International Journal of Photoenergy

International Journal of Photoenergy / 2011 / Article

Research Article | Open Access

Volume 2011 |Article ID 532578 | 14 pages | https://doi.org/10.1155/2011/532578

TiO2/ZnS/CdS Nanocomposite for Hydrogen Evolution and Orange II Dye Degradation

Academic Editor: Panagiotis Lianos
Received18 Jan 2011
Revised14 Apr 2011
Accepted21 May 2011
Published01 Aug 2011

Abstract

TiO2/ZnS/CdS composites for photocatalytic hydrogen production from water were prepared by homogeneous hydrolysis of aqueous solutions mixture of TiOSO4, ZnSO4, and CdSO4 with thioacetamide. Hydrogen evolution was observed in the presence of palladium and platinum nanoparticles deposited on TiO2/ZnS/CdS composites. The morphology was obtained by scanning electron microscopy, the nitrogen adsorption-desorption was used for determination of surface area (BET) and porosity. The method of UV-VIS diffuse reflectance spectroscopy was employed to estimate band-gap energies of prepared TiO2/ZnS/CdS nano-composites. The photocatalytic activity of the prepared samples were assessed by photocatalytic decomposition of Orange II dye in an aqueous slurry under UV irradiation at 365 nm wavelength and visible light up to 400 nm wavelength. Doped titanium dioxide by the CdS increased band-gap energy and doping with ZnS increased photocatalytic activity. The best photocatalytic activity for H2 evolution shows sample named TiZnCd7 on surface deposited with palladium, which contains 20.21% TiO2, 78.5% ZnS, and 1.29% CdS.

1. Introduction

Nano-sized TiO2 photocatalytic water-splitting technology has great potential for low-cost and environmentally friendly solar-hydrogen production to support the future hydrogen economy [1]. Semiconductor photocatalysis (TiO2, ZnO) has been intensively studied in recent decades for a wide variety of application such as hydrogen production from water splitting and water and air treatment. Nevertheless, the majority of photocatalysts are, however, wide band-gap semiconductors which are active only under UV irradiation. On the other hand, in order to effectively utilize visible solar radiation, the present work investigates various types of visible-light active photocatalysts including metal ion-doped TiO2, nanocomposites of TiO2 and ZnS or CdS, and a mixed-phase ZnS-CdS matrix interlinked with elemental Pt deposits.

The vast majority of today’s researches are focused on TiO2 thin films that have been synthesized by radio-frequency magnetron sputtering and/or sol-gel method to study the hydrogen generation by photocatalytic water splitting under visible light irradiation. It is necessary to mention that there is quite a long time period to obtain the constant H2 generation on such TiO2 films. This phenomena both because of the separated evolution of H2 and O2 gases, and consequently thanks to the back-reaction effect [2]. Following the work of authors using different approaches to resolve some or all of the disadvantages H2 production by using TiO2 films, Cr or Fe ion-doped TiO2 thin films have been synthesized by radio-frequency magnetron sputtering and a sol-gel method to study hydrogen generation by photocatalytic water splitting under visible light irradiation. The doping method, dopant concentration, charge transfer from metal dopants to TiO2, and type of dopants used for modification of TiO2 were investigated for their ability to enhance photocatalytic activity [3].

Highly dispersed iridium and cobalt metal particles (average 3 nm) were introduced in the texture of the synthesized titania nanotube by ion-exchange method, which were found to be effective photocatalysts for the production of stoichiometric hydrogen and oxygen by the splitting of water under the visible light irradiation [4].

Fe3+-doped titania photocatalysts were prepared by hydrothermal treatment for the photocatalytic water splitting to produce stoichiometric hydrogen and oxygen under visible light irradiation [5]. Sensitized photocatalytic production of hydrogen from water splitting is investigated under visible light irradiation over mesoporous-assembled titanium dioxide nanocrystal photocatalysts, without and with Pt loading [6]. ZnO-CdS core-shell nanorods with a wide absorption range were designed and synthesized by a two-step route. The ZnO-CdS core-shell nanorods exhibit stable and high photocatalytic activity for water splitting into hydrogen in the presence of S2− and as sacrificial reagents [7].

