From Blue-Emitting Copper Nanoclusters to Red ... - ACS Publications

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Matrix Sputtering into Liquid Mercaptan: From Blue-Emitting Copper Nanoclusters to Red-Emitting Copper Sulfide Nanoclusters Matteo Porta, Mai Thanh Nguyen, Tomoharu Tokunaga, Yohei Ishida, Wei-Ren Liu, and Tetsu Yonezawa Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03017 • Publication Date (Web): 31 Oct 2016 Downloaded from http://pubs.acs.org on November 2, 2016

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Matrix Sputtering into Liquid Mercaptan: From

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Blue-Emitting Copper Nanoclusters to Red-Emitting

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Copper Sulfide Nanoclusters

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Matteo Porta,a Mai Thanh Nguyen,a Tomoharu Tokunaga,b Yohei Ishida,a Wei-Ren Liu,a,c and

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Tetsu Yonezawaa,*

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a

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Kita 13 Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan

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b

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Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8603, Japan

Division of Materials Science and Engineering, Faculty of Engineering, Hokkaido University,

Department of Quantum Engineering, Graduate School of Engineering, Nagoya University,

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c

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Rd., Zhongli District, Taoyuan City, 32023 Taiwan

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KEYWORDS Photoluminescence, Copper Nanoclusters, Copper Sulfide Nanoparticles, Matrix

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Sputtering.

Department of Chemical Engineering, Chung Yuan Christian University, No. 200, Chung Pei

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ABSTRACT A modified magnetron sputtering technique using pentaerythritol tetrakis(3-

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mercaptopropionate) (PEMP) as a stabilizing agent and liquid dispersion medium was developed

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to generate photoluminescent copper nanoclusters. The results reveal that over time, the as-

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prepared blue-emitting copper nanoclusters were converted to red-emitting copper sulfide

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nanoclusters. The formation of copper oxide as an intermediate during the conversion of copper

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to copper sulfide nanoclusters was demonstrated. Furthermore, based on the mechanism of

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formation of copper sulfide, the kinetics of the conversion process could be controlled via

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ultraviolet (UV) irradiation of the as-synthesized dispersion. These findings shed light on the

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formation and conversion of nanoclusters obtained via sputtering into liquid, demonstrating that

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the method is highly versatile for producing metal nanoclusters and compounds with tailorable

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composition and optical properties.

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INTRODUCTION

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Nanoclusters of copper and its compounds have attracted much attention in recent years due to

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their low cost and optophysical properties, which are comparable to those of the most widely

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used and studied species, gold and silver.1-3 Instability to the environment, and towards oxidation

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in particular, has been a long-term issue limiting the use of copper.3-5 On the other hand, copper

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can form numerous stable compounds, providing great versatility for possible applications.2,3,5-15

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The great interest in copper has led to the development of numerous approaches towards

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controlled syntheses of copper nanoparticles. Reduction of precursors triggered in various ways

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(chemically, thermally, radiolytically, sonochemically, etc.,) not only facilitated great progress in

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the field, but also opened up new challenges. However, for most chemical methods, copper salts,


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reducing agents, and protecting agents are required, causing problems related to impurity of the

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generated copper nanoclusters. Recently, sputtering using a liquid substrate has become

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recognized as one of the clean methods for direct production of nanoparticles and nanoclusters

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dispersed in liquid media.16-31 The combination of a top-down technique under vacuum

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conditions and the use of a liquid medium offers many advantages in terms of producing

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nanoparticles with high purity and providing the capability for tailoring their physicochemical

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properties. Thus, this method allows for control of the particle size, composition, surface

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functionalization, dispersion, and hence the nanoparticle properties.17-21 The choice of liquid

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media and stabilizing agents has been demonstrated to be an important factor for creating

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nanoparticles with desired characteristics, such as surface plasmon resonance or fluorescent

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emission phenomena, based on the particle size.18-27 By combining a top-down technique under

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vacuum conditions and a liquid medium, the formation of fluorescent metal nanoclusters of Au

