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Nanoscale

PAPER

Cite this: Nanoscale, 2015, 7, 6216

Received 4th January 2015,Accepted 23rd February 2015

DOI: 10.1039/c5nr00041f

www.rsc.org/nanoscale

New insights into organicinorganic hybridperovskite CH3NH3PbI3 nanoparticles.An experimental and theoretical study of doping inPb2+ sites with Sn2+, Sr2+, Cd2+ and Ca2+

Javier Navas,*a Antonio Snchez-Coronilla,*b Juan Jess Gallardo,a

Norge Cruz Hernndez,c Jose Carlos Piero,d Rodrigo Alcntara,a

Concha Fernndez-Lorenzo,a Desire M. De los Santos,a Teresa Aguilara andJoaqun Martn-Callejaa

This paper presents the synthesis of the organicinorganic hybrid perovskite, CH3NH3PbI3, doped in the

Pb2+ position with Sn2+, Sr2+, Cd2+ and Ca2+. The incorporation of the dopants into the crystalline struc-

ture was analysed, observing how the characteristics of the dopant affected properties such as the crystal-

line phase, emission and optical properties. XRD showed how doping with Sn2+, Sr2+ and Cd2+ did not

modify the normal tetragonal phase. When doping with Ca2+, the cubic phase was obtained. Moreover,

DR-UV-Vis spectroscopy showed how the band gap decreased with the dopants, the values following the

trend Sr2+ < Cd2+ < Ca2+ < CH3NH3PbI3 Sn2+. The biggest decrease was generated by Sr2+, whichreduced the CH3NH3PbI3 value by 4.5%. In turn, cathodoluminescence (CL) measurements confirmed the

band gap obtained. Periodic-DFT calculations were performed to understand the experimental structures.

The DOS analysis confirmed the experimental results obtained using UV-Vis spectroscopy, with the

values calculated following the trend Sn2+ Pb2+ > Cd2+ > Sr2+ for the tetragonal structure and Pb2+ >Ca2+ for the cubic phase. The electron localization function (ELF) analysis showed similar electron localiz-

ations for undoped and Sn2+-doped tetragonal structures, which were different from those doped with

Sr2+ and Cd2+. Furthermore, when Cd2+ was incorporated, the CdI interaction was strengthened. For

Ca2+ doping, the CaI interaction had a greater ionic nature than CdI. Finally, an analysis based on the

non-covalent interaction (NCI) index is presented to determine the weak-type interactions of the CH3NH3groups with the dopant and I atoms. To our knowledge, this kind of analysis with these hybrid systems has

not been performed previously.

1. Introduction

Organicinorganic hybrid perovskite is being widely studiednowadays as it has a variety of interesting optical, magneticand electronic applications.13 Specifically, perovskites basedon methylammonium lead halide (MAPbI3, where MA =CH3NH3) are being studied after they caused a revolution inthe world of photovoltaic energy due to their high efficiency,which reaches 15%.46 Electronic absorption spectra for thiskind of perovskite shift to a longer wavelength, and this is oneof the reasons for its good performance.5 In turn, from a struc-tural perspective, the reported X-ray diffraction patterns forthis system have been interpreted as tetragonal structures.7,8

Thus, their increased photonic absorption towards the infraredregion and their crystalline structure are two vital factors inthe performance of this kind of compound. The substitutionof some elements of MAPbI3 with dopants to control the band

Electronic supplementary information (ESI) available: Elemental analysisobtained by ICP-AES, and XRF (Table S1). XRD patterns of the samples syn-thesized (Fig. S1). XRD patterns of commercials PbI2 and CdI2 used as reagents(Fig. S2). XRD patterns of MAPb0.5Sn0.5I3 (Fig. S3). UV-Vis spectra, in modereflectance diffuse, of the samples synthesized (Fig. S4). UV-Vis spectra, in modereflectance diffuse, of MAPbI3 and MAPb0.5Sn0.5I3 samples (Fig. S5). XPS spectrafor MAPbI3 and MAPb1xBxI3 (B: Sn, Sr, Cd, Ca) with x = 0.10 (Fig. S6). UV-Visspectra, in mode reflectance diffuse, of commercial PbI2 used as a reagent(Fig. S7). Details of the ELF for the simulated structures (Fig. S8). Starting con-figurations of the MA groups in the structure (Fig. S9). See DOI: 10.1039/c5nr00041f

aDepartamento de Qumica Fsica, Facultad de Ciencias, Universidad de Cdiz,

E-11510 Puerto Real (Cdiz), Spain. E-mail: javier.navas@uca.esbDepartamento de Qumica Fsica, Facultad de Farmacia, Universidad de Sevilla,

41012 Sevilla, Spain. E-mail: antsancor@us.escDepartamento de Fsica Aplicada I, Escuela Tcnica Superior de Ingeniera

Informtica, av. Reina Mercedes, Universidad de Sevilla, 41012 Sevilla, SpaindDepartamento de Ciencias de los Materiales e Ingeniera Metalrgica y Qumica

