Phase controlled synthesis of bifunctional TiO2 nanocrystallites: Via d-mannitol for dye-sensitized solar cells and heterogeneous catalysis

Article abstract of DOI:10.1039/d0ra01366h

The crystal architecture of TiO₂ was successfully tailored via a low-temperature (≤200 °C) hydrothermal process in the presence of d-mannitol for feasible applications in dye-sensitized solar cells (DSSCs) and heterogeneous catalysis. In the development of anatase-TiO₂ (A-TiO₂), d-mannitol does not merely acts as a complexing agent to manage the zigzag chains of octahedral TiO₆^2- with dominant edge sharing but also performs as a capping agent by influencing the hydrolysis process during nucleation, as confirmed by Fourier-transform infrared spectroscopy and dynamic light scattering studies. After physical measurements, the as-synthesized nanocrystallites (NCs) of A-TiO₂ were used in DSSCs, where a fascinating power conversion efficiency (PCE) of 6.0% was obtained, which showed excellent performance compared with commercial anatase-TiO₂ (CA-TiO₂: 5.7%) and rutile-TiO₂ (R-TiO₂) obtained without d-mannitol (3.7%). Moreover, a smart approach was developed via the A-TiO₂ catalyst to synthesize pharmaceutically important C-3 alkylated 4-hydroxycoumarins through different activated secondary alcohols under solvent-free, and heat/visible light conditions. In addition, the catalytic activity of the so-produced A-TiO₂ catalyst under solvent-free conditions exhibited remarkable recyclability with up to five consecutive runs with negligible reduction, which is superior to existing reports, and clearly reveals the novelty, and green, sustainable nature of the as-synthesized A-TiO₂ catalyst. A plausible reaction mechanism of both coupling partners was activated through the interaction with the A-TiO₂ catalyst to produce valuable C-3 alkylated 4-hydroxycoumarins with 95% yield and high selectivity.

Full text of DOI:10.1039/d0ra01366h

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PAPER
Phase controlled synthesis of bifunctional TiO2
nanocrystallites via D-mannitol for dye-sensitized
solar cells and heterogeneous catalysis†
Cite this: RSC Adv., 2020, 10, 14826
a
a
Abdullah M. Al-Enizi,
T. A. J. Siddiqui,b Shoyebmohamad F. Shaikh,
a
b
Ayman Yousef,c Rajaram S. Mane
and Abu ul Hassan Sarwar Rana
*
*
Mohd Ubaidullah,
d
*
The crystal architecture of TiO2 was successfully tailored via a low-temperature (#200 ꢀC) hydrothermal
process in the presence of D-mannitol for feasible applications in dye-sensitized solar cells (DSSCs) and
heterogeneous catalysis. In the development of anatase-TiO2 (A-TiO2), D-mannitol does not merely acts
2ꢁ
as a complexing agent to manage the zigzag chains of octahedral TiO6 with dominant edge sharing
but also performs as a capping agent by influencing the hydrolysis process during nucleation, as
confirmed by Fourier-transform infrared spectroscopy and dynamic light scattering studies. After
physical measurements, the as-synthesized nanocrystallites (NCs) of A-TiO2 were used in DSSCs, where
a fascinating power conversion efficiency (PCE) of 6.0% was obtained, which showed excellent
performance compared with commercial anatase-TiO2 (CA-TiO2: 5.7%) and rutile-TiO2 (R-TiO2)
obtained without D-mannitol (3.7%). Moreover, a smart approach was developed via the A-TiO2 catalyst
to synthesize pharmaceutically important C-3 alkylated 4-hydroxycoumarins through different activated
secondary alcohols under solvent-free, and heat/visible light conditions. In addition, the catalytic activity
of the so-produced A-TiO2 catalyst under solvent-free conditions exhibited remarkable recyclability with
up to five consecutive runs with negligible reduction, which is superior to existing reports, and clearly
reveals the novelty, and green, sustainable nature of the as-synthesized A-TiO2 catalyst. A plausible
reaction mechanism of both coupling partners was activated through the interaction with the A-TiO2
catalyst to produce valuable C-3 alkylated 4-hydroxycoumarins with 95% yield and high selectivity.
Received 12th February 2020
Accepted 27th March 2020
DOI: 10.1039/d0ra01366h
rsc.li/rsc-advances
photovoltaic cells, and photocatalysis in the last few decades.2 For
instance, A-TiO2 is largely preferred in photovoltaics and photo-
Introduction
catalysis applications over the other two polymorphs, due to its
abundance, environmental friendliness, low cost, excellent elec-
tron mobility, catalytic activity, and availability of several
cleanroom-free chemical/physical synthesis methods. Generally,
the higher performance of the A-TiO2 over R-TiO2 because of its
maximum dye-loading capacity and superior incident photocur-
rent conversion efficiency. Moreover, the high refractive index and
UV absorptivity of R-TiO2 make it a potential candidate in the
application of optical communication devices.3 The crystal phase
transition of TiO2 polymorphs using simple, cost-effective, and
low-temperature solution-based methods with different
morphologies has attracted considerable research interest for
technologically important and practical applications. Normally,
peptization, a modier/surfactant/chelating agent, capping agent,
cohesive agent, and annealing temperature play crucial roles
during the phase transition pathways of these TiO2 polymorphs, in
addition to intrinsic parameters for instance pore size, concen-
tration of the Ti precursor, energy on the surface, aggregation rate,
pH value of solution, and crystal growth dynamics.4,5 Many
solution-based methods have been reported for the synthesis of
Titanium dioxide (TiO2) mainly exists in three polymorphs:
anatase (tetragonal), i.e., A-TiO2; brookite (orthorhombic), and
rutile (tetragonal), i.e., R-TiO2; these are regarded as different
orientations of the shared edges/corners of the [TiO62ꢁ] octa-
hedra.1 The TiO2 polymorphs play an important role due to their
outstanding electrical, optical, and catalytic properties, as well as
their chemical stability under both acidic and alkaline conditions,
which have enabled their applications in energy storage,
aDepartment of Chemistry, College of Science, King Saud University, Riyadh 11451,
Saudi Arabia. E-mail: mtayyab@ksu.sa
bSwami Ramanand Teerth Marathwada University, Nanded-431606, MS, India.
