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The Role of Defects in Carbon Nanotube Walls in Deposition of CdS Nanoparticles from a Chemical Bath Lyubov Gennadievna Bulusheva, Yu. V. Fedoseeva, Alexander G. Kurenya, Denis V. Vyalikh, and Alexander Vladimirovich Okotrub J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b07549 • Publication Date (Web): 27 Oct 2015 Downloaded from http://pubs.acs.org on October 31, 2015

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The Journal of Physical Chemistry

The Role of Defects in Carbon Nanotube Walls in Deposition of CdS Nanoparticles from a Chemical Bath L. G. Bulusheva,1,2,* Yu. V. Fedoseeva,1,2 A. G. Kurenya,1 D. V. Vyalikh,3,4 and A.V. Okotrub1,2 1

Nikolaev Institute of Inorganic Chemistry, SB RAS, 3 Acad. Lavrentiev Ave., 630090

Novosibirsk, Russia 2

Novosibirsk State University, 2 Pirogova Str., 630090 Novosibirsk, Russia

3

Institute of Solid State Physics, Dresden University of Technology, 01062 Dresden, Germany

4

St. Petersburg State University, 198504 St. Petersburg, Russia

ABSTRACT Multi-wall carbon nanotubes (MWCNTs), as-prepared and annealed in an inert atmosphere, have been used as templates for growth of CdS nanoparticles from an aqueous ammonium solution of cadmium chloride and thiourea. Transmission electron microscopy and Raman spectroscopy revealed that formation of CdS nanocrystals on the raw MWCNT surface proceeds markedly faster than on the annealed one. X-ray photoelectron spectroscopy showed a strong difference in the chemical state of the deposited CdS depending on the defect density in the MWCNTs. An interaction of Cd2+ ions and mixed Cd(II) complexes with a defective graphene fragment was studied using density functional theory. The calculations indicated that for attachment of Cdcontaining species to the graphitic surface, defects with the dangling carbon bonds are needed. By combining the experimental and theoretical results, a model of CdS nucleation on the MWCNTs was proposed.

KEYWORDS: CdS nanoparticles, carbon nanotubes, chemical bath deposition, XPS, NEXAFS, DFT calculations 1 ACS Paragon Plus Environment


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1. Introduction Owing to high surface area, chemical inertness, and mechanical stability, carbon nanotubes (CNTs) are attractive as the template for deposition of different inorganic species.1 A conjugation of one-dimensional conductive nanotubes with semiconductor nanoparticles, whose electronic and optical properties are strongly size-dependent, could open up a new avenue in the design of optoelectronic and light-energy conversion devices.2 Cadmium sulfide (CdS) is an important semiconductor because of its high photosensitivity and direct band gap energy of 2.42 eV that is optimal for optical windows. For efficient decoration of CNTs with CdS nanoparticles the nanotube surface is usually functionalized by oxygen-containing groups,3–6 thiol groups,7 and thiophene8 and then these functionalities serve as the binding sites for Cd2+ ions3–5,

7

or

interlinkers for attachment of pre-formed nanoparticles.6, 8 Recently, we have showed that multi-wall CNTs (MWCNTs) produced by catalytic chemical vapor deposition (CCVD) method can be used for CdS deposition without surface pretreatment.9 The CdS nanoparticles were grown from an aqueous solution containing CdCl2, ammonia NH3, and thiourea SC(NH2)2, and it was found that their sizes can be controlled by tuning the temperature of solution and residence time of MWCNTs in solution.10 Probably, defects existing in the MWCNT surface layer assisted in nucleation and growth of CdS. This suggestion was supported by transmission electron microscopy (TEM) analysis, which has detected the bigger CdS nanoparticles grown on the nanotube caps as compared to the nanoparticles formed on the nanotube sidewalls.11 Under applied electric field, the nanoparticles located at MWCNT tips emitted green light with an admixture of red and orange colors.12, 13 A chemical bath deposition (CBD) procedure, where a cadmium salt and thiourea are taken as sources of cadmium and sulfur ions, while ammonia works as a complexing agent and makes basic medium for thiourea hydrolysis, is widely used for the formation of CdS thin films.14, 15 The structure and properties of deposited film depend on the temperature and duration of synthesis,16,

