Preparation, characterization and application of Cu-Ni/TiO2 in Orange II photodegradation under visible light: Effect of different reaction parameters and optimization

Article abstract of DOI:10.1039/c6ra10371e

Bimetallic Cu-Ni/TiO₂ photocatalysts were prepared using a wet impregnation method. The physicochemical and morphological properties of the photocatalysts were studied using different characterization techniques. FTIR analysis showed the nitrate peaks are still present but did not show an effect on the catalytic performance of the photocatalysts. Photocatalysts were of nanosize and their morphologies are spherical and slightly agglomerated. From DR-UV-visible analysis, it was proved that incorporation of Cu and Ni onto TiO₂ has successfully shifted the optical absorption to the visible region with reduced bandgap energies. Furthermore, by increasing the calcination temperature the bandgap energy was reduced. The lowest band gap energy (2.74 eV) was reported for 9Cu:1Ni-300. Photocatalytic degradation of Orange II was studied under visible light. The photocatalyst performance of bimetallic Cu-Ni/TiO₂ for Orange II decoloration and mineralization is promising compared to bare TiO₂ and the monometallic photocatalysts. Compared to other Cu:Ni mass compositions, results for photodegradation studies showed that a 9Cu:1Ni mass composition was observed with 100% Orange II decoloration and 89.8% and 100% TOC removal in 1 h and 1.5 h of irradiation duration, respectively. The optimum pH value was 6.8. The main identified intermediates and by-products of Orange II photodegradation under visible light irradiation during reaction as a function of time were oxalic acid, formic acid, formaldehyde, benzyl alcohol and benzaldehyde as measured by HPLC analysis.

