Chemical Engineering Journal 247 (2014) 152–160

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Chemical Engineering Journal

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CaFe2O4 sensitized hierarchical TiO2 photo composite for hydrogenproduction under solar light irradiation

http://dx.doi.org/10.1016/j.cej.2014.02.0761385-8947/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +91 40 27193165; fax: +91 40 27160921.E-mail address: durgakumari@iict.res.in (D.K. Valluri).

Police Anil Kumar Reddy a, Basavaraju Srinivas a, Valluri Durga Kumari a,⇑, Muthukonda V. Shankar b,Machiraju Subrahmanyam a, Jae Sung Lee c

a Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad 500 607, Indiab Department of Materials Science and Nanotechnology, Yogi Vemana University, Kadapa 516 003, Indiac School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology, Ulsan 689-798, Korea

h i g h l i g h t s

� Hierarchical spheres of TiO2

nanosheets are prepared & coupledwith CaFe2O4 for the first time.� The TiO2 spheres of nanosheets show

improved charge separation andcharge carrier mobility.� The CaFe2O4/TiO2 composite

photocatalyst showed highphotocatalytic activity under sunlight for H2 production.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:Received 27 November 2013Received in revised form 13 February 2014Accepted 25 February 2014Available online 12 March 2014

Keywords:CaFe2O4/TiO2

TiO2 hierarchical spheresWater splittingHydrogen production

a b s t r a c t

Hierarchical spheres of self organized nanosheets of TiO2 are prepared by solvothermal method. Thespheres comprising nanosheets are expected to exhibit high conducting properties. The present workis an attempt to explore the conducting properties of these spheres of TiO2 nanosheets that facilitatecharge transfer and charge mobility. And at the same time to extend the absorption of TiO2 to visiblelight, it is combined with a low bandgap semiconductor CaFe2O4. In this perspective, CaFe2O4/TiO2 com-posite photocatalyst consisting of CaFe2O4 and TiO2 hierarchical spheres of nanosheets is prepared bysolid state dispersion (SSD) method. The photocatalysts are characterized by thermo gravimetric analysis(TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy(TEM), N2 adsorption desorption, photoluminescence (PL) and UV–Vis diffuse reflectance spectra (DRS).SEM and TEM images show TiO2 spheres and the mesoporous structure is substantiated by the N2 adsorp-tion desorption studies. UV–Vis DRS of the composites show visible light absorption confirming thesensitization of TiO2 by CaFe2O4. XRD shows the crystallinity of the prepared composites is anataseand SEM confirms that the TiO2 spheres are intact at 400 �C calcination temperature. However, deforma-tion of spheres is seen at higher temperatures. Photocatalytic activity of the composites is studied usingmethanol water mixtures and optimum conditions for hydrogen production are established. By compar-ing the activity of CaFe2O4/TiO2 composite under visible and solar light irradiation, the possible chargetransfer processes that are responsible for the synergistic activity are visualized. Based on the results,a mechanism highlighting the structure activity correlation has been proposed.

� 2014 Elsevier B.V. All rights reserved.

A.K.R. Police et al. / Chemical Engineering Journal 247 (2014) 152–160 153

1. Introduction

For the past few years, semiconductor based composites arebeing extensively studied in the field of photocatalysis and solarenergy conversion. Many studies have been reported to developefficient photocatalysts using synthesis techniques like hydrother-mal, sol–gel, mechanical mixing, chemical vapor deposition,impregnation, etc. to bring interactions between semiconductors[1–6]. TiO2 is a better photocatalyst compared to the other photocatalysts because of its band positions that are feasible for bothoxidation as well as reduction of water. However, its photocatalyticactivity is limited due to its high bandgap that requires UV light forexcitation of charge carriers. Since its discovery, it has been modi-fied in many ways by doping with metals like Si, Ni, Co, Bi [7–10]and non-metals like C, N, S [11,12] to improve its visible absorp-tion. Besides expanding absorption edge of the photocatalysttowards visible region, it is also important to have control overthe electron–hole recombination.

