Novel synthesis of Ag decorated TiO2 anchored on zeolites derived from coal fly ash for the photodegradation of bisphenol-A

Article abstract of DOI:10.1039/c7nj02885g

The disposal of millions of tons of coal fly ash (CFA) threatens the environment, hence means to reuse CFA are highly sought after. In this study, CFA was reused to make materials which were tested for water purification. Zeolitic material (CFA-Zeo) was derived from CFA by a 2-step alkali-fusion hydrothermal method and then composited with TiO₂ nanoparticles using a novel resin-gel technique. CFA-Zeo loadings were 15 and 30 wt% in the resulting TiO₂/CFA-Zeo composites. These composites were then loaded with 1 wt% Ag nanoparticles by a deposition-precipitation technique using NaOH and urea. CFA-Zeo rods (morphology confirmed by TEM) were confirmed by PXRD to be sodium aluminum silicate hydrate. TEM analyses of the CFA-Zeo rods in the composites revealed them to be completely coated with TiO₂ nanoparticles that had Ag nanoparticles on their surfaces. The photoluminescence emission peak of TiO₂ was found to be significantly higher than that of TiO₂/CFA-Zeo composites, with the TiO₂/CFA-Zeo composites that were loaded with Ag having even lower emission intensities. UV-vis DRS spectra showed that CFA-Zeo had no effect on the band gap of TiO₂, while composites that contained Ag had a wide absorption band in the visible region. The photocatalytic efficiency of these materials was then determined using bisphenol-A (BPA) as a model compound under both UV and visible light. Except for the 30 wt% TiO₂/CFA-Zeo composites without Ag, all of the composites had superior photoactivity to uncomposited TiO₂ under both UV and visible light. On the other hand, composites with Ag nanoparticles showed the best photoactivities. The superior photoactivities of these composites under UV-light were mainly attributed to the separation of charge carriers, whereas under visible light it was attributed to the ability of silver to harvest visible light through surface plasmon resonance (SPR).

Full text of DOI:10.1039/c7nj02885g

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Novel synthesis of Ag decorated TiO2 anchored on zeolites derived
from coal fly ash for the photodegradation of bisphenol-A
Lerato Hlekelele,*a,b Paul J. Franklyna, Farai Dziikea,b, and Shane H. Durbacha,b
Received 00th January 20xx,
Accepted 00th January 20xx
The disposal of millions of tons of coal fly ash (CFA) threatens the environment, hence means to reuse CFA are highly
sought after. In this study, CFA was reused to make materials which were tested for water purification. Zeolitic material
(CFA_Zeo) was derived from CFA by a 2-step alkali-fusion hydrothermal method and then composited with TiO2
nanoparticles using a novel resin-gel technique. The CFA_Zeo loadings were 15 and 30 % wt. in the resulting TiO2/CFA_Zeo
composites. These composites were then loaded with 1 % wt. Ag nanoparticles by deposition precipitation using NaOH
and urea. The CFA_Zeo rods (morphology confirmed by TEM) were confirmed by PXRD to be sodium aluminum silicate
hydrate. TEM analyses of the CFA_Zeo rods in the composites showed them to be completely coated with TiO2
nanoparticles that had Ag nanoparticles on their surfaces. The photoluminescence emission peak of TiO2 was found to be
significantly higher than that of the TiO2/CFA_Zeo composites, with TiO2/CFA_Zeo composites that were loaded with Ag
having even lower emission intensity. UV-Vis DRS spectra showed that CFA_Zeo had no effect on the band gap of TiO2,
while composites that contained Ag had a wide absorption band in the visible region. The photocatalytic efficiency of
these materials was then determined using bisphenol–A (BPA) as a model compound under both UV and visible light.
Except for the 30 % wt. TiO2/CFA_Zeo composites without Ag, all of the composites had superior photoactivity to
uncomposited TiO2 under both UV and visible light. On the other hand, composites with Ag nanoparticles showed the best
photoactivities. The superior photoactivities of these composites under UV-light was mainly attributed to the separation of
charge carriers, whereas under visible light it was attributed to the ability of silver to harvest visible light through surface
DOI: 10.1039/x0xx00000x
www.rsc.org/
plasmon
resonance
(SPR).
Furthermore, zeolites have been derived from CFA using direct
and non-direct techniques.1,16,17
Introduction
Zeolites are aluminosilicates with group 1 and 2 metals as
counter ions. The framework that makes up their structure is
composed of [SiO4]-4 and [AlO4]-5 arranged in a tetrahedral
geometry. The two framework components are linked to each
other through the corners of a tetrahedron, sharing an oxygen
atom. The 3-D network that results from such structures has
cavities/voids which are responsible for the properties of
zeolites and by extension the large scope of their applications.
