2D Heterojunctions

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SrTiO3 nanoparticles/SnNb2O6 nanosheets 0D/2D Heterojunctions with Enhanced Interfacial Charge Separation and Photocatalytic Hydrogen Evolution Activity Yu Jin, Deli Jiang, Di Li, Peng Xiao, Xiaodong Ma, and Min Chen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b01548 • Publication Date (Web): 21 Sep 2017 Downloaded from http://pubs.acs.org on September 22, 2017

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ACS Sustainable Chemistry & Engineering

SrTiO3 nanoparticles/SnNb2O6 nanosheets 0D/2D Heterojunctions with Enhanced Interfacial Charge Separation and Photocatalytic Hydrogen Evolution Activity† Yu Jina, Deli Jianga,*, Di Lib, Peng Xiaoa, Xiaodong Maa, Min Chena,* a

School of Chemistry and Chemical Engineering, Jiangsu University, 301 Xuefu

Road, Zhenjiang 212013, China b

Institute for Energy Research, Jiangsu University, 301 Xuefu Road, Zhenjiang

212013, China *Corresponding

author

E-mail: [email protected]; [email protected]

ACS Paragon Plus Environment


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ABSTRACT: Exploration of high performance and stable metal-oxide-based hybrid photocatalysts for hydrogen evolution is highly desirable. In this work, novel SrTiO3 nanoparticles/SnNb2O6 nanosheets hybrid 0D/2D heterojunctions with an interfacial interaction were constructed by a facile two-step wet chemistry strategy. Different characterization techniques were adopted to investigate the microscopic structures and physicochemical properties of the as-prepared hybrid heterojunctions. The optimal weight percent of SrTiO3 loading is 20 wt%, generating the highest H2 evolution amount of 17.16 µmol, which are 298 and 2 times higher than that of bare SrTiO3 and SnNb2O6. It can be suggested that an interfacial interaction among SrTiO3 and SnNb2O6 could result in efficient charge separation and enhanced H2-generation activity, which was confirmed by photoelectrochemical analyses. This work implies that the construction of 0D/2D metal-oxide-based hybrid heterojunctions with an interfacial interaction is an effective way to fabricate high-performance photocatalyst for solar fuel production. KEYWORDS: photocatalysis, hydrogen-evolution, heterojunction, charge separation, mechanism INTRODUCTION Owing to the depletion and combustion of fossil fuels, it is a worldwide priority target to develop renewable, ecofriendly and economical energy technologies to alleviate energy and environmental crisis.1-4 As a promising strategy, heterogeneous semiconductor photocatalytic hydrogen production driven by renewable solar energy has attracted a great deal of attention around the world, which could achieve the


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sustained solar-to-fuel conversion and ease the above encountered problems.5-9 Therefore, various types of heterogeneous semiconductors aimed at H2-generation have been developed. The thin two-dimensional (2D) crystal with exceptional electronic, optical and mechanical properties is a significant category of nanostructured materials to which tremendous attention has been paid recently.11-14 It is noteworthy that 2D nanomaterials with outstanding photocatalytic properties benefit from high two-dimensional anisotropy and atomic-layer thickness which can provide more reactive sites and efficient charge separation.15-17 As a typical 2D nanosheet materials, SnNb2O6, consisting of a two-octahedron-thick sheet built by corner-sharing NbO6 octahedral flat and edge-sharing SnO8 square antiprisms flat. Besides, SnNb2O6 possess the narrow band gap (~2.3 eV) and proper CB position, which is beneficial for the visible light harvesting and photocatalytic H2 generation.18-21 Recently, Seo et al. synthesized SnNb2O6 nanoplates by a facile template-free solvothermal method, and the synthesized nanoplates exhibited enhanced photocatalytic H2-evolution rate of 18.4 µmol/g·h-1.22 Zhou et al. reported the ultrathin SnNb2O6 nanosheets, which was synthesized via a facile hydrothermal method and showed improved visible-light photocatalytic H2-production rate of 13.2 µmol·h-1.23 Yuan et al. fabricated SnNb2O6-GR nanocomposite by an efficient electrostatic self-assembly method and almost 100% of RhB was degraded after 60 min.24,25 Nevertheless, the photocatalytic efficiency of SnNb2O6 is limited by its high recombination rate of photogenerated electron-hole pairs, which greatly strict its practical applications in photocatalytic H2 production. For the sake of further



