1

Enhancement of photocatalytic activity by site-poisoning platinum doped titanium dioxide

Yang ChuThe University of Akron

Committees: Dr. Mesfin Tsige Dr. Toshikazu MiyoshiAdvisor: Dr. Steven S.C. Chuang

Research Presentation

2

Overview of my Master project

MotivationAir pollution

http://news.cutv.com/tupian/2013-12-6/1386305085865.shtmlhttp://www.asyousow.org/health_safety/energy.shtml

Water pollution

3

Coal-fired power plant Oil refinery

4

Water Wind 

Nuclear Geothermal 

http://en.wikipedia.org/wiki/Power_station

Solar

Clean energy

5

• A solar cell that can interconvert chemicals and electricity with the energy of light by the photovoltaic effect.

• Performance of PEC are determined by the hydrogen production and organic degradation.

Photoelectrochemical cell

6

Gratzel, M., Nature, 2001, 414, 338

TiO2 Pt

Jsc: short circuit current densityVoc: open circuit voltagePmax: maximum power of PECPin: power of UV light

7

Photocatalyst: semiconductor

http://www.powerguru.org/semiconductor-doping/M. Gratzel, Nature, 2001, 414, 338

Cyclic voltammetry

Epc: energy level of conduction bandEpa: energy level of valance bandBand gap = Epc-Epa

8

Titanium dioxideThree main crystal structure: rutile, anatase and brookite

http://en.wikipedia.org/wiki/Titanium_dioxideWoodley, S. M.; Catlow, C. R. A. Computational Materials Science 2009, 45, 84-95.

RutileBand gap: 3.0 eV

AnataseBand gap: 3.2 eV

BrookiteBand gap: 3.4 eV

Ti atoms are grayO atoms are red

Slow electrontransfer

Fast Recombination

Surface

9

Challenge of PECs Low performance – Electrolyte– Photocatalyst

Slow electron transfer to acceptorFast electron-hole recombination

10Antoniadou, M.; Bouras, P.; Strataki, N.; Lianos, P., Int J Hydrogen Energy 2008, 33, 5045-5051.

Electrolyte

H2SO4 (aq.)NaOH (aq.)+EtOH

Chemical bias:∆V=0.059 ∆pH

11

Ethanol enhancement

Antoniadou, M.; Bouras, P.; Strataki, N.; Lianos, P., Int J Hydrogen Energy 2008, 33, 5045-5051.

The anode electrolyte: 1.0 mol/L NaOH The cathode electrolyte: 1.0 mol/L H2SO4

20%

12

Easier to be oxidized than H2O due to low oxidized potential

Proposed reaction pathway

Antoniadou, M.; Kondarides, D.; Lianos, P., Catal Lett 2009, 129, 344-349.

OrE0= +1.229V

E0= +0.197VE0= +0.580VE0= +0.42V

13

Organics enhancement

Antoniadou, M.; Lianos, P., Catalysis Today 2009, 144 (1–2), 166-171.

14

Photocatalyst

Enhancement of TiO2

• Increase the porosity • Extend its absorption spectrum from UV to the

visible range: N-TiO2

• Increase photocatalytic activity: Pt-TiO2

15

Mechanism of Pt-TiO2

• Facilitate the interfacial electron transfer to electron acceptors

• Decrease fast electron-hole recombination by serving as an electron sink

A. Yamakata; T. Ishibashi; H. Onishi, J. Phys. Chem. B 2001, 105, 7258.

16

Photocatalytic degradation of various organics over Pt-TiO2

Toluene

Young, C.; Lim, T. M.; Chiang, K.; Scott, J.; Amal, R., Applied Catalysis B: Environmental 2008, 78, 1-10.

CO2 concentration over time

Methylene blue (MB)

Z. Yu and S. S. C. Chuang, Appl. Catal., B, 2008, 83, 277-285

CO2

Pt-TiO2

UV

IR intensity over time

17

Challenge

• Low performance– Electrolyte– Photocatalyst

Slow electron transfer to acceptorFast electron-hole recombination

18

Hypothesis

Fast hole-electron recombination

Significantly decrease or inhibit electron-hole recombination

Block electron generating site

19

Site poisoning: H2S

Sulfur

1. Hydrogen production is decreased.2. Organics degradation is increased.

Sulfur

Electron trap

Electron trap

20

Preparation of photocatalystPt-TiO2

Pt-TiO2/H2S

0.5 wt.% Pt-TiO2

6.6 mg H2PtCl6

Z. Yu and S. S. C. Chuang, Appl. Catal., B, 2008, 83, 277-285

21

Thin film preparation

TiO2 Pt-TiO2 Pt-TiO2/H2S

Characterization of photocatalyst

22

4000 3500 1500 10000.0

0.5

1.0

1.5Pt-TiO

2/H

2S

Pt-TiO2

Sing

le b

eam

inte

nsity

(a.u

.)

