Plasmon-Enhanced Self-Powered UV Photodetectors Assembled by

Subscriber access provided by FLORIDA STATE UNIV

Article

Plasmon-enhanced self-powered UV photodetectors by incorporating Ag@SiO2 core–shell nanoparticles into TiO2 nanocube photoanodes Yuewu Huang, Qingjiang Yu, Jinzhong Wang, Jianan Wang, Cuiling Yu, James Taban Abdalla, Zhi Zeng, Shujie Jiao, Dongbo Wang, and Shiyong Gao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02697 • Publication Date (Web): 04 Dec 2017 Downloaded from http://pubs.acs.org on December 4, 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 Sustainable Chemistry & Engineering 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 34 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 Sustainable Chemistry & Engineering

Plasmon-Enhanced Self-Powered UV Photodetectors by Incorporating Ag@SiO2 Core–Shell Nanoparticles into TiO2 Nanocube Photoanodes Yuewu Huang,a Qingjiang Yu,*ab Jinzhong Wang,*a Jianan Wang,a Cuiling Yu,c James Taban Abdalla,a Zhi Zeng,a Shujie Jiao,a Dongbo Wang,a and Shiyong Gao a a

Department of Opto-electronic Information Science, School of Materials Science and Engineering,

Harbin Institute of Technology, No.92, Xidazhi Street, Nangang District, Harbin, China. b

Key Laboratory for Photonic and Electric Bandgap Materials, Ministry of Education, Harbin

Normal University, No.1 South of shida RD Limin Development Zone, Harbin, 150025, China. c

Department of Physics, Harbin Institute of Technology, No.92, Xidazhi Street, Nangang District,

Harbin, 150001, China. Corresponding Authors *E-mail: [email protected] (Q.Y.); [email protected] (J.W.)

1

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 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

ABSTRACT: A novel photoelectrochemical self-powered ultraviolet photodetector (UVPD) has been assembled employing the Ag@SiO2 core-shell nanoparticles (NPs) incorporated TiO2 nanocubes (NCs) as the photoanode. The incorporating of the plasmonic core-shell NPs can boost the photocurrent of the self-powered UVPD. The Finite difference time domain (FDTD) and transient absorption spectroscopy (TAS) were employed to understand plasmonic enhancement processes. By optimizing the incorporated ratio of Ag@SiO2 NPs, the photocurrent of UVPD with 2 wt.% Ag@SiO2 NPs reaches the maximum value in view of the enhanced light harvesting and effective inhibition of charge recombination. More importantly, the UVPD with 2 wt.% Ag@SiO2 NPs achieves a high on/off ratio of 8212 and a remarkable responsivity of 0.151 A W−1, combined with a rapid response time, prominent spectral selectivity and photosensitivity linearity response.

KEYWORDS: UV photodetector, TiO2 nanocube, core-shell nanoparitcles, localized surface plasmon resonance, transient absorption spectroscopy

2


Page 2 of 34

Page 3 of 34 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


INTRODUCTION Ultraviolet photodetectors (UVPDs) have drawn extensive attentions, as the detection of UV rays is of prime importance in many civil and military applications such as ozone hole monitoring, chemical/biological agent sensing, flame detection, missile warning, and secure inter-satellite communication.1−4 Mainstream UVPDs are falled into two categories: photoconductive type and photovoltaic type. The photoconductive UVPDs need additional power sources to drive the entire system, which not only greatly increases the overall circuitry network size but also extremely limits their flexible applications. By contrast, the photovoltaic UVPDs designed on the p-n junction5−9 and Schottky junction10−12 may generate built-in electric field with no need for a driving bias voltage and exhibit superior performance. However, their complicated fabrication processes require fairly precise equipments, accompanying with high additional economic costs. Therefore, it is still urgent to develop a novel UVPD with a high performance and simple preparation process. Recently, a new emerging photoelectrochemical (PEC) self-powered UVPD has aroused increasing concerns due to its inexpensive material consumption and simple technological process.13−24 In addition to the absence of sensitizers, the PEC UVPDs have a similar structure to dye-sensitized solar cells (DSCs). As a result, the composition, structure and morphology of the photoanode material take significant roles in PEC UVPDs. Over recent years, various strategies about the photoanodes have been proposed in order to optimize the PEC self-powered UVPDs. Li and co-workers15 reported a TiO2 nanocrystalline PEC self-powered UVPDs, and exhibited a sensitivity of 2698.5% as well as an ultrafast rise time (τr) of 0.08 s and a decay time (τd) of 0.03 s. Xie et al.18 fabricated TiO2 nanorod and nano-branched arrays on a FTO substrate and applied them as photoanodes to assemble a PCE self-powered UVPD. In contrast to the bare TiO2 nanorod arrays 3



based UVPD, the photosensitivity of the UVPD based on TiO2 nano-branched arrays increases from 0.03 to 0.22 A W−1. However, the TiO2 nano-branched arrays based UVPD exhibits a relatively slow response time (τr=0.15 s and τd=0.05 s). Although some exciting results have been obtained in the PEC self-powered UVPDs, more effective nanostructure photoanodes that can increase light harvesting capacities and reduce interfacial charge recombination are still desirable to optimize for higher-performance UVPDs. Localized surface plasmon resonance (LSPR) of noble metal nanostructures has emerged as an effective method to enhance the performance in diverse scientific fields due to its unique optical properties.25−29 Ray et al.26 utilized Ag nanoparticles interacted with graphene sheets in flexible piezoelectric nanogenerator, and demonstrated an enhancement of the energy conversion efficiency up to ∼46.6%. Moreover, an impactful work by Ray and co-works27 showed amazing results when Au nanoparticle-embedded silk protein was incorporated into ZnO nanorod array hybrid photodetector. The Au nanoparticles gave rise to photo-generated hole trapping. This LSPR induced photo-generated hole trapping led to a higher photoresponse and specific detectivity for the ZnO nanorod array hybrid photodetector. Recently, Bardhan et al.28 reported significant carrier generation and injection enhancement when Au/Ag bimetallic nanostructures were incorporated into MAPbBr3-based perovskite solar cells, resulting in a 26% increase in the efficiency. It is well known that LSPR can enhance the light harvesting and power conversion efficiencies of DSCs by adjusting the shape, size, composition, and dielectric environment of metal nanostructures.30−34 Snaith et al.30 demonstrated the plasmon-enhanced efficiency for DSCs by incorporating core-shell Au-SiO2 nanoparticles (NPs) with strong LSPR into the mesoporous TiO2 photoanode. Kelly et al.32 reported that the incorporation of Ag@SiO2 core-shell nanoprims in the photoanode boost an 4


