Ultrathin Lanthanum Tantalate Perovskite Nanosheets Modified by

Ultrathin Lanthanum Tantalate Perovskite Nanosheets Modified by Nitrogen Doping for Efficient Photocatalytic Water Splitting Meilin Lv,† Xiaoqin Sun,† Shunhang Wei,† Cai Shen,‡ Yongli Mi,†,§ and Xiaoxiang Xu*,† †

Shanghai Key Lab of Chemical Assessment and Sustainability, School of Chemical Science and Engineering, Tongji University, 1239 Siping Road, Shanghai, 200092, China ‡ Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences. 1219 Zhongguan Road, Zhenhai District, Ningbo, Zhejiang 315201, China § The Hong Kong University of Science and Technology, Department of Chemical and Biomolecular Engineering, Kowloon, Hong Kong, China S Supporting Information *

ABSTRACT: Ultrathin nitrogen-doped perovskite nanosheets LaTa2O6.77N0.15− have been fabricated by exfoliating Dion−Jacobson-type layered perovskite RbLaTa2O6.77N0.15. These nanosheets demonstrate superior photocatalytic activities for water splitting into hydrogen and oxygen and remain active with photon wavelengths as far as 600 nm. Their apparent quantum efficiency under visible-light illumination (λ ≥ 420 nm) approaches 1.29% and 3.27% for photocatalytic hydrogen and oxygen production, being almost 4-fold and 8-fold higher than bulk RbLaTa2O6.77N0.15. Their outstanding performance likely stems from their tiny thickness (single perovskite slab) that essentially removes bulk charge diffusion steps and extends the lifetime of photogenerated charges. Theoretical calculations reveal a peculiar 2D charge transportation phenomenon in RbLaTa2O6.77N0.15; thus, exfoliating RbLaTa2O6.77N0.15 into LaTa2O6.77N0.15− nanosheets has limited impact on charge transportation properties but significantly enhances the surface areas which contributes to more reaction sites. KEYWORDS: photocatalysis, water splitting, perovskite nanosheets, doping, exfoliating, visible-light photocatalyst doping and particle size reduction.17−20 The former has been quite effective in extending light absorption of various wide band gap semiconductors, while the latter is very useful in shortening the migration pathways for photogenerated charges as well as enhancing surface reactive sites (e.g., nanosized particles).21−26 Nevertheless, their side effects are evident. For instance, dopants per se are defects which may induce additional charge recombination events, and nanosized particles generally have cross-section too small to harvest adequate solar photons.21,27 How to balance these two strategies in a single semiconductor photocatalyst so as to minimize their side effects remains a challenge. 2D semiconductors with thickness of a few nanometers are promising materials for this purpose. Their ultrathin thickness ensures very short charge migration distance while their large cross-section guarantees ample photon collections and conversions.21 This is extremely important as water oxidation is a four-holes, four-proton reaction which

H

ow to fulfill the ever-growing energy need of our modern economies is one of the major tasks for this century, not only because traditional fossil-fuel-based energy feedstock is essentially unsustainable but also due to various environmental issues associated with fossil fuel usage. 1−3 There is now an increasing consensus on searching/developing clean and renewable energy resources and technologies that can ultimately free us from fossil-fuel reliance.4−6 In these contexts, water splitting into hydrogen and oxygen by means of photocatalytic reactions offers a promising scenario to build a clean and renewable energy infrastructure.7−12 This is because hydrogen is a clean energy vector whose recycling involved only water and solar insolation guarantees inexhaustible photon fluxes that are widely accessible all over the world.7 The practical deployment of this intriguing technique awaits critical breakthrough on the efficiency of converting solar energy into hydrogen fuel.13 Most photocatalysts, however, are subject to poor visible light absorption or the short lifetime of photogenerated charges, etc., bearing a low solar-to-hydrogen efficiency.14−16 Strategies to overcome or mitigate above shortcomings generally include © 2017 American Chemical Society

Received: August 29, 2017 Accepted: November 1, 2017 Published: November 1, 2017 11441

