Interaction of l-Cysteine with ZnO: Structure, Surface Chemistry, and

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Interaction of L-cysteine with ZnO: structure, surface chemistry and optical properties Alice Sandmann, Alexander Kompch, Viktor Mackert, Christian H. Liebscher, and Markus Winterer Langmuir, Just Accepted Manuscript • DOI: 10.1021/la504968m • Publication Date (Web): 08 May 2015 Downloaded from http://pubs.acs.org on May 11, 2015

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Interaction of L-cysteine with ZnO: structure, surface chemistry and optical properties Alice Sandmann,*,† Alexander Kompch,† Viktor Mackert,† Christian H. Liebscher ‡and Markus Winterer.† †

Nanoparticle Process Technology and CENIDE, University of Duisburg-Essen, Duisburg

47057, Germany ‡

Interdisciplinary Center for Analytics on the Nanoscale and CENIDE, University of

Duisburg-Essen, Duisburg 47057, Germany present address: Max-Planck-Institut für Eisenforschung, Düsseldorf 40237, Germany

ZnO nanoparticles, ZnS, colloidal dispersion, L-cysteine, EXAFS, XANES

ABSTRACT: Zinc oxide (ZnO) nanoparticles (NPs) were stabilized in water using the amino acid L-cysteine. A transparent dispersion was obtained with an agglomerate size on the level of the primary particles. The dispersion was characterized by dynamic light scattering (DLS), pH dependent zeta potential measurements, scanning transmission electron microscopy (STEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, photoluminescence (PL) spectroscopy and X-ray absorption fine structure (EXAFS, XANES) spectroscopy. Cysteine acts as a source for sulfur to form a ZnS shell


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around the ZnO core and as a stabilizer for these core-shell NPs. A large effect on the photoluminescent properties is observed: the intensity of the defect luminescence (DL) emission decreases by more than two orders of magnitude, the intensity of the near band edge (NBE) emission increases by 20% and the NBE wavelength decreases with increasing cysteine concentration corresponding to a blue shift of about 35 nm due to the Burstein-Moss effect.

1. INTRODUCTION ZnO is a semiconducting material with a direct wide band gap of approximately 3.3 eV.1, 2 It is applied as thin film for example in light emitting diodes3, varistors4, transparent electrodes5 for solar cells and flat panel displays, photocatalysts6 and gas sensors7. For electronic and optoelectronic applications films with a high degree of crystallinity (low defect density), with controlled impurity incorporation, density and homogeneity are required. The surface atoms in nanocrystals dominate their properties due to the high surface to volume ratio. These surface sites are one of the origins of charge trapping states and can be passivated by inorganic shells or by adsorption of organic molecules affecting the chemical and optoelectronic properties of the particles. The binding of thiols to semiconductor NPs strongly influences the electron and hole wavefunctions. Thiol molecules can easily be oxidized and quench band edge emission by extracting holes from the surface. The observed effects depend on the relative position of the HOMO and LUMO of the thiol to the valence band maximum and conduction band minimum and the Fermi-level in the semiconductor particles.8 In case of ZnO semiconductor nanocrystals DL – typically a broad band in the green part of the visible spectrum – in addition to NBE emission is observed in PL. For example, Felbier et al. obtained ligand free quantum dots of 2.1 to 3.4 nm diameter with quantum yields up to 60% in the visible spectral range, i. e.


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the defect luminescence. The NBE emission intensity increased by a factor of three in vacuum on the expense of the defect luminescence probably due to the removal of surface hydroxyl groups.9 Van Dijken et al. proposed a model in which the visible emission is due to the recombination of a shallowly trapped electron with a deeply trapped hole.10 According to Norberg et al. green luminescence involves trapping of photogenerated holes on surface defect sites and is directly correlated to the surface hydroxyl concentration. Dodecylamine and trioctylphosphine quench the green trap emission almost completely due to complete removal of such surface defects. The growth of ZnO in dodecylamine produces low energy surfaces and together with a high surface coverage of the capping agent also increases the excitonic emission intensity.11 Inorganic semiconductor films typically require high processing temperatures which are acceptable for deposition on substrates like Si, GaAs, GaN and sapphire. In contrast polymer substrates with low melting points do not tolerate high temperatures and alternative deposition methods are required. Such an alternative deposition method is inkjet printing of highly crystalline ZnO NPs produced by a process like chemical vapor synthesis (CVS) that allows for weakly agglomerated NPs with a narrow size distribution and a very good control of dopant incorporation.7, 12, 13 Solution based deposition and processing is suitable for large area, flexible substrates and makes more efficient use of materials, allows processing at lower temperatures but requires ‘inks’.14 Therefore, it is necessary to prepare stable colloidal dispersions of NPs with a low degree of agglomeration. Colloidal stabilization of small NPs is a challenge due to the higher surface energy compared to larger particles, which favors agglomeration. According to DLVO theory pure electrostatic stabilization of NPs is difficult as the electrostatic repulsion becomes too small to generate a sufficiently high barrier in the total particle interaction energy15 due to the decreasing number


