Ultrasonically-assisted surface modified TiO2/rGO/CeO2 heterojunction photocatalysts for conversion of CO2 to methanol and ethanol

Article abstract of DOI:10.1016/j.ultsonch.2019.104657

Converting CO₂ to usable fuel may contribute to lowering of global warming, thus this study developed effective heterojunction photocatalysts for the photoreduction of CO₂ with water into methanol and ethanol fuels. The photocatalysts were prepared from combining surface modified titanium dioxide (TiO₂) nanoparticles with reduced graphene oxide (rGO) and cerium oxide (CeO₂). The TiO₂ surfaces were firstly modified via the sono-assisted exfoliation, with high intensity ultrasonic waves (ultrasonic horn, 20 kHz, 150 W/cm²) in 10 M NaOH for 1 h. Highly reactive nanosheets delaminated from outer surfaces of the primary TiO₂ crystals leading to an increase in specific surface active area, light absorption and decrease in electron-hole recombination rate, which enhanced photocatalytic activity. Then, 0.75 wt% rGO and 1 wt% CeO₂ were incorporated into the surface modified TiO₂ to promote photogenerated charge separation, electron mobility and CO₂ absorptivity. The modified TiO₂/rGO/CeO₂ photocatalysts exhibited superior photocatalytic performance by producing methanol at 641 μmol/g_cath and ethanol at 271 μmol/g_cath, almost 7 times higher than rates from pure TiO₂. The significant improvement in CO₂ photoconversion activity was mainly attributed to the high interfacial contact area and strong connection between the reactive delaminated TiO₂ nanosheets, rGO and CeO₂, which, in turn, facilitated the flow of large number of photogenerated charge carriers to react with the absorbed species, and the multi-step charge transportation due to the heterojunction effect that effectively retarded electron-hole recombination.

Full text of DOI:10.1016/j.ultsonch.2019.104657

Accepted Manuscript
Ultrasonically-assisted surface modified TiO /rGO/CeO heterojunction photo-
2
2
catalysts for conversion of CO to methanol and ethanol
2
Panpailin Seeharaj, Panyata Kongmun, Piyalak Paiplod, Saowanee Prakobmit,
Chaval Sriwong, Pattaraporn Kim-Lohsoontorn, Naratip Vittayakorn
PII:
S1350-4177(19)30766-7
https://doi.org/10.1016/j.ultsonch.2019.104657
104657
DOI:
Article Number:
Reference:
ULTSON 104657
To appear in:
Ultrasonics Sonochemistry
Received Date:
Revised Date:
Accepted Date:
17 May 2019
21 June 2019
22 June 2019
Please cite this article as: P. Seeharaj, P. Kongmun, P. Paiplod, S. Prakobmit, C. Sriwong, P. Kim-Lohsoontorn, N.
Vittayakorn, Ultrasonically-assisted surface modified TiO /rGO/CeO heterojunction photocatalysts for conversion
2
2
of CO to methanol and ethanol, Ultrasonics Sonochemistry (2019), doi: https://doi.org/10.1016/j.ultsonch.
2
2
019.104657
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Ultrasonically-assisted surface modified TiO /rGO/CeO heterojunction
2
2
photocatalysts for conversion of CO to methanol and ethanol
2
1
1
1
1
Panpailin Seeharaj *, Panyata Kongmun , Piyalak Paiplod , Saowanee Prakobmit ,
1
2
1
Chaval Sriwong , Pattaraporn Kim-Lohsoontorn , Naratip Vittayakorn .
1Advanced Materials Research Unit, Department of Chemistry, Faculty of Science,
King Mongkut’s Institute of Technology Ladkrabang, 10520 Ladkrabang, Bangkok, Thailand
2Center of Excellence on Catalysis and Catalytic Reaction Engineering, Department of
Chemical Engineering, Faculty of Engineering, Chulalongkorn University, 10330
Pathumwan, Bangkok, Thailand.
Corresponding author: e-mail address: panpailin.se@kmitl.ac.th
Tel.: +66 8496-9664
Abstract
Converting CO to usable fuel may contribute to lowering of global warming, thus this study
2
developed effective heterojunction photocatalysts for the photoreduction of CO with water
2
into methanol and ethanol fuels. The photocatalysts were prepared from combining surface
modified titanium dioxide (TiO ) nanoparticles with reduced graphene oxide (rGO) and
2
cerium oxide (CeO ). The TiO surfaces were firstly modified via the sono-assisted
2
2
2
exfoliation, with high intensity ultrasonic waves (ultrasonic horn, 20 kHz, 150 W/cm ) in
1
0 M NaOH for 1 h. Highly reactive nanosheets delaminated from outer surfaces of the
primary TiO crystals leading to an increase in specific surface active area, light absorption
2
and decrease in electron-hole recombination rate, which enhanced photocatalytic activity.
Then, 0.75 wt% rGO and 1 wt% CeO were incorporated into the surface modified TiO to
2
2
1

promote photogenerated charge separation, electron mobility and CO absorptivity. The
2
modified TiO /rGO/CeO photocatalysts exhibited superior photocatalytic performance by
2
2
producing methanol at 641 mol/g h and ethanol at 271 mol/g h, almost 7 times higher
cat
cat
than rates from pure TiO . The significant improvement in CO photoconversion activity was
2
2
mainly attributed to the high interfacial contact area and strong connection between the
reactive delaminated TiO nanosheets, rGO and CeO , which, in turn, facilitated the flow of
2
2
large number of photogenerated charge carriers to react with the absorbed species, and the
multi-step charge transportation due to the heterojunction effect that effectively retarded
electron-hole recombination.
Keywords: Sono-assisted exfoliation; photocatalyst; titanium dioxide; CO photoconversion
2
2

