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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Chemical Bonding and Electronic Structure in CdS/GO and CdSSe/GO Multilayer Films Roberto A. Colina-Ruiz, Rafael Valentin Tolentino-Hernandez, Cesia GuarnerosAguilar, José Mustre de León, Francisco J. Espinosa-Faller, and Felipe Caballero-Briones J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b03328 • Publication Date (Web): 10 May 2019 Downloaded from http://pubs.acs.org on May 12, 2019

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The Journal of Physical Chemistry

Chemical Bonding and Electronic Structure in CdS/GO and CdSSe/GO Multilayer Films Roberto A. Colina-Ruiz§, Rafael V. Tolentino-Hernández†, Cesia Guarneros-Aguilar‡, José Mustre de León§, Francisco J. Espinosa-Faller$,*, Felipe Caballero-Briones†,* §

Departamento de Física Aplicada, CINVESTAV Unidad Mérida, Km 6 antigua carretera a

Progreso, 97310 Mérida, Yucatán, México. †

Instituto Politécnico Nacional, Materiales y Tecnologías para Energía, Salud y Medio

Ambiente (GESMAT), CICATA Altamira, Km 14.5 Carretera Tampico-Puerto Industrial, 89600 Altamira, Tamaulipas, México. ‡

CONACYT- Instituto Politécnico Nacional, Materiales y Tecnologías para Energía, Salud y

Medio Ambiente (GESMAT), CICATA Altamira, Km 14.5 Carretera Tampico-Puerto Industrial, 89600 Altamira, Tamaulipas, México. $

Escuela de Ingeniería, Universidad Marista de Mérida, Periférico Norte Tablaje 13941, 97300

Mérida, Yucatán, México.

ABSTRACT: The local atomic structure of multilayer films of graphene oxide, cadmium sulfide, and cadmium sulfide-selenide was analyzed by X-ray absorption fine structure spectroscopy. The films were prepared by sequential electrophoretic deposition and successive ion layer adsorption

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and reaction onto F-doped SnO2 substrates. The obtained films were characterized by scanning electron microscopy, energy dispersive spectroscopy, Raman spectroscopy, UV-Vis spectroscopy, and step-potential photocurrent response. From the transmittance data analysis, a direct band gap ranging from 2.13 to 2.3 eV was deduced. From photocurrent measurements, we conclude that films of CdS and CdSSe have photovoltaic properties suitable for utilization as an active layer in multilayer thin film hybrid solar cells. X-ray absorption near-edge structure spectra show a difference in amplitude between CdS/GO and CdSSe/GO samples, ascribed to a change in the local electronic structure around S. Extended X-ray absorption fine structure spectra show a direct C-S bond between the CdS/CdSSe nanoparticles and the graphene oxide layer. Band diagrams for the CdS/GO and CdSSe/GO electrolyte interfaces are proposed.

1. INTRODUCTION Multilayer films of graphene oxide (GO), cadmium sulfide nanoparticles (CdS NPs) and cadmium sulfide-selenide nanoparticles (CdSSe NPs) are currently being studied as potential materials in optical, electronic and catalytic applications.1–5 The chemical and physical properties of these multilayers depend on its constituents and interactions. Nanoparticle properties often differ from the bulk material due to an increased surface to volume ratio, the optical absorption spectra behavior is strongly related to size-dependent properties.6,7 In addition, a significantly larger number of under-coordinated sites occur in NPs compared to bulk materials that potentially affect the interatomic interactions that usually occur when modifications to the atomic structure are present. The organic/inorganic mixture in nanocomposites or cluster-semiconductor-decorated form also provides a combination of unique properties between NPs and GO.8 The GO structure consists of a two-dimensional, one-atom thick, sp2 hybridized carbon material closely packed in a honeycomb


