Highly-active direct Z-scheme Si/TiO2 photocatalyst for boosted CO2 reduction into value-added methanol

Article abstract of DOI:10.1039/c4ra10670a

In the present study, direct Z-scheme Si/TiO₂ photocatalyst was synthesized via a facile hydrothermal reaction using tetrabutyl titanate and Si powder prepared from magnesiothermic reduction of SiO₂ nanospheres. The Si/TiO₂ nanospheres were composed of porous Si nanospheres with a diameter of ～300 nm and TiO₂ nanosheets with a diameter of 50 nm and thickness of 10 nm, and demonstrated superior visible light harvesting ability to either Si nanospheres or TiO₂ nanosheets. CO₂ photocatalytic reduction proved that Si/TiO₂ nanocomposites exhibit high activity in conversion of CO₂ to methanol with the maximum photonic efficiency of 18.1%, while pure Si and TiO₂ catalyst are almost inactive, which can be ascribed to the integrated suitable band composition in the Si/TiO₂ Z-scheme system for CO₂ reduction. The enhanced photocatalytic property of Z-scheme Si/TiO₂ nanospheres was ascribed to the formation of Si/TiO₂ Z-scheme system, which improved the separation efficiency of the photogenerated carriers, prolonged their longevity, and therefore boosted their photocatalytic activity.

Full text of DOI:10.1039/c4ra10670a

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Highly-active direct Z-scheme Si/TiO2
photocatalyst for boosted CO2 reduction into
value-added methanol
Cite this: RSC Adv., 2014, 4, 56961
a
a
Yousong Liu, Guangbin Ji, Mohammed Abdulkader Dastageer,b Lei Zhu,a
*
a
a
a
b
*
Junyi Wang, Bin Zhang, Xiaofeng Chang and Mohammed Ashraf Gondal
In the present study, direct Z-scheme Si/TiO2 photocatalyst was synthesized via a facile hydrothermal
reaction using tetrabutyl titanate and Si powder prepared from magnesiothermic reduction of SiO2
nanospheres. The Si/TiO2 nanospheres were composed of porous Si nanospheres with a diameter of
ꢀ300 nm and TiO2 nanosheets with a diameter of 50 nm and thickness of 10 nm, and demonstrated
superior visible light harvesting ability to either Si nanospheres or TiO2 nanosheets. CO2 photocatalytic
reduction proved that Si/TiO2 nanocomposites exhibit high activity in conversion of CO2 to methanol
with the maximum photonic efficiency of 18.1%, while pure Si and TiO2 catalyst are almost inactive,
which can be ascribed to the integrated suitable band composition in the Si/TiO2 Z-scheme system for
CO2 reduction. The enhanced photocatalytic property of Z-scheme Si/TiO2 nanospheres was ascribed to
the formation of Si/TiO2 Z-scheme system, which improved the separation efficiency of the
photogenerated carriers, prolonged their longevity, and therefore boosted their photocatalytic activity.
Received 19th September 2014
Accepted 23rd October 2014
DOI: 10.1039/c4ra10670a
www.rsc.org/advances
and the effective electron–hole pairs are generated.12 Unfortu-
nately, most of the effective electron–hole pairs are recombined
1. Introduction
and dissipated as heat before they arrive at the photocatalyst
surface, which makes TiO2 an inefficient photogenerated carrier
hampering its charge separation ability.
High consumption of fossil fuels is creating not only an energy
crisis but also environmental pollution and climate change due
to excessive green house gas emissions to the atmosphere. As
carbon dioxide (CO2) is a major contributor to green house
gases, attention is focused on its mitigation all over the world.
Solar-driven photocatalytic conversion of CO2 into added value
fuels is the most promising proposition as it does not only
remove CO2 from effluent gases but also produces hydrocarbon
fuels, which could be used to meet future energy needs. Thus, a
lot of research efforts have focused on developing efficient
catalysts for CO2 photocatalytic reduction. Semiconductor
photocatalysts (such as ZnO,1,2 CdS,3 ZnGa2O4,4 Zn2GeO4,5,6
WO3,7 TiO2 (ref. 8) etc.) have been explored. Among them, tita-
nium dioxide (TiO2) has been extensively studied as an impor-
tant photocatalyst because of its low toxicity, low cost, superior
photocatalytic activity and long-term chemical stability.9–11
However, the wide band gap of TiO2 (ꢀ3.2 eV) limits its efficient
utilization for solar energy conversation as TiO2 only absorbs
light with wavelengths shorter than ꢀ387 nm in the ultraviolet
region. Moreover, aer photoabsorption, the electrons are
excited from the valence band of TiO2 to the conduction band
Research scientists have devoted extensive efforts to address
these problems. Introducing doping elements (such as S,13 N,14
and C (ref. 15)) into TiO2 has been proven to be an effective
approach to narrow the band gap, improve the visible light
absorption and enhance the photocatalytic activity in CO2
reduction. TiO2 modication with metal particles (e.g., Ag,16
Au,17 Pt,18 and Cu (ref. 19)) has been reported to inhibit charge
recombination probability because these metals serve as elec-
tron traps to suppress the recombination of the photogenerated
electron–hole pairs and hence improve the photocatalytic
activity. In addition, coupling TiO2 with a narrower band gap
semiconductor to construct heterojunctions is another effective
approach to accommodate the visible-light photon energy and
improve the photogenerated charge separation and CO2
conversion efficiency. This coupling takes advantage of both the
heterojunction to improve charge separation rate, and the
narrow band gap of the coupled semiconductor to expand the
light absorption region. TiO2 based heterojunctions such as
PbS/TiO2,20 CuO/TiO2,21 FeTiO3/TiO2 (ref. 22) have been repor-
ted in recent years.