Page 4 of 34

Page 5 of 34 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


overall 32±17% enhancement in the efficiency, which is because the triangular Ag nanoprism core can induce large improvement in the light harvesting efficiency at longer wavelengths. Subsequently, Bardhan and co-works34 reported that SiO2 coated bimetallic Au core/Ag shell nanostructures embedded in the photoanods of DSCs achieved a higher efficiency of 7.51% in comparison with a reference DSC (5.97%), which is attributed to the improved light harvesting for DSCs. Owing to the similar device structure between DSCs and PEC UVPDs, the semiconductor light trapping layer modified by plasmonic metal nanostructures may be beneficial to the utilization of UV light in PEC UVPDs. Herein, we present a new approach to improving the performance of PEC self-powered UVPDs by incorporating the Ag@SiO2 core-shell NPs into the TiO2 nanocube (NC) photoanode. Compared to the pure TiO2 NCs based UVPD, the UVPD with the Ag@SiO2 NPs exhibits an obvious improvement in the photocurrent density (J). The impact of Ag@SiO2 NPs on the J of UVPDs is systematically investigated. Furthermore, the photosensitivity, response time, spectral response and the change activity of J signal to light intensity are also measured to evaluate the performance of the UVPD based on Ag@SiO2 NPs incorporated TiO2 NC photoanode.

EXPERIMENTAL Materials. Tetrabutyl titanate, tetramethylammonlum hydroxide, sodium dodecylsulfonate, tri-sodium citrate, Polyvinylpyrrolidone (PVP, Mw=58000), titanium (IV) chloride TiCl4, 1,3-dimethylimidazolium iodide (DMII), LiI, I2, 4-tert-butylpyridine (TBP), guanidinium thiocyanate (GNCS), acetonitrile, valeronitrile, ethyl cellulose powders, α-terpineol were purchased

5



from Aldrich. Ethylene glycol, isopropanol, tetraethyl orthosilicate, ammonia hydroxide, acetone, and ethanol were obtained from Sinopharm Chemical Reagent Co., Ltd. Synthesis of TiO2 NCs. The TiO2 NCs were synthesized under hydrothermal conditions.35 In a typical experiment, tetrabutyl titanate (0.05 mol) was dissolved in deionized water (30 mL), then stirred at 50 oC for 1 h. The titanium hydrate was mixed by adding the tetramethylammonlum hydroxide (0.017 mol) dropwisely into the tetrabutyl titanate solution at 0 oC. Subsequently the mixed solution was heated at 135 oC for 5 h. After reflux cooling, the titanium hydrate was transferred to autoclave at 230 oC for 5 h in an electric oven. The obtained precipitate was collected for the following pastes preparation. Synthesis of Ag NPs. Ag NPs were prepared in large scale building on the previous reported method.36 200 mL of 2.5 g of PVP (Mw=58000) ethylene glycol solution was brought to 130 oC at a 10 oC/min rate under vigorous stirring. Then, 0.5 g of AgNO3 was quickly added and kept at 130 oC for 1 h. Wait until the temperature dropped to room temperature, the Ag NPs can be precipitated by adding acetone (800 mL). Finally, the Ag NPs were redispersed in 4 mL of ethanol to obtain the 0.05 g (Ag NPs)/mL solution. Preparation of Ag@SiO2 core-shell NPs. The Ag@SiO2 NPs were prepared according to the Stӧber method.37 Typically, Ag NPs (1.0g) were added into the solution containing isopropanol (20 mL) and ammonium hydroxide (28 wt.%, 0.2 mL) under vigorous stirring. Then, tetraethyl orthosilicate (10 µL) was added slowly into the above solution and retained at 23 °C for 6 h. After the reaction, the Ag@SiO2 NPs were centrifuged and collected for further using.

6


Page 6 of 34

Page 7 of 34 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


Preparation of TiO2 and Ag@SiO2 incorporated TiO2 Pastes. The as-prepared TiO2 NCs (1.0 g) was added into ethanol (80 mL) containing α-terpineol (4.34 mL) and ethyl cellulose (0.5 g) under vigorous stirring for 1 hour, and then removed the solvent with rotary evaporator for 40 min to prepare TiO2 paste. A certain content of Ag@SiO2 NPs (1, 2 and 3%, w/w) incorporated TiO2 NCs pastes were followed the same pattern. Assembling of PEC UVPDs. Prior to the fabrication of PEC UVPDs, electrolyte was prepared similar with our previous work.38,39 The sandwich structure was assembled by placing a platinized counter electrodes on the unsensitized screen-printed film, and the two electrodes were clipped together for sealing with a 30 µm thick surlyn gasket under a hot-pressing condition. After injecting the electrolyte by a vacuum pump, the infusing hole was sealed hermetically by a mini hot press. Characterizations. The morphology and crystallinity of the samples were characterized by field emission scanning electron microscopy (FESEM, JEOL JEM-6700F), X-ray diffraction (XRD, Rigaku D/max-2500) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100). X-ray photoelectron spectroscopy (XPS) measurement was done on the spectrometer (ESCALAB 250Xi, Thermo Scientific Escalab). The photocurrent density-voltage (J−V) characteristics of the UVPDs were tested with a Source Meter (Keithley Instruments). The external quantum efficiency (EQE) spectra were estimated using a 500 W xenon lamp (Zolix) with a monochromator operated in direct current (DC) mode. The absorbance spectra and diffuse reflectance

spectra

were

taken

employing

a

Perkin-Elmer

Lambda

850

UV-visible

spectrophotometer. Femtosecond transient absorption experiments were performed using a HELIOS TAS setup (Ultrafast Systems, Helios fire). The bulk of the fundamental 800 nm (35 fs, 1 kHz, 7 mJ/pulse) output from a Coherent ultrafast Ti: Sapphire amplifier (Astrella, Coherent) was split into 7



two beams. The larger portion was led to a Coherent OPerA Solo optical parametric amplifiers (OPAs) to produce the pump beam (300 nm, 5 µJ/cm2 ). The photocurrent response and electrochemical impedance spectroscopy (EIS) measurements for the UVPDs were conducted on the Metrohm electrochemical workstation (Autolab-PGSTAT302N). The EIS measurements were conducted in the dark with a perturbation modulation of 20 mV from 10 mHz to100 kHz. The UV irradiation source was provided by a UV LED (365 nm, Nichia).