17

type of cadmium salt,18,

19

concentration and ratio of the reagents,20,

21

pH of 2

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solution20, 22 as well as the used substrate.23, 24 Despite long time of investigation of this process, the mechanism of CdS deposition is still unclear. At present, two ways of CdS formation are considered in literature: (1) nucleation and growth directly on the substrate, heterogeneous or socalled “ion-by-ion” process and (2) solution growth of CdS nanoparticles accompanied by their precipitation and aggregation on the substrate, homogeneous or so-called “cluster-by-cluster” process.25, 26 In an ammonia-thiourea bath, cadmium ion may attach ammonia, hydroxide ion, and thiourea thus forming a single or mixed ligand complex.21, 27 The complexes are adsorbed on the substrate, and a heterogeneous growth process takes place by an ionic exchange reaction with sulfur ions28 or through a breaking of S–C bond in the thiourea coordinated to cadmium.22 The hydroxide ions presented in the coordination sphere of cadmium are supposed to help in thiourea hydrolysis.26,

29

Thus, the “ion-by-ion” mechanism is better represented by more general

“molecule-by-molecule” denomination. The usually discussing points are the way of CdS deposition from a chemical bath, the necessity of cadmium hydroxide Cd(OH)2 for the CdS formation, and the structure of the adsorbed complexes. Obviously, that these points depend on the synthetic conditions mentioned above. In the present work, we use the methods of high-resolution TEM (HRTEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy in order to reveal a mechanism of CdS growth on the MWCNTs from an aqueous ammonium solution containing CdCl2 and SC(NH2)2. We changed only duration of synthesis and quality of the MWCNT surface, while maintaining constant the concentrations of reagents and temperature of the solution. The raw MWCNTs synthesized by a CCVD technique and those after annealing in a nitrogen flow were taken for the CdS deposition. The experimental results were supported by quantum-chemical calculations of models within density functional theory (DFT).

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2. Materials and Methods 2.1 Synthesis Arrays of aligned MWCNTs were grown perpendicularly to Si(100) substrates using aerosolassisted CCVD method. The details of the synthesis are described elsewhere.30 Si substrates 10×10 mm2 in size were located in the central part of horizontal tubular reactor, the reactor was pumped, filled by argon gas and heated up to 800°C. A 2 wt% solution of ferrocene in toluene was injected into reactor with a rate of 0.14 ml/min. The pyrolysis was performed at atmospheric pressure in an argon flow of 150 cm3/min for half hour. As the result, the Si substrates were covered by black films. Annealing of MWCNT samples was carried out at 1000°C in a nitrogen flow for two hours. CBD of CdS was carried out using a procedure especially developed for the formation of thin uniform films.31 SC(NH2)2 and CdCl2 were dissolved in a 7 wt%-ammonia solution in water in two different glasses. Substrate with MWCNT array was fixed to a holder and placed in the glass with SC(NH2)2, then the CdCl2 solution was quickly added to that solution and the obtained mixture was accurately stirred. The initial concentrations of CdCl2 and SC(NH2)2 in the reaction mixture were 0.015 mol/l and 0.15 mol/l, respectively. After a certain period of the reaction (from 2 to 10 min) at room temperature (26°C), substrate with MWCNTs was taken out, washed twice in distilled water and dried at ambient conditions. An examination of the synthesis products by the SEM showed a preservation of vertical orientation of MWCNTs after the used CBD procedure (Fig. S1(a)). 2.2 Characterization The structure of the samples was characterized by SEM on a Hitachi S-3400N microscope, HRTEM on a JEM 2010 microscope and Raman spectroscopy on a SPEX triple spectrometer with an excitation of 488 nm. The XPS and NEXAFS experiments were performed at the Berliner Elektronenspeicherring für Synchrotronstrahlung (BESSY) using radiation from the 4 ACS Paragon Plus Environment

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Russian-German beamline. Base pressure in the analysis chamber was ~10–8 Pa. NEXAFS spectra near the C K-edge were acquired in total-electron yield mode. The spectra were normalized to the primary photon current from a gold-covered grid recorded simultaneously. The monochromatization of the incident radiation was ~80 meV in FWHM. The π* resonance of the annealed CNT sample at a photon energy of 285.4 eV was taken as an internal standard. The overall XPS spectrum as well as C 1s, N 1s, S 2p, and Cd 3d lines were measured using monochromatized synchrotron radiation at 800 eV. Moreover, the S 2p lines were recorded at an excitation energy of 400 eV. The energy resolution of the lines was better than 0.4 eV (full width at a half maximum (FWHM)). All the binding energy was calibrated with an Au 4f7/2 peak. The S 2p XPS spectra were resolved into Gaussian/Lorentzian symmetric components by a nonlinear least squares procedure after proper subtraction of a Shirley-type baseline. Owing to a high roughness of the surface of the used arrays and entangling of MWCNTs on the array top (Fig. S1(b)), the XPS probed the CdS nanoparticles grown on the tips as well as the side walls of the nanotubes. 2.3 Computational Details Since the outer diameter of MWCNTs used for the CdS deposition was ~38‒62 nm (Fig. S2) the nanotube surface was modeled by a graphene fragment. A similarity in the electronic structure of graphite and MWCNTs has been demonstrated by comparing their NEXAFS spectra,32 which give an information about empty electronic levels, and X-ray emission spectra,33 which provide the partial electron density distribution in the valence band. An initial perfect fragment had a symmetry of D2h and a composition of C120H28, where hydrogen atoms saturated the dangling bonds at fragment edges (Fig. S3). The models of CdS nanoparticles and graphene fragments with deposited Cd2+ ions and Сdcomplexes were calculated using the three-parameter hybrid functional of Becke34 and LeeYang-Parr35 correlation functional (B3LYP method) included in the Jaguar program package.36 The LACVP basis set was used to approximate the atomic orbitals in CdS nanoparticle models, 5 ACS Paragon Plus Environment


