himmelspach - us china 2009
TRANSCRIPT
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Chemical and physical synthesisof TiO2-based nanocomposites for
solar energy production and otherenvironmental applications
US China Workshop
17 October 2009
Kimberly A. Gray
Institute for Catalysis in Energy Processes,Civil & Environmental Engineering, Chemical & BiologicalEngineering,
Northwestern UniversityEvanston, IL
CB
VB
e-
h+
et
ht
O2
O2-
OH-
OH
E hv
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The Photocatalysis Challenge: SolarFuels
Basic Research Needs for Solar Energy Utilization", Report of the 2005 BasicEnergy Sciences Workshop on Solar Energy Utilization, US Department of Energy.
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Shift photoresponse into visible light region
Increase photoefficiency Retard
recombination
Target chemical reaction - CO2 reduction to CO,CH4 or
CH3OH (catalytic active sites or hot spots)
CO2 Redox Potential: - 1.9 V NHE (single electron
transfer)
- 0.103 V NHE (presence of H+-donor)
Challenge for Solar Energy Capture,Conversion & Storage
CO2 + 2H+ + 2e- CO + H2O
CO2 + CH3CH=CH2 CO + CH3CH-CH2
O
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CO2 adsorption/ CO2 speciation
Role of water
- Hole scavenger
- Proton donor
- Hydroxylate catalyst surface
- Solvate CO2 (H2CO3, HCO3-, CO3
2- )
Hydrogen management (H+
, H
, H-
) Reaction mechanism & retard back-reactions
Multiple electron transfer reaction/product
control
Photoefficiency how high can we push it. . .
But, there are some other issues, as
well:
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H2O H2 + O2 CO2 CH4 + CH3OHFujishima and Honda, Nature1972, 238, 37
Inoue et al., Nature1979, 277, 637Gratzel, Nature2001, 414, 338
TiO2 Photocatalysis for Solar Fuel Generation
Anatase
Ebg = 3.2 eV; m = 385 nm
Rutile
Ebg = 3.0 eV; m = 415 nm
ruby.colorado.edu/~smyth/min/tio2.htmlhttp://www.che.kyutech.ac.jp/chem23
Brookite
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-0.85 CO2/HCOOH
-0.76 CO2/CO
-0.72 CO2/HCHO
-0.66 H+/H2
-0.62 CO2/CH3OH
-0.48 CO2/CH4
pH = 7Anatase
(bulk)
Potential
/V
vsSCE
+2.50
3.23 eV-0.50
-1.00 -0.85 CO2/HCOOH
-0.76 CO2/CO
-0.72 CO2/HCHO
-0.66 H+/H2
-0.62 CO2/CH3OH
-0.48 CO2/CH4
pH = 7Anatase
(bulk)
Potential
/V
vsSCE
+2.50
3.23 eV-0.50
-1.00
Inoue et al, Nature1979, 277, 637; Yoneyama, Catal. Today1997, 39, 169
Relative energies for CO2 reduction
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Conceptual Picture
e-
e-
h+
CB
VB
hv
ht
CB
VB
et
ht
et
Rutile Anatase
e-h+
1 2
3
Solid-Solid Interface -
Key to Highly Efficient & Reactive Photocatalysts
Location of adlineation or defect sites
4
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EPR studies of P25 photochemistry
Electron trapping sites are rutile and anatase lattice sites
2.10 2.05 2.00 1.95 1.90 1.85
g
anatase, UV(black) and visible (red)
rutile, visible illumination
Degussa P25 Visible
UV
anatase: g= 1.990
rutile: g= 1.975
anatase: g =1. 957
rutile:g =1.940
Hurum, Gray, Rajh, Thurnauer. J. Phys. Chem. B2003, 107, 4545
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~0.45 m
2.05 2.00 1.95 1.90
g
~1.5 m
P25 Slurry
~0.2 m
Hurum, Gray, Rajh, Thurnauer, J. Elect. Spect. 2006, 150, 155 ; J. Phys. Chem. B2003, 107,
4545
Better electron transfer
Higher photoactivityEPR spectra of size-fractionated P25
Photochemistry of P25 (1): NanostructuredAssembly
Anatase
1.990Rutile1.975
Visible light
(> 400 nm)
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Direct evidence of surface dominated recombination
Intensity =517.6 au
Intensity =1197.5 au
Intensity=922.3 au
Intensity=1354.3 au
2.10 2.05 2.00 1.95 1.90 1.85
g
Interfacialg=1.979
Anatase
surfaceg=1.930
Recombination is dominated by surfacereactions.
Experimental evidence of an interfacial electrontrapping site.
The mechanisms of recombination is randomflight.
