Solution grown single-unit-cell quantum wires affording self-powered

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Solution grown single-unit-cell quantum wires affording self-powered solar-blind UV photodetectors with ultrahigh selectivity and sensitivity Dong Li, Simeng Hao, Guanjie Xing, Yunchao Li, Xiaohong Li, Louzhen Fan, and Shihe Yang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b10791 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 4, 2019

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Journal of the American Chemical Society

Solution Grown Single-Unit-Cell Quantum Wires Affording Self-Powered Solar-Blind UV Photodetectors with Ultrahigh Selectivity and Sensitivity Dong Li†, §, Simeng Hao†, §, Guanjie Xing†, Yunchao Li†, *, Xiaohong Li†, Louzhen Fan†, *, and Shihe Yang‡, * † College of Chemistry, Beijing Normal University, Beijing 100875, China ‡ Guangdong Key Lab of Nano-Micro Material Research, School of Chemical Biology and Biotechnology, Shenzhen Graduate School, Peking University, Shenzhen 518055, China ABSTRACT: As crystalline semiconductor nanowires are thinned down to a single-unit-cell thickness, many fascinating properties could arise pointing to promising applications in various fields. A grand challenge is to be able to controllably synthesize such ultrathin nanowires. Herein we report a strategy, which synergizes soft template with oriented attachment (ST-OA), to prepare high-quality single-unit-cell semiconductor nanowires (SSNWs). Using this protocol, we are able to synthesize for the first time ZnS and ZnSe nanowires (NWs) with only a single-unit-cell thickness (less than 1.0 nm) and a cluster-like absorption feature (i.e., with a sharp, strong and significantly blue-shifted absorption peak). Particularly, the growth mechanism and the single-unit-cell structure of the as-prepared ZnS SSNWs are firmly established by both experimental observations and theoretical calculations. Thanks to falling into the extreme quantum confinement regime, these NWs are found to only absorb the light with wavelengths shorter than 280 nm (i.e., solar-blind UV absorption). Utilizing such a unique property, self-powered photoelectrochemical-type photodetectors (PEC PDs) based on the ZnS SSNWs are successfully fabricated. The PDs after interface modification with TiO2 exhibit an excellent solar-blind UV photoresponse performance, with a typical on/off ratio of 6008, a detectivity of 1.5 × 1012 Jones and a responsivity of 33.7 mA/W. This work opens the door to synthesizing and investigating a new dimension of nanomaterials with a wide range of applications.

INTRODUCTION Single-unit-cell semiconductor nanowires (SSNWs) refer to those with a diameter of only a single unit cell thickness (typically below 1.0 nm), essentially approaching the fundamental limit of the thinnest crystalline nanowires (NWs).1-4 Benefiting from their ultrathin diameters, ultrahigh structural anisotropies and huge surface-to-volume ratios, SSNWs can push quantum confinement effects and interface effects to the limit, thus gaining much more attractive optoelectronic properties, and much more novel physical/chemical properties at large than regular ultrathin SNWs (USNWs) (typically with diameters of 1.0 to 3.0 nm).5-9 As a result, they are regarded as the ideal material base for applications in diverse fields ranging from optoelectronic nanodevices,10 high-performance catalysts,11 photothermal theraputics,12 to high-performance sensors.13 Although great efforts have been made so far to synthesize and utilize regular USNWs, it still remains a big challenge to prepare high-quality USNWs with diameters below 1.0 nm, not to mention the type of SSNWs, by solution processes. The challenge lies in the lack of effective means to not only generate the tiny nuclei that are only and exactly one-unit-cell in diameter at the initial nucleation stage, but also direct the as-formed nuclei to assemble or grow along a specific crystalline direction exclusively during the subsequent growth process. Apparently, to obtain such special nanostructures, morphology constraining factors (e.g., ligand passivation or

