A facile UV-light mediated synthesis of l -histidine stabilized silver nanocluster for efficient photodegradation of methylene blue

Journal of Molecular Catalysis A: Chemical 404 (2015) 27–35

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A facile UV-light mediated synthesis of l-histidine stabilized silver nanocluster for efficient photodegradation of methylene blue Nabin Kumar Pal, Carola Kryschi ∗ Department of Chemistry and Pharmacy and ICMM, Friedrich-Alexander University of Erlangen-Nuremberg, D-91058 Erlangen, Germany

a r t i c l e

i n f o

Article history: Received 3 December 2014 Received in revised form 10 January 2015 Accepted 11 April 2015 Available online 14 April 2015 Keywords: l-histidine stabilized silver nanocluster Titania-modified alumina particles Photocatalysis Photo-oxidation Methylene blue

a b s t r a c t In this paper, we have reported of a facile one-pot UV-light mediated synthesis of highly luminescent, nearly monodisperse silver nanocluster using l-histidine as the stabilizing agent. The as-synthesized silver nanocluster were characterized using diverse spectroscopy and microscopy techniques such as UV/Vis absorption and photoluminescence spectroscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. A new photocatalyst was engineered by depositing the l-histidine stabilized silver nanocluster on titania-modified alumina particles that were synthesized via a surface sol–gel method. The photocatalytic activity was tested using methylene blue as substrate for the titania-modified alumina supported silver nanocluster. In order to elucidate the effect of the photocatalyst on the photodegradation reaction of methylene blue the concentrations of the catalyst and dye as well as the UV-light irradiation time were systematically varied. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Noble metal nanocluster with a core diameter in the range of 1–2 nm have attracted tremendous attention over the past few years, mainly because of their high photo-stability, low toxicity, bio-compatibility and huge application potential for optoelectronics [1], optical sensing [2–5], bio-imaging [6], and catalysis [7–10]. These noble metal nanocluster usually consist of several to hundreds of atoms in the core and exhibit molecular-like size-tunable photo-physical properties [11–13]. Unlike gold nanocluster that have been extensively investigated over the past few decades, the synthesis of chemically stable, highly luminescent, monodisperse silver nanocluster (AgNC) is still a challenge. However, research efforts on the synthesis of luminescent AgNC have appreciably increased in recent years, and novel chemical procedures using a broad variety of reducing agents and surfactants were developed. The template-assisted synthesis of AgNC is one of the most recognized techniques until now, where, for instance, polymers [7,15,16], polyelectrolytes [17], DNA [18], dendrimers [19,20], polymer microgels [21], and proteins [22] were used as template. In contrast, the common methods are based on the formation of a

∗ Corresponding author. Tel.: +49 91318527307. E-mail address: [email protected] (C. Kryschi). http://dx.doi.org/10.1016/j.molcata.2015.04.004 1381-1169/© 2015 Elsevier B.V. All rights reserved.

monolayer around the Ag core which occurs via physisorption or chemisorption of smaller molecules such as thiols [23]. Recently, several amino acids, for instance, l-histidine [24], lysine [25], and L-2,3-dihydroxyphenylalanine [26] were successfully employed as surfactant for the synthesis of luminescent gold nanocluster. The use of amino acids as stabilizing agents have several benefits: first, in most cases the reactions do not require any external addition of toxic reducing agents such as sodium borohydride. Second, the amino-acid stabilized AgNC can readily be dissolved in aqueous solvents which facilitate their potential applications in biology and medicine. Thus, a facile one-pot synthesis of water-soluble, luminescent AgNC using low-cost starting materials is highly desirable and required. Supported noble-metal nanoparticles are highly qualified for catalytic applications owing to their ultra-small sizes and large surface-to-volume ratio when compared to their bulk equivalent [27,28]. Among various kinds of support, TiO2 is considered to be the material with excellent potential for photocatalytic purposes (e.g., remediation of polluted air/water by organic compounds) because of its remarkably high thermal and chemical stability, unique optical and electronic properties, non-toxicity, and low cost [29,30]. The photocatalytic efficiency of TiO2 mainly depends on parameters such as, the light absorption capacity, the migration rate of the photogenerated charge carriers to the surface of the crystallite, and the rate of surface reduction and oxygen processes.

