- Email: [email protected]
porous brookite TiO2 nanoflutes
Nano Energy xxx (xxxx) xxx
Contents lists available at ScienceDirect
Nano Energy journal homepage: http://www.elsevier.com/locate/nanoen
Full paper
Photocatalytic pure water splitting with high efficiency and value by Pt/ porous brookite TiO2 nanoflutes Shuang Cao a, 1, Ting-Shan Chan b, 1, Ying-Rui Lu b, 1, Xinghua Shi c, Bing Fu a, Zhijiao Wu a, Hongmei Li d, Kang Liu d, Sarah Alzuabi e, Ping Cheng c, **, Min Liu d, f, ***, Tao Li g, h, ****, Xiaobo Chen e, *****, Lingyu Piao a, * a
CAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing, 100190, PR China National Synchrotron Radiation Research Center, Hsinchu, 30076, Taiwan c CAS Key Laboratory for Nanosystem and Hierarchy Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Chinese Academy of Sciences, Beijing, 100190, PR China d School of Physics and Electronics, Central South University, Changsha, Hunan, 410083, PR China e Department of Chemistry, University of Missouri, Kansas City, MO, 64110, USA f State Key Laboratory of Powder Metallurgy, Central South University, 932 South Lushan Road, Changsha, Hunan, 410083, PR China g Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, Illinois, 60115, USA h X-ray Science Division, Argonne National Laboratory, Lemont, IL, 60439, USA b
A R T I C L E I N F O
A B S T R A C T
Keywords: Photocatalytic water splitting Hydrogen production Hydrogen peroxide Porous brookite TiO2 nanoflutes Seawater
We report the remarkable production of H2 and H2O2 from pure water using porous brookite TiO2 nanoflutes decorated with Pt nanoparticles through two-electron photocatalytic intermediate water splitting (PIWS). The H2 production rate from deionized water (pH ~7.0) was 9.8 � 0.6 μmol mg 1 h 1, more than 12-fold and 230-fold higher than the reported TiO2-based photocatalytic overall water splitting (POWS) and H2/H2O2 production systems, respectively. The apparent quantum yields (AQY) value can achieve 43.4%, which is the highest value compared with the reported TiO2-based POWS systems and the commercial value for PIWS is ~9-fold higher than that for POWS even ignoring the separation cost. The mechanism of the distinct performance was confirmed by DFT and experimental measurements. The present work provides a feasible strategy to significantly improve the process efficiency and value of photocatalytic water splitting in both pure water and seawater using natural sunlight.
1. Introduction The utilization of solar-driven photocatalytic overall water splitting (POWS: 2H2O → 2H2 þ O2) for clean energy hydrogen generation has been recognized as a promising way to address the current energy crisis [1–4]. Despite the great progress achieved over the past decades, this approach has encountered many obstacles due to low efficiency, poor stability, high complexity, operation safety and cost concerns on the
mixing of the products of H2 and O2 [5]. The undesirable back-reaction of H2 and O2 in the POWS process significantly reduce efficiency and require rigorous high cost gas separation in practical applications [6,7]. Some of these problems have been addressed by the approach of pho tocatalytic partial water splitting (PPWS), in which sacrificial reagents are frequently used to consume the photo-generated holes or electrons [8]. However, this process is only considered as “semi-reaction” of water splitting, and is economically and environmentally unfavorable due to
* Corresponding author. ** Corresponding author. *** Corresponding author. School of Physics and Electronics, Central South University, Changsha, Hunan, 410083, PR China. **** Corresponding author. Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, Illinois, 60115, USA. ***** Corresponding author. E-mail addresses: [email protected] (P. Cheng), [email protected] (M. Liu), [email protected] (T. Li), [email protected] (X. Chen), [email protected] (L. Piao). 1 These authors contribute equally to this work. https://doi.org/10.1016/j.nanoen.2019.104287 Received 6 September 2019; Received in revised form 31 October 2019; Accepted 8 November 2019 Available online 13 November 2019 2211-2855/© 2019 Published by Elsevier Ltd.
Please cite this article as: Shuang Cao, Nano Energy, https://doi.org/10.1016/j.nanoen.2019.104287
S. Cao et al.
Nano Energy xxx (xxxx) xxx
the continuous consumption of valuable and expensive chemicals (such as methanol, ethanol, sodium sulfite, etc.). Photocatalytic intermediate water splitting (PIWS) has recently been shown as a promising method to avoid the aforementioned issues, where H2O is split into H2 and high value H2O2 (2H2O → H2 þ H2O2) [9]. H2O2 is one of the intermediate species in traditional photocatalytic water splitting processes. By contrast, POWS and PPWS are the processes that split H2O into H2 and O2, H2 or O2 in the presence of sacrificial reagents, respectively. The significant advantages of PIWS include: (1) Unlike the four-electron reaction in POWS, PIWS is a more kinetically feasible two-electron reaction [9]. (2) The back-reaction of H2 and O2 was significantly inhibited and the conversion efficiency was further enhanced. (3) PIWS generates more high value products of H2 and H2O2. H2O2 is widely used in electronics, refining, mining, packaging, pulp and textile bleaching industries [10]. (4) PIWS is a simple and cost-effective approach. Since the products are H2 gas and H2O2 liquid, they are automatically separated without using any separation facility. There fore, PIWS holds a great potential for practical applications. Previously, the formation of H2O2 has been found in the Pt/Degussa P25 and Pt/C3N4 photocatalysis systems, however, only with poor H2 and H2O2 production efficiencies [11,12]. Besides, the produced H2O2 was considered as an inhibitor for H2 production and the highlights of the H2O2 were ignored. Very recently, production of H2 and H2O2 has been achieved with photoelectron-catalytic (PEC) approaches, whereas, not only light irradiation is required, but also highly concentrated electrolytes (such as HCO-3) and electrical bias are needed [13]. Compared with photocatalysis process, it is a higher energy consuming process and belongs to a different research field. Based on this, we explicitly proposed the PIWS concept and put it into practice for the first time [14]. Our primary research indicated that the anatase nano particles possess the ability to oxide water to H2O2. Howbeit, the effi ciency and stable performance are far from satisfactory, so is the understanding of the PIWS mechanism. Therefore, it is highly desirable to explore effective PIWS catalysts that can produce H2 and H2O2 with
high efficiency and stability under sunlight from pure water without the assistance of any other additives. In this work, we firstly obtained the nano-scale brookite TiO2 nanoflutes catalyst with ~25 nm holes by a novel method. Although brookite TiO2 is commonly considered as a photocatalytically less active material, recent research has proved that brookite TiO2 has moderate depth of the electron trap that can promote both of photocatalytic oxidations and reduction reactions compared with anatase and rutile [15]. When decorated with Pt nanoparticles, it can achieve efficient H2 and H2O2 production from water with the PIWS mechanism. Besides, the brookite TiO2 nanoflutes in nanoscale with good crystallinity possess the advantages of enhanced light absorption ability, improved photogenerated charge separation efficiency, shorter carrier migration distance, and more active sites to promote the pho tocatalytic efficiency. 2. Results and discussion Fig. 1A shows the PIWS and POWS processes. Contrary to the POWS which is a four-electron reaction to produce H2 and O2, PIWS is a twoelectron reaction to produce H2 and H2O2. Density functional theory (DFT) calculations suggested that production of H2O2 and H2 was feasible in the Pt/brookite TiO2 system after revealing the possible water splitting mechanisms: (1) dissociation of water into surface in termediates (Fig. 1B); (2) formation of H2 or oxidation products from intermediates. During H2O dissociation process, H2O molecules prefer to adsorb at the top site of Ti, with the newly formed O-Ti bond tilting to the surface (Fig. 1B, S1) [16,17]. The adsorbed H2O molecule (H2Oad) donates a H to a nearby oxygen (O) site of TiO2 forming a H* and OH* overcoming an energy barrier of 0.395 eV. However, as shown in Fig. 1C, the two co-adsorbed H atoms are difficult to combine to form H2 due to a large energy barrier of 2.381 eV on brookite TiO2. This barrier is reduced to 1.009 eV on the Pt (111) surface. Out of the three possible ends for OH*: (I) OH* dissociates to form adsorbed O* and H* [18,19]; (II) OH* combines with each other to form an OOH* intermediate and
Fig. 1. Photocatalytic production of H2 and H2O2 from pure water. (A) Comparison of PIWS and POWS processes. (B) Reaction energy diagram from adsorbed H2O to OH* and H*. (C) Energy profiles of H2-production on the brookite TiO2 (210) and Pt (111) surfaces. (D) Calculated energy profile of converting OH* to O*, H* (route I), OOH* (route II) and H2O2* (route III). The red and gray spheres are the O and Ti atoms in TiO2, respectively; the pink and white spheres are the O and H atoms in H2O, respectively; the blue spheres are the Pt atoms. 2
S. Cao et al.
Nano Energy xxx (xxxx) xxx
release a H; (III) two OH* combine to form H2O2* (Fig. 1D), DFT results indicated that route (III) is energetically more favored, as it only needs to overcome an energy barrier of 0.769 eV, in contrary to the 1.250 and 1.463 eV for route (I) and (II), respectively. These results indicate that it
is possible to obtain H2O2 and H2 on the brookite TiO2/Pt system through PIWS. The DFT results indicates OH* plays a significant role during PIWS pathway and based on the guidance, we prepared mesoporous brookite
Fig. 2. Preparation and characterization of photocatalyst. (A) Schematic illustration of the synthesis process of Pt/porous b-TiO2 nanoflutes. (B) XRD, (C) Raman spectrum of b-TiO2 nanoflutes and Pt/porous b-TiO2 nanoflutes. (D) TEM, (E) HRTEM and (F) SAED of b-TiO2 nanoflutes. (G) TEM and (H) HRTEM of the Pt/porous b-TiO2 nanoflutes (the black dots in Fig. 2G are the Pt nanoparticles on TiO2 surface). 3
