Applied Catalysis A: General 466 (2013) 32–37 Contents lists available at SciVerse ScienceDirect Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata Solvent-free hydrothermal synthesis of anatase TiO2 nanoparticles with enhanced photocatalytic hydrogen production activity Ayad F. Alkaim a,b , Tarek A. Kandiel c,∗ , Falah H. Hussein d , Ralf Dillert a , Detlef W. Bahnemann a a Institut für Technische Chemie, Leibniz Universität Hannover, Callinstrasse 3, D-30167 Hannover, Germany Department of Chemistry, College of Girls Sciences, Babylon University, Hilla, Iraq c Department of Chemistry, Faculty of Science, Sohag University, Sohag 82524, Egypt d Department of Chemistry, College of Science, Babylon University, Hilla, Iraq b a r t i c l e i n f o Article history: Received 28 March 2013 Received in revised form 28 May 2013 Accepted 23 June 2013 Available online xxx Keywords: TiO2 Photocatalytic hydrogen production EDTA Photocatalysis Anatase nanoparticles a b s t r a c t TiO2 nanoparticles exhibiting large surface area were synthesized by the hydrothermal treatment of the water soluble titanium(IV) bis(ammoniumlactato) dihydroxide (TALH) complex in the presence of aqueous ammonia. The obtained powders were characterized by X-ray diffraction, scanning electron microscopy, diffuse reflectance spectroscopy, and nitrogen adsorption. Their photocatalytic activities were assessed by the photocatalytic hydrogen evolution from aqueous EDTA solutions. The effects of Ptand photocatalyst loading, EDTA concentration, light intensity, pH, and temperature on the H2 evolution rate were studied in detail. The highest reaction rate was obtained for the TiO2 photocatalyst loaded with 0.4–0.5 wt.% Pt at pH 5 and this was found to be 18 and 34% higher than that of TiO2 P25 and TiO2 UV100, respectively. The reaction rate increased substantially with increasing the temperature from 5 ◦ C to 45 ◦ C. 1. Introduction Recently, growing environmental concern and an increasing energy demand are driving the search for new and sustainable sources of energy. In particular, solar generated molecular hydrogen has attracted much attention because it can be regarded as a renewable energy source and because its combustion produces only water as a by-product without the emission of greenhouse gases [1–3]. Hydrogen gas is currently produced from a variety of primary sources, such as natural gas, naphtha, heavy oil, methanol, biomass, wastes, coal, wind energy, hydropower, and nuclear energy [4,5]. Nowadays, the photocatalytic hydrogen production resulting from the water splitting reaction using metal oxide semiconductors, e.g. TiO2 , has attracted much attention because it is an ideal process, utilizing the clean and abundant resources of water and solar energy [6–11]. Unfortunately, the efficiency of the photocatalytic water splitting reaction employing TiO2 is still low, mainly due to the high recombination rate of photo-generated electron/hole pairs and the fast backward reaction of hydrogen and oxygen to form water [12–14]. Loading the TiO2 surface with noble metal islands, typically Pt, creates sinks for the electrons as well ∗ Corresponding author. Tel.: +2 093 4570000x2342. E-mail address: kandiel@science.sohag.edu.eg (T.A. Kandiel). 0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apcata.2013.06.033 © 2013 Elsevier B.V. All rights reserved. as active sites for the H2 formation, thus facilitating the separation of e− /h+ pairs photogenerated in TiO2 and promoting the formation of H2 gas. Although the loading with a noble metal can reduce the charge carrier recombination to some extent, hydrogen production from pure water-splitting is difficult to achieve since the recombination of the electron–hole pairs cannot be completely eliminated and the backward reaction of H2 and O2 to form H2 O is thermodynamically favorable. [15–19] Therefore, electron donors or carbonate salts as well as other mediators are usually required to avoid this problem. Most research groups employ methanol as electron donor for the photocatalytic H2 production on Pt loaded TiO2 [20] while the hydrogen-rich EDTA molecule is, in comparison, rarely being employed [21]. For the synthesis of anatase TiO2 nanoparticles, in most cases, TiCl4 or titanium alkoxides are used as precursors. However, the hydrolysis