Hydrogen generation and degradation of trypan blue using fern-like structured silver-doped TiO2 nanoparticles

This journal is©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014 New J. Chem.

Cite this:DOI: 10.1039/c4nj01403k

Hydrogen generation and degradation of trypanblue using fern-like structured silver-dopedTiO2 nanoparticles

Thammadihalli Nanjundaiah Ravishankar,a Thippeswamy Ramakrishnappa,*a

Hanumanthappa Nagabhushana,b Virginia S. Souza,c Jairton Dupontc andGanganagappa Nagaraju*ad

TiO2:Ag nanoparticles have been successfully prepared at 120 1C in one day using an ionic liquid assisted

hydrothermal method with methoxyethyl methyl imidazolium methanesulfonate as the ionic liquid. The

obtained product was characterized using various techniques. The XRD pattern indicated the formation of

TiO2:Ag nanoparticles, the average crystallite size was found to be 40 nm. XPS confirmed the formation of

TiO2:Ag nanoparticles. The UV-Vis spectrum indicated a maximum absorbance at 362 nm which is red

shifted compared to nano-sized TiO2. The surface morphology was analyzed using SEM, which shows a

leaf-like structure for the TiO2:Ag nanoparticles. TEM images showed almost elliptically (rice pellet) shaped

nanoparticles with an average particle size of about 60 nm. The EDS spectrum revealed the presence of

Ti, O and Ag with atomic percentages of 42.6, 43.7 and 2.3%, respectively. The TiO2:Ag nanoparticles

generate 2230 mmol H2 per 1 g of photocatalyst in 2.5 h via the water splitting reaction. They also show

good photocatalytic activity in the degradation of trypan blue.

1. Introduction

Nanocrystalline transition metal oxides have attracted wideattention due to their unique properties compared to bulk metaloxides, which are technologically very useful in nanodevicefabrication.1,2 Amongst these metal oxides, TiO2 nanoparticlesare very interesting due to their semiconducting properties, wideband gap, low costs, non-toxicity, good oxidizing power andunique properties making them e.g. promising photocatalysts,sensors and solar cells.3 Literature reports suggested that TiO2

has some practical limitations, for example bare TiO2 nano-particles are less efficient under near UV irradiation (4% of thesolar spectrum) for effective photocatalysis and recombinationof electron–hole pairs than doped TiO2 nanoparticles. Hence,bare TiO2 exhibited less photocatalytic activity than the dopedTiO2 nanoparticles.4 In order to enhance the photocatalyticactivity, scientists are working on the synthesis of doped TiO2

nanoparticles using various methods such as hydrothermal,ionic liquid assisted hydrothermal, sol–gel, co-precipitation,

combustion and other strategies.5 Amongst these methods,the ionic liquid assisted hydrothermal method is consideredthe most prominent, as room temperature ionic liquids (RTILs)have received extensive attention from both academic andindustrial researchers over the past two decades. RTILs possessunique properties such as negligible vapor pressure, a wideliquid temperature range, high thermal stability, dissolubilityin both organic and inorganic compounds, high ionic conduc-tivity and a wide electrochemical window. An important aspectof the interaction of RTILs with nanoparticle precursors involvesthe nucleation and growth of the nanoparticles.6 Metal dopantslike Ag, Al, Ce, Nd, Eu, Mo, and Fe as well as non-metal dopantslike O, N, and S, have been used to enhance the photocatalyticactivity of TiO2 nanoparticles.7 Ag is one of the most promisingdopants which modifies the surface of TiO2 during its fabrica-tion by decreasing the total volume of the particles which in turndecreases the recombination probability making more carriersavailable for oxidation or reduction processes on the surface.8

Due to these properties, Ag-doped TiO2 nanoparticles show anenhanced photocatalytic activity (H2 generation via the watersplitting reaction and degradation of organic dyes) compared toundoped TiO2 nanoparticles.

