A review on the visible light active titanium dioxide photocatalysts for environmental applications

TL;DR: In this paper, the development of different strategies to modify TiO2 for the utilization of visible light, including non metal and/or metal doping, dye sensitization and coupling semiconductors are discussed.

Abstract: Fujishima and Honda (1972) demonstrated the potential of titanium dioxide (TiO2) semiconductor materials to split water into hydrogen and oxygen in a photo-electrochemical cell. Their work triggered the development of semiconductor photocatalysis for a wide range of environmental and energy applications. One of the most significant scientific and commercial advances to date has been the development of visible light active (VLA) TiO2 photocatalytic materials. In this review, a background on TiO2 structure, properties and electronic properties in photocatalysis is presented. The development of different strategies to modify TiO2 for the utilization of visible light, including non metal and/or metal doping, dye sensitization and coupling semiconductors are discussed. Emphasis is given to the origin of visible light absorption and the reactive oxygen species generated, deduced by physicochemical and photoelectrochemical methods. Various applications of VLA TiO2, in terms of environmental remediation and in particular water treatment, disinfection and air purification, are illustrated. Comprehensive studies on the photocatalytic degradation of contaminants of emerging concern, including endocrine disrupting compounds, pharmaceuticals, pesticides, cyanotoxins and volatile organic compounds, with VLA TiO2 are discussed and compared to conventional UV-activated TiO2 nanomaterials. Recent advances in bacterial disinfection using VLA TiO2 are also reviewed. Issues concerning test protocols for real visible light activity and photocatalytic efficiencies with different light sources have been highlighted.

Summary

1.2 Electronic processes in TiO 2 photocatalysis

• Alternatively, the charge carriers can migrate to the catalyst surface and initiate redox reactions with adsorbates [27] .
• Positive holes can oxidize OHor water at the surface to produce OH radicals (Eq. 1.2) which, are extremely powerful oxidants (Table 2 ).
• The hydroxyl radicals can subsequently oxidize organic species with mineralization producing mineral salts, CO 2 and H 2 O (Eq. 1.5) [28] .

1.3 Recombination

• Recombination of photogenerated charge carriers is the major limitation in semiconductor photocatalysis as it reduces the overall quantum efficiency [30] .
• When recombination occurs, the excited electron reverts to the valence band without reacting with adsorbed species (Eq. 1.2) [31] non-radiatively or radiatively, dissipating the energy as light or heat [6, 32] .
• Doping with ions [35] [36] [37] , heterojunction coupling [38] [39] [40] and nanosized crystals [41, 42] have all been reported to promote separation of the electron-hole pair, reducing recombination and therefore improve the photocatalytic activity.
• The conduction band potential of rutile is more positive than that of anatase which means that the rutile phase may act as an electron sink for photogenerated electrons from the conduction band of the anatase phase.

1.4 Strategies for improving TiO 2 photoactivity

• Which will be reviewed in the next section, their overall efficiencies have been significantly enhanced by controlling the semiconductor morphology.
• The most commonly used TiO 2 morphology is that of monodispersed nanoparticles wherein the diameter is controlled to give benefits from the small crystallite size (high surface area, reduced bulk recombination) without the detrimental effects associated with very small particles (surface recombination, low crystallinity) [44] .
• One dimensional (1D) titania nanostructures (nanotubes, nanorods, nanowires, nanobelts, nanoneedles) have been also formed by hydrothermal synthesis but high emphasis was given in titania self-assembled nanotubular films grown by electrochemical anodization on titanium metal foils.
• Advantages of such structures is their tailored morphology, controlled porosity, vectorial charge transfer [45, 46] and low recombination at grain boundaries that result in enhanced performance in photoinduced applications, mainly in photocatalysis [45, 47, 48 ].

