Simultaneous photocatalytic and catalytic activity of p–n junction NiO@anatase/rutile-TiO2 as a noble-metal free reusable nanoparticle for synthesis of organic compounds

Article abstract of DOI:10.1016/j.catcom.2017.02.023

Photocatalytic and catalytic activity of NiO loaded on mixture of anatase/rutile phases of TiO₂ nanoparticles were investigated for some organic reactions including selective oxidation of alcohols, synthesis of benzimidazoles and preparation of imines at room temperature. The p–n junction formed between NiO and TiO₂ as well as difference between band gaps in anatase/rutile phases facilitated electron transfer to TiO₂ and successfully promoted synthesis of compounds. The use of a reusable photocatalyst without noble metal co-catalysts, along with the mild reaction conditions and use of light energy, make this method as an environmentally benign and energy saving chemical procedure.

Full text of DOI:10.1016/j.catcom.2017.02.023

Accepted Manuscript
Simultaneous photocatalytic and catalytic activity of p–n junction
NiO@anatase/rutile-TiO2 as a noble-metal free reusable
nanoparticle for synthesis of organic compounds
Abolfazl Ziarati, Alireza Badiei, Ghodsi Mohammadi Ziarani,
Hamed Eskandarloo
PII:
DOI:
Reference:
S1566-7367(17)30074-2
doi: 10.1016/j.catcom.2017.02.023
CATCOM 4951
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
26 October 2016
24 January 2017
23 February 2017
Please cite this article as: Abolfazl Ziarati, Alireza Badiei, Ghodsi Mohammadi Ziarani,
Hamed Eskandarloo , Simultaneous photocatalytic and catalytic activity of p–n junction
NiO@anatase/rutile-TiO2 as a noble-metal free reusable nanoparticle for synthesis of
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for all authors. Please check if appropriate. Catcom(2017), doi: 10.1016/
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ACCEPTED MANUSCRIPT
Simultaneous photocatalytic and catalytic activity of p-n junction
NiO@anatase/rutile-TiO2 as a noble-metal free reusable
nanoparticle for synthesis of organic compounds
Abolfazl Ziaratia, Alireza Badiei*,a,b, Ghodsi Mohammadi Ziaranic, Hamed Eskandarlooa
aSchool of Chemistry, College of Science, University of Tehran, Tehran, Iran
bNanobiomedicine Center of Excellence, Nanoscience and Nanotechnology Research Center, University
of Tehran, Tehran, Iran
cDepartment of Chemistry, Faculty of Science, Alzahra University, Tehran, Iran
Corresponding author. Tel.: +98-21-61112614; Fax: +98-21-61112614
E-mail address: abadiei@khayam.ut.ac.ir (A. Badiei)
1

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Abstract
Photocatalytic and catalytic activity of NiO loaded on mixture of anatase/rutile phases of TiO2
nanoparticles were investigated for some organic reactions including selective oxidation of alcohols,
synthesis of benzimidazoles and preparation of imines at room temperature. The p-n junction formed
between NiO and TiO2 as well as difference between band gaps in anatase/rutile phases facilitated
electron transfer to TiO2 and successfully promoted synthesis of compounds. The use of a reusable
photocatalyst without noble metal co-catalysts, along with the mild reaction conditions and use of light
energy, make this method as an environmentally benign and energy saving chemical procedure.
Keywords: Heterogeneous catalysis, Photocatalysis; Titanium dioxide; Anatase/Rutile; Organic reactions;
Nanoparticles

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1. Introduction
Synthetic organic chemists pursuit towards novel reactions and chemo-selective transformations
under green, mild and energy saving conditions are a continuous process [1-3]. Almost a century ago, in
1912, Giacomo Ciamician understood that light is a renewable and sustainable energy source for
performing energy saving and green chemical reactions [4]. Since then, photochemistry has found wide
utility in synthesis of organic compounds. But, the lack of light absorption via many organic compounds
has limited the usage of photochemical synthesis. Therefore, employing light absorbing photocatalysts
and employing their electron-energy transfer operations to produce organic compounds to accomplish
required photochemical reactions would serve as an efficient tool for overcoming this barrier [5-8].
Among different types of semiconductors, titanium (IV) oxide is widely accepted as the best
photocatalyst. TiO2 has been broadly investigated lately as a potentially valuable material in the
photocatalytic organic reactions due to its strong oxidizing ability, suitable band gap, high resistance to
photo corrosion and chemical corrosion, low price and easy preparation [9-11]. But, the high
recombination rate (about 90%) of photon induced electron-hole pairs reduces the photocatalytic
efficiency of TiO2 [12].
Previous reports have indicated that the physico-chemical factors of titanium dioxide materials,
such as surface area, particle sizes and phase compositions, strongly influenced the photocatalytic
efficiencies by restraining the charge recombination [13]. Especially, the crystallite phase dependent
synergistic effect is often encountered in titanium dioxide photocatalysts, in which blended phases of
rutile and anatase have been published to display higher efficiencies than their pure anatase compositions
[14-18] However, photocatalytic activity of pure TiO2 especially in organic synthesis is low and its good
performance is mainly achieved in the presence of noble metal co-catalysts, such as Pt, Pd, Au, etc [19-
26]. To reduce the cost of organic synthesis, it is necessary to explore alternative co-catalysts based on
3

