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