of TiCl4 and titanium alkoxides will inevitably take place in water, even in moist air. Therefore, ice-cooled water baths or organic solvents are often used to ensure a control of the synthesis conditions. Hence, it is important to develop a simple single step method for the preparation of anatase TiO2 nanoparticles in an aqueous environment at low temperatures. In the present work, anatase TiO2 nanoparticles exhibiting a large surface area were readily synthesized by the hydrothermal treatment of aqueous solutions of commercially available titanium bis(ammoniumlactato) dihydroxide (TALH) in the presence A.F. Alkaim et al. / Applied Catalysis A: General 466 (2013) 32–37 of aqueous ammonia. The advantages of the use of TALH as a TiO2 precursor are that it is a water-soluble precursor and, thus, does not require an alcohol based solution and that it is stable at ambient temperature in air, hence eliminating the need of an inert atmosphere during hydrolysis and condensation procedures. The obtained TiO2 nanoparticles were employed as photocatalysts for the hydrogen production from aqueous EDTA suspensions. EDTA is widely used in industries and agriculture [22,23] and it is frequently present in sewage effluents, rivers, lakes, and groundwater [24]. Thus, the photocatalytic removal of EDTA from water and the simultaneous hydrogen production were studied here. The effects of Pt- and photocatalyst loading, EDTA concentration, light intensity, pH, and temperature on the H2 evolution rate were studied in detail. 2. Experimental 2.1. Materials Titanium(IV) bis(ammoniumlactato) dihydroxide (TALH, 50% aqueous solution), aqueous ammonia solution (28.0–30.0% NH3 ), sodium hydroxide, Nitric acid (70%), and Ethylenediaminetetraacetic acid disodium salt dihydrate (99%) were purchased from Sigma–Aldrich and used as received. All aqueous solutions were prepared employing deionized water obtained from a SARTORIUS ARIUM 611 apparatus (resistivity = 18.2 ␮S cm−1 ). 2.2. Hydrothermal synthesis TiO2 nanoparticles were prepared by the thermal hydrolysis of titanium(IV) bis(ammoniumlactato) dihydroxide (TALH). Typically, 10 mL of an aqueous titanium(IV) bis(ammoniumlactato) dihydroxide solution was mixed with an aqueous ammonia solution. The resulting solution volume was 100 mL and the concentration of aqueous ammonia was adjusted to be 0.1, 1, 3, and 5.0 M, respectively. The resulting solution was transferred into a 250 mL Teflon cup. Afterwards, the Teflon cup was sealed in an autoclave and placed into an electric furnace held at 160 ◦ C for 24 h. Finally, the autoclave was naturally cooled in air. The resulting TiO2 nanoparticles were separated by centrifugation, washed with water three times and dried overnight in an oven at 60 ◦ C. The TiO2 samples are denoted as TiO2 -x where x represents the aqueous ammonia concentration. 33 scale factors, one background parameter, specimen displacement and the zero point error were optimized. Profile shape calculations were carried out on the basis of standard instrumental parameters using the fundamental parameter approach implemented in the program, varying also the average crystal size (integral breadth) of the reflections. Structural data for the known phases were taken from the PDF-2 database with the following PDF number: anatase [21–1272]. Field-emission scanning electron microscopy (FE-SEM) measurements were carried out on a JEOL JSM-6700F field-emission instrument, using a secondary electron detector (SE) at an accelerating voltage of 2.0 kV. A Varian Cary 100 Scan UV–visible spectrophotometer system equipped with a labsphere diffuse reflectance accessory was used to obtain the reflectance spectra. Labsphere USRS-99–010 was employed as a reflectance standard. Single-point standard BET surface area measurements were carried out employing a Micromeritics AutoMate 23 instrument. The gas mixture used for the adsorption determinations was 30% nitrogen and 70% helium. The TiO2 samples were previously heated to 150 ◦ C for approximately 30 min in order to clean the surface of adsorbed organic compounds and humidity. 