Full text of DOI:10.1039/c6ra10371e

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DOI: 10.1039/C6RA10371E
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ARTICLE
Preparation, characterization and application of Cu-Ni/TiO2 in
Orange II photodegradation under visible light: Effect of different
reaction parameters and optimization
Received 00th January 20xx,
Accepted 00th January 20xx
Nadia Riaz,a,† F. K. Chong,b,† Z. B. Man,c R. Sarwar,d U. Farooq,d A. Khand and M. S. Khana
DOI: 10.1039/x0xx00000x
www.rsc.org/
BimetallicCu-Ni/TiO2photocatalysts were prepared using wet impregnation method. The physicochemical and
morphological properties of the photocatalsyts were studied using different characterization techniques. FTIR analysis
showed the nitrate peaks are still present but did not showed effect on the catalytic performance of the photocatalysts.
Photocatalysts were nanosize and morphologies are spherical and slightly agglomerated. From DRUV-Visible analysis, it
was proved that incorporation of Cu and Ni onto TiO2 has successfully shifted the optical absorption to the visible region
with reduced bandgap energies. Furthermore, increasing the calcination temperature bandgap energy was reduced. The
lowest band gap energy (2.74 eV) was reported for 9Cu:1Ni-300. Photocatalytic degradation Orange II was studied under
visible light. The photocatalyst performance of bimetallic Cu-Ni/TiO2 for Orange II decoloration and mineralization is
promising compared to bare TiO2 and the monometallic photocatalysts. Compared to other Cu:Ni mass compositions,
results for photodegradation studies showed that 9Cu:1Ni mass composition was observed with 100 % Orange II
decoloration and 89.8% and 100% TOC removal in 1h and 1.5 of irradiation duration, respectively. The optimum pH value
was 6.8. The main identified intermediates and by products of Orange II photodegradation under visible light irradiation
during reaction as a function of time were oxalic acid, formic acid, formaldehyde, benzyl alcohol and benzaldehyde as
measured by HPLC analysis.
of residual dyes has been an important issue of research. Although
textile dyes can be disposed via some physical and chemical
processes, these methods are usually incomplete and ineffective.
Introduction
The textile industry is the major contributor responsible for the
aquatic ecosystems pollution due to the generated wastewaters.
Such industrial effluents are highly colored and their release in the
Orange II does not undergo biological degradation6. Additionally,
biological processes exhibited limited efficiency due to xenobiotic
and non-biodegradable characteristics of textile dyes. Furthermore,
some physical and chemical treatments can generate secondary
pollution resulting from toxic products7.
ecosystem is
a dramatic source of aesthetic pollution and
perturbation in the aquatic life. For this reason, the international
environmental standards become more and more stringent1-3. Azo
dyes are the largest group of dyes, constituting 60–70% of all
dyestuffs produced and used in textile industry4, 5.Orange II was
used as model azo dye, an anionic monoazo textile dye of the acid
class. The high stability of Orange II is useful in textile
manufacturing due to its resistant to light degradation, action of O2
and common acids or bases but problem arises later due to
difficulty in managing its removal from the wastewaters. Treatment
Semiconductor photocatalysis has been investigated extensively for
light-stimulated
degradation
of
pollutants8-14
.
Several
semiconductors exhibit band-gap energies suitable for
photocatalytic degradation of contaminants. Among the
photocatalysts applied, titanium dioxide (TiO2) is one of the most
widely employed photocatalytic semiconducting materials because
of the peculiarities of chemical inertness, non-photocorrosion, low
cost and non-toxicity. Carp et al.,9 pointed out that doping
semiconductors
with
various
metal
ions,
composite
a.
Department of Environmental Sciences, COMSATS Institute of
Information Technology, Abbottabad, Pakistan.
Fundamental&Applied Sciences Department, Universiti Teknologi
PETRONAS, Tronoh, Malaysia
Chemical Engineering Department, Universiti Teknologi PETRONAS,
31750 Tronoh, Malaysia
Department of Chemistry, COMSATS Institute of Information
Technology, Abbottabad, Pakistan
semiconductors, deposition of group VIII metals, and oxygen
reduction catalysts can be employed to enhance photocatalytic
efficiency.
The use of Cu and Ni as bimetallic catalyst supported on different
semiconductor materials has been reported as the effective method
to improve the efficiency of various reactions like carbondioxide
hydrogenation15, steam reforming of methane16, Liquid-phase
b.
c.
d.
†
Correspondence: nadiariazz@gmail.com; chongfaikait@petronas.com.my
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glycerol hydrogenolysis by formic acid over Ni–Cu/Al2O3 catalysts17, prepared using wet impregnation (WI) method with TiO2
decomposition of methane over Ni-SiO2 and Ni-Cu-SiO2 catalysts18 support. To prepare photocatalysts using WI method, support was
DOI: 10.1039/C6RA10371E
and for photocatalytic reduction of nitrate19. Only few studies have added into the metal salt solution. The suspension was stirred for 1
been reported using Cu, Ni or Cu-Ni photocatalysts for azo dye hour before the solvent was evaporated in a water bath at 80ºC
degradation like Cu-Zn/TiO220, Ni/TiO221 and Cu-Fe/TiO222 for methyl until a thick paste was obtained. This paste was then dried in an
orange degradation and Cu/TiO2 for Orange II degradation with 90% oven at 120ºC for 18 hours. The dried photocatalyst was ground
color removal in the presence of UVC light and O2 after 150 min with a mortar and pestle, kept in air-tight glass bottle as raw
reaction23.
photocatalyst and stored in a desiccator at room temperature prior
In our previous studies Cu-Ni/TiO2 photocatalysts were prepared to calcination.
employing deposition-precipitation (DP) method24 and modified co–
Photocatalyst Characterization
precipitation method (CP)25. Depending on the method of
preparation and calcination temperatures have prominent
influence on the activity of the prepared photocatalysts. In
comparison to our previously reported methods, WI photocatalysts
displayed the best performance for Orange II decoloration among
DP and CP photocatalysts. Higher calcination temperature (300:C)
resulted TiO2 photocatalysts with lower bandgap energy 2.74 eV for
WI-9Cu;1Ni-300 and high crystallinity. The addition of metals onto
the surface of TiO2 has effectively led to the better photocatalytic
performance for Orange II photodegradation under visible light
irradiation effectively as compared to the blank experiment without
adding any photocatalyst indicating the merely photolysis.
The present work deals with the bimetallic TiO2 photocatalyst
preparation via wet impregnation method (WI). The introduction of
Cu and Ni was with the intention to reduce the band gap of the
photocatalyst for enhanced visible light absorption. The objective of
present study was to determine the photocatalytic activity of
bimetallic Cu-Ni/TiO2 for Orange II photodegradation under visible
light source, using Orange II as a model azo dye. Different
parameters studied were; different Cu:Ni mass composition, metal
loading and the effect of calcination temperatures on the
photocatalyst performance. The photocatalysts were further
characterized using different characterization techniques to
understand the chemical and physical properties and then to relate
these properties to their photocatalytic performance.
It is important to characterize the calcined photocatalysts in order
to determine mainly their chemical and physical properties and
then to relate these properties to their photocatalytic performance.
The thermal stability of the raw photocatalysts was determined by
thermal gravimetric analysis (Perkin Elmer, Pyris 1TGA) instrument.
The dried photocatalyst was loaded in a sample cup and weighed
using a built-in microbalance attached to the instrument which
automatically provides the weight of the sample (in the range of 5-
10 mg). The sample was heated in flowing N2 as background
atmosphere from 30°C to 800°C at a ramp rate of 20°C/min. Fourier
Transform Infrared spectroscopy (FTIR, Shimadzu FTIR-8400S)
analyses was carried out to identify species present in the
photocatalysts. A small amount of photocatalyst was grained
together with 50 mg of IR-grade KBr and pressed into pellet using a
hydraulic hand press. Later the pellet was placed in the sample
holder and scanned at 4000 cm-1 to 400 cm-1. KBr was used as the
background file. Point of zero charge (PZC) was determined by mass
titration method 26. Photocatalysts were weighted and mixed with
distilled water to give concentration of 0.1, 0.5, 1, 2, 5 and 10 %
(w/v). The suspensions were agitated for 24 h to equilibrate the
adsorption-desorption processes, after which pH of the suspensions
were determined using pH meter. PZC was estimated by measuring
the pH at which further addition of solid catalyst did not change the
pH of the suspension. The crystal phases present in the prepared
photocatalysts were investigated using X-ray diffractometer (XRD,
Bruker D8 Advance Diffractometer), with CuKα radiation (40 kV, 40
mA) at 2θ angles from 10° to 80°, with a scan speed of 4°min-1. The
morphology of the photocatalysts such as crystallite particle shape
and size distribution wereanalyzed using FESEM (Supra55VP) and
HRTEM (Zeiss Libra 200). The Brunauer-Emmett-Teller (BET) analysis
was conducted to determine surface area and pore size distribution
of the photocatalyst by adsorption of nitrogen at 130 °C, by using a
Micromeritics ASAP 2000 instrument. Reflectance spectrums were
recorded at 190–800 nm wavelength using DR-UV-Vis.
Spectrophotometer (Shimadzu Lambda 900). The band gap energies
of the photocatalysts were determined from the reflectance using
Kubelka-Munk function, F(R), and the extrapolation of Tauc plot,
which is a plot of (F(R).h)1/2 against h. F(R) is Kubelka-Munk
function which derived from reflectance spectra following F(R) = (1-
R)2/2R equation, and hѵ is the photon energy. Barium sulphate
(Ba2SO4) powder was used as a standard, an internal reference. The
temperature programmed reduction (TPR) analyses were
conducted in order to determine reducibility of the photocatalysts
and metal dispersion in photocatalysts using ThermoFinnigan
Experimental
Materials
Copper nitrate trihydrate, Cu(NO3)2.3H2O and nickel nitrate
hexahydrate, Ni(NO3)2.6H2O (Acros brand >98% purity) were used
as dopant metal salts. Titanium dioxide, TiO2 (Degussa P25 80%
anatase, 20% rutile) was used as the support which also acts as the
semiconductor in photocatalysis. Orange II (Acros, pure) was used
as the model azo dye for photocatalytic degradation study. The
chemical structure and specifications of Orange II are shown in
S1.(see supplementary materials). All chemicals were used as
received.
Photocatalyst preparation
A series of bimetallic Cu-Ni/TiO2 photocatalysts with different total
metal loading (5, 10 and 15 wt%) and different mass composition of
copper to nickel (10:0, 9:1, 7:3, 5:5, 3:7, 1:9 and 0:10) were
2 | J. Name., 2012, 00, 1-3
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equipment (TPDRO 1100). Prior to reduction, the sample was where: C₀ = initial concentration of dye in ppm and Ct is the
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pretreated under nitrogen at 110 ºC with a flow rate of 20 mLmin-1 concentration of dye at different time intervals during reaction in
DOI: 10.1039/C6RA10371E
and ramp rate of 10 ºC min-1, and finally holding at 110 ºC for 30 ppm. At the end of a particular reaction, centrifuged samples were
minutes to eliminate moisture before cooling to room temperature. further analysed for the total organic carbon (TOC). The TOC
TPR analysis was carried out in 5% H2 in N2 with a flow rate of 20 mL removal (%) was calculated as follows:
min-1. Sample were heated with a ramp rate of 20 ºC min-1 from 40
TOC  TOC f








