Journal of Nanomaterials

Volume 2013 (2013), Article ID 852139, 8 pages

http://dx.doi.org/10.1155/2013/852139

Research Article

Preparation, Characterization, and Enhanced Photocatalytic Hydrogen Evolution Activity of -Based Compounds under Simulated Sunlight Irradiation

Li Zhang,1,2,3 Jianhui Yan,1,2,3 Minjie Zhou,1,3 and Younian Liu²

¹School of Chemistry and Chemical Engineering, Hunan Institute of Science and Technology, Yueyang, Hunan 414000, China

²College of Chemistry and Chemical Engineering, Central South University, Changsha, Hunan 410083, China

³Hunan Province Key Laboratory of Catalysis and Separation of Speciality Petrochemicals, Hunan Institute of Science and Technology, Yueyang 414006, China

Received 28 November 2012; Accepted 18 January 2013

Academic Editor: Feng Yan

Copyright © 2013 Li Zhang et al. 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.

Abstract.

photocatalyst was successfully prepared via solid state reaction and further combined with TiO₂ by a sol-gel method and a solid phase method, respectively. For comparison, Pt particles were loaded to prepare Pt- via a hydrogen reduction method. All the samples were characterized by thermogravimetry and differential thermal analysis (TG/DTA), X-ray diffraction (XRD), UV-vis diffuse reflectance spectroscopy (UV-vis DRS), and scanning electron microscopy (SEM) techniques. Photocatalytic H₂ evolution activities of the as-obtained samples were evaluated from aqueous oxalic acid solution under simulated sunlight irradiation. The effects of photocatalyst concentration, TiO₂ content, and composite method on the H₂ evolution activities of the as-obtained photocatalysts were investigated. The results show that, when the concentration of photocatalyst is 0.8 , the TiO₂- composite photocatalyst prepared by a sol-gel method exhibits the optimized photocatalytic activity, and the H₂ production rate is 4.35 mmol  with 30 wt.% content of TiO₂.

1. Introduction

Water splitting with photon energy has been extensively studied since the Honda-Fujishima effect was reported [1, 2]. TiO₂-based photocatalytic reactions such as organic matter degradation [3, 4], H₂ evolution [5], and disinfection [6] have received great attention around the world. Conventional TiO₂ photocatalysts possess excellent activity and stability, but require near-ultraviolet (UV) irradiation (about 4% of the solar energy) for the effective photocatalysis, which severely limits their practical applications [7–10]. One of the promising methods to solve the above problem for TiO₂ photocatalysts is to modify TiO₂ with metals or nonmetals [11–13]. However, although the doping leads to significant visible light absorption for TiO₂, the role of dopant metals or nonmetals on the photocatalytic activity is still under investigation. Indeed, the charges recombination processes largely depend on both dopant concentration and type. Low concentration is not sufficient to significantly increase the absorption of the visible light in TiO₂. High dopant concentration causes an efficient narrowing of the TiO₂ band gap but at the same time it probably introduces new recombination centers for the photo-generated charges. From these consideration, a higher efficiency photocatalyst is needed to be developed. Another method is to design efficient non-TiO₂ visible-light activated photocatalysts. These novel kinds of photocatalysts, such as K₂Ti₄O₉ [14, 15], K₂La₂Ti₃O₁₀ [16], K₄Nb₆O₁₇ [17, 18], and KLaNb₂O₇ [19], show potentially applications in the decomposition of pure water due to their unique structures such as layered or tunnel structures [20]. However, these photocatalysts, in general, display small absorption in the visible light region. Y₂Cu₂O₅ is a good p-type semiconductor with a special layered structure and exhibit good electromagnetic properties [21, 22]. However, there is comparatively little attentions given to photocatalytic activity of Y₂Cu₂O₅. Due to its unique interlayer, where the electron-hole recombination process could be retarded by physical separation of the electron-hole pairs generated by photo-absorption. We try to prepare the novel Y₂Cu₂O₅-based photocatalysts for water splitting. Furthermore, the photocatalysts are modified by noble metal Pt and TiO₂ nanoparticles to enhance the photocatalytic activity.