1
. Introduction
As a concern on global warming caused by carbon dioxide (CO ) emissions, the
2
research and development on photoconversion of CO to useful product e.g. fuels and
2
valuable chemicals have received great attention [1-5]. The photoconversion of CO is
2
economical and environmentally friendly technology which can provide simultaneous
benefits in reducing the amount of CO emissions into atmosphere and producing new source
2
of sustainable energy. The photocatalytic conversion of CO is an artificial photosynthesis
2
process based on the use of semiconducting or transition metal complex photocatalysts to
promote chemical reactions. Generally, the CO conversion occurs in presence of water and
2
combines both water splitting and CO reduction processes which can be used to produce
2
various carbonaceous molecules in gaseous or liquid forms e.g. carbon monoxide (CO),
formaldehyde (HCHO), methane (CH ), formic acid (HCOOH), methanol (CH OH), and
4
3
ethanol (C H OH), depending on types of photocatalysts, reductants and charge transfer
2
5
reactions [1-3]. In term of materials development, the photocatalysts with high CO2
photoconversion efficiency and high selectivity are required in order to make this technology
feasible for practical applications.
Titanium dioxide (TiO ) is considered as one of the most promising photocatalysts for
2
CO conversion, due to its high photocatalytic property, stability, non-toxicity, reusability,
2
and low cost [1-8] . Although, TiO offers great potential for the CO photoconversion, its
2
2
photoactivity is relatively low due to some drawbacks such as a large intrinsic band gap
energy, E , 3.18 eV for anatase, 3.03 eV for rutile and 3.54 eV for brookite structures [9, 10],
g
a rapid recombination of photogenerated electron-hole pairs, and a slow transfer rate of
electrons to oxygen [9-11]. To overcome these limitations, TiO has been manipulated to
2
optimize performance by varying the crystal structure, defect disorder, morphology and
surface characteristic or by incorporating various non-metal, metal, and metal oxides [1-9,
3

1
1-15]. Among these attempts, surface modification of TiO particles by the sono-assisted
2
exfoliation method was reported to be a simple and effective approach to improve TiO2
photocatalytic activity [16]. This method makes used of the acoustic cavitation phenomena
(formation, growth and implosive collapse of bubbles) generated by irradiating a solution
with a high intensity ultrasonic waves (frequency 20 kHz to 15 MHz with acoustic
wavelengths of 100 mm to 100 µm) as a driving force for the intercalation and exfoliation
[
17-21]. The formation of highly reactive TiO nanosheets on outer surface of the bulk TiO
2
2
crystals was found to increase surface reactive sites, decrease E and electron-hole
g
recombination rate, which resulted in a significant improvement of methylene blue dye
degradation efficiency [16]. However, this simple and effective approach for optimizing the
photocatalytic property of TiO has never been applied to CO photoconversion applications
2
2
yet.
Another interesting strategy that can be used to enhance the photocatalytic activity of
TiO , is to integrate TiO with small amounts of reduced graphene oxide (rGO) [8, 22-26]
2
2
and CeO [11, 27, 28] to form complex heterojunction photocatalysts. rGO is a graphene
2
derivative which is a 2 dimensional (2D) sheet of carbon atoms constructed from π-π
conjugated hexagonal packed structure, interspersed with a few oxygen-containing functional
groups such as epoxide, carbonyl, carboxyl and hydroxyl groups [22, 29]. When rGO is
loaded into TiO , rGO can act as an electron acceptor, due to the delocalized π-π conjugation
2
structure, which leads to many beneficial outcomes including enhanced quantum efficiency
and the surface adsorption of CO through the conjunction between CO molecule and
2
2
graphene, accelerating the electron-hole pair separation and increasing the electron transfer
rate for reduction reaction [22, 23, 29]. In general, when large specific surface area 2D
graphene based materials are coupled with 0D nanosized TiO particles, only a small part of
2
the TiO nanoparticle is connected with the 2D graphene structure and this can restrict
2
4