2.5. Photocatalytic H2 production The photocatalytic molecular hydrogen production tests were carried out in a double jacket Duran glass reactor (110 cm3 ) equipped with three outlets. The inner part of the reactor was a cylindrical tube with a diameter of 4 cm and a height of 6.0 cm [16]. In a typical run, the desired amount of Pt loaded TiO2 photocatalyst was suspended in 75 mL of an aqueous EDTA solution (1–5 mmol L−1 ) by sonication. The resulting suspension was transferred into the photoreactor and purged with Ar for 30 min to remove dissolved O2 . The reactor was sealed with a silicone rubber septum and repeatedly flushed with Ar for another 30 min until no O2 and N2 were detected by gas chromatography in the headspace above the suspension. Subsequently, the stopcocks were closed and the photoreactor was connected to the cooling system. The 2.3. Preparation of Pt-loaded TiO2 Pt-loaded TiO2 was prepared by suspending 0.5 g of TiO2 powder in 100 mL water by sonication, followed by the addition of the desired amount of hexachloroplatinic acid solution containing 5.0 × 10−4 –5.0 × 10−3 g of Pt [25]. The resulting suspension was purged with Ar and illuminated by UV(A) light (1 mW cm−2 ) for 2 h under Ar atmosphere. Afterwards, 1 mL methanol was injected and the suspension was subsequently illuminated overnight. The Pt-loaded powders were separated by centrifugation, washed three times with deionized water, and dried overnight in an oven at 60 ◦ C. 2.4. Characterizations X-ray diffraction (XRD) data for the Rietveld phase analysis of TiO2 have been recorded on a Phillips PW1800 diffractometer using a reflection geometry with variable divergence slits, Cu-K␣1, and a secondary monochromator. Three thousand data points were collected with a step width of 0.02◦ and 2 measurement times per step in the 2 range from 20 to 80◦ . The phase analysis by the Rietveld method was carried out by using the TOPAS 2.0 (Bruker AXS) software. During the refinements, general parameters such as Fig. 1. XRD diffraction patterns of TiO2 prepared by thermal hydrolysis of the TALH complex in the presence of different concentrations of aqueous ammonia, and of 0.4 wt.% Pt loaded TiO2 , prepared in the presence of 1 M aqueous ammonia. Label A indicate the Bragg positions for anatase. 34 A.F. Alkaim et al. / Applied Catalysis A: General 466 (2013) 32–37 Table 1 Properties of anatase TiO2 nanoparticle powders prepared from the TALH complex in the presence of different concentrations of aqueous ammonia. Photocatalyst Aqueous ammonia (M) Rutile (wt.%) Anatase (wt.%) Crystallite size anatase (nm) Crystallite size rutile (nm) SBET (m2 g−1 ) TiO2 -0.1 TiO2 -1 TiO2 -3 TiO2 -5 TiO2 P25 TiO2 -UV100 0.1 1 3 5 – – – – – – 18 – 100 100 100 100 82 100 7 8 7 7 31 9 – – – – 49 – 248 189 177 178 52 301 photoreactor was irradiated from the outside using an Osram XBO 1000 W Xenon lamp in a Müller LAX 1000 lamp housing. The evolved gas was sampled at a constant rate through the silicone rubber septum using a locking-type syringe. The sampled gas was quantitatively analyzed using a gas chromatograph (Shimadzu 8A, TCD detector). The GC was equipped with a molecular sieve 5 Å packed column. For hydrogen analysis, the employed carrier gas was Ar. The intensity of the UV(A) illuminations was controlled by changing the distance of the reaction vessel from the light source; this intensity was measured at the entrance window of the photoreactor by a UV light meter (ultraviolet radiometer LTLutron UVA-365). 3. Results and discussion 3.1. Characterization of the TiO2 photocatalysts The thermal hydrolysis of the TALH complex in the presence of aqueous ammonia leads to the formation of anatase TiO2 nanoparticles as revealed from the XRD diffraction patterns presented in Fig. 1. It can be seen from Fig. 1 that all the diffractions can be indexed to the anatase phase. Moreover, the diffraction data were analyzed by the Rietveld method considering the whole pattern and not only single peaks. Thus, a higher sensitivity for low phase contents is possible even when peak broadening due to small crystallite sizes occurs. The Rietveld analysis proves that no rutile or brookite is present in the powders synthesized under the present conditions. The quantitative phase composition and crystallite diameters of the nanocrystalline TiO2 powders as evident from the Rietveld analysis of the XRD data are given in Table 1. No diffraction related to Pt was detected in the XRD patterns of Pt loaded TiO2 powders, possibly because the Pt