RESULTS AND DISCUSSION The morphological and structural characterizations of TiO2 NCs are listed in Figure 1a and b. The TEM image (Figure 1a) displays that the obtained TiO2 sample composes of cubic structures with an average size of about 14 nm, which is consistent with the FESEM result (inset of Figure 1a) of the TiO2 sample. To further study the fine structure of NCs, representative HRTEM images of the NCs are shown in Figure 1b and Figure S1(Supporting Information). The clear lattice fringes in the HRTEM images were observed, suggesting that the TiO2 NC was constructed with a single-crystal structure. The corresponding fast Fourier transform (FFT) pattern (inset of Figure 1b) turns out to be dot matrix instead of concentric rings, which further proves its single crystalline features of the TiO2 NCs. The interplanar spacings are 0.35 and 0.48 nm, which is identical to the (101) and (001) facets of anatase TiO2, respectively. Figure 1c reveals the TEM image of Ag NPs. The Ag NPs are close to spherical in shape and ∼35 nm in diameter. The typical TEM characterization (Figure 1d) conﬁrms the core-shell nanostructures of Ag@SiO2 NPs. The SiO2 layer is uniformly coated around the metallic Ag NPs and ∼3 nm in thickness. The SiO2 layer serves several purposes for the PEC UVPDs: (i) ensuring the electrical isolation as well as impeding Ag NPs as the charge recombination center, (ii) enhancing the thermal stability of Ag NPs and reducing Ostwald ripening 8


Page 8 of 34

Page 9 of 34 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 the annealing process, (iii) preventing the direct contact of the electrolyte with the Ag NPs and providing the resistance to degradation in the corrosive electrolyte. The absorbance spectra of the Ag and Ag@SiO2NPs dispersed in ethanol are shown in Figure S2. The introduction of silica shell causes a change in refractive index surrounding the Ag NPs, explaining that the LSPR peak of the Ag@SiO2 NPs centered at 402 nm.40,41 The TiO2 NCs and Ag@SiO2 NPs incorporated TiO2 NCs were applied as the photoanodes of UVPDs. The size distribution of TiO2 NCs and Ag@SiO2 NPs in the mixed film is shown in Figure S3. Figure 2 shows the XRD patterns of the TiO2 NC films with different contents of Ag@SiO2 NPs. The TiO2 diffraction peaks of all films can be indexed to the anatase phase of TiO2 (JCPDS No. 21-1272). In addition, the Ag@SiO2 NPs incorporated films also exhibit four diffraction peaks of Ag NPs, which appear at 2θ= 38.1°, 44.2°, 64.4°, and 77.5°, corresponding to the (111), (200), (220), and (311) planes of face-centered cubic (fcc) Ag (JCPDS No. 04-0783), respectively. The intensity of Ag diffraction peaks increases with the increment of the Ag content. The AgO diffraction peaks cannot be observed for the Ag@SiO2 NPs incorporated films, indicating that the Ag cores exist as Ag0 in the core-shell structures. The XPS measurement was also used to analyze the chemical components of the Ag@SiO2 NPs incorporated TiO2 NCs film. As shown in Figure S4, the Ag, Si, Ti and O atoms present in the film with 3 wt.%Ag@SiO2 NPs. To further investigate the elemental distribution in the Ag@SiO2 NPs incorporated TiO2 NCs film, the element-mapping characterization gotten from EDS is performed on the film with 3 wt.% Ag@SiO2 NPs (Figure S5). Based on the above results, it indicates that the SiO2 shell is existent in the Ag@SiO2 NPs incorporated TiO2 NCs film. To investigate the roles of Ag@SiO2 NPs in affecting the performance of the series UVPDs, their 9



J-V characteristics with light intensity of 20 mW cm−2 (λ=365 nm) were measured (Figure 3a). The characteristic photovaltatic parameters were listed in Table 1. It is found that the short-circuit current density (Jsc) and open-circuit voltage (Voc) of the Ag@SiO2 NPs based UVPDs are significantly enhanced in comparison with the UVPD without Ag@SiO2 NPs. The highest Jsc of 1.51 mA cm−2 and Voc of 598 mV are achieved for the UVPD with 2 wt.% Ag@SiO2 NPs. Further, the EQE spectra of these UVPDs were measured to understand the Jsc increment of the UVPDs with Ag@SiO2 NPs. As shown in Figure 3b, the EQE curves have a good match trend with the Jsc of UVPDs. The maximum EQE value is 54% for the UVPD with 2 wt.% Ag@SiO2 NPs, leading to a higher Jsc. The reasons for EQE variation can be consistently correlated with the light-harvesting capabilities of photoanodes and interfacial charge recombination.42 To gain an insight into the functions of Ag@SiO2 NPs on the light-harvesting capabilities, the diffused reflectance spectra of TiO2 photoanodes with different contents of Ag@SiO2 NPs were first measured. As shown in Figure S6, these photoanodes show a similar diffused reflectance spectrum, indicating that the far-field scattered light of Ag@SiO2 NPs can not give rise to significant influence on the light harvesting of photoanodes. We further tested the absorbance spectra of these photoanodes, as presented in Figure 4a. Compared to the pure TiO2 NC photoanode, the light absorbance of the Ag@SiO2 NPs based photoanode exhibits a gradual enhancement with increasing the Ag@SiO2 NPs content in the spectra from 300 to 400 nm. To assess the effect of LSPR of Ag@SiO2 NPs, Finite difference time domain (FDTD) method was used to estimate the distribution of the electric field intensity around an Ag NP attached with a TiO2 NC under illumination at 365 nm. As shown in Figure 4b, the intense electromagnetic near-field caused by the LSPR effect is localized at the 10