Wavenumber (cm-1)

TiO2

3650

3629

3677

3414

4000 3500 1500 10000

2

4

Pt-TiO2/H

2S

Pt-TiO2

Abs

orba

nce

(a.u

.)

Wavenumber (cm-1)

TiO2

3650

3629

3677

3414

4000 3500 1500 10000.0

0.5

1.0

1.5 3711

Pt-TiO2/H2SPt-TiO

2Sing

le b

eam

inte

nsity

(a.u

.)

Wavenumber (cm-1)

TiO2

3650

3629

3677

3414

4000 3500 1500 10000.0

0.5

1.0

1.5

2.0 3711

Pt-TiO2/H2SPt-TiO2TiO

2

Abs

orba

nce

(a.u

.)

Wavenumber (cm-1)

3650

3629

3677

3414

IR of powder

IR of thin film

H2 O

• Pt-TiO2 and Pt-TiO2/H2S has more hydroxyl group in powder.• TiO2 has more hydroxyl group in thin film.

Single beam: I0Absorbance=log(1/ I0)

23

200 300 400 500 600 700 8000.0

0.5

1.0

1.5

2.0

Pt-TiO2/H

2S

Abs

orba

nce

(a.u

.)

Wavelength (nm)

Pt-TiO2

TiO2

418

200 300 400 500 600 700 8000.0

0.5

1.0

Abs

orba

nce

(a.u

.)

Wavelength (nm)

Pt-TiO2/H

2S

Pt-TiO2

TiO2

418

Characterization of photocatalyst

UV-vis of powder UV-vis of thin film

• Band gap is not changed because of the same onset point.

24

Atom %Ti O Pt S

TiO2 30.89 69.11 0 0Pt-TiO2 35.03 64.77 0.2 0Pt-TiO2/H2S 34.55 64.99 0.2 0.26

15 20 25 30 35 40 45

R

AR

A

R

2

Cou

nts (

a.u.

)

Pt-TiO2/H

2S

Pt-TiO2TiO

2

Pt (111)

Characterization of photocatalystXRD of powder

EDS (Energy-dispersive X-ray spectroscopy) of thin film

• Sulfur is only covered on the platinum.

• Crystal structure of TiO2 is not changed.

• Platinum is too little to be detected by XRD.

25

Characterization of photocatalystTEM of powder

• Particle size of TiO2 is 15~25 nm, Pt is 3~5 nm.• Sulfur cannot been due to the non-conductivity.

26

Characterization of photocatalystSide view: SEM of thin film

Top view: SEM of thin film

• The thickness of Pt-TiO2/H2S is much less than TiO2 and Pt-TiO2.• The optimize thickness is around 10~15 µm.

• Higher porosity causes the higher performance.

27

Experimental setup of PECs

0.1% MB

TiO2

Pt-TiO2

Pt-TiO2/H2S 100 mW/cm2

TiO2, Pt-TiO2 and Pt-TiO2/H2S tested in PECs

28

Current density vs. Voltage

0 10 20 30 40 50 60

0.0

0.5

1.0

1.5

UV

on

UV

off

UV

off

UV

on

UV

off

UV

on

TiO2

Pt-TiO2

Pt-TiO2/H

2S

Cur

rent

den

sity

(mW

/cm

2 )

Time (min)

1.0 V

0.5 V

0.0 V

• UV irradiation has significant effect on the current.• Pt-TiO2 has highest current at 0V because of the higher photocatalytic activity.• Pt-TiO2/H2S has lowest current because of the block electron generating site.

0 10 20 30 40 50 60

1.0 V0.5 V0 V

CO2

CO2

H2

H2

H2

CO2

TiO2

Pt-TiO2

Pt-TiO2/H

2S

Ion

inte

nsity

(a.u

.)

Time (min)

8.88E-13 1.14E-12 1.57E-12

7.71E-13 8.33E-13 1.24E-121.53E-12 1.85E-12

7.54E-13 6.65E-13 5.12E-138.47E-13 6.56E-13 4.90E-13

3.43E-13 3.15E-13

29

MS: H2 and CO2

CO2 blubbing

• Pt-TiO2 and Pt-TiO2/H2S produced much more CO2 than TiO2.

30

Catalyst Forward bias (V)

Average current density

(mW/cm2)

MS H2 intensity

area

H2 volume

(µL)

MS CO2 intensity

area

CO2 volume

(µL)

TiO2

0 0.41 0 0.00 0 0.000.5 0.89 1.53E-12 22.71 3.43E-13 0.511 1.49 1.85E-12 29.38 3.15E-13 0.47

Pt-TiO2

0 0.51 7.71E-13 6.90 8.47E-13 1.240.5 1.16 8.33E-13 8.19 6.56E-13 0.961 1.37 1.24E-12 16.67 4.90E-13 0.72

Pt-TiO2/H2S0 0.38 8.88E-13 9.33 7.54E-13 1.11

0.5 0.64 1.14E-12 14.58 6.50E-13 0.961 0.77 1.57E-12 23.54 5.12E-13 0.76

• TiO2 produced the most H2.• Pt-TiO2 and Pt-TiO2/H2S produced the similar amount of CO2.