The properties of zeolites that make them potentially suitable
materials for photocatalytic applications includes their: i)
Photochemical stability, ii) Chemical inertness, iii) Thermally
stability, iv) High surface area, and iii) Ability to contribute in
electron-transfer processes as either electron donors or
acceptors.18
Since the burning of coal for electricity generation scaled up in
the 1920s, an increase in coal combustion by-products is being
witnessed. It is estimated that only 16 % of the 600 Mt of coal
fly ash (CFA) produced annually is utilized,1 the rest of it is
disposed of in landfills and lagoons.2
CFA is a powdery material that is principally composed of
glassy, hollow or solid spherical particles with diameters in the
micron range. The chemical composition of CFA is 65-95 %
alumina and silica whereas the other 5-35 % is made up of
other metal oxides and unburned carbonaceous material.1,2
The physical and chemical properties of CFA, in particular its
rich mineral content, makes it a potentially useful material to
study. Research has been done on the potential uses of CFA to
obviate an increasing threat to the environment, while
minimizing the costs associated with its disposal.1,3 For
instance, different types of carbon nanomaterials have been
synthesized by chemical vapor deposition (CVD), when CFA
was used as a catalyst.4–9 Amongst other applications, CFA has
also been used as adsorbents and catalyst supports.10–15
Several authors have reported on the use of zeolites as
support materials for TiO2 for use in photocatalysis.18–21 For
example, Anpo et al. observed an improved photocatalytic
efficiency and selectivity for TiO2 and Y-zeolite composites
relative to TiO2, for the production of methanol.19 On the other
hand, Tayade et al. showed that photodegradation efficiency
of methylene blue using zeolites coated with TiO2 was higher
than that of bare TiO2.18 However, the synthetic method used
to prepare supported TiO2 based photocatalysts has been
shown to influence the morphological properties of the
resulting
material
and
thus
their
photocatalytic
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efficiencies.22,23 For example, Gao et al. reported that Separately, 10 g of polyethylene glycol (PEG, Mw, 8000 Sigma-
composites of TiO2 and carbon nanotubes (CNTs) that were Aldrich) was added to 50 ml ethanol and heated to 100 °C to fully
synthesized by the surfactant wrapping sol-gel method dissolve the polymer. The ethanoic molten polymer solution was
showed superior photocatalytic activity relative to those that added to the TiCl4 and zeolite mixture whilst vigorously stirring. At
were synthesized by the conventional sol-gel method.22
this stage, CFA_Zeo was covered with the polymer that had Ti4+ ions
stabilized onto by electrostatic interactions. The Ti4+ ions were then
hydrolyzed by slowly adding 5 ml of ammonium solution (25 %,
Fluka) while stirring. The mixture was stirred for a further 30 min
and kept under a 200 W IR lamp until a hard wax was formed. The
mixture was then transferred into a ceramic crucible, placed on a
sand heating bath and heated to ca. 500 °C. A Bunsen burner was
used to ignite the sample and it was left to burn until the flame
ceased. The resulting black granular powder product was collected,
ground and calcined at 450 °C for 2 h to crystallize TiO2 and remove
the residual polymer material and soot.
Another useful way of improving the photocatalytic activity of
TiO2 is by doping it with metals, nonmetals or metal-oxides.23–
25 For instance, doping TiO2 with noble metals has been shown
to increase visible light driven photocatalysis. Silver has been
investigated in greater detail because of its low cost relative to
other noble metals, strong surface plasmon resonance and
easy shape control.23,26–28
In this work, we report a 2-step alkali-fusion hydrothermal
synthesis of zeolites (CFA_Zeo) from CFA. Various quantities of
these zeolites (CFA_Zeo) were then used to make composites
with TiO2 using a novel resin-gel method. These composites
were then loaded with Ag nanoparticles and tested as
photocatalysts for their photodegradation efficiency of BPA
under both ultraviolet and visible light. Leading on from our
previous research, this work serves to further show how CFA, a
The samples were synthesized such that they contained 15 or 30 %
CFA_Zeo relative to TiO2 and as such are labeled as
TiO2/CFA_Zeo(15 %) and TiO2/CFA_Zeo(30 %) respectively in the
text. TiO2 nanoparticles were synthesized using the same procedure
except that CFA_Zeo was not added.
toxic waste material can be made useful for the Ag nanoparticles (1 %) were loaded onto TiO2 and TiO2/CFA_Zeo
decontamination of water.
composites by the deposition-precipitation technique using NaOH
and urea, as follows: A mass (0.500 g) of TiO2 or TiO2/CFA_Zeo
composite was dispersed in 100 ml distilled water by sonication for
30 min. To this suspension, 8.030 mg of silver nitrate (Sigma-
Aldrich) was added along with 0.2840 g of urea (Merck). The pH of
the mixture was then increased to 10 using dilute sodium hydroxide
(Sigma-Aldrich). The resulting mixture was then heated to 80 °C for
24 h. The product was washed with ethanol and centrifuged. The
resulting solid was dried at 60 °C for 12 h in an oven and then
calcined at 450 °C for 2 h in a furnace. A percent (1 %) of Ag
nanoparticles was deposited on TiO2/CFA_Zeo(15 %),
TiO2/CFA_Zeo(30 %), and TiO2, and as such the resulting composites
were labeled: Ag/TiO2/CFA_Zeo(15 %), Ag/TiO2/CFA_Zeo(30 %) and
Ag/TiO2.
Methodology
Synthesis of zeolites from CFA
A modified alkali fusion hydrothermal method of synthesizing
zeolites from CFA as was adopted from Shigemoto and Hayashi
was carried out as follows:29 10 g of NaOH pellets (Sigma-
Aldrich) was mixed with 10 g of CFA (as received from the
ESKOM, Research and Innovation Centre, Rosherville in South
Africa). The mixture was ground into fine powder and heated
on a tempered iron jaffle maker to 700 °C in a furnace. The
resultant mixture was allowed to cool down to 350 °C and
transferred into ice-cold water whilst stirring magnetically. The
slurry was transferred into a Teflon cup, placed in an autoclave
and heated to 200 °C for 12 h while stirring. The product was
then centrifuged, washed with distilled water and the
precipitate was oven dried at 60 °C for 12 h. The dry powder
was then calcined at 500 °C for 2 h in a furnace. The product
was labeled and is referred to as CFA_Zeo in the text.
Catalyst characterization
The minerals in CFA were quantified by X-ray fluorescence
spectrometer (XRF
- PANalytical PW2404). The elemental
composition of CFA and CFA_Zeo as well as the presence of Ag were
determined by using an energy dispersive X-ray spectrometer (EDS)
coupled to a field emission scanning electron microscope (SEM -
ZEISS SIGMA) which was operated at a practical voltage of 10 kV.
The morphologies of raw CFA, CFA_Zeo, TiO2 and all the composites
were studied by a transmission electron microscope (TEM - FEI
Tecnai G2 Spirit electron microscope FEICo., Hillsboro, OR, USA) and
SEM. The specimen for SEM analysis was coated with a layer of
gold-palladium. The surface area of the materials was studied using
a Brunauer-Emmett-Teller (BET) Micrometrics Tristar 3000 surface
area and porosity analyzer. The crystallinity of the materials was
studied using powder X-ray diffraction (PXRD) using a Bruker D2
phaser (Bruker AXS, Karlsruhe, Germany) in Bragg-Brenton
geometry with a Lynexe detector using Co-Kα radiation at 30 kV and
10 mA. The optical properties of TiO2 and the composites were
studied by UV-Vis diffuse reflectance spectroscopy (DRS) measured
on a commercial Praying Mantis DRS Cary 500. Photoluminescence
Synthesis of Photocatalysts
TiO2 and CFA_Zeo composites were synthesized using the resin-gel
technique. Here a predetermined mass of CFA_Zeo (0.135 and
0.315 g for composites consisting of 15 and 30 % wt. CFA_Zeo
respectively) was dispersed in 20 ml ethanol (96 %, Sigma-Aldrich)
by sonication followed by vigorous magnetic stirring. The mixture
was then kept in ice at temperatures below 5 °C and 1 ml of
titanium tetrachloride (Fluka) was added slowly and stirred for 30
min.
