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Abdou Lachgar
Semiconductor Heterojunctions
for Enhanced Photocatalytic Hydrogen production
NCEAC/University of Sindh, Jamshoro, Pakistan, Feb. 20-23, 2017
Center for Energy, Environment, and Sustainability
Department of Chemistry
Introducing Wake Forest University
Location
Founded in 1834
Enrollment
Undergraduate: ~5,000
Graduate and professional schools: ~2,500
Total enrollment: ~7,500
Faculty: ~400 Faculty/Student ratio: 1/10
Rankings: 27th among 300 national
Cost (2016-17)
Tuition: ~ $49,000
Room and Board and other fees: ~$15,000
Total: ~$64,000
Financial aid: 75%
Endowment: $2.25 billion
Data
Research is Performed at Three Campuses
Biomedical Sciences Campus
Reynolda Campus Innovation Quarters
People
• 16 tenured/tenure track faculty
• 3 lecturers
• 3 full-time instrumentation managers
• 1 Instructional Technology Specialist
• ~32 full-time graduate students
• Postdoctoral associates/research assistant
professors/visiting scholars
• Undergraduate researchers
Research Projects in my lab
Molecular Building Block Approach
NSF and WFU
Catalysts for Waste-to-Fuel Conversion
NC Biofuel Center NAS and USAID
Heterogeneous Photocatalysis
Center for Energy,
Environment, and Sustainability
Background:
Hydrogen as fuel
Photocatalysis
Challenges and Potential Solutions
Visible-light-active heterojunctions as photocatalysts
Case study: g-C3N4/Sr2Ta2O6
Synthesis
Characterization
Photocatalytic activity
Proposed mechanism
Summary
Outline
Hydrogen as Fuel
Largest mass-specific energy content:
2 H2 (g) + O2(g) 2 H2O (g) DGrxn=-285kJ/mol
119.93 MJ/kg, compared to 44.5 MJ/kg for gasoline.
~8 kg of H2 to drive a range of 400 kms
Clean – H2O is the only product
Most common fuel for fuel cells Natu
ral Gas 48 %
Oil 30 %
Coal 18 %
Electrolysis 4 %
However, 96% of hydrogen is produced from fossil fuels.
Low energy density: 8 kg H2 occupy 90 m3 at 1 atm
Photosynthesis vs. Photocatalysis
Both systems are uphill processes
A Kudo and Y Miseki, Chemical society reviews 2009, 38, 1, 253-278.
Photosynthesis
6CO2 + 6H2O C6H12O6 + 6O2
2H2O 2H2 + O2
Photocatalytic
water splitting
A. Kudo and Y. Miseki, Chemical Society Reviews 2009, 38, 1, 253-278.
(i) Excitation (ii) Migration (iii) Surface chemical rxn.
Semiconductor photocatalysis?
Semiconductors absorb energy to generate electrons and holes in CB and VB.
The photo-generated carriers can be used in electrochemical reactions
Semiconductor photocatalysis - History
A. Fujishima, K. Honda, Nature 1972, 238, 37–38
Photocatalytic water slitting was first reported by Fujishima and Honda in 1972
o Electron hole generation)
TiO2 + h e- + h+
o Oxidation at the TiO2 electrode
2H2O + 4h+ O2 + 4H+
o Reduction at the Pt electrode
2H+ + 2e- H2
Overall reaction
2H2O 2H2 + O2
Semiconductor photocatalysis - History
After 1972, lots of photocatalytic materials have been discovered.
. B. Adeli, F. Taghipour, ECS J Solid state sci technol 2013;2:Q118-Q126
Large band gap (>3.0 eV) Unstable Unsuitable band positions
H+/H2
O2/H2O
Two Major Challenges
Absorption range
Lifetime of photogenerated carriers
Most photocatalysts so far studied are active only in
the UV which represents about 5% of solar spectrum.
Recombination of photogenerated carriers is
thermodynamically favored.
