Visible-Light Self-Powered Photodetector and Recoverable

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Article

Visible Light Self-Powered Photodetectors and Recoverable Photocatalyst Fabricated from Vertically Aligned SnO Nanoflakes on Carbon Paper 3

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Weiwei Xia, Haoyu Qian, Xianghua Zeng, Jing Dong, Juan Wang, and Qin Xu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05520 • Publication Date (Web): 31 Jul 2017 Downloaded from http://pubs.acs.org on July 31, 2017

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The Journal of Physical Chemistry

Visible-light Self-powered Photodetector and Recoverable Photocatalyst Fabricated from Vertically Aligned Sn3O4 Nanoflakes on Carbon Paper Weiwei XiaŦa, Haoyu QianŦa, Xianghua Zenga*, Jing Donga, Juan Wangb, Qin Xub a

College of Physics Science and Technology & Institute of Optoelectronic Technology, Yangzhou University, Yangzhou 225002, P.R. China b

College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, P.R. China

Abstract Self-powered photodetectors (SPPDs) are promising candidates for high-sensitivity and high-speed applications because they do not require batteries as an external power source. It is a challenge to fabricate visible-light photodetectors. Herein, vertically aligned two-dimensional (2D) Sn3O4 nanoflakes on carbon fiber paper were prepared by a modified hydrothermal approach and used as a self-powered photoelectrochemical cell-type visible-light detector. The detector exhibits reproducible and flexible properties, as well as an enhanced photosensitive performance. The improved photoresponse was attributed to the synergistic effects of the vertically grown Sn3O4 nanoflakes and carbon fiber paper substrate; the former provided efficient active sites, as it exposed more catalytic sites to the electrolyte and absorbed more light scattered among the nanoflakes, and the latter benefited charge transport. The photocatalytic activity of the three-dimensional (3D) Sn3O4 hierarchal structure on rhodamine B under visible-light irradiation was investigated and shown to have a degradation rate constant of 3.2×10−2 min−1. The advantage over ordinal materials for use in an SPPD device is that this material is flexible and easily recoverable as a photocatalyst.

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INTRODUCTION

Photodetectors have attracted great attention due to their wide applications in communications, chemical/biological sensing, environmental monitoring, remote control, binary switches in imaging techniques, etc. Recently, self-powered photodetectors (SPPD) have become promising candidates for use in high-sensitivity and high-speed applications because they do not require batteries as an external power source. Metal-oxide semiconductors based on low-dimensional nanostructures (LDN) are regarded as ideal materials for photodetectors (PDs) due to their chemical stability and optoelectronic characteristics. Metal-oxide semiconductors with different structures have been used as self-powered ultraviolet photodetectors (UVPDs), for example, TiO2 nanorods1, n-ZnO/p-NiO core-shell nanowires2, ZnO microwire/p-Si heterojunctions3, ZnO nanoneedles4, TiO2 nanorod arrays5, SnO2 -TiO2 core-shell structures6, ZnO/ZnS core-shell structures7, Ag-modified ZnO nanowires 8 , etc. In addition to photoconductive-type photodetectors, photovoltaic-type photodetectors have been recently developed, and photoelectrochemical (PEC) cell-based self-powered photodetectors9,10,11 are composed of an active photoanode, an electrolyte, and a catalytic counter electrode. Using this simple and effective PEC structure, complicated and expensive microfabrication techniques are avoided, and photodetectors based on PEC cells can work in a self-powered mode. Therefore, these devices are regarded as promising strategies due to their low cost, facile fabrication process and fast time response. Until now, the most widely studied SPPDs have been photoconductive-type SPPDs based on p-n junctions12 and Schottky junctions 13 . However, few visible-light self-powered photodetectors have been reported14,

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, and there are no reports of the visible-light self-powered photodetectors based on