Mixed semiconductor CdS-ZnS-TiO2 () mixture system over different supports like MgO, CaO, γ-Al2O3, SiO2, and modified MgO and CaO have been prepared, characterized and tested for H2 production in a S2−/ mixture solution [8]. In order to efficiently use the UV-vis light in the photocatalytic reaction, a novel (CdS/ZnS)/Ag2S + RuO2/TiO2 was synthesized by chemical coprecipitation and metal ion implantation, and this composite exhibited much higher photocatalytic activity for the generation of hydrogen [9]. Powdered and immobilized Pt/CdS/TiO2 photocatalysts were used to oxidize model inorganic (S2−/) and organic (ethanol) sacrificial agents-pollutants in water. Powdered Pt/CdS/TiO2 photocatalysts of variable CdS content (0–100%) were synthesized by precipitation of CdS nanoparticles on TiO2 (Degussa P25) followed by deposition of Pt (0.5 wt%) [10].

As it has been shown earlier [11] the homogeneous precipitation with urea leads to anatase nanoparticles assembled into rather big (1-2 μm) porous clusters consist of small 4-5 nm crystallites embodied good photocatalytic properties. For substitution of urea, we used the modified homogeneous precipitation with thioacetamide to prepare TiO2/ZnS/CdS nanocomposites. In the same way as the urea method, the homogeneous precipitation of metal sulphides by thermal decomposition of thioacetamide (TAA) can be used [12]. Thioacetamide at temperature higher than 60°C in acidic solution released hydrogen sulphide: The reaction of products are nanosized spherical particles [13] with a well-developed microstructure, but different from homogeneous urea precipitation. These products have high specific surface area and are properly washed and filtered. In the reaction conditions, the spherical agglomerates of titania are formed by thermal hydrolysis of titanyl sulphate [14] and agglutinated with spherical agglomerates of ZnS and CdS, respectively, precipitated with thioacetamide. As-prepared TiO2/ZnS/CdS nanocomposites by homogeneous hydrolysis with thioacetamide exhibited new optical properties concerning the absorption, which were different from those of the bulk anatase [15], CdS [16], or sphalerite [17].

During our study, ten samples were synthesized, labeled as TiZnCd1–TiZnCd10. As a preparation method, was select homogeneous hydrolysis of titania, zinc, and cadmium sulphates with thioacetamide (TAA). The photocatalytic activity of TiO2/ZnS/CdS nanocomposites was assessed by the photocatalytic decomposition of Orange II dye in an aqueous slurry under irradiation of 365 nm and up to 400 nm wavelength. Subsequently, the above prepared samples were deposited by noble metal palladium and/or platinum nanoparticles. And as such they were used for the photocatalytic hydrogen evolution.

2. Experimental

2.1. Synthesis of ZnS and CdS Doped Titania

All used chemicals, titanium oxosulphate (TiOSO4), zinc(II) sulphate (ZnSO4·7H2O), cadmium(II) sulphate (CdSO4·8/3H2O), sodium borohydride (NaBH4), and thioacetamide (TAA), were of analytical grade and were supplied by Aldrich. The noble metal, palladium, platinum, gold, silver, iridium, rhenium, and ruthenium as standard solution for atomic absorption spectroscopy (AAS, 1000 mg/L, TraceCERT) were obtained from Fluka.

Titanium oxosulphate, zinc(II) sulphate, and cadmium(II) sulphate were dissolved in 4 L of distilled water and 50 g of TAA was added (see Table 1). The reaction mixture was adjusted to with sulphuric acid. The reaction mixture was heated at the temperature of 80°C under stirring for 4 hours. The thus synthesized TiO2/ZnS/CdS samples were washed with distilled water with decantation, filtered off, and dried at 105°C in a drying kiln. Using with this method ten samples labeled TiZnCd1–TiZnCd10 were prepared.


SampleTiOSO4(g)ZnSO4 (g)CdSO4(g)TAA (g)

TiZnCd11001001050
TiZnCd2100501050
TiZnCd3100100550
TiZnCd410050550
TiZnCd5100100150
TiZnCd610050150
TiZnCd750100150
TiZnCd850100550
TiZnCd9501001050
TiZnCd1050505050

2.2. Noble Metal Deposition

1 g photocatalyst (prepared TiO2/ZnS/CdS or P25) was sonicated 30 min in 100 mL of water in ultrasonic bath (300 W, 35 kHz) and added 1 mL of noble metal 1000 mg L−1 AAS solution. The reaction mixture was mixed on a magnetic stirrer and very slowly the 0.001 M solution of sodium borohydride was added. The formation of Me0 was monitored using UV-VIS spectrophotometer. The noble metal AAS solution shows an intense characteristic absorption band in UV region at 296 nm of Pd3+ and Pt4+ ions, at 300 nm for Cu2+ and Ag+ ions, 315 nm for Au3+ ion, in visible region at 450 nm of Ru3+ ion, and 488 nm of Ir3+ ion. Noble metal deposition on the surface of photocatalyst was monitored by disappearance of these intense absorption band.

