Photocatalysis Fundamental and Applications. Environmental Pollution u Atmosphere pollution u Green...

PhotocatalysisPhotocatalysis

Fundamental and Applications

Environmental PollutionEnvironmental Pollution

Atmosphere pollution Green house effect (CO2) Acid rain

Water pollution Soil pollution

Air PollutionAir PollutionSmog

Acid rain

Burning of fossil fuelsBurning of fossil fuels

Water PollutionWater Pollution

Waste water from textile industry

Soil PollutionSoil Pollution

Contaminated soil

Pesticides buried with strong odor

Advanced Oxidation TechnologyAdvanced Oxidation Technology

O3/H2O2

O3/UV

O3/CATALYSTS

Fenton reaction (H2O2/Fe2+)

Photo-Fenton reaction (H2O2/Fe2+/UV)

H2O2/UV

O3/H2O2/UV

UV/TiO2 (Photocatalysis)••OHOH

Nature’s CleanerNature’s Cleaner ：：•• OHOH

In Atmosphere:1) O3+ h(λ < 320 nm) →O2(1∆g) + O (1D)O (1D) + H2O →2•OH2) HONO + h(λ < 400 nm) →NO + •OH [•OH]avg 10∼ 6radicals cm-3 (< 0.1 ppt!!)

In Water:1) FeIII(OH)2+ (aq) + h(λ< 400 nm) →Fe2+ (aq) + •OH

2) NO3-(aq) + h →NO2+ O-

O-+ H2O →OH-+ •OH

Oxidation Potentials of Common Oxidation Potentials of Common Chemical OxidantsChemical Oxidants

Oxidation Potentials (V vs NHE)

HO• 2.80O3 2.07H2O2 1.78HO2• 1.70ClO2 1.57HOCl 1.49Cl2 1.36

What is Photocatalysis?What is Photocatalysis?

The definition of photocatalysis is basically the acceleration of a photoreaction in the presence of a catalyst.

Principle of TiOPrinciple of TiO22 Photocatalysis Photocatalysis

1) Hoffmann, M. S.; Martin, T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69-96.

OH

OH

CB

VB

-0.5V

+2.7V

TiO2

e- / H+

H2O2

O2

OH-OH./OH-

O2-

UV light (< 387.5nm)

3.2eV

O2-/O2

e-

h+

Important Reactions during Important Reactions during PhotocatalysisPhotocatalysis

TiO2UV h+ + e-

e- + O2 O2-

O2- + 2H+ + e- H2O2

H2O2 + O2- •OH + OH- + O2

h+ + H2O •OH + H+

Three Parameters Affecting Three Parameters Affecting Photocatalytic ActivityPhotocatalytic Activity

Light absorption Property Light absorption spectrum and coefficient

Rate of reduction and oxidation of reaction substrate by e- and h+, respectively

Rate of e- and h+ recombination

Enhancement of Photocatalytic Enhancement of Photocatalytic ActivityActivity

Enhancing interfacial charge-transfer Improving charge separation Inhibiting charge carrier recombination

Common Semiconductor Common Semiconductor PhotocatalystPhotocatalyst

TiO2

Why TiO2? Strong oxidizing power of valance band hole Excellent chemical and photochemical stability Availability: One of top 50 chemicals

Band gap: 3.2 eV Only active under UV light (4% of the incoming solar

energy)

Crystal Structure of TiOCrystal Structure of TiO22

Anatase Rutile Brookite

Anatase is the most active one!

Approaches to Improve the Approaches to Improve the Activity of TiOActivity of TiO22

To enlarge band gap by reducing crystal sizes (quantum size effect)

To increase surface area (mesoporous structure) To reduce crystal defects ( high crystallinity ) To dope metal ions To deposit noble metal nanoparticles To couple two kinds of semiconductors

Hot Research Topics of Hot Research Topics of PhotocatalysisPhotocatalysis

How to enhance the efficiency Preparation of nanostructured

photocatalysts Extension of absorption of TiO2 to the

visible region Design of novel non-titania based visible

Light photocatalysts

Nanostructured PhotocatalystsNanostructured Photocatalysts

Nanocrystals Nanoporous materials

Preparation Methods of Preparation Methods of Nanostructured TiONanostructured TiO22

Thermal decomposition method Sol-gel method Microemulsion method Hydrothermal (or solvothermal) method Combustion method Other methods

microwave nonhydrolytic sonochemical

Approaches to Improve Approaches to Improve the Activity of TiOthe Activity of TiO22

Photocatalytic Activity Photocatalytic Activity Enhancement by Noble Metal Enhancement by Noble Metal DepositionDeposition

OH

CB

VB

-0.5V

+2.7V

TiO2

e- / H+

H2O2

O2

OH-OH./OH-

O2-

UV light (< 387.5nm)

3.2eV

O2-/O2

e-

h+

Au

Inhibition of the recombination of h+ and e-!

OH

Photocatalytic Activity Photocatalytic Activity Enhancement by Semiconductor Enhancement by Semiconductor CouplesCouples

CB

VB

-0.5V

+2.7V

TiO2

CB

VB

e-

h+

Inhibition of the recombination of h+ and e-!

