The threat of global warming is high due to the extensive use of fossil fuels.Using non-renewable resources is a viable solution. Sunlight can be converted in two ways - into electrical energy and into chemical energy. Water splitting and CO2 are two important methods which can be used in solar cells.

1. Photocatalytic reduction
of CO2
Apratim K, Karthick M, Manohar K.H.

2. Introduction
 The threat of global warming is high due to the extensive use of fossil fuels.
 Using non-renewable resources is a viable solution.
 Sunlight can be converted in two ways - into electrical energy and into chemical
energy
 Water splitting and CO2 are two important methods which can be used in solar cells.

3. Artificial Photosynthesis
The basic process can be split into four steps:
 Generation of charge carriers (electron–hole pairs) upon absorption of photons with
suitable energy from light irradiation
 Charge carrier separation and transportation
 Adsorption of chemical species on the surface of the photocatalyst
 Chemical reactions between adsorbed species and charge carriers

4. Pictorial representation of Artificial
photosynthesis
Courtesy of Toshisba Corporate Research & Development Center
Fig 1: Mechanism of Artificial photosynthesis

5. Photocatalysis
A good photocatalytic material should be able to-
 Separate electron - hole pairs generated and prevent recombination.
 Transfer electrons to the surface for chemical reaction
 Provide catalytic surface for the chemical reaction to take place.
Materials which generate catalytic activity when exposed to light are
called photocatalysts.

6. Possible Redox reactions
Chem. Commun., 2016, 52, 35; DOI: 10.1039/c5cc07613g
Table 1: Some possible reactions related to photocatalytic conversion of CO2 with H2O

7. Drawbacks
 Mismatch between the absorption ability of semiconductor and the solar spectrum
 Charge recombination or poor charge carrier separation
 Low solubility of CO2 in water (approximately 33 µmol in 1 ml of water at 100 KPa and room
temperature)
 Problem of back reactions during CO2 reduction
 Water reduction to hydrogen is a competing reaction

8. TiO2 as Photocatalyst
High efficiency in UV irradiation.
Wide availability
Lack of Toxicity
Durability and Stability
Easy to synthesize at nanoscale (TiO2 nanotubes)
The PROBLEM?
Lack of Photo-response under visible light irradiation
Avelino Corma , Hermenegildo Garcia; Photocatalytic reduction of CO2 for fuel production: Possibilities and Challenges; Journal of Catalysis 308 (2013) 168–175

9. TiO2 - Modifications
• Retarding fast charge recombination
• Reducing the band gap: improving
photoresponse
Doping
&
Creating Heterojunction
• Selectivity towards a single product
• Increasing the recombination time:
Permanent effect
• Solution to photocorrosion
Loading a
Co-catalyst
Eg: Au/Ag NPs on TiO2

10. 1. Doping
Metallic Non-Metallic
Replacing Ti4+ by Fe3+, Pt4+ or Pd2+
Photocorrosion- Leaching and deactivation
Replacing Oxygen/Oxygen vacancies by C, N
or S
More stable TiO2 photocatalysts
Reproducibility problems due to variation in dopant concentrations and its location.
1) A. Fujishima, X. Zhang, D.A. Tryk, Surf. Sci. Rep. 63 (2008) 515–582.
2) C. Burda, Y.B. Lou, X.B. Chen, A.C.S. Samia, J. Stout, J.L. Gole, Nano Lett. 3 (2003) 1049–1051.

11. 2. Forming a Heterojunction
Fig 2: Schematic Representation of ‘surface
heterojunction’ effect in TiO2 , {001} and {101} facets
 Improve light absorption and charge separation
 Autonomous effect: due to the field generated
 Semiconducting QDs can be used to create
heterojunctions
 Results in better visible light response
J. Yu, J. Low, W. Xiao, P. Zhou and M. Jaroniec, J. Am. Chem. Soc., 2014, 136, 8839–8842.

12. 3. Loading a Co- Catalyst
Fig 3: Proposed mechanism for the photocatalytic hydrogen generation assisted by
Au NPs on the TiO2 surface
 Surface plasmon band characteristic: Absorption of visible light
 Supplying e- s to the CB of TiO2
 State of charge separation
 Reproducibility X Water oxidation X Redundant reactions Product Selectivity
Avelino Corma , Hermenegildo Garcia; Photocatalytic reduction of CO2 for fuel production: Possibilities and Challenges; Journal of Catalysis 308 (2013) 168–175

13. Desirable properties
Int. J. Mol. Sci. 2014, 15, 5246-5262; doi:10.3390/ijms15045246
How to accomplish the property Property Effect
Small particle size High surface area High adsorption
Crystalline material Single site structure Homogeneity
Engineering the band gap Light absorption Higher efficiency
Preferential migration along
certain direction
Efficient charge separation Low recombination
Presence of co-catalysts Long lifetime of charge separation Possibility of chemical reactions
High crystallinity High mobility of charge carriers More efficient charge separation
Adequate co-catalysts Selectivity towards single product Efficient chemical process
Table 2: Compendium of all the desired properties and necessary modifications for efficient photocatalysis.

14. TiO2 nanotubes via anodization
 Titanium foils - degreased using acetone, methanol, rinsed with DI water and blow
dried
 Ethylene glycol + Ammonium fluoride is used as electrolyte, Pt as counter electrode
 Electrochemical anodization carried out at 50V for 3 hours
 Annealed at around 450℃ for 2 hours for obtaining the crystalline phase
 Free standing nanotubes can also be obtained by methanol evaporation
Chem. Mater. 2008, 20, 1257–1261

15. Biomaterials and Biotechnology Schemes Utilizing TiO2 Nanotube Arrays, By Karla S. Brammer, Seunghan Oh, Christine J. Frandsen and
Sungho Jin ISBN 978-953-307-609-6, Published: September 15, 2011
Anodization mechanism
Fig 4: Schematic illustration of TiO2 nanotube formation

16. Fig 5: Schematic of experimental setup
Nanoscale, 2014, 6, 14305; DOI: 10.1039/c4nr05371k
CO2 photoreduction experiment

17. CO2 Photoreduction experiment
 Before the start of the experiment the steel chamber is heated to 80℃ to remove
the desorbed gases
 Photocatalyst is kept inside a steel chamber along with water droplets covered with
an optical grade Quartz window
 The chamber is vacuumed and desired pressure of CO2 is filled
 The solar simulator is switched on to irradiate the sample for a period of time
 After the experiment is over, the output valve can be connected to Mass
Spectrometer to analyse the reaction products
Nanoscale, 2014, 6, 14305; DOI: 10.1039/c4nr05371k

18. Conclusion and further work
 The efficiency of artificial photocatalysis is generally lower than in natural
photosynthesis
 Alcohols/amines can be used in place of water to generate H+ ions
 Need for optimal catalyst for CO2 reduction to be applied commercially
 Lack of a single measure of efficiency which would allow for an unequivocal
comparison of heterogeneous photocatalytic systems
 In-situ spectroscopic techniques for understanding elementary steps
 Developing efficient co-catalysts for activation and selective reduction of CO2