Lead-free organic-inorganic hybrid perovskite heterojunction composites for photocatalytic applications

Article abstract of DOI:10.1039/c7cy00389g

Heterojunction structured MASnI₃/TiO₂ photocatalysts (MA represents CH₃NH₃⁺) are prepared via a facile wet-chemical method and characterized by various techniques. The characterization demonstrates the formation of a heterojunction structure and the good crystallinity of MASnI₃/TiO₂ (1:9) (mass ratio of MASnI₃ and TiO₂ is 1:9). Moreover, the results also display the improved visible light absorption of MASnI₃/TiO₂ (1:9) and low recombination of photoinduced carriers. As a result, MASnI₃/TiO₂ (1:9) could completely degrade rhodamine B (97%) in 40 min, which has higher photocatalytic ability than pure MASnI₃ and pure TiO₂. The improved photocatalytic activity could be ascribed to the improved light harvesting ability and easy transfer of photogenerated carriers of the materials generated by the formation of a heterojunction structure and the suitable energy band position between MASnI₃ and TiO₂. In particular, coupling of MASnI₃ and TiO₂ could also avoid oxidation and hydrolysis of MASnI₃ so that the long-term stability of the composite could be efficiently maintained. It is the first report on MASnI₃ and TiO₂ composites being used as photocatalysts for dye degradation. A possible mechanism for the enhancement of photocatalytic activity under light irradiation is also proposed.

Full text of DOI:10.1039/c7cy00389g

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Lead-free organic-inorganic hybrid
perovskite heterojunction composite for
photocatalytic applications
Received 00th January 20xx,
Accepted 00th January 20xx
Weining Zhanga,b,c,1, Qingguo Zhaoa,b,c,1, Xiaohong Wanga,b,c*,
Xiaoxia Yana, Jiaqiang Xua,b,c*,Zhigang Zengd
DOI: 10.1039/x0xx00000x
www.rsc.org/
The heterojunction structured MASnI3/TiO2 photocatalysts (MA represents
CH3NH3+) are prepared via a facile wet-chemical method and characterized
by various techniques. The characterizations demonstrate the formation of
heterojunction structure and well crystallinity of MASnI3/TiO2 (1:9) (Mass
ratio of MASnI3 and TiO2 is 1:9). Moreover, the results also display the
improved visible light absorption of MASnI3/TiO2 (1:9) and low
recombination of photoinduced carriers. As a result, the MASnI3/TiO2 (1:9)
could completely degrade rhodamine B (97%) in 40 min, which has higher
photocatalytic ability than pure MASnI3 and pure TiO2. The improved
photocatalytic activity could be ascribed to the high light harvesting ability
and easy transfer of photogenerated carriers of the materials generated by
the formation of heterojunction structure and the suitable energy band
position between MASnI3 and TiO2. Specially, coupling of MASnI3 and TiO2
could also avoid oxidation and hydrolysis of MASnI3 so that it could keep the
long-term stability of the composite efficiently. It is the first report that
MASnI3 and TiO2 composites are used as photocatalysts for dye degradation.
A possible mechanism for the enhancement of photocatalytic activity under
light irradiation is also proposed.
Introduction
The environmental pollutions and energy demands of society
have become the serious problems all over the world 1-5. Under this
circumstance, the semiconductor-based photocatalysis technology
has attracted much attention with many fields such as degradation
6-8
of organic pollutants, environmental remediation and solar cells
.
The application of organic-inorganic hybrid perovskite MAMX3
+
(MA=CH3NH3 , M=Pb, Sn et al., X=Cl, Br, I et al.) as light harvesters
and photogenerated carrier-transport layers have facilitated the
development of emerging photovoltaic technologies in recent years
9-13. For organic-inorganic lead halide materials, they achieve high
solar photovoltaic conversion efficiency over 20% and become one
14,
of the most promising photovoltaic devices
15. However, the
toxicity of lead is a serious defect, which limits their commercial
applications to a great degree. As the same IV group element, tin is
likely
a perfect substitute with nontoxicity. As a direct-gap
semiconductor, MASnI3 has an energy band gap of 1.3 eV, and it has
a wide range of optical absorption when applied in photocatalysis
16-18
because of the narrow band gap
. However, the high
photocatalytic activity can’t achieve perhaps due to the fast
recombination of photogenerated electron-hole pairs with the low
band gap during photocatalytic process 19. In terms of materials
design, coupling with a wide band gap semiconductor with a more
positive conduction band level to form heterojunction structure has
been pointed to be feasible ways to allow the inhibition of
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recombination of electron-hole pairs and thus further improve the
Synthesis of MASnI3 perovskite materials. The materials were
synthesized by a solution method according the report of Mercouri
G. Kanatzidis et al. 13. Typically, 8.00 mL HI aqueous and 1.00 mL
H3PO2 aqueous were mixed into a 100 mL three-necked round
bottom flask. The mixture was degassed under the nitrogen
condition for 1 min and maintaining it under a nitrogen atmosphere
during all processes. 1 mmol SnI2 was added into the solution after
heating to 120 ˚C by an oil bath with continuous magnetic stirring,
the color of the solution became bright yellow. Then 1mmol MAI
was introduced into the hot solution and the stirring was stopped
until the solution volatilizing to half of its original volume. When the
system was cooled to ambience temperature, black, elongated
crystals precipitate from the mixed solution. Finally, before
filtration and rinse by degassed ethanol, the crystals should grow in
the original liquid for 24 h under N2 circumstance.
