# Synthesis of Visible-Light-Activated Yellow AmorphousTiO2 Photocatalyst

Authors: Chamnan Randorn; John T. S. Irvine; Peter Robertson
Journal: International Journal of Photoenergy
 (2008)

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## Abstract

Visible-light-activated yellow amorphousTiO2 (yam-TiO2) was synthesised by a simple and organic-free precipitation method. TiN, an alternative precursor for TiO2 preparation, was dissolved in hydrogen peroxide under acidic condition (pH∼1) adjusted by nitric acid. The yellow precipitate was obtained after adjusting pH of the resultant red brown solution to 2 with NH4OH. The BET surface area of this sample was 261 m2/g. The visible light photoactivity was evaluated on the basis of the photobleaching of methylene blue (MB) in an aqueous solution by using a 250 W metal halide bulb equipped with UV cutoff filter (λ>420 nm) under aerobic conditions. Yam-TiO2 exhibits an interesting property of being both surface adsorbent and photoactive under visible light. It was assigned to the η2-peroxide, an active intermediate form of the addition of H2O2 into crystallined TiO2 photocatalyst. It can be concluded that an active intermediate form of titanium peroxo species in photocatalytic process can be synthesised and used as a visible-light-driven photocatalyst.


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## Body

## 1. Introduction

Titanium dioxide (TiO2) has been the prevailing
material in the fields of photocatalysis and solar energy conversion due to being chemically and biologically inert and
environmentally friendly. Although TiO2 is now used in various
practical applications, only a small UV fraction of solar light can be utilised
because of its large band gap of 3.2 eV. The
development of a titania-related photocatalyst that shows a high level of
activity under visible light would therefore be a major advance. Yellow
TiO2 prepared by nitrogen doping has been regarded as a visible-light-sensitive
photocatalyst [1–4] as well as Cr or V doped TiO2 [5, 6].
Additionally, metal ion doped SrTiO3 [7–9], some metal sulfides such
as AgGaS2 and AgInZn7S9 [10], (Ga1−yZny)(N1−xOx) [11],
and Sm2Ti2O7 [12] have been reported as
visible-light-driven photocatalysts. There may be some disadvantages of doped
materials, for instance, low surface area because of using high temperature,
the requirement of the expensive chemicals and synthetic methods, and an
increase in carrier recombination.Amorphous TiO2 is of interest for
photocatalysis because of its high surface area and high adsorption. Moreover, simple synthesis with elimination
of the calcination step would mean lower costs for chemicals and energy
consumption. There have been only a few works that studied or mentioned
amorphous TiO2. Among them, it was found that commercial amorphous
TiO2 had negligible photoactivity because it contains high
concentrations of defects which cause rapid e−-h+ recombination [13]. However, synthetic amorphous TiO2 has been
recently shown to exhibit significantly photoactivity rates in aqueous methanol
solutions under full solar spectrum (200 to >1000 nm) using a 400 W Xe arc
lamp [14].Hydrogen peroxide (H2O2)
has been reported as an important factor in the photocatalytic process because
it has occurred as an intermediate in the mechanism of the photocatalytic
process using TiO2 as photocatalysts, and it has been reported that
the reactivity can be accelerated by the addition of H2O2 [15–17].
Moreover, titanium-peroxo species were found to be the active species in the
titanium silicalite-1 (TS-1)/H2O2/H2O system
for partial hydrocarbon oxidation [18, 19]. In general, titanium-peroxo species
or aqueous peroxotitanate are yellow or yellow-green depending on pH. There
have been some works that used titanium peroxo complex as a starting material
for TiO2 preparation [20–22].As mentioned in the above discussion,H2O2 is important in photocatalysis at TiO2, amorphous TiO2 offers some enhancement of activity and noting the yellow colour of the
catalytically active titanium-peroxo species in the TS-1/H2O2/H2O
system, it would be interesting to see if yellow amorphous titanium-peroxo
species can act as a photocatalyst under visible light. In this work,
therefore, we investigate the visible-light-driven
photoactivity of yellow amorphous TiO2 (hereinafter referred to as “yam-TiO2”) prepared by a peroxide-based
route, using titanium nitride (TiN) as a precursor.