Efforts are made to extend the photo response of TiO2 throughcharge transfer interactions with narrow bandgap ferrites. Degrada-tion of pollutants in aqueous solutions on TiO2 based CoFe2O4, NiFe2-

O4 and ZnFe2O4 [13–15] and hydrogen production on CuFe2O4

nanoparticles are reported in literature [16]. Recently, photo electro-chemical water splitting to hydrogen and oxygen is observed onCaFe2O4 and TiO2 electrodes [17]. In general, ferrites are known fortheir magnetic properties, yet they also show photocatalytic proper-ties for hydrogen or oxygen evolution from water. These materialshave narrow bandgap of nearly 2 eV that allow the absorption of alarge portion of visible light. Owing to their narrow bandgap, how-ever, recombination of photogenerated charge carriers is high andtheir photocatalytic activity is low. Several researchers have ob-tained good photocatalytic activity by coupling these narrow band-gap semiconductors with wide bandgap semiconductors [18–20].

Based on our earlier experience in making photocatalysts forhydrogen production [21–26], in the present investigation, CaFe2-

O4 is dispersed over TiO2 spheres. These TiO2 spheres preparedunder solvothermal conditions are of hierarchical structures con-sisting nanosheets of TiO2 crystallites. These nanosheets arereported to facilitate high conductivity of the charge carriers alongwith improved quantum efficiency due to multiple reflections onTiO2 spheres [27,28]. The composite material prepared from TiO2

spheres and CaFe2O4 is used for the first time for water splittingunder solar irradiation. The solid state dispersion method usedretained the morphology of TiO2 spheres at the same time creatinga contact with CaFe2O4 through which a synergism has been estab-lished resulting in enhanced hydrogen production activity.

Fig. 1. Thermogravimetric analysis of TO.

2. Experimental

2.1. Preparation of catalysts

Hierarchical spheres of TiO2 nanosheets (TO) is prepared bysolvothermal method as reported earlier [28] wherein, diethylene-triamine (DETA) is used as stabilizing agent and isopropyl alcohol(IPA) is used as solvent for dissolving titanium isopropoxide. Thereaction solution contains Titanium (IV) isopropoxide (3.6 ml)dissolved in DETA (0.7 ml) and 100 ml IPA. The reaction mixturewas taken in a 250 ml Teflon container placed in a stainless steelautoclave and was kept in a furnace at 200 �C for 24 h. After thereaction the autoclave was taken out from the oven and allowedto cool to the room temperature. The white precipitate obtainedwas collected by centrifugation and washed thoroughly with etha-nol, and dried overnight in the oven at 100 �C. Thus obtained sam-ple was subjected to thermogravimetric analysis (TGA) in order toensure the temperature of calcinations as shown in Fig. 1. Gradual

weight loss till 400 �C was observed due to the organic matterpresent in the precursor. Based on TGA analysis the samples werecalcined at 400 �C for 2 h.

CaFe2O4 (CFO) was prepared by polymerizable complex (PC)method [29]. Citric acid of 6.4 g dissolved in 100 ml deionizedwater. Then, 7.9 g calcium acetate and 40.4 g ferric nitrate wereadded to the citric acid solution and stirred for 30 min. To theabove solution, 4.3 g ethylene glycol was added. The final solutionwas stirred on a hot plate at 80 �C until it converts into a polymericgel. Finally the gel obtained was oven dried at 100 �C, crushed andcalcined at 300 �C. The resultant powder was further calcined at1000 �C for 2 h.

