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Highly-active dir

aCollege of Materials Science and Technolog

Astronautics, Nanjing 210016, P. R. C

+86-25-5211-2902bPhysics Department, King Fahd Universit

31261, Saudi Arabia. E-mail: magondal@kf

Cite this: RSC Adv., 2014, 4, 56961

Received 19th September 2014Accepted 23rd October 2014

DOI: 10.1039/c4ra10670a

www.rsc.org/advances

This journal is © The Royal Society of C

ect Z-scheme Si/TiO2

photocatalyst for boosted CO2 reduction intovalue-added methanol

Yousong Liu,a Guangbin Ji,*a Mohammed Abdulkader Dastageer,b Lei Zhu,a

Junyi Wang,a Bin Zhang,a Xiaofeng Changa and Mohammed Ashraf Gondal*b

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.

1. Introduction

High consumption of fossil fuels is creating not only an energycrisis but also environmental pollution and climate change dueto excessive green house gas emissions to the atmosphere. Ascarbon dioxide (CO2) is a major contributor to green housegases, attention is focused on its mitigation all over the world.Solar-driven photocatalytic conversion of CO2 into added valuefuels is the most promising proposition as it does not onlyremove CO2 from effluent gases but also produces hydrocarbonfuels, which could be used to meet future energy needs. Thus, alot of research efforts have focused on developing efficientcatalysts for CO2 photocatalytic reduction. Semiconductorphotocatalysts (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, superiorphotocatalytic activity and long-term chemical stability.9–11

However, the wide band gap of TiO2 (�3.2 eV) limits its efficientutilization for solar energy conversation as TiO2 only absorbslight with wavelengths shorter than �387 nm in the ultravioletregion. Moreover, aer photoabsorption, the electrons areexcited from the valence band of TiO2 to the conduction band

y, Nanjing University of Aeronautics and

hina. E-mail: gbji@nuaa.edu.cn; Tel:

y of Petroleum and Minerals, Dhahran,

upm.edu.sa; Tel: +966-38602351

hemistry 2014

and the effective electron–hole pairs are generated.12 Unfortu-nately, most of the effective electron–hole pairs are recombinedand dissipated as heat before they arrive at the photocatalystsurface, whichmakes TiO2 an inefficient photogenerated carrierhampering its charge separation ability.

Research scientists have devoted extensive efforts to addressthese problems. Introducing doping elements (such as S,13 N,14

and C (ref. 15)) into TiO2 has been proven to be an effectiveapproach to narrow the band gap, improve the visible lightabsorption and enhance the photocatalytic activity in CO2

reduction. TiO2 modication with metal particles (e.g., Ag,16

Au,17 Pt,18 and Cu (ref. 19)) has been reported to inhibit chargerecombination probability because these metals serve as elec-tron traps to suppress the recombination of the photogeneratedelectron–hole pairs and hence improve the photocatalyticactivity. In addition, coupling TiO2 with a narrower band gapsemiconductor to construct heterojunctions is another effectiveapproach to accommodate the visible-light photon energy andimprove the photogenerated charge separation and CO2

conversion efficiency. This coupling takes advantage of both theheterojunction to improve charge separation rate, and thenarrow band gap of the coupled semiconductor to expand thelight absorption region. TiO2 based heterojunctions such asPbS/TiO2,20 CuO/TiO2,21 FeTiO3/TiO2 (ref. 22) have been repor-ted in recent years.