## 2. Experimental

### 2.1. Materials

Yam-TiO2 was synthesised by a peroxide-based
route using TiN as precursor. 50 mL of 30%H2O2 (Fisher Scientific, Leicestershire, UK) was added to
1.0 g of TiN (Alfa Aesar, Lancashire, UK)
under acidic condition (pH≤1) adjusted by HNO3 acid solution. After
aging at room temperature without any stirring for 24 hours, a clear red-brown
solution was obtained. The obtained precursor solution was stable for several
days under ambient atmosphere. Yellow precipitate is formed from this
solution after adjusting the pH of the solution to 2 by slowly adding ammonia
solution with the constant stirring. The precipitate was filtered and washed
with distilled water several times. It was then dried at room temperature. The
dried powder was also calcined at a constant heating rate of 5C°/min at various temperatures and held at these
temperatures for 2 hours.XRD data were
collected with a Stoe Stadi-P Transmission X-ray diffractometer, using Cu Kα1 radiation in the range 2θ = 20–80°. The BET surface area measurement and pore analysis were carried out by
nitrogen adsorption with use of Micromeritics ASAP 2020 V3.00 H surface area
analyser. The measurement was carried out at liquid nitrogen temperature after
degassing the powder sample at 120C°. TEM imaging was obtained using a JEOL-JEM 2011 electron
microscope. Diffuse reflectance spectroscopy was carried out with a Perkin
Elmer Lambda35 UV/Vis spectrometer using
BaSO4 as a reference.


### 2.2. Evaluation of Photocatalytic Activity

Visible light activity was evaluated on the basis of the
decomposition of methylene blue (MB) in an aqueous solution. The sample powder was suspended in 200 mL of
an 1 × 10−4 M MB solution by air bubbling. This mixture was first
suspended in the dark for 2 hours to reach the adsorption equilibrium before
irradiation with a high intensity discharge 250 W iron doped metal halide UV
bulb (UV Light Technology
Ltd., Bermingham, UK) equipped with UV cutoff filter, λ≥420 nm (Borosilicate Coated Glass HM07, UQG(optic)Ltd., Cambridge UK).
The mixture temperature was controlled at about 25C° using a water bath for infrared radiation and lamp
heating removal. After irradiation, 2 mL of the mixture was collected and
centrifuged at the irradiation time intervals (hourly). The photoactivity was
examined by monitoring the reduction of the absorbance at 665 nm. The
spectrophotometric measurements were carried out using Perkin Elmer Lambda35
UV/Vis spectrometer. A ten-fold dilution was used for the high absorbance
solutions.


## 2.1. Materials

Yam-TiO2 was synthesised by a peroxide-based
route using TiN as precursor. 50 mL of 30%H2O2 (Fisher Scientific, Leicestershire, UK) was added to
1.0 g of TiN (Alfa Aesar, Lancashire, UK)
under acidic condition (pH≤1) adjusted by HNO3 acid solution. After
aging at room temperature without any stirring for 24 hours, a clear red-brown
solution was obtained. The obtained precursor solution was stable for several
days under ambient atmosphere. Yellow precipitate is formed from this
solution after adjusting the pH of the solution to 2 by slowly adding ammonia
solution with the constant stirring. The precipitate was filtered and washed
with distilled water several times. It was then dried at room temperature. The
dried powder was also calcined at a constant heating rate of 5C°/min at various temperatures and held at these
temperatures for 2 hours.XRD data were
collected with a Stoe Stadi-P Transmission X-ray diffractometer, using Cu Kα1 radiation in the range 2θ = 20–80°. The BET surface area measurement and pore analysis were carried out by
nitrogen adsorption with use of Micromeritics ASAP 2020 V3.00 H surface area
analyser. The measurement was carried out at liquid nitrogen temperature after
degassing the powder sample at 120C°. TEM imaging was obtained using a JEOL-JEM 2011 electron
microscope. Diffuse reflectance spectroscopy was carried out with a Perkin
Elmer Lambda35 UV/Vis spectrometer using
BaSO4 as a reference.


## 2.2. Evaluation of Photocatalytic Activity

Visible light activity was evaluated on the basis of the
decomposition of methylene blue (MB) in an aqueous solution. The sample powder was suspended in 200 mL of
an 1 × 10−4 M MB solution by air bubbling. This mixture was first
suspended in the dark for 2 hours to reach the adsorption equilibrium before
irradiation with a high intensity discharge 250 W iron doped metal halide UV
bulb (UV Light Technology
Ltd., Bermingham, UK) equipped with UV cutoff filter, λ≥420 nm (Borosilicate Coated Glass HM07, UQG(optic)Ltd., Cambridge UK).
The mixture temperature was controlled at about 25C° using a water bath for infrared radiation and lamp
heating removal. After irradiation, 2 mL of the mixture was collected and
centrifuged at the irradiation time intervals (hourly). The photoactivity was
examined by monitoring the reduction of the absorbance at 665 nm. The
spectrophotometric measurements were carried out using Perkin Elmer Lambda35
UV/Vis spectrometer. A ten-fold dilution was used for the high absorbance
solutions.