aCollege of Materials Science and Technology, Nanjing University of Aeronautics and
Astronautics, Nanjing 210016, P. R. China. E-mail: gbji@nuaa.edu.cn; Tel:
+86-25-5211-2902
Similar to the heterojunction photocatalytic system, the Z-
scheme photocatalytic system also features the spatial isola-
tion of photogenerated electrons and holes, which reduces the
bulk electron–hole recombination.23 However, a Z-scheme
bPhysics Department, King Fahd University of Petroleum and Minerals, Dhahran,
31261, Saudi Arabia. E-mail: magondal@kfupm.edu.sa; Tel: +966-38602351
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ꢁ
photocatalytic system is generally constructed by employing a deionized water and ethanol 3 times and then dried at 100 C
conductor as the electron mediator to form the known Ohmic for 12 h.
contact with low contact resistance. Until now, many Z-scheme
systems have been reported, such as TiO2–Au–CdS system,24
2.2 Synthesis of Si nanospheres
AgBr–Ag–AgI,25 and ZnO–Au–CdS,26 etc. Recently, studies on
0.5 g SiO2 nanospheres and 0.42 g Mg powder were ground for 5
direct Z-scheme photocatalysts (such as g–C3N4–TiO2 and WO3–
minutes and then transferred into a crucible to calcine at 750 ꢁC
NaNbO3 system) have been investigated for photocatalytic water
for 5 h under N2 atmosphere. Aer cooling to room tempera-
splitting,
CO2
conversion
and
photocatalytic
ture, the powder was then added to 50 mL 32 wt% HCl solution
with stirring for 24 h. Si nanospheres were then cleaned several
times by centrifugaꢁtion and water dispersion and nally dried
into powders at 60 C in a vacuum oven for 12 h.
decomposition.27–29
A narrow band gap semiconductor, silicon (1.12 eV), has
drawn considerable interest because of its potential application
in optoelectronic devices and integrated microelectronics.
Recently, studies of silicon materials have reported its prom-
ising photocatalytic activity. Shao et al. prepared hydrogen-
2.3 Synthesis of direct Z-scheme Si/TiO2 nanospheres
terminated Si nanowires (Si NWs) and noble metal-modied Direct Z-scheme Si/TiO2 nanospheres were fabricated via a
(Pt, Pd, Au, Rh, Ag) Si NWs substrates by oxide-assisted facile hydrothermal reaction with tetrabutyl titanate and Si
growth method, and investigated their performance for the nanospheres. In a typical process, 25 mL of Ti(OBu)4 and 1.5 g Si
degradation of Rhodamine B and oxidation of benzyl alcohol to nanospheres were added in a 100 mL Teon pot and 3 mL of
benzoic acid under visible light irradiation.30 Independently, hydrouoric acid was added dropwise with stirring. Aer stir-
Megouda et al. investigated the performance of hydrogen- ring for 15 min at room temperature, the Teon pot was sealed
terminated Si NWs and two kinds of metals (Ag, Cu) deco- and kept at 200 ꢁC for 24 h. Finally, the as-prepared Si/TiO2
rating Si NWs for the degradation of dye molecules.31 Further- nanospheres were obtained aer the resulting precipitate was
more, literature about Si/TiO2 heterojunctions that achieve centrifuged three times, washed with ethanol to remove the
enhanced photochemical and photocatalytic properties have hydrouoric acid and organics, and then dried in a vacuum
been reported.32 Wang et al. successfully deposited TiO2 onto Si oven for 12 h. Pure TiO2 sample was prepared using the same
nanowire arrays to construct Si/TiO2 heterojunctions using a hydrothermal reaction without Si nanospheres.
surface reaction-limited pulsed chemical vapor deposition
method, and tailored the electrical properties of TiO2 for wider
spectrum solar energy harvesting and conversion.33 Li et al.
attained a novel composite material of TiO2 and porous silicon
using a sol–gel method and found that it exhibits much higher
photocatalytic activity for the degradation of RhB.34 Recently,
direct Z-scheme Si/TiO2 tree-like heterostructures were con-
structed by Yang,35 which is demonstrated to greatly improve
the photocatalytic activity of H2 evolution. Research on the
direct Z-scheme system is just a recent work, and still needs
further study.