TiOTiO22-based Photocatalysts -based Photocatalysts

Responding to Visible LightResponding to Visible Light Sensitization of TiO2

Organic dyes Metal complexes Narrow band gap semiconductors Polymers

Ion-doped TiO2 Metal ions Non-metal ions

Sensitization of TiOSensitization of TiO22-Dye-Dye

CB

VB+2.7V

TiO2

Dye

Dye*

Dye+•

O2

O2-

e-

Visible light

OH

e- / H+

H2O2

This is also the fundamental of dye-sensitized solar cell!

CB

VB

-0.5V

+2.7V

TiO2

O2-/O2

Sensitization of TiOSensitization of TiO22-Narrow Band--Narrow Band-

Gap SemiconductorGap Semiconductor

CB

VB

CdS band-gap:2.4eV

h+

e-

OH

H2O2 O2-

e- / H+

O2

e-

Visible light

Environmental ApplicationsEnvironmental Applications

Water PurificationWater Purification

Water purification (Purifics environmental technologies)

Air CleanerAir Cleaner

Self-Cleaning GlassSelf-Cleaning Glass

Photo-Induced Superhydrophilicity Photo-Induced Superhydrophilicity of TiOof TiO22 Coating Coating

UV

Anti-Bacterial MaterialsAnti-Bacterial Materials

0 min

30 min

60 min

Photo-Electricity Conversion

Strategies of Solar Energy Strategies of Solar Energy ConversionsConversions

LightFuel

Electricity

Electricity

Photovoltaics

sc

e

M

Photosynthesis

Fuels

CO

Sugar

H O

O

2

2

2

H O

O H

2

22

sc M

e

Semiconductor/LiquidJunctions

Traditional Silicon Solar CellTraditional Silicon Solar Cell

GratzelGratzel CellCell

Dye Sensitized Solar Cell

Gratzel, Nature 414, 338 (2001)

Characteristics ofCharacteristics of GratzelGratzel CellCell

Inexpensive 1/10 of amorphous silicon

Flexible Efficiency not high enough Solid electrolyte

20001950 1960 1970 1980 1990

5

10

15

20

25

Effi

cie

ncy

(%

)

Year

crystalline Siamorphous Sinano TiO2

CIS/CIGSCdTe

Efficiency of Photovoltaic Efficiency of Photovoltaic DevicesDevices

Water Splitting Utilizing Solar Energy

-Hydrogen Production-Hydrogen Production

Water Splitting Utilizing Solar Water Splitting Utilizing Solar EnergyEnergy

H2

anodecathode

O2

membrane

MOxMSxe-

H+

4H+ + 4e- 2H2

2H2O O2 + 4H+ + 4e-

纳米二氧化钛光催化性能研究

实验目的

1. 了解纳米光催化材料的性质；2. 确定纳米二氧化钛光催化降解罗丹明 B

水溶液的反应速率常数；3. 了解光催化剂催化性能评价的一般方法

。

仪器与药品

分光光度计，离心机，电动搅拌器，光催化反应器 ( 自制 ) ，卤钨灯（ 220V 500W ）

罗丹明 B ，纳米二氧化钛 P25 （德国Degussa 公司产品）。

实验步骤

1. 取罗丹明 B 水溶液 100 mL 置于光催化反应器 ( 自制 )中，加入 0.1 g P25 ，避光，开启冷凝水，搅拌。

2. 2 h 后，取 6 mL 反应液，离心分离，测上层清液的吸光度 A0 。

3. 0.5 h 后，取 6 mL 反应液，离心分离，测上层清液的吸光度 A0 ，将其与第 2 步测定的吸光度进行比较，判断罗丹明 B 在催化剂上是否达到吸附平衡。

4. 确认罗丹明 B 在催化剂上是否达到吸附平衡后，打开卤钨灯，每隔 1 h 取样 6 mL 反应液，离心分离，取上层清液用分光光度法测定其吸光度 A 。

5. 实验完毕，关闭卤钨灯，停止搅拌，清洗反应器，将仪器恢复原位，桌面擦拭干净。

注 释

1 ．数据处理 lnA 对 t 作图，求出 k 及 t1/2 。2 ．注意事项 实验前仔细阅读离心机说明书，使用时

一定要遵守操作规程。

思考题

1 ．如何确定光催化剂的暗态吸附达到稳定时间？2 ．简述 TiO2 做为光催化剂降解有机污染物 的原理。3 ．欲提高 TiO2 的光催化活性，你认为可采取哪些措施

？

参考文献

[1] Fujishima A ， Honda K. Nature[J]. 1972 ， 37:238~239.[2] Piscopo A ， Robert D ， Weber J V. Journal of Photo

chemistry and Photobiology A: Chemistry[J]. 2001 ， 139 (2):253~256.

[3] 李越湘，吕功煊，李树本等 . 分子催化 [J]. 2002 ， 16(4):241~246.

[4] 黄东升，陈朝凤，李玉花，曾人杰 . 无机化学学报 [J]. 2007 ， 4(4):738 ～ 742.

[5] 张立德，牟季美 . 纳米材料和纳米结构 [M]. 北京 : 科学出版社， 2001.