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improving the visible-light harvesting property of SnNb2O6 as well as to suppress the photogenerated charge recombination, extensive modification strategies have been undertaken including morphological control,22,23 noble metal deposition,26 and coupling with the others semiconductors to construct the heterojunction. 24,27,28 Particularly, cooperating different semiconductor materials with matched band gap to form heterojunction is an effective strategy to improve the photocatalytic performance because of the synergistic effect, which could lead to the enhanced visible light absorption ability, improved the efficient separation and immigration of photoexcited charge.29-32 Heretofore, some semiconductors such as GR,24 g-C3N427 and WO328 have been employed to couple with SnNb2O6 to form heterojunctions with enhanced photocatalytic activity. However, despite these great progresses in constructing heterojunction photocatalysts, their efficiency is not enough for its applications in photocatalytic environmental remediation due to their limited interfacial contact and low charge separation efficiency. Thus, further stimulating the development of novel SnNb2O6-based heterojunctions with high-performance photocatalytic activities for hydrogen generation is still a huge challenge. Strontium titanate (SrTiO3) as one of the promising perovskite-type photocatalyst candidates have been widely investigated and found to be favorable and catalytically active in water splitting, due to its outstanding corrosion resistance, heat resistance, chemical and structural stability.33-41 In this contribution, SrTiO3 is probably an ideal candidate to construct heterojuntions with SnNb2O6 nanosheet owing to the following reasons: the band-edge positions of SnNb2O6 match well with those of SrTiO3, which


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could theoretically form the II-type heterojunction and therefore improve the separation efficiency of photoinduced electron-hole pairs, consequently enhancing the photocatalytic activity. To our knowledge, there is no research on the construction of SrTiO3 nanoparticles/SnNb2O6 nanosheets hybrid photocatalyst with high hydrogen production efficiency under visible light irradiation. Herein, we report a facile fabrication of SrTiO3/SnNb2O6 (denoted as STO/SNO) 0D/2D hybrid heterojunction with an interfacial interaction. The photocatalytic hydrogen-evolution of as-prepared 0D/2D hybrid heterojunction was investigated. The results demonstrated that the STO/SNO hybrid heterojunction exhibit remarkably enhanced photocatalytic H2 generation efficiency compared with bare STO and SNO. It is postulated that the excellent photocatalytic activity for H2 production of STO/SNO can be mainly ascribed to the formed interfacial contact and enhanced interfacial charge separation between STO and SNO heterojunction during the photocatalytic reaction. EXPERIMENTAL Materials. Potassium hydroxide (KOH), niobium oxide (Nb2O5), hydrochloric acid (HCl), stannous chloride dehydrate (SnCl2·2H2O), tetrabutyl titanate (Ti(C4H9O)4), ethylene glycol (EG), strontium nitrate (Sr(NO3)2), sodium hydroxide (NaOH), absolute ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd., China and were used as received with no purification. Deionized water was used in the experiments. Synthesis of STO/SNO Heterojunction. STO was synthesized by a simple