Anatase
TiO O
O O
O
O
TiO O
O OO
OTi
OO
O
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Conceptual Picture
e-
e-
h+
CB
VB
hv
ht
CB
VB
et
ht
et
Rutile Anatase
e-h+
1 2
3
4
Extend photoresponse into visible. Spatially separate and stabilize charge. Create interfacial sites having unique chemistry and
reactivity - adlineation (defect) sites.
The high activity of mixed phase TiO2 due to
rutile-anatase interactions that:
c
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Current Work
Hypothesis:
The solid-solid interface in nanocomposite materials iskey to overcoming the three grand challenges of TiO2
photocatalysis
(high activity, tailored chemistry, visiblephotoresponse)*
Methods to synthesize materials with high densitiesof
solid-solid interface & controlled defects
- Solvothermal method for TiO2nanocomposites; Low T, tune phase composition by HCl:H2O**
- Reactive DC magnetron sputtering - target power,substrate bias, oxygen partial pressure, and
deposition angle***
* G. Li, K.A. Gray (2007). The Solid-Solid Interface: Explaining the High and Unique Reactivity of TiO2-based NanocompositeMaterials. Chemical Physics,
339:173-187.
**G. Li, K.A. Gray (2007). Preparation of Mixed-phase Titanium Dioxide Nanocomposites via Solvothermal Processing. Chemistry of Materials, 19:1143-
1146.
***Chen, Graham, Li and Gray, Fabricating Highly Active Mixed Phase TiO2 Photocatalysts by Reactive DC Magnetron Sputter Deposition,Thin Solid Films
2006, 515, 1176-1181.
0
20
40
60
80
100
0 10 20 30 40 50
A%R%B%
WeightPerc
HCl/Ti molar ratio
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Thermal treatmentAnatase (A)
RA
A
R
Rutile (R)
Tetrahedral Ti Sites During PhaseTransformation
Li, Dimitrijevic, Chen, Nichols, Graham, Rajh and Gray, JACS.2008, 130:5402-5403.
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CO2Sampling
Port
Cooling Water
Stir Bar
CO2Sampling
Port
Cooling Water
Stir Bar
Mixed-Phase TiO2: Highly Active
PhotocatalystsUV, isopropanol
A+R 773 K
A+R 373 K
3100 3200 3300 3400 3500 3600
A R
Field (Gauss)
UV
Li, Ciston, Saponjic, Chen, Dimitrijevic, Rajh and Gray, J. Catal.2007, 253:105-110.
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Preparation of mixed phase TiO2 thin film by
magnetron sputtering
Cryo Pump
Load Lock
Rotating Substrate Holder
Valve
FlowMeter
FlowMeter
Valve
O2
BaratronFlowController
Ar
Valve
MassSpectrometer
Slave FlowController
Slave FlowController
MasterController
FlowMeter
CryoPump
Target Target
Pulsed dcPower
rf/Pulseddc Power
Pulsed dcPower
Schematic of dual-cathode unbalanced
magnetron system
Cryo-pumped chamberand load lock
Closed-field unbalancedmagnetron targetarrangement (13cm x 38cm targets)
Arc suppression (pulsedpower) for targets
RF power for substrate Mass spectrometer control
of reactive gas partialpressure
Rotating substrate table
Target: pure titanium; Substrates: glassslides
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Process Effects for TiO2Conditions XRD results
Low angle deposition,
Power: 3kW, Bias: 300W,
TP: 6mTorr, Anneal: 400C (1 hr)
Pure anatase
Normal deposition,
Power: 5.8kW, Bias: 150W,
TP: 3.5mTorr, No anneal
Almost pure rutile
Low angle deposition,
Power: 5.8kW, Bias: 120W,
TP: 3.5mTorr, No anneal
Mixed phase.More anatase
Normal deposition,
Power: 5.8kW, Bias: 120W,
TP: 3.5mTorr, No anneal
Mixed phase,more rutile
xrd f or pure anatase made by pvd metho
0
1000
2000
3000
4000
5000
6000
20 30 40 50 60
2 thet a
intensity(counts)
xrd for ruti l e PVD sampl
0
100
200
300
400
500
600
700
800
900
1000
20 30 40 50 60
2 theta
intensity(counts)
xr d f or mi xed phase PVD (
0
500
1000
1500
2000
2500
3000
20 30 40 50 60
2 thet
intensity(counts)
xr d f or mi xed phase PVD(
0
500
1000
1500
2000
20 30 40 50 60
2 thet
intensity
(counts)
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The morphology and structure of the films
TEM figures
TEM results
A(110)
A(112)
R(101)
TEM plan-view image of themixed phase film preparedby magnetron sputtering
(JEOL JEM-2100F FAST TEM )
TEM plan-view selected areadiffraction pattern of the sputteredmixed phase film
(Hitachi H-8100 )
Anatase crystals and rutile crystals are completely mixedtogether, indicating a high density of rutile-anataseinterfaces were created.
