Hollow Sr/Rh-codoped TiO2 photocatalyst for efficient sunlight-driven organic compound degradation

Article abstract of DOI:10.1039/c6ra25987a

Sunlight-driven photocatalysis has emerged as a potential technology to address organic pollutant issues. Here, we report the first Rh-doped hollow-structured TiO₂ photocatalyst, which is highly active in the photocatalytic decomposition of organic pollutants under solar light. We achieved this by introducing Sr²⁺ as a co-doping agent, which stabilized the hollow structure at high temperatures and enabled us to control the oxidation state of Rh. The designed photocatalyst exhibited strong visible light absorption (up to 600 nm), and a very high surface area (up to 140 m² g^?1). As a result, the Sr/Rh-doped TiO₂ hollow photocatalysts demonstrated a photocatalytic efficiency (PE) of 0.242%, which was at least 8 times higher than that of commercial TiO₂ (0.03%) and 25 times higher than that of bulk Sr/Rh-TiO₂ (0.01%), in the photocatalytic decomposition of isopropanol under solar light irradiation.

Full text of DOI:10.1039/c6ra25987a

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Hollow Sr/Rh-codoped TiO photocatalyst for
2
efficient sunlight-driven organic compound
degradation†
Cite this: RSC Adv., 2017, 7, 3480
a
b
a
Chinh-Chien Nguyen, Cao-Thang Dinh and Trong-On Do*
Sunlight-driven photocatalysis has emerged as a potential technology to address organic pollutant issues.
Here, we report the first Rh-doped hollow-structured TiO photocatalyst, which is highly active in the
photocatalytic decomposition of organic pollutants under solar light. We achieved this by introducing
2
2+
Sr as a co-doping agent, which stabilized the hollow structure at high temperatures and enabled us to
control the oxidation state of Rh. The designed photocatalyst exhibited strong visible light absorption (up
Received 28th October 2016
Accepted 23rd November 2016
2
ꢀ1
). As a result, the Sr/Rh-doped TiO
to 600 nm), and a very high surface area (up to 140 m
photocatalysts demonstrated a photocatalytic efficiency (PE) of 0.242%, which was at least 8 times
higher than that of commercial TiO (0.03%) and 25 times higher than that of bulk Sr/Rh–TiO (0.01%), in
the photocatalytic decomposition of isopropanol under solar light irradiation.
g
2
hollow
DOI: 10.1039/c6ra25987a
2
2
www.rsc.org/advances
numerous attempts to overcome the poor light absorption of
Introduction
TiO
2
under sunlight by methods including the sensitization of
As a consequence of rapid global economic growth, environ- TiO
mental pollution by organic compounds has become one of the coupling TiO
2
with dyes or small bandgap semiconductors, doping, and
7
2
with plasmonic nanostructures. Among them,
most critical issues for human society over the past four doping TiO
decades. Photocatalytic decomposition of organic species in air enhancement of visible light absorption.
and water offers an efficient solution to solve this problem introduce sub-bands into the forbidden band of TiO . Rh
with Rh has emerged as a viable technique for the
2
1
8–12
3
+
4+
Both Rh and Rh
3
+
2
through the utilization of solar energy and high performance contributes a donor level to the valence band, thus reducing the
2
photocatalysts. Among the various photocatalysts developed to band energy and shiing light absorption to the visible region,
4
+
date, TiO -based photocatalysts continue to receive consider- whereas Rh introduces an electron acceptor level below the
2
able attention because of their abundance, nontoxicity, and conduction band, which serves as a recombination site, reducing
6,13
stability under photochemical conditions, which makes them the activity of the materials, as shown in Fig. SI 1.† Therefore,
3
highly suitable for scale-up in environmental remediation.
controlling the oxidation state of Rh in Rh-doped TiO is critical
2
Currently, most TiO
2
photocatalysts are based on the anatase to the photocatalytic enhancement of TiO
2
.
4
phase, which is generally the most active phase of TiO
2
.
Another approach to improve the performance of TiO
2
14
Compared to the anatase phase, the rutile phase is oen over- photocatalysts is through nanostructuring. For example,
looked due to its relatively low specic surface area originating photocatalysts based on hollow structures have attracted
from its high temperature synthesis, and its poor light considerable attention because they improve the photoactivity
absorption. Nevertheless, it has been demonstrated that the of the catalyst by enhancing the separation of photogenerated
rutile phase is a promising candidate for the degradation of charge carriers, shortening the charge diffusion length, and
15
organic pollutants because of its strong reduction potential and increasing the accessibility of active sites for reactants. In
5,6
high O
2
adsorption capacity.
addition, multiple reections within the hollow cavity could
One of the biggest limitations of TiO
2