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measurements were done on
a Jobin-Yvon T64000 Raman accounted for more than 90 % of the CFA composition. The silica to
spectrometer armed with an Olympus BX40 microscope, at an alumina ratio was found to be 2.04. This meant that the raw CFA
excitation wavelength of 290 nm.
used was class F, according to the American society for testing of
materials (ASTM).3
Photocatalytic Measurements
Powder X-ray diffraction (PXRD) analysis was conducted in order to
investigate if zeolization of CFA had been achieved (Fig 1).
The photocatalytic activity of these materials was evaluated by the
photodegradation of BPA. Individual slurries consisting of 25 mg of
the each of the materials to be tested and a BPA solution (50 ml, 60
ppm) were first magnetically stirred for 2 h in the dark to ensure
that adsorption equilibrium had been reached. Each mixture was
then exposed to light for 60 min with a solar simulator (Newport
model 69911), with and without a separate UV light filter (λ> 400
nm, Zeiss 95mm UV filter) equipped with a 300 W Xenon Short Arc
lamp (Eurolux) giving an irradiance of 100 W/cm2 at 25 °C. An Oriel
PV reference system fitted with a 2 cm x 2 cm monocrystalline
silicon photovoltaic cell was used to set the solar simulator to the
equivalence of 1 sun irradiance.
CFA_Zeo
CFA
X
X_Zeolite x
M-mulite
X
X
Q_quartz
S_Sodalite
X
X
X
X
X
X
S
S
Q
M
M
Q
MM
M
20
M
Q Q
M
H
Volumes of 1 ml were sampled at 15 min intervals and analyzed by
a high-performance liquid chromatography (HPLC) coupled to a
photodiode array detector (UltiMate 3000). The photodegradation
was monitored at 225 nm. The samples were eluted using a
gradient method at a flow rate of 0.2 ml/min using a Phenomenex
LUNA 5µ C18 reverse phase column (150 x 4.60 mm). At first 70 %
of ultra-pure water and 30 % of acetonitrile were used for elution
for 6 min and the ratio of acetonitrile was increased from 30 % to
90 % over 2 min and held for 1 min. The ratio of acetonitrile was
then reduced back to 30 % over an 8 min period and was finally
held constant for 3 min.
10
30
40
50
60
70
80
90
2
θ
Fig 1. PXRD patterns of CFA and CFA_Zeo
The PXRD pattern of raw CFA showed that quartz (Q), mullite (M)
and hematite (H) were the major minerals in CFA. PXRD analysis of
the material formed after the 2-step alkali-fusion hydrothermal
reaction with the CFA showed the quartz and mulite peaks had
completely disappeared, while peaks indexed to sodium aluminum
silicate hydrate and sodalite had appeared. These showed that
zeolization had occurred. Indeed, Molina and Poole have previously
investigated two methods of synthesizing zeolites from CFA and
have found that the alkali-fusion process was crucial for the
formation of zeolite-X under short reaction times.30 The PXRD
pattern of the zeolite-X they obtained, along with several others
reported as zeolite-X in literature,31–33 were similar to that which is
reported in this work (Fig 1). This along with peak indexing led to
the conclusion that the synthesis procedure that had been used in
this work had yielded zeolite X.
The Total Organic Content (TOC) measurements were done on two
samples: sample that was photodegraded using the sample that
was photodegraded for 48 h using Ag/TiO2/CFA_Zeo(15 %). This
sample was vacuum filtered in its entirety and 40 ml of the filtrate
was analyzed for TOC by a TOC analyzer (Tekmar Dormann Apollo
9000) relying on CO2 quantification.
The photodegradation intermediates were analyzed by an
LC/MS/MS. The intermediates were separated by liquid
chromatography (LC) on an LC-20Ad XR model using the same
column that was used for the HPLC analyses. The mobile phase
system was also the same as was used for HPLC analyses. The
photodegradation intermediates were identified using a mass-
spectrometry (3200 Q TRAP LC/MS/MS system) coupled to the
aforementioned LC system. MS was run in the negative ion mode in
the m/z range of 50-300. The sample analyzed for the identification
of the photodegradation intermediates was first photodegraded
using Ag/TiO2/CFA_Zeo(15 %) then filtered to remove the catalyst.
The filtrate was freeze dried (Telstar LyoAlfa 10) then dissolved in 4
ml water before analysis.
A semi-quantitative measurement of the chemical composition of
CFA and CFA_Zeo was obtained through EDS. Here it was observed
that the major components of CFA were Si and Al, with very low Na
content (S2 and S3). Significantly, it was observed that upon
zeolization, the Si/Na ratio had decreased (Si/Na=1.31) compared to
that of CFA (Si/Na=22.7). This was plausible because Na ions (NaOH)
had been used in the process of converting CFA into a zeolitic
material. On the other hand, the Si/Al ratio of CFA_Zeo was found
to be 1.87 (similar to that of CFA, 2.13) which indicated that an
insignificant amount of alumina and silica had been lost during the
zeolization process.
Results and discussion
Fourier transform infrared spectroscopy (FTIR) analysis was
conducted on both the raw CFA and CFA_Zeo in order to further
investigate if zeolization occurred (Fig 2).
The chemical composition of the raw CFA was determined by XRF
(S1), where it was observed that silica, alumina and iron oxide
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CFA_Zeo
CFA
2500
2000
1500
1000
500
-1
cm )
Wavenumber
(
Fig 2. FTIR spectra of CFA and CFA_Zeo
The FTIR spectrum of the raw CFA was comprised of two focal
intense and wide peaks, which were typical of internal vibrations in
MO4 tetrahedra, where M=Si/Al. The peak centered at 1090 cm-1
was attributed to the vibrations of the M–O asymmetric stretching.