In the 21st century, environmental issues became more promi-nent because of the population explosion and rapid urbanizationresulting in an energy crisis. Hence, researchers are trying to findnew alternative routes to prepare energy-generating sources

a Centre for Nano and Material Sciences, Jain University, Jakkasandra, Kanakapura

(T), India. E-mail: [email protected], [email protected] CNR Rao Center for Advanced Materials, Tumkur University, Tumkur, Indiac Laboratory of Molecular Catalysis, Institute of Chemistry, UFRGS, Porto Alegre,

Brasild Department of Chemistry, Siddaganga Institute of Technology, Tumkur,

Karnataka, India

Received (in Montpellier, France)20th August 2014,Accepted 27th November 2014

DOI: 10.1039/c4nj01403k

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using different methods.9,10 Amongst various energy sources,H2 produced by water splitting is an eco-friendly and highlyefficient fuel. Photocatalysis has been considered to be apromising technique for solving energy and environmentalissues using abundant sunlight.11 Over the past several dec-ades, significant progress has been achieved on semiconductor-based photocatalytic H2 generation through water splitting,and many excellent reviews have been published.12,13 Literaturereports state that Ag-doped TiO2 nanoparticles show enhancedhydrogen generation via the water splitting reaction comparedto bare TiO2 nanoparticles.14,15

Trypan blue has been extensively used in the textile, foodand paint industries for dyeing nylon, wool, cotton, silk andalso for coloring oil, fats, waxes, varnish, plastics, etc. Dyescreate several environmental problems by releasing highlycarcinogenic molecules into bodies of water. Hence, it isnecessary to remove/degrade the industrial effluents containingtrypan blue to non-hazardous substances. Based on this, manyarticles are available on the enhanced photocatalytic activity ofAg-doped TiO2 nanoparticles for the degradation of organicdyes.16,17

In the present research work, we have used methoxyethylmethyl imidazolium methanesulfonate (MOEMIMS) as thereaction medium for the synthesis of TiO2:Ag nanoparticles.These nanoparticles are used as photocatalysts for H2 genera-tion via a water splitting reaction and also for the degradationof trypan blue dye.

2. Experimental2.1. Preparation of ionic liquid – methoxyethyl methylimidazolium methanesulfonate (MOEMIMS)

The above ionic liquid was prepared using an earlier report.18

2-Methoxyethyl methanesulfonate (5.38 g, 32.0 mmol) was mixedwith 1-methyl imidazole (2.62 g, 32.0 mmol) and the reactionmixture was heated to 60 1C for 30 h. The resulting liquid waswashed twice with ethyl acetate (5 mL) and dried under vacuum,and 1-(2-methoxyethyl)-3-methylimidazolium methanesulfonate wasobtained as a colorless and hygroscopic liquid. This IL (MOEMIMS)was used for the preparation of TiO2:Ag nanoparticles and thestructure is shown in Scheme 1.

2.2. Preparation of TiO2:Ag nanoparticles

0.5 mL TiCl4 was added to the beaker containing 12 mg silvernitrate and 10 mL MOEMIMS under constant stirring for homo-genization. After 5 min, 1 mL water was added to hydrolyzeTiCl4. The white precipitate was transferred into a Teflon tube,

and heated at 120 1C for one day. When the reaction/durationwas completed, the autoclave was naturally cooled to roomtemperature. The obtained product was mixed with acetonitrileand stirred overnight to remove the ionic liquid. Finally,TiO2:Ag nanoparticles were retrieved via centrifugation. Thefinal product was calcined at 400 1C for 3 h.

2.3. Characterization

Powder X-ray diffraction data was recorded on Philips X’pertPRO X-ray diffractometer with graphite monochromatizedCu-Ka (1.5418 Å) radiation operated at 40 kV and 30 mA. X-rayphotoelectron spectroscopy (XPS) analysis was carried out on anESCALAB 250 (Thermo-VG Scientific) using Al Ka radiation asthe excitation source. The instrument was standardized againstthe C1s spectral line at 284.6 eV. The Fourier transform infraredspectra (FTIR) of the samples were collected using a BrukerAlpha-P spectrometer. The absorption spectra of the sampleswere measured by dispersing the nanoparticles in water andmeasured using a Perkin Elmer Lambda-750 UV-Vis spectro-meter. The morphology was examined using scanning electronmicroscopy (SEM) (table top Hitachi 3000). The nanostructure ofthe product was observed using transmission electron microscopy(TEM) which was performed with a JEOL JEM 1200 Ex operating at100 kV. Samples for TEM were prepared by dropping a dispersionof metal oxide nanoparticles in 2-propanol on a holey carbon gridand drying the grids under vacuum for 24 h. The water/organiccontent present in the sample was investigated via thermogravi-metric analysis (TGA) using an SDT Q600 V20.9 thermo-microbalance under N2 atmosphere from room temperature to800 1C at a heating rate of 10 1C min�1.