o
TOC removal(%)
100
(2)
ºC-500 ºC and holding at 500 ºC for 10 minutes. The reduction
profile was showed in a plot of hydrogen consumption as a function
of linearity temperature.
TOCo
where: TOC₀ = initial TOC in ppm and TOC f = final TOC in ppm.
High Performance Liquid Chromatography Analysis (HPLC)
Measurements of photocatalytic activities
Orange II photodegradation products were analyzed using high
performance liquid chromatography (HPLC Agilent 1100 series)
equipped with auto sampler. The analysis was carried out using a
Zorbex SB- C18 5um, 150 mm x 10 mm column with a Mobile phase
70:30 aqueous solution of ammonium acetate (20 mM) and
acetonitrile at a flow rate: 1 mL.min-1, while retention time was 20
min at ambient temperature. The injection volume was 5 µL, room
temperature 22.2°C, column temperature 26.6°C and 117 bar pump
pressure. Detector was operated at three different wavelengths
485nm, 254nm and 215nm (DAD-UV Lamp). Standard solutions
were prepared in the eluent with different known concentrations.
Photocatalytic activity of the prepared photocatalysts was
evaluated by monitoring the photocatalytic degradation of Orange
II in aqueous solution under visible light irradiation. For a typical
experiment, photocatalysts were weighed and mixed with distilled
water and then ultrasonicated for 10 min using an ultrasonicator.
Afterwards, Orange II solution was added with a final concentration
of 50ppm with photocatalyst loading 1 g·L-1 and total volume of 30
ml. The suspension was stirred using a magnetic stirrer for in the
dark (2 h) and later this suspension was illuminated for 1 h using
the 500 W halogen lamp as the visible light source at a distance of
25 cm (46.1615 W·m–2). Reaction study was carried out at
atmospheric pressure and room temperature (at 25±2:C) that was
controlled by continuous cooling air. Prior to absorbance
measurement (at wavelength of 485 nm, corresponding to
maximum absorption wavelength of Orange II), the reaction
samples were immediately centrifuged twice at 3500 rpm for 10
min to remove suspended solid photocatalyst and monitored for
the remaining Orange II concentration and further analysis. Orange
II specifications, the model azo dye, and the experimental setup
used for photodegradation are shown in S1 and S2, respectively
(see supplementary materials).
28
30
Some intermediates27,
products29,
of Orange II
photodegradation were used as standards for analysis and the
results were used to determine their compositions during reaction.
The standards are benzyldehyde (99.5%, Fluka), benzyl alcohol
(Fluka), formaldehyde (Assay ≥ 36.5 %) and formic acid (≥ 98%),
oxalic acid (≥ 99.0%, Fluka), propionic acid (≥ 99.8%), sulfanilic acid
(≥ 99.0%, Fluka), 2-naphthol (99 %, Aldrich). Calibration curves were
obtained for all the standards with known concentrations.
Results and Discussions
The degradation of a dye can be characterized in two ways: percent Resultsfrom TGA were reported as thermograms which are plots of
decoloration and percent mineralization. Decoloration refers to the
the relative weight of the photocatalyst versus temperature. The
reduction in concentration of the parent dye molecule under calcined photocatalysts were given denotation: xCu–yNi–T (the
consideration at its characteristic wavelength, but does not refer to dash is from insert symbol) where ‘x’ and ‘y’ represent the mass
the complete removal of the organic carbon content. This is due to composition of Cu and Ni, respectively, with ‘x’+’y’=10; while ‘T’
the formation of colored dye intermediates, which absorb at represents calcination temperature in °C.
different wavelengths. Hence, complete degradation or
mineralization occurs when all the organic carbon is converted to
CO2. Therefore, analyzing the mineralization of the dyes in terms of
the total organic carbon (TOC) content assumes importance. TOC is
the amount of carbon bound in an organic compound and is often
used as a non-specific indicator of water quality or cleanliness of
pharmaceutical manufacturing equipment. A typical analysis for
TOC measures both the total carbon present as well as the so called
"inorganic carbon" (IC), the latter representing the content of
dissolved carbon dioxide and carbonic acid salts. Subtracting the
inorganic carbon from the total carbon (TC - IC) yields TOC. A total
organic carbon analyzer (VCSH Shimadzu Co.) was used to measure
the TOC concentration (ppm) for the reaction samples.
Photocatalyst characterization
Thermal analysis. Prior to calcination, pre-treatment of the raw
photocatalysts was carried out to estimate the suitable calcination
temperatures to activate the raw photocatalysts material. This step
was conducted to monitor the decomposition temperature of the
photocatalysts at which the raw photocatalyst began to decompose
and the temperature range at which the photocatalyst has
achieved thermal stability. The thermogram of the raw
photocatalyst (10 wt%-9Cu:1Ni) is shown in Figure 1. The TG curves
showed two decomposition steps: the first weight loss step (30 to
150oC) depicts the evaporation of physically adsorbed water
31-33
while thesecond abrupt step from 150 to 400oC and onwards
represents the decomposition of the Cu(NO3)2 and Ni(NO3)2, to form
copper oxide, CuO31 and NiO, respectively. Total weight loss was
In order to determine the decoloration efficiency of photocatalyst,
Orange II decoloration (%) was calculated as follows:








Co  Ct
Co
Orange II decoloration (%)
100
(1)
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26.45% (S3). Proposed decomposition steps for the raw WI pH and ionic strength of the solution. It is necessaryVtieowdAretitcelerOmnilninee
DOI: 10.1039/C6RA10371E
photocatalyst are shown in Equations 3 and 4.
the property of the photocatalyst-solution interface as the research
on the photocatalyst is intended for the degradation of dyes in
aqueous solution. For the photocatalyst samples containing Cu and
Ni, the PZC shifts to more basic values. The PZC value of the Cu-
Ni/TiO2 photocatalysts in the present study is towards alkaline (7.7).
The loading of metal oxides onto the surface of TiO2 can remarkably
change the PZC values. PZC for TiO2 was about 3.8 comparable to
those reported in the literature for TiO2 at 3.5- 6.540.
Cu

NO32  CuO s  2NO2
 O2
(3)
(4)
 
