Synthesis and applications of graphene based ti o2 photocatalysts

Synthesis and Applications of
Graphene-Based TiO2 Photocatalysts

Obsession of GRAPHENE
•Excellent affinity to form composites with other materials.
•Large specific surface area.(2630 m2
g-1
)[1]
•High thermal conductivity.(5000 wm-1
K-1
)[2]
•Superior electron mobility. (15000 m2
V-1
S-1
) [3]
•TiO2
alone gives low efficiency and narrow light response range
hence it is coupled with graphene.
•Good optical transparency (97.7%) [4]
•High Young’s modulus (1Tpa) [5]

Methods for preparation of graphene and its
derivatives
•Through chemical and physical routes.
•Micromechanical cleavage of a graphite crystal.
•Chemical vapor deposition on metal surfaces. [6]
•Epitaxial growth on single crystal SiC. [7,8]
•Top-down exfoliation of graphite by means of oxidation.
•Intercalation and/or sonication. [9,10]
•Oxidation of graphene to graphene oxide and subsequent reduction.

Semiconductor Photocatalysts
TiO2
[11]
ZnO[12]
SnO2
[13]
MnO2
[14]
Fe2O3
[15]
NiO[16]
ZrO2
[17]
Cu2O [18]
CuS[19]
ZnS[20]
ZnFeO4
[21]
BiWO6
[22]
CdS[23]
CdSe[24]
Ag3PO4
[25]
•Most researched photocatalyst is TiO2 . Reasons being its long
term thermodynamic stability, strong oxidizing power, low cost
and relative nontoxicity. [26,27]
•But TiO2 cannot be used as lone component. When it is used as
single component its efficiency is found to be very low. Also the
range of wavelength absorb is very less. Hence to increase its
efficiency it has to be coupled with graphene or GO.

•Change in textural design of photocatalyst. To increase porosity and
the surface area. [28]
•Metallic [29]
and non-metallic [30]
doping. Noble-metal loading [31]
.
•Metal oxide [32]
and metal hydroxide loading [33]
.
•Incorporating carbonaceous nonmaterial such as carbon nanotubes
(CNTs)[34]
and fullerenes [35]
.
•Recent development on nanocarbon - TiO2 by Leary and Westwood
covered activated carbon, carbon doping, CNTs, graphene, C60
fullerenes and other novel carbonaceous materials.
Photocatalytic Enhancement

Need of enhancing photocatalytic activity!
•Recombination time 10-9
s
•Time for chemical reaction 10-8
to 10-3
s[36-38]
.
•The two common phases of TiO2 are anatase and rutile. They
exhibit band gap of 3.2eV to 3eV. Hence it gets excited only under
UVlight. Inability to absorb the visible light. Low solar photo
conversion.[39]
.
•The combination of TiO2 and graphene allows to enhance the
photocatalytic activity by a) increasing adsorptivity of pollutant b)
facile charge transportation and separation c) extended light
absorption range. [40]
.

Role of graphene in semiconductor assisted
photocatalyst.
•Excellent acceptor material because of two dimensional π-conjugation structure.
•In graphene - TiO2 system:
Excited electron of TiO2 could transfer from CB to graphene through
percolation mechanism.
Calculated work function of graphene is 4.42eV and CB position of anatase
is -4.21eV with a band gap of 3.2eV.
 Photo induced electrons from CB will flow into graphene sheets.
Graphene effectively separates electron hole pairs.
Generates large amount of radical species with strong oxidation ability for
degradation of pollutant.

Preparation of Graphene
•Hummers’ method.[41]
Oxidation of natural graphite powder in conc. H2SO4, NaNO3, and KMnO4.
The addition of H2O2 to reduce residual permanganate and manganese
dioxide and subsequent washing.
•Tour’s modification:[42]
.
 Exclude NaNO3, increase KMnO4, perform reaction in 9:1 mixture of
H2SO4 /H3PO4.
Efficiency of oxidation process is increased.
Process is simple, higher yield, no toxic gas evolved during preparation
and equivalent conductivity upon reduction.
•NOPG( non oxidative preparation of graphene): [43]
Natural graphite in suspension with a mixture of distilled water and EG,
was irradiated with intense cavitations' field in hp ultrasonic reactor.
Anisotropy affects the spread of acoustic waves in the material.
Graphene produced from this method is of higher quality .