photoactivity and electron transfer from TiO to rGO [29-31]. Using of surface modified TiO
2
2
particles, in which the relative surface area to volume ratio is increased by forming 2D
reactive nanosheets, with larger exposed surfaces, compared to the typical TiO2
nanoparticles, may overcome this obstacle, by increasing the interfacial contact area between
the TiO and rGO.
2
CeO is another effective additive that has been reported to improve photocatalytic
2
activity of TiO [11, 27, 28]. CeO is an n-type semiconductor with a 2.3-3.4 eV band gap
2
2
[
11, 27]. The incorporation of CeO not only increases light absorption, but also suppresses
2
charge recombination, due to the unique ability of cerium to change its oxidation state
between Ce(III) and Ce(IV), and form a heterojunction at the interface [11, 27, 28].
The combination of surface modification of TiO particles, by sono-assisted
2
exfoliation, and incorporation of rGO and CeO into the modified TiO to form complex
2
2
heterojunction photocatalysts is a promising strategy to optimize CO photoconversion
2
activity. Therefore, we prepared and characterized modified TiO /rGO/CeO composite
2
2
photocatalysts and measured their CO photoconversion efficiency for production of
2
methanol and ethanol fuels.
2
. Experimental
2
.1. Materials preparation
Surface modified titanium dioxide (Modified TiO ): The experimental procedure
2
for modifying the surface of TiO particles by sono-assisted exfoliation was modified from
2
®
the one described by Thasirisap et al. [16]. Firstly, 1 g of TiO particles (Evonik Aeroxide
2
P25; purity 99.8%, Sigma Aldrich, consisting of 20 wt% rutile and 80 wt% anatase phases
[32]) were dispersed in 80 ml of 10 M sodium hydroxide aqueous solution (NaOH; purity
9
8%, Carlo Erba) followed by shaking in an orbital shaker at 150 rpm for 24 h. Next, the
suspension was irradiated with a high intensity ultrasonic wave (Ti-horn, 20 kHz, 150 W/cm2
5

Sonics Model VCX 750, Vibracell) for 1 h. After the reaction was completed, TiO was
2
centrifugally separated and washed with 0.1 M nitric acid (HNO ; purity 65%, Merck) and
3
distilled water until neutral (pH 7). The surface modified TiO was finally obtained after
2
drying in an oven at 90 °C for 24 h.
Reduced graphene oxide (rGO): rGO was prepared by modified Hummers’ method
[33, 34]. In brief, graphite powders (purity 99%, Sigma Aldrich) were oxidized with
concentrated sulfuric acid (H SO ; purity 99.8%, Carlo Erba) and potassium permanganate
2
4
(
KMnO ; purity 99%, Merck) to obtain graphite oxide. Then 500 mg of the as-prepared
4
graphite oxide dispersed in 50 ml distilled water was irradiated with a high intensity
ultrasonic wave for 1 h, and then centrifuged to obtain the brown supernatant of graphene
oxide (GO) suspension. After, the GO was reduced to rGO via the modified chemical
reduction method of Li et al. [35] in which 8 ml of the obtained GO (1 mg/ml) suspension
was diluted with 8 ml distilled water followed by addition of 0.2 ml ammonia solution
(
NH OH; 30%, Sigma Aldrich) and 0.02 ml hydrazine hydrate (N H ; 60%, Sigma Aldrich).
4
2
4
The brown suspension was subsequently heated and stirred at 90 °C in a water bath for
2
0 min to form a black stable suspension of rGO with a concentration of 0.5 mg/ml.
Cerium oxide (CeO ): CeO was prepared via a precipitation method modified from
2
2
the one described by Gawande et al. [36]. Firstly, 100 ml of 0.1 M ammonium cerium(IV)
nitrate ((NH ) Ce(NO ) ; purity 99%, Acros organics) aqueous solution was adjusted to
4
2
3 6
pH 12 with 0.5 M NaOH followed by magnetic stirring at room temperature for 2 h. After
that the precipitate was centrifuged, washed with distilled water until neutral, and finally
dried in oven at 90 °C for 24 h to obtain nanocrystalline CeO2.
Modified TiO /rGO/CeO / photocatalysts: The required amounts of modified TiO
2
2
2
and CeO (1 wt%) [11] were ultrasonically dispersed in 100 ml de-ionized water for 30 min.
2
To protonate the metal oxide surfaces, the mixture pH was adjusted to 3-4 with 0.5 M
6

hydrochloric acid (HCl, 35.4%, Loba Chemie). Then, 0.75 wt% rGO suspension was added
dropwise to the mixture of modified TiO and CeO under magnetic stirring for 1 h. The
2
2
mixture was subsequently dried at 90 ºC for 24 h to obtain the modified TiO /rGO/CeO
2
2
composite photocatalysts.
2
.2 Characterization
Sample morphology was investigated by transmission electron microscopy (TEM;
Tecnai 20 Twin) and scanning electron microscope (SEM; FEI Quanta 250). Elemental
composition was evaluated from energy-dispersive spectroscopy (EDS, Oxford instrument X-
MAX 20) equipped with SEM. The particle size was estimated from measuring at least 200
particles observed in TEM images using the ImageJ program [37]. Crystal and molecular
structures were identified by X-ray diffractometry (XRD; Bruker AXS: D8 Advance, 40 kV,
Cu Kα with indexing from the JCPDS (ICDD) database) and Raman spectrometry (Thermo
Scientific: DXR Smart, 532 nm excitation solid state laser). The crystallite size, D, was
estimated from the XRD data using the Debye-Scherrer equation [21]:
푘휆
퐷 = 훽
(Eq. 1)
ℎ푘푙푐표푠휃
where k is a constant taken as 0.94, λ is the wavelength of the X-ray radiation (CuKα
radiation 0.15418 nm), β is the full width at half maximum (FWHM) of intensity of the
hkl
diffraction peak at the Miller indices hkl and θ is the diffraction angle. The specific surface
area was evaluated from nitrogen (N ) absorption and desorption analysis from the Bruanuer-
2
Emmett-Teller model (BET; Autosorp-1). Optical properties were measured using diffuse
reflectance UV-Vis spectrophotometry (DRS; Thermo Scientific evolution 201) and
fluorescence spectrophotometry (Jasco FP-6300; excitation at 325 nm). The optical band gap
energy (E ) was calculated using the Kubelka–Munk method combined with the Tuac’s
g
relation [38, 39]:
αh = A(h - Eg)n
(Eq. 2)
7