DOI: 10.1021/acsnano.7b06131 ACS Nano 2017, 11, 11441−11448

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Figure 1. (a) X-ray powder diffraction patterns of freshly prepared RbLaTa2O7 before and after nitridation, vertical bars of standard RbLaTa2O7 patterns (JCPDS: 01-089-0389) are shown at the bottom for comparisons. (b) Observed and calculated XRD patterns for RbLaTa2O6.77N0.15; the refinement converged with good R factors (Rp= 3.91%, Rwp = 5.09%, χ2 = 1.473). Inset: schematic representation of refined crystal structure; unit cell is marked by blue lines. (c) Calculated band structures, total density of states (DOS), and partial density of states (PDOS) of constituent elements of RbLaTa2O7. Fermi level is marked by dotted orange line. (d) Density contour maps of RbLaTa2O7 at VBM and CBM. Unit cell is marked by black lines.

requires accumulation of charges at single reaction site.28−30 In addition, all dopants in 2D semiconductors will now lie close to the surface so that bulk charge recombination induced by dopants is largely removed.21 Previous studies on 2D semiconductors such as exfoliated perovskite nanosheets have witnessed long-lived photogenerated charges (nanosecond to microsecond time scale)31−33 and superior photocatalytic activity.34−36 For instance, Rh-doped calcium niobate nanosheets exhibit apparent quantum efficiency as high as 60% at 300 nm for photocatalytic hydrogen production in the presence of aqueous methanol solution.37 Nevertheless, those exfoliated perovskite nanosheets mostly contain triple layers of metal− oxygen octahedrons (MO6, M = Nb, Ta or Ti), e.g., Ca2Ta3O10−;34 further decreasing the thickness of nanosheets down to a double layer of MO6 is most appealing as all MO6 settle at the surface and all charges will be generated instantaneously at the surface without bulk diffusion steps. In this work, we fabricated nitrogen-doped lanthanum tantalate nanosheets (LaTa2O6.77N0.15−) containing a double layer of TaO(N)6 octahedrons by exfoliating Dion−Jacobson (DJ)-type layered perovskite RbLaTa2O6.77N0.15. LaTa2O6.77N0.15− nanosheets demonstrate efficient photocatalytic activity for water splitting under both UV and visible light illumination and remain active with photon wavelength as far as 600 nm.

RESULTS AND DISCUSSION DJ type layered perovskite RbLaTa2O7 was prepared by conventional solid-state reactions, and nitrogen doping was realized by burning RbLaTa2O7 in flowing ammonia gas at high temperatures. Samples before and after nitrogen doping illustrate a XRD pattern similar to that of standard RbLaTa2O7 (JCPDS: 01-089-0389) (Figure 1a), indicating single phase formation. Rietveld refinement on these XRD patterns reveals a slight shrinkage on the unit cell after nitrogen doping (Figure 1b, Figure S1, and Table S1) and can be attributed to the formation of oxygen vacancies during nitrogen substitution, i.e., 2NH3 + 3O×O → 2N′O + V··O + 3H2O↑. Typical refined XRD patterns of nitrogen-doped RbLaTa2O7 are illustrated in Figure 1b. Reasonable goodness-of-fit parameters (Rp, Rwp, and χ2) were achieved by setting the constraints that O and N atoms occupy the same crystallographic positions. Thus, nitrogen doping likely occurs randomly in the O sublattice. The nitrogen-doping level at oxygen sites was determined to be 2.2% according to TGA analysis (Figure S5); therefore, the chemical formula of N-doped sample can be written as RbLaTa2O6.77N0.15. The crystal structure of RbLaTa2O7 and its nitrogen-doped congener have typical layered architectures where perovskite slabs, i.e., LaTa2O7− layers, are intervened by Rb+ layers along [001]p direction (Figure 1b, inset). This peculiar laminated crystal structure has important implications 11442

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Figure 2. (a) UV−vis light absorption spectra (converted from diffuse reflectance spectra) of freshly prepared RbLaTa2O7 and RbLaTa2O6.77N0.15. (b) Kubelka−Munk transformation of diffuse reflectance data. Inset: digital photograph of sample powders for visual inspection. The band gap value can be deduced by extrapolating the linear region down to the energy axis. (c) X-ray photoelectron spectra (XPS) of N 1s state for RbLaTa2O7 and RbLaTa2O6.77N0.15. (d) XPS valence band scan for RbLaTa2O7 and RbLaTa2O6.77N0.15.