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Comparison of Acetaldehyde Decay (UVirradiation)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 10 20 30 40 50
time(min)
C/C0
mixed phase magnetron sputtering (71%1.12um2/um2
anatase magnetron sputtering1.10um2/u
P25 dip coating 2.18um2/um2
mixed phase solgel (70%A) 1.09um2/um
L. Chen, M.E. G raha m, G. Li, K.A. Gra y (2006) Fabrica ting Highly Ac tive Mixed Phase T iO2
Photocatalysts b y React ive D C M agnetron Sputter Deposition,Thin Solid Films
, 515(3):1176-1181.
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0. 00E+00
1. 00E+04
2. 00E+04
3. 00E+04
4. 00E+04
5. 00E+04
6. 00E+04
7. 00E+04
420 440 460 480 500 520 540 560 580 600
energ loss/ eV
counts
TEM results
100 nm
TEM Cross section, 70% A
(Hitachi HF-2000 )
Electron Energy
Lossspectroscopy
results
T
N
T
N
T: in the column
N: at the interfaces of the columns
Ti-L edge
(mixture of Ti3+ and Ti4+ )
O-K edge
More oxygen at point T (Ti:O=44.2: 55.8)
than point N (Ti:O=70:30);
Ti-L edge is shifted to low energy state
for point N lower Ti-valence.
More oxygenvacancies at the
interfaces of the
columns
50nm
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CO2 reduction results
UV condition (mercury vapor lamp
100W)+ water added as holescavenger
Film (1): O2: 0.07 pa,with minimumnitrogen
Film (2): O2: 0.07 pa,with no nitrogenFilm (3): O2: 0.08 paFilm (4): O2: 0.035 paFilm (5): O2: 0.12 pa
No detectionof methanefor P25coated film
Film(1)
Film(3)
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Photoefficiency Comparison
Kaneco, S., et al., Photocatalytic reduction of high pressure carbon dioxide using TiO2 powderswith a positive hole scavenger. Journal of Photochemistry and Photobiology a-Chemistry, 1998.115(3): p. 223-226.
0.00%
0.01%
0.02%
0.03%
0.04%
0.05%
0.06%
1 2 3 4 5
Photoefficiency
1. Mixed phase low angle film with oxygen vacancies under UVlight
2. High angle mixed phase film under UV light
3. Titania nanotubes (TiNT) under UV light4. Mixed phase film with oxygen vacancies under visible light5. P25 suspended in isopropyl alcohol illuminated with light
>345nm with about 28atm of CO2 (Kaneco, et. al)
Continued work with TiNT - > 0.1% conversion efficiency, butcatalyst deactivation
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X-ray diffraction patterns for TiO2 and
Ti1-x NbxO2 thin films. Films are prepared
under idential sputtering conditions withvarying concentrations of Nb.
UV-visible absorption spectra for TiO2
andTi1-x NbxO2 thin films
Nb additions promote the growth of
rutileAS PREPARED films are amorphousabove 32%NbCrystalline films with 40%Nb have beenprepared upon post deposition annealingat 550C
0% Nb, 81% anatase
15% Nb, rutile
9.6% Nb, 17% anatase
45%Nb, amorphous
Nb-doped films have red-shifted
photoresponsePhotocurrent measurements confirmbandgap excitation with visible light
Film growth and phaseformation
Optical response
Nb-doping to red-shift photoresponse
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Conclusions Solid-solid interface
- facilitates charge transfer and separation; hindersrecombination, increases photoefficiency
- location hot spots (tetrahedral Ti4+ , oxygen deficiencyTi3+ )
By varying sputtering power, substrate bias, totalpressure, depostion angle of reactive DC magnetronsputtering
- prepared a variety of TiO2 films having high interfacialdensities & with different phases, structures,
function.
Mixed phase sputtered film proved to be superior tothe other films as measured by oxidation andreduction reactions.
- The surface of the mixed phase sputtered TiO2 film has
columnar structure composed of well mixed and
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Techniques to synthesize TiO2-based composites -
Second Generation TiO2-x photocatalysts
- Oxygen vacancies help to extend the catalysts light responseto the visible light range while maintaining the high activity of thecatalysts. They are located mainly at the interfaces of the columns.
- There is an optimum pN2 stabilizes oxygen control and films
The ability to reduce CO2
to fuel under visible light provides thelarge potential for both environmental and energy areas.