2. Development of Visible Light Active (VLA) Titania Photocatalysts

• 1 Non metal doping 2.1.1 Nitrogen doping Ultraviolet light makes up only 4-5 % of the solar spectrum, whereas approximately 40 % of solar photons are in the visible region.
• Non-metal doping of TiO 2 has shown great promise in achieving visible light active (VLA) photocatalysis, with nitrogen being the most promising dopant [52, 53] .
• Nitrogen containing precursors used include aliphatic amines, nitrates, ammonium salts, ammonia and urea [74] [75] [76] .
• In a different approach N-TiO 2 , was synthesized via two successive steps: synthesis of TiO 2 and then nitrogen doping using various nitrogen-containing chemicals (e.g. urea, ethylamine, NH 3 or gaseous nitrogen) at high temperatures [53, [83] [84] [85] or inductively coupled plasma containing a wide range of nitrogen precursors [86] .
• Visible light active N-TiO 2 with anatase-rutile mixed phase has also been prepared by tuning the parameters of the sol-gel synthesis.

2.1.4 Oxygen rich TiO 2 modification

• Following another approach, recently the visible light active photocatalytic properties have been achieved by the in-situ generation of oxygen through the thermal decomposition of peroxo-titania complex [119] .
• Increased Ti-O-Ti bond strength and upward shifting of the valence band (VB) maximum were responsible for the visible light activity.
• The upward shifting of the VB maximum for oxygen rich titania is identified as another crucial reason responsible for efficient visible light absorption.
• Typical band gap structures of control and oxygen rich titania samples obtained are represented in Figure 5 .

2.2 Metal Deposition 2.2.1. Noble metal and transition metal deposition.

• In addition, metals remaining on the TiO 2 surface block reaction sites [124] .
• The incorporation of transition metals in the titania crystal lattice may result in the formation of new energy levels between VB and CB, inducing a shift of light absorption towards the visible light region.
• Possible limitations are photocorrosion and promoted charge recombination at metal sites [127] .
• Seery et al., showed enhanced visible light photocatalysis with Ag modified TiO 2 [141] .

2.3 Dye sensitization in photocatalysis

• In addition to the mentioned species, singlet oxygen may also be formed under certain experimental conditions.
• The subsequent radical chain reactions can lead to the degradation of the dye [150] .
• Ultrafast electron injection has been reported for many dye-sensitized TiO 2 systems.
• This injection depends on the nature of the sensitizer, the semiconductor, and their interaction.

2.4 Coupled semiconductors

• These parameters strongly depend on the manner with which the couples are prepared.
• Synthesis methods normally require high temperatures, long times, strict inert atmosphere protection and complex multistep reaction process.
• TiO 2 -coated nanoparticles with a core-shell structure have been prepared with ultrasound treatment.
• It is found that modification of TiO 2 with CdS particles extends the optical absorption spectrum into the visible region in comparison with that of pure TiO 2 .
• Such core-shell nanocomposites may bring new insights into the design of highly efficient photocatalysts and potential applications in technology.

3.1. Reactive oxygen species and reaction pathways in VLA TiO 2 photocatalysis

• As a model, the reaction pathways of visible light-induced photocatalytic degradation of acid orange 7 (AO7) in the presence of TiO 2 has been investigated [181] , monitoring the formation and the fate of intermediates and final products in solution and on the photocatalyst surface as a function of irradiation time.
• During the irradiation of AO7-TiO 2 suspension with visible light different intermediates such as compounds containing a naphthalene ring, phthalic derivatives, aromatic acids, and aliphatic acids were identified.
• This indicates that the superoxide radical is an active oxidative intermediate.
• The spin-trap 2,2,6,6tetramethyl-4-piperidone-N-oxide (TEMP) is generally used as a probe for singlet oxygen in EPR studies.
• The degradation of phenol by adding i-PrOH or MeOH was decreased by about 60 percent which indicated that both of them seriously inhibited the photocatalytic degradation of phenol [186] .