S. Cao et al.
Nano Energy xxx (xxxx) xxx
TiO2 nanoflutes. Although the synthesis of mesoporous TiO2 crystals have been reported, most of them were in micron scale and more easily leads to carrier recombination. Besides, all the reported mesoporous TiO2 crystals are anatase and rutile. Up to now, the synthesis of pure mesoporous brookite TiO2 nanoflutes is technically difficult. In this work, the novel nano-sized mesoporous brookite TiO2 nanoflutes were good to provide large surface area for reaction and high crystallinity was made to reduce the charge recombination in the bulk [20]. Meanwhile, surface hydrophilicity was made with abundant hydroxyl -OH groups to provide good water adsorptivity to facilitate the activation, which was confirmed by the infrated (IR) spectroscopy (Fig. S2) and water contact angle measurements (Fig. S3) [21]. Diffuse reflectance ultra violet–visible spectra and Mott-Schottky (M-S) plots (pH ¼ 7) revealed its band-gap and valence are 3.21 eV (Figure. S4 and S5) and 2.76 V (NHE, Fig. S6), respectively, indicating the favorable band gap structure for -OH activation during PIWS [22]. The porous TiO2 nanoflutes and the Pt/TiO2 were obtained through a template method as illustrated in Fig. 2A. The formation of the brookite phase was confirmed with the characteristic X-ray diffraction (XRD) pattern and Raman spectrum (Fig. 2B–C) [23]. The sharp XRD diffrac tion peaks of each sample match well with those of standard diffraction data of brookite (JCPDS no. 65–2448), which demonstrates that the as-synthesized TiO2 is highly crystalline and phase pure. The Raman spectroscopy, which is a phase-sensitive technique, further confirms it. The Raman spectra of the samples (Fig. 2C) show the presence of the bands typical of brookite, including A1g (152, 195, 247, 412, 637 cm 1), B1g (213, 322, 412, 502 cm 1), B2g (366, 395, 460, 584 cm 1), and B3g (287, 545 cm 1). The good agreement of the observed Raman bands with the active modes of brookite reported previously, confirmed that the final powder was pure brookite TiO2 [24]. Further scrutiny spectra shows no difference between TiO2 and Pt-loaded TiO2, implying that Pt nanoparticles are incorporated onto the surface of TiO2 rather than into the lattice (Fig. 2B and C). The large surface area was realized with nanoflutes’ tube size of 40–50 nm, pore size around 25 nm and length of 250 � 50 nm (Fig. 2D, Fig. S7-S8). They were grown via a hydrothermal-assisted penetration sol-gel process on assemblies of silica (SiO2) nanoparticles with an average diameter of ~25 nm (Fig. S9) and obtained final products after etching the SiO2 nanoparticle template in aqueous NaOH solution. The clear lattice fringes with plane space of 0.35 nm from the (210) crystal plane in the high-resolution transmission electron microscopy (HRTEM) image in Fig. 2E and the single crystalline pattern of the corresponding selected area electron diffraction (SAED) pattern in Fig. 2F revealed the high crystallinity and the exposed surface of the active (210) facet suitable for H2O2 and H2 production [25]. The noticeable disordered layer near the surface (Fig. 2E) may provide effective charge trapping and photocatalytic active sites for the re actions. The porosity and surface area were seen with the N2 adsorption from Brunauer-Emmett-Teller (BET) measurements of 25–40 nm and 54.2 m2 g 1, respectively (Fig. S10). Pt nanoparticles were loaded on brookite TiO2 using an in situ photo-deposition method. It means the brookite TiO2 nanoflutes were obtained firstly and then Pt nanoparticles were loaded on it. Therefore, the Pt nanoparticles are loaded on the surface rather than embedded in the nanostructure of TiO2. The above XRD and Raman result also confirm it. It can be clearly observed that the black dots of Pt nanoparticles with 3–5 nm (Fig. 2G) and exposed (111) surface facets (Fig. 2H) are well-dispersed on the surface of TiO2. Their mass percentages were determined using inductively coupled plasma mass spectrometry (ICP-MS) (Table S1). The photocatalytic activity was tested under Hg lamp irradiation (λ � 300 nm, 500 W, Fig. S11) at room temperature. The H2 production rate from deionized water (pH ~7.0) was 9.8 � 0.6 μmol mg 1 h 1 after 5 h of irradiation (Fig. 3A, Fig. S12-13), which is over 220 and 900-fold higher than that with the Pt/P25 TiO2 (0.04 μmol mg 1 h 1) [11] and Pt/C3N4 (0.01 μmol mg 1 h 1) [12] H2/H2O2 production systems, respectively, 12-fold higher than the reported highest rutile TiO2 POWS system (0.8 μmol mg 1 h 1) [26] (Fig. 3B–C, Table S2). The AQY value
of H2-production can achieve 43.4%. As far as we known, it is the highest value compared with the reported TiO2-based POWS systems. Apart from the higher efficiency, the catalyst also shows better stability than non-TiO2-based POWS systems (Tables S2 and S3). The H2O2 production rate were 8.2 � 0.8 μmol mg 1 h 1. The price per electron for H2O2 is ~140-fold higher than that for O2 and ~18-fold higher than that for H2 [9]. Therefore, the commercial value for PIWS is ~9-fold higher than that for POWS even ignoring the separation cost. Besides, the low concentration of H2O2 possesses more advantages: (a) The low concentration of H2O2 has a wider range of applications. For example, most chemical synthesis and pulp bleaching require <9 wt % of H2O2 and only <0.1 wt% H2O2 is necessary for water treatment. (b) The low centration of H2O2 is safer than high concentrated H2O2 for trans portation. In this work, the Pt nanoparticles act as the role of cocatalyst, which could not only promote the charge separation of TiO2, but also act as the active center for proton reduction. Then, the H2 was generated on Pt nanoparticles, and H2O2 was formed on TiO2. To further confirm the cocatalytic role of Pt nanoparticles, controlled experiment was con ducted, and the result indicates that the H2O2 were not derived from catalytic reaction by Pt nanoparticles (Fig. S14). The pH value main tained 7.0 during the entire reaction, indicating both Hþ and OH from water are consumed at the same time. H2O2 instead of O2 was the only oxidation product from pure water on the Pt/brookite TiO2 system. The preferable formation of H2O2 instead of O2 was likely due to the uniqueness of the Pt/brookite TiO2 system which kinetically favored the formation of H2O2 as our DFT calculation revealed (Fig. 1). During the PIWS process, will the formed H2O2 decompose into oxygen? Although it is generally acknowledged that H2O2 could decompose on Pt NPs, the absolute concentration of H2O2 in this system is relatively low because of the diluted reaction solution. Most of the produced H2O2 can maintain steadily for the following separation and application. As the reaction prolonged, the amount of H2O2 decreased and the ratio of H2O2: H2 was lower than 1:1 (Fig. S15), however, no O2 was detected in gas and liquid (Fig. S16). It is proposed that even small amount of H2O2 decomposed to O2, the O2 can be readily reduced because of the close adsorption (Figs. S17–18). DFT results confirmed that desorption of the absorbed O2 from the TiO2 (210) surface required a large energy barrier of 3.18 eV (Fig. S19). Therefore, it can be concluded that even if a small portion of H2O2 decompose into O2, the adsorbed O2 could be readily reduced by the photogenerated electrons, which does not have negative effects on the separation of H2 and PIWS process. Continuous production of hydrogen is attractive for practical appli cations. H2 was time-proportionally produced from pure water under UV-irradiation for 30 h (Fig. 3D). Although a slight rate decrease ap pears after that, restoration can be easily achieved by regenerating the catalyst in situ via heating at 120 � C for 5 h under Ar flow. The crystal morphology and composition of the nanoflutes were maintained after the reaction based on the results of XRD, Raman, XPS and TEM (Figs. S20–S23) measurements, indicating the good stability of the brookite TiO2 nanoflutes. The slight rate decrease maybe due to the larger size of Pt nanoparticles (Fig. S23) and decreased amount of absorbed -OH on the surface of TiO2 (Fig. S22). To demonstrate the possibility of large-scale application, we further tested the H2 produc tion efficiency in seawater. The H2 production rate of 7.2 and 3.6 μmol mg 1 h 1 were achieved, under UV and natural sunlight irra diation (12:00 a.m.–13:00 p.m., 2018-05-03, Beijing, China), respec tively (Fig. 3E). The efficiency under UV light is 9-fold higher than the reported highest TiO2-based POWS systems (0.8 μmol mg 1 h 1) [26] (Fig. 3B, Table S2) and as far as we known, it is also the highest value compared the reported TiO2-based systems in seawater. More impor tantly, the system can sustain for more than 10 days (12 h/day, average rate ~2.0 μmol mg 1 h 1) when a photocatalytic plate was constructed (Fig. 3F and Fig. S24), demonstrating the excellent stability for practical application. To experimentally understand the photocatalytic mechanism, we 4
S. Cao et al.
Nano Energy xxx (xxxx) xxx
Fig. 3. Photocatalytic performance of Pt/b-TiO2 porous nanoflutes. (A) Reaction time course of photocatalytic H2 and H2O2 production of Pt/b-TiO2 porous nanoflutes. (B) Comparison of photocatalytic H2 and H2O2 production efficiencies of Pt/b-TiO2 porous nanoflutes with reported H2/H2O2 production systems. (C) Comparison of photocatalytic H2 production of Pt/b-TiO2 porous nanoflutes with other reported TiO2-based POWS systems (the detail information is shown in Table S2). (D) Photocatalytic H2-production durability test for Pt/b-TiO2 porous nanoflute powders. (E) Photocatalytic H2 production activities of Pt/b-TiO2 porous nanoflutes in pure water and seawater under UV light or sunlight irradiation. (F) A 10-day photocatalytic H2 production test using a 4 cm � 6 cm ground glass panel coated Pt/b-TiO2 porous nanoflute powders.
