Effects of the hybrid plasmonic Ag/SrTiO₃ nanocubes for efficient photo-catalytic of H₂ generation and RhB decomposition

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  1. Science & Technology Development Journal, 24(3):2011-2018 Open Access Full Text Article Research Article Effects of the hybrid plasmonic Ag/SrTiO3 nanocubes for efficient photo-catalytic of H2 generation and RhB decomposition Ton Nu Quynh Trang1,2, Le Lam Anh Phi1,2, Le Thi Ngoc Tu3, Vu Thi Hanh Thu1,2,* ABSTRACT Introduction: Designing a novel photocatalyst toward the photo-catalytic degradation of dye molecules and hydrogen evolution reaction (HER) based on semiconductors has been drawn enor- Use your smartphone to scan this mous attention as a potential strategy in the clean energy field and environmental treatment. QR code and download this article Herein, a SrTiO3 (STO) nanocubes decorated with co-catalysts Ag nanoparticles (NPs) for H2 gen- eration and organic dye Rhodamine B (RhB) photodegradation activity under UV and visible light irradiation was fabricated. Method: The crystallinity, morphology, and chemical components of the photocatalyst were characterized through X-ray powder diffraction (XRD), scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectroscopy (EDX), respectively. The op- tical properties were evaluated through the UV-Visible absorbance spectra. Results: As a result, the photo-catalytic performance of the obtained Ag/STO hybrid photocatalyst was higher than that of the pristine STO nanocubes due to i) the efficient charge separation and transfer of photogenerated electron-holes pairs; ii) the improvement of the light-harvesting efficacy in the visible light. The high photo-catalytic RhB decomposition of the achieved Ag/STO hybrid photo-catalytic was obtained compared to the pure SrTiO3 nanocubes under UV and visible light exposure. The H2 generation of Ag/STO photocatalyst was 2- and 12-time higher than that of the pure STO nanocubes under UV and visible light illumination, respectively. Conclusion: This study gives a general strategy for 1 Faculty of Physics and Physics photocatalyst design towards water splitting for photo-catalytic H2 evolution and environmental Engineering, University of Science, Ho deterioration. Chi Minh City 700000, Vietnam Key words: SrTiO3, hybrid structures, photo-catalytic activity, Ag nanoparticles 2Vietnam National University, Ho Chi Minh City 700000, Vietnam 3 Dong Thap University, Cao Lanh City, to the structural and compositional flexibility allow- Dong Thap Province 870000, Vietnam INTRODUCTION ing A- and B-site replacement in the crystal struc- Achieving the global energy demand and environ- Correspondence 7,8 mental sustainability is an ideal approach for the ture . Moreover, CB of STO has more negative po- + Vu Thi Hanh Thu, Faculty of Physics and tential than the H /H2 potential (0 V vs. NHE, at Physics Engineering, University of growth of clean energy to address the global en- pH = 0), therefore supplies favorable redox potential Science, Ho Chi Minh City 700000, ergy crisis and environmental concerns 1,2. There- Vietnam fore, many technologies have been proposed for a for photo-catalytic water splitting into H2 gas, leading Vietnam National University, Ho Chi cleaner environment and fossil energy alternatives. to enhanced photo-catalytic efficiencies. From that Minh City 700000, Vietnam Among these, the production of hydrogen from solar- time, many efforts based on perovskite materials have Email: vththu@hcmus.edu.vn 9–12 driven water-splitting reactions employing semicon- been explored . However, the energy conversion History ductor nanomaterials as photocatalyst has a huge po- efficiency of photo-catalytic water splitting into H2 is • Received: 2021-05-24 tential for replacement from conventional fossil fuels still low due to i) STO can only be activated by UV • Accepted: 2021-07-25 light because of its wide forbidden energy of 3.2 eV; • Published: 2021-08-02 to renewable energy sources since the work of Honda and Fujishima was reported in the 1970s 3. In princi- ii) the recombination rate of photogenerated electron- DOI : 10.32508/stdj.v24i3.2568 ple, the activity of the water-splitting process based on hole couples is rapid. Several effective strategies to semiconductor nanomaterials is highly influenced by address these intrinsic drawbacks of STO have been the appropriate alignment of the valence band (VB) proposed, such as the use of co-catalysts 13,14, doping and the conduction band (CB) edges, thigh mobility metal into the bandgap 15,16, incorporating with other Copyright of the charge carriers, the stability in an aqueous envi- oxide metal 17,18. Among these approaches, the use of © VNU-HCM Press. This is an open- ronment, and harvesting solar light capability 4–6. In co-catalysts has been considered as one of the most access article distributed under the terms of the Creative Commons this sense, the perovskite oxide family, with its ABO3 promising approaches for enhanced photo-catalytic Attribution 4.0 International license. stoichiometry, especially SrTiO3 (STO), were exten- H2 generation. Recently, plasmonic nanomaterials sively postulated as a potential photocatalyst to pro- have attached considerable concerns in the field of duce photo-catalytic H2 from aqueous solutions due photo-catalytic activity related to the localized surface Cite this article : Trang T N Q, Phi L L A, Tu L T N, Thu V T H. Effects of the hybrid plasmonic Ag/SrTiO3 nanocubes for efficient photo-catalytic ofH2 generation and RhB decomposition. Sci. Tech. Dev. J.; 24(3):2011-2018. 2011
  2. Science & Technology Development Journal, 24(3):2011-2018 plasmon resonance (LSPR) and the formation of the 98% Schottky barrier at the interface between metal and purity, Merck) and silver nitrate (AgNO3, Merck, semiconductor. In view of these characteristics, noble >=99). The chemical reagents were employed with- metals using as co-catalyst are expected to advantage out any further purification. Double distilled water photo-catalytic H2 evolution as well. The vital role was used overall in the experiment. of the co-catalyst is to enable visible light absorption and prolong the lifetime of photoinduced charge car- Synthesis of SrTiO3 (STO) nanocubes riers. Some previous studies reported that coupling STO nanocubes have been fabricated by hydrother- plasmonic nanostructures such as Ag, Au for water 22 splitting could achieve higher photo-catalytic rates. mal approach as described in our study . The For example, Yan et al. reported that loading Au onto schematic for the fabrication of STO is presented in Schematic 1. In a typical synthesis, 20 mL of a 0.5 M CaTiO3 composites exhibit highly enhanced photo- catalytic performance compared to pristine CaTiO3 TiCl4 aqueous solution was dissolved in 60 mL of DI due to an effective separation and transportation of and vigorously stirred for 30 min. After that, a suit- photoinduced electron/hole pairs in CaTiO3 and the able amount of Sr(NO3)2 was put into the above solu- LSPR effect of Au 19. Guo et al. showcased that Al tion and continually stirred. Then, 10 ml of 2M KOH incorporated with BaTiO3 exhibited excellent photo- solution was dropped into the above reaction mixture catalytic water splitting and pollutant degradation and stirred for 30 minutes. The achieved suspension based on the piezoelectric polarization of BaTiO3 and was transferred in a closed Teflon-lined autoclave at 20 ◦ plasmonic activities of Al nanoparticles . Another 210 C for 4 h. The precipitates were collected using work reported that Aux/BaTiO3 plasmonic photocat- the centrifugation process and washed several times alysts endowed a large light absorption from 300 to with double-distilled water. In the end, the sample 600 nm. The results exhibited a high photo-catalytic ◦ was dried at 100 C to obtain the final specimen. performance for MO decomposition under solar light 21 illumination of Aux/BaTiO3 nanocubes . Based on Synthesis of Ag loaded onto the STO photo- these phenomena, herein, we demonstrate that load- catalyst (Ag/STO) ing Ag nanoparticles on well-defined STO nanocubes (Ag/STO) can effectively improve the photo-catalytic Ag/STO photocatalyst was achieved by the photore- H2 evolution from