CFTO composite was prepared by solid state dispersion method.The method involves mixing of CaFe2O4 and TiO2 in requiredamount using solvent ethanol in an agate mortar till the solventis evaporated. The samples obtained were dried in oven at 100 �Cand calcined at 400 �C for 2 h. The amounts of 0.5, 1.0, 1.5, 2.0,and 3.0 wt% of CaFe2O4 are loaded over TiO2 and designated as0.5 CFTO, 1.0 CFTO, 1.5 CFTO, 2.0 CFTO and 3.0 CFTO respectively.For comparison, CFTO composites are also prepared by loadingsimilar amounts of CFO over commercial TiO2 (Degussa P-25)and designated as CFTO-P25. Further, 1CFTO composite was mod-ified with silver by photo-deposition method. The catalyst (1CFTO)was suspended in aqueous methanol solution containing AgNO3

and was subjected to UV irradiation using 250 W high pressuremercury vapor lamp (Phillips) for 2 h. After the illumination thedark grey colored powder was collected and washed with thedeionized water and dried overnight in the oven at 100 �C anddenoted as AgCFTO.

2.2. Characterization

The catalysts prepared were characterized by techniques likeTGA, XRD, SEM, TEM, UV–Vis DRS, N2 adsorption/desorption andPhotoluminescence Spectra. The TGA was obtained by Leeds andNorthup (USA) unit at a heat rate of 10 �C/min/N2 flow using Pt &Pt-Rh (10%) differential thermocouple. Powder X-ray diffractionpatterns were recorded on a Rigaku diffractometer using Cu Karadiation (0.1540 nm). Surface area of the catalysts was measuredon Autosorb 1C Quantachrome physical adsorption apparatus. UV–Vis DRS measurements were recorded in the wavelength range200–800 nm using a GBC UV–Visible Cintra 10e spectrometer withan integration sphere diffuse reflectance attachment. For SEM

154 A.K.R. Police et al. / Chemical Engineering Journal 247 (2014) 152–160

analysis, samples were mounted on an aluminum support using adouble adhesive tape, coated with gold in HUS-SGB vacuum coat-ing unit and observed in Hitachi S-520 SEM unit. The TEM andHRTEM studies were made on a TECNAI G2 TEM microscopeequipped with a slow-scan CCD camera and at an accelerating volt-age of 200 kV. The preparation of samples for this analysis involvedsonication in ethanol for 10 min and deposition on a copper grid.The photoluminescence spectra were obtained from Elico Spectro-fluorometer-SL174.

2.3. Photocatalytic activity

Photocatalytic hydrogen production from water splitting wascarried out over TiO2, CaFe2O4 and CaFe2O4/TiO2 photocatalystsunder solar light and visible light irradiation. The reaction was per-formed in a 100 ml tube like quartz reactor with round bottom.50 mg of catalyst was added to 50 ml of reaction solution in aquartz reactor and sealed with air tight rubber septum. The reactorwas evacuated for 30 min and then, the solution was purged withN2 gas for another 30 min. The reaction was carried out underbright sun light irradiation from 10:00 AM to 3:00 PM. For the vis-ible light reactions, the reactor was exposed to a Xenon arc lampwith a filter to remove light of wavelength below 400 nm. Theamount of H2 gas produced during the course of reaction was mon-itored hourly by taking the gas samples in an airtight syringe andthe analysis of the sample was carried out using a gas chromato-graph (Shimadzu GC-2014) equipped with TCD detector andmolecular sieve 5A column and N2 as a carrier gas.

3. Results and discussion

3.1. Characterization

3.1.1. XRDX-ray diffraction patterns of the catalysts prepared are shown in

Fig. 2. XRD pattern of CaFe2O4 shown in Fig. 2a matches with thedocumented XRD data (PCPDF No. 32-0168), indicating crystalline

Fig. 2. XRD patterns of CFO (a), TO (b) and 1CFTO400 (c), 1CFTO500 (d) 1CFTO600 (e).

phase of CaFe2O4. Fig. 2b is the XRD pattern of TiO2 calcined at400 �C, which is in agreement with the anatase phase (PCPDF71-1167). Fig. 2(c–e) are the XRD patterns of 1 wt% CaFe2O4/TiO2

photocatalysts calcined at 400, 500 and 600 �C respectively. Theanatase phase is intact even up to 600 �C calcination temperature.The TiO2 crystallite size of the composite is seen increasing withcalcination temperature as reported in Table 1.