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while the LACVP** basis set, where polarization functions are included for all atoms except of transition metals, was taken for calculation of interaction of Cd-containing species with a graphene fragment. Geometry of the models was relaxed by analytic method to the gradient value of 5·10–5 Hartree/Bohr. During the optimization procedure, atomic positions at the graphene fragment boundary were fixed. The interaction energy between Cd-complex and graphene fragment was calculated as: Eint = Etot(model) – Etot(graphene) – Etot(Cd-complex) + δBSSE, where Ex is a total energy of the optimized model, the optimized isolated graphene fragment, and the optimized free Cd-complex, respectively, and δBSSE is the basis set superposition error (BSSE) correction calculated using the counterpoise method proposed by Boys and Bernardi.37

3. Results 3.1 Structural aspects A study of the samples using the HRTEM detected that placing of as-prepared MWCNTs in a room-temperature CdCl2-NH3-SC(NH2)2 aqua bath for 2 min is sufficient for deposition of CdS nanoparticles. The size of nanoparticles may vary from ~5 to 36 nm (Fig.1 a,b). The small CdS nanoparticles have amorphous-like structure (Fig. 1a), while the large nanoparticles are partially crystallized (Fig. 1b). Increase in the reaction duration to 5 min enlarges the deposited CdS nanoparticles in general. The smallest nanoparticle, observed by the HRTEM, has a size of ~20 nm (Fig. 1c) and the largest one is ~40 nm (Fig. 1d). It can be seen that nanoparticles have a polyctrystalline structure and they are elongated along the nanotube axis. Moreover, the surface of the nanoparticles is usually not smooth, and many irregularities like the cavities or protruding edges are visible in the images. Note, the MWCNTs are convenient substrates for deposition, since they help much in the visualization of small-size nanoparticles by the HRTEM. However, to provide the formation of nanoparticles on the sidewalls of the nanotubes, they should be sufficiently spaced in an array.


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Figure 1. HR TEM images of CdS nanoparticles grown on as-prepared MWCNTs after 2 min (a, b) and 5 min (c, d) of synthesis and on annealed MWCNTs after 5 min (e) and 10 min (f) of synthesis. Insert demonstrates polycrystalline structure of CdS nanoparticle.

Annealing of the MWCNTs in a nitrogen atmosphere makes their surface significantly more inert to the CdS deposition. Actually, we did not find any nanoparticles in the sample, which was 7 ACS Paragon Plus Environment


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in the chemical bath for 2 min, and detected only a single nanoparticle on the TEM images when the synthesis was continued for 5 min. A nanoparticle of ~12 nm in size grew on a very defective site of MWCNT surface, such as a negative curvature of graphene layers (Fig. 1e). The number of CdS nanoparticles markedly increased in the sample after 10 min of the reaction. The HRTEM image of typical CdS nanoparticle from that sample shows that the nanoparticle in fact is composed of many nanocrystals orientated randomly relatively to each other (Fig. 1f). A blur between well-ordered regions (see the magnified image in insert) indicates a growth of nanocrystals in different directions and, hence, not from the same nucleus. A necessity of the inductive period more than 5 min for decoration of the surface of the annealed MWCNTs with CdS nanoparticles was supported by Raman spectroscopy data. The longitudinal optical phonon mode of CdS38 at ~300 cm–1 appeared only in the Raman spectrum of the sample synthesized for 10 min (see Fig. S4). To reveal the structural features, which could promote growth of CdS nanoparticles on the MWCNT surface, we comparatively examined the MWCNT arrays before and after annealing by means of NEXAFS, XPS, and Raman spectroscopy. The C K-edge NEXAFS spectra of both samples showed two main resonances located at ~285.4 and 291.7 eV and corresponding to the C 1s→π* and C 1s→σ* transitions, respectively (Fig. 2a). A slight enhancement in the intensity between these resonances evidences a presence of sp3 hybridized carbon atoms in the nanotube structure. These atoms could be involved particularly in the bonding with oxygen-containing groups or form linkages between the layers. As compared to the annealed sample, the C K-edge spectrum of the as-prepared sample is characterized by broadening of the π* and σ* resonances that is typical for the CCVD-synthesized MWCNTs.39 From theoretical modeling, the dangling bonds and non-hexagonal rings in the graphitic network of nanotubes are responsible for the π* resonance broadening.40 A heating of MWCNT sample in an inert atmosphere heals these defects.41 Increase of atomic ordering in the layers of the annealed MWCNTs is supported by sharpness of the σ* resonance in the C K-edge spectrum. 8 ACS Paragon Plus Environment

