Novel TiO2/C3N4 Photocatalysts for Photocatalytic Reduction of CO2 and for Photocatalytic Decomposition of N2O

Article abstract of DOI:10.1021/acs.jpca.6b07236

TiO₂/g-C₃N₄ photocatalysts with the ratio of TiO₂ to g-C₃N₄ ranging from 0.3/1 to 2/1 were prepared by simple mechanical mixing of pure g-C₃N₄ and commercial TiO₂

Full text of DOI:10.1021/acs.jpca.6b07236

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Article
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Novel TiO/CN Photocatalysts for Photocatalytic Reduction
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of CO and for Photocatalytic Decomposition of NO
Martin Reli, Pengwei Huo, Marcel Šihor, Nela Ambrožová, Ivana Troppová, Lenka Mat#jová,
Jaroslav Lang, Ladislav Svoboda, Piotr Ku#trowski, Michal Ritz, Petr Praus, and Kamila Ko#í
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b07236 • Publication Date (Web): 05 Oct 2016
Downloaded from http://pubs.acs.org on October 9, 2016
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Novel TiO2/C3N4 Photocatalysts for Photocatalytic
Reduction of CO2 and for Photocatalytic
Decomposition of N2O
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Martin Reli1*, Pengwei Huo1, Marcel Šihor1, Nela Ambrožová1, Ivana Troppová1, Lenka
Matějová1, Jaroslav Lang1, Ladislav Svoboda1,2, Piotr Kuśtrowski4, Michal Ritz2, Petr Praus1,2,
Kamila Kočí1,3
1 Institute of Environmental Technology, 2 Department of Chemistry, 3 Centre ENET, VŠBꢀTU
Ostrava, 17. listopadu 15/2172, 708 33 Ostrava, Czech Republic
4 Faculty of Chemistry, Jagiellonian University in Kraków, ul. Ingardena 3, 30ꢀ060 Kraków,
Poland
Abstract
TiO2/gꢀC3N4 photocatalysts with the ratio of TiO2 to gꢀC3N4 ranging from 0.3/1 to 2/1 were
prepared by simple mechanical mixing of pure gꢀC3N4 and commercial TiO2 Evonik P25. All the
nanocomposites were characterized by Xꢀray powder diffraction, UVꢀvis diffuse reflectance
spectroscopy, photoluminescence, Xꢀray photoelectron spectroscopy, Raman spectroscopy,
infrared spectroscopy, transmission electron microscopy, photoelectrochemical measurements
and nitrogen physisorption. The prepared mixtures along with pure TiO2 and gꢀC3N4 were tested
for the photocatalytic reduction of carbon dioxide and photocatalytic decomposition of nitrous
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oxide. The pure gꢀC3N4 exhibited the lowest photocatalytic activity in both cases pointing to a
very high recombination rate of charge carriers. On the other hand, the most active photocatalyst
towards all the products was (0.3/1)TiO2/gꢀC3N4. The highest activity is achieved by
combination of number of factors: (i) specific surface area, (ii) adsorption edge energy, (iii)
crystallite size and (iv) efficient separation of the charge carriers. Where the efficient charge
separation is the most decisive parameter.
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1. Introduction
The greenhouse effect and global warming are the most discussed topics of present days. A
number of initiatives is trying to come up with a way how to decrease emissions of greenhouse
gases. CO2, CH4, N2O and fluorinated gases belong among anthropogenic greenhouse gases.
Carbon dioxide and nitrous oxide is one of the highest contributors to global warming based on
their radiative forcing and concentration at the atmosphere 1.
Carbon dioxide’s radiative forcing is around 1.74 W mꢀ2 and it is estimated CO2 is the largest
contributor in global warming accounting for 63 % of total radiative forcing 2. Carbon dioxide is
mainly produced from fossil fuels combustion and it is estimated more than 31 billion tons of
CO2 is produced annually all over the world 3. The CO2 concentration in the atmosphere already
crossed 400 ppm in most places and keeps increasing approximately about 2 ppm per year 4. One
of the closely watched methods how to utilize carbon dioxide is its photocatalytic reduction. The
reaction is mostly done under UV irradiation and the reaction scheme is as follows:
ꢀꢁꢂ + ꢃꢂꢁ ꢎꢄꢅꢏ,ꢏꢆꢏꢄꢏꢇꢏꢈꢇꢏꢉꢏꢊꢏꢈꢏꢊꢏꢋꢏꢌꢐꢍꢈ ꢀꢑꢒꢓꢔꢕꢑꢖꢗꢔꢘꢙ ꢚꢒꢔꢛꢘꢖꢜꢙ + ꢁꢂ
(1)
Nitrous oxide is the third highest contributor to global warming, accounting approximately for
6.2 % of global radiative forcing. The concentration of N2O at the atmosphere is quite low
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(320 ppb) but its contribution to global warming potential is nearly 300 times higher than the one
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of CO2 . The photoinduced decomposition is focused on both indoor and outdoor abatement of
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nitrous oxide. The reaction is carried out under ambient conditions and UV or visible irradiation
as:
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ꢝꢂꢁ ꢎꢄꢅꢏ,ꢏꢆꢏꢄꢏꢇꢏꢈꢏꢇꢏꢉꢊꢏꢈꢏꢊꢏꢋꢏꢌꢐꢍꢈ ꢝꢂ + ꢂꢞ ꢁꢂ
(2)
The photocatalytic reduction of carbon dioxide attracted great attention after the successful
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photoelectrocatalytic reduction of CO2 by Innue et al. . Since then the number of publication
focusing on the photocatalytic reduction of CO2 rises exponentially every year. Due to high
attention of this topic there are several thorough reviews summing up the state of art 6ꢀ9. Nitrous
oxide on the other hand is attracting only partial attention of the photocatalytic society and to the
best of authors´ knowledge there are no reviews summarizing achievements in the field of
photocatalytic reduction of nitrous oxide. Matsuoka et al. conducted the photocatalytic reduction
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of N2O in the presence of zeolite photocatalysts in the beginning of this millennia
but after
that there have not been reported any papers dealing with photocatalytic reduction of nitrous
oxide. Our group published several papers about photocatalytic reduction of N2O in the presence
of ZnS immobilized on montmorillonite and TiO2 based photocatalysts 12ꢀ14
.
