Perspectives on photocatalysis to
the water and wastewater
treatment
Prof Regina de F P M Moreira
Departamento de Engenharia Química e Engenharia de Alimentos
Universidade Federal de Santa Catarina
Florianópolis - SC
regina@enq.ufsc.br
Photocatalysis
Number of papers in Photocatalysis:
1975–1980: 249
2000–2010: 16.757
2500
photocatalyst
2000
photocatalysis
1500
TiO2 - the most used photocatalyst (non-toxic,
stable and not expensive)
Air treatment
Water and
wastewater
treatment
1000
500
Self cleaning surfaces
2012
2011
2010
2009
2008
2007
2006
2005
2004
2003
2002
0
Number of papers/year (www.sciencedirect.com)
Number of patents in photocatalysis
Publications about
“nanoparticle
photocatalysts”
Balkus Jr, K., New and Future Developments in Catalysis Catalysis by Nanoparticles, 2013, Pages 213–244
Photocatalysis
(1975–1985)
semiconductor/solution interface under UV irradiation
several semiconductors
 Polycristalline materials were the most suitable.
Photocatalysts in aqueous suspensions.
(1986 - 2000)
Thin films;
Doping of semiconductors to explore visible light;
Dye sensitization (photocatalysts in aqueous
suspension).
Industrial activities.
(2006–2010):
Nanophotocatalysts
Photocatalysts
•Semiconductors
•Conduction Band (CB)  electrons have a chemical potential of + 0.5 to -1.5 V
vs NHE  hence they can act as reductants.
•Valence Band (VB)  holes exhibit a strong oxidative potential + 1.0 to + 3.5 V
vs NHE
Photocatalytic activity and semiconductor properties
Energy band configuration  determinates the absorption of incident photons,
photoexcitation of electron-hole pairs, migration of carriers, and redox capabilities of
excited-state electrons and holes.
Band-edge positions of semiconductor
photocatalysts relative to the energy levels of
various redox couples in water.
Energy bands engineering
H Tong, S Ouyang, Y Bi, N Umezawa, M Oshikiri, J Ye, Nanophotocatalytic materials: possibilities and challenges, Adv Mater 2012, 24,
229-251.
Photocatalysts
ENERGY BAND ENGINEERING
Some important aspects:
- Optical absorption: direct and narrow bandgap semiconductors are more likely to
exhibit high absorbance  suitable for the efficient harvesting of low energy photons.
- Disadvantages:
- recombination electron/hole
- Band-edge positions are frequently incompatible with the electrochemical
potential necessary to trigger specific redox reactions
- Modulate the band gap and band-edge positions
in a precise manner  different strategies
- Improvement of light sensitization by the inclusion
of quantum dots, plasmon-exciton coupling
between anchored noble metal nanoparticle cocatalysts and the host semiconductor, and photon
coupling in semiconductor photonic crystals.
Energy Band Engineering
I. Modiulation of VB
II. Adjustment of the CB
III. Continuous modulation of the VB and/or CB
Photocatalytic degradation of pollutants in water or
wastewater  oxygen as electron acceptor
• VB  Redox potential should be
sufficiently positive in order to the
holes act as electron acceptor ; 
oxidation reaction
• CB: Redox potential should be
sufficiently negative in order to the
oxygen act as electron acceptor 
reduction reaction
A. Millis and S. L. Hunte J. Photochem. Photobiol. A: Chem 180 (1997) 1
Energy Band Engineering
Oxide semiconductors 
CB slightly negative ;
VB significantly positive with respect to the oxidization
of H2O (vs NHE).
Therefore
For the consideration of stability of materials,
raising to top of the VB to narrow the bandgap
takes precedence over all other methods of
energy-band modulation.
To adjust the level of the VB: the most effective strategies:
I. Doping with 3d transition elements
II. Cations with d10 our d10s2 configurations
III. Non-metal elements
Energy band engineering
•
A) TiO2  Doping N, S, C, metals  strategies to raise the VB maximum
•
B) TiO2  Dye surface sensitization
•
C) Surface modification to increase stability
•
D) Coupled semiconductors
•
E) Novel semiconductor containing 3d metals.
