Efficient hydroxylation of benzene to phenol by H2O2using Ni-doped CuWO4on carbon nitride as a catalyst under solar irradiation and its structure-activity correlation

Article abstract of DOI:10.1039/d0ta03729j

A series of environmentally benign and highly efficient Z-scheme Ni-doped CuWO4 nanoparticles on graphitic carbon nitride (g-C3N4) were synthesized. The Ni-CuWO4 nanoparticles were prepared via the substitution of Ni2+ on a wolframite CuWO4 crystal. The photocatalytic activity of these nanocomposites was investigated for the hydroxylation of benzene to phenol, considering the importance of phenol and carcinogenicity of benzene. An excellent benzene conversion of 98.5percent, with 82.7percent selectivity and 81.5percent yield of phenol, was achieved over 0.2percent Ni-CuWO4/g-C3N4 in 15 minutes under sunlight using H2O2 as an oxidant in water, which is higher than those of pristine g-C3N4 and Ni-CuWO4. The high yield of phenol is mainly attributed to the narrow band gap of the semiconductor and enhanced visible light absorption capacity over a specific range of wavelength by the introduction of g-C3N4, which minimized the rapid recombination of photogenerated holes and electrons. The computational study related to this work also implied the high optical property and stability of the photocatalyst. This journal is

Full text of DOI:10.1039/d0ta03729j

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Journal of
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Materials for energy and sustainability
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Guha, S. Borthakur, L. Kalita, P. Chetia, K. Sonowal and L. Saikia, J. Mater. Chem. A, 2020, DOI:
10.1039/D0TA03729J.
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Number 12
28 March 2018
Pages 4883-5230
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Efficient hydroxylation of benzene to phenol by H O using Ni doped
DOI: 10.1039/D0TA03729J
2
2
CuWO4 on carbon nitride as catalyst under solar irradiation and its
structure activity correlation
Purashri Basyach,a,b Ankur Kanti Guha,c Sukanya Borthakur,a,b Lisamoni Kalita,a,b Pubali
Chetia,a,b Karanika Sonowala,b and Lakshi Saikia*a,b
a Advanced Materials Group, Materials Sciences & Technology Division, CSIR-North East Institute
of Science and Technology, Jorhat-785006, Assam, India
b Academy of Scientific and Innovative Research, New Delhi, India
c Department of Chemistry, Cotton University, Panbazar, Guwahati-781001
Abstract: A series of environmentally benign and highly efficient Z-scheme Ni doped
CuWO4 on graphitic carbon nitride (g-C3N4) were synthesized. The Ni-CuWO4 nanoparticles
were prepared through substitution of Ni2+ on wolframite CuWO4 crystal. The photocatalytic
activity of these nanocomposites was investigated for the hydroxylation of benzene to phenol
considering the importance of phenol and carcinogenicity of benzene. Excellent benzene
conversion of 98.5% with 82.7% selectivity and 81.5% yield of phenol was achieved over
0.2% Ni-CuWO4/g-C3N4in 15 minutes under sunlight using H2O2 as oxidant in water which
is higher than those of pristine g-C3N4 and Ni-CuWO4. The high yield of phenol was mainly
attributed to the narrow band gap of the semiconductor and enhanced visible light absorption
capacity over a specific range of wavelength by the introduction of g-C3N4 which minimized
the rapid recombination of photogenerated holes and electrons. The computational study
related to this work also implied the high optical property and stability of the photocatalyst.
Keywords: Ni-CuWO4/g-C3N4, photocatalysis, Z-scheme, nanocomposites, computational
study
1

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1. Introduction:
DOI: 10.1039/D0TA03729J
Photocatalytic organic transformations through semiconductor based nanocatalysts have
recently gained very promising and challenging interest in the field of catalytic territory.1-4
Photocatalytic hydroxylation of benzene to phenol is one of the most important reactions
regarding this field. Benzene and phenol both act as very important intermediate in many
chemical reactions.5 But being a polluting and carcinogenic, conversion of benzene to some
other valuable chemicals is necessary for environmental benefit. Moreover, phenol is also a
very important precursor or intermediate used for the synthesis of some pharmaceutical
compounds, dyes, fibers, polymer resins, plastics etc.6,7 Phenol can also be used as
intermediate in the synthesis of fungicides and can be used as a preservative.8,9 Phenol is
mainly industrially prepared from benzene via three step cumene process.10,11 However, the
yield of phenol is very low (only 5%) under this process which also requires high temperature
and pressure in the presence of Lewis acid catalysts.12,13 These drawbacks lower the
efficiency of cumene process, so, it is highly required to develop a new economic and
environment friendly process for the production of phenol from benzene with high yield
using homogeneous and heterogeneous catalysts.14-16
Catalytic aerobic oxidation of benzene to phenol in the liquid phase can be performed by
using molecular oxygen as an oxidant. However, the requirement of sacrificial reducing
agents such as hydrogen, CO, ascorbic acid and NADH analogues are the main drawback of
this process.17-21 Because of this, oxidation system based on H2O2 as an oxidant has recently
gained tremendous interest in catalytic oxidative process, as this process is green, economic
and environment friendly and produces only water or dioxygen as products.22
Development of a photocatalytic process using visible light as a source of light for the
efficient conversion of benzene to phenol has drawn more concern in research field. Instead
of using artificial source of light use of natural sunlight as an active source has earned great
challenge in organic transformation reactions.23 As most of the photocatalytic reactions are
performed under conservative reaction conditions, developing a suitable photocatalyst which
is of low cost and can be used to conduct the reactions under controlled reaction conditions
with high phenol yield is very challenging.24,25 Many reports have been found to be
articulated on visible light driven hydroxylation benzene to phenol using H2O2 as an
oxidant.26-31 Recently, Hutchings and co-workers have reported the photocatalytic oxidation
of benzene to phenol under UV light using titania loaded monometallic Au and Pd as
catalysts.32 Again, Hosseini et.al have reported the visible light oxidation of benzene to
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phenol using bimetallic AuPd@g-C3N4 photocatalyst.22 Zhao et. al. has reportedDtOhI:e10g.1-0C393/DN04TA03729J
supported single metal atom catalyst for oxidation of benzene to phenol using H2O2.33
Metal tungstates, MWO4 (M=Ni, Cu, Fe, Mg etc.) with wolframite like structure, relatively
high photocatalytic activity and good stability have recently attracted potential interest in
photocatalytic reactions like CO2 reduction, C-H activation and dye degradation etc.34-37
Recently, CuWO4, a narrow band gap semiconductor has gained more importance as it can be
used as photocatalyst in many reactions.38,39 However the photocatalytic performance of
CuWO4 is not that satisfactory due to the rapid recombination of photogenerated holes and
electrons. The incorporation of other metal ions into CuWO4 may able to enhance its
photocatalytic activity. Nickel salts being cost effective and easily available, various forms of
nickel can be used in many photocatalytic reactions. Again, considering the similar ionic
radius with Cu2+, Ni2+ may easily replace Cu2+ ion in CuWO4 crystal thereby improving its
photocatalytic activity. Therefore, the first aim of this study is to develop a metal tungstate
like CuWO4 through the substitution of Ni2+ for better photocatalytic results.