g

 
g
NiNO32

NiOs  2NO2
 O2
 
g

g
Fig.1 The thermogram of the raw photocatalyst (10 wt%-9Cu:1Ni)
From 400oC onwards there was a straight horizontal line, indicating
no further decomposition and the photocatalyst was thermally
stable.
Fig.2 Plot of pH versus the photocatalyst/water mass percentage for
TiO2 and WI-9Cu:1Ni/TiO2 photocatalyst
Fourier Transform Infrared Spectroscopy (FTIR). FTIR spectra of
10wt% 9Cu:1Ni-200 photocatalysts is shown in S4. Bare TiO2, raw
and calcined photocatalysts (at 180oC, 200oC and 300oC) were
compared. Several absorption peaks were observed. The broad
band around 3400 cm-1 was attributed to O–H stretching, and the
peak near 1600 cm–1 to H–O–H bending and related to physically
absorbed moisture 31, 34. The IR band observed from 400–900 cm–
1corresponds to the Ti-O stretching vibrations35-37. Similar results
were displayed by the photocatalysts calcined at 180oC and 300oC.
If the pH of the suspension is lower than PZC, the surface charge is
positive. On the other hand if pH is higher than PZC, the surface
charge is negative 26.These results are in agreement with Di Paola et
al. 26, 41, who reported that for samples containing Co, Cu and Fe,
the PZC moves to more basic values compared to that of the
support (PZC 7.1). Di Paola et al.26, 42, suggested the PZC value of 7.1
for TiO2. This could be due to the difference in the synthesis
procedure as PZC value can be affected by heat treatment on TiO2.
X- Ray Powder Diffraction (XRD). The XRD patterns of the bare TiO2
and Cu-Ni/TiO2photocatalysts calcined at different calcination
temperature are shown in Figure 3. Two main phases were
observed: anatase and rutile. The peaks at 2θ = 25.34° and 2θ =
27.46° appeared for bare TiO2 as well as Cu-Ni doped
photocatalysts, corresponding to the main peak of anatase and
rutile, respectively 31, 43-45. However, no diffraction lines of Cu or Ni
containing phases were observed indicating that the dopants were
well dispersed onto TiO2. Zhu et al. 46, reported that TiO2 with metal
dopant concentration above 5wt% could display small peaks
attributable to a separate oxide phase in addition to the peaks for
anatase TiO2. The metal dispersion is confirmed by the results
displayed from FESEM-EDX images.
–
The intense peak at 1384 cm-1 was ascribed to nitrate (NO3 ) group
which is present in all the spectra. The presence of nitrate band was
also observed by Mohan 38, Li and Inui 39. They mentioned that the
nitrate will always be present when nitrate salts are used as
precursors. Since the color of the calcined photocatalysts was
different compared to the raw photocatalysts but not the same as
the color of the oxides, it can be deduced that the calcination
conditions were not enough to remove completely the nitrates.
However, in the present study, the presence of nitrate anions in
photocatalysts did not affect the catalytic activity of the
photocatalysts. Possible assignment of the peaks observed in FTIR
spectra of bare TiO2 and Cu:Ni/TiO2 photocatalysts is shown in S5.
Point of Zero Charge determination. PZC is defined as the pH at A major variable in photocatalyst preparation is the calcination
which the net charge is zero, at which coverage of the H+ equals the temperature or subsequent thermal treatment because it affects
47
coverage of OH–. In the present study, PZC was determined by the resulting TiO2 crystalline phase, its porosity and surface area
.
In the calcined samples, no phase transition from anatase to rutile
mass titration method. Plots of pH versus mass percentage WI-
9Cu:1Ni-200 and bare TiO2 are shown in Figure 2. Surface change of was observed. This might be due to the low calcination temperature
the oxide is a result of the acid-base equilibrium. It is a function of as the transition of anatase to the rutile phase normally occurs at
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temperature above 600°C. Lopez et al. 48, indicated that the phase
transition temperature from anatase to rutile started at calcination
temperature about 550:C and complete conversion to the rutile
phase was realized at 900:C. However in the current work the
highest calcination temperature was 300:C and no phase
transformation was observed.
View Article Online
DOI: 10.1039/C6RA10371E
Fig.4 FESEM micrographs of Cu-Ni/TiO2 photocatalysts (100 nm, at
magnification 150KX)
Elemental mapping of the 10wt%-WI-9Cu:1Ni-200 photocatalyst is
shown in Figure 5. The respective elements detected in EDX were
shown as colored spots in the mapping images. Although the
presence of copper, nickel, or both copper and nickel alloy were not
detected by XRD, but from the results it is very clear that the small
clusters of doped metal (copper and nickel) were highly dispersed
and incorporated onto TiO2 surface. Similar morphology with
spherical shape and agglomeration of metal particle was also
Fig.3 XRD patterns of bare TiO2 and 9Cu-1Ni/TiO2 calcined at
different temperatures (180, 200 and 300°C)
Field Emission Scanning Electron Microscopy (FE-SEM) and High
Resolution Transmission Electron Microscopy (HRTEM). Field
Emission Scanning Electron Microscopy (FESEM) was used to
evaluate the surface morphologies of photocatalyst such as
crystallite shape, size, and metal dispersion. Energy Dispersive X-
Ray Spectroscopy (EDX) is used to qualitatively and quantitatively
analyze the elements present in a selected area of the FESEM
image. The morphology of Cu-Ni/TiO2 photocatalysts as analyzed
by HRTEM and FESEM microscopy, was observed with typically
spherical particles of nanometer size and the photocatalysts
samples is partly composed of clusters containing composite
nanoparticles adhering to each other, slightly agglomerated ranging
from 10- 40 nm. The agglomeration may be due to sintering during
calcination process. FESEM micrographs of WI-Cu-Ni/TiO2
photocatalysts (100nm, at magnification 150KX) are shown in Figure
4.
50
observed by Riaz et al.24,25, 49 and Nurlaela et al. for Cu-Ni/TiO2
while Yoong et al.31 for Cu/TiO2 photocatalyst. The highly dispersed
metal particle on TiO2 led to better activity thus further enhances
the photocatalytic activity51.
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DOI: 10.1039/C6RA10371E
Fig.6 HRTEM micrographs of (a) fresh 9Cu:1Ni-200 and (R) recycled
9Cu:1Ni-200 photocatalysts
Surface Area and Porosity Analysis. The BET surface area, pore
volume, and pore diameter of bare TiO2 and 10wt% photocatalysts
with different Cu:Ni mass composition are shown in the Table 1
while the isotherm plots are shown in Figure 7. The N2 adsorption-
desorption isotherm for all the Cu-Ni/TiO2 photocatalysts showed
IUPAC of type IV pattern with H1 hysterisis loop indicating capillary
53
condensation in mesoporous adsorbate52,
. TiO2 however,
displayed type III pattern, which is typically ascribed to non-porous
products with weak interactions between the adsorbent and the
adsorbate. The surface area of the photocatalysts was found to
decrease slightly due to the increase in calcination
temperature. Figure 8 shows the pore size distribution of 10wt%-
9Cu:1Ni-200, the sharp decline in desorption curve may be a good
indicator of the non-uniform pore size of this specimen54. The
values for BET surface area, pore volume, and pore diameter of Cu-
Ni/TiO2 photocatalysts is shown in Table 2.
Fig.5 EDX and Mapping image of 10wt% -WI-9Cu:1Ni-200
HRTEM micrographs of the Cu-Ni/TiO2 photocatalysts (Figure 6)
showed a very good crystalline morphology of the particles with
different crystallite shapes. The particle size was from 11–35 nm for
Cu-Ni/TiO2 photocatalyst while 11nm–35 nm for bare TiO2.
In terms of calcination temperature, the surface area and pore
volume decreases with increasing temperature while average pore
diameter of photocatalyst increase as the temperature increases.
However, the high metal dispersion on the TiO2 surface was
reported when the total surface area of photocatalysts was
increased 55
.
Table 1 BET surface area, pore volume, and pore diameter of
Cu-Ni/TiO2 photocatalysts
Surface
area
Total pore
volume
Average pore
diameter
(nm)
Photocatalyst
(m2·g–1)
(cm3·g–1)
Bare TiO2
WI-9Cu:1Ni-180
43.1
72.2
0.20
0.43
18.5
23.9
WI-9Cu:1Ni-200
WI-9Cu:1Ni-300
35.7
46.3
0.24
26.4
0.32
27.4
Different Cu:Ni calcination temperatures
6 | J. Name., 2012, 00, 1-3
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activities of the photocatalysts. In terms of calcination temperature
View Article Online
DOI: 10.1039/C6RA10371E
it was found that with an increase in the calcination temperature
there was a decrease in the bandgap energy of the photocatalyst
was observed (S6-b). The calculated bandgap energies for bare
TiO2was 3.16eV while 2.96, 2.95 and 2.74eV for photocatalysts
calcined at different temperature 180, 200 and 300°C, respectively.
This statement suggested that different calcination temperature
affected the bandgap energy as proposed by Sakhtivel et al. 58
.
Temperature Programmed Reduction (TPR). Temperature
Programmed Reduction (TPR) was used in order to characterize the
Cu-Ni/TiO2 photocatalyst with respect to the type of metal oxide
species present, either copper oxide, nickel oxide, or copper-nickel
mixed oxide, and the degree of interaction of the oxides with TiO2
support 31. The summary of the amount of hydrogen consumed and
reduction temperature of different photocatalysts is shown in Table
