Semiconductor nanomaterials

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Catalysts for Environmental and Energy
Applications

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Types of Catalyst

3. Semiconductors in Photocatalysis
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In 1972, Fujishima and Honda discovered the
phenomenon of a photocatalytic splitting of water
on a TiO2 electrode under ultraviolet (UV) light.
Steps in photocatalysis:
(i) light absorption by the semiconductor, (ii) formation of photogenerated electron–hole
pairs, iii) migration and recombination of the photogenerated electron–hole pairs, (iv)
adsorption of reactants and desorption of products, and (v) occurrence of redox reactions
on the semiconductor surface.

4. TiO2 as a Photocatalyst
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The main advantages of TiO2 are its high chemical
stability when exposed to acidic and basic
compounds, its nontoxicity, its relatively low cost
and its highly oxidizing power.
The larger band gap of conventional unmodified
anatase TiO2 (3.2 eV) makes it less economical for
practical visible light applications.
Different crystalline forms: anatase, rutile and
brookite.
Mostly acts as a n-type semiconductor due to oxygen
vacancies (related to presence of Ti3+). This oxygen
vacancies sites are ideal for oxygen adsorption on
the catalyst surface. In TiO2 photocatalysis, surface
adsorbed O2 acts as the primary electron acceptor
and no photocatalytic organic degradation is
possible in the absence of O2 (reduction of O2 is the
rate limiting step in photocatalysis).
Non-metal doped TiO2 generally is P-type.

5. Heterojunction Photocatalysts
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Fg is the gravitational force, G is the
gravitational constant, M is the Earth’s mass, m
is the mass of the man, R is the distance
between the center of the Earth and the man,
Fc is the Coulombic force, k is the Coulomb
constant, qe and qh are the charge magnitudes
of the electron and the hole, respectively, and r
is the distance between the centers of these
charges.
Coulomb constant (8.99 × 109 N m2 C−2) is
much larger than that of the gravitational
constant (6.67 × 10−11 N m2 kg−2). Man will fall
back to the ground on the timescale of seconds,
while electron–hole pairs will recombine much
faster, i.e., in the range of nanoseconds.
A heterojunction is defined as the interface between two different semiconductors with
unequal band structure

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(CB) 2H+ + 2e-  H2
(VB) H2O + 2h+  ½ O2 + 2H+
At pH = 0
The doted lines are the energy levels for the water-splitting half-cell reactions. To make the
reaction thermodynamically favorable, the CB edge should have higher energy (more
negative potential) than the hydrogen-evolution potential and the VB edge should be lower
in energy (more positive potential) than the oxygen-evolution potential.
The electrons of VB can lower their energy and being transferred to H+ in
solution, similarly the holes lower their energy and being transferred to H2O molecules
through a short-circuited reaction for balancing the charges transferred to the solution.
Finally, H2 and O2 molecules are formed. Similarly, other photochemical reactions are take
place.

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The band gap, CB (ECB) and VB (EVB) edge positions of commonly used
semiconductors at pH zero versus normal hydrogen electrode (NHE)

8. Types of Heterojunctions
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Three types of heterojunction:
(i) Type-I or Straddling gap
(ii) Type-II or Staggered gap
(iii) Type-III or Broken gap

9. Different Heterojunctions: Type I
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• Type-I (Straddling) heterojunction photocatalyst: the conduction band (CB) and
the valence band (VB) of semiconductor A are respectively higher and lower
than the corresponding bands of semiconductor B. The electrons and holes will
accumulate at the CB and the VB levels of semiconductor B, respectively. Since
both electrons and holes accumulate on the same semiconductor, the electron–
hole pairs cannot be effectively separated for the type-I heterojunction
photocatalyst. A redox reaction takes place on the semiconductor with the lower
redox potential, thereby significantly reducing the redox ability of the
heterojunction photocatalyst.
• Ex. Core/Shell NPs
• CdSe (bandgap:1.74 eV) /CdS (bandgap:2.42 eV)
CdSe/ZnS, InAs/CdSe, and ZnO/MgO
• Reverse Type I:
• CdS/HgS, CdS/CdSe, ZnSe/CdSe and MgO/ZnO