Sn3O4 materials. To better use the solar light for energy conversion, developing visible-light-responsive SPPDs is highly desirable because visible light comprises approximately 43% of the solar spectrum. Since the morphology and structure of a metal-oxide semiconductor greatly influence both the optoelectronic properties and absorption of incident light, it is a necessary challenge to fabricate hierarchical-structured materials, which could provide a large surface area to facilitate charge and mass transfer, and much effort needs to be made to produce an efficient PEC-type self-powered photodetector. 2


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Tin oxides, as earth-abundant and non-toxic materials, have a strong resistance against acidic/alkaline solutions and exhibit good photocatalytic activity without generating secondary pollutants under irradiation with ultraviolet (UV) light. Hence, tin oxides (SnO, SnO2) and mixed-valence tin oxides (Sn3O4, Sn2O3)16 have been applied in the optoelectronic field.17, 19

18,

Among them, the mixed-valence tin oxide Sn3O4 can absorb light at visible wavelengths and

has been used as an efficient visible-light-driven photocatalyst20,

21

and anode material for

lithium-ion batteries.22 The mixed-valence tin oxide Sn3O4 is thus a desirable material for PEC-type photodetectors. In this paper, 3D hierarchical Sn3O4 nanoflakes grown on carbon paper were synthesized via a hydrothermal route without any template, in which the Sn3O4 nanoflakes were vertically aligned on the surface of carbon fiber paper and each individual nanoflake had a thickness of 30 nm and a width of approximately 800 nm. A hybrid PEC-type photodetector was designed using the 3D Sn3O4 nanostructures as a photoanode. The visible-light SPPD device exhibits an improved performance. The photocatalytic activity of the 3D Sn3O4 hierarchal structures on rhodamine B under a visible-light irradiation was investigated, and the mechanism was discussed.

Experimental Section

Materials All reagents were of analytical grade and used as received without further purification. Tin (II) chloride dehydrate (SnCl2·2H2O, 98%) and sodium citrate dihydrate (Na3C6H5O7·2H2O, 99%)

were supplied by Sinopharm Chemical Reagent Company, Limited. Commercial carbon fiber papers were provided by FuelCellStore. Prior to deposition, the commercial carbon fiber papers were cut into the desired sizes and treated with ethanol and deionized water. Sn3O4 nanoflakes were grown on the carbon fiber paper by a modified hydrothermal approach using tin chloride dehydrate and sodium citrate dehydrate as precursors. In detail, 1.073 g of SnCl2·2H2O and 2.940 g of Na3C6H5O7·2H2O

were dissolved in a mixture of 20 mL deionized water and 20 mL alcohol under constant magnetic stirring for approximately 60 min. The obtained homogeneous solution was transferred to a Teflon-lined autoclave of 50 mL capacity, with the carbon fiber paper vertically 3



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immersed in the reaction solution, and kept sealed at 180 °C for 12 h. After the reaction, the carbon fiber paper, coated with a brown product, was further washed with deionized water and absolute ethanol several times. The final sample was dried in an oven at 60 °C before characterization. Characterization The phase identification of the as-prepared product was characterized by powder X-ray diffraction (XRD, Bruker D8 advance) with Cu Kα ( λ = 1.5406 Å). The morphology and microstructure of the synthesized sample was examined by a Hitachi S-4800 field emission scanning electron microscope (SEM) and a Tecanai G2 F30 field emission transmission electron microscope (TEM) operated at an accelerating voltage of 300 kV. UV-visible absorption spectra (200-800 nm) were recorded with a Varian Cary 50 UV-visible spectrophotometer. Raman measurements were performed at room temperature on a Renishaw inVia Reflex Raman spectrometer with 532 nm lines. A 532 nm laser output of 20 mW and a 50× objective lens were used, which resulted in an incident power at the sample of approximately 3 mW. The photoluminescence (PL) measurements were performed on a Britain Renishaw inVia spectrophotometer, with the 325 nm line of a He-Cd laser as the excitation light source. X-ray photoelectron spectroscopy (XPS) was conducted on an ESCALAB-250Xi photoelectron spectroscope to obtain information on the valence state of the Sn and O ions. Photocatalytic Activity Measurement Photocatalytic experiments were conducted on the as-prepared samples to decompose RhB aqueous solutions (30 mL, 10-5 M) using the following procedure: carbon fiber paper-supported Sn3O4 nanoflakes with dimensions of 2×2 cm2 were evaluated for the photodegradation of RhB with a 350 W Xe arc lamp equipped with a UV cut-off (λ > 420 nm) as the visible-light source. Prior to visible-light irradiation, the suspensions were magnetically stirred in the dark for 30 min to reach the desorption-adsorption equilibrium. Then, the solution was exposed to visible-light irradiation. At 15 min intervals, 5 mL samples were collected and centrifuged to remove the residual photocatalyst, and the upper clear liquid was analyzed by recording the maximum absorption band (554 nm for RhB) using a Shimadzu UV-3600 spectrophotometer. 4