2.4 Characterizations
The crystal structure of all the samples was examined by means
of X-ray diffraction analysis (XRD, Bruker D8 ADVANCE with Cu-
Ka radiation, l ¼ 1.5418 A). The morphology and particle size
˚
were determined by eld emission scanning electron micros-
copy (FE-SEM, Hitachi S4800) and transmission electron
microscopy (TEM, JEOL JSM-2010) with an accelerating voltage
of 200 kV. UV-Vis absorption spectra were obtained using a UV-
Vis spectrometer (Shimadzu UV-3600). Photoluminescence (PL)
spectra were obtained with an Edinburgh Instrument FLS 920
spectrometer. The excitation wavelength, lex, was 360 nm, and
both the bandwidths of excitation and emission were 5 nm. The
Brunauer–Emmet–Teller (BET) specic surface area of the
samples were determined by a high speed automated area and
pore size analyzer (ASAP 2010).
In this work, we report a novel direct Z-scheme Si/TiO2
photocatalyst synthesized via a facile hydrothermal method
with tetrabutyl titanate and Si powder prepared from the mag-
36
¨
nesiothermic reduction of SiO2 Stober nanospheres. The
enhanced photocatalytic conversion of CO2 reduction into
value-added methanol is investigated.
2.5 Photocatalytic reduction of CO2 under 355 laser
irradiation
2. Experimental section
2.1 Synthesis of SiO2 nanospheres
The photocatalytic reaction cell and its setup have been
described in our earlier publication.37 The photocatalytic
The monodisperse silica spheres were prepared by hydrolysis reactor is a cylindrical stainless steel cell with quartz windows
and condensation of tetraethoxysilane (TEOS) in a mixture of on the top to enable the transmission of 355 nm pulse laser
water, ammonia, and ethanol. In a typical synthesis process, 9 radiations. At the bottom of the cell there is a gas inlet with a
mL 28 wt% ammonia water was mixed with 16.25 mL ethanol needle valve which lets the CO2 gas pass through the distilled
and 24.75 mL deionized water under a stirring condition water in the cell and there is an outlet xed with the rubber
(solution A). 4.5 mL TEOS was added to 45.5 mL ethanol under septum in order to dispense the sample through the syringe
stirring (solution B). Here, we added B to A drop by drop and almost at the same level at the opposite side. The whole cell is
strewed for another 2 h at room temperature. SiO2 nanospheres kept on a magnetic stirrer that constantly replenishes the
were centrifuged from the mixture, alternately washed with photocatalyst in the path of laser radiations. Care has to be
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taken not to let the water level go much higher than the level of complete. During the irradiation, about 1 mL of gas was
the catalyst platform in order to have better interaction of continually taken from the reaction cell at given time intervals
radiation with the photocatalyst. Since the quantity of sample for subsequent CH4 concentration analysis by using a gas
taken for gas chromatographic analysis at each time was around chromatograph (GC-2014, Shimadzu Corp., Japan). All samples
4.0 mL, the water level did not decrease due to sample with- were treated at 300 ꢁC in nitrogen atmosphere for 2 h for
drawing from the reaction cell. The reaction cell was cleaned, removal of organic adsorbates before the photocatalysis
dried, then was tightly closed and checked for leaks up to 50 psi reaction.
pressure aer 1.0 g catalyst was loaded along with 100 mL
distilled water. High purity CO2 gas (99.99%) was introduced
through reactor inlets and the reactor pressure was maintained
The photocatalytic activity was measured as follows: 0.100 g of as-
at 50 psi. Prior to turning on the pulsed laser, CO2 gas was
purged into 100 mL water containing 1.0 g of catalyst for 30 min
in order to saturate the contents of the reactor with CO2. Aer a
predetermined irradiation time, water samples were withdrawn
TrustTech Co. Ltd.) with an illumination intensity of
from the reactor using a syringe without opening the reactor
2.7 Photocatalytic degradation of aqueous RhB solution
prepared TiO2 and Si/TiO2 samples were added to a 250 mL Pyrex
glass vessel which contained 200 mL RhB solution (7.5 mg Lꢃ1).
The light source was a 300 W Xe arc lamp (CHF-XM500W, Beijing
400 mW cmꢃ2. Prior to irradiation, RhB solution suspended with
photocatalysts was stirred in the dark for 30 min to ensure that the
surface of photocatalysts reaches the adsorption–desorption
equilibrium. 3 mL of the suspension was withdrawn throughout
and subjected to GC analysis.
