Controlled Synthesis, Formation Mechanism and Applications of

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C: Energy Conversion and Storage; Energy and Charge Transport

Controlled Synthesis, Formation Mechanism and Applications of Colloidal AgSnS Nanoparticles and AgSnS/AgS Heterostructured Nanocrystals 8

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Jiajia Ning, and Bo Zou J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00730 • Publication Date (Web): 05 Mar 2018 Downloaded from http://pubs.acs.org on March 5, 2018

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 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 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.

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The Journal of Physical Chemistry

Controlled Synthesis, Formation Mechanism and Applications of Colloidal Ag8SnS6 Nanoparticles and Ag8SnS6/Ag2S Heterostructured Nanocrystals Jiajia Ning, Bo Zou* State Key Laboratory of Superhard Materials and College of Physics, Jilin University, Qianjin Road No.2699, Changchun, 130012, P. R. China Email: [email protected]

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ABSTRACT: Semiconductor nanocrystals (SC NCs) have attracted scientists’ attention because of their size and shape dependent optical properties. Now, many scientists’ research is focused on the synthesis of the earth abundant and environment compatible SC NCs and their applications for solar energy conversion. Herein, colloidal silver based I-IV-VI SC NCs, Ag8SnS6 nanoparticles (NPs) were synthesized. Ag8SnS6 NPs showed strong quantum size confinement effect from optical measurement. The photocurrent response measurement showed an n-type property for Ag8SnS6 NPs. It is very interesting that colloidal Ag8SnS6/Ag2S heterostructured nanocrystals (HSNCs), containing drop-shaped and little bowtie-shaped, can also be synthesized via changing the adding process of precursors in the beginning of experiment. The first formation of Ag NPs is very important to form Ag8SnS6/Ag2S HSNCs. A seeded growth mechanism was proposal to explain the formation of Ag8SnS6/Ag2S HSNCs. The polycrystalline structure in Ag2S NPs seed played an important role to synthesize drop-shaped HSNCs. The synthetic method for Ag8SnS6/Ag2S HSNCs can provide an approach to synthesize branched HSNCs with polycrystalline seeds. Moreover, Ag8SnS6 NPs showed potential for solar energy conversion.

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INTRODUCTION Semiconductor nanocrystals (SC NCs) have attracted scientists’ attention because of their size and shape dependent properties. Based on their special properties, SC NCs have been widely used for display, optoelectronic device and biomedicine.1,2 During the past two decades, the colloidal synthesis of SC NCs has been intensively studied, which opened up new possibilities to address the challenges in fabricating low cost device.3-6 High quality colloidal SC NCs, such as CdSe, CdS, PbS, CdTe and CuInSe2, have been produced for colloidal NCs-derived optoelectronic device.7-10 Based on these reported SC NCs, different compound can grow on these SC NCs to form heterostructured nanocrystals (HSNCs), such as semiconductor and metal. Semiconductor-semiconductor HSNCs and metal-semiconductor HSNCs show excellent properties for charge separation and transfer, which can be widely used in solar energy conversion and optoelectronic device.11-12 Moreover, the semiconductor-semiconductor HSNCs can be designed for different p-n junction, the p-n junction is very important to optoelectronic device.13-14 Design, synthesis and formation mechanism of different semiconductor based HSNCs is another focus in nanoscience and nanotechnology. However, the reported high quality SC NCs and semiconductor-based HSNCs are focused on Cd and Pb based materials. The toxic components in these materials present a serious threat to both environment and worker safety. It is an original impetus to develop low cost and environmentally compatible semiconductor nanomaterials. I-IV-VI semiconductors, an important ternary type of semiconductor compound, are potential materials for optoelectronic device, which exhibit suitable band gaps, high absorption coefficient and carrier motilities in bulk materials. Very attractively, they are environmentally compatible and abundant in earth. Copper based I-IV-VI semiconductor NCs, such as Cu2SnSe3, Cu2GeS3 NCs have been synthesized and investigated,15-16 however, high quality silver based I-IV-VI semiconductor NCs have been rarely reported yet.17-19 Bulk Ag8SnS6 with canfieldite crystal structure (Space group: Pna21(33), a=15.298, b=7.548, c= 10.699), shows a band gap of 1.2 eV, which is 3