Other environmental applications for Titania nanocomposites:
- Photocatalytic ceramic ultrafiltration membranes for watertreatment
- Photocatalysts for gas phase chemical oxidation for airtreatment
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Northwestern University Le Chen, Gonghu Li, Shannon Ciston, Paul Desario Dr. Michael Graham (Materials Science and Engineering) Yuan Yao, Prof. Richard Lueptow (Mechanical Engineering) Drew Gentner (undergrad), Jamie Nichols (RET program) NUANCE, MRSEC, ASL
Argonne National Laboratory Dr. Nada Dimitrijevic Dr. Tijana Rajh Dr. Zoran Saponjic
Degussa for their generous donation of P25U.S. National ScienceFoundationU.S. Department of Energy
Honeywell Corporation
Acknowledgements
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TiO2 can only utilize ~5% of the solar spectrum
http://www.globalwarmingart.com/images/thumb/4/4c/Solar_Spectrum.png/400px-Solar_Spectrum.png
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CB
VB
e-
h+
et
ht
O2
O2-
OH-
OH
Ehv
Artificial Photosynthesisvs.
Water splitting or PV
Why TiO2
- new focus
(solid-solid interface &reduction)
Understand relationshipbetween
synthesis-structure-function
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Why do mixed phase TiO2
materials show such highphotoactivity?
- Degussa P25 used as model- Flame hydrolysis of TiCl4
- Composition approximately80% Anatase20% Rutile
- Anatase generally considered the
active phase (385 nm, 3.2 eV)- Rutile comparatively less activecatalytically (410 nm, 3.0 eV)
Why does mixing an inactivewith active phase yield higheractivity material?
Previous Work:
TiO O
O O
O
O
TiO O
O OO
1.980
(Ti-O)
1.949 (Ti-O)
TiO O
O O
O
O
TiO O
O OO
1.980 (Ti-O)
1.934 (Ti-O)
a,b =3.7842 c= 9.5146
Anatase
Rutile
a,b =4.5845 c= 2.9533
University of Colorado Mineral Structure Database
T ti t l d l f
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Two competing conceptual models ofactivity
Rutile acts as an electron sink for anatase improving
activity by separating the hole and the electron. OR Rutile acts as an electron source for anatase
improving activity by both separating charges andextending the photoresponse of the catalyst.
h+
CB
VB
e-
hv
ht
CB
VB
et
ht
e-et
Rutile
Anatase
Anatase Anatase
Hurum, D. et.al. J. Phys. Chem. B,107, 4545 (2003)
Rutile
Rutile
Anatase
e -
+
Bickley, R. et al. J. Solid State Chem. 92, 178-190 (1991)
Mixed PhaseTiO2
Leytner, S., Hupp, J. Chem. Phys. Letter,330, 231 (2000)
Mi d Ph TiO Hi hl A ti
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Mixed-Phase TiO2: Highly Active
Photocatalysts
A+R 373 K
500 nm
Aldrich
Anatase
A+R 773 K
Hydrothermal
773 K
Li, Ciston, Saponjic, Chen, Dimitrijevic, Rajh and Gray, J. Catal.2007, 253:105-110.
Mixed Phase TiO : Magnetron
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Mixed-Phase TiO2: Magnetron
Sputtering
Chen, Graham, Li and Gray, Thin Solid Films2006, 515, 1176
Sampleholder
Titarget
20 25 30 35 40 45 50 55 60
2-Theta (degree)
A
R
200 nm 200 nm
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Conceptual Picture
Solid-Solid Interface Non-stoichiometryLocation of Undercoordinated Ti & Oxygen-
deficiency
e-
e-
h+
CB
VB
hv
ht
CB
VB
et
ht
et
Rutile Anatase
e-h+
1 2
3
4
TiO2Ti
TiO2(phase
1)
MOx(phase
2)
TiO2-x
Ti
O
M
Chen et al. 2006. Thin Solid Films, 515(3):1176-1181; Chen et al. 2009. Thin Solid Films, in press; Chen et al 2009.
JVST-A, in press; Li & Gray (2007). Chem. Mater., 19:1143-1146; Li & Gray, 2007. Chemical Physics, 339:1-3:173-187; G. Li, N.M. Dimitrijevic, L. Chen, J.M. Nichols, T. Rajh, K.A. Gray (2008)JACS, 130:5402-5403.