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The surface-sensitive XPS method also indicated the narrowing of the C 1s line with the annealing of sample (Fig. 2b). Additional intensity at the low-energy and high-energy sides of the C 1s spectrum of as-prepared MWCNTs is provided by carbon atoms being respectively negatively and positively charged as compared to the atoms constituting the perfect graphitic network. Particularly, it has been shown that negative and positive charges are induced on the pentagonal and heptagonal rings.33 The oxygen content estimated from the overall XPS spectra was ~5 at% in the as-prepared sample and ~6 at% in the annealed sample (Fig. S5). Since the annealed sample was stored in the laboratory before the XPS study, we conclude that oxygencontaining species on the MWCNT surface are originated from surrounding air.

π∗

b

NEXAFS CK-edge

XPS C 1s, 800 eV

c

Raman, 488 nm G

sp 2 raw MWCNTs

annealed MWCNTs

raw MWCNTs

annealed MWCNTs 288 296 304 312 320 328 Photon energy (eV)

292 290 288 286 284 282 Binding energy (eV)

Int ensity (arb. un. )

σ∗

Int ensi ty (arb. un. )

a Intensity (arb. un.)

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raw MWCNTs D

annealed MWCNTs 1300 1400 1500 1600 1700 -1 Raman shift ( cm )

Figure 2. NEXAFS C K-edge spectra (a), XPS C 1s spectra (b), and Raman spectra (c) measured for raw MWCNTs and those after annealing at 1000°C in a nitrogen flow for 2 h.

It was surprising, that Raman spectra measured in an interval from 1200 to 1800 cm–1 showed only a minor difference for two samples (Fig. 2c). After the sample annealing, the D and G peaks narrow and downshift by ~8 and ~4 cm–1, respectively. The most changes were observed between 1370 and 1542 cm–1, where spectral intensity decreased. Probably, such defects as dangling bonds and non-hexagonal rings are responsible for this scattering region. Summarizing, we showed experimentally, that dipping of MWCNTs into the CdCl2-NH3SC(NH2)2 aqua bath at room temperature leads to a deposition of CdS nanoparticles already after 2 min, whiles this period increases significantly, when the annealed MWCNTs are taken. This supposes an importance of structural defects present in the MWCNT layers for nanoparticle 9 ACS Paragon Plus Environment


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nucleation. To reveal the nature of the defects, we performed the DFT calculations of interactions of Cd2+ ion (the simplest Cd species existing in solution) with a perfect graphene fragment and that with topological defects, single vacancy, and non-saturated edge bonds (Fig. S3). The results indicated that the ion attaches only to the sp-hybridized carbon atoms, which can constitute the edges of the nanotube layers or be located on the boundary of a vacancy. 3.2 XPS study Electronic state of CdS nanoparticles in the investigated samples was examined by XPS. The spectra measured in the regions of Cd 3d and S 2p binding energies are compared in Fig. 3. The Cd 3d line is presented by a doublet, where the 3d5/2 and 3d3/2 spin-orbit components are separated by ~6.7 eV and have a ratio of 3:2. In literature, position of the 3d5/2 component varies from 404.4 to 406.6 eV,42–46 and such a big difference in the reported values could be related to different structure and surface composition of CdS nanoparticles and films. The 3d5/2 binding energy of CdS nanoparticles grown on the as-prepared MWCNTs for 5 min is ~406.0 eV (Fig. 3a) and it lowers to ~405.6–405.7 eV with decrease of the CBD and use of the annealed MWCNTs (Fig. 3b). The value, which we obtained for crystalline CdS, is 405.6 eV (Fig. S6(a)). Below the Cd 3d5/2 component, the spectra of all samples showed a signal at 400.1–400.2 eV (Fig. 3a,b). This binding energy can be attributed to amine groups and/or ammonia.47,

48

Relatively to the intensity of the Cd 3d line, the N 1s signal decreases significantly with the staying of as-prepared MWCNTs in chemical bath, while it is almost unchanged in the case of the annealed MWCNTs. We associate this effect with the size of CdS nanoparticles, which especially noticeably increases with the duration of deposition on the raw MWCNT surface. Since the nitrogen-containing species are most likely accommodated at the surface of CdS nanoparticles, their amount should be greater for the smaller nanoparticles, i.e. formed for 2 min.