## 3. Results and Discussion

### 3.1. Characterisation

The XRD pattern of the as-prepared powder confirmed the
amorphous structure of yam-TiO2 as no diffraction peaks can be
observed, Figure 1. Yam-TiO2 was quite stable under air atmosphere (the
sample used throughout this work was kept for several months in the aerobic
vial). However, colour and phase transformation of yellow amorphous to white
anatase/rutile occurred on heat treatment. Anatase was presented after
calcination at 400C° and remained as a major phase until 900C°. This indicated that a high temperature stable anatase
phase can be produced by this method. Phase transformation from anatase to
rutile can be observed at about 800–850C° which was higher than TiO2 prepared by
conventional methods.Figure 1
XRD patterns of as-prepared yam-TiO2 (a)
and after calcined at temperatures of (b) 400C°, (c) 800C°,
(d) 850C°, (e) 900C°, (f) 1100C°
(∗: Vaseline specific peaks, A: anatase, R: rutile).Furthermore, it was found that the particle
sizes of TiO2 after calcination at 600 and 900C° calculated by Scherrer equation were 31 and 51 nm, respectively. The obtained phase transformation temperature
and particle size were
consistent with the previous work that synthesised TiO2 by peroxide-based
route and using H2TiO3 as precursor [20]. However, the
stability of anatase phase in this work was higher than TiO2 synthesised by the peroxide-based route using TiCl3 as precursor
[21]. The peroxide-based route, therefore, is interesting for TiO2 preparation because of the high temperature anatase-rutile phase
transformation; however, it depends on precursor and synthesis condition. TiN
is interesting to be used
as an alternative precursor in a peroxide-based route because of its air and
moisture stability, organic and chloride ion-free route, simplicity, and low
cost.According
to our focus on evaluation of the visible light photoactivity of yellow
amorphous TiO2, TEM, BET and diffuse reflectance were only examined
with the as-prepared powder. Figure 2 shows the TEM image of the as-prepared
powder. The ultrafine particles with amorphous structure can be seen. As a
result, a high BET surface area of 261 m2/g was obtained. The
diffuse reflectance spectrum shows the red shift of the absorption edge into
the visible region (Figure 3(a)) compared with commercial rutile TiO2 (TIOXIDE)
(Figure 3(b)).Figure 2
TEM image of as-prepared yam-TiO2.Figure 3
The UV-Vis diffuse
reflectance of (a) yam-TiO2, (b) commercial rutile TiO2.Yam-TiO2 is attributed to the η2-peroxide (η2-TiOOH) because of the similar
red shift and that yam-TiO2 transformed to white powder on heating
similar to the η2-titanium peroxo species in
the TS-1/H2O2/H2O system reported by Bonino et al. [18]. Lin and Frei [19] found that the η2-peroxide species obtained upon
loading H2O2 into TS-1 was photodissociated efficiently under irradiation
with visible or near UV light. Moreover, η2-peroxide was assigned to the
adsorption structure formed by the addition of H2O2 on
the surface of rutile TiO2. This structure was preferable to produce
hydroxyl radical that accelerates the photoactivity [17].