Obtained suspension of photocatalyst (Degussa P25) with noble metals (Pt, Pd, Ru—light grey powder, Au—light purple powder, Ag—light brown purple powder, Ir, Rh, Cu—slightly yellow powder) deposition were centrifuged and washed in water.

2.3. Characterization Methods

Diffraction patterns were collected with diffractometer PANalytical X’Pert Pro equipped with conventional X-ray tube (Cu Kα radiation, 40 kV, 30 mA) and a linear position sensitive detector PIXcel with an antiscatter shield. A programmable divergence slit set to a fixed value of 0.5 deg, Soller slits of 0.02 rad, and mask of 15 mm were used in the primary beam. A programmable antiscatter slit set to fixed value of 0.5 deg., Soller slit of 0.02 rad, and Niβ-filter were used in the diffracted beam. Qualitative analysis was performed with the DiffracPlus Eva software package (Bruker AXS, Germany) using the JCPDS PDF-2 database [18]. For quantitative analysis of XRD patterns we used Diffrac-Plus Topas (Bruker AXS, Germany, version 4.1) with structural models based on ICSD database [19]. This program permits to estimate the weight fractions of crystalline phases and mean coherence length by means of Rietveld refinement procedure.

Scanning electron microscopy (FESEM) was performed with a high-resolution, field-emission gun SEM microscope Quanta 200 FEG (FEI, Czech Republic) equipped with an energy dispersive X-ray spectrometer (EDS); specimens for morphological investigations were prepared by evaporation of a droplet of samples dispersion on a carbon support. The specimens were then imaged in the low-vacuum mode using accelerating voltages of 30 kV.

The surface areas of samples were determined from nitrogen adsorption-desorption isotherms at liquid nitrogen temperature using a Coulter SA3100 instrument with outgas 15 min at 150°C. The Brunauer-Emmett-Teller (BET) method was used for surface area calculation [20], the pore size distribution (pore diameter, pore volume, and micropore surface area of the samples) was determined by the Barrett-Joyner-Halenda (BJH) method [21].

Diffuse reflectance UV/VIS spectra for evaluation of photophysical properties were recorded in the diffuse reflectance mode (R) and transformed to absorption spectra through the Kubelka-Munk function [22]. A Perkin Elmer Lambda 35 spectrometer equipped with a Labsphere RSAPE-20 integration sphere with BaSO4 as a standard was used.

Photocatalytic activity of samples was assessed from the kinetics of the photocatalytic degradation of 0.02 M Orange II dye (sodium salt 4-[(2-hydroxy-1-naphtenyl)azo]-benzene-sulfonic acid) in aqueous slurries. The azo-dyes (Orange II, Methyl Red, Congo Red, etc.) are not absorbed on titania surfaces in contrast to methylene blue. For azo-dye degradation, the complete mass balance in nitrogen indicated that the central –N=N– azo-group was converted in gaseous dinitrogen, which is ideal for the elimination of nitrogen-containing pollutants, not only for environmental photocatalysis but also for any physicochemical method [23]. Direct photolysis employing artificial UV light or solar energy source cannot mineralize Orange II [24]. Kinetics of the photocatalytic degradation of aqueous Orange II dye solution was measured by using a self-constructed photoreactor [25]. The photoreactor consists of a stainless steel cover and quartz tube with fluorescent lamp Narva with power 13 W and light intensity ~3.5 mW cm−2. Black light (365 nm) for UV and warmwhite (upon 400 nm) for visible light irradiation were used. Orange II dye solution was circulated by means of membrane pump through flow cuvette. The concentration of Orange II dye was determined by measuring absorbance at 480 nm with VIS spectrophotometer ColorQuestXE. The 0.5 g of titania sample was sonicated for 10 min with a ultrasonic bath (300 W, 35 kHz) before use. The pH of the resulting suspension was taken as the initial value for neutral conditions and under the experiment was kept at value 7.0.