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transition metals. Among them, Nickel oxide (NiO) as a p-type co-catalyst, with an indirect band gap of
about 3.55 eV, is considered as the most effective choice because of its unique catalytic effect, low cost
and so on Refs [27-30]. As above mentions, to improve the catalytic and photocatalytic efficiency, the p-
type semiconductor NiO (hole-rich), can be coupled to an n-type semiconductor TiO2 (electron-rich)
which produce an n–p junction in the interface as a space charge region [31, 32]. This n–p junction along
with mixing of anatase and rutile phases on TiO2 occurs as an inner electric field by the equilibrium of
electron and hole diffusion, which improves the catalytic and photocatalytic activity.
As part of our continuing efforts to develop green synthetic methods [33-35], herein we wish to
report an energy saving and efficient protocol for the synthesis of benzimidazoles, imines and selective
oxidation of alcohols at room temperature using NiO@anatase/rutile-TiO2 as an efficient, cost-effective
and reusable photocatalyst (Scheme 1).
2. Experimental
2.1. Materials
All the chemical reagents used in our experiments were purchased from Merck and Sigma-Aldrich
and were used as received without further purification.
2.2. Preparation of NiO@anatase/rutile-TiO2 and NiO@anatase-TiO2 nanoparticles
NiO@Anatase/rutile-TiO2 and NiO@anatase-TiO2 NPs were prepared from a powder mixture of
the corresponding component solid oxides, according to the following steps. First, 1 g of TiO2 and NiO
powders were grounded thoroughly in an agate mortar. Then the mixed oxides were dispersed in 100 mL
boiling deionized water and sonicated for 5 min using a probe sonicator. After that the suspension
solution was stirred for 24 h and then dried in an air oven at 80oC for 12 h. Finally, the dried solids were

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calcined at different temperatures (see Fig. 2) for 1 h to prepare catalysts with anatase/rutile phases in
different ratios.
2.3. Photocatalytic activity of NiO@anatase/rutile-TiO2 in organic reactions
The photocatalytic reaction processes were carried out at room temperature in a batch quartz
reactor. Artificial irradiation was provided by a 55W (UV-C) mercury lamp (Philips, Holland) emitting
around 254 nm, positioned on top of the batch quartz reactor.
2.3.1. Selective oxidation of alcohols
The catalyst (3 mol%) was added to alcohol (5 mL) and dispersed by ultrasonication for 5 min.
Then the suspension was added to a batch quartz reactor and photo-irradiated under magnetic stirring for
appropriate times (see Table S1). After completion of reaction, the catalyst was recovered by
centrifugation. The crude products were determined by 1H-NMR and FT-IR.
2.3.2. Synthesis of benzimidazoles
The catalyst (3 mol%) was added to alcohol (5 mL) containing ortho-phenylenediamine (1 mmol)
and dispersed by ultrasonication for 5 min. Then the suspension was added to a batch quartz reactor and
photo-irradiated under magnetic stirring for appropriate times (see Table S2). After completion of
1
reaction, the catalyst was recovered by centrifugation. The crude products were determined by H-NMR,
FT-IR and melting point.
2.3.3. Synthesis of Imines
The catalyst (3 mol%) was added to alcohol (5 mL) containing corresponding imine (1 mmol) and
dispersed by ultrasonication for 5 min. Then the suspension was added to a batch quartz reactor and
photo-irradiated under magnetic stirring for appropriate times (see Table S3). After completion the
reaction, the catalyst was recovered by centrifugation. The organic solvent was removed under reduced