template confinement), kinetic controlling factors (e.g., precursor concentration and reactivity, interparticle interaction) and thermodynamic driving factors (e.g., product stabilization, reaction temperature) are highly imperative, and should be controlled to work synergistically. Considering such a particularity and complexity involved in making SSNWs, it stands to reason that adopting of only one type of synthetic strategy, e.g., templating confinement,14,15 ligand control,16,17 and oriented attachment,18-20 will not make the synthesis possible, particularly for the USNWs with a diameter below 1.0 nm. Herein, we report a novel soft template-cum-oriented attachment (ST-OA) strategy in synergy to synthesize unprecedented ZnS and ZnSe SSNWs with a diameter below 1.0 nm and a cluster-like absorption feature. Their special growth mechanisms and single-unit-cell structures were firmly established by both experimental observations and theoretical calculations. More importantly, the unique solar-blind UV selective absorption properties of the as-prepared ZnS SSNWs have enabled us to develop a new type of high-performance self-powered PEC PDs for the solar-blind UV detection with ultrahigh selectivity and sensitivity.

EXPERIMENTAL SECTION Chemicals. Chloroform, acetonitrile, ethanol, toluene and Zinc nitrate hexahydrate (Zn(NO3)2·6H2O) were purchased from Beijing Chemical Reagent Company. Sulfur (S) powder,

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Selenium (Se) powder (200 mesh, 99.99%), 1, 2-dimethyl3-propylimi-dazolium iodide, oleylamine (OLA), titanium (diisopropoxide) bis(2,4-pentanedionate) (TIPD) isopropanol solution, manganese acetate tetrahydrate (Mn(Ac)2·4H2O), poly(methyl methacrylate) (PMMA), LiI (99.9%), I2 (99.8%), and 4-tert-butylpyridine were obtained from Aladdin Chemistry Co., Ltd.. All reagents were of analytical grade and were used without further purification. Synthesis of ZnS SSNWs. ZnS USNWs were prepared by using a synergized soft template - oriented attachment strategy. Specifically, to prepared ZnS SSNWs (with a diameter of 0.8 nm), 0.2~0.5 mmol Zn(NO3)2 and 0.2~0.5 mmol S powder were predissolved in 10 and 5.0 mL of OLA, respectively. The two precursor solutions were then mixed together and transferred to a Teflon-lined stainless-steel autoclave (25 mL) at room temperature, which were finally heated at 100 °C for 2.0~12 h. Similarly, ZnS USNWs with diameters of 1.2 and 2.2 nm were prepared by following the identical procedure, except the reaction temperature was enhanced to 120 °C and 140 °C, respectively. Mn2+-doped ZnS SSNWs were prepared by using a synthetic procedure identical to that of the ZnS SSNWs except that a small amount of Mn(Ac)2 (5 mol%, referring to the Mn/Zn molar ratio) was added into the reaction system in the beginning. After reactions, the as-prepared ZnS samples were purified by using a typical washing, precipitation and re-dispersion protocol, and were finally dissolved in toluene/chloroform or dried under vacuum for further characterizations. Synthesis of ZnSe SSNWs. ZnSe SSNWs (with a diameter of 0.9 nm) were prepared by a synthetic procedure similar to that used for the ZnS SSNWs, except that Se powder was used as the non-metal precursor instead of S powder and meanwhile the reactions were performed at 120°C. Fabrication of self-powered PEC PDs based on ZnS NWs. To obtain ZnS NW film-based photoanodes, the as-prepared ZnS NWs of different diameters (e.g., 0.8, 1.2 and 2.2 nm) were spin-coated on fluorine-doped tin oxide (FTO) glasses, respectively. To obtain the films with similar thickness, the ZnS NW products of similar mass were adopted to prepare such films. They were then treated at 80 °C for 24 h in a vacuum oven. The counter electrodes were prepared by pasting Pt nets on quartz glasses. The photoanodes and the counter electrodes were finally assembled into sandwich-type cells. Their interelectrode spaces were then filled with a liquid electrolyte consisting of 50 mM I2, 0.1 M LiI, 0.6 M 1, 2-dimethyl-3-propylimidazolium iodide and 0.5 M 4-tertbutylpyridine in acetonitrile. To improve the response performance of the PDs, FTO glasses were modified with a TiO2 layer before spin-coating ZnS NWs thereon. To do so, 10 mL of TIPD/ethanol (1: 9) solutions were specially sprayed onto FTO glasses using oxygen as the carrier gas, and the FTO glasses were then sintered at 450 °C for 30 min to obtain a compact TiO2 layer. Preparation of Mn2+-doped ZnS SSNW film-based photochromic cards. Before film fabrication, 0.6 g of PMMA powder was added into 10 mL of toluene under stirring to form a clear solution. Afterwards, 0.02 g of Mn-doped ZnS SSNWs was dispersed into the PMMA solution under stirring. The as-formed hybrid solution was then spin-coated on quartz sheets and dried at room temperature to form ZnS SSNW film-based photochromic cards.