water splitting under visible light- duction method. In detail, STO (30mg) was dispersed responsive photocatalyst. The obtained photocatalyst into the AgNO3 aqueous solution under stirring. The are prepared via a facile and cost-effective route, in reaction mixture was irradiated under a 300 W Xe which STO nanocubes are fabricated by hydrother- lamp at room temperature for 1 hour. After the irradi- mal approach and loaded Ag nanoparticles through ation process, the grey products were gathered using chemical reduction method under UV light irradia- the centrifugation process, washed with double water ◦ tion. The morphological and crystal characteristics, several times, and dried at 60 C overnight. optical properties, compositions, and photo-catalytic activity of the achieved Ag/STO heterojunctions were Materials characterization evaluated in detail. The results reveal that the het- Powder X-ray diffraction (XRD) patterns of the as- erostructured photocatalyst shows a greatly enhanced obtained photocatalyst were acquired using X-ray photo-catalytic efficiency on H2 generation and the degradation of organic dyes under the illumination of diffractometry (Bruker D8 ADVANCE) equipped α α UV and visible light compared with bare STO due to with Cu K radiation ( = 1.54056 Å). Morpho- an efficient separation and transfer of photogenerated logical characterization of the samples was evaluated charge carriers and an improvement in the optical ab- using a transmission electron microscope (TEM), sorption ability. field-emission scanning electron microscopy (FE- SEM) equipped with energy disperse X-ray spec- MATERIALS - METHODS trometer (EDX). The optical absorption of the pho- Materials tocatalyst was assessed by UV−visible diffuse re- Titanium dioxide (TiO2, Merck, 99%), strontium ni- flectance spectroscopy (V-650, JASCO) at room tem- trate (Sr(NO3)2, Sigma-Aldrich <99%), hydrochloric perature. Raman scattering spectra of the as-prepared acid fuming (HCl, Merck, <37%), sodium hydrox- was recorded by a Horiba XploRA PLUS Raman Sys- ide (NaOH, Merck, 99%), methanol (Merck, CH3OH, tem at an excitation wavelength of 532 nm. 2012
  3. Science & Technology Development Journal, 24(3):2011-2018 The photo-catalytic activity for H2 evolu- as STO, indicating that Ag assembly did not change tion reaction (HER) the morphological features of STO structures, which The photo-catalytic HER activity of samples was car- is consistent with SEM images. The EDX mapping im- ried out in a Pyrex cell closed gas-circulation system. ages show the presence of Sr, Ti, O, and Ag elements A Xe lamp (Cermax, 300 W) was utilized as the light of Ag/STO photocatalyst as presented in Figure 1(d- source. In a typical process, 10 mg of the photocata- h), suggesting that Ag nanoparticles were deposited lyst was dispersed in 40 mL of the aqueous solution successfully on STO nanocubes via photoreduction method under UV light irradiation. containing 25 vol % methanol as the sacrificial agent The crystal information of the pristine STO and and the reaction mixture was sonicated for 30 min Ag/STO photocatalyst was assessed through XRD to achieve a well-dispersed particle suspension. Be- analyses, as displayed in Figure 2. The XRD patterns fore the light irradiation, the suspension was sealed of the samples exhibit typical diffraction peaks of cu- and vacuumed for 30 min to ensure an inert envi- bic perovskite STO, in which the characteristic peaks ronment. Then, the suspension was exposed under at 2θ of 24.807, 26.507, 28.182, 36.620, 43.681, and a 300 W xenon lamp equipped with a band-cutoff fil- 47.839 are assigned to (100), (002), (101), (102), (110), ter (420 nm). The H2 evolution was monitored by gas and (103) crystal planes respectively (JCPDS no. 74- chromatography with a thermal conductivity detector ◦ 1296). Furthermore, the peak appearing at 2θ = 38 (TCD). is produced in STO/Ag nanocubes belonging to the Photo-catalytic decomposition of the RhB Ag phase about the JCPDS card no. 04-0783. This molecules Dye demonstrated that the AgNPs were deposited success- fully on the surface area of STO, which is consistent