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Proposal: A possible
pathway of CO2 reduction
33
Series of surfaces (progressing from model (110 rutile) to morerelevant (101, 001 anatase, nanocubes)
H+ H
H-
HCOOH
H- H-H+e-
H
O
H
O
H
O
TiO2
e-
e-
e-
H+
H+
H+
H+
e-
e-e- e-
-H2O H2COCO2
H
e- H+
H+ H+H
H
CH3OH CH4-H2O
H
e-
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Standard Magnetron
Intensifies plasma at target
surface for higher sputtering
rate Minimizes heating of
substrate from ions and
electrons
Unbalanced Magnetron
Increases plasma density
close to substrate Increases numberof ions
hitting the substrate
Closed field further intensifies
plasma at substrate
(-) V BiasPump
VacuumChamber
Plasma
Ar O2
-V -VN S
substrate
Target(cathode)
Argon
atoms
MagnetAssembly
EDislodged Tiatoms
Argon ions and
electrons form
plasma
Preparation of mixed phase TiO2 thin film by
magnetron sputtering
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Changing the band-gap structure of
TiO2
N. Serpone / J. Phys Chem B 2006, 110, 24287-24293
a) Bandgap of TiO2b) localized dopant levels near VB and CBc) band gap narrowing due to broadening of VB,d) localized dopant levels and electronic transitions to CBe) electronic transitions from localized levels near VB to corresponding excited states for Ti3+ and
F+ centers
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SEM results
SEM (Leo)
Low angle, 70% A film
Low angle, 70% A film,
with oxygen vacancies
High angle, 70% A film
High angle, N-doped film
Columnar structure
Similar to Anpos
structures -
mixedphase column;
Anisotropy key to
optical and electronic
properties;
Increasing visible light
absorption withdecreasing O:Ti
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Solar FuelGeneration
Methane production under UV light for films with different
oxygen partial pressure
0
20
40
60
80
100
120140
160
0 50 100 150 200
time (min)
m
ethaneproduction(umol)
O 4 E-8 minium N 1.3 E-9
O 3.7 E-8 minimum N: 2.3E-9
O 3.7 E-8 N 8E-9
O 4E-8, no N, not stable
O 4E-8, N 4E-9
O 4.2E-8, No N
Methane production after 4 hours under
visible light illumination
8.90
13.90
18.60
22.62
15.01
9.64 9.94
5.57
0.00
4.60
0
5
10
15
20
25
30
O:5E-
8
O:4
.6E-
8
O:4
.2E-
8
O:4E-
8,not
stable
O:4E-
8,N:1
.3E-
9
O:4E-
8,N:4E-
9
O:3
.5E-
8,N:3E-
9
O:3E-
8,N:6E-
9N>
OP2
5
O and N partial pressure gradient,65-80%A
mehaneproduction(umol)
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GOAL Delineate details of mechanism
39
COgas CH4CH3OH
C2H4, C2H6HCOO-
CO2H+
(H)(e-) C3, > C3(H)
(:CH2)
n
H2C CH 2
:CH2
2e- 2H+
(H)H2C
-H2O
4H+ + 4e-(H)
slow
2H+ + 2e-
-H2O
e-H
H
CO22e-
H+
O
C
COads
(H)
Starting point:
Simplified Reaction Network at an ElectrodeSurface
?
Centi et al. 2007, Green Chemistry, 9, 671-678; Gattrell, Gupta & Co, 2006, J. Electroanal.Chem., 594, 1-19.
eac on resu s - e ane
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eac on resu s e aneProduction
CH4 producti on
0
10
20
30
40
50
60
70
80
90
100
0 50 100 150
ti me (mi n)
CH4umol/surfacearea(
70%A sputtered(H)
5%A sputtered
P25
70%A sputtered(L)
pure A sputt ered (L)
With addition of I-PrOH
Normalized by surface
area
CH4 Production @180 min.
0
10
20
30
40
50
60
70
80
90
100
P25 A (L) 70% A (L) 70% A (H) 5% A (H)
CH4production(nmo/mm2)
Figure 3. CH4 production of sputtered films and P25
UVexcitation
Chen, Graham, Li, Gentner, Dimitrijevic and Gray, Thin Solid Films (2009). Photoreduction of CO2 by TiO2 Nanocomposites
Synthesized through Reactive DC magnetron Sputter Deposition, Thin Solid Films
Optimization of Methane Production
0
50
100
150
200
250
0 0.5 1 1.5 2 2.5 3 3.5 4
Time (hr)
Methane(umol)
Exp 1
Exp 2
Exp 3
Exp 4
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Objectives:1. Determine effect of Nb on film growth and phase formation2. Characterize films structurally and optically
XRD, SEM, AFM, XPS, DR, etc.1. Determine solubility limit for Nb in TiO
2
Goal:
To determine if Nb substitution in the TiO2 lattice is aneffective way to red shift the photo response of thematerial without deleteriously modifying itsphotochemical properties.