photocatalytic property 9, 20-27
.
In this work, a novel heterojunction MASnI3/TiO2 composites are
synthesized by simple and facile wet-chemical method
a
successfully. On the one hand, the suitable energy band position
and well contact interface favor the smooth transfer of
photogenerated carriers between MASnI3 and TiO2, which
effectively separates the electron-hole pairs and consequently
enhances the relatively higher photocatalytic activity in the
degradation of rhodamine B (RhB). On the other hand, the
improved light harvesting ability of the heterostructure materials is
another factor for higher photocatalytic activity. Specially, coupling
of MASnI3 and TiO2 could avoid oxidation and hydrolysis of MASnI3
in atmosphere so that it keeps the long-term stability of
heterostructure material efficiently.
Synthesis of MASnI3/TiO2 heterojunction structured materials.
The MASnI3/TiO2 ptotocatalysts were prepared by a facile wet-
chemical process. Typically, 50 mL hexane and TBOT were added
into a 100 mL three-necked flask, then the as-preparaed MASnI3
were poured into a mixture of hexane and TBOT. Next, the system
was heated at 70 ◦C for 8 h in nitrogen atmosphere under
continuous stirring, followed by centrifugation and further rinsed by
absolute ethanol to remove remained TBOT. After that, the
deionized water was introduced into the system for hydrolysis and
subsequently the acquired precipitates were transferred into
vacuum oven dried at 80 ◦C for 24 h. Finally, the samples were
calcined at 500 ◦C for 2 h in muffle furance. The obtained
Experimental Detials
Chemicals
I2 solids (purity of 99.8%), Sn granules, HI aqueous solutions (45%
in water), H3PO2 aqueous solutions (50% in water), hexane
solutions, absolute ethanol, HCl aqueous solutions (2mol/L) and
CH3NH2 solutions (30% in ethanol) were purchased by Sinopharm
chemical regent Co., Ltd (Shanghai, China). And tetrabutyl titanate
was purchased by Sigma chemical regent Co., Ltd. All the chemicals
were analytical grade and used as received without any purification.
The deionized water for all processes was purified followed by ion
exchange and distillation.
composites were defined as MASnI3/TiO2 (x:y), where
x : y
representing the mass ratio of MASnI3 and TiO2. Futhermore, pure
TiO2 materials were also prepared using the same method.
Catalysts preparation
Preparation of CH3NH3I (MAI). MAI was synthesized by
a
modified method of Michael Gratzel et al. 23. 33.57 mL CH3NH2
solution and 48.50 mL HI solution were mixed in a 250 mL round-
bottom flask and stirred at 0 °C for 2 h. The precipitate was
recovered according to rotary evaporator to remove the solvents at
75 °C. The product was purified by recrystallizing from a mixed
solvent of diethyl ether and ethanol, and the final
methylammonium iodide (MAI) was dried at 60 °C in a vacuum oven
for 24 h.
Photocatalytic activity measurements
The method to measure the properties of photocatalyst could be
found in publication of Yan et al. 28. The photocatalytic degradation
of RhB was carried out under visible light irradiation from 300 W
xenon lamp (PLS-SXE 300, Beijing Perfectlight Co., Ltd.) with a cutoff
filter that restrict the illumination in a range of 400–800 nm, and
300 W xenon lamp without cutoff fiter was used to provide the
illumination in a range of 200-800 nm. Briefly, 80 mg catalysts were
added into 80 mL of RhB solution (10 ppm). Prior to illumination,
the suspension was magnetically stirred in the dark for 30 min to
establish an adsorption-desorption equilibrium. At certain
irradiation time intervals, a 3 mL liquid was taken, centrifuge and
analyze on UV-vis spectrophotometer. The degradation percentage
is reported as C/C0, where C0 is the absorption intensity of RhB after
adsorption equilibrium and C is the absorption intensity of RhB for
each irradiated time interval at wavelength 552 nm. After each
photoreaction, the catalyst was filtered, rinsed and dried at 80 °C
for 4 h. Then the retrieved sample was employed to degrade a new
10 ppm RhB aqueous for stability test under the same condition.