2.4. Photocatalytic H2 production setup

Photocatalytic H2 production reactions were carried out in a closedgas-circulating system. TiO2:Ag nanoparticles were suspended inaqueous ethanol solution by sonicating for about 20 min within aninner irradiation-type reactor made of Pyrex glass. A 240 W Hg–Xearc lamp (Cermax) was used as the excitation source. Prior tothe reaction, the mixture was deaerated by purging with Ar gasrepeatedly to minimize the oxygen content. The liberated H2

was periodically analyzed using an Agilent 6820 GC chromato-graph equipped with a thermal conductivity detector and a 5 Åmolecular sieve-packed column with argon as the carrier gas.Using a gas tight syringe with a maximum volume of 50 mL, theamount of H2 produced was measured at 0.5 h time intervals.The amount of gas liberated is plotted as a function of UV exposuretime. During the entire experiment, the reaction temperature waskept at 25 1C by eliminating the IR radiation with the circulation ofwater in the water jacket of the reactor.

2.5. Photocatalytic degradation of trypan blue

Photocatalytic experiments were carried out in a 150 � 75 mmbatch reactor in the months of November–December between11 am and 2 pm under sunlight (B750 W m�2 intensity) inBangalore, India, and under UV-light (mercury lamp as radia-tion source with 125 W m�2 intensity). An aqueous suspensionwas prepared by adding a known quantity (0.1–0.5 g) of TiO2:AgScheme 1 Structure of 2-methoxyethyl methyl imidazolium methanesulfonate.

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nanoparticles to 100 mL trypan blue dye solution at appropriateconcentrations (5–25 mg L�1). For reactions in different pHmedia, the initial pH was adjusted by adding either 0.05 MNaOH or 0.05 M H2SO4. During the photocatalytic experiments

the slurry composed of dye solution and catalyst was put intoa reactor and placed on a magnetic stirrer with a distance of8–10 cm to the light and stirred magnetically at 400 rpm for auniform distribution with simultaneous exposure to UV light/sunlight. A known volume (5 mL) of the exposed solution waswithdrawn at specific time intervals (15 min). The TiO2:Ag nano-particles were removed from the solution via centrifugation to assessthe extent of degradation. The dye concentration was measuredusing a spectrophotometer at 592 nm; various parameters likedye concentration, catalytic load, irradiation time, pH, differentlight sources, etc., were studied to identify their effect on the rateof photo degradation of the dye.

3. Results and discussion3.1. Characterization of the prepared TiO2:Ag nanoparticles

XRD is mainly used to know the phase composition and crystal-lite size of the prepared materials. Fig. 1 shows the XRD patternsof pure TiO2 and the TiO2:Ag nanoparticles prepared at 120 1Cfor 1 day using an ionic liquid assisted hydrothermal method.The most intense diffraction peak at 2y = 25.31 confirms theanatase phase of TiO2 (Fig. 1(b)) and all other diffraction peaks

Fig. 1 XRD patterns of (a) TiO2 and (b) TiO2:Ag nanoparticles (* = Ag).

Fig. 2 XPS spectra of (a) Ti, (b) O, and (c) Ag. (d) Broad XPS spectrum of TiO2:Ag nanoparticles.

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also confirmed the tetragonal anatase phase of the TiO2 nano-particles with the lattice parameters a = 3.775 Å, and b = 9.490 Åand the space group = 141/amd ( JCPDS no. 2-387). The twopeaks at 44.21 and 64.21 indicate the presence of Ag particles(JCPDS no. 1-1167). The average crystallite size (Fig. 1(b)),calculated using the Debye–Scherer equation for most the intensediffraction peak 2y = 25.31, was found to be 40 nm. Fig. 1(a) showsthe XRD pattern of the pure tetragonal anatase phase TiO2

nanoparticles prepared at 120 1C for 1 day using an ionic liquidassisted hydrothermal method.

