Hydrogen Generation over RuO2 Nanoparticle-Decorated LaNaTaO3 Perovskite Photocatalysts under UV Exposure.

The efficacy of LaNaTaO3 perovskites decoration RuO2 at diverse contents for the photocatalytic H2 generation has been explored in this study. The photocatalytic performance of RuO2 co-catalyst onto mesoporous LaNaTaO3 was evaluated for H2 under UV illumination. 3%RuO2/LaNaTaO3 perovskite photocatalyst revealed the highest photocatalytic H2 generation performance, indicating that RuO2 nanoparticles could promote the photocatalytic efficiency of LaNaTaO3 perovskite significantly. The H2 evolution rate of 3%RuO2/LaNaTaO3 perovskite is 11.6 and 1.3 times greater than that of bare LaNaTaO3 perovskite employing either 10% CH3OH or pure H2O, respectively. Interestingly, the photonic efficiency of 3%RuO2/LaNaTaO3 perovskite was enhanced 10 times than LaNaTaO3 perovskite in the presence of aqueous CH3OH solutions as a hole sacrificial agent. The high separation of charge carriers is interpreted by the efficient hole capture using CH3OH, hence leading to greater H2 generation over RuO2/LaNaTaO3 perovskites. This is attributed to an adjustment position between recombination electron-hole pairs and also the reduction of potential conduction alignment as a result of RuO2 incorporation. The suggested mechanisms of RuO2/LaNaTaO3 perovskites for H2 generation employing either CH3OH or pure H2O were discussed. The photocatalytic performances of the perovskite photocatalyst were elucidated according to the PL intensity and the photocurrent response investigations.

Introduction

With the growth of the industrial and scientific community, the photocatalyst as a favorable semiconductor material is considered as a promising and hot theme of research studies owing to its wide implementations in considerable fields, particularly for energy saving and environmental protection.[1−3] Photocatalytic production of molecular hydrogen through semiconductor materials as efficient photocatalysts is considered as a promising avenue to produce sustainable and clean energy,[1−3] and promoting semiconductor materials under visible light with a high photonic efficiency for the conversion of solar energy to molecular hydrogen is ultimately desired for potential applications.[4−6] Recently, water splitting to generate molecular hydrogen employing perovskite oxide materials (ABO3) has attracted increasing attention with a high photonic efficiency. Among ABO3 perovskite oxide materials, the NaTaO3 photocatalyst has been realized for hydrogen generation from H2O using UV irradiation.[7−16] The band gap of NaTaO3 is 4.0 eV, and it can be synthesized by diverse approaches, for instance, solid-state,[7−9,13,16] molten salt,[17] sol–gel,[11] and hydrothermal methods.[10,13−15]

To promote the photocatalytic activity of NaTaO3 particles, numerous scientists have made great effort to employ other synthetic avenues to obtain NaTaO3 nanoparticles as efficient photocatalysts. NaTaO3 as a colloidal array was synthesized using carbon mesopores as a direct structure agent for casting that was reproduced using silica nanosphere configuration.[14] The mesoporous carbon matrix was eliminated by calcination, and then NaTaO3 nanoparticles as a colloidal array were obtained with a 34 m2 g–1 surface area and a 20 nm particle size. The obtained NaTaO3 prepared by this approach exhibited a 3 times higher photocatalytic efficiency than that prepared from the traditional hydrothermal synthesis for overall water splitting.[14] NaTaO3 nanoparticles with ∼25 nm crystallite size, synthesized by an exo-template method, exhibited an ∼20 times higher hydrogen production rate than those synthesized using the solid-state approach.[16]