Miao Zhang et al, Angew. Chem. Int. Ed. 2008, 47, 9730 –9733
A) Doping with non-metal: C, N, P, B, S
A.1.1 Doping with sulfur
A.1.2 Doping with nitrogen
Successful example of band-edge control for the
utilization of visible light  mechanism under
debate.
- Hybridization of the N-related states with
the host VB;
- N-doping in TiO2 is accompanied by
formatin of Ti3+ via donor-type deffects
Mechanism of photocatalytic activity of TiO2
doped with S
S.X. Liu, X.Y. Chen, J. Hazard. Mater. 152, 48–55 (2008)
K. HASHIMOTO et al. Jpn. J. Appl. Phys., Vol. 44, No. 12 (2005)
• Doping with N, C, S narrows the bandgap by less than 0.3 V.
• Significant extension of visible light absorption via anion doping remains a big challenge.
Photocatalytic degradation of Phenol in aqueous solution using nanowires of Ndoping TiO2
Ilha, José, Moreira, Degradação fotocatalítica de fenol utilizando nanofios
de dióxido de titânio modificados com nitrogênio). UFSC, 2012
Nanofio dopado Nanofio
com nitrogênio
Photocatalyst
Bandgap (eV)
TiO2 P25
3,05
Nanowire TiO2
2,62
N-doped TiO2 nanowired
2,53
Pseudo first order kinetic constant for the phenol
minearlization using different photocatalysts
Catalisador
k' (10-3min-1)
P25
2,6
nanowireTiO2
0,6
N doped TiO2 nanowired
1,1
Phenol initial concentration: 100 mg/ L; Photocatalyst dosage 1g/L.
N doped TiO2
Effect of nitrogen content
Theoretical studies: only 1%
atomic% N (0.53 % w/w) on TiO2 is
necessary to activate
photocatalytic reactions under
visible light.
Fu, Zhang, Zhang, Zu, J Phys Chem B 2006, 110, 3061.
Decomposition of rhodamine B after 1 h using TiO2 or N- TiO2 (different N/Ti ratio) under
visible light.
Ye Cong et al., J. Phys. Chem. C, Vol. 111, No. 19, 2007, 6976-6982
B – Metal doping
CB
e-(M)  M+e-e- e- e- e- e- e- e- e- e- e- e- eEg
Recombination
e-/h+
VB
h+ h+ h+ h+ h+ h+ h+ h + h+ h+
Metal promoter: attracts the electrons to the CB  recombination is inhibited.
B – Metal Doping
• ionic radius of the metal  similar to the Ti4+ ,
• Exhibit 2 or more oxidation states.
• Energy levels Mn+ /M(n+1)  similar to Ti3+ /Ti4+ ,
• Electronegativity: higher than Ti
• incomplete/parcial electronic configuration
Ionic radius
B – Noble metals doping
Fotoactivity of TiO2 doped with Pt  effect of the metal concentration on the production
of methane by the photoreaction: CO2 + H2O  CH4 + O2
Effect of Pt-metal content in Pt/TiO2 (P25) catalysts on CH4 yield for photocatalytic reduction
of CO2 after 7 h UV irradiation at 323 K, H2O/CO2 = 0.02.