2.4. Photocatalytic H2 Production

Photocatalytic H2 production reaction was carried out in a self-constructed quartz batch photocatalytic reactor with closed gas-circulation system (see Figure 1). The photocatalyst (1 g) was suspended in distilled water by means of magnetic stirrer within an inner irradiation-type reactor. A high-pressure Hg lamp (400 W) was utilized as the light source. Prior to the reaction, the mixture was deaerated by purging with Ar gas repeatedly. For H2 measurement the Xgard Type 6 gas detector from firm CROWCON was used. The gas detector measure in range 0–10 vol% H2 in air on current output ~4–20 mA. The calibration curve of output current of hydrogen sensor is shown in Figure 2. The measured values were recalculated to concentration c (vol%) = where is current of detector, is current of detector in the saturated state, is current of detector at zero concentration of hydrogen, and is maximum range of detector. The basic measurements and calibrations of the apparatus for photocatalytic H2 evolution were made in the commercial photocatalyst P25 with deposition of Pt, Pd, Ru, Au, Ag, Ir, Rh, and Cu. The evolution of hydrogen was achieved only in samples P25 with surface deposition of platinum, palladium, and gold (see Figure 3.). P25 after deposition of other noble metals (Ru, Ag, Ir, Rh, and Cu) showed no effect of hydrogen evolution and therefore for deposition of TiZnSCdS composites only Pt, Pd, and Au were used. However, samples TiZnSCdS deposited with the Au also showed no effect of hydrogen evolution.

3. Results and Discussion

The powder XRD patterns of the TiO2/ZnS/CdS composite prepared by homogeneous hydrolysis of TiOSO4 and Zn and Cd sulphates with thioacetamide are shown in Figures 4 and 5, the phase composition and crystallite size are presented in Table 2. According to SEM the prepared nanocomposites are a physical mixture of anatase, Zn, and Cd sulphides, particle sizes are only a few micrometers, and hence Rietveld refinement can be safely used for quantitative phase analysis. From the XRD patterns and the corresponding characteristic values of the diffraction peaks, it can be confirmed that TiO2 in as-prepared samples is identified as anatase-phase (ICDD PDF 21-1272) while the ZnS is sphalerite-phase (ICDD PDF 5-0566) and CdS is hawleyite-phase (ICDD PDF 10-0454). No other polymorph of titania are observed.


SampleAnatase (%)Anatase (nm)Sphalerite (%)Sphalerite (nm)Hawleyite (%)Hawleyite (nm)

TiZnCd159.5415.7 28.7821.511.6815.0
TiZnCd277.9815.211.521.110.9711.5
TiZnCd339.889.756.520.53.6113.5
TiZnCd462.4612.034.9220.52.6219.8
TiZnCd586.6115.012.7121.10.686.9
TiZnCd637.4415.261.5126.21.056.3
TiZnCd720.2112.578.528.71.296.1
TiZnCd828.2913.066.3123.85.4014.3
TiZnCd927.499.257.8622.414.6623.5
TiZnCd107.8233.46.8623.785.3231.8

The average size of crystallites was calculated from the peak half-width, using the Scherrer equation [26], where is a shape factor of the particle (it is 1 if the spherical shape is assumed) and and are the wavelength and the incident angle of the X-rays, respectively. The peak width was measured at half of the maximum intensity. The crystallite size was calculated from diffraction plane (1 0 1) of anatase, diffraction plane (1 1 1) of sphalerite, and diffraction plane (1 1 1) of hawleyite. The relative amount of anatase, sphalerite, and hawleyite phase was calculated from XRD patterns by Diffrac-Plus Topas v.4.1.

The solid solutions of , respectively, may arise under hydrothermal conditions [27], at temperature range 150–700°C [28] or under microwave irradiation [29]. The X-ray diffraction lines show no distinct shift that suggests the creation of solid solution Cd1−xZnxS [30].

The specific surface area of the samples, calculated by the multipoint Brunauer-Emmett-Teller (BET) method and total pore volume, micropore surface area, and micropore volume are listed in Table 3. For the all samples, the isotherm of a type IV isotherm characteristic of mesoporous material with type H2 hysteresis is typical, which is a characteristic of mesoporous materials and can be ascribed to capillary condensation in mesopores (see Figure 6). The size of mesopores is on the border with micropores, corresponding to size of ~3-4 nm. According de Bore’s characterization [31], all samples have characteristic of Type E hysteresis loop. This hysteresis type is connected with ink-bottle pores or with interconnected capillaries. All samples have a microporous surface area in interval 5–40 m2 g−1.