Page 10 of 34

Page 11 of 34 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


interface between the Ag NP and the TiO2 NC, which can enhance the effective absorption cross-section of TiO2 NCs, thereby increasing the light absorption.25 Moreover, additional FDTD simulations (Figure S7 and S8) to verify the plasmon resonance properties of Ag structures in the photoanodes, which reveal that Ag@SiO2 NP with ∼35 nm in diameter is more suitable for the plasmon-enhanced UVPDs. Interestingly, the light absorption peak positions show red shift with increasing the content of Ag@SiO2 NPs. This might be related to the aggregation of Ag@SiO2 NPs in the photoanode, leading to the plasmon resonance to the longer wavelength.34 Furthermore, the plasmon resonance of Ag@SiO2 NPs has a good overlap with the light absorption of TiO2 NCs (Figure S2). This may induce the plasmon resonance energy transfer (PRET) from the metal nanostructures to the adjacent semiconductor, which can also improve the light harvesting of photoanodes.34 Considering the mechanism of carrier dynamics at Ag@SiO2 NP/ TiO2 NC interfaces and the photodetector performance, the 1, 2 and 3 wt.% Ag@SiO2 NPs within the TiO2 NC films were investigated by means of femtosecond transient absorption spectroscopy (TAS). All samples were performed using a 300 nm pump impulses, and the absorption dynamics was probed with a white light continuum from 320 to 550 nm. The corresponding normalized TA spectra at 1 and 500 ps time delay are presented in Figure 5a and b, respectively. For the three films, negative absorption wing observed at 390 nm is deduced from the photobleaching (PB) state of the plasmon mode. This figure feature reflects the well-known band filling effect,43 which is consistent well with the LSPR band of the Ag@SiO2 NP, as shown in Figure S2. On the other hand, a positive absorption band signified a result of stimulated absorption above the populated states in the conduction band,34,43 which is attributed to photoinduced absorption (PIB). More insight into the transient decay kinetics 11



in the three samples is presented in the temporal profiles by bi-exponential function fitting (Figure 5c), and the fitting data are listed in Table 2. In all films, the first time constant (τ1) is attributed to the electron-electron scattering, which gives rise to a "hot" electron transfer (HET).44 Subsequently, the "hot" electron cools thermally equilibrium with nanocrystal lattice by electron-phonon interactions. A longer time constant (τ2) represents the stored photon energy releasing to the surrounding by phonon-phonon interactions, which is contributed by the plasmon resonance energy transfer (PRET) to TiO2.45 The fitting results imply that both HET and PRET may describe the plasmonic enhancement mechanism for Ag@SiO2 NPs incorporated TiO2 NCs films. It is found that τ1 is relatively constant for all the samples, suggesting that the HET can scarcely occur through the insulating silica layer of Ag@SiO2 NPs.30 However, the variations in τ2 and amplitude weighted lifetimes (τavg) were caused by increasing the density of Ag@SiO2 NPs in the film, indicating that the PRET from Ag@SiO2 NPs to the TiO2 conduction band and thus giving rise to a high photocurrent. As expected, due to a good overlap between the plasmon resonance of Ag@SiO2 NPs and the light absorption of TiO2 NCs, which weights toward the overall enhancement dominantly represented by the PRET.46 As shown in the Table 2, compared with the film with 1 wt.% Ag@SiO2 NPs, the carrier lifetime reflected in the TAS exhibits a significant decreasing trend for the film with 2 wt.% Ag@SiO2 NPs, which is believed to derive from the PRET facilitating the rapid transmission of electrons to the TiO2 conduction band.47 This rapid transmission of electrons is manifested in the PB decay kinetics, both as a shorter τ2 as well as τavg. At a high concentration (3 wt.% Ag@SiO2 NPs), the aggregation of Ag@SiO2 NPs in the TiO2 NCs is exposed to a damping of the plasmon resonance and generates a weaker PRET process,34 resulting in an increment trend in the corresponding τ2 and τavg. 12


Page 12 of 34

Page 13 of 34 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


To comprehend the effect of Ag@SiO2 NPs in TiO2 photoanodes on the recombination dynamics at the TiO2 NCs/electrolyte and Ag@SiO2 NPs incorporated TiO2 NCs/electrolyte interfaces, the EIS of UVPDs was executed. Figure 6a displays the Nyquist plots of UVPDs with various contents of Ag@SiO2 NPs at −0.57 V. Normally, two arcs can be exhibited in Nyquist plots and fitted based on an equivalent circuit (inset in Figure 6a). From left to right, the first small arc represents the resistance with respect to the reduction reaction of I3− ions in the electrolyte (Rct1), and the second arc reflects the resistance related to the charge recombination process at the TiO2/electrolyte interface (Rct2).48−50 The inset in Figure 6a is the equivalent circuit model which simulate the electrochemical and photoelectric processes in the UVPDs. In this study, we mainly consider that the introduction of Ag@SiO2 NPs influences the interfacial charge recombination process for different UVPDs. Therefore, Rct2 was discussed in detail. Figure 6b is the fitted Rct2 under different biases. By contrast, the UVPDs with Ag@SiO2 NPs exhibit a larger Rct2 value than the pure TiO2 based UVPD, revealing that the existence of Ag@SiO2 NPs contributes to suppressing the interfacial charge recombination. Moreover, the Rct2 value shows an increasing to decreasing trend with the introduction of Ag@SiO2 NPs, achieving the maximum value at 2 wt.%. Additionally, the values of photoelectrons lifetime (τn) in the photoanodes were simultaneously determined by the Rct2 and constant phase element (CPE) and expressed as τn= Rct2×CPE2,51 as presented in Figure 6c. At a given bias, the larger value of the τn is, the slighter photoelectrons recombination rate is. The maximum τn of photoelectrons is also obtained at the 2 wt.%, which is well consistent with the Rct2 results and will favor the generated superior EQE and Jsc. In addition, we also investigated the charge recombination process in the UVPDs by an open-circuit voltage decay (OVD) technique.52 The decay curves of the four UVPDs are shown in Figure 6d. The UVPD with 2 wt.% Ag@SiO2 13



Page 14 of 34

NPs exhibits the slowest Voc decay rate among the four UVPDs, indicating the longest electron lifetime. However, the excess Ag@SiO2 NPs embedded in the TiO2 networks can impair the structural integrity of photoanodes and increase the charge recombination,53 resulting in the reduction of EQE as well as Jsc. In order to explain the Voc variation for the UVPDs, we used the equation of ideal diode to fit the J−V curves in dark conditions, as given in eq. (1).54

  q (V − JARs)   V + JARs J = Jph(V ) − J 0 exp   − 1 − ARsh nkT   

(1)

where J is the current density, Jph(V) is the voltage-dependent photogenerated current density, V is the applied bias, J0 is the reverse saturation current density, q is the elementary charge, Rs is the series resistance, n is the ideality factor, k is the Boltzmann constant, T is the absolute temperature, A is the area of the device, and Rsh is the shunt resistance. According to the fitting results of J0 (Table 1), it is found that the J0 values of UVPDs with Ag@SiO2 NPs reduce with respect to the pure TiO2 based UVPD. The minimum J0 of 1.33×10−6 mA cm−2 is obtained for the UVPD with 2 wt.% Ag@SiO2 NPs. Moreover, under open-circuit conditions (V=Voc, J=0), the eq. (1) may be described by the following modified expression:55,56

