# Lead-free CsSnCl3 perovskite nanocrystals: Rapid synthesis, experimental
characterization and DFT simulations

Md. Shahjahan Ali, Subrata Das, Yasir Fatha Abed and M. A. Basith
<EMAIL_ADDRESS>Nanotechnology Research Laboratory, Department of
Physics, Bangladesh University of Engineering and Technology, Dhaka-1000,
Bangladesh.

DOI: 10.1039/D1CP02666F

###### Abstract

Lead-free metal halide perovskites have drawn great attention as light
harvesters due to their promising optoelectronic and photovoltaic properties.
In this investigation, we have successfully synthesized thermally stable cubic
phase cesium tin chloride (CsSnCl3) perovskite nanocrystals (crystal size
$\sim$300 nm) with improved surface morphology by adopting a rapid hot-
injection technique. An excellent crystalline quality of these cubic shaped
nanocrystals was confirmed by high-resolution transmission electron microscopy
imaging. The binding of organic ligands on the surface of the sample was
identified and characterized by the nuclear magnetic resonance spectroscopy.
The UV-visible spectroscopy ensured that CsSnCl3 nanocrystals have a direct
band gap of $\sim$2.98 eV which was further confirmed by steady-state
photoluminescence spectroscopy. The band edge positions calculated using the
Mulliken electronegativity approach predicted the potential photocatalytic
capability of the as-prepared nanocrystals which was then experimentally
corroborated by the photodegradation of rhodamine-B dye under both visible and
UV-visible irradiation. Our theoretical calculation by employing
experimentally obtained structural parameters within generalized gradient
approximation (GGA) and GGA + U methods demonstrated 90% accurate estimation
of experimentally observed optical band gap when Ueff = 6 eV was considered.
The ratio of the effective masses of the hole and electron expressed as D =
$m_{h}^{*}/m_{e}^{*}$ was also calculated for Ueff = 6 eV. Based on this
theoretical calculation and experimental observation of the photocatalytic
performance of CsSnCl3 nanocrystals, we have proposed a new interpretation of
the “D” value. We think that a “D” value of either much smaller or much larger
than 1 is the indication of low recombination rate of the photogenerated
electron-hole pairs and the high photocatalytic efficiency of a photocatalyst.
We believe that this comprehensive investigation might be helpful for the
large-scale synthesis of thermally stable cubic CsSnCl3 nanocrystals and also
for a greater understanding of their potential in photocatalytic,
photovoltaics and other prominent optoelectronic applications.

##  I Introduction

Halide perovskites of ABX3 stoichiometry where A is a monovalent organic
(e.g., CH3NH3+) or inorganic cation (e.g., Cs+ or Rb+), B is a divalent metal
ion (e.g. Sn2+, Pb2+, Mn2+ or Ge2+) and X is a halogen (e.g. Cl, Br or I),
have gained significant research focus over the past few years due to their
promising optoelectronic applications, including semiconducting thin-film
transistors, Mao _et al._ (2018); Lin _et al._ (2017) light-emitting diodes
(LED), Yantara _et al._ (2015); Xu _et al._ (2019); Tian _et al._ (2016);
Ghosh _et al._ (2021) solar cells, Pandey _et al._ (2019); Jung _et al._
(2017); Wang _et al._ (2021); Haris _et al._ (2021) photodetectors Saleem _et
al._ (2021); Wu _et al._ (2019) and photovoltaics. Teng _et al._ (2021); Sun
_et al._ (2015); Guo _et al._ (2021) Recently, a good number of all-inorganic
metal halide perovskites (MHPs), especially cesium (Cs) based perovskites,
CsBX3 have gained a particular attraction due to their fascinating thermal
stability Jung _et al._ (2017) as well as intriguing optoelectronic
properties. De Roo _et al._ (2016); Protesescu _et al._ (2015); Liang _et al._
(2018); Kang and Han (2018) To date, various divalent metal ions such as Pb2+,
Sn2+, Mn2+ as the B-site cation in CsBX3 perovskites have been studied
comprehensively and particularly, lead (Pb2+) based halide perovskites have
shown a record power conversion efficiency (PCE) of 25.5% after years of
research. J. Jia and Yang (2021) However, from an environmental standpoint,
the toxicity of Pb is still a frustrating issue for the large-scale production
and industry-level applications of lead-based halide perovskites. Hence, from
commercial prospects, recently a significant amount of research has been
conducted by substituting Pb with non-toxic Sn to fabricate lead-free CsSnBr3
and CsSnI3 MHPs and thereby to investigate their potential applications in
photodetectors, Han _et al._ (2019) LEDs, Mu _et al._ (2020) LASERs, Xing _et
al._ (2016) X-rays, Li _et al._ (2018) memristors Siddik _et al._ (2021) and
synaptic devices. Zhou (2020) Compared to Br and I based MHPs, the
experimental and theoretical investigations on the optical properties and
electronic band structure of CsSnCl3 perovskite are less explored.
Particularly, the photocatalytic properties of CsSnCl3 perovskite are yet to
be investigated to the best of our knowledge.

Notably, the fabrication of thermally stable CsSnCl3 nanocrystals with desired
morphology is quite challenging. A previous investigation reported a
fabrication process of cubic CsSnCl3 nanocrystals which required temperature-
controlled mechanical milling and further 12 hour-long high temperature heat
treatment. Xia _et al._ (2020) In a separate investigation, Wu _et al._ (2019)
CsSnCl3 perovskite of phases I and II were also synthesized successfully,
however, the synthesis technique required high temperature processing steps.
Apart from these, in a number of other investigations, Yamada _et al._ (1998);
Peedikakkandy and Bhargava (2016) CsSnCl3 nanocrystals were synthesized by
employing organic solvent-assisted treatment along with energy consuming high
temperature annealing which are not suitable for time and cost-effective
synthesis of halide perovskites. Therefore, a simple and fast alternative
synthesis approach like hot-injection technique Chen _et al._ (2016) is
essential to fabricate CsSnCl3 nanocrystals with desired structure, phase
purity and surface morphology. Notably, the hot-injection technique has been
widely employed to synthesize lead-free CsSnBr3 and CsSnI3 perovskite
nanocrystals. Wang _et al._ (2017); Kang _et al._ (2021)