During the photocatalysis process, the active species were trapped
by adding 10 mmol scavenger, including benzoquinone (BQ),
Synthesis of SnI2. The synthesis method of SnI2 in this study could
13
be found in the publication of Mercouri G. Kanatzidis et al.
.
Typically, 5g granulated Sn and 30 mL of 2mol/L aqueous HCl were
added into a three-necked 100mL bottom round flask, and the
mixed solution was flushed with nitrogen gas (N2) for 1-2 min. The
mixture turned to dark brown after adding of 7g I2. The system was
heated to 170 ˚C in an oil bath and condensed at the same time. It
was noted that the rate of N2 flow should be minimum to avoid I2
fumes to escape. 10 mL of 2mol/L HCl was added to transfer the
remained I2 back into the flask when I2 vapors reacted absolutely.
After 1 h, the solution color turned yellow, 0.1g Sn was added to
the system within 15 min to react with any traces of I2 present.
When the solution color turned to bright yellow and the added Sn
retained its lustre, the reaction was defined to be complete. The
bright yellow solution was transferred to a 100 mL three-necked
flask, which was heated to 70 ˚C under a flow of N2 with a water
bath. The hot solution was slowly cooled to room temperature and
stood overnight, the large red needles of SnI2 formed. The dried
samples have a color of bright red and could store for several weeks
under dry ambience.
−
AgNO3, t-BuOH, ammonium oxalate (AO), which could trap •O2
radicals, e- species, •OH radicals and h+ species, respectively 29
.
electrochemical impedance spectra (EIS) measurement
The EIS measurement was measured on an electrochemical
analyzer (CHI660D, Chenhua, China) with a homemade standard
three-electrode cell. The prepared photocatalyst/ITO electrodes,
platinum foil and saturated calomel electrode (SCE) were used as
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the working electrode, counter electrode and reference electrode, demonstrates that MASnI3 could prohibit the preferential
respectively. The working electrodes were prepared by drop- crystallization of TiO2. Among all the samples, the XRD peaks of
coating sample suspensions onto an indium-tin oxide (ITO) glass (2 MASnI3 are weaked possibly due to the low crystallinity of MASnI3
× 2 cm) surface and then dried at 100 ◦C for 2 h in vacuum oven. that induced by the presence of TiO2.
The electrolyte was 0.1 mol/L Na2SO4 aqueous solution. A 300 W
In oder to test the stability of as-prepared materials during
calcination, we present the XRD patterns of pure MASnI3 and
MASnI3/TiO2 (1:9) before and after calcinations. As shown in Fig.1b,
the pure CH3NH3SnI3 can be decomposed under the condition of
500◦C for 2 h. As for the decomposition product of pure MASnI3, the
peaks at 2θ = 26.61o, 33.89 o, 37.95 o, 51.78 o, 54.76 o, 65.94 o could
be assigned to (110), (101), (200), (211), (220), (301) facets of SnO2
(JCPDS No. 41-1445) respectively. It means that Sn2+ is oxidized to
Sn4+ during the calcination process. However, for MASnI3/TiO2 (1:9)
composite, there is no appearance of new phase after calcination. It
shows the higher stability than pure MASnI3 possibly due to the
formation of heterojunction structure.
Xenon lamp was used for excitation.
Characterizations
X-ray diffraction (XRD) patterns were collected on a DLMAX-
2200 instrument. Diffraction patterns were recorded with Cu Kα
radiation (40 mA, 40 kV, λ=0.154056 nm) over a 2θ-range of 5° to
90°. The high-resolution transmission electron microscopy (HRTEM)
images were obtained using
a JEM-2010F high-resolution
transmission electron microscope operating at 200 kV. The
morphology of the samples was observed with a field emission
scanning electron microscope (FESEM, Zeiss, ULTRA55). The diffuse
reflection spectra (DRS) and photocatalytic performance test were
recorded using
a
UV-vis spectrophotometer (UV-2501PC).
Photoluminescence (PL) spectra were obtained through a RF-5301
fluorescence spectrometer with the excitation wavelength at 300
nm. The X-ray photoelectron spectroscopy (XPS) characterization
was performed using an ESCALAB 250XI spectrophotometer (Al Kα
radiation, hν = 1486.6 eV) with a pass energy of 30 eV. All the
binding energy values were referred to the surface adventitious
carbon (C1s) at 284.8 eV. The Fourier transform infrared
spectroscopy (FTIR) of samples was acquired with Nicoler IS10 FTIR
spectrometer. The electrical properties of MASnI3/TiO2TcMomposites
were measured with CAFM (Dimension Edge
Corporation) operating in contact mode.