where A is a constant, which does not depend on phonon energy, α is absorption, n is a
constant dependent on the band transition, n = ½ for the indirect transition in TiO , h is
2
Planck’s constant (6.626×10-34 Js) and  is the photon frequency (s ). The average bandgap
-1
transition energy, E in unit of eV, was estimated from the intercept of linear region with the
g
1
/2
x-axis in the plot of (αh) in y-axis versus h in x-axis.
2
.3.
CO photocatalytic conversion
2
CO photoconversion was evaluated by dispersing 0.15 g of photocatalysts in 150 ml
2
distilled water. The photoreactor was sealed during the reaction and the solution was
magnetically stirred to prevent sedimentation of the photocatalysts. CO (purity 99.9 %,
2
Praxair) was continuously bubbled through the suspension at 0.25 l/min during the whole
reaction. After elimination of dissolved air and saturation of CO in the aqueous suspension
2
for 30 min, UV light (a 15 W UV-C mercury lamp, peak light intensity 254 nm, Tokiva)
irradiated the solution to stimulate the photocatalytic reaction for 6 h. 2 ml liquid suspension,
containing the generated fuel, was collected at 2 h intervals, and centrifuged to remove the
solid photocatalysts. The composition and quantity of products in liquid form were
determined by gas chromatography (GC; flame ionization detector (FID); Varian CP-3800,
DB-WAX column) using operating conditions following U.S. Environmental Protection
Agency (EPA) method 308 [40] (ref). Methanol and ethanol yields, Y (mol/g of
catalyst.hour), were calculated:
퐶푉
푌
=
(Eq. 3)
퐶퐻 푂퐻 표푟 퐶 퐻 푂퐻
3
2
5
푀푐푎푡 ℎ
where C is the methanol or ethanol concentration (mmol/l), V is the volume of reactant
solution (l), M is the mass of photocatalysts (g) and h is the reaction time (hour)
cat
8

3
. Results and Discussion
3
.1 Surface modification of TiO2
Fig. 1 shows TEM images of pure TiO particles and TiO particles after modifying
2
2
surface with sono-assisted exfoliation. The pure TiO particles (Fig. 1(a)) had smooth
2
surfaces and distorted spherical morphology. The particle size distribution varied between 5-
4
0 nm with an average particle size of 20 ± 7 nm. After irradiation with the ultrasonic wave
in 10 M NaOH solution (Fig. 1 (b)), TiO nanosheets delaminated from the outer surface of
2
the primary TiO particles and some TiO particles appeared with rough surfaces confirming
2
2
the preliminary delamination had occurred. The delamination of TiO nanosheets from layer
2
+
of octahedral TiO crystals was caused by intercalation of Na ions between layers of TiO
2
6
octahedral lattice driven by the acoustic cavitation stimulated by high intensity sonication.
This led to an expansion along the cleavage plane in the TiO crystals and consequently
2
exfoliated the titanate layers from the outmost surface of the parent TiO particles [16, 41].
2
Fig. 1 (c) is a TEM image of the magnified area corresponding to the marked area in Fig. 1
(
b), which clearly shows the formation of TiO nanosheets, after surface modification by
2
sono-assisted exfoliation.
The BET surface areas of the pure and modified TiO , listed in Table 1, shows that
2
2
2
the TiO surface area increased from 59 ± 5 m /g to 101 ± 5 m /g after surface modification.
2
XRD patterns of the pure and modified TiO in Fig. 2 shows that the main diffraction peaks
2
were indexed to a major phase of anatase TiO (JCPDS card No. 73-1764, tetragonal
2
structure) and a minor phase of rutile TiO (JCPDS card No. 21-1276, tetragonal structure)
2
which are typical characteristics of TiO Aroxide P25 (80 wt% anatase and 20 wt% rutile
2
phases [32]). After the surface modification, XRD pattern had a slightly lower peak intensity
and broader peak width indicating a decrease of degree of crystallinity, due to the
delamination of TiO nanosheets from the bulk crystal. The average crystallite size,
2
9