Furthermore, the experimental investigation of the material properties of
halide perovskites at the molecular level are very complicated and sometimes
unattainable due to the unavailability of the required experimental
facilities. These complexities can be circumvented to a greater extent by
implementing the density function theory (DFT) based first-principles
calculation. By using the conventional DFT method, theoretical investigations
of optical and electronic properties of CsSnCl3 perovskite were carried out.
Idrissi _et al._ (2021); Körbel _et al._ (2016) In the aforementioned
investigations, the theoretically calculated optical band gap of CsSnCl3
perovskite was underestimated significantly if compared with the
experimentally obtained band gap. Idrissi _et al._ (2021); Körbel _et al._
(2016) In a previous investigation, Körbel _et al._ (2016) computationally
more costly hybrid exchange-correlation functional of Heyd, Scuseria, and
Ernzerhof (HSE06) Heyd _et al._ (2003) was used in DFT calculation. However,
the theoretically calculated band gap of CsSnCl3 deviated 45.9% from the
experimentally observed value. Huang and Lambrecht (2013) The accuracy of the
optical properties and electronic band structure calculation of CsSnCl3 is
required to be improved for precise calculation of the optical and electronic
band structure of CsSnCl3 perovskite. In this regard, the effect of on-site
Coulomb and exchange interaction might be considered within the generalized
gradient approximation (GGA) by implementing the GGA+U method. Das _et al._
(2021); Lin _et al._ (2021); Prasad _et al._ (2020) In GGA+U method the
Hubbard Ueff correction is implemented in a form of Ueff = U – J, where the
effects of the on-site Coulomb and exchange interaction are denoted by U and
J, respectively. Wang _et al._ (2006); Dudarev _et al._ (1998)

Therefore, in this investigation, CsSnCl3 perovskite was synthesized by hot-
injection technique at temperature 200 ∘C without performing any subsequent
mechanical milling or any other energy-consuming high-temperature pre/post-
processing. We observed that the chemical compositions and optical band gap
were in good agreement with other published results, Siddik _et al._ (2021);
Peedikakkandy and Bhargava (2016); Huang and Lambrecht (2013) which further
confirmed the successful synthesis of CsSnCl3 nanocrystals via the hot-
injection method. Moreover, the synthesized CsSnCl3 nanocrystals exhibited
excellent absorption in the visible spectrum, which indicated its usability as
a solar absorber and photocatalyst. Therefore, as a case study of its
practical application, the photocatalytic dye degradation test of a well-known
organic dye, rhodamine-B (RhB), was performed under both visible and UV-
visible light irradiation. The photocatalytic dye degradation efficiency of up
to 58% was observed by irradiating solar light for 180 minutes using a solar
simulator. Moreover, using both GGA and GGA+U methods, the effects of the on-
site Coulomb and exchange interaction on absorption coefficient, band gap,
band structure, the effective masses of charge carriers, charge density etc.
have been investigated. The outcome of our theoretical calculations
demonstrate that the GGA+U method incorporated with Perdew-Burke-Ernzerhof
(PBE) functional provides reasonably accurate results close to the
experimentally obtained values when Ueff = 6 eV was considered. Through
theoretical investigation, the ratio of the effective masses of electron and
hole was also evaluated to get a deep insight into the charge carrier
recombination rate within the photocatalyst.

Figure 1:  Schematically the synthesis steps of CsSnCl3 perovskite using hot-
injection technique.

##  II Experimental Details

###  II.1 Sample preparation

In the present investigation, we have synthesized CsSnCl3 perovskite by
adopting a facile, low-temperature hot-injection technique. Murray _et al._
(1993); Yu and Peng (2002) Fig. 1 depicts schematically the synthesis steps of
CsSnCl3 perovskite. At first, to prepare Cs-oleate, 0.40 mmol of Cs2CO3 was
added into the mixture of 15 mL of 1-octadecene and 1.25 mL of oleic acid.
Then the mixture was rigorously stirred for 15 minutes and dried at 120 ∘C for
1 hour in vacuum. Afterward, the resultant mixture was heated for 1 hour at
180 ∘C in argon (Ar) atmosphere which led to the precipitation of the desired
Cs-oleate. The obtained Cs-oleate precipitation was heated at 80 ∘C for 30
minutes before using it for the synthesis of CsSnCl3.

Next, for the synthesis of CsSnCl3 perovskite, 225 mmol of SnCl2.2H2O, 30 mL
of 1-octadecene, 3 mL of oleic acid and 3 mL of oleylamine were mixed together
in a beaker by magnetic stirring and then dried at 120 ∘C for 1 hour in
vacuum. Afterward, 2.5 mL of Cs-oleate solution was injected rapidly into the
beaker and then, the beaker was quickly transferred to a furnace working at
inert atmosphere. Thereafter, the solution was heated at 200 ∘C for 1.5 hours
in Ar atmosphere. The mixture was then rapidly cooled using an ice bath.
Finally, CsSnCl3 nanocrystals were separated from the solution by
centrifugation at 5000 rpm and were washed three times by n-hexane. Prior to
further characterization, the obtained powder sample was dried at 80 ∘C for 4
hours in vacuum.

###  II.2 Material characterization

The crystal structure of the as-prepared sample was investigated by obtaining
its powder X-ray diffraction (XRD) pattern using a diffractometer (Rigaku
SmartLab) with a Cu X-ray source (wavelength, $\lambda$: Kα1 = 1.54059 Å and
Kα2 = 1.54180 Å). Further, Rietveld refinement analysis of the XRD data was
carried out by the FullProf computer program Rodriguez-Carvajal (1990) to
determine the crystallographic parameters. Thermal analysis of the as-prepared
CsSnCl3 perovskite was performed by Thermogravimetric analysis (TGA) and
Differential Scanning Calorimetry (DSC) measurement (NETZSCH, STA 449 F3
Jupiter) in N2 atmosphere (heating rate 10 ∘C/min). Fourier transform infrared
(FTIR) spectrum of the sample was collected in the wavenumber range of 650
cm-1 to 4000 cm-1 using a Spectrum Two FT-IR Spectrometer (PerkinElmer).
Further, the surface morphology of the sample was examined by a Field Emission
Scanning Electron Microscope (FESEM) (XL30SFEG; Philips, Netherlands and
S4300; HITACHI, Japan). In addition, elemental analysis was carried out by
energy-dispersive X-ray (EDX) spectroscopy. Further, Tecnai G2 FE1 F12
transmission electron microscope (TEM) was used at an accelerating voltage of
200 kV to obtain the bright field images and selected area electron
diffraction (SAED) pattern of the as-synthesized CsSnCl3 perovskite. To
determine the chemical binding energies and valence states of different
compositional elements at the surface of the CsSnCl3, X-ray photoelectron
spectroscopy (XPS) was conducted by a PHI5600 ESCA 120 (ULVAC PHI) system. The
XPS spectrum was extracted using the monochromatized Al Kα X-ray (1486.6 eV)
and 121 hemispherical mirror analyzer (total energy resolution $\sim$0.13 eV).
The Shirley 123 procedure was applied to subtract the background of the XPS
spectra and the Gaussian-Lorentzian function was used for peak fitting. For
assessing the nuclear magnetic resonance (NMR) spectrum, a Bruker Avance III
HD spectrometer was used with the 1H frequency of 400.17 MHz and temperature
of 297.6 K. The spectral width was set to be as 14 ppm and 64k data points
were sampled with a relaxation delay of 4 seconds. The absorbance spectrum of
the material was obtained using an ultraviolet-visible (UV-visible)
spectrophotometer (UV-2600, Shimadzu). The steady-state photoluminescence (PL)
spectrum of the sample was obtained on an RF 6000 luminescence spectrometer
made by Shimadzu, Japan. The photoluminescence lifetime of the sample was
measured using a PicoQuant Fluotime 300 equipped with a PMA 175 detector and a
LDH-P-C-440 diode laser with an excitation wavelength of 440 nm. The
photocatalytic efficiency of the CsSnCl3 nanocrystals was evaluated in the
photodegradation of RhB dye under solar irradiation. The details of the
photocatalytic experiments are provided in electronic supplementary
information (ESI).