E-mail: rajarammane1970@srtmun.ac.in
cDepartment of Mathematics and Physics Engineering, Faculty of Engineering at
Mataria, Helwan University, Cairo 11718, Egypt
dDepartment of Intelligent Mechatronics Engineering, Sejong University, Seoul, South
Korea. E-mail: rana@sejong.ac.kr
† Electronic supplementary information (ESI) available: FTIR, X-ray diffraction
pattern, Raman analysis, FE-SEM images, HR-TEM, UV-Vis, JV, IPCE,
electrochemical impedance spectroscopy, BET analysis NMR spectral data etc.
NMR spectral data etc. See DOI: 10.1039/d0ra01366h
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different TiO2 polymorphs such as solvothermal, wet chemical, ethanol (96%), and a-terpineol (90%) were purchase from
and sol–gel.6 Among these, hydrothermal synthesis is used to Sigma Aldrich company. Acetonitrile (99.8%), tert-butanol
obtain phase homogeneity and morphological control at low (99.5%), and ethyl cellulose were purchased from Alfa Aesar.
temperatures.7 Zhou et al. reported the hydrothermal crystalliza- The N-719 dye (Ruthenium-535, bis TBA) and iodide electrolyte
tion process for the synthesis of TiO2 via a dissolution–crystalli- (Iodolyte AN-50), were commercially available and used as
zation mechanism.8 At a specic temperature, hydrated Ti ions received.
begin to dissolve to produce Ti4+ hydrated octahedral monomers,
which, eventually, form polymers via a condensation reaction.
Consequently, connection with octahedral monomers, the
Hydrothermal synthesis of A- and R-TiO2
formation and growth of the TiO2 nuclei are initiated.9 Thus, the
architecture of Ti4+ hydrated octahedral monomer and nucleation
process under hydrothermal conditions are major components
involved in the fabrication of TiO2 polymorphs. Recently, Oskam
et al. reported the nucleation and growth mechanism of different
TiO2 polymorphs by applying precursor chemistry, which was
mainly dependent on the utilization of reactants.10 Joo et al.
studied the crystal structures of TiO2 polymorphs that are
controlled from rutile to anatase without and with D-sorbitol
acting as a complexing agent.11 Yanqing et al. examined the impact
of the TiCl4 concentration and pH of solution on nucleation
processes of TiO2 polymorph.12 Until now, the formation of TiO2
polymorphs with a denite crystal structure, shape, surface
orientation, and nucleation process under hydrothermal condi-
tions remain uncharted; thus, the correlation between crystal
structure formation, and the hydrolysis ratio needs to be investi-
gated for various applications.13
The NCs of A- and R-TiO2 polymorphs were produced by
a simple and elegant one-step hydrothermal route. Typically, for
the A-TiO2 NCs, 1 M TiCl4 and 2.5 g of D-mannitol were dissolved
in DI-water water (DI, Milli-Q water; 18.2 MU cm). The reaction
mixture was transferred to high pressure Teon reactor and
kept at constant temperature of 180 ꢀC for 24 h. The as-procured
yellowish-white precipitate was separated on centrifugation at
4000 rpm for 10 min, followed air annealing at 550 ꢀC for 30 min
to get white powder of A-TiO2NCs (Scheme 1). A similar exper-
imental condition was applied for the synthesis of R-TiO2NCs,
except there was no D-mannitol in the precursor solution. A
100% A-TiO2 paste of ꢂ20 nm sized NCs, named CA-TiO2, was
purchased from ENB Korea and used without any modication
as the reference.
Fabrication of the DSSCs
A uorine-tin-oxide substrate (FTO, TEC15, Pilkington, 15 ohm/
sq.) was sonicated through detergent in DI-water, acetone, and
isopropanol for 20 min, in sequence, and dried under nitrogen
gas ow, followed by 15 min ultraviolet–ozone treatment to
remove presence of any unwanted residues. Typically, the TiO2
paste was made by mixing of 1.0 g of TiO2 NCs, 2.0 g of a-
terpineol, and 0.3 g of ethyl cellulose in 5.0 mL of ethanol with
continuous stirring to obtain homogeneous paste for 6 h.
Subsequently, the as-prepared NCs of A-, R-, and CA-TiO2 pastes
were coated on the FTO substrate via a simple doctor-blade
In the present work, we successfully synthesized pure phase A-
TiO2 NCs via a facile, low-temperature, and economical hydro-
thermal method in the presence of D-mannitol, which play role of
both capping as well as complexing agent. The D-mannitol was
selected due to its non-toxic biological origin, environmental
friendliness, low-cost, capping ability, and capability to form an
intermediate complex. The possible reaction mechanism to the
formation of A-TiO2 is based on the different orientation of shared
2ꢁ
corners and edges of the TiO6
octahedral. The nucleation
process of TiO2 has been derived by [Ti(OH)x(D-mannitol)yClz]nꢁ
,
where, 4 # x + y + z # 6 during the hydrolysis process, and the
relationship between the crystal structure formation and hydro-
lysis process has been explored. Furthermore, for comparison, we
conrmed that the reactions that proceed without D-mannitol
result in a phase of pure R-TiO2 rather than A-TiO2. The effect of
the hydrothermal reaction conditions on the crystal architecture
formation as well as the photovoltaic and photocatalytic proper-
ties was investigated, where A-TiO2, synthesized using D-mannitol,
exhibited an enhanced DSSCs performance compared to that of
the R-TiO2 prepared without it. The presented method for
obtaining both A-TiO2 and R-TiO2 polymorphs is facile, environ-
mentally friendly, exhibits long-term chemical stability, and
involves a single-step. The TiO2 polymorphs synthesized through
this method are used in various applications, including in DSSCs
and heterogeneous catalysis.