This peak signifies the extent of crystallinity of the material.34 The
other less intense band centered at 787 cm-1 was attributed to the
presence of Al atoms in the tetrahedral form.35 Each type of zeolite
has distinguishing IR signatures even though there are some
common characteristics.34 The broad and intense peak observed on
the FTIR spectra of CFA_Zeo, at 1001 cm-1 was attributed to the
asymmetric stretching of bridge bonding between M–O–M, where
M = Si or Al. The smaller peak at 670 cm–1 was attributed to the
symmetric stretching vibrations of bridge bonds of M–O–M. The
differences in the FTIR spectra of CFA and CFA_Zeo along with the
data obtained from EDS and PXRD confirmed that zeolization had
occurred upon fusing CFA with NaOH and then hydrothermally
treating this product.
Fig 3. SEM image of (a) CFA and (b) CFA_Zeo
TEM was also used to investigate the structure of the CFA_Zeo
where it was observed that the needle-like morphology observed
on SEM, appeared more like nano-rods (Fig 4a) which when
measured had an average diameter of 95 nm. The unbound TiO2
nanoparticles (Fig 4b) were observed to be grain-like and had
average diameters of 9 nm (calculated by measuring ca. 200
particles fitted using the Lorentz data fitting model).
SEM was also used to investigate if zeolization had occurred. The
morphology of raw CFA was found to be dominated by spherical
particles with diameters that ranged between 1 and 100 µm (Fig
3a). However, the 3-D morphology of CFA_Zeo was found to be
predominantly needle-like, with these needles having aggregated
together to form patterns that resembled skein of wool (Fig 3b).
Here, it was hypothesized that the crystallinity nature of the
resulting material, CFA_Zeo was the reason why its morphology was
different from that of CFA. CFA_Zeo crystallization occurred
following a three-step process: heating CFA in the presence of
NaOH resulted in the dissolution of Si4+ and Al3+ of CFA. Upon
condensation of Si4+ and Al3+, a molten aluminosilicate gel which is a
starting material for zeolite synthesis was formed. Under
hydrothermal conditions, the aluminosilicates along with Na+
crystallized to form predominantly zeolite-X as was shown by PXRD
(Fig 1).
TiO2 was synthesized in the presence of the CFA_Zeo in order to
maximize the likelihood of complete coating of CFA_Zeo rods with
TiO2 and forming chemical interactions between the two materials.
Indeed, it was observed from TEM micrographs (Fig 4(c and d)) that
the CFA_Zeo rods were completely coated with TiO2. The images
shown on Fig 4c and Fig 4d were those of composites consisting of
30 % CFA_Zeo, i.e. TiO2/CFA_Zeo(30 %) and Ag/TiO2/CFA_Zeo(30 %)
respectively. Morphological characteristics of composites consisting
of 15 and 30 % CFA_Zeo were found to have been similar, except
for those that consisted of 15 % CFA_Zeo had more TiO2 particles
that were not supported on CFA_Zeo nano-rods (TEM image of
TiO2/CFA_Zeo(15 %), supporting information S4a). To further
illustrate that the CFA_Zeo rods were completely coated with TiO2,
SEM image of Ag/TiO2/CFA_Zeo(15 %) was shown in supporting
information S4b.
Certainly, the morphological characteristics of the composites by
virtue of the CFA_Zeo nano-rods being totally coated with TiO2
nanoparticles indicated that there could be a possibility of chemical
interactions. It has been shown that close proximity coating of
substrates such as zeolites, graphene, and CNTs indicate the
existence of chemical interactions.22,23,36–38 Chemical interactions
are desirable in photocatalysis as they bring about channels for the
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separation of photo-generated charge carriers thus possibly Brunauer–Emmett–Teller (BET) analysis was conducted in order to
increasing the photocatalytic efficiency.22,36
measure the BET surface area of CFA, CFA_Zeo, and the various
composites (Table 1).
Table 1. BET surface areas and crystallite sizes of the various
materials.
Sample
BET surface TiO2 particle
2
size (nm)
area (m /g)
Raw CFA
2.9
79
CFA_Zeo
TiO2
43
48
55
68
59
73
10.2
9.6
8.5
8.4
8.4
8.3
Ag/TiO2
TiO2/CFA_Zeo(15 %)
TiO2/CFA_Zeo(30 %)
Ag/TiO2/CFA_Zeo(15 %)
Ag/TiO2/CFA_Zeo(30 %)
The surface area of the CFA_Zeo was found to be significantly
higher than that of CFA. The increase in surface area was attributed
to that many smaller rods that were formed upon zeolization as
opposed to the few large fly ash spherical particles (refer back to Fig
3). The BET surface areas of TiO2 and Ag/TiO2 were found to be
similar. However, they were lower than that of CFA_Zeo but higher
than that of CFA. The surface area of the rest of the composites,
with the exception of Ag/TiO2 was found to be significantly higher
than that of CFA and TiO2, but lower than that of CFA_Zeo (which
had the highest surface area). The decrease in the surface area of
CFA_Zeo composited with Ag and/or TiO2 was attributed two
possible factors: 1) The blockage of the CFA_Zeo pores and 2) The
agglomeration of these nanoparticles.
Fig 4. TEM images of (a) CFA_Zeo, (b) TiO2 and a histogram showing
the particle size distribution of TiO2 (c) TiO2/CFA_Zeo(30 %), (d)
Ag/TiO2/CFA_Zeo(30 %) and (e) an EDS plot of Ag/TiO2/CFA_Zeo(30
%).
Nevertheless, it was observed that the surface area of the
composites was higher than that of unbound TiO2. Similarly it was
further observed that the surface area increased with increased wt.
% of CFA_Zeo in the composite, with the composites containing Ag
nanoparticles having slightly higher surface areas. The increase in
the BET surface area of such composites, suggests that increased
wt. % loadings of CFA_Zeo lessened the agglomeration of TiO2
nanoparticles during hydrolysis. This was expected to be
advantageous in photocatalysis, as more pollutant molecules would
have been expected to be on the larger surface, where the reaction
would occur.
TEM micrographs of composites loaded with Ag, i.e.