The photo-catalytic activity of the as-achieved pho- with the results of the TEM image in Figure 1. tocatalyst was controlled under UV light (365 nm) The optical properties of as-prepared specimens are and visible light irradiation for 150 min (Xe lamp, 300 important processes for photo-catalytic reactions. W). Prior to light illumination, the mixture of aque- Therefore, the UV−vis spectra were used to inves- ous RhB solution and the photocatalyst was placed in tigate the optical characteristic of the photocatalysts a dark. The remaining RhB concentration was inves- (Figure 3). As reflected by Figure 3, the pristine STO − tigated through a UV vis spectrophotometer at the photocatalyst shows absorption edges at 388 nm re- wavelength of 554 nm. The % decomposition of RhB lated to a charge transfer from the valence band (VB) dye was evaluated using the equation: to the conduction band (CB), which corresponds to RhB decomposition (%) = [(C0-Ct )/C0] ´100 a bandgap energy of 3.2 eV for pristine STO (Fig- Where, Co and Ct are the RhB concentration at the ure 3(b)). For the Ag/STO sample case, the Ag/STO initial time and after irradiation for time “t”, respec- photocatalyst exhibits visible-light absorption with an tively. absorption edge at ∼ 450 nm due to the LSPR of RESULTS Ag NPs, which is beneficial for enhancing the photo- catalytic HER activity in the visible light. The detailed morphological characteristics of the as- To evaluate the effect of decorating with Ag on the prepared STO and Ag/STO nanocubes were inves- surface of the SrTiO3 nanocubes for photo-catalytic tigated via SEM and TEM analyses. As shown in H2 evolution activities. The photo-catalytic H2 evolu- Figure 1(a), the pristine STO exhibits 3D nanostruc- tion performance of photocatalyst was accomplished tures with an average particle size of ca.300 nm and using water/methanol solution under UV light and an almost perfect cubic structure with six isotropic visible light, as shown in Figure 4. As observed, the {100} facets. As mirrored in Figure 1(b), structures H2 production of the Ag/SrTiO3 sample exhibited of Ag/STO are analogous to those of STO. Moreover, significantly higher than that of the pure STO un- after photo-deposition of the Ag NPs on STO, a large der experimental conditions. Under UV light, the number of Ag nanoparticles with the particle size of Ag/STO sample achieved the H2 evolution of ~ 414 − − ca.30 nm was assembled on the whole STO surface, as µmol.h 1.g 1, which is ~ 2 times that photocata- marked by the yellow arrows (Figure 1(b)). As shown lyst with the pure STO nanocubes due to efficient in Figure 1(b), the Ag atoms were not easily discov- charge separation. Interestingly, the amount of H2 ered as separate particles on the surface of STO, im- produced by the Ag/STO photocatalyst was ~ 316 − − plying the Ag atoms were homogeneously deposited µmol.h 1.g 1, roughly 12-times that of the pure STO on the STO. The TEM image in Figure 1(c) reveals under visible light, whereas, no appreciable H2 pro- the nanocubes of Ag/STO with almost the same size duction was observed under visible light irradiation 2013
  4. Science & Technology Development Journal, 24(3):2011-2018 Figure 1: The morphological features and chemical elements of the pristine STO and the Ag/STO photocat- alyst (a, b) SEM image of the pristine STO, and Ag/STO photocatalyst, respectively, (c) TEM image of the Ag/STO specimen, and (d-g) elemental mapping of Sr, Ti, O, and Ag, respectively, (h) EDX spectrum of the Ag/STO heterostructures. Figure 2: The crystallinity using XRD patterns of the obtained Ag/STO and pristine STO photocatalyst. 2014