3.1.2. SEMSEM image in Fig. 3a shows aggregated particles of CaFe2O4

whereas, Fig. 3(b and c) shows the spherical morphology of TiO2

with particle size of 1–2 lm. Fig. 3(d–f) are the SEM images ofthe 1 wt% CaFe2O4/TiO2 calcined at 400, 500, 600 �C respectively.The SEM evidences the fact that the spherical structure of theTiO2 is deformed at high calcination temperatures.

3.1.3. TEMTEM image of the bare TiO2 (TO) is shown in Fig. 4(a and b). TiO2

has spherical morphology with small crystal projections on thesurface of the spheres. These structures are established to be hier-archical spheres formed from ultrathin TiO2 nanosheets [28]. Asthese nanosheets are highly flexible they can readily self organizedinto hierarchical structures. The selected area electron diffraction(SAED) image of TiO2 (Fig. 4c) shows ring patterns, confirmingthe polycrystalline nature of TiO2 spheres. The patterns are in-dexed to the anatase phase of TiO2, thus substantiate the XRD data.The TEM images of 1CFTO in Fig. 4d and e shows the CaFe2O4 nanocrystals on the surface of TiO2 spheres. HRTEM image shown in theFig. 4(f) depicts the (020) plane of CaFe2O4 crystallite with a ‘‘d’’spacing of 0.465 nm and the (101) plane of TiO2 with a ‘‘d’’ spacingof 0.351 nm.

3.1.4. UV–Vis DRSUV–Vis DRS of the TiO2, CaFe2O4 and CaFe2O4/TiO2 composites

are shown in Fig. 5. Bare TiO2 is showing an absorption edgearound 390 nm. The deep brown color of calcium ferrite powdersevidence its absorption in visible light region forming absorptionedge around 670 nm. The bandgap of CaFe2O4 is 1.85 eV calculatedaccording to the equation Eg = 1239.8/kmax, where kmax and Eg arethe absorption maximum and the bandgap of the semiconductor.Owing to its small bandgap, CaFe2O4 absorbs a wide range of visi-ble spectrum. As it can be seen from the spectra, the CaFe2O4 addi-tion to TiO2 leads to the expansion of absorption of TiO2 towardsthe visible light region. The silver deposition on CFTO gives peakwith absorption maxima at 480 nm and confirms the presence ofnanosilver particles on the surface of CFTO.

3.1.5. N2 adsorption/desorptionFig. 6a illustrates the N2 adsorption/desorption isotherms and

the corresponding pore size distribution curves of the TiO2 (TO).TiO2 shows Type IV isotherms with a H2 hysteresis loop. The poresize distribution confirms that TiO2 spheres have uniform distribu-tion of mesopores with a pore-size of about 4–7 nm. Fig. 6b showsN2 adsorption/desorption isotherms of 1 wt% CaFe2O4/TiO2

(1CFTO) calcined at 600 �C. CaFe2O4/TiO2 composite shows TypeIV isotherms with a H3 hysteresis that describes the increase inpore size at high temperature calcination. The wide range distribu-tion of pore-sizes (4–15 nm) depicts the trend of irregular mesop-ores system.

3.1.6. Photo-luminescence spectraThe photo luminescence (PL) spectra are useful to disclose the

information regarding the efficiency of charge carrier trapping,migration and transfer of photo excited free charge carriers. SincePL emission results from the recombination of photo excitedcharge carriers, it is a useful technique to understand the

Fig. 3. SEM images of CFO (a), TO (b and c), 1CFTO400 (d), 1CFTO500 (e), and 1CFTO600 (f).