Characterization Methods. The absorption spectra of the as-prepared ZnS NW products were measured on a UV-2600 spectrophotometer. The room-temperature photoluminescence (PL) spectra of as-prepared ZnS NW products were measured on a PerkinElmer-LS55 luminescence spectrometer. The lowtemperature PL spectra and time-resolved PL spectra of as-prepared ZnS NW products were measured on an Edinburgh Instruments FLS980 steady-state fluorescence spectrometer. To measure the free Zn precursor amount in the reaction system and the Zn element amount in the as-formed ZnS NWs, inductively coupled plasma atomic emission spectroscopy (ICP-AES, Spectro Arcos Eop) was employed to analyze the aliquots taken from the reaction flasks at a given moment. Before ICP measurements, the aliquots must suffer from a standard HCl/HNO3 digestion. The dynamic light scattering (DLS) characterization was carried out on a DLS instrument (Zeta-Plus, Brookhaven Instruments Corp.). Before analysis, samples were diluted by pre-filtered chloroform to have an appropriate scattering intensity. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) observations were performed using a FEI Talos F200S operated at 200 kV, accompanying with two energy disperse X-ray spectrometers (Super-EDS). Atomic-resolution aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images were collected on a JEOL ARM 200F with probe corrector operating at 200 kV. The specimens for TEM observation were prepared by depositing a drop of dilute toluene solution of NW samples on a carbon-coated copper grid and drying at room temperature. For HAADF-STEM observations, ultrathin carbon films supported by lacey on 400 mesh copper grids were used instead. XPS data were obtained on an ESCALAB MK spectrometer equipped with a hemisphere analyzer. X-ray diffraction (XRD) patterns were recorded by a Rigaku D/Max-2400 diffractometer operated at 40 kV and 200 mA current under CuKa radiation (λ=1.5418 Å). The small angle X-ray diffraction (SAXRD) patterns were obtained on Rigaku D/MAX-2000 diffractometer using Cu Ka radiation as well. The samples for XRD and XPS measurements were all purified solid powders. Thermogravimetry-differential scanning calorimetry (TG-DSC) measurements were performed on a TGA/DSC 1/1100 SF (METTLER-TOLEDO) under a nitrogen gas flow at a heating rate of 10 K/min. The thicknesses of ZnS NW films were measured by a Dektak 6 M surface profilometer. The photocurrent responses of ZnS NW-based photodetectors were tested by a controlled intensity modulated photoresponse spectroscopy system (CIMPS, Zahner, PP211) coupled with a Zahner Zennium electrochemical workstation, with using intensity modulated light emitting diodes (265 or 312 nm) as light sources. Electrochemical impendence spectra (EIS) was carried out on a Zahner Zennium electrochemical workstations with sweeping the frequency from 100 mHz to 1 MHz at open-circuit voltage with 10 mV AC amplitude. The current-voltage (I-V) characteristics of ZnS NW-based PDs were measured by using liner sweep voltammetry (LSV) also on such a photoelectrochemical workstation in dark or under 265 nm (0.1 mW cm-2) UV illumination, at a scanning speed of 100 mV s-1.