Similar to the heterojunction photocatalytic system, the Z-scheme photocatalytic system also features the spatial isola-tion of photogenerated electrons and holes, which reduces thebulk electron–hole recombination.23 However, a Z-scheme

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photocatalytic system is generally constructed by employing aconductor as the electron mediator to form the known Ohmiccontact with low contact resistance. Until now, many Z-schemesystems have been reported, such as TiO2–Au–CdS system,24

AgBr–Ag–AgI,25 and ZnO–Au–CdS,26 etc. Recently, studies ondirect Z-scheme photocatalysts (such as g–C3N4–TiO2 andWO3–

NaNbO3 system) have been investigated for photocatalytic watersplitting, CO2 conversion and photocatalyticdecomposition.27–29

A narrow band gap semiconductor, silicon (1.12 eV), hasdrawn considerable interest because of its potential applicationin optoelectronic devices and integrated microelectronics.Recently, studies of silicon materials have reported its prom-ising photocatalytic activity. Shao et al. prepared hydrogen-terminated Si nanowires (Si NWs) and noble metal-modied(Pt, Pd, Au, Rh, Ag) Si NWs substrates by oxide-assistedgrowth method, and investigated their performance for thedegradation of Rhodamine B and oxidation of benzyl alcohol tobenzoic acid under visible light irradiation.30 Independently,Megouda et al. investigated the performance of hydrogen-terminated Si NWs and two kinds of metals (Ag, Cu) deco-rating Si NWs for the degradation of dye molecules.31 Further-more, literature about Si/TiO2 heterojunctions that achieveenhanced photochemical and photocatalytic properties havebeen reported.32 Wang et al. successfully deposited TiO2 onto Sinanowire arrays to construct Si/TiO2 heterojunctions using asurface reaction-limited pulsed chemical vapor depositionmethod, and tailored the electrical properties of TiO2 for widerspectrum solar energy harvesting and conversion.33 Li et al.attained a novel composite material of TiO2 and porous siliconusing a sol–gel method and found that it exhibits much higherphotocatalytic 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 improvethe photocatalytic activity of H2 evolution. Research on thedirect Z-scheme system is just a recent work, and still needsfurther study.

In this work, we report a novel direct Z-scheme Si/TiO2

photocatalyst synthesized via a facile hydrothermal methodwith tetrabutyl titanate and Si powder prepared from the mag-nesiothermic reduction of SiO2 Stober nanospheres.36 Theenhanced photocatalytic conversion of CO2 reduction intovalue-added methanol is investigated.

2. Experimental section2.1 Synthesis of SiO2 nanospheres

The monodisperse silica spheres were prepared by hydrolysisand condensation of tetraethoxysilane (TEOS) in a mixture ofwater, ammonia, and ethanol. In a typical synthesis process, 9mL 28 wt% ammonia water was mixed with 16.25 mL ethanoland 24.75 mL deionized water under a stirring condition(solution A). 4.5 mL TEOS was added to 45.5 mL ethanol understirring (solution B). Here, we added B to A drop by drop andstrewed for another 2 h at room temperature. SiO2 nanosphereswere centrifuged from the mixture, alternately washed with

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deionized water and ethanol 3 times and then dried at 100 �Cfor 12 h.

2.2 Synthesis of Si nanospheres

0.5 g SiO2 nanospheres and 0.42 g Mg powder were ground for 5minutes and then transferred into a crucible to calcine at 750 �Cfor 5 h under N2 atmosphere. Aer cooling to room tempera-ture, the powder was then added to 50 mL 32 wt% HCl solutionwith stirring for 24 h. Si nanospheres were then cleaned severaltimes by centrifugation and water dispersion and nally driedinto powders at 60 �C in a vacuum oven for 12 h.

2.3 Synthesis of direct Z-scheme Si/TiO2 nanospheres

Direct Z-scheme Si/TiO2 nanospheres were fabricated via afacile hydrothermal reaction with tetrabutyl titanate and Sinanospheres. In a typical process, 25mL of Ti(OBu)4 and 1.5 g Sinanospheres were added in a 100 mL Teon pot and 3 mL ofhydrouoric acid was added dropwise with stirring. Aer stir-ring for 15 min at room temperature, the Teon pot was sealedand kept at 200 �C for 24 h. Finally, the as-prepared Si/TiO2

nanospheres were obtained aer the resulting precipitate wascentrifuged three times, washed with ethanol to remove thehydrouoric acid and organics, and then dried in a vacuumoven for 12 h. Pure TiO2 sample was prepared using the samehydrothermal reaction without Si nanospheres.