Now-a-days, graphitic carbon nitride (g-C3N4), a 2D layered like semiconductor, has attracted
enormous interest in photocatalysts because of its suitable band gap (2.7 eV) electronic
structure, superior visible light response, high thermal stability and non-toxicity.40 However,
like other photocatalysts, the g-C3N4 has also encountered with high recombination of
photogenerated holes and electrons which diminishes its photoactivity. To avoid this
circumstances g-C3N4 has been blended with different narrow band gap materials to build a
direct Z-scheme system for enhanced photocatalytic performance.41 Thus, the second aim of
this work is to construct a novel g-C3N4 supported CuWO4 nanocomposite through the
substitution of Ni2+ (simply denoted as Ni-CuWo4/g-C3N4) for efficient photocatalytic
hydroxylation of benzene to phenol with high yield using natural sunlight as a promising
source of light.
2. Experimental section:
2.1 Materials:
Urea (Himedia, 99.0%), CuCl2.2H2O (Sigma Aldrich, ≥ 99.0%), NiCl2 (anhydrous, Alfa
Aesar, 98.0%), polyethylene glycol 4000 (SRL), benzene (LobaCheme, 99.5%), phenol
(Sigma Aldrich, ≥99.0%) chemicals were used for this research purpose. All the purchased
chemicals are of analytical grade and were used directly without further treatment.
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2.2 Catalyst preparation:
2.2.1 Synthesis of g-C3N4:
DOI: 10.1039/D0TA03729J
The bulk g-C3N4 was synthesized by thermal polymerisation of urea as reported earlier.41
Urea being less costly and easily available can be used as the most valuable precursor for the
synthesis of g-C3N4. In a typical process, 20 g urea taken in a silica crucible covered with a
lid was calcined in a muffle furnace at 5500C for 2 hrs with heating rate 50C min-1under air
atmosphere. After cooling to room temperature yellow coloured bulk g-C3N4 was formed as
the desired product.
2.2.2 Synthesis of Ni-CuWO4 (NCW):
Ni-CuWO4nanocompound can be synthesized by chemical precipitation method. In a typical
synthesis, 1 mmol of CuCl2.2H2O was added to the solution of Na2WO4.2H2O dissolved in
2% PEG-4000. A blue precipitate was found to be observed. After sonicated for half an hour,
1 mmol of NiCl2 was added to it and mixed well. The blue precipitate turned bluish green
after the addition of NiCl2. After sonicated for half an hour, 1 mmol of NiCl2 was added to it
and mixed well. A bluish green precipitate was obtained. The mixture was then again
sonicated in an ultrasonicator bath for 2 h and stirred for 6 h. The bluish green precipitate
obtained was filtered and washed with deionized water followed by ethanol. The precipitate
was dried under vacuum at 600C andfinally calcined at 5000C in a muffle furnace for 4 h to
get the desired product.
2.2.3 Synthesis of Ni-CuWO4/g-C3N4nanocomposite (NCWCN):
For the synthesis of Ni-CuWO4/g-C3N4nanocomposite, the highly porous bulk g-C3N4 (340
mg) was first dispersed in 50 mL of methanol. Then the solution of g-C3N4 was
ultrasonicated for almost 10 hrs for exfoliation. Subsequently, a certain amount of as
prepared Ni-CuWO4 (60 mg) was added to this solution and was sonicated for almost 5 hrs.
The solution was then kept stirring for about 24 h. The product obtained was filtered and
washed with distilled water followed by ethanol several times. Finally, the product was dried
under vacuum at 600C. The products obtained were labelled as x NCWCN, where x was
denoted as the value of mass ratio of Ni-CuWO4 and g-C3N4. Thus the as synthesized
nanocomposite would contain mass ratio of 0.2. In this manner, different mass ratio
containing Ni-CuWO4/g-C3N4 such as 0.05, 0.1, 0.3, 0.4, 0.5 etc. nanocomposites were also
synthesized. The percentage composition of main metal elements Cu, Ni, W in the catalysts
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with differentmass ratio were included in supporting information (Table S1).The synthesis
DOI: 10.1039/D0TA03729J
method is represented in Figure 1.
Figure 1. Synthesis of Ni-CuWO4/g-C3N4
2.3 Activity test:
Catalytic experiments were done to investigate the activity of the photocatalysts for the
conversion of benzene to phenol. In a typical procedure, 20 mg of prepared Ni-CuWO4/g-
C3N4 was taken in a 10 mL round bottom flask. To this 1 mL benzene and 200 μL distilled
H2O were added and the reaction mixture was stirred in dark for half an hour to establish
adsorption-desorption equilibrium. 500 μL of 30% H2O2 was then added to the reaction
mixture and was stirred in presence of sunlight. The reaction was stopped after a certain
period of time and the catalyst was separated from the reaction mixture by centrifuge. The
biphasic system appeared to be colourless before the reaction and it changed to light
yellowish like colour in a single phase at the end of the reaction. The reactions were carried
out on sunny days from 10 am to 12 pm with an intensity of about 160 watt/m2. The liquid
products were analysed by using GC 700 series (Thermo Scientific) equipped with a FID
detector and capillary column (GC graph Figure S5). The GC oven temperature was set at
2500C for this sample and it was run for 4 mins. The required product was also further
1
confirmed by doing H NMR and 13C-NMR spectroscopy in a 500 MHz FT-NMR
spectrometer(Figure S4). The reaction was performed with a series of catalyst of Ni-
CuWO4/g-C3N4 having different mass ratio of Ni-CuWO4. But 0.2Ni-CuWO4/g-C3N4 catalyst
was found to give better result for this photocatalytic hydroxylation of benzene to phenol.
The reaction pathway is shown in Scheme 1.The effects of reaction time, peroxide amount,
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and light were also studied for this photocatalytic hydroxylation reaction by varying these
DOI: 10.1039/D0TA03729J
parameters and keeping constant amount of benzene and aqueous medium. Some of the
reactions were also carried out with molecular oxygen instead of using H2O2 under same
reaction conditions to study their effects on the reaction by bubling dioxygen (3 mL/min) to
the bottom of the reaction mixture for 30 minutes before irradiation.
OH
0.2 Ni-CuWO4/g-C3N4
1. 30% H2O2
2. H2O, Sunlight, 15 min
Benzene
Phenol
Conversion 98.5%
Yield: 81.5%
Scheme 1. Reaction pathway for photocatalytic hydroxylation of benzene to phenol
2.4 Catalyst characterisation:
The synthesized catalysts were characterised by PXRD (Powder X-ray Diffractometer), UV-
Visible spectrophotometer, FTIR spectrometer, SEM (Scanning Electron Microscope), TEM
(Transmission Electron Microscope), BET adsorption isotherm etc. The PXRD measurement
was carried out on a Rigaku, ultima IV X-ray diffractometer of 2θ range 2-800 using Cu K∞-
source of wavelength, λ= 1.54 Å. Nitrogen adsorption measurements were performed on
Autosorb-iQ (Quantachrome USA) adsorption analyser which measures adsorption isotherm,
specific surface area, pore volume mechanically. The samples were degassed at 2500C for 4
hrs. The FTIR spectra of the prepared catalysts were measured on SHIMADZU IR Affinity-1
spectrometer in the range of wave no 4000-400 cm-1 using the KBr pellet technique. The UV-
Visible spectroscopy of the synthesized catalysts was recorded on SHIMADZU UV-1800
spectrometer and the fluorescence spectroscopy was recorded on Horiba Scientific Flurolog 3
spectrometer to generalise the optical properties. For analysing the surface, scanning electron
microscopy images (SEM) were obtained Carl Zeiss SIGMA scanning electron microscope.