RESULTS AND DICUSSION


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Journal of the American Chemical Society Synthetic Protocol and Mechanism. Figure 1a schematizes the synthetic conception in accordance with the protocol we developed. As an amphiphilic molecule with a

long alkyl chain and a headgroup, oleylamine (OLA) is known for its ability to assemble into crossed lamellar structures with

Figure 1. (a) Schematic illustration of the preparation process of ZnS SSNWs by using a synergized soft template-oriented attachment strategy. (b) SAXRD patterns of the Zn-oleylamine (i.e., Zn-OLA) complex prepared at different temperatures. (c) TEM images of Zn-OLA complexes formed by heating Zn(NO3)2 and OLA together at 100 C. (d) The DSC curves showing the influence of precursor predissolution on the ZnS formation reaction, I: Zn(NO3)2, and S powder predissolved in OLA separately before mixing together, II: only Zn(NO3)2 predissolved in OLA before mixing with S powder. (e) Temporal evolution of the absorption spectra of ZnS SSNWs formed at 100C with different reaction time. The inset in (c) is the corresponding size distribution diagram.

ultranarrow interlayer and intralayer spacings upon binding with metal ions (e.g., Zn ions) at certain temperatures, thus affording a double-lamellar template.21-23 Once nonmetal precursors (e.g., S) are introduced, ZnS primary SSNWs with an ultrathin diameter and a length limited by the template size are expected to form in the highly confined cylindrical nanocavities within such templates. With a proper thermolysis treatment, the multilayer templates can be exfoliated into single-layer nanosheets and as a result, the embedded SSNWs therein will be released. Under proper conditions, the free ZnS primary SSNWs can then be longitudinally assembled by oriented attachment into longer SSNWs under the assistance of ligand protection. To confirm this assumption, we availed ourselves of various techniques to intensively investigate the growth mechanism of the ZnS USNWs through the above-mentioned synthetic route. The small angle X-ray diffraction (SAXRD) patterns (Figure 1b) reveal the formation of Zn-OLA complexes at temperatures above 100 C; all of them possess three distinct diffraction peaks assignable to the characteristic “00L” peaks (L = 1, 2, 3), confirming the genuine formation of lamellar structures after heating the Zn salts and OLA together to a certain temperature. Transmission electron microscopic (TEM) images (Figure 1c) further confirm that such complexes have a pseudoplanar morphology with a lateral size of ca. 70 nm. Notably, the complexes formed at 100 C display the smallest interlayer spacing (ca. 3.3 nm, including the OLA bilayer and the cavity thus-formed), implying that it may serve as the most suitable template for directing the growth of the ZnS primary USNWs. Furthermore, differential scanning calorimetry (DSC) curves

confirm that the Zn and S precursors started to react with each other precisely at 100 C if they were both pre-dissolved in OLA before mixing together, evidenced by the appearance of a distinct exothermic peak with an onset temperature of 100 C (Figure 1d). More importantly, the absorption spectra (Figure 1e) further unveil that this reaction took place in a well-controlled fashion producing extremely tiny products of constant “effective size”, as witnessed by the appearance of a non-shifted sharp excitonic absorption peak during the entire growth process. In contrast, when one of the precursors was not pre-dissolved in OLA, the reaction between them became slightly more facile (started to react at 94 C) (Figure 1d), resulting in the formation of much thicker NWs (Figure S1 and S2) due evidently to the less effective template protection. We followed the entire formation process of ZnS USNWs by continuous TEM observations over a fixed time interval. For case I with the Zn and S precursors being separately pre-dissolved in OLA, we observed ZnS primary USNWs with a poor crystallinity packed side by side in the pseudoplanar templates after reacting for 30 min (Figure 2a and Figure S3). Interestingly, only after 30 min of reaction, well-defined single crystalline ZnS primary USNWs were generated and released from the templates (Figure 2b). Such a splitting process was also reflected by the dynamic light scattering (DLS) measurement, which showed a clear decrease in hydrodynamic diameter of the products within the initial 1.0 h of reaction (Figure S4). After that, the well-separated ZnS USNWs of different lengths but identical diameter (ca. 0.8 nm) were produced as the reaction proceeded (Figure 2c-2e). More