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of charges at the particle surface at constant surface (rsp. zeta) potential with decreasing particle size. Additionally, the primary minimum becomes shallower and, therefore, already small functionalizing molecules can prevent coalescence and coagulation of NPs. Chemisorbed molecules provide a steric barrier and for particles below 10 nm this will be sufficient to offset van der Waals attraction if the surface potential is large enough to prevent the formation of loose agglomerates.16 However, the particle volume fraction in such particleligand systems becomes very small due to their large surface to volume ratio.17 Therefore, small, chemisorbed molecules should be used to functionalize NPs16 thereby preventing reagglomeration and generating longterm stable dispersions. Ultrasonic treatment or milling processes are often used to break agglomerates in liquids, i.e. to provide sufficient energy to cross the barrier between primary and secondary minimum and the energy barrier should be increased by adjusting the pH to maximize the zeta potential. Degen et al. report that the electrostatic stabilization of ZnO NPs is difficult because the pH decreases into the neutral to basic pH regime due to transformation of colloidal zinc hydroxide particles to ions and at acidic pH ZnO is dissolved forming Zn2+.18-20 ZnO can be dissolved in acidic or basic aqueous media. The dissolution at circumneutral pH of 7.5 is even enhanced for smaller ZnO NPs. Citric acid enhances the dissolution at all sizes.20 For biomedical applications amino acids are often used as stabilizers or linker molecules between inorganic NPs and biomolecules.21,

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Cysteine is a neutral amino acid with carboxylic -COOH, amino -NH2 and thiol –SH groups. Therefore, cysteine can be potentially used as a polydentate ligand and may interact via three chemically different functional groups with the ZnO particle surface. Its isoelectric point is about pH 5.1. In neutral media cysteine is present in the zwitterionic form.23 Liu et al. report that cysteine reacts via the thiol group with Zn2+ of the ZnO NPs leading to a ZnO-cysteine


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complex.24 However, Hiromitsu et al. did not observe a ZnO-cysteine-tetraphenylporphyrine binding.25 These contrary results demonstrate that the interaction of cysteine with ZnO NPs is complex and not completely understood. According to Gondikas et al. the Zn-cysteine system is also a model for natural organic matter in wastewater. They investigated the precipitation of ZnS from aqueous solutions of zinc nitrate by sodium sulfide in the presence of cysteine and found that the adsorption of the thiol group of cysteine slows down aggregation due to a change in interfacial chemistry. XRD and EXAFS data showed the formation of small cubic ZnS cysteine capped NPs where the first Zn-S coordination shell is mostly due to ZnS and not chemisorbed cysteine.26 Sol-gel synthesis of ZnO quantum dots coated in situ by cysteine provides stable optical emission for cell labeling and stable colloidal dispersion over a wide pH range due to zwitterionic nature of the surface. A hierarchical assembly of spherical nanostructures via H-bonding is observed.27 Tamang et al. found that nanocrystals coated with zwitterionic ligands such as cysteine produce small hydrodynamic diameters even at physiological pH. This is necessary for in vitro and in vivo studies. The deprotonation of the thiol function by adjusting the pH is important to obtain strong ligand binding to the nanocrystal surface consistent with Gondikas et al.. Fluorescence quenching is observed for cysteine capped quantum dots (QDs, small semiconductor nanocrystals) and probably related to cysteine dimer (cystine) formation to which a hole may be transferred from cysteine.28, 29 In this paper we want to solve three major challenges to generate stable colloidal dispersions of nanocrystalline ZnO, (i) dispersion down to the primary particle size, (ii) relatively high particle to ligand ratio and (iii) limited etching or dissolution in aqueous media. We report the successful colloidal stabilization of ZnO NPs prepared in the gas phase with the amino acid cysteine in water. The pH of our dispersion was adjusted to pH 9 where cysteine exists mainly


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in the mononegative anionic form but also in the double anionic form.23 A detailed structural analysis and optical properties provides a better understanding of the complex interaction of cysteine with ZnO. 2. EXPERIMENTAL SECTION 2.1. Preparation of the L-cysteine capped ZnO Dispersions. ZnO NPs were synthesized by chemical vapor synthesis (CVS) as described elsewhere.30 This method has the benefit that highly crystalline and weakly agglomerated particles with a narrow size distribution are obtained. 0.82 mmol of L-cysteine (ABCR, 98%) was dissolved in 10 ml of pure water and the pH was adjusted to pH 9 using 1 molar NH4OH (Alfa Aesar). Thereafter, the L-cysteine solution was added to 50 mg of as synthesized ZnO NP and dispersed for 10 min. at room temperature using an ultrasonic horn (Hielscher UP200S, 200 W, 24 kHz). For the PL measurements dispersions with increasing cysteine concentration from 0 – 0.08 mol/l were prepared. 2.2. Characterization. 2.2.1. Colloidal properties. The particle size distribution of the resulting cysteine stabilized ZnO dispersion was determined by dynamic light scattering (DLS) and the zeta potential was measured by laser Doppler electrophoresis, both performed with a Zetasizer Nano ZS (Malvern) using a He-Ne laser emitting at 633 nm. For the pH dependent zeta potential measurements an autotitrator (MPT-2, Malvern) with diluted HCl and NaOH as titrants was used. The zeta potential measurement consists of two titration measurements: one titration into the acidic and the other into the basic pH region, both starting from the current pH of the dispersion (pH 9). An aberration corrected JEOL 2200FS scanning transmission electron


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microscope (STEM) operated at 200 kV was utilized to obtain bright-field (BF) and annular dark-field (ADF) micrographs. The cysteine stabilized ZnO dispersion was drop cast onto a holey carbon Cu-grid followed by subsequent drying for TEM analysis. 2.2.2. Optical and structural characterization. Photoluminescence measurements were performed at room temperature using a Fluorolog 3 fluorescence spectrometer (Fl3-22, Jobin– Y, Horiba) with an excitation wavelength of 350 nm. For the Raman spectra presented herein, a commercial Raman microscope system (Senterra, Bruker) was used. The spectra were collected using an excitation wavelength of 785 nm with a laser power of 40 mW and an acquisition time of 30 s. A Bruker spectrometer (Model IFS 66v/s) was used to carry out Fourier transform infrared spectroscopy (FTIR). Crystallographic phase and crystallinity of the ZnO NPs were determined by analysis of X-ray diffraction (XRD) data obtained from the original as-synthesized ZnO powder and a powder obtained from a dried ZnO-cysteine dispersion on a silicon substrate using a PANalytical X'pert Pro multi purpose diffractometer with Cu K radiation. The original powder was measured in - geometry in the range 25 to 75°2 with a step size of 0.03°2 and 200 s integration time in continuous scan mode using a real time multiple strip detector (X´Celerator). For the dried dispersion on the silicon substrate a grazing incidence angle of 2.3 degree  was set. The scan range was identical to the original powder but with a step size of 0.1°2 and 1 s integration time using a proportional counter. The background was determined from a Rietveld fit that included a 6th order polynomial for the background in the case of the as-synthesized powder and a linear interpolation of 8 points for the dried dispersion. From both samples the background was subtracted and the diffractograms were normalized to the most intense Bragg- reflection.