performed X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements. Compared with Pt foil and PtO2 references, the white line peaks (2p→5d) of Pt display an increased intensity in both Pt/b-TiO2 nanoflutes and nanoparticles, indicating that the Pt is mildly oxidized (Fig. 4A) [27,28]. However, the Pt L3-edge XANES spectra of Pt/b-TiO2 nanoflutes display a weaker white line peak, which indicates more electrons occupied on Pt 5d or bitals in b-TiO2 nanoflutes than that of nanoparticles suggesting electron transfer from b-TiO2 nanoflutes to Pt are easier than that of nano particles [29,30]. More information can be obtained from EXAFS spectra of the Pt L3-edge (Fig. 4B). The main peaks near 2.5 Å are derived from neatest coordination shells of Pt atoms and the weaker intensity in both Pt/b-TiO2 nanoflutes and nanoparticles indicate lesser first coordination
number than that of Pt foil due to the size effects. The similar intensity indicates the similar coordination environment of Pt in Pt/b-TiO2 nanoflutes and nanoparticles [31]. However, for the small shoulder peaks near 1.5 Å, which belong to Pt-O coordination, indicates that Pt on b-TiO2 nanoparticles is more serious oxidized than that of on nanoflutes, agreeing well with the XANES curves of Pt. In the Ti K-edge X-ray absorption near-edge structure (XANES) spectra in Fig. 4C, three typical pre-edge peaks corresponding to quadruple-allowed 1s → 3d transitions were observed [31], with larger intensities of the Ti pre-edge peaks due to larger local structural distortion. A left shift of K-edge indicates a lower valence state likely due to oxygen vacancy (VO) in the nanoflutes, as confirmed by the decreased “white line” intensities and the observed disordered structure near the 5
S. Cao et al.
Nano Energy xxx (xxxx) xxx
Fig. 4. Mechanism exploration for the PIWS process. XANES (A) and EXAFS (B) analysis of Pt L3-edge spectra for Pt/b-TiO2 nanoflutes and nanoparticles. (C) Ti K-edge XANES spectra and (D) Fourier transforms of Ti K-edge EXAFS spectra for Pt/b-TiO2 nanoflutes and nanoparticles. (E) XPS spectra of O1s for the Pt/b-TiO2 nanoflutes and nanoparticles after 1 h of UV light irradiation. (F) ESR responses of the DMPO–OH spin adduct in water suspension of Pt/b-TiO2 nanoflutes and nanoparticles before and after 60’s UV light irradiation in an Ar atmosphere.
surface in the HRTEM in Fig. 2E. The benefits of disordered structure near the surface on photocatalytic activity were widely acknowledged in previous studies [32]. The larger percentage of VO in the nanoflutes were confirmed by the EXAFS spectra (Fig. 4D). The peak located at 1.5 Å is derived from Ti-O coordination and the weaker intensity of the peak indicates a lower Ti-O coordination number than that of nano particles. The more VO may provide more active sites for the -OH adsorption and H2O2 formation. The abundant adsorbed -OH and subsequent formation of �OH for mation were confirmed by X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR) measurements. The amount of -OH (OOH, 531.7 eV) on the surface of Pt/TiO2 nanoflutes increased after ultravi olet light irradiation (Fig. 4E) [32], although there were no obvious difference in the dark, from the O 1s XPS spectra (Fig. S25). Meanwhile,
a new peak at 533.2 eV belonging to physisorbed water emerged indi cating improved hydrophilicity after irradiation [33], in contrary to the unchanged peak for the nanoparticles after irradiation (Fig. 4E and Fig. S26). Similar conclusion was drawn from the Ti 2p XPS spectra before and after irradiation (Figs. S27–28) [34,35]. The large formation of �OH was confirmed with ESR results. A much higher 5,5-dimethyl-1- pyrroline-N-oxide (DMPO), DMPO-OH radical adduct signal (1:2:2:1, aN ¼ aH ¼ 15.4 G) was observed after 60 s of UV light irradiation for the Pt/TiO2 nanoflutes than for the nanoparticles [36], indicating the facilitated formation of �OH (Fig. 4F). Based on the above analysis, the catalytic pathway can be proposed in Scheme S1. H2O2 formation is a two-electron transfer process by the combination of two �OH. By contrast, water oxidation to O2 involves four electron and four proton transfer for the eventual formation of an O-O bond, which is much more 6
S. Cao et al.
Nano Energy xxx (xxxx) xxx
difficult than the H2O2 formation. Therefore, H2O2 generation is kinet ically more favorable than O2 formation. In conclusion, we have demonstrated the remarkable production of H2 and H2O2 from pure water using porous brookite TiO2 nanoflutes decorated with Pt nanoparticles through PIWS. The H2 and H2O2 pro duction rates can reach up to 9.8 � 0.6 μmol mg 1 h 1 and 8.2 � 0.8 μmol mg 1 h 1, respectively, in pure water (pH~7). And the AQY value of H2-production can achieve 43.4%. Continuous H2 pro duction and H2 production from seawater have also been displayed. DFT and experimental results confirm that the distinct performance proceeds by a kinetically favorable two-electron reaction on the surface of brookite TiO2 and its unique surficial chemistry micro-circumstance is beneficial to adsorb activate hydroxyl groups under light and further form hydrogen peroxide.
[23] T. Tachikawa, M. Fujitsuka, T. Majima, J. Phys. Chem. C 111 (2017) 5259–5274. [24] A. Naldoni, T. Montini, F. Malara, M.M. Mr� oz, A. Beltram, T. Virgili, C.L. Boldrini, M. Marelli, I. Romero-Oca~ na, J.J. Delgado, V.D. Santo, P. Fornasiero, ACS Catal. 7 (2017) 1270–1278. [25] M. Choi, J. Lim, M. Baek, W. Choi, W. Kim, K. Yong, ACS Appl. Mater. Interfaces 9 (2017) 16252–16260. [26] R. Li, Y. Weng, X. Zhou, X. Wang, Y. Mi, R. Chong, H. Han, C. Li, Energy Environ. Sci. 8 (2015) 2377–2382. [27] J. Mao, W. Chen, D. He, J. Wan, J. Pei, J. Dong, Y. Wang, P. An, Z. Jin, W. Xing, H. Tang, Z. Zhuang, X. Liang, Y. Huang, G. Zhou, L. Wang, D. Wang, Y. Li, Sci. Adv 3 (2017) e1603068. [28] L. Tao, Y. Shi, Y.C. Huang, R. Chen, Y. Zhang, J. Huo, Y. Zou, G. Yu, J. Luo, C. L. Dong, S. Wang, Nano Energy 53 (2018) 604–612. [29] N. Becknell, Y. Kang, C. Chen, J. Resasco, N. Kornienko, J. Guo, N.M. Markovic, G. A. Somorjai, V.R. Stamenkovic, P. Yang, J. Am. Chem. Soc. 137 (2015) 15817–15824. [30] J. Li, C.H. Liu, M.N. Banis, D. Vaccarello, Z.F. Ding, S.D. Wang, T.K. Sham, J. Phys. Chem. C 121 (2017) 24861–24870. [31] A. L�eon, G. Yalovega, A. Soldatov, M. Fichtner, J. Phys. Chem. C 112 (2008) 12545–12549. [32] X. Chen, L. Liu, P.Y. Yu, S.S. Mao, Science 331 (2011) 746–750. [33] A. Naldoni, M. Allieta, S. Santangelo, M. Marelli, F. Fabbri, S. Cappelli, C. L. Bianchi, R. Psaro, V. Dal Santo, J. Am. Chem. Soc. 134 (2012) 7600–7603. [34] B. Erdem, R.A. Hunsicker, G.W. Simmons, E.D. Sudol, V.L. Dimonie, M.S. El-Aasser, Langmuir 17 (2001) 2664–2669. [35] J. Zheng, Y. Lyu, R. Wang, C. Xie, H. Zhou, S.P. Jiang, S. Wang, Nat. Commun. 9 (2018) 3572. [36] R. Shi, H.F. Ye, F. Liang, Z. Wang, K. Li, Y. Weng, Z. Lin, W.F. Fu, C.M. Che, Y. Chen, Adv. Mater. 30 (2018) 1705941.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements We acknowledge the financial support by the National Natural Sci ence Foundation of China (No 21703046, 21972028 and 11672079) and the Ministry of Science and Technology of China (No 2016YFA0200902). X. Chen appreciates the support from the U.S. Na tional Science Foundation (DMR-1609061), and the College of Arts and Sciences, University of Missouri - Kansas City. The authors would like to thank Natural International Children’s Growth Center for their help in painting the TOC.