XPS studies help to determine the chemical composition ofthe prepared metal oxide nanomaterial. The XPS spectra ofTiO2:Ag nanoparticles, prepared at 120 1C for 1 day using anionic liquid assisted hydrothermal method, are shown in Fig. 2.Fig. 2(a) shows the XPS spectrum of Ti2p, it is a doublet withthe Ti2p3/2 peak at 458.5 eV and the Ti2p1/2 peak at 461.67 eV.Fig. 2(b) shows the XPS spectrum of O1s with three peakscentered at 530.1, 533 and 535.2 eV.19 Fig. 2(c) represents theXPS spectrum of the Ag3d core levels.

The Ag3d5/2 and Ag3d3/2 core level binding energies appeared at369 and 375 eV, respectively, which are in good agreement withthose reported for metallic silver.20 Fig. 2(d) shows the broad XPSspectrum of the TiO2:Ag nanoparticles and it shows the character-istic peaks of Ti, O and Ag. In addition to these, we have alsoobserved the presence of C1s (284.5 eV) and N1s (398.5 eV) suggest-ing the presence of amino groups. It indicates that a small amountof ionic liquid is still present even after calcination.21

FT-IR spectroscopy is mainly used to check the presence offunctional groups present in the synthesized nanomaterial. TheFTIR spectrum of P-25 shows a peak at around 400 cm�1 indicat-ing the characteristic vibrational mode of Ti–O. Pure TiO2 nano-particles (Fig. 3(b)) also show a significant peak at around400 cm�1 due to the stretching vibration mode of Ti–O and thespectral region from 1000 to 1700 cm�1 can be assigned to theC–H, C–C, CQC and N–H stretching/bending vibrations due tothe presence of ionic liquid22–24 and it is in good agreement withthe XPS study discussed earlier. The FT-IR spectrum of theTiO2:Ag nanoparticles (Fig. 3(c)) shows various vibrational modes

of Ag–O25,26 at 500–680 cm�1 in addition to the characteristicstretching vibration modes of Ti–O at around 415 cm�1.

UV-Vis spectroscopy was used to evaluate the optical proper-ties of the prepared materials. In general, nanomaterials show ablue shift compared to bulk materials, due to the quantumconfinement effect or reduction in size of the materials. TheUV-Vis spectra (Fig. 4) were taken after sonicating doped/undopedTiO2 nanomaterials in water for about 20 min. P-25 shows a broadabsorption peak at 400 nm (Fig. 4(a)). The pure TiO2 nanomaterial(Fig. 4(b)) shows an absorption peak at 335 nm which is blue-shifted compared to the bulk material which may be due to thesize effect. The TiO2:Ag nanoparticles show an intense narrowabsorption peak at 360 nm (Fig. 4(c)) which is red-shifted com-pared to the TiO2 nanomaterials. It clearly indicates that the Agnanoparticles were successfully associated with the TiO2 nano-particles. This red shift indicates that the TiO2:Ag nanoparticleshave much stronger absorption than the TiO2 nanoparticles,i.e., TiO2:Ag nanoparticles possess a higher photocatalytic activitythan the undoped TiO2 nanoparticles.27

TG analysis was employed to determine the thermal stabilityof the as-prepared materials. Fig. 5 shows the TGA plot of theTiO2:Ag nanoparticles, prepared at 120 1C for 1 day using an

Fig. 3 FT-IR spectra of (a) P-25, (b) TiO2 nanoparticles and (c) TiO2:Agnanoparticles.

Fig. 4 UV-visible spectra of (a) P-25, (b) TiO2 nanoparticles and (c)TiO2:Ag nanoparticles.

Fig. 5 Thermogravimetric analysis of TiO2:Ag nanoparticles.

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ionic liquid assisted hydrothermal method. Two weight lossstages were observed. The weight loss from room temperatureto 100 1C is due to the presence of adsorbed water molecules.The second weight loss from 100 to 400 1C can be attributed to

the thermal decomposition of organic substances (ionicliquid).28 Above 400 1C almost no weight loss occurred whichindicates that the TiO2:Ag nanoparticles are thermally stable upto 700 1C.

Fig. 6 SEM images and EDS spectrum of TiO2:Ag nanoparticles.