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hydrothermal method process according to a previously report.42 Typically, 10 mmol of Ti(C4H9O)4 was added into 20 mL of ethylene glycol (EG) to form a clear solution. Subsequently, 20 mL of 0.5 M Sr(NO3)2 aqueous solution and 10 mL of 5 M NaOH aqueous solution were dissolved in the obtained solution under stirring for 30 min. The obtained suspension was transferred to a 100 mL Teflon-lined vessel, followed by heat-treating at 473.15 K for 24 h, then washed and collected with distilled water and absolute ethanol until the pH value was 7, and dried at 343.15 K overnight. SNO nanosheets were prepared similar to the former report.18,27 In detail, a certain amount of Nb2O5 and KOH were mixed in 40 mL distilled water respectively. The mixed solution was stirred vigorously for 10 min and transferred into a 50 mL Teflon-lined stainless steel autoclave, and heat-treated at 453.15 K for two days. HCl aqueous solution (2 mol·L−1) was added to the obtained clear solution until the pH value was adjusted to 7. After that, SnCl2·2H2O was quickly dispersed into the solution and HCl (2 mol·L−1) aqueous solution was dropped into the solution till pH reach 2 by continuous stirring. The obtained suspension was transferred to a 100 mL Teflon-lined vessel, hydrothermally treated at 473.15 K for 24 h. Eventually, the resultant yellow sample was centrifuged and washed thoroughly with DI water and anhydrous alcohol, then drying at 343.15 K overnight. A facile two-step wet chemistry strategy is proposed to synthesis the STO/SNO heterostructures. The different weight ratios of the STO to SNO samples (1/10, 1/5, 1/3, 2/5) were separately dispersed into 80 ml absolute ethanol. When the mixed solution was ultrasonically treated for 30 min and then stirred continously for 4 h to


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form a homogeneous solution. The solution was moved into a 100 mL Teflon-lined vessel, followed by heat-treating at 433.15 K for 12 h. The as-formed samples were rinsed with DI water, then drying at 343.15 K overnight. The resultant STO/SNO composites were labeled as 10%-STO/SNO, 20%-STO/SNO, 30%-STO/SNO, and 40%-STO/SNO, respectively. Characterization. X-ray diffraction were recorded collected on a Bruker D8 Advance X-ray diffractometer using Cu Kα radiation. The morphology of the products was also examined by transmission electron microscopy (TEM) which was recorded on a Tecnai 12 (The Dutch Philips company). X-ray photoelectron spectroscopic technique (XPS, ESCA PHI500) was used to determine the metal oxidation states and surface element compositions of the samples. The nitrogen adsorption-desorption isotherms at 77 K were researched using a TriStar II 3020 surface area and porosity analyzer (Micromeritics Instrument Corporation, USA). Furthermore, the ultraviolet-visible (UV–vis) diffuse absorbance and reflectance spectra of the samples in the range of 200–800 nm were determined with a Shimadzu UV-2450 spectrophotometer using BaSO4 as a reference. The photoluminescence (PL) spectra of the photocatalysts were obtained by a Varian Cary Eclipse spectrometer. The transient photocurrent responses (I–t curves) were performed on an electrochemical analyzer (CHI 660B Chenhua Instrument Company) with a 420nm optical filter. Photocatalytic

Hydrogen

Production.

The

photocatalytic

H2-evolution

experiments were performed in a Pyrex flask, and a closed gas circulation and



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evacuation system are connected to it for sampling. A 300 W Xe lamp with a UV-light cutoff filter (420 nm) was used as the visible light source. In a typical reaction, 50 mg as-prepared catalysts were suspended in 50 mL of aqueous solution containing 20 vol% methanol under stirring conditions. The 1.0 wt % Pt-loaded photocatalyst were conducted by an in situ photodeposition method using H2PtCl6 as a precursor before H2 evolution. Prior to irradiation, the suspension of the photocatalyst was dispersed under ultrasonically treated for 5 min and then completely degassed to remove oxygen by bubbling N2 for 20 min and ensured an aerobic condition. The produced hydrogen gas was sampled and measured by using gas chromatography (GC-14C, Shimadzu, Japan, TCD, with nitrogen as a carrier gas and 5 Å molecular sieve column). The apparent quantum efficiency (QE) estimated by the equation (1)： ×

QE=

× 100%

(1)