Shuang Cao received her PhD degree in Technical Institute of Physics and Chemistry, Chinese Academy of Sciences in 2016. She joined National Center for Nanoscience and Technology as a assistant research fellow in 2016. Her research focuses on designing and optimizing photocatalytic catalysts.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.nanoen.2019.104287.
Ting-Shan Chan is currently an associate research scientist fellow at National Synchrotron Radiation Research Center. The job responsibilities are spokesperson of beamline 01C1 and 16A1. He got his Ph.D. degree in the Department of Chemistry from National Taiwan University, Taiwan, in 2003. His research focuses on the energy and environmental materials.
References [1] Z. Pan, G. Zhang, X. Wang, Angew. Chem. Int. Ed. 58 (2019) 7102–7106. [2] J. Meng, H. Zhou, Y. Zou, R. Wang, Y. Lyu, S.P. Jiang, S. Wang, Energy Environ. Sci. 12 (2019) 2345–2347. [3] J. Huang, H. Gao, Y. Xia, Y. Sun, J. Xiong, Y. Li, S. Cong, J. Guo, S. Du, G. Zou, Nano Energy 46 (2018) 305–313. [4] X. Du, J. Huang, J. Zhang, Y. Yan, C. Wu, Y. Hu, C. Yan, T. Lei, W. Chen, C. Fan, J. Xiong, Angew. Chem. Int. Ed. 58 (2019) 4484–4502. [5] A. Meng, L. Zhang, B. Cheng, J. Yu, Adv. Mater. 31 (2019), 1807660. [6] J. Cai, M. Wu, Y. Wang, H. Zhang, M. Meng, Y. Tian, X. Li, J. Zhang, L. Zheng, J. Gong, Chem 2 (2017) 877–892. [7] X. Chen, R. Shi, Q. Chen, Z. Zhang, W. Jiang, Y. Zhu, T. Zhang, Nano Energy 59 (2019) 644–650. [8] Z. Wang, W. Wu, Q. Xu, G. Li, S. Liu, X. Jia, Y. Qin, Z.L. Wang, Nano Energy 38 (2017) 518–525. [9] K. Sayama, ACS Energy Lett 3 (2018) 1093–1101. [10] M.N. Young, M.J. Links, S.C. Popat, B.E. Rittmann, C.I. Torres, ChemSusChem 9 (2016) 3345–3352. [11] V.M. Daskalaki, P. Panagiotopoulou, D.I. Kondarides, Chem. Eng. J. 170 (2011) 433–439. [12] J. Liu, Y. Zhang, L. Lu, G. Wu, W. Chen, Chem. Commun. 48 (2012) 8826–8828. [13] K. Fuku, K. Sayama, Chem. Commun. 52 (2016) 5406–5409. [14] L. Wang, S. Cao, K. Guo, Z. Wu, Z. Ma, L. Piao, Chin. J. Catal. 40 (2019) 470–475. [15] J.J.M. Vequizo, H. Matsunaga, T. Ishiku, S. Kamimura, T. Ohno, A. Yamakata, ACS Catal. 7 (2017) 2644–2651. [16] R. Mu, Z.J. Zhao, Z. Dohn� alek, J. Gong, Chem. Soc. Rev. 46 (2017) 1785–1806. [17] D. Wang, T. Sheng, J. Chen, H.F. Wang, P. Hu, Nat. Catal 1 (2018) 291–299. [18] J. Huang, Y. Sun, Y. Zhang, G. Zou, C. Yan, S. Cong, T. Lei, X. Dai, J. Guo, R. Lu, Y. Li, J. Xiong, 30(2018) 1705045. [19] J. Huang, Y. Sun, X. Du, Y. Zhang, C. Wu, C. Yan, Y. Yan, G. Zou, W. Wu, R. Lu, Y. Li, J. Xiong, Adv. Mater. 30 (2018) 1803367. [20] E.J.W. Crossland, N. Noel, V. Sivaram, T. Leijtens, J.A. Alexander-Webber, H. J. Snaith, Nature 495 (2013) 215–219. [21] B.F. Iskandar, A.B.D. Nandiyanto, K.M. Yun, C.J. H Jr., K. Okuyama, P. Biswas, Adv. Mater. 19 (2017) 1408–1412. [22] X. Li, J. Yu, M. Jaroniec, Chem. Soc. Rev. 45 (2016) 2603–2636.
Ying-Rui Lu is currently a postdoctoral research fellow at Na tional Synchrotron Radiation Research Center. He got his Ph.D. degree in the program for science and technology of accelerator light source from National Chiao Tung University, Taiwan, in 2017. His research focuses on the developments of X-ray spectroscopy instruments for in-situ monitoring the chemical processes in gas–solid and liquid–solid interfaces.
7
S. Cao et al.
Nano Energy xxx (xxxx) xxx Xinghua Shi received his Bachelor degree from Peking Uni versity, Master degree from Institute of Mechanics, Chinese Academy of Sciences and PhD from Brown University. From 2011 to 2016 back in Institute of Mechanics, CAS as an asso ciate professor, he studied nanoparticle-cell interaction with multiscale modeling. In the beginning of 2016 he moved to National Center for Nanoscience and Technology as full pro fessor and principle investigator. There he focused on the simulation of catalysis, self-assembly and drug delivery.
Sarah Alzuabi is a graduate student in Chemistry at University of Missouri – Kansas City (UMKC). She got her B.S. degree from the University of Hafr Albatin, Saudi Arabia. Her research in terest including nanomaterials synthesis, characterization and applications in photocatalysis, electrocatalysis and microwave absorption.
Bing Fu received his B.S. degree in chemistry from Capital Normal University, Beijing, China, in 2016. He is currently in a Ph.D.‘s course under the supervision of Professor Lingyu Piao at National Center for Nanoscience and Technology, Chinese Academy of Sciences. His research interests are engineering and optimizing semiconductor photocatalysts for water splitting reactions.
Ping Cheng received her Bachelor degree from Yantai Univer sity, Master degree and PhD from Beijing University of Chem ical Technology. From 2016 to 2019, she do research in National Center for Nanoscience and Technology as the post doctor. In the May of 2019 she moved to University of Shanghai for Science and Technology as a lecturer. She focused on the simulation of the structure, electronic and catalytic properties of materials.
Zhijiao Wu received her M.S. degree from University of Chinese Academy of Sciences in 2011. She joined National Center for Nanoscience and Technology from 2011 as an engineer. She mainly focuses on nano-standardization and the reference ma terials development of nanomaterials.
Min Liu is currently a professor in School of Physics and Elec tronic Sciences, Central South University (CSU). He joined CSU in 2017 after receiving the National 1000 Young Talents award in China. He obtained the Ph.D. degree (Excellent Doctoral Thesis) in 2010 from Institute of Electrical Engineering, Chi nese Academy of Sciences. From 2010 to 2015, he worked as a project researcher and a senior researcher in Tokyo University. From 2015 to 2017, he work as research associate at the Uni versity of Toronto. His research focused on developing efficient catalysts for CO2 reduction and water splitting.
Hongmei Li received her bachelor and Ms.C. in School of Physics and Electronics from Hunan Normal University, China in 2004 and 2007. From 2008 to 2017, she worked as a lectures in Shaoyang University and a visiting researcher at Univeristy of Toronto. Now she is a lecturer at Center South University. Her research focused on developing efficient catalysts for photoelectrochemical water splitting.
Dr. Li earned his bachelor’s degree from the East China Uni versity of Science and Technology (ECUST) in 2003. He completed his Ph.D. at the University of South Caro lina—Columbia in 2009. He is currently an assistant professor in the Department of Chemistry and Biochemistry at Northern Illinois University and holds a joint assistant scientist position at the Advanced Photon Source at Argonne National Lab. His research interests focus on using advanced X-ray techniques to study the self-assembly of nanoparticles as well as energy ma terials including catalyst and battery. Dr. Li has authored/ coauthored more than 100 peer-reviewed research papers.
Kang Liu is currently a Ph.D. candidate in Prof. Min Liu’s Group at Central South University. He received M.S. degree in school of physics and optoelectronics from Xiangtan University in 2017. Now, his research interests focus on the density func tional theory calculation, electrocatalytic CO2 reduction and synchrotron-based techniques.
Xiaobo Chen, Ph.D., is an Associate Professor in Chemistry at University of Missouri – Kansas City (UMKC). After he obtained his Ph.D. degree from Case Western Reserve University in 2005, he spent six years in University of California – Berkeley and Lawrence Berkeley National Laboratory, exploring various renewable energy projects. He joined UMKC in 2011 and his research interests include nanoscience, photocatalytic and electrochemical hydrogen production, photocatalytic pollutant removal and organic synthesis, microwave-absorbing mate rials, AI chemistry, etc. He has published over 160 peerreviewed, high-profile articles with a total citation number over 43,000.
8
S. Cao et al.
Nano Energy xxx (xxxx) xxx Lingyu Piao received her Ph.D. in Institute of Chemical Engi neering from Tianjin University, China in 2002. From 2002 to 2005, she worked as a postdoctor in Peking University and Pierre and Marie Curie University. Now she is a professor at National Center for Nanoscience and Technology. Her research interests include synthesis, characterization and optical prop erties of functionalized nanomaterials and functional nano materials in the field of energy and environment.
