Effect of method of synthesis and different metal doping on the photocatalytic activity of titania

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Effect of method of synthesis and different
metal doping on the photocatalytic activity of
titania
P. Vijayan

• Department of Chemistry
• Anna University
• Chennai

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Photocatalysis

• In the year 1972, Fujishima and Honda discovered
  the photocatalytic splitting of water on titania
  semiconductor electrode. Later on Sato and White
  decomposed water on platinum loaded TiO2 powder.
  
• After that researchers concentrated towards the
  usage of semiconductor photocatalysis for
  environmental protection.
• Photocatalysis has been exploited for various
  environmental process such as deodorization,
  water purification, air purification,
  sterilization and soil proof.
• Many researchers have put their effort on
  degradation and mineralization of variety of
  toxic substances by photocatalytic oxidation.
  Carbon dioxide has been reduced efficiently with
  water by photocatalysis.
• Different types of semiconductors were used as
  photocatalyst. Among these titania is the widely
  used photocatalyst due to its strong oxidizing
  power, stable at different pH and favorable band
  gap energy.

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Photocatalysts and their band gap energy
Semiconductor Valence band Conductance band Band gap (eV) Band gap wavelength (nm)
SnO2 4.1 0.3 3.8 318
ZnO 3.0 -0.2 3.2 390
ZnS 1.4 -2.3 3.7 336
CdS 2.1 -0.4 2.5 497
CdSe 1.6 -0.1 1.7 730
TiO2 3.1 -0.1 3.0 380
GaP 1.3 -1.0 2.3 540
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• Even though titania is the widely used
  semiconductor, it has some disadvantage like low
  surface area, fast recombination and wavelength
  maximum lies in UV region.
• Commercial titania P-25 has average particle size
  of 40-50nm. Since larger particles increases the
  specific surface area of the catalyst, reduction
  of particle size become essential.
• Recombination are of two types (i) surface
  recombination (ii) volume recombination.
• Surface recombination can be avoided by doping
  some foreign material like metal, non-metals and
  coupling titania with another semiconductor.
  Volume recombination can be reduced by preparing
  titania in nanoscale. Wavelength of maximum can
  also increased to higher wavelength by doping
  metals or non-metals.
• It is well known that wavelength and surface area
  are mainly depend on particle size. Altering the
  size of the particle alters the degree of
  confinement of the electrons, and affects the
  electronic structure of the solid, in particular
  band edges, which are tunable with particle
  size.
• From the above fact that the synthesis of titania
  with controlled nanometer dimension and doped
  with altervalent ion is essential.

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Size Dependence of Light Absorption

• Ismat shah etal studied the effect of particle
  size for the degradation of 2-chlorophenol
  using titania synthesized by metallo organic
  chemical vapour deposition method with different
  particle size.
• Particle size decrease from 29 to 17 nm showed
  red shift and further decrease i.e less than 17nm
  leads to blue shift.
• Titania with 17nm have shown remarkable catalytic
  activity towards the degradation of
  2-chlorophenol than the catalysts having 23nm and
  12nm.
• Comparison of band gaps
• B17nm lt B12nm lt B23nm

Appl.Catal.BEnviron, Vol. 68, 2006, Pages. 1-11
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Methods used

• Sol-gel
• Liquid phase deposition
• Pulse laser deposition
• Chemical vapour deposition
• Magnetic sputtering
• Direct hydrothermal method

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Sol-gel

• Generation of a dispersion of colloidal particles
  suspended in Brownian motion within a fluid
  matrix.
• Colloids are suspension of particles of linear
  dimensions between 1nm and 1 ?m. The colloidal
  suspensions can subsequently convert to viscous
  gels and then to solid materials
• Sol-gel preparation leads to the greatest
  possible homogeneous distribution of the dopant
  ion in the host matrix.
• Products have high purity and homogeneity, ease
  of processing and composition control
• Sol-gel synthesis involving following steps
• Ageing
• Gelation
• Drying
• Densification