The recombination of photogenerated holes and electrons of large NaTaO3 nanoparticles was faster than those in smaller NaTaO3 nanoparticles with high crystallinity. On the other hand, much effort was made to perform a greater photonic efficiency of lanthanide-doped NaTaO3.[18−22][18−22] The photonic efficiency of NaTaO3 is greatly promoted by employing a co-catalyst such as Ru, NiO, Pt, or Rh, loaded on the NaTaO3 surface.[23−28] In general, loading of co-catalysts at different contents onto the photocatalyst surface led to a significant boost of molecular H2 production compared with pure photocatalysts. A co-catalyst serves as a trapping agent of electrons, which produces a prolonged lifetime of photoinduced charge carriers, reducing their recombination rate. In terms of the co-catalyst-loaded semiconductor photocatalyst preparation, it is concluded that the crystalline structure of the prepared photocatalysts is very susceptible to synthetic approaches such as solid-state,[11,29,30] solvothermal,[30] sol–gel,[30] hydrothermal,[12,31,32] alkalide reduction,[33] flux,[34] and electrospinning methods.[35] NaTaO3-based photocatalysts were synthesized via the traditional solid-state and sol–gel approaches. The conventional solid-state approach needs elevated annealing temperatures to produce NaTaO3 with orthorhombic structure, whereas the sol–gel avenue requires low temperatures during the preparation to obtain NaTaO3 with a monoclinic structure.[10,36,37] Also, NaTaO3-based photocatalysts could be prepared by the hydrothermal process.[37,38] Efficient separation and inhibition recombination of charge carriers are paramount for H2O splitting to create molecular H2. In addition, separation and fabrication of active sites for H2 generation are indispensable. Obviously, incorporation of RuO2 co-catalysts onto NaTaO3 perovskite surface is substantial for boosting their photonic efficiency for the production of molecular hydrogen. The photonic efficiency of NaTaO3-based photocatalysts could be considerably calculated by doping foreign ions in the NaTaO3 lattice.

Therefore, in the present proposal, synthesis of mesoporous RuO2/LaNaTaO3 perovskites at different RuO2 contents for molecular H2 generation was investigated employing the CH3OH/H2O system. The H2 evolution rate of 3%RuO2/LaNaTaO3 perovskite is 11.6 and 1.3 times greater than that of the LaNaTaO3 perovskite employing 10% methanol pure H2O, respectively. Interestingly, the photonic efficiency of 3%RuO2/LaNaTaO3 perovskite was enhanced 10 times compared to LaNaTaO3 perovskite in the presence of aqueous CH3OH solutions. The suggested mechanisms of RuO2/LaNaTaO3 perovskites for H2 evolution employing aqueous CH3OH solutions and pure H2O were discussed. The photocatalytic performances of perovskite photocatalyst were evaluated according to the PL intensity and the photocurrent response investigations.

Results and Discussion

Perovskite Investigations

X-ray diffraction patterns of LaNaTaO3and RuO2/LaNaTaO3 perovskites at different RuO2 contents were investigated, as shown in Figure . The XRD pattern of LaNaTaO3 perovskite was assigned as the monoclinic structure of the synthesized LaNaTaO3 perovskite. The peaks at 22.91, 32.36, 40.04, 46.71, 52.61, 58.17, 68.08, 72.9, and 77.4° (Figure a) have corresponded to the planes of (100), (101), (111), (200), (102), and (121) (JCPDS no. 74-2478). After addition of RuO2 at different contents of 0.5, 1, 3, and 5%, the intensity of the mean peak is gradually decreased with increasing RuO2 content (Figure b–d). It is documented that the ionic radii of La3+ (1.36 Å) and Na+ (1.39 Å) ions are equivalent.[39] In addition, the ionic radius of the Ta5+ ion (0.64 Å) is notably smaller than that of the La3+ ion (1.032 Å).[39] If Ta5+ ions were replaced with La3+ ions at the B site position in the perovskite structure, a considerable shift should be recognized. Interestingly, there was no crystalline phase involving RuO2 at different RuO2 concentrations of 0.5–5% that could be detected, indicating that RuO2 nanoparticles are highly contributed over the mesoporous La0.02Na0.98TaO3 network with a small particle size. This is attributed to the adsorption of the Ru(III)–acetylacetonate complex onto the La0.02Na0.98TaO3 surface, and then the obtained powder was annealed at 450 °C and the adsorbed Ru(III)–acetylacetonate complex was decomposed to RuO2 nanoparticles onto the surface of the LaNaTaO3 perovskite network and inside the walls of the pores. The possibility of interaction (substitution of Ru4+ for Ta5+) between equivalent ionic radii materials Ru4+ (0.62 Å) and Ta5+ (0.64 Å) could partly explain this observation.