### 3.2. Visible Light Photocatalytic Activity

The photobleaching of MB has been widely studied in
heterogeneous photocatalysis. However, there are some concerns about the
bleaching of MB due to the photoreduction to colourless form and
photoabsorption of MB [23–25]. There are two main forms of MB, the blue colour
of oxidised form (MB) and the colourless reduced form (leuco form, LMB). MB can
be photoreduced to LMB by TiO2 under UV light under anaerobic
condition and in the presence of a sacrificial electron acceptor (SED). It was
found that MB itself can act as an SED. However, this process is reversed on
the addition of O2 to the anaerobic system. Although LMB is
readily oxidised back to MB by oxygen, this reaction depends on pH. The rate
increases with pH, LMB is moderately stable under acidic condition but reacts
rapidly with air under basic condition [23]:(1)MB+SED⟶TiO2+UVLMB+SED2+;2LMB+O2⟶2MB+2H2O.As continuous air bubbling of a neutral
solution was utilised in this work and no reducing agent was added to the
system, it is likely that the dominant bleaching process is the
photomineralisation of MB rather than the reductive formation of LMB. This system, MB/TiO2/air-saturated
water system, has widely been utilised for the demonstration of semiconductor
photocatalysis, as reviewed by Mills and Wang [23]. It is possible or even probable that partial
rather than complete mineralisation into CO2, NH4+, NO3−, and SO42 is
occurring as the main observable in bleaching MB.The photoabsorption of MB can be an issue for
visible-light-driven photocatalyst investigation because MB can absorb visible
light, especially, in the range 600–700 nm. Yan et al. [25] concluded that MB
was not an appropriate substrate for a visible-light photocatalytic activity
test particularly in the range of 540–680 nm irradiation. However, the most important and intense
emitting wavelength of the metal halide lamp used in this work is in the range of about 200–580 nm [26]. After equipping with a UV cutoff filter, the emitting wavelength is in the range of
420–580, which has little overlap with the absorption of MB as shown in Figure 4.Figure 4
The emitting wavelength of metal halide
lamp (a) and UV-Vis absorption spectrum of MB (b).As a result, the photoabsorption of MB should
not significantly effect the determination of visible light photocatalytic
activity test in this work.Figure5 shows the photocatalytic degradation of MB comparing between amorphous TiO2 prepared in acidic condition (yam-TiO2) and in basic condition
(labeled byam-TiO2), including blank test (2 × 10−5 M MB).
It was found that 10% bleaching of MB was shown after visible light
irradiation of only MB for 5 hours (Figure 5(a)), whereas slightly higher, 15%
photobleaching of MB, was obtained by using 0.5 g/L byam-TiO2 as
photocatalyst (Figure 5(b)). However, the latter amount was obtained after 45% of 1 × 10−4 M MB was adsorbed, that indicated that the initial concentration of MB before
photobleaching was about 5.5 × 10−5 M. Comparision with using 0.5 g/L
yam-TiO2, about 97% adsorption and 2% photocatalytic activity was
measured (Figure 5(c)). It can be concluded that the adsorption capacity of yam-TiO2 was significantly higher than byam-TiO2; however, the extent of
adsorption must hinder the hetereogeneous photoactivity. Therefore, a
decreased amount of yam-TiO2 to 0.2 g/L was performed for photoactivity comparison (Figure 5(d)). It can be seen that
about 46% photobleaching of MB after adsorption was obtained, which was
considerably higher than using byam-TiO2 in spite of using less
amount and higher concentration remained after adsorption. Moreover, only 9%
photobleaching was observed for 0.5 g/L of Degussa P25 in 1 × 10−5 MB under
visible light for 2 hours (compared to 93% under UV for 1 hour) showing that
the yam-TiO2 was much more effective, ~40% under visible light for 2
hours. An important point to note is that the adsorbed MB seemed much less
intense in colour and indeed the colour had changed to pale purple/violet. This could indicate an oxidative adsorption
process in the dark with the oxidised form, MB•+ being formed and
adsorbed onto the yam-TiO2; however,
confirmation requires further study.Figure 5
% decolourisation of MB under the
condition of dark 2 hours and light 5 hours of (a) MB, (b) MB + 0.5 g/L byam-TiO2, (c) MB + 0.5 g/L yam-TiO2,
(d) MB + 0.2 g/L yam-TiO2.The irreversibility of the photobleaching
process on continuous air bubbling for 24 hours in the dark after complete
decolourisation of MB and the disappearance of the bands associated with MB
(294 nm and 665 nm) with no appearance of band associated with LMB (256 nm) [24] as shown in Figure 6. were observed,
further confirming that photoreduction of MB to LMB was not the dominant
process.Figure 6
UV-Vis absorption spectra of MB.These results show that yam-TiO2 can
act as a visible-light-driven photocatalyst better than byam-TiO2,
in parallel with a red shift of the absorption edge into the visible region as
shown in Figure 7. However, an increase in visible absorption edge does not