Low Efficiency
Potential Solutions
1. Cocatalyst loading
2. Band gap engineering
– Metal ion doping
– Anion doping
3. Sensitization
– Dye sensitization
– Semiconductor Heterojunctions
S. Patnaik et al, RSC Adv., 2016,6, 46929-46951. M Ni et. al, Ren Sus En Rev 2007, 11, 401-425.
Three Different possible combinations
Semiconductors heterojunctions as photocatalyst
Two semiconductors to form heterojunction.
p-n or non p-n junction
Visible-light-active Semiconductor Heterojunctions
S. Adhikari, A. Lachgar, Renewable & Sustainable Energy Reviews, Submitted.
Type 2: Two visible light
active components
Type 1: Visible light active
and UV active components Type 3: Z-type of mechanism
Three types of visible light active semiconductor heterojunctions
Bi2O3/WO3 and Bi2O3/Ta3N5 Heterojunctions
S. Adhikari et al., RSC Adv., 2015, 5, 91094-91102 S. Adhikari et al., RSC Adv., 2015, 5, 54998–55005
Pseudo first order rate constants for
photocatalytic degradation process of RhB or
4-NA under visible light ( 420 nm).
Scheme for electron-hole
separation at the
Bi2O3/WO3 heterojunction
Example of Type II
Bi2O3/WO3
Amount of hydrogen gas evolved for different
samples in 4 hours (50 mg of catalyst in 50 mL
of 20 % aqueous methanol solution irradiated
with visible light ( 420 nm)
Example of Type III Z-Scheme
Bi2O3/TaON and Bi2O3/Ta3N5
RHydrogen CN/SNO (per mole of CN) = 11 X pristine CN
S. Adhikari et al., ChemSusChem, 2016, 9,1869-1879.
g-C3N4 / SrTa2O6 Heterojunction - Rationale
g-C3N4 /Sr2Nb2O7 (CN/SNO)
Heterojunction
In-situ: g-C3N4/SrTa2O6 heterojunction, in
which one component is metastable oxide
Heterojunction: Relatively low temperature
by Chemie Douce (soft chemistry) method
Photocatalytic study of K2SrTa2O7,
H2SrTa2O7, and metastable, SrTa2O6 .
No previous report: Photocatalysis of
metastable oxide
g-C3N4 / SrTa2O6 Heterojunction - Rationale
P. J. Ollivier and T. E. Mallouk , Chem. Mater., 1998, 10 (10), pp 2585–2587
g-C3N4 / SrTa2O6 Heterojunction - Synthesis
Melamine
Hydrothermal
treatment
Grinding followed
by sonication
200 oC
24 hr
Synthesis
S. Adhikari et al., Applied Catalysis B: Environmental, Submitted.
Synthesis of K2SrTa2O7 by
SSR
Proton exchange to obtain
H2SrTa2O7
Hydrothermal treatment
Calcination to obtain
CN/STO heterojunction
Key Steps
550 oC 4 hr
g-C3N4 / SrTa2O6
Heterojunction
g-C3N4 / SrTa2O6 Heterojunction
K2SrTa2O7 H2SrTa2O7 SrTa2O6 g-C3N4
Crystal structure parameters
Tetragonal Tetragonal Cubic Hexagonal
S. Adhikari et al., Applied Catalysis B: Environmental, Submitted.
PXRD patterns of KSTO,
hydrated KSTO and proton
exchanged form HSTO
PXRD patterns of CN,
STO and CN/STO
heterojunction
PXRD patterns of HSTO
heated at different
temperatures
• Upon heating, HSTO converts to metastable cubic phase of SrTa2O6 (STO) at ~500 oC
• STO converts to Tetragonal Tungsten bronze phase of SrTa2O6 at ~ 900 oC.
• The Heterojunction CN/STO is made of metastable SrTa2O6 and CN.
g-C3N4 /SrTa2O6 Heterojunction - PXRD
10 20 30 40 50 60 70
(200)
(11
0)
(105)(101)
(001)
(22
4)
(11
0)
(2010)(1110)(215)(200)
(0010)(107)
(105)
(101)(004)
(002)
(200)(110)(100)(002)
(001)
KSTO hydrated
In
ten
sit
y,
a.u
.
2degree
KSTO
HSTO
10 20 30 40 50 60 70
(220)(211)(210)(111)
(002)
(100)
(200)
(110)(100)
STO
In
ten
sit
y,
a.
u.
2degree
CN/STO
CN
10 20 30 40 50
STO TTB phase
(200)(110)(100)(002)(001)
900 oC
850 oC
750 oC
550 oC
450 oC
350 oC
(620
)
(311
)
(540
)
(321
)
(410
)
(211
)
(320
)
(001
)(3
10
)
Inte
ns
ity
, a
. u
.