2. The TPR profiles of the bimetallic 9Cu:1Ni/TiO2 photocatalysts
with different calcination temperature are presented in S7. The
higher reduction peaks at 295oC was ascribed to bulk CuO phases
that include large clusters and bulk CuO. The reduction profile of
bimetallic 9Cu:1Ni-200 showed a shoulder around 220–240oC and
one main reduction peaks, 289oC, that might be attributed to the
reduction of Cu–Ni mixed oxide instead of individual oxide. Higher
reduction temperatures are attributed to the reduction of NiO with
strong interaction with TiO259. The distinct peak observed at 289oC
for 9Cu:1Ni-180 might be attributed to the reduction of Cu-Ni mixed
oxide instead of individual oxide 55. It was also found that the
Fig.7 Isotherm plot for 10wt%-WI-9Cu:1Ni-200
presence of Cu lowered the reduction temperature of Ni. Li et al. 60
,
observed the same behavior as the bimetallic Cu-Ni/TiO2. The
addition of Cu enhanced the reduction of Ni, thereby it can be
concluded that the reducibility of bimetallic Cu–Ni is controlled by
the amount of Cu. However, the presence of Cu–, Ni–, as well as
mixed Cu–Ni species were not detected by XRD analysis due to high
Fig.8 Pore size distribution of 10wt%-WI-9Cu:1Ni-200
Diffuse Reflectance UV-Visible Spectroscopy (DR-UV-Vis).The DR-
UV-Vis absorption spectrum of bare TiO2 and photocatalysts with
different calcination temperature are presented in (S6-a). dispersion of the metal particle on TiO2. The TPR profile of
Modification with metal can shift the absorption spectrum of
TiO2into visible region. The shift in the absorption spectrum has
been observed for the synthesized photocatalysts. Impregnation
method has also been reported previously that this method can
also shift the absorption spectrum of TiO2 42. The shift is not
possibly caused by the change in the bandgap, but rather by the
impurity of energy level as the metal is spread on the surface of
monometallic Cu/TiO2 was in good agreement with the results
showed by the XRD and FESEM, that the Cu- species was highly
dispersed on TiO2 photocatalyst.
Table 2 Summary of the hydrogen consumption and the reduction
temperature of the photocatalysts
TiO2 and not incorporated into TiO2 framework. Nevertheless, bulk
56
Reduction
peak (°C)
289
Amount of hydrogen
consumed (μmol·g–1)
1879.3
properties of TiO2 are as important as surface properties of TiO2
.
Photocatalyst
The DR-UV-Vis spectra of bare TiO2 showed absorption peaks
ranging from 190 nm and 400 nm, similar to that observed in our
previous studies 24, 31, 50. The absorption at 323 nm is generally
related to the electronic excitation from the valence band O 2p
WI-9Cu:1Ni-200
357
295
1938.2
1877.0
WI-9Cu:1Ni-180
WI-9Cu:1Ni-300
361
216
276
355
1939.9
345.79
1436.2
158.98
electron to conduction band Ti 3d orbital indicating the Ti is in the
57
form of tetrahedral Ti4+ species
. Better activity of the
photocatalysts was reported which clearly shows that the surface
modification of TiO2 Cu-Ni, has significantly enhanced the
absorption properties of photocatalyst. Calcination temperature
and duration play important role in shaping the characteristics and
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for all the photocatalysts. 10wt%-WI-9Cu:1Ni-300 was reported
DOI: 10.1039/C6RA10371E
with the lowest bandgap energy (2.74eV) and 100% Orange II
Photocatalytic activity of Cu-Ni/TiO2 photocatalysts
View Article Online
decoloration. The same photocatalyst was observed with 22.8 ppm
TOC value that was higher compared to the best performing
10wt%- WI-9Cu:1Ni-200. This shows that the low bandgap energy
alone was not the only factor affecting the photocatalytic activity.
Cu:Ni mass compositions. By fixing the total metal loading, 10 wt%
Cu-Ni/TiO2 photocatalysts prepared with different Cu-Ni mass
compositions calcined at 200oC were studied for Orange II
decoloration at pH 6.8. Bimetallic photocatalysts with 9:1, 5:5 and
1:9 Cu:Ni mass compositions were observed with highest
percentage of Orange II decoloration compared to mono metallic
and bare TiO2. In case of different Cu:Ni mass compositions
9Cu:1Ni-200 photocatalysts showed best performance with 100%
Orange II decoloration compared to other mass compositions while
bare TiO2 showed only 21% Orange II decoloration. Among 9:1, 5:5
and 1:9 Cu:Ni mass compositions 1:9 Cu:Ni mass composition
calcined at 200 oC gave 99.2% while 10:0 and 0:10 Cu: Ni mass
compositions gave only 92.7% and 45.3% Orange II decoloration
respectively. Comparison of Orange II decoloration vs %TOC
removal with different Cu:Ni mass compositions and bare TiO2 is
shown in Figure 9.
Effect of calcination temperature. All the Cu-Ni/TiO2 photocatalysts
displayed better dye decoloration activity regardless of the
calcination temperatures. The photocatalysts calcined at 180 :C,
with 9:1 and 5:5 Cu:Ni mass compositions were observed with 100%
Orange II decoloration and 1:9 Cu:Ni mass compositions calcined at
200 oC gave 99.2%. Although the photocatalyst performance was
100 % but the difference in performance was identified as the TOC
removal efficiency. Comparison of % TOC removal for photocatalyst
with different Cu:Ni mass compositions is shown in Figure 10. The
best values obtained for photocatalyst calcined at 200o C with 100 %
Orange II decoloration and 89.9 % TOC removal (3.18 ppm TOC
value). The TOC % removal for different calcination temperature
It is well accepted that 100% decoloration of organic dyes such as
Orange II does not mean that 100% mineralization of the dye. This
is because during decoloration, many colorless long-lived
intermediates, which might not be environmentally acceptable, are
formed. Therefore, from the point of view of discharge standards,
mineralization of Orange II could be equally important23. Detailed
TOC analysis was conducted by further screening of the best
performing photocatalysts for photodegradation studies to check
the photocatalyst with highest performance with lower TOC values.
In case of different Cu:Ni mass compositions 9Cu:1Ni-200
photocatalysts showed best performance with 100% Orange II
decoloration and lowest TOC values 3.18 ppm (89.9 %) for 1 h
reaction study compared to 5Cu:5Ni-200, 1Cu:9Ni-200, 10Cu:0Ni-
200 and 0Cu:10Ni-200 photocatalysts with TOC values 22.9ppm,
12.9 ppm, 20.5 ppm and 25.8 ppm, respectively. After 1.5 h of
irradiation duration, 9Cu:1Ni-200 photocatalyst was reported with
the 100% TOC removal.
o
was 56.3 % and 27.2 % for photocatalysts calcined at 180 and 300
C, respectively. The results also indicate that the photocatalytic
activity of synthesized titania decreased with the increased
61
calcination temperature. Yu et al.
also reported a significant
decrease in the photocatalytic activity of titaniananopowder
calcined at higher temperature may be attributed to the growth of
particle and result in the reduction of contact area of particles for
photocatalytic reaction. The improvement of photocatalytic activity
compared with commercial materials can be associated to the
combined increase of crystallinity with the preservation of a
relatively large surface area based on the existence of mesopores62.
Fig.10 Effect of Cu-Ni mass composition on TOC for WI
photocatalysts
Effect of initial pH. The interpretation of pH effects on the
efficiency of dye photodegradation process is a very complex
parameter task because of its multiple roles. 10 wt% Cu-Ni/TiO2
photocatalysts with 9:1 Cu:Ni mass composition calcined at 200oC
displayed the lowest dye degradation at pH 3 (22.3%) and the
highest dye degradation at pH 6.8 (Figure 11). The effect of pH is
generally dependent on the type of the pollutant and surface
properties of the photocatalysts 27. The effect of pH on the
Fig.9 Effect of Cu:Ni mass compositionon % Orange II decoloration
and %TOC removal (Reaction duration=1h)
Comparing the bandgap energies with the % Orange II decoloration
during the photocatalytic reaction, results showed that there was
not much significant difference for the calculated bandgap energies
8 | J. Name., 2012, 00, 1-3
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photocatalytic degradation of Orange II can be explained by the effect of excessive coverage which results in limitVaiteiwoAnrticolef Olinglihnet
9, 64
DOI: 10.1039/C6RA10371E
. Thus the number of
surface charge of Cu-Ni/TiO2 photocatalysts (positive) and its penetration reaching the surface
relation to the acid dye Orange II. In this study at pH 6.8, while the photogenerated e– and h+ reduced and subsequently lowered the
pH is neutral, a strong adsorption of the acid dye onto the photocatalytic activity65. Furthermore, as the metal loading
photocatalysts particle as a result of electrostatic attraction of increases, agglomeration of metal particles is believed to occur that
positively charged TiO2 with the ionized dyes is observed. This can decrease the photocatalytic activity of photocatalyst31, 55. The
also be seen with the naked eye, while the photocatalyst turns optimum photocatalysts loading was obtained for photocatalysts
orange-brown from light aquamarine color. On the other hand, with 10wt% that showed 89.8% TOC removal compared to 29.8ppm
above pH 6.8, a decrease in the reaction rate has been observed (4.7% TOC removal) and 35.3ppm ( 0% TOC removal) for 5 wt% and
with a minimum at pH 12, reflecting the difficulty of anionic dye in 15 wt% respectively.
approaching the negatively charged TiO2 surface when increasing Effect of photocatalysts loading. TiO2 loading in slurry