2. Experimental

2.1. Synthesis of Photocatalyst

2.1.1. Preparation of Y₂Cu₂O₅ Microparticles

All the chemical reagents were analytically pure and used without further purification. The photocatalyst Y₂Cu₂O₅ was prepared by a solid phase method. CuO and Y₂O₃ powders were weighed according to the stoichiometric ratio, mixed together and grinded fully in an agate mortar, then heated for 12 h in air, and slowly cooled to room temperature. After milling and mixing, the powders were calcined at a certain temperature for 12 h three times in air. Y₂Cu₂O₅ microparticles were then obtained.

2.1.2. Preparation of Pt-Y₂Cu₂O₅ Microparticles

Y₂Cu₂O₅ (3 g) was added into a 1 mM H₂PtCl₆ aqueous solution, and the total volume of the mixture was 100 mL by adding water. After ultrasonic treatment for 30 minutes, the mixture was stirred at 80°C with H₂ passing through (flow rate of 330–340 mL min⁻¹) for 2 h to reduce Pt⁴⁺. After being filtered, the precipitates were washed by deionized water and dried at 100°C to obtain Pt-Y₂Cu₂O₅ microparticles.

2.1.3. Preparation of TiO₂-Y₂Cu₂O₅ Composite Microparticles

The photocatalyst TiO₂-Y₂Cu₂O₅ was prepared via two routes.

(i)Sol-gel approach: 10 mL Ti(OBu)₄ and 20 mL ethanol were vigorously stirred to get pale yellow transparent solution (A) at room temperature. 1.4 mL HNO3 was slowly dropped into the mixture of 6 mL water and 20 mL ethanol under stirring to get the mixture (B). Solution (A) was dropwise added into the mixture (B) under vigorous stirring over 1 h, and TiO₂-Y₂Cu₂O₅ sol was prepared by using appropriate amount of Y₂Cu₂O₅. The as-obtained gel was subsequently transferred into an oven and kept at 100°C for 1 h. The as-prepared precursor was then annealed at 400°C for 2 h. The crystalline phase of TiO₂ is mainly due to the anatase phase. (ii)Solid state reaction: The as-prepared Y₂Cu₂O₅ and P25 TiO₂ with a certain ratio were added into 100 mL ethanol under stirring. After that, the precipitates from the mixture were collected by centrifugation, and then rinsed with ethanol several times. The as-prepared sample was dried in an oven at 80°C for 2 h. The final product was prepared via calcination at 400°C for 2 h.

2.2. Photocatalyst Characterization

Thermogravimetric and differential thermal (TG-DTA) patterns of the samples were recorded by using thermogravimetric-differential thermal analyzer to determine the temperature of possible decomposition and phase changes. The crystal structures of the as-prepared photocatalysts were identified by powder X-ray diffraction (XRD, Bruker D8) using Cu Kαradiation (=1.5418 Å) at a scan speed of 0.05° s⁻¹, a voltage of 40 kV and a current of 300 mA. The surface morphologies of the samples were observed with a JEOL JSM-5600LV scanning electron microscope (SEM) operated at 25 kV. The UV-vis diffuse reflectance spectra (UV-vis DRS) of the as-prepared photocatalysts were measured on a UV-vis spectrometer (Shimadzu UV-2500). Brunauer-Emmett-Teller (BET) surface areas of the samples were determined by nitrogen adsorption using a Micromeritics ASAP 2000 system.