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PEC Measurements The solid-liquid heterojunction-based visible-light-driven detector was examined using a three-electrode electrochemical cell controlled by a Zanner CIMPS electrochemical workstation (Germany). PEC measurements were performed in 0.2 M Na2SO4 using Pt wire as the counter electrode and Ag/AgCl in saturated KCl as a reference electrode. The carbon-paper-supported Sn3O4 nanoflakes were used as the working electrode and placed in the cell with an area of 1×1 cm2 exposed to the electrolyte. LED irradiation with suitable wavelengths in the visible-light spectral region (564 ± 60 nm) was employed as the illumination source by directly irradiating the surface of the photoanode placed in the quartz PEC cell. The maximum output power of the visible-light LED was 120 mW/cm2. The Mott−Schottky experiments were also conducted on the Zanner CIMPS electrochemical workstation. ■ RESULTS AND DISCUSSION Morphology and Structure Scanning electron microscopy (SEM) measurements were carried out to examine the morphologies of the as-prepared Sn3O4 nanomaterials. To clearly see the growth process, we present the SEM images of the original carbon paper in Figure 1a and the low-magnification SEM and high-magnification SEM images of the Sn3O4 nanomaterials in Figure 1b and 1c. Comparing Figure 1a and 1b, one can find that the Sn3O4 nanoflakes were vertically grown on the carbon paper, and the vertically aligned nanoflakes were densely and uniformly distributed on the surface of the carbon paper. Each nanoflake had a thickness of 30 nm and a diameter of approximately 800 nm, and some were crossover connected, creating an amount of mesopores with sizes of several hundred nanometers, as shown in Figures 1c and S1 (see Supporting Information, Figure S1). The crystalline structure of the Sn3O4 nanomaterials was identified from the XRD patterns, shown in Figure 1d. The crystalline structure of the Sn3O4 nanomaterials was regarded as triclinic Sn3O4 (JCPDS No. 20-1293) with lattice parameters of a=5.88, b=8.20 and c=4.86 Å, as the intense peaks at 2θ=24.10˚, 27.08˚, 31.73˚, 32.32˚, 33.01˚ and 37.07˚ well match the (101), (111), (-210), (-121), (210) and (130) planes23. The peaks marked with the symbol ♣ resulted from the carbon fiber paper (see Supporting Information, 5



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Figure S2). The microstructure and morphology of the products were studied with transmission electron microscopy (TEM), as shown in Figure 1e and 1f. Figure 1f displays the high-resolution TEM (HRTEM) images of the prepared sample, where the lattice spacing of ~3.69 Å corresponds to the (101) plane of Sn3O4.

Figure 1 (a) SEM image of the carbon fiber paper; (b) and (c) Low-magnification SEM and magnified SEM images of hierarchical Sn3O4 structures composed of nanoplate building blocks; (d) XRD patterns; (e) TEM image of a single nanoplate; (f) HRTEM image of the hierarchical Sn3O4 structures.