will be significantly increased. On the other hand, pristine RbLaTa2O7 is a typical wide band gap semiconductor with band gap as large as 4.26 eV (Figure 2b). Such a large band gap essentially prevents it from usage in solar energy conversions due to limited light absorption. However, nitrogen doping considerably extend its light absorption to the visible light region (Figure 2a). The absorption edge of RbLaTa2O7 is redshifted by almost 300 nm after nitrogen doping which effectively turns its color from white to orange (Figure 2a,b). XPS spectra confirm the existence of N 1s signal at bind energy around 395 eV in sample RbLaTa2O6.77N0.15, corresponding to lattice N3− species.36,39 The Ta 4p3/2 state, however, remains almost intact before and after nitrogen doping, suggesting that the color change does not stem from reduction of Ta5+ such as Ta4+ species.28,29,40 The intense visible light absorption as well as band gap reduction for RbLaTa2O6.77N0.15 compared with pristine one origin primarily from the uplifting of the VBM, as can be seen from the XPS valence band scan (Figure 2d). DFT calculation suggests that VBM for RbLaTa2O6.77N0.15 has

to the electronic structure in which an anisotropic charge transportation phenomenon can be seen (Figure 1c): the band dispersion along [001] direction (from M to A), i.e., perpendicular to perovskite slabs, is negligible for both conduction band (CB) and valence band (VB), corresponding to enormously large effective mass for both electrons and holes.38 On the contrary, band dispersion for CB along [100] direction (from Γ to X), i.e., parallel to perovskite slabs, covers a large energy range (>2 eV), indicating a much smaller effective mass for electrons. Considering the fact that both conduction band minimum (CBM) and valence band maximum (VBM) settles within perovskite slabs (Figure 1d, specifically, CBM on Ta 5dxy orbitals and VBM mainly on O3 2px and 2py orbitals), charges are only allowed to migrate within perovskite slabs and transportation across the Rb+ layers is practically forbidden. In other words, an individual perovskite slab can function as an independent 2D semiconductor photocatalyst, and exfoliating them into single perovskite nanosheets is highly beneficial as surface area/reaction sites 11443

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Figure 3. (a) Transmission electron microscopy (TEM) image of exfoliated LaTa2O6.77N0.15− nanosheets. Inset: selected area electron diffraction (SAED) patterns. (b) High-resolution TEM image of single LaTa2O6.77N0.15− nanosheet; local area is enlarged for visual inspections. (c) Atomic force microscopy (AFM) image of LaTa2O6.77N0.15− nanosheets. Inset: digital photograph of LaTa2O6.77N0.15− nanosheet suspension. (d) Cross-sectional profile of selected LaTa2O6.77N0.15− nanosheet marked in (c).

seems to increase the negative charge on the nanosheets. More importantly, the orange color is maintained after RbLaTa2O6.77N0.15 is exfoliated into nanosheets (Figure 3c and Figure S6, inset), confirming again that the strong visible light absorption is due to nitrogen doping rather than the secondary phases. The suspensions were further inspected by different microscopic techniques. TEM analysis shows the suspension is made up of nanosheets with lateral size up to several hundred nanometers (Figure 3a). Selected area electron diffraction (SAED) on individual nanosheet displays spot patterns with tetragonal symmetry, being consistent with the structure of single perovskite slab peeled off from RbLaTa2O6.77N0.15. The high-resolution TEM image clearly reveals the local fine structures of a single nanosheet in which discontinuity of lattice fringes due to defect formation can be easily identified (Figure 3b). The thicknesses of LaTa2O6.77N0.15− nanosheets were further examined under AFM conditions. A typical AFM image of LaTa2O6.77N0.15− nanosheets is illustrated in Figure 3c. The nanosheets normally have a lateral dimensions up to microns and cross-sectional profile of single nanosheets suggests a thickness around 2 nm (Figure 3d). This value is approximately 0.9 nm larger than the theoretical thickness of single perovskite slab determined from crystallographic data. The extra thickness is probably due to water molecules or TBA+ cations adsorbed at the surface of these nanosheets.34 To the best of our knowledge, this is the thinnest perovskite nanosheets ever reported, which contains only two layers of TaO(N)6 octahedrons. Such thin perovskite nanosheets are extremely beneficial for photocatalytic reactions, not only because of a larger surface area therein more reaction sites but also because all TaO(N)6 octahedrons are now accommodated at the surface, photogenerated charges can be formed instantaneously at the surface without bulk diffusion steps.

