Photocatalytic hydrogen generation of Pt-Sr(Zr_1-xY_x)O_3-δ-TiO₂ heterojunction under the irradiation of simulated sunlight

Abstract

The Pt-Sr(Zr_1-xY_x)O_3-δ-TiO₂ (Pt-SZYT) heterojunction photocatalysts were prepared by a photodeposition method. The composite particles were characterized by XRD, SEM, UV-Vis DRS, and PL techniques. Photocatalytic hydrogen generation in H₂C₂O₄ aqueous solution under the irradiation of simulated sunlight was used as a probe reaction to evaluate the photocatalytic activity of the photocatalysts. The effects of the content of Pt loading and the concentration of oxalic acid on the photocatalytic activity of the catalyst were discussed. The continuous photocatalytic activity of the Pt-SZYT and the relationship between PL intensity and hydrogen generation were also discussed. The results show that Pt-SZYT catalysts had high photocatalytic activity of hydrogen generation. The content of Pt loading and the concentration of oxalic acid have important influence on the photocatalytic hydrogen generation. The optimal loading content of platinum was 0.90 mass%. Under this condition, the average rate of photocatalytic hydrogen generation was 1.68 mmol•h^-1 when the concentration of oxalic acid was 50 mmol•L^-1. The higher the photocatalytic activity, the weaker the PL intensity, which was demonstrated by the analysis of PL spectra.

Keywords

heterojunction / photocatalysis / hydrogen generation / Pt / oxalic acid

Introduction

Recently, the rapid development of heterogeneous photocatalytic technique has provided a promising approach for semiconductor photocatalytic technique to convert solar energy to renewable hydrogen energy and solve the energy crisis. However, narrow light response spectrum and low quantum efficiency restrict the further development and application of photocatalytic technique. In order to settle these stumbling blocks, many researchers have made great efforts and received certain achievements [1-3], but there are still many problems that should be solved for the practical application of the photocatalytic technique. Our previous work [4] indicated that heterojunction composite with p-n structure is of excellent photocatalytic activity and can effectively restrict the recombination of photo-generated electrons and holes due to its special energy band structure and specific transmission for carries so as to improve quantum efficiency [5-8]. Here, Sr(Zr_1-xY_x)O_3-δ-TiO₂ (SZYT) heterojunction photocatalyst was modified by noble metal platinum loading, and the photocatalytic activity of the as-obtained photocatalyst was investigated for photocatalytic H₂ generation in oxalic acid aqueous solution under the irradiation of simulated sunlight.

Experiments

Photocatalysts preparation

The preparation of SZYT photocatalyst was referred to reference [4]. Here, the optimal mass ratio of the heterojunction photocatalysts (TiO₂∶Sr(Zr_1-xY_x)O_3-δ) was 70∶30, which was marked as SZYT-70 and served as the photocatalyst substrate before modification.

The preparation of Pt loaded photocatalyst was referred to reference [9]. The SZYT-70 was ultrasonic dispersed in methanol solution (V_{H_{\mathrm{2}}O}∶V_MeOH=99∶1) containing chloroplatinic acid for 15 min, and after further agitation for 30 min, the system was directly irradiated with 250W high-pressure Hg lamp in N₂ atmosphere for 10 h. The photo-reduction of chloroplatinic acid resulted in the formation of highly dispersed Pt particles deposited on the surface of SZYT-70 photocatalyst. After filtration and wash with distilled water, the Pt loaded photocatalyst was calcined at 200°C for 2 h to remove the methanol and other impurity.

Photocatalysts characterization

The morphology of the as-prepared photocatalyst was characterized with a Japan JEOL JSM-5600LV Scan Electron Microscope (SEM). The crystal phase of the as-obtained photocatalyst was identified by powder X-ray Diffraction Method (XRD, Rigaku D/max 2550 VB⁺ 18 kW) using Cu Kα radiation from 5°-85° and operating conditions of 40 kV and 300 mA with graphite as filter. The UV-Vis Diffuse Reflectance Spectrum (UV-Vis DRS) of the as-prepared photocatalysts was measured with an UV-Vis Spectrometer (UV-3101PC). The photoluminescence measurements (PL) were obtained with an LS-55 Fluorescence Spectrometer.