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and Ag in a single-step synthesis was possible via introduction of a stabilizing agent with a thiol

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functional group into the liquid substrate.18-24 However, creation of copper nanoclusters remains

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an open research field as the cheaper and more abundant copper is more prone to oxidation

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compared to the noble metals.6 Therefore, in our research, using pentaerythritol tetrakis(3-

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mercaptopropionate), PEMP, with high viscosity, low vapor pressure, and high capping ability

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for copper via the four mercaptan groups in each molecule, we aim to synthesize

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photoluminescent copper nanoclusters. The stability of the resulting nanoclusters in the rich thiol

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medium is also addressed herein.

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The main finding of this study is that copper nanoclusters dispersed in PEMP undergo

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transformation to copper sulfide nanoclusters. Through this mechanism, it is possible to tune the

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photoluminescence properties from blue emission of the copper nanoclusters32-34 to red emission


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of the copper sulfide nanoclusters.35,36 The underlying mechanism was found to be the oxidation

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of copper, followed by conversion of copper oxide to copper sulfide in the thiol-rich

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environment.37 Furthermore, control of the kinetics of formation of the copper sulfide

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nanoclusters was achieved via UV (365 nm) irradiation of the as-synthesized samples. The UV

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absorption of copper oxide may make it a self-catalyst for the transformation to copper sulfide.

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The results of this study shed light on the transformation of nanoclusters obtained from

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sputtering onto liquid. Further, our findings demonstrate that sputtering-onto-liquid is highly

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versatile for creating various types of inorganic-organic photoluminescent hybrid materials and

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controlling their optical properties based on judicious choice of the liquid for sputtering and post-

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treatment.

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EXPERIMENTAL

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Materials.

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purchased from Sigma Aldrich. PEMP was vacuum dried at 100 °C for 2 h to remove volatile

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substances immediately before the synthesis. A copper (99.5% pure) sputtering target (50 mmϕ,

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3 mm thickness) was purchased from Nilaco, Japan and used as received. Acetonitrile and

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chloroform were used as received.

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Sputtering Apparatus. A modified magnetron sputtering device (Figure S1) was used for

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preparation of the nanoclusters in the low vapor pressure liquids. A chemically inert stainless

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steel bar was installed to stir the liquid during sputtering. The sputtering chamber was evacuated

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by using an oil rotary pump and a turbo molecular pump. A temperature control system was

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installed for both the sputtering copper target and the liquid substrate. Prior to sputtering, the

Pentaerythritol tetrakis(3-mercaptopropionate) (PEMP, Mw = 488.66) was


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chamber was evacuated to 10-3 Pa and Ar gas was injected up to 101 Pa multiple times. During

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the sputtering experiment, Ar was continuously injected to maintain a constant pressure of 2.0 Pa

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in the sputtering chamber. In some experiments, instead of Ar, air was used to sputter the copper

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target.

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Preparation of Cu nanoclusters in PEMP. The surface of the target was cleaned by pre-

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sputtering for 10 min. The synthesis was performed at a sputtering current of 20 mA for 120 min,

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whereas PEMP (7 g) was stirred at 120 rpm at 25 °C. After sputtering, the samples were stored at

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25 °C for various times. For comparative analysis, some of the samples were divided, placed in

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different glass containers, and stored in air under three different conditions as follows: 1, stored

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in a glass container (with a cap) at room temperature, 25°C, in the dark; 2, stored in a refrigerator

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at -20 °C in the dark; 3, kept in a quartz cuvette and continuously irradiated from the side wall of

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the cuvette for several hours at room temperature using a UV lamp (9.0 W, 60 Hz, 365 nm

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wavelength).

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Characterization.