guarantee consistency in visible light photoactivity, the recombination and
surface area are also important factors.Figure 7
The UV-Vis diffuse reflectance of (a)
yam-TiO2, (b) byam-TiO2.Recyclability and stability of photocatalyst are important factors for any practical
applications. Therefore, we have attempted to investigate cyclability and
stability. In general, the photoactivity of crystalline TiO2 is
observed by monitoring the degradation of MB in an aqueous solution, and
adsorbed MB is not an issue probably due to small amount of MB that was
adsorbed on crystallined TiO2. The recyclability of crystallined TiO2 normally performed by elimination of adsorbed MB on TiO2 by
oxidation at temperatures higher than about 200C°, which is the decomposition temperature of MB. However, removal of adsorbed MB by
calcination cannot be used in the case of amorphous form because
crystallisation will be induced.As a result, the cyclability of yam-TiO2 was measured by two different methods, with and without removal of adsorbed MB.
The first way, to completely remove adsorbed MB on the surface by using
photoactivity itself until the original colour of yam-TiO2 was
recovered. Due to a large amount of MB being adsorbed on the surface, 48 hours
UV irradiation was used for adsorbed MB removal, after colourless solution was
obtained by visible light photoactivity test, in order to reduce the duration
of adsorbed MB removal and to evaluate the stability of yam-TiO2 under UV light simultaneously. Figure 8 shows the recyclability by using UV
irradiation for adsorbed MB removal. The 0.5 g/L ratio of catalyst to volume of
MB was chosen because of the incomplete MB adsorption, hence, the visible-light
photoactivity can be monitored, and a quantity of catalyst will be sufficient
for removal of the high amount of adsorbed MB in 48 hours. About 97% adsorption and 2% photoactivity in
5 hours were obtained in the preliminary photoactivity test (Figure 8(a)). After the
mixture was irradiated with UV light for 48 hours, the violet powder of visible
light irradiated/adsorbed MB was changed to yellow powder as the original
colour with a small amount of violet specks. Some violet specks indicated
incomplete removal of adsorbed MB. Thereafter, the colourless aqueous solution
was removed from the mixture by rinsing and the remaining powder left to dry at
room temperature.Figure 8
Recyclability
with removal of adsorbed MB (a) MB + 0.5 g/L yam-TiO2, (b) 1st reuse,
(c) 2nd reuse, (d) only photoactivity of 1st reuse, (e) only photoactivity of 2nd
reuse.The first reuse was performed by readding 200 mL of1×10−4 M MB. A slight decrease of adsorption and photoactivity
was obtained for the first and second recycles (Figure 8(b), (c)). A small decrease of
adsorption capacity and photoactivity presumably caused by incomplete adsorbed
MB removal and decrease of efficiency or stability of amorphous structure after
prolonged UV
irradiation, which the colour changing from yellow to
pale yellow can be observed by the naked eye (XRD data showed that it remained
amorphous structure). However, if considering only photoactivity by using MB concentration after adsorption, about 2 × 10−5 M estimated from the
percentage of bleaching, as an initial concentration, about 80%
bleaching of MB under visible light irradiation can be reached in 5 hours both
in the first and second recycles (Figure 8(d), (e)).The second
way of recyclability testing was performed by readding 200 mL1×10−4 M MB without removal of adsorbed MB. The results are shown in Figure 9. The
adsorption capacity significantly decreased in the first and second reuses as expected, 75%
and 22%, respectively (Figure 9(b), (c)), due to MB was adsorbed 97% of 1 × 10−4 M MB in the preliminary photoactivity test before recycling (Figure 9(a)). The
photobleaching of MB in the first reuse, including adsorption and
photoactivity, was 85% in 4 hours and 99% in 24 hours (Figure 9(b)), while 51% and 95%,
respectively, in the second reuse (Figure 9(c)). However, considering only the
photoactivity, the
percentage of bleaching of MB under visible light irradiation can be
about 40% in 5 hours and more than 90% in 24 hours both in the first and second
recycles (Figure 9(d), (e)). These results show the reusability of yam-TiO2.Figure 9
Recyclability
without removal of adsorbed MB (a) MB +
0.5 g/L yam-TiO2, (b) 1st reuse, (c) 2nd reuse, (d) only
photoactivity of 1st reuse, (e) only photoactivity of 2nd reuse.As mentioned above about the gradual colour
changing of yam-TiO2 from yellow to pale yellow after irradiation,
it was found that it remained pale yellow and can be a visible-light-driven
photocatalyst even on prolonged irradiation, although the reaction rate
decreased. The color change probably due to some η2-peroxide being converted to a
hydroxide form upon photo irradiation [17, 19]. The intense yellow can be
recovered after further addition of H2O2 as well known for crystalline TiO2. For this reason, yam-TiO2 can be reused several times with addition of H2O2.