Synthesis of Graphene/TiO2 Composites
1) Mounting of TiO2 on GO by the hydrolysis of titanium.
2) Reduction of GO by using chemical, hydro/solvothermal,
microwave, photocatalytic methods to yield rGO/TiO2
composites.

.
•Zhang employed sol-gel method: Tetrabutyl titanate and GO as starting
material. GO was reduced to graphene sheets by using sodium borohydride. [44]
•Zhou used one-pot solvothermal method : Same starting material. The
reduction of GO to graphene and formation of TiO2 particles on graphene is
simultaneously done. [45]
•Reduction of GO in solvothermal is more effective in reducing oxygen and
defect level in graphene, compared with using strong reductant e.g. hydrazine,
hydroquinone and sodium borohydroxide. [46]
• Shen introduced ecofriendly method. Used glucose reducing agent. The process
is simple, scalable and intrinsically pure as it involves only glucose and water. [47]
•Park synthesized highly photoactive graphene-wrapped anatase nanoparticles,
by sol-gel. [48]
Negatively charged GO sheets were attached to functionalized TiO2
particles. Finally GO was reduced to graphene by hydrothermal treatment
forming graphene- TiO2 hybrid nanoparticles.
•Wang used anionic sulfate surfactant for uniform coating of TiO2 on graphene.
•Microwaves: Heats the reactant to high temp uniformly and rapidly.
Simultaneous reduction of GO in TiO2 suspension results in intimate connection
between two components. It is crucial for interelectron transfer at the interface. [49]
Liu synthesized rGO/ TiO2 composite by applying microwave system. [50]

•Photocatalytic method provides uniform reducing environment. No need to
add extra reducing agents. Greener process than others. Kamat fabricated
rGO/TiO2 by UV assisted photocatalytic reduction of GO in presence of TiO2
nanoparticles.[51,52]
•Highest photocatalytic activity for H2 evolution was obtained with 5%
graphene. Excessive graphene content in composite increases the probability
of collision between electrons and holes. But high graphene content exhibits
light harvesting between graphene and TiO2 this reduces the photocatalytic
performance. [44]
•Graphene- TiO2 composite calcined in nitrogen atmosphere shows higher
photocatalytic activity than those calcined in air. Nitrogen atmospheres
facilitates the formation of oxygen vacancies which acts as electron traps. [53]

•Zhang and Choi fabricated GO/TiO2 nanocomposite by facile electrostatic attraction
method. Hydrophilic TiO2 nanoparticles was modified by functionalization with HCl
before applied onto GO surface by negative-positive electrostatic attractive force.
• Park prepared a hybrid of P25-TiO2 by spontaneous exfoliation and recognition of
graphite oxides without employing any thermal or chemical treatments. This shows
enhanced activities for both photocurrent generation and H2 evaluation as compared to
lone TiO2 under UV-light irradiation.
• Facile one step hydrothermal method is used for preparation of chemically bonded
P25/graphite. The nanocomposite are retained in its 2D sheet structure with micrometer-
long wrinkles. Because of carboxylic groups on GO, P25 nanoparticles dispersed on the
carbon support were observed to accumulate along the wrinkles and edges. P25/graphene
nanocomposite efficiently photodegrades methylene blue (MB) compared with bare P25.
The composite shows higher activity than that of P25/CNTs with same carbon content.
The enhancement of P25/graphene over P25/CNTs forms giant 2D planar structure of
graphene this increases adsorption of dyes and charge transportation.