calculated using the Debye-Scherrer equation, was 21 ± 3 nm for the pure TiO and 19 ± 3
2
nm for the modified TiO . The XRD and BET surface area analyses agreed well with the
2
TEM study, in which the formation of TiO nanosheets on the outer surface of the primary
2
TiO particles, was induced by sono-assisted exfoliation and led to a decreased degree of
2
crystallinity and corresponding increase of specific surface area.
3
.2 rGO and CeO nanoparticles
2
As described previously, the rGO was prepared by modified a Hummers’ method. The
rGO sheets can be dispersed in water and remain stable for several weeks, as shown in Fig.
(a). The crystal disorder, degree of hybridization and electronic structure of graphene are
3
very sensitive with Raman spectroscopy, Fig. 3(b) displays the Raman spectrum obtained for
rGO sheets. The spectrum exhibits a D band corresponding to the doubly resonant disorder-
-1
induced mode at 1350 cm and a G band arising from the first-order scattering of the E
2
g
-1
mode at 1590 cm [42]. The rGO sheets show a prominent D band with a I /I of 1.11
D
G
indicative of significant structural disorder created due to the presence of oxygen functional
groups and lower defect density after the reduction of GO to rGO [26, 33]. Fig. 3(c) clearly
shows that the rGO sheets exhibit almost transparent, which is due to good exfoliation of rGO
sheets. However, it is observed that the prepared rGO are slightly multilayer in nature. XRD
pattern of rGO is shown in Fig. 3(d). The well-resolved diffraction peaks of the (002) plane
o
were observed clearly in rGO at 25.9 , indicating lattice spacing of 0.34 nm. A small
diffraction peak at 2θ of 42.2° is associated with the (100) plane of rGO structure [42]. XRD
pattern of CeO nanoparticles obtained by the precipitation method (Fig. 4(a)) showed broad
2
peaks indicating either the particles are semi-crystalline in nature or particles of very small
crystalline size [43]. The XRD peaks observed at 2θꢀ=ꢀ28.5, 33.0, 47.4, and 56.3
correspond to the (111), (200), (220), and (311) planes of CeO2 with cubic fluorite
structure (JCPDS card No. 34-0394). The calculated crystallite size of CeO using the
2
1
0

Debye-Scherrer equation was 2 ± 1 nm. The obtained reflections corresponding to the face-
centered cubic phase with the lattice parameter of a = 5.4223 ± 0.0056 ꢀÅ. Fig. 4(b) shows
optical absorbance of the CeO nanoparticles investigated by diffuse reflectance UV-visible
2
1
/2
spectroscopy. The band gap can be estimated from a plot of (αhν) versus photon energy
(
hν). The obtained band gap for the as-prepared CeO nanoparticles is 2.82 eV which is
2
slightly smaller than 3.19 eV for the bulk CeO due to the contribution of quantum size effect
2
of CeO [43]. Fig. 4(c) shows bright field TEM images illustrating the morphology of the
2
CeO nanoparticles inset with the particle distribution. TEM image shows aggregation of the
2
CeO nanoparticle with non-uniform quasi-spherical nanoparticle with a diameter in the
2
range of 1 - 8 nm.
3
.3 Modified TiO /rGO/CeO / photocatalysts
2 2
The composite photocatalysts were prepared by incorporating rGO and CeO into
2
TiO . For comparison, both pure and surface modified TiO particles were used as main
2
2
active phases for the photocatlytic reaction. Fig. 5 shows XRD patterns of composite
photocatalysts including TiO /rGO modified TiO /rGO, TiO /rGO/CeO , and modified
2
,
2
2
2
TiO /rGO/CeO The XRD patterns of the composites showed only the diffraction peaks
2
2.
corresponding to the major phase of TiO P25 (JCPDS card No. 73-1764 for the anatase and
2
JCPDS card No. 21-1276 for the rutile phases): no diffraction peaks, related to the minor
phases of rGO and CeO , were observed due to the low loading of rGO (0.75 wt%) and CeO
2
2
(
1 wt%) and high dispersion of both species in the composites.
Raman spectroscopy was used to further confirm phase compositions. Raman spectra
of all samples displayed in Fig. 6 showed the characteristic Raman-active modes of TiO P25
2
-1
-1
-1
consisting of the E (144, 195 and 638 cm ), B (397 cm ) and A (517 cm ) for the
g
1g
1g
-1
anatase structure (space group of I41/amd) and E (445 cm ) for the rutile structure (space
g
group of P42/mnm) [44].The Raman-active modes of graphitized structure, rGO, for the D
1
1