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TiO2 has a large band-gap energy (3.2 eV) and is thus optically active under UV light exposure but cannot be optically excited with visible light. This limits its optoelectronic applications to the small UV spectrum of the sunlight. Moreover, as in most of semiconductor materials, a high electron–hole pair recombination rate results into a low quantum efficiency. Recent studies could show that deposition of noble-metal nanoparticles (Au, Ag, Pt, and Pd) on TiO2 surfaces extends its photocatalyst activity into the visible light region due to surface plasmon resonance absorption of the noble metal nanoparticles [31–36]. The combination of TiO2 with noble metal nanoparticles was reported to exhibit an enhanced photocatalytic activity by trapping the photoinduced charge carriers that promotes the interfacial charge-transfer processes [37,38]. However, the deposition of larger noble metal nanoparticles on TiO2 surfaces often causes accumulation of photogenerated electrons on the noble metal nanoparticle surfaces. Therefore large noble metal nanoparticles may act as recombination centers for photogenerated charge carriers which reduces the photocatalytic efficiency. A way to overcome this drawback is to use ultra-small noble metal nanocluster instead of nanoparticles. For example, Turner et al. could demonstrate the excellent catalytic behaviour of a noble metal nanoparticle-TiO2 composite system with noble metal nanoparticle sizes smaller than 2 nm [39]. However, use of pure TiO2 as a support for noble metal nanocluster often results in a deactivation of the photocatalyst due to agglomeration of smaller nanocluster into larger but less active nanoparticles [40,41]. It has been suggested that the use of binary mixed oxides as supports may impede merging of deposited noble metal nanocluster [42,43]. With these in mind, we prepared alumina–titania mixed oxides (Al2 O3 :TiO2 ) using a surface sol–gel method. The Al2 O3 :TiO2 substrate was shown to exhibit improved phase stability and enhanced photocatalytic activity when loaded with silver nanocluster (AgNC). Compared to other, more expensive noble-metal (Au, Pt, and Pd) nanocluster, AgNC deposited on Al2 O3 :TiO2 were emerged as a similarly promising photocatalyst due to its low-toxicity, easy availability and low cost. In this contribution, we report of a novel facile one-pot photochemistry procedure for the synthesis of l-histidine stabilized AgNC. The as-synthesized AgNC were characterized using versatile microscopy and spectroscopy methods. Subsequently, the AgNC were deposited on titania-modified alumina supports and tested for the removal of a model pollutant namely methylene blue (MB) dye in aqueous solution. The as prepared Ag:Al2 O3 :TiO2 photocatalyst was found to exhibit a remarkably high photoactivity in comparison with catalysts composed of Ag:TiO2 , Ag:Al2 O3 , and Al2 O3 :TiO2 .

2.2. Synthesis of AgNC An aqueous solution of AgNO3 (0.5 ml of 10 mM) was mixed with an aqueous solution of l-histidine monochloride monohydrate (1.5 ml of 0.1 M) under vigorous stirring at room temperature. Immediately, white colored Ag-l-histidine complexes were formed. After 10 min, 50 ␮l of a 1 M NaOH solution were added to the reaction mixture that was subsequently exposed to UV light at 254 nm (100 W, Muller Electronic-Optic) for 60 min. The reaction mixture was then centrifuged at 12,000 rpm for 30 min for three times in order to remove the insoluble impurities, excess l-histidine, and large Ag nanoparticles. The supernatant obtained from this procedure was twice dialyzed using a cellulose ester membrane tube with a cut-off molecular-weight (MWCO) of 1 KDa in 100 ml of water. Finally a light yellow colored powder of the AgNC was obtained after evaporation of the dialysate using a rotavapor. 2.3. Synthesis of the titania-modified alumina support (Al2 O3 :TiO2 ) The titania-modified alumina support was prepared using a surface sol–gel method. Therefore pre-heated (at 120 ◦ C for 24 h) alumina powder (1.5 mg) was first loaded into a dried round bottom flask under an inert atmosphere, and then 2.7 ml of titanium(IV) n-butoxide, 15 ml of anhydrous toluene and 15 ml of anhydrous methanol were added very carefully one-by-one. The whole mixture was then refluxed at 90 ◦ C for about 3 h. Care must be taken to prevent the entry of moisture inside the flask. The obtained solid precipitate was then filtered, repeatedly washed with absolute ethanol and finally with deionised water. The as-synthesized product was then dried at 80 ◦ C inside an oven for 12 h. 2.4. Synthesis of the Ag:Al2 O3 :TiO2 photocatalyst