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Intensity (arb. un.)

Raw MWCNTs

412.7

Cd 3d3/2406.0 Cd(OH)2

N 1s 400.1

CdS 2 min

405.7 412.4

415

410

163.9

405

CdS 2 min

170

400


405.7

CdS 5 min

412.4

164.1

I

168

166

164

162

160

170

CdS 10 min

168

166

164

f S 2p, 400 eV II 162.8 III I

164.0

162.2

162

160

Binding energy (eV)

d S 2p, 800 eV

CdS 10 min

400.2

CdS 2 min 163.8

Binding energy (eV)

N 1s

412.3

III

III I


Cd 3d3/2

Cd 3d5/2 405.6

162.3

163.5

CdS 5 min

169.0

Binding energy (eV)

b

162.5

CdS 5 min Cd(OH)2

e S 2p, 400 eV III I

c S 2p, 800 eV III I

Cd 3d5/2 CdS 5 min Intensity (arb. un.)

a

Annealed MWCNTs

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CdS 10 min

III

II

162.7

163.9

I

161.6

168.8

161.5

168.7

II

III

III

CdS 5 min

I

II I

CdS 5 min

161.6

161.8

415

410

405

400

Binding energy (eV)

170

168

166

164

162

Binding energy (eV)

160

170

168

166

164

162

160

Binding energy (eV)

Figure 3. XPS spectra of CdS nanoparticles deposited on raw MWCNT surface (a,c,e) and annealed MWCNT surface (b,d,f) for different time. (a,b) – Cd 3d and N 1s regions and (c,d) - S 2p region at excitation photon energy of 800 eV, (e,f) – S 2p region at excitation photon energy of 400 eV.

While the Cd 3d spectra showed a little change among the investigated samples, the S 2p spectra were found to vary significantly depending on the defect density in MWCNT surface and duration of the CdS deposition (Fig. 3c–e). The S 2p spectra exhibited two or three main peaks for the raw or heat-treated MWCNT surface. According to the number of peaks, two or three doublets consisting of S 2p3/2 and S 2p1/2 components with a ratio of the intensities of 2:1 and separation of ~1.2 eV were used for approximation of the experimental spectra. The energy position and integral intensity ratio for 2p3/2 components in the spectra measured at excitation energies of 800 and 400 eV are collected in Table 1. The higher excitation energy provides the larger probing depth of a nanoparticle.



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Table 1. Energy position and intensity ratio of the S 2p3/2 components in the XPS spectra of CdS nanoparticles deposited on raw and annealed MWCNTs for different period. The spectra recorded at excitation photon energy of 800 eV and 400 eV are shown in Figs. 3c,d and 3e,f respectively. Synthesis conditions 2 min raw MWCNT 5 min 5 min annealed MWCNT

10 min

S 2p3/2 components position (eV) I; III ratio (I:III) position (eV) I; III ratio (I:III) position (eV) I; II; III ratio (I:II:III) position (eV) I; II; III ratio (I:II:III)

800 eV 162.5; 164.1 1:2.7 162.5; 163.9 1:0.8 161.8; 162.8; 164.0 1:1.5:1 161.6; 162.8; 164.0 1:1.4:0.8

400 eV 162.2; 163.8 1:6.7 162.3; 163.5 1:1.7 161.6; 162.7; 163.9 1:2.3:2.1 161.5; 162.7; 163.9 1:1.5:1.6

As compared to the component with the lowest binding energy (labeled by I in Fig. 3c–f and Table 1), the higher energy components II and III grow in the intensity with decrease of the excitation energy. Hence, these components correspond to the sulfur species located at surface of CdS nanoparticles, while the component I should be attributed to the subsurface layers. Position of this component in the spectrum of crystalline CdS is 162.0 eV (Fig. S6(b)) and it shifts to the higher binding energy (162.2–162.5 eV) or to the lower binding energy (161.5–161.8 eV) depending on whether the nanoparticles were grown on the as-prepared MWCNTs or on the annealed ones. The component III at 163.5–164.1 eV is present in the spectra of both kinds of the samples and attributed to the S–S bonding in Sn0 species.49 The component II with a binding energy of 162.7–162.8 eV appears only in the spectra of the CdS nanoparticles deposited on the annealed MWCNTs. Intensity of this component rises in the spectra measured at 400 eV not so significantly as compared to the component III intensity. We relate the component II with the – SH groups.50 The ratio of such groups can be large when the CdS nanoparticle is an agglomerate of small-size crystals, as it is evident from the HRTEM image presented in the insert of Fig. 1f. The low-intensity component at 168.7–169.0 eV is due to oxidized sulfur states. The quantity of these sulfur species is negligible in the surface of CdS nanoparticles formed on the raw MWCNTs (Fig. 3c,e) and is ~3‒8% of the total sulfur content for the nanoparticles grown on the annealed MWCNTs (Fig. 3d,f). Since the basic media of the chemical bath does not allow 12 ACS Paragon Plus Environment