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The amount of silver leached/discharged from the TiO2:Agphotocatalyst was estimated using the direct air-acetyleneflame method. In the case of 0.1 g of TiO2:Ag in 100 mL water,the amount of doped silver discharged from the photocatalyst(TiO2:Ag nanomaterial) was found to be 0.26 mg L�1; for 0.2 g ofphotocatalyst in 100 mL water, the amount of silver dischargedfrom the photocatalyst was 0.33 mg L�1; for 0.3 g of the TiO2:Agnanomaterial in 100 mL water, the amount of discharged silverwas 0.37 mg L�1. Fig. 6(a) and (b) show the SEM images of theTiO2:Ag nanoparticles, prepared using an ionic liquid assistedhydrothermal method, which closely resemble a fern-likephyllotaxy. i.e. they show hyperbranched structures (100 mm)grown with pronounced trunks with a dimension of 30 mm. TheTiO2:Ag nanoparticles have corrugations and ordered branchesthat are symmetrically distributed on the opposite sides of thetrunks. The low magnification SEM image (Fig. 6(c)) shows thatthe TiO2:Ag nanoparticles are agglomerated to from a cloud-likemorphology. The high magnification SEM images (Fig. 6(d) and (e))clearly show the presence of nanoparticles. The EDS spectrum ismainly used to identify the elements present in the preparedmaterials and Fig. 6(f) clearly shows the presence of Ti, O and Ag.

Fig. 7 shows the TEM images of the TiO2:Ag nanoparticles,prepared at 120 1C for 1 day, in which the nanoparticles showshapes very similar to rice grains and the size of the nano-particles was found to be B60 nm. Due to the large atomicradius of the silver atom, they cannot interpenetrate into theTiO2 crystal lattice to form a solid solution alloy. Therefore,the Ag nanoparticles distribute in the TiO2 matrix. In general,the TiO2 particles appear almost transparent. The dark regions

indicated by arrows clearly show the presence of almost spheri-cally shaped Ag nanoparticles.29,30

3.2. Hydrogen generation

Fig. 8 shows the hydrogen generation of P-25, and the TiO2 andTiO2:Ag nanoparticles as well as a water–ethanol system in theabsence of catalyst. Liu et al. reported that Ag deposited onto aTiO2 nanosheet film showed an 8.5 times higher photocatalyticwater splitting activity for hydrogen generation than the bareTiO2 nanomaterial.29 From the graph it can be seen that theTiO2:Ag nanoparticles liberate 2230 mmol H2 generated per 1 gof photocatalyst in 2.5 h compared to bare TiO2 nanoparticles

Fig. 7 TEM images of TiO2:Ag nanoparticles.

Fig. 8 Hydrogen generation of (a) P-25, and the (b) TiO2 and (c) TiO2:Agnanoparticles.

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(1200 mmol H2 generated per 1 g of photocatalyst in 2.5 h) andP-25 (707 mmol H2 generated per 1 g of photocatalyst in 2.5 h),respectively. The control experiment clearly suggests that theliberation of H2 is due to the presence of the catalyst.

3.3. Photocatalytic degradation of trypan blue

3.3.1. Effect of dye concentration. The effect of dye concen-tration on the photocatalytic activity of the TiO2:Ag nanoparticles,prepared at 120 1C for 1 day using an ionic liquid assisted hydro-thermal method, is shown in Fig. 9. The concentration of trypanblue was varied from 5 to 25 mg L�1 and the catalytic load and thepH were maintained at 400 mg and pH 8. As the concentrationof dye increased, the time taken for complete degradation alsoincreased. At higher concentrations more dye molecules areadsorbed onto the surface of the catalyst resulting in a decreasein active sites on the catalyst (fewer number of hydroxyl andsuperoxide radicals) and, thereby, in reduced light penetration.

3.3.2. Effect of catalyst dosage. The effect of catalyst loadon the photocatalytic degradation of the dye is shown in Fig. 10.In order to determine the optimal dosage of the catalyst, the catalyticload was varied from 100–500 mg per 100 mL. The optimal loadwas found to be 400 mg per 100 mL. Above the optimal load, the

turbidity of the solution increased, light penetration decreasedand thus the availability of hydroxides and super oxides becameminimal.