The most studied photocatalyst is, without any doubt, titanium dioxide. Nevertheless, carbon
nitride has attracted huge attention in last couple years. Especially graphitic carbon nitride (gꢀ
C3N4) has been used as photocatalyst quite extensively. The gꢀC3N4 is one of materials with
medium sized band gap (2.7 – 2.8 eV) capable of visible light absorption. It is also relatively
chemically, photochemically and thermally stable semiconductor. The main drawback of gꢀC3N4
is its high recombination rate of generated charge carriers which is resulting in low
photocatalytic performance. The extensive review of gꢀC3N4 of Dong et al. summarizes its
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properties and utilization as photocatalyst . The review of Ye et al. goes even further and
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summarizes the fabrication methods of C3N4 and its utilization for photocatalytic water splitting
and CO2 reduction 16.
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In order to slow down the recombination rate of charge carriers the creation of heterojunction
on the semiconductor’s surface has been studied. Various different heterojunctions have been
studied and tested to improve the photocatalytic activity of gꢀC3N4 17ꢀ19. Yu et al. successfully
prepared ZnO/gꢀC3N4 heterostructure and used it for the photocatalytic reduction of carbon
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dioxide and obtained results twice higher than in the presence of pure ZnO or gꢀC3N4
.
Although there are many possibilities for creating the heterostructure the TiO2/gꢀC3N4 is one of
the most extensively studied one. There have been many studies focused on TiO2/gꢀC3N4 for its
superior visible light absorption 21ꢀ33. The most of the mentioned studies tested the photocatalytic
activity of the TiO2/gꢀC3N4 composite on degradation of organic pollutants in liquid phase. This
work was focused on testing the photocatalytic activity of mechanically created mixtures of TiO2
and gꢀC3N4 for two different reactions, the photocatalytic reduction of CO2 and photocatalytic
reduction of N2O, respectively.
2. Experimental
2.1. Preparation of TiO2/g-C3N4 photocatalysts
The gꢀC3N4 was synthesized by simple heating of melamine in the muffle furnace. 10 g of
melamine powder as precursor were heated to 550 °C at a heating rate of 10 °C/min in a covered
crucible. The temperature was kept at 550 °C for 2 h and then cooled down to room temperature.
A yellow product of gꢀC3N4 was obtained and was ground to a fine powder.
As a precursor of TiO2 commercial P25 (Evonik) was used. The TiO2/gꢀC3N4 composites were
prepared by mechanical mixture and calcined in a muffle furnace at 450 °C for 2 h with
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temperature increasing at the rate of 10 °C/min. The samples were labelled according to weight
ratio of TiO2/gꢀC3N4 as follows: (0.3/1), (0.5/1), (1/1) and (2/1) TiO2/gꢀC3N4.
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2.2. Characterization of photocatalysts
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Xꢀray powder diffraction patterns (XRD) were measured using a Bruker D8 (Bruker AXS)
diffractometer equipped by a detector sensitive for the placement (VÅNTEC 1). The CoKα lamp
was used as a source of irradiation of wavelength λ = 0.178897 nm. Measurements were done in
the conventional Bragg–Brentano setup.
Nitrogen physisorption at 77 K was performed on a 3Flex automated volumetric apparatus
(Micromeritics Instruments, USA) after degassing of materials at 150 °C for more than 24 h
under vacuum below 1 Torr. Degassing at low temperature was applied to remove physisorbed
water, but having no influence on the porous morphology of the developed materials. The
specific surface area was calculated according to the classical Brunauer–Emmett–Teller (BET)
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theory for the p/p0 range of 0.05–0.30 . As the specific surface area, SBET, is not a proper
parameter in the case of mesoporous solids containing micropores 35, the mesopore surface area,
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Smeso, and the micropore volume, Vmicro, were also evaluated based on the tꢀplot method with
the Cmodif constant 35, 37. The net pore volume, Vnet, was determined from the nitrogen adsorption
isotherm at maximum p/p0 (~0.99). The poreꢀsize distribution was evaluated from the adsorption
branch of the nitrogen adsorption desorption isotherm by the Barrett–Joyner–Halenda (BJH)
method 38 using the de Boer standard isotherm and assuming cylindrical pore geometry.
IR spectra of samples were measured by potassium bromide pellets technique. Exactly 1.0 mg
of sample was ground with 200 mg dried potassium bromide. This mixture was used to prepare
the potassium bromide pellets. The pellets were pressed by 8 tons for 30 seconds under vacuum.
The IR spectra were collected using FTꢀIR spectrometer Nexus 470 (ThermoScientific, USA)
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with DTGS detector. The measurement parameters were the following: the spectral region of
4000ꢀ400 cmꢀ1, spectral resolution 4 cmꢀ1; 64 scans; the HappꢀGenzel apodization. Treatment of
spectra: polynomial (second order) baseline, subtraction spectrum of pure potassium bromide.