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1
pressure to afford the respective imines. The crude products were determined by H-NMR, FT-IR and
melting point.
3. Results and discussion
3.1. Preparation and characterization of NiO@anatase/rutile-TiO2 nanoparticle
NiO@anatase/rutile-TiO2 nanoparticles were prepared from a powder mixture of the corresponding
component solid oxides by using an impregnation method (Fig. S5) [28]. The synthesized materials were
characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM),
scanning electronic microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and Fourier
transform infrared spectroscopy (FT-IR).
(1)
Where FWHM (full-width at half-maximum or half-width) is in radians and θ is the position of the
maximum of diffraction peak, K is the so-called shape factor, which usually takes a value about 0.9, and λ
is the X-ray wavelength. Crystallite size of NiO@anatase/rutile-TiO2 nanoparticle has been found to be
42 nm.
Results in Fig. 2a show that changing the phase content of coupled sample critically depends on
calcination temperature. As confirmed by X-ray diffraction analysis in Fig. 2b, with increasing of
calcination temperature, rutile phase ratio increases and anatase phase ratio decreases, so that in the
sample calcined at 550oC crystalline phase is anatase and in the sample calcined at 850oC major
crystalline phase is rutile to be ∼54%. The (101) reflection (2θ = 25.28°) of anatase, the (110) reflection
(2θ = 27.42°) of rutile and the (200) reflection (2θ = 43.31o) of NiO were used for comparisons.

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The phase content of prepared nanoparticles was calculated by the following equation, where, IA is
integrated intensity of the anatase (101) diffraction peak and IR is integrated intensity of the rutile (110)
diffraction peak.34
(2)
o
HRTEM image of the NiO@anatase/rutile-TiO2 nanoparticles calcined at 650 C (contain 87:13
anatase:rutile ratio) with 30 wt% content of NiO are shown in Fig. 3a. The average particle size of the
TiO2 nanoparticles in mixed phases system is assessed to be approximately 50 nm, which is in
compromise with the average crystallite size determined from the XRD patterns. Furthermore, the small
NiO particles with average diameter of 10 nm were observed on the surface of TiO2 particles by the
HRTEM image.
The DRS spectra of pure NiO, pure TiO2, and optimized 30%NiO@anatase/rutile-TiO2 nanoparticles are
illustrated in Fig. S1 (Supporting Information). The values of band gap energy, via analogizing the linear
part of the spectra and determining its intersection with energy axis, were determined to be 3.55, 3.28,
and 3.31 eV, respectively. The attained results specified that with NiO coupling, the band gap energy of
pristine TiO2 increases by 0.05 eV, displaying a blue shift. So, the electron hole separation time is
comparatively longer in the NiO@anatase/rutile-TiO2 than pure TiO2.
The Raman spectra of 30%NiO@anatase/rutile-TiO2 is shown in Fig. S9 (Supporting Information). The
Raman peaks located at 198, 394, 514, 634 cm-1 correspond to the anatase phase and the peaks located at
240, 443, 610 cm-1 correspond to the rutile phase of TiO2. Moreover, the broad peak observed at 508 cm-1
corresponds to NiO [36, 37].
3.2. Photocatalytic activity of NiO@TiO2 nanoparticle in organic reactions

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In order to investigate the effects of this nano photocatalyst, catalytic and photocatalytic
performance of NiO@TiO2 were considered in the different organic reactions including synthesis of
benzimidazoles, synthesis of imines and selective oxidation of alcohols. In these reactions ethanol, aniline
and ortho-phenylenediamine were used as initial material for model reactions. To optimize the synthesis
conditions the NiO content of photocatalyst and ratio of anatase:rutile phases of TiO2 were investigated in
dark and light conditions.
Firstly to evaluate the effect of the NiO quantity on the catalytic and photocatalytic activity of
NiO@anatase/rutile-TiO2 NPs, Various contents of NiO were used in the preparation process. As shown
in Fig. S6 the product yields in all of the reactions were increased with the NiO weight ratio to 30 wt % in
coupled nanoparticles. Also the result of Fig. S7 is shown the optimum amount of the catalyst was (3
mol%) of NiO@anatase/rutile-TiO2 NPs which increasing of this amount did not show any significant
change in yield of reactions.
Subsequently, for comparison of photocatalytic activity in NiO@anatase-TiO2 and
NiO@anatase/rutile-TiO2, the reactions in the dark and light at room temperature are provided in Table 1.
Also, control experiments using pure anatase TiO2 as the catalyst was performed.
The results in table 1 show an interesting trend. For all reactions conducted in the dark, the
NiO@anatase/rutile-TiO2, NiO@anatase-TiO2 and pure anatase TiO2 didn’t exhibit substantial activity.
Moreover, the obtained results were indicated that in absence of catalyst, trace amounts of products were
generated. But the NiO@anatase/rutile-TiO2 NPs exhibited superior photocatalytic activity to both the
NiO@anatase-TiO2 and the pure anatase TiO2 NPs in the presence of light irradiations. As can be seen in
Table 1, the NiO@anatase/rutile-TiO2 NPs increased the product yield of the benzimidazoles to 98%
(from 80% using NiO@anatase-TiO2), and the yield of imines to 99% (from 81% using NiO@anatase-
TiO2), and the yield of benzyl alcohols to >99% (from 84% using NiO@anatase-TiO2).