2. Experimental

The deposition of the AgNC onto the surface of titania-modified alumina support was achieved by deposition-precipitation method. 1 g of the Al2 O3 :TiO2 particles were added to an aqueous solution of the AgNC (10 mg in 50 ml) under vigorous stirring. The mixture was heated to 80 ◦ C and left there for 2 h. Then the mixture was cooled down to room temperature and kept there for 1 h, in order to facilitate the complete precipitation of the solid phase. The solid phase mainly consisting of AgNC-covered Al2 O3 :TiO2 particles was separated from the liquid phase by centrifugation and was washed twice with de-ionised water and finally with absolute ethanol. The Ag:Al2 O3 :TiO2 photocatalyst was obtained by drying the AgNC-covered Al2 O3 :TiO2 particles under vacuum at 40 ◦ C for 12 h and subsequent calcination at 250 ◦ C. The Ag:TiO2 (P25) and Ag:Al2 O3 photocatalysts were synthesized using the same deposition-precipitation method.

2.1. Materials

2.5. Photocatalysis experiments

Silver nitrate (99.9999% AgNO3 ) was purchased from Sigma–Aldrich, sodium borohydride (97% NaBH4 ) from Fluka, l-histidine monohydrochloride monohydrate (≥99%) from Merck, sodium hydroxide (≥97%) from Aldrich, aluminium oxide nanopowder from Aldrich, titanium (IV) n-butoxide (98%) from Alfa Aesar, titanium(IV) oxide nanopowder (anatase, ≥99.7%, <25 nm particle size) from Aldrich, AEROXIDE® TiO2 P25 from Evonic Industries, absolute ethanol from Merck, toluene (≥99.9) from Merck, methanol (≥99.9) from Merck, and Membra-Cel® dialysis tubing (regenerated cellulose, MWCO 1 KDa) from SERVA. All materials and solvents were used as received from the suppliers without further purification. Milli-Q deionised water (18 M´) was used throughout the whole work.

The photocatalytic efficiency of the as-prepared Ag:Al2 O3 :TiO2 was examined employing the heterocyclic aromatic dye methylene blue (MB) as the model pollutant. At first, 25 ml of an aqueous suspension of the dye (10−5 M) and the photocatalyst (20 mg) were poured into a 100 ml Pyrex glass vessel and were stirred for 30 min in the dark so that the adsorption–desorption equilibrium was reached. The suspension was then irradiated with a visible light source (incandescent light bulb, 100 W/m2 , Philips). The distance between the light source and the vessel was maintained at 10 cm throughout the reaction. A 3 ml of the aliquot was taken at various time intervals, centrifuged to separate the solid catalyst from the dye solution and subsequently analysed using UV/Vis absorption spectroscopy. The same experiment was also performed

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using ␥-Al2 O3 , TiO2 (P25), TiO2 (A), Al2 O3 :TiO2 , Ag:TiO2 (P25) and Ag:TiO2 (A) as the photocatalyst.

TEM images of the clusters and the photocatalysts were taken using a Zeiss EM 900 instrument operating at 120 kV accelerating voltage. HRTEM images were recorded using a Phillips CM 300 UltraTwin microscope. The measurements were carried out at an accelerating voltage of 300 kV in the bright-field mode. The samples for the TEM and HRTEM were prepared by dispersing the AgNC and Ag:Al2 O3 :TiO2 photocatalyst in ethanol and evaporating in air multiple drops of the suspension onto an ultrathin carbon-coated copper grid. 2.7. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) X-ray diffraction data were obtained employing a Philips PW1800 diffractometer with high-intensity Cu K␣ radiation ( = 1.5418 Å). The samples were scanned in the 2 range of 20◦ –80◦ . The chemical compositions of the AgNC and Ag:Al2 O3 :TiO2 photocatalyst were investigated using X-ray photoelectron spectroscopy (XPS, PHI 5600 XPS spectrometer). Monochromatic Al K␣ (h␯ = 1486.71 eV) X-rays was used as the excitation source. 2.8. Steady-state optical spectroscopy The photoluminescence (PL) spectra were recorded on a Jobin–Yvon FluoroMax-3 spectrofluorometer using the magicangle polarization configuration and a slit width of 5 nm for both, the excitation and emission spectrum. UV/Vis absorption spectra were taken with a PerkinElmer UV/Vis spectrometer Lambda 2. All experiments were performed at room temperature using quartz cuvettes with a thickness of 10 mm. The PL quantum yields of the lhistidine-terminated AgNC were determined using a fluorescence standard (s ). The PL quantum yield of the sample with t was obtained utilizing the following equation: ˚t = ˚s