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producing SOx2‒ (x = 3 or 4), the CdS nanoparticles are likely partially oxidized in air. A higher reactivity of the nanoparticles deposited on the annealed MWCNTs could be due to the smaller size of nanocrystals. This conclusion is in the line with the HRTEM examination of the samples (Fig. 1).

Figure 4. Initial Cd28S29H2 cluster with hexagonal arrangement of Cd and S atoms and hydrogen atoms linked to the surface S atoms (a), the cluster after geometry relaxation at the B3LYP/LACVP level: side view (b) and top view (c). The yellow and blue balls show sulfur and cadmium, respectively. The limiting distance for the connection of atoms was 2.8 Å.

At the end of this part, we analyzed displacement of the component I in the S 2p3/2 spectra of CdS nanoparticles relative to the S 2p3/2 component of the crystal. For this purpose, the difference in the experimental energies was compared with the shift of one-electron Kohn-Sham energies for structurally distinguishable S atoms in a cluster Cd28S29H2. The cluster was composted of three CdS layers along the c axis with five hexagons in the (0001) plane (Fig. 4a). Hydrogen atoms were attached to two surface S atoms. After optimization procedure, geometry of the cluster was distorted substantially as compared to the hexagonal CdS lattice. Initially zigzag-like CdS layers became almost flat and the edge atoms moved to the oppositely charged neighbors to increase the coordination number (Fig. 4b). Examination of the Kohn-Sham energies showed that the 2p level energies of non-hydrogenated S atoms are varied within ~1.3 eV that well agrees with the difference of ~1.0 eV between extreme positions of the component I in the XPS spectra (Fig. 3c–f). The atoms constituting S-rich and Cd-rich faces (Fig. 4c) have correspondingly the highest and the lowest S 2p binding energy. The energy for the atoms 13 ACS Paragon Plus Environment


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located on the most flat areas of the (0001) planes has an intermediate value. Hence, large variation in the S 2p binding energies observed experimentally could be attributed to different preferential orientations of CdS nanocrystals on the substrate. The calculations predict an increase in the S 2p binding energy of a surface sulfur atom after its bonding with hydrogen thus confirming assignment of component II in the XPS S 2p spectrum of CdS nanoparticles to the – SH groups. 3.3 Modeling CNT interaction with Cd-complexes from chemical bath A study of mechanism of the formation of CdS in a water solution of CdCl2, thiourea, and ammonia revealed an important role of the mixed complexes [Cd(NH3)iSC(NH2)2]2+ in this process.51 It was found that at the ratio of the reagents close to that used in the present work, the complex with i=3 prevails at the first stage of the reaction and then thiourea is hydrolyzed with retention of Cd–S bond in complex. Based on these experimental data and results of quantumchemical modeling showing that the Cd2+ ion prefers bonding with a two-fold coordinated carbon atom (Fig. S3), we calculated an interaction of [Cd(NH3)iSH]1+ (i=0–3) species with a graphene fragment having a monovacancy. Moreover, we examined a possibility of attachment of [Cd(NH3)3SC(NH2)2]2+ complex to such defect. However, as the result of geometry optimization

thiourea

left

the

complex

(Fig.

S7)

and

this

result

excludes

the

[Cd(NH3)3SC(NH2)2]2+ complex from consideration as a source for CdS nucleation on the CNT surface. Optimized structures of the models of [Cd(NH3)iSH]1+ complexes attached to the graphene fragment are presented in Fig. 5. The distances between Cd and its neighboring C, N, and S atoms and interaction energy of the complexes to defective graphene are collected in Table 2.


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Figure 5. The models of graphene fragments with [Cd(NH3)iSH]1+ complexes (i=3, 2, 1, 0, from top to bottom) attached to the two-coordinated carbon atom at monovacancy boundary (highlighted). Geometry of the models was optimized at the B3LYP/LACVP** level. The structures of free complexes are shown at the left.