3.2 Photoelectrochemical methods for determining visible light activity

• The latter will not have any direct relation with the redox potential such as E eq (OH/H 2 O) but will have a strong relation with the basicity of H 2 O or the energy of an intermediate radical [Ti-O HO-Ti] s that is roughly giving the activation energy for the reaction.
• They concluded that the observed photocurrent in the presence of reductants strongly depends on the reaction mechanism of oxidation and more knowledge is needed concerning the mechanism.
• The addition of iodide partially suppressed the recombination due to hole scavenging.
• Photoelectrochemical measurements can contribute significantly to the understanding of the mechanisms involved in the visible light activity of doped TiO 2 and other photocatalytic materials and can be combined with direct measuring the spectral dependence of the quantum efficiency for different pollutants [197] .
• More research is required to fully elucidate the mechanisms involved.

4.1 Water treatment and air purification with VLA photocatalysis

• Conventional TiO 2 has been extensively studied for water treatment and air purification and it is well known to be an effective system to treat several hazardous compounds in contaminated water and air.
• The diverse group of substances, which are commonly detected at low concentration in the aqueous media and often are difficult to quantitatively remove from the water by conventional water treatment processes, can produce important damages in human health and in the aquatic environment, even at low concentrations.
• Composite materials, such as nitrogen-doped TiO 2 supported on activated carbon (N-TiO 2 /AC), have also been tested and proven to have a dual effect on the adsorption and photocatalytic degradation of bisphenol-A under solar light [203] .
• N-TiO 2 photocatalyst, described in section 2.1 as a one step process synthesis with DDAC as pore template and nitrogen dopant, efficiently degraded MC-LR under visible light.
• A pH dependence was observed in the initial degradation rates of MC-LR where acidic conditions (pH 3.0) were favorable compare to higher pH values [116] .

4.2 Water disinfection with VLA photocatalysis

• A 5-log inactivation was observed after ca. 30 min irradiation, although disinfection was observed in the dark controls due to the biocidal properties of Ag ions.
• Some of the most comprehensive studies on VLA TiO 2 disinfection have been undertaken by the Pulgarin group at EPFL, Switzerland.
• Suspension reactor studies using E. coli showed that the doped Tacya materials were slightly less active that the non-doped powders during UV excitation, however, under visible light excitation (400-500 nm) the N, S co-doped powders outperformed the undoped powders, with those annealed at 400 o C resulting in 4-log E. coli inactivation following 75 min treatment [216] .
• S co-doped P25, under solar excitation the main species responsible for E. coli inactivation was the hydroxyl radical produced by the UV excitation of the parent material .

5. Assessment of VLA photocatalyst materials 5.1 Standardization of test methods

•  r = rate of disappearance of substrate / rate of disappearance of phenol (5.4) However, Ryu and Choi reported that the photocatalytic activities can be represented in many different ways, and even the relative activity order among the tested photocatalysts depends on what substrate is used [227] .
• If the application is purely a visible light driven process e.g. self-cleaning surfaces for indoor applications, then a visible light source should be utilized for the test protocol.
• Various photocatalytic test systems with different model pollutants/substrates have been reported.
• An example of this dye sensitised phenomenon was reported with the apparent photocatalytic "disappearance" of indigo carmine dye [229] .
• The test system should utilize the catalyst in the same form -suspension or immobilized.

5.2 Challenges in commercializing VLA photocatalysts

• Some VLA TiO 2 photocatalytic products, like Kronos® VLP products, have already appeared in the market.
• Deactivation occurs when partially oxidized intermediates block the active catalytic sites on the photocatalyst [235] .
• The photocatalytic degradation of many organic compounds also generates unwanted by-products, which may be harmful to human health [23] .
• Carboxylic acids formed from alcohol degradation are also believed to strongly be adsorbed to the active sites of a catalyst and cause deactivation [23] .
• Strongly adsorbed intermediate species appear to commonly cause deactivation of a photocatalyst and it is certainly an area where further improvement is essential before TiO 2 can be considered a viable option for continuous photocatalytic applications.

Conclusions

• Titanium dioxide is introduced as a promising semiconductor photocatalyst due to its physical, structural and optical properties under UV light.
• Other non metals including carbon, fluorine and sulphur for doping and co-doping with nitrogen have been also investigated and shown visible light photo-induced activity.
• The reactive oxygen species generated with VLA TiO 2 under visible light indicate a different mechanism of photoactivation compared to UV light.
• Therefore, these results are promising for further development of sustainable environmental remediation technologies, based on photocatalytic advanced oxidation processes driven by solar light as a renewable source of energy.