2.3. Photocatalytic Activity Measurement

The photocatalytic activities of the as-prepared photocatalysts were evaluated by monitoring the amount of hydrogen evolution from aqueous oxalic acid solution under simulated sunlight irradiation. Typically, the photocatalytic reactor consists of two parts, a quartz cell (600 mL) with a circulating water jack and a Xe lamp (150 W, wavelength of 290–800 nm (unfiltered), main wavelength of 400–700 nm) placed inside the quartz cell. In all experiments, the reaction temperature was kept at room temperature by using the circulating water jack to prevent any thermal effect. In all experiments, 100 mL of deionized water containing a specified amount of the as-prepared photocatalyst and 0.05 M H₂C₂O₄ mixed sacrificial agent was added into the reactor. Subsequently, the reactor was left in dark and N₂ was bubbled through the reaction mixture for 30 min to remove O₂. Afterwards, the photocatalytic reaction system was closed and exposed to the light irradiation. During the irradiation, the mixture was suspended by using a magnetic stirrer within the quartz cell. The as-produced H₂ was collected in a water manometer and analyzed by gas chromatography (TCD, N₂ as gas carrier, zeolite NaX column). An eliminator containing saturated NaOH solution was placed before the collector of H₂ to remove CO₂. The volume of H₂ was determined by the volume of the displaced water at different intervals of irradiation time. Some experiments were repeated three times and the results were reproducible within the experiments errors (±4%).

3. Results and Discussion

3.1. TG/DTA Curves

As seen in Figure 1, the weight and heat changes of samples are relatively uniform, and the weight loss is only 2% under 800°C; suggesting that the precursors CuO and Y₂O₃ have no mutual reaction under this temperature. However, there exists an obvious weight loss stage with the weight loss of 4% at about 800~1000°C, the corresponding small exothermic DTA is observed also, probably due to the transformation of CuO and Y₂O₃ into Y₂Cu₂O₅ [23], and a certain amount of other compounds, such as YCuO₂ (shown in Figure 2), which result in weight loss.

Figure 1: TG/DTA curves for the Y₂Cu₂O₅ precursors.

Figure 2: XRD patterns of samples calcined at different temperature.

3.2. XRD Patterns

Figure 2 shows the XRD patterns of the samples calcined at 800°C, 850°C, 900°C, 950°C, and 1000°C, respectively. From Figure 2, most diffraction peaks from this pattern can be indexed to reflections of CuO and Y₂O₃ except for a minor peak ascribed to a small amount of Y₂Cu₂O₅ at 800°C. This indicates that a solid-state reaction starts around 800°C, resulting in the formation of Y₂Cu₂O₅. The results are in good agreement with the reported data [23]. With further increasing the calcination temperature, the peaks of both CuO and Y₂O₃ start to decrease in intensity and more Y₂Cu₂O₅ is formed. While calcination temperature increase to 950°C and 1000°C, respectively, the diffraction peaks are mainly indexed to Y₂Cu₂O₅ (JCPDS Card No. 84-1853), and a certain amount of other compound, such as YCuO₂ (JCPDS Card No. 76-1422), indicating that most of CuO and Y₂O₃ are transferred to Y₂Cu₂O₅. But the diffraction peaks of Y₂O₃ and CuO still exist, although at very small intensities.

3.3. Specific Surface Area

The specific surface areas of Y₂Cu₂O₅ photocatalysts calcined at 800 and 950°C are listed in Table 1. With the increase of calcination temperatures, the specific areas increase from 1.781 to 2.151 m² g⁻¹, while the mean pore size and the pore volume of Y₂Cu₂O₅ particles decrease. The photocatalytic activity relates to the particle size and the specific surface area for photocatalyst [24]. Although the increase of calcination temperature can cause particle size growth and aggregation among the particles, Y₂Cu₂O₅ remains the major component in the sample calcined at 950°C, which was confirmed by XRD analysis (see Figure 2). That is, most of CuO and Y₂O₃ are transferred to Y₂Cu₂O₅ at 950°C. Furthermore, in the initial synthesis stage, the as-produced Y₂Cu₂O₅ crystallite size is also small.

Table 1: Specific surface area of Y₂Cu₂O₅ nanoparticles.