SampleBET surface area (m2 g−1)Total pore volume (cm3 g−1)Micropore surface area (m2 g−1)Micropore volume (cm3 g−1)EDX Ti (wt%)EDX Zn (wt%)EDX Cd (wt%)

TiZnCd1133.8 0.1167 36.00.0153945.0613.333.15
TiZnCd2129.9 0.097740.90.0176051.985.334.99
TiZnCd394.6 0.076616.20.0062735.8134.460.55
TiZnCd4147.80.133116.70.0058546.8712.510.94
TiZnCd5193.70.176835.90.0148247.407.330.69
TiZnCd6144.80.178615.80.0060029.9442.790.81
TiZnCd7106.20.12245.50.0014623.2554.730.75
TiZnCd8105.60.11496.30.0019228.4043.281.77
TiZnCd9107.40.112211.10.0040827.1542.192.22
TiZnCd10103.80.094013.60.0052819.0910.6847.35

The SEM micrographs of the prepared TiO2/ZnS/CdS nanocomposites are presented in Figures 7(a)7(j), Ti, Zn, and Cd content is presented in Table 3, as obtained from EDX analysis. The product of homogeneous precipitation of thioacetamide and zinc sulphate consists of approximately spherical round particle agglomerates of diameter about 1-2 μm (Figure 7(d)) are formed from laminar nanoparticles of size 16 nm joined to the chains [17]. The products of homogeneous hydrolysis of thioacetamide and titanium oxosulphate are 2-3 μm spherical agglomerates formed with 6 nm nanoparticles (Figure 7(i)) [15]. The TiO2/ZnS/CdS nanocomposites are formed as mixture of single TiO2, ZnS, and CdS agglomerates and overgrown TiO2, ZnS, and CdS agglomerates (see Figures 7(b) and 7(h)).

The reflectance data obtained was relative percentage reflectance to a nonabsorbing material (BaSO4) which can optically diffuse light. The Kubelka-Munk theory is generally used for the analysis of diffuse reflectance spectra obtained from weakly absorbing samples. It provides a correlation between reflectance and concentration. The concentration of an absorbing species can be determined using the Kubelka-Munk formula: where is the reflectance, is the scattering coefficient, is the molar absorption coefficient, is the concentration of the absorbing species, and is absorbance [32].

Compared with the pure titania sample, obvious absorption edge red-shifts are observed in the results of the doped samples, among which the best response to visible-light is obtained in the codoped sample. The anatase has a wide absorption band in the range from 200 to 385 nm, the ZnS has an absorption band in the range from 200 to 310 nm, and CdS has absorption edge at 490 nm [33]. For the TiO2/ZnS/CdS nanocomposites, an absorption edge red-shift is presented and the absorption tail extends to ~400 nm as Figure 8 shows. With increasing content of Cd the prepared samples are beginning to have a yellowish tinge, the sample marked TiZnCd10 is already dark orange.

The method of UV-VIS diffuse reflectance spectroscopy was employed to estimate band-gap energies of the prepared TiO2/ZnS/CdS nanocomposites. Firstly, to establish the type of band-to-band transition in these synthesized particles, the absorption data were fitted to equations for direct band-gap transitions. The minimum wavelength required to promote an electron depends upon the band-gap energy of the photocatalyst and is given by where is the wavelength in nanometers [34]. The band gap values were calculated using the UV-VIS spectra from the following equation: where is the absorption coefficient and is the photon energy. In case the fundamental absorption of the titania crystal possesses indirect transitions between bands, then , for direct transition between bands [35, 36]. The energy of the band gap is calculated by extrapolating a straight line to the abscissa axis, when α is zero, then [37]. Figure 9 shows the versus for a direct band-gap transition, where α is the absorption coefficient and is the photon energy. The value of extrapolated to gives an absorption energy, which corresponds to a band-gap energy (see Table 4). The value of 3.20 eV is reported in the literature for pure anatase nanoparticles [38]. ZnS has a wider energy band gap [39] than CdS has [40], which results in the transmission of more high-energy photons [41]. The value of band-gap energy decreases with increasing the content of CdS dopant. Low values of in samples with higher content of CdS or ZnS may be caused by formation of sulphur compounds such as polysulphides, which were already observed in zinc sulphide or cadmium sulphide. The explanation of this effect was the agglomeration of nanoparticles [42].


