Ultrathin Lanthanum Tantalate Perovskite Nanosheets Modified by

Subscriber access provided by University of Birmingham

Article

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 ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b06131 • Publication Date (Web): 01 Nov 2017 Downloaded from http://pubs.acs.org on November 2, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Nano is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

Ultrathin Lanthanum Tantalate Perovskite Nanosheets Modified by Nitrogen Doping for Efficient Photocatalytic Water Splitting Meilin Lva, Xiaoqin Suna, Shunhang Weia, Cai Shenb, Yongli Mia,c and Xiaoxiang Xua,* a

Shanghai Key Lab of Chemical Assessment and Sustainability, School of Chemical Science and Engineering, Tongji University, 1239 Siping Road, Shanghai, 200092, China. Email: [email protected], telephone: +86-21-65986919

b

Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences. 1219 Zhongguan Road, Zhenhai District, Ningbo, Zhejiang, China c

The Hong Kong University of Science and Technology, Department of Chemical and Biomolecular Engineering, Kowloon, Hong Kong, China

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 wavelength 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 lifetime of photo-generated charges. Theoretical calculations reveal a peculiar 2D charge transportation

phenomenon

in

RbLaTa2O6.77N0.15,

thereby

exfoliating

RbLaTa2O6.77N0.15 into LaTa2O6.77N0.15- nanosheets has limited impact on charge

ACS Paragon Plus Environment

ACS Nano

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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

How to fulfil 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 short lifetime of photo-generated charges, etc. bearing a low solar-to-hydrogen efficiency.14-16 Strategies to overcome or mitigate above shortcomings generally include doping and particle size reduction.17-20 The former has been quite effective in extending light


Page 2 of 25

Page 3 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

absorption of various wide band gap semiconductors whilst the latter is very useful in shortening the migration pathways for photo-generated charges as well as enhancing surface reactive sites (e.g. nano-sized particles).21-26 Nevertheless, their side effects are evident. For instance, dopants per se are defects which may induce additional charge recombination events and nano-sized 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 whilst their large cross-section guarantees ample photon collections and conversions.21 This is extremely important as water oxidation is a four-holes-four-protons reaction which requires accumulation of charges at single reaction site.28-30 Besides, 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 photo-generated 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 double layer of MO6 is most


ACS Nano

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

appealing as all MO6 now 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 double layer of TaO(N)6 octahedrons by exfoliating Dion-Jacobson type (DJ) layered perovskite RbLaTa2O6.77N0.15. LaTa2O6.77N0.15- nanosheets demonstrate efficient photocatalytic activity for water splitting both under UV and visible light illumination and remain active with photon wavelength as far as 600 nm.

Results and Discussions

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


Page 4 of 25

Page 5 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

(Rp= 3.91%, Rwp = 5.09%, χ2 = 1.473), a schematic representation of refined crystal structure is shown as inserted image, 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.

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 similar XRD patterns to 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. 2 NH 3 + 3OO× → 2 N O' + VO'' + 3H 2O ↑ . 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. Thereby, 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


ACS Nano

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

direction (Figure 1b, inserted image). This peculiar laminated crystal structure has important implications to the electronic structure in which 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 Contrarily, 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 small 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, individual perovskite slab can function as independent 2D semiconductor photocatalyst and exfoliating them into single perovskite nanosheets is highly beneficial as surface area/reaction sites 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 red-shifted by almost 300 nm after nitrogen doping which effectively turns its color from white to orange (Figure 2a and 2b). XPS spectra confirm the existence of N 1s signal at bind energy around 395 eV in sample


Page 6 of 25

Page 7 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

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 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 disk-like appearance of sample particles remains after nitrogen doping, although the smooth surface becomes somewhat crumbled.