solution pH.At pH higher than PZC the photocatalyst surface was photocatalytic processes is an important factor that can influence
negatively charged, thus preventing the negatively charged dye as dye degradation38. The effect of catalyst loading was studied for
well as the hydroxide anion from adsorbing onto the surface, this Orange II decoloration in the range 100–1000 mg·L–1 and the
would explain the reduced degree of degradation recorded at comparison of Orange II decoloration and %TOC removal is shown
alkaline conditions. On the other hand, adsorption of negatively in S9. It was observed that photocatalysts loading is directly
charged dye was favored at pH lower than the PZC due to the proportional to the Orange II decoloration28, 66, 67. With increasing
positively charged photocatalyst surface. In this study the higher concentration of photocatalysts, Orange II decoloration was also
percentage of dye decoloration was achieved at pH 6.8 and PZC increased while TOC was decreased with increasing concentration
value for Cu-Ni/TiO2 is above pH 7.40 for higher metal oxides of photocatalyst.
loading. Findings of others 27, 28 showed that degradation of anionic Effect of irradiation time. Orange II decoloration (%) as a function
dyes is more in acidic medium, compared to pH equal or higher of time (120 min dark reaction followed by 60 min light reaction) for
than PZC value of TiO2. Furthermore using ZnO photocatalysts, WI photocatalysts with 9:1 Cu:Ni mass composition calcined at
degradation of Orange II are much faster in neutral medium 63. So it different temperature is displayed in Figure 12. During dark
can be said that pH equals to PZC of TiO2 would favor the best reaction, decoloration was the fastest for photocatalyst calcined at
results for Orange II decoloration with lower TOC values. After the 180ºC with 95.3% after the dark reaction without exposure to
photodegradation (60 min duration), the remaining TOC was irradiation indicating high dye adsorption rate. For photocatalyst
different for different photocatalysts compared to their initial TOC calcined at 200°C, Orange IIdecoloration was observed even in the
values. In case of different initial pH for the reaction sample using dark, and the decoloration progressed to 89.4% after 120 min in the
9Cu:1Ni-200, 100% decoloration with 89.8% TOC removal was dark. The degradation progressed further to 100% after 60 min
observed at pH 6.8, the best performance as compared to other pH irradiation. However, for photocatalyst calcined at 300 ºC and bare
values.
TiO2, degradation rate during dark reaction was very slow (61.8%
and 37.4%) compared to other two photocatalysts which further
increased to 100% and 21% after irradiation of 60 min, respectively.
It is evident that the percentage of decoloration and
photodegradation increases with irradiation time for photocatalysts
calcined at different temperatures. Photocatalysts calcined at 300ºC
showed less adsorption during dark reaction but the rate of
reaction during irradiation was comparable to that for
photocatalyst calcined at 200°C. The reaction rate during light
reaction decreases with irradiation time as it follows the pseudo
first-order kinetics and additionally a competition for degradation
may occur between the reactant and the intermediate products.
The slow kinetics of dye degradation after certain time limit is
mainly attributed to: (a) the difficulty in converting the N-atoms in
Orange II into oxidized nitrogen compounds 68, (b) the slow reaction
Fig.11 Effect of initial pHon Orange II decoloration
69
of short chain aliphatics with •OH radicals and (c) the short life-
Effect of metal loading. By fixing the Cu-Ni mass composition
(9Cu:1Ni-200), photocatalysts with different metal loading (5, 10
and 15 wt%) were screened for Orange II decoloration. For different
metal loading 10wt% 9:1 Cu-Ni/TiO2 photocatalysts showed best
performance with 100% Orange II decoloration compared to 5wt%
photocatalysts with 52.8% and 15wt% photocatalysts with 55.2%
Orange II decoloration respectively (S8). As the metal loading
increased, blockage of the active sites of TiO2 may occur as the
time of photocatalyst because of active sites deactivation by strong
by-products deposition (carbon etc.) 70
.
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representative peak for Orange II concentration71-73. After 30 min of
View Article Online
DOI: 10.1039/C6RA10371E
irradiation time and onwards, the visible band the disappearance of
the visible band was observed that might be due to the
fragmentation of the azo links by oxidation 74. In addition to this
rapid decoloration effect, the decay of the absorbance at 310 nm
was considered as evidence of aromatic fragment degradation in
the dye molecule and its intermediates 72
.
Fig.12 Orange II decoloration in the dark and under visible light
irradiation using bare TiO2 and WI photocatalysts with 9Cu:1Ni mass
compostion calcined at different temperatures.
Table 3 Effects of different physical parameters on Orange II
decoloration and TOC using selective photocatalysts and bare TiO2
[Org II]f Decoloration TOCf
TOC
Photocatalysts
(ppm)
39.5
48.49
23.6
22.4
3.6
0
(%)
21
3.0
(ppm) Removal(%)
Bare TiO2
NoPhotocatalyst
55.7
36.7
29.8
35.3
20.5
3.19
22.9
12.9
25.9
13.6
16.08
8.61
4.19
22.8
4.0
18.4
4.7
5wt%-9Cu:1Ni-200
15wt%-9Cu:1Ni-200
10wt%-10Cu:0Ni-200
10wt%-9Cu:1Ni-200
10wt%-5Cu:5Ni-200
10wt%-1Cu:9Ni-200
10wt%-0Cu:10Ni-200
10wt%-9Cu:1Ni-180
10wt%-5Cu:5Ni-180
10wt%-3Cu:7Ni-180
10wt%-1Cu:9Ni-180
10wt%-9Cu:1Ni-300
52.8
55.2
92.7
100
100
99.2
45.3
100
100
100
100
100
0.0
34.5
89.9
26.6
58.5
17.3
40.5
29.4
40.9
83.5
27.2
0
0.41
27.4
0
0
0
0
0
[Org II]f= Final Orange II conc. in ppm after 1h; TOCf = Final TOC value in
ppm after 1h, pH=6.8]
Fig.13 UV–vis spectra changes with reaction time (0 min- 60 min)
for Orange II decoloration during (a) dark reaction and (b) light
reaction
UV–visible spectral changes. In order to understand and clarify the
changes in molecular and structural characteristics of Orange II as a
result of photodegradation using Cu-Ni/TiO2 photocatalysts under
visible light irradiation, representative UV–visible spectral changes
in the dye solution as a function of irradiation time were observed
and the corresponding spectra are shown in Figure 13 for 9Cu:1Ni-
200 photocatalysts. It can be observed from UV–vis spectra at
different time interval, the absorption spectrum of Orange II in
water was characterized by one main band in the visible region,
with its maximum absorption at 485 nm and by the other band in
the ultraviolet region located at 310 nm, respectively. The peaks at
310 nm were associated with “benzene-like” structures in the
molecule, and that at 485 nm originated from an extended
chromophore, comprising both aromatic rings, connected through
the azo bond. The absorbance peak at 485.0 nm was used as the
Analysis of intermediates and by-products
Reaction intermediate and final products at specific time interval of
photocatalytic degradation reactions were identified by HPLC.
Degradation products formed during the photodegradation process
using visible light source were analyzed using HPLC. There are
several separation peaks appearing during the degradation
experiment, which can be ascribed to the intermediates of the
Orange II dye degradation. Peaks were obtained for the
intermediates compounds and final products such as Oxalic acid,
formaldehyde, benzyl alcohol, benzaldehyde, salfanilic acid and
27, 28
formic acid
and Salfanilic acid. From the Chromatogram of
reaction samples before and after the photo-reaction (S10), it is
clear that 1h of irradiation time was enough to remove Orange II
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completely from the samples. The retention time of identified products is also fully described in literature27,
ARTICLE
75-77
.In bimetallic
View Article Online
DOI: 10.1039/C6RA10371E
photodegradation compounds (using bimetallic photocatalysts and photocatalysts, the addition of small amount of Ni, enhanced the
bare TiO2 photocatalysts) detected after 1 h of irradiation time are activity of photocatalyst. Photocatalysts with higher Cu contents
presented in Table 4. The identified intermediates and by products (e.g 9Cu:1Ni mass composition) showed higher activity compared to
are at RT 1.2 (oxalic acid), RT 1.4 (formic acid), RT 1.7 those of bimetallic photocatalyst with lower Cu content and
(formaldehyde), RT 3.8 (benzyl alcohol) and RT 7.8 (benzaldehyde).
monometallic photocatalysts. Cu2+also acts as electron trap, adding
small amount of Ni can enhance the hole trapping process thus
retarding recombination reaction. However, as the amount of Ni
increased, it becomes hole accumulation site. Such hole
accumulation further attracted the negatively charged species,
either from one embodied in Cu2+ (as electron trap) or the mobile
electron from the conduction band of TiO2. Therefore, Ni2+ itself
becomes the recombination reaction center thus decreasing the
activity of photocatalyst. A decrease in Cu content led to lower
Orange II decoloration. Meanwhile, for monometallic 10Cu:0Ni-200
and 0Cu:10Ni-200, the activities were found to be lower than those
Table 4Identified intermediates and by products obtained by
Orange II decoloration
Products identified in
different reaction
Products
RT
samples
WI

