vis-active n-p-n photocatalytic structures · for wastewater treatment 1. efficient in the complete...

Vis-active n-p-n photocatalytic structures

IC-ANMBES, 13-15 June, Brasov, Romania

A.Duta, A. Enesca, C. Bogatu Transilvania University of Brasov, Romania R&D Center: Renewable Energy Systems and Recycling

6th International Workshop

"Advanced optical and X-ray characterization techniques of multifunctional materials for information and communication technologies, health and renewable energy applications"

Diaspora in Cercetarea Stiintifica si Invatamantul Superior din Romania, Timisoara, 25-28.04.2016

Outline

Problem 1: the water stress

Problem 2: barriers in scaling up AOPs

Material (VIS-active photocatalysts)

Process (heterogeneous photocatalysis)

Integration (material – process – equipment)

Conclusions

Transilvania University of Brasov www.unitbv.ro 19000 students

18 faculties (9 technical and 9 with non-technical profile), Study Programs: B.Sc. 107; M.Sc. 70, Ph.D. 17 fields

28 departments

29 R&I Centres In the Research Institute of the Transilvania University

(ICDT)

http://www.unitbv.ro/

The R&I Centre Renewable Energy Systems and Recycling

Sustainable Built

Environment

Renewable Energy

Systems

Hybrid systems

Materials for environment and energy

Wastes recycling and

reuse

Sustainable Community

Education and Training

Research and Innovation

Awareness (stakeholders,

community)

The water stress

• Sustainable built environment

• Sustainable water use

• Wastes management

Water and wastewater – sustainable use Fresh water -

sanitation Wastewater –

treatment for recycling, reuse

Using the sludge for energy production

Sanitation

Water recycling Water reuse

Sustainable (waste)water treatment

Bio-treatment Bioaccumulation

(using energy plants)

Membrane processes (UF, RO)

- mature technology - affordable - long duration

- highly efficient - allow water reuse - high cost

Solar driven advanced oxidation

processes (AOP)

- highly efficient - optimized for water reuse - high cost

IUPAC definition: „Change in the rate of a chemical reaction or its initiation: - Under the action of ultraviolet, visible, or infrared radiation - In the presence of a photocatalyst, that absorbs light and is involved in the chemical transformation of the reaction partners.” Silvia E. Braslavskyet al., Pure Appl. Chem. , Vol. 83, No. 4, pp. 931–1014, 2011.

Homogeneous photocatalysis:

UV/O2; UV/H2O2/O2; Fe2+/3+ + UV (Photo-Fenton Systems)

Large pilot plants: tertiary treatment, dissinfection

(Almeria, Valencia)

Sludge + potential polluting by-products

Photocatalysis for wastewater treatment

Almeria, Spain Solar water treatment plant

Heterogeneous photocatalysis („semiconductor-assisted photoreactions”)

UV, VIS, Semiconductor/H2O2 Small laboratory installations

Heterogeneous photocatalysis for wastewater treatment

1. Efficient in the complete removal (mineralization) of pollutants at low concentrations

2. Efficient in mineralizing recalcitrant pollutants: industrial organic by-products, natural matter, micro-organisms, innorganic pollutants

3. No solid and liquid by-products (no sludge)

Advantages

Future prospects: WATER RE-USE

Heterogeneous photocatalysis for wastewater treatment

Barriers to scaling up

The cost

Photocatalyst Photocatalytic

process

Composition Opto-electric

properties Long term stability

Radiation: UV, Vis, solar

Process conditions:

pH, H2O2, …

The installation The photoreactor

Processes based on UV activated TiO2 are about four times more expensive as RO Al-Bastaki, 2004, Chem. Eng. Proces., 43, 935-940

Efficiency

Wide band gap semiconductors: TiO2, ZnO, WO3, SnO2, ZrO2

1. Inert in the working environment - Water stable - Stable over a broad pH range - No leaching

2. Active over long operation periods 3. Easy recovareable 4. Avoiding critical materials 5. Low cost

1. High photoactivity - Reduced recombination - Wide band gap

2. High VIS activity

- Lower band gap

+

The Photocatalyst

The wish-list

Narrow band gap semiconductors: CuxS, CuInS2

Wide band gap semiconductors: TiO2, SnO2 Thin films

Vis- active thin photocatalytic films - Anion doping - Cation doping (?) - Loading with metals (Ag, Pt) Schottky diode?

The possible solutions

State of the art : Enhance the photo-response of wide band gap semiconductors. … and beyond: Associating semiconductors into VIS-active systems

- Diode type systems: n-p (n-p-n-...) semiconductor composites - Tandem systems: n-n semiconductor composites

The Photocatalytic Process

Eg, rutile = 3.02 eV (λg < 413 nm) CB = -0.6 eV VB = +2.4 eV Eg, anatase = 3.2 eV CB = - 0.4 eV VB = + 2.8 eV (λg < 387 nm) Eg, brookite = 3.54 eV (λg < 357 nm)

E hc/λg

Parallel process: recombination h+ + e-

Solution 1: unbalanced consumption of the charge carriers

H+ and HO- concentrations pH

λ < 254 nm

The Photocatalytic Process

Eg, rutile = 3.03 eV (λg < 413 nm) CB = -0.62 eV VB = +2.41 eV Eg, anatase = 3.2 eV CB = - 0.4 eV VB = + 2.8 eV (λg < 387 nm)

E hc/λg

Parallel process: recombination h+ + e-

Solution 2: insure fast charge separation (in situ)

e.g. Degussa P25 (Evonik, 71% anatase, 27% rutile, 2% amorphous)

e-

h+

Addapted from Scanlon D.O., et al., Nature Materials, 2013, 12, 798-801

2.81 eV Efficient charge flow

Lower effective Eg (2.81 eV)

n-p composite photocatalytic systems

Fig. 10. Energy levels diagram for the tandem structures (inset: Eg values and EDX

spectra).