Table 1Physical properties of the catalysts.

Catalysta Crystallite size by XRD (nm) BET surface area (m2/g) Average pore dia (nm) Pore volume (cm3/g)

TO 12.51 173.62 6.42 0.179CFO 26.47 2.25 – –1CFTO400 12.93 158.32 – –1CFTO500 17.25 129.94 – –1CFTO600 25.40 89.54 10.51 0.084

a Subscript shows the calcinations temperatures.

A.K.R. Police et al. / Chemical Engineering Journal 247 (2014) 152–160 155

recombination nature of electron–hole pairs in semiconductorparticles. The PL spectrum of 1CFTO and AgCFTO is shown in theFig. 7. The spectra clearly indicate that the luminescence intensityof 1CFTO was substantially reduced by Ag addition. This revealsthat electron–hole recombination minimization is evidenced inAgCFTO by the deposition of silver on CFTO surface. The slowerrecombination process of photogenerated charge carriers willbenefit the photocatalytic reaction that result in enhanced hydro-gen production.

3.2. Photocatalytic activity

3.2.1. Effect of CFO loading on TOPhotocatalytic activity on TO, CFO and CFTO is evaluated for the

production of hydrogen under solar light irradiation using

methanol:water mixtures. Hydrogen production is not observedunder solar light on bare CaFe2O4 (CFO) whereas, 200 lmole H2

h�1 g�1 is observed on bare TiO2 (TO). Fig. 8a depicts the hydrogenproduction in 10% methanol over different wt% of CaFe2O4/TiO2

photocatalysts as a function of irradiation time. 1CFTO is showingmaximum photocatalytic activity i.e., 2100 lmole H2 h�1 g�1 inaqueous methanol.

Under the similar experimental conditions in the absence ofscavenger i.e., in pure water, the optimum activity observed isaround 92 lmole H2 h�1 g�1 on 2CFTO catalyst (Fig. 8b). Furtherincrease in the CaFe2O4 amount resulted in decreased photocata-lytic activity of CaFe2O4/TiO2 composite. When the CaFe2O4

crystallites are dispersed by SSD over high surface area TiO2, thecrystallite size of CaFe2O4 tends to decrease. The highly dispersednano crystallites show properties different from bulk such as

(f)

(a) (b) (c)

(d) (e)

Fig. 4. TEM (a and b) and SAED pattern (c) of TO; TEM (d and e) and HRTEM (f) images of 1CFTO.

300 400 500 600 700

(g)

(f)

(e)

(d)(c)

(b)(a)

Abso

rban

ce (a

.u)

Wavelength (n.m)

Fig. 5. UV–Vis DRS of TO (a), 0.5CFTO (b), 1CFTO (c), 2CFTO (d), 3CFTO (e), AgCFTO(f), and CFO (g).

0.0 0.2 0.4 0.6 0.8 1.00

102030405060708090

100110120

(b)

(a)

5 10 15 20 25 30 35 40 45 50

0.0

0.2

0.4

0.6

0.8

1.0

1.2

(b)

(a)

dV/d

log

(D)

Pore dia (n.m)

Volu

me

adso

rbed

(cm

3 .g-1

) STP

Relative Pressure (P/P0)

Fig. 6. N2 adsorption/desorption isotherms and corresponding pore size distribu-tion curves of TO (a) and 1CFTO600 (b).

250 300 350 400 450 500 550 600 650 700

6080

100120140160180200220240260280300

(b)

(a)

PL in

tens

ity (c

ount

s)

Wavelength (n.m)

Fig. 7. Photoluminescence spectra of 1CFTO (a) and AgCFTO (b).

156 A.K.R. Police et al. / Chemical Engineering Journal 247 (2014) 152–160

bandgap broadening (due to size quantization) resulting in mini-mal electron hole recombination. As the loading increases the crys-tallites become bulky and tend to show the bulk characteristicssuch as absorption of shorter wavelengths and fast electron holerecombination.