The laser (wavelength ¼ 355 nm) used for this study was the
third harmonic of the pulsed Nd:YAG laser (Model Spectra
Physics GCR 250-10) operated at 10 Hz and the pulse width of
the experiment aer every 10 min. The samples were analyzed by a
ꢀ8 ns. Throughout this study, a laser pulse energy of 40 mJ was
UV-Vis spectrophotometer aer removing the catalyst powders by
used. The laser beam was routed with high power UV reecting
centrifugation.
mirrors/dichroic mirrors so that the beam enters from the top
of the cell and an appropriate lens was also used to slightly
expand the beam to the same diameter of the catalyst platform.
Although the laser pulse energy was quite stable it was moni-
tored throughout the experiment with the 50–50 beam splitter
3. Results and discussion
3.1 Phase and morphology analysis
and the laser energy meter supplied by Coherent USA.
X-ray diffraction patterns of as-prepared Si nanospheres, TiO2
The water samples were analyzed for methanol and other nanosheets and direct Z-scheme Si/TiO2 nanospheres are
hydrocarbons using a gas chromatograph equipped with ame depicted in Fig. 1. The diffraction peaks in the XRD curve
ionization detector (FID). The separation was carried out on Rtx- marked in red at 28.4ꢁ, 47.3ꢁ, 56.1ꢁ, 69.1ꢁ, 76.3ꢁ and 88.0ꢁ can be
Wax column (dimensions: 30 m ꢂ 0.32 mm ꢂ 0.32 mm) assigned to (111), (220), (311), (400), (331) and (422) planes of Si
obtained from Restek, using temperature programmed condi- (JCPDS Card 27-1402), respectively. The narrow broadness of
tions. For the analysis of end products, 4.0 mL of the laser diffraction peaks for Si nanospheres indicates that the Si
irradiated sample was injected into the gas chromatograph and nanospheres prepared via
a magnesiothermic reduction
the operating conditions were as follows: oven temperature wꢃas1 method have a high purity and crystallinity. The diffraction
set at 40 ꢁC and was then increased to 90 ꢁC at 5 ꢁC min
peaks in the XRD curve marked in black at 25.3ꢁ, 37.8ꢁ, 48.0ꢁ,
heating rate and increased to 180 ꢁC at the rate of 50 ꢁC minꢃ1 to 55.0ꢁ, 62.6ꢁ, 70.3ꢁ, 75.0ꢁ and 82.1ꢁ are indexed to the (101),
elute all the components from the column before injecting (004), (200), (211), (204), (220), (215) and (303) planes of TiO2
another sample. The injector and detectors were both set at 200 (JCPDS Card 21-1272), respectively. From the diffraction peaks
ꢁC and helium was used as the carrier gas. The total analysis run in the XRD curve of direct Z-scheme Si/TiO2 nanospheres
time was 11.8 min. A calibration plot was established for marked in blue, which contain both the Si and TiO2 diffraction
methanol standard solution in distilled water for calculating peaks, it can be seen obviously that the Si/TiO2 product
the amount of methanol produced as a function of irradiation obtained via a hydrothermal method is composed of Si and
time.
TiO2. The diffraction peaks of Si in the XRD curve of direct
Z-scheme Si/TiO2 nanospheres are very low, which may result
from its low content.
2.6 Photocatalytic reduction of CO2 under Xe arc lamp
irradiation
As depicted in Fig. 2(a), the SiO2 nanospheres prepared via
¨
the Stober method are monodisperse and uniform with a
In a typical process, 0.1 g of the sample was uniformly dispersed smooth surface and an average diameter of about 300 nm. Aer
on the glass reactor (4.2 cm2). A 300 W Xenon arc lamp was used the magnesiothermic reduction process, Si nanospheres were
as the light source. The reaction system (230 mL in volume) was obtained with a porous structure and rough surface, resulting
vacuum-treated several times, and then the high purity of CO2 from the loss of O atoms from SiO2 nanospheres captured by Mg
gas was followed into the reaction setup to reach ambient under high temperature. The diameter of Si nanospheres
pressure. 0.4 mL of deionized water was injected into the remained mainly unchanged (as shown in Fig. 2(b)). The SEM
reaction system as reducer. The as-prepared photocatalysts were images of the contrast TiO2 samples prepared by a hydro-
allowed to equilibrate in the CO2/H2O atmosphere for several thermal method without adding Si nanospheres are depicted in
hours to ensure that the adsorption of gas molecules was Fig. 2(c). It can be seen that the TiO2 nanoparticles displayed a
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Fig. 1 XRD patterns of the as-prepared Si nanospheres, TiO2 nano-
sheets and direct Z-scheme Si/TiO2 nanospheres.
Fig. 3 TEM images: (a) TiO2 nanosheets, (b, c) direct Z-scheme Si/
TiO2 nanospheres, (d) high-magnification TEM image of direct Z-
scheme Si/TiO2 nanospheres.
0.31 nm observed in the HRTEM image match those of the (001)
and (111) crystallographic planes of anatase TiO2 and Si nano-
particles, indicating the formation of Si/TiO2 interfaces via a
hydrothermal method, which may improve the photocatalytic
properties of TiO2.