2.4 Characterizations

The crystal structure of all the samples was examined by meansof X-ray diffraction analysis (XRD, Bruker D8 ADVANCE with Cu-Ka radiation, l ¼ 1.5418 A). The morphology and particle sizewere determined by eld emission scanning electron micros-copy (FE-SEM, Hitachi S4800) and transmission electronmicroscopy (TEM, JEOL JSM-2010) with an accelerating voltageof 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 920spectrometer. The excitation wavelength, lex, was 360 nm, andboth the bandwidths of excitation and emission were 5 nm. TheBrunauer–Emmet–Teller (BET) specic surface area of thesamples were determined by a high speed automated area andpore size analyzer (ASAP 2010).

2.5 Photocatalytic reduction of CO2 under 355 laserirradiation

The photocatalytic reaction cell and its setup have beendescribed in our earlier publication.37 The photocatalyticreactor is a cylindrical stainless steel cell with quartz windowson the top to enable the transmission of 355 nm pulse laserradiations. At the bottom of the cell there is a gas inlet with aneedle valve which lets the CO2 gas pass through the distilledwater in the cell and there is an outlet xed with the rubberseptum in order to dispense the sample through the syringealmost at the same level at the opposite side. The whole cell iskept on a magnetic stirrer that constantly replenishes thephotocatalyst 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 ofthe catalyst platform in order to have better interaction ofradiation with the photocatalyst. Since the quantity of sampletaken for gas chromatographic analysis at each time was around4.0 mL, the water level did not decrease due to sample with-drawing from the reaction cell. The reaction cell was cleaned,dried, then was tightly closed and checked for leaks up to 50 psipressure aer 1.0 g catalyst was loaded along with 100 mLdistilled water. High purity CO2 gas (99.99%) was introducedthrough reactor inlets and the reactor pressure was maintainedat 50 psi. Prior to turning on the pulsed laser, CO2 gas waspurged into 100 mL water containing 1.0 g of catalyst for 30 minin order to saturate the contents of the reactor with CO2. Aer apredetermined irradiation time, water samples were withdrawnfrom the reactor using a syringe without opening the reactorand subjected to GC analysis.

The laser (wavelength ¼ 355 nm) used for this study was thethird harmonic of the pulsed Nd:YAG laser (Model SpectraPhysics GCR 250-10) operated at 10 Hz and the pulse width of�8 ns. Throughout this study, a laser pulse energy of 40 mJ wasused. The laser beam was routed with high power UV reectingmirrors/dichroic mirrors so that the beam enters from the topof the cell and an appropriate lens was also used to slightlyexpand 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 splitterand the laser energy meter supplied by Coherent USA.

The water samples were analyzed for methanol and otherhydrocarbons using a gas chromatograph equipped with ameionization detector (FID). The separation was carried out on Rtx-Wax column (dimensions: 30 m � 0.32 mm � 0.32 mm)obtained from Restek, using temperature programmed condi-tions. For the analysis of end products, 4.0 mL of the laserirradiated sample was injected into the gas chromatograph andthe operating conditions were as follows: oven temperature wasset at 40 �C and was then increased to 90 �C at 5 �C min�1

heating rate and increased to 180 �C at the rate of 50 �Cmin�1 toelute all the components from the column before injectinganother sample. The injector and detectors were both set at 200�C and helium was used as the carrier gas. The total analysis runtime was 11.8 min. A calibration plot was established formethanol standard solution in distilled water for calculatingthe amount of methanol produced as a function of irradiationtime.