Voc =

nkT  J ph (Voc)  nkT  J ph (Voc)  ln 1 + ≈   q J0  q  J0  

(2)

where Jph (Voc) represents the dark current density at Voc. Jph (Voc) is 1.63 and 2.24 mA cm−2 for the UVPD with 2 wt.% Ag@SiO2 NPs and without Ag@SiO2 NPs, respectively. According to the eq. (2), the ratio of the Voc of the two UVPDs can be calculated. Since the variation in n values is negligible,

the

expected

ratio

of

the

Voc

for

the

two

UVPDs

is

ln(1.63/1.33×10−6)/ln(2.24/2.31×10−6)=1.02, which is approximate to the ratio (598/571=1.05) of 14


Page 15 of 34 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


the measured Voc. Therefore, the higher Voc in the UVPDs may be interpreted by the smaller J0. The decrement of J0 suggests that the interfacial charge recombination is restrained, which is consistent with the EIS and OVD results. To evaluate the performance of the UVPDs based on the Ag@SiO2 NPs incorporated TiO2 photoanodes, time-dependent photoresponse was carried out during repetitive on/off illumination (365 nm) at zero bias. Figure 7a presents the time-resolved J signal response over several turning on/off cycles measured with a power density of 20 mW cm−2. It is worth noting that that the periodical switch of J signals from the “on” state to the “off” state, indicating these UVPDs possess good reproducibility. The on/off ratio of the J signal can reach 8212 for the UVPD with 2 wt.% Ag@SiO2 NPs, which is significantly higher than that (5332) of the UVPD with pure TiO2. In general, the τr and τd are defined as the time required for the J to transition from 0 to 1−1/e or recovery to 1/e.19 As seen from Figure 7b, the τr and τd of the 2 wt.% Ag@SiO2 incorporated TiO2 UVPD are 0.003 and 0.008 s, respectively. By contrast, the τr of this UVPD is faster than that (0.005 s, Figure S10) of the pure TiO2 based UVPD. However, the τd of this UVPD shows a relative delayed time compared with pure TiO2 UVPD (0.005 s, Figure S10), which may be caused by restraining the interfacial electron recombination. In addition, the spectral response (Rλ) of the UVPD may also be calculated by the relationship: Rλ = EQE%λ/1240. As presented in Figure 7c, the peak responsivity (0.151 A W−1) is evaluated for the UVPD with 2 wt.% Ag@SiO2 NPs. Furthermore, the UV/visible rejection ratio of this UVPD is more than 103, which qualifies this UVPD to be applied as a visible-blind UVPD. For a better comparison, a comprehensive survey of TiO2-based PEC self-powered UVPDs with critical parameters has been summarized in Table S1. Both the photosensitivity and response time of the UVPD with 2 wt.% Ag@SiO2 NPs are better as 15



compared to those of many other UVPDs. Intensity of incident light dependent Jsc responses of the UVPDs is also a significant property to evaluate device performance. The Jsc as a function of incident light power is presented in Figure 7d. Notably, the Jsc shows an excellent linear relationship with increasing the incident light power. A higher absorbed photon flux would produce more photogenerated charge carriers, suggesting that the plasmon-enhanced UVPDs are more sensitive and accurate for UV light measurement. In addition, the detectivity (D*) as one of important parameters to evaluate the sensitivity of UVPDs is shown in Figure S11. The 2 wt.% Ag@SiO2 NPs incorporated UVPD (D*= 5.81×109 Jones) exhibits higher detectivity than that of TiO2 NCs, which further proves the capability of the introduction of Ag@SiO2 NPs for self-powered UVPDs to respond to a light signal.

CONCLUSIONS In summary, the Ag@SiO2 core–shell NPs incorporated TiO2 NC films were applied as photoanodes to construct the PEC self-powered UVPDs. We demonstrated that plasmonic core–shell NPs can significantly enhance the J of UVPDs. For the UVPDs with different contents of Ag@SiO2 NPs, the highest J was achieved at 2 wt.% Ag@SiO2 NPs. The J increment is attributed to the improved light harvesting by the strong near-field coupling of Ag@SiO2 NPs with TiO2 NCs and the PRET from Ag@SiO2 NPs to neighboring TiO2 NCs and the suppressed charge recombination at the TiO2/electrolyte interface. The advantage over pure TiO2 NCs UVPD is that self-powered UVPD with 2 wt.% Ag@SiO2 NPs can dramatically enhance response time and responsivity, along with outstanding visible-blind characteristic and photosensitivity linearity. More importantly, this methodology offers a feasible strategy to obtain prominent PEC self-powered UVPD in future practical applications. 16


Page 16 of 34

Page 17 of 34 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


ASSOCIATED CONTENT Supporting Information Supplementary date related to this article can be found via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Q.Y.); [email protected] (J.W.) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 51502056 and 51202046), the National Science and Technology Support Project (Nos. 2015BAI01B05), the Fundamental Research Funds for the Central Universities (Nos. HIT.NSRIF.2013006 and HIT.BRETIII.201403), the Natural Science Foundation of Heilongjiang Province of China (No. F2016013), and the Open Project Program of Key Laboratory for Photonic and Electric Bandgap Materials, Ministry of Education, Harbin Normal University, China.

REFERENCES (1) Razeghi, M.; Rogalski, A. Semiconductor Ultraviolet Detectors. J. Appl. Phys. 1996, 79, 7433−7473. (2) Liao, M.; Koide,Y. High-Performance Metal-Semiconductor-Metal Deep-Ultraviolet Photodetectors Based on Homoepitaxial Diamond Thin Film. Appl. Phys. Lett. 2006, 89, 113509.