Figure 1

X-ray diffraction peaks around 32.5° of (a) LaNaTaO3 and LaNaTaO3 doped with RuO2: (b) 1%, (c) 3%, and (d) 5%.

Figure shows SEM images of (a) bare LaNaTaO3 perovskite and RuO2/LaNaTaO3 at 0.5% (b), 1% (c), 3% (d), and 5% (e) loadings. The ordered surface nanostructure of the LaNaTaO3 perovskite was self-constructed as shown in Figure a. The particle sizes of the LaNaTaO3 perovskite were enlarged on increasing the RuO2 content from 0.5 to 5% (Figure b–e). These characteristics are advantageous in terms of small particle size and high crystallinity for the enhanced photocatalytic efficiency of perovskite photocatalysts. EDS analysis showed the presence of Ru, La, Na, and O and proved that the RuO2/LaNaTaO3 perovskite consisted of the precursor ratios employed in the starting mixtures. The EDS quantitative analysis of 1%RuO2/LaNaTaO3 shows that the weight percents of Ru, La, Na, Ta, and O are 0.08, 0.42, 16.91, 18.90, and 63.68, respectively. Figure displays the TEM images of the structure and morphology of mesoporous LaNaTaO3, and 3%RuO2/LaNaTaO3 perovskite. The LaNaTaO3 perovskite particles were highly dispersed with uniform shape and size (∼10 nm) as clearly displayed in Figure a. The morphology of the 3%RuO2/LaNaTaO3 NPs is similar to the bare LaNaTaO3 perovskite in terms of shape and size (Figure b). The atomic planes of RuO2 and NaTaO3 NPs were estimated at 3.2 and 3.80 Å, respectively, which matches to the lattice spacing of (110) and (111), as obviously depicted in Figure c, and the NaTaO3 and RuO2 NPs are connected, along with the well matching of selected area electron diffraction of NaTaO3 perovskite with the orthorhombic crystal (Figure d). The high crystallinity of the synthesized RuO2/LaNaTaO3 perovskite was confirmed by clear lattice spacing of atomic planes (Figure d).

Figure 2

Scanning electron microscope images of (a) LaNaTaO3 and LaNaTaO3 doped with RuO2: (b) 0.5%, (c) 1%, (d) 3%, and (e) 5%. (f) EDS pattern of 1%RuO2-doped La/NaTaO3.

Figure 3

TEM images of bare LaNaTaO3 (a) and 3%RuO2/LaNaTaO3 nanocomposite (b). HRTEM image of mesoporous 3%RuO2/LaNaTaO3 nanocomposite (c). Selected area electron diffraction of 3%RuO2/LaNaTaO3 (d).

Nitrogen adsorption isotherms of the bare LaNaTaO3 and 3%RuO2/LaNaTaO3 perovskites are depicted in Figure . The adsorption isotherms of both LaNaTaO3 and 3%RuO2/LaNaTaO3 perovskites are of typical reversible type IV. The inflection sharpness was obtained at relative pressures in the capillary condensation range of 0.45–0.7, resulting in mesostructured materials. The mesopores were formed as a result of interparticle voids between prepared nanoparticles. The mesoporosity can be explained by the formation of irregular voids between LaNaTaO3 particles. In addition, the existence of voids among LaNaTaO3 NPs participates in boosting the surface area of the prepared LaNaTaO3 photocatalyst. The BET surface area of 3%RuO2/LaNaTaO3 perovskite was calculated to be 34 m2 g–1.

Figure 4

N2 sorption isotherms of the mesoporous LaNaTaO3 and 1%RuO2/LaNaTaO3.

