Noble Metal Free, Visible Light Driven Photocatalysis Using TiO2 Nanotube Arrays Sensitized by P-Doped C3N4 Quantum Dots

Copyright WILEY-VCH Verlag GmbH & Co. KGaA, 69469 Weinheim, Germany, 2019.
Supporting Information
for Adv. Optical Mater., DOI: 10.1002/adom.201901275
Noble Metal Free, Visible Light Driven Photocatalysis Using
TiO2 Nanotube Arrays Sensitized by P-Doped C3N4 Quantum
Dots
Pawan Kumar,* Piyush Kar, Ajay P. Manuel, Sheng Zeng,
Ujwal K. Thakur, Kazi M. Alam, Yun Zhang, Ryan Kisslinger,
Kai Cui, Guy M. Bernard, Vladimir K. Michaelis, and Karthik
Shankar*

ELECTRONIC SUPPLEMENTARY INFORMATION
Noble Metal Free, Visible Light Driven Photocatalysis Using TiO2 Nanotube Arrays
Sensitized by P-doped C3N4 Quantum Dots
Keyword: Noble metal free photocatalyst
Pawan Kumar,1‡
* Piyush Kar1‡
Ajay P. Manuel,1
Sheng Zeng,1
Ujwal K. Thakur,1
Kazi M.
Alam,1
Yun Zhang,1
Ryan Kisslinger,1
Kai Cui,2
Guy M. Bernard,3
Vladimir K. Michaelis,3
and
Karthik Shankar1
*

Copyright WILEY-VCH Verlag GmbH & Co. KGaA, 69469 Weinheim, Germany, 2018.
Supporting Information
Pawan Kumar,1
†* Piyush Kar,1
† Ajay P. Manuel,1
Sheng Zheng,1
Ujwal K. Thakur,1
Kazi M.
Alam,1
Yun Zhang,1
Ryan Kisslinger,1
Kai Cui,2
Guy M. Bernard,3
Vladimir K. Michaelis,3
and
Karthik Shankar1,2
*
1
Department of Electrical and Computer Engineering, University of Alberta, 9211 - 116 St,
Edmonton, Alberta, Canada T6G 1H9
2
NRC National Institute for Nanotechnology, 11421 Saskatchewan Dr NW, Edmonton, AB T6G
2M9, Canada
3
Department of Chemistry, University of Alberta, Edmonton, Alberta, T6G 2G2, Canada
*Email: Karthik Shankar (kshankar@ualberta.ca); Pawan Kumar (pawan@ualberta.ca)
†Contributed equally
Section S1
1.0 Materials. Analytical grade citric acid (≥99.5%), urea (98%), 1-Butyl-3-
methylimidazolium hexafluorophosphate (≥97.0%), ammonium fluoride (>98%), aqueous
hydrofluoric acid (49%), ethylene glycol, EG (>99%) acetic acid (>99%), KOH (90%) and
sodium sulfate (≥99%) were of analytical grade and used as received after being procured from

Sigma Aldrich. All other solvents were of HPLC grade and used without further purification.
Deionized water was used throughout all experiments. Titanium foil (99 %, 0.89 mm thickness)
was purchased from Alfa Aesar. Ti foil was cut into 1 cm x 2.5 cm pieces and degreased by
sonication in water, methanol and acetone for 10 min each.
1.1 Structural and physicochemical characterization.
Field emission scanning electron microscopy (FESEM) was used to investigate the surface
morphology of materials. The FESEM images were acquired using a Zeiss Sigma FESEM
operating at an accelerating voltage of 5 kV. The ultrafine morphological attributes of the
samples were determined using high resolution transmission microscopy (HRTEM), on a JEOL
2200 FS TEM/STEM with EDX and EELS (electron energy-loss spectroscopy) detectors, and
operating at an acceleration voltage of 200 kV. The samples for HRTEM analysis were prepared
by scratching the samples from the surface of Ti foil using a razor and depositing directly on a
lacey carbon coated copper TEM grid. The acquired .dm3 HR-TEM images were processed with
Gatan micrograph to calculate size and interplanar d-spacing. Elemental mapping data were
further processed with INCA Energy software and extracted 16-bit depth files were processed in
Gatan micrograph to derive RGB images. Further, inner shell ionization edge (core loss) EELS
spectra for various elements were collected at different sample spots and EELS scanning was
performed in regular mode and in line scan mode to verify the presence of all the constituent
elements. The EELS data for spectra and mapping was processed in GATAN Pro. software and
later plotted in Origin 8.5. To validate the formation of the carbon nitride framework and
succcessful phosphorous doping, 15
N and 31
P solid-state nuclear magnetic resonance (NMR)
spectra were collected on a Bruker Avance 500 NMR spectrometer (B0 = 11.75 T) equipped with
a 4 mm double resonance MAS NMR probe. The 15
N and 31
P NMR spectra were acquired using