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2.2.3. EXAFS/XANES. X-ray absorption spectra at the zinc K-edge of a powder from a dried ZnO-cysteine dispersion and the as-synthesized ZnO powder were recorded in transmission mode at beamline 20-BM-B at the Advanced Photon Source at 80 K for a detailed analysis of the coordination in the ZnO-cysteine system. Energy calibration of the Si(111) beamline monochromator was performed using a metal zinc foil (as internal standard) and assigning the point of inflection of the absorption step to 9659 eV. The powder samples were diluted with wheat starch to limit the absorption of the pellets pressed from the sample material. For data reduction and extraction of the EXAFS signal the program xafsX31 is used. For the analysis of EXAFS spectra using Reverse Monte Carlo simulations a structural model is needed to describe the sample. Initial configurations based on the wurtzite lattice are generated using structural information from Garcia-Martinez et al.32 and lattice parameters of a, b = 3.252(1) Å, c = 5.216(3) Å obtained by Rietveld refinement of the diffractogram. A cylindrical cluster was generated with a radius of 23 Å and a height of 44 Å. Inside a shell thickness of 5 Å all oxygen atoms were replaced by sulfur and subsequently inside a shell thickness of 2 Å all zinc atoms were removed to obtain a cluster with ZnO core and ZnS shell terminated by sulfur. The single scattering paths, amplitudes and phases were calculated using FEFF 9.6.433 from corresponding ZnO and ZnS clusters. The program rmcxas34 then uses these FEFF data to compute the EXAFS spectrum of the model structure and attempts to move every atom once during a Monte Carlo cycle to fit the experimental spectrum. The final results of the RMC analysis are the partial pair distribution functions (p-PDF).


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3. RESULTS

ZnO

ZnO + cysteine

Figure 1. Comparison of the size distribution of pure ZnO NPs in water and cysteine capped ZnO in water measured with DLS (a). A monolayer of cysteine would add about 0.6 nm to the primary particle diameter. pH dependent zeta potential measurements of ZnO NPs in water and cysteine capped ZnO NPs in water (b) 3.1. Size distribution and zeta potential. Comparing the two volume weighted particle size distributions in Fig. 1 (a) shows that the particle size of the cysteine capped ZnO NP is significantly smaller than that of pure ZnO NPs dispersed in water. Cysteine prevents the NPs from agglomeration and the mean diameter in the dispersion (dagg = 24 nm) is close to the primary (dpri) (crystallite) particle size of 13 nm for the ZnO NPs as determined from the line broadening in XRD data corresponding to a degree of agglomeration of (dagg/dpri)3  6 which is a rough estimate for the number of primary particles in an agglomerate. It should be emphasized that the particle size measured by DLS is always larger because a hydrodynamic diameter is determined. The good dispersibility is also visible on the STEM images in Fig. 2 where the particles are homogenously distributed and just very few agglomerates are detectable. Images (a) and (b) of Fig. 2 show the same region of the sample in bright-field and


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annular dark-field mode, respectively. Fig. 2 (c) reveals a matrix around the ZnO NPs. Within the matrix the particles seem to be assembled. Further characterizations of the ZnO-cysteine dispersion were carried out to determine what the matrix consists of and are discussed later. Figure 1 (b) shows the pH dependent zeta potential curves of pure ZnO dispersed in water and cysteine capped ZnO in water. Zeta potential measurements reflect the surface charge of NPs in dispersion. The IEP is the pH where the particles have zero net-charge and the dispersion is least stable. Dispersions with zeta potential values greater than ± 30 mV are considered stable.35 The zeta potential curves of the dispersions show an entirely different shape indicating a change of the ZnO NP surface chemistry by interaction with cysteine. For the ZnO-water dispersion the IEP is at pH 9.6, in good agreement with literature data18, while the IEP for the cysteine capped ZnO dispersion is shifted to pH 4.7. The most striking difference between the curves is the inversion of the surface charge. In case of the ZnO-water dispersion the charge is positive over a large pH range with zeta potential values only slightly greater than + 30 mV at

Figure 2. STEM images of a ZnO-cysteine dispersion dried on a holey carbon supported Cugird. Images (a) and (b) show the same region on the grid in bright-field and dark-field mode, respectively. Image (c) gives a closer view of the ZnO nanoparticles with the surrounding matrix in bright-field mode.