Optical Properties To determine the phase transformation of tin oxides, Raman spectroscopy was performed using an excitation wavelength of 532 nm, as shown in Figure 2a. The main patterns centered at 143, 173 and 242 cm-1 are the characteristic peaks of the phonon modes of Sn3O4, which is consistent with the literature24. Clearly, no other impurities were found in the Sn3O4 samples, indicating the purity and high crystallinity of the prepared samples25. The UV-Vis absorption spectra were obtained by measuring the optical absorption spectra on a UV-Vis spectrophotometer (Cary-5000), as shown in Figure 2b. From Figure 2b, we find that there is a visible-light absorption from 450-800 nm, and from the relations of the absorption coefficient (R) near the absorption edge and the optical direct band gap (Eg)26, the band gap of the semiconductor can be extracted from the formula (α hv )=c(hv -E g )1/2 , where c and Eg are the constant and the band gap, respectively. The direct band gap estimated from a plot of ( α hv )2 6


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versus photon energy hv is equal to 435 nm (2.85 eV), as shown in Figure 2b (inset). Photoluminescence (PL) measurements were carried out to study the optical properties of the tin oxides, as presented in Figure 2c. Two emission peaks at ~442 and ~486 nm were observed; the former is the near band related emission, and the latter wide band emission is the oxygen-deficient related emission27. Examining the absorption and PL spectra, visible-light absorption could be determined to be related to the deficient emission of Sn3O4.

Figure 2 (a) Raman spectrum with excitation at 532 nm; (b) UV-Vis spectrum and (αhν)1/2 versus photon energy (hν); (c) PL spectrum with excitation at 325 nm; (d) Survey XPS spectrum of Sn3O4; (e) High-resolution Sn 3d spectra; (f) High-resolution O 1s spectra.

XPS measurements were used to investigate the surface composition of the as-prepared product. Figure 2d shows the XPS survey spectrum, in which all of the peaks can be ascribed to the elements Sn, O, and C only; the binding energies were calibrated using the carbon C 1s peak (285.0 eV) as a reference. The binding energies at ~486 and ~494 eV were ascribed to Sn 3d5/2 and 3d3/2, respectively. The lower energy at 486 eV can be deconvoluted into 486.02 eV (Sn2+) and 486.66 eV (Sn4+) in Sn3O4, and the higher energy at 494 eV can be deconvolved into 494.44 eV (Sn2+) and 495.07 eV (Sn4+).28 The integrated area ratio of I (Sn2+)/[I (Sn4+) +I (Sn2+)] is approximately 0.71, which is close to the theoretical value of 0.67 in Sn3O4. A small peak at 496.97 eV is not clear. In the O 1s spectrum, there are two sharp peaks at 530.13 and 531.31 eV, which can be ascribed to O-Sn2+ and O-Sn4+, respectively. The peak at 532.35 eV 7



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was indexed to oxygen vacancies (VO)29. In addition, the extra component at 535.97 eV was assigned to oxygen atoms chemisorbed/adsorbed at the surface30,

31

. Therefore, the deficient

emissions mainly come from oxygen vacancies and surface-state-related emission.

Figure 3 The schematic illustration of a self-powered PEC-type detector; (b) Photocurrent response under on/off cycling of 20 s at 0.0 V vs. Ag/AgCl for incident intensity equal to 30, 60, 90 and 120 mW/cm2; (c) The photocurrent (I) with incident intensity (P) satisfying I∝Pθ, θ=0.69; (d) Enlarged rising and (e) decaying edges of the photocurrent response; (f) Mott–Schottky plots of the prepared samples in a 0.2 M Na2SO4 electrolyte solution (0.1 M; pH 7).