Experimental details
Chemicals
Scheme 1 The hydrothermal synthesis process of A-TiO2 in presence
of D-mannitol for DSSCs and heterogeneous catalysis applications.
AR grade chemicals were utilized without any renement.
Titanium(IV) chloride (TiCl4, 99.9%), D-mannitol (>98%),
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method using an adhesive tape on both side as a spacer. About INC). The amount of dye adsorbed over A-, R-, and CA-TiO2
eight-ten-micron thickness TiO2 lms for each case were photoelectrode was conrmed by desorbing in a 0.1 M NaOH
ꢀ
prepared and annealed at 550 C in a muffle furnace under an water–ethanol (v/v ¼ 1 : 1) mixed solution, followed by UV-Vis
air environment at a constant heating 5ꢀ minꢁ1 rate for 30 min. spectrophotometer (Varian Cary 5000 spectrophotometer)
For sensitization, these TiO2 photoanodes were heated to 60 ꢀC measurement by UV-Vis spectrophotometer (Shimadzu UV-
and kept for adsorbing 0.3 mM N-719 dye solution of 1 : 1 tert- 2450) at 540 nm. Simulated sunlight provided by a solar simu-
butanol : acetonitrile for 24 h at 27 ꢀC in the dark. The Pt lator (Sun 2000 ABET 5 Technologies, USA) tted with an A.M.
counter electrode having mirror nish, deposited by DC 1.5G lter was used, and the intensity of one sun (100 mW
magnetron sputtering on the conducting FTO substrate, was cmꢁ2) was calibrated on standard silicon solar cell. A Keithley
used as the counter electrode. The two electrodes, i.e., TiO2 with 2400 source meter was used to estimate the current density–
N-719 dye and Pt were clipped together, and to prevent the applied voltage (J–V) spectra of the A-, R-, and CA-TiO2 photo-
leakage of the iodine electrolyte solution heat resistant subli- anodes. The computer-controlled electrochemical impedance
mation tape was used as a sealant. The purpose of mask is to spectroscopy (EIS) measurement spectra were recorded on
allow light to fall only onto a active area of all the devices, i.e. the IviumStat Technologies, Netherland at a bias voltage of 0.70 V in
area was 0.5 cm2 dened by the mask.
a frequency range of 150 kHz to 0.1 Hz under dark condition.
Catalyst activity
NMR spectral data
A combination of 4-hydroxycoumarin (1 mmol) and 4-methyl-
phenylethanol (1 mmol) and A-TiO2 NCs powder (20 mg) was
stirred at 70 ꢀC for 5 h. Aer completion of the reaction
(monitored by TLC) ethyl acetate was added to the reaction
mixture and the catalyst was collected by ltration. The organic
solvent was concentrated under reduced pressure. Aer puri-
cation by recrystallization with hot ethanol, C-3 alkylated 4-
hydroxycoumarins were obtained. Moreover, to achieve the best
reaction conditions, the effects of various amount of catalyst,
reaction time, optimal temperature, and solvent were meticu-
(1) 4-(1-p-Tolylethoxy)-2H-chromen-2-one (3a). 1H (400 MHz;
CdCl3-d6): d 1.59–1.63 (d, 3H, J ¼ 16 Hz), 2.33 (s, 3H), 4.67–4.71
(q, 1H, J ¼ 16 Hz), 6.04 (S, 1H), 7.22–7.63 (m, 8H).
(2) 3-(1-(4-Bromophenyl)ethyl)-4-hydroxy-2H-chromen-2-one
(3b). 1H (400 MHz; CdCl3-d6): d 1.653–1.671 (d, 3H, J ¼ 0.),
4.696–4.750 (q, 1H, J ¼ 7.2 Hz, 14.4 Hz), 6.33 (s, 1H), 7.198–7.676
(m, 8H).
(3) 3-(1-(4-Chlorophenyl)ethyl)-4-hydroxy-2H-chromen-2-one
(3c). 1H (400 MHz; CdCl3-d6): d 1.65–1.69 (d, 3H, J ¼ 16 Hz),
4.65–4.75 (q, 1H, J ¼ 12 Hz and J ¼ 28 Hz), 6.33 (bs, 1H), 7.22–
7.72 (m, 8H).
lously investigated.
A model reaction between 4-hydrox-
ycoumarin and activated alcohols was monitored to ascertain
(4) 4-(1-(2,4-Dichlorophenyl)ethoxy)-2H-chromen-2-one (3d).
1H (400 MHz; CdCl3-d6): d 1.65–1.69 (d, 3H, J ¼ 16 Hz), 4.65–4.72
(q, 1H, J ¼ 28 Hz), 7.11–7.69 (m, 9H), 6.33 (s, 1H).
the optimal reaction parameters. Aer completing reaction,
1
obtained products were conrm and characterized by H NMR
spectral data.
Results and discussion
Measurements and characterizations
Reaction mechanism
The morphology observation of A-TiO2 and R-TiO2 were
2ꢁ
conrmed from the surface digital images by eld emission The TiO6 octahedral with different orientations of shared
scanning electron microscopy (FE-SEM, FEI Nova NanoSEM). edges and corners are responsible for affording A- and R-TiO2.