XPS spectroscopy was used to examine the states and composition of the 1%RuO2/LaNaTaO3 photocatalyst as displayed in Figure . Figure a shows two peaks located at 838.45 and 834.45 eV for La 3d3/2 and La 3d5/2, respectively, which are comparable to the existence of La3+ in LaNaTaO3. As displayed in Figure b, the Ru 3d spectrum exhibited two mean peaks centered at 284.44 and 279.62 eV referred to Ru 3d3/2 and Ru 3d5/2, respectively, emphasizing the presence of Ru in the Ru4+ form. Figure c shows two peaks at 1 and 27.9 eV for the Ta 4f spectrum, confirming the existence of Ta in the Ta5+ form.[40] It is attributed to one mean peak for the O 1s spectrum at 530.1 eV, which is confirmed to the presence of O atoms in the LaNaTaO3 crystal lattice; besides, there are other two peaks centered at 531.4 and 532.5 eV, leading to the presence of –OH surface and adsorbed O (Figure d), respectively.[41] The Na 1s peak is located at ∼1071.3 eV, identifying the Na+ oxidation state, as obviously seen in Figure e. The XPS results confirmed that the prepared perovskite was composed of Ru4+, Na+, La3+, Ta5+, and oxygen in the crystal lattice, and their atomic percentages were determined to be approximately 0.98, 7.87, 1.98, 70.82, and 19.05%, respectively.

Figure 5

XPS analysis of 1%RuO2/LaNaTaO3 exhibiting the high-resolution spectra for La 3d (a), Ru 3d (b), Ta 4f (c), O 1s (d), and Na 1s (e).

The UV–vis spectra of bare LaNaTaO3 and RuO2/LaNaTaO3 perovskites were examined to demonstrate the effects of Ru4+ doping on the band gap structure modulation of the LaNaTaO3 perovskites shown in Figure . The DRS of the prepared photocatalysts displayed a broad absorption in the UV region (250–320 nm), leading to the electronic transformation from O 2p to the Ta 5d orbitals. The absorption spectrum of Ru4+-doped LaNaTaO3 perovskite is different from that of LaNaTaO3 perovskite (Figure a). The Ru4+-doped LaNaTaO3 perovskite sample revealed a superficial peak in the range of 450–600 nm with higher intensities (Figure a). The direct optical band gap energy of RuO2/LaNaTaO3 photocatalysts at different RuO2 contents can be calculated as follows: αhν = A(hν – Eg)1/2, where α, Eg, hν, A, and n are the absorption coefficient, band gap energy, photon energy, constant, and incident light, respectively.[24] Band gap energy was estimated to be ∼4.08–4.01 eV corresponding to the absorption in the 307–310 nm region with the increase of RuO2 content as depicted in Figure b. The calculated band gap energies of the RuO2 loading LaNaTaO3 perovskite photocatalysts with various RuO2 contents are listed in Table . The addition of RuO2 did not change the absorption band for LaNaTaO3; thus, the band gap values are very close.

Figure 6

(a) Diffuse reflectance spectra of LaNaTaO3 and LaNaTaO3 doped with RuO2 at varying contents. (b) Plot of transferred Kubelka–Munk versus energy of LaNaTaO3 and LaNaTaO3 doped with RuO2 at varying contents.

Table 1

Hydrogen Production from Methanol and Water over Mesoporous RuO2/LaNaTaO3 Photocatalyst at Different RuO2 Contents

		H2 evaluation rate (μmol h–1)		PE (%)
photocatalysts	band gap (eV)	H2O	CH3OH	H2O	CH3OH
LaNaTaO3	3.98 ± 01	0.00	0.99	0.00	0.02
0.5%RuO2/LaNaTaO3	4.08 ± 01	0.88	10.96	0.02	0.19
1%RuO2/LaNaTaO3	4.08 ± 01	1.07	9.66	0.02	0.16
3%RuO2/LaNaTaO3	4.18 ± 01	1.26	11.54	0.02	0.20
5%RuO2/LaNaTaO3	4.18 ± 01	1.29	8.81	0.17	0.15