The compositional analysis of the sample was performed by Energy Dispersive X-ray
spectroscopy on Oxford EDS attached with the same instrument. TEM images were recorded
on JEOL, JEM-2100 Plus Electron Microscope. XPS analysis was performed on an X-ray
Photoelectron Spectrometer (ESCALAB Xi+, Thermo Fischer Scientific Pvt. Ltd., UK) using
monochromatised AlK∞ radiation. The reaction products were analysed with a Trace GC 700
series GC system (Thermo Scientific) equipped with a FID detector and a capillary column.
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DOI: 10.1039/D0TA03729J
1
13
The products formed were further confirmed by H and C NMR using 500 MHz FT-NMR
spectrometer.
2.5 Computational Study
3 X √3 and √3 X 3 single g-C3N4 layer containing 18 carbon and 24 nitrogen atoms were
used to match a (3 X 2) twelve atomic layer stoichiometric CuWO4 (010) surface slab
containing 108 atoms of W, Cu and O atoms. Density functional calculations with hybrid
functional PBE0 with relativistic small-core effective core-potential basis set of
Stuttgart/Dresden (SDD) were used for all atoms using the periodic boundary condition as
implemented in GAUSSIAN16 suite of program.42-44 The projected density of states (PDOS)
were calculated on the optimized geometries using 128 k-space for the Brillouin zone
interaction. Plots of the PDOS were obtained using a broadening energy parameter of 0.1 eV
using Multiwfn program code.45
3. RESULTS AND DISCUSSION
3.1 Catalyst characterisation:
The purity of phase and crystal structures of the as synthesized samples were analysed by
powder XRD technique as shown in Figure 2. Two well-defined diffraction peaks arefound in
the samples, which are the characteristic peaks of typical g-C3N4. The strongest peak at
around 27.40 depicts a characteristic interlayer stacking structure of conjugated aromatic
system triazine ring, indexes as the (002) crystal plane of the graphitic materials.46 The small
peak at 13.20 reflects the (100) plane of g-C3N4. Again the observed diffraction peaks at
15.40, 190, 28.70, 30.10, 31.50 corresponding to the planes (010), (100), (-1-11), (111), (020)
show the crystalline nature of CuWO4nanocompound confirming the formation of triclinic
phase of CuWO4 (JCPDS NO. 01-070-1732).47 After the addition of Ni2+ species the intensity
of the major diffraction peaks of CuWO4 undergo a slight weakening and broadening due to
incorporation of Ni2+ ion into CuWO4 crystal replacing some of the Cu2+ ions during the
synthesis process and thus implies that there is a close combination between Ni2+ species with
CuWO4. The lattice parameters of composite 0.2 Ni-CuWO4/g-C3N4 are also found to be less
than that of 0.2CuWO4/g-C3N4which may be due to substitution effect of Ni2+ into CuWO4.48
Moreover with the increase in mass ratio of Ni-CuWO4/g-C3N4 the intensities of the major
diffraction peaks of g-C3N4 are also found to be diminished depicting the formation of
nanocomposite with g-C3N4 (Figure S1).
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DOI: 10.1039/D0TA03729J
g-C N
3 4
0.2 Ni-CuWO /g-C N
4
3 4
0.2 CuWO /g-C N
4
3 4
(002)
Ni-CuWO
4
(100)
CuWO
4
(020)
(100)
(010)
20
40
2 (degree)
60
80
Figure 2. PXRD pattern of as synthesized catalysts
For the further confirmation of the formation of nanocomposite FTIR spectra of g-C3N4, pure
Ni-CuWO4 and Ni-CuWO4/g-C3N4 were studied and depicts in Figure 3. The intense band
within 1230-1650 cm-1 correspond to the typical stretching vibration mode of carbon nitride
heterocycles.49 The peak at 809 cm-1 is associated with the characteristic mode triazine unit
indicating the presence of typical structure of g-C3N4. The broad bands at 3020-3660 cm-1 are
the evidence of the presence of NH or NH2 groups. For pure Ni-CuWO4, the absorption band
below 600 cm-1 indicates the deformation modes of W-O-W bridges or W-O in the WO4
tetrahedra.50 The band appearing at around 1400 cm-1 is associated with δ(OH) or υW-O
bond.51 The bands around 700-800 cm-1 can be attributed to the stretching of Cu-O bond.50
From the figure it is seen that the intensity of the FTIR spectrum of the as synthesized Ni-
CuWO4/g-C3N4 is found to be slightly decreased compared to pure g-C3N4 which confirms
that Ni-CuWO4nanocompound is chemically coordinated to the g-C3N4 support.
g-C N
3 4
Ni-CuWO
4
Ni-CuWO /g-C N
4
3 4
4000 3500 3000 2500 2000 1500 1000
Wavenumber (cm-1)
500
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DOI: 10.1039/D0TA03729J
Figure 3.FTIR spectra of as synthesized catalysts
The BET adsorption isotherm of pure g-C3N4, 0.2 Ni-CuWO4/g-C3N4 and 0.2 CuWO4/g-C3N4
catalysts are shown in the Figure 4. It is observed that pure g-C3N4 catalyst has a Brunauer-
Emmet-Teller (BET) surface area of 50 m2g-1 and a pore volume of 0.18 cm3g-1, while the
BET surface area of 0.2 Ni-CuWO4/g-C3N4 and 0.2 CuWO4/g-C3N4 are found to be 103.2
m2g-1 and 81.6 m2g-1 with pore volume 0.66 cm3g-1 and 0.42 cm3g-1 respectively (Table 1).
The surface area of Ni-CuWO4/g-C3N4is found to be higher compared to CuWO4/g-C3N4 and
pure g-C3N4 implying that incorporation of Ni2+ into CuWO4 leads to increase in surface area
of Ni-CuWO4. The increase in surface area of Ni-CuWO4/g-C3N4with respect to bare support
isdue to doping effect and the creation ofnew pores due to an effective diffusion of Ni-
CuWO4 species in the g-C3N4 support during the preparation method which causes increase in
surface roughness without clogging pores.52
g-C3N4
0.2 Ni-CuWO4/g-C3N4
0.2 CuWO4/g-C3N4
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0)
Figure 4. Nitrogen adsorption-desorption isotherms of the as prepared samples.