##  III Results and discussions

###  III.1 Experimental Analysis

####  III.1.1 Crystal structure analysis

The crystallographic structure and phase composition of the as-synthesized
CsSnCl3 perovskite were investigated by analyzing its Rietveld refined powder
XRD spectrum. As shown in Fig. 2 (a), the Bragg reflected peaks exhibited by
the sample can be indexed at (110), (200), (211), (220), (301), (222), (401),
(411) and (420) planes (JCPDS card file no 22-0200) which conform to the cubic
crystal phase (_Pm3m_ space group) of CsSnCl3 perovskite. Xia _et al._ (2020)
Notably, the XRD pattern of as-synthesized CsSnCl3 did not exhibit any
undesirable peaks which indicates its high-phase purity. By Rietveld
refinement analysis, the lattice parameters of as-synthesized CsSnCl3
perovskite were determined as $a$ = $b$ = $c$ = 5.583 Å, and $\alpha$ =
$\beta$ = $\gamma$ = 90∘ with unit cell volume of 174.02 Å3. For a further
understanding, the atomic positions and bond length of CsSnCl3 as well as the
reliability (R) factors of Rietveld refinement were inserted in Table 1.

Figure 2:  (a) Rietveld refined powder XRD patterns of CsSnCl3 perovskite. (b)
TGA and DSC curves of cubic CsSnCl3 perovskite. (c) FTIR spectrum recorded at
room temperature. (d) FESEM image of CsSnCl3 perovskite nanocrystals. Inset: A
magnified view of the as-synthesized nanocrystals.

The thermal stability of as-synthesized CsSnCl3 sample was investigated by
conducting TGA and DSC measurements. The outcomes of these measurements are
presented in Fig. 2 (b). As can be observed from the TGA curve, the weight
loss of CsSnCl3 perovskite due to the increase of temperature from 30 ∘C to
280 ∘C was only 2.15% which is an indication of its excellent thermal
stability. Such a small percentage of weight loss indicates the presence of
nominal amount of binding surface ligands i.e. oleylamine and oleic acid on
the as-synthesized nanocrystals. De Roo _et al._ (2016); Dastidar _et al._
(2017); Hoffman _et al._ (2017)

Further, as illustrated in Fig. 2 (b), no endothermic or exothermic peak was
observed in the DSC curve of synthesized CsSnCl3 perovskite indicating that
any crystallographic phase transition did not occur in our fabricated sample
within the temperature range of 30 ∘C to 280 ∘C. The observed initial decrease
in the DSC curve can be attributed to the moisture removal from the sample.
Here, it is intriguing to note that according to a recent investigation, Xia
_et al._ (2020) CsSnCl3 crystals synthesized by mechanical milling technique
undergoes monoclinic to cubic phase transition at around 120 ∘C and hence,
heat treatment at 120 ∘C is required to obtain stable cubic phase using this
technique . In contrast, the as-synthesized cubic-phase CsSnCl3 perovskite has
demonstrated high structural stability over a wide temperature range which
eliminates the necessity of post-processing heat treatment and justifies the
significance of employing rapid hot-injection technique to synthesize CsSnCl3
with a stable crystallographic phase.

Table 1: Structural parameters of CsSnCl3 nanocrystals and the value of R factors. Atom | Wyc. Positions | $x$ | $y$ | $z$ |  $a$ (Å) | $\alpha$ |  volume (Å3) | Bond length (Å) | R factors
---|---|---|---|---|---|---|---|---|---
Cs | 1a | 0.0 | 0.0 | 0.0 |  |  |  |  |  Rexp=5.48
Sn | 1b | 0.5 | 0.5 | 0.5 | 5.583 |  90∘ | 174.02 | Sn-Cl = 2.791 |  Rwp=7.24
Cl | 3c | 0.0 | 0.5 | 0.5 |  |  |  |  |  $\chi^{2}$=2.53

####  III.1.2 FTIR analysis

Further, FTIR spectroscopy was conducted to identify the presence of different
functional groups within the CsSnCl3 perovskite. As shown in Fig. 2 (c), the
FTIR spectrum of as-prepared CsSnCl3 nanocrystals exhibited a broad band at
590 cm-1 which can be attributed to the in-plane O-H group bending vibration.
Degen (1997); Feng _et al._ (2009) The two weak absorbance peaks observed at
1406 and 1462 cm-1 are ascribed to the CH2 bending. Delgado-Mellado _et al._
(2018) The absorption band at 1637 cm-1 might have arisen due to the Sn-Cl
bond formation. Feng _et al._ (2009) The discerned peak around 2360 cm-1 can
be associated with the C=O vibrations. Hills-Kimball _et al._ (2017) The
vibrational bands at 2856 and 2925 cm-1 have emerged due to the C-H stretching
vibrations at the methylene groups of the precursors i.e. oleylamine and oleic
acid, respectively. Kim _et al._ (2021); Morales _et al._ (2013) In addition,
the broad absorption band at 3417 cm-1 can be imputed to the stretching
vibration of O-H functional group. Thus, the outcomes of FTIR analysis
conspicuously justify the successful formation of phase-pure CsSnCl3
perovskite as was also evident from the XRD analysis.

####  III.1.3 Morphological and elemental analysis

The FESEM image of CsSnCl3 perovskite is presented in Fig. 2 (d) which
demonstrates that the surface of our as-prepared sample was satisfactorily
homogeneous and non-porous. Noticeably, in a previous investigation,
agglomeration in CsSnCl3 perovskite was observed although the material was
synthesized by mechanical milling under inert atmosphere and subsequent heat
treatment for 12 hours. Xia _et al._ (2020) On the contrary, the hot-injection
technique employed in this investigation yielded non-agglomerated CsSnCl3
perovskite crystals successfully as shown in Fig. 2 (d). For a better insight,
we have presented a magnified view of as-synthesized sample in the inset of
Fig. 2 (d). The Fig. S1 inserted in the ESI shows the distribution of particle
size. From these two figures, it can be estimated that the average particle
size lies in the sub-micrometer region which confirms the successful synthesis
of CsSnCl3 nanocrystals. Further, the elemental composition of as-prepared
CsSnCl3 nanocrystals was assessed from EDX analysis at room temperature. The
mass and atom percentages of each element in CsSnCl3 as obtained by EDX are
provided in ESI Table S1. For comparison, we have also tabulated the
theoretically calculated values in ESI Table S1. Clearly, the mass and atom
percentages of the desired elements i.e. Cs, Sn and Cl in CsSnCl3 nanocrystals
are in good agreement with the theoretical values which provides further
evidence for the successful synthesis of CsSnCl3 perovskite.