very close to the ideal band gap (1.4 eV) for single junction photovoltaic device.20 Till now, colloidal synthesis of high quality Ag8SnS6 NCs and related HSNCs have not been reported, hindering the fundamental study of their intrinsic properties. Herein, we developed a simple way to synthesize colloidal Ag8SnS6 nanoparticles (NPs) and Ag8SnS6/Ag2S HSNCs. Silver (I) acetate and tin (IV) acetate were used as metal precursors, sulfur powder was dissolved into oleylamine (OLA) as sulfur precursor. OLA was used for solution and ligand. The optical and electronic properties of Ag8SnS6 NPs were characterized. The UV/Vis/NIR absorption spectra gave 1.26 eV for indirect band gap in Ag8SnS6 NPs and these obtained Ag8SnS6 NPs showed n-type semiconductor properties in photocurrent response measurement. If we changed the adding process of precursors, Ag8SnS6/Ag2S HSNCs were produced. A seeded growth mechanism was proposed to explain the formation mechanism of Ag8SnS6/Ag2S HSNCs. The synthetic method for Ag8SnS6 NPs and Ag8SnS6/Ag2S HSNCs can provide a new way to silver based SC NCs and their related HSNCs. The application of Ag8SnS6 NPs for solar energy conversion was investigated. EXPERIMENTAL SECTION Chemicals. Silver(I) acetate (AgOAc, 98%), sulfur (S, 99.9%) and oleylamine (OLA, >70%) were purchased from Sigma-Aldrich. Tin(IV) acetate (Sn(OAc)4, 98%) were obtained from Alfa Asear. Toluene, chloroform, ethanol and methanol were purchased from Beijing Chemical Company. All chemicals were used as-received without further purification. Synthesis. All experiments were carried out using a standard airless technique: a vacuum/dry nitrogen gas Schlenk line was used for synthesis and a nitrogen glove-box for storing and handling air-and moisture-sensitive chemicals. S/OLA solution (1 mmol/mL) was prepared in a glove-box by dissolving S in OLA at room temperature. (1) Synthesis of Ag8SnS6 NPs: 0.1 mmol of Sn(OAc)4 and 10 ml of OLA were placed into a 50 ml three-neck flask and the flask was evacuated and flushed with nitrogen for a number of times to remove oxygen. This reaction 4


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solution was kept at 120 °C for 30 minutes to remove any water. Then the solution was subsequently heated to 200 °C under nitrogen protection. 3 ml OLA solution containing AgOAc (0.8 mmol) and S (0.6 mmol) was injected by a syringe. The solution temperature was then dropped to 190 °C. After the injection of the AgOAc (0.8 mmol) and S (0.6 mmol)/OLA solution, the color of the reaction mixture became to black as soon as. The reaction was terminated typically for 1.5 minutes, and the solution was quenched by toluene to room temperature. (2) Synthesis of Ag8SnS6/Ag2S HSNCs: The synthetic process was basically identical to the synthesis of Ag8SnS6 NPs except that injection of 1 ml of S/OLA solution (1mmol/mL) into 10 ml OLA solution in the presence of 0.8 mmol of AgOAc and 0.1 mmol of Sn(OAc)4 at 220 °C, and that the reaction time was lasted for 2 min. And growth of the large Ag2S/Ag8SnS6 HSNCs was lasted for 10 min. Purification of NCs: Methanol with equal volume to solution was added and Ag8SnS6 NCs or Ag8SnS6/Ag2S HSNCs were precipitated from the solution by centrifuging. The Ag8SnS6 NCs or Ag8SnS6/Ag2S HSNCs were additionally purified twice by re-dispersing them in chloroform and precipitating with methanol. Finally, Ag8SnS6 NPs or Ag8SnS6/Ag2S HSNCs were dissolved in toluene to form a stable colloidal solution for further characterizations. Characterizations: Transmission electronic microscopy (TEM) images were obtained using a FEI TECNAI G2 Spirit microscope, operating at an accelerating voltage of 120 kV. High-resolution transmission electronic microscopy (HRTEM) images were obtained with a FEI TECNAI F30 microscope operating at 200 kV. Elemental analysis was conducted using an energy dispersive X-ray spectroscopy (EDX) analyzer attached to FEI TECNAI G2 Spirit. X-ray diffraction (XRD) analyses were performed at a scanning rate of 2°/min on a Rigaku RINT D/Max-2500 powder diffraction system using Cu Kα radiation source (λ = 1.54 Å) operating at 40 kV and 200 mA. XRD samples were prepared by evaporation of the colloidal NPs onto a piece of glass slide to produce a thin film in air. UV-Vis-NIR absorption spectra of the NP suspensions in toluene were acquired in 1 cm path length quartz 5