  5. Science & Technology Development Journal, 24(3):2011-2018 Figure 3: The optical characteristics of the photocatalyst using the UV–Vis diffuse reflectance spectra (a) and plot of (ah )1/2 vs. photon energy (b) of the pure STO and the Ag/STO specimen. of the pure STO nanocubes. This could be attributed DISCUSSION to the larger bandgap energy of the single STO that The morphological and crystal characteristics of STO cannot be reached under visible light, leading to low did not significantly change after the introduction H2. On the other hand, compared with that of the to Ag nanoparticles by hydrothermal pathway and pure STO, the Ag/STO photocatalyst exhibited out- photo-reduction method as shown in Figure 1 and standing H2 photo-catalytic activity. Additionally, Figure 2. The photocatalytic degradation of dye the reusability of a photocatalyst is also a critical fac- molecules and hydrogen evolution reaction (HER) of tor for practical applications. The heterostructure of the Ag/STO photocatalyst was higher than that of the Ag/STO was thus employed for H2 production un- pristine STO under both UV and visible light expo- der UV light (Figure 5). The H2 performance of the sure. This could be explained by i) improvement of Ag/STO remained unchanged after 4 cycles, implying absorption ability in the visible light region based on that the high durability of the as-obtained photocata- the LSPR effect of AgNP, which efficiently prompted lyst. the absorption and separation of the photoinduced The photo-catalytic performance of the single STO electron-hole pairs; ii) the effective separation and and Ag/STO photocatalyst was evaluated under pho- transport of the photogenerated charge carriers based todecomposition of RhB aqueous solution with the on the formation of a Schottky barrier between the existence of UV light and visible light irradiation af- Ag and STO, leading to an enhanced photo-catalytic ter 120 min, as shown in Figure 6. It could be H2 generation and photodegradation of the RhB dye. Under irradiation with an photon energy equal or clearly observed that no decomposition of the RhB greater than the bandgap energy of semiconductor, was achieved without the photocatalyst under UV and the photoinduced electron–hole pairs are generated. visible light irradiation, which may be assigned to the These photoinduced electrons can reduce organic pol- low self-degradation of RhB. In contrast, the effective lutants or water to form H2 production. Meanwhile, decomposition of RhB in the presence of photocat- the photoinduced holes react with dye molecules to alyst and light energy source was obtained, thereby generate a strong oxidizing agents, which improve the indicating that the photocatalyst and light irradiation decomposition of dye molecules into nontoxic prod- plays a significant role in the photodegradation activ- ucts 22. Based on the above results, to better un- ity of RhB solution. Compared with the pure STO, the derstand the photo-catalytic improvement of the hy- Ag/STO photocatalyst showed a higher photodegra- brid photocatalyst compared with the pure photocata- dation of the RhB dye under both UV and visible light lyst, a mechanism of their photo-catalytic activity was exposure. As presented in Figure 6, the Ag/STO pho- discussed. Upon exposure to UV light, STO is ex- tocatalyst exhibited outstanding RhB decomposition cited and generated the photoinduced electron-hole with degradation rates of RhB of approximately ~80 pairs. These electrons jump to the CB and transfer to + % and ~70 % under exposure to UV light and visible Ag NPs. They reduce protons (H ) to generate H2. light, respectively. Meanwhile, the photogenerated holes on the VB of 2015
  6. Science & Technology Development Journal, 24(3):2011-2018 Figure 4: The photo-catalytic H2 evolution of the obtained photocatalyst under the different irradiation (a) under UV light, (b) visible light of the STO and Ag/STO photocatalyst. Figure 5: The recyclability performance for H2 activity under visible light illumination after 4 cycles of the Ag/STO photocatalyst. STO can oxidize the absorbed H2O molecules into hy- nontoxic products. These outcomes from this work • droxyl radicals (OH ) 23. Under visible light irradia- provide new insight into the water-splitting perfor- tion, STO cannot be activated because of its large en- mance and photo-catalytic RhB degradation in prac- ergy gap. Meanwhile, the AgNPs can strongly absorb tical applications based on the incorporation of per- under the visible light based on the effect of LSPR to ovskite and noble metal. form the hot electrons 24. These hot electrons move to + CONCLUSION the CB of STO and reduce H to produce H2 gas. The holes on the Ag surface can react with H2O to form In summary, Ag/STO heterostructures were success- • OH in a successive reaction route. These formed fully synthesized by the hydrothermal method and reactive species decompose RhB molecule dyes into photo-reduction under assisted-UV light. An out- 2016