Fig. 3 (a) demonstrates the bright field TEM image of CsSnCl3 nanocrystal
which is of cubic shape. The high resolution TEM (HRTEM) image of Fig. 3 (b)
shows the crystallinity of this nanocrystal with a 3.70 Å interplanar spacing.
The inset of Fig. 3 (b) reveals the selected area electron diffraction (SAED)
pattern of the CsSnCl3 nanocrystal. The SAED pattern demonstrates the presence
of (202), (022), and (220) planes of the face centered cubic phase which
further ensures the formation of the cubic structure of this nanocrystal.
Jellicoe _et al._ (2016); Lin _et al._ (2019)

Figure 3:  (a) Bright field TEM image of synthesized CsSnCl3 perovskite
nanocrystals. (b) HRTEM image of CsSnCl3 perovskite. Inset: SAED pattern of
CsSnCl3 nanocrystals. Figure 4:  (a) XPS full spectra of CsSnCl3 nanocrystals
illustrating the existence of constituent elements (Cs, Sn and Cl) in the
fabricated sample. Core level XPS spectra for (b) Cs 3d (c) Sn 3d (d) Cl 2p
demonstrating the purity of the sample and valence states of the constituent
elements, respectively.

####  III.1.4 Chemical state analysis

The chemical composition of as-prepared CsSnCl3 nanocrystals was investigated
by performing XPS analysis and the obtained full survey XPS spectra has been
demonstrated in Fig. 4 (a). In this survey spectra, strong peaks for Cs, Sn
and Cl core levels were identified which confirms the successful synthesis of
CsSnCl3 nanocrystals with the desired surface chemical states. Notably, two
additional peaks corresponding to C 1s and O 1s orbitals were also noticed in
the XPS spectrum of CsSnCl3 [Fig. 4 (a)]. The emergence of these peaks may be
due to the molecular adsorption of oxygen and carbon on the surface of as-
prepared CsSnCl3 nanocrystals during the exposure to the atmosphere which is
common in the XPS investigation of related perovskites. Musazay (2015);
Jellicoe _et al._ (2016) For a further insight, we have presented the high
resolution XPS core spectra of Cs 3d, Sn 3d and Cl 2p orbitals in Fig. 4
(b)-(d), respectively. In Fig. 4 (b), two distinct peaks can be observed at
the binding energies of 723.9 eV and 737.8 eV in the Cs 3d XPS spectrum which
can be ascribed to the presence of Cs 3d5/2 and Cs 3d3/2 states, respectively.
Zhang _et al._ (2019) As shown in Fig. 4 (c), the core level spectrum of Sn
was distinguished by the two pronounced peaks at 485.6 eV and 494 eV
corresponding to the states of Sn 3d5/2 and Sn 3d3/2, respectively. Allioux
_et al._ (2020) In Fig. 4 (d), the Cl 2p core level of CsSnCl3 exhibits an
asymmetric peak which has been de-convoluted into two symmetric Gaussian peaks
at 197.1 eV and 198.5 eV. Notably, the peak at the lower binding energy, i.e.
197.1 eV is associated with the Cl 2p3/2 state, while the higher binding
energy peak can be attributed to the existence of Cl 2p1/2 state in as-
prepared CsSnCl3 nanocrystals. Bera _et al._ (2017); Cheng _et al._ (2012 DOI:
10.1155/2012/593245)

Further, to identify the binding of surface ligands of the CsSnCl3 perovskite
nanocrystals, we have performed the one-dimensional (1D) 1H NMR spectroscopy.
Prior to the characterization, the CsSnCl3 nanocrystals were disseminated in
deuterated CDCl3 solvent. The 1D 1H NMR spectrum of the nanocrystals obtained
under the aforementioned condition are demonstrated in the ESI Fig. S2.
Interestingly, the peak formation is comparable with that of analogous
perovskite nanocrystals. However, an up-field chemical shift in the peaks can
be detected which might be attributed to the electronegativity of the Cl
atoms. De Roo _et al._ (2016) For deeper speculation, we have compared the
obtained NMR spectrum with previously reported NMR spectra of oleic acid,
oleylamine, and octadecene. De Roo _et al._ (2016); Ravi _et al._ (2017) It
can be clearly seen that the peaks below 2.2 ppm appear because of the oleic
acid, octadecene, and oleylamine ligands, but as their peaks are overlapping
owing to their chemical shift, it is difficult to separate their individual
peaks in this regime. Around 2.44 ppm, we found a relatively weak peak which
is the characteristic peak of the oleic acid. Evidently, at around 3.25 ppm, a
very wide peak is obtained, which can be attributed to the CH2-N and
similarly, around 6.87 ppm, we have obtained another broad peak which
corresponds to the $\text{NH}_{3}^{+}$ originated from the oleyammonium. These
two peaks are unambiguously correlated to the oleylamine, which is also in
well agreement with the previous reports. De Roo _et al._ (2016) Notably, the
peak at 5.35 ppm is associated with the alkane proton, which might have
originated from either the oleylamine or oleic acid or from both of them. Such
observation eventually conforms to the presence of protonated oleylamine. De
Roo _et al._ (2016) Interestingly, we did not observe the characteristic
resonances of a terminal alkene presumably octadecene which provides evidence
for the surface purity of the as-synthesized CsSnCl3 nanocrystals unlike
CsPbBr3 nanocrystals reported in a previous investigation.De Roo _et al._
(2016)

Figure 5:  (a) Absorption spectrum of CsSnCl3 nanocrystals. (b) Tauc plot of
CsSnCl3 perovskite demonstrating its direct band gap as $\sim$2.98 eV. (c)
Steady-state PL spectrum of CsSnCl3. (d) Time resolved PL decay curve under
pulsed 440 nm excitation at room temperature for CsSnCl3 nanocrystals.

####  III.1.5 Experimentally obtained optical properties

The optical properties of the CsSnCl3 nanocrystals were studied by conducting
UV-visible absorption spectroscopy. From Fig. 5(a), it can be seen that the
CsSnCl3 nanocrystals show sufficiently higher absorption in the visible region
as compared to the infrared region. Moreover, a strong absorption peak is
noticeable in the ultraviolet region of the absorption spectrum of CsSnCl3
which suggests its applicability for solar energy harvesting. Further, we
employed the absorbance data to determine the optical band gap of our as-
synthesized CsSnCl3 nanocrystals using the Tauc relation. Tauc _et al._ (1966)
Since a number of previous investigations have reported the band gap of
CsSnCl3 to be direct, Huang and Lambrecht (2013); Zhang _et al._ (2019) hence
we have used the Tauc relation for direct band gap materials as shown in Fig.
5(b). From the Tauc plot, the direct optical band gap of as-synthesized
CsSnCl3 can be estimated as $\sim$2.98 eV which is well consistent with the
literature. Siddik _et al._ (2021); Peedikakkandy and Bhargava (2016); Huang
and Lambrecht (2013) We have also measured the PL spectra of CsSnCl3 to have
an insight into the charge carrier recombination phenomenon. The PL peak shown
in Fig. 5(c) is an indication of radiative recombination of photoexcited
electron-hole (e--h+) pairs. The direct band gap value can also be estimated
from the PL peak as $\sim$2.96 eV which agrees well with the outcome of the
Tauc plot (Fig. 5(b)).