Q.-H. Zhang et al. / Catalysis Today 148 (2009) 335–340
B – Non noble metal doping
Capítulo 6
Copper, zinc and Chromium
De Bem Luiz et al., Journal of Photochemistry and Photobiology A: Chemistry 246 (2012) 36– 44
• Photocatalyst synthesis: photodeposition by controllingl of precursor metals
solubility
Capítulo 6
B - Non-noble metals doping
Copper, zinc and Chromium
De Bem Luiz et al., Journal of Photochemistry and Photobiology A: Chemistry 246 (2012) 36– 44
• Photocatalytic denitrification:
– Photoreduction of NO3- to produce N2
– Hole scavanger: Formic acid (electron donor)
– Nitrate  electron acceptor
• Theoretical molar ratio to reduce nitrate to nitrogen CHOOH:NO3- = 8:1
𝑁𝑂3− + 2𝑒 − + 𝐻2 𝑂 → 𝑁𝑂2− + 2𝑂𝐻−
𝑁𝑂2− + 6𝑒 − + 7H + → 𝑁𝐻3 + 2𝐻2 𝑂
2𝑁𝐻3 → 𝑁2 + 3𝐻2
2𝐻𝐶𝑂𝑂 − + ℎ+ → 𝐶𝑂2 + 𝐶𝑂2°− + 𝐻2
De Bem Luiz et al., Journal of Photochemistry and Photobiology A: Chemistry 246 (2012) 36– 44
B – Non-noble metal doped TiO2
NH4+
main
byproduct
No
N-byproducts
Time, min
Kinetics of photocatalytic degradation of nitrate and formic acid (measured as TOC), and formation of
products (ammonia and nitrite)
pH 2.5. TiO2, Zn-TiO2, Cr-TiO2 e Cu-TiO2 = 1g L-1. NO3- = 0.6 mM (9 mg N L-1); CHOOH = 9.8 mM (117.4 mg COT
L-1).
Moreira., Journal of Photochemistry and Photobiology A: Chemistry 246 (2012) 36– 44
B – Non noble metal doping
Capítulo 6
• Copper, zinc or chromium:
– Zn-TiO2: higher photocatalytic activity than Cr-TiO2 or Cu-TiO2, and lower byproducts
formation.
– Zn action  To promote efficient charge separation (e-/h+)
Moreira et al., Journal of Photochemistry and Photobiology A: Chemistry 246 (2012) 36– 44
B – Doping with non noble metals
• Effect of dissolved oxygen on the photocatalytic activity
of Zn-TiO2
– O2 competes with NO3- ions, acting as electron acceptor
Photocatalytic nitrate reduction using 4.4% Zn–TiO2 as photocatalyst
Presence of
O2 (air)
By purging
argon
(without O2
Selectivity [%]
Nitrate
conversion after Activity [µmolNO3- (min gcatalisador) -1]
2 h[%]
77.2
87.5
4.11
95.4
91.7
14.24
Moreira et al., Journal of Photochemistry and Photobiology A: Chemistry 246 (2012) 36– 44
C) Coupling semiconductors
Ensemble of nanoparticles may exhibit
new collective properties resulting from
the inter-particle coupling of surface
electrons (excitons), plasmons or magnetic
moments.
Illustration of an electronic bond formed between (A) two
atoms and (B) two nanocrystals.
- induce a substantial alteration of the
electronic structures of the
nanoparticle ensemble  bonding and
anti-bonding levels are formed, yielding
a new electronic structure.
Tong, Ouyang, Bi, Umezawa, Oshikiri, Ye, Adv Mater 2012, 24, 229.
C) Coupling semiconductors
Interesting way to increase the efficiency of a photocatalytic process:
- by increasing the charge separation
- by extending the energy range of photoexcitation for the system
- by extending
The potential of VB or CB of coupled semiconductors should be more negative or less
positive, respectively, than pure TiO2
Hole produced in the VB  remains in the CdS particle
Electron  it is transferred to the CB of TiO2 particle.
The electron transference from CdS to TiO2 increase the charge separation and the
photocatalytic efficiency.
Sclafani, A.; Mozzanega, M.-N.; Pichat, P. J. Photochem. Photobiol. A: Chem. 1991, 59, 81.
Hybrid seminconductors– TiO2/graphene
is promising to simultaneously possess excellent adsorptivity, transparency, conductivity,
and controllability, which could facilitate effective photodegradation of pollutants.
Graphene  increase the electric conductivity, charge transfer and chemical stability
- Decrease recombination electron/hole due to the high electronic conductivity of
graphene;
- High active site concentration, due to the high ratio area:volume, and bidimensional
structure
- High range of light absorption
-TiO2/graphene composites Strong interaction aromatic rings of graphene and organic
molecules
Bond Ti-O-C  graphene acts as co-catalyst (Lv et al., Procedia Engineering 27 (2012) 570-576.