•Fan and co-workers: Produced P25/rGO nanocomposite using three different ways:
1. UV-assisted photocatalytic reduction
2. Hydrazine reduction: composite least effective for H2 evolution.
3. Hydrothermal method: composite most effective for H2 evolution.
 P25/rGO is more effective in H2 evolution than P25/CNTs.
 But Zhang’s paper’s result conflicted with Fan’s. This showed TiO2/graphene was
similar to TiO2/carbon(CNTs, fullerenes and activated carbon ) with enhancement of
photocatalytic activity.•Rational design and engineering of graphene/TiO2 nanocomposites:
 Hard integration of solid P25 particles and rGO has slight edge of photocatalytic
performance over CNT/TiO2.
The unique 2D mat of rGO was interfacially engineered by using a facile two-step wet
chemistry approach and thus synthesized rGO/TiO2.
This method provides excellent interfacial contact between graphene and TiO2.,
similar approach has failed to produce same results for CNT/TiO2.
GO has advantage over CNT in controlling the morphology of as-formed

Modification of TiO2/graphene composites
•Tuning the selectivity of TiO2 photocatalyst:
conversion and purification of targeted organics.
Modifying the catalyst chemistry with specific surface chemistry and surface charge
Texturally design a catalyst with controllable pore structure that allow selective
adsorption of substrates.
•Li explained direct growth of well dispersed mesoporous anatase nanospheres on
graphene sheets by template free self-assembly process. Hydroxyl and epoxy functional
groups act as heterogeneous nucleation sites to anchor anatase nano particles on dispersed
surface sites. Self assembling of TiO2 nanoparticles around the pre-anchored nuclei
particle on graphene which results in formation of mesoporous nanospheres with sizes of
around 100nm.

•Du :
Prepared hierarchically ordered porous TiO2 films with 2D hexagonal
mesostructure and well interconnected periodic macropores by a confinement self
assembly method.
Interconnected macropores enhances mass transport through the film. Increases the
accessible surface area of the thin film and improved catalytic performances.
•Chen:
Prepared unique hybrid structure of hollow TiO2 particles wrapped with
graphene sheets.
Electroactive egg like TiO2 hollow particles were synthesized. After
functionalizing them with positive surface charges, TiO2 particles were wrapped
in GO sheets through simple electrostatic interactions. Reduction of GO to
graphene sheets by thermal treatment results in production of graphene wrapped
TiO2 hollow particles.

•Design and morphological control of the crystal facets of anatase: (Method to optimize
photocatalytic performance):
Anatase {001} or {100} facets have a considerably higher photocatalytic reactivity
than those of thermodynamically stable {101} facets.
Yang produced micron-sized anatase crystals with approximately 47% exposed
{001} facets. Extensive efforts have been directed to the synthesis, properties and
modifications of anatase with high energy {001}facets.
•Wang: Synthesis of graphene/ TiO2 nanocomposites with controlled crystal facets
through one pot hydrothermal process.
Starting material: (NH4)2TiF6 and GO. Hydrolysis of (NH4)2TiF6 produced fluoride
ions, which controls TiO2 crystal facets and reduces oxygen content in GO.
•Liu : Also synthesized chemically bonded {001}-facet exposed TiO2 /graphene
composites by a simple hydrothermal reaction of a monolayer GO dispersion and HF.
TEM images revealed nanocrystals with different shapes like squares, hexagons, and
truncated rhombuses. HF acts as morphology controlling agent.
Based on symmetries of anatase particles square surface should be {001}facets and the
four trapezoidal surfaces should be {101} facets.

Fabrication of graphene-modified TiO2
nanotubes arrays
•Nanoparticles have tendency to agglomerate and have poor interfacial contact with
graphene surfaces because of their nearly spherical shapes.
•Recently developed TNT composite, in contrast to TiO2 nanoparticles, TNT possesses high
surface area(inner and outer) with large number of active sites. High aspect ratio also
enhances the photocatalytic activities.
•Parera: Reported easy approach for growth of TNTs on rGO sheets by a hydrothermal
process under basic conditions.
GO layers are decorated by commercially available TiO2 nanoparticles (P90).
Alkaline hydrothermal treatment was then employed to convert nano particles into
smaller diameter (9nm) TNTs on the GO surface. Simultaneous reduction of GO to
rGO was observed.
TNT-graphene based composite exhibits excellent photocatalytic activity towards the
degradation of malachite green.