Photoelectrochemical Measurement. Transient photocurrent measurements of STO/SNO heterojunctions were measured on an electrochemical analyzer in a standard three-electrode configuration. Irradiated with a 500 W Xe arc lamp with a 420 nm light filter, the prepared FTO/samples, the Ag/AgCl electrode, and Pt wire were used as the working, reference, and counter electrodes, respectively. The modified electrodes were prepared by a dip-coating method: 4 mg STO, SNO, 20%-STO/SNO heterojunction and 20 µL of 5 wt % Nafion solution dispersed in 1 mL of ethanol by ultrasonication, which then dip-coated directly onto 1 cm×2 cm FTO slice. A 0.2 M Na2SO4 aqueous solution was used as the electrolyte. RESULTS AND DISCUSSION


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The TEM and HRTEM characterization provides insights into the morphology and microstructure of the obtained photocatalysts. The typical TEM images of SNO and STO/SNO heterojunctions were displayed in Figure 1. As can be seen, SNO possessed typical 2D nanosheet structure with a smooth surface (Figure 1a). As for STO/SNO heterojunctions, it could be clearly seen that the STO irregular nanoparticles dispersed on the surface of SNO nanosheets. An unexpected aggregation of these STO nanoparticles was also observed (Figure 1b-e). The more detailed information about the interfacial structure of the STO/SNO heterojunctions was further confirmed by the HRTEM image. As shown in Figure 1f, the distinct fringe with an interval of 0.27 nm can be perfectly indexed to (110) lattice plane of STO and that of 0.30 nm and 0.36 nm coinciding with the (311), (-111) lattice plane of SNO, respectively. Furthermore, there was a large and intimate contact interface between STO and SNO, which is beneficial for photogenerated charge separation in photocatalytic process. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray spectroscopy (EDS) elemental scanning are used to reveal the composition and microstructure of binary 20%-STO/SNO heterojunction, which are shown in Figure 1g-m, confirming the existence of Sr, Ti, O, Sn and Nb elements in the 20%-STO/SNO heterojunction, and the STO deposited on the surface of SNO nanosheets can also be confirmed by the signals intensity of different elements.



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Figure 1 TEM images of (a) SNO, (b) 10%-STO/SNO, (c) 20%-STO/SNO, (d) 30%-STO/SNO, (e) 40%-STO/SNO, (f) HRTEM image of 20%-STO/SNO, (g-m) HAADF-STEM image and the corresponding EDS elemental mapping images of the 20%-STO/SNO heterojunction. The surface elemental compositions and bonding configuration of the STO, 20%-STO/SNO heterojunction, and SNO can be evidenced by X-ray photoelectron spectra (XPS), as displayed in Figure 2. The XPS survey spectra (Figure 2a) confirmed the co-existence of Sr, Ti, O, Sn, Nb elements in STO/SNO heterojunction and ruled out the possibility of the existence of other impurity elements. The small C peak at 284.6 eV was ascribed to surface adventitious reference carbon during the XPS measurement, which is unavoidable.43 The high-resolution spectrum of Sr 3d (Figure 2b) presented two characteristic peaks centered at approximate 132.5 eV and


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134.2 eV, which could be assigned to Sr 3d5/2 and Sr 3d3/2, respectively. The peak located at around 132.5 eV was in accordance with reports for STO perovskite materials and the higher energy peak can be attributed to SrO complexes.45,46 Titanium existed as Ti4+ state was verified by two separate peaks at 457.8 eV (Ti 2p3/2) and 463.7 eV (Ti 2p1/2) due to the spin-orbit splitting (Figure 2c). The curves of Sn 3d show that the binding energies of Sn 3d5/2 and Sn 3d3/2 appeared at 485.7 eV and 494.1 eV (Figure 2d), revealing that the oxidation state of Sn present +2 in the sample.46 The spin orbit separation of Nb 3d is 2.8 eV and the distinct peaks at 206.4 eV in Nb 3d5/2 are feature of the Nb5+ in the SNO (Figure 2e).47 Additionally, in the spectrum of O 1s (Figure 2f), the characteristic peaks centered at 529.8 eV and 530.8 eV. The binding energy peak of 529.8 eV was associated with the the O 1s core level of the O2- anions in the SNO crystal lattice, and the binding energy peak of 530.87 eV could be ascribed to titanium bonded oxygen (Ti-O) and hydroxides presented on the surface.48-50 Notably, the peaks of Sr 3d, Ti 2p, Sn 3d, Nb 3d, O1s in 20%-STO/SNO shift toward the higher binding energies. This result indicate that there was an interfacial interaction between STO nanoparticles and SNO nanosheets in the heterojunction, which was driven from the probable electron transfer and delocalization of firmly contacted STO and the SNO nanosheets.51