Photocatalytic Performance

Photocatalytic tests were conducted on mesoporous RuO2/LaNaTaO3 perovskites for H2 generation from either CH3OH or pure H2O. The RuO2 loading LaNaTaO3 perovskite at different contents (0–5%) was assessed for H2 generation from either pure H2O or CH3OH (10 vol %). The illumination time of the photocatalytic H2 evolution was conducted over the obtained photocatalysts employing pure H2O and CH3OH, as illustrated in Figure a,b. The findings exhibited that the H2 evolution immediately started as the UV lamp was turned on. H2 evolution rates were reached to steady state within 30 min. At this stage, the photocatalytic reaction was illuminated for 6 h to detect and determine the H2 evolution rate. Finally, the UV lamp was turned off, and the H2 evolution abruptly declined to reach the baseline (Figure a,b). The H2 evolution rates were calculated by subtracting the baseline and average of the values obtained from the curve with almost steady rates of H2 evolution, as shown in Figure . The findings indicated that there was no H2 evolution without using the photocatalysts. It can be seen that the mesoporous LaNaTaO3 perovskite photocatalyst exhibits the minimum photocatalytic performance. The H2 evolution ultimately increased when RuO2 was grafted onto LaNaTaO3 perovskite surface. In addition, the photocatalytic efficiency of the LaNaTaO3 perovskite was enhanced with the increment of the RuO2 content, achieving the highest H2 evolution at 3% RuO2.

Figure 7

Time course of photocatalytic H2 evolution over LaNaTaO3 perovskite loading different RuO2 contents (0.5, 1, 3, and 5%), from pure water (a) and 10% methanol (b).

Figure a exhibits H2 evolution rates evolution over LaNaTaO3 perovskite loading different RuO2 contents (0, 0.5, 1, 3, and 5%), from pure H2O and from 10% CH3OH. The H2 evolution rate was increased from 0 to 1.29 μmol h–1 when pure H2O was used with the increase of RuO2 content from 0 to 5%. However, in the case of 10% methanol, the H2 evolution rate was improved from 0.99 to 11.54 μmol h–1 with the increase of RuO2 content from 0 to 5%. Interestingly, the H2 evolution rate of 3%RuO2/LaNaTaO3 perovskite is the fastest among all of the synthesized photocatalysts. Besides, the H2 evolution rate of 3%RuO2/LaNaTaO3 perovskite is 11.6 times greater than that of LaNaTaO3 employing 10% methanol; however, in the case of pure H2O, the H2 evolution rate of 3%RuO2/LaNaTaO3 perovskite was enhanced 1.3 times than LaNaTaO3. Also, the H2 evolution rate of 3%RuO2/LaNaTaO3 employing 10% methanol is 9 times higher than employing pure H2O. Figure b shows the photonic efficiency of RuO2/LaNaTaO3 perovskite at different RuO2 contents (0.5, 1, 3, and 5%), from pure water and 10% methanol. The results revealed that the photonic efficiency was increased from 0 to 1.5 with the increase of the RuO2 content from 0 to 5% employing pure water; however, the photonic efficiency was increased from 0.2 to 2% with increasing the RuO2 content from 0 to 3%; then, it was decreased to 1.7% at 5%RuO2 using 10% methanol. Interestingly, the photonic efficiency of 3%RuO2/LaNaTaO3 perovskite was enhanced 10 times than bare LaNaTaO3 perovskite. Table summarizes the comparison between the synthesized photocatalysts and other samples for photocatalytic H2 generation.

Figure 8

(a) H2 evolution rates evolution over LaNaTaO3 loading different RuO2 contents (0.5, 1, 3, and 5%) from pure water and from 10% methanol. (b) Photonic efficiency of LaNaTaO3 and RuO2 loading LaNaTaO3 at different contents (0.5, 1, 3, and 5%) from pure water and from 10% methanol.