Calcination temperature/°C

Surface area (m² · g⁻¹)
Pore volume (cm³ · g⁻¹)

Pore size (nm)

800
1.7811
0.01383
31.06

950
2.1509
0.00724
13.46

3.4. UV-Vis DRS Spectra

The UV-vis DRS spectra of Y₂Cu₂O₅, TiO₂ and 1 wt.% Pt/Y₂Cu₂O5 are compared in Figure 3. The Y₂Cu₂O₅ photocatalyst shows low reflectance compared to TiO₂, revealing good absorption ability. The absorption edge of Y₂Cu₂O₅ is located at 500 nm, and the energy band gap of Y₂Cu₂O₅ can be calculated to be 2.48 eV according to the following formula [25]. The results show that the Y₂Cu₂O₅ photocatalyst shows excellent response performance under visible light.

Figure 3: UV-Vis diffuse reflectance spectra of the samples.

The reflectance of 1 wt.% Pt/Y₂Cu₂O₅ decreases a little compared to Y₂Cu₂O₅, indicating the superior absorption ability of 1 wt.% Pt/Y₂Cu₂O₅ compared to Y₂Cu₂O₅, which is favorable for the enhancement of the photocatalytic activity. The order of absorption abilities of the samples is as follows: TiO₂ < Y₂Cu₂O₅ < 1 wt.% Pt/Y₂Cu₂O₅, which results from the contribution of metal nanoparticles plasma absorption peak and the formation of heterojunction called Schottky junction with the semiconductor Y₂Cu₂O₅ loaded by a certain amount of Pt.

3.5. SEM Morphology

The SEM images of the as-prepared photocatalysts calcined at 950°C were shown in Figure 4. From Figure 4(a), the TiO₂-Y₂Cu₂O₅ photocatalyst prepared by the sol-gel approach is of small spherical shape with agglomeration. The surfaces of some large Y₂Cu₂O₅ particles are covered by tiny TiO₂ particles to form heterogeneous cladding structures. Furthermore, the rough surfaces of particles can offer a large amount of sites for the photocatalytic reaction. Meanwhile, the TiO₂-Y₂Cu₂O₅ particles prepared by the solid phase method are more irregular than that prepared by the sol-gel approach (see Figure 4(b)). The Y₂Cu₂O₅ photocatalyst in Figure 4(c) is of irregular shape with less aggregation.

(a)

(b)

(c)

Figure 4: SEM images of the as-prepared photocatalysts calcined at 950°C: (a) TiO₂-Y₂Cu₂O₅ preared by the sol-gel approach; (b) TiO₂-Y₂Cu₂O₅ prepared by the solid state reaction; (c) Y₂Cu₂O₅ prepared by the solid state reaction.

3.6. Photocatalytic H₂ Evolution Activity

3.6.1. Effect of the Photocatalyst Concentration

The H₂ evolution activities of Y₂Cu₂O₅ photocatalysts calcined at 950°C with different concentrations are shown in Figure 5. The H₂ evolution activity increases with the increase of the photocatalyst concentrations. However, the H₂ evolution activity reaches its maximum at the photocatalyst concentration of 0.8 g L⁻¹. The highest photocatalytic H₂ evolution activity of 3.78 mmol h⁻¹ g⁻¹ is obtained, when the initial concentration of 0.05 mol L⁻¹ of oxalic acid is used as the sacrificial reagent. It is well known that the active center number on catalyst particle surfaces and the penetration ability of incident light in reactor are extremely important for the photocatalytic H₂ evolution activity [26]. Generally, larger catalyst concentration provides more catalytically active centers for the absorption of photons, and then more electrons and holes are generated. However, the excess photocatalyst may act as an optical filter and impede the further penetration of incident light into the suspension. We observed that the photocatalytic H₂ evolution activity increases with increasing the Y₂Cu₂O₅ concentrations when the concentration is lower than 0.8 g L⁻¹, whereas it decreases with further increasing Y₂Cu₂O₅.

Figure 5: Effect of the photocatalyst concentration on photocatalytic activity.