(210
)2degree
25 oCHSTO
S. Adhikari et al., Applied Catalysis B: Environmental, Submitted.
High-resolution STEM images
g-C3N4/SrTa2O6 Heterojunction: Microscopy
STEM images for (a) CN, (b) STO and (c, and
d) CN/STO heterojunction. Color codes for
EDS mapping in (d): N (purple),Ta (yellow), Sr
(pink)
SEM images
SEM images for (a) KSTO, (b) HSTO, (c) STO, (d) CN, (e, and
f) CN/STO heterojunction. (g, h and i) are the elemental
mappings for Sr, Ta and N in image (f) .
g-C3N4 / SrTa2O6 Heterojunction – DRS and TGA
KSTO (Eg= 3.92 eV), HSTO (3.96 eV), STO (3.94 eV) : UV light region
Heterojunction : Extended absorption range up to 450 nm
DRS for different catalysts
TGA/DSC
CN and STO
1:1 mass ratio
S. Adhikari et al., Applied Catalysis B: Environmental, Submitted.
800 600 400 200 0
Binding energy, eV
C1s
282 284 286 288 290 292 294
Binding energy, eV
Survey spectra: Hetrojunction CN and STO only
C1s and N1s peaks in heterojunction: shifted towards lower binding energy
XPS Study
g-C3N4 / SrTa2O6 Heterojunction - XPS
N1s
396 398 400 402
Binding energy, eV
g-C3N4 / SrTa2O6 Heterojunction - Activity
Sample
UV light,
µmol/h
Visible light,
µmol/h
KSTO 2280 0
HSTO 2980 0
STO 216 0
CN 174 156
CN/STO 874 744
Photocatalytic hydrogen production for different catalysts.
Heterojunction = 137 mmol/h/mole of CN
= 9 X pristine CN
[AQY = 0.35 % for CN, 2.62 % for CN/STO heterojunction]
Intimate contact matters!!!
Blend sample does not
show any enhancement
Amounts of hydrogen per g of catalyst
g-C3N4 / SrTa2O6 Heterojunction - Mechanism
VB of STO is lower than that of CN, and CB of CN is higher than that of STO.
STO (Eg = 3.94 eV) cannot be excited upon visible light irradiation.
Photocatalytic hydrogen production by CN/STO: photogenerated electrons in CN.
PL
g-C3N4 / SrTa2O6 Heterojunction - Mechanism
The suppressed PL intensity: less recombination
Small resistance: easy migration of photogenerated electrons from CN to STO
EIS Nyquist plot
DCN= ~ 1200 Ω DCN/STO = ~ 400 Ω
S. Adhikari et al., Applied Catalysis B: Environmental, Submitted.
Summary
Semiconductor heterojunction: To address two major problems in Semiconductor
photocatalysis: Absorption range and recombination rate
The H2 evolution for CN/STO heterojunction(per mole of CN) = 9 x than that of pristine CN
(under visible light irradiation)
Enhanced activity of CN/STO: Efficient charge separation
A well designed heterojunction: three major factors
Selection of materials: nature and stability
Suitable band positions: band alignment
Proper synthetic methods: intimate contact
Acknowledgements
Shiba Adhikari, Cynthia Day, Marcus Wright,
Wake Forest University
Zili Wu, Rui Peng, Karren More, Ilia Ivanov,
Oak Ridge National Lab (ORNL)
Carrie L. Donley, for XPS at UNC Chapel Hill
Zachary D. Hood, Vincent Chen, for TRPL at
Georgia Institute of Technology
Thank you
Additional slides
Activity of CN/SNO vs other heterojunctions
Catalyst Reaction Conditions Co-catalyst
loading
Amount of Hydrogen
per mol of C3N4
Enhancement
factor Reference
g-C3N4/Sr2Nb2O7 aqueous methanol (10 %
vol) solution 2.5 wt. % Pt 107 mmol/h
11 times than the
pristine g-C3N4 This work
g-C3N4/Sr2Nb2O7
blend sample
aqueous methanol (10 %
vol) solution 2.5 wt. % Pt 13.8 mmol/h
1.3 times than the
pristine g-C3N4 This work
g-C3N4/TiO2 aqueous methanol (12.5 %
vol) solution 0.5 wt. % Pt 14 mmol/h
2 times than the pure
g-C3N4
J. Alloys Compd.
2011, 509, L26–L29.
g-C3N4/In2O3 0.1 M L-ascorbic solution 0.5 wt, % Pt 20 mmol/h 5 times than the pure
g-C3N4
Appl. Catal. B Environ.