High-resolution transmission electron microscopy and selected Basically, R-TiO2 is connected by ten octahedra, in which eight
area electron diffraction (SAED) measurements analyses were are corner-shared and two are edge-shared; the crystal nuclei for
2ꢁ
conducted by (HR-TEM, FEI TECNAI G2 20 S-TWIN) by using rutile TiO2 is made up of linear chains of TiO6 octahedra
a 200 kV accelerating voltage. The phases of the TiO2 NCs were connected by corner sharing.14 In contrast, A-TiO2 is connected
validated by powder X-ray diffraction patterns (Shimadzu, XRD- by eight octahedra, in which four are corner-shared and four are
6000, Japan) obtained using l ¼ 0.1542 nm. The phase was edge-shared, making zigzag chains of TiO62ꢁ octahedra that are
conrmed by Raman microscopy (Renishaw, inVia Raman linked to each other through edge-sharing bonding.15 The
microscope, UK) also. The laser beam (l ¼ 532 nm) was focused density of A-TiO2 (3.84 g cmꢁ3) less than R-TiO2 (4.26 cmꢁ3) due
using a lens to produce a spot on the TiO2 lm surfaces. The to more edge-shared, and larger interstitial spaces between the
Fourier-transform infrared (FTIR) spectroscopy spectra were octahedra.16 Notably, the crystal structure of TiO2 can be
obtained to conrm intermediate reaction state from 500 to agglomerated through different edge-shared and corner-shared
4000 cmꢁ1 using Nicolet, iS10, Smart MIRacle, Thermo Scien- TiO6
octahedra orientations during the condensation
2ꢁ
tic. X-ray photoelectron spectroscopy (XPS) spectra were processes. Generally, when Ti precursor reacts with water
acquired using a PHI 5000 VersaProbe (Ulvac-PHI) to conrm molecules, the coordination number of Ti4+ increases from four
the surface elements and the chemical states. The binding to six, owing to the acceptance of oxygen lone pairs from
energies were used for internal calibration of C 1s peak (284.6 nucleophilic ligands through its vacant d-orbitals. These six-
eV). The specic surface areas of A-TiO2 and R-TiO2 powders fold octahedra structural units can be combined into the nal
were measured through Brunauer–Emmett–Teller spectra on crystal structure formation through condensation.17 Scheme 2
nitrogen physisorption instrument (BET, Belsorp II, BEL Japan has illustrated the formation of A-TiO2 reaction mechanism. It
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was found that D-mannitol anions can be substituted for Cl stretching vibration peaks at 3366, 1080, and 1015 cmꢁ1, as well
anions during the hydrolysis process to form [Ti(OH)x(D- as the peaks indicative of the C–H stretching vibration at 2983,
mannitol)yClz]nꢁ intermediate complexes during crystallization. 2947, and 2901 cmꢁ1. The peak indicative of –CH2 scissor
Similarly, Chen et al. demonstrated that BiOCl surface selec- vibrations is noted at 1417 cmꢁ1
.
Fig. S1b† shows the FTIR
20
tively adsorb molecules of D-mannitol to from hydrogen bonds spectrum of the intermediate complex between Ti4+ and D-
with exposed oxygen atoms to decrease the surface energy mannitol ions, where the molecular interaction of D-mannitol
during the crystal growth formation.18 Effect of D-mannitol on ions with Ti ions, in addition to overlapping, is shied posi-
the crystal structure formation and nucleation process of A-TiO2 tively and negatively, thereby validating the construction of
is worth for future investigations. In addition, the hydrochloric a complex structure. The FTIR spectrum of the intermediate
acid present in the TiCl4 aqueous solution could act as a catalyst complex showed a broad peak at 3167 cmꢁ1, which coincides to
not only facilitating the formation of crystal growth via –OH stretching vibrations. A broad peak of the –OH stretching
condensing the intermediate complex but also nucleation vibrations validated the presence of hydroxyl groups over
process and related species.19
different sites. The high-intensity FTIR peak near 600 cmꢁ1
could be assinged to TiO2.21
Intermediate complexes
Hydrolysis rate estimation
To conrm the formation of intermediate complexes, the FTIR
spectrum was investigated. On completing hydrothermal reac-
tion between TiCl4 and D-mannitol, the white precipitate of
intermediate complex was transferred into a centrifuge tube at
8000 rpm for 10 min, and washed many times with DI-water,
and subsequently dried at room temperature before FTIR
measurement. Fig. S1a in ESI† shows the FTIR spectrum of
standard D-mannitol consisting of strong –OH and –C–O
The effect of D-mannitol (0.1 M) on the hydrolysis process of
TiCl4 (1 M) was measured through dynamic light scattering
technique. As shown in Fig. 1a, in the presence of D-mannitol,
TiCl4 in aqueous medium under goes hydrolysis to develop
slow particle agglomeration with a small size distribution of
ꢂ18 nm. TiCl4 in aqueous solution, in the absence of D-
mannitol undergoes hydrolysis to form fast particles agglom-
eration with ꢂ70 nm size distribution (Fig. 1a), conrming the
interaction of D-mannitol with TiCl4 that prevents the rapid
hydrolysis (Fig. 1b). The coordination of D-mannitol anions
with TiCl4 inhibit the crystal growth resulting the formation of
small-sized A-TiO2 NCs. For an improved understanding of the
hydrolysis reaction, we maintained both samples for more
than thirty days (TiCl4 and TiCl4-0.1 M D-mannitol) at room
temperature (Fig. 1c). Aer two days, aqueous medium of TiCl4
solution changed to white coloured precipitate with sedi-
ments. This white coloured precipitate subsequently agglom-
erated, and settle down slowly on the surface of bottom.
However, the TiCl4-0.1 M D-mannitol solution remained
transparent without precipitated even keep at one month,
Fig. 1 DLS spectra of; (a) 1 M TiCl4, (b) TiCl4-0.1 M D-mannitol in
aqueous solution, and (c) diagram showing 30 days stability at a room-
Scheme 2 Plausible reaction mechanism to the formation of A-TiO2 temperature where hydrolysis takes place in aqueous solution of TiCl4
NCs.
without D-mannitol (R-TiO2) and TiCl4-0.1 M D-mannitol (A-TiO2).
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Fig. 4 N2 adsorption desorption isotherms with inset depicts the pore
size distribution of R-TiO2 (black and brown dot), and A-TiO2 (blue and
purple dot), NCs in the presence of D-mannitol. The black and blue dot
denoted adsorption and brown and purple dot denoted the desorption
of R- and A-TiO2 NCs.