Photocatalytic activity testing

The photocatalytic activity of the as-prepared photocatalysts was evaluated by monitoring the amount of H₂ evolution from aqueous oxalic acid solution under simulated sunlight irradiation. The photocatalytic reaction was carried out in self-made tube-shaped quartz reactor (600 mL), including three layers connected with gas collecting devices, and water was used as the external circulation cooling and wind as the internal cooling. A 250 W xenon lamp was used as the light source (wavelength, 200-1100 nm; main wavelength, 400-700 nm; UV light<5%; without filter). Before the photocatalytic reaction, the photocatalyst was ultrasonic dispersed, and system was deaerated with nitrogen for 30 min to remove the oxygen in the system. During the photocatalytic reaction, the photocatalyst was kept in suspension state by a magnetic stirrer. After the photocatalytic reaction, the photocatalyst was centrifugal separated. The hydrogen gas product was analyzed by gas chromatography with ICD detector using a molecular sieve 13X and nitrogen as the carrier gas.

Results and discussion

Photocatalyst characterizations

Morphology analysis

The morphology of the as-prepared photocatalyst was characterized with SEM, as shown in Fig. 1. It can be seen that there exist larger aggregations in Pt loaded heterojunction. However, the surface of the catalyst particles is composed of many uniform nanoparticles to form a loose surface that provides better adsorption circumstance and photocatalytic reaction sites.

XRD analysis

Figure 2 depicts the XRD patterns of the as-prepared photocatalysts. From the diffraction lines, it can be observed that the photocatalyst is mainly composed of anatase TiO₂ and SrZrO₃. There are also some rutile TiO₂ and ZrO₂ impurity that can be detected. After Pt deposition, intensity of the diffraction peaks of SrZrO₃ decreases to a certain extent. No diffraction peaks of Pt were observed in the XRD patterns. This is because the platinum particles were highly uniform dispersed on the surface of the photocatalyst particles, and the low content of Pt (w(Pt)=0.90%) is under the detection limit.

UV-Vis DRS analysis

The UV-Vis DRS of the as-prepared Pt-SZYT-70, TiO₂, and Sr(Zr_1-xY_x)O_3-δ are described in Fig. 3. It is clear that TiO₂ shows light absorption below 400 nm. The slight absorption of Sr(Zr_1-xY_x)O_3-δ at long wavelength spectrum is due to the formation of defects by Y³⁺ doping [7]. After Pt loading, the heterojunction shows obvious absorption at long wavelength spectrum, which indicates that the heterojunction can exhibit certain visible light response, and the deposition of Pt on the surface of photocatalyst particles can help to the absorption and reflection of light. Mei et al. [10] showed that the deposition of noble metal particles on the surface of photocatalyst particles can form a narrow metal band potential between the band gaps of the composite semiconductors, and this narrow metal band potential can act as transfer location to absorb visible light with long wavelength, enhance the visible light absorption ability, and enlarge the light response spectrum.

PL analysis

PL is an effective method for researching on the electronic structure and optical properties of the semiconductor nanoparticles and for showing the information of the transfer, trap, and recombination of photo-generated carriers [11, 12]. Fig. 4 presents the PL spectra of TiO₂, SZYO, SZYT-70, and Pt-SZYT-70. The excitation wavelength is 250 nm, and the excitation and emission slits are all 5 nm.