## 3.1. Characterisation

The XRD pattern of the as-prepared powder confirmed the
amorphous structure of yam-TiO2 as no diffraction peaks can be
observed, Figure 1. Yam-TiO2 was quite stable under air atmosphere (the
sample used throughout this work was kept for several months in the aerobic
vial). However, colour and phase transformation of yellow amorphous to white
anatase/rutile occurred on heat treatment. Anatase was presented after
calcination at 400C° and remained as a major phase until 900C°. This indicated that a high temperature stable anatase
phase can be produced by this method. Phase transformation from anatase to
rutile can be observed at about 800–850C° which was higher than TiO2 prepared by
conventional methods.Figure 1
XRD patterns of as-prepared yam-TiO2 (a)
and after calcined at temperatures of (b) 400C°, (c) 800C°,
(d) 850C°, (e) 900C°, (f) 1100C°
(∗: Vaseline specific peaks, A: anatase, R: rutile).Furthermore, it was found that the particle
sizes of TiO2 after calcination at 600 and 900C° calculated by Scherrer equation were 31 and 51 nm, respectively. The obtained phase transformation temperature
and particle size were
consistent with the previous work that synthesised TiO2 by peroxide-based
route and using H2TiO3 as precursor [20]. However, the
stability of anatase phase in this work was higher than TiO2 synthesised by the peroxide-based route using TiCl3 as precursor
[21]. The peroxide-based route, therefore, is interesting for TiO2 preparation because of the high temperature anatase-rutile phase
transformation; however, it depends on precursor and synthesis condition. TiN
is interesting to be used
as an alternative precursor in a peroxide-based route because of its air and
moisture stability, organic and chloride ion-free route, simplicity, and low
cost.According
to our focus on evaluation of the visible light photoactivity of yellow
amorphous TiO2, TEM, BET and diffuse reflectance were only examined
with the as-prepared powder. Figure 2 shows the TEM image of the as-prepared
powder. The ultrafine particles with amorphous structure can be seen. As a
result, a high BET surface area of 261 m2/g was obtained. The
diffuse reflectance spectrum shows the red shift of the absorption edge into
the visible region (Figure 3(a)) compared with commercial rutile TiO2 (TIOXIDE)
(Figure 3(b)).Figure 2
TEM image of as-prepared yam-TiO2.Figure 3
The UV-Vis diffuse
reflectance of (a) yam-TiO2, (b) commercial rutile TiO2.Yam-TiO2 is attributed to the η2-peroxide (η2-TiOOH) because of the similar
red shift and that yam-TiO2 transformed to white powder on heating
similar to the η2-titanium peroxo species in
the TS-1/H2O2/H2O system reported by Bonino et al. [18]. Lin and Frei [19] found that the η2-peroxide species obtained upon
loading H2O2 into TS-1 was photodissociated efficiently under irradiation
with visible or near UV light. Moreover, η2-peroxide was assigned to the
adsorption structure formed by the addition of H2O2 on
the surface of rutile TiO2. This structure was preferable to produce
hydroxyl radical that accelerates the photoactivity [17].