cuvettes using a Cary 5000 spectrophotometer. Cyclic Voltammetry: Cyclic voltammetry was recorded on a CHI760C electrochemical workstation (CH Instruments), using 5 mm glassy carbon as the working electrode, a Pt wire as the counter electrode, a Ag/Ag+ as the reference electrode and tetrabutylammonium hexafluorophosphate (TBAPF6) dissolved in acetonitrile (0.1 M) as supporting electrolyte, respectively. The as-prepared Ag8SnS6 NPs were purified thoroughly and the capping ligands were replaced by pyridine before each measurement. The sample was prepared by drop-casting a concentrated suspension of Ag8SnS6 NPs in hexane onto the working electrode. This film was dried under vacuum for 5 minutes. The scan rate was set at 30 mV/s, and the electrolyte solutions were thoroughly deoxygenated by bubbling with high purity nitrogen for 15 min before measurement and a nitrogen atmosphere was maintained over the solutions. The NPs remain stable through 10 voltage sweeps, suggesting that electrochemical degradation of the NPs during the CV measurement does not occur. The LUMO energy levels were calculated from the onset reduction potential (E´red), according to: ELUMO = -Ea = -( E´red + 4.71) eV,21 where potential values are relative to the Ag/Ag+ reference electrode. The cyclic voltammetry experiment herein produced a clear reduction peak near -1.28 V with an onset reduction potential near -0.95 V from the Ag/Ag+ reference electrode, corresponding to a LUMO of -3.76 eV from vacuum level. Photocurrent Measurement: A suspension of the Ag8SnS6 NPs was spin-coat at 3000 rpm for 10 s to form a thin black film on ITO. Photocurrent measurement were conducted in a quartz cell under nitrogen in aqueous 0.01 M Eu(NO3)3/0.1 M KCl using a white LED illuminated (WL101, Zahner, 6.42 mW/cm2) with a Ag wire pseudo-reference electrode and a Pt wire counter electrode. The photoelectrochemical response of the Ag8SnS6 NC films was assessed at several positive potentials under 10 s on/off chopped illumination. Device Fabrication and Characterization: The hybrid solar cells were assembled in a configuration of 6


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ITO/PEDOT:PSS (~30 nm)/P3HT:Ag8SnS6 (~80 nm)/LiF (0.5-1 nm)/Al (100 nm). PEDOT:PSS (Baytron P AI 4083) was spun cast onto a glass substrate with pre-patterned ITO electrodes. Chlorobenzene (CB) was mixed with the Ag8SnS6 NPs, sonicated and filtered to get clear solution. Then, filtered region-regular electronics grade P3HT (Reike Metals) in CB solution was blended with the Ag8SnS6 NPs solution with different ratios. Hybrid and neat P3HT films were spin-coated onto the target substrate in a nitrogen-filled glove box. The samples were then annealed at 140 °C for 10 min. Finally, the samples were transferred into a vacuum chamber and the top LiF/Al contact was evaporated through a shadow mask to generate an array of patterned electrodes. The final active area was 0.12 cm2 for each device. Current density-voltage (J-V) characteristics of the solar cells were measured using a computer-controlled Keithley 2420 source meter under AM 1.5 illumination from a calibrated solar simulator with irradiation intensity of 100 mW/cm2. RESULTS and DISCUSSIOINS