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STEM-HAADF) were analyzed for as-synthesized and for stored samples over time, in order to

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understand NPs transformations. Chemical state and composition (XPS) characterization as also

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carried on to have a full picture of the evolution process NPs underwent to. UV-Vis extinction

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and photoluminescence (PL) spectra were respectively acquired using a Shimadzu UV-1800 or a

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Perkin-Elmer spectrophotometer and a Jasco FP-6600 spectrofluorometer using a liquid sample

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in a fused quartz cuvette with a 1-cm optical path. Transmission electron microscope (TEM)

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measurements were performed using a JEOL JEM-2000FX (200 kV) instrument. HAADF

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images were acquired using a JEOL JEM-ARF200F scanning transmission electron microscope

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(STEM, 200 kV). TEM and STEM samples were prepared by adding acetonitrile to dilute the

Optical properties (PL, UV-Vis) as well as size and crystal structure (TEM,


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nanocluster dispersions, after which a droplet was placed on carbon-coated copper grids. Particle

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size distribution plots were calculated by measuring more than 100 nanoparticles’ diameters for

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each of the samples. XPS measurements were performed using a JEOL JPS-9200 instrument. For

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the XPS measurements, nanoclusters dispersed in PEMP were mixed with chloroform (sample:

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CHCl3 = 1:4 (w/w)) then separated from the matrix by ultracentrifugation (Hitachi CS150GX) at

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100,000 rpm for 30 min. The obtained nanoclusters were again washed with chloroform,

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centrifuged again, then deposited on a pre-treated Si wafer for the XPS measurement. Surface

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treatment of the Si wafers was performed by dipping the Si wafers in an aqueous solution of

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hydrofluoric acid (5 wt%) followed by rinsing with water and ethanol with subsequent drying

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and ultraviolet-ozone cleaning.38 The Si 2p binding energy was used as the reference for

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correcting the charging effect.

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RESULTS AND DISCUSSION

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As-synthesized Cu nanoclusters.

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transparent and almost colorless (Figure 1a). There was no absorption linked to surface plasmon

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resonance of the Cu nanoparticles, revealing the formation of small copper nanoclusters in the

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dispersion. Moreover, the synthesized nanoclusters showed sharp photoluminescence with

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excitation and emission maxima at 400 and 445 nm respectively, which is typical for copper

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nanoclusters (Figure 1b).32-34 The Stokes’ shift of sputtered copper nanoclusters is similar to that

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of copper nanoclusters obtained via chemical reduction.1 Copper metal nanoclusters with a

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fluorescent emission at 445 nm were reported being composed of sixteen metal atoms as

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estimated from SE1.3,39 The particle diameter of 1.1 ± 0.2 nm observed by TEM (Figures 1c,1d,

The as-synthesized nanoclusters dispersed in PEMP were


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and Figure S3) is consistent with the theoretically calculated value obtained from the above

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mentioned equation.

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Figure 1. UV-Vis spectrum (a), PL excitation and PL emission spectra respectively represented

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by a dashed and a solid line (b), TEM image (c), and size distribution (d) of the as-synthesized

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copper nanoclusters dispersed in PEMP.

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Optical properties of nanocluster dispersions: UV-Vis extinction and photoluminescence

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spectra. Storing the samples at room temperature without light induced a slow change in the

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optical properties: an absorption peak appeared at 480 nm after two days of storage (Figure 2a).

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This peak became visible and the intensity continued to increase up to eleven days of storage,

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after which the increase slowed, indicating that the transformation reached near completion. The


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change in the optical absorbance corresponded to a change in the PL properties over time (Figure

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2). With storage time, the strong PL emission of the as-synthesized samples at 445 nm gradually

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decreased in intensity (band I, excitation wavelength: 400 nm); simultaneously a new PL

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emission band appeared at 630 nm (band II, excitation wavelength: 500 nm. Figure S2). Notably,

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neither the position of the original peak at 445 nm nor that of the new emission band at 630 nm

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changed during storage, meaning that the sources of the PL did not change. The increase in the

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intensity of band II the PL spectrum indicated formation of more PL centers. Unfortunately, the

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PL intensity was not high enough for the measurement of the quantum yield, that was confirmed

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several times. To clarify the origin of the new photoluminescence emission band, we further

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analyzed the particle size, structure, and composition during storage.