Fig. 2 (a) X-ray diffraction, (b) Raman spectrum, (c) XPS analysis of
Ti2p, and (d) O1S of A-TiO2 NCs with D-mannitol.
Structural elucidation
which is also supported by the dynamic light scattering
method, where to preventing the fast hydrolysis process Ti
cations could bond with D-mannitol anion at room tempera-
ture. We assumed that the slow hydrolysis process taking place
between TiCl4-0.1 M D-mannitol solutions plays important role
in the formation of small particles size, which ultimately
supports the phase transformation process, from rutile to
anatase. Though, to initiate the nucleation process during the
hydrolysis of A-TiO2NCs from the D-mannitol comprising
solution, suitable temperature is required. Xiong et al. re-
ported, in their BiOCl study, that D-mannitol not only behaved
as a cohesive agent to prompt the assembly of the structural
morphology but also acted as a capping agent to prevent the
crystal anisotropic growth.22 To validate the comparison, the
acidity (pH ¼ 0.6) of both solutions were kept the same for the
dynamic light scattering measurement.
XRD patterns were used to characterize the crystallite struc-
tures, phases, and average sizes of the as-obtained A- and R-
TiO2. The XRD trends of the as-synthesized TiO2 NCs with and
without D-mannitol are respectively shown in Fig. 2a and S2a.†
The prior XRD pattern conrmed peaks at 25.3ꢀ, 37.8ꢀ, 48.0ꢀ,
53.8ꢀ, 55.0ꢀ, 62.6ꢀ, and 68.8ꢀ, which are ascribed to the 101, 004,
200, 105, 211, 204, and 116 reection planes of A-TiO2 (JCPDS
no. 21-1272),23 respectively, whereas in later XRD pattern, the
peaks at 27.4ꢀ, 36.0ꢀ, 39.1ꢀ, 41.2ꢀ, 44.0ꢀ, 54.3ꢀ, 56.6ꢀ, 62.7ꢀ,
64.0ꢀ, 69.7ꢀ, and 72.4ꢀ in association with (110), (101), (200),
(111), (210), (211), (220), (002), (310), (112), and (311) for R-TiO2
(JCPDS data le no. 21-1276) are respectively evidenced.24 There
were no difficulties in separating the A and R phases of TiO2; as
for anatase, the rst two-theta peak was at 25.3ꢀ for d101, and for
rutile, it was at 27.4ꢀ for d110. From the XRD patterns it was clear
that, though both A- and R-TiO2 NCs were synthesized by the
same chemical method, different phases were obtained just by
Fig. 3 (a) FE-SEM, (b) EDX mapping for Ti and O. (c) HR-TEM, and (d) Fig. 5 (a) UV-Vis, (b) J–V, (c) IPCE, and (d) s (vs. Voc) measurements of
SAED pattern of A-TiO2 NCs.
R-, A- and CA-TiO2 photoanodes.
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adding D-mannitol. The average crystal size was calculated facets, and the HR-TEM image also has evidenced clear and
using the Debye–Scherrer equation. For the as-synthesized A-, sharp rings in reference to the (101), (004), (200), (105), and
CA-, and R-TiO2, the average crystallite sizes were 12.0, 16.2, and (211) lattice planes of A-TiO2 in the SAED pattern (Fig. 3d).
19.5 nm, respectively. Raman spectroscopy is an effective tool
for identifying the molecules in A-TiO2 NCs obtained from D-
mannitol. In Fig. 2b, the major Raman shi peaks at 144, 199,
400, 519, and 635 cmꢁ1 were attributed to the Eg, Eg, B1g (B1g/
A1g), and Eg symmetries of A-TiO2.25 In the Raman spectrum, no
additional peaks, except distinctive vibrations, were found,
which conrms the formation of pure phase A-TiO2. The Raman
spectrum demonstrated at 142, 235, 443, and 610 cmꢁ1 are
attributed to the B1g, two-phonon scattering, Eg, and A1g modes
of the rutile phaseof TiO2 NCs, respectively (Fig. S2b†). The
spin-orbital splitting of Ti 2p1/2 and Ti 2p3/2 photoelectrons
corresponds to a doublet situated at binding energies of 463.9
and 458.1 eV, respectively, as shown in Fig. 2c. The peak sepa-
ration of Ti 2p1/2 and Ti 2p3/2 was 5.8 eV, conrming the Ti4+
chemical state which is in magnicent consensus with the
literature data for A-TiO2,.26 The O1S singlet of oxygen at 529.3 eV
is shown in Fig. 2d. The Lorentzian–Gaussian mix function
value was employed in the curve resolution of the individual O1S
peaks in the two spectra. The curve resolved O1S signal revealed
small sized secondary peak that is positioned at a binding
energy of 531.86 eV for –OH species present on the surface.27
Specic surface area and pore-size distribution measurements
The specic surface area and porosity of both R-TiO2 and A-TiO2
NCs were investigated from the BET and BJH spectra. A type-IV
isotherm (Brunauer–Deming–Deming–Teller classication) was
due to their mesoporous nature. The hysteresis loop of type H1
suggested that the porosity of A-TiO2 is derived from inter-
connected NCs.29 Fig. 4 indicates 77.8 m2 gꢁ1 as the specic
surface area of A-TiO2 NCs. Furthermore, its narrow pore-size
distribution centered at 13.0 nm was ascribed to the presence
of relatively small inner pores between NCs and a large pore
constructed by an independent structure, which agrees with the
FE-SEM and HR-TEM analyses as well. Additionally, the stan-
dard multi-point BET technique was employed to calculated the
specic surface area of 14.2 m2 gꢁ1 for R-TiO2, which was only
one-h that of A-TiO2. The narrow pore-size distribution
exhibited in the as-prepared R-TiO2 was centered at 60.2 nm.