content on the TiO2 surface is low and the particle size of Pt is small [26,27]. The BET surface area of the as-prepared powders was measured by nitrogen adsorption. The results are also given in Table 1. It was found that when the aqueous ammonia concentration employed in the synthesis process increases, the surface area decreases. This trend might be explained by the rapid hydrolysis of the TALH complex at high aqueous ammonia concentration and the growth of the TiO2 particles, or by the different aggregation nature of TiO2 nanoparticles. For comparison, the characteristic parameters of two commercial photocatalysts, i.e., Evonik TiO2 P25 and Sachtleben Hombikat UV100, are also presented in Table 1. Fig. 2. SEM micrographs of anatase TiO2 nanoparticles prepared by the thermal hydrolysis of the TALH complex in the presence of (a) 0.1, (b) 1, (c) 3, and (d) 5 M aqueous ammonia at 160 ◦ C for 24 h. A.F. Alkaim et al. / Applied Catalysis A: General 466 (2013) 32–37 35 In order to investigate the particles’ morphology, the asprepared powders were investigated by field-emission scanning electron microscopy (FE-SEM). Fig. 2 shows the micrographs of the anatase TiO2 powder obtained by thermal hydrolysis of the TALH precursor in the presence of different concentrations of aqueous ammonia. It is obvious from these micrographs that the pure anatase powder aggregates from fine TiO2 nanoparticles in the size range of 10 nm or even less. This finding is in good agreement with the crystallite size predicted from the XRD analysis. The bandgap energy of the as-prepared TiO2 powders was determined by diffuse reflectance measurements. Assuming that TiO2 has an indirect optical transition [28,29], the plots of the modified Kubelka–Munk function [F(R)E]1/2 calculated from the optical absorption spectra versus the energy of the exciting light E indicate that TiO2 nanomaterials exhibit a bandgap energy of 3.2 eV in good agreement with the value usually encountered in the literature. 3.2. Photocatalytic hydrogen production 3.2.1. Effect of Pt and photocatalyst loading Despite the fact that TiO2 generally exhibits relatively high photocatalytic activity toward the degradation of organic compounds under UV illumination in the presence of molecular oxygen, it usually does not show any ability to photocatalyze the H2 evolution in oxygen free systems even in the presence of an electron donor. When TiO2 absorbs a photon the energy of which exceeds its bandgap energy, an electron (e− )/hole (h+ ) pair is generated. In the presence of an electron donor, such as EDTA, and in the absence of O2 , the excess holes will be consumed and the photogenerated electrons will be trapped near the surface forming tri-valent titanium (Ti3+ ) sites instead of reducing H+ [30]. Loading the TiO2 surface with small Pt islands creates sinks for the electrons thus facilitating the separation of the e− /h+ pairs photogenerated in the TiO2 and promoting the formation of H2 gas [31]. Fig. 3 (a) shows the effect of Pt loading on the photocatalytic hydrogen production rate from aqueous EDTA solutions on TiO2 nanoparticles prepared in the presence of 1 M aqueous ammonia. It can be seen that the increase of the Pt content from 0.1 wt.% to 0.3 wt.% leads to an increase of the H2 production rate, then the latter levels off and reaches plateau. Further increase of the Pt content resulted in a slight decrease of the H2 production rate which might be attributed to the fact that at high Pt loading Pt can act as a recombination site and also to alterations of the light absorption capacity due to the gray coloration of the Pt-loaded TiO2 nanoparticles. A similar dependence of the photocatalytic activity on the amount of Pt loading has been reported previously [32,33]. A higher concentration of the photocatalyst is expected to result in an increased absorption of UV energy, leading to a higher photocatalytic H2 evolution activity. However, the activity usually starts to decline when the concentration of the photocatalyst exceeds a certain value, indicating that the concentration of the photocatalyst has to be optimized. Fig. 3(b) shows the effect of the Pt-TiO2 loading on the photocatalytic hydrogen production rate. The highest rate was obtained employing 1 g L−1 , subsequently the rate is decreasing with an increase