SampleRate constant OII 365 nm TiO2/ZnS/CdS (min−1)Rate constant OII 400 nm TiO2/ZnS/CdS (min−1)Band gap (eV)

TiZnCd10.017780.003752.25
TiZnCd20.009390.002512.20
TiZnCd30.007420.001552.35
TiZnCd40.009020.002482.35
TiZnCd50.034460.004502.50
TiZnCd60.008560.002712.65
TiZnCd70.008040.001512.55
TiZnCd80.005240.001322.20
TiZnCd90.002970.001122.10
TiZnCd100.003970.000782.05

The photocatalytic activity of the prepared TiO2/ZnS/CdS nanocomposites was determined using the degradation of 0.02 M Orange II dye aqueous solutions under UV radiation at 365 nm (UV-A, black lamp). In regions in which the Lambert-Beer law is valid, the concentration of the Orange II dye is proportional to absorbance: where is absorbance, c is concentration of absorbing component, is length of absorbing layer, and is molar absorbing coefficient. Orange II dye was not a subject of photolysis, and any change in Orange II dye concentration can be attributed only to the heterogeneous photocatalysis. Photodegradation experiments of Orange II dye by catalysts process exhibited first-order kinetics with respect to the concentration of the organic compound. The time dependence of Orange II dye decomposition can be described using (10) for a reaction following first-order kinetics: where is the concentration of Orange II dye, is the initial concentration Orange II dye, and is the rate constant. The rate constant of photocatalytic degradation of Orange II dye under UV and visible light are shown in Table 4 and the kinetics of the degradation are presented in Figure 10. As shown in the figure, the highest photocatalytic activity in the degradation of Orange II dye under UV and visible light have the sample labeled TiZnCd5. This seems appropriate given the high specific surface area of the sample and the highest TiO2 content in the sample. For comparison, the rate constant for the P25 under UV is  min−1 and under visible light is  min−1 [43].

A heterojunction is created when two different layers of crystalline semiconductors are placed in conjunction or layered together with alternating or dissimilar band gaps. Heterojunction has been widely studied for effective decomposition of organic compounds on CdS/TiO2 [44], [45], and for photocatalytic water splitting [46]. CdS functions as a sensitizer while TiO2 works as a substrate in the heterojunction system. When the TiO2-CdS heterojunction was excited by UV or visible light with a photon energy higher or equal to the band gaps of TiO2 and CdS, the electrons in the valence band could be excited to the conduction band with simultaneous generation of the same amount of holes in the valence band. In this way, the photoinduced electron-hole pairs in the two catalysts are effectively separated and the probability of electron-hole recombination is reduced. In the samples series TiO2/ZnS/CdS highest photocatalytic activity for UV and visible light was observed in the sample denoted TiZnCd5, which is probably the optimum ratio of TiO2 : CdS With increasing content of cadmium, too much CdS on the TiO2 surface hinder the contact of TiO2 with Orange II dye solution, resulting in too many electrons accumulating on the TiO2 surface under light irradiation, leading to lower photocatalytic activity.

The sample TiZnCd5 with optimal Cd content also has the highest specific surface area (around 193 m2) and the highest amount of anatase in the structure. Both facts contribute to significant photocatalytic activity. However, optimal cadmium content also causes beneficial activity in visible light region. With increasing amount of Cd in the structure the photoactivity decreases, as seen by samples TiZnCd9 and TiZnCd10 with lowest photocatalytic activity and high content of cadmium. These two samples have also lower specific surface area and amount of anatase, proving that both properties affect photoactivity.

The TiO2/ZnS/CdS composites deposited on surface with Pd and Pt nanoparticles were used for hydrogen evolution from water. The hydrogen evolution on series samples TiZnCd deposited with Pd is presented in Figure 11 and samples TiZnCd deposited with Pt is presented in Figure 12. For the composites TiO2/ZnS/CdS deposited with nano-Pt were well active only samples labeled as TiZnCd3, TiZnCd7, and TiZnCd8. The best activity for photocatalytic hydrogen evolution showed the sample marked TiZnCd7 deposited on surface with palladium.