PEC Properties PEC Measurements were performed in 0.2 M Na2SO4 using Pt wire as a counter electrode, Ag/AgCl in saturated KCl as a reference electrode, and carbon-paper-supported Sn3O4 nanoflakes as an active photoanode, as shown in Figure 3a. The incident light source was an LED with a wavelength of 564 ± 60 nm, and the maximum output power of the visible-light LED irradiation was ~120 mW/cm2. The measurement of the self-powered PEC-type detector was carried out using a continuous visible-light pulse with an on-off interval of 10 s (or 20 s) at different intensities. The photocurrent was maintained at up to 85% after 20 repeated cycles of on/off irradiation switching, and the maximal photosensitivity (ratio of photocurrent to dark current) is approximately 81, as shown in Figures 3b, S3 and S4 (see Supporting Information, Figure S3 and S4), indicating that the photodetector has an excellent reproducible and 8


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high-photosensitivity performance. The photocurrent increases with incident intensity, which is equal to 28, 59, 78 and 108 µA with an on-off interval of 10 s at a bias of 0 V for 30, 60, 90 and 120 mW/cm2, respectively; this is equal to 33, 53, 70 and 79 µA with an on-off interval of 20 s for the same respective incident intensity. The photocurrent (I) with incident intensity (P) satisfies I∝Pθ, with θ=0.69 (Figure 3c). From Figure 3a, 3b, and 3c, one can find that the current of the Sn3O4 device increased quickly upon visible-light illumination in the first half second, and after 3 seconds, the current saturated until 20 s of continuous illumination was reached. Turning off the light, the current decreased to 60% of its maximum in one second and, several seconds later, reached the dark current. This behavior occurs because the H2O readsorption process in the electrolyte is very slow, and a long time is required to recover the initial state (before UV illumination). Therefore, the photocurrent decay time after turning off the UV light is very long; the depletion layer at the surface of Sn3O4 with H2O decreases the conductivity of Sn3O4, and the charge carriers accumulated for a longer time, resulting in a longer current saturation time32. The time response is characterized by the decaying and rising time, corresponding to 0.35 and 0.284 s, respectively, shown in Figure 3d and 3e. The lowest photocurrent is two orders of magnitude larger than the results of the 3D Sn3O4 nanostructures reported by He et al.33 Additionally, the device can function well in self-powered mode with a photosensitivity ((Ilight − Idark)/Idark) of 80, which is comparable to that of a detector based on plasmonic titanium nitride (TiN)34, and the responsivity of the hybrid device is ~0.1 mA/W, which is larger than the highest responsivity of 88.5 nA/W of a visible photodetector prepared via a monolithically integrated technique35. The capacitance-voltage curve (Figure 3f) was obtained to show the interface state comprehensively. According to the Mott-Schottky equation, the capacitance of a semiconductor (C) is related to the applied potential (E) according to

1 2(E - Efb ) =− 2 2 C A qεε0 Ns

(1)

where Efb is the flat band potential and E, q, A, ε, ε0 and Ns are the external applied potential, the elementary charge, the area of the metal contacts, the dielectric constant, the vacuum permittivity, and the carrier concentration, respectively. In Figure 3f, linear regions with 9



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positive slopes can be observed between −0.3 and 0.6 V. The lines share a common x-axis intercept at approximately −0.79 V, which is the typical behavior of an n-type semiconductor.36 The flat-band potential derived from electrochemical analysis (Mott-Schottky plot) is −0.79 V vs. Ag/AgCl at pH 7 for Sn3O4, and thus, the maximal conduction band energy of the Sn3O4 nanoflakes is determined to be −0.6 V vs. NHE at pH 7. Here, the extracted flat-band potential is slightly higher than −1.1 V, as reported by He et al.33 The different results probably resulted from the difference in the carrier concentration, conductivity of the samples, and the instrument. The Sn3O4 hierarchal nanoflakes were used for the degradation of RhB under visible-light irradiation, as shown in Figure 4. The absorption spectra of RhB solutions as a function of wavelength in the presence of Sn3O4 show that with increasing time, the intensity of the peak at 550 nm decreased gradually, indicating that RhB was gradually photodegraded. After one hour of visible-light degradation, the Sn3O4 hierarchal nanoflakes degraded up to 95% of the RhB dye. Defining the degradation efficiency as C/C0, where C0 is the initial concentration and C the concentration during the reaction, we obtained a degradation rate constant of k=3.2×10−2 min−1 from the relation ln(C0/C)=kt, as shown in Figure 4c. The obtained degradation rate constant is nearly one order of magnitude larger than that of 2.3×10–3 and 3.6×10−3 min−1 for SnO/Sn3O4 and SnO2/Sn3O4 semiconductors under UV irradiation, respectively. smaller than 1.73×10