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Solubility Preparation
Chargecompensation
Reference
>10at.% in anatase Sol-gel - Sacerdoti, Ruiz,Traversa
>40at.% in anatase DC co-sputtering
Low conc.: cationvacancy
High conc.: both Nb andTi reduced
Sheppard
>1at.% in rutile singlecrystals Diffusionaltechnique Low conc.: Ti reduced(Ti3+ )High conc.: Nb & Ti
reduced
Morris
40at.% in rutile MBE Nb substitutes as Nb4+ Gao
6.6at.% in rutile heattreatment
Nb substitutes as Nb5+ , Tireduced (Ti3+ )
Valigi
>10at.% in anatase,6% in rutile
Sol-gel Cation vacancies RuizNb doping: charge compensation1) One Ti4+ vacancy for every four Nb5+ ions2) One Ti4+ reduced to Ti3+ for every Nb5+ ion (or Nb5+ reduced to Nb4+ )3) One oxygen interstitial for two Nb5+ ions
L. Sheppard et al. / Thin Solid Films 510 (2006) 119-124
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Future Research Directions Does Nb segregate in rutile phase or at interphases?
LEAP (Local-Electrode Atom-Probe) tomography
Can we synthesize films at high concentrations of Nb withhigh anatase content? Post-deposition treatment
What is the mechanism of charge compensation in ourfilms?
Does red-shift in absorption lead to higher visible lightactivity? Photoreduction of CO
2and photooxidation of acetaldehyde
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What is EPR?
Electron ParamagneticResonance (EPR) is aspectroscopic method whichcan be used to detect,identify, and quantifyparamagnetic species.
A single electron has a spinof +1/2 or -1/2, a doubletstate.
Any nonzero spin can beobserved. ie... triplet statesin phosphorescence, organicradicals, and transitionmetals.
O
Cl
Cl
Cl H
H
Ti
O
Ti
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Titanium Dioxide/Carbon Nanotube
Composites for Photo-reactive Filtration
Speaker: Yuan Yao
Advisor: Prof. Richard M. Lueptow
Co-advisor: Prof. Kimberly A. Gray
Colleagues: Dr. Gonghu Li
Shannon Ciston
Funded by NSF
Background
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Not all membrane filtration processes can remove organic compoundseffectively.
Organic compounds
Problem 1 BiofoulingAccumulation and growth of microbial communities (biofilms) on the
membrane surfaces.
Ref:
Biofouling in water treatment, in Biofouling and Biocorrosion in Industrial Water Systems, edited by H.C. Flemming (Springer-Verlag,Berlin, 1991), pp. 47-80.
www.micromemanalytical.com/ bacAA/bactAA.htm
Membrane filtration
EPS(extracellular polymeric substances)
Microorganisms
algaeTEM image of biofilm(Transmission Electron Microscropy)
widely used for water purification.
Disinfection by-products (DBP)
Biofouling
Problem 2 Organic compound removal
Background
http://www.micromemanalytical.com/bacAA/bactAA.htmhttp://www.micromemanalytical.com/bacAA/bactAA.htmhttp://www.micromemanalytical.com/bacAA/bactAA.htm -
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TiO2
Electrons transfer reduced recombination
enhanced reactivity
Energ
y(eV)
Ref:D. C. Hurum, et al. J. Phys. Chem. B 107:4545-4549, 2003.
Example: Degussa P25 (composed of 70-80% anatase, 20-30% rutile)
High photocatalytic activity
Mixed-phase TiO2:
Mechanism:
TiO2 photocatalysis
CB
VB
e-
h+
Carbon nanotube
?
Background
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Synergistic effect enhanced photocatalysis
Oxidation of organic compounds & inactivation of microorganisms
Porous structure as filtration media
Reduction of biofouling
Objective:
TiO2coated on MW-CNTsMixture of TiO2& MW-CNTs
New
SW-CNTs attached on TiO2
To fabricate a TiO2/CNT composite with enhanced photocatalytic action
for membrane filtration.
10mg photocatalyst
Results
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Phenol degradation testPhenol degradation test
in a suspension:in a suspension:
10mg photocatalyst
100ml phenol (C0=400M)
Mercury UV lamp , (100W, with intense
lines at 366, 436 and 549nm)
0
0.2
0.4
0.6
0.8
1
0 15 30 45 60 75 90
Irradiation Time (minutes)
C/C0
P25 control
TiO 2 (100nm) control
20:1
SWCNTs control
TiO2 (100nm) /SW-CNTs
Results
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0
0.2
0.4
0.6
0.8
1
0 15 30 45 60 75 90
Irradiation Time (minutes)
C/C0
P25 control
10:1
TiO 2 (100nm) control
20:1100:1
SWCNTs control
Mixture 20:1 control
TiO2 (100nm) /SW-CNTs
Phenol degradation testPhenol degradation test
in a suspension:in a suspension:
10mg photocatalyst
100ml phenol (C0=400M)
Mercury UV lamp
Results
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TiO2(100nm)/MW-CNTs TiO2(100nm)/SW-CNTs
Lower degree of contact Greater degree of contact
MW-CNT diameter:
20-30nm
SW-CNT diameter:
~1.4nm (individual)
2-10nm (bundles)
Results
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TiO2(5nm)/SW-CNTs TiO2(5nm)/MW-CNTs
TiO2(5nm)
huge clumps bad reactivityTiO2 (5nm) and its composites
Results
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Laser activation
PL spectra collected
TiO2 has a broad
peak at ~ 540nm due
to recombination
Photoluminescence (PL) test:Photoluminescence (PL) test:
TiO2(100nm)/CNTs composites have reduced recombination.