, Bruker
Results and discussions
Structural, morphology and surface composition characterization
Figure 1. a. XRD patterns of the samples with different mass ratios
of MASnI3 and TiO2. b. XRD patterns of pure MASnI3 and
MASnI3/TiO2 (1:9) before and after calcinations.
Fig.1a shows the XRD spectra of the samples with different mass
ratios of MASnI3 and TiO2. As observed on the patterns of
MASnI3/TiO2 (1:9), the peaks at 2θ = 25.21o, 37.91 o, 48.31o, 53.91 o,
55.01 o, 62.71 o could be attribute to anatase TiO2 (JCPDS No. 21-
1272) 4. No diffraction peaks assigned to MASnI3 are recogonized,
possibly because of the low content of MASnI3 in MASnI3/TiO2 (1:9)
and low crystallinity. The strongest diffraction is located at the (101)
facets, which demonstrates a preferential growth of TiO2 along the
(101) direction in the composite. With the increase of MASnI3
content, the broad XRD peaks appeared in MASnI3/TiO2 (5:5) and
MASnI3/TiO2(9:1) and the intensities of all peaks are decreased at
the same time, especially for (101) facets of TiO2 , which indicates
the gradual disappearance of preferential growth and the low
crystallinity of MASnI3/TiO2(5:5) and MASnI3/TiO2(9:1). It
Figure 2. SEM, TEM and HRTEM images of MASnI3/TiO2 (1:9)(a),
MASnI3/TiO2 (5:5)(b) and MASnI3/TiO2 (9:1)(c) and the TEM-EDX
patterns(d), diameter distribution patterns(e), FTIR spectra(f) of
MASnI3/TiO2 (1:9).
To investigate the morphological features of as-prepared
materials, SEM and TEM measurements are carried out. Fig.2a-0, b-
0 and c-0 show the SEM images of MASnI3/TiO2 (1:9), MASnI3/TiO2
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(5:5) and MASnI3/TiO2 (9:1) composites. As shown in Fig.2a-0, the formation of p-n heterojunction at the interface of MASnI3 and
MASnI3/TiO2 (1:9) samples have a granular morphology and the TiO2. In addition, the MASnI3/TiO2 (1:9) material has better
particles are almost mono dispersed in size of hundreds of rectifying property compared to MASnI3/TiO2 (5:5) and MASnI3/TiO2
nanometers, and the diameter distribution of the particles (9:1) composites, it can be explained that MASnI3/TiO2 (1:9)
concentrate mainly on the range of 500-700 nm (Fig.2e). With the composite has a better contacted interface than MASnI3/TiO2 (5:5)
weight ratios of MASnI3 increasing, the aggregation of the particles and MASnI3/TiO2 (9:1) samples, which benefits the transfer of
occurred (Fig.2b-0 and Fig.2c-0), and the particles gradually grow to photoinduced carriers between MASnI3 and TiO2, thus further
irregular shapes. TEM images shown in Fig.2a-1, b-1 and c-1 suppressing the recombination of electron-hole pairs.
displayed the well dispersion and smaller size of MASnI3/TiO2 (1:9)
compared to MASnI3/TiO2 (5:5) and MASnI3/TiO2 (9:1), which
consistents with the results of SEM images.
In order to confirm the trends displayed in XRD patterns, HRTEM
images are mentioned. As shown in Fig.2a-2, two different lattice
fringes can be dramaticlly visualized over MASnI3/TiO2 (1:9). The
clear interplanar spacing of ca. 0.35 nm is ascribed to (101) plane of
TiO2 nanoparticle, which is the most thermodynamically stable facet
that used as the storage of electrons30-32. For another lattice fringe,
the lattice spacing has the value of ca. 0.31 nm, which is derived
from (200) plane of MASnI333. A interface also forms and contacting
each other preferably between MASnI3 and TiO2 in Fig.2a-2, which
benefits the transfer of photogenerated carriers between MASnI3
and TiO2, thus inhibiting the recombination of electron-hole pairs.
The lattice fringes of MASnI3/TiO2 (5:5) and MASnI3/TiO2 (9:1)
become fuzzy shown in Fig.2b-2 and Fig.2c-2, the trend is
particularly apparent with the increased weight ratios of MASnI3,
which agrees the XRD results shown in Fig.1.
In addition, the FTIR spectra of MASnI3/TiO2 (1:9) is also shown in
Fig.2f. Two peaks emerged at ca. 3428 cm-1 and 1078 cm-1 in the
spectrum are accordance with the stretching vibration of N-H and
C-N bond respectively. The Peaks around 1590 cm-1 and 1650 cm-1
could be assigned to the scissoring vibration of CH3NH3+. The peak
at ca. 1200 cm-1 should be ascribed to C-O bond. The peaks at range
4, 34, 35
of 500~700 cm-1 could be assigned to the absorption of TiO2
All these information confirmed the co-existence of MASnI3 and
TiO2. Meanwhile, the EDX spectroscopy also shows the existence of
Ti, O, C and Sn elements in Fig.2d, which further confirmed the co-
existence of MASnI3 and TiO2.