A 180° sampling was used as measurement technique of Raman spectroscopy. The Raman
spectra were measured at dispersive Raman spectrometer DXR SmartRaman (ThermoScientific,
USA) with CCD detector. The measurement parameters were as follows: excitation laser 780
nm, grating 400 lines/mm, aperture 50 ꢀm, exposure time 1 second, number of exposures 1000,
the spectral region of 1800ꢀ50 cmꢀ1. An empty sample compartment was used for background
measurement. Treatment of spectra: fluorescence correction (6th order).
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The UV–visible spectra of prepared nanocomposites were recorded using a UVꢀ2600
spectrophotometer with an ISRꢀ2600 integrating sphere attachment (Shimadzu Scientific Co.,
Japan) from 220 to 1400 nm. Sampling interval was 1 nm and width of slit was 2 nm. Barium
sulphate (BaSO4) was used as reference material.
Photoluminescence (PL) spectra were measured by a spectrometer FLS920 (Edinburgh
Instrument Ltd, UK). The spectrometer was equipped with a 450 W Xenon lamp (Xe900). The
excitation wavelength was 325 nm. The width of excitation and emission slits was 0.5 nm.
Gas chromatography with isotope ratio mass spectrometry (GC/IRMS) DELTA Advantage
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(Thermo Scientific, USA) was used for the detection of C in gas samples before and after
photocatalytic reaction.
Xꢀray photoelectron spectra (XPS) were recorded using a hemispherical VG SCIENTA R3000
analyzer with constant pass energy of 100 eV, a monochromatized aluminum source Al Kα (E =
1486.6 eV) and a low energy electron flood gun (FS40AꢀPS) to compensate the charge on the
surface of nonconductive samples. The base pressure in the analytical chamber was 5×10ꢀ9 mbar.
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The binding energies were referenced to C 1s core level (Eb = 284.6 eV). The composition and
chemical surrounding of the sample surface were investigated on the basis of the areas and
binding energies of Ti 2p, O 1s, N 1s and C 1s photoelectron peaks. The fitting of high resolution
spectra was provided through the Casa XPS software.
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The morphology of particles was observed on a transmission electron microscope (TEM)
JEOL 2010 HC (JEOL Ltd., Japan). The particles were dispersed in ethanol and using an
ultrasonic sprayer deposited on a TEM grid with carbon holey support film.
Photoelectrochemical measurements were performed using a photoelectric spectrometer
equipped with the 150 W Xe lamp and coupled with the PꢀIF 1.6 potentiostat (Instytut
Fotonowy, Poland). The photocurrent responses were recorded using a classical three electrode
setup. The platinum wire and Ag/AgCl were used as the auxiliary and reference electrodes,
respectively. The working electrode consisted of photocatalyst powder deposited onto indiumꢀtin
oxide (ITO) foil coated by polyethylene terephthalate. The 0.1 M KNO3 was used as an
electrolyte solution. The photocurrent spectra were recorded within the range of 240 – 500 nm
with the step of 10 nm in the potential range of ꢀ 0.2 to 0.8 V, step 0.1 V. Before the
measurement itself the electrolyte was purged by argon to ensure oxygen free environment. The
argon purge was also kept constant during the measurement.
2.3. Photocatalytic reduction of CO2
The photocatalytic reduction of carbon dioxide was carried out in a homemade apparatus (
Figure 1). A cylindrical stirred batch reactor with a photocatalyst spread on the bottom of the
reactor was illuminated by an 8 W Hg lamp (λ = 254 nm; intensity = 0.5 mW/cm2) placed in
horizontal position on top of the quartz glass visor. The reactor shell was made from stainless
steel. The internal volume was 355 cm3. The photocatalyst powder loading was 0.1 g. Before the
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irradiation was turned ON, a supercritical fluidꢀgrade CO2 with a certified maximum of
hydrocarbons less than 1 ppm (SIAD Technical Gases, CZ) to avoid any hydrocarbon
contamination was purged through distilled water reservoir to saturate it by humidity (50 rel.%).
Afterwards the water saturated CO2 was purged through the reactor in order to displace the air
from the reactor. The CO2 pressure was maintained at 120 kPa through the purging for 20
minutes. The pressure inside the reactor was continuously monitored. The reactor was
pressurized to 140 kPa and sealed. The sample at 0 hour was taken and immediately analyzed on
barrier discharge ionization detector (GC/BID). The photocatalytic reaction was started by
switching on the Hg lamp. Gas samples were discontinuously taken at various times during the
irradiation (8 hours). Gas sampling was performed using a gasꢀtight syringe (10 ml) through a
septum. GC/BID equipped by ShinCarbon ST micropacked column was used for the analysis of
reaction products in the gas phase.
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Humidity sensor
UV lamp
CO2
cylinder
Discontinuous sampling
Figure 1: The schematic description of the CO2 photocatalytic reduction apparatus.
The accuracy of the experiment was verified by a series of repeated measurements, and the
relative error of product yields (ꢀmol/gcat) was determined to be less than 10 %. Blank tests were
carried out to guarantee that the methane production was due to the photocatalytic reduction of
CO2 and to eliminate the surrounding interference. During the first test, the reactor was UVꢀ
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illuminated without the photocatalyst, the second blank test was conducted in the dark with the
photocatalyst under the same experimental conditions, and the third test consisted of irradiating
the photocatalyst in the absence of CO2. No hydrocarbons were detected in the above blank tests.
To double check the products are really coming from the photocatalytic reduction of CO2 the
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measurement with CO2 containing C isotope was conducted. Samples were taken at various
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times and the increase of C amount in methane was observed with increasing irradiation time.
These measurements confirmed that the methane is a product of the CO2 photocatalytic
reduction.