ACS Nano

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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, a digital photograph of sample powders is inserted for visual inspection, band gap value can be deduced by extrapolating the linear region down to energy axis; (c) X-ray photoelectron spectra (XPS) of N 1s state for RbLaTa2O7 and RbLaTa2O6.77N0.15 and (d) XPS valence band scan for RbLaTa2O7 and RbLaTa2O6.77N0.15


Page 8 of 25

Page 9 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

Figure 3. (a) Transmission electron microscopy (TEM) image of exfoliated LaTa2O6.77N0.15- nanosheets, selected area electron diffraction (SAED) patterns are inserted; (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, a digital photograph of LaTa2O6.77N0.15- nanosheet suspension is also inserted; (d) cross-sectional profile of selected LaTa2O6.77N0.15nanosheet marked in (c)

LaTa2O6.77N0.15- nanosheets were then fabricated by step-by-step inserting large cations into the interlayers and exfoliating the compound ultrasonically. A schematic


ACS Nano

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

representation of 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 is 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 seems to increase the negative charge on the nanosheets. More importantly, the orange color is maintained after exfoliating RbLaTa2O6.77N0.15 into nanosheets (Figure 3c and Figure S6, inserted image), 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. High-resolution TEM image clearly reveals the


Page 10 of 25

Page 11 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

local fine structures of single nanosheet in which discontinuity of lattice fringes due to defect formation can be easily identified (Figure 3b). The thickness 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, photo-generated charges can be formed instantaneously at the surface without bulk diffusion steps.


ACS Nano

7000 6000

H2 evolved / µ mol⋅g-1

(b)

Nanosheet Bulk

400

5000

λ ≥ 250 nm 4000 3000 2000

300

200 150 100 50

0

0 0.5

1.0

1.5

2.0

λ ≥ 420 nm

250

1000

0.0

Nanosheet Bulk

350

H2 evolved / µ mol⋅g-1

(a)

2.5

0.0

0.5

1.0

Time / h

1.5

2.0

2.5

Time / h

0.8 80

-1

-1

100

0.6 60

40

0.4

20 0.2 0 0.0

O2 evolved / µ mol⋅g-1

(d)

120

1.0

H2 evolution (µ mol⋅ g ⋅ h )

(c) Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 25

2500

Nanosheet Bulk 2000

1500

λ ≥ 420 nm

1000

500

0 -20

200

300

400

500

600

700

800

0.0

0.5

Wavelength (nm)

1.0

1.5

2.0

2.5

3.0

Time / h

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 was loaded as cocatalyst; (b) hydrogen production under visible light illumination (λ ≥ 420 nm), 1 wt% Pt was loaded as cocatalyst; (c) dependence of photocatalytic hydrogen production of LaTa2O6.77N0.15- nanosheets on the wavelength of incident light, 1 wt% Pt was loaded as cocatalyst; (d) oxygen production under visible light illumination (λ ≥ 420 nm), 1 wt% CoOx was 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


Page 13 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

agents under light illumination. Proper amounts of Pt or CoOx were used as cocatalysts and were loaded onto sample powders by photo-deposition 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 and 4b display 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 bulk counterpart, highlighting the benefit of using nanosheets for photo-reduction 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. 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


ACS Nano

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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 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 stochiometrically 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 photo-generated charges have been noticed for ultrathin semiconductors in previous literatures and are likely responsible for their high activity.31-33 We have also


Page 14 of 25

Page 15 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

investigated the charge generation and separation situations in LaTa2O6.77N0.15nanosheets by open-circuit voltage decay (OCVD) experiments (see supporting information for more details). Clearly, LaTa2O6.77N0.15- nanosheets show a much slower OCVD processes and own a much longer charge lifetime than bulk RbLaTa2O6.77N0.15 (Figure S11). These results well explain the superior photocatalytic activities observed after exfoliating bulk RbLaTa2O6.77N0.15 into LaTa2O6.77N0.15nanosheets.