Table 2

Comparison between Photocatalytic H2 Generation over the Synthesized Photocatalyst in the Present Work and Other LaNaTaO3 Photocatalysts

photocatalysts	reaction medium	light source	generation H2 rate	references
NiO/LaxNa1–xTaO3	CH3OH	UV	26.94 mmol g–1 h	(23)
2%Ag/La0.02Na0.98TaO3	glycerol	UV	332.43 μmol g–1 h–1	(24)
1%Pt/La0.02Na0.98TaO3	glycerol	UV	86.16 μmol g–1 h–1	(25)
0.6%Nd2O3/LaNaTaO3	glycerol	UV	95 μmol g–1 h–1	(26)
1%In2O3/La0.02Na0.98TaO3	glycerol	UV	235 μmol g–1 h–1	(27)
3%RuO4/La0.02Na0.98TaO3	CH3OH	UV	11.54 μmol h–1	this work

It is supposed that the high RuO2 content can cover LaNaTaO3 perovskite surface, suggesting reduction of the photoexciting capability of the LaNaTaO3 perovskite photocatalyst.[42] In addition, it could be caused by the agglomeration and growth of RuO2 onto mesoporous LaNaTaO3 perovskite surface and hence weakened the role of the co-catalyst.[24,43] The 3%RuO2/LaNaTaO3 perovskite revealed the maximum photocatalytic performance among all of the synthesized photocatalysts, indicating that the incorporation of RuO2 could promote the photocatalytic activity of LaNaTaO3 perovskite significantly. The improved photocatalytic performance of the RuO2/LaNaTaO3 perovskite photocatalyst was explained by the effective separation of charge carriers in the present RuO2/LaNaTaO3 perovskite that is accomplished by exciting the electrons from the VB to the CB of LaNaTaO3. Then, the photogenerated electrons migrate to RuO2 NPs (Scheme ). The addition of RuO2 nanoparticles onto the LaNaTaO3 perovskite leads to prepared materials possessing Brønsted acids with the distinguishing interaction of the Ru–O···H bond, However, the acid strength onto the surface of RuO2 attributes to its capability to eliminate a proton. It is documented that RuO2 possesses the highest electronegativity, small particle size, and the highest oxidation state (IV).[44] Therefore, RuO2 has the strongest Brønsted acid and shows the maximum photocatalytic performances for H2 evolution in both CH3OH solution and pure H2O due to the prohibition of the unwanted backreaction of O2 with H2 resulting in H2O onto the RuO2 surface.[44,45]

Scheme 1

Schematic Demonstration of Hydrogen Production over Mesoporous RuO2/La0.02Na0.98TaO3 Photocatalyst in the Presence of Methanol

To confirm the reason for the promotion of the photocatalytic activity of RuO2/LaNaTaO3 perovskites, photocurrent response and photoluminescence (PL) were measured. The photocurrent response over LaNaTaO3 and RuO2/LaNaTaO3 perovskites is depicted in Figure a in the dark and under illumination. In the dark, there was no response current; however, upon illumination, bare LaNaTaO3 perovskite revealed the lowest photoresponse. With the increase of RuO2 from 1 to 3%, the photocurrent intensity was increased gradually decreased at 5%RuO2/LaNaTaO3 perovskite, implying the high tendency upon illumination to facilitate the separation of photo-created electrons and holes. This result is consistent and explained the photocatalytic H2 generation. The PL of bare LaNaTaO3 and RuO2/LaNaTaO3 perovskites at diverse RuO2 percentages is displayed in Figure b. The PL peak of bare LaNaTaO3 perovskite was assigned at λ ∼ 469.34 nm with a higher PL intensity. However, the PL intensity of the RuO2/LaNaTaO3 perovskites revealed a lower intensity than bare LaNaTaO3 perovskite. The RuO2/LaNaTaO3 perovskites exhibited a low exciton emission owing to the expedition of charge carrier separation. Interestingly, the PL intensity of RuO2/LaNaTaO3 perovskites decreased with the increase of RuO2 content, presenting photoinduced electron transfer from the CB of LaNaTaO3 perovskites to the close contact RuO2 NPs.

Figure 9

(A) Photocurrent density response of (a) LaNaTaO3 and LaNaTaO3 doped with RuO2: (b) 1%, (c) 3%, and (d) 5%. (B) PL spectra of (a) LaNaTaO3 and LaNaTaO3 doped with RuO2: (b) 1%, (c) 3%, and (d) 5%.