Figure 3. Rectifying current vs voltage
I-V characteristic of
MASnI3/TiO2 composites at room temperature.
Figure 4. Nitrogen adsorption isotherms and pore size distribution
of MASnI3/TiO2 composites (inset in fig 4 showed the pore size
As shown in Fig.3, the MASnI3/TiO2 composites display rectifying
characteristics with CAFM measurement, which indicates the
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distribution of composites) : a. MASnI3/TiO2 (1:9) composite; b.
MASnI3/TiO2 (5:5) composite; c. MASnI3/TiO2 (9:1) composite.
In order to investigate the surface areas and pore structure of the
composites, the nitrogen adsorption-desorption isotherms are
measured. As shown in Fig.4, all of composites display type IV
isotherms, which demonstrates the presence of mesopores and the
mesopores could be assigned to interstitial pores between
nanoparticles. This could be confirmed by Barret–Joyner–Halenda
(BJH) pore size distribution. For MASnI3/TiO2 (1:9) material, the pore
size distribution mainly on 4.5 nm, while another two materials
have the wide pore distribution. In addition, the Brunauer–
Emmett–Teller (BET) surface areas of MASnI3/TiO2 (1:9),
MASnI3/TiO2 (5:5), MASnI3/TiO2 (9:1) are 108.96 m2/g, 53.47 m2/g,
10.95 m2/g, respectively. The higher surface area of MASnI3/TiO2
(1:9) material will supply larger reaction field and more active sites
for photocatalytic reactions.
Figure 7. High-resolution XPS spectra of Sn 3d for MASnI3/TiO2 (1:9)
composite (a) and pure MASnI3 (b).
To study the surface composition of MASnI3/TiO2 (1:9), XPS
analysis is implemented. Fig.5 shows the XPS survey spectrum of
MASnI3/TiO2 (1:9), which indicates the existence of C, N, Sn, I, Ti and
O elements in the composite. The chemical states of MASnI3/TiO2
(1:9) was analysed by High-resolution XPS. In Fig.6a, the peak at
284.8eV might be ascribed to adventitious carbon from XPS
instrument itself, and other two peaks at 286.3eV and 289.1eV are
assigned to C-O or O-C-O bond, which agrees the FTIR observation
in Fig.2f, indicating the formation of Ti-O-C bond between MASnI3
and TiO2 components 36, thus forming the interface that contacting
closely and promoting the smooth transfer of photogenerated
carriers on the as-formed interface.
The expanded pattern of Ti 2p and O 1s are shown in Fig.6b and
Fig.6c. TiO2 shows two peaks at 459.1eV and 464.9eV, which
consistent with the Ti 2p3/2 and Ti 2p1/2 of Ti4+, respectively. Fig.5c
shows the O1s signal, it displays two peaks around 530.5eV and
532.1eV due to the Ti-O bond and Ti-OH bond, respectively 37
.
Sn2+ is easily oxidized to Sn4+ in atomosphere, and the stability of
Sn2+ also becomes the challenge when applied in practice. As shown
in Fig.7a, the Sn 3d5/2 and Sn 3d3/2 are located at ca. 487.0eV and ca.
495.5eV, respectively, which accordance with 3d orbital of Sn2+ 38. It
demonstrates that the stability of MASnI3 in atomosphere has been
improved by combining with TiO2. To confirm this conclusion, the
Sn 3d of pure MASnI3 also be given (shown in Fig.7b). The Sn 3d3/2
and Sn 3d5/2 are located at 494.8eV and 486.3eV, respectively,
Figure 5. XPS survey spectrum of MASnI3/TiO2 (1:9) composite.
which indicates that Sn2+ is oxidized to Sn4+ after calcination39
.
photoelectric properties of materials
Figure 6. High-resolution XPS spectra of C 1s (a), Ti 2p (b), O 1s (c)
and Sn 3d (d) for MASnI3/TiO2 (1:9) composite.
Figure 8. DRS spectra of pure TiO2, pure MASnI3, MASnI3/TiO2(1:9),
MASnI3/TiO2(5:5) and MASnI3/TiO2(9:1) samples.