2.4. Photocatalytic reduction of N2O
The photocatalytic reduction of N2O was carried out in a homemade apparatus at ambient
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temperature with and without the photocatalyst (photocatalysis and photolysis, respectively) .
The 0.1 g of photocatalyst was placed on an adhesive tape inserted on the bottom of the batch
reactor (635 ml volume). In both photochemical and photocatalytic experiments, the reactor was
filled with a N2O/He mixture (968 ppm) and irradiated by an 8 W Hg lamp (λ = 254 nm;
intensity = 0.5 mW/cm2). Pressure was continuously monitored inside the photoreactor during
the experiments. A gas chromatograph equipped by ShinCarbon ST micropacked column and
GC/BID was used for the analysis of N2O decrease. The concentration of N2O was measured
before switching on the UV lamp to determine initial concentration and during the reaction. The
reproducibility of the photocatalytic experiments was verified by repeated tests in the interval of
0–18 h. The accuracy of the measurement was certified by a series of repeated measurements,
and the relative error of the N2O concentration was determined to be less than 5 %.
3. Results and discussion
3.1. Photocatalysts characterization
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Results from nitrogen physisorption, UVꢀVis DRS and XRD are summarized in Tab. 1. The
specific surface area of mixed photocatalysts increased significantly compared to pure gꢀC3N4.
Due to very organized arrangement of gꢀC3N4 crystal lattice its specific surface area is very low.
Usually the value is somewhere around 10 m2/g 21, 39. Excluding pure gꢀC3N4 all the prepared
photocatalysts and TiO2 Evonik P25 had very similar specific surface area with a value close to
50 m2/g.
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Absorption edge energy is one of the key aspects of the photocatalysis (Tab. 1). Graphitic
C3N4 has relatively low band gap energy which is one of the reasons why this material attracted
great attention of the photocatalytic society. The band gap energy of commercial TiO2 Evonik
P25 is quite high, 3.26 eV in our case and the addition of gꢀC3N4 decreased the absorption edge
energy only slightly (Table 1). The absorption edge energies of photocatalysts were determined
from UVꢀVis DRS spectra (Figure 2b). Based on the UVꢀvis DRS spectra the blue shift can be
observed after addition of TiO2 to pure gꢀC3N4 (Figure 2a). The absorption edges of all TiO2/gꢀ
C3N4 composites are the same within the experimental error. Therefore, the UVꢀvis DRS spectra
of all composites are the same but different from pure TiO2 and gꢀC3N4.
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0
gꢀC3N4
a)
2
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(0.3/1)TiO2/gꢀC3N4
(0.5/1)TiO2/gꢀC3N4
(1/1)TiO2/gꢀC3N4
(2/1)TiO2/gꢀC3N4
TiO2
TiO2
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(2/1)TiO2/gꢀC3N4
(1/1)TiO2/gꢀC3N4
(0.5/1)TiO2/gꢀC3N4
(0.3/1)TiO2/gꢀC3N4
gꢀC3N4
0.5
0
b)
2.1
2.6
3.1
220
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420
520
620
Wavelength (nm)
hν (eV)
Figure 2: a) UVꢀVis DRS spectra of TiO2/gꢀC3N4 photocatalysts and b) the determination of the
absorption edges of TiO2/gꢀC3N4 photocatalysts.
Table 1: Specific surface area, absorption edge energy and crystallite size of prepared
photocatalysts.
Photocatalyst
Specific surface
area (m2/g)
Absorption edge
energy (eV)
Crystallite size
(nm)
Anatase
20
Rutile
29
TiO2 (Evonik P25)
(2/1)TiO2/gꢀC3N4
(1/1)TiO2/gꢀC3N4
(0.5/1)TiO2/gꢀC3N4
(0.3/1)TiO2/gꢀC3N4
gꢀC3N4
51
43
48
49
53
11
3.26
3.13
3.16
3.16
3.13
2.78
23
42
24
41
23
43
23
28
ꢀꢀꢀꢀꢀ
ꢀꢀꢀꢀꢀ
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The commercial TiO2 is composed from anatase and rutile phase which was confirmed by
Raman spectroscopy (Figure 3a). The Ti–O vibrational modes of anatase are located around 395,
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515 and 640 cmꢀ1. The Ti–Ti vibrational modes of anatase are located at 140 and 195 cmꢀ1 40, 41
.
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Rutile’s Ti–O vibrational modes are located around 445, 610, 825 cmꢀ1 and Ti–Ti vibrational
mode of rutile is located around 140 cmꢀ1 41. Since the rutile phase is representing the minority,
the only recognizable peaks were at 445 and 791 cmꢀ1. The other ones are being overlapped by
the anatase vibrational modes.
1000
900
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400
300
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0
3000
2500
2000
1500
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500
143
a)
638
b)
706
1232
100
515
396
749
472
500
196
977
791
0
1,500
1,000
0
1,000
500
Raman shift (cmꢀ1)
0
Raman shift (cmꢀ1)
Figure 3: Raman spectra of a) commercial TiO2 Evonik P25 and b) gꢀC3N4.
The interpretation of pure gꢀC3N4 Raman spectra is a little bit more complicated (Figure 3b).
The number of vibrational modes and their intensity depends on calcination temperature and
temperature ramp rate. Characteristic peaks for gꢀC3N4 calcined at 550°C can be found at 472,
712, 751, 980, 1152 and 1226 cmꢀ1 42, 43
.