TiO2

A
B
C
D
E
Oxalic Acid (OA)
1.2
1.4
1.7
1.8
2.2


Formic Acid (FA)

Formaldehyde (FD)
X
of some bimetallic Cu-Ni/TiO2 photocatalysts 9, 78
.


N
N
X
Degradation of Orange II was found to increase with increasing the
photocatalyst concentration and at neutral pH of the solution. As
discussed previously the photocatalytic degradation by products
can be classified into three groups: (i) naphthalene like compounds
such as 2-napthole which is a primary degradation intermediates

Benzyl Alcohol
(BA)


F
3.8


X
G
H
I
N
2.4
4.3
4.8
7.5
X

X
75,
accompanying Orange II cleavage in the vicinity of the azo bond
N
76. Its formation has also been corroborated in several previous
studies concerning Orange II photocatalytic degradation 28, (ii)
aromatic intermediates such as benzyl alcohol, benzaldehyde, 2-
methyl phenol, toluene and benzophenone and (iii) ring cleavage
compounds such as 2-ethyl-1-hexanol. Photocatalytically treated
Orange II solution mainly consisted of aromatic intermediates as
confirmed by HPLC analysis. The identified intermediates and by
products are benzaldehyde, benzyl alcohol, formaldehyde, oxalic
acid and formic acid. In photocatalytic oxidation process, active
electron contribute to the decoloration and mineralization of the
N
J
Benzaldehyde (BD)