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As shown in Fig.8, the heterojunction structured materials with lower recombination rate of electron-hole pairs. As shown in Fig.9c
different mass ratios of MASnI3 and TiO2 present different optical and Fig.9d, the MASnI3/TiO2 (1:9) and MASnI3/TiO2 (5:5) samples
properties in range of 200-800 nm. Pure TiO2 (Fig.8c) shows light show the lower PL signals compared to pure TiO2, indicating that
absorption at ultraviolet region mainly due to the intrinsic band gap the formation of heterojunction structure promote the carriers
of TiO2. The intense absorption with wavelengths below 400nm transfer. Appropriate MASnI3 could be used as capture centre to
because of the electronic promotion from O 1s valence band to Ti trap holes from TiO2, and thus improve the separation ability of
2p conduction band 40. As displayed in Fig.8e, pure MASnI3 samples photoinduced carriers and future inhibit the recombination of
have a wide range of optical absorption, which could be attributed electron-hole pairs. Moreover, MASnI3/TiO2 (1:9) samples have
to the narrow band gap. However, the absorption intensities are minimum signals possibly due to the well crystallinity (shown in
slightly lower perhaps due to the fast recombination of Fig.1), which decrease the density of surface defects and oxygen
photoinduced electron-hole pairs. Fig.8a and Fig.8b show the light vacancies to some extent, and therefore the samples may have
absorption of MASnI3/TiO2 (1:9) and MASnI3/TiO2 (5:5) samples, higher photocatalytic activity for target reaction. However, the
which demonstrate distinct improvement of the visible light MASnI3/TiO2(9:1) samples display higher PL intensity than pure TiO2
absorption in range of 400~800 nm. The formation of Ti-O-C in Fig.9a, which demonstrates the excessive MASnI3 could act as
structure and the special heterojunction structure can narrow the recombination centre of electron-hole pairs to prohibit the charge
41
band gap of TiO2 and enlarge the photoresponse to visible light
.
carriers separation.
Nevertheless, the light absorption of MASnI3/TiO2 (9:1) samples
(Fig.8d) display the lower absorption in visible region possibly due
to the poor crystallinity and immature hetercojuntion structure,
indicating that excess MASnI3 may inhibit the formation of
heterojunction structure and reduce the crystallinity of composite,
which agree with the results of XRD and TEM images in Fig.1 and
Fig.2.
Figure 10. Electrochemical impedance spectroscopy (EIS) of
MASnI3/TiO2(1:9), MASnI3/TiO2(5:5), MASnI3/TiO2(9:1) and pure
MASnI3 samples. a. EIS of different composites in the dark. b.
Enlarged images of Fig a-1 and Fig a-2. c. EIS of different composites
under xenon light irradiation. d. Enlarged images of Fig c-1 and c-2.
As an effective technology, the electrochemical impedance
spectroscopy (EIS) is usually used to study the transfer of interfacial
charges in composite. Thus, the EIS images of different composites
are investigated in the dark and under xenon light irradiation. As
shown in Fig.10a and Fig. 10b, the smallest diameter of arc of
MASnI3/TiO2 (1:9) materials demonstrates the fast charge transport
at contact interface between MASnI3 and TiO2, and the results
further prove the formation of heterojunction structure and good
interaction between MASnI3 and TiO2. When switch on the Xenon
lamp, MASnI3/TiO2 (1:9) composites also have the smallest
semicircle of arc (shown in Fig. 10c-1) compared to other samples
(shown in Fig. 10c-2, 10c-3, 10c-4 respectively), which indicates the
efficient separation of photoinduced electron-hole pairs and faster
carriers transfer at interface over MASnI3/TiO2(1:9) composites.
Figure 9. Photoluminescence (PL) spectra of pure TiO2,
MASnI3/TiO2(1:9), MASnI3/TiO2(5:5) and MASnI3/TiO2(9:1) using
excitation of 300 nm.
As a non-destructive technology, PL spectra has been employed
widely in photocatalysis field to study the surface defect, carrier
transfer and photochemical properties of semiconductor
photocatalysts. Most of all we could understand the recombination
and seperation efficiency of photoinduced electron-hole pairs. Fig.9
shows the PL spectra of MASnI3/TiO2 materials with different mass
ratios, and the excitation wavelength of PL analysis is 300 nm. It can
be seen that PL spectra of all samples are similar, and four emission
peaks at 451nm, 468nm, 483nm and 492nm appear in Fig.9. They
could be attributed to the surface defects and oxygen vacancies.
The peaks around 451nm and 468nm are assigned to band edge
free excitons, another two peaks located at 483nm and 492nm are
Moreover, it could be seen that the semicircles arc of
MASnI3/TiO2 (5:5) and MASnI3/TiO2 (9:1) are bigger than pure
MASnI3 possibly due to the inhibiting interaction between MASnI3
and TiO2 with excessive MASnI3, thus prohibiting the carriers
transfer at contact interface, which accordance with the
conclusions of XRD and TEM images in Fig.1 and Fig.2.
ascribed to bound excitons 42, 43
.
In addition, the different samples have different PL intensities.