As shown in Fig. 4, TiO₂ exhibits high PL spectrum intensity at 360-420(460)? nm, and there are obvious peaks at 396 and 420 nm. The peak at 396 nm responds to the band gap of TiO₂ (E_g=3.2 eV), resulting from the recombination of photo-generated holes and electrons, which directly go back from conduct band to valence band [13]. The peak at 420 nm is because that the excited electrons at higher conduct band first transit to metastable state on the conduct band and then transit to a valence band to recombine with the excited holes [14]. The PL peaks of TiO₂ are broadened, which is because the defect structure in nanoparticles can form defect energy band in band gap, and the excited electrons are first trapped by the defect energy band in the transition process and then transited to a valence band [15]. The PL peaks of SZYO are weak. This is mainly because that the band gap of SrZrO₃ is broad (E_g=5.0 eV), and the energy of the excitation photons is not enough to excite the electrons from valence band to conduct band. As a result, SrZrO₃ only shows weak PL spectrum at 350~430 nm resulted from the defect structure. The PL spectrum of SZYT-70 shows that the coupling of TiO₂ and SZYO can reduce the PL intensity, which means that the formation of p-n heterojunction structure can help to separate the photo-generated electron and hole, so as to reduce the recombination of the photo-generated carriers [4]. Pt-SZYT-70 shows much lower PL intensity, suggesting that the deposition of Pt particles on the surface of catalyst particles further enhances the separation of photo-generated electron and hole and reduces the recombination of the photo-generated carriers.

Photocatalytic H₂ evolution activity

Effect of photocatalyst type

Figure 5 presents the effect of photocatalyst type on the H₂ evolution activity (insert is the local magnification of three similar curves). As can be seen, without oxalic acid, there is nearly no H₂ generated by adding SZYT-70 into distilled water under simulated sunlight irradiation. On the other side, without photocatalyst, there is also only trace H₂ generated from oxalic acid solution under simulated sunlight irradiation for 8 h, whereas the addition of SZYT-70 can improve the H₂ evolution rate to a certain extent, and the addition of Pt-SZYT-70 can effectively improve the H₂ evolution rate (the average rate is 1.68 mmol•h^-1). The loading of Pt on the surface of the photocatalyst particles can build a Schottky energy barrier, promoting the separation of photo-generated carriers. Besides, the existence of Pt can also reduce the overpotential of hydrogen on the surface of the photocatalyst particles [16]. There is almost no H₂ generated without oxalic acid in solution, indicating that the addition of electron donor is crucial. Industrial pollutant oxalic acid is a low-cost and excellent electron donor (E₀ (CO₂/HO₂CCO₂H)=-0.49 V). Oxalic acid can reversibly(? irreversibly) combine with the photo-generated holes or surface hydroxyl and restrict the recombination of electrons and holes, so as to efficiently enhance the photocatalytic H₂ evolution efficiency.

The photocatalytic mechanism of the Pt loaded composite photocatalyst is explored as follows: Without Pt loading, when the p-n heterojunction composite photocatalyst is irradiated, there exist surplus holes and surplus electrons on the p-type semiconductor Sr(Zr_1-xY_x)O_3-δ and n-type semiconductor TiO₂, respectively. The photo-generated electrons and holes temporarily separate with the effect of heterojunction, but they recombine soon. Moreover, the potential between the sides of heterojunction with carriers enrichment can hinder the production of photo-generated charges. However, the loaded small Pt clusters on the TiO₂ surface act as an improvement function for the transfer of the photo-generated charges. Because the work function of Pt (Φ_m) is higher than that of TiO₂ (Φ_s), the electrons on the TiO₂ continuously transfer to the loaded Pt; meanwhile, the contact of TiO₂ and Pt can bend the energy band to create a wasting layer and form a shallow trap Schottky energy barrier for electron trapping between the interface of Pt-catalyst. Therefore, when the composite photocatalyst was irradiated by simulated sunlight, the small Pt clusters on the n-type semiconductor TiO₂ act as good trap and collector to collect the surplus electrons, which react with the absorbed H⁺ to generate H₂. The Pt loading can help to the separation and transfer of photo-generated charges, effectively reduce the potential in the heterojunction, prolong the life-span of photo-generated carriers, restrict the recombination, and improve the quantum efficiency of the photocatalytic reaction.