17



(3) Oder, T. N.; Li, J.; Lin, J. Y.; Jiang, H. X. Photoresponsivity of Ultraviolet Detectors Based on InxAlyGa1-x-yN Quaternary Alloys. Appl. Phys. Lett. 2000, 77, 791−793. (4) Jiang, Y.; Zhang, W. J.; Jie, J. S.; Meng, X. M.; Fan, X.; Lee, S.-T. Photoresponse Properties of CdSe Single-Nanoribbon Photodetectors. Adv. Funct. Mater. 2007, 17, 1795−1800. (5) Bie, Y.-Q.; Liao, Z.-M.; Zhang, H.-Z.; Li, G.-R.; Ye, Y.; Zhou, Y.-B.; Xu, J.; Qin, Z.-X.; Dai, L.; Yu, D.-P. Self-Powered, Ultrafast, Visible-Blind UV Detection and Optical Logical Operation Based on ZnO/GaN Nanoscale p-n Junctions. Adv. Mater. 2011, 23, 649−653. (6) Hatch, S. M.; Briscoe, J.; Dunn, S. A Self-Powered ZnO-Nanorod/CuSCN UV Photodetector Exhibiting Rapid Response. Adv. Mater. 2013, 25,867−871. (7) Jin, W.; Ye, Y.; Gan, L.; Yu, B.; Wu, P.; Dai, Y.; Meng, H.; Guo, X.; Dai, L. Self-Powered High Performance Photodetectors Based on CdSe Nanobelt/Graphene Schottky Junctions. J. Mater. Chem.2012, 22, 2863−2867. (8) Ni, P.-N.; Shan, C.-X.; Wang, S.-P.; Liu, X.-Y.; Shen, D.-Z. Self-Powered Spectrum-Selective Photodetectors Fabricated from n-ZnO/p-NiO Core–Shell Nanowire Arrays. J. Mater. Chem. C. 2013, 1, 4445−4449. (9) Yang, S.; Tongay, S.; Li, S.-S.; Xia, J.-B.; Wu, J.; Li, J. Environmentally Stable/Self-Powered Ultraviolet Photodetectors with High Sensitivity. Appl. Phys. Lett. 2013, 103, 143503. (10) Wu, D.; Jiang, Y.; Zhang, Y.; Yu, Y.; Zhu, Z.; Lan, X.; Li, F.; Wu, C.; Wang, L.; Luo, L. Self-Powered and Fast-Speed Photodetectors Based on CdS:Ga Nanoribbon/Au Schottky Diodes. J. Mater. Chem. 2012, 22, 23272−23276. (11) Yang, Y.; Guo, W.; Qi, J.; Zhao, J.; Zhang, Y. Self-Powered Ultraviolet Photodetector Based on a Single Sb-doped ZnO Nanobelt. Appl. Phys. Lett. 2010, 97, 223113.

18


Page 18 of 34

Page 19 of 34 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


(12) Lu, S.; Qi, J.; Liu, S.; Zhang, Z.; Wang, Z.; Lin, P.; Liao, Q.; Liang, Q.; Zhang, Y. Piezotronic Interface Engineering on ZnO/Au-Based Schottky Junction for Enhanced Photoresponse of a Flexible Self-Powered UV Detector. ACS Appl. Mater. Interfaces 2014, 6, 14116−14122. (13) Lee, W-J.; Hon, M-H. An Ultraviolet Photo-Detector Based on TiO2/Water Solid-Liquid Heterojunction. Appl. Phys. Lett. 2011, 99, 251102. (14) Gao, C.; Hu, C.; Wang, X.; Wang, S.; Tian, Y.; Zhang, H. UV Sensor Based on TiO2 Nanorod Arrays on FTO Thin Film. Sens. Actuators. B. Chem. 2011, 156, 114−119. (15) Li, X.; Gao, C.; Duan, H.; Lu, B.; Pan, X.; Xie, E. NanocrystallineTiO2 Film Based Photoelectrochemical Cell as Self-Powered UV-Photodetector. Nano Energy 2012, 1, 640−645. (16) Xie, Y.; Wei, L.; Wei, G.; Li, Q.; Wang, D.; Chen, Y.; Yan, S.; Liu, G.;Mei, L.; Jiao, J. A Self-Powered UV Photodetector Based on TiO2 Nanorod Arrays. Nanoscale Res. Lett. 2013, 8,188. (17) Xie, Y.; Wei, L.; Li, Q.; Chen, Y.; Liu, H.; Yan, S.; Jiao, J.; Liu, G.; Mei, L. A High Performance Quasi-Solid-State Self-Powered UV Photodetector Based on TiO2 Nanorod Arrays. Nanoscale 2014, 6, 9116−9121. (18) Xie, Y.; Wei, L.; Li, Q.; Chen, Y.; Yan, S.; Jiao, J.; Liu, G.; Mei, L. High-Performance Self-Powered UV Photodetectors Based on TiO2 Nano-Branched Arrays. Nanotechnology 2014, 25, 075202. (19) Zhou, J.; Chen, L.; Wang, Y.; He, Y.; Pan, X.; Xie, E. An Overview on Emerging Photoelectrochemical Self-Powered Ultraviolet Photodetectors. Nanoscale 2016, 8, 50−73. (20) Majumder, T.; Hmar, J. J. L.; Debnath, K.; Gogurla, N.; Roy, J. N.; Ray, S. K.; Mondal, S. P. Photoelectrochemical and Photosensing Behaviors of Hydrothermally Grown ZnO Nanorods. J. Appl. Phys. 2014, 116, 034311.

19



(21) Li, Q.; Wei, L.; Xie, Y.; Zhang, K.; Liu, L.; Zhu, D.; Jiao, J.; Chen, Y.; Yan, S.; Liu, G.; Mei, L. ZnO Nanoneedle/H2O Solid-Liquid Heterojunction based Self-Powered Ultraviolet Detector. Nanoscale Res. Lett. 2013, 8, 415. (22) Gao, C.; Li, X.; Wang, Y.; Chen, L.; Pan, X.; Zhang, Z.; Xie, E. Titanium Dioxide Coated Zinc Oxide Nanostrawberry Aggregates for Dye-Sensitized Solar Cell and Self-Powered UV-Photodetector. J. Power Sources 2013, 239, 458−465. (23) Li, X.; Gao, C.; Duan, H.; Lu, B.; Wang, Y.; Chen, L.; Zhang, Z.; Pan, X.; Xie, E. High-Performance Photoelectrochemical-Type Self-Powered UV Photodetector Using Epitaxial TiO2/SnO2 Branched Heterojunction Nanostructure. Small 2013, 9, 2005−2011. (24) Huang, Y.; Yu, Q.; Wang, J.; Li, X.; Yan, Y.; Gao, S.; Shi, F.; Wang, D.; Yu, C. A High-Performance Self-Powered UV Photodetector Based on SnO2 Mesoporous Spheres@TiO2. Electron. Mater. Lett. 2015, 11, 1059−1065. (25) Atwater, H. A.; Polman, A. Plasmonics for Improved Photovoltaic Devices. Nat. Mater. 2010, 9, 205−213. (26) Sinha, T. K.; Ghosh, S. K.; Maiti, R.; Jana, S.; Adhikari, B.; Mandal, D.; Ray, S. K. Graphene-Silver-Induced Self-Polarized PVDF-Based Flexible Plasmonic Nanogenerator Toward the Realization for New Class of Self Powered Optical Sensor. ACS Appl. Mater. Interfaces 2016, 8, 14986−14993. (27) Gogurla, N.; Kundu, S. C.; Ray,S. K. Gold Nanoparticle-Embedded Silk Protein-ZnO Nanorod Hybrids for Flexible Bio-Photonic Devices. Nanotechnology 2017, 28, 145202. (28) Zarick, H. F.; Boulesbaa, A.; Puretzky, A. A.; Talbert, E. M.; DeBra, Z. R.; Soetan, N.; Geoheganc, D. B.; Bardhan, R. Ultrafast Carrier Dynamics in Bimetallic Nanostructure-Enhanced Methylammonium Lead Bromide Perovskites. Nanoscale 2017, 9, 1475−1483.