2.6 Photocatalytic reduction of CO2 under Xe arc lampirradiation

In a typical process, 0.1 g of the sample was uniformly dispersedon the glass reactor (4.2 cm2). A 300W Xenon arc lamp was usedas the light source. The reaction system (230 mL in volume) wasvacuum-treated several times, and then the high purity of CO2

gas was followed into the reaction setup to reach ambientpressure. 0.4 mL of deionized water was injected into thereaction system as reducer. The as-prepared photocatalysts wereallowed to equilibrate in the CO2/H2O atmosphere for severalhours to ensure that the adsorption of gas molecules was

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complete. During the irradiation, about 1 mL of gas wascontinually taken from the reaction cell at given time intervalsfor subsequent CH4 concentration analysis by using a gaschromatograph (GC-2014, Shimadzu Corp., Japan). All sampleswere treated at 300 �C in nitrogen atmosphere for 2 h forremoval of organic adsorbates before the photocatalysisreaction.

2.7 Photocatalytic degradation of aqueous RhB solution

The photocatalytic activity was measured as follows: 0.100 g of as-prepared TiO2 and Si/TiO2 samples were added to a 250 mL Pyrexglass vessel which contained 200 mL RhB solution (7.5 mg L�1).The light source was a 300 W Xe arc lamp (CHF-XM500W, BeijingTrustTech Co. Ltd.) with an illumination intensity of400 mW cm�2. Prior to irradiation, RhB solution suspended withphotocatalysts was stirred in the dark for 30min to ensure that thesurface of photocatalysts reaches the adsorption–desorptionequilibrium. 3 mL of the suspension was withdrawn throughoutthe experiment aer every 10min. The samples were analyzed by aUV-Vis spectrophotometer aer removing the catalyst powders bycentrifugation.

3. Results and discussion3.1 Phase and morphology analysis

X-ray diffraction patterns of as-prepared Si nanospheres, TiO2

nanosheets and direct Z-scheme Si/TiO2 nanospheres aredepicted in Fig. 1. The diffraction peaks in the XRD curvemarked in red at 28.4�, 47.3�, 56.1�, 69.1�, 76.3� and 88.0� can beassigned to (111), (220), (311), (400), (331) and (422) planes of Si(JCPDS Card 27-1402), respectively. The narrow broadness ofdiffraction peaks for Si nanospheres indicates that the Sinanospheres prepared via a magnesiothermic reductionmethod have a high purity and crystallinity. The diffractionpeaks in the XRD curve marked in black at 25.3�, 37.8�, 48.0�,55.0�, 62.6�, 70.3�, 75.0� and 82.1� are indexed to the (101),(004), (200), (211), (204), (220), (215) and (303) planes of TiO2

(JCPDS Card 21-1272), respectively. From the diffraction peaksin the XRD curve of direct Z-scheme Si/TiO2 nanospheresmarked in blue, which contain both the Si and TiO2 diffractionpeaks, it can be seen obviously that the Si/TiO2 productobtained via a hydrothermal method is composed of Si andTiO2. The diffraction peaks of Si in the XRD curve of directZ-scheme Si/TiO2 nanospheres are very low, which may resultfrom its low content.

As depicted in Fig. 2(a), the SiO2 nanospheres prepared viathe Stober method are monodisperse and uniform with asmooth surface and an average diameter of about 300 nm. Aerthe magnesiothermic reduction process, Si nanospheres wereobtained with a porous structure and rough surface, resultingfrom the loss of O atoms from SiO2 nanospheres captured byMgunder high temperature. The diameter of Si nanospheresremained mainly unchanged (as shown in Fig. 2(b)). The SEMimages of the contrast TiO2 samples prepared by a hydro-thermal method without adding Si nanospheres are depicted inFig. 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. 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).

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.

Fig. 4 UV-Vis absorption spectra of Si nanospheres, TiO2 nanosheetsand direct Z-scheme Si/TiO2 nanospheres.