(a)

Sn 3d

(b)

O 1s

(c)

Sr 3d

Ti 2p

20%-STO/SNO STO

O 1s Ti 2p Sr 3d C 1s 0

200

400

600

STO

800

1000

128

1200

20%-STO/SNO

130

132

134

136

138

140

454

456

Binding energy(eV)

SNO

(f)

Nb 3d

SNO

460

202

204

206

208

210

462

464

466

O 1s

SNO

20%-STO/SNO

20%-STO/SNO 482 484 486 488 490 492 494 496 498

458

Binding energy(eV)

Intensity (a.u.)

(e)

Sn 3d

Binding energy(eV)

STO

20%-STO/SNO

Binding energy(eV)

(d)

Intensity (a.u.)

SNO

Intensity (a.u.)

C 1s

Intensity (a.u.)

Intensity (a.u.)

Nb 3d

Intensity (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

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212

214

20%-STO/SNO 526

Binding energy(eV)

528

530

532

534

Binding energy(eV)

Figure 2 XPS spectra of the STO, SNO, and 20%-STO/SNO heterojunction: (a) survey spectrum, (b) Sr 3d, (c) Ti 2p, (d) Sn 3d, (e) Nb 3d, (f) O 1s. Pristine STO, SNO, and STO/SNO heterojunctions were determined by XRD patterns, as depicted in Figure 3a. The diffraction peaks of pristine STO located at 32.4°, 39.9°, 46.3°, 57.6°, 67.7° and 77.0°, assigned to the characteristic diffraction peaks of (110), (111), (200), (211), (220) and (013) planes, which were consistent with the previously reported values for the perovskite SrTiO3 (JCPDS, No. 79-0176). It suggests that pure phase STO was formed and well crystallized with cubic structure.52 For the pure SNO, all peaks are in good agreement with monoclinic SNO (JCPDS, No. 84-1810). The main diffraction peaks at 24.9°, 29.0°, 31.3°, 33.8°, 36.9°, 49.5°, 51.0°, 52.9°, 54.3°, 60.0°, 61.7°, and 65.6° can be well-indexed to the (-111), (311), (600), (202), (020), (022), (222), (-113), (911), (-821), (331), and (-713) planes, respectively.27 In addition, after the deposition of STO on the surface of SNO, no trace of any impurity phased can be discovered from the XRD patterns of STO/SNO heterojunctions, indicating the successful combination of STO/SNO heterojunctions. Moreover, the intensity of the diffraction peak of STO gradually increased with the increase of STO content, while the peak intensity of SNO gradually weakened.


30%

-111 311 600 202 020

10% SNO 10

20

30

40

022 222 -113 911 -821 331 -713

20%

50

60

2 Theta (degree)

70

80

180 160 140 120 100 80 60 40 20

SNO 20%-STO/SNO STO

0.006

dV/dD (cm3/g·nm)

40%

(b) Volume adsorbed (cm3 STP g-1)

013

220

012 211

STO

200

100

110

(a)

Intensity (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


111

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0.005 0.004 0.003 0.002 0.001

2 4 6 8 10 12 STO Pore Diameter (nm) 20%-STO/SNO SNO

14

0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (p/p0)