The results of the microstructural analysis of the as-synthesized Ag8SnS6 NPs were presented in Figure 1. Transmission electron microscopy (TEM) images (in Fig. 1a) show the morphology of the NCs. The synthesized Ag8SnS6 NCs are dot-shaped and the mean size of NPs is ~10.7 nm with narrow size distribution (Fig. S1 in Supporting Information (SI)). The crystal nature of the NPs was also confirmed by high-resolution TEM (Fig. 1b). The lattice fringes of 0.206nm, 0.218 nm and 0.246nm are in line with (431), (024) and (41-3) interplanar spacings of the canfieldite Ag8SnS6, respectively. The angles between these planes have been labeled, which are according to the simulation results given in Fig. S2 in SI. The FFT (Fast Fourier Transform) of HRTEM inset in Fig. 1b gives similar results, the electronic diffraction spots from this single nanoparticle are responding to (024), (431) and (41-3) plane of canfieldite Ag8SnS6. All of the above analysis indicates the synthesized canfieldite Ag8SnS6 have canfieldite Ag8SnS6 crystal structure (Space group: Pna21(33)). Selected area electron diffraction (SAED) of as-synthesized Ag8SnS6 NPs shows that the diffraction rings, which can be attributable to the (121), (402), (412), 7



(322), (611) and (132) lattice planes of canfieldite Ag8SnS6, as shown in Fig. S3 in SI.

Powder X-ray diffraction (XRD) pattern in Fig. 1d shows some broad diffraction peaks, which is typical for nanocrystals. The peaks are well indexed with major peaks of canfieldite-structured Ag8SnS6 (JCPDS No. 38-0434), which agree with the result given by HRTEM. There is no evidence of secondary phase observed. Fig. S4 in SI gives the EDS for the synthesized nanocrystals, which show Ag, Sn and S element in the nanocrystals, which is according to the results gotten from HRTEM and XRD.

Figure 1. Crystal structure, shape and chemical composition measurement of Ag8SnS6 NPs. (a) TEM image of Ag8SnS6 NPs, (b) HRTEM image of Ag8SnS6 NPs and FFT images of this single Ag8SnS6 NP, and (c) XRD pattern of Ag8SnS6 NPs.

In order to exploit the potential applications of the Ag8SnS6 NPs, it is important to understand the intrinsic physical properties of the Ag8SnS6 NPs. The optical absorption spectrum of the clear solution of the Ag8SnS6 NPs shows a continuous absorption spanning the whole visible spectrum to NIR (near infrared) region (Fig. 2a), meaning the indirect band gap in Ag8SnS6 NPs. Kubelka-Munk transformations were performed to determine the optical band gaps through a plot of [αhν]1/2 versus energy for an indirect band gap (Fig. 2b) and a plot of [αhν]2 versus energy for a direct band gap (Fig. S5 in SI).22 The indirect band gap and the direct band gap of the Ag8SnS6 NPs were determined to be 1.26 eV and 2.62 eV, respectively. Both band gaps are larger than that of 1.2 eV in bulk

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material, probably due to quantum confinement effect in Ag8SnS6 NPs.

Further information about the band gap structure in Ag8SnS6 NPs can be obtained by electric cyclic voltammetry (CV) measurements. The onset reduction potential of the cyclic voltammogram corresponds to the bottom of conduction band (LUMO) for semiconductors.23 Fig. 2c presents the CV curve of the Ag8SnS6 NPs, exhibiting a clear reduction peak at -1.28 V with onset reduction potential at -0.95 V relative to the Ag/Ag+ reference electrode. The bottom of the conduction band was then determined to be -3.76 eV from vacuum level based on the equation of E(LUMO) = -Ea = -( E´red + 4.71) eV, 21 and the top of the valence band (HOMO) was estimated to be -5.02 eV, according to the indirect band gap of 1.26 eV given by above optical absorption measurements, as shown in the figure inset in Figure 2c.

Figure 2. (a) UV-vis-NIR absorption spectra of Ag8SnS6 NPs, (b) the dependence of (αhv)1/2 on photon energy (hv) of Ag8SnS6 NPs, (c) cyclic voltammogram of Ag8SnS6 NPs using Ag/Ag+ reference electrode, the figure inset in (c) is the electronic structure from vacuum level, and (d) photocurrent response of Ag8SnS6 NPs film at different potential (n-type semiconductor).