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TiO2
b)
a)
TiO2
(2/1)TiO2/gꢀC3N4
(1/1)TiO2/gꢀC3N4
(0.5/1)TiO2/gꢀC3N4
(0.3/1)TiO2/gꢀC3N4
gꢀC3N4
(2/1)TiO2/gꢀC3N4
(1/1)TiO2/gꢀC3N4
(0.5/1)TiO2/gꢀC3N4
(0.3/1)TiO2/gꢀC3N4
gꢀC3N4
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1,500
1,000
500
0
190
170
150
130
110
90
Raman shift (cmꢀ1)
Raman shift (cmꢀ1)
Figure 4: a) Raman spectra of all prepared photocatalysts; b) evolution of the Raman spectra
focused on TiO2 characteristic peak at 143 cmꢀ1.
Figure 4a shows the Raman spectra of all investigated photocatalysts. The decrease of TiO2
characteristic peaks intensity and increase of gꢀC3N4 peaks intensity can be observed with
decreasing ratio of TiO2/gꢀC3N4. Also no peak shifts were observed which means no structural
changes occurred during preparation of mixed photocatalysts regarding to pure TiO2 and gꢀC3N4.
For better resolution the most intense anatase peak part is shown in Figure 4b.
The FTꢀIR spectra of prepared gꢀC3N4, TiO2 Evonik P25 and TiO2/gꢀC3N4 photocatalysts are
shown in Figure 5. In the FTꢀIR spectra of gꢀC3N4 the strong band region from 1200 to 1650 cmꢀ
1 can be found. The peak at 1640 cmꢀ1 is attributed to C–N stretching, while three bands at 1555,
1461 and 1405 cmꢀ1 are the result of the typical stretching vibrations of C–N heterocycles
.
44, 45
The last two bands at 1319 and 1243 cmꢀ1 of the region belong to stretching vibration of
connected units of C–NH–C, which is also supported by the broad band at 2900 – 3500 cmꢀ1.
This broad band is attributed to N–H and O–H stretches due to the free amino groups and
adsorbed hydroxyl species, respectively 45. The peak at 806 cmꢀ1 is attributed to the characteristic
breathing mode of sꢀtriazine 44, 45, which increases with the increase of the gꢀC3N4 content. All
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the gꢀC3N4 characteristic bands are also present in the mixtures of TiO2/gꢀC3N4 with decreasing
intensity as the ratio of TiO2/gꢀC3N4 increase. The following bands are typical for TiO2 based
materials. The broad band in the region of 3200 – 3600 cmꢀ1 can be assigned to OH stretching
modes. The two very small hard recognizable bands at 1600 and 1400 cmꢀ1 are attributed to
bending OH modes of hydroxyl groups. The broad band below at 900 cmꢀ1 belongs to Ti–O
vibrations 46.
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TiO2
(2/1)TiO2/gꢀC3N4
(1/1)TiO2/gꢀC3N4
(0.5/1)TiO2/gꢀC3N4
(0.3/1)TiO2/gꢀC3N4
gꢀC3N4
3400
2400
1400
400
Wavenumber (cmꢀ1)
Figure 5: FTꢀIR spectra of TiO2/gꢀC3N4 photocatalysts.
The photoluminescence spectra of the TiO2/gꢀC3N4 samples are shown on Figure 6. All
samples exhibit similar profiles with a broad emission band from 435 to 600 nm under an
excitation wavelength of 325 nm. These spectra imply that PL intensities of the prepared
nanocomposites are strongly dependent on recombination of charge carriers mainly in gꢀC3N4.
The pure gꢀC3N4 has strong PL emission at wavelength between 440 ꢀ 500 nm which is caused
by fast radiative recombination of electron and holes in the planar structure of gꢀC3N4. Position
47
of emission band of gꢀC3N4 depends strongly on preparation conditions . Weaker PL emission
intensity is caused by electrons and holes transfer between gꢀC3N4 and TiO2 48, 49. With
increasing content of TiO2 the probability of recombination between electrons and holes in
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prepared samples significantly dropped. Similar results was observed on the gꢀC3N4ꢀTiO2
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heterojunction systems by other groups 28, 50, 51
.
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350000
300000
250000
200000
150000
100000
50000
0
gꢀC3N4
(0.3/1)TiO2/gꢀC3N4
(0.5/1)TiO2/gꢀC3N4
(1/1)TiO2/gꢀC3N4
(2/1)TiO2/gꢀC3N4
TiO2
425
475
525
575
625
Wavelength (nm)
Figure 6: Photoluminescence spectra of TiO2/gꢀC3N4 photocatalysts.
gꢀC3N4 (002)
TiO2
(2/1)TiO2/gꢀC3N4
(1/1)TiO2/gꢀC3N4
(0.5/1)TiO2/gꢀC3N4
(0.3/1)TiO2/gꢀC3N4
gꢀC3N4
5.00
15.00
25.00
35.00
45.00
2θ (°)
55.00
65.00
75.00
85.00
Figure 7: XRD patterns of TiO2/gꢀC3N4 photocatalysts.
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Two main diffraction peaks for gꢀC3N4 located at around 14.89 and 31.95°, were observed and
indexed as (100) and (002) planes, respectively (Figure 7). These two diffraction peaks are in
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50
good agreement with the gꢀC3N4 (JCPDS no. 87–1526) . Titanium dioxide was identified in
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composite samples in anatase (JCPDS no. 084–1285) and rutile (JCPDS no. 079–6029) forms.
The crystallite size of the TiO2 was determined from the halfꢀwidth of peaks using Scherrer’s
formula (where K=0.89) on the anatase (101) and rutile (110) peaks. The intensity of gꢀC3N4
peaks in TiO2 composites was attenuated in a simile way as reported 52. The presence of gꢀC3N4
did not have a significant influence on the phase structure of TiO2.