Fig. 10. Energy levels diagram for the tandem structures (inset: Eg values and EDX

spectra).

CuInS2 – SnO2 CuInS2 – TiO2 – SnO2 (solid state solar cell: FTO/TiO2/CIS) Nanu et al., Thin solid films, 492-496, 2003

Eg = 1.45 Eg = 1.22


CuInS2 – SnO2 CuInS2 – TiO2 – SnO2 Mineralization: Pollutant: MB 0.0125mM Radiation: 10 W/m2 15% UV (λ= 365nm) + 85%Vis (λ= 565nm)

Stability, 3 consecutive working cycles

Stability, 3 consecutive working-rinsing cycles

Enesca A et al., , Appl. Cat. B, 69-76, 2016


CuInS2 – SnO2 CuInS2 – TiO2 – SnO2

Poluant: MB 0.0125mM Effect of Radiation Radiation source: 1Vis: 18W 1UV: 18W

Radiation sources 2Vis+ 1UV

4Vis+ 2UV

5Vis+ 2UV

4Vis+ 3UV

2Vis 4UV

7Vis

Irradiance [W/m2] 10 20 23 21 9 9 32

Bleaching efficiency [%] 93,5 94,8 96,4 94,2 80,2 81,5 93,6

Stability (T%)

81% 34%

Composite photocatalysts CuxS – SnO2 ZnO – CuxS(CuxO) – SnO2 TiO2 – CuxS(CuO) – SnO2 (n-type – p-type – n-type, Elecrolyte: the wastewater and the active ionic species) Material design

A. Enesca et al., Appl. Cat. B., 2015, 162, 352 - 363

Sustainable n-p - composite photocatalytic systems Replacing In-based compounds

Eg: 1.47 eV 1.19 eV 1.19 eV

Sustainable n-p - composite photocatalytic systems

Pollutant: MB 0.0125mM (4ppm) Radiation (10 W/m2): 15% UV + 85%Vis

Efficiency: 77% Efficiency: 66%

pH = 6.8

Composite photocatalysts CuxS – SnO2 ZnO – CuxS(CuxO) – SnO2 TiO2 – CuxS(CuO) – SnO2 (n-type – p-type – n-type, Elecrolyte: the wastewater and the active ionic species) Process design

Composite photocatalysts TiO2 – CuxS(CuO) – SnO2 (n-type – p-type – n-type, Elecrolyte: the wastewater and the active ionic species) Process design: trapping the charge carriers

Pollutant: MB 0.0125mM (4ppm) Radiation: 15% UV + 85%Vis

H+ and HO- concentrations pH


Composite photocatalysts TiO2 – CuxS(CuO) – SnO2 (n-type – p-type – n-type, Elecrolyte: the wastewater and the active ionic species) Process design: - Trapping the charge

carriers - Increasing the

efficiency

Acidic and neutral pH

pzc, TiO2 = 6.2

Thiazine dye, MB

Colourless in the absence of oxygen

Side effects: photo-corrosion




efficiency

Efficiency: 75%

Acidic and neutral pH

pzc, TiO2 = 6.2

Efficiency: 60%

Efficiency: 70%




efficiency

Alkaline pH

pzc, TiO2 = 6.2

TiO2(h+) +HO - → HO•

TiO2 -O - + MB(+) → TiO2 - O - MB

E0 = +2.53V




efficiency

Alkaline pH

pzc, TiO2 = 6.2 Initial

Efficiency: 88% Efficiency: 65%


Incident solar radiation

Transmitted solar radiation

Wavelength [nm]

Wavelength [nm] Wavelength [nm] E

ffic

ien

cy [

%]

Eff

icie

ncy

[%

]

Eff

icie

ncy

[%

]

Efficiency [%]

Key component: The photoreactor Thickness Light penetration in water

Dimensions Irradiation duration

Flow Thin film stability Process efficiency Stagnation Inhomogeneous flow

List of pre-requisites - Use solar radiation - Allow pollutants mineralization - Able to treat large amounts of water - Technically feasible - Economic affordable - Minimal changes in the WWT plant

Sustainable n-p - composite photocatalytic systems The photreactor

v=0.01m/s v=0.005m/s

v=0.0025m/s v=0.001m/s

v=0.0025m/s v=0.0001m/s

Key component: The photoreactor Thickness Light penetration in water

Dimensions Irradiation duration

Flow Thin film stability Process efficiency Stagnation Inhomogeneous flow

Sustainable n-p - composite photocatalytic systems The photreactor and the thin film additional prerequisites

Conclusions

1. Heterogeneous photocatalysis has a broad range of applications (WWT,

atmospheric decontamination, surfaces decontamination, etc.)

2. Upscaling photocatalysis in WWT requires market acceptance.

3. Key-barriers need to be jointly approached :

The materials fundamentals: design and development of VIS-active

photocatalytic systems, mimicking SSSC

Photocatalytic process design and optimization

Feasible equipment

4. Interdisciplinary projects can support complex and complete results

Acknowledgements The structural founds project PRO-DD (POS-CCE, O.2.2.1., ID 123, SMIS 2637, No 11/2009) for providing the infrastructure used in this work The PNII-Cooperation project NANOVISMAT, contract no. 162/2012 financed by UEFISCDI which supported the latest research hereby presented.

Thank you!

www.unitbv.ro/icdt; [email protected]

http://www.unitbv.ro/icdt



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vis-active n-p-n photocatalytic structures · for wastewater treatment 1. efficient in the complete...

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