the cross polarization[1]
technique, with contact times of 5 ms (15
N) or 3 ms (31
P), a 4.0 µs π/2
pulse (ƔB1/2π = 62.5 kHz) and a recycle delay of 3.0 s, and with broadband proton decoupling
(ƔB1/2π = 62.5 kHz) via two-pulse phase modulation (TPPM).[2]
For the measurement CNPQDs
a powdered sample was packed into a 4 mm zirconia rotor and all spectra were acquired under
magic-angle spinning (MAS) conditions using a spinning frequency of 10 to 14 kHz. 15
N NMR
spectra were referenced to nitromethane δ(15
N) = 0.00 ppm by setting the isotropic peak of a
glycine sample (98 % 15
N) to −347.6 ppm. NB: Liquid NH3 is also a common 15
N reference
compound (δ(15
N) = -380 ppm wrt nitromethane). One can easily convert the values reported
here with respect to liquid NH3 at 0 ppm by adding 380 ppm to all reported values (δwrtNH3 =
δreported + 380 ppm). 31
P NMR spectra were referenced to 85% phosphoric acid (δiso = 0.0 ppm)
by setting the isotropic peak of ammonium dihydrogen phosphate to 0.81 ppm.
The surface chemical composition, binding energy and oxidation state of various elements in
the samples were determined using X-ray photoelectron spectroscopy (XPS) employing an Axis-
Ultra (Kratos Analytical) instrument equipped with a monochromatic Al-Kα source (15 kV, 50
W) and a photon energy of 1486.7 eV in ultrahigh vacuum (∼10−8
Torr). The binding energy of
the C1s XPS peak of adventitious carbons (BE ≈ 284.8 eV) was used as standard (carbon
correction) to assign the binding energies of other elements. System-generated raw data in .vms
format was deconvoluted into various peak components using CasaXPS software and the
exported .csv files were plotted in Origin 8.5. To determine the band structure, work function
and valence band spectra of samples, ultraviolet photoemission spectroscopy (UPS) was
performed using a 21.21 eV He lamp as excitation source.
The crystalline features and phase properties of materials were determined by glancing angle
X-ray diffraction (XRD) recorded on a Bruker D8 Discover instrument using Cu-Kα radiation

(40 kV, λ = 0.15418 nm), and equipped with a LynxEYE 1-dimensional detector. The spectra
were collected in the 4–60° range of 2θ values with a scan size of 0.02°. To determine the optical
properties of the materials, UV-Vis absorption spectra were measured in diffuse reflectance
mode using a Perkin Elmer Lambda-1050 UV–Vis-NIR spectrophotometer equipped with an
integrating sphere accessory. Fourier transform infrared (FT-IR) spectra to discern the infrared-
active vibrational features of various functional groups in the synthesized materials were
recorded on a Digilab (Varian) FTS 7000 FT-Infrared spectrophotometer equipped with a UMA
600 Microscope using a ZnSe ATR accessory. To measure the spectra, scratched samples were
deposited on a ZnSe crystal by maintaining nitrogen gas flow and spectra was acquired by
averaging 64 scans in the frequency range 450–4000 cm-1
.
Figure S1. Average particle size distribution of CNPQDs in water determined with dynamic
light scattering (DLS).