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 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


ACS Nano

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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 Materials synthesis Dion-Jacobson type (DJ) 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. Typical calcining temperature is 1373 K and 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 oxygen site according to thermogravimetric analysis and the chemical formula of the compound can be written as RbLaTa2O6.77N0.15. LaTa2O6.77N0.15- nanosheets were fabricated by step-by-step exchanging large cations


Page 16 of 25

Page 17 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

into RbLaTa2O6.77N0.15 and exfoliating the compound ultrasonically (Figure S5): first, about 1 g RbLaTa2O6.77N0.15 was suspended in concentrated HCl aqueous solution (3 mol·L-1). The suspensions were magnetically stirred for one week with refreshing 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 labelled as EA-LaTa2O6.77N0.15 and were collected by centrifugation. Third, EA-LaTa2O6.77N0.15 was further suspended in 150 mL 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 is 0.01° and 0.1 s. Rietveld refinement on the collected data was performed using General Structure Analysis System (GSAS) software package.44 A field emission scanning electron microscope (Hitachi S4800) and a transmission electron microscope


ACS Nano

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(JEOL JEM-2100) were used to examine the microstructure of as-prepared samples. The thickness of nanosheet samples was analyzed by atomic force microscopy (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 JASCO software suite. The reference non-absorbing 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 adventitious carbon C 1s peak at 284.7 eV45. Nitrogen content in the N doped sample was determined using thermogravimetric analysis (TGA) (Labsysevo, SETARAM, France). Zeta potential of nanosheets were analyzed on 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 sacrificial agent solution. Either sodium sulfite (0.05 M) or silver nitrate (0.05 M) was used as sacrificial agent to promote photo-reduction 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


Page 18 of 25

Page 19 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

using an on-line 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 photo-deposition 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. 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 ∼668.5µmol·m-2·s-1 for visible light irradiation (λ ≥ 420 nm). The apparent quantum efficiency (AQE) is then calculated using the following equation:

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%


ACS Nano

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Theoretical calculations The electronic structures of RbLaTa2O7 and its nitrogen doped congener were studied by theoretical calculation using density functional theory (DFT) implemented in the Vienna Ab initio simulation package (VASP).46 We applied the Perdew, Burke and Ernzerhof (PBE) exchange-correlation functional within the generalized gradient approxiamtion (GGA)

47

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 13 × 13 × 5 Monkhorst-Pack k-point grid.49

Associated content Supporting Information The supporting information is available online: details of 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


Page 20 of 25

Page 21 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

gap values of samples and zeta potential of nanosheets before and after nitrogen doping.

Acknowledgements We thank Young Scientists Fund of the National Natural Science Foundation of China (Grant No. 21401142) for funding and Recruitment Program of Global Youth Experts (1000 plan). The 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.

2.

Dudley, B. BP Statistical Review of World Energy; 2016.

3.

Nocera, D. G. Living Healthy on a Dying Planet. Chem. Soc. Rev. 2009, 38, 13-15.

4.

Turner, J. A. A Realizable Renewable Energy Future. Science 1999, 285, 687-689.

5.

Lewis, N. S.; Nocera, D. G. Powering the Planet: Chemical Challenges in Solar Energy Utilization. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15729-15735.

6.

Reece, S. Y.; Hamel, J. A.; Sung, K.; Jarvi, T. D.; Esswein, A. J.; Pijpers, J. J. H.; Nocera, D. G. Wireless Solar Water Splitting Using Silicon-Based Semiconductors and Earth-Abundant Catalysts. Science 2011, 334, 645-648.

7.

Turner, J. A. Sustainable Hydrogen Production. Science 2004, 305, 972-974.

8.

Liu, J.; Liu, Y.; Liu, N. Y.; Han, Y. Z.; Zhang, X.; Huang, H.; Lifshitz, Y.; Lee, S. T.; Zhong, J.; Kang, Z. H. Metal-free Efficient Photocatalyst for Stable Visible Water Splitting via a Two-Electron Pathway. Science 2015, 347, 970-974.

9.

Wang, X. C.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.; Domen, K.; Antonietti, M. A Metal-Free Polymeric Photocatalyst for Hydrogen Production from Water under Visible Light. Nat. Mater. 2009, 8, 76-80.