TiO2 (P25)-graphene  photocatalytic activity is higher than pure TiO2 P25 (Zhang et al., 4 (2010) 380)
TiO2/Graphene
Scheme of the Photocatalytic Degradation of methylene blue (a) TiO2 (b) TiO2/Graphene
E. Lee et al. / Journal of Hazardous Materials 219– 220 (2012) 13– 18
High activity results from:
Kinetic of photocatalytic
degradation of Rhodamine B
Kinetic constant for the photocatalytic
degradation of Rhodamine B
• Strong coupling between TiO2
on graphene oxide  facilitate
interfacial change transfer;
• (GO ) acts as electron acceptor
and
inhibits
the
e/h
recombation.
Liang et al, Nano Res,2010.
Huimin et al., Chinese Journal of Catalysis, 33 (2012) 777-782.
ZnFe2O4/Magnetic graphene
Spinel ZnFe2O4 (Eg= 1.90 eV)
 Magnetic semiconductor material
Nanosheets of graphene and ZnFe2O4 nanocrystals
Comparing ZnFe2O4 and ZnFe2O4/grafeno
• Composite ZnFe2O4/grafeno  catalyst for photodegradation
• Generation of HO* radicals via photochemical reactions of H2O2 under visible light
ZnFe2O4 – with (a) and without (b)
magnetic field
The photogenerated electrons of excited ZnFe2O4 were transferred instanteously from the
conduction band of ZnFe2O4 to graphene at the site of generation via a percolation mechanism,
resulting in a minimized charge recombination  enhanced photocatalytic activity
Fu e Wang, Ind Eng Chem Res 50 (2011) 7210-7218.
Lanthanide modified semiconductor photocatalystss
The biggest difference between the transition metal ion
and the lanthanide ions  nature of the 4f orbitals
Lanthanide  excellent optical properties
• Incorporation of Rare-Earths metal ions leads to the
formation of multi energy levels below the
conduction band edge of TiO2
• Lanthanide ions may act as electron scavenger
and suppress e/h recombination;
Photocatalytic activity of Ln3+/TiO2
Weber, Grady and Kookdali, Cat Sci & Tech 2012, 2, 683.
• Lanthanite ions also can faciliate the
adsorption of organics or act as electron
acceptors (minimizing e/h recombination)
General enhancement in the photocatalytic activity:
- Enhanced adsorption of the organics;
- Effective separation of e/h
- High intrinsic absorptivity under UV irradiation  due to the ability of RE metal ions to trap
electrons and minimize e/h recombination
CeO2/TiO2
TOC removal efficiencies (Methylene
blue) during visible light irradiation
(t=180 min)
(a) UV vis absorption spectra fo undoped
and Ce-doped TiO2 microspheres
(b) Photographs of Ce-doped TiO2 samples
Effect of cerium doping the photocatalytic activity to degrade methylene blue:
From 1 – 5% cerium  excess Ce4+ dopants may introduce the indirect recombination
of electrons and holes to reduce the photocatalytic activity.
J. Xie et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 372 (2010) 107–114
CeO2/TiO2
Photocatalytic degradation of
methylene blue – different catalysts
and P25
Photocatalytic degradation of Rhodamine B–
different catalysts and P25
J. Xie et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 372 (2010) 107–114
Compósitos de TiO2 dopados com Er3+:YAlO3/Fe- e Co
•Fe and Co ions doped into TiO2 powder to restrain the recombination
•Er3+:YAlO3  upconversion luminescence agent  can transform the visible light into UV
light more efficiently
Degradation of organic compounds in the
presence of Er3+:YAlO3/(Co or Fe)/TiO2 under
visible light
• Visible light is converted luz UV pelo Er3+:YAlO3.