In Figure 8 the examples of highꢀresolution XPS spectra of elements identified on the surface
of TiO2/gꢀC3N4 photocatalyst with the highest gꢀC3N4 loading are shown in comparison to those
measured for pure TiO2 and gꢀC3N4. The C 1s spectrum of (0.3/1)TiO2/gꢀC3N4 can be
deconvoluted into two peaks at 284.6 eV (C1) and 288.1 eV (C2) attributed to CꢀC
(contaminated carbon) and N=CꢀN (sp2ꢀbonded carbon in carbon nitride) coordination,
respectively 53, 54. Moreover, three components are identified in the XPS N 1s spectrum of this
sample at 398.5 eV (N1), 399.8 eV (N2) and 401.1 eV (N3), corresponding to the N atoms in the
C=NꢀC groups, the sp2 hybridized N atoms and the sp3 hybridized terminal N atoms in the
heptazine rings (ꢀNH2), respectively 53, 54. A slight shift (0.2ꢀ0.3 eV) towards lower binding
energies for the C2, N1 and N2 peaks related to the N=CꢀN species is detected after the addition
of TiO2 to gꢀC3N4. This finding indicates changes in electron cloud density in the nitrogen and
carbon atoms, which can result from the interaction of carbon nitride with titania. On the other
hand, the Ti 2p spectra of commercial titania before and after doping with gꢀC3N4 show only two
photoelectron peaks at 458.2ꢀ458.3 eV and 464.0ꢀ464.1 eV, with a peak separation of 5.7ꢀ5.8 eV,
which correspond to Ti4+ 2p3/2 and Ti4+ 2p1/2, respectively 55. The detectable changes in the peak
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positions are observed in the highꢀresolution XPS O 1s spectra for pure TiO2 and TiO2 mixed
with gꢀC3N4. Among two main O 1s components found in pure TiO2, attributed to lattice O2ꢀ
(529.5 eV) and the O2ꢀ ions/hydroxyl groups (530.6 eV) 56, the latter one is shifted to 530.9 eV in
the case of (0.3/1)TiO2/gꢀC3N4, confirming the occurrence of interactions between TiO2 and gꢀ
C3N4 in the formed composites.
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N 1s
O 1s
(0.3/1)TiO2/g-C2N4
TiO2
g-C2N4
(0.3/1)TiO2/g-C2N4
g-C2N4
535
534
533
532
531
530
529
528
527
404
402
400
398
396
Binding energy [eV]
Binding energy [eV]
C 1s
Ti 2p
(0.3/1)TiO2/g-C2N4
g-C2N4
(0.3/1)TiO2/g-C2N4
TiO2
TiO2
292
290
288
286
284
282
468
466
464
462
460
458
456
Binding energy [eV]
Binding energy [eV]
Figure 8: XPS N 1s, O 1s, C 1s and Ti 2p spectra measured for TiO2, (0.3/1)TiO2/gꢀC3N4 and gꢀ
C3N4.
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The morphology of the photocatalysts was examined by TEM. Figure 9 shows the TEM
images of gꢀC3N4, TiO2, and (0.3/1)TiO2/gꢀC3N4. It can be seen that pure gꢀC3N4 (Figure 9a)
indicates a 2D lamellar structure while pure TiO2 (Figure 9b) shows spherical particles. For
(0.3/1)TiO2/gꢀC3N4 (Figure 9c,d), the TiO2 particles are embedded in the gꢀC3N4 lamellar
structure. This finding suggests the creation of a heterojunction between TiO2 and gꢀC3N4, which
could lead to improved separation of electron and hole.
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b)
a)
c)
d)
Figure 9: TEM images of gꢀC3N4(a), TiO2 (b), and (0.3/1)TiO2/gꢀC3N4 (c, d).
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The photocurrent measurements were conducted to predict the photocatalytic activity of the
prepared photocatalysts (Figure 10). It is evident all the prepared photocatalysts except pure gꢀ
C3N4 show large increase of generated photocurrent after switching the light source ON. The
results revealed the pure gꢀC3N4 has the highest rate of charge carriers’ recombination which in
agreement with experimental data. On the other hand, the measurements suggest the
(0.3/1)TiO2/gꢀC3N4 and pure TiO2 possess the most efficient separation of electrons and holes.
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g-C3N4
g-C3N4
TiO2
(0.5/1)TiO2/g-C3N4
(0.3/1)TiO2/g-C3N4
(2/1)TiO2/g-C3N4
(1/1)TiO2/g-C3N4
a)
(0.5/1)TiO2/g-C3N4
(2/1)TiO2/g-C3N4
TiO2
1.2
1.0
0.8
0.6
1.2
1.0
0.8
0.6
b)
(0.3/1)TiO2/g-C3N4
(1/1)TiO2/g-C3N4
325
330
335
340
345
350
300
350
400
450
Wavelength (nm)
Wavelength (nm)
Figure 10: a) Photocurrent generation at electrodes made from the studied photocatalysts and b)
cutoff interval of wavelengths recorded at 0.8 V vs. Ag/AgCl in deoxygenated 0.1 M KNO3.
3.2. Photocatalytic reduction of CO2
The effect of irradiation time on the formation of gaseous products in the photocatalytic
reduction of carbon dioxide was studied in the presence of all prepared TiO2/gꢀC3N4
photocatalysts including pure TiO2 Evonik P25 and gꢀC3N4. Figure 11 demonstrates the
evolution of CO2 photocatalytic reduction products as the function of irradiation time. Methane
and carbon monoxide are the direct products of CO2 reduction. The methane production rate is
more or less linear in the first 8 hours of irradiation (Figure 11a); carbon monoxide yields on the
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other hand, rapidly increase in first 2 hours and then are almost constant (Figure 11b). The last
detected product was hydrogen which is coming from the photocatalytic water (vapor) splitting
(Figure 11c). The highest yields of the three products were reached in the presence of
(0.3/1)TiO2/gꢀC3N4. On the other hand, the lowest products production was achieved over pure
gꢀC3N4.