10. Xu, X. X.; Randorn, C.; Efstathiou, P.; Irvine, J. T. S. A Red Metallic Oxide Photocatalyst. Nat. Mater. 2012, 11, 595-598. 11. Zou, Z. G.; Ye, J. H.; Sayama, K.; Arakawa, H. Direct Splitting of Water under Visible Light Irradiation with An Oxide Semiconductor Photocatalyst. Nature 2001, 414, 625-627. 12. Maeda, K.; Teramura, K.; Lu, D. L.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K. Photocatalyst Releasing Hydrogen from Water - Enhancing Catalytic Performance Holds Promise for Hydrogen Production by Water Splitting in Sunlight. Nature 2006, 440, 295-295.


ACS Nano

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

13. Chen, Z. B.; Jaramillo, T. F.; Deutsch, T. G.; Kleiman-Shwarsctein, A.; Forman, A. J.; Gaillard, N.; Garland, R.; Takanabe, K.; Heske, C.; Sunkara, M.; McFarland, E. W.; Domen, K.; Miller, E. L.; Turner, J. A.; Dinh, H. N. Accelerating Materials Development for Photoelectrochemical Hydrogen Production: Standards for Methods, Definitions, and Reporting Protocols. J. Mater. Res. 2010, 25, 3-16. 14. Chen, X. B.; Shen, S. H.; Guo, L. J.; Mao, S. S. Semiconductor-Based Photocatalytic Hydrogen Generation. Chem. Rev. 2010, 110, 6503-6570. 15. Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Environmental Applications of Semiconductor Photocatalysis. Chem. Rev. 1995, 95, 69-96. 16. Kudo, A.; Miseki, Y. Heterogeneous Photocatalyst Materials for Water Splitting. Chem. Soc. Rev. 2009, 38, 253-278. 17. Zhou, Y. G.; Zhang, Y. F.; Lin, M. S.; Long, J. L.; Zhang, Z. Z.; Lin, H. X.; Wu, J. C. S.; Wang, X. X. Monolayered Bi2WO6 Nanosheets Mimicking Heterojunction Interface with Open Surfaces for Photocatalysis. Nat. Commun. 2015, 6, 8340 18. Jiang, L.; Ni, S.; Liu, G.; Xu, X. X. Photocatalytic Hydrogen Production over Aurivillius Compound Bi3TiNbO9 and Its Modifications by Cr/Nb Co-Doping. Appl. Catal. B-Environ. 2017, 217, 342-352. 19. Liu, G.; Wang, L. Z.; Sun, C. H.; Yan, X. X.; Wang, X. W.; Chen, Z. G.; Smith, S. C.; Cheng, H. M.; Lu, G. Q. Band-to-Band Visible-Light Photon Excitation and Photoactivity Induced by Homogeneous Nitrogen Doping in Layered Titanates. Chem. Mater. 2009, 21, 1266-1274. 20. Liu, G.; Wang, L. Z.; Yang, H. G.; Cheng, H. M.; Lu, G. Q. Titania-Based Photocatalysts-Crystal Growth, Doping and Heterostructuring. J. Mater. Chem. 2010, 20, 831-843. 21. Ida, S.; Ishihara, T. Recent Progress in Two-Dimensional Oxide Photocatalysts for Water Splitting. J. Phys. Chem. Lett. 2014, 5, 2533-2542. 22. Maeda, K. Rhodium-Doped Barium Titanate Perovskite as a Stable p-Type Semiconductor Photocatalyst for Hydrogen Evolution under Visible Light. ACS Appl. Mater. Interfaces 2014, 6, 2167-2173. 23. Lu, L. W.; Lv, M. L.; Wang, D.; Liu, G.; Xu, X. X. Efficient Photocatalytic Hydrogen Production over Solid Solutions Sr1-xBixTi1-xFexO3 (0

Ultrathin Lanthanum Tantalate Perovskite Nanosheets Modified by

Recommend Documents