Figure 3 (a) XRD patterns of the as-prepared samples; (b) N2 adsorption isotherms and the corresponding pore size distribution (inset) for SNO, STO and the 20%-STO/SNO heterojunction. N2 adsorption–desorption isotherms and the corresponding pore-size distribution curves of as-obtained samples were shown in Figure 3b. The specific surface area, pore volumes and pore sizes data of as-prepared samples calculated by BET and BJH methods, as summarized in Table 1. All the samples possess a classical type IV isotherm with a H3 adsorption hysteresis loops in terms of the International Union of Pure and Applied Chemistry (IUPAC) classification, indicating the existent of mesoporous (2-50 nm) structures.53-55 As displayed in Table 1, SNO, STO, 20%-STO/SNO exhibit the pore size distributions with mean pore diameters of 4.74, 6.43, and 5.39 nm, respectively, further suggesting their mesoporous structures. The Brunauer-Emmett-Teller (BET) specific surface area were calculated to be 22.84 m2·g-1, 30.31 m2·g-1, and 25.35 m2·g-1 for pure SNO, STO, and 20%-STO/SNO, respectively, following the order of STO > 20%-STO/SNO > SNO. This result is inconsistent with the photoactivity order of these samples. Therefore, it is reasonable to deduce that the surface area was not the primary factor to affect the photocatalytic



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performance in this study. Table 1 BET surface areas and pore parameters of different samples. Sample

Surface area (m2/g)

Pore volume (cm3/g)

Average pore size (nm)

SNO

22.84

0.028

4.74

STO

30.31

0.048

6.43

20%-STO/SNO 25.35

0.032

5.39

The optical characterizations of all samples were recorded by the UV–Vis diffuse reflectance spectra (UV–Vis DRS), as depicted in Figure 4a. Pristine STO harvested majority of light below 400 nm can be clearly observed, and SNO have a strong absorption edge located at 550 nm. When SNO was combined with STO, the absorption edges of STO/SNO heterojunctions are gradually expanded towards high wavelength with increasing SNO amount, indicating that there is optical property enhancement via hybridized with SNO compared to bare STO. The enhanced visible light absorption indicated that the composite photocatalysts could utilize more visible light, which will be favorable for a photocatalytic reaction. The band gap energies of both STO and SNO were calculated by the Tauc plot.56 As shown in Figure 4b, where the intercept of the tangents on horizontal axis gives the value of the bandgap energies, the bandgap energies for the STO was measured to be 3.2 eV (n is equal to 2 for indirect transition), and that of the SNO was 2.59 eV (n is equal to 1/2 for direct transition), similar to the previous reports. 27,37


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(b) SNO 10%-STO/SNO 20%-STO/SNO 30%-STO/SNO 40%-STO/SNO STO

14

2.0

12

1.8 1.6

10

(ahv) 2 (eV) 2

(a)

Intensity (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


1.4

8

SNO

1.2

6

STO

0.8

4

0.6 2

2.59 eV

3.2 eV

0 200

1.0

(ahv) 1/2 (eV) 1/2

Page 15 of 34

300

400

500

600

700

800

0.4 0.2

2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2

Wavelength (nm)

hv (eV)

Figure 4 (a) UV–vis diffused reflection spectra of pristine STO, SNO nanosheet, and STO/SNO heterojunctions; (b) Plot of (αhʋ)1/2 vs. hʋ for the band gap energy of STO and plot of (αhʋ)2 vs. hʋ for the band gap energy of SNO. To give more information about the charge transfer properties of the as-obtained samples, the time-resolved photocurrent behaviors were performed under intermittent visible light irradiation (λ>420 nm). As displayed in Figure 5, the photocurrent densities climbed up to a high current level when the light was activated and rapidly decreased as soon as the light was turned off. It is observed that photoresponse of 20%-STO/SNO heterojunction is much higher compared to that of pristine STO and SNO, indicating the heterostructure formed between STO and SNO significantly facilitates the photo-induced charge transfer, which should contribute to the obvious enhanced photocatalytic hydrogen production.