## 3.2. Visible Light Photocatalytic Activity

The photobleaching of MB has been widely studied in
heterogeneous photocatalysis. However, there are some concerns about the
bleaching of MB due to the photoreduction to colourless form and
photoabsorption of MB [23–25]. There are two main forms of MB, the blue colour
of oxidised form (MB) and the colourless reduced form (leuco form, LMB). MB can
be photoreduced to LMB by TiO2 under UV light under anaerobic
condition and in the presence of a sacrificial electron acceptor (SED). It was
found that MB itself can act as an SED. However, this process is reversed on
the addition of O2 to the anaerobic system. Although LMB is
readily oxidised back to MB by oxygen, this reaction depends on pH. The rate
increases with pH, LMB is moderately stable under acidic condition but reacts
rapidly with air under basic condition [23]:(1)MB+SED⟶TiO2+UVLMB+SED2+;2LMB+O2⟶2MB+2H2O.As continuous air bubbling of a neutral
solution was utilised in this work and no reducing agent was added to the
system, it is likely that the dominant bleaching process is the
photomineralisation of MB rather than the reductive formation of LMB. This system, MB/TiO2/air-saturated
water system, has widely been utilised for the demonstration of semiconductor
photocatalysis, as reviewed by Mills and Wang [23]. It is possible or even probable that partial
rather than complete mineralisation into CO2, NH4+, NO3−, and SO42 is
occurring as the main observable in bleaching MB.The photoabsorption of MB can be an issue for
visible-light-driven photocatalyst investigation because MB can absorb visible
light, especially, in the range 600–700 nm. Yan et al. [25] concluded that MB
was not an appropriate substrate for a visible-light photocatalytic activity
test particularly in the range of 540–680 nm irradiation. However, the most important and intense
emitting wavelength of the metal halide lamp used in this work is in the range of about 200–580 nm [26]. After equipping with a UV cutoff filter, the emitting wavelength is in the range of
420–580, which has little overlap with the absorption of MB as shown in Figure 4.Figure 4
The emitting wavelength of metal halide
lamp (a) and UV-Vis absorption spectrum of MB (b).As a result, the photoabsorption of MB should
not significantly effect the determination of visible light photocatalytic
activity test in this work.Figure5 shows the photocatalytic degradation of MB comparing between amorphous TiO2 prepared in acidic condition (yam-TiO2) and in basic condition
(labeled byam-TiO2), including blank test (2 × 10−5 M MB).
It was found that 10% bleaching of MB was shown after visible light
irradiation of only MB for 5 hours (Figure 5(a)), whereas slightly higher, 15%
photobleaching of MB, was obtained by using 0.5 g/L byam-TiO2 as
photocatalyst (Figure 5(b)). However, the latter amount was obtained after 45% of 1 × 10−4 M MB was adsorbed, that indicated that the initial concentration of MB before
photobleaching was about 5.5 × 10−5 M. Comparision with using 0.5 g/L
yam-TiO2, about 97% adsorption and 2% photocatalytic activity was
measured (Figure 5(c)). It can be concluded that the adsorption capacity of yam-TiO2 was significantly higher than byam-TiO2; however, the extent of
adsorption must hinder the hetereogeneous photoactivity. Therefore, a
decreased amount of yam-TiO2 to 0.2 g/L was performed for photoactivity comparison (Figure 5(d)). It can be seen that
about 46% photobleaching of MB after adsorption was obtained, which was
considerably higher than using byam-TiO2 in spite of using less
amount and higher concentration remained after adsorption. Moreover, only 9%
photobleaching was observed for 0.5 g/L of Degussa P25 in 1 × 10−5 MB under
visible light for 2 hours (compared to 93% under UV for 1 hour) showing that
the yam-TiO2 was much more effective, ~40% under visible light for 2
hours. An important point to note is that the adsorbed MB seemed much less
intense in colour and indeed the colour had changed to pale purple/violet. This could indicate an oxidative adsorption
process in the dark with the oxidised form, MB•+ being formed and
adsorbed onto the yam-TiO2; however,
confirmation requires further study.Figure 5
% decolourisation of MB under the
condition of dark 2 hours and light 5 hours of (a) MB, (b) MB + 0.5 g/L byam-TiO2, (c) MB + 0.5 g/L yam-TiO2,
(d) MB + 0.2 g/L yam-TiO2.The irreversibility of the photobleaching
process on continuous air bubbling for 24 hours in the dark after complete
decolourisation of MB and the disappearance of the bands associated with MB
(294 nm and 665 nm) with no appearance of band associated with LMB (256 nm) [24] as shown in Figure 6. were observed,
further confirming that photoreduction of MB to LMB was not the dominant
process.Figure 6
UV-Vis absorption spectra of MB.These results show that yam-TiO2 can
act as a visible-light-driven photocatalyst better than byam-TiO2,
in parallel with a red shift of the absorption edge into the visible region as
shown in Figure 7. However, an increase in visible absorption edge does not
guarantee consistency in visible light photoactivity, the recombination and
surface area are also important factors.Figure 7
The UV-Vis diffuse reflectance of (a)