-1
2
-1
band (disordered carbon) at 1350 cm and G band (graphitic sp carbon) at 1590 cm were
also observed in the spectra of the composites. Nevertheless, the characteristic F active
2
g
-1
mode of CeO , which typically occurs at ~460 cm [45] was not seen.
2
A TEM image of modified TiO /rGO/CeO photocatalysts, in Fig. 1(d), showed a
2
2
good dispersion of the modified TiO particles on the rGO sheet. Notably, the existence of
2
CeO phase could not be observed clearly in the TEM image and this was likely due to the
2
extremely small size (1-8 nm) and low concentration of nanocrystalline CeO . An SEM
2
image and EDS mapping spectrum of the modified TiO /rGO/CeO composites are presented
2
2
in Fig. 1(e). The EDS elemental spectrum showed X-ray peaks, corresponding to Ti, O, C,
and Ce, confirming the presences of TiO , rGO, and CeO phases in the composites.
2
2
The BET surface area of the composite photocatalysts showed that the contribution
2
from the high specific surface area phases, rGO (293 m /g [42]) and nanocrystalline CeO
2
2
(
228 m /g), led to composites with much higher surface area compared to bare TiO - see
2
Table 1. Despite the increased surface area, after surface modification of TiO , the
2
composites prepared from the modified TiO (modified TiO /rGO (71 m²/g) and
2
2
modified TiO /rGO/CeO (83 m²/g)) possessed lower surface area compared to those of the
2
2
composites prepared from the pure TiO (TiO /rGO (102 m²/g) and TiO /rGO/CeO
2
2
2
2
(
92 m²/g)). The reduced surface area for the modified TiO based composites was attributed
2
to the strong bonding between the protonated Ti-O and carbon-oxygen functional groups on
the rGO surface, caused by breaking the Ti-O-Ti bonding networks, to form nanosheet
structures [16, 22]. Although interaction between the surface modified TiO and rGO led to
2
decreased surface area, the strong connection, between the modified TiO and rGO phases,
2
was suggested to enhance photocatalytic activity by facilitating the flow of electrons from
TiO to rGO during photoexcitation [22, 25].
2
1
2

The optical properties of the photocatalysts were investigated using diffuse
reflectance UV-visible and fluorescence spectroscopy. Fig. 7 shows the reflectance spectra of
all samples with the Tuac plot of the Kubelka–Munk function for estimating the optical band
gab energy (E ) inset. The onset absorption onset and E values are summarized in Table 1. It
g
g
is interesting to note that E of the pure TiO P25 was 3.14 eV, comparable to those reported
g
2
elsewhere (3.1 to 3.28 eV) [46, 47]. After sono-assisted exfoliation, the absorption edge for
the modified TiO had a slight red shift and lower E at 3.07 eV. The band gap reduction for
2
g
the modified TiO was attributed to a variation of defects in the TiO network and
2
2
stoichiometric deficiency in the Ti/O ratio caused by formation of nanosheets [16, 48]. For
the composite photocatalysts, the contribution from the lower E phases (rGO 1-1.69 eV
g
[
49, 50] and CeO 2.82 eV) led to slightly lower E 3.05-3.02 eV in the TiO composites with
2 g 2
the modified TiO /rGO/CeO had the lowest E at 3.02 eV.
2
2
g
The fluorescence spectra of the pure TiO , surface modified TiO and their
2
2
composites measured in range of 350-600 nm with excitation energy at 325 nm are presented
in Fig 8. All samples showed the characteristic emission bands of TiO P25 centered around
2
3
64, 401, 452 and 498 nm [31, 38, 51]. As the emission intensity was related to
photogenerated electron-hole recombination, the weaker emission of the surface modified
TiO compared to the pure TiO likely indicated the better charge separation resulting from
2
2
defect formation of oxygen vacancies along the exposed TiO nanosheets facets that acted as
2
electron traps to separate the charge carriers [48, 52, 53]. For composites, incorporation of
rGO and CeO in TiO tended to lower the emission intensity and this was attributed to a
2
2
multi-step charge transfer due to heterojunction effect.
3
.4 CO photoconversion
2
The efficiency for conversion of CO to liquid fuels under UV irradiation using the
2
photocatalysts was measured: the production yields at 6 h reaction time are presented in
1
3

Fig. 9 and Table 2. The pure TiO , surface modified TiO and their composites can be used
2
2
for converting CO to methanol and ethanol, where the methanol production is ~2 times
2
higher than that of ethanol . For the surface modified TiO methanol (351 mol/g h) and
2
,
cat
ethanol (134 mol/g h) yields were ~3 times higher than for pure TiO (methanol 208
cat
2

mol/g h and ethanol 143 mol/g h). This showed that surface modification of TiO
cat
cat
2
particles via sono-assisted exfoliation significantly enhanced the CO photoconversion
2
activity. This improvement was ascribed to the unique mophology of the modified TiO2
induced by irradiating with high intensity ultrasonic waves that yielded two effects (a) the
high surface density of reactive sites of the nanosheet structure and (b) the high degree of
crystallinity and good optical properties of the bulk crystal structure. This is consistent with
previous study of the photocatalytic degradation of methylene blue dye [16]. Production
yields from pure TiO reported here are comparable to those reported previously [1-8, 54].
2
However, the formation rates obtained from different studies may vary over a wide range due
to differences in photoreaction conditions and experimental settings.
Comparing the composite photocatalysts, including TiO /rGO, modified TiO /rGO,
2
2
TiO /rGO/CeO , and modified TiO /rGO/CeO , the production yields of the modified TiO
2
2
2
2
2
based composites were higher than those of the pure TiO based composites, because of the
2
superior photocatlytic properties of the modified TiO . The composites containing both rGO
2
and CeO species also showed higher production yields than composites containing only
2
rGO. Among these composites, the modified TiO /rGO/CeO photocatalysts exhibited the
2
2
highest methanol (641 mol/g h) and ethanol (271 mol/g h) yields, which were almost 7
cat
cat
times higher than those of the pure TiO . The large enhancement of the photocatalytic
2
activity of modified TiO /rGO/CeO composites was due to the combined benefits of the
2
2
surface modified TiO and the heterojunctions occurring in the interface of modified TiO ,
2
2
rGO, and CeO phases.
2
1
4