The lower signal might reveal higher separation efficiency and
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photocatalytic activity and stability
0.0084 and 0.0066 min-1 respectively, which indicates the reaction
rate of MASnI3/TiO2 (1:9) is fastest compared to other samples.
In general, the performance of pure TiO2 decreased significantly
under visible light irradiation. When add MASnI3 into TiO2, the
performance of composites increased immediately, and the
MASnI3/TiO2 (1:9) materials have the best property. With the
increase of MASnI3 content, the photocatalytic activity of
composites gradually decreased, and even lower than pure MASnI3
over MASnI3/TiO2 (9:1) materials, it can be explained that the
coupling of two components play the main role in photocatalytic
process and it also has the optimal proportion between MASnI3 and
TiO2.
Figure 11. Photcatalytic performace of pure TiO2, pure MASnI3,
MASnI3/TiO2 (1:9), MASnI3/TiO2 (5:5) and MASnI3/TiO2 (9:1) samples.
a. Phtocatalytic activity of different samples under UV and visible
light. b. Phtocatalytic activity of different samples under Visible
light. c. Corresponding dependence of ln(C/C0) on irradiation time.
The photocatalytic activities of different photocatalysts are
investigated by degradation of RhB under light irradiation at range
of 200-800 nm and 400-800 nm, respectively. Fig.11a shows the
residue RhB percentage over different samples under UV and visible
light. It can be seen that MASnI3/TiO2 (1:9) composites have the
best photocatalytic activity for degradation of RhB under light
irradiation in Fig.11a-6 in contrast with MASnI3/TiO2 (5:5) and
MASnI3/TiO2 (9:1) composites (shown in Fig.11a-4 and Fig.11a-3
respectively). The residual RhB percentage declines to about 3%
after 40 min illumination, which illustrates an approximately
completed degradation of RhB on MASnI3/TiO2 (1:9) samples.
However, the degradation of RhB is lower than 10% without
catalysts. Thus, the results illustrate that the degradation of RhB
increases significantly in the presence of catalysts because of
photodegradation only instead of the photosensitization of RhB44,
Figure 12. a. Stability test of MASnI3/TiO2 (1:9) sample; b. XRD
patterns of MASnI3/TiO2 (1:9) sample before and after recycle
performance test; c. High-resolution XPS spectra of Sn 3d for used
MASnI3/TiO2 (1:9) composite.
45
.
As shown in Fig.11b, When visible light irradiate, the
photocatalytic activity of all samples over RhB are reduced,
especially for TiO2 and MASnI3/TiO2 (9:1) composites (shown in
Fig.11b-2 and Fig.11b-3 respectively). It can be explained that they
have the poor absorption of visible light, which agree with the DRS
results in Fig.8. Compared with other samples, MASnI3/TiO2 (1:9)
materials have higher photocatalytic activity possibly due to the
lower recombination rate of electron-hole pairs and smooth
carriers transfer with heterojunction structure. In addition, the
improved light absorption and enlarged photoresponse in visible
region (shown in Fig.8) are another factors to facilitate the
photocatalysis of MASnI3/TiO2 (1:9) materials. Meanwhile, the
relationship of ln(C/C0) and irradiation time under visible light was
also studied. The results in Fig.11c demonstrate that the reactions
are accordance with the pseudo-first-order kinetics, and other
samples also follow it, just not shown in Fig.11c. The rate constants
k over MASnI3/TiO2 (1:9), pure TiO2 and pure MASnI3 are 0.0456,
Figure 13. a. SEM image of used MASnI3/TiO2 (1:9) sample (inset in
fig a showed the SEM image of fresh MASnI3/TiO2 (1:9) sample); b.
The elemental mapping of MASnI3/TiO2 (1:9) sample after
utilization.
In order to investigate the stability of MASnI3/TiO2(1:9)
composite, recycle performance tests are carried out. As shown in
Fig.12a, no obvious reduce of photocatalytic activity is observed
after six runs under visible light irradiation, which demonstrates the
excellent stability of MASnI3/TiO2 (1:9) composite during the
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photodegradation process. At the same time, the morphology of degradation is partial inhibited, and they have small contribution in
−
used MASnI3/TiO2 (1:9) composite was also displayed in Fig.13a. It current photodegradation process. As a result, •O2 radicals is the
can be seen that no distinct variation of structure is observed. dominant species in the degradation of RhB, electrons have the
Fig.13b shows the elemental mapping of MASnI3/TiO2 (1:9), which secondary position in this system, holes and •OH radicals have
indicates the distribution of Sn, I, O, Ti, N and C elements are minor extent.
relatively homogeneous in MASnI3/TiO2 (1:9) composite after
utilization.