Ag/TiO2/CFA_Zeo(30 %) and TiO2/CFA_Zeo(15 %) are shown in
Fig 4d and S4a respectively. Ag nanoparticles were identified as the
dark particles on the images (because Ag is heavier than Ti) circled
in white on Fig 4d. The average particle size of the Ag nanoparticles
(1 % loaded) was determined to be 19 nm from the TEM
micrographs, with several agglomerates that had particle sizes over
40 nm and some particles as small as 8 nm (Fig 4d and S4a). These
Ag nanoparticles were observed to have been deposited on the
surface of TiO2 nanoparticles. This was desired as it had the
potential to increase the likelihood of forming SPR and hence the
ability of the composites to absorb visible light.39–42 EDS analysis
was conducted on the Ag/TiO2/CFA_Zeo(15 %). Here peaks of Ag
were observed (Fig 4e), were observed which further confirmed
that Ag had been loaded onto the composite. The Au and Pd peaks
were present because the sample had been coated with a layer of
Au-Pd prior to EDS analysis.
The PXRD analysis was also conducted on unbound TiO2 and its
composites (Fig 5). Here the crystallite size of TiO2 alone and when
composited was calculated via the Scherrer equation using the
(101) reflection from the PXRD patterns (Fig 5). The crystallite sizes
of the unbound TiO2 nanoparticles and TiO2 nanoparticles in
Ag/TiO2 were found to be larger than that of TiO2 in composites
(Table 1). It was also observed that the peak widths of the TiO2
(101) reflections of the unbound TiO2 and Ag/TiO2 were narrower
than that of the composites (Fig 5(a,b)) These observations
indicated that the CFA_Zeo had played a role in the crystallization
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of TiO2 and that the CFA_Zeo had affected the crystal development
of TiO2.36,43,44 This further suggested that there was a possibility
that chemical interactions between TiO2 nanoparticles and CFA_Zeo
had occurred, and that CFA_Zeo had affected the crystal
development of TiO2.36 The TiO2 (101) reflection on the Ag/TiO2 was
not broadened as was the case with the other composites because
Ag nanoparticles were precipitated on the TiO2 surface after these
had already been crystallized. Apart from the broadening of the
(101) reflection, there was no evidence of the presence of CFA_Zeo
in the PXRD patterns of the composites. This was attributed to the
high crystallinity of TiO2 nanoparticles and the fact that the
composites consisted of at least 70
% TiO2. Since the Ag
nanoparticles were loaded to a level below the detection level of
the instrument (i.e. 1 % wt. of the composite) this explains why
their characteristic peaks were not observed.
Fig 6 (a,b). UV-vis DRS absorption spectra of TiO2 and composites
The steep absorption edge is typical of TiO2 nanoparticles,
indicating that compositing TiO2 with CFA_Zeo had had no influence
on the optical properties of TiO2. However, the UV-vis DRS profiles
of the catalysts containing Ag nanoparticles i.e. Ag/TiO2,
Ag/TiO2/CFA_Zeo(15 %) and Ag/TiO2/CFA_Zeo(30 %) were similar as
shown in Fig 6b, but different from those shown in Fig 6a i.e. while
a steep absorption edge was also present and a band that extended
from 600 nm in the visible range was also observed (circled in blue
in Fig 6b). This wide absorption band was attributed to the SPR of
Ag nanoparticles which have been reported to be responsible for
the improved photocatalytic activities of other noble metal-based
26,41,45–48
TiO2 composites.
The centroids of the SPR peaks of the
Fig 5. (a) PXRD patterns of unbound TiO2, Ag/TiO2 and the various
composites and (b) insert showing the (101) reflection.
various materials were not identified as the UV-vis DRS analyses
were only collected up to 800 nm, but it was clear that the peak
extended beyond 800 nm. In other reports, the shape, peak
positioning and intensity of SPR peaks have been shown to vary
with the concentration of Ag nanoparticles, the morphology of the
Ag nanoparticles, and the environment in which they are in, i.e. the
other materials in the composite, amongst other things. For
instance, Wu et al. observed broad peaks extending from 380 nm
to, it seemed beyond 800 nm when they did the UV-Vis analyses of
TiO2 nanotubes doped with Ag nanoparticles.49
UV-vis DRS analysis was conducted in order to ascertain if
compositing TiO2 with CFA_Zeo and Ag had had any influence on its
optical properties. Here a solitary band gap absorption with a steep
absorption edge in the UV region (380 nm) was observed for TiO2,
TiO2/CFA_Zeo(15 %) and TiO2/CFA_Zeo(30 %) (Fig 6a).
Photoluminescence (PL) emission spectroscopy is a significant
instrument for investigating the efficiency of charge carrier
separation. PL emission is primarily
a consequence of the
recombination of photo-generated electron–hole pairs, and a low-
intensity emission peak signposts a reduction in the recombination
rate.19,24,50 In order to study the effect of CFA_Zeo and Ag
nanoparticles on the recombination of electron-hole produced by
photo-ionizing TiO2, the PL profiles of the various photocatalysts
were recorded (Fig 7).
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Ag/TiO2/CFA_Zeo(15 %)
Ag/TiO2/CFA_Zeo(30 %)
3.4
6.3
96
89
67
46
Here it was found that TiO2 and Ag/TiO2 adsorbed the least amount
of BPA at equilibrium i.e. 0.8 ppm, and 0.9 ppm respectively.
TiO2/CFA_Zeo(15 %) and Ag/TiO2/CFA_Zeo(15 %) adsorbed 3.2 and
3.4 ppm, respectively. The composites consisting of 30 % wt
CFA_Zeo showed the largest adsorption capacity values i.e. 5.9 ppm
for TiO2/CFA_Zeo(30 %) and 6.3 ppm for Ag/TiO2/CFA_Zeo(30 %)
compared to the other composites. The trends observed here were
found to be consistent with the surface properties previously
observed (Table 1) i.e. those materials with larger surface areas
adsorbed more BPA at equilibrium. The highest adsorption of BPA
was observed for CFA_Zeo alone, probably due to its high surface
area and large pore volume (Table 1).
Fig 7. Photoluminescence spectrum of TiO2 and composites
Generally organic molecules adsorb onto zeolites via three
possible means: i) Van der Waals forces with lattice oxygen, ii)
Electrostatic interactions between organic compounds and the
extra framework cations, and iii) π-π connections of the sp2
bonds in organic compounds with transition metal cations if
present as extra framework cations.18 Of these, option iii
wasn’t applicable in this work as the zeolitic materials reported
here had sodium as cations for charge balance. However,
opitions i) and ii) were applicable in our study and were
considered as possible mechanisms by which BPA could have
interacted with the various zeolitic composites. It is
noteworthy that the adsorption of organic compounds
contributes in enhancing the photocatalytic activity of the
coated TiO2 as it brings the organic molecules in closer
proximity.