3.6.2. Effect of Pt Loading Content on Photocatalytic Activity

The effect of Pt loading content on the H₂ evolution activity of Pt-loaded Y₂Cu₂O₅ is shown in Figure 6. After loading Pt, the photocatalytic activity is obviously improved. The H₂ evolution rate increases from 3.78 mmol h⁻¹ g⁻¹ to 4.12 mmol h⁻¹ g⁻¹ at first and then decreases with increasing the Pt loading content and reaches the summit of 4.12 mmol h⁻¹ g⁻¹ when the Pt loading content is 1 wt.%, which increases by ca. 9% in comparison with that of the pure Y₂Cu₂O₅ (3.78 mmol h⁻¹ g⁻¹). Noble-metal nanoparticles (NPs) such as Pt, Au, and Ag can respond to visible light due to the localized surface plasmon resonance (LSPR). LSPR is produced by the collective oscillations of the surface electrons, exhibiting great potential for extending the light absorption range of semiconductors [27–29]. The enhancement of H₂ evolution activity at low fraction of Pt deposition can be ascribed to the formation of the heterojunction called Schottky junction between metal Pt and semiconductor Y₂Cu₂O₅, and the electrons and holes generated by light irradiation can be localized at Pt and Y₂Cu₂O₅, respectively. It is supposed that the loaded noble metal particles could serve as efficient electron traps, which suppressed the recombination rate of photogenerated electron-hole pairs and therefore enhanced the photocatalytic activity [30]. It is clear that there exists an optimum loading amount for Pt particles. The appropriate amount of Pt particles loaded on sample surface can trap the larger number of photoexcited electrons, resulting in the enhanced photocatalytic activity. However, excess Pt particles would serve as the recombination centers that decrease the photocatalytic activity. Moreover, the excess metal particles can mask the Y₂Cu₂O₅ surface and reduce the light absorption capability of the catalyst and therefore reduce the photoexcitation to generate the active electrons. In addition to the shadow effect (i.e., cover on the surfaces of Y₂Cu₂O₅), there are a few other possible reasons that affect the hydrogen generation due to the excessive Pt. It can also affect the crystalline formation, particle boundary and agglomeration, and so forth. Further detailed studies are in progress to understand the mechanism.

Figure 6: Effect of Pt loading content on photocatalytic activity.

3.6.3. Effect of TiO₂ Content on Photocatalytic Activity

After Y₂Cu₂O₅ is combined with TiO₂, the formation of heterojunction at the interfaces of Y₂Cu₂O₅ (p-type semiconductor) loaded with TiO₂ (n-type semiconductor) can improve the separation of electrons and holes, sequentially, enhancing the adsorbed water splitting into H₂. As shown in Figure 7, the H₂ evolution activities of TiO₂-Y₂Cu₂O₅ prepared by a sol-gel method with different TiO₂ contents are in the order of 30 wt.% TiO₂-Y₂Cu₂O₅ > 20 wt.% TiO₂-Y₂Cu₂O₅ > 40 wt.% TiO₂-Y₂Cu₂O₅ > 50 wt.% TiO₂-Y₂Cu₂O₅ > 70 wt.% TiO₂-Y₂Cu₂O₅. It can be concluded that the introduction of small amount of TiO₂ onto the Y₂Cu₂O5 surfaces results in the effective heterojunction formation with forming good transfer channels to separate electrons and holes. The TiO₂ content of 30 wt.% in Y₂Cu₂O₅ provides the highest efficiency for photocatalytic H₂ evolution. However, when the amount of TiO₂ in Y₂Cu₂O₅ increases to a certain extent, excess TiO₂ cannot contribute to the heterojunction formation, it only displays its photocatalytic activity, which can only use about 3%–5% of UV light in sunlight for the H₂ evolution.

Figure 7: Effect of TiO₂ content on photocatalytic activity.

3.6.4. Effect of Composite Method on Photocatalytic Activity

Y₂Cu₂O₅ combined with TiO₂ via two different methods of sol-gel and solid phase methods in the experiments. The effect of the two different composite methods on H₂ evolution activities of photocatalysts is shown in Figure 8. The H₂ evolution rate of TiO₂-Y₂Cu₂O₅ prepared by the sol-gel approach reaches 4.35 mmol h⁻¹ g⁻¹, which is higher than that of TiO₂-Y₂Cu₂O₅ prepared by the solid state method (3.99 mmol h⁻¹ g⁻¹). It may be due to the fact that in the sol-gel process, TiO₂ can combine sufficiently with Y₂Cu₂O₅ to form enough p-n heterojunction, and therefore improve the photocatalytic H₂ evolution activity.