of the photocatalyst concentration. The obtained results can be rationalized in terms of the availability of active sites on the TiO2 surface and on the penetration of the activating light into the suspension. The availability of active sites increases with the increase in photocatalyst concentration in the suspension, while the light penetration and consequently the photoactivated volume of the suspension shrinks resulting in a decrease of the hydrogen production rate at very high photocatalyst loadings [34]. The photocatalytic hydrogen evolution observed for different TiO2 preparations obtained in the presence of different concentrations of aqueous ammonia is presented in Fig. 4. The TiO2 powders prepared employing 1 M aqueous ammonia showed the highest Fig. 3. Effect of Pt (a) and photocatalyst (b) loadings on the photocatalytic H2 evolution rate on TiO2 prepared in the presence of 1 M aqueous ammonia. Conditions: photocatalyst concentration = 1 g L−1 , EDTA concentration = 1 mM, temperature 298 K, initial pH 5, Pt loading = 0.4 wt.% (part b), and light intensity 33 mW cm−2 . photoactivity which even exceeded the photocatalytic activity of TiO2 P25 and Hombikat UV100. The latter is pure anatase and exhibits large surface area, i.e., 300 m2 g−1 . The higher photocatalytic activity of the TiO2 prepared in presence of 1 M aqueous ammonia as compared with that of TiO2 prepared in the presence of 3 and 5 M aqueous ammonia can be explained by the former’s higher surface area and crystallinity (see Table 1). Fig. 4. Rate of H2 evolution on 0.4 wt.% Pt loaded TiO2 prepared in the presence of different concentrations of aqueous ammonia. Conditions: photocatalyst concentration = 1 g L−1 , EDTA concentration = 1 mM, temperature = 298 K, light intensity = 30 mW cm−2 , and initial pH 5. 36 A.F. Alkaim et al. / Applied Catalysis A: General 466 (2013) 32–37 Fig. 5. Effect of EDTA concentration (a) and initial pH (b) on the rate of H2 evolution on 0.4 wt.% Pt loaded TiO2 prepared in the presence of 1 M aqueous ammonia. Conditions: photocatalyst concentration = 1 g L−1 , EDTA concentration = 2 mM (part b), initial solution pH 5 (part a), temperature = 298 K, and light intensity 17 mW cm−2 . Fig. 6. (a) Relation between the rate of H2 evolution and the square root of the employed light intensity and (b) linearized form according to the Arrhenius equation. Conditions: photocatalyst concentration = 1 g L−1 , EDTA concentration = 2 mM, light intensity = 22 mW cm−2 (b), temperature 298 K (a), initial pH 5. 3.2.2. Effect of EDTA concentration and initial pH Besides its dependence on the Pt and the photocatalyst loading, the photocatalytic hydrogen production rate also strongly depends on the concentration of the electron donor, i.e., EDTA in the present study. Fig. 5(a) shows the effect of the EDTA concentration on the rate of the hydrogen production on 0.4 wt.% Pt loaded TiO2 prepared in the presence of 1 M aqueous ammonia. Fig. 5(a) evinces that with an increase in the EDTA concentration, the photocatalytic activity increases and reaches a maximum at an EDTA concentration of 2 mM. Beyond this optimum EDTA concentration, a further increase in the EDTA concentration leads to a decrease in the photocatalytic activity. It is rather expected that at relatively high concentrations of EDTA beyond the optimum point, the hydrogen evolution rate will reach a plateau since the surface active sites and the absorbed photons are assumed to remain constant at a constant loading of photocatalyst and at a fixed light intensity. The observed decrease in the photocatalytic H2 evolution activity might be explained by a blockage of the adsorption of hydronium ions at surface active sites [35]. The same phenomenon has previously been observed when methanol [36] and glucose [32,37] were used as electron donors. The effect of the initial suspension pH on the photocatalytic hydrogen production rate was also investigated and the results are presented in Fig. 5(b). It can be seen that the rate of H2 evolution increases with an increase of the pH from 2 to 5. Further increase of the pH beyond 5 leads to a decrease of the H2 evolution rate. The results also indicate a very small amount of hydrogen evolution at pH 10. The solution pH affects the charge of both reactants in solution as well