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Finally, to achieve NiO@TiO2 photocatalyst with high catalytic and photocatalytic activity, we
have screened the effect of the rutile and anatase phases ratio in the catalytic and photocatalytic activity of
the NiO@anatase/rutile-TiO2 NPs in model reactions. Results in figure S8 show that an anatase/rutile
ratio of TiO2 with 87:13 ratio is the best choice for these reactions.
Therefore, the study was extended to prepare benzimidazoles, imines and selective oxidation of
benzyl alcohol using anatase/rutile-TiO2@30%NiO NPs with 87:13 of anatase/rutile ratio. The detailed
data of these three reactions are provided in Tables S1-S3.
The results in Tables S1-S3 show that, NiO@anatase/rutile-TiO2 under UV light irradiation at room
temperature is the suitable catalyst for maximizing the yield of these reactions. This catalyst promoted
selective oxidation of alcohols and synthesis of benzimidazoles and imines in high yields and the yields
were higher than those of obtained NiO@anatase-TiO2 under UV at room temperature.
The reasons of the excellent photocatalytic activity of NiO@anatase/rutile-TiO2 NPs may be the
phenomenon of p-n junction formed between NiO and TiO2 as well as difference between band gaps in
rutile and anatase phases. When NiO as a p-type hole-rich semiconductor is in contact with TiO2 as an n-
type electron-rich semiconductor, at p-n junction interfacial contact produces a space charge region. In n–
p junction interfacial contact the holes are diffused from TiO2 to NiO and electrons are diffused from NiO
to TiO2. On the other hands, the phase junction between anatase and rutile provides a synergistic effect,
easing effective electron-hole separation. As previous reports [14,38], a band alignment of about 0.4 eV
exists between rutile and anatase (with rutile possessing a lower electron affinity) enabling photo-
generated CB electrons flow from rutile phase to anatase phase. As result, a strong separation of electron-
hole pairs between the two phases as well as n–p junction between TiO2 and NiO improvement of the
photocatalytic activity.
3.3. Mechanism of Photocatalytic activity of the NiO@anatase/rutile-TiO2 in organic
reactions

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On the basis of the cases described above, the mechanism for these reactions on the
NiO@anatase/rutile-TiO2 photocatalytic is described by the following mechanism, as summarized in Fig.
4 and scheme 2 [15,16,39]. As shown in Fig. 4 the electron injection from NiO nanoparticles to rutile
accumulates electron in its conduction band and results in the negative shift of ECB. This promotes the
continuous electron transfer from the NiO particles to rutile and then to anatase and efficient O2 reduction
on the anatase surface. Therefore, the NiO@anatase/rutile joint active site promotes the reactions.
Mechanism of reactions at the joint active site is explained as in Scheme 2. (a) p-n junction of NiO
particles transfers electrons to the tightly bound rutile; then the electron is moved to well-conjugated
anatase; and O2 is reduced thereby electron (formation of O−O− species). (b) The O−O− species
probably attracts the H atom of the alcohol while the empty orbital on NiO adsorbs the lone pair of
oxygen in alcohol (c). Subsequent removal of the H atom from this species produces the aldehydes 1.
In the next step, the produced aldehydes 1 are activating by the Lewis acidity of NiO coupled with
TiO2 and then condensation with ortho-phenylenediamine 2 or imines 3 takes place to afford
corresponding benzimidazoles 4 or imines 5.
3.4. Recyclability Study of NiO@anatase/rutile-TiO2 photocatalyst
The activity and stability of the catalyst was assessed by reusing and recycling experiments for the
synthesis of benzimidazoles, imines and selective oxidation of alcohols under optimized reaction
conditions and the results are shown in Fig. 5. After the experiment, the NiO@TiO2 photocatalyst was
separated from the reaction mixture by centrifugation, and washed with hot ethanol for three times to
remove adsorbed organic impurities from its surface. The recovered photocatalyst was dried at 100 °C for
3 h and reused for the next runs under optimized reaction conditions. It was found that the
NiO@anatase/rutile-TiO2 photocatalyst could be recycled and reused for six times without significant loss
in its photocatalytic and catalytic activity and its selectivity, as depicted in Fig. 5.
4. Conclusions