The mechanism of photocatalytic enhancement for CO conversion over the modified
2
TiO /rGO/CeO composites driven by UV radiation in the presence of water to obtain
2
2
methanol and ethanol is shown in Figure 10. The energy band theory, based on the relative
positions of conduction band (CB), valence band (VB) and oxidation potentials, was used to
explaining the diagram. The band edges of the conduction band (E ) and valance band (E )
CB
VB
of the modified TiO and CeO were estimated from [55]:
2
2
EVB = - E + 0.5E
(Eq. 4)
(Eq. 5)
e
g
ECB = E – E
VB
g
where  is the absolute electronegativity of the semiconductor (5.81 eV for TiO and 5.56 eV
2
for CeO [55]), E is the free electron energy on the hydrogen scale (4.5 eV [55]) and E is
2
e
g
the semiconductor band gap (3.07 eV for modified TiO and 2.82 eV for CeO ). Therefore,
2
2
the EVB and ECB as the normal hydrogen electrode (NHE) of modified TiO were 2.85 eV and
2
-
0.23 eV and, for CeO were 2.47 eV and -0.35 eV, respectively. In general, the
2
photocatalytic conversion of CO in the presence of water as a reducing agent is a two-step
2
process, including water splitting (photooxidation) and CO photoreduction [2, 5]. During
2
photoconversion, both modified TiO and CeO were activated under UV light, forming
2
2
+
-
photogenerated holes (h ) in the VB and electrons (e ) in the CB. Next, the holes from the
modified TiO VB were transferred to the CeO VB, through the interface as E (TiO ) =
2
2
VB
2
2
.85 eV > EVB (CeO ) = 2.47 eV. Then the photogenerated holes subsequently oxidized H O
2 2

+
(
photoreduction process) into hydroxyl radical (OH ), proton (H ) and oxygen (O ), which
2
would further interact with intermediate species obtained from the photoreduction process
see later). Meanwhile, the photogenerated electrons at the CeO CB were simultaneously
(
2
transferred to the modified TiO CB as the modified TiO E (-0.23 eV) is more positive
2
2
CB
than that of CeO (-0.35 eV). Then electrons in the modified TiO CB spontaneously
2
2
migrated to its surface and then to the rGO sheet as the calculated work function () of rGO
1
5

(
-4.42 eV in vacuum) is less positive than that of the ECB of TiO (-4.2 eV in vacuum) [22,
2
2
3]. Then multiple electrons are collected and transported along the rGO sheet to participate
in photoreduction with the absorbed CO at the conjugation between CO and rGO to form
2
2
•
•
+
the intermediate species e.g. CO and CO, which finally reacted with H from
2
photooxidation to obtain CH OH and C H OH [29].
3
2
5
This mechanism, with the combination of modified TiO , rGO, and CeO to form
2
2
complex heterojunction photocatalysts, provides several advantages over the pure TiO . They
2
are:
(a) The narrowed band gap of the modified TiO /rGO/CeO composites at 3.02 eV
2 2
allowed the photocatalysts to be activated with lower energy light and thus increased
the light absorption capability and the number of photogenerated charge carriers.
b) The multi-step charge transportation, resulting from the formation of heterojunction,
effectively separated the charge carriers and hindered electron-hole recombination,
which, in turn, lengthened charge carrier lifetimes to participate in photocatalytic
reactions.
(
(c) The excellent photocatalytic properties of each individual component i.e. the low
electron-hole recombination rate of CeO , due to the two oxidation states of cerium
2
3
+
4+
(
Ce and Ce ) [11, 27, 28], the large specific surface area and good electron mobility
of rGO, and especially the high surface reactive sites and good optical properties of
the surface modified TiO also played an important role to boost the photocatalytic
2
activity.
(
d) The high interfacial contact area between the reactive TiO nanosheets exposed on the
2
surface of the bulk TiO crystal, CeO and rGO and the strong connection, e.g.
2
2
chemical bond interaction between d orbitals (CB) of TiO and the π orbitals of rGO
2
to form d-π electron orbital overlap [8, 22, 29], facilitated the flow of large numbers
1
6