Inorgnica, Facultad de Ciencias, Universidad de Cdiz, E-11510 Puerto Real

(Cdiz), Spain

6216 | Nanoscale, 2015, 7, 62166229 This journal is The Royal Society of Chemistry 2015

www.rsc.org/nanoscalehttp://crossmark.crossref.org/dialog/?doi=10.1039/c5nr00041f&domain=pdf&date_stamp=2015-03-24

gap and electronic properties has been studied from bothexperimental and theoretical points of view, for example thecontrolled substitution of I with Cl or Br,4,7,913 or Pb2+

with Sn2+.5,6,14,15

In this paper, we synthesized CH3NH3Pb1xBxI3 species,where B represents Sn2+, Sr2+, Cd2+ and Ca2+ ions and x = 0.05,0.10, 0.15. To our knowledge, the doping of MAPbI3 with Sr

2+,Cd2+ and Ca2+ has not been reported previously. Thus, theeffect of this doping on the band gap and crystalline phase ofperovskite was analysed. To do this, synthesized samples werecharacterized using elemental analysis techniques such as theCHNS technique or X-ray fluorescence (XRF) to corroborate thelevels of dopant introduced; X-ray diffraction (XRD) deter-mined the crystalline phases present in the samples; X-rayphoton spectroscopy (XPS) determined the chemical bondingstates; UV-Vis spectroscopy in the diffuse reflectance mode(DR-UV-Vis) was used to establish the band gap of thesamples; scanning electron microscopy (SEM) was used toobserve the morphology of the nanoparticles obtained; andthe cathodoluminescence (CL) technique was used to analysethe optical emission of the samples. In addition, from atheoretical perspective, periodic density functional theory(DFT-periodic) calculations were performed to rationalize theexperimental information on this topic. The structures(MA)8Pb7SnI24, (MA)8Pb7SrI24, (MA)8Pb7CdI24 and(MA)8Pb7CaI24 were optimized to simulate the experimentalstoichiometries (with x = 0.10). Density of state (DOS) resultsagreed with the experimental ones, showing a slight decrease inband gap energy. Moreover, the analyses of both the electronlocalization function (ELF)1620 and non-covalent interaction(NCI)21 shed light on the bonding interactions establishedwithin these hybrid systems. An NCI analysis with these hybridsystems has not been reported before, to our knowledge.

2. Results and discussion2.1. Elemental analysis

The elemental analysis of the synthesized samples (see theExperimental section) was performed using XRF to determinethe amount of I, Pb and the dopant incorporated into thestructure, and with the CHNS technique to ascertainthe amount of C, H and N. The weight percentages of each ofthe elements in the synthesized samples were obtained (seeTable S1) and the atomic percentages were determined andare shown in Table 1. From these data, only small deviationswere found in x compared with the nominal composition.

2.2. X-Ray diffraction

The XRD patterns for the undoped sample, MAPbI3, and forthe samples doped with x = 0.10 are shown in Fig. 1. The diffr-actograms of the rest of the samples synthesized are shown inFig. S1 in the ESI. The pattern for MAPbI3 showed severaldiffraction peaks, which can be indexed to planes of the tetra-gonal I4/mcm space group, as reported for MAPbI3 perovs-kite.4,8,10,11,22,23 The peak that corresponds with the reflection

of the plane (211) of the tetragonal phase establishes the maindifference with the peaks that would be observed if there werea cubic phase (Pm3m space group). But, four peaks that cannotbe indexed to the tetragonal and cubic phases are observed atabout 2 = 12.8, 26.0, 34.2 and 39.5. These peaks are

Table 1 Results in atomic percentage (at%) obtained from the elemen-tal analysis performed using the CHNS and XRF techniques

Samplea

MAPb1xBxI3at%C

at%H

at%N

at%Pb

at%I

at%B xreal

MAPbI3 8.4 50.0 8.4 8.3 24.9 0.00B = Sn 0.05 8.3 50.1 8.3 7.9 24.9 0.5 0.05

0.10 8.3 50.0 8.3 7.6 25.0 0.8 0.090.15 8.3 50.0 8.3 7.1 25.0 1.3 0.15

B = Sr 0.05 8.3 50.0 8.3 7.9 25.0 0.5 0.050.10 8.3 50.0 8.3 7.5 25.0 0.9 0.100.15 8.4 49.9 8.3 7.2 25.0 1.2 0.14

B = Cd 0.05 8.3 50.1 8.3 7.9 24.9 0.5 0.050.10 8.4 49.9 8.4 7.5 25.0 0.8 0.100.15 8.3 50.0 8.3 7.1 25.0 1.3 0.15

B = Ca 0.05 8.4 49.9 8.3 7.9 25.0 0.5 0.060.10 8.3 49.9 8.4 7.5 25.1 0.8 0.100.15 8.3 50.0 8.3 7.1 25.0 1.3 0.15

a The doped samples are named according to the dopant and thenominal value of x. The real value of x is shown in the last column ofthe table.

Fig. 1 XRD patterns of the MAPbI3 sample, and MAPb1xBxI3, with B =Sn, Sr, Cd, Ca, and x = 0.10. Te: tetragonal phase, Cu: cubic phase.

Nanoscale Paper

This journal is The Royal Society of Chemistry 2015 Nanoscale, 2015, 7, 62166229 | 6217

assigned to PbI2, used as a reagent in the synthesis. For thepurpose of comparison, the pattern of the commercial PbI2used is shown in Fig. S2A in the ESI.

The patterns with Sn2+ and Sr2+ show reflections that canbe assigned to planes of the tetragonal phase with the spacegroup I4/mcm, the same as the undoped sample. In turn, PbI2is not present. Thus, incorporating Sn2+ and Sr2+ into thestructure of MAPbI3 perovskite did not affect the

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