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pH 4.5 – 6 and at pH 11. In contrast, the charge of the ZnO-cysteine NPs is mostly negative and stable over a large pH range from pH 5.5 reaching zeta potential values up to – 55 mV at pH 7. Cysteine binds to the ZnO surface and acts as an electrosteric stabilizer with negatively charged functional groups. Therefore, this dispersion should be of enhanced stability compared to pure ZnO in water as can already be seen from the smaller particle size in dispersion. Pure ZnO particles in water aggregate and sediment within several hours while the ZnO-cysteine dispersion is stable for one week. 3.2. Photoluminescence. Cysteine adsorption has a strong effect on the photoluminescence properties of ZnO as demonstrated in Fig. 3 where PL spectra of ZnO dispersions with different cysteine concentration are compared. Two emission bands are observed in the PL spectra for pure ZnO NPs. There is a sharp NBE emission band in the UV regime at around 380 nm and a broad DL band in the visible between 460 and 680 nm with a

Figure 3. Photoluminescence spectra (excitation at 350 nm) with different cysteine concentration as shown in Figure 4 (a). Enlargement of the NBG region (b). The PL spectra are normalized to the maximum of the NBG emission band.


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mode in the green regime. The PL spectra were fit using five pseudo-Voigt lines, two for the NBE regime and three for the broad DL. The results for the emission wavelength and the intensities are plotted in Fig. 4. With increasing cysteine concentration substantial changes for both the NBE and DL features are observed. Especially, the NBE emission band displays a significant blue shift of about 35 nm probably due to the Burstein-Moss effect. The DL band disappears, the NBE emission intensity first decreases and then increases with increasing cysteine concentration reaching a maximum at 0.05 mol/l cysteine about 20% above the value for pure ZnO. The ratio of NBE and DL intensity is increasing by a factor of seven till a cysteine concentration of 0.07 mol/l and is correlated to the Stern-Volmer representation plotting the ratio of the DL of the sample without ligands to the DL with increasing cysteine

Figure 4. Blue shift of NBG and DL as a function of cysteine concentration determined by fitting the spectra with up to five pseudo-Voigt lines (a). Relative emission intensities for NBG and DL (c) and analysis of the Stern-Volmer plot of the defect luminescence quenching with increasing cysteine concentration using the approach by Keizer (model). Absolute (integrated) intensities for NBG and DL (b) as a function of cysteine concentration determined by fitting the spectra of the left figure with up to five pseudo-Voigt lines.


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concentration and is increasing up to factor of 120. This strong quenching of the DL shows a positive curvature and was analyzed using the method of Keizer.36 This analysis provides order of magnitude estimates for the equilibrium constant (9·10-2 m3/mol), fluorescence lifetime (1 ns), encounter radius (0.4 nm), diffusion coefficient (10-12 m2/s) and rate constant for the quenching reaction (108 m3/mol·s). 3.3. FTIR- and Raman-spectroscopy. Fig. 5 shows the Raman and FTIR spectra of ZnO in water, L-cysteine in water (pH 9) and cysteine capped ZnO dispersion (pH 7). In the Raman spectrum of the aqueous ZnO dispersion bands at 1629 cm-1 and 2313 cm-1 can be assigned to OH stretching modes of water. The band at 2331 cm-1 is due to N2 and present in all spectra. The characteristic bands of cysteine are observed at 3213 cm -1 (NH2, O-H), 2577 cm-1 (S-H), 1430- 1403 cm-1 (COO-) and 1098-1065 cm-1 (C-N). The strong band observed at 2577 cm-1 for aqueous cysteine is due to the -SH stretching mode.37 This band disappears in the Raman spectrum of the dispersion which indicates a deprotonation of the thiol group. A new

Figure 5. Raman spectra of ZnO in water, cysteine in water and cysteine capped ZnO in water (S-H band at 2577 cm-1) (a). FTIR spectra of ZnO in water, cysteine in water and cysteine capped ZnO in water (S-H band at 2540 cm-1) (b).


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absorption band at 504 cm-1 which is attributed to the characteristic vibration absorption of adisulfide bridge S-S would provide evidence of the transformation of cysteine to the dimer cystine38 but was not observed. Therefore, we can assume that cysteine binds via the thiol group to the ZnO surface and no free cysteine is present in the dispersion. The FTIR spectra are in good agreement with the Raman spectra. The absorption band at 2540 cm-1 of cysteine is assigned to the stretching vibration of -SH. This absorption band disappears in the spectrum of the ZnO-cysteine dispersion which confirms that the thiol group interacts with the ZnO NPs. Additionally, the intensities of the COO--bands at 1430 and 1403 cm-1 change in the Raman spectra and in case of the FTIR bands at 1396 and 1513 cm-1 they are strongly depressed, which indicates that there is also an interaction of the carboxylic group with the ZnO NPs. For the amino group no changes can be observed in both spectra. Therefore, cysteine binds either only via the thiol group or as a chelate over the thiol- and the carboxylic group to the ZnO surface. 3.4. Structural characterization by XRD and XAFS. Figure 6 shows X-ray diffractograms of the original synthesized zinc oxide powder used in the dispersion and a powder obtained from a dried ZnO-cysteine dispersion on a silicon substrate. Comparing both diffractograms clearly shows that the powder from the dried dispersion is still consisting of zinc oxide in the wurtzite structure. Intensities as well as peak widths compare very well suggesting that the nanocrystalline microstructure is conserved in the dispersion. However, there are additional reflections in the diffractogram of the dried dispersion below 30°2 and between 50 and 55°2 which could be caused by wurtzite or zinc blende type ZnS. Both ZnO and ZnS exist in wurtzite or zinc blende phase of ZnS having larger lattice parameters than ZnO. The Bragg reflections (010), (002) and (011) for wurtzite ZnS fall between 26 and 31°2