of charge carriers (both electrons and holes) to react with the absorbed species, which
required for the photoconversion of CO to methanol and ethanol as shown in
2
equations (6) and (7):
+
-
CO + 6H + 6e  CH OH + H O
-0.38 eV
-0.33 eV
(Eq. 6)
(Eq. 7)
2
3
2
+
-
2
CO + 9H + 12e  C H OH + 3H O
2 2 5 2
4
. Conclusion
Ultrasonically-assisted surface modified TiO /rGO/CeO composite photocatalysts
2
2
were developed and shown to be successful in photoconversion of CO to methanol and
2
ethanol. The TiO particle surfaces were firstly modified to enhance surface reactive area and
2
optical properties via the sono-assisted exfoliation method. Then 0.75 wt% rGO and 1 wt%
CeO were incorporated into the modified TiO to further increase the photocatalytic
2
2
performance. The modified TiO /rGO/CeO showed much better CO photoconversion
2
2
2
efficiency compared to the pure TiO by providing high yields of methanol (641 mol/g h)
2
cat
and ethanol (271 mol/g h). The combination of modified TiO /rGO/CeO led to complex
cat
2
2
heterojunction photocatalysts with narrowed band gap, high interfacial contact area and
strong connection between each phase: all of these factors combined to significantly enhance
CO photoconversion rates.
2
Acknowledgements
This study was supported by Thailand Research Fund [MRG618236], Malaysia-
Thailand Joint Authority (MTJA), Research Cess Fund. N. V. was supported by Faculty of
Science, King Mongkut's Institute of Technology Ladkrabang (KMITL) [2562-01-05-46].
Authors thank E. Thasirisap, J Sawasdee, P. Wuttiyanan, S. Sarnjaikha, P. Soranakom, P.
Keadsomboon and M. Saengsawang for laboratory assistance.
1
7

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2
1

List of Tables
Table 1. Surface areas and optical properties
Table 2. Yields of methanol and ethanol for the photocatalytic conversion of CO over
2
various photocatalysts.
List of Figures
Fig. 1. TEM images of (a) TiO ; inset particle size distribution, (b) modified TiO (c)
2
2,
modified TiO (magnified image of the marked area in (b)), (d) modified TiO /rGO/CeO , and
2
2
2
(
e) SEM image and EDS spectrum of modified TiO /rGO/CeO .
2
2
Fig. 2. XRD patterns of (a) TiO and (b) modified TiO .
2
2
Fig. 3. (a) photograph of rGO suspension, (b) Raman spectrum, (c) TEM image, and (d) XRD
pattern of rGO.
Fig. 4. (a) XRD pattern, (b) UV-visible diffuse reflectance spectrum; inset Tuac plot of the
Kubelka–Munk function for E determination, and (c) TEM image of CeO nanoparticles;
g
2
inset particle size distribution.
Fig. 5. XRD patterns of (a) rGO, (b) CeO , (c) TiO /rGO, (d) modified TiO /rGO,
2
2
2
(
e) TiO /rGO/CeO , and (f) modified TiO /rGO/CeO .
2 2 2 2
Fig. 6. Raman spectra of photocatalysts.
Fig. 7. UV-visible diffuse reflectance spectra; inset Tuac plot of the Kubelka–Munk function
for E determination of photocatalysts.
g
Fig. 8. Fluorescence spectra of photocatalysts.
Fig. 9. Yields of methanol and ethanol conversion over photocatalysts.
Fig. 10. Mechanism for photocatalytic conversion of CO with H O to metanol and ethanol
2
2
over modified TiO /rGO/CeO photocatalysts.
2
2
2
2

Table 1. Surface areas and optical properties.
Surface area
m²/g)
Absorption edge
(nm)
Eg
(eV)
Photocatalyst
(
TiO2
Modified TiO2
59 ± 5
101± 5
102 ± 5
71 ± 5
92 ± 5
83 ± 5
385
395
387
396
392
398
3.14
3.07
3.05
3.04
3.03
3.02
TiO /rGO
2
Modified TiO /rGO
TiO /rGO/CeO
2
2
2
Modified TiO /rGO/CeO
2
2
Table 2. Yields of methanol and ethanol for the photocatalytic conversion of CO over
2
various photocatalysts.
CO conversion (mol/g h)
2
cat
Photocatalyst
Methanol
Ethanol
TiO2
92
41
Modified TiO2
351
208
320
401
641
134
143
181
147
271
TiO /rGO
2
Modified TiO /rGO
2
TiO /rGO/CeO
2
2
Modified TiO /rGO/CeO
2
2
2
3

Fig. 1. TEM images of (a) TiO ; inset particle size distribution, (b) modified TiO (c)
2
2,
modified TiO (magnified image of the marked area in (b)), (d) modified TiO /rGO/CeO , and
2
2
2
(
e) SEM image and EDS spectrum of modified TiO /rGO/CeO .
2
2
2
4

Fig. 2. XRD patterns of (a) TiO and (b) modified TiO .
2
2
2
5

Fig. 3. (a) photograph of rGO suspension, (b) Raman spectrum, (c) TEM image, and (d) XRD
pattern of rGO.
2
6

Fig. 4. (a) XRD pattern, (b) UV-visible diffuse reflectance spectrum; inset Tuac plot of the
Kubelka–Munk function for E determination, and (c) TEM image of CeO nanoparticles;
g
2
inset particle size distribution.
2
7

Fig. 5. XRD patterns of (a) TiO /rGO, (b) modified TiO /rGO, (c) TiO /rGO/CeO , and (d)
2
2
2
2
modified TiO /rGO/CeO .
2
2
2
8

Fig. 6. Raman spectra of photocatalysts.
2
9