as of the photocatalyst surface ultimately changing the electrostatic interaction between the reactants and the TiO2 surface [38,39]. The increased rate of hydrogen production with increasing pH can thus be explained by a better adsorption of EDTA at pH 5 due to the fact that EDTA is partially ionized and negatively charged whereas the TiO2 surface is positively charged. Further increase of the pH above the zero point of proton condition of TiO2 , i.e., pHZPC 5.8–6.0, leads to a negatively charged TiO2 surface [40] resulting in an electrostatic repulsion between this negatively charged surface and the dissociated EDTA molecules, subsequently, decreasing the photocatalytic hydrogen production rate. 3.2.3. Effect of light intensity and temperature The investigation of the effect of the light intensity on the rate of the H2 evolution indicates that the rate is increasing with increasing UV light intensity as more radiation is available to excite the catalyst and hence more charge carriers are generated resulting in a higher rate of H2 evolution. However, the relation between the light intensity and the hydrogen production rate was found to be nonlinear due to the fact that at high photon flux the recombination rate of the charge carriers also increases as compared with a lower photon flux. Hence, a square root dependence of the rate of hydrogen evolution on the light intensity was actually observed as shown in Fig. 6(a) in good agreement with the behavior usually reported, e.g., for the photocatalytic degradation of organic pollutants in the presence of molecular oxygen [41]. Investigations of the temperature effect on the hydrogen production rate indicate that the amount of H2 evolution increases with an increase of the temperature. This behavior which cannot be related A.F. Alkaim et al. / Applied Catalysis A: General 466 (2013) 32–37 to light-induced reaction steps which usually do not exhibit any temperature dependency, may be rationalized in terms of the facile desorption of the photocatalytic products, i.e., molecular hydrogen and/or oxidized EDTA, with increasing temperature. Moreover, it is also possible that increasing solution temperature results in higher reaction rates between the photo-generated holes or other intermediate oxidants and the sacrificial agent EDTA and, therefore, to higher rates of the photo-induced hydrogen production [42]. Based on the average steady-state rates measured at different temperatures, the apparent activation energy of the reaction is calculated from the slope of the Arrhenius plot presented in Fig. 6(b) to be 11.4 ± 1.2 kJ mol−1 . The existence of an activation energy barrier in a photocatalytic process has so far been mainly linked with an increase of the desorption of adsorbed products with increasing temperature [43]. 4. Conclusions Anatase TiO2 nanoparticles with high surface area were readily prepared by thermal hydrolysis of the water soluble titanium(IV) bis(ammoniumlactato) dihydroxide (TALH) precursor at 160 ◦ C for 24 h in the presence of different aqueous ammonia concentrations. The obtained nanomaterials were characterized by X-ray diffraction, scanning electron microscopy, diffuse reflectance spectroscopy, and nitrogen adsorption. Their photocatalytic activities were assessed by the photocatalytic hydrogen production from aqueous EDTA solutions. Investigations of the effect of different parameters, e.g., Pt and photocatalyst loading, pH, temperature, and EDTA concentration on the photocatalytic hydrogen production rate indicate that the optimum conditions for the hydrogen evolution employing these newly synthesized anatase nanoparticles are a Pt loading of 0.4 wt.%, a photocatalyst concentration of 1 g L−1 , an EDTA concentration of 2 mM, and pH 5. A nonlinear relationship between the initial hydrogen production rate and the light intensity as well as an increase of the rate of hydrogen formation with increasing suspension temperature was observed. The obtained TiO2 nanoparticles exhibit higher photocatalytic activity than the commercial photocatalysts TiO2 P25 and Hombikat UV100. Acknowledgements We thank Dr. L. Robben (Solid State Chemical Crystallography, University of Bremen) for the XRD measurements and the Rietveld phase analysis. A. F. 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