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Figure 6. X-ray diffractograms of the as-synthesized powder (red) and the powder obtained after drying a dispersion of ZnO with cysteine (blue). The vertical marks denote the Bragg reflection positions corresponding to wurzite ZnS (top row), cubic ZnS (middle row) and wurzite ZnO (bottom row).and the (013) reflection is observed at 51.7°2. For cubic ZnS Bragg reflection (111) is detected at 29°2 Fig. 7 presents XANES spectra of ZnS, the as-synthesized ZnO nano powder and the powder from the dried ZnO cysteine dispersion. The spectrum of the ZnO cysteine sample is similar to a broadened linear combination of about 50% ZnO and ZnS. The first peak (A) corresponds to the white line of ZnS and the second peak to the white line of ZnO. The spectrum of the ZnOcysteine sample follows in general the features of the original ZnO nano powder. The intensity of the individual features is however significantly different. The first peak (A) beyond the edge is more pronounced while the intensity of the white line (B) is strongly reduced. The decreased white line intensity suggests a filling of empty 3d states that might be caused by electron transfer from the cysteine to the ZnO. There is also a shift of 0.9 eV in the Zn-K edge position between the original ZnO powder and the powder from the dried dispersion consistent with the


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Figure 7. XANES spectra of the as-synthesized ZnO powder (grey), the powder obtained after drying a dispersion of ZnO with cysteine (red) and ZnS powder (blue). The derivative of the absorption coefficient is plotted in dashed lines in the corresponding color. Additionally, linear combinations of the experimental ZnO and ZnS spectra are plotted for comparison with a ZnO fraction of 0.75, 0.5 and 0.25. Burstein-Moss effect observed in the PL spectra. XANES spectra obtained by Guglierie and Chaboy of nano-ZnO, bulk ZnO and thiol capped nano-ZnO show analogous features. However, we observed more broadening in our spectrum probably due to larger disorder. XANES simulations show that capping ZnO NPs with dodecanethiol leads to the formation of a wurtzite ZnS shell surrounding the ZnO core.39 Considering that thiols have already been used as sulfur source to convert ZnO to ZnS40 we further look into the local structure of our ZnO-cysteine nano powder by Reverse Monte Carlo simulations of the EXAFS spectra.34 The experimental spectrum (Fig. 8) could not be fitted satisfactorily by either a ZnO or ZnS model alone. However, a cylindrical core (ZnO) shell (ZnS) model agrees well with the experimental data. The EXAFS spectrum from the dried dispersion sample is shown in Fig. 8 along with its fit from RMC simulation and difference plot. The fit misses the first feature around 1.5 Å-1 and


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Figure 8. EXAFS spectrum from the dried dispersion sample along with its fit from RMC simulation and difference plot (a). Magnitude of the phase corrected Fourier transform of EXAFS data of Fig. 8a (b). starting at 11 Å-1 noise in the spectrum becomes increasingly significant. The discrepancy in the beginning of the spectrum (XANES-regime) is probably due to multiple scattering effects which our RMC simulation does not take into account. The resulting R-value of 21.0 % represents a good fit of the spectrum. For comparison, a fit of a standard, bulk-ZnO powder resulted in an R-value of 25.5 %. Additionally models of ZnO, ZnS, a core of ZnO and an outer shell of ZnS with ZnO lattice parameters and a core of ZnO and an outer shell of zinc sulfide with zinc sulfide lattice parameters were tested but gave generally worse fit results (Rvalues between 24 % and 46 %). From the refined cluster model the p-PDF are obtained and plotted in Fig. 9 along with the initial values. The orange line represents the Zn-S distances in a zinc sulfide structure derived from Kisi and Elcombe.41 The Zn-O and Zn-Zn distances are broadened around their initial values while the Zn-S nearest neighbor distance moves from its initial position to the distance expected in a ZnS structure. The rather broad first Zn-S peak between 1 and 2 Å is probably an artifact as cysteine is modeled only as sulfur, the rest of the molecule is neglected. In wurtzite ZnO and ZnS Zn atoms are coordinated tetrahedrally to


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Figure 9. p-PDF obtained by RMC analysis of EXAFS data shown in Fig. 8. oxygen respectively sulfur atoms with a coordination number of 4. According to our RMC/EXAFS results Zn is coordinated by 2.4 oxygen atoms and 1.7 sulfur atoms on average yielding again a total coordination number of 4. The reduced Zn-Zn coordination number of 9.1 compared to bulk wurtzite ZnS and ZnO is due to missing Zn atoms at the surface of the ZnO-ZnS model.

4. DISCUSSION AND CONCLUSIONS 4.1 Interaction of cysteine with ZnO: colloidal chemistry / surface chemistry. According to our experimental observations, part of the ZnO NPs is converted to ZnS. Cysteine is the only sulfur-source available in our samples. The abstraction of sulfur from a thiol (group) is observed for example in the biosynthesis of Fe/S clusters which relies on cysteine as sulfur source through cysteine desulfurase catalyzing the conversion of cysteine into alanine and sulfane sulfur.42 In – industrially used – desulfurization reactions sulfur is also removed catalytically. Methanethiol (CH3SH) absorbs for example dissociatively already at 300 K on ZnO(0001).43