0.25

Photocurrent (µA)

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

on

off

160

180

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STO SNO 20%-STO/SNO

0.20 0.15 0.10 0.05 0.00 200

220

240

260

280

300

Irradiation time(s)

Figure 5. Transient photocurrent response for pristine STO, SNO, and 20%-STO/SNO heterojunction. Figure 6 presented the photocatalytic H2-generation performances of as-prepared photocatalysts deposited with 1 wt.% Pt using methanol as a scavenger, and irradiated by visible-light irradiation (λ > 420 nm). Before the practical photocatalytic experiment, control-experiment was carried out to demonstrate that H2 was generated by photocatalytic reaction as there was no appreciable H2 production under the non-irradiation or no photocatalysts. As observed in Figure 6a, the photoactivity trends over all the samples are the same during the whole photocatalytic H2 evolution reaction, implying that photocatalysts are rather stable under light irradiation. Figure 6b showed the comparison of the H2 evolution amount of all the photocatalysts (50 mg) during 3 h reaction time. Apparently, bare STO showed a negligible H2 evolution amount, ca. 0.06 µmol, and bare SNO showed a clearly lower H2 evolution amount (8.99 µmol). When a certain amount of STO attached on the surface of SNO nanosheet, caused a dramatic increase in H2 production as compared to the pure STO and SNO. Especially, it was found that among a series of STO/SNO heterojunctions,


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the 20%-STO/SNO possesses the maximum H2 evolution amount of ca. 17.16 µmol with apparent quantum efficiency of about 3.2%, which represented an increase of up to 298 and 2 times higher H2-generation amount with respect to bare STO and SNO, respectively. When further increasing of the content of STO, the H2-generation activity of the heterojunction samples decreased, possibly due to the fact that overloading of STO could restrain the separation of photo-induced charges. It is worth noting that the present STO/SNO heterojunction exhibits the higher H2-evolution activity as compared with the other STO-based photocatalysts reported previously (Table 2). 40,41,57 In addition, we also examined the stability and recyclability of the 20%-STO/SNO heterojunction. As displayed in Figure 7, no clear inactivation can be found after 12 h, suggesting that the as-prepared STO/SNO heterojunction is very stable in the photocatalytic reaction.

H2 evolu tion (µmo

16 12 8 4 0 3

40 %

H2 evolution (µmol)

(b) l)

(a)

m Ti

20 17.16

16

2

20 %

10 %

1

ST O

SN O

16.17

15.05 13.13

12 8.99

8 4

30 %

h) e(

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


0

0.06

STO SNO 10% 20% 30% 40%

Figure 6 (a) Time courses of photocatalytic H2 evolution over different photocatalysts under visible light (λ > 420 nm); (b) Comparison of photocatalytic H2 evolution activity over different photocatalytsts in 3 h.



400 1 run

350

H2 evolution (µmol g-1)

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 18 of 34

2 run

3 run

4 run

300 250 200 150 100 50 0 0

3

6

9

12

Time (h)

Figure 7 Stability test of 20%-STO/SNO heterojunction under visible-light irradiation. Table 2. The comparison of photocatalytic H2-evolution behavior over our work and the other STO-based photocatalysts. Light

Reactant

source

solution

Cocatalyst

Cr-SNO

300W Xe Lamp (λ> 420 nm)

methanol

0.6 wt.% pt

82.6

N-STO


methanol

0.5 wt.% pt

1.2

Cr/N-STO


methanol

0.5 wt.% pt

106.7

Rh-STO


methanol

0.5 wt.% pt

47

STO/SNO


methanol

1 wt.% pt

114.4

Photocatalyst

Activity (µmol·h-1)

QE

Ref

2.95%

39 40

3.1%

56 3.2 %

The processes of separation, transfer and recombination of photoinduced e-h pairs in photocatalysts could be detected by PL spectra.58,59 The PL spectrum of the STO, SNO, 10%-STO/SNO, 20%-STO/SNO, 30%-STO/SNO, and 40%-STO/SNO were presented in Figure 8a, excited with a wavelength of 397 nm. Since PL emission primarily arises from the recombination of charge carriers, the low PL intensity indicates inhibited recombination of charge carriers.60 All the samples showed strong emission peak around 533 nm. In particular, STO/SNO nanocomposites displayed obviously diminished PL intensity, which indicated the recombination rate of e-h


40

Our sample

Page 19 of 34

pairs was efficiently inhibited and the photocatalytic hydrogen production activities was improved. These results are in good agreement with the corresponding photocatalytic H2-evolution activities, further suggesting that the interfacial electron transfer is beneficial for the improvement of the H2-generation activity over the STO/SNO heterojunctions. SNO 40%-STO/SNO 10%-STO/SNO 30%-STO/SNO 20%-STO/SNO

520

530

540

550

560

570

(b) PL intensity (a.u.)