•Song: Has developed a novel hybrid material composed of GO network on the
surface of TNT arrays.
•Electrochemical anodization was employed to fabricate TNT arrays on a Ti
substrate. This was followed by assembly of GO on the self organized arrays by
simple impregnation method. Consequently the photoconversion efficiency of
GO-modified TNT arrays relative to pristine TiO2 under visible light was
increased by 15 folds.

Photocatalytic Applications of Graphene/
TiO2 Composites
•Main challenge is of rapid recombination of photogenerated electron hole pairs.
This process causes dissipation of energy input as heat which results in low
photocatalytic efficiency.
•Graphene has shown great potential in standing against this challenge and
provide high performance support for photocatalysis.
•Graphene has ability to store and shuttle electrons by a step-wise electron transfer
process, effectively avoiding charge recombination.
•Graphene/ TiO2 is applicable in
Degradation of pollutants.
Water splitting of H2 evolution.
Reduction of CO2 for hydrocarbon fuel production.

Photocatalytic Degradation of Pollutants
•This technology concerns with environmental photocatalysis including air
cleansing, water disinfection and purification, hazardous waste remediation, self
cleaning and deodorization.
•The composite fabricated by Park (highly photoactive graphene-wrapped
anatase nanoparticles) has showed enhanced photocatalytic properties, as
compared to bare TiO2 nanoparticles, for degrading MB under visible light
condition.
•Band gap has been narrowed from 3.2eV to 2.8eV of TiO2 in the composite
which would allow greater absorption of visible light and efficient electron
transfer.

•Photodegradation process occurs through two pathways:
1) Electrons from excited MB flow to the CB of TiO2 through graphene sheets.
2) Valence electrons of TiO2 are excited to the CB due to the sufficiently low band
gap of the graphene/ TiO2 hybrid material.
•The excited electron can be trapped by O2 molecules leading to formation of singlet (.
O2
-
)
and hydroxyl radicals (.
OH) . These reactive oxygen species (ROS) then participate in the
oxidative decomposition of adsorbed MB on the surface.
•Photogenerated holes participate in the formation of .
OH through adsorption of water or
surface hydroxyl groups.
•The route is
TiO2 + hν (e-
CB) + (hν+
VB) graphene (e-
) + h+
TiO2 (e-
CB) + O2ads
.
O2
-
+ TiO2
H
2
O .
OH + TiO2
TiO2 (hν+
VB ) + OH-
ads
.
OH + TiO2
graphene (e-
) + O2ads
.
O2
-
+ graphene H
2
O .
OH + graphene
.

•Liu and co workers:
• Reported application GO/ TiO2 nanorod composites on Photodegradation of MB under
UV light irradiation.
•Excellent interfacial contact allows efficient electron transfer, thus avoiding charge
recombination and increasing photocatalytic performance.
•This increases the photocatalytic activity of graphene/ TiO2 composites by optimizing
the morphology and distribution of TiO2 nano particles on graphene sheets.
• DU:
• Reported the production of macro/mesoporous TiO2 film. This significantly degrades MB
relative to pure mesoporous TiO2 films.
•Incorporation of macropores into mesoporous films improve the accessibility of MB to the
films, thus increasing the adsorption of the contaminant and enhancing their photocatalytic
activities.

• Zhang:
• Emphasized the importance of smart design and engineering of graphene/ semiconductor
photocatalysts.
•Soft integration of TiO2 and GO by the hydrolysis process of TiF4 resulted in composites
with higher photocatalytic activity than those prepared by physically mixing graphene and
P25.
•Visible light-driven photocatalysis of graphene/ TiO2 composites also present clear merits
on the utilization of solar energy.
• Chen: Used GO/ TiO2 composites with p/n heterojunctions in the photodegradation of MO.
•GO forms p-type semiconductors which can be excited by visible light. (λ > 510nm). They act
as sensitizer and enhance the visible-light photocatalytic activity of the composite.
•Liang produced graphene/ TiO2 nanocrystals hybrid efficiently degrade RhB.
•Neppolian used GO/Pt/ TiO2 to degrade anionic surfactant, dodecylbenzenesulfate (DBS).
•Photodegration of following organics has been achieved: malachite green, phenol, reactive
brilliant red dye, X-3B, photocatalytic reduction of Cr, decomposition of butane and 2-