9
Contents lists available at ScienceDirect
Nano Energy journal homepage: http://www.elsevier.com/locate/nanoen
Full paper
Photocatalytic pure water splitting with high efficiency and value by Pt/ porous brookite TiO2 nanoflutes Shuang Cao a, 1, Ting-Shan Chan b, 1, Ying-Rui Lu b, 1, Xinghua Shi c, Bing Fu a, Zhijiao Wu a, Hongmei Li d, Kang Liu d, Sarah Alzuabi e, Ping Cheng c, **, Min Liu d, f, ***, Tao Li g, h, ****, Xiaobo Chen e, *****, Lingyu Piao a, * a
CAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing, 100190, PR China National Synchrotron Radiation Research Center, Hsinchu, 30076, Taiwan c CAS Key Laboratory for Nanosystem and Hierarchy Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Chinese Academy of Sciences, Beijing, 100190, PR China d School of Physics and Electronics, Central South University, Changsha, Hunan, 410083, PR China e Department of Chemistry, University of Missouri, Kansas City, MO, 64110, USA f State Key Laboratory of Powder Metallurgy, Central South University, 932 South Lushan Road, Changsha, Hunan, 410083, PR China g Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, Illinois, 60115, USA h X-ray Science Division, Argonne National Laboratory, Lemont, IL, 60439, USA b
A R T I C L E I N F O
A B S T R A C T
Keywords: Photocatalytic water splitting Hydrogen production Hydrogen peroxide Porous brookite TiO2 nanoflutes Seawater
We report the remarkable production of H2 and H2O2 from pure water using porous brookite TiO2 nanoflutes decorated with Pt nanoparticles through two-electron photocatalytic intermediate water splitting (PIWS). The H2 production rate from deionized water (pH ~7.0) was 9.8 � 0.6 μmol mg 1 h 1, more than 12-fold and 230-fold higher than the reported TiO2-based photocatalytic overall water splitting (POWS) and H2/H2O2 production systems, respectively. The apparent quantum yields (AQY) value can achieve 43.4%, which is the highest value compared with the reported TiO2-based POWS systems and the commercial value for PIWS is ~9-fold higher than that for POWS even ignoring the separation cost. The mechanism of the distinct performance was confirmed by DFT and experimental measurements. The present work provides a feasible strategy to significantly improve the process efficiency and value of photocatalytic water splitting in both pure water and seawater using natural sunlight.
1. Introduction The utilization of solar-driven photocatalytic overall water splitting (POWS: 2H2O → 2H2 þ O2) for clean energy hydrogen generation has been recognized as a promising way to address the current energy crisis [1–4]. Despite the great progress achieved over the past decades, this approach has encountered many obstacles due to low efficiency, poor stability, high complexity, operation safety and cost concerns on the
mixing of the products of H2 and O2 [5]. The undesirable back-reaction of H2 and O2 in the POWS process significantly reduce efficiency and require rigorous high cost gas separation in practical applications [6,7]. Some of these problems have been addressed by the approach of pho tocatalytic partial water splitting (PPWS), in which sacrificial reagents are frequently used to consume the photo-generated holes or electrons [8]. However, this process is only considered as “semi-reaction” of water splitting, and is economically and environmentally unfavorable due to
* Corresponding author. ** Corresponding author. *** Corresponding author. School of Physics and Electronics, Central South University, Changsha, Hunan, 410083, PR China. **** Corresponding author. Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, Illinois, 60115, USA. ***** Corresponding author. E-mail addresses: [email protected] (P. Cheng), [email protected] (M. Liu), [email protected] (T. Li), [email protected] (X. Chen), [email protected] (L. Piao). 1 These authors contribute equally to this work. https://doi.org/10.1016/j.nanoen.2019.104287 Received 6 September 2019; Received in revised form 31 October 2019; Accepted 8 November 2019 Available online 13 November 2019 2211-2855/© 2019 Published by Elsevier Ltd.
Please cite this article as: Shuang Cao, Nano Energy, https://doi.org/10.1016/j.nanoen.2019.104287
S. Cao et al.
Nano Energy xxx (xxxx) xxx
the continuous consumption of valuable and expensive chemicals (such as methanol, ethanol, sodium sulfite, etc.). Photocatalytic intermediate water splitting (PIWS) has recently been shown as a promising method to avoid the aforementioned issues, where H2O is split into H2 and high value H2O2 (2H2O → H2 þ H2O2) [9]. H2O2 is one of the intermediate species in traditional photocatalytic water splitting processes. By contrast, POWS and PPWS are the processes that split H2O into H2 and O2, H2 or O2 in the presence of sacrificial reagents, respectively. The significant advantages of PIWS include: (1) Unlike the four-electron reaction in POWS, PIWS is a more kinetically feasible two-electron reaction [9]. (2) The back-reaction of H2 and O2 was significantly inhibited and the conversion efficiency was further enhanced. (3) PIWS generates more high value products of H2 and H2O2. H2O2 is widely used in electronics, refining, mining, packaging, pulp and textile bleaching industries [10]. (4) PIWS is a simple and cost-effective approach. Since the products are H2 gas and H2O2 liquid, they are automatically separated without using any separation facility. There fore, PIWS holds a great potential for practical applications. Previously, the formation of H2O2 has been found in the Pt/Degussa P25 and Pt/C3N4 photocatalysis systems, however, only with poor H2 and H2O2 production efficiencies [11,12]. Besides, the produced H2O2 was considered as an inhibitor for H2 production and the highlights of the H2O2 were ignored. Very recently, production of H2 and H2O2 has been achieved with photoelectron-catalytic (PEC) approaches, whereas, not only light irradiation is required, but also highly concentrated electrolytes (such as HCO-3) and electrical bias are needed [13]. Compared with photocatalysis process, it is a higher energy consuming process and belongs to a different research field. Based on this, we explicitly proposed the PIWS concept and put it into practice for the first time [14]. Our primary research indicated that the anatase nano particles possess the ability to oxide water to H2O2. Howbeit, the effi ciency and stable performance are far from satisfactory, so is the understanding of the PIWS mechanism. Therefore, it is highly desirable to explore effective PIWS catalysts that can produce H2 and H2O2 with
high efficiency and stability under sunlight from pure water without the assistance of any other additives. In this work, we firstly obtained the nano-scale brookite TiO2 nanoflutes catalyst with ~25 nm holes by a novel method. Although brookite TiO2 is commonly considered as a photocatalytically less active material, recent research has proved that brookite TiO2 has moderate depth of the electron trap that can promote both of photocatalytic oxidations and reduction reactions compared with anatase and rutile [15]. When decorated with Pt nanoparticles, it can achieve efficient H2 and H2O2 production from water with the PIWS mechanism. Besides, the brookite TiO2 nanoflutes in nanoscale with good crystallinity possess the advantages of enhanced light absorption ability, improved photogenerated charge separation efficiency, shorter carrier migration distance, and more active sites to promote the pho tocatalytic efficiency. 2. Results and discussion Fig. 1A shows the PIWS and POWS processes. Contrary to the POWS which is a four-electron reaction to produce H2 and O2, PIWS is a twoelectron reaction to produce H2 and H2O2. Density functional theory (DFT) calculations suggested that production of H2O2 and H2 was feasible in the Pt/brookite TiO2 system after revealing the possible water splitting mechanisms: (1) dissociation of water into surface in termediates (Fig. 1B); (2) formation of H2 or oxidation products from intermediates. During H2O dissociation process, H2O molecules prefer to adsorb at the top site of Ti, with the newly formed O-Ti bond tilting to the surface (Fig. 1B, S1) [16,17]. The adsorbed H2O molecule (H2Oad) donates a H to a nearby oxygen (O) site of TiO2 forming a H* and OH* overcoming an energy barrier of 0.395 eV. However, as shown in Fig. 1C, the two co-adsorbed H atoms are difficult to combine to form H2 due to a large energy barrier of 2.381 eV on brookite TiO2. This barrier is reduced to 1.009 eV on the Pt (111) surface. Out of the three possible ends for OH*: (I) OH* dissociates to form adsorbed O* and H* [18,19]; (II) OH* combines with each other to form an OOH* intermediate and
Fig. 1. Photocatalytic production of H2 and H2O2 from pure water. (A) Comparison of PIWS and POWS processes. (B) Reaction energy diagram from adsorbed H2O to OH* and H*. (C) Energy profiles of H2-production on the brookite TiO2 (210) and Pt (111) surfaces. (D) Calculated energy profile of converting OH* to O*, H* (route I), OOH* (route II) and H2O2* (route III). The red and gray spheres are the O and Ti atoms in TiO2, respectively; the pink and white spheres are the O and H atoms in H2O, respectively; the blue spheres are the Pt atoms. 2
S. Cao et al.
Nano Energy xxx (xxxx) xxx
release a H; (III) two OH* combine to form H2O2* (Fig. 1D), DFT results indicated that route (III) is energetically more favored, as it only needs to overcome an energy barrier of 0.769 eV, in contrary to the 1.250 and 1.463 eV for route (I) and (II), respectively. These results indicate that it
is possible to obtain H2O2 and H2 on the brookite TiO2/Pt system through PIWS. The DFT results indicates OH* plays a significant role during PIWS pathway and based on the guidance, we prepared mesoporous brookite
Fig. 2. Preparation and characterization of photocatalyst. (A) Schematic illustration of the synthesis process of Pt/porous b-TiO2 nanoflutes. (B) XRD, (C) Raman spectrum of b-TiO2 nanoflutes and Pt/porous b-TiO2 nanoflutes. (D) TEM, (E) HRTEM and (F) SAED of b-TiO2 nanoflutes. (G) TEM and (H) HRTEM of the Pt/porous b-TiO2 nanoflutes (the black dots in Fig. 2G are the Pt nanoparticles on TiO2 surface). 3