Further, in order to measure the excited state lifetime of the charge carriers
in the CsSnCl3 nanocrystals, we have employed the time-correlated single
photon counting (TCSPC) technique and presented the time-resolved PL decay
curve in Fig. 5 (d). The average lifetime ($\tau_{av}$) of the charge carriers
was calculated as 4.27 ns by fitting the decay curve with the single-
exponential function. Notably, the photoluminescence quantum efficiency (PLQY)
of the sample was determined to be 91%. Radiative lifetime was also calculated
from $\tau_{av}$ and PLQY using equation (1). The non-radiative lifetime
($\tau_{nr}$) and the radiative ($k_{r}$) /non-radiative ($k_{nr}$)
recombination rate constants were finally obtained using equation (2) and (3).
Notably, the calculated values of $k_{r}$ and $k_{nr}$ of as-synthesized
CsSnCl3 nanocrystals were 0.213 ns-1 and 0.021 ns-1, respectively which are
comparable with related previous investigations. Jellicoe _et al._ (2016);
Koscher _et al._ (2017); Ghosh _et al._ (2019)

$PLQY=\frac{\tau_{av}}{\tau_{{}_{r}}}$ (1)
$\frac{1}{\tau_{av}}=\frac{1}{\tau_{r}}+\frac{1}{\tau_{nr}}$ (2)
$k_{r}=\frac{1}{\tau_{r}},k_{nr}=\frac{1}{\tau_{nr}}$ (3)

In order to assess the potential of CsSnCl3 nanocrystals for photocatalytic
applications, we have determined their band edge positions by adopting the
Mulliken electronegativity approach. The conduction band minimum (CBM) and
valence band maximum (VBM) potential (vs. normal hydrogen electrode potential,
NHE) of CsSnCl3 were calculated as -0.44 and 2.54 eV, respectively. An energy
band diagram is provided in ESI Fig. S3 consolidating the obtained values of
CBM and VBM along with the redox potentials of different redox half-reactions
of our interest. According to thermodynamics, to act as an effective catalyst
of photocatalytic dye degradation, a material must possess a VBM $>$ 2.38 eV
and CBM $<$ -0.16 V for executing reduction and oxidation half-reactions,
respectively. Tama _et al._ (2019) Clearly, both the CBM and VBM potentials of
as-synthesized CsSnCl3 nanocrystals meet the aforementioned requirements which
indicate their promising potential for the photocatalytic degradation of
different organic dyes. Moreover, the calculated band edge positions of
CsSnCl3 nanocrystals shown in ESI Fig. S3 are also favorable for
photocatalytic hydrogen evolution via water splitting. Hence, further, we had
assessed the photocatalytic efficiency of CsSnCl3 nanocrystals which will be
discussed in the next section.

####  III.1.6 Photocatalytic degradation activity

The photocatalytic performance of the CsSnCl3 nanocrystals was investigated as
a case study towards the degradation of RhB under both visible and UV-visible
illumination. Initially, for both experimental conditions, a typical RhB blank
test and dark-adsorption test were carried out. The outcome of these tests
confirmed the negligible self-photolysis and chemi-adsorption potential of the
RhB dye molecules implying that the decomposition of RhB is solely attributed
to photocatalysis. Fig. 6(a) and 6(b) demonstrate the change in the absorbance
spectra of RhB photodegraded over as-synthesized CsSnCl3 nanocrystals under
the irradiation of visible and UV-visible light, respectively. Clearly, under
visible light illumination (Fig. 6(a)), the intensity of the characteristic
absorption peak at 553 nm decreased gradually up to 240 minutes indicating the
successful decomposition of RhB. On the other hand, for UV-visible irradiation
(Fig. 6(b)), the peak decreased more rapidly up to 180 minutes but no
significant degradation could be observed after that. Fig. 6(c) provides the
degradation efficiency vs. irradiation time curves where C0 and C denote the
initial and residual concentrations measured at 30 minutes of interval,
respectively. It can be seen that CsSnCl3 degraded $\sim$35% of RhB dye after
240 minutes of visible light irradiation. Interestingly, the degradation
percentage of RhB increased to $\sim$58% within only 180 minutes under UV-
visible irradiation. Such enhancement in photocatalytic efficiency of as-
synthesized CsSnCl3 nanocrystals can be attributed to their reasonably wide-
band gap value, $\sim$2.98 eV (Fig. 5(d)) along with good crystallinity and
excellent surface morphology. It can be anticipated that the photoexcitation
of charge carriers was enhanced significantly under the illumination of high
energy UV photons which eventually resulted in a higher RhB dye degradation
rate.

Figure 6:  Time-dependent absorption spectra of RhB solution for different
times up to 240 minutes under the irradiation of the (a) visible and (b) UV-
visible light. (c) Degradation efficiency of RhB as a function of the
irradiation time for CsSnCl3 photocatalyst. (d) The wavelength shifts of the
absorption spectra of the RhB dye solution under identical conditions of
figure (c).

It is worth noting that RhB dye typically degrades via two competitive and co-
existing pathways i.e. the cleavage of its whole conjugated chromophore
structure and N-deethylation process. Watanabe _et al._ (1977); Hu _et al._
(2006) In the first pathway, the absorption peak intensity decreases while the
peak position remains constant. On contrary, during N-deethylation, the
characteristic peak gradually shifts to shorter wavelengths with constant
intensity. Therefore, in Fig. 6(a) and 6(b), the decrement in the RhB
absorption peak intensity can be attributed to the cleavage of the aromatic
ring of the RhB dye during photocatalysis. Another intriguing feature to be
noticed from these figures is the shift of the absorption peak to shorter
wavelengths during photocatalysis in the presence of CsSnCl3 nanocrystals.
Such hypsochromic shifts can be imputed to the formation of a series of
N-deethylate intermediates of RhB during photocatalysis. The trend of this
blue-shift under visible and UV-visible irradiation is demonstrated in Fig.
6(d). As shown in this figure, the maximum absorption band of RhB solution
significantly shifted from 553 nm to 498 nm under 240 minutes of UV-visible
irradiation, whereas the peak wavelength shift was relatively insignificant
under visible irradiation. Hence, it can be inferred that the effect of
N-deethylation process on photocatalysis was more prominent under UV-visible
light as compared to the visible light illumination.