Table 1. BET surface area analysis of the synthesized catalysts
Catalyst
BET surface area
(m2g-1)
50.2
Pore volume (cm3g-1)
Pore radius
(nm)
1.8
g-C3N4
0.2 CuWO4/g-C3N4
0.2 Ni-CuWO4/g-C3N4
0.18
0.42
0.66
81.6
103.2
1.9
1.9
Figure 5a illustrates the UV-Visible absorption abilities of pure g-C3N4, 0.2 NiWO4/g-C3N4,
0.2 CuWO4/g-C3N4, pure Ni-CuWO4 and 0.2 Ni-CuWO4/g-C3N4 composites. From the
Figure 5a it is observed that pure g-C3N4 possesses a blue shifted absorption edge at around
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450 nm which is associated to the n-π*electronic transition of lone pair electrons oVfiewNAr-ticle Online
DOI: 10.1039/D0TA03729J
atoms.35 It can be observed that the UV-Visible absorption capacity of the as prepared
composites are higher than that of pure g-C3N4. The energy band gaps of the synthesized
materials estimated via a Tauc plot based on UV-Visible absorption results are shown in the
Figure 5b. The Tauc plot is based on the following formula:
αhυ= A(hυ-Eg)n/2
Where αis the absorption coefficient, h is the Planck’s constant, A is a constant, υis the
frequency of light, Eg is band gap energy, and the n value is determined by the type of optical
transition of semiconductor (n=1 and 4) with direct or indirect band characteristics.53 The
energy gap of the composite Ni-CuWO4/g-C3N4 (2.2 eV) isfound to be narrower than that of
composite CuWO4/g-C3N4 (2.5 eV) as evident from the Figure 5b. The incorporation of Ni2+
ioninto CuWO4 crystal may affect the band edge positions of CuWO4 i.e. top of CB and
bottom of VB helping in generating an intermediary level between CB and VB of the
CuWO4/g-C3N4 which causes decrease in band gap of Ni-CuWO4/g-C3N4 and enhances its
visible light absorption capacity.54 Owing to narrow band gap energy the photocatalysts Ni-
CuWO4 and g-C3N4 upon irradiation of sunlight absorbs incident visible light which helps in
transfer of electron from VB to CB creating holes in its VB. As a Z-scheme material, the
photoinduced electrons in the CB of Ni-CuWO4 tends to recombine with the photogenerated
holes in the VB of g-C3N4 maintaining electron in the CB of g-C3N4 and holes in the VB of
Ni-CuWO4. This process helps in minimizing the fast recombination of electron and hole
pairs which intensifies the optical property of Z-scheme narrow band gap semiconductor
composites like Ni-CuWO4/g-C3N4.
The photoluminescence property of the synthesized materials g-C3N4, 0.2 NiWO4/g-C3N4, 0.2
CuWO4/g-C3N4 and 0.2 Ni-CuWO4/g-C3N4 composites are shown in the Figure 5c. A
compound having a lower PL intensity helps in better charge separation of photoinduced
holes and electrons which is observed from the Figure 5c.35 The g-C3N4 has a strong emission
peak at around 450 nm indicating that there is fast recombination of photogenerated holes and
electron pairs in g-C3N4 which lowers its photoactivity to some extent. On the other hand, it
can be said that combination of g-C3N4 with other material by forming a heterojunction helps
in enhancing the charge separation efficiency of photogenerated holes and electrons. Again,
the PL intensity of composite 0.2 Ni-CuWO4/g-C3N4 is found to be the least amongst the all
composites like 0.2 CuWO4/g-C3N4, 0.2 NiWO4/g-C3N4. This may be due to substitution of
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Ni2+ into CuWO4 which helps in creating an intermediary level between CB DaOnI:d10.V103B9/Do0fTA03729J
CuWO4 thus minimizing the recombination process of electron and hole pairs of the material
Ni-CuWO4/g-C3N4 and enhanced its phocatalytic activity.
To further investigate the charge transfer properties of the as synthesized materials, time
resolved PL spectra of pure g-C3N4 and the composite 0.2 Ni-CuWO4/g-C3N4 were also
performed and is depicted in Figure 5d. Generally, the shorter the life time, faster is the
recombination of photogenerated electron and hole pairs; on the contrary, longer life time is
associated with the lower recombination i.e. fast separation of electron-hole pairs.55 It is
observed from the decay curves that the composite 0.2 Ni-CuWO4/g-C3N4 had longer life
time compared to its support g-C3N4. The reason for prolonged life time of the composite
may be due to interfacial electron transfer with g-C3N4 indicating the formation of
heterojunction with g-C3N4 thereby improving its photocatalytic activity.
g-C3N4
(b)
0.2 NiWO4/g-C3N4
0.2 Ni-CuWO4/g-C3N4
Ni-CuWO4
(a)
0.2 Ni-CuWO4/g-C3N4
0.2 CuWO4/g-C3N4
0.2 NiWO4/g-C3N4
Ni-CuWO4
0.2 CuWO4/g-C3N4
g-C3N4
400
500
Wavelength (nm)
600
700
2.0
2.5
Energy (eV)
3.0
3.5
0.2 CuWO4/g-C3N4
g-C3N4
0.2 Ni-CuWO4/g-C3N4
(d)
(c)
g-C3N4
0.2 NiWO4/g-C3N4
0.2 Ni-CuWO4/g-C3N4
450
500
Wavelength (nm)
550
600
40
60
80
100
Time (ns)
Figure 5.(a) UV-DRS spectra, (b) Tauc plot, (c) PL spectra and (d) life time spectra of as
synthesised catalysts.
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The thermal stability of the synthesized materials g-C3N4 and 0.2 Ni-CuWO4/g-C3N4 was
DOI: 10.1039/D0TA03729J
examined by thermogravimetric analysis (TGA) under N2 atmosphere with a heating rate of
100C/minat temperature range ambient to 8000C (Figure 6). The g-C3N4 is stable up to 5500C
under N2 atmosphere and rapid weight loss of the material is observed 5500C which can be
attributed to the decomposition of g-C3N4. On the other hand, the composite 0.2 Ni-
CuWO4/g-C3N4 shows its thermal stability up to 4900C which is lower than that of pure g-
C3N4. The initial decomposition of the composite starts at 4900C and the complete
decomposition occurs at 6150C indicating the combustion of g-C3N4 in the composite under
N2 atmosphere. The residual content of Ni-CuWO4in the composite Ni-CuWO4/g-C3N4 can
be easily calculated from the weight remainder after heating the sample up to 8000C and is
found to be ∽9.8%. This may be due to the absorption and activation of atmospheric oxygen
on the catalyst considering the oxidative property of the material.56
g-C N
3 4
120
100
80
60
40
20
0
0.2 Ni-CuWO /g-C N
4
3 4
-20
100
200
300
400
500
600
700
800
0
Temperature ( C)
Figure 6. TGA curves of pure g-C3N4 and composite 0.2 Ni-CuWO4/g-C3N4
The as synthesized 0.2 Ni-CuWO4/g-C3N4 was further investigated by XPS analysis, which is
shown in Figure 7. The XPS data shows the presence of C, N, O, Cu, Ni and W on the
surface of the catalyst and their concentration were 42.1%, 41.8%, 11.5%, 1.7%, 1.1% and
1.8% respectively. The photoelectron peaks of these elements are distinctly observed from
the XPS spectra at binding energies of 286.9 eV (C1s), 398.1 eV (N1s), 530.2 eV (O1s),
935.1 eV (Cu2p), 874.8 eV (Ni2p) and 35.4 eV (W4f) respectively (Figure 7a). The high
resolution C1s spectra (Figure 7b) can be fitted into several C species with different binding
energies of 283.7 eV and 287.1 eV, corresponding to graphitic carbon (C-C) and sp2-bonded
carbon (N-C=N) respectively.57 The high resolution N1s spectra (Figure 7c) for 0.2 Ni-
CuWO4/g-C3N4 exhibits three N states including triazine rings, C=N-C (397.5 eV), tertiary