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At basic pH-values the thiol group is deprotonated (reaction pH 9.3) and strong ligand binding to nanocrystal surface is observed.28 Duran et al. report an isoelectric point of ZnS of about pH 5.5.44 However, isoelectric points of zinc sulfide vary largely. Radhika et al. observe isoelectric points of pH 3 for high purity ZnS, of pH 7.8 for chemically synthesized ZnS, of pH 2.8 for biogenically, and of pH 8 for bacterially produced ZnS.45 The ZnS attachment efficiency for aggregation decreases by three orders of magnitude with an increasing amount (0.1 µmol/m2 to 0.5 µmol/m2) of cysteine adsorbed.29 Arslan et al. observe the hierarchical assembly of spherical nanostructures via H-bonding for ZnO quantum dots coated in situ by cysteine during sol-gel synthesis. They find stable colloidal dispersions over a wide pH range due to the zwitterionic nature of the surface.27 This zwitterionic nature of cysteine can also lead to an etching of the surface of the ZnO nanocrystals by the carboxylic group, especially at our small particle size with large specific surface area, as was for example observed for the interaction of ZnO NPs with citric acid.20 This can lead to low energy particle surfaces and a (partial) removal of charge carrier traps as well as an increase in NBE emission intensity (Norberg and Gamelin).11 Our observation of an inversion of the particle charge between pH 4 and pH 10 and a shift in the isoelectric point from pH 9.6 for ZnO nanocrystals to pH 4.7 for the ZnO-cysteine system is consistent with results of Gondikas et al. for the system ZnScysteine which indicates that the surface chemistry of the ZnO nanocrystals has been dramatically changed due to interaction with cysteine molecules.26 In conclusion, we propose that cysteine is dissociatively adsorbed partially to the surface of the ZnO NPs with alanine as byproduct thereby converting the ZnO surface to ZnS with the released oxygen from ZnO going into to solvent. The remaining – excess – cysteine molecules are adsorbed on the ZnS surface stabilizing the thus formed core-shell NPs.


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4.2 Optical properties and localization of the defects. The PL spectra (Fig. 3) of the ZnOcysteine samples are remarkable due to an increasing blue shift of the NBE emission and a decreasing intensity for the DL with increasing cysteine concentration (Fig. 4). For a ZnO NP of 13 nm in diameter the number of ZnO formula units in one surface monolayer is about 4500 and the number of adsorbed cysteine molecules in one monolayer is roughly 400. This corresponds to a cysteine concentration of 5·10-4 mol/l for one monolayer adsorbed at the surface of the ZnO NPs, 6·10-3 mol/l for the conversion of the top ZnO monolayer into ZnS and 0.06 mol/l for the complete conversion of all ZnO into ZnS as estimated from a simple spherical model and molar volumes of ZnO, ZnS and cysteine. We observe the largest effect between pure ZnO and ZnO with 0.02 mol/l cysteine and a reversal in the trend of the optical properties for more than 0.07 mol/l cysteine (Fig. 4) which could mean that at first, a ZnS surface monolayer is formed and for large cysteine concentrations the reaction of ZnO with cysteine to ZnS progresses towards complete conversion. The experimentally determined band gap energies of both zinc blende and wurtzite type ZnS at room temperature vary from about 3.56 eV to 3.78.46 Since the spectral range of our spectrometer is limited, we cannot directly observe the NBE emission of ZnS formed with 3.54 eV corresponding to 350 nm and 3.78 eV corresponding to 328 nm. Cysteine itself does not contribute to the PL intensity as cysteine is predicted to absorb at 200 nm and fluoresce at 300 nm. Cystine fluoresces at 700 nm outside of the range of our spectrometer.47 DL of ZnS in the visible between 500 nm and 600 nm should be readily observable but is overlapping with the DL of ZnO. We do not observe DL of ZnS at about 450 nm.48 A shift of the NBE emission band due to size effects can be excluded as DLS and STEM of the dispersion yield a particle size of about 20 nm and XRD analysis shows primary particle size of 13 nm which is far too large to observe size effects in case of our ZnO


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NPs even when considering etching and formation of a ZnS shell.49 Quantum confinement can be excluded as explanation for the blue shift as the observed particle size is far too large. In case of ZnO, the quantum confinement effect widens the band gap substantially only for sizes below 6 nm.50 The cause for the blue shift is more likely the Burstein-Moss effect observed in doped ZnO51 and also observed for S doped ZnO.52,

53

The Fermi level moves towards the

conduction band with increasing carrier concentration leading to a widening of the effective band gap. The suppression of the DL band can be explained as a result of cysteine saturating surface defects like oxygen vacancies with increasing cysteine concentration. Geng et al. and Shen et al. observed such a blue shift in sulfur-doped ZnO nanomaterials by oxidizing ZnS nanowires or synthesizing ZnO nanowires in a sulfur atmosphere. However, they observe an increase in the defect luminescence due to defect formation upon S incorporation.52, 53 Sulfur behaves as isovalent donor since it is more electropositive than oxygen. PL and absorbance blue shifts were also observed for ZnO nanorods with Au nanoparticles attached by dithiol as a linker and are explained by the Burstein-Moss effect. In this case the visible PL is quenched and the UV PL is enhanced.54 Adsorption of methanethiol on ZnO(0001) leads to a 0.7 eV increase in the work function which is explained by an electron transfer from the substrate to the adsorbate. The formation of a zinc-sulfur bond, possibly filling oxygen vacancies quenches the defect luminescence.55 Our observation of a blue shift is also in agreement with the experimentally observed blue shift of the NBE in ZnO/graphene composites56 but opposite to the theoretical results on ZnO/ZnS heterostructures which are explained by a closing band gap due to strain effects between ZnO and ZnS.57, 58 We observe a substantial quenching of DL by more than two orders of magnitude (Figs. 3 and 4). According to Norberg and Gamelin and van Dijken et al. the green emission is directly