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form lattice arrangement,9 with oxygen functional groups located on the basal plane or at the edges of the sheet that exhibit exceptional chemical, thermal, and mechanical properties including a good electron mobility, high thermal conductivity, large surface area and strong mechanical stiffness.10,11 CdS is a II-VI group semiconductor with the well-known cubic Zinc-blende (ZB) structure with lattice parameter a = 5.818 Å12 and a wide band gap of  2.42 eV. On the other hand, CdS1-xSex is a pseudo-binary alloy whose NPs are unstable and easy to aggregate due to the small size and high surface energy. CdS1-xSex has a tunable band gap and a lattice parameter that depend on the value of x, ranging from 1.7 to 2.4 eV for 0 < x < 1.13 CdS and CdSe are both n-type semiconductors with absorption coefficients above 104 cm-1.14,15 We note that the recombination rates of the photogenerated carriers in these nanoparticles are usually large, compromising the performance of photovoltaic, photoelectrochemical or photocatalytic devices based on these semiconductors.16–18 Experimentally, the addition of graphene, which serves as the supporting matrix and has large electron conductivity, allows the charge separation rate to overcome the charge recombination enhancing the charge transfer process.4,19–21 The local atomic structure around a specific element can be probed with X-ray absorption fine structure (XAFS) spectroscopy. A one-dimensional partial pair distribution is obtained from the extended X-ray absorption fine structure (EXAFS) spectra, while the X-ray absorption near-edge (XANES) probes unoccupied electron states around the absorbing species. Recent research by Liu et al. in MoS2 nanosheets decorated on free-standing single wall carbon nanotube films indicates the possible formation of interfacial Mo–S–C bonds in the hybrid material.22 Ji et al. probed that GO is a S immobilizer in high-performance Li/S cells.23 Zhang et al. studied the electronic


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structure of GO-S nanocomposites prepared by direct S-anchoring onto the GO sheet, confirming partial GO reduction indicated by the presence of C-S and C-O bonds.24 In the present work, CdS and CdSSe films prepared by the successive ionic layer adsorption and reaction (SILAR) method were deposited onto GO prepared by electrophoretic deposition (EPD) onto SnO2:F/Soda-lime glass (FTO/SLG) substrates forming hybrid multilayer structures of ten successive layers (GO/CdS)10/GO and (GO/CdSSe)10/GO, called herein CdS/GO and CdSSe/GO samples respectively. The hybrid multilayer structures were analyzed in order to address the local atomic environment around S and evaluate the S/Se ratio effects in the interface between the CdS and GO as well as CdSSe and GO to reveal chemical coordination, neighbor distances and structural disorder in this hybrid system. Additionally, the structural, optical and photoelectrochemical characterization was performed and related to proposed band diagrams.

2. EXPERIMENTAL AND ANALYTICAL PROCEDURES Films deposition and characterization details are given in the Supporting Information. Briefly, GO prepared by a modification of the Hummers method was deposited onto cleaned FTO/SLG substrates by EPD (Figure S1). CdS and CdSSe films were deposited by SILAR onto the GO/FTO/SLG from CdCl2 and Na2SSe precursors. Ten successive layers of (GO/CdS)10 and (GO/CdSSe)10 were deposited (Figure S2). Films were characterized by Scanning Electron Microscopy/Energy Dispersive Spectroscopy (SEM/EDS) (Figure S4 and Figure S5), Raman spectroscopy (Figure S6), UV-Vis spectroscopy and Photocurrent Response (PR). Film thickness estimation was performed following reference25 using optical transmittance and a combination of a calibration curve of CdS films prepared by chemical bath deposition (Figures S7 and S8). The thickness was finally determined using the SCOUT software.26


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XAFS measurements at the S K-edge were performed at beamline 4-3 of the Stanford Synchrotron Radiation Lightsource (SSRL) in the low energy configuration mode. The X-ray photon energy was selected using an LN2 cooled Si (111) double crystal monochromator. A flat Rh coated mirror in the beam path was used for harmonic rejection. Measurements were carried out in fluorescence yield mode using a 4-element Ge detector. The intensity of the incident beam was monitored by an ionization chamber. The absorption spectra were obtained by calculating the ratio between the fluorescence from the sample and the incident beam intensity. Energy calibration was accomplished by defining the first inflection point in K-edge spectra of sodium thiosulfate powder at 2472.02 eV. The ionization energy, E0, was set to the calibration energy such that the photoelectron wave vector is 𝑘 = (2𝑚/ℏ2)(𝐸 ― 𝐸0).27 IFEFFIT, ARTEMIS and ATHENA packages were used for data reduction and analysis.28 Data were normalized setting the value of a second order polynomial fit over the pre-edge to zero and a third order polynomial over the region above the edge to unity at E0. Theoretical scattering amplitude and phases for CdS ZB type structure were calculated using the ab-initio code FEFF8.529 in a cluster size of 8 Å that contains 87 atoms. Nonlinear least squares curve fitting in momentum space (k) were performed with data weighted by k3. Metrical parameters were obtained from fits to the data in real space in a range between 1.0 Å and 3.5 Å. This range includes the first coordination shell from CdS ZB structure. In addition to the single scattering path S-Cd from CdS, we considered some possible paths outside of the established crystal structure, due to the presence of GO, to get high quality fits. These paths involved S-C, S-O, S-S, and the S-Se path for the CdSSe/GO sample. The fit quality was estimated from the value of the R-factor, a measure of the goodness of fit relative to the theoretical model. A good value of R-factor is

GO

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