Figures

Table 2.

Table 1.

Full text

Technological University Dublin Technological University Dublin
ARROW@TU Dublin ARROW@TU Dublin
Articles ESHI Publications
2012-08-15
A Review on the Visible Light Active Titanium Dioxide A Review on the Visible Light Active Titanium Dioxide
Photocatalysts for Environmental Applications Photocatalysts for Environmental Applications
Miguel Pelaez
University of Cincinnati
Nicholas Nolan
Technological University Dublin
Suresh Pillai
Technological University Dublin
, suresh.pillai@tudublin.ie
See next page for additional authors
Follow this and additional works at: https://arrow.tudublin.ie/ehsiart
Part of the Environmental Sciences Commons, Materials Chemistry Commons, and the Polymer
Chemistry Commons
Recommended Citation Recommended Citation
Pelaez, M. et al (2012). A Review on the Visible Light Active Titanium Dioxide Photocatalysts for
Environmental Applications.
Applied Catalysis B:Environmental
, vol. 125, pp. 331– 349. doi:10.1016/
j.apcatb.2012.05.036
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Authors Authors
Miguel Pelaez, Nicholas Nolan, Suresh Pillai, Michael Seery, Polycarpos Falaras, Athanassios G. Kontos,
Patrick S.M. Dunlop, Jeremy W.J. Hamiltone, J. Anthony Byrne, Kevin O’Shea, Mohammad H. Entezari, and
Dionysios D. Dionysiou
This article is available at ARROW@TU Dublin: https://arrow.tudublin.ie/ehsiart/2

Accepted Manuscript
Title: A review on the visible light active titanium dioxide
photocatalysts for environmental applications
Authors: Miguel Pelaez, Nicholas T. Nolan, Suresh C. Pillai,
Michael K. Seery, Polycarpos Falaras, Athanassios G. Kontos,
Patrick S.M. Dunlop, Jeremy W.J. Hamilton, J.Anthony
Byrne, Kevin O’shea, Mohammad H. Entezari, Dionysios D.
Dionysiou
PII: S0926-3373(12)00239-1
DOI: doi:10.1016/j.apcatb.2012.05.036
Reference: APCATB 12094
To appear in: Applied Catalysis B: Environmental
Received date: 28-3-2012
Revised date: 21-5-2012
Accepted date: 25-5-2012
Please cite this article as: M. Pelaez, N.T. Nolan, S.C. Pillai, M.K. Seery, P.
Falaras, A.G. Kontos, P.S.M. Dunlop, J.W.J. Hamilton, J.A. Byrne, K. O’shea, M.H.
Entezari, D.D. Dionysiou, A review on the visible light active titanium dioxide
photocatalysts for environmental applications*, Applied Catalysis B, Environmental
(2010), doi:10.1016/j.apcatb.2012.05.036
This is a PDF file of an unedited manuscript that has been accepted for publication.
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Page 1 of 54
Accepted Manuscript
1
A review on the visible light active titanium dioxide photocatalysts for 1
environmental applications* 2
3
4
MIGUEL PELAEZ,
1
NICHOLAS T. NOLAN,
2
SURESH C. PILLAI,
2
MICHAEL K. SEERY,
3
5
POLYCARPOS FALARAS,
4
ATHANASSIOS G. KONTOS,
4
PATRICK S.M. DUNLOP,
5
JEREMY 6
W.J. HAMILTON,
5
J.ANTHONY BYRNE,
5
KEVIN O‟SHEA,
6
MOHAMMAD H. ENTEZARI
7
and 7
DIONYSIOS D. DIONYSIOU
1,
§
8
9
10
1
Environmental Engineering and Science Program, School of Energy, Environmental, Biological, and 11
Medical Engineering, University of Cincinnati, Cincinnati, Ohio 45221-0071, USA 12
2
Center for Research in Engineering Surface Technology (CREST) 13
DIT FOCAS Institute, Kevin St, Dublin 8, Ireland 14
3
School of Chemical and Pharmaceutical Sciences, Dublin Institute of Technology, Kevin St., Dublin 8, 15
Ireland 16
4
Institute of Physical Chemistry, NCSR Demokritos, 15310 Aghia Paraskevi, Attiki, Greece 17
5
Nanotechnology and Integrated BioEngineering Centre, School of Engineering, University of Ulster, 18
Northern Ireland, BT37 0QB, United Kingdom 19
6
Department of Chemistry and Biochemistry, Florida International University, University Park, Miami, 20
Florida 3319, USA 21
7
Department of Chemistry, Ferdowsi University of Mashhad, Mashhad, 91775, IRAN 22
23
* All authors have contributed equally to this review. 24
25
§Corresponding author phone: (513) 556-0724; fax: (513) 556-2599; e-mail: 26
dionysios.d.dionysiou@uc.edu. 27
*Manuscript