Effect of Pt loading content

From the discussion above, it can be concluded that Pt loading is significant for the improvement of the H₂ evolution rate. In order to get the optimal Pt loading content, the H₂ evolution activity experiments of Pt loaded heterojunction with different Pt loading content were performed under the same conditions as mentioned above. From Table 1, it can be seen that the heterojunction shows the best H₂ evolution activity at Pt loading content w(Pt)=0.90%, and the total H₂ evolution is 10.1 mmol after irradiated for 6 h. Meanwhile, according to the PL analysis of different Pt loading content in Fig. 6, the PL intensity of the catalyst shows the “decrease-increase-decrease” trend with the increase of Pt loading content. The change of the H₂ evolution activity of the Pt loaded catalyst is adverse as a whole to that of the PL intensity. The PL spectrum results from the recombination of the electron-hole [17-19], and the PL intensity of the catalyst reflects the recombination extent of the photo-generated carriers. Below the optimal content, the increase of Pt loading content can reduce the recombination of the photo-generated carriers exhibiting the decrease of PL intensity and increase of catalytic activity. Nevertheless, overmuch Pt particles will act as the recombination center of the photo-generated carriers; meanwhile, the light scatter caused by the overmuch Pt particles may reduce the light absorption by composite catalyst and depress the adsorption of oxalic acid on the surface of the catalyst, resulting in the decrease of the photocatalytic activity.

Effect of oxalic acid concentration

From the discussion above, it concludes that the oxalic acid concentration is also significant to the H₂ evolution activity. Therefore, the effect of oxalic acid concentration was also investigated, as illustrated in Fig. 7. Because the continuous change of the oxalic acid concentration during the reaction, the reaction time should be as short as the product can be detected so as to determine the initial reaction rate. It is clear that the H₂ evolution rate is independent of the oxalic acid concentration when c₀(oxalic acid)>10.0 mmol•L^-1, whereas the H₂ evolution rate reduces with the decrease of the oxalic acid concentration when c₀(oxalic acid)<10.0 mmol•L^-1. The effect of oxalic acid concentration on the H₂ evolution activity is consistent with Langmuir-type isotherm, implying that the decomposition of oxalic acid takes place in the adsorbed phase on the surface of the catalyst.

Durability of the photocatalysts

In order to test the durability of the photocatalysts, w(0.90%)Pt-SZYT-70 was chosen as the representation to perform the continuous photocatalytic H₂ evolution experiment, as shown in Fig. 8. After the former run, certain amount of oxalic acid was added directly into the reaction system to recover the oxalic acid concentration to its initial value kept in the dark with agitation for 30 min, and then turned on the lamp to continue the next H₂ evolution experiment.

The results show that the H₂ evolution activity decreases a little during the continuous H₂ evolution experiment. It may be due to the rudimental product produced in the former run on the surface of the catalyst if it had not been desorbed completely before the next run. It affects the transfer of the charges and carriers on the surface of the catalyst, the further adsorption of the reactant, and the latter photocatalytic reaction, resulting in the decrease of the photocatalytic activity [20]. After the third run, the reactor was ultrasonicly cleaned for 10 min and then the reaction was continued by adding new oxalic acid. It can be seen that the H₂ evolution rate of the fourth run is higher than that of the second run and third run, indicating that the adsorption of catalyst on the inner wall of the reactor and the gather between the catalyst particles affect the H₂ evolution activity. Though the initial H₂ evolution rate decreases to a certain extent in the fourth run, the total amount of H₂ evolution (2.15 mmol) for 4 h is a little higher than that of the first run (2.04 mmol). It may be because the unreacted oxalic acid in the former runs can act as the electron donor in the fourth run, enhancing the total amount of H₂ evolution. The results show that the Pt loaded catalyst is a stable photocatalyst that has high catalytic performance and has a good life span.

Conclusions

(1) The as-obtained Pt-SZYT-70 photocatalyst shows excellent photocatalytic H₂ evolution activity in oxalic acid aqueous solution under the irradiation of simulated sunlight. The average H₂ evolution rate of w(0.90%)Pt-SZYT-70 was 1.68 mmol•h^-1 when the mass concentration of photocatalyst and the concentration of oxalic acid were 1.0 g•L^-1 and 50 mmol•L^-1, respectively.

(2) The content of Pt loading and the concentration of electron donor oxalic acid show various effects on the photocatalytic activity of the catalyst. The optimal loading content of platinum was 0.90 mass%, and the higher the photocatalytic activity, the weaker the PL intensity.

(3) The continuous photocatalytic activity experiment indicates that the as-obtained Pt-SZYT-70 photocatalyst is of stable catalytic performance and with long life span.

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