12

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nitrogen, N-(C)3 (399.3 eV) and the amino functions, N-H (402.8 eV) which aDreOI:t1h0e.10t3h9/rDe0eTA03729J
units that constituted the heptazine ring of g-C3N4.58 It infers that the framework of g-C3N4is
not altered with the loading of Cu, Ni and tungsten species. There are two main peaks of O1s
with binding energies of 529.5 eV, 531.4 eV and 533.1 eV as shown in Figure 7d. The peak
at 529.5 eV can be assigned to the lattice oxygen of Ni-CuWO4. Another peak at 531.4 eV
indicates the presence of oxygen vacancies in the photocatalyst and the peak at 533.1 eV
corresponds to the oxygen of –OH in water molecules which may be due to loosely bound
moisture on the surface of the solid.59 The higher resolution Cu2p XPS spectra are shown in
Figure 7e. The Cu2p3/2 and Cu2p1/2 are observed at 933.4 eV and 953.2 eV respectively and
these doublet peaks with ratio 1:2 are occurred due to spin orbit coupling. Because of the
inelastic scattering of the core level electrons strong shake-up peaks are observed at 940.1 eV
and 942.4 eV for Cu2p3/2 and another one at 962.0 eV for Cu2p1/2 implying the presence of
Cu2+ species in the composite.60 As shown in Figure 7f, the Ni2p spectra mainly depicts the
peaks of Ni2p3/2 and Ni2p1/2 orbits owing to spin orbit coupling at binding energies of 855.2
eV and 873.4 eV respectively, as well as two shake-up satellite peaks mainly indicating the
presence of Ni2+ species.61 The deconvolutedhigh resolution XPS spectra of W4f (Figure 7g)
shows two main peaks of W4f at 34.5 eV and 36.4 eV correspond to W4f7/2 and W4f5/2
respectively as well as two satellite peaks at 35.4 eV for W4f7/2 orbit and 37.5eV for W4f5/2
confirming the presence of tungsten in W6+ state.62
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Page 14 of 39
N1s
C 1s
(c)
O1s
(b)
(a)
N1s
(d)
Cu2p
O1s
Ni2p
C1s
W4f
394
396
398
400
402
404
406
408
526
528
530
532
534
536
0
200
400
600
800
1000
1200
285
290
Binding energy (eV)
Binding energy (eV)
Binding energy (eV)
Binding energy (eV)
Ni2p
Cu2p
(f)
(e)
Ni2p3/2
W4f
(g)
W4f7/2
Satellite peak
Ni2p1/2
Cu2p3/2
Satellite peak
Cu2p1/2
W4f5/2
Satellite peak
30
32
34
36
38
40
42
44
850
860
870
880
930
940
950
960
Binding energy (eV)
Binding energy (eV)
Binding energy (eV)
Figure 7.(a) XPS survey, (b) C1s XPS spectra, (c) N1s XPS spectra, (d) O1s XPS spectra, (e) Cu2p XPS spectra, (f) Ni2p XPS spectra and (g)
W4f XPS spectra of catalyst 15% Ni-CuWO4/g-C3N4
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Journal of Materials Chemistry A
Morphological structures of pure g-C3N4 and 0.2 Ni-CuWO4/g-C3N4 are furtherDvOeI:r1i0f.1i0e3Vd9ie/wDbA0yrTtiAcl0e3O7n2li9nJe
SEM, TEM and HRTEM analysis. The SEM image of pure g-C3N4 sample shows closely
arranged staking of the conjugated aromatic system with some small scattered particles at the
surface which is depicted in the inset of Figure 8a. The reason may be due to the light layer
crack during the crystal growth.63 Figure 8b showed the Ni-CuWO4 particles appears as
clusters having polyhedral shapes. The elemental composition of this nanocomposite is
obtained from the EDX pattern of 0.2 Ni-CuWO4/g-C3N4 which confirms the presence of C,
N, O, Cu, Ni & W in the compound (Figure 8c). The TEM, HRTEM images of the pure g-
C3N4 and synthesized 0.2 Ni-CuWO4/g-C3N4 are shown in Figure 8d, 8e & 8f. Figure 8d
shows a TEM image of the synthesized pure g-C3N4 nanosheets. It is observed that the TEM
image showed free standing nanosheets with diameters of few nanometres. Figure 8e shows
an enlarged image of the composite 0.2 Ni-CuWO4/g-C3N4. Uniform distribution of
supported Ni-CuWO4 nanoparticles having polyhedral morphology was seen in the TEM
image. The HRTEM images of Ni-CuWO4/g-C3N4 along with pure CuWO4/g-C3N4 are also
depicted in Figure 8f and 8g. It can be observed from the HRTEM images that the lattice
spacing of (010) plane of CuWO4 in the Ni-CuWO4/g-C3N4 nanocomposite is somewhat
different from that of pure CuWO4/g-C3N4. This is because of the incorporation of Ni2+ into
CuWO4 crystal leading to disorder in the crystal. Besides this the HRTEM images also
revealed an integration heterojunction of Ni-CuWO4/g-C3N4 or CuWO4/g-C3N4 which
enabled the formation of Z-direct scheme enhancing the photocatalytic performance by
preventing the charge recombination of these materials. Furthermore, the selected area
electron diffraction (SAED) pattern is also shown in the inset of Figure 8f. The diffused rings
in the SAED pattern implied the formation of crystalline phases over the support due to
interaction of Ni-CuWO4 nanoparticles with the g-C3N4 support. The STEM image of the
composite 0.2 Ni-CuWO4/g-C3N4 is also shown in Figure 8h and EDX mapping images of
composite 0.2 Ni-CuWO4/g-C3N4 are depicted in Figure 9a-9f which also confirmed the
existence of C, N, O, Cu, Ni and W elements in the composite. The presence of lower content
of Cu, Ni and W elements in the composite can be ensured from the low intensity signal of
these elements.
15

Journal of Materials Chemistry A
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DOI: 10.1039/D0TA03729J
(b)
(a)
(d)
(e)
(f)
(g)
(h)
Figure 8. (a), (b) FESEM images of g-C3N4 and Ni-CuWO4 respectively, (c) EDX pattern of
composite 0.2 NCWCN, (d) and (e) TEM images of g-C3N4 and the composite 0.2 NCWCN
respectively, (f) and (g) HRTEM images of 0.2 NCWCN and 0.2 CWCN respectively, (h)
STEM image of composite 0.2 Ni-CuWO4/g-C3N4
16

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Journal of Materials Chemistry A
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DOI: 10.1039/D0TA03729J
(b)
(c)
(e)
(d)
(f)
Figure 9. (a) to (f) elemental mapping of composite 0.2 Ni-CuWO4/g-C3N4
3.3. Catalyst screening for photocatalytic hydroxylation of benzene to
phenol:
3.3.1 Effect of metal loading:
The activity of the as synthesized catalysts were investigated for the photocatalytic
hydroxylation of benzene to phenol under sunlight irradiation using H2O2(30% V/V) as an
oxidant in aqueous medium. The results are summarized in Table 2. The amount of benzene
was kept constant in all the reactions.
As predicted no benzene conversion is observed when the reaction was carried out without
any catalyst (entry 1). With pristine Ni-CuWO4 (NCW) 25.8% benzene conversion was
observed (entry 7). Again with pure g-C3N4 very poor conversion of benzene was obtained
with low yield of phenol (entry 10). In contrast, when the hydroxylation reaction was
conducted with g-C3N4 supported Ni-CuWO4 (Ni-CuWO4/g-C3N4) having different mass
ratio of Ni-CuWO4/g-C3N4, an increase in the conversion of benzene was observed (entries 2-
6). With 0.05 Ni-CuWO4/g-C3N4, 68.7% conversion of benzene is obtained with 70.2%
selectivity of phenol. However, the yield of phenol is not satisfactory (48.2%). The low yield
of phenol is observed because of the lower conversion of benzene and this may be due to
presence of less amount of active catalytic sites in the 0.05 mass ratio of Ni-CuWO4/g-C3N4.