3.3.3. Effect of pH on photocatalytic degradation. Fig. 11shows the effect of pH on the photocatalytic degradation oftrypan blue. From the figure it is clear that the photocatalyticprocess strongly depends on the pH of the dye solution.31 ThepH of the solution is adjusted by adding either 0.05 M NaOH or0.05 M H2SO4. The degradation efficiency is higher in basicmedia than in acidic ones. This observation matches well withreported studies.32 The variation of pH alters the surfaceproperties of the TiO2:Ag nanoparticles, which in turn couldlead to the dissociation of dye molecules. At pH 8, perhydroxylradicals are formed, leading to the formation of hydrogenperoxide which in turn, produces more number of hydroxylradicals. The highest photocatalytic degradation is observed atpH 8 which is in good agreement with the literature.33

3.3.4. Effect of different light sources on photocatalyticactivity. Fig. 12 shows the effect of different light sources onthe photocatalytic degradation. Two different light sources areused namely sunlight and UV light. From the figure, it is clearthat the photo catalytic degradation of the dye was higher underUV light than that under sunlight. This is due to the fact that UVlight has a higher intensity (lower wavelength or higher energy),hence the light can easily penetrate to the surface resulting in

Fig. 9 Effect of dye concentration on the photocatalytic activity.

Fig. 10 Effect of catalytic load on the photocatalytic activity.

Fig. 11 Effect of pH on photocatalytic degradation of trypan blue.

Fig. 12 Effect of different light sources on photocatalytic degradation.

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the formation of a higher number of radicals, which increasesthe rate of photocatalytic degradation of the azo-dye.

3.3.5. Effect of recycling of TiO2:Ag nanoparticles onphotocatalytic degradation. Fig. 13 shows the effect of recyclingof TiO2:Ag nanoparticles, prepared at 120 1C for 1 day using anionic liquid assisted hydrothermal method, on the photocata-lytic degradation of trypan blue. One of today’s main industrialwastewater treatment strategies is focused on the developmentof green technologies. Recycling of the TiO2:Ag nanoparticles canbe foreseen as good practice for sustainable industrial wastetreatment. Consequently, it is necessary to demonstrate whetherafter a photocatalytic treatment the catalyst can be reused. Thenanoparticles were used and recycled consecutively three times.The photocatalytic reaction was carried out at a constant dyeconcentration (10 mg L�1) and constant catalytic load (400 mgper 100 mL of trypan blue). Once the first set of experimentsis completed, the catalyst is retrieved from the solution viacentrifugation and dried in an oven. The recycled catalyst wasused for the second cycle and so on. This experimental studyindicated that the rate of photo degradation using TiO2:Agdecreased as the number of cycles increased. This could bedue to aggregation and sedimentation of the dye around TiO2:Agafter each cycle of photocatalytic degradation. Each time thecatalyst is reused, new parts of the catalyst surface becomeunavailable for dye adsorption and, thus, photon absorption,reducing the efficiency of the catalytic reaction.34

4. Conclusions

We have successfully synthesized TiO2:Ag nanoparticles using anionic liquid assisted hydrothermal method with an imidazolium-based functionalized ionic liquid at 120 1C for one day. XRDand XPS indicated the presence of Ag nanoparticles on anataseTiO2 nanoparticles. EDS also clearly shows the presence of Ag,in addition to Ti and O. The FTIR spectrum indicates thepresence of Ag–O in addition to Ti–O vibration modes. TheUV-Vis spectrum of the TiO2:Ag nanoparticles shows a band gapof 3.41 eV which is red-shifted compared to that of the TiO2

nanoparticles. The SEM images resembled a leaf-like structure.The TEM images clearly showed the presence of Ag in thecomposite, evidenced by dark regions, and the size of the nano-particles was found to be B60 nm. TGA showed the presence ofionic liquid which was observed by the weight loss up to 400 1C.The TiO2:Ag nanoparticles showed enhanced H2 generation(2230 mmol H2 generated per 1 g of photocatalyst per 2.5 h)compared to bare TiO2 (1200 mmol H2 generated per 1 g ofphotocatalyst per 2.5 h) and P-25 (707 mmol H2 generated per 1 gof photocatalyst per 2.5 h), indicating that these nanoparticlesare a promising candidate for photocatalytic H2 generation viawater splitting reaction. It also shows a well-enhanced photo-catalytic activity for the degradation of the toxic azo-dye trypanblue compared to bare TiO2 nanoparticles.

Acknowledgements

One of the authors, T. N. Ravishankar, wishes to acknowledgethe Jain University for support and funding to carry out theresearch work.

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Fig. 13 Effect of recycling of TiO2:Ag nanoparticles on photocatalyticdegradation.

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