Figure S2. Zeta potential distribution of CNPQDs in water for determination of surface charge

Figure S3. FE-SEM image of (a) top-view of TNA (b) cross-sectional view of TNA (c) top-view
of STNA and (d) cross-sectional view of STNA.
Figure S4. STEM-EDX pattern of TNA samples showing presence of Ti and O elements.

Figure S5. STEM-EDX pattern of CNPQDs-TNA samples showing presence of C, N, P Ti and
O elements.

Figure S6. a) XRD patterns of TNA, CNPQD-TNA, STNA, and CNPQD-STNA; anatase peaks were
assigned in accordance with JCPDS Card # 21-1272, and rutile peaks were indexed to JCPDS Card #
21-1276. b) Raman spectra of TNA, CNPQD-TNA, STNA. Colour: TNA (blue), CNPQD-TNA
(green), STNA (red) and CNPQD-STNA (wine red).
Figure S7. High resolution core level XPS spectra of CNPQDs-STNA in O1s regions.

Figure S8. a) XPS survey scan spectra of CNPQDs-TNAs (black) and CNPQDs-STNAs (red),
HR-XPS spectra of CNPQDs-TNAs nanohybrid in b) C1s, c) N1s, d) P2p, e) Ti2p, and f) O1s
regions.

Fourier transform infrared (FTIR) spectroscopy was employed to investigate vibrational features
and to probe the presence of various functional groups in the material (Figure 7) The FTIR
spectra of CNPQDs show two absorption bands at 3335 and 3433 cm-1
accredited respectively to
the N-H stretching vibration of NH2 groups decorated at the edge of CNPQDs and the -OH
stretch of -COOH and surface adsorbed moisture respectively.[3]
The combination of surface
adsorbed H2O bending vibration and C=O stretch of COOH groups gave FTIR peak at 1680 cm-
1
. The IR absorption peaks at 1592 , 1453 and 1148 cm–1
were due to C=N vibration, C–N stretch
and trazine ring (C3N3) deformation respectively.[4]
The FTIR spectra of CNPQDs-TNA and
CNPQDs-STNA displayed all the peaks corresponded to phosphorus doped carbon nitride
quantum dots along with the observance of some additional peaks which might be due to less
stacked configuration of QDs on the surface of TiO2.
Figure S9. FTIR spectra of CNPQDs (black), CNPQDs-TNA (green) and CNPQDs-STNA
(wine red).

Figure S10. Electric field intensity along the cross-section of a single nanotube in an array of
titania nanotubes (a) CNFQDs-TNA at 450 nm (b) CNFQDs-TNA at 500 nm (d) CNFQDs-
STNA at 450 nm and (e) CNFQDs-STNA at 500 nm; (c) and (f) Simulated optical spectra of
CNFQDs-TNA and CNFQDs-STNA respectively.

Figure S11. Photocurrent density vs applied potential plot during light On-Off cycle for CNPQDs-
TNA under dark conditions (navy blue), under solar simulated AM1.5 G irradiation with 420 nm cut-
off filter (purple), under AM1.5 G irradiation without UV cut-off filter (green) All the measurements
were carried out in 1.0 M KOH solution at a scan rate of 0.1 mV/sec.

Figure S12. Photocurrent density vs applied potential plot for CNPQDs-TNA under dark conditions
(navy blue), under 500 nm LED irradiation (power density 40.58 mW cm-2
) All the measurements
were carried out in 1.0 M KOH solution at a scan rate of 0.1 mV/sec.
Section S2
Calculation of Efficiencies
1. Applied bias photon-to-current efficiency (ABPE):
The photon conversion efficiency of materials under applied bias is expressed by applied bias
photon-to-current efficiency percentage (ABPE%). The plot between ABPE% and applied
voltage on reversible hydrogen electrode (RHE) scale gave value of ABPE efficiency. Following
expression was use to calculate ABPE%
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.
From the ABPE vs RHE plot the calculated maximum ABPE% for TNA, STNA, CNPQDs-TNA
and CNPQDs-STNA was estimated to be 0.043, 0.078, 0.110, 0.142 under AM1.5 G irradiation
with UV filter while ABPE% without UV filter was found to be 0.060, 0.323, 0.604 and 0.929
respectively (Figure xxb). From these result the ABPE% of CNPQDs-STNA was 2.87 times of
STNA and 15.48 times of TNA respectively under solar irradiation without filter. Further the
calculated ABPE% of TNA, STNA, CNPQDs-TNA and CNPQDs-STNA samples under 450 nm
irradiation was found to be 0.005, 0.065, 0.033 and 0.273 which was changed to 0.002, 0.045,
0.008 and 0.066 under 500 nm irradiation (Figure xxc). The highest ABPE% was measure at 450
nm for CNPQDs-STNA samples (54.6 times of TNA).
2. Incident photon-to-current efficiency (IPCE):
The IPCE which is also referred as external quantum efficiency (EQE) demonstrate
photocurrent obtained (numbers of electrons collected) per incident photon flux as a
function of wavelength). The IPCE% of all samples was calculated at 450 nm and 500
nm at an applied bias of 0.6 V vs Ag/AgCl (1.23 V vs RHE, thermodynamic water
splitting potential) The IPCE values was calculated using the following expression.