20


Page 20 of 34

Page 21 of 34 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


(29) Gogurla, N.; Sinha, A. K.; Santra, S.; Manna, S.; Ray, S. K. Multifunctional Au-ZnO Plasmonic Nanostructures for Enhanced UV Photodetector and Room Temperature NO Sensing Devices. Sci. Rep. 2014, 4, 6483. (30) Brown, M. D.; Suteewong, T.; Sai Santosh Kumar, R.; D’Innocenzo, V.; Petrozza, A.; Lee, M. M.; Wiesner, U.; Snaith, H. J. Plasmonic Dye-Sensitized Solar Cells Using Core-Shell Metal-Insulator Nanoparticles. Nano Lett. 2011, 11, 438−445. (31) Qi, J. F.; Dang, X. N.; Hammond, P. T.; Belcher, A. M. Highly Efficient Plasmon-Enhanced Dye-Sensitized Solar Cells through Metal@Oxide Core-Shell Nanostructure. ACS Nano 2011, 9, 7108−7116. (32) Gangishetty, M. K.; Lee, K.E.; Scott, R. J.; Kelly, T. L. Plasmonic Enhancement of Dye Sensitized Solar Cells in the Red-to-Near-Infrared Region using Triangular Core−Shell Ag@SiO2 Nanoparticles. ACS Appl. Mater. Interfaces 2013, 5, 11044−11051. (33) Zarick, H. F.; Hurd, O.; Webb, J. A.; Hungerford, C.; Erwin,W. R.; Bardhan, R. Enhanced Efficiency in Dye-Sensitized Solar Cells with Shape-Controlled Plasmonic Nanostructures. ACS Photonics 2014, 1, 806−811. (34) Zarick,H. F.; Erwin, W. R.; Boulesbaa, A.; Hurd, O. K.; Webb, J. A.; Puretzky, A. A.; Geohegan, D. B.; Bardhan, R. Improving Light Harvesting in Dye-Sensitized Solar Cells UsingHybrid Bimetallic Nanostructures. ACS Photonics 2016, 3, 385−394. (35) Burnside, S. D.; Shklover, V.; Barbe, C.; Comte, P.; Arendse, F.; Brook, K.; Grӓzel, M. Self-Organization of TiO2 Nanoparticles in Thin Films. Chem. Mater. 1998, 10, 2419−2425. (36) Sun, Y.; Xia, Y. Mechanistic Study on the Replacement Reaction betweenSilver Nanostructures and Chloroauric Acid in Aqueous Medium. J. Am. Chem. Soc. 2004, 126, 3892−3901.

21



(37) Stӧber, W.; Fink, A. Controlled Growth of Monodisperse Silica Spheres in the Micron Size Range. J. Colloid Interface Sci.1968, 26, 62−69. (38) Yu, Q.; Yu, C.; Guo, F.; Wang, J.; Jiao, S.; Gao, S.; Li, H.; Zhao, L. A Stable and Efficient Quasi-Solid-State Dye-Sensitized Solar Cell with a Low Molecular Weight Organic Gelator. Energy Environ. Sci. 2012, 5, 6151−6155. (39) Li, X.; Yu, Q.; Yu, C.; Huang, Y.; Li, R.; Wang, J.; Guo, F.; Zhang, Y.; Gao, S.; Zhao, L. Zinc-Doped SnO2 Nanocrystals as Photoanode Materials for Highly Efficient Dye-Sensitized Solar Cells. J. Mater. Chem. A 2015, 3, 8076−8082. (40) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. The Optical Properties of Metal Nanoparticles: The Influence of Size, Shape, and Dielectric Environment. J. Phys. Chem. B 2003, 107, 668−677. (41) Fernandez, J. R.; Santos, I. P.; Juste, J. P.; Javier Garcia de Abajo, F.; Marzan,L. L. The Effect of Silica Coating on the Optical Response of Sub-Micrometer Gold Spheres. J. Phys. Chem. C 2007, 111, 13361−13366. (42) Snaith, H. J.; Ducati, C. SnO2-Based Dye-Sensitized Hybrid Solar Cells Exhibiting Near Unity Absorbed Photon-to-Electron Conversion Efficiency. Nano Lett. 2010, 10, 1259−1265. (43) Burda, C.; Link, S.; Green, T. C.; El-Sayed, M. A. New Transient Absorption Observed in the Spectrum of Colloidal CdSe Nanoparticles Pumped with High-Power Femtosecond Pulses. J. Phys. Chem. B 1999, 103, 10775−10780. (44) Sum, T. C.; Mathews, N.; Xing, G.; Lim, S. S.; Chong, W. K.; Giovanni, D.; Dewi, H. A. Spectral Features and Charge Dynamics of Lead Halide Perovskites: Origins and Interpretations. Acc. Chem. Res. 2016, 49, 294−302.