X
= Identified; RT= Retention time (min), N = Not identified; X= Not
detected
Intermediates and products appeared during reaction at different
time interval and pH value 6.8 may change in different trends, and
therefore their contents as a function of time were measured and
the plot of changing concentrations of the identified organic
intermediates is shown in Figure 14.
•–
dye due to the producing of oxidative species such as O2 and
•OOH, but also contribute to the decoloration of Orange II as
reductive species. After 60 min of irradiation, the reaction mixture
consists of aliphatic compounds presumably due to the oxidation of
Orange II and its ring intermediates. It is also assumed that the
differences in the identified by-products of photocatalysis are due
to the different photodegradation mechanisms involved.
On the other hand, the formation of these acids suggests the
degradation process after the opening of aromatic and naphthalene
rings. Once the azo bond is cleaved, the conjugated structure of
Orange II dye is destructed, accompanied by the complete color
removal rather than TOC removal. The most possible reduction
mechanism is generally presented in the Figure 15. These results
indicate that many intermediate compounds are converted into CO2
via either formic acid. However since formic acid is
Fig.14 Intensity Changes in concentration of the organic compounds
as a function of time at different retention time (irradiation
duration= 1h)
photocatalytically more degradable than other aliphatic acids
77
reported previously
, the more intermediate compounds
apparently degraded through formic acid. The complete
degradation pathway of Cu-Ni/TiO2 mediated photodegradation
under visible light can be briefly summarized as:
Proposed Mechanism of Orange II photodegradation
Orange II is the most studied compound among the azo dyes as far
as its photocatalytic degradation under several experimental
conditions. The degradation pathways and the formation of by-
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Dyeintermediates aliphaticandaromaticacidsCO2, NH 4, NO3, NO2,SO42
View Article Online
DOI: 10.1039/C6RA10371E
Fig.16 Photocatalyst recycling study: Orange II % decoloration
(irradiation duration= 1h)
Conclusions
In this study,
a series of monometallic and bimetallic Cu-
Ni/TiO2photocatalysts were successfully prepared wet impregnation
(WI) methods for Orange II removal. Photocatalysts with different
Cu-Ni mass compositions (10:0, 9:1, 7:3, 5:5, 3:7, 1:9 and 0:10) with
different total metal loading (5, 10 and 15 wt%) were prepared. The
physical and chemical properties of the modified Cu-Ni doped
photocatalysts and bare TiO2 were investigated and it was
concluded that the addition of metal onto the surface of TiO2 has
successfully modified the physical and chemical properties of bare
TiO2 which led to better photocatalytic performance for Orange II
photodegradation activity under visible light irradiation. For the
photocatalysts samples containing Cu and Ni, the PZC shifts to more
basic values (7.7) compared to that of the support (for TiO2 was
about 3.8). In XRD analysis, no phase transition was observed for
anatase into rutile for the calcined photocatalysts and also no
diffraction lines of Cu or Ni containing phases were observed
indicating that the dopants were well dispersed onto TiO2. FESEM
and HRTEM analysis for all the samples showed spherical and
agglomerated morphologies. DRUV-Vis analysis showed that
incorporation of Cu and Ni onto TiO2 has successfully shifted the
optical absorption to the visible region with reduced bandgap
energies.
Fig.15 Proposed photodegradation mechanism of Orange II under
visible light using Cu-Ni/TiO2 photocatalyst
Photocatalyst recycling study
To investigate the recyclability/reusability of the photocatalyst after
reaction study, photocatalytic degradation study was carried out for
multiple times using recycled photocatalyst and the results for the
Orange II decoloration and TOC removal (%) are presented in the
Figure 16. It was found that the activity of photocatalyst decreased
as the recycle number increased which was showed by the
decreased % Orange II photodegradation. Results for Orange II
decoloration was 100% with 89.8% TOC removal, when
photocatalysts was used for the first time and reduction peaks at
289 and 357:C with 1879.4 and 1938.2µmol·g–1 hydrogen
consumption. After the first reaction study, the Orange II %
decoloration was decreased up to 83.98% and 58.4% TOC removal.
The percent dye decoloration was still higher (75.6%Orange II
decoloration and 46.3% TOC removal) compared to that obtained
with bare TiO2.
The synthesized Cu-Ni/TiO2 photocatalysts were screened for
Orange II dye decoloration efficiency against different reaction
parameters. The photocatalyst activity is directly related to
photocatalysts loading while the optimum pH for Orange II
decoloration was pH 6.8 with the best performance (100% Orange II
decoloration). Higher calcination temperature (300:C) resulted TiO2
In S11, the reduction peak for recycled photocatalyst was found at
lower temperature (185 and 263oC with 163.3 and 142.0 µmol·g–1,
low hydrogen consumption) compared to fresh photocatalyst. The
enhanced reducibility of the photocatalyst might be ascribed to the
interaction between CuO and NiO with TiO2. The interaction
between CuO and NiO with TiO2 reduce as the photocatalyst used
for sequence of reaction. In S12 FTIR spectra comparison of Orange
II dye, fresh and used photocatalysts are shown, indicating that
even after two washings still a small amount of dye was detectable.
The decrease in the efficiency of the recycled catalyst may be
attributed to the deposition of photoinsensitive hydroxides
with lower bandgap energy 2.74 eV for 9Cu:1Ni-300.
photocatalysts
Despite of some advantages, some detrimental effects such as
decreased performance for Orange II mineralization were observed.
For different Cu:Ni mass compositions and metal loading, 10wt%-
9Cu:1Ni photocatalysts showed the best performance with 100%
Orange II decoloration with 10 wt% optimum metal loading and
9Cu:1Ni mass composition. The improvement in catalytic
performance can also be attributed to the synergistic effect of
(Fouling) on the photocatalysts surface blocking its active sites 79
.
12 | J. Name., 2012, 00, 1-3
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ARTICLE
16. T.-J. Huang and S.-Y. Jhao, Applied Catalysis A: General,
reduced bandgap energy, high crystallinity, smaller particle size,
increase in surface area and to the higher number of OH groups
exposed on the surface of the Cu-Ni-metalized TiO2. Therefore,
enhanced photocatalytic activity of the Cu-Ni/TiO2 photocatalysts
was observed under the present experimental conditions. The main
identified intermediates and by products of Orange II
photodegradation under visible light irradiation during reaction as a
function of time were oxalic acid, formic acid, formaldehyde, benzyl
alcohol and benzaldehyde as measured by HPLC analysis. The as-
prepared Cu-Ni/TiO2 photocatalysts have a great potential for
photocatalytic water purification, particularly the elimination of
toxic aromatic compounds. Highly stable Cu-Ni/TiO2 photocatalysts
with excellent responsiveness to visible light (wavelength and the
efficiency of maximum absorbance) might be beneficial for solar-
driven applications in the treatment of hazardous aqueous
pollutants and might play an important role as ‘‘green’’ and
inexpensive photocatalysts for the improvement of water quality.
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Acknowledgments
The authors would like to acknowledge Universiti Teknologi
PETRONAS for the funding of this research work.
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DOI: 10.1039/C6RA10371E
Graphical Abstract