(a) Intensity (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


STO SNO 20%-STO/SNO

132

134

Wavelength (nm)

136

138

140

Time (ns)

Figure 8 (a) Photoluminescence (PL) spectra of different photocatalysts at room temperature with an excitation wavelength of 397 nm; (b) The time-resolved fluorescence spectra for as-prepared photocatalysts monitored at 469 nm using 337 nm laser at room temperature. The enhanced efficiency in prolonging the lifetime of photogenerated charge carriers caused by the interface interaction between STO and SNO can be confirmed by the time-resolved photoluminescence. For semiconductor materials, the dynamic electron migration process can be estimated by time-resolved photoluminescence experiments, and the normalized decay profiles were shown in Figure 8b, the TRPL decay signals were fitted by single-exponential decay kinetics function61: I (t) = I0 • exp (−t/τ)

(2)

The calculated lifetime values of bare STO, SNO and 20%-STO/SNO are 0.58,



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

0.60 and 0.72 ns, respectively. Obviously, the 20%-STO/SNO heterojunction possessed a longer lifetime. The prolonged lifetime can prove that the 20%-STO/SNO heterojunction retarded the recombination of photo-generated e-h, thereby increasing the activity of STO/SNO heterojunction for photocatalytic H2 evolution. Accordingly, a reasonable reaction mechanism for photocatalytic H2 evolution under visible light irradiation (λ> 420 nm) over the STO/SNO is illustrated in Figure 9. ECB and EVB of STO were determined to be -0.77 and 2.43 eV, and those of SNO were -0.99 and 1.6 eV. When SNO is excited by visible-light, the excited e- in the valence band (VB) of SNO would easily inject into the conduction band (CB) of SNO and then immigrated to the more positive conduction band (CB) of STO through the close contact interface. At last, the excited electrons tend to be trapped by the Pt nanoparticles due to its high Fermi level62, which could reduce H+ and produce H2 molecules effectively.63-65 Simultaneously, the corresponding holes accumulated at the VB of SNO can oxidize the electron donor (CH3OH). Hence, the STO/SNO 0D/2D heterojunction with matched energy level and interfacial interaction could increase the charge separation efficiency, leading to an enhancement of photocatalytic H2 production activity.


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Figure 9 Proposed mechanism of charge transfer in the STO/SNO 0D/2D heterojunctions under visible light irradiation. CONCLUSIONS In summary, novel STO/SNO 0D/2D heterojunctions with an interfacial interaction for photocatalytic H2 evolution were successfully prepared by means of a facile hydrothermal method. It is demonstrated that the optimal 20%-STO/SNO 0D/2D heterojunctions exhibited the highest hydrogen evolution of 17.16 µmol under visible-light irradiation, which is about 298 and 2 times higher than those on the pure STO and SNO. This enhanced photocatalytic H2 production activity can be mainly ascribed to the improved interfacial charge-transfer efficiency, which is resulted from the constructed 0D/2D heterojunctions. This work is expected to afford more valuable guidance for the rational design and preparation of novel 0D/2D heterojunctions photocatalyst for highly efficient H2 production and further applications in the field of environment and energy. AUTHOR INFORMATION



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

Corresponding Author *E-mail: [email protected]; [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

This work was supported by the financial supports of National Nature Science Foundation of China (No. 21406091, 21576121 and 21606111), Natural Science Foundation of Jiangsu Province (BK20140530 and BK20150482), China Postdoctoral Science Foundation (2015M570409).

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Table of Contents

Novel 0D/2D SrTiO3/SnNb2O6 heterojunctions showed enhanced interfacial charge separation and visible-light-driven photocatalytic hydrogen evolution activity.


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2D Heterojunctions

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