  It /At 2t   2, Is /As s

where  is the quantum yield, I the integral area under the PL spectrum,  the refractive index of the solvent, and A the absorption at the selected excitation wavelength. The subscripts “t” and “s” represent the test sample and the standard sample, respectively. 2.9. Time-correlated single-photon counting photoluminescence spectroscopy The PL decay profiles were recorded using the time-correlated single-photon counting (TCSPC) PL technique. The time resolved measurement of the PL intensity was carried out on a TCSPC spectrometer Fluorolog-3 (Jobin–Yvon) equipped with a microchannel plate (Hamamatsu, R3809U-50) which provides a time resolution of about 60 ps. 3. Results and discussion 3.1. l-histidine-terminated AgNC UV-light mediated syntheses of noble metal nanocluster have been well demonstrated by various research groups in the past. Recently Yang et al. [44] synthesized atomic gold nanocluster by blending an aqueous solution of HAuCl4 with l-histidine followed

1.0 0.8

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2.6. TEM and HRTEM analysis

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L-histidine-terminated AgNCs pure L-histidine reaction mixture without UV

0.6 0.4 0.2 0.0 250

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wavelength [nm] Fig. 1. UV/Vis absorption spectra of l-histidine-terminated Ag NC in aqueous solution (solid line), pure l-histidine in water (dotted line), and the reaction mixture without UV-light irradiation (dashed line).

by incubation for 2 h at room temperature. However, this procedure did not work for the synthesis of AgNC. For the preparation of l-histidine-terminated AgNC we have modified the procedure proposed by Yang et al. by performing the reaction in a basic medium under UV-light irradiation. Apart from NaOH and UV light, no other reducing agent, no catalyst or any template were used in course of the whole synthesis process. Moreover, the absence of any reducing agent and template enables to control the reduction reaction. Thus, the separation and the purification of the as-synthesized AgNC become much simpler. In our synthesis simple mixing of an appropriate concentration ratio of AgNO3 and l-histidine in a basic medium produces a white-colored complex which upon UV-light irradiation changes to a yellowish colored solution that exhibits an intense bluish–green PL. Before UV-light irradiation the reaction mixture shows no absorption in the wavelength range of 250 nm–600 nm (Fig. 1). However, the AgNC exhibit two narrow, molecular-like absorption bands at 260 nm and 330 nm, respectively. The absence of the surface plasmon resonance (SPR) peak at 397 nm, as being characteristic for larger-sized Ag nanoparticles, suggests that the here prepared AgNC have a core diameter of less than 2 nm. Recently, employing an electrochemical procedure Gonzalez et al. [45] synthesized AgNC the UV/Vis absorption spectrum of which is composed of broad featureless bands at 260 nm, 310 nm and 375 nm. Other authors [14,23] also reported of AgNC with different sizes which exhibit absorption bands between 230 and 700 nm. The spectral peak positions were observed to depend on the type of capping ligand and the respective nanocluster size. These results justify our assignment of the observed narrow UV/Vis absorption bands as emerging from AgNC. Moreover, almost no change in the absorption intensities of the AgNC solution was observed over a month which indicates its great stability towards air and moisture. The contour plot of the PL excitation vs. emission spectra exhibits a broad maximum between 360–380 nm (excitation) and 455–470 nm (emission) (Fig. 2a). Exemplary PL excitation (red dashed line) and emission spectra (blue solid line) of l-histidineterminated AgNC are shown in Fig. 2b. The PL quantum yield of the l-histidine-terminated AgNC was found to be 3.9% with quinine sulfate used as reference dye. The evolution of PL emission spectra of the AgNC dispersion measured for different UV-light irradiation times is depicted in Fig. 3. The non-irradiated reaction solution shows no visible emission. However, after 10 min of UV irradiation, a PL emission band emerges at 465 nm when the solution had been excited at 373 nm. With rising irradiation time, the PL emission intensity increases and

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1.0

350 nm 355 nm 360 nm 365 nm 370 nm 375 nm 380 nm 385 nm

PL intensity

0.8 0.6 0.4 0.2 0.0 400

450

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wavelength [nm] Fig. 4. PL emission spectra of the l-histidine-terminated AgNC recorded at different excitation wavelengths.