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a)

Absorbance

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As synthesized 1 day 2 days 3 days 6 days 8 days 11 days 14days

0.5

0

100

I)

II)

b)

550 Wavelength/nm 200

PL Intensity a.u.

200

450

c)

650

750

445 nm emission intensity 630 nm emission intensity

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0

0

1

Intensity

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Intensity

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600650700 350400450500550600 700 Wavelength/nm Wavelength/nm

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2 4 6 8 10 12 14 Number of days of storage

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Figure 2. (a) UV-Visible spectra and (b) photoluminescence emission (solid lines) and excitation

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(dashed lines) spectra of copper nanoclusters sputtered into PEMP and stored for different times

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(kept for 1−14 days at room temperature in the dark). The purple curves correspond to the PL

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excitation and emission of the as-synthesized sample, whereas the blue and the orange curves

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correspond to samples stored for 3 days and 14 days (at room tempearture in the dark),

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respectively. Area I corresponds to the excitation at 400 nm and the emission at 445 nm; area II

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corresponds to excitation at 500 nm and the emission centred at 630 nm. Figure 2c shows the

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peak intensity evolution-to-time.

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TEM, HR-TEM and STEM images of nanoclusters. Images of the nanoclusters and size

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distribution diagrams were obtained for the stored samples (Figure 3) to evaluate if the change in

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the optical properties was accompanied by particle growth. The average particle diameter

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increased over time, moving from 1.1 ± 0.2 nm for the as-synthesized sample (Figure 1) to 1.6 ±


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0.2 for the sample stored for three days at room temperature without light, to 1.9 ± 0.3 nm for the

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sample stored for fourteen days (Figures 3a and 3b). The average particle size increased

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gradually over the first few days and reached the final value after about five days (Figure 3c).

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The STEM-HAADF image of the particle acquired after seven days of storage clearly showed

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the formation of monoclinic Cu2S with respective d-spacings of 2.4 Å, 2.7 Å, and 3.3 Å for the

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(204), (240), and (104) planes. The increase in the particle size with time is in accordance with

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the conversion of copper to Cu2S. A 93% volume increase is reportedly associated with this

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conversion.40,41 We further confirmed the formation of Cu2S via XPS analysis of the samples.

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Figure 3. TEM images and relative particle size distributions of the obtained nanoclusters

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dispersed in PEMP for samples stored for (a) 3 days and for (b) 14 days at room temperature in

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the dark. (c) Average particle size of nanoclusters dispersed in PEMP as a function of storage

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time.

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Figure 4. STEM-HAADF image of nanoclusters after 7 days of storage at room temperature in

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the dark.

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Chemical state of the resulting nanoclusters with storage time. The Cu 2p3/2 and S 2p XPS

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spectra of the sample stored for eight days at room temperature in the dark are shown in Figure

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5. No characteristic secondary peaks of Cu(II) oxide between 942 eV and 945 eV were observed

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in the XPS spectrum. The Cu 2p3/2 binding energies for Cu(0) and Cu(I) are in the range between

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932.4 − 932.7 eV with an energy difference of only few decimal points, as shown in the Wagner

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plot in Figure S4. Therefore, it is impossible to distinguish between the two states simply on the

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basis of the position of the Cu 2p3/2 peak in the XPS spectrum. In addition, the excess of unbound

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thiols increased the binding energy of the photoelectrons for both copper and sulfur. In order to

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address this issue, it is a common practice to use the modified Auger parameter (calculated from

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the position of the Cu LMM Auger peak and Cu 2p3/2 XPS peak), which is free from charging

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effects, and allows differentiation of Cu(I) from Cu(0). The modified Auger parameter for the


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present sample is 1848.5 eV, indicating the presence of Cu(I).37 Moreover, the S 2p XPS peak

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was significantly broadened and could not be fit to a single chemical state of S. The peak could

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be deconvoluted into two components: the peak shown in green (165.4 eV and 166.6 eV)

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corresponds to the state of excess PEMP molecules, whereas that indicated in orange (164.7 eV

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and 165.9 eV) corresponds to S 2p in copper sulfide.376 These XPS data reveal the presence of

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copper sulfide in the present samples. Considering the ease of oxidation of copper nanoclusters

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and the high amount of thiol present in PEMP, formation of copper sulfide nanoclusters from the

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oxidized copper nanoclusters is plausible.