The calculations show that hydrolyzed [Cd(NH3)3SH]1+ complex interacts readily with twofold coordinated carbon with an energy gain of ~2.56 eV. Graphene network pulls electronic density from the complex that causes elongation of the Cd–S and Cd–N bonds as compared to the corresponding values in the free complex (Table S1). Two NH3 molecules in the attached complex are structurally equivalent, while third molecule is located at the larger distance from Cd (Table 2). Particularly this molecule can leave the complex that by the results of calculation should result in shortening of the Cd–C and Cd–S bonds and hence, the stronger bonding of [Cd(NH3)2SH]1+ complex with graphene network. Curiously, that the Cd–N bonds in this complex is longer than corresponding bonds in the previous complex. This may initiate further detaching of NH3 molecules up to their complete removal from the complex. With each 15 ACS Paragon Plus Environment


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subsequent detachment of the NH3 molecule, the Cd–S bond is strengthened and the remaining species may be a nucleus for the growth of CdS nanocrystal. Table 2. Bond lengths in [Cd(NH3)iSH]1+ complexes (i=3, 2, 1, 0) attached to a monovacancy in graphene fragment (Fig. 5) and interaction energy between the components. Attached complex [Cd(NH3)3SH]1+ [Cd(NH3)2SH]1+ [Cd(NH3) SH]1+ [CdSH]1+

Cd–C 2.429 2.325 2.293 2.257

Bond length (Å) Cd–S Cd–N 2.627 2.482; 2.483; 2.523 2.514 2.491; 2.497 2.467 2.476 2.432 -

Interaction energy (eV) –2.560 –3.392 –4.151 –5.196

4. Discussion The point, which is most often considered in the CBD studies of CdS films, is an influence of synthesis conditions on the film structure, particularly, surface uniformity, crystallite size, and adhesion to the substrate. Some authors believe that cadmium hydroxide is a first precipitating species, which provides good adhesion of growing film.15, 17, 29, 52 Hydroxide ions are substituted for S2– ions and CdS grows by the “ion-by-ion” mechanism.52 The Cd(OH)2 phase was observed on a silicon substrate hosted in a chemical bath for 3 min and this was followed by the CdS phase appearance.17 Moreover, formation of metastable complexes of Cd(OH)2 with thiourea and/or ammonia20, 52, 15, 21 in solution as well as on substrate is proposed. In this case, hydroxide ions could facilitate hydrolysis of thiourea. In contrary to these results, we detected Cd(OH)2 only in one sample prepared after 5 min deposition of CdS on the raw MWCNT surface (Fig. 3a). Hence, the presence of Cd(OH)2 is not a necessary condition for nucleation and growth of CdS nanoparticles on a CNT. In our synthesis, formation of cadmium hydroxide can be suppressed due to high ammonia content in the chemical bath. Actually, it has been shown that a competition between Cdx(OH)y2x–y+ and Cd(NH3)x2+ depends on the pH and pNH3 of solution.20, 29, 53

It was shown previously, that an excess of ammonia and thiourea in solution as in our case leads to formation of mixed Cd‒S complexes, mainly of [Cd(NH3)3SC(NH2)2]2+ composition.51 16 ACS Paragon Plus Environment

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Based on the quantum-chemical calculations, we found that the complexes of Cd2+ should not be attached to a perfect graphene network. However, Cd2+ species readily interact with two-fold coordinated carbon atoms constituting the boundaries of graphene planes and vacancies. Importance of such types of defects for nucleation of CdS nanoparticles is confirmed by increase of inductive period from 2 to 5 min after annealing of the MWCNTs in an inert atmosphere. The data of NEXAFS, XPS, and Raman spectroscopy indicated that the annealing improves atomic ordering in the MWCNT layers. A quantum-chemical attempt attaching [Cd(NH3)3SC(NH2)2]2+ complex to two-fold coordinated carbon atom in a graphene fragment resulted in thiourea removal (Fig. S7). This is due to weakening of Cd–S interaction in a consequence of strong Cd– C bonding. However, [Cd(NH3)3SH]1+ complex, obtained after hydrolysis of thiourea, binds to the defect (Fig. 5) and it could be a nucleus for CdS nanoparticle. The suggestion is in line with the XPS data, which detected a presence of –SH and NH3 species on the surface of CdS nanoparticles (Fig. 3). We emphasize a high reactivity of the CVD-produced MWCNTs, where the deposition of CdS nanoparticles occurred at room temperature already after 2 min. HRTEM images showed that the nanoparticles were poorly crystalized and had an amorphous surface layer (Fig. 1 a,b), which can be composed of the cadmium complexes deposited from chemical bath. Our quantumchemical calculations indicate that the surface of CdS nanoparticle has higher reactivity than graphene even if it contains defects. Actually, [Cd(NH3)3SC(NH2)2]2+ complex interacts with sulfur atom on the sulfur-rich face of CdS nanoparticle (Fig. 6a). Herewith, the Cd–S bond in complex elongates, but thiourea adheres to the nanoparticle surface through amine groups. Existence of thiourea in the investigated samples could be confirmed by the NEXAFS data. CKedge spectra of the CdS/MWCNT samples show an increase of the peak located around 288 eV and corresponding to functionalized carbon (Fig. 6b). This increase is especially noticeable when CdS was deposited on the raw surface of MWCNTs and this can not be explained by the