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Hydrothermal treatment

• The hydrothermal process including aqueous
  solvents as reaction medium is eco-friendly since
  it is carried out in a closed system and the
  contents can be recovered and reused after
  cooling down to room temperature.
• The equipment and processing required are simpler
  and reaction is low energy consumption,
• By controlling hydrothermal temperature and
  duration of the treatment, various crystalline
  products with different composition, structure
  and morphology could be obtained.
• Fine particle size can be obtained with more
  uniform distribution and high dispersion either
  in polar and nonpolar solvents
• In this way the energy band structure becomes
  discrete and titania nanoparticles exhibit
  improved optical and photocatalytic properties.

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Sol-gel hydrothermal

• Materials can be obtained with high purity
• Particle size can be reduced and controlled
• Highly crystalline material can be obtained
• Material with higher light harvesting character
• Phase formation can be decided before the heat
  treatment i.e, calcination
• Eco-friendly

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2,4,6-Trichlorophenol

• Chlorophenols are deadly toxic compounds, present
  in wastewater which mainly arise from chemical
  intermediates or by-products in petrochemical,
  paper making, plastic, pesticidal and water
  disinfection
• PCDD/PCDF are the most toxic chlorinated
  substances which are 1000 times poisonous than
  arsenic. Due to handling problem of these
  chemicals in the laboratory, the model pollutant
  such as chlorophenol and chlorobenzene which are
  having structural similarities with them are
  taken
• 19 different chlorophenols have been listed by
  the US-EPA as priority pollutants
• Among these 2,4,6-trichlorophenol is one of the
  most vulnerable water pollutants, which causes
  serious damage to the vital organs of human
  beings . Thus, the removal of the chlorophenols
  from the waste water is highly imperative.
• In this work, we have used a modified sol-gel
  followed by hydrothermal treatment to synthesize
  metals (Fe, Cr, Co and Zr ) doped titania and
  used them for the oxidative degradation of
  2,4,6-TCP.

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Experimental

• Pure titania was synthesized by sol-gel,
  hydrothermal and combination of these two methods
  
• Parameters affecting synthesis such as Water
  Ethanol ratio, hydrothermal temperature and
  duration were optimized for effective synthesis.
• Doped titania were synthesized by sol-gel
  hydrothermal method with various weight
  percentage of loading (0.3, 0.5, 0.7 and 1wt).

• Reactor
• Slurry photoreactor
• 8x8w lamp
• Emits 365nm wave length

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Procedure adopted------Sol-gel
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Procedure adopted----hydrothermal
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Procedure adopted----sol-gel hydrothermal
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Effect of synthesis parameters
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XRD pattern of (a) sol-gel (b) hydrothermal and
(c ) sol-gel hydrothermal synthesized titania

• In sol-gel, hydrothermal and combination of
  sol-gel and hydrothermal methods, only anatase
  phase is formed.
• Particle size and Crystallinity of titania with
  different method of synthesis follows the order
  sol-gellt sol-gel hydrothermal lt hydrothermal.
• Most of the particle are in same size in both the
  hydrothermal and sol-gel hydrothermal methods
• In sol-gel restriction of particle size within a
  range is not possible. Particles are in wide
  range of sizes.

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Histogram of (a) sol-gel (b) hydrothermal and (c)
sol-gel hydrothermal
(c)
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UV-Vis DRS spectra of (a) sol-gel (b)
hydrothermal and (c )sol-gel hydrothermal
synthesized titania

• Titania synthesized by sol-gel (11nm),
  hydrothermal (24 nm) and combination of these two
  methods (17nm) show different electrical and
  optical properties
• Titania synthesized by combination of sol-gel
  followed by hydrothermal show red shift while
  titania synthesized by sol-gel and hydrothermal
  separately show blue shift
• From the above observation it can be concluded
  that the particle size greatly influenced on the
  electrical and optical properties of the catalysts

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XRD pattern of metals (Fe, Cr, Co and Zr) doped
titania

• Metals doping does not alter the phase formation
• Metal doping reduces the crystallite growth
• There is no peak observed for the formation of
  metal oxide
• Particle size was calculated using scherrer
  equation. Among the metals doped, cobalt doped
  titania having the smaller size.