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correlated to the surface hydroxyl concentration. In analogy to their experiments with dodecylamine and trioctylphosphine ligands we propose that the DL can be quenched if O2- or OH- at the particle surface is replaced by a thiol group.10, 11 According to Hines and Kamat, binding of thiols strongly influences both electron and hole wavefunctions of the NPs. The observed effects depend on the relative position of the HOMO and LUMO of the thiol to valence band maximum and conduction band minimum and the Fermi-level in the semiconductor particles. In case of CdSe the HOMO of a thiol is above the HOMO of the semiconductor particle. Therefore, thiol-groups on CdSe QDs can easily be oxidized and quench band edge emission by extracting holes from the surface.8 In our case the redox potential of cysteine is close to the conduction band minimum of ZnO (Fig. 10) and can fill all defect states in the band gap or even unoccupied states in the conduction band leading to the Burstein-Moss effect. For an n-type semiconductor adsorbed donors can effectively return surface trapped charge to the semiconductor, thereby reducing the surface electric field. The result is a contraction of the depletion width and an increase in PL intensity59. This could be the reason for the increase in NBE emission intensity with increasing cysteine concentration. Green emission can originate from anion vacancies at the ZnO surface. Kamat et al. observed the quenching by adsorption of hole scavengers like SCN- and S2- adsorption and explain it by the extraction of electrons from the valence band creating a localized hole which are deep traps for the conduction band electrons acting as DL centers. By coupling two semiconductor systems such as ZnO and ZnSe, it is possible to enhance charge separation and improve efficiency of the interfacial charge transfer. The quenching of the ZnO emission in the visible spectrum continues with increasing H2Se until the ZnO surface is completely covered with ZnSe indicating a transfer of photogenerated holes from ZnO to the ZnSe layer.60 According to


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Tamang et al., the fluorescence quenching of cysteine capped InP/ZnS QDs is related to the cysteine dimer (Cystin) formation to which a hole may be transferred. 28 These observations are consistent with our PL data and indicate that a ZnS shell is formed on the ZnO particles. Cysteine adsorbed at the ZnS surface can then transfer electrons to the ZnO core through the ZnS shell rsp. the ZnO core transfers a hole to the cysteine ligand oxidizing the ligand and reducing the ZnO particle. We observe a nonlinear decrease of the DL intensity as a function of cysteine concentration with a positive curvature (Fig. 4). Rabani et al. observe that ZnS quenches the visible emission spectrum of ZnO by hole transfer in ZnO particles coated with ZnS by reacting ZnO colloidal particles with H2S. At 14 µM H2S and 5·10-4 M ZnO the defect luminescence is quenched by about a factor of 10 and a linear Stern-Volmer plot is observed providing an estimate for the apparent quenching rate constant of 1.06·1010/[ZnO] M-1s-1 about an order of magnitude smaller than our estimate.61 We could fit our Stern-Volmer plot according to a stationary nonequilibrium distribution of quencher molecules around a fluorophore which causes a positive curvature (‘static quenching’) according to Keizer.36 The encounter radius of 0.4 nm could be explained by cysteine attached to a ZnO-ZnS-core-shell particle with a thickness of the ZnS shell of 0.4 nm corresponding to about 1 monolayer or by cysteine contained in the electrolyte double layer of the colloidal particle. The diffusion coefficient is about two orders of magnitude lower than for cysteine in pure water (estimated from the molar volume of cysteine using the Stokes-Einstein equation) and about a factor of five lower than the diffusion coefficient of a spherical 24 nm ZnO particle in pure water which can probably be explained by a fractal structure of the agglomerate and the interaction with of cysteine with the surrounding ions and molecules in the electrolyte double layer of the ZnO/ZnS core-shell NP, which probably forms the matrix observed in the STEM image in


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dried, condensed state. This would mean that the mechanism of the substantial quenching of the defect luminescence with increasing cysteine concentration is a diffusion controlled reaction of the ZnO-ZnS core- shell particles with cysteine. Since the reduction potential of the cysteine-cystine reaction is close to the conduction band minimum of ZnO it could be a reversible reaction, which could explain why we do not observe cystine in FTIR- and Ramanspectra. We conclude that the PL observations can be explained by the formation of a thin ZnS shell. The observed blue shift of all PL lines cannot be explained by a confinement effect since the (primary) particles are too large even when considering some etching of the particles and the formation of a thin ZnS shell. The small thickness of the ZnS shell could also be a reason that we do not observe a tail of the NBE emission of ZnS. The quenching of the DL is probably due to a diffusion-controlled electron transfer reaction of the core-shell particle with cysteine in the electrolyte double layer. 4.3 Complex structure. Our discussion so far indicates that the system ZnO-cysteine is structurally rather complex. Additionally, ZnS can form not only wurtzite but also zinc blende type structures as observed for ZnS quantum dots of 3 nm diameter which show a blue shifted band gap of 4.11 eV due to the quantum size effect.62 Gilbert et al. investigated the structural phase transition in 3 nm ZnS in methanol following the addition of water.63 The Zn-K-edge XANES spectrum of dodecanethiol capped ZnO NPs is similar to ZnS in methanol after addition of water and is interpreted by the formation of a ZnO-ZnS interface with both ZnO core and ZnS shell in the wurtzite structure. The main XANES absorption peak for ZnO at about 7.5 eV above the absorption edge is strongly reduced in the thiol-capped sample. The variation of the white-line intensity is often attributed to charge transfer effects.39 Our XANES