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uniform sheet shape with an average edge length of about 100nm 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 clearlynoticed from Fig. 3(a) that the as-prepared TiO2 samples arecomposed of a large quantity of square nanosheets with anaverage edge length of about 100 nm and a thickness of about10 nm, which is in good agreement with the result obtainedfrom the SEM images. The TEM image of Si/TiO2 direct Z-scheme nanocomposites in Fig. 3(b) clearly shows that the Sinanospheres were coated by TiO2 nanosheets. Moreover, it canbe observed that the Si nanospheres appear with a porousmorphology resulting from the O element captured byMg in themagnesiothermic reduction process. The magnied TEM imageof the nanostructure of the Si/TiO2 nanocomposites in Fig. 3(c)presents the TiO2 nanosheets aggregation morphology on the Siporous nanospheres, revealing the formation of the Si/TiO2 Z-scheme system. The high-magnication 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

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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 ahydrothermal method, which may improve the photocatalyticproperties of TiO2.

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-preparedTiO2 and Si/TiO2 nanocomposites exhibit similar absorptionbehaviour in the ultraviolet region. However, Si/TiO2 Z-schemenanospheres show an enhanced absorbance throughout thevisible light region due to the existence of Si which is a visiblelight responsive material with a band gap of 1.12 eV. Theimproved visible light absorption explains the enhanced

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Table 1 Relevant parameters of Ti, O, Si atoms (ionization energy,atomic electron affinity and absolute electronegativity) and TiO2, Si

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photocatalytic properties of the direct Z-scheme Si/TiO2 nano-spheres, as described later.

semiconductors (absolute electronegativity, band gap and electro-chemical potentials of CB/VB band edges)a

Element Si Ti O

Atomic ionization energy (eV) 8.15168 6.82812 13.6182Atomic electron affinity (eV) 1.38952 0.079 1.46111Absolute electronegativity (eV) 4.7704 3.45356 7.53958

a The relevant data were selected from handbook.39

Catalyst Si TiO2

Band gap (eV) 1.1 3.2Absolute electronegativity (eV) 4.7706 5.81193CB band edge electrochemicalpotential (V vs. NHE)

�0.38 �0.29

VB band edge electrochemicalpotential (V vs. NHE)

0.721 2.912

3.3 Photocatalytic reduction of CO2

To evaluate the photocatalytic activity of Si nanospheres, TiO2

nanosheets and direct Z-scheme Si/TiO2 nanospheres, theconversion of CO2 into hydrocarbon fuels in distilled water wasinvestigated using a high power pulsed laser as light source at355 nm wavelength. As we know, a number of reaction products(such as HCHO, CH3OH, HCOOH, CO, CH4, etc.) can beobtained during the CO2 photoreduction process. The followingreactionsmay be the pathways of CO2 photoreduction into valueadded hydrocarbons.

Catalyst + hn / e�cb + h+vb

2H2O + 4h+ / 4H+ + O2 +0.82 V

CO2 + 2H+ + 2e� / HCOOH �0.61 V

CO2 + 2H+ + 2e� / CO + H2O �0.53 V

CO2 + 4H+ + 4e� / HCHO + H2O �0.48 V

CO2 + 6H+ + 6e� / CH3OH + H2O �0.38 V

CO2 + 8H+ + 8e� / CH4 + H2O �0.24 V

In this study, we were selective to obtain CH3OH as the mainproduct of CO2 photocatalytic reduction over Si/TiO2 nano-composites. Our comparative tests demonstrated that nearly noproduct was found by using Si nanospheres or TiO2 nanosheetsas photocatalysts. This may be due to the low conductive bandpotential of TiO2 and low valance band potential of Si, respec-tively. The conduction band (CB) and valence band (VB) edge ofSi and TiO2 semiconductors were calculated by the equation asfollows38 and are presented in Table 1.

ECB ¼ X � EC � 1/2Eg

EVB ¼ Eg + ECB

where X is the absolute electronegativity of the semiconductor;EC is the energy of free electrons on the hydrogen scale (4.5 eV);and Eg is the band gap of the semiconductor.