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In this study tandem photocatalytic and catalytic activity of NiO@anatase/rutile-TiO2 nanoparticles
were investigated for the synthesis of some organic compounds including aldehydes, benzimidazoles and
imines under UV-light irradiation. The photocatalytic activity strongly depends on the loadings of NiO on
the TiO2 (the amount of NiO was optimized to be 30 wt%) and the content of anatase/rutile phases in
TiO2 (the optimized content of anatase:rutile phases was 87%:13%). The significant advantages of this
photocatalytic process over the previous reported processes for these organic reactions are: [18-24] (i)
noble-metal free and inexpensive heterogeneous catalyst (NiO@TiO2), (ii) reusable and recyclable
catalyst (six times), (iii) safe and inexpensive reducing agent (alcohol), (iv) wide ranges of substrates
(benzylic and aliphatic alcohols) and (v) mild reaction conditions (room temperature and atmospheric
pressure). This method therefore, may help open a new strategy toward the design of more effective
photocatalysts and the creation of new methods for photocatalysis based green organic synthesis.
Acknowledgments
The authors thank University of Tehran for the support of this work.
Appendix A. Supplementary data
Supplementary data associated with this article can be found in the online version.
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Table 1. Comparison of the photocatalytic activity in organic reactions
Yield of reactions (%)
Reaction
Light a
Without TiO2 NiO@anatase NiO@anatase
catalyst
trace
-TiO2
83
/ rutile-TiO2
>99
UV
48
21
31
14
1) Selective oxidation of alcohol
O
OH
cat.
H
Dark
UV
0
trace
0
25
80
22
26
98
23
R.T. ; Air, I atm
2) Synthesis of benzimidazole
NH2
N
cat. ,
OH
NH2
Dark
R.T. ; Air, I atm
N
H
UV
trace
0
34
17
81
23
99
25
3) Preparation of imine
NH2
N
cat. ,
R.T. ; Air, Iatm
OH
Dark
aThe data of the reactions in the dark are provided for comparison
N
R1
NH2
N
H
NH2
OH
O
anatase/rutile-TiO2@NiO
UV photo-irradiation
R
H
R1
N
H
1 H
R2
NH2
R1
R2
Scheme 1. Photocatalytic and catalytic activity of the NiO@anatase/rutile-TiO2 in organic reactions
15

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Scheme 2. Proposed mechanism for the synthesis of benzimidazole, imines and aldehydes promoted by
an NiO@anatase/rutile-TiO2 under photo-irradiation

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Fig. 1. XRD patterns of TiO2, NiO and NiO@anatase/rutile-TiO2 nanoparticles
Fig. 2. Anatase and rutile phase content of NiO@TiO2 in different calcination temperature

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Fig. 3. (a) HRTEM image of the NiO@anatase/rutile-TiO2 (the arrows indicate NiO nanoparticles); (b)
SAED pattern of the NiO@anatase/rutile-TiO2 nanoparticles
Fig. 4. Schematic of the valence and conduction band alignment mechanism for anatase–rutile interface
coupled by NiO

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Fig. 5. (a) Reuseability Study of NiO@anatase/rutile-TiO2 photocatalyst in organic reactions. (b) XRD
patterns of photocatalyst before used and after six times reused.

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Graphical Abstract
A noble-metal free reusable NiO@anatase/rutile-TiO2 nanoparticles photocatalyzed
selective synthesis of organic compounds under UV-irradiation at room temperature
Highlights
 NiO coupled with anatase/rutile phases of TiO2 were prepared by impregnation route
 The p-n junction NiO@anatase/rutile-TiO2 was employed as efficient photocatalyst
 This nanostructure exhibited superior photocatalytic activity in organic reactions
 30%NiO coupled with 87:13 of anatase:rutile phases ratio showed highest activity
 The activity of this nanocatalyst was nearly the same after six times of recycling