–1

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The rate is

min–1, as reported by Song et al.21, where the irradiation source was

light (UV + visible) from a solar simulator. The cycling performance was tested, showing that after five cycling runs for the photocatalytic degradation of RhB over Sn3O4 under visible-light irradiation, the degradation efficiency was well maintained, as shown in Figure 4d. Here, the better degradation performance can be ascribed to the vertically aligned structures. Usually, nanoparticles have a larger specific surface area, which can adsorb more dye molecules, while more adsorbed dye molecules on the surface of nanomaterials will inevitably prevent the light absorption of the semiconductors.38 Here, the nanoflakes not only improve the adsorption of dye molecules but also increase light absorption by Sn3O4 via increasing light scattering between the shells.

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Figure 4 (a) Absorption spectra of a RhB solution as a function of wavelength in the presence of Sn3O4; (b) Degradation efficiency with irradiation time under visible-light irradiation, where the inset is a photograph of the color change of RhB during the photodegradation process; (c) The fitting of the degradation efficiency with degradation time by the relation ln(C0/C)=kt; (d) Five cycling runs for the photocatalytic degradation of RhB over Sn3O4 under visible-light irradiation.

Figure 5 (a) UPS secondary edge spectrum; (b) Positions of the band gap and oxygen vacancies (VO) for Sn3O4.

Mechanism To explain the enhanced photosensitivity of the visible-light self-powered photodetector, we present the mechanism for the carrier-transfer process of the PEC under illumination, as shown in Figure 5. As discussed above, by combining both the band gap (2.85 eV) estimated from optical absorption and the maximal conduction band energy of −0.6 V vs. NHE at pH 7, both the valence band position and the oxygen vacancy (2.55 eV) position were obtained at 2.25 and −0.3 V vs. NHE at pH 7, as displayed in Figure 5b. At the same time, a work function of 3.9 11



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eV was obtained by ultraviolet photoemission spectroscopy (UPS), as shown in Figure 5a, which is 0.46 eV above the reduction potential for water (−4.44 eV). Therefore, the reduction potential for water is situated below the conduction band minimum and the oxygen vacancy of Sn3O439. This indicates that reduction can easily occur using Sn3O4 as the anode electrode, as shown in Figure 5b. In comparison with ordinal materials, 3D Sn3O4 hierarchal structures can absorb more incident light due to the existence of mesopores. Incident light will scatter between the Sn3O4 nanoflakes and be trapped by the mesopores, resulting in reduced light reflection and enhanced light absorption. As an n-type semiconductor, electrons from the n-type Sn3O4 nanoflakes reduce water to form H2 and