In a control experiment, it was observed that CFA_Zeo did not have
an emission profile when excited at 290 nm (not shown). In the case
of, TiO2 nanoparticles a broad and intense peak centered at
ca. 400 nm was observed, this was indicative of a high electron-hole
recombination rate.
TiO2/CFA_Zeo(15 %) had a significantly lower emission peak as
compared to that of TiO2, indicating that the CFA_Zeo was effective
at reducing the electron-hole recombination rate. Indeed, the
emission peak of TiO2/CFA_Zeo(30 %) was further decreased,
showing that the greater the wt% loading of CFA_Zeo the lower the
electron-hole recombination rate. The PL emission peak observed
for Ag/TiO2 was the same level as that TiO2/CFA_Zeo(30 %),
showing that Ag nanoparticles also have the ability to reduce the
electron-hole recombination rate. The emission peaks observed for
composites containing Ag nanoparticles (i.e. Ag/TiO2/CFA_Zeo(15
%) and Ag/TiO2/CFA_Zeo(30 %)) were much lower than those of the
other catalysts, with that of Ag/TiO2/CFA_Zeo(30 %) being almost
flat. This suggested the existence of a synergy between CFA_Zeo
and Ag nanoparticles that was effective for reducing the electron-
hole recombination rate. It was proposed that this occurred
through the transfer of the photo-induced electron from TiO2 into
the zeolite matrix. The electrons upon coming into contact with the
zeolite matrix became delocalized through the zeolitic skeletal
structure or in clusters of charge balancing sodium cations.18
Control experiments were conducted in order to check if the
photolysis of BPA would occur in the absence of the various
materials that were prepared. Here BPA was irradiated with the
solar simulator (with and without a UV-light filter) for 60 min.
Analyses showed that the concentration of BPA in solution had not
changed under these conditions. The photoactivity of CFA_Zeo
under solar simulator irradiation (with and without a UV-light filter)
was then determined, where it was found that the CFA_Zeo was
almost inert (Table 2). Experiments conducted with the solar
simulator without a filter were labeled X60min_UV in Table 2, while
those that were conducted with a UV-filter were labeled X60min_Vis
.
From there experiments conducted under a full light spectrum
(X60min_UV) were referred to as UV light experiments and those that
were conducted with a UV-filter (X60min_Vis) were referred to as
visible light experiments.
The adsorption equilibrium at 2 h (Q) was determined (Table 2).
Table 2. Adsorption equilibrium and photocatalytic activities of the
various photocatalysts
The photocatalytic activities of TiO2 and the composites under UV
and visible light were evaluated using BPA as a model compound.
The curves for BPA degradation under UV light radiation were
shown in Fig 8 and were tabulated in Table 2. The photocatalytic
activity of TiO2 was found to be 49 %, superior to the 33 % obtained
for TiO2/CFA_Zeo(30 %) but inferior to the 64 % obtained for
TiO2/CFA_Zeo(15 %). The improved photoactivity observed for
TiO2/CFA_Zeo(15 %) was attributed to the reduced electron-hole
Sample
Q (ppm)
9.2
X
8
60min_UV (%)
X60min_Vis (%)
CFA_Zeo
3
TiO2
0.8
49
69
64
33
24
91
24
21
Ag/TiO2
0.9
TiO2/CFA_Zeo(15 %)
TiO2/CFA_Zeo(30 %)
3.2
5.9
recombination rate (Fig 7).
A
discrepancy was observed for
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TiO2/CFA_Zeo(30 %), which displayed an inferior photoactivity
relative to that of TiO2. This was attributed to that the ratio of the
photoactive TiO2 was reduced and the photo-inactive CFA_Zeo
blocked the light from reaching TiO2. Photoactivity of 69 % was
obtained for Ag/TiO2, 89 % for Ag/TiO2/CFA_Zeo(30 %) and 96 % for
Ag/TiO2/CFA_Zeo(15 %). The superior photoactivity of composites
containing both Ag nanoparticles and CFA_Zeo was attributed to
the significantly reduced electron-hole recombination rate.
1,0
0,9
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0,0
Ag/TiO2/CFA_Zeo(30 %)
Ag/TiO2/CFA_Zeo(15 %)
Ag/TiO2
1,0
0,9
0,8
0,7
0,6
TiO2
TiO2/CFA_Zeo(15 %)
TiO2/CFA_Zeo(30 %)
0
10
20
30
40
50
60
Time (min)
0,5
Ag/TiO2
0,4
Ag/TiO2/CFA_Zeo(15 %)
Fig 9. Photodegradation of BPA under visible light.
0,3
0,2
0,1
0,0
Ag/TiO2/CFA_Zeo(30 %)
TiO2/CFA_Zeo(30 %)
TiO2/CFA_Zeo(15 %)
TiO2
The superior photoactivity of Ag/TiO2/CFA_Zeo(15 %) compared to
Ag/TiO2/CFA_Zeo(30 %) was justified by that the larger amounts of
the CFA_Zeo blocked the incident light thus decreasing the amount
of irradiation for the photoionization of TiO2. Ag/TiO2 (91 %) was
found to have the highest photocatalytic activity than all the other
composites under visible light irradiation. This was attributed to
that photoionization occurred in the bulk of Ag nanoparticles
0
10
20
30
40
50
60
Time (min)
Fig 8. Photodegradation of BPA under full solar simulator spectrum. whereby electrons were transferred directly from Ag nanoparticles
into TiO2 nanoparticles. This phenomenon also existed in the case
of Ag/TiO2/CFA_Zeo(15 %) and Ag/TiO2/CFA_Zeo(30 %), but here a
The photoactivity curves of the photocatalysts under visible light
were shown in Fig 9 and Table 2. The photocatalytic activity of the
composites consisting of TiO2 and CFA_Zeo, i.e. TiO2/CFA_Zeo(15 %)
and TiO2/CFA_Zeo(30 %) showed photocatalytic activities of 24 and
21 % respectively, similar to that obtained for pristine TiO2 (24 %).