• UV light can excite TiO2 -> electrons transfer from VB to CB
•e/h pairs  no recombination due to presence of Fe or Co ions
R. Xu et al. / Solar Energy Materials & Solar Cells 94 (2010) 1157–1165
TiO2 composites doped with Er3+:YAlO3/Fe- or Co
25% Er3+YAlO3/Fe/TiO2
25% Er3+YAlO3/Co/TiO2
10% Er3+YAlO3/Co/TiO2
5% Er3+YAlO3/Fe/TiO2
Fe/TiO2
Co/TiO2
Photocatalytic degradation of azo fuchsine int the presence of photocatalysts Fe
or Co/TiO2 and different amouns of Er3+:YAlO3
R. Xu et al. / Solar Energy Materials & Solar Cells 94 (2010) 1157–1165
Bismutum Spinels
BiWO6, Bi4Ti3O12, BIOX (X=Cl, Br, I), Bi2O3  photocatalytic activity under UV and visible light
Eg = 2,9 a 3,5 eV, depending on the preparation method (Chen et al., 2012).
Bi2S3 Eg= 1,3 a 1,7 eV (Mesquita e Silva, 34ª Reunião SBQ,
2011).
* Bi2O2CO3  High activity: morphology, low band gap energy. (Chen et al., 2012)
* CdBiYO4 (Du and Juan, Solid State Sciences, 14 (2012) 1295-1305)  spinel
Copper nanowires
CuO  Eg ~1.2 eV
Nanowires  CuO e Cu(OH)2
FESEM images of sample
Nanowires of CuO
Efficient charge separation
and increase of
photocatalytic activity
UV absorption spectra of CuO
nanowires
Photocatalytic degradation of Rhodamine B using different
photocatalysts under UV light
Yu Li, Xiao-Yu Yang, Joanna Rooke, Guastaaf Van Tendeloo, Bao-Lian Su. Ultralong Cu(OH)2 and CuO nanowire bundles: PEG200-directed crystal growth for
enhanced photocatalytic performance, Journal of Colloid and Interface Science 348 (2010) 303–312
Tungstenium oxides
WO3 + co-catalyst(Pt, Cu, or Pd): high photocatocalytic efficency to degrade organics
WO3 --> Conduction Band ( +0.5 V vs NHE) is more positive than that for O2 reduction
O2 + e = O2*- (aq)
0.284 V vs NHE;
O2 + H+ + e = HO2* (aq), 0.046 V vs NHE
WO3  can act as photocatalyst sensible to visible light in the presence of an electron acceptor
(ozônio  +2.07 V vs NHE).
Ozone reacts with the photoexcited electrons  oxidation of organic compounds
WO3
 Eg
= 2,5 ev
S. Nishimoto et al. / Chemical Physics Letters 500 (2010) 86–89
Photocatalytic degradation of Phenol
TOC initial = 130 ppm
S. Nishimoto et al. / Chemical Physics Letters 500 (2010) 86–89
Photocatalytic degradation of Phenol
TOC initial = 130 ppm
E) Photocatalysts
d0 e d10 Óxidos metálicos
Domen et al. New Non-Oxide
Photocatalysts Designed for Overall
Water Splitting under Visible Light.
J. Phys. Chem. 2007
d0
Ti4+: TiO2, SrTiO3, K2La2Ti3O10
Zr4+: ZrO2
Nb5+: K4Nb6O17, Sr2Nb2O7
Ta5+: ATaO3(A=Li, Na, K), BaTa2O6
W6+: AMWO6 (A=Rb, Cs; M=Nb, Ta)
d10
Ga3+: ZnGa2O4
In3+: AInO2 (A=Li, Na)
Ge4+: Zn2GeO4
Sn4+: Sr2SnO4
Sb5+: NaSbO7
Generally, the band gap energy is high
Photocatalytic activity of oxides and nitrides d10 metals  it is associated with the CB of
the hybridized sp-orbitals, that are able to produce photoexcited eletrons with high
mobility.
Final Remarks
 The function and engineering of co-catalysts is one of the most important subjects in
photocatalysis.
 Challenge and perspectives  photocatalysts sensible to visible light and high activity
 Promissor materials
Graphene
Rare earths
Composites and doped co-catalyts
 Reactor design is still a big challenge
Thank you
regina@enq.ufsc.br