Intensity (a.u.)

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Intensity (a.u.)

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950 945 940 935 930 Binding energy/eV

168 167 166 165 164 163 Binding energy/eV

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Figure 5. Narrow scan XPS Cu 2p3/2 and S 2p spectra of nanoclusters stored in PEMP after 8

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days of storage at room temperature in the dark. The grey and black curves are for the raw and

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summation spectra, respectively. The S 2p spectrum comprises two components: green curves

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for S 2p in free PEMP and orange curves for S 2p in copper sulfide (for each component, the

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curve at higher binding energy is S 2p1/2 and the curve at lower binding energy is S 2p3/2).


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Mechanism of formation of copper sulfide nanoclusters. Analysis of the change in the optical

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properties over time based on TEM, STEM-HAADF, and XPS data consistently indicate the

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formation of copper sulfide nanoclusters with red photoluminescence emission. It is suspected

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that the formation of copper sulfide from copper particles can occur via copper oxide as an

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intermediate in the presence of thiol moieties. The conversion of thiolate capped copper

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nanoparticles to copper sulfide requires thermal heating (~130 °C),37 whereas the conversion

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from copper oxide to copper sulfide is more feasible, even near room temperature.42-45 In order to

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understand this transformation, sputtering was performed with injection of air into the sputtering

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chamber instead of Ar. Copper nanoclusters coated with copper oxides were deposited on a Si

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wafer, as evidenced by the XPS spectra (Figure S5).

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Figure 6. PL (solid line) and PL excitation (dashed line) spectra for sample sputtered while

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injecting air into the sputtering chamber (orange line) and of same sample after 10 days of

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storage at room temperature in the dark chamber (blue line).

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The as-synthesized liquid sample generated by sputtering with injection of air showed

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photoluminescence emission at 662 nm (Figure 6) and an excitation peak at 435 nm. This

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emission arises from Cu2O generated during sputtering in air.46 After ten days of storage at room

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temperature without light, the emission peak shifted to 671 nm (with excitation at 500 nm),

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derived from Cu2S nanoclusters. The slight difference in the emission maximum compared with

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the sample sputtered in Ar and then stored is thought to be due to some difference in the particle

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size (Figure S6). The excitation peak at 500 nm, which was not present for the as-sputtered

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samples in air, and which appeared after storage for ten days at room temperature in the dark

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(Figure 6), corresponds to the presence of Cu2S, as also observed when sputtering using Ar. This

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result suggests that oxidation of the as-synthesized copper nanoclusters during storage served as

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an intermediate step for formation of the copper sulfide nanoclusters. The mapping image

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showed that after forty days, the photoluminescence from Cu2O (excitation peak at 435 nm,

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emission peak at 662 nm) diminished, indicating complete transformation of Cu2O to copper

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sulfides (Figure S2).

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From the observations discussed above, we propose a mechanism for transformation of

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the copper nanoclusters during storage in PEMP as depicted in Figure 7. The first step involves

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the oxidation of copper to copper oxides after maintaining the Cu nanocluster dispersion under

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ambient conditions. The second step corresponds to the conversion of copper oxides to copper

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sulfide nanoclusters. Formation of the copper sulfide nanoclusters results in an increase of the

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particle size in comparison with that of the as-synthesized copper nanoclusters. This

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transformation is accompanied by the appearance of an absorption maximum at 480 nm in the

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UV-Vis spectra6 and appearance of a new red emission peak in the photoluminescence spectra of

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the sample stored at room temperature for three days or more. Moreover, it was noticed that


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storage of the samples at low temperature (i.e., -20 °C) resulted in significant reduction of the

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particle growth and eventual reactions, and the samples showed negligible change (Figure S7).

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Storage at room temperature induced the oxidation of copper and formation of copper sulfide.