Basically, both DSSCs and heterogeneous catalyst applications
are porosity dependent due to the increase in the dye adsorp-
tion and active centers.
DSSC performance
Surface morphology evolution
The DSSC performance of CA-TiO2 (ꢂ20 nm particle-size, 100%
The surface morphology evolution of A-TiO2 NCs is shown in anatase, ENB Korea) NCs was also measured as a reference. The
Fig. 3a. The FE-SEM images of A-TiO2 conrmed the formation performances of A- and CA-TiO2 NCs based DSSCs are given in
of small-sized spheres, where all the grains are well-interlinked Fig. 5. The UV-Vis absorption spectra of both photoanodes, with
to one another, which would offer various facile transporting N-719 dye, were obtained in a wide visible wavelength range.
channels for electrons (or charge carriers) through the TiO2 The characteristic peak hump appeared at 530 nm was due to
NCs. The FE-SEM images demonstrate the well-interconnected the N-719 dye (Fig. 5a). The N-719 dye adsorption over A-TiO2-
ꢂ60 nm average diameter of nanocrystallites, when D- NCs (0.86 ꢃ 10ꢁ7 mol cmꢁ2) was higher than those over CA-TiO2
mannitol was absent in the precursor solution as shown in (0.81 ꢃ 10ꢁ7 mol cmꢁ2) and R-TiO2 NCs (0.42 ꢃ 10ꢁ7 mol
Fig. S2c.† The elemental stoichiometry determined by EDX cmꢁ2), which possibly as a result of the higher surface area and
mapping (Fig. S3†) has evidenced 30.8 and 69.1% atomic narrow particle size of A-TiO2. To quantify the dye-loading
proportions for Ti and O, respectively, suggesting the formation amounts over two photoanodes, these sensitized photoanodes
of TiO2. The HR-TEM results indicate the high crystallinity of were immersed into 0.1 M sodium hydroxide solution to desorb
TiO2 interconnected NCs of about 15 nm in sizes (Fig. 3b). The the adsorbed dye molecules, followed by measuring the UV-Vis
0.36 nm interplanar distance in compliance with the (101) plane absorbance spectra. Using the Beer–Lambert law, the amounts
of A-TiO2 NCs (Fig. 3c), and 0.32 nm interplanar distance in of dye absorbed over these electrodes were determined and are
compliance with the (110) plane of R-TiO2 NCs (Fig. S2d†) were listed in Table 1. The current density–voltage curves of both
approved.28 The (101) facet of the A-TiO2 NCs has a relatively low DSSCs, i.e., CA-TiO2 and, A-TiO2 NCs photoanodes, under one
surface energy; thus, it has better stability than those of other sun illumination were measured and are plotted in Fig. 5b.
Table 1 Summary of DSSCs average efficiency were obtained using six devices under AM 1.5G illumination (100 mW cmꢁ2
)
Dye adsorption mol
Crystal size
(nm)
Photoanodes
JSC (mA cmꢁ2
)
VOC (V)
ff (%)
PCE (%)
cmꢁ2
R-TiO2
Std. dev.
A-TiO2
Std. dev.
CA-TiO2
Std. dev.
7.7
0.089
12.9
0.082
12.0
0.088
0.71
0.023
0.70
0.020
0.70
0.022
69
0.018
67
0.011
65
0.016
3.7
0.019
6.0
0.012
5.8
0.017
0.42 ꢃ 10ꢁ7
0.86 ꢃ 10ꢁ7
0.81 ꢃ 10ꢁ7
19.5
12.0
16.2
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Table 1 presents a summary of the DSSCs outcome based on
various parameters for both photoanodes, where, in both cases,
the open circuit voltage (VOC) of 0.70 V was nearly same because
VOC mainly is determined by the difference between the oxida-
tion potential of the electrolyte and the Fermi level of the
photoelectrode.30 Although, the as-obtained short-circuit
current densities (JSC) of both DSSCs conrmed higher perfor-
mance in A-TiO2 NCs photoanode compared to that in CA-TiO2,
could be assigned to a narrow particle size as A-TiO2 demon-
strates more dye molecules than that of CA-TiO2.31
The presence of an interconnected architecture of NCs in A-
TiO2 electrode could provide a long-range charge-transfer chan-
nels, resulting in 12.9 mA cmꢁ2 JSC, 0.70 V VOC, 67% ff, and 6.0%
PEC, which were considerably higher than those of CA-TiO2 (12.0
mA cmꢁ2 JSC, 0.70 V VOC, 65% ff, and 5.8% PEC) and R-TiO2
electrodes (7.7 mA cmꢁ2 JSC, 0.71 V VOC, 69% ff, and 3.7% PEC),
Fig. 5. The signicant enhancement in the DSSCs performance of
A-TiO2NCs photoanode could be due to its unique morphology,
high electrical conductivity (2.96 ꢃ 10ꢁ7 U cm), relatively high
specic surface area (77.8 m2 gꢁ1), and optimum electron
mobility (0.124 cm2 Vꢁ1 sꢁ1). Fig. 5c reveals the incident photon-
to-electron conversion efficiency (IPCE) spectra of R-TiO2, A-TiO2,
and CA-TiO2 photoanodes, where two peaks, one at around
350 nm (TiO2) and the other at around 520 nm, indicative of the
absorbed N-719 dye, were observed.32 The as-synthesized A-TiO2
photoanode conrmed a maximum IPCE of 22.08%, which was
20.31% in CA-TiO2, and 15.12% in R-TiO2 photoanodes, revealing
a higher N-719 dye adoption of A-TiO2 over CA-TiO2 and R-TiO2.