###  III.2 Theoretical Analysis

To explain the underlying mechanism behind the photocatalytic activities of
as-synthesized CsSnCl3 nanocrystals, a comprehensive investigation is further
required on their optical and electronic properties. However, the complexity
of the experimental conditions and to some extent, the unavailability of the
required experimental set-up pose difficulty to carry out such measurements
precisely. Therefore, we have analyzed both the optical and electronic
properties i.e. band structure, effective mass, density of states, atomic bond
formation and electron charge density of as-synthesized CsSnCl3 nanocrystals
via density functional theory (DFT) based first-principles calculation by
employing the experimentally obtained crystallographic parameters. Das _et
al._ (2021)

####  III.2.1 Computational details

The DFT based first-principles calculation was carried out within the plane
wave pseudopotential (PWPP) framework as implemented in the Cambridge Serial
Total Energy Package (CASTEP). Segall _et al._ (2002) We have adopted both the
standard generalized gradient approximation (GGA) and GGA+U methods based on
the PBE gradient corrected exchange-correlation functional. Perdew _et al._
(1996) The crystallographic structural parameters obtained from the Rietveld
refined powder XRD spectrum of as-synthesized CsSnCl3 were employed for DFT
calculation. The Cs 5s25p66s1, Sn 5s2 5p2 and Cl 3s23p5 electrons were treated
as the valance electrons. To relax the configuration in terms of atomic
locations and lattice parameters, the Broyden-Fletcher-Goldfarb-Shannon (BFGS)
algorithm was implemented. Fischer and Almlof (1992) The atomic structures
were completely relaxed before the residual forces fell below 0.01 eV/Å with a
self-consistent field (SCF) tolerance of 5.0$\times$10-7 eV/atom.

Prior to calculation, convergence study and geometry optimization were carried
out to select the optimum plane-wave cutoff energy and k-points. Fig. S4(a) in
the supplementary information shows the plane-wave cut-off energy convergence
result for structural optimization where the dashed line indicates the default
cut-off energy. Noticeably, 600 eV cut-off energy was found to be sufficient
to converge the ground state energy. Therefore, for the subsequent
calculations, we have applied the Vanderbilt type ultrasoft pseudopotential to
the plane wave basis sets with the optimized cut-off energy of 600 eV.
Moreover, from Fig. S4(b), it can be observed that a 15 x 15 x 15 Monkhorst-
Pack k-point mesh is sufficient to obtain the ground state energy of the
CsSnCl3 perovskite and hence, 15 x 15 x 15 k-point grid was employed for
Brillouin-zone integration. Monkhorst and Pack (1976) The convergence
thresholds for the geometry optimization were: (a) total energy
5.0$\times$10-6 eV/atom (b) maximum force 0.01 eV/Å(c) maximum stress 0.02 GPa
(d) maximum displacement 5.0$\times$10-4 Å.

Finally, to investigate the effect of on-site Coulomb and exchange
interactions on the structural, optical and electronic properties of CsSnCl3
perovskite, the GGA+U calculation was conducted with different Ueff values. To
be specific, Ueff was varied from 1 to 7 eV for the s and p orbitals of all
the atoms of CsSnCl3.

####  III.2.2 First-principles calculation of crystal structure

The structural parameters of CsSnCl3 perovskite have been determined via
first-principles calculation using GGA (Ueff = 0 eV) and GGA+U (Ueff = 1 to 7
eV) approaches. The calculated lattice constant $a=b=c$ along with the unit
cell volume for Ueff = 1 to 7 eV are presented in Table 2. For comparison, the
lattice parameters and unit cell volume obtained from Rietveld refined powder
XRD spectrum of CsSnCl3 are also included in Table 2. As can be observed, the
calculated structural parameters were found to be slightly larger than the
experimentally obtained ones at all values of Ueff except Ueff = 7 eV for
which the lattice parameter and unit cell volume deviated largely from the
experimental values. Moreover, within GGA+U approximation, the calculated
lattice parameters and unit cell volume increased with the increment of Ueff
from 1 to 7 eV. Notably, such a variation of structural parameters are
consistent with previous investigations of analogous materials. Das _et al._
(2021); Shenton _et al._ (2017)

Table 2: Lattice parameters and unit cell volume of CsSnCl3 for different values of Ueff obtained through first-principles calculation along with the corresponding experimental values. | Experimental | Ueff = | Ueff = | Ueff = | Ueff = | Ueff = | Ueff = | Ueff = | Ueff =
---|---|---|---|---|---|---|---|---|---
| value | 0 eV | 1 eV | 2 eV | 3 eV | 4 eV | 5 eV | 6 eV | 7 eV
$a=b=c$ (Å) | 5.583 | 5.613 | 5.605 | 5.605 | 5.605 | 5.625 | 5.739 | 5.890 | 6.348
Volume (Å3) | 174.02 | 176.84 | 175.89 | 175.89 | 175.89 | 177.97 | 189.02 | 204.33 | 255.80
Figure 7:  (a) Variation of theoretical absorption coefficient as a function
of wavelength for different Ueff. (b) Direct optical band gap as a function
Ueff. For Ueff = 6 eV the theoretically calculated direct optical band gap
matches well with the experimentally obtained direct band gap of $\sim$2.98 eV
(square marked).

####  III.2.3 Theoretical analysis of optical properties

We have calculated the optical absorption coefficient of CsSnCl3 perovskite by
first-principles calculation via GGA (Ueff = 0 eV) and GGA+U (Ueff = 1 to 7
eV) methods using the following equation:

$\alpha(\omega)=\sqrt[]{2}\omega\leavevmode\nobreak\
\sqrt[]{-\varepsilon_{1}\left(\omega\right)\leavevmode\nobreak\
\sqrt[]{\varepsilon_{1}^{2}\left(\omega\right)+\varepsilon_{2}^{2}\left(\omega\right)}}$
(4)

where $\alpha(\omega)$ is the absorption coefficient,
$\varepsilon_{1}\left(\omega\right)$ and $\varepsilon_{2}\left(\omega\right)$
are the frequency dependent real and imaginary parts of the dielectric
function, $\omega$ is photon frequency. Basith _et al._ (2018); Deng _et al._
(2019) The real part of the dielectric function
$\varepsilon_{1}\left(\omega\right)$ was evaluated from the imaginary part
$\varepsilon_{2}\left(\omega\right)$ by the Kramers-Kronig relationship. Peng
_et al._ (2016)

Fig. 7(a) demonstrates the calculated absorption coefficient of CsSnCl3 as a
function of wavelength for different values of Hubbard Ueff parameter. It can
be noticed that for all values of Ueff, the absorption peak of CsSnCl3 lies in
the UV region which is consistent with the absorbance spectrum obtained by UV-
visible spectroscopy [Fig. 5(a)]. Clearly, with the increase of Ueff, the
theoretically calculated absorption coefficient spectra of CsSnCl3 underwent a
blue shift. Moreover, it is worth noticing that the trend of coefficient
spectrum calculated using Ueff = 7 eV is reasonably different than the other
spectra for Ueff = 0 to 6 eV. Such observation suggests that the Ueff value
should be kept below 7 eV for the GGA+U approximation to determine the optical
properties and electronic band structure of CsSnCl3 nanocrystals.