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UV-Visible DRS of metals doped titania

• Cr, Fe and Co doping shifted the wavelength
  maximum towards visible region. Whereas zirconium
  showed blue shift when compared with bare
  titania.
• The band gap energies of Co, Cr, Fe and Zr are
  2.85, 2.92, 2.95 and 3.18 eV respectively.
• Among the different metal doping, cobalt doped
  titania shows an extra absorption at 500-700nm.

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EPR spectral analysis of metals doped titania

• Spectra shows the EPR signal of (A) Fe/TiO2 (B)
  Co/TiO2 and (C) Cr/TiO2 catalyst
  respectively
• All the spectra show only one signal
  corresponding to (A) Fe3, (B) Co2 and (C) Cr3
  in anatase matrix respectively.

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Transmission Electron Microscopy
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Extent of degradation

• Among the different metal ion doped, cobalt doped
  titania which is having smaller particle size and
  increased wavelength absorption showed higher
  activity
• Iron and chromium doped titania are almost same
  in catalytic activity. Among these two, iron
  doped titania shows better activity.
• Zirconium doped titania showed poor catalytic
  activity when compared with others. Wavelength
  maximum shifts to blue line of the spectrum as
  zirconium concentration is increased may be the
  reason for this.

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Extent of mineralization

• Mineralization profile follows the same order
  with degradation profile
• Rate of Mineralization follows the order Cobalt
  gt Iron gt Chromium gt Zirconium

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Conclusion

• Sol-gel method can be used for the preparation of
  nanosized titania with high purity but size
  restriction is not possible anywhere in the
  preparation procedure. Sol-gel method does not
  produce phase purity
• Titania synthesized by hydrothermal method have
  more uniform distribution in particle size. High
  purity and reduced particle size is not possible
  in hydrothermal methods.
• Titania synthesized by combination of these two
  methods have the advantages of sol-gel and
  hydrothermal methods and overcame the
  disadvantages.
• Catalysts synthesized by combination methods have
  lower band gap energy and controlled particle
  size ( 15-20 nm) than sol-gel and hydrothermal
  methods.
• These observations conclude that titania
  synthesized by the combination method is the opt
  one for photocatalytic activity.

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• Iron, cobalt and chromium doping shifts the
  wavelength absorption maximum in to visible
  region whereas zirconium doping shifts towards UV
  region.
• From the XRD patterns of metal doped titania,
  metal doping doesnt affect the phase formation
  of core species i.e titania. But restricted the
  crystallite growth and thereby reduced the
  particle size.
• From the EPR spectra, all the dopant ion present
  in anatase matrix and not in the surface is
  confirmed.
• From the TEM histogram, the uniform distribution
  of particle size acquired by combination methods
  is confirmed. From the TEM picture, particle size
  of all the doped catalyst is ranging between
  15-20nm is observed.
• Cobalt doped titania have smaller particle size
  and higher wavelength of absorption than the
  other doped titania.
• Above synthesized catalysts was tested for the
  photocatalytic oxidative degradation of 2, 4, 6-
  Trichlorophenol in aqueous suspension.

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• From the degradation and mineralization profiles,
  cobalt shows higher catalytic activity than other
  doped catalysts.
• Iron and chromium doped titania shows almost
  similar activity profile, but iron doping shows
  higher catalytic activity.
• Zirconium doped titania which showed blue shift
  as the metal content increased shows poor
  catalytic activity.
• On comparison with other dopant ion, cobalt shows
  higher catalytic activity even at higher
  concentration (1 wt).

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Thank you