The open-circuit voltage decay plots of both electrodes measured
under applying open circuit voltage conditions with respect to
time are shown in Fig. 5d. At zero current, there was an expo-
nential decrease in VOC, which supports the recombination of
electrons.33 In the graph, the A-TiO2 photoanode exhibited a slow
decrement in VOC, which is due to the electron recombination
mitigation. The calculated lifetime of the electron, i.e., the time
taken by an excess minority carrier to recombine, of A-TiO2,
determined by the VOC decay method, was higher than those of
the others, suggesting that the electrons are more likely to reach
the working electrode than CA-TiO2 and R-TiO2 photoanodes.34
Fig. 6a–d summarizes the average performances of CA-TiO2
and A-TiO2 photovoltaic devices. The as-synthesized A-TiO2
photoanode exhibited a high reproducibility of JSC, VOC, ff, and
PEC with a comparatively small standard deviation of 1.72%,
which is substantially higher than those for the CA-TiO2
photoanode-based DSSCs devices (1.20%), suggesting the
possibility of its reusability compared to the case with others.
Fig. 6 Summary of average performance of six devices: (a) Jsc, (b) Voc
(c) ff, and (d) PCE of CA- and A-TiO2-based. The maximum/minimum
and average values are denoted by — and C, respectively.
,
binding energy onset (Eonset) was ꢁ5.30 eV, relative to the Fermi
level of Au (at 0 eV). The valence band minimum (VBM) calcu-
lated using VBM ¼ hn ꢁ (Ecut-off ꢁ Eonset) equation, whereas, n is
the frequency of vibration of light and h is the Planck's constant
estimates VBM of ꢁ7.35 eV for A-TiO2 NCs. The band gap energy
denoted by (Eg) of the A-TiO2 NCs electrode was estimated using
the following equation:
(ahn)2 ¼ A(hn ꢁ Eg),
(1)
where, a is the absorption coefficient, and A is the proportion-
ality constant. Fig. S4† presents a UV-Vis spectrum of the A-TiO2
photoanode, where 3.2 eV Eg was obtained for A-TiO2NCs lm,
which closely matches to the reported value.35 Using this band
Energy levels
To conrm the energy alignment of dye and TiO2, UPS
measurement was performed, which is shown in Fig. 7a, where
the various energy edge ranges such as the cut-off region, survey
binding energy spectra, and onset region for A-TiO2 NCs are
clearly indicated. A schematic energy band diagram of the
DSSCs based on the A-TiO2 NCs photoanode charge transfer
process is shown in Fig. 7b. The high binding energy cut-off
Fig. 7 (a) UPS spectra shown three different energy edge ranges: cut-
off region, survey binding energy spectra, and onset region of A-TiO2.
(b) The schematic energy bandgap diagram of FTO/A-TiO2/N-719
(Ecut-off) of the A-TiO2 NCs photoanode was 8.56 eV. The dye/electrolyte/Pt photoanode device calculated from UPS analyses.
14832 | RSC Adv., 2020, 10, 14826–14836
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Scheme
3
Synthesis of the substituted C-3 alkylated 4-hydrox-
ycoumarins using the A-TiO2 catalyst.
Effects of the catalyst amount, temperature, time, and solvent
system
For the synthesis of C-3 alkylated 4-hydroxycoumarins, we
successfully analyzed the reaction conditions, for instance
catalyst amount, temperature, and required time for the
completion of the reaction (Scheme 3, Table 3). In rst, of A-
TiO2 catalyst amount was tested; among them a model amount
of 20 mg exhibited a notable yield. Although, when the catalyst
amount was increased to 30 mg, no obvious change in the yield
was observed, this may be due to the exhaustion of the cata-
lytic sites or the maximum conversion of the catalyst
efficiency.
Under the optimized experimental conditions, we then per-
formed the reactions with different solvents and varying reac-
tion conditions (reux, 50 ꢀC, 70 ꢀC, and under visible light
irradiation) for different time periods (1–10 h). In the presence
of 20 mg of A-TiO2 catalyst, at 70 ꢀC for a reaction time of 4.5 h,
95% yield was obtained, which is one of the maximum values,
so far, reported in the literature.38–47 It was observed that aer
extended reaction time and high temperature, the reaction did
not proceed in the forward direction. In our study, 20 mg of A-
TiO2 catalyst under heat/visible light and solvent free condi-
tions demonstrated a notable product yield of 95% with stipu-
lated time period of 4.5 h. Similarly, the nature of the solvent
can interrupt the reaction environment by affecting the catalytic
stability, solubility, and rate of the reaction. Aer careful
examination of all the results, a solvent-free condition was
preferred for the synthesis of further derivatives. Thereaer,
this solvent-free specic condition was used for all the reac-
tions. Table 3 summarizes the percentage yield of C-3 alkylated
4-hydroxycoumarins under distinct reaction condition i.e.
catalyst amount, reaction temperature, various solvents, and
time.
Fig. 8 EIS plot of the R-, A- and CA-TiO2 NCs photoanodes.
gap energy, the conduction band minimum (CBM) of A-TiO2
was calculated using the following equation:
CBM ¼ VBM + Eg.
(2)
The calculated CBM of the A-TiO2 NCs electrode was ꢁ4.1 eV.
To understand the electron transfer pathways of this photo-
anode, the energy band diagram of the FTO/A-TiO2/N-719 dye/
electrolyte/Pt DSSCs was also designed. In DSSCs, on
capturing photons over the N-719 dye layer, the electron–hole
pairs are created, and the excited electrons are injected into the
CBM of the A-TiO2NCs, which are then collected at the FTO
substrate of the ꢁ4.6 eV work function more easily.36
Electrochemical charge transfer process
The photogenerated charge transfer and recombination
processes are crucial in DSSCs performance, which can be
explained with EIS measurement operated bias potential 0.70 V
under dark condition. Fig. 8 shows the Nyquist curve of the A-
TiO2 NCs photoanode which demonstrates one semicircle at
high-frequency region (>1 kHz), and another semicircle at a low-
frequency region (100–0.1 Hz). A relatively small semicircle at
high-frequency region corresponds to the charge-transfer resis-
tance (R1), and the capacitance (CPE1) at the counter electrode
(platinum)/redox electrolyte interface. The second semicircle, at
the low-frequency range, corresponds to the charge-transfer
interactions occurring at the photoanode-dye/electrolyte inter-
face (R2), and the chemical capacitance (CPE2) at the TiO2/dye/
redox electrolyte interface.37 The equivalent circuit and the cor-
Synthesis of substituted C-3 alkylated 4-hydroxycoumarins
using the A-TiO2 catalyst
responding values obtained by the Z view soware are shown as Under the optimal re substituted C-3 alkylated 4-hydrox-
an inset of Fig. 8, and also in Table 2. ycoumarins using the A-TiO2 catalyst (Table 4).