OH —

anions, and a positive center will be formed at the interfacial surface of the

Sn3O4 nanoflakes. On one hand, because the Fermi level of n-type Sn3O4 is higher than the redox potential of the aqueous electrolyte, electrons will initially flow into the aqueous side at the interfacial area between the n-type Sn3O4 nanoflakes and aqueous electrolyte until a new electric equilibrium is reached. Then, the built-in electric field is formed immediately, which works in Schottky barrier mode. Thus, the energy band of Sn3O4 is bent upward, and the dark current can be observed40. On the other hand, incident photons with energy exceeding that of the Sn3O4 band gap (2.85 eV) will be absorbed, and electron–hole pairs will be generated. The generated holes are driven from the valence band of Sn3O4 into the interface of Sn3O4 nanoflakes/electrolyte and captured by the reduced form of the redox molecule ( h+ +OH- → OH ⋅), and the photogenerated electrons transport from the Sn3O4 nanoflakes to the carbon paper. The fast removal of holes can be expected across the heterojunction due to the large surface area. Due to the good conductivity of the carbon fiber paper, more generated electrons can be collected by the carbon paper (electrode) and transfer to the working electrode (Pt/FTO) by the external circuit, and the hydroxyl radicals OH⋅ are then reduced to O H - anions ( e − + OH ⋅ → OH − ) at the counter electrode (Pt/FTO) by the electrons returned

from the external circuit. The circuit was completed in this manner, demonstrating self-powered UV detection. In this paper, although the incident energy (2.48 eV) of the LED source is smaller than the band gap of the 3D hierarchal Sn3O4 nanostructures (2.85 eV), the SPPD device exhibits a much better visible-light photoresponse than ordinal Sn3O4, which can be explained as due to the existence of oxygen vacancies in Sn3O4, where the deficient 12


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emission light peak ranges from near-band emission to 700 nm. This wide visible emission helps to provide more electrons under LED irradiation. Similar explanations of the photocatalytic activity can be found in a previous report [37]. To further study the stability of Sn3O4 for PEC reaction, we carried out SEM and XPS measurements of the samples after the PEC reactions (see Supporting Information, Figure S5). The low-magnification SEM and magnified SEM images show that the structure is well maintained (Figure S5a and S5b), and the XPS spectra showed that Sn2+ and Sn4+ ions still existed in the samples (Figure S5c), and oxygen-related states were are observed (Figure S5d).

■ CONCLUSIONS Compared with an ordinal electrode on FTO, the Sn3O4 electrode grown in situ on carbon fiber paper is binder-free and demonstrates flexibility and long-term stability, as well as enhanced performance. As reported by Li et al.41, the mesoporous SnO2 nanoflakes grown on carbon cloth was used an electrode to reduce CO2 and demonstrated excellent performance. Both the LiFePO4-carbon paper hybrid cathode for lithium-ion batteries 42 and MoS2-carbon paper hybrid electrode for hydrogen evolution 43 also exhibited an improved performance. The common point is the usage of carbon cloth (or paper), which provides large surface areas and facilitates charge and mass transfer. Here, vertically aligned Sn3O4 nanoflakes on carbon paper can ensure enough light absorption and dye molecule adsorption and provide a good conductive channel for electron transport, therefore leading to an enhancement in the photoresponse.

Considering

its

uncomplicated

low-cost

fabrication

process

and

environmentally friendly features, this self-powered detector is a promising candidate for visible-light device applications. The advantage over ordinal materials for applications in SPPD devices is that this material is flexible and easily recoverable as a photocatalyst. Such hybrid material could achieve a complementary effect in lithium-ion batteries and a superior band gap match in photovoltaic devices; these works are under way.

■ ASSOCIATED CONTENT Supporting Information Additional XRD data; TEM and SEM images; Photocurrent response; XPS data. This material is available free of charge via the Internet at http://pubs.acs.org.

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■ AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected].

Author Contributions Ŧ

These authors contributed equally to this work.

Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Program of China (Grant No. 2017YFB0403101), the National Natural Science Foundation of China (Grant 61474096, 61604127 and 21675140), the Natural Science Foundation of Jiangsu Province (Grant No. BK20150453) and the Doctoral Program of Jiangsu Province (Grant No.1501144B). The authors would like to thank Dr. Rong-Bin Wang and Dr. Steffen Duhm from Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO), Chinese Academy of Sciences for assistance with the XPS measurements.

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Visible-Light Self-Powered Photodetector and Recoverable

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