Fig. 2. Contour plot of the PL excitation vs. emission spectra of l-histidineterminated AgNC (a); PL excitation (dashed line) and emission (solid line) spectra of l-histidine-terminated AgNC (b).

finally reaches a maximum after 60 min of UV irradiation. Irradiation for more than 60 min did not change the PL emission intensity. This suggests that the photo-reduction reaction of the Ag+ solution had come to an end after 60 min irradiation with UV light. Fig. 4 shows the PL emission spectra of the l-histidineterminated AgNC which were excited at different wavelengths in the range of 350–385 nm. Independently of the excitation wavelength the PL emission spectra exhibit their maximum at 466 ± 1 nm. This observation demonstrates that the AgNC is composed of one emitting species only. It has to be mentioned that

neither the pure l-histidine solution nor solutions containing lhistidine with adsorbed or embedded Ag ions were observed to emit detectable PL emission under the same photo-excitation conditions. PL lifetime measurements of the l-histidine-terminated AgNC were performed by detecting their emission at 470 nm. The decay curves are obviously non-exponential and were therefore analyzed performing bi-exponential fits (Fig. 5). The longer lifetime with 4.6 ns (69%) is ascribed to the PL emission of silver clusters, whereas the shorter lifetime with 0.7 ns (29%) may be attributed to emission of charge-transfer states. X-ray photoelectron spectroscopy was used to examine the valence state of the Ag atoms in the cluster. The XPS spectrum in Fig. 6 displays binding energy values of 368.73 eV and 374.57 eV that were obtained for Ag 3d5/2 and Ag 3d3/2 , respectively. These binding energy values are very close to the binding energy values of Ag(0), implying that the photo-reduction reaction of the Ag+ ions was completely accomplished. The TEM image in Fig. 7a illustrates that the as-prepared l-histidine-terminated AgNC are nearly monodisperse with an average size of 1.6 nm. As been shown by the HRTEM image (Fig. 7b) the AgNC exhibit a poorly resolved spherical shape (exemplary AgNC are marked with a white circle). The sizes of the AgNC were estimated to be in the range of 1.7 nm to 1.8 nm.

10 min irradiation 20 min irradiation 40 min irradiation 50 min irradiation 60 min irradiation 70 min irradiation without irradiation

PL intensity

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experimental data bi-exp fit: A= 0.31, 1= 0.7 ns

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wavelength [nm] Fig. 3. PL emission spectra of l-histidine-terminated AgNC in dependence on the UV-light irradiation time.

Fig. 5. PL emission decay curves of l-histidine-terminated AgNC in aqueous solution detected at 470 nm; the excitation wavelength was 403 nm. The black dots are the experimental data, whereas the solid line represents the best fit.

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Fig. 8. The XRD pattern of Ag:Al2 O3 :TiO2 and pure ␥-Al2 O3 .

Fig. 6. XPS spectrum of the Ag 3d states in the l-histidine-terminated AgNC.