attractively, statistical analysis reveals their lengths were actually increased in a stepwise manner within the time span of 1.0 to 12 h of reaction, i.e., from the initial 70 nm, then to 200 nm and 280 nm, and finally to 340 nm when prolonging the reaction time (Figure 2f and Figure S5). Given the fact that the lengths of all the as-formed ZnS NWs are nearly multiples of 70 nm (the length of the primary ZnS NWs), we believe that the primary ZnS NWs are the building block of them. This polymerization-like process is striking and could be made possible by the “living” nature of the two distinct edges (i.e., the S or Zn atoms-dominated terminals) of the primary ZnS NWs under proper conditions. Actually, such an assembly is an energy favorable process. Taking the assembly of two-unit-cell ZnS nanorods (building blocks) as an example, density functional theory (DFT) calculation shows that the formation energy of the assembly product (four-unit-cell ZnS nanorod) is 15.82 eV lower than the sum of that of the two individual building blocks (Figure S6, Table S1 and S2). ICP analysis further discloses not only the free Zn precursor amount in the reaction system but also the Zn element amount in the as-formed ZnS NWs was kept nearly constant during such a reaction period (Figure S7), strongly implying the growth of ZnS NWs was at the cost of consumption of the existing ones. Apparently, TEM images along with ICP analysis clearly confirm the formation of long ZnS USNWs really goes through an initial template-confined growth of ZnS primary NWs followed by an oriented assembly of them into much longer counterparts.

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proposed synthetic protocol encourages us to fully utilize it for the preparation of high-quality SSNWs. As expected, with this protocol, we were able to prepare wurtzite ZnS NWs with a diameter as small as 0.8 nm and an average length of 340 nm (Figure 2e, Figure 3 and Figure S8). Significantly, this special ZnS NWs display a cluster-like absorption feature, with a sharp and strong absorption peak (at 273 nm) and a

Figure 3. UV-vis absorption spectrum (a), HAADF-STEM image (b) and HRTEM images (c, d) of the as-prepared ZnS USNWs; (c) imaging at normal condition, (d) imaging with tilting the specimen ca. 30 in clockwise direction. The insets in (a) and (c-d) are the corresponding Tauc plot and FFT images, respectively.

Figure 2. TEM images (a-e) and statistical analysis of the length and diameter variations (f) of ZnS USNWs prepared at 100 C for different reaction times.

Optical Properties and Structural Characterizations of ZnS SSNWs. The confirmation of the feasibility of the

steep absorption edge (at 285 nm) (Figure 3a), indicating the product having an ultrasmall and highly uniform diameter. A closer analysis reveals that their absorption edges are significantly blue-shifted by ca. 0.55 eV relative to that of bulk wurtzite ZnS, demonstrating the existence of extremely strong quantum confinement effect.8,24 More importantly, their optical absorptions mainly cover the wavelength range shorter than 280 nm (i.e., UVC), thus endowing them with the ability to selectively absorb solar-blind UV light. In addition, photoluminescence (PL) measurements reveal that the emission spectra of the ZnS USNWs are dominated by trap emission without band-edge emission,25,26 evidenced by the appearance of a broad emission peak (comprising several shoulder peaks, spanning from 320 to 450 nm), excitationdependent peak shift and biexponential decay (Figure S9). Such an emission feature can be rationally explained by considering that the ZnS USNWs possess a huge specific surface area and are thus prone to forming various point defects (e.g., interstitial atoms or vacancies). Furthermore, the fine structures and local atomic arrangements of the ZnS USNWs were intensively characterized and confirmed by high resolution TEM (HRTEM) and structural simulation, respectively. For example, Figure 3c and 3d both confirm the ZnS USNWs are single crystalline in nature and grow along the [0001] direction. A closer analysis unveils that the _

projections of (0001) planes along [1010] direction are


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Journal of the American Chemical Society composed of five lattice points and show a line width of 0.8 nm, while only three lattice points and a line width of 0.68 nm _

_

can be observed when projected along [2110] direction (by tilting the specimen ca. 30 in clockwise direction). Because the diameters of the ZnS USNWs observed from _

_

_

[1010] and [2110] directions are both rather close to that of ideal unit-cell of wurtzite ZnS NWs (e.g., 0.76 and 0.66 nm _