It can be inferred from the table that, although the calcu-lated VB band edge electrochemical potential of TiO2 (2.91 V vs.NHE) is high enough to initiate the reaction of H2O and h+ toform O2 and H+ (0.82 V vs. NHE), the calculated CB band edgeelectrochemical potential (�0.29 V vs. NHE) is lower than thereaction potential needed for CO2 transformation to CH3OHwith H+ and photogenerated e� (�0.38 V vs. NHE), resulting inthe thermodynamic impossibility of CO2 photoreduction intoCH3OH over TiO2 catalyst under 355 nm laser irradiation. In thesame way, pure Si catalyst has an appropriate CB band edgeelectrochemical potential to reduce CO2 into CH3OH with H+

and e�, but its low VB band edge electrochemical potential (0.72

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V vs. NHE) cannot transform any H+ from the reaction of H2Ooxidation, which enables the photoreduction of CO2 intoCH3OH.

Gas chromatography (GC) was employed to verify andquantify the methanol products from the CO2 reduction overdirect Z-scheme nanospheres. It is depicted in Fig. 5(a) that theretention time for methanol standard is 2.46 min for theselected GC parameters and the used column. The relationshipbetween GC peak area and methanol concentration wasconrmed by using known methanol concentrations in a stan-dard sample for calibration as depicted in Fig. 5(b), whichexhibits a linear trend.

Fig. 6(a) depicts the GC peaks of products for samples fromCO2 photoreduction, which are taken in 30 min intervals ofirradiation 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 GCpeaks were detected, which suggests that the methanol is theonly product obtained through the laser induced photocatalyticreduction of CO2, possibly because of the sharp line width of thelaser beam centred around 355 nm (highly monochromatic) ofthe pulsed laser radiation. Furthermore, it was shown that theGC peak areas of methanol from CO2 photoreduction increasewith the irradiation time (30 min, 60 min, 90 min, 120 min, 150min) and reach the maximum in 150 min of irradiation aerwhich they start to fall. Fig. 6(b) shows the concentration vari-ation of the photocatalytic process of converting CO2 intomethanol with laser irradiation. It can be observed that theproduced methanol concentration increases with laser irradia-tion time and reaches its maximum (197 mM/100 mL) at 150min, but aerwards it declines. The decrease of the methanolconcentration may be caused by the existence of photocatalyticoxidation effect of Si/TiO2 composed semiconductor withpositive VB position. When the methanol was produced in asubstantial amount, it will be adsorbed on the surface of thephotocatalysts 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.

Fig. 6 (a) GC peaks of methanol for sample taken every 30 min ofirradiation with a laser pulse energy of 40 mJ per pulse at 355 nmradiation with 600 mg catalyst in 100 mL distilled water, 50 psi CO2

pressure. (b) Concentration of produced CH3OH and conversionefficiency with time.

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agreement with the results obtained by, and explanation of,other groups.37,40,41

In order to estimate the efficiency of CO2 conversion intomethanol using direct Z-scheme Si/TiO2 nanospheres as thephotocatalyst with 355 nm laser irradiation, the process of theCO2 conversion efficiency was estimated. The amount of CO2

dissolved in distilled water under our experimental conditionscan 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 photocatalyticmeasurement is 50 psi (3.4 atm), the total CO2 dissolved in 100mL of water is 11.56 mmol. Once we know the methanol

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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 methanoland CO2 concentration (as shown in Fig. 6(b)). For that themaximum concentration of methanol is 197 mM/100 mL aer150 minutes of laser irradiation, the maximum CO2 conversionefficiency is calculated (197/11 560) to be about 1.71%.However, according to Schuler’s experimental result,42 the CO2

solubility in binary mixtures of water and methanol increaseswith increasing methanol content, resulting in the actual CO2

conversion efficiency being slightly lower than the calculatedefficiency (1.71%).

Moreover, we can calculate the photonic efficiency of thephotocatalytic reduction of CO2 from the number of methanolmolecules produced for certain irradiation time and thenumber of consumed photons in the reaction. The number ofmethanol molecules can be estimated from the molar concen-trations and Avogadro’s number. In the case of laser, thenumber of photons at 355 nm wavelength with the laser pulseenergy of 40 mJ per pulse and repetition rate of 10 Hz can becalculated to be 4.286 � 1019 photons per min. The maximumrate of methanol is 1.294 � 1018 molecules per min at theirradiation time interval from 60 min to 90 min. As a singlemethanol molecule needs 6 photogenerated electrons, themaximum photonic efficiency (P.E.) of photoreduction of CO2

can be calculated (6 � 1.294 � 1018/4.286 � 1019) to be about18.1%. The achievement of high photonic efficiency of direct Z-scheme Si/TiO2 nanospheres may be due to the constructionnature of Si/TiO2 direct Z-scheme system.