Page 2 of 54
Accepted Manuscript
2
Abstract 1
2
Fujishima and Honda (1972) demonstrated the potential of titanium dioxide (TiO
2
) 3
semiconductor materials to split water into hydrogen and oxygen in a photo-4
electrochemical cell. Their work triggered the development of semiconductor 5
photocatalysis for a wide range of environmental and energy applications. One of the 6
most significant scientific and commercial advances to date has been the development of 7
visible light active (VLA) TiO
2
photocatalytic materials. In this review, a background on 8
TiO
2
structure, properties and electronic properties in photocatalysis is presented. The 9
development of different strategies to modify TiO
2
for the utilization of visible light, 10
including non metal and/or metal doping, dye sensitization and coupling semiconductors 11
are discussed. Emphasis is given to the origin of visible light absorption and the reactive 12
oxygen species generated, deduced by physicochemical and photoelectrochemical 13
methods. Various applications of VLA TiO
2
, in terms of environmental remediation and 14
in particular water treatment, disinfection and air purification, are illustrated. 15
Comprehensive studies on the photocatalytic degradation of contaminants of emerging 16
concern, including endocrine disrupting compounds, pharmaceuticals, pesticides, 17
cyanotoxins and volatile organic compounds, with VLA TiO
2
are discussed and 18
compared to conventional UV-activated TiO
2
nanomaterials. Recent advances in bacterial 19
disinfection using VLA TiO
2
are also reviewed. Issues concerning test protocols for real 20
visible light activity and photocatalytic efficiencies with different light sources have been 21
highlighted. 22
23
24
Keywords: TiO
2
, visible, solar, water, treatment, air purification, disinfection, non-metal 25
doping, anatase, rutile, N-TiO
2
, metal doping, environmental application, reactive 26
oxygen species, photocatalysis, photocatalytic, EDCs, cyanotoxins, emerging pollutants. 27
28
29
1. Titanium dioxide- an Introduction 30
31
1.1 TiO
2
structures and properties 32
33
Titanium dioxide (TiO
2
) exists as three different polymorphs; anatase, rutile and brookite 34
[1]. The primary source and the most stable form of TiO
2
is rutile. All three polymorphs 35
can be readily synthesised in the laboratory and typically the metastable anatase and 36
brookite will transform to the thermodynamically stable rutile upon calcination at 37
temperatures exceeding ~600 °C [2]. In all three forms, titanium (Ti
4+
) atoms are co-38
ordinated to six oxygen (O
2-
) atoms, forming TiO
6
octahedra [3]. Anatase is made up of 39
corner (vertice) sharing octahedra which form (001) planes (Figure 1a) resulting in a 40
tetragonal structure. In rutile the octahedra share edges at (001) planes to give a 41
tetragonal structure (Figure 1b), and in brookite both edges and corners are shared to give 42
an orthorhombic structure (Figure 1c) [2,4-7]. 43
44
Titanium dioxide is typically an n-type semiconductor due to oxygen deficiency [8]. The 45
band gap is 3.2 eV for anatase, 3.0 eV for rutile, and ~3.2 eV for brookite [9-11]. Anatase 46