_

_

along [1010] and [2110] projection direction respectively, Figure S10), we speculate that they may possess single-unit-cell structures. However, we also note that the ideal wurtzite ZnS SSNW model exhibits a slightly different atomic arrangement in comparison with that observed in HRTEM images (Figure 3c and 3d). Considering its abundant dangling bonds and rather high formation energy (-2.63 eV per pair of ZnS), this ideal SSNW model (Figure S10) should not be able to exist stably. Hence, we further established a more stable SSNW model (i.e., surface-optimized ZnS SSNW model) by cutting the bulk wurtzite ZnS crystal and adding additional Zn and S atoms to passivate some highly unsaturated surface dangling bonds, thus endowing it with a much lower formation energy (-3.39 eV per pair of ZnS) (Figure 4a, Figure S10, Figure S11 and Table S3). Notably, both the atomic arrangement and the overall width of this _

_

_

model along [1010] (Figure 4b) and [2110] direction (Figure 4c) are highly consistent with the HRTEM observations (Figure 3c and 3d), implying the ZnS USNWs are actually of the single-unit-cell structure. This unprecedented structure was further confirmed by atomic-resolution aberration-corrected high-angle annular dark field scanning TEM (HAADF-STEM). As shown in Figure 4d, it is clear that almost all the projections of the (0001) planes of the ZnS

Figure 4. Surface-optimized atomic structure model (only showing a segment of two unit–cell length) of wurtzite ZnS _ SSNWs (a), and the corresponding atom arrangement along [101 _ _ 0] (b) and [2110] direction (c). (d) Atomic-resolution aberrationcorrected HAADF-STEM image of the as-prepared ZnS SSNWs. The inset in (d) is the corresponding FFT image.

ZnSe SSNWs. Impressively, this synthetic strategy can be easily extended to the preparation of wurtzite ZnSe NWs with a diameter of ca. 0.9 nm and a length of ca. 530 nm (Figure 5a-5b and Figure S12), demonstrating its generality. We have confirmed that not only the diameter but also the atomic arrangements of the as-obtained ZnSe USNWs (Figure 5b) are well consistent with that of wurtzite ZnSe SSNW model (Figure 5c and 5d), suggesting again that these ZnSe NWs are also the single-unit-cell NWs. Similar to above well-characterized ZnS SSNWs, absorption spectra, TEM images, along with ICP analysis reconfirm that the growth of this special ZnSe NWs also follows a mechanism similar to that of ZnS SSNWs (Figure S13-S16). Therefore, we believe that this synthetic strategy is potentially general for preparing SSNWs.

_

USNWs along [1010] direction, possess five bright points (corresponding to five Zn atoms directly) and a cross width of ca. 0.8 nm. It should be pointed out that the ZnS SSNWs are the thinnest ZnS NWs synthesized so far, which have never been reported until now.

Figure 5. UV-vis absorption spectrum (a) and HRTEM image (b) of ZnSe SSNWs (with a diameter of 0.9 nm). Surface-optimized atomic structure model of wurtzite ZnSe SSNWs (c) and the _

corresponding atomic arrangement along [1010] direction (d). The inset in (b) is the corresponding diameter distribution histogram.

Solar Blind UV Photodetectors. Considering their unique absorption properties, the ZnS SSNWs were specially chosen as the photoactive elements to fabricate self-powered solar-blind PEC PDs, which have a sandwich-type device configuration (Figure 6a).27, 28 To appraise the photoresponsive characteristics, I-V curves of the PDs were firstly recorded in dark or under 265 nm light illumination (with a power density of 0.1 mW cm-2). As shown in Figure S17, the I-V curve