-200
0
200
400
600
800
1000
475 500 525 550 575 600 625 650
Wavelength (nm)
Luminescen
ceIntensity(arb.
units)
TiO2(100nm)TiO2(100nm)/SWCNTs
SWCNTs
100:1 20:1TiO 2(100nm)/MWCNTs
Results
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Dip-coating results:Dip-coating results:
TiO2(100nm)/SW-CNTs 100:1
TiO2(100nm)
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Experiment
Exhaust GC FID He
He
CO2
Pump
Six-port Valve
Reactor
H2O
Exhaust GC FID He
He
CO2
Pump
Six-port Valve
Reactor
H2O
CO2 reduction reactor
Light condition:
a Black Ray UV lamp: The UV lamp provided light primarily at a
wavelength of 365nm and an energy density of ~1mW/cm2
A solar light lamp (SVLVANIA, 20W)A solar light lamp (SVLVANIA, 20W)
I-PrOH was added in some cases as a hole scavenger
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increasing magnetic field
h = gHE
In the absence of a magneticfield, the two states for theelectron are at the sameenergy.
In the presence of themagnetic field, the interactionof the magnetic moment withthe external field splits the
two states apart. The energyseparation of the states cannow be probed.
E=h =g H
Magnetic resonance
S TiO N it
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Summary: TiO2 Nanocomposites
TiO2 Nanocomposites as Highly Active Photocatalysts
How to fabricate highly active TiO2 nanocomposites?Solution phase method, magnetron sputtering
Importance of contact: simple mixing does not work
What is the origin of interfacial sites in mixed-phase TiO2?
Phase transformation by thermal treatment
Photoreduction of CO2 with H2O
Magnetron sputtering: oxygen deficiency
Role of Defect Sites in Photocatalysis
Corners, edges, steps, interfacial sites
Li and Gray, Chem. Phys.2007, 339, 173
H2
EvolutionRe
action
Jaramillo et al., Science2007, 317, 100
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0
2
4
6
8
10
12
14
0 2 4
ti me(hr)
methane(umol)
P25
sputtered f i l mwi th
O2 di f f i ci ency, 60%A
sputtered f i l m70%A
N doped f i l m(N/Of l owrate i s 3. 5)
CO2 reduction for sputtered
films under visible light
condition
Interfacial Sites in Mixed-Phase
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Interfacial Sites in Mixed PhaseTiO2
1. Baiker et al., Phys. Chem. Chem. Phys.2002, 4, 3514; 2. Penn and Banfield,Am. Miner.1999,
84, 871; 3. Zhang et al., J. Phys. Chem. B 2006, 110, 927
Mixed-phase TiO2: highly distorted, tetrahedrally coordinated interfacial sites
Tetrahedrally coordinated Ti species in flame synthesized materials(P25) 1
Distorted clusters with rutile-like character at anatase-anatasecontacts during phase transformation2
Phase transformation interfaces between anatase particles 3
Interfacial Sites in Thermally Treated
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Interfacial Sites in Thermally TreatedP25
EPR spectra in the presence of TCP
Li, Dimitrijevic, Chen, Nichols, Graham, Rajh and Gray, JACS,2008, 130:5402-5403.
20 25 30 35 40 45 50 55 60
2-Theta (degree)
P25
R
773 K
873 K
973 KA
3200 3250 3300 3350 3400 3450 3500 3550
Magnetic Field (Gauss)
R R
A AS
P25
773 K
873 K
973 K
Interfacial Sites in Synthesized Mixed-
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Interfacial Sites in Synthesized MixedPhase TiO2
Rutile-like character in A (773 K) Interfacial sites in A+R (873 K)
20 25 30 35 40 45 50 55 60
2-Theta (degree)
873 K
773 K
A
R
Li, Dimitrijevic, Chen, Nichols, Graham, Rajh and Gray, JACS,2008, 130:5402-5403.
3200 3250 3300 3350 3400 3450 3500 3550
Magnetic Field (Gauss)
R R
A
A H2O
A+R H2O
A+R TCP
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Coating Micro-/nano-structure
Typically columnar
structure, highly textured,
tens to hundreds of
nanometers grain dia.