10. Type II
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• Type-II (staggered) heterojunction photocatalyst: The CB and the VB levels of
semiconductor A are higher than the corresponding levels of the
semiconductor B. Thus, the photogenerated electrons will transfer to
semiconductor B, while the photogenerated holes will migrate to
semiconductor A under light irradiation, resulting in a spatial separation of
electron–hole pairs.
• The redox ability of the type-II heterojunction
photocatalyst will be also reduced because the
reduction reaction and the oxidation reaction take
place on semiconductor B with lower reduction
potential and on semiconductor A with lower
oxidation potential, respectively
• Generally, type-II heterojunction photocatalysts
exhibit good electron–hole separation efficiency,
wide light-absorption range, and fast mass
transfer.
• SnO2-TiO2, TiO2-g-C3N4, g-C3N4 - WO3

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Type III
Type-III is similar to that of the type-II, except that the staggered gap becomes so
extreme that the bandgaps do not overlap. Therefore, the electron–hole migration
and separation between the two semiconductors cannot occur for the type-III
heterojunction, making it unsuitable for enhancing the separation of electron–hole
pairs.
GaSb-InAs, WO3-TiO2

12. • The photogenerated electrons and holes in the p-type and
n-type semiconductors will migrate under the influence of
the internal electric field to the CB of the n-type
semiconductor and the VB of the p-type semiconductor,
respectively, which results in the spatial separation of the
electron–hole pairs.
• Migration continues till the Fermi level equilibrium.
• The electron–hole separation efficiency in p–n
heterojunction photocatalysts is faster than that of type-II.
NiS (p)/CdS(n).
p–n Heterojunctions
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Although the type-II heterojunction can ideally separate electron–hole pairs in space, the achieved
enhancement in the electron–hole separation across a type-II heterojunction is not sufficient to
overcome the ultrafast electron–hole recombination on the semiconductor. Thus, a p–n
heterojunction photocatalyst concept was proposed, which is able to accelerate the electron–hole
migration across the heterojunction for improving the photocatalytic performance by providing
an additional electric field.
Photocatalytic H2 generation

13. Anatase TiO2
Surface Heterojunctions
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It is well-known that the different crystal facets on a single semiconductor can have different
band structures. Since a heterojunction is formed by combining two semiconducting materials
with different band structures, it is possible to create a heterojunction between two crystal
facets of a single semiconductor, namely a surface heterojunction.
• In fact, the basic principle of the surface heterojunction is
similar to that of the type-II heterojunction, in which the CB
and VB levels of the {001} facets are higher than the
corresponding levels of the {101} facets of anatase TiO2.
• Electrons and holes can be spatially separated on the {101}
facets for reduction reactions and on the {001} facets for
oxidation reactions, respectively.
• The photocatalytic CO2 reduction activity of anatase TiO2
with an optimal ratio of exposed {001} and {101} facets was
3.5 times higher than that of commercial TiO2.

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Mechanism of natural photo-synthesis/ Light reactions
The “Z‐scheme” describes the
oxidation/reduction changes
during the light reactions of
photosynthesis. The vertical
axis in the figure represents
the reduction potential of a
particular species—the higher
the position of a molecular
species, the more negative its
reduction potential, and the
more easily it donates
electrons.
In the Z‐scheme, electrons are removed from water (to the left) and then donated to the lower (non‐excited)
oxidized form of P680 (PS-II). Absorption of a photon ( = 680 nm) excites P680 to P680*, which “jumps” to a
more actively reducing species. P680* donates its electron to the quinone‐cytochrome bf chain, with proton
pumping. The electron from cytochrome b6f is donated to PS-I, converting P700 to P700*. This electron, along
with others, is transferred to NADP, forming NADPH. Alternatively, this electron can go back to cytochrome
b6f in cyclic electron flow.
OEC: Oxygen-evolving (Mn-Ca-O) complex.
PQ/PQH2: Q-cycle
Cyt b6f: Cytochrome b6f complex
PC: Plastocyanine; PS: Photosystem
Fd: Ferredoxin
FNR: Ferredoxin NADP* Oxidoreductose
2H2O  O2 + 4H+ + 4e
Red
Red
P700*/P700
P680*/P680
e-
e-
Oxidation
Reduction
Ox. Pot.
Water splitting, Ox.
Red. Pot.