Table 2 The R-TiO2 and A-TiO2 NCs photoanodes fitted electronic parameters of Nyquist plots
RS
R1
R2
CPE1-T
CPE1-P
CPE2-T
CPE2-P
s (ms)
R-TiO2
A-TiO2
5.2
5.9
4.4
2.4
31.0
19.7
5.32 ꢃ10ꢁ3
1.06 ꢃ10ꢁ3
0.59
0.73
7.90 ꢃ10ꢁ3
21.0 ꢃ10ꢁ3
0.91
0.87
1.58 ꢃ 10ꢁ4
2.24 ꢃ 10ꢁ4
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Table 3 The optimization of reaction conditions determines
Amount of catalyst
(mg)
No
Temperature (ꢀC)
Solvent
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
0
5
Reux
Reux
MeCN
MeCN
MeCN
MeOH
EtOH
THF
DCM
IPA
Toluene
Solvent free
Solvent free
Solvent free
Solvent free
10
1
1
1
1
1
1
1
1
5
5
5
5
—
0
0
0
0
0
0
0
0
75
95
95
95
10
15
20
20
20
20
20
20
20
30
20
50 ꢀ
C
50 ꢀC
50 ꢀ
50 ꢀ
50 ꢀ
C
C
C
50 ꢀC
9
50 ꢀ
50 ꢀ
70 ꢀ
70 ꢀ
C
C
C
C
10
11a
12
13
Visible light
a
b
Reaction conditions: 4-hydroxycoumarin (1 mmol), 4-methylphenylethanol (1 mmol), catalyst (20 mg), solvent free. Isolated yields.
was observed using the reaction between 4-hydroxycoumarin
and 4-methyl-phenylethanol as a model reaction under the
optimal conditions. Typically, aer completion of reaction, the
A-TiO2 catalyst was separated from the reaction mixture
through a simple ltration method. The reusability of the
The plausible reaction mechanism
A-TiO2-catalyzed synthesis of C-3 alkylated 4-hydroxycoumarins
occurred by removal of a water molecule, which results in the
formation of a stable carbocation, as alkylating species that
were derived from alcohol on the Lewis acidic sites of A-TiO2 (C-
3 alkylation). The enolic hydroxy group of 4-hydroxycoumarin
was activated by Ti metal via Lewis acid catalysis to increase the
nucleophilicity of the 3-position of 4-hydroxycoumarin. The
nal product C-3 alkylated 4-hydroxycoumarins was produced
through an electrophilic attack activated secondary benzyl
alcohols on the nucleophilic enolic hydroxy group of 4-hydrox-
ycoumarin (Scheme 4).
ꢀ
catalyst was assessed aer activating the catalyst at 200 C for
1 h; the catalyst was employed for ve runs to test the consis-
tency of its activity. Aer ve runs, the product yields obtained
over the A-TiO2 catalyst exhibited no noticeable decrease, sug-
gesting a high consistency of the activity of the as-synthesized A-
TiO2 catalyst. This reusability study reveals the high stability
and turnover of A-TiO2 nanocatalysts under optimal operating
conditions. The reusability of the catalysts is one and signicant
advantage that makes them benecial for marketable
applications.
Recyclability of the A-TiO2 nanocatalyst
Aer evaluating the activity and exibility of the A-TiO2 catalyst
for different kinds of reactions, the recyclability of the catalyst
Table 4 C-alkylation of 4-hydroxycoumarin with various alcohols
No
Alcohol
Product
Time (h)
Yield %
1
4.5
97
2
3
5
5
95
94
4
5
90
Scheme 4 Plausible reaction mechanism for the synthesis of C-3
alkylated 4-hydroxycoumarins.
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´
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2ꢁ
regulates the zigzag chains of the TiO6 octahedral by domi-
nant edge sharing, and (b) capping agent that inuences the
nucleation for NCs during the hydrolysis process. The FTIR
spectrum impeccably suggests the formation of an intermediate
complex structure between Ti4+ and D-mannitol ions. A PCE of
6.0% is achieved from A-TiO2 NCs photoanode, which is about
1.6 times higher than R-TiO2 (3.7%); this may be due to the
difference in the structure, surface area, dye intake capacity,
and charge transfer resistance. In addition, A-TiO2 catalyst was
used for synthesizing of pharmaceutically signicant C-3 alky-
lated 4-hydroxycoumarins under solvent-free conditions. An
excellent yield of 95% at very less reaction time of 4.5 h is ob-
tained. The all synthesized compounds are conrmed by 1H
NMR spectroscopy. The catalytic activity of produced A-TiO2 has
remarkable recyclability up to ve consecutive runs with slight
change in formed product yields, which clearly demonstrates
the novel, efficient, recyclable, and economical nature of the as
synthesized A-TiO2 catalyst. Therefore, the A-TiO2 catalyst can
be a potential candidate, over other expensive and rare-earth
catalysts, during organic transformation processes.
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Conflicts of interest
There are no conicts to declare.
´
´
25 G. C. Vasquez, M. A. Peche-Herrero, D. Maestre, B. Aleman,
Acknowledgements
´
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J. Ramırez-Castellanos, A. Cremades, J. M. Gonzalez-Calbet
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The authors would like to extend their sincere appreciation to
the Researchers supporting project number (RSP-2019/55), King
Saud University, Riyadh, Saudi Arabia for funding this research.
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