The optoelectronic properties of the Ag8SnS6 NPs were investigated by measuring the photocurrents of 9



Ag8SnS6 NP films in an aqueous photoelectrochemical cell under illumination of a white-light emitting diode (LED). The Ag8SnS6 NP film produces an anode photocurrent that increases gradually with the rising of positive potentials (Fig. 2d), indicative of n-type semiconductor property.24 The photocurrents quickly increase upon tuning on the LED, and drop to their pre-illumination values upon tuning off the LED over many cycles without apparent degradation. These results demonstrate that the Ag8SnS6 NPs are stable and have quick response to light illumination and, which would be favorable for further exploiting their application for photodetector. An interesting phenomenon was found that the final products were directly affected by the addition sequence of the reaction precursors in the experiment. While Ag8SnS6 NPs were produced by injecting the mixture of AgOAc and sulfur/OLA into (Sn(OAc)4)/OLA solution, if only sulfur/OLA was added into the AgOAc and (Sn(OAc)4)/OLA solution, the products would be changed to HSNCs. Fig. 3a shows the TEM image of the produced HSNCs, most of HSNCs show drop-shape. These drop-shaped HSNCs exhibit an average size of 13.9 nm. Each drop-shaped HSNC consists of two segments with different contrasts, the little darker tip and the brighter bottom, indicating that different compounds are integrated in one drop-shaped HSNCs. Moreover, a little bowtie-shaped HSNCs can be observed in TEM image (marked in Fig. 3a), the little center in bowtie-shaped HSNCs have the darker contrast, and the two wings in this bowtie-shaped HSNCs have brighter contrast in TEM image.

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Figure 3. Shape and composition characterizations of Ag8SnS6/Ag2S HSNCs. (a) TEM image of Ag8SnS6/Ag2S HSNCs, the marked NCs in (a) are bowtie-shaped HSNCs, (b) HRTEM image of drop-shaped Ag8SnS6/Ag2S HSNCs, (c) EDS analysis of area A marked in (b) and (d) EDS analysis of area B marked in (b). The XRD of HSNCs shows a few diffraction peaks from canfieldite Ag8SnS6 (Fig. S6 in SI), which is similar to XRD of pure Ag8SnS6 NPs given in Fig. 1c. The same diffraction peaks indicate the main composition is Ag8SnS6 in HSNCs. To reveal the detail composition and crystal structure of the two segments of the HSNCs, HRTEM was performed. As given in XRD, the main compound is Ag8SnS6 in HSNCs, so the big segment with brighter contrast in TEM image maybe Ag8SnS6. HRTEM image of the larger segment with brighter contrast in the drop-shaped HSNCs was shown in Fig. 3c (Part B). Part B in Fig. 3c shows the planar distance of 0.393 nm, agreeing with the (311) lattice fringe of the canfieldite Ag8SnS6. Ag, Sn and S elements can be detected in part B from EDS (Fig.3e), which can further support the above conclusion. The larger segment with brighter contrast in drop-shaped HSNCs is Ag8SnS6. It is clearly shown that the smaller segment with darker contrast in the drop-shaped HSNCs (part A in Fig. 3c) has polycrystalline structure, which possess planar distance of 0.29 nm, in accordance to the ( 1 03) lattice fringe of Ag2S (JCPDS No. 65-2356). EDS analysis in area A shows that this segment consist of only Ag and S elements (Fig. 3d), which is according to the result given by HRTEM. It should be noted that the Ag2S part in HSNCs is polycrystalline structure, which is aggregated by a few very small Ag2S clusters, as shown in Fig.3c. The absence of diffraction peaks from Ag2S in HSNCs (Fig. S7 in SI) would be induced by the strong diffraction peaks from big Ag8SnS6 part and the small size of Ag2S clusters in HSNCs. Moreover, the HRTEM image of one bowtie-shaped HSNC was given in Figure S6 in SI, the center in HSNC with darker contrast has the same structure and elements to the little tip in drop-shaped HSNCs, and the center is Ag2S NPs with polycrystalline structure. The big two wings with brighter contrast in bowtie-shaped HSNCs are Ag8SnS6 NPs.

