Self-assembled mesoporous metal oxide thin filmsPurdue University MSE REU August 5, 2004

Heidi Springer

Advisor: Dr. Hugh HillhouseVikrant Urade

What are mesoporous materials?

Porous materials whose pores are between 2 and 50 nm in diameter.

The mesoporous materials investigated here have uniformly ordered pores.

Silica based materials first synthesized by scientists at the Mobil Corporation in 1992.

Non-silica mesoporous materials first reported at UC Santa Barbara in 1994.

Applications

Molecular sieves High surface area catalysts Gas sensors Dye sensitized photovoltaic solar cells

Mesoporous materials are templated by amphiphilic molecules.

An amphiphillic molecule has a hydrophillic (water loving) head and a hydrophobic (water loathing) tail.

A micelle is an association of amphiphillic molecules.

Mesoporous structure is created by the ordered packing of micelles.Courtesy of Brian Eggiman

Why do amphiphiles form micelles? Gibb’s Free Energy (ΔG) is a thermodynamic quantity which

predicts the spontaneity of a reaction. A decrease in Gibb’s Free Energy indicates a spontaneous reaction.

Water molecules create structure by forming hydrogen bonds with one another.

The hydrophobic tails of individual amphiphiles placed in solution force water molecules to associate in a particular way. This decreases the entropy (ΔS) of the system.

When the hydrophobic tails associate to each other (form micelles) in order to minimize their interaction with water molecules they increase the entropy of the system.

ΔG = ΔH - TΔS An increase in entropy, decreases Gibb’s Free Energy of the system

therefore the reaction will occur spontaneously.

What determines micelle shape?

Diagrams compiled from work by Brian Eggiman and Soler-Illia et al, “Chemical Strategies To Design Textured Materials: from Microporous and Mesoporous Oxides to Nanonetworks and Hierarchical Structures”, Chem. Rev. 2002, 102, 4093-4138.

How do we use micelles to form mesoporous metal oxides?

Courtesy of Brian Eggiman

Pore Structure Characterization

X-Ray DiffractionSmall angle XRD

0.6 to 3.0 degrees 2θ

Peaks in XRD pattern show d-spacing between micelles (pores) in parallel planes.

Transmission Electron Microscopy (TEM)

Example of silica XRD pattern indicating mesostructure

Research Goal

Synthesize mesoporous tin oxide thin films with ordered cubic structure. Explore impact of surfactant concentration on

mesostructure formation using amphiphilic triblock copolymers (PEO-PPO-PEO).

Experimental Method Surfactant (amphiphile source)

solution is mixed with metal oxide precursor.

Combined solution is deposited onto glass slides using dip coating.

As slides are withdrawn from solution, solvent evaporates leaving hybrid surfactant/metal oxide thin film.

Experimental Method

Samples are subjected to humidity or thermal treatments to aid structure condensation.

Samples are calcined to remove organic template leaving a uniformly structured mesoporous material.

Surfactant Concentration P123

Pluronic P123 (EO20PO70EO20)Maintained consistent solution of:

5.4 grams tin (IV) chloride 5.4 grams water 41 grams ethyl alcohol.

Varied P123 content from 1.2g – 1.8g(EO):Sn molar ratio of 0.4 – 0.6.

P123 Results

P123 Surfactant Concentration

0

2000

4000

6000

8000

10000

12000

14000

16000

0.6 1.1 1.6 2.1 2.6

Degrees 2-theta

Co

un

ts P

er

Se

co

nd

(C

PS

)

1.2g P123

1.5g P123

1.8g P123

Best mesostructure observed at 1.8g P123.(EO):Sn molar ratio of 0.6.

Surfactant Concentration F127

Pluronic F127 (EO106PO70EO106)Maintained consistent solution of:

5.4 grams tin (IV) chloride 5.4 grams water 41 grams ethyl alcohol.

Varied F127 content from .75 - 2.0 grams.

F127 Results

No XRD peaks observed at lowest concentrations.

Lowest Concentrations F127

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

1 2 3

Degrees 2-theta

Co

un

ts P

er S

eco

nd

(C

PS

)

.75g F127

1.0g F127

F127 Results

Unresolved peaks, indicating poor mesostructure begin to appear at 1.25g F127.

Low Angle XRD at 1.25g F127

0

2000

4000

6000

8000

10000

12000

14000

16000

1 2 3

Degrees 2-theta

Co

un

ts P

er

Se

co

nd

(C

PS

)

XRD patterns of mesostructured tin oxide displaying an Im3m derived structure. (a) after thermal treatment at temperatures up to 250°C and (b) after calcination at 400°C for four hours.

Best structure is observed at 1.75g F127.(EO):Sn molar ratio of 1.4.

F127 ResultsC

ourtesy of Vikrant

Urade

F127 Results

Courtesy of Vikrant Urade

Future Work

Expand synthesis methods to other metal oxides.Synthesize p-type mesoporous metal oxide

for dye-sensitized solar cells.

Acknowledgments

Professor Hillhouse Vikrant Urade and Brian Eggiman Members of the Hillhouse group 2004 MSE REU group

Thank You