Fig. 2 SEM images of as-prepared SiO2 nanospheres (a), Si nano-
spheres (b), TiO2 nanosheets (c), and direct Z-scheme Si/TiO2 nano-
spheres (d).
3.2 Optical absorption properties
Fig. 4(a) shows the UV-Vis absorption spectra of the Si nano-
spheres, TiO2 nanosheets and direct Z-scheme Si/TiO2 nano-
spheres. It can be noticed from the spectra that the as-prepared
TiO2 and Si/TiO2 nanocomposites exhibit similar absorption
behaviour in the ultraviolet region. However, Si/TiO2 Z-scheme
nanospheres show an enhanced absorbance throughout the
visible light region due to the existence of Si which is a visible
light responsive material with a band gap of 1.12 eV. The
improved visible light absorption explains the enhanced
uniform sheet shape with an average edge length of about 100
nm and a thickness of about 10 nm. Fig. 2(d) shows the repre-
sentative SEM image of direct Z-scheme Si/TiO2 nanospheres,
clearly indicating the Si nanospheres were coated with TiO2
nanosheets.
In order to obtain further information on the structure of the
samples, TEM observation of the TiO2 nanosheets and direct Z-
scheme Si/TiO2 nanospheres was carried out. It can be clearly
noticed from Fig. 3(a) that the as-prepared TiO2 samples are
composed of a large quantity of square nanosheets with an
average edge length of about 100 nm and a thickness of about
10 nm, which is in good agreement with the result obtained
from the SEM images. The TEM image of Si/TiO2 direct Z-
scheme nanocomposites in Fig. 3(b) clearly shows that the Si
nanospheres were coated by TiO2 nanosheets. Moreover, it can
be observed that the Si nanospheres appear with a porous
morphology resulting from the O element captured by Mg in the
magnesiothermic reduction process. The magnied TEM image
of the nanostructure of the Si/TiO2 nanocomposites in Fig. 3(c)
presents the TiO2 nanosheets aggregation morphology on the Si
porous nanospheres, revealing the formation of the Si/TiO2 Z-
scheme system. The high-magnication TEM in Fig. 3(d)
depicts many different lattice fringes of the Si/TiO2 nano-
composites. The fringes with lattice spacing of ca. 0.235 nm and
Fig. 4 UV-Vis absorption spectra of Si nanospheres, TiO2 nanosheets
and direct Z-scheme Si/TiO2 nanospheres.
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Table 1 Relevant parameters of Ti, O, Si atoms (ionization energy,
atomic electron affinity and absolute electronegativity) and TiO2, Si
semiconductors (absolute electronegativity, band gap and electro-
chemical potentials of CB/VB band edges)a
photocatalytic properties of the direct Z-scheme Si/TiO2 nano-
spheres, as described later.
3.3 Photocatalytic reduction of CO2
Element
Si
Ti
O
To evaluate the photocatalytic activity of Si nanospheres, TiO2
nanosheets and direct Z-scheme Si/TiO2 nanospheres, the
conversion of CO2 into hydrocarbon fuels in distilled water was
investigated using a high power pulsed laser as light source at
355 nm wavelength. As we know, a number of reaction products
(such as HCHO, CH3OH, HCOOH, CO, CH4, etc.) can be
obtained during the CO2 photoreduction process. The following
reactions may be the pathways of CO2 photoreduction into value
added hydrocarbons.
Atomic ionization energy (eV)
Atomic electron affinity (eV)
Absolute electronegativity (eV)
8.15168
1.38952
4.7704
6.82812
0.079
3.45356
13.6182
1.46111
7.53958
a
The relevant data were selected from handbook.39
Catalyst
Si
TiO2
Band gap (eV)
1.1
3.2
Absolute electronegativity (eV)
CB band edge electrochemical
potential (V vs. NHE)
VB band edge electrochemical
potential (V vs. NHE)
4.7706
ꢃ0.38
5.81193
ꢃ0.29
Catalyst + hn / eꢃ + h+
cb
vb
0.721
2.912
2H2O + 4h+ / 4H+ + O2 +0.82 V
CO2 + 2H+ + 2eꢃ / HCOOH ꢃ0.61 V
CO2 + 2H+ + 2eꢃ / CO + H2O ꢃ0.53 V
CO2 + 4H+ + 4eꢃ / HCHO + H2O ꢃ0.48 V
CO2 + 6H+ + 6eꢃ / CH3OH + H2O ꢃ0.38 V
CO2 + 8H+ + 8eꢃ / CH4 + H2O ꢃ0.24 V
V vs. NHE) cannot transform any H+ from the reaction of H2O
oxidation, which enables the photoreduction of CO2 into
CH3OH.
Gas chromatography (GC) was employed to verify and
quantify the methanol products from the CO2 reduction over
direct Z-scheme nanospheres. It is depicted in Fig. 5(a) that the
retention time for methanol standard is 2.46 min for the
selected GC parameters and the used column. The relationship
between GC peak area and methanol concentration was
conrmed by using known methanol concentrations in a stan-
dard sample for calibration as depicted in Fig. 5(b), which
exhibits a linear trend.