substantial N 2p characters and band gap reduction is due to the hybridization between N 2p orbitals with O 2p orbitals which uplifts the VBM (Figure S3). It is also worth mentioning that surface hydroxyl species is considerably enhanced after nitrogen doping, attributing to a more hydrophilic surface (Figure S2). Thereby, nitrogen doping serves as an effective approach to tailor the optical and surface properties of layered perovskite compound RbLaTa2O7 with layered crystal framework maintained. This is also confirmed from the SEM image of RbLaTa2O7 before and after nitridation (Figure S4). The disklike appearance of sample particles remains after nitrogen doping, although the smooth surface becomes somewhat crumbled. LaTa2O6.77N0.15− nanosheets were then fabricated by step-bystep insertion of large cations into the interlayers and exfoliating the compound ultrasonically. A schematic representation of an individual step is illustrated in Figure S6. The Coulombic interactions between perovskite slabs (negatively charged) and interlayer cations (positively charged) is gradually weakened upon exchanging larger cations into the interlayer space. Thereby, the periodic stacking of perovskite slabs along [001] direction can be eventually broken down with aid of sonication and self-standing LaTa2O6.77N0.15− nanosheets are formed. The degradation of periodicity along [001] direction upon ion exchange is monitored by XRD techniques where (001) peak is clearly broadened and finally depressed by substituting Rb+ with hydrated protons and ethylamine (Figure S7). This is also accompanied by a slight pulverization of sample particles as well as a coarser particle surface (Figure S8). Suspensions of LaTa2O6.77N0.15− nanosheets obtained by this means can last up to several days without severe agglomeration. The negative charge of these nanosheets has been confirmed by their negative zeta potentials (Table S2), and nitrogen doping 11444

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Figure 4. Temporal photocatalytic activities for bulk sample RbLaTa2O6.77N0.15 and LaTa2O6.77N0.15− nanosheets: (a) hydrogen production under full range illumination (λ ≥ 250 nm), 1 wt % Pt loaded as cocatalyst; (b) hydrogen production under visible light illumination (λ ≥ 420 nm), 1 wt % Pt loaded as cocatalyst; (c) dependence of photocatalytic hydrogen production of LaTa2O6.77N0.15− nanosheets on the wavelength of incident light, 1 wt % Pt loaded as cocatalyst; (d) oxygen production under visible light illumination (λ ≥ 420 nm), 1 wt % CoOx loaded as cocatalyst.

Photocatalytic activities of LaTa2O6.77N0.15− nanosheets were then evaluated by monitoring hydrogen or oxygen production from water in the presence of sacrificial agents under light illumination. Proper amounts of Pt or CoOx were used as cocatalysts and were loaded onto sample powders by photodeposition method. Control experiments with either sample powders or light illumination missing gave indiscernible hydrogen or oxygen signals, precluding any spontaneous hydrogen or oxygen production reactions. Immediate hydrogen or oxygen signals were detected by illuminating the sample powders, confirming real photocatalytic processes. Figure 4a ,b displays temporal hydrogen production under full range (λ ≥ 250 nm) and visible light illumination (λ ≥ 420 nm). In both cases, LaTa2O6.77N0.15− nanosheets exhibit much higher activity than the bulk RbLaTa2O6.77N0.15. More than 6-fold and 4-fold enhancement in hydrogen production was noticed for nanosheets under full range and visible light illumination in comparison to the bulk counterpart, highlighting the benefit of using nanosheets for photoreduction reactions. The activity of these nanosheets for hydrogen production shows a clear dependence on the wavelength of incident photons which matches well with their light absorption curve (Figure 4c). Therefore, LaTa2O6.77N0.15− nanosheets are efficient photocatalysts for hydrogen productions with photon wavelength as far as 600 nm. The performance of these nanosheets was further optimized by altering the amounts of Pt cocatalysts