But, it is observed that with the increase in the mass ratio of Ni-CuWO4/g-C3N4 from 0.1-0.4
17

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the conversion of benzene is also found to be enhanced from 82.6% to >99% (Table 1, entries
DOI: 10.1039/D0TA03729J
3-6).
Table 2.Photocatalytic hydroxylation of benzene to phenol over various catalystsa
Entry
Catalyst
Benzene
Phenol
Phenol yield (%)
conversion (%) selectivity (%)
1
2
No catalyst
0.05 NCWCN
0.1 NCWCN
0.2 NCWCN
0.3 NCWCN
0.4 NCWCN
Ni-CuWO4
0.2 NWCN
0.2 CWCN
g-C3N4
0
0
0
68.7
82.6
98.5
>99
>99
25.8
30.2
32.5
≈2
16.8
4.8
70.2
75.3
82.7
74.5
60.6
62.5
54.5
56.3
<1
48.2
62.2
81.5
73.2
60.3
16.1
16.5
18.3
-----
3
4
5
6
7
8
9
10
11b
12b
13b
0.2 NCWCN
Ni-CuWO4
g-C3N4
94.3
95.2
-----
15.8
4.5
-----
≈1
aReaction conditions: catalyst (20 mg), benzene (1 mL, 11.3 mmol), H2O (0.2 mL), H2O2 (0.5
mL), reaction time (15 min), sunlight, breactions performed with molecular oxygen
The best result is observed with 0.2 Ni-CuWO4/g-C3N4 which provided 98.5% conversion of
benzene with 82.7% selectivity and 81.5% yield of phenol for 15 minutes of reaction under
sunlight and is worth mentioning. However, with the increase in the mass ratio of the
composite Ni-CuWO4/g-C3N4 up to 0.4 a decrease in selectivity and yield of phenol are found
to be attained (entry 5, 6). This may be due to increase in the active catalytic sites with the
increase in the mass ratio of Ni-CuWO4/g-C3N4 produces more hydroxyl radicals. This
enables more benzene conversion and produced phenol, but the presence of excess amount
hydroxyl radicals further oxidizes phenol and produces other product like p-Benzoquinone.
Again with individual NiWO4/g-C3N4 (0.2 NWCN) and CuWO4/g-C3N4 (0.2 CWCN), lower
benzene conversion with low phenol yield are obtained (entries 8, 9). Thus Ni-CuWO4/g-
C3N4 (0.2 mass ratio of Ni-CuWO4/g-C3N4) is found to be the best catalyst for conversion of
benzene to phenol with high yield and this may be due to the incorporation of Ni2+ ions into
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CuWO4 crystal which leads to increase in the active catalytic sites in the composites for
DOI: 10.1039/D0TA03729J
photocatalytic performance.
3.3.2 Effect of catalyst amount:
The effect of 0.2 Ni-CuWO4/g-C3N4 catalyst amounts on the sunlight driven hydroxylation of
benzene to phenol is also surveyed and the results are depicted in Figure 10a. When the
reaction is carried out with 10 mg of catalyst 72.4% conversion of benzene with 92.5%
selectivity and 67% yield of phenol are found to be observed. With the increase of catalyst
amount upto 30 mg a gradual increment the conversion of benzene is observed and with 20
mg of catalyst the reaction shows its best result of 98.5% conversion of benzene with 81.5%
yield of phenol. However, further increasing the catalyst amount upto 30 mg a decrease in the
yield of phenol is observed. This is because of the formation of other product like p-
benzoquinone by the over oxidation of phenol. It can be narrated that with the increase in
catalyst amount in the reaction mixture would lead to increase in the active catalytic sites
which produced more hydroxyl radicals. This enhances the conversion of benzene molecules
and produces phenol, but the presence of excess amount hydroxyl radicals accelerates the
over oxidation of phenol and produces other product like p-benzoquinone. Thus 20 mg of
catalyst amount is found to be the appropriate amount for the production of high yield of
phenol.
115
% Benzene conversion
% Phenol yield
(a)
110
105
100
95
90
85
80
75
70
65
60
10
15
20
25
30
Catalyst amount (mg)
Figure 10a. Effect of catalyst amount in photocatalytic hydroxylation of benzene to phenol
under sunlight irradiation. Reaction conditions: catalyst from 10 mg to 30 mg, benzene (1
mL, 11.3 mmol), H2O (0.2 mL), H2O2 (0.5 mL), reaction time (15 min), sunlight
19

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3.3.3 Effect of reaction time:
DOI: 10.1039/D0TA03729J
Reaction time has significant influence on photocatalytic hydroxylation of benzene to phenol
and this is depicted in Figure 10b. It is clear from the figure that a lower benzene conversion
of 25.6% is observed with high selectivity of phenol, 95.2% when the reaction is carried out
for five minutes under natural sunlight. The low conversion also affects the yield of phenol
and was found to be a bit low (24.3%). However, with the subsequent increase in time from
10 minutes to 60 minutes, conversion of benzene is also found to be boosted from 60.2% to
>99% with increase in phenol yield upto 15 minutes. However the selectivity of phenol is
observed to be low and the reaction reaches its optimization condition with 98.5% conversion
of benzene and 82.7% selectivity and 81.5% yield of phenol after 15 minutes of reaction. It is
observed that with gradual increase in time yield of phenol is also found to be diminished.
This is due to more exposure to sunlight with increase in time which causes more H2O2
decomposition and produces more hydroxyl radicals and because of the surplus amount of
hydroxyl radicals some of phenol produced in the reaction gets over oxidized and produces
other hydroxylated benzene product. Thus these results indicate that phenol is more stable
under our optimized reaction condition i.e. for 15 minutes of reaction.
140
% Benzene conversion
% Phenol selectivity
% Phenol yield
(b)
130
120
110
100
90
80
70
60
50
40
30
20
10
0
10
20
30
40
50
60
Reaction time ( min)
Figure 10b. Effect of catalyst amount in photocatalytic hydroxylation of benzene to phenol
under sunlight irradiation. Reaction conditions: catalyst (20 mg), benzene (1 mL11.3 mmol),
H2O (0.2 mL), H2O2 (0.5 mL), reaction time from 5 to 60 minutes, sunlight
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3.3.4 Effect of H2O2 and molecular oxygen:
DOI: 10.1039/D0TA03729J
The effect of H2O2 in hydroxylation of benzene to phenol is also studied and this is
summarized in Figure 10c. It is observed that without H2O2 there is no benzene conversion
within 15 minutes. With lower amount of H2O2 (30% V/V) such as 100 μL, very low
conversion of benzene (15%) is observed with 95% selectivity of phenol. However, the yield
of phenol is not convincing (only 14.2%). When the reaction was carried out for 1h the
conversion was found to be increased to around 80.5% with phenol yield of 67.2%. With the
increase in the peroxide amount from 200 μL to 1000 μL gradual enhancement in the
conversion of benzene is marked upto >99%, but with a decrease in selectivity from 92.5% to
55.6%. The reaction shows its better result with 500 μL of H2O2 and is found to be optimized
with 98.5% conversion of benzene and with 81.5% phenol yield. The increase in phenol yield
with the increase in the amount of peroxide upto 500 μL is the indication of the presence of
more hydroxyl radicals in the reaction mixture. These hydroxyl radicals lead to more benzene
conversion by attacking benzene rings and produce more hydroxylated species of benzene
like phenol. However, when the peroxide amount is increased upto 1000 μL a decline in the
yield of phenol is happened to be noticed although the conversion of benzene is remarkably
high. This is because of the formation of other hydroxylated product p-Benzoquinone due to
over oxidation of phenol. Thus, it is worth mentioning that H2O2 has significant effects on
hydroxylation of benzene to phenol. The results of the reactions which were carried out with
molecular oxygen (dioxygen, O2) are illustrated in Table 2 (entries 11-13). It is observed
from the results that there is about 16.8% conversion of benzene with 0.2NCWCN
photocatalyst for 15 minutes of reaction (Table 2, entry 11) and the phenol yield is also in the
lower side. This can be explained on the basis of oxidising property of molecular oxygen.