3.2. Ag:Al2 O3 :TiO2 photocatalyst The composition of the photocatalyst (Ag:Al2 O3 :TiO2 ) was examined upon conducting XRD measurements. Fig. 8 shows the XRD pattern of pure ␥-alumina and the Ag:Al2 O3 :TiO2 photocatalyst. The reflection peaks located at the 2 values 25.37, 37.88, 48.06, 53.48, 62.14 and 70.85 were assigned to the (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 0 4) and (2 2 0) diffractions of titania, respectively. The strong diffraction peaks at 25.37 and 48.06 identify titania in the anatase phase (JCPDS: 21-1272). The appearance of the peaks at the 2 values 19.01, 32.5, 36.67, 45.25, 61.2 and 67.27 can be attributed to the (1 1 1), (2 2 0), (3 1 1), (4 0 0), (5 1 1) and (4 4 0) diffractions of ␥-alumina, respectively. The broad and intense diffraction peaks reflect that the photocatalyst are highly crystalline in nature. The crystalline properties were further confirmed by determining the crystallite size of alumina (for 440 planes) and titania (for 101 planes) using the Debye–Scherrer formula. The crystallite size was estimated to be 21.7 nm and 16.3 nm for alumina and titania, respectively. The absence of XRD peaks for Ag is explained with the relatively low concentration and too small sizes of the AgNC on the photocatalyst. The XRD detection limit is up to 5 nm. The TEM and HRTEM images in Fig. 9a and b of the photocatalyst illustrate that the AgNC are homogeneously dispersed on the surface of the support. The presence of lattice fringes of the crystallographic planes demonstrates the crystallinity of the photocatalyst (Fig. 9b). A mean diameter of 17.2 nm was calculated for

the Al2 O3 :TiO2 nanoparticles from the analysis of the HRTEM image which is consistent with the XRD results. The average size of AgNC deposited on the surface of Al2 O3 –TiO2 support was estimated to be 1.8 nm which is slightly larger when compared to the size of free-standing l-histidine-terminated AgNC. This small reduction of the size may occur because of slow agglomeration of AgNC on the support surface due to Ostwald ripening [46]. XPS measurements were performed to examine the oxidation states of Ti, Al, and Ag in the photocatalyst. Fig. 10a shows a representative survey scan for the calcined photocatalyst. Ti 2p1/2 and Ti 2p3/2 peaks appear at 465.07 eV and 459.28 eV, respectively (Fig. 10c). The 5.8 eV difference in the binding energy values between Ti 2p1/2 and Ti 2p3/2 confirms the presence of Ti in its tetravalent state. The peaks corresponding to O1s and Al 2p appear at 531.42 eV and 73.67 eV, respectively (Fig. 10b and d). In comparison with the binding energy of Al 2p (74.2 eV) in pure ␥-alumina, a 0.7 eV reduction in the binding energy of Al 2p in the photocatalyst was observed. This reduction may be due to the presence of Ti which has a stronger electron affinity than Al. The binding energy values at 374.53 eV for Ag 3d3/2 and 368.57 eV for Ag 3d5/2 (Fig. 10e) prove the presence of elemental Ag(0) on the support surface. 3.3. Photocatalytic degradation of methylene blue (MB) The activity of the Ag:Al2 O3 :TiO2 photocatalyst was evaluated by monitoring the photo-induced decolonization kinetics of methylene blue (MB) in aqueous solution under visible-light irradiation.

Fig. 7. TEM (a) and HRTEM (b) images of the l-histidine-terminated AgNC.

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Fig. 9. TEM (a) and HRTEM (b) images of the photocatalyst Ag:Al2 O3 :TiO2 .

MB displays strong optical absorption in the range of 500–700 nm. Nevertheless, photodegradation of MB by irradiation with sun light does not work in the absence of any catalyst. Here, the reaction kinetics of the photodegradation of MB in presence of the Ag:Al2 O3 :TiO2 photocatalyst was examined by detecting the decay of the absorption of MB at 665 nm. The spectroscopic data are depicted in Figs. 11–13 shows the time evolution of the UV/Vis absorption spectra of MB dissolved in water at a concentration of 5 ppm and irradiated with a tungsten–halogen lamp in the presence of Ag:Al2 O3 :TiO2 photocatalyst (0.8 g/l). The irradiation time was varied between 0 and 30 min. The maximum absorbance at 665 nm clearly decreases with rising irradiation time. After 30 min of irradiation the absorbance becomes nearly zero which indicates the complete degradation of MB. The decrease of the MB concentration, when irradiated in the presence of a photocatalyst, is shown as a function of the irradiation time in Fig. 12. According to these results, the photocatalytic activity of the Ag:Al2 O3 :TiO2 photocatalyst is much higher than that of the other photocatalysts under visible-light irradiation. Pure TiO2 and ␥-Al2 O3 exhibit negligible photocatalytic activity because of their large band-gap energy (>3 eV). A histogram that represents the degradation of MB in percentage by various supported photocatalysts under identical experimental conditions is shown in Fig. 13. In order to optimize the photocatalyst loading concentration, a series of experiments were performed using different Ag:Al2 O3 :TiO2 photocatalyst concentrations ranging from 0.2 g/l to 1.0 g/l. The initial MB concentration was kept at a constant value. The degradation rates of MB (c/co ) as a function of the irradiation time for various photocatalyst concentrations are shown in Fig. 14a. These results show that the degradation rate of MB initially increases linearly with the photocatalyst concentration to reach its maximum value at 0.8 g/l. The further increase of the photocatalyst concentration to 1.0 g/l slightly decreases the degradation rate. The increase of the degradation rate with rising photocatalyst concentration presumably arises from the continuously growing MB adsorption on the photocatalyst surface until the maximum photocatalyst concentration with 0.8 g/l will be reached. At larger photocatalyst concentrations the visible light will be increasingly absorbed by photocatalyst particles without any adsorbed MB which was observed as a small decrease of the degradation rate. The impact of the initial MB concentration on the degradation rate was examined by carrying out photodegradation experiments with varied initial MB concentrations. 99% MB was degraded after