3.4 Mechanism analysis of the enhanced photocatalyticactivity of Si/TiO2

As shown in Fig. 7, photoluminescence spectroscopy wasemployed 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 540nm and a weak, broad peak from 400 to 520 nm, which areattributed to Si and TiO2, respectively. Moreover, it can be seenclearly that the peak intensities in photoluminescence intensityof 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.

Fig. 8 Photocatalytic degradation rates (a) and the ln(C0/C) vs. irra-diation time curves of RhB (b).

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that of Si nanospheres and TiO2 nanosheets. As the PL emissionresulted from the recombination of photo-induced chargecarriers and information regarding the efficiency of chargecarrier trapping, and their recombination kinetics can be drawnfrom the PL spectra,43 it can be inferred that direct Z-scheme Si/TiO2 nanospheres have a higher efficient separation rate ofphotogenerated charge carriers than that of TiO2 nanosheets,which can be attributed to the formation of the Si/TiO2 direct Z-scheme system.

In order to conrm the effect of the Si/TiO2 direct Z-schemesystem on improving the charge separation efficiency, RhB wasselected 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 anillumination intensity of 400 mW cm�2. As shown in Fig. 8(a), itcan be clearly seen that RhB molecules were completelydecomposed by Si/TiO2 direct Z-scheme nanospheres aer 1 hXe arc lamp irradiation, while only 88.5% were decomposed byTiO2 nanosheets and 6.6% by Si nanospheres, indicating theenhanced photocatalytic activity of the Si/TiO2 Z-schemecompared with the TiO2 nanosheets and Si nanospheres.Fig. 8(b) depicts the kinetic study of photocatalytic degradationof RhB solution over the three photocatalytic materials. Thelinear relationship of ln(C0/C) vs. irradiation time suggests thatdegradation of RhB is a rst order reaction. The calculated rateconstants for Si nanospheres, TiO2 nanosheets and Si/TiO2 Z-scheme nanospheres are 0.000772, 0.0354 and 0.074 min�1,respectively, from which it can be seen that the Si/TiO2 Z-scheme has the best photocatalytic activity, which is 2.09times that of the TiO2 nanosheets and 95.8 times the Si nano-spheres. Therefore, construction of the Si/TiO2 Z-scheme isbenecial 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 inuence oflight source and experimental conditions on the product.Fig. 9(a) shows that CO2 can be photoreduced to CH4 by usingall the prepared samples as photocatalysts. It is obvious that thedirect Z-scheme Si/TiO2 nanocomposites exhibit much higheractivity than pure TiO2 nanosheets and Si nanospheres. Thehigher conversion is attributed to the improved photogenerated

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carriers separation efficiency of the Si/TiO2 Z-scheme system. Tofurther 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 otherexperiments. It should be noted that the extremely low CH4

concentrations in non-normal conditions are from the naturallyoccurring CH4 in air (1–2 ppm). In other words, CH4 can beproduced only in the case of possessing all the conditionsincluding catalyst, CO2, H2O and light. It can be inferred fromthe contrast experiment that C in the photoreduction productCH4 is from CO2, while H is from H2O. Therefore, it can beconrmed that the detected CH4 is from the photoreduction ofCO2, 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

specic surface areas of the prepared TiO2 nanosheets and Si/TiO2 nanocomposites were also measured by BET to study theactual exposed surface area in the photocatalytic reaction. Asshown in Fig. 10, the Si/TiO2 sample possesses a slightly higherspecic surface area (165.4 m2 g�1) than TiO2 nanosheets(154.3 m2 g�1) due to the surface structuring effect by theformation of spherical heterostructures with TiO2 nanosheetswell dispersed. In the photocatalytic reduction of CO2 experi-ments, our results show that only the Si/TiO2 sample is capableof producing methanol under laser irradiation due to thermo-dynamics, indicating that the exposed surface area of the

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Fig. 9 (a) CH4 evolution with time under Xe arc lamp irradiation, (b)comparative experiments of the CO2 photoreduction under differentconditions.