S. Cao et al.
Nano Energy xxx (xxxx) xxx
TiO2 nanoflutes. Although the synthesis of mesoporous TiO2 crystals have been reported, most of them were in micron scale and more easily leads to carrier recombination. Besides, all the reported mesoporous TiO2 crystals are anatase and rutile. Up to now, the synthesis of pure mesoporous brookite TiO2 nanoflutes is technically difficult. In this work, the novel nano-sized mesoporous brookite TiO2 nanoflutes were good to provide large surface area for reaction and high crystallinity was made to reduce the charge recombination in the bulk [20]. Meanwhile, surface hydrophilicity was made with abundant hydroxyl -OH groups to provide good water adsorptivity to facilitate the activation, which was confirmed by the infrated (IR) spectroscopy (Fig. S2) and water contact angle measurements (Fig. S3) [21]. Diffuse reflectance ultra violet–visible spectra and Mott-Schottky (M-S) plots (pH ¼ 7) revealed its band-gap and valence are 3.21 eV (Figure. S4 and S5) and 2.76 V (NHE, Fig. S6), respectively, indicating the favorable band gap structure for -OH activation during PIWS [22]. The porous TiO2 nanoflutes and the Pt/TiO2 were obtained through a template method as illustrated in Fig. 2A. The formation of the brookite phase was confirmed with the characteristic X-ray diffraction (XRD) pattern and Raman spectrum (Fig. 2B–C) [23]. The sharp XRD diffrac tion peaks of each sample match well with those of standard diffraction data of brookite (JCPDS no. 65–2448), which demonstrates that the as-synthesized TiO2 is highly crystalline and phase pure. The Raman spectroscopy, which is a phase-sensitive technique, further confirms it. The Raman spectra of the samples (Fig. 2C) show the presence of the bands typical of brookite, including A1g (152, 195, 247, 412, 637 cm 1), B1g (213, 322, 412, 502 cm 1), B2g (366, 395, 460, 584 cm 1), and B3g (287, 545 cm 1). The good agreement of the observed Raman bands with the active modes of brookite reported previously, confirmed that the final powder was pure brookite TiO2 [24]. Further scrutiny spectra shows no difference between TiO2 and Pt-loaded TiO2, implying that Pt nanoparticles are incorporated onto the surface of TiO2 rather than into the lattice (Fig. 2B and C). The large surface area was realized with nanoflutes’ tube size of 40–50 nm, pore size around 25 nm and length of 250 � 50 nm (Fig. 2D, Fig. S7-S8). They were grown via a hydrothermal-assisted penetration sol-gel process on assemblies of silica (SiO2) nanoparticles with an average diameter of ~25 nm (Fig. S9) and obtained final products after etching the SiO2 nanoparticle template in aqueous NaOH solution. The clear lattice fringes with plane space of 0.35 nm from the (210) crystal plane in the high-resolution transmission electron microscopy (HRTEM) image in Fig. 2E and the single crystalline pattern of the corresponding selected area electron diffraction (SAED) pattern in Fig. 2F revealed the high crystallinity and the exposed surface of the active (210) facet suitable for H2O2 and H2 production [25]. The noticeable disordered layer near the surface (Fig. 2E) may provide effective charge trapping and photocatalytic active sites for the re actions. The porosity and surface area were seen with the N2 adsorption from Brunauer-Emmett-Teller (BET) measurements of 25–40 nm and 54.2 m2 g 1, respectively (Fig. S10). Pt nanoparticles were loaded on brookite TiO2 using an in situ photo-deposition method. It means the brookite TiO2 nanoflutes were obtained firstly and then Pt nanoparticles were loaded on it. Therefore, the Pt nanoparticles are loaded on the surface rather than embedded in the nanostructure of TiO2. The above XRD and Raman result also confirm it. It can be clearly observed that the black dots of Pt nanoparticles with 3–5 nm (Fig. 2G) and exposed (111) surface facets (Fig. 2H) are well-dispersed on the surface of TiO2. Their mass percentages were determined using inductively coupled plasma mass spectrometry (ICP-MS) (Table S1). The photocatalytic activity was tested under Hg lamp irradiation (λ � 300 nm, 500 W, Fig. S11) at room temperature. The H2 production rate from deionized water (pH ~7.0) was 9.8 � 0.6 μmol mg 1 h 1 after 5 h of irradiation (Fig. 3A, Fig. S12-13), which is over 220 and 900-fold higher than that with the Pt/P25 TiO2 (0.04 μmol mg 1 h 1) [11] and Pt/C3N4 (0.01 μmol mg 1 h 1) [12] H2/H2O2 production systems, respectively, 12-fold higher than the reported highest rutile TiO2 POWS system (0.8 μmol mg 1 h 1) [26] (Fig. 3B–C, Table S2). The AQY value
of H2-production can achieve 43.4%. As far as we known, it is the highest value compared with the reported TiO2-based POWS systems. Apart from the higher efficiency, the catalyst also shows better stability than non-TiO2-based POWS systems (Tables S2 and S3). The H2O2 production rate were 8.2 � 0.8 μmol mg 1 h 1. The price per electron for H2O2 is ~140-fold higher than that for O2 and ~18-fold higher than that for H2 [9]. Therefore, the commercial value for PIWS is ~9-fold higher than that for POWS even ignoring the separation cost. Besides, the low concentration of H2O2 possesses more advantages: (a) The low concentration of H2O2 has a wider range of applications. For example, most chemical synthesis and pulp bleaching require <9 wt % of H2O2 and only <0.1 wt% H2O2 is necessary for water treatment. (b) The low centration of H2O2 is safer than high concentrated H2O2 for trans portation. In this work, the Pt nanoparticles act as the role of cocatalyst, which could not only promote the charge separation of TiO2, but also act as the active center for proton reduction. Then, the H2 was generated on Pt nanoparticles, and H2O2 was formed on TiO2. To further confirm the cocatalytic role of Pt nanoparticles, controlled experiment was con ducted, and the result indicates that the H2O2 were not derived from catalytic reaction by Pt nanoparticles (Fig. S14). The pH value main tained 7.0 during the entire reaction, indicating both Hþ and OH from water are consumed at the same time. H2O2 instead of O2 was the only oxidation product from pure water on the Pt/brookite TiO2 system. The preferable formation of H2O2 instead of O2 was likely due to the uniqueness of the Pt/brookite TiO2 system which kinetically favored the formation of H2O2 as our DFT calculation revealed (Fig. 1). During the PIWS process, will the formed H2O2 decompose into oxygen? Although it is generally acknowledged that H2O2 could decompose on Pt NPs, the absolute concentration of H2O2 in this system is relatively low because of the diluted reaction solution. Most of the produced H2O2 can maintain steadily for the following separation and application. As the reaction prolonged, the amount of H2O2 decreased and the ratio of H2O2: H2 was lower than 1:1 (Fig. S15), however, no O2 was detected in gas and liquid (Fig. S16). It is proposed that even small amount of H2O2 decomposed to O2, the O2 can be readily reduced because of the close adsorption (Figs. S17–18). DFT results confirmed that desorption of the absorbed O2 from the TiO2 (210) surface required a large energy barrier of 3.18 eV (Fig. S19). Therefore, it can be concluded that even if a small portion of H2O2 decompose into O2, the adsorbed O2 could be readily reduced by the photogenerated electrons, which does not have negative effects on the separation of H2 and PIWS process. Continuous production of hydrogen is attractive for practical appli cations. H2 was time-proportionally produced from pure water under UV-irradiation for 30 h (Fig. 3D). Although a slight rate decrease ap pears after that, restoration can be easily achieved by regenerating the catalyst in situ via heating at 120 � C for 5 h under Ar flow. The crystal morphology and composition of the nanoflutes were maintained after the reaction based on the results of XRD, Raman, XPS and TEM (Figs. S20–S23) measurements, indicating the good stability of the brookite TiO2 nanoflutes. The slight rate decrease maybe due to the larger size of Pt nanoparticles (Fig. S23) and decreased amount of absorbed -OH on the surface of TiO2 (Fig. S22). To demonstrate the possibility of large-scale application, we further tested the H2 produc tion efficiency in seawater. The H2 production rate of 7.2 and 3.6 μmol mg 1 h 1 were achieved, under UV and natural sunlight irra diation (12:00 a.m.–13:00 p.m., 2018-05-03, Beijing, China), respec tively (Fig. 3E). The efficiency under UV light is 9-fold higher than the reported highest TiO2-based POWS systems (0.8 μmol mg 1 h 1) [26] (Fig. 3B, Table S2) and as far as we known, it is also the highest value compared the reported TiO2-based systems in seawater. More impor tantly, the system can sustain for more than 10 days (12 h/day, average rate ~2.0 μmol mg 1 h 1) when a photocatalytic plate was constructed (Fig. 3F and Fig. S24), demonstrating the excellent stability for practical application. To experimentally understand the photocatalytic mechanism, we 4
S. Cao et al.
Nano Energy xxx (xxxx) xxx
Fig. 3. Photocatalytic performance of Pt/b-TiO2 porous nanoflutes. (A) Reaction time course of photocatalytic H2 and H2O2 production of Pt/b-TiO2 porous nanoflutes. (B) Comparison of photocatalytic H2 and H2O2 production efficiencies of Pt/b-TiO2 porous nanoflutes with reported H2/H2O2 production systems. (C) Comparison of photocatalytic H2 production of Pt/b-TiO2 porous nanoflutes with other reported TiO2-based POWS systems (the detail information is shown in Table S2). (D) Photocatalytic H2-production durability test for Pt/b-TiO2 porous nanoflute powders. (E) Photocatalytic H2 production activities of Pt/b-TiO2 porous nanoflutes in pure water and seawater under UV light or sunlight irradiation. (F) A 10-day photocatalytic H2 production test using a 4 cm � 6 cm ground glass panel coated Pt/b-TiO2 porous nanoflute powders.
