Energetics of Nanomaterials Energetics of Nanomaterials

and Zeolitesand Zeolites

Alexandra Navrotsky

UC Davis

Control of Polymorphism Control of Polymorphism at the Nanoscaleat the Nanoscale

Competition between polymorphism and surface energy

Free energy crossovers as function of sizeMore metastable polymorphs have lower

surface energies in general

0

4

8

12

16

0 4000 8000 12000

Surface area (m2/mol)

En

thalp

y (

kJ/m

ol)

0 50 100 150

Surface area (m2/g)

brookiterutile

anatase

Enthalpy of titania polymorphs as a function of surface area (8).

Energetics of Nanocrystalline Zirconia

0

20

40

60

80

100

120

140

0 10000 20000 30000 40000 50000

Surface area measured (m2/mol)

H w

.r.t

. bu

lk m

-ZrO

2(k

J/m

ol)

monoclinic

tetragonal

amorphous

ZEOLITES: NANOMATERIALS WITH INTERNAL SURFACES

• Many different framework types, all of enthalpy 8 - 14 kJ/mol above quartz

• Molar volume changes by a factor of two because of large internal pores and channels

• Internal surfaces generated by pores, can be modeled using Cerius2 software

• Can one define a physically meaningful surface energy from slope of trend between enthalpy and internal surface area?

7.20

10.90

9.30

11.40

10.50

13.60

6.60

13.90

8.20

6.80

8.70

0.00

y = 0.6992x - 17.5 R

2 = 0.7147

0

4

8

12

16

25 30 35 40 45 50

Molar Volume (cm 3 /mol)

H

tra

ns (

kJ

/mo

l)

quartz

MTW

MFI

FER

MEL

AFI

AST

BEA

CHA

EMT

FAU

MEI

Surface Energy of 40 nm Particle

Material Enthalpy relative to bulk(kJ/mol)

__________________________________________

Silicalite 0.5

Corundum 10

-alumina 6.5

Rutile 6.2

Brookite 3.1

Anatase 1.2

Low value of surface energy (internal and external) may be what allows many open polymorphs, the manganese oxides may be a test case.

2 4 6 8 10 12 14 16 18 20 22 24 26 280

4

8

12

16

20

24

28

32

Ent

halp

y of

for

mat

ion

rela

tive

to q

uart

z (k

J/m

ol)

Pore size (nm)

Enthalpies of formation of pure-silica mesoporous materials relative to quartz as a function of pore size. represents SBA-15 and MCM-41 materials (Trofymluk et al. 2005); - MCM-48 and SBA-16 materials; - MCM-41 (Navrotsky et al. 1995) - MCM-41 materials from Lee, B. MS thesis 2003, UC Davis

ChallengesChallengesof Hydrationof Hydration

Detailed structural rearrangements at surface and in frameworks related to degree of hydration

Energetics Is hydration a major driving force or

a by-product? Which is the tail, which the dog?

Hydration control of growth

• High energy surface sites have highest heats of hydration, hold on to water

• Hydrated surface layers for enhanced reactivity, less hydration and more order as particle grows, e.g. apatite

• Hydrophilic-hydrophobic competition

• Control of shape

Scanning heat flow curves of a zeolite synthesis mixture (5.15Na2O-1.00Al2O3-

3.28SiO2- 165H2O at a

constant heating rate of 0.10 ºC/min in a Setaram C-80 heat flux microcalorimeter. Repeated in situ experiments were performed and stopped at the selected temperatures denoted by capital letters. Apparent peaks below 30 oC are artifacts. Peaks between 40 and 70 oC represent several steps of gel formation

Crystal Growth from Nanoclusters

• Attachment of nanoclusters, rather than atoms or molecules, to growing crystal

• Elimination of surface area and eentually of surface-adsorbed species

• Classical nucleation and growth not applicable

• Ostwald step rule rationalized

Insight into Zeolite Growth Mechanisms

Alexandra Navrotsky

University of California at Davis, DMR-01-01391

A

B

C D

E

F

Increase of pH in solution

Increase of surface charge density

Vessel set used inin situ calorimetry

Teflon liner

Stainless steel vessel

Synthesis mixture

Exo Endo

Tim

e

Cal

orim

etri

c cu

rve

Framework structure of MFI zeolite

Schematic representation of zeolite crystal growth by aggregation of the pre-assembled nano-precursor particles from exothermic stage to endothermic stage.