loaded. A 2 wt % Pt loading was found to give the best performance both under full range and visible-light illumination (Figure S9), corresponding to apparent quantum efficiency (AQE) as high as 3.88% and 1.29%, respectively. It is worth mentioning that the AQE gives the lowest estimate of the real quantum efficiency as long-wavelength photons and photons scattered/reflected by sample powders and electrolyte are all included during calculation. More interestingly, LaTa2O6.77N0.15− nanosheets also demonstrate superior photocatalytic activity for oxygen productions under visible light illumination (λ ≥ 420 nm). Figure 4d illustrate the temporal photocatalytic oxygen production of LaTa2O6.77N0.15− nanosheets and their bulk counterpart in the presence of silver nitrate aqueous solution. Likewise, nanosheets display much higher activity than bulk counterpart. The AQE of nanosheets approaches 3.27% for 2.5 h illumination, which is almost 8 fold higher than bulk RbLaTa2O6.77N0.15. The oxygen evolution reaction is generally considered as a rate-limiting step for water splitting as it needs the participation of four holes and four protons, being energetically and mechanistically difficult. Such a high activity for oxygen production under visible light illumination signifies the usefulness of LaTa2O6.77N0.15− nanosheets for solar water splitting. It is also worth mentioning that AQE and hydrogen production rate of LaTa2O6.77N0.15− nanosheets in this work is much better or at least comparable to a number of highly active photocatalysts reported in the 11445

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LaTa2O6.77N0.15− nanosheets were fabricated by step-by-step exchange of large cations into RbLaTa2O6.77N0.15 and exfoliating the compound ultrasonically (Figure S5): first, about 1 g of RbLaTa2O6.77N0.15 was suspended in concentrated HCl aqueous solution (3 mol·L−1). The suspensions were magnetically stirred for 1 week with fresh HCl solution every 24 h to ensure complete protonation. The protonated compound was designated as HLaTa2O6.77N0.15 and was collected by centrifugation. Second, HLaTa2O6.77N0.15 was then suspended in 1 M ethylamine aqueous solution under magnetic stirring for 15 days. The resultant powders were labeled as EA−LaTa2O6.77N0.15 and were collected by centrifugation. Third, EA−LaTa2O6.77N0.15 was further suspended in 150 mL of 0.025 M tetrabutylammonium hydroxide (TBAOH) aqueous solution under magnetic stirring for 5 days. LaTa2O6.77N0.15− nanosheets were obtained by sonicating the suspensions for 2 h and centrifuging at 3000 rpm for 10 min. The supernatant was collected for further analysis. The concentration of LaTa2O6.77N0.15− nanosheets was about 0.05 g·mL−1 and was determined by drying the supernatant and weighing the residuals. Materials Characterization. Phase purity and crystal structure were inspected using X-ray powder diffraction (XRD) techniques on a Bruker D8 Focus diffractometer. The incident X-ray radiation is Cu Kα1 (λ = 1.5406 Å) and Cu Kα2 (λ = 1.5444 Å). Typical step size and collection time are 0.01° and 0.1 s. Rietveld refinement on the collected data was performed using the General Structure Analysis System (GSAS) software package.44 A field emission scanning electron microscope (Hitachi S4800) and a transmission electron microscope (JEOL JEM-2100) were used to examine the microstructure of asprepared samples. The thickness of nanosheet samples was analyzed by AFM (Veeco, USA). Optical absorption data of sample powders were collected on a UV−vis spectrophotometer coupled with integrating sphere (JASCO-750) and were analyzed using the JASCO software suite. The reference nonabsorbing material is BaSO4. Surface conditions and valence band states were probed by X-ray photoelectron spectroscopy (Thermo Escalab 250 with a monochromatic Al Kα X-ray source). All bonding energies were adjusted according to the energy shift of the adventitious carbon C 1s peak at 284.7 eV.45 Nitrogen content in the N-doped sample was determined using thermogravimetric analysis (TGA) (Labsysevo, SETARAM, France). Zeta potential of nanosheets were analyzed on a Zetasizer Nano ZSP (Malvern). Photocatalytic Activity. Photocatalytic experiments were carried out in a top-illumination-type reactor connected to a gas-closed circulation and evacuation system (Perfect Light, Labsolar-IIIAG). In a typical experiment, 50 mg sample powders were ultrasonically dispersed into 100 mL of sacrificial agent solution. Either sodium sulfite (0.05 M) or silver nitrate (0.05 M) was used as a sacrificial agent to promote photoreduction or photo-oxidation reactions. The suspensions were then sealed within the reactor and were subjected to evacuation for 30 min in order to remove air dissolved. The photocatalytic reactions were initiated by illuminating the reactor with full range or visible light and simultaneously monitoring the gas component within the reactor using an online gas chromatograph (TECHCOMP, GC7900) coupled with a TCD detector (5 Å molecular sieve columns and Ar carrier). The temperature of the reactor during the whole experiments was maintained at 293 K using a water jacket. Appropriate amounts of either Pt or CoOx were used as cocatalysts to promote H2 or O2 evolutions and were loaded onto sample powders by photodeposition method: proper amounts of H2PtCl6 or Co(NO3)3 was added to the sample powder suspensions. The suspensions were subsequently irradiated by full range illumination for 1 h to full convert H2PtCl6 or Co(NO3)3 into Pt nanoparticles or CoOx. The light source in these experiments was a 500 W high-pressure mercury lamp (NbeT, Merc-500). Visible-light illumination was generated by filtering the output of the lamp with a UV cutoff filter (λ ≥ 420 nm). The photon flux of the lamp was calibrated using a quantum meter (Apogee MP-300). The recorded photon flux was ∼1543.9 μmol·m−2·s−1 for full range irradiation (λ ≥ 250 nm) and ∼668.5 μmol·m−2·s−1 for visible light irradiation (λ ≥