performed X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements. Compared with Pt foil and PtO2 references, the white line peaks (2p→5d) of Pt display an increased intensity in both Pt/b-TiO2 nanoflutes and nanoparticles, indicating that the Pt is mildly oxidized (Fig. 4A) [27,28]. However, the Pt L3-edge XANES spectra of Pt/b-TiO2 nanoflutes display a weaker white line peak, which indicates more electrons occupied on Pt 5d or bitals in b-TiO2 nanoflutes than that of nanoparticles suggesting electron transfer from b-TiO2 nanoflutes to Pt are easier than that of nano particles [29,30]. More information can be obtained from EXAFS spectra of the Pt L3-edge (Fig. 4B). The main peaks near 2.5 Å are derived from neatest coordination shells of Pt atoms and the weaker intensity in both Pt/b-TiO2 nanoflutes and nanoparticles indicate lesser first coordination
number than that of Pt foil due to the size effects. The similar intensity indicates the similar coordination environment of Pt in Pt/b-TiO2 nanoflutes and nanoparticles [31]. However, for the small shoulder peaks near 1.5 Å, which belong to Pt-O coordination, indicates that Pt on b-TiO2 nanoparticles is more serious oxidized than that of on nanoflutes, agreeing well with the XANES curves of Pt. In the Ti K-edge X-ray absorption near-edge structure (XANES) spectra in Fig. 4C, three typical pre-edge peaks corresponding to quadruple-allowed 1s → 3d transitions were observed [31], with larger intensities of the Ti pre-edge peaks due to larger local structural distortion. A left shift of K-edge indicates a lower valence state likely due to oxygen vacancy (VO) in the nanoflutes, as confirmed by the decreased “white line” intensities and the observed disordered structure near the 5
S. Cao et al.
Nano Energy xxx (xxxx) xxx
Fig. 4. Mechanism exploration for the PIWS process. XANES (A) and EXAFS (B) analysis of Pt L3-edge spectra for Pt/b-TiO2 nanoflutes and nanoparticles. (C) Ti K-edge XANES spectra and (D) Fourier transforms of Ti K-edge EXAFS spectra for Pt/b-TiO2 nanoflutes and nanoparticles. (E) XPS spectra of O1s for the Pt/b-TiO2 nanoflutes and nanoparticles after 1 h of UV light irradiation. (F) ESR responses of the DMPO–OH spin adduct in water suspension of Pt/b-TiO2 nanoflutes and nanoparticles before and after 60’s UV light irradiation in an Ar atmosphere.
surface in the HRTEM in Fig. 2E. The benefits of disordered structure near the surface on photocatalytic activity were widely acknowledged in previous studies [32]. The larger percentage of VO in the nanoflutes were confirmed by the EXAFS spectra (Fig. 4D). The peak located at 1.5 Å is derived from Ti-O coordination and the weaker intensity of the peak indicates a lower Ti-O coordination number than that of nano particles. The more VO may provide more active sites for the -OH adsorption and H2O2 formation. The abundant adsorbed -OH and subsequent formation of �OH for mation were confirmed by X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR) measurements. The amount of -OH (OOH, 531.7 eV) on the surface of Pt/TiO2 nanoflutes increased after ultravi olet light irradiation (Fig. 4E) [32], although there were no obvious difference in the dark, from the O 1s XPS spectra (Fig. S25). Meanwhile,
a new peak at 533.2 eV belonging to physisorbed water emerged indi cating improved hydrophilicity after irradiation [33], in contrary to the unchanged peak for the nanoparticles after irradiation (Fig. 4E and Fig. S26). Similar conclusion was drawn from the Ti 2p XPS spectra before and after irradiation (Figs. S27–28) [34,35]. The large formation of �OH was confirmed with ESR results. A much higher 5,5-dimethyl-1- pyrroline-N-oxide (DMPO), DMPO-OH radical adduct signal (1:2:2:1, aN ¼ aH ¼ 15.4 G) was observed after 60 s of UV light irradiation for the Pt/TiO2 nanoflutes than for the nanoparticles [36], indicating the facilitated formation of �OH (Fig. 4F). Based on the above analysis, the catalytic pathway can be proposed in Scheme S1. H2O2 formation is a two-electron transfer process by the combination of two �OH. By contrast, water oxidation to O2 involves four electron and four proton transfer for the eventual formation of an O-O bond, which is much more 6
S. Cao et al.
Nano Energy xxx (xxxx) xxx
difficult than the H2O2 formation. Therefore, H2O2 generation is kinet ically more favorable than O2 formation. In conclusion, we have demonstrated the remarkable production of H2 and H2O2 from pure water using porous brookite TiO2 nanoflutes decorated with Pt nanoparticles through PIWS. The H2 and H2O2 pro duction rates can reach up to 9.8 � 0.6 μmol mg 1 h 1 and 8.2 � 0.8 μmol mg 1 h 1, respectively, in pure water (pH~7). And the AQY value of H2-production can achieve 43.4%. Continuous H2 pro duction and H2 production from seawater have also been displayed. DFT and experimental results confirm that the distinct performance proceeds by a kinetically favorable two-electron reaction on the surface of brookite TiO2 and its unique surficial chemistry micro-circumstance is beneficial to adsorb activate hydroxyl groups under light and further form hydrogen peroxide.
[23] T. Tachikawa, M. Fujitsuka, T. Majima, J. Phys. Chem. C 111 (2017) 5259–5274. [24] A. Naldoni, T. Montini, F. Malara, M.M. Mr� oz, A. Beltram, T. Virgili, C.L. Boldrini, M. Marelli, I. Romero-Oca~ na, J.J. Delgado, V.D. Santo, P. Fornasiero, ACS Catal. 7 (2017) 1270–1278. [25] M. Choi, J. Lim, M. Baek, W. Choi, W. Kim, K. Yong, ACS Appl. Mater. Interfaces 9 (2017) 16252–16260. [26] R. Li, Y. Weng, X. Zhou, X. Wang, Y. Mi, R. Chong, H. Han, C. Li, Energy Environ. Sci. 8 (2015) 2377–2382. [27] J. Mao, W. Chen, D. He, J. Wan, J. Pei, J. Dong, Y. Wang, P. An, Z. Jin, W. Xing, H. Tang, Z. Zhuang, X. Liang, Y. Huang, G. Zhou, L. Wang, D. Wang, Y. Li, Sci. Adv 3 (2017) e1603068. [28] L. Tao, Y. Shi, Y.C. Huang, R. Chen, Y. Zhang, J. Huo, Y. Zou, G. Yu, J. Luo, C. L. Dong, S. Wang, Nano Energy 53 (2018) 604–612. [29] N. Becknell, Y. Kang, C. Chen, J. Resasco, N. Kornienko, J. Guo, N.M. Markovic, G. A. Somorjai, V.R. Stamenkovic, P. Yang, J. Am. Chem. Soc. 137 (2015) 15817–15824. [30] J. Li, C.H. Liu, M.N. Banis, D. Vaccarello, Z.F. Ding, S.D. Wang, T.K. Sham, J. Phys. Chem. C 121 (2017) 24861–24870. [31] A. L�eon, G. Yalovega, A. Soldatov, M. Fichtner, J. Phys. Chem. C 112 (2008) 12545–12549. [32] X. Chen, L. Liu, P.Y. Yu, S.S. Mao, Science 331 (2011) 746–750. [33] A. Naldoni, M. Allieta, S. Santangelo, M. Marelli, F. Fabbri, S. Cappelli, C. L. Bianchi, R. Psaro, V. Dal Santo, J. Am. Chem. Soc. 134 (2012) 7600–7603. [34] B. Erdem, R.A. Hunsicker, G.W. Simmons, E.D. Sudol, V.L. Dimonie, M.S. El-Aasser, Langmuir 17 (2001) 2664–2669. [35] J. Zheng, Y. Lyu, R. Wang, C. Xie, H. Zhou, S.P. Jiang, S. Wang, Nat. Commun. 9 (2018) 3572. [36] R. Shi, H.F. Ye, F. Liang, Z. Wang, K. Li, Y. Weng, Z. Lin, W.F. Fu, C.M. Che, Y. Chen, Adv. Mater. 30 (2018) 1705941.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements We acknowledge the financial support by the National Natural Sci ence Foundation of China (No 21703046, 21972028 and 11672079) and the Ministry of Science and Technology of China (No 2016YFA0200902). X. Chen appreciates the support from the U.S. Na tional Science Foundation (DMR-1609061), and the College of Arts and Sciences, University of Missouri - Kansas City. The authors would like to thank Natural International Children’s Growth Center for their help in painting the TOC.