Further, we have theoretically calculated the optical band gap of CsSnCl3
nanocrystals for different values of Ueff by employing the calculated
absorption coefficient spectra in Tauc relation. The calculated direct band
gap values are presented in Fig. 7(b) as a function of Ueff. It is worth
noting that upon varying the Ueff values from 0 to 4 eV, we observed nominal
change in the calculated band gap. Moreover, for this range of Ueff values
i.e. for Ueff = 0 to 4 eV, the calculated direct band gap remained within 1.1
eV which is significantly smaller as compared to the band gap obtained
experimentally. Interestingly, the calculated band gap value increased
significantly when we applied Ueff$>$ 4 eV. As can be seen in Fig. 7(b), the
direct optical band gap value of CsSnCl3 was calculated to be $\sim$2.69 eV
for Ueff = 6 eV which matches well with the experimental one $\sim$2.98 eV
(marked by a square in the figure). This confirms the high i.e. 90% accuracy
of band gap estimation by considering GGA+U method in first-principles
calculation. For a further increase of Ueff to 7 eV, the optical band gap
increased abruptly to $\sim$4.4 eV which is much higher than the experimental
value. This observation implies that among all the Ueff values employed for
GGA+U approximation, Ueff = 6 eV most accurately localized the orbitals of
CsSnCl3 nanocrystals.

####  III.2.4 Electronic band structure

Initially, with a view to bench-marking, we have determined the electronic
band structure of CsSnCl3 perovskite using the hybrid exchange-correlation
i.e. HSE06 functional which has been illustrated in Fig. S5 of ESI. As shown
in the figure, using this functional, the electronic band structure of CsSnCl3
was determined to be direct with a band gap value of 1.534 eV. Notably, this
outcome agrees quite well with the previously reported HSE06 functional based
electronic band structure of CsSnCl3 Körbel _et al._ (2016) and thus, validate
the simulation framework of the present investigation. However, this band gap
value (1.534 eV) is almost 45.9% smaller than the experimentally obtained
optical band gap of CsSnCl3 nanocrystals. Notably, according to a previous
investigation, Bredas (2014) the electronic band gap of any material would be
comparable with its optical band gap. Such an underestimation of the band gap
of CsSnCl3 nanocrystals reveals the severe limitation of HSE06 functional
based first-principles calculation to accurately analyze the electronic band
structure of related all-inorganic halide perovskites.

Therefore, further, we have analyzed the band structure of CsSnCl3 perovskite
using the GGA (Ueff = 0 eV) method incorporating PBE functional as
demonstrated in ESI Fig. S6(a). The dashed line between the valence and
conduction bands indicates the Fermi level. As can be seen, we obtained a
direct electronic band gap of 0.95 eV which is much smaller than the direct
optical band gap $\sim$2.98 eV of fabricated CsSnCl3 nanocrystals as obtained
by UV-visible and PL spectroscopic analyses. Hence, we may infer that the band
structure formation of CsSnCl3 was erroneous for Ueff = 0 eV. Therefore, we
have calculated the electronic band structure via GGA+U method with Ueff = 1
to 7 eV and presented the result in ESI Fig. S6(b) to S6(h). It can be clearly
seen in Fig. S6 that for all values of Ueff, both the VBM and CBM lie at the
same symmetry point R confirming the direct band structure of CsSnCl3.
However, the band gap enlarged with increasing Ueff which can be associated
with the enhanced localization of the Cl 3p and Sn 5p orbitals owing to
increased Ueff. It is also worth mentioning that the calculated electronic
band structure was semiconducting while the Ueff value remained within 6 eV.
However, for Ueff = 7 eV, the band structure became insulating which is
contradictory to the experimental result. Therefore, we may recommend an Ueff
value of 6 eV for the first-principles calculation of CsSnCl3 nanocrystals.

####  III.2.5 Effective mass

It is well known that along with optical characteristics and electronic band
structure, the photocatalytic performance of a typical semiconductor
photocatalyst is also significantly influenced by the mobility of its
photogenerated electrons and holes. Hence, to get insight into the carrier
transport property of CsSnCl3 nanocrystals, we have calculated its charge
carrier effective masses ($m^{*}$) using the following equation- Deng _et al._
(2019); Yu _et al._ (2018)

$m^{*}=\hbar^{2}\left(\frac{\mathrm{d}^{2}E}{\mathrm{d}k^{2}}\right)^{-1}$ (5)

Here, E is the band-edge energy as a function of wave-vector k and $\hbar$ is
the reduced Planck constant. The effective mass of electrons (me∗) and holes
(mh∗) were evaluated by parabolic fitting of the E-k curve within the small
region of wave-vector near the CBM and VBM, respectively. Fig. 8 illustrates
the variation of me∗ and mh∗ of CsSnCl3 nanocrystal as a function of Ueff. It
can be observed that mh∗ did not change significantly with the increase of
Ueff up to 6 eV. However, a steeper increase can be noticed in mh∗ from 2.39
mo to 4.89 mo for Ueff = 6 eV to Ueff = 7 eV which suggests a decrease in
curvature at the VBM. Similarly, in Fig. 8, we can observe that the change in
me∗ was not notable with the increase of Ueff up to 6 eV. However, the
calculated me∗ value significantly increased from 9.77 mo to 12.64 mo when we
applied Ueff = 7 eV which can be attributed to the decrease in the curvature
of the CBM. Such variation trend again justifies that it would be rational to
consider Ueff below 7 eV for CsSnCl3 perovskite .

Typically, lower effective mass of a semiconductor results in higher
transportability of photogenerated charge carriers from its bulk to surface
which can mitigate the recombination of e--h+ pairs and thus, enhance
photocatalytic activity. In order to get a deep insight into the recombination
rate within photocatalysts, a number of previous investigations Zhang _et al._
(2012, 2015) have defined the ratio of effective mass of the electron and hole
using the following formula-

$D=\frac{\mathrm{m}_{h}^{*}}{\mathrm{m}_{e}^{*}}$ (6)

Notably, the aforementioned investigations have reported that materials with
“D” values much greater than 1 demonstrate excellent photocatalytic
activities. Zhang _et al._ (2012) Interestingly, in this present
investigation, the “D” value of CsSnCl3 perovskite can be calculated as 0.25
for Ueff = 6 eV which is much smaller than 1. However, despite possessing such
a small “D” value, our as-synthesized CsSnCl3 nanocrystals have exhibited
notable photocatalytic performance without the assistance of any co-catalyst
or re-agents under both visible and UV-visible irradiation [Fig. 6(c)]. Based
on these outcomes, it can be inferred that as the me∗ value is larger than mh∗
in CsSnCl3, a “D” value much smaller than 1 indicates a large difference in
its electron and hole mobility which has facilitated the separation of
photoexcited e--h+ pairs, inhibited recombination and consequently enhanced
photocatalytic activity. Therefore, it might be worthwhile to propose a
modification for the interpretation of Eqn. [6] as follows. A “D” value either
much smaller or much larger than 1 is the indication of low recombination rate
and high photocatalytic efficiency. Here, for clarification, it should be
mentioned that since for CsSnCl3, an Ueff value of 6 eV most accurately
determined the optical band gap and electronic band structure, hence, we used
the effective masses obtained for Ueff = 6 eV to evaluate the “D” value.