Photocatalytic water splitting for H2
generation.
• Fuel of the future..which is not to far. Excellent alternative to conventional and
fossil fuels, which are depleting nowadays.
•It is storable, clean and ecofriendly. It also possesses high energy capacity per
unit mass.
•Photocatalysis can be used to split the water and generate the hydrogen energy.
But here also the main challenge is of rapid recombination of photogenerated
electron-hole pairs. This can be solve by charge separation.
•Decomposition of Pt on TiO2 can significantly enhance H2 generation efficiency
when water is split in presence of sacrificial reagents (electron donors and hole
scavengers.)

• Park compared three composite catalyst for hydrogen generation under UV light
irradiation.
Catalyst H2 Evolution Remarks
Pt/TiO2 medium High cost. Pt is consumed a
lot.
GO/TiO2 low TiO2 not catalytically active
GO/Pt/TiO2 high Pt on TiO2 gives high yield
of H2, when GO serves as co
catalyst. Reduces cost.

•Xiang:
•By using graphene-modified TiO2 nanosheets, he evaluated the water-splitting
performance with methanol as a sacrificial agent under UV light irradiation. Composite
exhibited 41 fold enhancement of photocatalytic H2 production activity relative to that of
pure TiO2 nanosheets.
• The graphene content was 1.0wt %. H2 production rate is 736 μmolh-1
g-1.
• High performance is due to face to face orientation between TiO2 nanosheets and
graphene., effectively separating electron-hole pairs.
•Potential of graphene/graphene-
is less negative than the CB of anatase and more negative
than the H+
/ H2 potential. Hence electrons from CB of TiO2 flow toward the graphene sheets
and then take part in reduction of a proton H2 molecules.
•The route in water – splitting mechanism are described by:
Graphene/ TiO2 graphene(e-
)/TiO2(h+
VB )
graphene(e-
) + 2H+
graphene + H2
hν

• Fan’s study on the effects of the preparation method (UV assisted photocatalytic
reduction, hydrazine reduction and hydrothermal method) and P25/rGO mass ratio on H2
generation showed remarkable improvement in photocatalytic performance over bare P25.
• Hydrothermal method proves to be the best in performance.
• Optimal P25/rGO mass ratio 1/0.2 gives H2 evolution rate of 74μmolh-1
, 10 times more
than bare P25.
• This process turns out to be better than sol-gel method of preparation of rGO/ TiO2
composites by using Na2S and Na2SO3 as sacrificial agent, reported by Zhang.
• Yeh and co-workers: Graphite oxide is intermediate state between graphite and
graphene. Graphite oxide with appropriate oxidized level can act as photocatalyst for
H2 generation.
•CB of graphite oxide is formed due to antibonding π* orbital. This has high energy level
required for H2 production. It injects the electron into the solution phase. This removes the
need of electron accepting co-catalyst. This reduces the cost significantly.

Few more Photocatalytic applications !
• CO2 content of atmosphere is increasing significantly. So by photocatalytic
activities, this CO2 can be efficiently converted into useful hydrocarbon products.
•Liang studied the defects of graphene on photoreduction of CO2 in presence of
water to produce CH4 under visible and uv light irradiation.
• Graphene/ TiO2 nanocomposite based on less-defective SEG exhibited excellent
CO2 photoreduction under visible light illumination.
• Photocatalytic oxidation is very effective in bactericidal and antifungal action.
• Graphene/ TiO2 thin films deactivate Escherichia coli bacteria in aqueous
solution under solar-light irradiation. This composite is 7 times better than pure
TiO2 in the antibacterial activity.

References
 References are mentioned in the word document attached along with this.

Synthesis and applications of graphene based ti o2 photocatalysts

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