Noble metals such as Pt, Au, and Pd have been shown to increase the photonic efficiency and inhibit electron-hole pair recombination. The optimum metal loading has major role of surface deposition of small metal clusters on TiO2 and is attributed due to the acceleration of hydroxyl radical formation and decreasing recombination which enhances the degradation rate of pollutants or active centers for hydrogen evolution. From the results [47], it is also observed that the photocatalytic efficiency increases with increase in the metal loading upto certain level (optimum metal loading) and then decreases. The excess loading of metal particles may cover active sites on the TiO2 surface thereby reducing photodegradation efficiency.

Sadeghi et al. [48] proposed that a large number of small epitaxial deposits of noble metals on semiconductor substrate energetically capable of trapping photoelectrons may decrease charge carrier space distance and thereby increase recombination. Negative effect was observed that of photocatalytic activity when the metal loading was higher than the optimum level [49]. Excess deposition of Pt on the surface TiO2/ZnS/CdS composite apparently causes the lower efficiency. It is observed from the experimental results that the photocatalytic efficiency for hydrogen evolution of the metal-doped TiO2/ZnS/CdS nanocomposites is in the order Pd > Pt > Au.

Due to rapid recombination of holes and electrons origins by photocatalysis, it is difficult to achieve water-splitting for hydrogen production using TiO2 photocatalyst in distilled water. Adding electron donors or sacrificial reagents or hole scavengers to react irreversibly with the photogenerated holes can enhance the photocatalytic electron-hole separation resulting in higher quantum efficiency. Since electron donors are consumed in photocatalytic reaction, continual addition of electron donors is required to sustain hydrogen production.

One of the most well-known semiconductor photocatalyst, CdS, has been widely used for hydrogen evolution. However, pure CdS is usually not very active and prone to photocorrosion [50]. Inorganic ions such as S2−/, Ce4+/Ce3+, and IO3−/I are often used as sacrificial reagents for hydrogen production. When CdS is used as photocatalyst for water-splitting hydrogen production, photocorrosion occurs as follows: Serving as a sacrificial reagent, S2− can react with two holes to form S. The aqueous ions added to reaction suspension, can dissolve S into in order to prevent any detrimental deposition of sulphur onto CdS. Therefore, photocorrosion of CdS is prevented. Combining CdS with the wide band-gap semiconductor ZnS to form the solid solution is demonstrated to be an effective way to solve the above problems. Alternative methods for improving the activity and stability of CdS include dispersion of small amounts of metal nanocrystallites (mainly Pt or Pd) on the photocatalyst surface and/or coupling with wide band-gap semiconductors with lower but closely lying conduction band level, such as TiO2, LaMnO3, and ZnS. The beneficial effect of Pd deposition on the rate of hydrogen production can be attributed to the ability of Pt or Pd crystallites to trap photogenerated electrons and to catalyze the reduction of water to hydrogen [10]. In summary, it was shown that the deposition of nano-Pd into surface of powder TiO2/ZnS/CdS nanocomposite can be prepared photocatalysts, which could be the basis for new efficient materials for the production of hydrogen from water.

4. Conclusions

The TiO2/ZnS/CdS nanocomposites deposited with nanoparticles of metal palladium were used for photocatalytic evolution from water. Doped titanium dioxide by the CdS reducing band-gap energy and doping with ZnS increasing photocatalytic activity. In [51] nanocomposites TiO2/ZnS with different ZnS : TiO2 ratios have been prepared using a chemical deposition method. The presence of small ZnS percentages on the nanocomposite surface (0.5% and 0.2%) promotes an increase in the catalyst photoactivity, when compared with the pure titania. The best photocatalytic activity for H2 evolution shows sample named TiZnCd7 deposited with palladium, which contains 20.21% TiO2, 78.5% ZnS, and 1.29% CdS. The TiO2/ZnS/CdS materials have been proposed as highly efficient photocatalysts for hydrogen production, in which the location of Pd on the TiO2/ZnS/CdS and the preparation method play an important role. The best photocatalytic activity under UV and visible light for Orange II dye degradation showed sample denoted TiZnCd5, which contains 47.4 wt% Ti, 7.33 wt% Zn, and 0.69 wt% Cd.

Acknowledgments

This work was supported by the Academy of Sciences of the Czech Republic (Project no. AV OZ 40320502) and Ministry of Industry and Trade of the Czech Republic (Project no. FT-TA5/134).

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Copyright © 2011 Václav Štengl and Daniela Králová. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


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