IPCE% = [1240 x J (mA cm–2
)/λ (nm) x P (mW cm–2
)] x 100 ……………… Eqn- (3)
Where, J is photocurrent density, λ is wavelength of incident light in nm and P is the power
density of incident light.
The IPCE% of TNA, STNA, CNPQDs-TNA and CNPQDs-STNA at 450 nm irradiation was
calculated to be 0.25, 1.02, 1.22 and 3.00 while IPCE% at 500 nm was found to be 0.18, 0.49,
2.02 and 4.06 respectively.
3. Absorbed photon-to-current efficiency percentage (APCE%):
Since in IPCE measurement only incident photon conversion efficiency is calculated
while the loss of fraction photons being unabsorbed by materials didn’t taken into
account. So, a more accurate measurement parameter is needed which involve photon
loss in calculation. Absorbed photon-to-current efficiency (APCE) also referred internal
quantum efficiency (IQE) defined as photocurrent collected per incident photon absorbed
is more promising parameter to calculate performance of devices. The APCE% can be
calculated by following formulas:
APCE% = (IPCE/LHE) x 100 ……………………………………………..… Eqn- (4)
Where, LHE is light harvesting efficiency or absorptance which is numbers of electron
hole pairs produced per incident photon flux. By presuming each absorbed photon
produce equal number of electron hole pairs, Beer’s law can be applied for calculating
LHE or absorptance by following expression.
LHE or absorptance = (1-10-A
)
So APCE% = [1240 x J (mA cm–2
)/λ (nm) x P (mW cm–2
) x (1-10-A
)] x 100 … Eqn- (5)
Where, J is photocurrent density, λ is wavelength of incident light in nm, P is the power density
of incident light, LHE is light harvesting efficiency and A is absorbance at measured wavelength.
4. Faradaic efficiencies
To validate the true origin of photocurrent from the photoelectrochemical water splitting not due
to side reactions or photocorrosion of electrode, the evolved hydrogen at the Pt counter electrode
was measured using photoelectrochemical water splitting H-cell. The samples containing
photoanode was irradiated under AM1.5 G solar simulated light and the evolved hydrogen at Pt
counter electrode was analyzed gas chromatography equipped with pulsed discharge detector
(GC-PDD). The observed hydrogen evolution rate per cm area for TNA, CNPQDs-TNA, STNA