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(0.3/1)TiO2/gꢀC3N4
(0.5/1)TiO2/gꢀC3N4
(1/1)TiO2/gꢀC3N4
TiO2
(2/1)TiO2/gꢀC3N4
gꢀC3N4
b)
(0.3/1)TiO2/gꢀC3N4
(0.5/1)TiO2/gꢀC3N4
TiO2
(1/1)TiO2/gꢀC3N4
(2/1)TiO2/gꢀC3N4
gꢀC3N4
a)
0
0
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4
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8
0
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4
6
8
Time (h)
Time (h)
350
(0.3/1)TiO2/gꢀC3N4
TiO2
(1/1)TiO2/gꢀC3N4
(0.5/1)TiO2/gꢀC3N4
(2/1)TiO2/gꢀC3N4
gꢀC3N4
300
250
200
150
100
50
c)
0
0
2
4
6
8
Time (h)
Figure 11: Time dependence of the a) methane yields, b) carbon monoxide yields and c)
hydrogen yields over TiO2/gꢀC3N4 photocatalysts in the CO2 photocatalytic reduction.
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Conditions: 8 W Hg lamp, CO2 pressure at carbonation of 120 kPa, humidity 50%, photocatalyst
loading of 0.1 g.
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3.3. Photocatalytic reduction of N2O
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The effect of irradiation time on the photochemical and photocatalytic decomposition of N2O
was investigated for all TiO2/gꢀC3N4 photocatalysts including pure TiO2 and gꢀC3N4 for a time
period of 0–16 h. The time dependences of N2O conversion for photocatalysis and photolysis are
shown in Figure 12. A gradual increase of N2O conversion with increasing time of irradiation
can be noticed. The highest N2O conversion of 57 % was observed for the (0.3/1)TiO2/gꢀC3N4
catalyst after 16 h. On the other hand, both pure TiO2 and gꢀC3N4 photocatalysts exhibited the
same N2O conversions as during the photolysis experiments. The products of the reaction are
oxygen and nitrogen; no other compounds were detected.
60
(0.3/1)TiO2/gꢀC3N4
(1/1)TiO2/gꢀC3N4
(2/1)TiO2/gꢀC3N4
50
(0.5/1)TiO2/gꢀC3N4
gꢀC3N4
40
P25
30
20
10
0
0
4
8
12
16
Time (h)
Figure 12: Time dependence of the N2O conversion over TiO2/gꢀC3N4 photocatalysts.
Conditions: 8 W Hg lamp, N2O (968 ppm)/He mixture, photocatalyst loading of 0.1 g.
3.4. Factors influencing the photocatalytic reduction of CO2 and N2O over different
TiO2/g-C3N4 photocatalysts
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The most active photocatalyst in both cases, the photocatalytic reduction of CO2 and N2O,
respectively, was the one composed of 0.3/1 ratio of TiO2/gꢀC3N4. The photocatalytic
experiments showed the ratio of TiO2 and gꢀC3N4 plays significant role in photocatalysis. It also
confirmed pure gꢀC3N4 is not very active photocatalyst due to its high recombination rate of
charge carriers and very low specific surface area 15. The very high recombination of gꢀC3N4 also
confirms photoluminescence spectra where the gꢀC3N4 band has the intensity several times
higher than the rest of the photocatalysts (Figure 6).
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Even though both studied reactions are reductions the reaction mechanism is different for each
of them. Photocatalytic reduction of carbon dioxide heavily depends on sufficient potentials of
photocatalyst’s conduction and valence bands. CO2 molecule is very stable and direct reduction
by electron (CO2/CO2 ) is limited by very negative potential (–1.9 V vs. NHE) 6, 57. The main
•
products of the photocatalytic reduction of CO2 are methane and carbon monoxide and also byꢀ
product hydrogen. Hydrogen is produced from water which is present in the form of vapor. The
most important redox potentials vs. NHE are given in Eqs. (3) – (8) 6:
2 H+ + 2 e – → H2
E0 = – 0.41 V
E0 = – 0.53 V
E0 = – 0.24 V
E0 = + 0.82 V
E0 = + 2.32 V
E0 = – 0.33 V
(3)
(4)
(5)
(6)
(7)
(8)
CO2 + 2 H+ + 2 e– → CO + H2O
CO2 + 8 H+ + 8 e– → CH4 + 2 H2O
2 H2O + 4 h+ → O2 + 4 H+;
H2O + h+ → OH• + H+;
•–
O2 + e– → O2 ;
The pure gꢀC3N4 proved to be the least efficient towards any of the products. This is in
agreement with gꢀC3N4 valence band not having sufficient potential of generated holes for
oxidation of water to hydroxyl radicals (5). Only the less effective water oxidation, due to more
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holes needed to achieve it, can be conducted. Because of this limitation, lower amounts of H+ are
formed and through these lower yields of all products were detected. The pure TiO2 on the other
hand gives the average yields of CH4 and CO and the higher yields of H2.
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In case of the nitrous oxide reduction the potential of N2O/N2 redox couple is around 1.35 V
which fits in between of valence and conduction bands of both, TiO2 and gꢀC3N4 as well.
Nevertheless, pure TiO2 and C3N4, respectively showed no photocatalytic activity, the results
were the same as in case of photolysis. The addition of TiO2 to C3N4 improved the conversion
about over 10 %.