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The formation process of Ag8SnS6/Ag2S HSNCs was studied by examining products taken at different reaction time with XRD and TEM. The sample was taken before addition of sulfur/OLA into the red solution. Some NPs can be detected with TEM, as shown in Fig. 4a. Diffraction peaks in XRD of this NPs are responding to the silver (Fig. 4d), indicating the formation of Ag NPs before injection of sulfur precursor. AgOAc can be reduced to Ag NCs in OLA, which have been widely reported.25 Since sulfur/OLA was injected, the red solution became black as soon as possible. The product obtained at 2 min after the injection was Ag8SnS6/Ag2S HSNCs with a mean size of 13.9 nm (Fig. 4b and histogram for size distribution inset in Fig.4). The Ag8SnS6/Ag2S HSNCs became apparently bigger with an average size of 21.8 nm after 10 min for the injection of sulfur precursor (Fig. 4c and histogram). Compared to the TEM images in Fig. 4b and 4c, the increasing size of HSNCs is mainly due to the growth of Ag8SnS6 segment while the size of Ag2S segment is essentially kept unchanged.

Figure 4. Formation mechanism of Ag8SnS6/Ag2S HSNCs. (a) TEM images of Ag NCs taken before adding sulfur precursor, (b) TEM images of Ag8SnS6/Ag2S HSNCs taken at 2min, (c) TEM images of Ag8SnS6/Ag2S HSNCs 12


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taken at 10 min, (d) XRD patterns of samples taken before and after adding sulfur precursor and (e) the proposal scheme illustration of formation mechanism to Ag8SnS6/Ag2S HSNCs, the histograms of size distribution inset in the figure are responding to the NCs in (b) and (c).

From the above characterizations and discussions, we can see Ag NPs played an important role in the formation of HSNCs. Compared to synthesis of Ag8SnS6 NPs, the AgOAc and Sn(OAc)4 were first added into flask to synthesize Ag8SnS6/Ag2S HSNCs, and AgOAc would be reduced to Ag NPs in the solution, Ag8SnS6/Ag2S HSNCs can be finally formed. In the synthesis of Ag8SnS6 NPs, AgOAc, sulfur and Sn(OAc)4 can react together to form homogenous silver based ternary compound, Ag8SnS6 NPs. Aim to explain the formation of Ag8SnS6/Ag2S HSNCs, a seeded growth mechanism was proposed. As the previous reported, seeded growth mechanism has been widely used to grow NCs, especially to one dimension (1D) nanostructures with catalyst-assisted mechanism.26-30 Recently, Wang and his co-workers reported the synthesis of ZnS nanorods with Ag2S NPs seeds, the formation mechanism was different from the catalyst-assisted process, ZnS precursor can absorb at the surface of Ag2S seeds to anisotropic grow along (001) direction to form ZnS rods.31

Herein, the formation of Ag8SnS6/Ag2S HSNCs showed a similar mechanism to Wang and his co-workers’ report.31 In the experiment, the firstly formed Ag NPs would react with sulfur precursor to form Ag2S NPs as soon as the injection. After the formation of Ag2S NPs, Ag2S NPs also acted as seeds for Ag8SnS6, the precursors absorbed at the surface of Ag2S seeds and grew to form Ag8SnS6/Ag2S HSNCs. However, because Ag2S NPs was formed simultaneously with Ag8SnS6, it was difficult to take the sample with Ag2S NCs only. In the following growth process, Ag2S NPs seeds were kept constant, and Ag8SnS6 part in HSNCs continued to grow till all of the precursors were used up (Figure 4b and 4c). Compared to the single crystalline Ag2S seeds for growth of ZnS nanorods along (011) direction,31 Ag2S seeds were aggregated via a few small Ag2S clusters in this work. Ag8SnS6 would grow at different Ag2S clusters to form this sphere-shaped Ag8SnS6. The polycrystalline structure of Ag2S 13



seeds played an important role to form drop-shaped and bowtie-shaped Ag8SnS6/Ag2S HSNCs.