Fig. 10 N2 adsorption–desorption isotherms of TiO2 nanosheets anddirect Z-scheme Si/TiO2 nanospheres.

Fig. 11 Schematic diagram of the enhanced photocatalytic propertyof the Si/TiO2 direct Z-scheme system for CO2 reduction.

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catalyst has no effect on its photocatalytic reduction activity. Inthe photocatalytic degradation experiment, 0.1 g of the as-prepared Si, TiO2 and Si/TiO2 catalysts was adopted, respec-tively. The slight difference of the catalysts in specic surfacearea is as small as 6.6%, which is suggested to not be the mainreason for the great improvement of photodegradation effi-ciency (more than twice). Hence, it can be concluded that theexcellent performance of the Si/TiO2 catalyst does not resultfrom a surface structuring effect, but from the improved

56968 | RSC Adv., 2014, 4, 56961–56969

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 canbe harvested by both Si and TiO2 to generate e�–h+ pairs. Asreported in Yang’s previous research,32 the photogenerated holein TiO2 (TiO2h

+) moves toward the TiO2/electrolyte interface andoxidizes OH� to oxygen, while photogenerated electrons in theTiO2 (TiO2e

�) move away from the interface between TiO2 andthe electrolyte due to the schottky barrier. The potential barrierat the Si/TiO2 interface reects holes back into the TiO2 layers.To complete the circuit, the photogenerated electrons in Si(Sie

�) move to the surface where the CO2 reduction reactiontakes place. The photogenerated hole in Si (Sih

+) moves towardsthe Si/TiO2 interface and recombines with the TiO2e

�. There-fore, the direct Z-scheme Si/TiO2 nanospheres show highactivity towards CO2 reduction into methanol since its bandalignment at the junction helps reduce recombination underillumination. However, for individual Si photocatalysts, its VBpotential (+0.721 V vs. NHE) is not high enough to achieve theoxidation reaction potential (O2/H2O 0.82 V vs. NHE). Similarly,the CB potential of TiO2 (�0.29 V vs. NHE) is too low to initiatethe CO2 reduction reaction into methanol (CH3OH/CO2 �0.38 Vvs. NHE). In other words, the individual Si nanospheres or TiO2

nanosheets lack suitable VB or CB potential to photoreduce CO2

into methanol.

4. Conclusion

In summary, we have presented a facile and low cost method toprepare a direct Z-scheme Si/TiO2 nanostructure via hydro-thermal reaction with tetrabutyl titanate and Si powder whichwas prepared from the magnesiothermic reduction of SiO2

Stober nanospheres. All the results indicate that Si/TiO2 nano-composites possess much higher photocatalytic activity thanindividual Si and TiO2 samples for the CO2 conversion anddegradation of RhB. This excellent performance could beattributed to the integrated suitable conductive band of Si andvalence band of TiO2 for CO2 reduction and improved lightabsorption ability, enhanced concentration of photogeneratedcarriers, and higher separation efficiency due to the elaborateconstruction of the Si/TiO2 direct Z-scheme system.

This journal is © The Royal Society of Chemistry 2014

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Acknowledgements

This work was nancially supported by the National NaturalScience Foundation of China (no. 51172109), the FundamentalResearch Funds for the Central Universities (no. NS2014057),Funding of Jiangsu Innovation Program for Graduate Education(no. CXLX12_0148), and the Fundamental Research Funds forthe Central Universities. This work is also supported by KFUPMthrough project # RG1011-1/2.

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