Fig. 6(a) depicts the GC peaks of products for samples from
CO2 photoreduction, which are taken in 30 min intervals of
irradiation with 355 nm laser by using Si/TiO2 as the photo-
catalyst. It can be seen clearly that all the GC peaks in Fig. 6(a)
appear at exactly 2.46 min of the retention time and no other GC
peaks were detected, which suggests that the methanol is the
only product obtained through the laser induced photocatalytic
reduction of CO2, possibly because of the sharp line width of the
laser beam centred around 355 nm (highly monochromatic) of
In this study, we were selective to obtain CH3OH as the main
product of CO2 photocatalytic reduction over Si/TiO2 nano-
composites. Our comparative tests demonstrated that nearly no
product was found by using Si nanospheres or TiO2 nanosheets
as photocatalysts. This may be due to the low conductive band
potential of TiO2 and low valance band potential of Si, respec-
tively. The conduction band (CB) and valence band (VB) edge of
Si and TiO2 semiconductors were calculated by the equation as
follows38 and are presented in Table 1.
ECB ¼ X ꢃ EC ꢃ 1/2Eg
EVB ¼ Eg + ECB
where X is the absolute electronegativity of the semiconductor; the pulsed laser radiation. Furthermore, it was shown that the
EC is the energy of free electrons on the hydrogen scale (4.5 eV); GC peak areas of methanol from CO2 photoreduction increase
and Eg is the band gap of the semiconductor.
with the irradiation time (30 min, 60 min, 90 min, 120 min, 150
It can be inferred from the table that, although the calcu- min) and reach the maximum in 150 min of irradiation aer
lated VB band edge electrochemical potential of TiO2 (2.91 V vs. which they start to fall. Fig. 6(b) shows the concentration vari-
NHE) is high enough to initiate the reaction of H2O and h+ to ation of the photocatalytic process of converting CO2 into
form O2 and H+ (0.82 V vs. NHE), the calculated CB band edge methanol with laser irradiation. It can be observed that the
electrochemical potential (ꢃ0.29 V vs. NHE) is lower than the produced methanol concentration increases with laser irradia-
reaction potential needed for CO2 transformation to CH3OH tion time and reaches its maximum (197 mM/100 mL) at 150
with H+ and photogenerated eꢃ (ꢃ0.38 V vs. NHE), resulting in min, but aerwards it declines. The decrease of the methanol
the thermodynamic impossibility of CO2 photoreduction into concentration may be caused by the existence of photocatalytic
CH3OH over TiO2 catalyst under 355 nm laser irradiation. In the oxidation effect of Si/TiO2 composed semiconductor with
same way, pure Si catalyst has an appropriate CB band edge positive VB position. When the methanol was produced in a
electrochemical potential to reduce CO2 into CH3OH with H+ substantial amount, it will be adsorbed on the surface of the
and eꢃ, but its low VB band edge electrochemical potential (0.72 photocatalysts and oxidized to inorganic matter, which is in
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Fig. 5 (a) GC peak position of methanol standard, (b) calibration curve for methanol concentration vs. GC peak area.
concentration at different irradiation times of the photo-
catalytic process, the CO2 conversion efficiency can be calcu-
lated from the quotient of actual concentrations of methanol
and CO2 concentration (as shown in Fig. 6(b)). For that the
maximum concentration of methanol is 197 mM/100 mL aer
150 minutes of laser irradiation, the maximum CO2 conversion
efficiency is calculated (197/11 560) to be about 1.71%.
42
¨
However, according to Schuler’s experimental result, the CO2
solubility in binary mixtures of water and methanol increases
with increasing methanol content, resulting in the actual CO2
conversion efficiency being slightly lower than the calculated
efficiency (1.71%).
Moreover, we can calculate the photonic efficiency of the
photocatalytic reduction of CO2 from the number of methanol
molecules produced for certain irradiation time and the
number of consumed photons in the reaction. The number of
methanol molecules can be estimated from the molar concen-
trations and Avogadro’s number. In the case of laser, the
number of photons at 355 nm wavelength with the laser pulse
energy of 40 mJ per pulse and repetition rate of 10 Hz can be
calculated to be 4.286 ꢂ 1019 photons per min. The maximum
rate of methanol is 1.294 ꢂ 1018 molecules per min at the
irradiation time interval from 60 min to 90 min. As a single
methanol molecule needs 6 photogenerated electrons, the
maximum photonic efficiency (P.E.) of photoreduction of CO2
can be calculated (6 ꢂ 1.294 ꢂ 1018/4.286 ꢂ 1019) to be about
18.1%. The achievement of high photonic efficiency of direct Z-
scheme Si/TiO2 nanospheres may be due to the construction
nature of Si/TiO2 direct Z-scheme system.