3.2.2. Comparison of CFTO under visible and solar irradiationThe photocatalytic activity of TO, CFO and 1CFTO under visible

& solar light irradiation is studied and compared (Fig. 9). Under vis-ible irradiation no hydrogen production is observed on CFO and asmall amount of hydrogen is produced on TO (36 lmole h�1 g�1)whereas, 1CFTO showed enhanced activity for hydrogen produc-tion (577 lmole h�1 g�1). However, the activity of 1CFTO undervisible light irradiation is seen decreased compared to sunlightirradiation. Though the observed activity is low under visible lightthan solar light, the results clearly show that both CFO and TO areplaying vital role in the enhanced activity of CFTO composite. Sincethe aim of this work is to effectively utilize solar light, furtherexperiments are conducted under solar light irradiation.

3.2.3. Effect of preparative conditions on CFTO activityThe activity comparison between the samples prepared by

mechanical mixing (1CFTO-MM) and solid state dispersion

0 1 2 3 4 5 6 7 8 9 100

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000(a)

H 2 pro

duct

ion

( µm

ole/

g)

Time (h)

1CFTO-MM 1CFTO-SSD

0.5 1 1.5 20

200

400

600

800

1000

1200

1400

1600

1800

2000

2200(b)

H 2 pro

duct

ion

( µm

ole/

g/h)

CaFe2O4 wt %

CFTO-P25 CFTO

100 400 500 6000

500

1000

1500

2000

2500(c)

H 2 pro

duct

ion

( µm

ole/

g/h)

Temperature (oC)

Fig. 10. Activity comparison of 1CFTO prepared by mechanical mixture 1CFTO-MMand solid state dispersion method 1CFTO-SSD (a) activity comparison of CFTO-P25and CFTO (b) and effect of calcination temperatures on photocatalytic activity of1CFTO catalyst (c).

1 2 3 4 5 6 7 8 9 10 15 20 250

2000

4000

6000

8000

10000

12000

20000300004000050000(a)

H 2 pro

duct

ion

(µm

ole/

g)

Time (h)

0 0.5 1.0 1.5 2.0 3.0

0 0.5 1 2 30

20

40

60

80

(b)

H 2 pro

duct

ion

(µm

ole/

g/h)

CaFe2O4 wt %

Fig. 8. Photocatalytic activity of CFTO composite with various CFO wt% loadingsover TO in 10% aqueous methanol (a) and pure water (b).

CFO TO 1CFTO0

500

1000

1500

2000

H 2 pro

duct

ion

(µm

oles

/h)

Catalyst

Visible light Solar light

Fig. 9. Activity comparison of catalysts under solar and visible light irradiation.

A.K.R. Police et al. / Chemical Engineering Journal 247 (2014) 152–160 157

(1CFTO-SSD) is shown in Fig. 10a. The higher activity of 1CFTO-SSDcompared to 1CFTO-MM confirms an established contact betweenCFO and TO during the process of grinding and calcination (SSD).The improved photocatalytic performance of the composite photo-catalysts is reported by several groups wherein a good contact be-tween the semiconductors facilitated enhanced charge separation[30–32]. Based on the experimental results, it may be consideredthat by using a simple solid state dispersion method a contactbetween two semiconductors can be achieved.

Since solid state dispersion method is establishing a contactbetween CFO and TO, it is proposed to study how the conducting

properties of TiO2 influence the charge separation in CFTO compos-ites. In general better crystallinity and high conductivity of a semi-conductor plays major role in the efficient charge separation.Nanotubes and sheet like structures are found to be efficient phot-ocatalysts because of their high conducting properties compared tothe normal crystalline particles [33]. Activity comparison of CFOloaded on commercial TiO2 (CFTO-P25) and TO spheres of nano-sheets (CFTO) is carried out and illustrated in Fig. 10b. Optimum

CFO AgCFO TO AgTO CFTO AgCFTO0

2000

4000

6000

8000

10000

H 2 pro

duct

ion

(µm

ole/

g/h)

Catalysts

Fig. 12. Effect of silver deposition on hydrogen production under solar lightirradiation.