The photocatalytic activity of semiconductor nanomaterials can be influenced by several
aspects such as: (i) specific surface area, (ii) adsorption edge energy, (iii) crystallite size and (iv)
efficient separation of the charge carriers. All these factors can be significantly influenced by the
formation of TiO2/gꢀC3N4 nanomaterials, which are formed by the addition of gꢀC3N4 to the
TiO2.
For (i), specific surface area of TiO2/gꢀC3N4 has increased with increasing amount of gꢀC3N4.
The increase of the BET specific surface area of TiO2/gꢀC3N4 could be explained due to TiO2
nanoparticles embedded in the gꢀC3N4 lamellar structure leads to the formation of thinner
lamellar gꢀC3N4 (Figure 9). The grinding may lead to the exfoliation of gꢀC3N4, which increases
the surface areas of the photocatalysts. The similar results were obtained by 50. The higher
specific surface area can offer more available active sites and due to that higher photoactivity.
The (0.3/1)TiO2/gꢀC3N4 has shown the highest specific surface area (SBET = 53 m2/g) in
comparison with pure gꢀC3N4 (SBET = 11 m2/g) and also the highest photoactivity. On the other
hand, the decrease of specific surface area with increase of TiO2 content can be explained by
agglomerating of TiO2 (Figure 9c, d).
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For (ii), all the prepared TiO2/gꢀC3N4 demonstrated lower absorption edge energy (Table 1)
and simultaneously suitable position of conduction and valence bands in comparison with pure
TiO2. The decrease of the absorption edge energy is leading to increased effectivity of electron
hole pair generation due to the absorption of lower energy photons.
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For (iii), crystallinity size of rutile can also influence the photocatalytic activity due to
formation of defects. Formation of lattice defects is enhancing electronꢀhole recombination
resulting in lower photocatalytic activity 58. The (0.3/1)TiO2/gꢀC3N4 proved to have the smallest
crystallite size of rutile from all the composites and this fact probably also contributed to the
higher photoactivity of this photocatalyst.
For (iv), the efficient separation of the charge carriers plays probably key role in photocatalytic
activity of TiO2/gꢀC3N4 nanocomposites. The electronic structure of the photocatalyst determines
whether the material is going to be active towards specific reaction or not. The conduction band
potentials of gꢀC3N4 and TiO2 are –1.15 V and –0.3 V, respectively. The valence bands of gꢀ
C3N4 and TiO2 have potentials at 1.5 V and 3.0 V, respectively. The quite big difference between
electronic bands potentials of gꢀC3N4 and TiO2 makes the combination of these two materials
very interesting. The electrons that are photogenerated from the semiconductor with a higher CB
edge (the gꢀC3N4) migrate to the one with a lower CB edge (TiO2). Furthermore, the
photogenerated holes are transported from the low VB of TiO2 to the high VB of gꢀC3N4
between the semiconductors 59. This leads to the accumulation of electrons on TiO2
semiconductor for a reduction reaction and accumulation of holes in gꢀC3N4 semiconductor for
an oxidation reaction (Figure 13). Therefore, the electrons and holes are spatially separated to
effectively repress charge recombination 59.
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RED
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ꢀ 1.15 V
CB
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OX
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CB
2.65 eV
ꢀ 0.3 V
C3N4
3.3 eV
VB
+ 1.5 V
+ 3 V
RED
VB
OX
TiO2
Figure 13: Schematic illustration showing the photogenerated charge separation in TiO2/gꢀC3N4
photocatalysts.
The enhancement in photocatalytic performance can be attributed to the synergetic effect of
the surface area, crystallite size of rutile, absorption edge energy and mainly separation of the
charge carriers which is confirmed by a higher activity of the prepared TiO2/gꢀC3N4
nanocomposites compared to pure gꢀC3N4. The optimal ratio of TiO2 to C3N4 have been found to
be 0.3 to 1. The similar result was observed by Li et. al. who found, that the most active gꢀ
C3N4/TiO2 composite sample investigated for methyl orange degradation contained 74.4 wt.% of
60
gꢀC3N4
.
4. Conclusion
A series of TiO2/gꢀC3N4 photocatalysts with different ratio of TiO2 to gꢀC3N4 (from 0.3/1 to
2/1) have been fabricated by mechanical mixing of gꢀC3N4 prepared from melamine and
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commercial TiO2 Evonik P25. The photocatalytic activity results from photocatalytic CO2
reduction and NO2 decomposition revealed that all of the synthesized TiO2/gꢀC3N4
photocatalysts had enhanced photocatalytic performance compared to their components
especially to gꢀC3N4. The highest photocatalytic activity of these photocatalysts can be explained
by the combination of several aspects: (i) specific surface area, (ii) adsorption edge energy, (iii)
crystallite size and (iv) efficient separation of the charge carriers. The most active photocatalyst
towards both oxides reduction was (0.3/1)TiO2/gꢀC3N4. Based on the obtained results it can be
assumed that the semiconductor nanocomposite formed between gꢀC3N4 and TiO2 can have
application for photocatalytic reaction in environmental remediation.
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Corresponding Author
*Martin Reli, Institute of Environmental Technology, VŠBꢀTU Ostrava, 17. listopadu 15/2172,
708 33 Ostrava, Czech Republic, eꢀmail: martin.reli@vsb.cz; tel.: +420 597 327 304
Acknowledgements
This work was supported by the Grant Agency of the Czech Republic (projects. reg. No. 14ꢀ
35327J and 16ꢀ10527S), by the Ministry of Education, Youth and Sports of the Czech Republic
within “National Feasibility Program I”, project LO1404 “TUCENET” and by EU structural
funding Operational Programme Research and Development for Innovation Project No.
CZ.1.05/2.1.00/19.0388.
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