2014, 147, 940–946
Amount of Hydrogen evolved from photocatalytic water reduction with different g-C3N4 based
photocatalysts under visible light irradiation.
The amount of hydrogen produced per mole of g-C3N4 in the CN/SNO is much higher than
the reported values for similar heterojunctions.
Three possible combinations of two semiconductors to form heterojunctions
Type 2 was chosen to explore the visible light active semiconductor heterojunctions.
Semiconductors Heterojunctions
• Use two or more semiconductors to form composites.
• Generally divided into p-n and non p-n junction
S. Adhikari, A. Lachgar, Renewable & Sustainable Energy Reviews, Submitted 2016.
g-C3N4 /Sr2Nb2O7 composite: XPS
High resolution XPS spectra
Shiba Adhikari et al., ChemSusChem, 2015, Submitted
AQY, TON, TOF
AQY, TON, TOF
Using the activity of 100 mg of sample for 1 hour of visible light irradiation ( = 420
nm), the apparent quantum yields (AQY) were calculated to be 0.38 % for CN and
2.45 % for the CN/SNO heterojunction. The amount of hydrogen produced after 15
hours of visible light irradiation (975 µmol by 100 mg of CN/SNO) was used to
determine the turnover number (TON) for CN/SNO, which was found to reach 1800
in 15 hours with a turnover frequency (TOF) of 120. The details of each equation
used for the calculation of AQY, TON and TOF are provided in the experimental
section.
AQY = 2.45 % for CN/SNO
TON = 1.8 and TOF = 1.2 in 15 hrs
0 25 50 75 100
0
10
20
30
Hyd
rog
en
am
ou
nt,
mm
ol/h
% (by mass) of CN in composite CN/SNO
Figure S5. Amounts of hydrogen generated by different composite (CN/SNO) samples varying the mass percentage of CN in final composite.
Conditions: 50 mg catalyst, 50 mL 10 % vol methanol aqueous solution, 300 W Xe-lamp with filter for visible light irradiation ( 420 nm).
Table S2. Photocatalytic overall water splitting on Sr2Nb2O7 (SNO) or g-C3N4 (CN) powder sample under UV or visible (
420 nm) light irradiation. The reaction was performed on 100 mg of catalyst in 50 mL of pure water (without any hole
scavengers like methanol) with or without 2.5 % (by weight) Pt cocatalyst.
Catalyst
Light source
Surface
area
m2/g
Co-catalyst Rate of gas evolution µmol/h
2.5% loading
(by weight)
H2
O2
SNO UV 64.7 none 5.2 trace
SNO UV 64.7 Pt 45.7 9.4
SNO Visible 64.7 Pt or none 0 0
CN Visible or UV 5.40 none 0 0
CN Visible 5.40 Pt 5.7 0
CN UV 5.40 Pt 8.8 0
CN/SNO visible 47.2 none 0 0
CN/SNO UV 47.2 none 2.8 1.0
CN/SNO UV 47.2 Pt 55.9 14.4
Figure S6. PXRD patterns of CN/SNO composite
photocatalyst before and after the photocatalytic test.
10 20 30 40 50 60 70
In
ten
sit
y,
a.u
.
2, degree
Before
After
0 2 4 6 8 10 12 14
0
50
100
150
200
250
300
3503rd
Hy
dro
ge
n e
vo
luti
on
, m
ol
Irradiation time, hr
1st 2nd
Figure 10. Recyclability test for the CN/SNO heterojunction: photocatalytic hydrogen generation for different cycles. The
photocatalytic setup was degassed with Ar for 30 minutes after each cycle.
Empirical equation used to calculate band positions
ECB = X− 0.5 Eg + E0
EVB = ECB + Eg
Eg is the band gap energy of the semiconductor,
E0 is a scale factor relating the reference electrode’s redox level to absolute vacuum scale
(E0 = −4.5 eV for NHE),
Х is the electronegativity of the semiconductor, which can be expressed as the mean of the
absolute electronegativities of the constituent atoms.
Example: X for Sr, Nb, O are 2.0, 4.40 and 7.54 eV respectively.
Electronegativity of Sr2Nb2O7 is (2.0)*2 + (4.40)*2 + (7.54)*71/11 = 5.36 eV.
Eg = 3.58 eV
ECB (Sr2Nb2O7) = 5.36 -1/2*3.58 - 4.5 = -0.93 eV
EVB (Sr2Nb2O7) = -0.93 + 3.58 = +2.65 eV