Figure 8:  Variation of the absolute effective masses of holes and electrons
in terms of the electron rest mass, mo with the increase of Ueff values.

####  III.2.6 Density of states

Figure 9:  The calculated total density of states (TDOS) of CsSnCl3 and the Cs
6s, Sn 5p and Cl 3p orbitals. The panels (a)–(h) show the DOS for Ueff = 0 eV
to 7 eV respectively. The zero position is set to the Fermi energy.

The total density of states (TDOS) and partial density of states (PDOS) for
the Cs 6s, Sn 5p, and Cl 3p orbitals of CsSnCl3 perovskite have been
determined using the GGA (Ueff = 0 eV) and GGA+U (Ueff = 1 to 7 eV) methods to
resolve each individual orbital’s contributions to its electronic bands. The
TDOS and PDOS obtained for Ueff = 0 to 7 eV are illustrated in Fig. 9(a) to
(h), respectively. As can be seen in these figures, for all values of Ueff,
the valence band (E-EF $<$ 0 eV) of CsSnCl3 is made up of the hybridization of
Cl 3p and Sn 5p orbitals with the major contribution from Cl 3p states. On the
other hand, the conduction band (E-EF $>$ 0 eV) had primarily the
characteristics of Sn 5p orbital with a minor contribution from Cs 6s. It
should be mentioned that for Ueff = 0 to 4 eV, the other orbital states of
CsSnCl3 had also contributed to form the TDOSs lying at the high energy region
i.e. from 4 eV to 6 eV along with the Cs 6s, Sn 5p, and Cl 3p orbitals.
However, to keep the figures simple and understandable, we did not demonstrate
the contribution from the other orbitals. Further, as can be clearly seen in
the Fig. 9(a) to (h), with the increase of Ueff from 1 to 7 eV, the conduction
band shifted to higher energy resulting in the enlargement of band gap.
Interestingly, at Ueff = 7 eV (Fig. 9(h)), we observed that the computed PDOS
for Cl 3p orbital was significantly different than the PDOSs calculated for
other Ueff values which is another indication for the limitation of Ueff $>$ 6
eV to explain the electronic band structure of CsSnCl3 perovskite accurately.

####  III.2.7 Electron charge density

Figure 10:  Electronic charge density along (110) plane of CsSnCl3 for (a)
Ueff = 0 eV, (b) Ueff = 3 eV, (c) Ueff = 6 eV, and (d) Ueff = 7 eV.

The electron charge density distribution of CsSnCl3 perovskite may provide
valuable information about its chemical bonding characteristics. Hence, we
have determined the charge density map of CsSnCl3 along the (110) plane using
the GGA (Ueff = 0 eV) and GGA+U (Ueff = 3, 6 and 7 eV) methods and illustrated
the result in Fig. 10. It is well-known that when the electron clouds from two
atoms overlap with each other and the electrons concentrate in the overlapping
region, a covalent bond is formed between them. Phillips (1968) In Fig. 10(a)
to 10(d), a significant overlapping of electron clouds can be noticed between
Sn and Cl atoms which confirms the formation of the sigma-type Sn-Cl covalent
bond in the CsSnCl3 perovskite. Dalpian _et al._ (2017) As illustrated by the
DOS plots in Fig. 9, the charge sharing between Sn-Cl can be associated with
the hybridization of Sn 5p and Cl 3p orbitals.

For a deeper insight into the chemical bonding nature, we have performed
Mulliken population analysis and calculated the effective atomic charge, bond
population and bond length in as-synthesized CsSnCl3 crystal structure. The
results have been explained in details in the ESI. Notably, our analysis
demonstrated that the degree of bond covalency in CsSnCl3 reduces with
increasing Ueff due to the enhanced effect of on-site Coulomb interaction.

##  IV Conclusions

We have demonstrated a rapid, facile hot-injection technique for the synthesis
of lead-free non-toxic CsSnCl3 perovskite nanocrystals having high structural
stability over a wide temperature range and excellent crystallinity. The as-
synthesized CsSnCl3 perovskite exhibited photocatalytic performance without
using any reagent under both visible and UV-visible irradiation owing to their
direct band structure and favorable band edge positions. Further, density
functional theory based analysis provided deep insight into the optical
absorption property, electronic band structure, charge carrier effective
masses and electron charge density of CsSnCl3 nanocrystals at atomic level.
The outcome of our theoretical analysis revealed the significance of
considering effect of on-site Coulomb and exchange interaction energy in GGA+U
approximation. Notably, when a Hubbard Ueff correction parameter of 6 eV was
employed, the GGA+U method incorporated with Perdew-Burke-Ernzerhof gradient
functional provided results with almost 90% accuracy which is much higher as
compared to the previously reported conventional Heyd, Scuseria, and Ernzerhof
(HSE06) hybrid exchange-correlation functional showing 45.9% accuracy.
Moreover, the ratio “D” of the theoretically calculated effective masses of
the hole and electron of CsSnCl3 nanocrystals shed light into the rationale
behind their satisfactory photocatalytic performance which led us to propose a
new interpretation of “D” value. We have suggested that low photogenerated
charge-carrier recombination rate and hence, high photocatalytic activity can
be predicted for materials with “D” value much larger or smaller than 1. The
outcome of this investigation might be useful for the theoretical analysis of
related halide perovskites in which the on-site Coulomb interaction energy is
expected to play a vital role. Moreover, the demonstrated rapid hot-injection
technique might be effectively employed to synthesized analogous all-inorganic
halide perovskites with excellent structural stability and crystallinity.

## Acknowledgments

The financial assistance from the Committee for Advanced Studies and Research,
Bangladesh University of Engineering and Technology (BUET) is acknowledged.
The computational facility provided by the IICT, BUET is also acknowledged.
Sincere gratefulness to Professor Tadahiro Komeda and Dr. Ferdous Ara,
Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku
University, Japan for XPS measurements and TEM imaging.

## Data availability

The raw and processed data required to reproduce these findings cannot be
shared at this time due to technical or time limitations.

## Supplementary Information

Additional electronic supplementary information (ESI) is available. See DOI:
10.1039/D1CP02666F

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