The mechanism of highly effective H2 evolution over RuO2/LaNaTaO3 photocatalysts in pure H2O and CH3OH was demonstrated in Scheme . After UV illumination, the generated electrons and holes move in a prolonged space to reach the active sites of the RuO2 surface. As the RuO2 nanoparticle is decreased in terms of size, the probability of the surface reaction of the generated electrons and holes with adsorbed methanol and water molecules is boosted compared to that of the bulk recombination of charge carriers.[27] At the conduction band of LaNaTaO3 perovskite, the adsorbed H2O molecules can be effectively reduced to molecular H2 onto RuO2 nanoparticles. The ordered surface RuO2/LaNaTaO3 perovskite with a small particle size has promoted the suppression of carrier recombination and of active site separation to prohibit the backward reaction of O2 with H2, indicating the highly effective H2O splitting. In the case of CH3OH as a sacrificial agent, the mechanism is not clear because it is not determined whether the movement of electrons from the reduction of •CH2OH radical or conduction band of LaNaTaO3 perovskite is the rate-limiting step or if the photocatalytic activity might be determined by transporting hole to the CH3OH.[44,46]

Conclusions

Synthesis of mesoporous RuO2/LaNaTaO3 perovskites at different RuO2 contents for the generation of molecular H2 was investigated employing the CH3OH/H2O system. The XRD findings show that mesoporous LaNaTaO3 perovskite was formed as the monoclinic structure. The adsorption isotherms of LaNaTaO3 perovskite type IV result in a mesopores structure. The H2 evolution rate in the case of pure H2O was increased from 0 to 1.29 μmol h–1 with the increase of RuO2 content from 0 to 5%. However, in the case of 10% methanol, the H2 evolution rate was increased from 0.99 to 11.54 μmol h–1 with the increase of the RuO2 content from 0 to 5%. The H2 evolution rate of 3%RuO2/LaNaTaO3 is the fastest among all of the synthesized photocatalysts. The H2 evolution rate of the 3%RuO2/LaNaTaO3 perovskite is 11.6 times higher than that of LaNaTaO3 employing 10% methanol; however, in the case of pure H2O, the H2 evolution rate of the 3%RuO2/LaNaTaO3 perovskite was enhanced 1.3 times than LaNaTaO3. The H2 evolution rate of the 3%RuO2/LaNaTaO3 perovskite employing 10% methanol is 9 times higher than employing pure H2O. The photonic efficiency of the 3%RuO2/LaNaTaO3 perovskite was enhanced 10 times than LaNaTaO3.

Experimental Section

Materials

Ruthenium(III) acetylacetonate, Ru(acac)3, sodium acetate CH3COONa, CH3COOH, Ti(OC(CH3)3)4 (TBOT), lanthanum nitrate, La(NO3)3·xH2O, tantalum(V) chloride, TaCl5, HCl, CH3OH, F-127 pluronic (EO106-PO70EO106, MW 12 600 g mol–1), and C2H5OH were procured from Sigma-Aldrich.

