanically exfoliated and chemical exfoliated g-C3N4 nanosheets. The Fermi levels of
CN and CNM are closer to the LUMO rather than the HOMO indicating that both bulk (CN) and
monolayered carbon nitride (CNM) exhibit n-type conduction. The n-type behavior of CN is also
reported in previous studies. Although the exact Fermi level position cannot be determined using
XPS VB spectra, considering the Fermi level to be the same for CN and CNM, the relative
conduction band values should be equivalent to -0.16 and -0.26 eV respectively. The more negative
CB of CNM compared to CN indicates better-reducing power to achieve CO2 reduction and water
splitting. Indeed, Mott-Schottky measurement displayed a positive slope in the 1/C2
−V curve
further corroborating the n-type nature of CN and CNM (Fig. S13). Furthermore, the flat band
potential (Vfb) values for CN and CNM calculated from the Mott-Schottky plot were found to be -
0.06 and -0.15 V vs Ag/AgCl confirming more negative CB of CNM. The electronic structure and
band edge energies of CNMBr were calculated from the UPS work function (WF) and valence

30. S30
band spectra (Fig. S20c-d). The valence band maximum for CNMBr calculated by linear
extrapolation of the leading edge in the UPS VB spectrum was found to be 1.96 eV below the
Fermi level (Fig. S20c). The subtraction of the energy of emitted secondary electrons (Ecut-off) from
the incident energy of UV light source (He lamp: 21.21 eV) by using the expression WF
(ϕ) = 21.21−Ecut-off provides the value of work function (Fig. 8d). The point of the intersection after
extrapolating the linear region of work function on the X and Y scale gives the cut-off energy
(Inset of Fig. S20d). The presence of two edges at 17.32 and 19.61 eV cut-off energies suggests
the presence of two Fermi edges corresponding to CNM and CsPbBr3. From Ecut-off the work
function values were obtained to be 3.89 and 1.60 eV. Additionally, Mott-Schottky measurement
demonstrates n-type character for CNMBr and flat band position was calculated to be -0.75 eV
(Fig. S13). The highly negative flat band potential and concomitantly more reductive conduction
band in heterostructure are well suited for water splitting and CO reduction. The unusually low
work function of 1.6 eV is likely due to a Cs-rich surface of the CsPbBr3 nanocrystals while the
work function of 3.89 eV corresponds well with carbon nitride whose electron affinity is reported
to be ~3.0 eV. It has been reported that a Cs-rich surface causes CsPbBr3 nanocrystals to exhibit
a very low or even negative electron affinity.[33] KPFM and Mott-Schottky data support the
existence of an interfacial built-in potential that promotes the extraction of photogenerated holes.
Integrating all this information, we constructed a band diagram for the CsPbBr3-CNM core@shell
heterojunction, which is shown in Fig. S21.

31. S31
Figure S20. XPS valence band spectra of (a) CN and (b) CNM; (c) UPS valence band spectrum of CNMBr showing
the estimated values of the valence band maximum below the Fermi level, (d) UPS work function spectra of
CNMBr and the inset shows cut-off energy (Ecut-off) of secondary electrons. The work function (WF) was
determined from the UPS work function spectra using the equation WF (ϕ) = 21.21 – Ecut-off, where 21.21 eV is
the energy of the photons emitted by the He lamp used for UPS.

32. S32
Figure S21. Band diagram of the CsPbBr3-CNM interface based on the XPS and UPS data, and a compilation
of the electron affinity values from the literature; Δ is the interface dipole at the hetero-interface.
Table S3. HRXPS peak deconvolution components and parameters obtained from CasaXPS fitting
software.
Element Backgrou
nd
Position RSF Eff. RSF Line shape Total area Peak
Area
Area constraint FWHM Position constraint %
concentratio
n
CN
C1s Shirley 288.2
284.8
286.3
1.0
1.0
1.0
0.980933
0.980933
0.980933
LA(1.53,243)
LA(1.53,243)
LA(1.53,243)
8936.90 5528.58
2142.49
1440.75
0.0 , 10000000.0
0.0 , 10000000.0
0.0 , 10000000.0
1.16963
1.20313
1.23617
290 , 278
290 , 278
290 , 278
60.67
23.51
15.81
N1s Shirley 398.7
399.7
401.3
404.3
1.8
1.8
1.8
1.8
1.81471
1.81471
1.81471
1.81471
GL(30)
GL(30)
GL(30)
GL(30)
12367.78 7207.24
3999.20
893.65
166.42
0.0 , 10000000.0
0.0 , 10000000.0
0.0 , 10000000.0
0.0 , 10000000.0
1.05328
1.85537
1.07908
1.24693
404 , 392
404 , 392
404 , 392
404 , 392
58.76
32.60
7.29
1.36
O1s Shirley 530.1
530.9
531.9
2.93
2.93
2.93
2.92783
2.92783
2.92783
LA(1.53,243)
LA(1.53,243)
LA(1.53,243)
5876.79 1776.55
2924.08
1180.34
0.0 , 16311.6
0.0 , 16311.6
0.0 , 16311.6
1.58088
1.25431
1.12617
535.36 , 524
535.36 , 524
535.36 , 524
30.21
49.72
20.07
CNM
C1s Shirley 284.8
288.1
285.8
1.0
1.0
1.0
1.0125
1.0125
1.0125
LA(1.53,243)
LA(1.53,243)
LA(1.53,243)
35473.93 4428.96
29795.77
812.88
0.0 , 153456.0
0.0 , 153456.0
0.0 , 153456.0
1.53363
1.92577
1.38883
294.2 , 279.8
294.2 , 279.8
294.2 , 279.8
12.64
85.04
2.32