• The rate of the gas production will be recorded in the future work.

31

Methylene blue degradation

278.5 nm 597.1 nm Average

Methylene blue Peak height

Degradation percentage

Peak height

Degradation percentage

Degradation percentage

Before degradation 0.265 0.00% 0.514 0.00% 0.00%After degradation by TiO2 0.194 26.79% 0.376 26.85% 26.82%After degradation by Pt-TiO2 0.098 63.02% 0.206 59.92% 61.47%After degradation by Pt-TiO2/H2S 0.142 46.42% 0.289 43.77% 45.09%

200 300 400 500 600 700 8000.0

0.5

1.0

597.1278.5

Abs

orba

nce

(a.u

.)

Wavelength (nm)

Before degradation After degradation by TiO

2

After degradation by Pt-TiO2

After degradation by Pt-TiO2/H

2S

Before degradation

After degradation byTiO2 Pt-TiO2 Pt-TiO2/H2S

UV-vis of electrolyte

• Pt-TiO2 has the best performance to degrade the methylene blue.Y. Chu; M. Lohrasbi; S. S. C. Chuang, Manuscript preparation

32

Possible reasons• Overloaded H2S covered the hole generating site. • The thickness of Pt-TiO2/H2S thin film is much less

than Pt-TiO2 thin film.

Solutions• Optimize the conditions of site poisoning.• Reduce the aggregation size of Pt-TiO2/H2S and make

thin films with the exactly same thickness.

33

Effect of UV light intensity

0

2

4

0.8

0.9

0 5 10 15 20 25

12.3

12.4

UV

on

UV

on

UV

on

UV

on

UV

off

UV

off

UV

off

UV

off

UV

off

UV

on

2.0V1.5V1.0V0.5V

Cur

rent

den

sity

(mA

/cm

2 )

0V

Base

Acid

Time (min)

pHpH

0 5 10 15 20 251.30

1.35

1.40

1.45

1.50

1.55

1.60

2.0 V1.5 V

1.0 V0.5 V

0.0 V

pH

Time (min)

1000 mW/cm2

100 mW/cm2

10 mW/cm2

0 5 10 15 20 2512.60

12.64

12.68

12.72

12.76

12.80

pH

Time (min)

2.0 V1.5 V1.0 V0.5 V0.0 V

UV light intensity affect pH change

• pH vibrate with the on and off of UV irradiation.• Higher UV light intensity causes larger amplitude.

34

UV light affect voltage and current density

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0

0.000

0.001

0.002

0.003 Reverse biasForward bias2000 mW/cm2

1000 mW/cm2

500 mW/cm2

200 mW/cm2

100 mW/cm2

75 mW/cm2

50 mW/cm2

30 mW/cm2

10 mW/cm2

Dark current

75 mW/cm2

100 mW/cm2

Cur

rent

den

sity

(A/c

m2 )

Voltage (V)

0.0 0.50.0000

0.0001

0.0002

0.0003

0.0004

• Current didn’t increase with the forward bias below 75 mW/cm2.• Current increase proportional to the forward bias above 100 nm/cm2.• Low UV light intensity has high Voc and high intensity has high Jsc.

Jsc

Voc

35

Summary

• Photoelectrochemical cell can not only generate hydrogen as clean energy, but also decompose organic wastes for polluted water treatment.

• Pt-TiO2 can enhance photocatalytic activity compared to TiO2.

• H2S is possible to poison Pt for blocking the electron generating site and significantly decrease the electron-hole recombination to increase the organics degradation.

36

Future work

• Performance of TiO2, Pt-TiO2 and Pt-TiO2/H2S thin films with the same thickness will be urgently compared.

• The doping amount of Pt and the conditions of H2S poisoning need to be optimized.

• In-situ infrared spectroscopy is strongly needed to study the photocatalytic degradation pathway of organics on Pt-TiO2/H2S.

37

• Ag and Fe will be tried to replace Pt because they have similar properties for enhancing the photocatalytic activity of TiO2 at low cost.

• Develop the applications of PEC in the utilization of the shale gas that can provide the energy with high efficiency and environmental friendly.

Future work

38

Acknowledgment

• Advisor: Dr. Steven S.C. Chuang• Committee members: Dr. Mesfin Tsige,

Dr. Toshikazu Miyoshi• Mehdi Lohrasbi, Piyapong Pattanapanishsawat, Dan

Huang, Jie Yu• Other group members• Families and friends