literatures,24,31,34,36,37,41−43 indicating that the strategies by nitrogen doping and exfoliating into nanosheets are quite effective. We then attempted to evaluate their photocatalytic activities for complete water splitting where no sacrificial agents were added. Quite interestingly, LaTa2O6.77N0.15− nanosheets can split water into H2 and O2 stoichiometrically under full range illumination, while bulk RbLaTa2O6.77N0.15 remains almost inactive (Figure S10), further confirming the superior photocatalytic activities of LaTa2O6.77N0.15− nanosheets. Long lived photogenerated charges have been noticed for ultrathin semiconductors in previous literatures and are likely responsible for their high activity.31−33 We have also investigated the charge generation and separation situations in LaTa2O6.77N0.15− nanosheets by open-circuit voltage decay (OCVD) experiments (see the Supporting Information for more details). Clearly, LaTa2O6.77N0.15− nanosheets show a much slower OCVD process and own a much longer charge lifetime than bulk RbLaTa2O6.77N0.15 (Figure S11). These results well explain the superior photocatalytic activities observed after exfoliation of bulk RbLaTa2O6.77N0.15 into LaTa2O6.77N0.15− nanosheets.

CONCLUSIONS In summary, ultrathin LaTa2O6.77N0.15− nanosheets have been successfully fabricated by exfoliating nitrogen-doped DJ-type perovskite compound RbLaTa2O7. These nanosheets are typical 2D semiconductors with a perovskite structure and strong visible-light absorption as far as 600 nm. Their intense light absorption stems from the hybridation between O 2p and N 2p orbitals which considerably uplift the valence band maximum. These nanosheets generally have a thickness of a single perovskite slab and are composed of two layers of TaO(N)6 octahedrons. Superior photocatalytic activities for water splitting have been noticed for these nanosheets both under full range (λ ≥ 250 nm) and visible light illumination (λ ≥ 420 nm). The apparent quantum efficiency under visible light illumination approaches 1.29% and 3.27% for photocatalytic hydrogen and oxygen production, being almost 4-fold and 8-fold higher than their bulk counterpart. Such an intriguing activity can be attributed to their peculiar microstructures that all TaO(N)6 octahedrons are accommodated at the surface, ensuring instantaneous charge generation at the surface and absence of bulk charge diffusion steps. This work here signifies the usefulness of 2D semiconductors for solar energy conversions. EXPERIMENTAL SECTION Materials Synthesis. DJ type layered perovskite RbLaTa2O7 was prepared by conventional solid-state reactions. Appropriate amounts of Rb2CO3 (Aladdin, 99%), La2O3 (Aladdin, 99.9%), and Ta2O5 (Aladdin, 99.99%) were mixed using an agate mortar and pestle. Raw powders were pretreated in a muffle furnace at 773 K for 5 h prior to weighing for moisture elimination. The admixtures were unaxially pressed into pellets under a pressure of 5 tons and were transferred into an alumina crucible for further calcination. The typical calcining temperature is 1373 K, and the duration time is 10 h. Extra Rb2CO3 (∼50%) was used in order to compensate Rb loss during high temperature calcination. The calcined pellets were pulverized using agate mortar and pestle. Nitrogen-doped RbLaTa2O7 was prepared by calcining freshly prepared RbLaTa2O7 in a tube furnace under flowing NH3 at 1073 K for 5 h. Typical NH3 flow rate was 200 mL·min−1. The nitrogen content was determined to be 2.2% at the oxygen site according to thermogravimetric analysis, and the chemical formula of the compound can be written as RbLaTa2O6.77N0.15. 11446