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spectra (Fig. 7) are similar to both published data with broadened features. This indicates that we have indeed a ZnO-ZnS core-shell system. The RMC analysis of the EXAFS data using a corresponding model corroborates this conclusion (Fig. 8). The XAFS (XANES and EXAFS) data support our interpretation of functionalized ZnO core shell particles contained within a (complex) matrix. They exclude the formation of a molecular Zn-cysteine complex. A molecular complex can never show an EXAFS spectrum with multiple oscillations in k-space corresponding to several substantial peaks in r-space as observed in our data (Fig. 8). This is because the backscatterer atoms in a molecular complex which are close enough and of well defined distance to be observed in EXAFS are only of low backscattering amplitude (carbon, oxygen, nitrogen, sulfur). Peaks at higher coordination distance can only be observed for heavy backscatterer atoms (zinc) which are only present in large enough particles. This is clearly observed in the raw data presented in Fig. 8 and in our data analysis where the Zn-Zn contribution to the EXAFS spectrum (Fig. 9) is the source for the corresponding signal in the experimental data. This is well supported by comparing our data with data of a Zn-cysteinecomplex published by Nicolis et al., where the EXAFS spectrum essentially contains only one oscillation in k-space corresponding to a single peak in r-space.64 4.4 Proposed Model. Combining all experimental results, we propose a core-shell-ligand model for our samples consisting of wurtzite type ZnO as core, wurzite or zinc blende type ZnS as shell and cysteine bound to sulfur surface vacancies in the ZnS shell via the thiol group (Fig. 10). Energetically, this would correspond to a staggered type-II band alignment where both valence and conduction band in the core are lower than in the shell. The electron could then be confined to the core and the hole is confined in the shell analogous to CdSe/ZnTe core shell QDs.65, 66 Guo et al. observed ZnS shell (20 nm) / ZnO core (70 nm) nanorods with a type


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II band alignment as determined by PLS and XPS with a valence band offset of 0.96 eV and a conduction band offset of 1.25 eV which enhances the separation of photo-generated carriers. Oxygen vacancies are filled by sulfur, thus the visible emission of oxygen vacancies at 575 nm is decreased and the UV emission is increased because a large band gap material can restrain tunneling of the charge carriers from the core to the shell confining more photo-generated electrons and holes inside the ZnO core. The remaining visible emission may be due to the electron-hole recombination at the interface.67 These observations are consistent with our experimental results. Figure 10 shows a band alignment diagram using values of -4.19 eV rsp. -3.46 eV for the conduction band minimum and -7.39 eV rsp. -7.06 eV for the valence band maximum of ZnO rsp. ZnS according to Xu and Schoonen.68 The redox potential of the cysteine-cystine couple (+0.19 V according to Joycelyn)69 is also plotted in Fig. 10. Although,

Figure 10. Band-alignment for the proposed core-shell-ligand- (ZnO-ZnS-cysteine) structure (right) including valence band maxima and conduction band minima for bulk ZnO and ZnS (Xu and Schoonen 2000)68 as well as the redox potential of cysteine-cystine reaction (Jocelyn 1967)69 and solid state (left) and electrochemical energy (center) scales.


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we do not observe the S-S bond of cystine in our Raman spectra, – possibly due to binding of the disulfur group to the particle surface or due to a transient formation of the molecule in the diffusion controlled redox reaction – the position of the redox potential relative to the conduction band minimum of ZnO shows that an electron transfer from cysteine to the particle is possible.69 However, the thin ZnS shell acts a tunnel barrier for the charge transfer. In conclusion, cysteine ligands may act as electron donor surface dopands on ZnO NPs. 5. SUMMARY We show that cysteine is a good, electrosteric stabilizer for ZnO NPs enabling the preparation of dispersions with particle sizes on the level of the primary particle size. It inverts the zeta potential, increases its absolute value and is a small but effective ligand. In addition to facilitating good NP dispersion, we observed that cysteine reduces the PL intensity in a broad band between 460 and 680 nm which is attributed to structural defects and blue shifts the near band edge emission band from 415 nm to 380 nm. STEM images show that the particles are homogeneously distributed and only few agglomerates are observed in good agreement with the results of the DLS measurements. The images also show that the particles are surrounded by a matrix. Structural characterizations show that the thiol group of cysteine acts as a stabilizer for the ZnO NPs probably binding via the thiol group to the ZnS shell and leading to a partial transformation of ZnO to ZnS as shown by RMC analysis of the EXAFS spectrum in combination with a Rietveld refinement of X-ray diffraction data. The chemisorption of cysteine effectively dopes the ZnO core and leads to a substantial quenching of the defect luminescence with positive curvature and increasing cysteine concentration. Cysteine has additional advantages as stabilizing ligand since it is biocompatible providing paths to


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biomedical applications and because the three functional groups – thiol-, amine- and carboxy group – make it possible to assemble the particles or link them to other surfaces. AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected].

ACKNOWLEDGMENT We gratefully acknowledge the financial support by the Ministry of Innovation, Science and Research of the German State of North Rhine-Westphalia (NanoEnergieTechnikZentrum, NETZ) and the DFG/NSF grant WI 981/11 in the ‘Materials World Network’ program. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. We are very thankful for provision of XAS beamtime at beamline 20-BM-B and for the generous help by Chengjun Sun. We thank Stefan Graß (group of Reinhard Zellner) for Raman, Sebastian Kluge (group of Hartmut Wiggers) for PL and Jens Theis (group of Axel Lorke) for FTIR measurements as well as Axel Lorke for his valuable discussions.

REFERENCES 1. Reynolds, D. C.; Look, D. C.; Jogai, B.; Litton, C. W.; Cantwell, G.; Harsch, W. C., Valence-band ordering in ZnO. Phys Rev B 1999, 60, (4), 2340-2344. 2. Janotti, A.; Van de Walle, C. G., Fundamentals of zinc oxide as a semiconductor. Rep Prog Phys 2009, 72, (12). 3. Neshataeva, E.; Kummell, T.; Bacher, G.; Ebbers, A., All-inorganic light emitting device based on ZnO nanoparticles. Applied Physics Letters 2009, 94, (9). 4. Barrado, C. M.; Leite, E. R.; Bueno, P. R.; Longo, E.; Varela, J. A., Thermal conductivity features of ZnO-based varistors using the laser-pulse method. Mat Sci Eng aStruct 2004, 371, (1-2), 377-381.


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Abstract Graphic


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Interaction of l-Cysteine with ZnO: Structure, Surface Chemistry, and

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