  7. Science & Technology Development Journal, 24(3):2011-2018 Figure 6: The photo-catalytic performance of the photocatalyst through decomposition of RhB organic dyes. standing photo-catalytic and yield in H2 evolution Le Thị Ngoc Tu and Le Lam Anh Phi has supported reaction of the Ag/STO photocatalyst in comparison the analytical techniques. with the pristine STO was observed. This could be at- tributed to the synergetic effect between STO and Ag ACKNOWLEDGEMENTS and the LSPR effect of Ag that was responsible for the Ton Nu Quynh Trang was funded by Vingroup Joint effective separation and transportation of the charge Stock Company and supported by the Domestic Mas- carriers and inhibition of the rapid recombination of ter/PhD Scholarship Programme of Vingroup Inno- the photoinduced charge carriers on the Ag/STO hy- vation Foundation (VINIF), Vingroup Big Data Insti- brid. As a result, the recombination of the photogen- tute (VINBIGDATA), code [VINIF.2020.TS.08]. erated electron-hole pairs on the photocatalyst was eliminated through the establishment of the Schottky REFERENCES channel, and the strong absorption in the visible light 1. Tian J, Zhang Y, Du L, He Y, Jin XH, Pearce S, Eloi JC, Harni- man RL, Alibhai D, Ye R, Phillips DL. Tailored self-assembled was enhanced via the LSPR effect of AgNPs, providing photo-catalytic nanofibres for visible-light-driven hydrogen an efficient charge separation and transfer of the hy- production. Nature Chemistry. 2020 Dec;12(12):1150-6;PMID: brid photocatalyst. Thus, the photo-catalytic activity 33219362. Available from: 020-00580-3. for the RhB photodegradation efficiency of Ag/STO 2. Guo L, Zhong C, Cao J, Hao Y, Lei M, Bi K, Sun Q, Wang was ~ 80% and ~ 70% under UV and visible light, re- ZL. Enhanced photo-catalytic H2 evolution by plasmonic and piezotronic effects based on periodic Al/BaTiO3 heterostruc- spectively. Furthermore, the H2 evolution rate of the tures. Nano Energy. 2019 Aug 1;62:513-20;Available from: achieved Ag/STO nanocubes was approximately 414 −1 −1 and 316 µmol.h .g under UV and visible light il- 3. Fujishima A, Honda K. Electrochemical photolysis of water lumination, respectively. This study can give an ef- at a semiconductor electrode. nature. 1972 Jul;238(5358):37- 8;PMID: 12635268. Available from: ficient pathway for designing the photo-catalytic sys- 238037a0. tem for environmental treatment and renewable en- 4. Liu Y, Sun Z, Hu YH. Bimetallic co-catalysts for photo-catalytic ergy production. hydrogen production from water. Chemical Engineering Jour- nal. 2020 Dec 24:128250;Available from: 1016/j.cej.2020.128250. ABBREVIATIONS 5. Zhang X, Peng T, Song S. Recent advances in dye-sensitized Ag: Silver semiconductor systems for photo-catalytic hydrogen pro- duction. Journal of Materials Chemistry A. 2016;4(7):2365- SrTiO3: Strontium titanate 402;Available from: 6. Trang TN, Nam ND, Ngoc Tu LT, Quoc HP, Van Man T, Ho VT, COMPETING INTERESTS Thu VT. In Situ Spatial Charge Separation of an Ir@TiO2 Mul- tiphase Photosystem toward Highly Efficient Photocatalytic The authors declare that there is no conflict of interest Performance of Hydrogen Production. The Journal of Physi- regarding the publication of this article. cal Chemistry C. 2020 Jul 10;124(31):16961-74;Available from: AUTHORS’ CONTRIBUTIONS 7. Li X, Zhao H, Liang J, Luo Y, Chen G, Shi X, Lu S, Gao S, Hu J, Liu Q, Sun X. A-site perovskite oxides: an emerging func- Ton Nu Quynh Trang has conceived of the present tional material for electrocatalysis and photocatalysis. Journal idea, carried out and written the manuscript with sup- of Materials Chemistry A. 2021;9(11):6650-70;Available from: port from Vu Thi Hanh Thu. 2017
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