Preparation of Mesoporous RuO2/LaNaTaO3 Perovskites

Mesoporous LaNa1–TaO3 (x = 0.02) perovskites were synthesized via a wet chemical approach employing F127 copolymer as a proper template. La and Na nanoparticles were homogeneously distributed into the tantalum oxide framework utilizing the assembly approach. To reduce possible changeability, the molar ratio of Ta5+:F127:C2H5OH:HCl:CH3COOH was maintained at 1:0.02:50:2.25:3.75. F-127 polymer surfactant (1.6 g) is added to 30 mL of C2H5OH using a magnetic stirrer at room temperature for 60 min; afterward, 0.74 mL of HCl and 2.3 mL of CH3COOH were added to the clear solution F127 in ethanol, and then 1.82 g of TaCl5 and 0.047 g of La(NO3)3·xH2O were added to the above mixture. Afterward, 3.5 g of CH3COONa was added with stirring for 60 min to obtain LaNaTaO3 perovskite. The mesophase was put in a Petri dish for drying at 110 °C for 24 h. The as-made mesophase was annealed at 450 °C for 4 h and then 650 °C for 4 h and annealed at 900 °C for 8 h in the air to obtain mesoporous LaNaTaO3 perovskite. The synthesized LaNaTaO3 perovskite (1 g) was suspended in 100 mL of ethanol, and a desired amount of ruthenium(III) acetylacetonate solutions containing the equivalent amount of Ru3+ was added to the suspension solution with sonication for 10 min to get 0.5, 1, 3, and 5% RuO2/LaNaTaO3 perovskites. The mixture was agitated magnetically for 3 h. The obtained samples were dried at 110 °C for 12 h and then annealed for 3 h at 450 °C to obtain mesoporous 0.5, 1, 3, and 5% RuO2/LaNaTaO3 perovskites.

Characterization of Mesoporous RuO2/LaNaTaO3 Perovskites

The detailed physicochemical characterization of the developed RuO2/LaNaTaO3 photocatalyst was performed to have a better understanding of composition, structure, and surface morphology of the perovskite photocatalysts. The X-ray diffraction pattern was measured through Cu Kα1/2, λα1 = 154.060 pm, λα2 = 154.439 pm radiation using a Bruker AXS D4 Endeavour X diffractometer. Field emission secondary electron microscopy (FE-SEM) was conducted with an FE scanning electron microanalyzer (JEOL-6300F, 5 kV). The N2 isotherm of the RuO2/LaNaTaO3 perovskites was performed at 77 K by analyzing adsorption isotherms with a Micromeritics ASAP 2010 volumetric adsorption unit. UV–vis diffuse reflectance spectra (DRS) of the RuO2/LaNaTaO3 perovskites were recorded on a UV–vis spectrophotometer (UV-2600, Shimadzu) at λ = 200–800 nm. A VG Escalab 200R electron spectrometer was applied to examine X-ray photoelectron spectra (XPS) for RuO2/LaNaTaO3 perovskites equipped with a Mg Kα X-ray source powered at 100 W. The C 1s peak at 284.8 eV was employed as calibration to estimate the binding energies (BE) of 1%RuO2/LaNaTaO3 perovskite.

H2 Generation Experiments

Hydrogen generation was conducted in a continuous flow setup containing gas supply with a mass flow controller and a 100 cm3 photoreactor quartz glass with a double jacket connecting a quadrupole mass spectrometer (QMS) for H2 and O2 detection. The QMS sampling rate is 1 cm3 min–1, facilitating a speedy H2 and O2 detection. Furthermore, this experimental setup provides an online recording of the whole course of the photocatalytic hydrogen generation with the utility of the simultaneous monitoring of the formation of H2 and O2 gases through the photocatalytic reaction. In the experimental series, 0.05 g of the synthesized LaNaTaO3 photocatalyst was mixed in 50 mL of pure H2O or 10 vol % CH3OH aqueous solution and was sonicated to disperse the photocatalyst. Afterward, the photoreactor was locked and connected to the QMS through the stainless steel valves. An Ar gas flux was employed to eliminate the dissolved oxygen from the reactor with the 50 cm3 min–1 flow rate for 10 min through the reactor to ensure there was O2 or H2 by the QMS. QMS was calibrated using standard H2 and O2 diluted in Ar. The flow rate of Ar gas at 10 cm3 min–1 was fixed throughout the photocatalytic system at 25 °C. Before turning on illumination, the photocatalytic reactions with magnetic stirring were kept for 40 min for stabilizing the background of photocatalytic reactions and the baseline was recorded by QMS. Afterward, the suspension was illuminated for 3 h employing an Osram XBO 1000 W Xe arc lamp as a UV source, and it stood inside a Müller LAX parallel photoreactor. During illumination, the obtained H2 or O2 gases were monitored under steady-state conditions. After 3 h illumination, the 1000 W Xe arc was turned off permitting the photocatalytic system to get the baseline again.