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The role of Z-scheme is to extract electrons from water and transfer those
electrons by using an electron transport system, ultimately convert NADP+ to
NADPH and which in turn helps in the photosynthesis process. The Z-scheme
is the plot of redox potential and the direction of electron flow.
Redox potential (voltage) is a measure of how easily a metal (or other ion) will
give up electrons or retain electrons. The more negative standard electrode
potential (E0 V) in a standard electrode potential series, more the position of
equilibrium towards left (ionized form) and opposite for more positive. G0
cell
= -nFE0
cell; If E°cell > 0, then the process is spontaneous.
In the previous diagram, the reduction potential of water is negative (+Ve
shown, because of oxidation potential;
𝟐𝑯 𝟐 𝑶 ↔
𝑬 𝒓𝒆𝒅
𝟎
=−𝟏.𝟐𝟑 𝑽
𝑬 𝒐𝒙
𝟎 = +𝟏.𝟐𝟑 𝑽
𝟒𝑯+ + 𝑶 𝟐 + 4e
Because of large difference in standard reduction potential of water splitting to
generate electron and the standard reduction potential of NADPH+ electron can
not directly combine. The process occur through PS II and PS I.
Mechanism of natural photo-synthesis

16. Artificial Z-scheme
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The artificial Z-scheme photocatalyst usually
consists of two connected semiconductor
photocatalysts: one is oxidation photocatalyst
and another is reduction photocatalyst.
The oxidation photocatalysts possess low VB
position and exhibit strong oxidation ability, while
the reduction photocatalysts usually have high CB
position and display strong reduction ability.
Types of Z-scheme:
(i) First-generation Z-scheme heterojunction. It
is also known as liquid-phase z-scheme
photocata-lytic system.It is built by combining
two different semi-conductors with a shuttle
redox mediator (viz. anelectron
acceptor/donor (A/D) pair).
(ii) Second-generation: It is also known as all-
solid-state (ASS) Z-scheme system. In order to
overcome the obvious problems identified in
the first generation, Tada et al. in 2006
synthesised the all-solid-state CdS/Au/TiO2 Z-
scheme.
(iii) Third-generation: This was developed in 2013,
which is a direct Z-scheme without A/D.

17. Direct Z-Scheme Heterojunctions
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• Problem: The heterojunction photocatalysts are efficient for enhancing electron–
hole separation, but the redox ability of the photocatalyst is sacrificed because the
reduction and oxidation processes occur on the semiconductor with the lower
reduction and oxidation potentials, respectively.
• Solution: The Z-scheme photocatalytic concept was proposed by Bard et al. in 1979
to maximize the redox potential of the heterojunction systems.
• Construction: Composed of two different semiconductors, and an acceptor/donor
(A/D) pair
• Photogenerated electrons migrate
from the CB of the PS II (Photo
system) to the VB of the PS I through
an A/D pair via redox reactions.
• Since electrons accumulate on the PS I,
with the higher reduction potential, and
holes accumulate on the PS II, with the
higher oxidation potential, a spatial
separation of electron–hole pairs and an
optimal redox ability can be achieved
• Limitation: can only be constructed in
liquid phase

18. All solid-state Z-scheme photocatalysts
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• In 2006, all-solid state Z-scheme photocatalyst
was developed, which consisted of two
different semiconductors (PS I and PS II) and
a solid electron mediator between them.
• The photogenerated electrons on the PS II
migrate to the VB of the PS I via an electron
mediator (such as Pt, Ag, and Au), and are
further excited to the CB of PS I. As a result,
photogenerated holes and electrons are
accumulated in the PS II, with a higher
oxidation potential, and in the PS I, with a
higher reduction potential, respectively, which
results in the spatial electron–hole separation
and optimization of the redox potential.
• All-solid-state Z-scheme photocatalysts can be
used in solution, gas, and solid media.

19. Direct Z-scheme without A/D
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In 2013, conventional all-solid-state Z-scheme heterojunction photocatalysts, except
that the rare and expensive electron mediators was proposed.
• The fabrication cost of this direct Z-scheme
heterojunction photocatalyst is low and
comparable to that of conventional type-II
heterojunction.
• Redox potential can be optimized for specific
photocatalytic reactions.
• The direct Z-scheme heterojunction
• photocatalyst is physically more favorable than
that on the type-II heterojunction
photocatalyst.

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