The low activity was attributed to that the illumination energy used
was lower than the band gap energy of the materials (Fig 6a). The
composites containing Ag nanoparticles and CFA_Zeo were shown
to have modest photoactivity. Ag/TiO2/CFA_Zeo(15 %) and
lower activity was observed. The low photoactivity of these
composites, compared to the superior of Ag/TiO2 was also
attributed to that since the same amount of catalyst (25 mg) was
used in all experiments, the ratio of Ag and TiO2 nanoparticles were
lower in these composites. This was adjudged to be crucial since
both Ag and TiO2 nanoparticles are required for SPR, the
phenomenon responsible for the visible light photoactivity.
Ag/TiO2/CFA_Zeo(30 %) were recorded to have photoactivity of 67 In order to investigate this hypothesis, an experiment using 29.45
% and 46 %. The relatively large photoactivity (compared to TiO2, mg of Ag/TiO2/CFA_Zeo(15 %) was used for the photodegradation
TiO2/CFA_Zeo(15 %) and TiO2/CFA_Zeo(30 %)) observed was of BPA under visible light irradiation. Using 29.45 mg of
attributed to the ability of the Ag containing composites to absorb Ag/TiO2/CFA_Zeo(15 %) compensated for the deficient amount of
visible light as a result of SPR (Fig 6b).41,42,51,49
TiO2 and Ag, such that the Ag/TiO2 part of the composite accounted
for 25 mg out of the total sample mass (29.45 mg). The results were
presented in S5. Here, it was observed that the photocatalytic
efficiency of 29.45 mg of Ag/TiO2/CFA_Zeo(15 %) was higher than
when 25 mg of the same material was used. Furthermore, it was
observed that the photocatalytic efficiency of 29.45 mg of
Ag/TiO2/CFA_Zeo(15 %) was slightly better than that of Ag/TiO2 (by
ca. 1 %). This showed that when photodegradation experiments
were performed under visible light, CFA was an insignificant
component of the composites.
Mindful of the different observations, postulations, and conclusions
made as to how the CFA_Zeo and Ag nanoparticles improved the
photodegradation efficiency of BPA, two separate mechanisms
were proposed: In order to account for the enhanced
photodegradation under UV-light brought about by CFA_Zeo, the
following mechanism was proposed. Upon UV-light irradiation of
TiO2/CFA_Zeo(15 %), the valence band electrons were excited into
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the conduction band where they were quickly removed from the
TiO2 surface before they could recombine with the holes as shown
in Fig 10 (Scheme 1). This was possibly achieved through the
delocalization photoinduced electrons through the zeolite
framework or in clusters of charge balancing sodium ions. A further
increase in photocatalytic activity was observed for composites
consisting of Ag nanoparticles. Herewith the proposed mechanism
was that electron transfer from the photo-excited TiO2 to Ag
nanoparticles occurred (Scheme 1). This was because electrons
easily transferred from the charge transfer excited state of TiO2
nanoparticles to the Ag nanoparticles while the holes remained
with TiO2 nanoparticles. This resulted in charge separation of
photoinduced electrons and holes pairs. Therefore, as
a
Fig 11. Schematic representations of SPR induced visible light driven
photocatalysis.
consequence of compositing TiO2 with CFA_Zeo and Ag,
photocatalytic reactions under UV light were more predominant as
opposed to the wasteful recombination of charge carriers’
reactions. The oxidation of BPA by holes and the reduction of BPA Having discussed the mechanism by which the materials
by electrons occurred separately from one another at different photodegraded BPA, the stability of one of the materials,
sites.
Ag/TiO2/CFA_Zeo(15
% over 7 experiments was studied. The
experiments were conducted using the same conditions that were
used for the photodegradation efficiency study, only for this study
Ag/TiO2/CFA_Zeo(15 %) was the only material tested. The
experiments were conducted under a full solar simulator spectrum,
as an ideal photocatalyst should show high photocatalytic efficiency
when irradiated with sunlight and remain stable.
The results of this study shown in Fig 12 are presented as the
amount of BPA concentration (ppm) photodegraded per amount
(mg) of Ag/TiO2/CFA_Zeo(15 %). This was done to compensate for
the differences in the mass of catalyst that was used as there were
small losses of material over the experiments. The exact mass of
material used for each experiment was tabulated on S6. Here, it
was observed that the photodegradation efficiency decreased
significantly (49 % efficiency) between day 1 and day 4. Even so, the
photodegradation efficiency (1.16) observed after 4 days was
similar to that of TiO2 on day 1 (1.19), showing the advantage of
compositing TiO2 with CFA_Zeo and Ag nanoparticles. Furthermore,
after day 4, the photodegradation efficiency stabilized between day
5 and day 7 at an efficiency that was 40 % the original efficiency.
Fig 10. Schematic representations of charge separation by Ag and
CFA_Zeo under UV-light
A different mechanism was postulated for the photocatalytic
activities of the materials observed under visible light (Fig 11). It
was different compared to that which was proposed to account for
the results which were obtained under UV light. The high activity
observed for composites that contained Ag nanoparticles was
attributed to SPR. Herewith, it was suggested that Ag absorbed
visible light and was photo-ionized, the resulting electrons were
then transferred into the empty valence band of TiO2 leaving
behind positively charged holes. As it was the case before, the
relevant oxidation and reduction reactions occurred at different
sites because the charge carriers were separated.26,39–41,46,47,52 So
essentially the addition of Ag nanoparticles achieved charge
separation and allowed for visible light driven photocatalysis.
Furthermore, it was proposed that CFA_Zeo did not play a positive
role in the improvement of TiO2 photocatalysis. This was because
SPR, in this case, it occurred between Ag nanoparticles and TiO2,
thus diluting TiO2 which occurred upon the addition of CFA_Zeo had
2,4
2,2
2,0
1,8
1,6
1,4
1,2
1,0
0,8
a
negative impact on photocatalytic activity. Indeed, it was
observed that Ag/TiO2 had higher activity than CFA_Zeo
a
containing composites and it was also observed that the activity
decreased with increasing CFA_Zeo.