33. S33
N1s Shirley 398.7
399.9
400.9
406.2
1.8
1.8
1.8
1.8
1.81715
1.81715
1.81715
1.81715
GL(30)
GL(30)
GL(30)
GL(30)
68061.94 36810.67
17217.46
9388.72
3995.95
0.0 , 253419.8
0.0 , 253419.8
0.0 , 253419.8
0.0 , 337301.7
1.48447
1.18832
1.15125
1.38036
407.25 , 393.75
407.25 , 393.75
407.25 , 393.75
407.25 , 393.75
54.60
25.54
13.93
5.93
O1s Shirley 529.9
531.5
532.8
2.93
2.93
2.93
2.86102
2.86102
2.86102
LA(1.53,243)
LA(1.53,243)
LA(1.53,243)
75501.59 42854.06
26622.22
7855.47
0.0 , 205702.8
0.0 , 205702.8
0.0 , 205702.8
1.4029
1.84521
1.62405
536.4 , 525.6
536.4 , 525.6
536.4 , 525.6
55.42
34.43
10.16
CNMBr
C1s Shirley 284.8
289.2
286.5
1.00
1.00
1.00
0.984171
0.984171
0.984171
LA(1.53,243)
LA(1.53,243)
LA(1.53,243)
16047.93 7460.63
5451.18
3391.62
0.0 , 10000000.0
0.0 , 10000000.0
0.0 , 10000000.0
1.37533
2.02735
2.47632
293 , 281
293 , 281
293 , 281
45.76
33.44
20.80
N1s Shirley 398.8
399.8
400.9
401.9
1.8
1.8
1.8
xx
1.86371
1.86371
1.86371
1.86371
GL(30)
GL(30)
GL(30)
GL(30)
7979.63 2014.04
2954.37
1977.79
669.17
0.0 , 10000000.0
0.0 , 10000000.0
0.0 , 10000000.0
0.0 , 10000000.0
1.48
1.34518
1.38917
1.13782
408 , 394
408 , 394
408 , 394
408 , 394
26.45
38.79
25.97
8.79
O1s Shirley 530.0
531.1
532.3
2.93
2.93
2.93
2.86095
2.86095
2.86095
LA(1.53,243)
LA(1.53,243)
LA(1.53,243)
16109.58 6790.15
5053.09
4646.78
0.0 , 58912.2
0.0 , 58912.2
0.0 , 58912.2
1.12547
1.48397
2.00891
537.4 , 526.6
537.4 , 526.6
537.4 , 526.6
41.18
30.64
28.18
Cs3d Shirley 724.2
738.1
725.9
740.0
40.22
40.22
40.22
40.22
44.0545
44.0545
44.0545
44.0545
LA(1.53,243)
LA(1.53,243)
LA(1.53,243)
LA(1.53,243)
16588.56 6912.94
5300.25
1761.97
1150.26
0.0 , 99682.0
0.0 , 99682.0
0.0 , 99682.0
0.0 , 99682.0
1.50926
1.6
1.76741
1.6
742.759 , 721.903
742.759 , 721.903
742.759 , 721.903
742.759 , 721.903
45.70
35.04
11.65
7.60
Pb4f Shirley 138.0
142.9
139.6
144.5
22.74
22.74
22.74
22.74
22.9564
22.9564
22.9564
22.9564
LA(1.53,243)
LA(1.53,243)
LA(1.53,243)
LA(1.53,243)
11905.94 3078.52
1745.85
4039.24
2906.42
0.0 , 33278.7
0.0 , 33278.7
0.0 , 33278.7
0.0 , 36606.6
1.29865
1.03718
2.55806
2.54138
148.502 , 135.555
148.502 , 135.555
148.502 , 135.555
148.502 , 135.555
26.16
14.83
34.32
24.69
Br3d Shirley 67.9
68.9
69.8
70.9
2.84
2.84
2.84
2.84
2.91582
2.91582
2.91582
2.91582
LA(1.53,243)
LA(1.53,243)
LA(1.53,243)
LA(1.53,243)
2148.78 881.76
701.52
357.85
150.48
0.0 , 10695.0
0.0 , 10695.0
0.0 , 10695.0
0.0 , 10695.0
1.06667
0.99518
1.23483
1.08876
73.7736 , 66.1971
73.7736 , 66.1971
73.7736 , 66.1971
73.7736 , 66.1971
42.16
33.54
17.11
7.19
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