and CNPQDs-STNA samples was calculated to be 2.74, 10.40, 6.34 and 22.08 µmol h-1
respectively. The excellent observed H2 evolution rate of CNPQDs-STNA clearly demonstrate
the photocatalytic performance of STNA samples was improved after decoration of CNPQDs.
Faradaic Efficiency (FE%) is a strong measure to determine the efficiency of the photocatalytic
system to drive photoelectrochemical water splitting to generate hydrogen and oxygen per unit
photocurrent produced. Faradaic Efficiency (FE%) can be defined as a ratio of observed
hydrogen in experimental condition to theoretically evolved H2 determined from photocurrent
density.
% =
! ! " #$
%& ' #$ ! ( " & '!
) · 100
% = -
#$ ! ! "
. /0 123$4.0 /12$4.6 781
$.8 9 .:0 2;<3=
> · 100
Where, J is the photocurrent density (A cm-2
), A is the irradiated area of the photoelectrode
(cm2
), T is time of measurement (seconds), e is the electronic charge (1.602 x 10-19
C) and NA is
the Avogadro number (6.02 x 1023
mol-1
), the amount of hydrogen generated duting PEC is
expressed in moles.
The theoretical hydrogen evolution rate calculated based on photocurrent density for TNA,
CNPQDs-TNA, STNA CNPQDs-STNA was found to be 3.16, 11.75, 6.99 and 23.69 µmol h-1
respectively. Further, from the theoretically calculated and experimentally observed H2 evolution
rate the Faradaic efficiency of the TNA, CNPQDs-TNA, STNA CNPQDs-STNA samples was
measured to be 86.86, 88.49, 90.67 and 93.20 % respectively (Figure S16, Table S1). The
excellent Faradaic efficiencies of the bare samples was slightly increased demonsrtating a
positive effect of quantum dots decoration on the surface of TiO2 which might be due to better
electron capture by the CNPQDs which improve charge carrier seapration.

Figure S13. GC chromatogram of gaseous product collected on Pt counter electrode, showing
peaks for evolved H2 in PEC water splitting using TNA, CNPQDs-TNA, STNA CNPQDs-STNA
as photocatalyst under AM1.5 irradiation.
Table S1. Photoelectrochemical H2 evolution rate calculated experimentally and theoretically
and associated Faradaic efficiencies by using TNA, CNPQDs-TNA, STNA CNPQDs-STNA
photocatalysts.
Sample H2 evolved
(µmol h-1 cm-2)
Theoretical H2
(µmol h-1 cm-2)
Faradaic Efficiency (%)
TNA 2.74 3.16 86.86
CNPQDs-TNA 10.40 11.75 88.49
STNA 6.34 6.99 90.67
CNPQDs-
STNA
22.08 23.69 93.20

Figure S14. GC chromatogram of gaseous product for (a) evolved hydrogen in electrocatalytic
reaction under dark (b) evolved hydrogen in photocatalytic reaction under AM1.5 G irradiation
and (c) evolved oxygen in photoelectrocatalytic reaction under AM1.5 G irradiation using TNA,
CNPQDs-TNA, STNA CNPQDs-STNA as photocatalyst.
Figure S15. GC chromatogram of oxygen in PEC experiment before and after irradiation under
AM1.5 irradiation.

Table S2. Electrochemical and photocatalytic H2 evolution rate calculated using TNA,
CNPQDs-TNA, STNA CNPQDs-STNA photocatalysts.
Sample Electrocatalytic H2 evolution
rate (µmol h-1 cm-2)
Photocatalytic H2 evolution
rate (µmol h-1 cm-2)
TNA 0.0070 0.0017
CNPQDs-TNA 0.0101 0.0177
STNA 0.0015 0.0151
CNPQDs-
STNA
0.0029 0.0683
Figure S16. Photograph of photoelectrochemical water splitting H-cell used for H2 evolution
testing irradiated under AM1.5 G light.
References
[1] A. Pines, M. Gibby, J. Waugh, J. Chem. Phys. 1972, 56, 1776.
[2] A.E, Bennett, C.M. Rienstra, M. Auger, K.V. Lakshmi, and R.G. Griffin, J. Chem. Phys.
1995, 103, 6951.
[3] a) J. Zhou, Y. Yang, C.-y. Zhang, Chem. Commun. 2013, 49, 8605; b) J. Yan, H. Wu, H.
Chen, Y. Zhang, F. Zhang, S. F. Liu, Appl. Catal. B 2016, 191, 130.
[4] a) A. Ferrari, S. Rodil, J. Robertson, Phys. Rev. B 2003, 67, 155306; b) B. Jürgens, E.
Irran, J. Senker, P. Kroll, H. Müller, W. Schnick, J. Am. Chem. Soc. 2003, 125, 10288.

Noble Metal Free, Visible Light Driven Photocatalysis Using TiO2 Nanotube Arrays Sensitized by P-Doped C3N4 Quantum Dots

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