1
2
3
4
5
6
7
Number of experiments
Fig 12. The efficiency at which BPA was photodegraded using
Ag/TiO2/CFA_Zeo(15 %) for 7 experiments over 7 days.
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One of the other factors that determine if a material is a viable In this work, it was demonstrated that coal fly ash could be used to
photocatalyst is if mineralization could be achieved using the
material under the given conditions. After 1 h of irradiation using a
full solar simulator spectrum, Ag/TiO2/CFA_Zeo(15 %) had only
mineralized 7 % of the total C content, this showed that even
though almost all of the BPA was photodegraded a small portion of
it was mineralized. Given the low levels of mineralization, in
another experiment under the same conditions, the reaction was
allowed to run for 48 h where 84 % mineralization was measured,
indicating high levels of mineralization efficiency. These results
were comparable to those that were reported by Fukahuri et al.
who reported a 78 % mineralization of BPA using TiO2: zeolite = 2:1
sheets after 24 h using a 3rd of the amount of BPA reported in this
work.53
synthesize a material used for cleaning water. This was achieved
through the zeolization of coal fly ash by the two 2-step alkali fusion
hydrothermal technique. The resulting material, CFA_Zeo was then
composited with TiO2 by the resin gel technique. Employing this
technique yielded total coverage of the CFA_Zeo nanorods with
TiO2 which was desirable as it increases the likelihood of charge
separation during photocatalysis experiments. Ag was loaded onto
TiO2 and the TiO2/CFA_Zeo composites by deposition precipitation
technique using urea and NaOH and were found to have been
loaded on top of TiO2 nanoparticles. It was found that compositing
TiO2 with CFA_Zeo increased the surface area of TiO2 and as such
the adsorption capability of the composites was superior to that of
TiO2, which is of significant advantage in photocatalysis as it is a
surface reaction. Compositing TiO2 with CFA_Zeo and/or Ag was
also shown to reduce the lifetime of photoinduced charge carriers;
one of the limitations of TiO2 photocatalysis is the high
recombination rate of the active electrons and holes. Furthermore,
it was observed on the UV-vis DRS that composites containing Ag
nanoparticles had and absorption peak in the visible part of the
spectrum, indicating that the composites could be used in visible
light driven photocatalysis experiments. The photocatalytic
efficiency of the materials was evaluated by photodegradation
Furthermore, from this sample (after a 48 h photodegradation
experiment using Ag/TiO2(CFA_Zeo(15 %)), the catalyst material
was then filtered and the remaining solution was then analyzed
using HPLC-MS in order to identify the main species that were
formed upon the photodegradation of BPA. The structures of some
of the compounds, retention times, M/Z (representing the negative
ion mode of HPLC/MS, i.e. the values are less by 1 of the actual M/Z
value), and some references are presented on S7. The HPLC-UV-
PDA chromatogram of this sample is shown on S8. The identified
compounds were (in order of appearance on the mass spectrum), 3- studies using bisphenol-A as a model compound under UV and
(4-hydroxyphenyl)-3-methyl-2-oxobutanoic acid, 4-(prop-1-en-2-yl)
phenol, 4-hydroxybenzaldehyde, 1-(-4-hydroxyphenyl) ethanone
and 4,4’-(propane-2,2-diyl)diphenol (BPA). Torres et al. used
ultrasound for the degradation of BPA; they identified 4 of the
compounds that were identified in this work as the degradation by-
products. They attributed their formation to degradation of BPA by
hydroxyl radicals. Indeed, even Fukahori et al. who used TiO2-
zeolites sheets for the UV-light photodegradation of BPA identified
some of the compounds which were identified in this work as the
photodegradation by-products. They also proposed that hydroxyl
radicals were the active species that broke down BPA. In relation to
this, in this work, TiO2 was immobilized on CFA_Zeo and Ag
nanoparticles were loaded onto TiO2 nanoparticles. As it was
suggested earlier that using Ag/TiO2/CFA_Zeo(15 %) to
photodegrade BPA under UV-light encouraged the trapping of
electrons by Ag nanoparticles and CFA_Zeo. It was then concluded
that the positively charged holes were the active species for
breaking down BPA via the oxidation of water molecules to form
the active hydroxyl radicals.
visible light. It was observed that in the case of experiments that
were conducted under UV light that compositing TiO2 with CFA_Zeo
and Ag was beneficial as those composites showed higher
photocatalytic efficiency relative to TiO2 and TiO2/CFA_Zeo
composites. Furthermore, it was observed that Ag/TiO2 and
TiO2/CFA_Zeo(15 %) performed better than TiO2, with
TiO2/CFA_Zeo(30 %) showing the lowest efficiency. The high
efficiency observed for most of the composites was attributed to
the decreased separation of charge carriers as it was inferred from
the photoluminescence data. In the case of experiments conducted
under visible light, only Ag containing composites showed improved
photocatalytic efficiency. This was because the presence of silver
through SPR allowed for the absorption of visible light. The lower
efficiency of composites consisting of 30 % CFA_Zeo was attributed
to the much dilution of TiO2.
It is noteworthy that although it was not determined if the resulting
sample had EDC effects, the remaining compounds (except for BPA)
are not been classed as EDCs. This indicated that the material used
in this experiment could possibly degrade BPA into compounds
which are not endocrine disruptors. Even so, this is notwithstanding
that in order to confirm this claim more experiments would need to
be conducted. Furthermore, the ultimate goal of photodegradation
is to remove organic pollutants from real water samples.
Nevertheless, this work gives a strong indication that zeolites
derived from CFA could play a pivotal role in designing materials
that could be used for the removal of organic pollutants in water at
the same time it is a way of finding uses for coal fly ash.
Acknowledgements
This project is built on the research supported in part by the
National Research Foundation of South Africa (Grant number
88076), ESCOM’s Tertiary Education Support Programme (TESP)
and the DST-NRF Centre of Excellence in Strong Materials (CoE-
SM) at the University of the Witwatersrand. The authors would
also like to express gratitude to the employees in the
Microscopy and Microanalysis Unit (MMU) at the University of
the Witwatersrand for assisting with microscopy analyses and Dr
R Erasmus based in the Raman and Luminescence Laboratory at
the University of the Witwatersrand.
Conclusions
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29
N. Shigemoto, H. Hayashi and K. Miyaura, J. Mater. Sci.,
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