158 A.K.R. Police et al. / Chemical Engineering Journal 247 (2014) 152–160

activity of 810 lmole H2 h�1 g�1 is achieved for CFTO-P25 compos-ite at 0.5 wt% loading whereas, maximum activity of2111 lmole H2 h�1 g�1 is observed for CFTO composite at 1 wt%loading. The possible reasons for better activity over CFTO maybe due to (i) fine dispersion of CaFe2O4 over the TiO2 spheres dueto its high surface area. (ii) High conductivity of TiO2 spheresformed by crystalline nanosheets that make charge separationmore facile. Moreover, multiple reflections in TiO2 spheres improvequantum efficiency.

Activity of 1CFTO catalyst calcined at different temperatures400, 500, and 600 �C is also studied. It was found that with the in-crease in calcination temperature, the activity is decreased asshown in Fig. 10c. At high calcination temperature the deformationof TiO2 spheres of nanosheets is taking place that is evidenced inSEM images (Fig. 3). Hence, the conducting nature of the TiO2

spheres is reduced that in turn minimizes charge separation andcharge mobility between semiconductors CFO and TO in the CFTOcomposite. This infers that to attain better activity the conductingnature of TiO2 is very important and should be retained to facilitatecharge separation between the two semiconductors.

3.2.4. Effect of methanol concentration and catalyst amountIn order to optimize the methanol concentration, reactions

were carried out with 1CFTO catalyst using methanol:water mix-tures. The results are summarized in Fig. 11a. It is clear from thefigure that the methanol concentration is affecting the productionof hydrogen and methanol is actively involved in the production ofH2. Maximum amount of H2 is produced in 5% methanol solutionand with increase in methanol concentration no increase is seenin H2 production. The reason may be seen as that at higher concen-trations the surface of the photocatalyst reaches saturation with nofurther increase in H2 production. Further, the catalyst amount0.05, 0.1, 0.2, 0.4 and 1.0 g l�1 of 1CFTO in 5% methanol:water solu-tion is investigated for hydrogen production as shown in Fig. 11b. Itis observed that, the rate of H2 production is decreased by increas-ing catalyst amount from 0.1 to 1.0 g l�1. This may be seen as thatthe higher amount of the catalyst makes the solution opaque andobstructs the light path reaching the catalyst particles. In the pres-ent study, 0.1 g l�1 is found to be the optimum catalyst amount formaximum production of hydrogen i.e. 7820 lmole h�1 g�1.

3.2.5. Effect of silver deposition on 1CFTO activityAn attempt was experimented to improve the hydrogen pro-

duction activity further by photo depositing silver as a co-catalyston CFO, TO and 1CFTO. The H2 production as a function of lightirradiation time over CFO, TO and 1CFTO composite photocatalystswith Ag modification is shown in Fig. 12. Ag deposition on TiO2

0 2.5 5 10 150

500

1000

1500

2000(a)

% methanol

H2 p

rodu

ctio

n (µ

mol

e/g/

h)

(

Fig. 11. Effect of methanol concentration (a)

surface gives marginal improvement in the hydrogen productionactivity. Nano silver particles play a role of electron sinks and actas active sites for proton reduction to decrease electron–holerecombination thereby increasing the production of H2. Moreoverthe enhancement in activity may be attributed to the fact thatthe loading of silver results in the formation of Schottky barriersat Ag–TiO2 contact regions, thus promoting charge transfer andinhibiting the recombination of electron–hole pairs. The decreasein the electron–hole recombination and an increase in the chargetransfer are also evidenced by PL spectral information of AgCFTOand CFTO composite depicted in Fig. 7. It is clear from the figurethat the intensity of AgCFTO is lesser than CFTO, which illustratesthat the recombination process is much slower in AgCFTO thanthat of CFTO.