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formation of Cd–C bonds only. While it may be related with the contribution of carbon from thiourea.54 Adhering and decomposition of cadmium complexes with thiourea and ammonia on the surface of CdS nanoparticle can help in the growth of this nanoparticle and also serve as a nucleus for the emergence and development of other nanocrystals. The latter may explain the polycrystalline organization of most of the nanoparticles formed on the MWCNT surface. Hexagonal structure of these crystals (Fig. S8) supports heterogeneous process of CdS deposition55 through the “molecule-by-molecule” interaction. Our investigations show that quality of carbon template affects not only the induction period for CdS nanoparticle formation, but also the surface composition of nanoparticles and their preferable orientation relative to template. Deposition on the well-graphitized surface results in a large relative amount of SH bonds. This can be related with the hydrolysis of thiourea units in most of the Cd-complexes before they will find an appropriate site for attachment on surface of CdS nanoparticles or MWCNTs.

Figure 6. Interaction of [Cd(NH3)3SC(NH2)2]2+ with a S-rich surface of Cd36S36H2 cluster (a). Thiourea unit adheres to the surface through NH2 groups and Cd from the complex interacts with two S atoms thus integrating to the cluster. The change in NEXAFS CK-edge spectra of asprepared MWCNTs (left panel) and annealed MWCNTs (right panel) after a certain time of CdS deposition (b).


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5. Conclusion In the excess of ammonia and thiourea in a chemical bath, Cd2+ ions form mixed complexes with these ligands. Such complexes or products of their partial decomposition were detected in samples obtained after a dipping of MWCNTs into the bath at room temperature. Quantumchemical modeling revealed that perfect graphene sheet or that with topological defects is inert to the Cd2+ species. The binding occurs only at two-fold coordinated carbon atoms, which can constitute graphene edges or vacancy boundaries. The theoretical assumption was supported by a noticeable increase of the induction period for CdS deposition, when the CNTs were annealed at 1000°C in a nitrogen flow. From the NEXAFS, XPS, and Raman study, annealing resulted in healing of

the

defects. Since

at the

used concentrations

of reagents a

mixed

[Cd(NH3)3SC(NH2)2]2+ complex should prevail in the solution, we considered its interaction with monovacancy of a graphene fragment. The calculations showed formation of strong Cd–C bond followed by thiourea removal from the complex. Hence, this complex can not be a nucleus for CdS formation, while the hydrolyzed counterpart [Cd(NH3)3SH]1+ may. XPS observed a large amount of SH species in the surface of CdS nanoparticles grown on the annealed CNT surface. We relate this with increase of a time needed for the complex to find a defective cite appropriate for deposition. During this time, most of the [Cd(NH3)3SC(NH2)2]2+ complexes are hydrolyzed. Thus, the chemical bath deposition of CdS on the CNT surface is a heterogeneous process presented by “molecule-by-molecule” mechanism. This finding may be useful in an understanding of the CBD mechanism for other semiconducting nanoparticles.

AUTHOR INFORMATION Corresponding Author *Dr. Lyubov G. Bulusheva e-mail: [email protected]



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ACKNOWLEDGMENT We are grateful to Mr. A. V. Ishchenko for the TEM measurements, Mr. S. I. Kozhemyachenko for the Raman spectra, Mrs. T. G. Larionova for the synthesis of CdS, and Prof. S. V. Larionov for fruitful discussion. The work was financially supported by the Russian Foundation for Basic Research (grant 13-03-12118) and the bilateral Program “Russian-German Laboratory at BESSY”.

ASSOCIATED CONTENT Supporting Information. SEM images of top surface and side view of MWCNT array with deposited CdS nanoparticles; low-magnification TEM image of MWCNTs; Raman spectra of the annealed MWCNTs before and after hosting in chemical bath; overall XPS spectra of raw and annealed MWCNTs; XPS spectra of crystalline CdS; HRTEM image of polycrystalline CdS nanoparticle grown on the surface of the annealed MWCNTs; optimized models of perfect and defective graphene fragments with deposited Cd2+ ions and [Cd(NH3)3SC(NH2)2]2+ complex; bond lengths in in free [Cd(NH3)iSH]1+ complex (i=3, 2, 1, 0) and that attached to a monovacancy in graphene fragment obtained from the B3LYP/LACVP** calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

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