30 min of irradiation time, when the initial MB concentration was 10−5 M and Ag:Al2 O3 :TiO2 was utilized as photocatalyst (Fig. 14b). A reduction of the degradation rate to about 30% was observed when the initial MB concentration was increased from 10−5 to 10−4 M. However, the higher dye concentration might cause that a fraction of the incoming photons was blocked by excess free dye molecules. This should lead to a reduced number of photons being available for interacting with the photocatalyst and surfaceadsorbed MB. As a consequence, the formation of reactive hydroxyl and superoxide radicals is expected to be significantly lowered so that the reaction became inhibited. The factors leading to enhanced activity of the Ag:Al2 O3 :TiO2 photocatalyst should be attributed to some changes of the physical and chemical properties of the TiO2 support when modified with ␥-Al2 O3 and covered with Ag atoms. In principle, both, crystallite size and chemical composition of the Al2 O3 :TiO2 support can exert a great impact on the recombination dynamics of the electron–hole pairs. Photoinduced electron transfer to adsorbed MB dye molecules crucially depends on the migration velocity of electrons and holes to the photocatalyst surface. Modification of TiO2 with ␥-Al2 O3 introduces additional energy states between the valence and conduction band of TiO2 which results to a significantly larger light absorption capacity of the Al2 O3 :TiO2 support. It is suggested that the reaction rate-determining step in the photocatalytic process taking place at the Ag:Al2 O3 :TiO2 photocatalyst surface is the electron transfer emerging from the photocatalyst surface to adsorbed oxygen molecules. The increased visible-light absorption capacity of Al2 O3 :TiO2 promotes the charge carrier diffusion to the catalyst surface. In addition, bulk electron–hole recombination in Al2 O3 :TiO2 is minimized due to its small crystallite size (17.2 nm, Fig. 9a). Owing to its small crystallite size, Al2 O3 :TiO2 has a large surface area and thus, a relatively high number of deposited AgNC that cause a small negative shift of the quasi–Fermi energy level. The lowered Fermi-energy level certainly enlarges the charge separation in the photocatalyst and thereupon, enhances the efficiency of the interfacial charge-transfer processes. Efficient interfacial charge-transfer processes provide for the highly reductive nature and therewith, for the enhanced photocatalyst performance of Ag:Al2 O3 :TiO2 . The main reason for the reductive nature is the electron deficiency of the AgNC which becomes larger the smaller the AgNC are. This is explained by the work function of silver which is larger than that of TiO2 . Hence, AgNC deposited on the surface of the Al2 O3 :TiO2 support act as a sink for the photo-generated

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Fig. 10. The complete XPS spectrum of the Ag:Al2 O3 :TiO2 photocatalyst (a); high resolution XPS spectra of the Al 2p (b), Ti 2p (c), O 1s (d) and Ag 3d (e) regions, respectively.