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420 nm). The apparent quantum efficiency (AQE) is then calculated using the following equations:

AQE for H2 production = 2 × moles of hydrogen production per hour /moles of photon flux per hour × 100 AQE for O2 production = 4 × moles of oxygen production per hour /moles of photon flux per hour × 100 Theoretical Calculations. The electronic structures of RbLaTa2O7 and its nitrogen-doped congener were studied by theoretical calculation using DFT implemented in the Vienna Ab initio simulation package.46 We applied the Perdew, Burke, and Ernzerhof exchangecorrelation functional within the generalized gradient approximation47 and the projector augmented-wave pseudopotential for the calculations.48 A tetragonal cell (a = b = 3.88 Å, c = 11.07 Å, α = β = γ = 90°) was built for simulations of RbLaTa2O7. Nitrogen doping was considered by substituting 3 O with 2 N and 1 vacancy randomly. All geometry structures were fully relaxed until the forces on each atom are less than 0.01 eV·Å−1. Static calculations were carried out using a 13 × 13 × 5 Monkhorst−Pack k-point grid.49

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b06131. Open-circuit voltage decay (OCVD) measurements, observed and calculated XRD patterns for RbLaTa2O7, XPS of O 1s state for RbLaTa2O7 before and after nitrogen doping, DFT calculation for RbLaTa2O6.77N0.15, SEM images of RbLaTa2O7 and RbLaTa2O6.77N0.15, TGA curve of RbLaTa2O6.77N0.15, exfoliating steps, XRD patterns of samples during exfoliation, SEM images of samples during exfoliation, effect on Pt loading on the photocatalytic activity, complete photocatalytic water splitting on LaTa2O6.77N0.15− nanosheets, OCVD curves and charge lifetime, unit cell parameters, BET surface area, and band gap values of samples and zeta potential of nanosheets before and after nitrogen doping (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel: +86-21-65986919. ORCID

Cai Shen: 0000-0001-5825-4028 Xiaoxiang Xu: 0000-0002-5042-9505 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank the Young Scientists Fund of the National Natural Science Foundation of China (Grant No. 21401142) for funding and the Recruitment Program of Global Youth Experts (1000 plan). This work was supported by the Shanghai Science and Technology Commission (14DZ2261100) and the Fundamental Research Funds for the Central Universities. REFERENCES (1) Annual Energy Outlook 2017; U. S. Energy Information Administration, 2017. 11447

DOI: 10.1021/acsnano.7b06131 ACS Nano 2017, 11, 11441−11448

Article

ACS Nano

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DOI: 10.1021/acsnano.7b06131 ACS Nano 2017, 11, 11441−11448