obtained under illumination shows an attractive photovoltaic behavior, yielding a short-circuit current (Isc) of ca. 0.29 μA and an open-circuit voltage (Voc) of ca. 0.56V. More importantly, the Isc under illumination is ca. three orders of magnitude larger than that in dark at zero bias, indicating the high potential of the devices for high-performance selfpowered photodetectors. Furthermore, the responses of PDs to different wavelengths of illumination were measured to disclose their spectral selectivity. As shown in Figure 6b, the solar-blind/UV (R275 nm/R290 nm) rejection ratios of the as-fabricated PDs are found to be typically around 75, which are much higher than those of previously reported PDs based on one-dimensional nanomaterials,29,30 manifesting the ultrahigh selectivity of the solar-blind UV response. In addition, the on/off response ratios of the PDs under 265 nm UV illumination are confirmed to be normally ca. 280 times higher than those under 312 nm UV illumination (Figure 6c), again demonstrating their excellent wavelength selectivity. Impressively, taking 265 nm illumination as a representative example, these PDs also exhibit rather attractive solar blind UV photoresponse (detailed information for the calculation can be found in Figure S18, S19, and Table S4): having a responsivity of 19.4 mA/W, a detectivity of 9.6 × 1011 Jones, an on/off response ratio of 4590, a rise/decay time of ca. 0.25 /0.20s, and a linear- response range from 20 to 280 μW cm-2 (Figure 6d). It should be noted that such parameters are much better than those of previously reported self-powered PDs. 29, 30 It is worth noticing that the response performances of our PDs are still slightly inferior to those of the best schottky- or pn junction-based PDs working under an external bias voltage (see Table S5). Such a limitation is probably attributable to our devices not only involving a solution-based ion diffusion process27, 28 but also adopting unmodified FTO glasses to support the photoanodes. Notably, the response performances of our devices can be substantially improved by only using TiO2 to modify the FTO glasses before spin-coating ZnS NWs thereon. Taking a representative modified PD as an example, its responsivity (33.7 mA/W), detectivity (1.5 × 1012 Jones), and rise/decay time (0.14 s /0.15 s) are found to be ca.1.7 times higher, 1.6 times higher, and 1.8 times/1.3 times faster, respectively, than that of the counterpart without the TiO2 modification (Figure S20 and Table S6). Similarly, when adopting a schottky29, pn junction30 or MSM type 31 device configuration, much better response performance is expected to be achieved, which is currently actively pursued in our laboratory. We contend that it is the extremely strong and highly selective solar-blind UV response that sets our ZnS SSNWs apart from the previously-reported solar blind materials.29, 32, 33 Therefore, the PEC PDs based on the ZnS SSNWs are ideal candidates for the sensitive and selective solar-blind UV detection. As a proof-of-concept, one of the PDs, combined with a multimeter, was specifically employed as a portable self-powered device for the solar-blind UV detection of the light leakage from a UV ozone cleaning system (mainly emitting 184 and 254 nm UV light) (Figure 6e-6f and S21). As expected, it worked very well and could easily and rapidly perceive such a light leakage. To meet the urgent need for rapid solar-blind UV detection in daily life, we further developed Mn2+-doped ZnS SSNW film-based photochromic cards. They can emit bright purple fluorescence (from Mn2+) when illuminated by short wavelength UV light (< 290 nm), thus enabling easy and rapid detection of such a radiation with the naked eye (Figure S22-S24).

To elucidate why the PDs based on the ZnS SSNWs could attain such an excellent photoresponse performance, we endeavored to investigate the absorption capacities and carrier transport properties of the ZnS NWs of different diameters (e.g., 0.8, 1.2 and 2.2 nm, Figure S25). As shown in Figure 7a and Fig S26, it is clear that the normalized absorbance in the UVC region of the ZnS SSNWs is significantly larger than that of the other two thicker NWs samples, no matter whether they are in a film or in a solution. Quite plausibly, this phenomenon is ascribable to the much higher oscillator strength of the ZnS SSNWs concentrated to the excitonic peak

Figure 6. Schematic diagram (a) and spectral responses (b) of a representative self-powered solar-blind PEC PD based on ZnS SSNWs. (c) Time response of the short-circuit photocurrent under 265 nm light illumination (with a power density of 0.3 mW cm-2) and 312 nm light illumination (with a power density of 0.5 mW cm-2) respectively, measured for light-on/off states. (d) Short-circuit photocurrent as a function of the incident light intensity under 265 nm UV illumination. (e-f) Photographs showing the operation of an as-fabricated PD combined with a multimeter to measure the light leakage from a UV ozone cleaning system (mainly emitting 184 and 254 nm UV light).