22


Page 22 of 34

Page 23 of 34 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


(45) Cushing, S. K.; Li, J.; Meng, F.; Senty, T. R.; Suri, S.; Zhi, M.; Li, M.; Bristow, A. D.; Wu, N. Photocatalytic Activity Enhanced by Plasmonic Resonant Energy Transfer from Metal to Semiconductor. J. Am. Chem. Soc. 2012, 134, 15033−15041. (46) Cushing, S. K.; Li, J.; Bright, J.; Yost, B. T.; Zheng, P.; Bristow, A. D.; Wu, N. Controlling Plasmon-Induced Resonance Energy Transfer and Hot Electron Injection Processes in Metal@TiO2 Core−Shell Nanoparticles. J. Phys. Chem. C 2015, 119, 16239−16244. (47) Zarick, H. F.; Boulesbaa, A.; Puretzky, A. A.; Talbert, E. M.; Debra, Z. R.; Soetan, N.; Geohegan, D. B.; Bardhan, R. Ultrafast Carrier Dynamics in Bimetallic Nanostructure-enhanced Methylammonium Lead Bromide Perovskites. Nanoscale 2017, 9, 1475−1483. (48) Bisquert, J. Theory of the Impedance of Electron Diffusion and Recombination in a Thin Layer. J. Phys. Chem. B 2002, 106, 325−333. (49) Wang, Q.; Ito, S.; Grätzel, M.; Fabregat-Santiago, F.; Mora-Seró, I.; Bisquert, J.; Bessho, T.; Imai, H. Characteristics of High Efficiency Dye-Sensitized Solar Cells. J. Phys. Chem. B 2006, 110, 25210−25221. (50) Bisquert, J.; Fabregat-Santiago, F.; Mora-Seró, I.; Garcia-Belmonte, G.; Giménez, S. Electron Lifetime in Dye-Sensitized Solar Cells: Theory and Interpretation of Measurements. J. Phys. Chem. C 2009, 113, 17278−17290. (51) Wu, W.; Xu, Y.; Rao, H.; Su, C.; Kuang, D. Multistack Integration of Three-Dimensional Hyperbranched AnataseTitania Architectures for High-Efficiency Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2014, 136, 6437−6445. (52) Zaban, A.; Greenshtein, M.; Bisquert, J. Determination of the Electron Lifetime in Nanocrystalline Dye Solar Cells by Open-Circuit Voltage Decay Measurements. ChemPhysChem 2003, 4, 859−864.

23



(53) Dong, H.; Wu, Z.; Lu, F.; Gao,Y.; Shafei, A. E.; Jiao, B.; Ning, S.; Hou, X. Optics–Electrics Highways: Plasmonicsilver Nanowires@TiO2 Core–Shell Nanocomposites for Enhanced Dye-Sensitized Solar Cells Performance. Nano Energy 2014, 10, 181−191. (54) Sheehan, S. W.; Noh, H.; Brudvig, G. W.; Gao, H.; Schmuttenmaer, C. A. Plasmonic Enhancement of Dye-Sensitized Solar Cells Using Core−Shell−Shell Nanostructures. J. Phys. Chem. C 2013, 117, 927−934. (55) Lee, J. H.; Cho, S.; Roy, A.; Jung, H. T.; Heeger, A. J. Enhanced Diode Characteristics of Organic Solar Cells using Titanium Suboxide Electron Transport Layer. Appl. Phys. Lett. 2010, 96, 163303. (56) Liu, Z.; Lee, E. C. Solvent Engineering of the Electron Transport Layer using 1,8-Diiodooctane for Improving the Performance of Perovskite Solar Cells. Org. Electron. 2015, 24, 101−105.

24


Page 24 of 34

Page 25 of 34 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 1. (a) TEM and FESEM (inset) image of TiO2 NCs. (b) HRTEM image and corresponding FFT patterns (inset) of TiO2 NCs. (c) TEM image of Ag NPs. (d) TEM image of Ag@SiO2 NPs.

25



Figure 2. XRD patterns of the TiO2 NC films with different contents of Ag@SiO2 NPs.

26


Page 26 of 34

Page 27 of 34 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 3. (a) J−V characteristics and (b) EQE spectra of the UVPDs with different contents of Ag@SiO2 NPs.

27



Figure 4. (a) Absorption spectra of the TiO2 NC films with different contents of Ag@SiO2 NPs. (b) The calculated distribution of the electric field intensity around an Ag NP attached with a TiO2 NC.

28


Page 28 of 34

Page 29 of 34 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 5. Femtosecond transient absorption spectra of the TiO2 NC films with different contents of Ag@SiO2 NPs at (a) 1 ps and (b) 500 ps upon laser excitation at 300 nm. (c) Decay kinetic at 390 nm for all films.

29



Figure 6. (a) Nyquist plots of EIS data of the UVPDs with different contents of Ag@SiO2 NPs at −0.57 V and the equivalent circuit applied to fit the impedance data (inset). (b) Interfacial charge recombination resistance and (c) Electron lifetime as a function of applied bias. (d) Open-circuit voltage decay curves of the UVPDs with different contents of Ag@SiO2 NPs.

30


Page 30 of 34

Page 31 of 34 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 7. (a) Photocurrent responses of the UVPDs with different contents of Ag@SiO2 NPs under on/off radiation of 20 mW cm−2 UV light illumination (λ=365 nm). (b) Enlarged rising and decaying edges of the photocurrent response and (c) spectral responsivity characteristic for the UVPD with 2 wt.% Ag@SiO2 NPs. (d) J as a function of the incident UV light intensity from 10 µW cm−2 to 40 mW cm−2 for the UVPD with 2 wt.% Ag@SiO2 NPs.

31



Page 32 of 34

Table 1. Detailed photovoltatic parameters of the UVPDs with different contents of Ag@SiO2 NPs. UVPD Pure TiO2 1 wt.% Ag@SiO2 2 wt.% Ag@SiO2 3 wt.% Ag@SiO2

Jsc (mA cm−2) 0.96 1.16 1.51 1.37

Voc (mV) 571 582 598 589

32


J0 (mA cm−2) 2.31×10−6 1.92×10−6 1.33×10−6 1.59×10−6

Page 33 of 34 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


Table 2. Amplitudes (A), time constants (τ), and amplitude weighted lifetimes (τavg) derived from bi-exponential fits of the transient absorption decay function for the different films. Film 1 wt.% Ag@SiO2 2 wt.% Ag@SiO2 3 wt.% Ag@SiO2

A1 −0.88 −0.90 −0.85

τ1 (ps) 18.06 22.03 20.05

A2 −0.079 −0.083 −0.090

33


τ2 (ps)

τavg (ps)

3365 1125 2209

293 114 227


TOC

Ag@SiO2 core–shell nanoparticles incorporated TiO2 nanocubes were prepared the photoanodes of self-powered UVPDs. The Ag@SiO2 NPs with strong localized surface plasmon resonance can enhance the light harvesting of UVPDs and suppress interfacial charge recombination.

34


Page 34 of 34

Plasmon-Enhanced Self-Powered UV Photodetectors Assembled by

Recommend Documents