yam-TiO2, (b) byam-TiO2.Recyclability and stability of photocatalyst are important factors for any practical
applications. Therefore, we have attempted to investigate cyclability and
stability. In general, the photoactivity of crystalline TiO2 is
observed by monitoring the degradation of MB in an aqueous solution, and
adsorbed MB is not an issue probably due to small amount of MB that was
adsorbed on crystallined TiO2. The recyclability of crystallined TiO2 normally performed by elimination of adsorbed MB on TiO2 by
oxidation at temperatures higher than about 200C°, which is the decomposition temperature of MB. However, removal of adsorbed MB by
calcination cannot be used in the case of amorphous form because
crystallisation will be induced.As a result, the cyclability of yam-TiO2 was measured by two different methods, with and without removal of adsorbed MB.
The first way, to completely remove adsorbed MB on the surface by using
photoactivity itself until the original colour of yam-TiO2 was
recovered. Due to a large amount of MB being adsorbed on the surface, 48 hours
UV irradiation was used for adsorbed MB removal, after colourless solution was
obtained by visible light photoactivity test, in order to reduce the duration
of adsorbed MB removal and to evaluate the stability of yam-TiO2 under UV light simultaneously. Figure 8 shows the recyclability by using UV
irradiation for adsorbed MB removal. The 0.5 g/L ratio of catalyst to volume of
MB was chosen because of the incomplete MB adsorption, hence, the visible-light
photoactivity can be monitored, and a quantity of catalyst will be sufficient
for removal of the high amount of adsorbed MB in 48 hours. About 97% adsorption and 2% photoactivity in
5 hours were obtained in the preliminary photoactivity test (Figure 8(a)). After the
mixture was irradiated with UV light for 48 hours, the violet powder of visible
light irradiated/adsorbed MB was changed to yellow powder as the original
colour with a small amount of violet specks. Some violet specks indicated
incomplete removal of adsorbed MB. Thereafter, the colourless aqueous solution
was removed from the mixture by rinsing and the remaining powder left to dry at
room temperature.Figure 8
Recyclability
with removal of adsorbed MB (a) MB + 0.5 g/L yam-TiO2, (b) 1st reuse,
(c) 2nd reuse, (d) only photoactivity of 1st reuse, (e) only photoactivity of 2nd
reuse.The first reuse was performed by readding 200 mL of1×10−4 M MB. A slight decrease of adsorption and photoactivity
was obtained for the first and second recycles (Figure 8(b), (c)). A small decrease of
adsorption capacity and photoactivity presumably caused by incomplete adsorbed
MB removal and decrease of efficiency or stability of amorphous structure after
prolonged UV
irradiation, which the colour changing from yellow to
pale yellow can be observed by the naked eye (XRD data showed that it remained
amorphous structure). However, if considering only photoactivity by using MB concentration after adsorption, about 2 × 10−5 M estimated from the
percentage of bleaching, as an initial concentration, about 80%
bleaching of MB under visible light irradiation can be reached in 5 hours both
in the first and second recycles (Figure 8(d), (e)).The second
way of recyclability testing was performed by readding 200 mL1×10−4 M MB without removal of adsorbed MB. The results are shown in Figure 9. The
adsorption capacity significantly decreased in the first and second reuses as expected, 75%
and 22%, respectively (Figure 9(b), (c)), due to MB was adsorbed 97% of 1 × 10−4 M MB in the preliminary photoactivity test before recycling (Figure 9(a)). The
photobleaching of MB in the first reuse, including adsorption and
photoactivity, was 85% in 4 hours and 99% in 24 hours (Figure 9(b)), while 51% and 95%,
respectively, in the second reuse (Figure 9(c)). However, considering only the
photoactivity, the
percentage of bleaching of MB under visible light irradiation can be
about 40% in 5 hours and more than 90% in 24 hours both in the first and second
recycles (Figure 9(d), (e)). These results show the reusability of yam-TiO2.Figure 9
Recyclability
without removal of adsorbed MB (a) MB +
0.5 g/L yam-TiO2, (b) 1st reuse, (c) 2nd reuse, (d) only
photoactivity of 1st reuse, (e) only photoactivity of 2nd reuse.As mentioned above about the gradual colour
changing of yam-TiO2 from yellow to pale yellow after irradiation,
it was found that it remained pale yellow and can be a visible-light-driven
photocatalyst even on prolonged irradiation, although the reaction rate
decreased. The color change probably due to some η2-peroxide being converted to a
hydroxide form upon photo irradiation [17, 19]. The intense yellow can be
recovered after further addition of H2O2 as well known for crystalline TiO2. For this reason, yam-TiO2 can be reused several times with addition of H2O2.


## 4. Conclusion

An active intermediate form of an addition ofH2O2 into crystallined TiO2 photocatalyst can be synthesised by peroxide-based
route. It can be used as a visible-light-driven photocatalyst itself with the
high surface area of amorphous form.



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*Source: 426872-2008-06-15.xml*