Zeolites are widely used in ion exchange, Catalysis and separation because of theirUniform cages and channels of nanometerDimension. Design of zeolite materials forApplications demands a detailed under-Standing of zeolite formation mechanisms.

Here we demonstrate that in situ calorimetry reveals a two-stage crystallization process for MFI-type zeolite

Chem. Mater. 14, 2803 (2002)

polymorph

nanoparticles

bulk phases

metastable polymorph

stable polymorph

species in solution or melt

nanoclusterscritical nucleus or cluster for assembly

.

Particle radius

Fre

e en

ergy

(sc

hem

atic

)

nanoparticlesnanoparticles

bulk phasesbulk phases

metastable polymorph

stable polymorph

species in solution or melt

nanoclusterscritical nucleus or cluster for assembly

.

Particle radius

Fre

e en

ergy

(sc

hem

atic

)

• Control of polymorphism• Selection of hydrous precursors with low surface energy• Storage, transport and attachment of nanoparticles rather than of individual ions• Specific surface-protein interactions• Non-classical reinterpretation of nucleation, growth, Ostwald step rule

Nanoparticles andBiomineralization

Other Possible Advantages of Nanoparticles

• Efficient concentration and storage of precursors, including sparingly soluble materials

• Tethering of particles to active sites

• Membrane transport

• Detox

Synthesis of Silver ThiolatesSynthesis of Silver Thiolates

Atul Parikh et al 1999

AgS CH3

 R-SH (sol)+Ag NO3(sol) → R-S Ag (solid)+HNO3(sol)

Self-assembled monolayersSelf-assembled monolayers

6 8 10 12 14 16 18 20

20

25

30

35

a, Å

number of carbons

Micellar (columnar) mesophase

Structure of silver thiolates. Structure of silver thiolates. Phase transitions.Temperature-Phase transitions.Temperature-dependent XRDdependent XRD

ainterlayer d-spacing

16 20 24 28 32 3625

30

35

40

45

50

55

d = 8.14+1.21*(2N)

inte

rlaye

r d-

spa

cing

, Å

2(N-1)

Phase Transitions in Silver Thiolates. DSC Phase Transitions in Silver Thiolates. DSC datadata

H, kJ/moln

9 130.5±0.5 35.3±0.5 86.2±1.0

11 131.1±0.3 39.3±2.7 97.2±6.2

15 131.0±0.5 53.7±1.2 132.9±3.8

T, °C S, J/K mol

17 131.1±0.3 58.8±2.2 145.5±7.5

0 50 100 150 200

20

40

60

Ent

halp

y, k

J/m

ol

Entropy, J/K mol

hydrocarbons

8 10 12 14 16 18 200

20

40

60

80

Ent

halp

y, k

J/m

ol

Number of carbons

8 10 12 14 16 18 2040

80

120

160

Ent

ropy

, J/K

mol

Number of carbons

6 8 10 12 14 16 18 20

0

-20

-40

-60

-80

-100

melting

solution in toluene

En

tha

lpy,

kJ/

mo

l

Number of carbons

0 20 40 60 80 100 120 140

0

1

2

3

4

5

Hea

t flo

w, a

r.un

.

Time, min

0 10 20 30 40 50 60 70-1

0

1

2

3

4

5

0 2 4 6 8 10 12 14 16 18 20-50

-100

-150

-200

-250

R2 = 0.98

Ent

halp

y, k

J/m

ol

Number of carbons

ConclusionsConclusions

Silver thiolates and zeolites both explore spatial confinement

The former show much stronger “tethering”

Both show enthalpy-entropy compensation