Figure 5. (a) Current-voltage (J-V) characteristics of P3HT/Ag8SnS6 nanoparticles with different weight ratio of P3HT/Ag8SnS6 hybrid solar cell under 100 mW/cm2 of simulated AM 1.5 irradiation. (b) Plot of the average PCE as a function of the Ag8SnS6:P3HT composition. Too little of Ag8SnS6 NPs in the hybrid blendes (e.. g. , the weight ratio of the NCs/P3HT is less than 1/1) would lead to the collapse of the PV performance.

As discussion in the above, Ag8SnS6 NPs showed the properties as an n-type semiconductor, which had 1.26eV for band gap. The band gap in Ag8SnS6 NPs is near to the ideal band gap for solar energy absorption (1.4eV). To evaluate the applicability of the n-type Ag8SnS6 NPs for solar energy conversion, Ag8SnS6 NPs were used as photovoltaic materials in solar cells device. P-type polymer P3HT was employed to blend with the Ag8SnS6 NPs under different weight ratios to form hybrid solar cells in typical sandwich geometry. The performance of Ag8SnS6/P3HT hybrid solar cells was dependent on the contents of the Ag8SnS6 NPs in the hybrid blends. With NCs/P3HT weight ratios of 5/1, 4/1, 3/1/ 2/1 and 1/1, respectively, Jsc of the corresponding hybrid device increase steadily (Figure 5 and Table S1), while the Voc keeps basically unchanged for all hybrid devices. And the maximum power conversion efficiency (PCE) of 0.019 % was obtained for the hybrid device with Ag8SnS6/P3HT weight ratio of 1/1. However, insufficient amount of Ag8SnS6 NPs in the hybrid blends (e. g., weight ratio of NCs/P3HT≤ 1/2) leads to collapse of the PV performance. These results suggest that the Ag8SnS6 NPs play important roles as the 14


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electron acceptors and in the charge transportation for the hybrid photovoltaic devices. The applications of Ag8SnS6/Ag2S HSNCs in solar energy conversion will be reported in the following work.

CONCLUSION

In summary, Ag8SnS6 NPs and Ag8SnS6/Ag2S HSNCs were controllably synthesized by varying the addition sequence of the reaction precursors. Ag8SnS6 NPs with 10nm showed the indirect band gap for 1.26ev, the quantum size confinement effect can be observed. The electrochemical measurement of Ag8SnS6 NPs showed the detail band gap structure and n-type semiconductor property. Two types of Ag8SnS6/Ag2S HSNCs can be observed, most of HSNCs are drop-shaped and a little of HSNCs show bowtie-shape. A seeded growth mechanism was proposed to explain the formation of Ag8SnS6/Ag2S HSNCs. The first formation of Ag NCs is very important to synthesize HSNCs. The polycrystalline structure in Ag2S seeds plays an important role to synthesize drop-shaped and bowtie-shape Ag8SnS6/Ag2S HSNCs. The synthesis and investigation on the mechanism of HSNCs can provide a way to synthesize HSNCs with controlled shape. The Ag8SnS6 NPs were also used in solar cell device, which showed potential for solar energy conversion materials.

ASSOCIATED CONTENT Supporting Information: TEM image and histograms of size distribution of Ag8SnS6 NPs, SAED of Ag8SnS6 NPs, EDS analysis of Ag8SnS6 NPs, HRTEM images of bowtie-shaped Ag8SnS6/Ag2S HSNCs, XRD patterns of samples taken at different reaction time in the synthesis of Ag8SnS6/Ag2S HSNCs and Table for Voc, Jsc, FF and η in solar cells device with different ratios of Ag8SnS6 to P3HT. AUTHOR INFORMATION Corresponding Author *B. Zou. Email:[email protected]. ORCID Bo Zou: 0000-0002-3215-1255 15



Jiajia Ning: 0000-0002-7922-5469 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Thanks to Prof. Kai Wang to simulate and discus the atomic model of canfieldite Ag8SnS6. This work is supported by the National Science Foundation of China (NSFC) (No. 21725304)

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Controlled Synthesis, Formation Mechanism and Applications of

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