Molecular oxygen is not as strong as H2O2 as an oxidising agent because of its lower
reduction potential as compared to H2O2. Additionally, molecular oxygen needs to form first
H2O2 to achieve the desired product phenol by capturing electron from the photocatalyst in a
prolonged way as discussed in the reaction mechanism (Figure 11b) thereby explaining the
role of oxygen in this photocatalytic hydroxylation.
21

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140
120
100
80
View Article Online
DOI: 10.1039/D0TA03729J
% Benzene conversion
% Phenol selectivity
% Phenol yield
(c)
60
40
20
0
0.0
0.2
0.4
0.6
0.8
1.0
Amount of H2O2 (mL)
Figure 10c. Effect of peroxide amount in photocatalytic hydroxylation of benzene to phenol
under sunlight irradiation. Reaction conditions: catalyst (20 mg), benzene (1 mL, 11.3 mmol),
H2O (0.2 mL), H2O2from 0.1 mL to 1 mL, reaction time (15 min), sunlight
3.3.5 Effect of light:
To study the effects of light some of the reactions were also executed in dark along with
sunlight irradiation. It is accessed from the GC results that no noticeable conversion of
benzene as well as phenol yield is obtained when the reactions are performed under dark
(Table 3, entries 1-5). The catalytic activity is found to be remarkably intensified with
exposure to sunlight. It is also monitored from the results that in dark only 2.4% conversion
of benzene is obtained with catalyst 0.2 Ni-CuWO4/g-C3N4 under optimised reaction
conditions (entry 3). The hydroxylation process seems to be enhanced with 98.5% conversion
of benzene when the reaction is performed in presence of sunlight irradiation. Under sunlight
irradiation the catalysts adsorbs sunlight and stimulates the decomposition of H2O2.
Consequently, large no. of hydroxyl radicals are generated which attack benzene molecules
and convert to phenol and other hydroxylated products. Thus it can be implied that light has
compelling influence in this hydroxylation reaction.
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Table 3. Effect of light in photocatalytic hydroxylation of benzene to phenol over various
DOI: 10.1039/D0TA03729J
catalystsa
Entry
Catalyst
In sunlight
Under dark
Benzene
Benzene
Phenol yield
(%)
conversion (%)
conversion (%)
<1
1
g-C3N4
…..
≈2
25.8
98.5
2
3
Ni-CuWO4
16.1
81.5
<1
≈2.4
<2
0.2 NCWCN
4
5
0.2 CWCN
0.2 NWCN
32.5
30.2
18.3
16.5
≈1
aReaction conditions: catalyst (20 mg), benzene (1 mL, 11.3 mmol), H2O (0.2 mL), H2O2 (0.5
mL), reaction time (15 min), reactions were performed under both sunlight and dark
3.3.6 Plausible reaction mechanism:
For mechanistic investigation, a series of trapping experiments were performed to explore the
.
effects of active species including OH, h+ and e- in the photocatalytic process. Therefore,
three different chemicals were chosen as scavengers for these trapping experiments and these
were isopropanol for .OH, EDTA for h+ and K2Cr2O7 for e-.
(a)
Initial
Isopropanol
EDTA
K2Cr2O7
140
120
100
80
60
40
20
0
0
10
20
30
40
50
60
Time (min)
Figure 11a. Photocatalytic hydroxylation of benzene to phenol with or without scavengers.
aReaction conditions: catalyst (20 mg), benzene (1 mL, 11.3 mmol), H2O (0.2 mL), H2O2 (0.5
mL), reaction time (15 min), scavengers (20 mmol).
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From the results depicted in Figure 11a it is observed that when isopropanol and EDTA are
DOI: 10.1039/D0TA03729J
used in the reaction system there is significant quenching in the photocatalytic activity,
indicating that .OH and h+ are the main active species. Thereby comparing the conversion of
benzene after inclusion of these scavengers the significance of these three types of species
follows a trend of .OH> h+> e-.
(b)
Figure 11b. Plausible mechanism of photocatalytic hydroxylation of benzene to phenol
Based on these results, a Z-scheme photocatalytic mechanism for Ni-CuWO4@g-
C3N4nanocomposite can be proposed and schematically depicts in Figure 11b. Under sunlight
irradiation both Ni-CuWO4 and g-C3N4 are excited and the photoinduced holes and electrons
are in their respective valance band (VB) and conduction band (CB). The energy band
positions of valence band (VB) and conduction band (CB) can be calculated by the following
equations:
ECB = χ―Ee― ½ Eg
EVB = ECB + Eg
Where X is the absolute electronegativity of the semiconductor, Eg is the energy of free
electrons on the hydrogen scale (4.5 eV) and Eg is the band gap of the semiconductor.64
According to this equation and DRS analysis, the CB and VB potentials of g-C3N4 can be
calculated to be -1.13 eV and 1.57 eV. Similarly, the CB and VB potentials of the compound
Ni-CuWO4 are found to be 0.51 eV and 2.41 eV respectively. It is seen from the Figure 11b
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Journal of Materials Chemistry A
that the CB and VB of Ni-CuWO4 is lower than that of g-C3N4. Again the energy differVeienwcAerticle Online
DOI: 10.1039/D0TA03729J
between the CB of Ni-CuWO4 and the VB of g-C3N4 is not too much. So, upon excitation,
the photoinduced electrons transferred from the CB of Ni-CuWO4 to the VB of g-C3N4
following a Z-scheme pathway thereby improving the separation process for photoinduced
electron-hole pairs. The photogenerated electrons on the CB of g-C3N4 can cause
.
decomposition of H2O2 under sunlight irradiation and produce OH- and OH radical. Besides
this, the electrons stored in the CB of g-C3N4 are captured by dissolved O2 molecules and
.-
produce O2 radicals which again react with H+ from surface water to form H2O2 and
.
subsequently produce OH radicals.65 Furthermore, the photoinduced holes in the VB of Ni-
.
CuWO4 also react with adsorbed water molecules to generate OH radicals. The generated
.OH radicals further attack the benzene molecules and thereby produce hydroxylated benzene
.
species in radical form. Upon further attacked by a OH radical, the hydroxylated benzene
radical is converted to our desired product phenol with H2O as only by-product. There is
supposed to be another minor route where surface holes react with benzene molecule to form
the corresponding intermediate which upon further reaction with dissolved oxygen molecule
produces our required product phenol.65
3.3.7 Structure Activity Correlation:
The geometry optimization, the g-C3N4 surface showed distortion due to the interaction of the
g-C3N4 surface with the CuWO4 (010) surface (Figure 12). The equilibrium distance between
the g-C3N4 surface and the top of the CuWO4 (010) surface is 2.215 Å. The calculated
equilibrium adhesion energy per formula unit of CuWO4 is -0.801 eV obtained from equation
1. The negative value of the adhesion energy suggests that the interaction of the g-C3N4
surface with CuWO4 (010) surface is stabilizing.