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Figure 7. Mechanism of formation of copper sulfide nanoclusters from copper nanoclusters

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dispersed in PEMP.

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Kinetic control of the formation of copper sulfides. The nanoparticles undergo compositional

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change naturally at room temperature over a few weeks. We observed, as shown in Figure S6,

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that sulfidation can be stopped by simply storing the sample at -20°C. Sulfidation to form Cu2S

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monodisperse nanoclusters is mediated by Cu2O as discussed above (Figure 7). Besides, Cu2O

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itself acts as a photocatalyst when irradiated at 600 nm or shorter wavelength.47 Therefore, it is

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possible to control the kinetics of the formation of copper sulfide via UV irradiation. This

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process can accelerate sulfide formation, and is therefore potentially important for practical

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applications. Using a UV source (9.0 W, 60 Hz, 365 nm wavelength), the as-sputtered sample

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inside a quartz glass cuvette was irradiated and the change in the sample over a short time was

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observed. The optical absorbance (Figure S8) and photoluminescence mapping (Figure S9) were

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monitored at intervals of 2 h. Figure 8a shows a comparison of the spectra of the stored and UV-


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irradiated samples. Five hours of UV irradiation was as effective as two weeks of storage for

2

conversion of the copper nanoclusters into copper sulfide with a similar particle size (Figures 8b

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and 8c). The average particle size for the sample exposed to UV irradiation for 6 h was 1.9 ± 0.3

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nm, which is in the same size range as that of the samples stored at room temperature for five to

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fourteen days in the dark.

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Figure 8. (a) UV-Vis absorption of nanocluster dispersion (in PEMP) for as-synthesized (blue)

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sample and samples irradiated with UV light for 5 h (brown) and 6 h (circle) and sample stored

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for 14 days (at room temperature in the dark, yellow). (b) and (c) TEM image and size

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distribution of the nanoclusters obtained after 6 h UV light irradiation, respectively.

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CONCLUSIONS

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In conclusion, using the one-step magnetron sputtering-into-liquid process, blue-emitting copper

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nanoclusters with dimensions of 1.1 ± 0.2 nm dispersed in PEMP were successfully obtained.

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Moreover, it was found that the blue-emitting copper nanoclusters can undergo transformation to

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red-emitting copper sulfide nanoclusters during storage. The abundance of thiol groups in

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conjunction with the catalytic properties of Cu2O were proposed to be the origins of the


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formation of the copper sulfide nanoclusters. These findings demonstrate that sputtering in liquid

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is a facile, versatile, and green synthesis technique for generation of small and monodisperse

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nanoclusters with controllable composition and optical properties. Furthermore, the kinetics of

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the conversion from copper to copper sulfide nanoclusters could be well controlled with UV-

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irradiation. This new finding affords a simple method of post-processing to tailor the structure

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and properties of nanoclusters in liquid obtained by sputtering.

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AUTHOR INFORMATION

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Corresponding Author

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*[email protected]

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Author Contributions

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The manuscript was written through contributions of all authors. All authors have given approval

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to the final version of the manuscript.

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ACKNOWLEDGMENT

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Authors thank Dr. Yu-ichi Kitagawa (Hokkaido Univ.) for the fruitful discussions and

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experimental assistances. This work is partially supported from Hokkaido University (to MTN,

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TY and YI). MTN thanks F3 program of Hokkaido University. MP and WRL thank the financial

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supports from Japanese government and Hokkaido University, respectively, for their stays in

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Sapporo. TY also thanks the partial financial support from Murata Foundation. A part of this

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work was conducted at Laboratory of Nano-Micro Material Analysis and Laboratory of XPS

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analysis, Joint-use facilities, Hokkaido University, supported by "Nanotechnology Platform"

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Program of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.


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ASSOCIATED CONTENT

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Supporting information is available free of charge via the Internet at http://pubs.acs.org/.

3 4

ABBREVIATIONS

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PEMP Pentaerythritol tetrakis(3-mercaptopropionate).

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