Figure 8: Effect of different composite methods on H₂ evolution activity.

3.6.5. Comparison of the Best Activities of the Four Types of Photocatalysts

The best activities of the four types of photocatalysts are listed in Table 2. The photocatalytic H₂ evolution activities of TiO₂-Y₂Cu₂O₅ prepared by a sol-gel method and solid state method, respectively, Pt-Y₂Cu₂O₅ and Y₂Cu₂O₅ are in the order of TiO₂-Y₂Cu₂O₅ (sol-gel method) > Pt-Y₂Cu₂O₅ > TiO₂-Y₂Cu₂O₅ (solid state method) > Y₂Cu₂O₅. It can be concluded that Pt and TiO₂ significantly enhanced photocatalytic activity of Y₂Cu₂O₅. Pt-Y₂Cu₂O₅ photocatalyst exhibits higher photocatalytic activity compared with TiO₂-Y₂Cu₂O₅ (solid state method), but lower than that of TiO₂-Y₂Cu₂O₅ (sol-gel method), indicating that composite method has a certain effect on photocatalytic activity of TiO₂-Y₂Cu₂O₅.

Table 2: Comparison of the activities of the four types of photocatalysts.

Type of photocatalysts
Amount of H₂ evolution/mmol 

h⁻¹ g⁻¹

TiO₂-Y₂Cu₂O₅ (sol-gel method)
4.35

Pt/Y₂Cu₂O₅
4.12

TiO₂-Y₂Cu₂O₅ (solid state method)
3.99

Y₂Cu₂O₅
3.78

3.7. Durability of the Y₂Cu₂O₅ Photocatalyst

The results of the durability for the Y₂Cu₂O₅ photocatalyst are depicted in Figure 9. After four cycles of photocatalytic H₂ evolution, the photocatalyst exhibits a little loss of activity, indicating that it is relatively unstable during the light photocatalysis process. In the first run of 12 h irradiation, the photocatalytic activity of Y₂Cu₂O₅ achieves the maximum value at 6 h and then decreases little because of the continuous consumption of the sacrificial reagent oxalic acid. However, with the supplement of new oxalic acid, a little increase of the photocatalytic H₂ evolution activity appears in the third run. But the amount of H₂ generated in the 4th run is relatively lower than the others because a small Y₂Cu₂O₅ will become Y₂(C₂O₄)₃·2H₂O during the long photocatalytic reaction process.

Figure 9: Durability of the as-obtained Y₂Cu₂O₅ nanoparticles.

4. Conclusions

In summary, Y₂Cu₂O₅ has been successfully prepared via solid state reaction. The as-prepared photocatalyst shows good photocatalytic H₂ evolution activity from aqueous oxalic acid solution when the photocatalyst concentration is 0.8 g L⁻¹ under simulated sunlight irradiation. Pt-loaded Y₂Cu₂O₅ shows higher H₂ evolution activity as compared to the pure Y₂Cu₂O₅. Furthermore, the combination of TiO₂ and Y₂Cu₂O₅ can effectively enhance the photocatalytic activity, and the photocatalytic H₂ evolution activity (4.35 mmol h⁻¹ g⁻¹) is obtained for TiO₂-Y₂Cu₂O₅ with 30 wt.% TiO₂ content prepared through a sol-gel approach. The as-prepared Y₂Cu₂O₅ is a promising photocatalyst for the H₂ evolution under sunlight irradiation.

Acknowledgments

This work was financially supported by National Nature Science Foundation of China (no. 21271071), the Natural Science Research Project of Hunan Province, China (no. 2012SK3174), the Natural Science Research Project of Yueyang City (no. 2012-3), and Aid Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province.

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