In order to further confirm the stability of MASnI3/TiO2 (1:9)
material, the chemical states of used MASnI3/TiO2 (1:9) is analysed
by High-resolution XPS. As shown in Fig.12c, the Sn 3d5/2 and Sn
3d3/2 are located at ca. 487.1eV and ca. 495.6eV, respectively, which
accordance with 3d orbital of Sn2+. This means that Sn2+ is not
oxidized to Sn4+ after utilization, indicating the good stability of
MASnI3/TiO2 (1:9) composite during photocatalytic process. In
addition, we also study the XRD patterns of MASnI3/TiO2 (1:9)
sample before and after recycle performance test shown in Fig.12b.
On the one hand, it is obvious that the used composite still have
strong peak intensities and well crystallinity compared to fresh
sample, indicating the excellent stability of MASnI3/TiO2 (1:9)
composites with heterojunction structure. On the other hand, there
are two emerging peaks at 2θ = 27.61 o, 41.10 o that seem coincide
with the (200) and (220) planes of MASnI3, which can be explained
as the recrystallization of MASnI3 during photocatalytic process and
it partly account for the high stability.
Figure 15. Scheme of photoexcitation of the MASnI3/TiO2 (1:9)
heterojunction composite and the subsequent photogenerated
carriers transfer process.
The mechanism for enhanced photocatalytic activity of the
heterojunction structured materials.
According to above investigation, a reasonable mechanism for
enhanced photocatalytic activity over MASnI3/TiO2 (1:9) composite
has been proposed. As illustrated in Fig.15, the electrons of MASnI3
are excited from the valence band (VB) to conduction band (CB),
thus producing the photogenerated electron-hole pairs. At the
same time, a small number of electrons of TiO2 are also excited due
to the suitable energy band position16 in the heterojunction
structure to some extent. The formation of p-n heterojunction
structure could further promote the transfer of carriers at contact
interface, therefore photogenerated holes from the VB of TiO2
could easily transfer to the VB of MASnI3, electrons from the CB of
MASnI3 would transfer to the CB of TiO2 with the most
46-48
thermodynamically stable (101) planes
smoothly. In addition,
the bottom of the conduction band (BCB) potential of TiO2
calculated by Mulliken electronegativity definition ( Equation (1) )49
:
EBCB = χ - Ee – 0.5Eg
(1)
Figure 14. The photocatalytic performance of MASnI3/TiO2 (1:9)
sample with different scavengers under visible light irradiation.
where χ is the electronegativity of semiconductor, Ee is the energy
of free electrons on the hydrogen scale (ca. 4.5 eV). According to
Equation (1), the EBCB of TiO2 is -0.29V vs. NHE at pH = 0, which is
more negative than EO2/•O2− (−0.046V vs. NHE at pH = 0) 48, thusthe
photoinduced electrons in CB of TiO2 could reduce O2 to •O2−. Last
but not least, it could be regarded as a repository for (101) facets to
reserve the photogenerated electrons and it could inhibit the
recombination of electron-hole pairs, resulting longer lifetime of
carriers. The separated electrons and holes would participate in the
production of active species, and therefore enhance the
photocatalytic activity for reaction of pollutants.
The formation of p-n junction is proved in Fig.3, the
heterojunction structure promote the transfer of photogenerated
carriers, which could further facilitate the production of radicals
and the redox reactions. In order to study the active species during
the photodegradation process, the experiment of trapping radicals
is carried out. Fig.14 shows the different quenchers on
photocatalytic activities of MASnI3/TiO2 (1:9) composite over the
degradation of RhB. It can be seen that the photocatalytic activity is
immediately decreased by trapping •O2− with 10 mmol BQ, which
−
demonstrates that •O2 is the main active specie in current
photocatalytic process. After adding AgNO3, OA, t-BuOH to trap e-,
h+, •OH, respectively, it could be observed that the rate of RhB
Conclusions
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In summary, the heterojunction MASnI3/TiO2 composites are
successfully synthesized via a wet-chemical method. To the best of 18.
our knowledge, it could be the first report that MASnI3 applied in
photocatalytic area. Under light illumination, MASnI3/TiO2 materials
show a higher photocatalytic activity in the degradation of RhB than
pure MASnI3 and pure TiO2 due to the improved optical absorption 19.
and the reduced recombination of photogenerated electron-hole
pairs. Last but not least, the couple of the two components increase
the stability of heterostructured composite and avoid the hydrolysis
and oxidation of MASnI3 in atmosphere, which could be supposed 20.
to make full use of solar energy to practice in environmental
applications. Futhermore, it provides guidelines for organic-
inorganic halide perovskite solar cells to solve the instability of 21.
devices in atmosphere.
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Acknowledgements
This research is supported by National Natural Science Foundation
of China (21103104, 61671284). We also acknowledge the
Instrumental Analysis and Research Center of Shanghai University
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