3.3. Structure vs. charge separation vs. activity

The present study describes composite photocatalyst developedby combining a visible light-driven material (CaFe2O4) with an UVlight driven photocatalyst (TiO2). Charges generated on the surfaceof visible active catalyst under visible light region are transferredto the surface of UV active catalyst and vice versa. In this way,the probability of recombination of electron and hole is minimizedto a greater extent as both the charge carriers (electron–hole) arelocalized on different surfaces of photo composite, which lead tothe synergistic photocatalytic activity.

Based on the results obtained, a Scheme 1 has been proposed toillustrate the photoactivity of CFTO composite under visible and

0.05 0.1 0.2 0.4 1.00

1000

2000

3000

4000

5000

6000

7000

8000b)

H2 p

rodu

ctio

n (µ

mol

e/g/

h)

catalysts amount (g/l)

and effect of 1CFTO catalyst amount (b).

Scheme 1. Photo excitations in CFTO under visible (a) and solar (b) light irradiation.

A.K.R. Police et al. / Chemical Engineering Journal 247 (2014) 152–160 159

solar light irradiation. Under visible light irradiation, the low en-ergy photons are harvested by CFO to produce electron and holein conduction band (CB) and valance bands (VB) respectively. Theelectron thus produced is transferred to the CB of TO therebyavoiding recombination of electron and hole in CFO. The electrontransferred to TO participates in the reduction of protons to pro-duce hydrogen. At the same time, the holes in the VB of CFO oxi-dize sacrificial agent methanol to yield carbon dioxide. However,the situation is different under solar-light irradiation. Under theseconditions both the semiconductors TO and CFO are excited bycapturing UV and visible part of solar light respectively. Thus, elec-trons are excited from the VB of both CFO and TO to the CB. Photo-generated electrons will be transferred from the CB of CFO into theCB of TO and holes at the VB of TO to the VB of CFO. In this case, thephotogenerated electron–hole pairs are effectively separated and ahigh concentration of electrons in the CB of TO and of holes in theVB of CFO is obtained. In addition, the high charge-carrier transferefficiency of nanosheets of TO spheres can be beneficial for theelectron transportation and effectively reduce H+ to produce H2.Meanwhile, the surface deposited CFO can function as an activeoxidation site, in which the assembled holes consumed by metha-nol sacrificial agent to produce CO2. This may be the possible rea-son for the activity under solar light irradiation compared to visiblelight irradiation. Whereas, under solar light, both semiconductorsare excited and involve in the redox reactions and under visiblelight only one semiconductor is excited and the other one facili-tates charge separation. The light absorption efficiency, electron–hole pair generation ability, effective charge transfer and chargemobility in the composite materials effect the photocatalytic prop-erties. Thus the mobility of photogenerated electrons and holesbetween the two semiconductors is the key factor in getting theoptimum activity of CFTO composite photo catalyst in the presentinvestigation.

4. Conclusion

Hierarchical spheres of self organized nanosheets of TiO2 areprepared by solvothermal method. A visible light active compositeconsisting these TiO2 spheres and CaFe2O4 is prepared. The com-posite prepared is found to be an effective catalyst for hydrogenproduction under solar light irradiation. The preparative conditionsare playing vital role in the enhancement of photocatalytic activity.A proper contact between the semiconductors of composite cata-lysts is achieved by SSD method in the present investigation. Also,it is found that the high conductive nature of TiO2 spheres com-posed of nanosheets is responsible for enhanced charge separationand charge mobility. The pronounced synergistic activity of thecomposite is explained by the possible mobility of charge carriers

under visible and solar irradiation. Thus a structure–activity corre-lation is arrived.

Acknowledgements

Authors thank CSIR, DST (Indo-Korea cooperation) and MNREfor funding the work.

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