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Figure 7. (a) Normalized absorption spectra of the three types of ZnS NW films of identical thickness. (b) Nyquist plots of the PDs, fabricated by using the films composed of ZnS NWs of different diameters as the photoactive layers (with a similar thickness), under 265 nm UV illumination. The inset in (b) is the corresponding Bode plots.

due to their huge quantum confinement effect,34, 35 thereby endowing them with the strongest UVC absorption ability. We next compare the carrier transport properties. As displayed in Figure 7b, EIS spectra reveal that, under 265 nm UV illumination, the PD based on the ZnS SSNWs possesses the smallest charge transfer resistance (ca. 345 Ω) and the longest

  1/ 2 f

r max electron lifetime (5.2 ms, according to , where fmax is the characteristic frequency of the charges transferring at NW films/electrolyte interface36, 37) among the three PDs, indicating its best carrier transport characteristics. We therefore propose that it is the excellent absorption ability along with the superior carrier transport property of the ZnS SSNWs that have enabled the PDs based them to have such a remarkable photoresponse performance (Figure S27 and Table S5).

CONCLUSION

I-V curves, detailed information for calculating response parameters, a proof-of-concept application of ZnS SSNW-based PDs, Mn2+-doped ZnS SSNW film-based photochromic cards. This material is available free of charge on the ACS Publications website at DOI: XXXXXXX.

AUTHOR INFORMATION Corresponding Author * Email: [email protected] * Email: [email protected] * Email: [email protected]

ORCID Dong Li: 0000-0003-2917-6520 Simeng Hao: 0000-0002-4048-2898 Guanjie Xing: 0000-0001-6829-0639 Yunchao Li: 0000-0002-5554-7252 Xiaohong Li: 0000-0003-2043-1206 Louzhen Fan: 0000-0002-1319-5412 Shihe Yang: 0000-0002-6469-8415

Author Contributions §

Dong Li and Simeng Hao contributed equally to this work.

Notes

In conclusion, we have proposed and demonstrated a soft template coupled with oriented attachment strategy, i.e., utilizing alkylamine lamellar template to direct the formation of primary SSNWs followed by oriented assembly (literally polymerization) into much longer NWs, to effectively synthesize high-quality SSNWs. With this protocol, we were able to prepare unprecedented ZnS and ZnSe NWs with single-unit-cell thickness (less than 1.0 nm) and an average length beyond 300 nm. Interestingly, the two kinds of SSNWs both display a cluster-like absorption feature, having a sharp, strong and significantly blue-shifted absorption peak, which almost keeps unchanged until reaction for 12h. More importantly, due to the existence of huge quantum confinement effect, the as-prepared ZnS SSNWs only absorb the light with a wavelength shorter than 280 nm. By taking advantage of such a unique property, self-powered PEC PDs based on the ZnS SSNWs were successfully fabricated and demonstrated, which have enabled ultraselective and ultrasensitive solar-blind UV detection. To our knowledge, this is the first report on the facile synthesis of high-quality SSNWs and the striking demonstration of their fascinating optoelectronic properties to the device level, which will definitely inspire the controllable preparation, deep understanding and wide application of this type of novel nanomaterials.

ASSOCIATED CONTENT Supporting Information ZnS NW amount normalization, TEM images, XRD patterns, XPS spectra, EDS patterns, UV−vis absorption spectra, ICP analysis, atomic structure models of ZnS SSNWs and ZnSe SSNWs, formation energy calculations, PL spectra of ZnS NWs,

The authors declare no competing financial interests.

ACKNOWLEDGMENT This work is financially supported by the Natural Science Foundation of China (NSFC, 21273020, 21872011, 21003012, 21573019 and 91023039), Shenzhen Peacock Plan Program (KQTD2016053015544057) and Nanshan Pilot Plan (LHTD20170001). The authors thank Prof. Run Long for his helpful discussion and Prof. Xinjun Xu, Prof. Zhan’ao Tan, Prof. Haizheng Zhong, Ting Meng and Qiang Guo for their helps with some measurements.

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