Fig. 6 (a) GC peaks of methanol for sample taken every 30 min of
irradiation with a laser pulse energy of 40 mJ per pulse at 355 nm
radiation with 600 mg catalyst in 100 mL distilled water, 50 psi CO2
pressure. (b) Concentration of produced CH3OH and conversion
efficiency with time.
agreement with the results obtained by, and explanation of,
other groups.37,40,41
3.4 Mechanism analysis of the enhanced photocatalytic
activity of Si/TiO2
In order to estimate the efficiency of CO2 conversion into
methanol using direct Z-scheme Si/TiO2 nanospheres as the
photocatalyst with 355 nm laser irradiation, the process of the
CO2 conversion efficiency was estimated. The amount of CO2
dissolved in distilled water under our experimental conditions
can be calculated from Henry’s law and the amount of CO2
dissolved in 1 L water at atmospheric pressure is 34 mmol
(Henry constant). As the pressure in the photocatalytic
measurement is 50 psi (3.4 atm), the total CO2 dissolved in 100
mL of water is 11.56 mmol. Once we know the methanol
As shown in Fig. 7, photoluminescence spectroscopy was
employed for further investigation of the photocatalytic activi-
ties of TiO2 nanosheets and direct Z-scheme Si/TiO2 nano-
spheres. Two major components of the spectrum of direct Z-
scheme Si/TiO2 nanospheres consisted of a strong peak at 540
nm and a weak, broad peak from 400 to 520 nm, which are
attributed to Si and TiO2, respectively. Moreover, it can be seen
clearly that the peak intensities in photoluminescence intensity
of Si/TiO2 Z-scheme nanospheres are much lower in contrast to
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Fig. 7 Photoluminescence spectra of TiO2 nanosheets and direct Z-
scheme Si/TiO2 nanospheres.
that of Si nanospheres and TiO2 nanosheets. As the PL emission
resulted from the recombination of photo-induced charge
carriers and information regarding the efficiency of charge
carrier trapping, and their recombination kinetics can be drawn
from the PL spectra,43 it can be inferred that direct Z-scheme Si/
TiO2 nanospheres have a higher efficient separation rate of
photogenerated charge carriers than that of TiO2 nanosheets,
which can be attributed to the formation of the Si/TiO2 direct Z-
scheme system.
Fig. 8 Photocatalytic degradation rates (a) and the ln(C0/C) vs. irra-
In order to conrm the effect of the Si/TiO2 direct Z-scheme
system on improving the charge separation efficiency, RhB was
selected to be the target pollutant for degradation by Si nano-
spheres, TiO2 nanosheets and direct Z-scheme Si/TiO2 nano-
spheres under 300 W Xe arc lamp irradiation with an
illumination intensity of 400 mW cmꢃ2. As shown in Fig. 8(a), it
can be clearly seen that RhB molecules were completely
decomposed by Si/TiO2 direct Z-scheme nanospheres aer 1 h
Xe arc lamp irradiation, while only 88.5% were decomposed by
TiO2 nanosheets and 6.6% by Si nanospheres, indicating the
enhanced photocatalytic activity of the Si/TiO2 Z-scheme
compared with the TiO2 nanosheets and Si nanospheres.
Fig. 8(b) depicts the kinetic study of photocatalytic degradation
of RhB solution over the three photocatalytic materials. The
linear relationship of ln(C0/C) vs. irradiation time suggests that
degradation of RhB is a rst order reaction. The calculated rate
constants for Si nanospheres, TiO2 nanosheets and Si/TiO2 Z-
diation time curves of RhB (b).
carriers separation efficiency of the Si/TiO2 Z-scheme system. To
further investigate whether CH4 is a product of CO2 photo-
catalytic reduction, we carried out a series comparative experi-
ments to investigate the source of C and H in the produced CH4
under Xe arc lamp irradiation. As shown in Fig. 9(b), the CH4
detected in normal conditions is much higher than that in other
experiments. It should be noted that the extremely low CH4
concentrations in non-normal conditions are from the naturally
occurring CH4 in air (1–2 ppm). In other words, CH4 can be
produced only in the case of possessing all the conditions
including catalyst, CO2, H2O and light. It can be inferred from
the contrast experiment that C in the photoreduction product
CH4 is from CO2, while H is from H2O. Therefore, it can be
conrmed that the detected CH4 is from the photoreduction of
CO2, but not a product of organic oxidation at the Si/TiO2
Z-scheme or release of surface bound organics.