Ead = Ecomp – Eg-C3N4 – ECuWO4 (010) (1)
(a)
(b)
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Figure 12. Model for simulating the interface between g-C3N4-CuWO4 heterojunction for
DOI: 10.1039/D0TA03729J
geometry optimization (a) before optimization (b) after optimization.
To have a better understanding of the interaction of the g-C3N4 with CuWO4 surface, it is
important to study the interface electronic structure. Figure 13 shows the projected density of
states (PDOS) of pure g-C3N4 surface, g-C3N4-CuWO4 heterojunction surface and Ni doped
g-C3N4-CuWO4 heterojunction surface. The valence band maximum (VBM) of pure g-C3N4
is mainly composed of N 2p states while the conduction band maximum (CBM) has
contribution from C 2p and N 2p hybridized state. In case of g-C3N4-CuWO4 heterojunction,
the VBM is composed of N 2p and O 2p hybridized state while the CBM is mainly composed
of W 5d state. The involvement of W 5d state in the CBM of g-C3N4-CuWO4 heterojunction
may be the electronic factor towards the stability of the heterojunction. Moreover, there is a
decrease in the band gap in the g-C3N4-CuWO4 heterojunction compared to the pure g-C3N4
surface. The decrease in the band gap is more in the Ni-doped heterojunction (Figure 13c)
which is expected to increase the optical activity of the heterojunction. This decrease in band
gap may play a key role in optical excitation.
Figure 13. Projected density of states (PDOS) of (a) pure g-C3N4 surface, (b) g-C3N4-CuWO4
heterojunction surface and (c) g-C3N4-Ni-CuWO4 heterojunction surface. The dotted line
represents the Fermi level.
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We have also calculated the highest occupied surface crystal orbital (HOSCO),DHOIO: 10S.1C03O9/-D10T,A03729J
HOSCO-2 and the lowest unoccupied surface crystal orbital (LUSCO, LUSCO+1 and
LUSCO+2) at the Γ-point to further understand the origin of visible light absorption. Figure
14 shows the respective orbitals. The HOSCO is mainly composed of N 2p orbital, in
agreement with the PDOS analysis. The HOSCO-1 has contribution from N 2p and O 2p
orbitals. The contribution of electron density of g-C3N4 vanishes at HOSCO-2. The LUSCO
is composed mainly of W 5d orbitals. The normal photoexcitation of CuWO4 takes place
from O 2p orbital (HOSCO) to W 5d states. However, upon the composite formation with g-
C3N4, some low energy gap bands forms which may enhance electron-hole pair separation
leading to higher photocatalytic activity of the heterojunction. This was also confirmed from
the optical studies like UV-DRS analysis, Tauc plot and PL spectroscopic analysis. Thus, our
theoretical calculations provided a rationale at the microscopic level towards understand the
mechanism of charge transfer and higher photocatalytic ability of the prepared composites.
Figure 14. Γ-point orbital-isoamplitude surface of the respective orbitals.
27

Journal of Materials Chemistry A
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3.3.7 Comparative study:
DOI: 10.1039/D0TA03729J
A comparative study of the optimised catalyst 0.2 Ni-CuWO4/g-C3N4 was surveyed with
other previously reported catalysts for the photocatalytic hydroxylation of benzene to phenol
which is shown in the Table 4.
Table 4. Comparative study of different reported catalysts with our catalyst
Catalyst
Benzene
conversion
27.9%
Phenol
yield
Reaction conditions
Refer
ences
5
CuII-based MOF
27.1%
Catalyst-10 mg, benzene-1 mmol,
H2O , H2O2 (30%)-1.25 mmol,
600Celcius, 6 h, 40 W LED bulb
Catalyst-50 mg, benzene-0.8 mL (9
mmol), H2O-4 mL, CH3CN-4 mL,
H2O2 (30%)-0.51 mL (5 mmol), 4
h, 300 W xenon lamp
Ferrocene
modified carbon
nitride
Not
14.4%
6
known
Au-Pd/g-C3N4
Au@g-C3N4
Fe-g-C3N4
24%
6%
23.52%
3.42%
0.39%
Catalyst-10 mg, CH3CN-5 mL,
H2O2 (25%)-2 mL, 2h, 100 W
mercury lamp
22
22
31
Catalyst-10 mg, CH3CN-5 mL,
H2O2 (25%)-2 mL, 2h, 100 W
mercury lamp
4.8%
Catalyst-50 mg, benzene-0.8 mL (9
mmol), H2O-4 mL, CH3CN-4 mL,
H2O2 (30%)-0.51 mL (5 mmol), 4
h, 500 W xenon lamp
Fe-g-
11.9%
5.8%
2.46%
5.75%
Catalyst-50 mg, benzene-0.8 mL (9
mmol), H2O-4 mL, CH3CN-4 mL,
H2O2 (30%)-0.51 mL (5 mmol), 4
h, 500 W xenon lamp
31
65
C3N4/SBA-15
BCW-4
Catalyst-50
mg,
benzene-0.5
mmol), H2O- 0.1 mL, CH3CN-3
mL, O2-3 mLmin-1, 3 h, 300 W
xenon lamp
28

Page 29 of 39
Journal of Materials Chemistry A
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DOI: 10.1039/D0TA03729J
Cu2O-8/dG
Au/dG
30.18%
30.06%
99%
19.30%
0.46%
Catalyst-5 mg, benzene-1 mmol,
66
H2O-5mL, CH3CN-5mL, H2O2
(30%)-1mmol, 16 h, white LED
lamp (130 mWxcm2)
Catalyst-5 mg, benzene-1 mmol,
H2O-5mL, CH3CN-5mL, H2O2
(30%)-1mmol, 16 h, white LED
lamp (130 mWxcm2)
66
67
68
CuAg@g-C3N4
g-C3N4/Cu/Pd
Not known Catalyst-25 mg, benzene-1 mmol,
CH3CH2OH -5mL, CH3CN-5mL,
H2O2 (30%)-1.1 mmol, 30 min, 20
W domestic bulb
98.1%
87.8%
Catalyst-20 mg, benzene-0.5 mL,
CH3CN: H2O-1:1 , H2O2 (30%)-5
mmol, 1.5 h, solar simulator
Ni-CuWO4/g-
C3N4 (0.2)
98.5%
81.5%
Catalyst-20 mg, benzene-1 mL This
(11.3 mmol), H2O-0.2 mL, H2O2 work
(30%)-0.50 mL (4.5 mmol), 15
min, natural sunlight
3.3.8 Catalyst recyclability:
Apart from the high catalytic activity, stability of the catalyst is also very important factor for
the heterogeneous catalysis. To study the stability, recyclability of the 0.2 Ni-CuWO4/g-C3N4
catalyst was performed. The reusability of the photocatalyst in the hydroxylation of benzene
to phenol was evaluated under optimized reaction conditions (20 mg catalyst, 1 mL benzene,
0.2 mL H2O, 0.5 mL H2O2) in presence of sunlight. Being heterogeneous in nature the
catalyst could be easily separated from the reaction mixture after each run by centrifugation.
The recovered catalyst was then directly used in the next catalytic run after washing and
drying under the same reaction conditions. The results in Figure 15a demonstrates that the
conversion of benzene decreases very slightly through five consecutive run resulting in
95.2% conversion of benzene with 78.1% selectivity and 74.3% yield of phenol in the fifth
29