Shuang Cao received her PhD degree in Technical Institute of Physics and Chemistry, Chinese Academy of Sciences in 2016. She joined National Center for Nanoscience and Technology as a assistant research fellow in 2016. Her research focuses on designing and optimizing photocatalytic catalysts.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.nanoen.2019.104287.
Ting-Shan Chan is currently an associate research scientist fellow at National Synchrotron Radiation Research Center. The job responsibilities are spokesperson of beamline 01C1 and 16A1. He got his Ph.D. degree in the Department of Chemistry from National Taiwan University, Taiwan, in 2003. His research focuses on the energy and environmental materials.
References [1] Z. Pan, G. Zhang, X. Wang, Angew. Chem. Int. Ed. 58 (2019) 7102–7106. [2] J. Meng, H. Zhou, Y. Zou, R. Wang, Y. Lyu, S.P. Jiang, S. Wang, Energy Environ. Sci. 12 (2019) 2345–2347. [3] J. Huang, H. Gao, Y. Xia, Y. Sun, J. Xiong, Y. Li, S. Cong, J. Guo, S. Du, G. Zou, Nano Energy 46 (2018) 305–313. [4] X. Du, J. Huang, J. Zhang, Y. Yan, C. Wu, Y. Hu, C. Yan, T. Lei, W. Chen, C. Fan, J. Xiong, Angew. Chem. Int. Ed. 58 (2019) 4484–4502. [5] A. Meng, L. Zhang, B. Cheng, J. Yu, Adv. Mater. 31 (2019), 1807660. [6] J. Cai, M. Wu, Y. Wang, H. Zhang, M. Meng, Y. Tian, X. Li, J. Zhang, L. Zheng, J. Gong, Chem 2 (2017) 877–892. [7] X. Chen, R. Shi, Q. Chen, Z. Zhang, W. Jiang, Y. Zhu, T. Zhang, Nano Energy 59 (2019) 644–650. [8] Z. Wang, W. Wu, Q. Xu, G. Li, S. Liu, X. Jia, Y. Qin, Z.L. Wang, Nano Energy 38 (2017) 518–525. [9] K. Sayama, ACS Energy Lett 3 (2018) 1093–1101. [10] M.N. Young, M.J. Links, S.C. Popat, B.E. Rittmann, C.I. Torres, ChemSusChem 9 (2016) 3345–3352. [11] V.M. Daskalaki, P. Panagiotopoulou, D.I. Kondarides, Chem. Eng. J. 170 (2011) 433–439. [12] J. Liu, Y. Zhang, L. Lu, G. Wu, W. Chen, Chem. Commun. 48 (2012) 8826–8828. [13] K. Fuku, K. Sayama, Chem. Commun. 52 (2016) 5406–5409. [14] L. Wang, S. Cao, K. Guo, Z. Wu, Z. Ma, L. Piao, Chin. J. Catal. 40 (2019) 470–475. [15] J.J.M. Vequizo, H. Matsunaga, T. Ishiku, S. Kamimura, T. Ohno, A. Yamakata, ACS Catal. 7 (2017) 2644–2651. [16] R. Mu, Z.J. Zhao, Z. Dohn� alek, J. Gong, Chem. Soc. Rev. 46 (2017) 1785–1806. [17] D. Wang, T. Sheng, J. Chen, H.F. Wang, P. Hu, Nat. Catal 1 (2018) 291–299. [18] J. Huang, Y. Sun, Y. Zhang, G. Zou, C. Yan, S. Cong, T. Lei, X. Dai, J. Guo, R. Lu, Y. Li, J. Xiong, 30(2018) 1705045. [19] J. Huang, Y. Sun, X. Du, Y. Zhang, C. Wu, C. Yan, Y. Yan, G. Zou, W. Wu, R. Lu, Y. Li, J. Xiong, Adv. Mater. 30 (2018) 1803367. [20] E.J.W. Crossland, N. Noel, V. Sivaram, T. Leijtens, J.A. Alexander-Webber, H. J. Snaith, Nature 495 (2013) 215–219. [21] B.F. Iskandar, A.B.D. Nandiyanto, K.M. Yun, C.J. H Jr., K. Okuyama, P. Biswas, Adv. Mater. 19 (2017) 1408–1412. [22] X. Li, J. Yu, M. Jaroniec, Chem. Soc. Rev. 45 (2016) 2603–2636.
Ying-Rui Lu is currently a postdoctoral research fellow at Na tional Synchrotron Radiation Research Center. He got his Ph.D. degree in the program for science and technology of accelerator light source from National Chiao Tung University, Taiwan, in 2017. His research focuses on the developments of X-ray spectroscopy instruments for in-situ monitoring the chemical processes in gas–solid and liquid–solid interfaces.
7
S. Cao et al.
Nano Energy xxx (xxxx) xxx Xinghua Shi received his Bachelor degree from Peking Uni versity, Master degree from Institute of Mechanics, Chinese Academy of Sciences and PhD from Brown University. From 2011 to 2016 back in Institute of Mechanics, CAS as an asso ciate professor, he studied nanoparticle-cell interaction with multiscale modeling. In the beginning of 2016 he moved to National Center for Nanoscience and Technology as full pro fessor and principle investigator. There he focused on the simulation of catalysis, self-assembly and drug delivery.
Sarah Alzuabi is a graduate student in Chemistry at University of Missouri – Kansas City (UMKC). She got her B.S. degree from the University of Hafr Albatin, Saudi Arabia. Her research in terest including nanomaterials synthesis, characterization and applications in photocatalysis, electrocatalysis and microwave absorption.
Bing Fu received his B.S. degree in chemistry from Capital Normal University, Beijing, China, in 2016. He is currently in a Ph.D.‘s course under the supervision of Professor Lingyu Piao at National Center for Nanoscience and Technology, Chinese Academy of Sciences. His research interests are engineering and optimizing semiconductor photocatalysts for water splitting reactions.
Ping Cheng received her Bachelor degree from Yantai Univer sity, Master degree and PhD from Beijing University of Chem ical Technology. From 2016 to 2019, she do research in National Center for Nanoscience and Technology as the post doctor. In the May of 2019 she moved to University of Shanghai for Science and Technology as a lecturer. She focused on the simulation of the structure, electronic and catalytic properties of materials.
Zhijiao Wu received her M.S. degree from University of Chinese Academy of Sciences in 2011. She joined National Center for Nanoscience and Technology from 2011 as an engineer. She mainly focuses on nano-standardization and the reference ma terials development of nanomaterials.
Min Liu is currently a professor in School of Physics and Elec tronic Sciences, Central South University (CSU). He joined CSU in 2017 after receiving the National 1000 Young Talents award in China. He obtained the Ph.D. degree (Excellent Doctoral Thesis) in 2010 from Institute of Electrical Engineering, Chi nese Academy of Sciences. From 2010 to 2015, he worked as a project researcher and a senior researcher in Tokyo University. From 2015 to 2017, he work as research associate at the Uni versity of Toronto. His research focused on developing efficient catalysts for CO2 reduction and water splitting.
Hongmei Li received her bachelor and Ms.C. in School of Physics and Electronics from Hunan Normal University, China in 2004 and 2007. From 2008 to 2017, she worked as a lectures in Shaoyang University and a visiting researcher at Univeristy of Toronto. Now she is a lecturer at Center South University. Her research focused on developing efficient catalysts for photoelectrochemical water splitting.
Dr. Li earned his bachelor’s degree from the East China Uni versity of Science and Technology (ECUST) in 2003. He completed his Ph.D. at the University of South Caro lina—Columbia in 2009. He is currently an assistant professor in the Department of Chemistry and Biochemistry at Northern Illinois University and holds a joint assistant scientist position at the Advanced Photon Source at Argonne National Lab. His research interests focus on using advanced X-ray techniques to study the self-assembly of nanoparticles as well as energy ma terials including catalyst and battery. Dr. Li has authored/ coauthored more than 100 peer-reviewed research papers.
Kang Liu is currently a Ph.D. candidate in Prof. Min Liu’s Group at Central South University. He received M.S. degree in school of physics and optoelectronics from Xiangtan University in 2017. Now, his research interests focus on the density func tional theory calculation, electrocatalytic CO2 reduction and synchrotron-based techniques.
Xiaobo Chen, Ph.D., is an Associate Professor in Chemistry at University of Missouri – Kansas City (UMKC). After he obtained his Ph.D. degree from Case Western Reserve University in 2005, he spent six years in University of California – Berkeley and Lawrence Berkeley National Laboratory, exploring various renewable energy projects. He joined UMKC in 2011 and his research interests include nanoscience, photocatalytic and electrochemical hydrogen production, photocatalytic pollutant removal and organic synthesis, microwave-absorbing mate rials, AI chemistry, etc. He has published over 160 peerreviewed, high-profile articles with a total citation number over 43,000.
8
S. Cao et al.
Nano Energy xxx (xxxx) xxx Lingyu Piao received her Ph.D. in Institute of Chemical Engi neering from Tianjin University, China in 2002. From 2002 to 2005, she worked as a postdoctor in Peking University and Pierre and Marie Curie University. Now she is a professor at National Center for Nanoscience and Technology. Her research interests include synthesis, characterization and optical prop erties of functionalized nanomaterials and functional nano materials in the field of energy and environment.
9