Because the photocatalytic process is surface orientated, the
specic surface areas of the prepared TiO2 nanosheets and Si/
TiO2 nanocomposites were also measured by BET to study the
actual exposed surface area in the photocatalytic reaction. As
shown in Fig. 10, the Si/TiO2 sample possesses a slightly higher
specic surface area (165.4 m2 gꢃ1) than TiO2 nanosheets
(154.3 m2 gꢃ1) due to the surface structuring effect by the
formation of spherical heterostructures with TiO2 nanosheets
well dispersed. In the photocatalytic reduction of CO2 experi-
ments, our results show that only the Si/TiO2 sample is capable
of producing methanol under laser irradiation due to thermo-
dynamics, indicating that the exposed surface area of the
scheme nanospheres are 0.000772, 0.0354 and 0.074 minꢃ1
,
respectively, from which it can be seen that the Si/TiO2 Z-
scheme has the best photocatalytic activity, which is 2.09
times that of the TiO2 nanosheets and 95.8 times the Si nano-
spheres. Therefore, construction of the Si/TiO2 Z-scheme is
benecial to improve the photocatalytic activity of Si and TiO2
extensively.
Furthermore, the CO2 photocatalytic reduction under Xe arc
lamp irradiation was carried out to investigate the inuence of
light source and experimental conditions on the product.
Fig. 9(a) shows that CO2 can be photoreduced to CH4 by using
all the prepared samples as photocatalysts. It is obvious that the
direct Z-scheme Si/TiO2 nanocomposites exhibit much higher
activity than pure TiO2 nanosheets and Si nanospheres. The
higher conversion is attributed to the improved photogenerated
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Fig. 11 Schematic diagram of the enhanced photocatalytic property
of the Si/TiO2 direct Z-scheme system for CO2 reduction.
photogenerated carriers’ concentration and separation effi-
ciency from the Si/TiO2 direct Z-scheme system construction.
Fig. 11 illustrates the schematic charge ow in the Si/TiO2
direct Z-scheme system under illumination. 355 nm lasers can
be harvested by both Si and TiO2 to generate eꢃ–h+ pairs. As
reported in Yang’s previous research,32 the photogenerated hole
in TiO2 (TiO2h+) moves toward the TiO2/electrolyte interface and
oxidizes OHꢃ to oxygen, while photogenerated electrons in the
TiO2 (TiO2eꢃ) move away from the interface between TiO2 and
the electrolyte due to the schottky barrier. The potential barrier
at the Si/TiO2 interface reects holes back into the TiO2 layers.
To complete the circuit, the photogenerated electrons in Si
(Sieꢃ) move to the surface where the CO2 reduction reaction
takes place. The photogenerated hole in Si (Sih+) moves towards
the Si/TiO2 interface and recombines with the TiO2eꢃ. There-
fore, the direct Z-scheme Si/TiO2 nanospheres show high
activity towards CO2 reduction into methanol since its band
alignment at the junction helps reduce recombination under
illumination. However, for individual Si photocatalysts, its VB
potential (+0.721 V vs. NHE) is not high enough to achieve the
oxidation reaction potential (O2/H2O 0.82 V vs. NHE). Similarly,
the CB potential of TiO2 (ꢃ0.29 V vs. NHE) is too low to initiate
the CO2 reduction reaction into methanol (CH3OH/CO2 ꢃ0.38 V
vs. NHE). In other words, the individual Si nanospheres or TiO2
nanosheets lack suitable VB or CB potential to photoreduce CO2
into methanol.
Fig. 9 (a) CH4 evolution with time under Xe arc lamp irradiation, (b)
comparative experiments of the CO2 photoreduction under different
conditions.
4. Conclusion
Fig. 10 N2 adsorption–desorption isotherms of TiO2 nanosheets and
direct Z-scheme Si/TiO2 nanospheres.
In summary, we have presented a facile and low cost method to
prepare a direct Z-scheme Si/TiO2 nanostructure via hydro-
thermal reaction with tetrabutyl titanate and Si powder which
was prepared from the magnesiothermic reduction of SiO2
¨
catalyst has no effect on its photocatalytic reduction activity. In Stober nanospheres. All the results indicate that Si/TiO2 nano-
the photocatalytic degradation experiment, 0.1 g of the as- composites possess much higher photocatalytic activity than
prepared Si, TiO2 and Si/TiO2 catalysts was adopted, respec- individual Si and TiO2 samples for the CO2 conversion and
tively. The slight difference of the catalysts in specic surface degradation of RhB. This excellent performance could be
area is as small as 6.6%, which is suggested to not be the main attributed to the integrated suitable conductive band of Si and
reason for the great improvement of photodegradation effi- valence band of TiO2 for CO2 reduction and improved light
ciency (more than twice). Hence, it can be concluded that the absorption ability, enhanced concentration of photogenerated
excellent performance of the Si/TiO2 catalyst does not result carriers, and higher separation efficiency due to the elaborate
from a surface structuring effect, but from the improved construction of the Si/TiO2 direct Z-scheme system.
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Acknowledgements
This work was nancially supported by the National Natural
Science Foundation of China (no. 51172109), the Fundamental
Research Funds for the Central Universities (no. NS2014057),
Funding of Jiangsu Innovation Program for Graduate Education
(no. CXLX12_0148), and the Fundamental Research Funds for
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through project # RG1011-1/2.
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