Nano-thicknessmultilayers possible.
Top surface can vary,
smooth or faceted, denseor open, and is controlled
by deposition conditions.
A
Film growth cross-section cartoon
Mixed phase TiO2
The growth of the films
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The growth of the films
Nucleation and film growth: creating solid-
solid interfaces among crystals or columns
Reactive sputtering: control reactive gas more control on defects formation.
http://www.alacritas-
consulting.com/thin_film_growth.html
Low mobilitycauses smallercolumns orgrains
The structurezone model of
Thornton
Our samples:dualcolumnar
structure!
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EPR Characterization of SputteredFil
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Films
Li, Chen, Graham and Gray, J. Mol. Catal. A2007, 275, 30
3200 3250 3300 3350 3400 3450 3500
Field (Gauss)
A
R
P25
MS Film
MS film: domain of crystallinity
200 nm
UV
1 m
3100 3200 3300 3400 3500 3600
Field (Gauss)
Ti3+
Ti2O3
MS Powder
MS powder: oxygen deficiency
(more like TiO2-x )
Dark
Photochemistry of P25 (1): Nanostructured
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~0.45 m
2.05 2.00 1.95 1.90
g
~1.5 m
P25 Slurry
~0.2 m
Hurum, Gray, Rajh, Thurnauer, J. Elect. Spect. 2006, 150, 155 ; J. Phys. Chem. B2003, 107,4545
Better electron transfer
Higher photoactivityEPR spectra of size-fractionated P25
Photochemistry of P25 (1): NanostructuredAssembly
Anatase1.990
Rutile1.975
Visible light
(> 400 nm)
Rutile
Rutile
Anatase
e-
+
h
Direct evidence of surface dominated recombination
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Direct evidence of surface dominated recombination
Intensity =517.6 au
Intensity =1197.5 au
Intensity=922.3 au
Intensity=1354.3 au
2.10 2.05 2.00 1.95 1.90 1.85
g
Interfacialg=1.979
Anatase
surfaceg=1.930
Recombination is dominated by surfacereactions.
Experimental evidence of an interfacial electrontrapping site.
The mechanisms of recombination is randomflight.
Hurum, Gray, Rajh and Thurnauer, J. Phys. Chem. B2005, 109, 977
Semiconductor assisted photocatalysis
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CB
VB
3.2 eV
TiO2Photoinduced charge separation
h
+
- -
+
Anodic part
Cathodic part
A/A-
D+/D
-
+
p y
+
-
Converting photons into chemicalenergy:
light-induced formation ofcharges
charge separation
charge-transfer reactions
Particulate TiO2 - behaves asminiature electrochemical cell
Background
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Background
Defects:
Fundamental issues in catalysis:What is the active site?
Schwab - 1930s; Adlineation of phases (metal clusters onsupport) creates active sites -Adlineation sites at solid-
solid interface.(Schwab, G. M.; Pietsch, E.Zeitschrift fuer PhysikalischeChemie, Abteilung B: Chemie der Elementarprozesse, Aufbau der Materie1928, 1,386-408.)
Somorjai (2006) catalytically active surfaces tend to bedisordered, while ordered surface are catalytically
inactive. (Somorjai, G. A.; Bratlie, K. M.; Montano, M. O.; Park, J. Y.Journal ofPhysical Chemistry B2006, 110, 20014-20022.)
Selloni & Diebold (2006)- Theoretical work models defectsites on specific crystal surfaces such as the anatase
TiO2 (001) surface. (Gong, X.-Q.; Selloni, A.; Batzill, M.; Diebold, U. NatureMaterials2006, 5, 665-670.)
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70
On rutile (110) single crystal - only observe CO2 adsorption &
reaction on oxygen deficient surface & in presence of H2O(Funk & Burghaus, 2006. Phys. Chem. Chem. Phys.,8, 4805-4813.)
- Ability to make anatase single crystals (101) & most reactivefacet (001)
Defects:
Yang et al. (2008) Nature453, 638-641;Selloni (2008)Nature Materials, 7, 613 615; P. Zapol, L. A. Curtiss,(2007)
J. Computational and Theor. Nanoscience. 4, 222.
Tetrahedrally Coordinated Titanium
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Anpo and Thomas, Chem. Comm.2006, 3273
Frei et al., J. Phys. Chem. B2004, 108, 18269
Tetrahedrally Coordinated TitaniumSpecies
TiO2 Nanocomposites as Highly Active Photocatalysts
What is the origin of interfacial sites in mixed-phase TiO2?
How to engineer the solid-solid interface for high density active sites?
Excitation: UV