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0.6

Absorbance

electrons and therefore, effectively impedes the recombination of electron–hole pairs. The trapped electrons are subsequently scavenged by adsorbed molecular oxygen to form highly reactive and oxidizing peroxy or superoxy species which results in the rise of photocatalytic activity. Furthermore, upon reaction with water, the photogenerated holes in the valence band produce highly oxidizing hydroxyl radicals (OH• ) that are assumed to rule the photooxidation process of methylene blue. When irradiated with visible light, the synergetic interplay between TiO2 and Al2 O3 , fortified by dense coverage of ultra-small AgNC, ultimately gives rise to a rather high concentration of electron–hole pairs in Ag:Al2 O3 :TiO2 which is much larger than that of electron–hole pairs generated by Ag:TiO2 , Ag:Al2 O3 and Al2 O3 :TiO2 photocatalysts. Therefore, the Ag:Al2 O3 :TiO2 photocatalyst exhibits the highest photocatalyst activity. Fig. 15 illustrates a scenario for the degradation reaction of MB occurring at the surface of the Ag:Al2 O3 :TiO2 photocatalyst under visible-light exposure.

0.4

0.2

0.0 300

400

500

600

700

800

Wavelength [nm] Fig. 11. UV/Vis absorption spectra of methylene blue as a function of the visiblelight irradiation time during the photodegradation activated by the Ag:Al2 O3 :TiO2 photocatalyst.

34

N.K. Pal, C. Kryschi / Journal of Molecular Catalysis A: Chemical 404 (2015) 27–35

1.0

Ag:Al2O3:TiO2 Ag:TiO2(P25)

0.8

Al2O3-TiO2 TiO2(P25)

C/Co

0.6

Ag:TiO2(A) TiO2(A)

0.4

γ-Al2O3

no catalyst 0.2 0.0 0

5

10

15

20

25

30

35

40

45

50

irradiation time [min] Fig. 12. Photodegradation of methylene blue using various supported photocatalysts under visible-light irradiation.

degradation of MB [%]

120

4. Conclusions

Ag:Al2O3:TiO2

100 80

Fig. 15. Schematic illustration of visible-light photoexcitation, photoelectron transfer, and photocatalytic degradation process of methylene blue (MB) dye.

Ag:TiO2 (P25)

Al2O3:TiO2 P25

Ag:TiO2 (A)

60

TiO2(A)

40 γ-Al2O3

20

no catalyst

0

photocatalysts Fig. 13. Histogram representing the maximum photo-degradation of methylene blue in percentage obtained for various supported photocatalysts under identical experimental conditions.

1.0

a

In summary, we developed a sophisticated method for an UV-light induced synthesis of highly luminescent l-histidineterminated AgNC. The AgNC emit intense blue–green PL with a lifetime on the ns time scale. The TEM and HRTEM studies demonstrate that the AgNC are nearly monodisperse and have an average size of 1.6 nm. The XPS results prove the complete photo-reduction of silver ions and thereupon, the presence of Ag as elemental Ag(0) in the clusters. Moreover, we could unambiguously show that AgNC after depositing on titania-modified alumina support oxide (Al2 O3 :TiO2 ) act as an efficient photocatalyst for the degradation of methylene blue in aqueous solution under visible-light illumination. Moreover, the Ag:Al2 O3 :TiO2 photocatalyst exhibits much higher photocatalytic efficiency in comparison with other commercially available catalysts (e.g., TiO2 , Al2 O3 , TiO2 P25, Al2 O3 :TiO2 , Ag:TiO2 , and Ag:Al2 O3 ). The significantly enhanced activity of the here developed Ag:Al2 O3 :TiO2 photocatalyst is attributed to efficient photo-induced charge-separation and charge-carrier transfer processes taking place at the interface between the AgNC and the Al2 O3 :TiO2 substrate. The kinetic data indicate that an initial dye concentration of 10−5 M and photocatalyst loading concentration

0.4 g/l

0.8

0.8

0.6 g/l 1.0 g/l 0.4

C/Co

0.8 g/l

0.6

C/Co

0.01 M 0.001 M 0.0001 M 0.00001 M

1.0

0.2 g/l

b

0.6 0.4

0.2 0.2

0.0 0

5

10

15

20

25

irradiation time [min]

30

0.0 0

5

10

15

20

25

30

irradiation time [min]

Fig. 14. The dependence of degradation rate of methylene blue (c/co ) on the irradiation time for various concentration of the Ag:Al2 O3 :TiO2 photocatalyst (a); the degradation rate c/co as a function of the irradiation time for various initial concentration of methylene blue.

N.K. Pal, C. Kryschi / Journal of Molecular Catalysis A: Chemical 404 (2015) 27–35

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