is its large bandgap improve the efficiency of light absorption, leading to the
>3 eV), which makes it active only in the UV region, which generation of additional electron–hole pairs.
16
(
accounts for less than 5% of the solar spectrum. There have been
It is reasonable that combining the aforementioned
approaches (i.e., Rh doping and a hollow structure) would result
2
in an efficient TiO -based photocatalyst with strong visible light
a
Department of Chemical Engineering, Laval University, Quebec, G1V 0A8, Canada.
absorption, good charge separation, and high surface active
area. However, the design of such materials remains chal-
lenging because the introduction of Rh into the TiO2 lattice
E-mail: trong-on.do@gch.ulaval.ca; Fax: +1-418-656-5993; Tel: +1-418-656-3774
b
Department of Electrical and Computer Engineering, University of Toronto, Toronto,
ON, M5S 1A4, Canada
ꢁ
requires high temperatures (>1000 C), at which the hollow
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c6ra25987a
structure collapses because high surface energy induces
1
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3
+
sintering effects. Therefore, using a stabilizer to maintain the be obtained by conventional methods. Therefore, Rh -doped
surface area is critical and so far has not been explored in the TiO2 materials with high surface areas are highly desired.
development of high performance photocatalysts.
Compared to traditional bulk Rh-doped TiO photocatalysts, Sr/
2
Here, we report the rst Rh-doped hollow-structured TiO2 Rh-codoped TiO2 offer several advantages. First, they exhibit
photocatalyst, which is highly active in the photocatalytic a high surface area, owing to the presence of nanoparticles on
decomposition of organic pollutants under sunlight. We ach- the hollow wall. Therefore, the reaction can proceed inside and
ieved this by introducing simultaneous rhodium and stron- outside of the hollow spheres. Second, the co-incorporation of
2
+
tium. We found that Sr is as a codoping agent which stabilized Sr cations enabled us to control the oxidation state of Rh, which
the hollow structure at high temperatures, and contribute to leads to a signicant enhancement in the visible light absorp-
16–18
control the chemical state of Rh in doped material.
The tion and charge separation. Third, the homogeneous distribu-
designed photocatalyst exhibited strong visible light absorption tion of dopants signicantly lowers the doping temperature as
2
ꢀ1
(
up to 600 nm), and a very high surface area (up to 140 m g ). compared to conventional synthetic routes.
As a result, the photoactivity of the photocatalyst was at least 8 Hollow Sr/Rh–TiO was prepared via a multistep pathway
times higher than those of commercial TiO and bulk Sr/Rh– (Scheme 1) comprised of the following steps: (i) the one-pot
TiO powders in the decomposition of isopropanol under solar synthesis of carbon colloidal spheres@Sr,Rh (C@Sr,Rh); (ii)
2
2
2
radiation.
2
coating C@Sr,Rh with TiO by controlling the hydrolysis of
tetraethyl titanium butoxide to obtain core–shell carbon
sphere@Sr,Rh–TiO2 microspheres (C@Sr,Rh–TiO ); (iii) two-
2
Results and discussion
ꢁ
step calcination at 550 C for 5 h to obtain anatase Sr/Rh–
ꢁ
TiO -500, followed by calcination at 900 C for 10 h to form
2
Traditional methods for the synthesis of Rh-doped TiO pho-
tocatalysts mainly focus on improving the doping of Rh in the
2
rutile Sr/Rh–TiO
substitution of Sr/Rh ions for Ti in the TiO
2
-900. The calcination steps are critical for the
4
+
2
crystal lattice and
2 2
TiO lattice to enhance the visible light response of TiO .
for the formation of the hollow structure with high porosity.
Fig. 1 shows the scanning electron microscopy (SEM) images
However, high-temperature calcination requirements for the
4
+
2
substitution of Rh for Ti in the TiO matrix results in a pho-
of C@Sr,Rh–TiO
2 2
, hollow Sr/Rh–TiO -550, and hollow Sr/Rh–
tocatalyst with a very low surface area, which increases the
charge recombination and decreases the number of active sites.
TiO -900. The size of the C@Sr,Rh–TiO microspheres was in
2
2
3
+
the range of 500 nm to 2 mm prior to the two-step calcination
Also, Rh , which serves a critical role in the photocatalytic
ꢁ
(
Fig. 1A). Aer calcination at 550 C, the hollow structure was
activity of Rh-doped TiO under visible light irradiation, cannot
2
2
Scheme 1 Schematic illustration of the synthesis of hollow Sr/Rh-doped TiO nanospheres: (1) one-pot synthesis of carbon colloidal spher-
2+
3+
ꢁ
ꢁ
es@Sr ,Rh , (2) loading of TiO
0 h.
2
from the titanium alkoxide precursor, and (3) calcination at 550 C for 5 h followed by calcination at 900 C for
1
ꢁ
ꢁ
Fig. 1 SEM images of (A) carbon colloidal spheres@Sr,Rh@TiO
2
and hollow Sr/Rh–TiO
2
after calcination at (B) 550 C and (C) 900 C.
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chemical composition of the hollow Sr/Rh–TiO
2
-900 was further
analyzed by inductively coupled plasma mass spectrometry
ICP-MS); Rh and Sr contents of 2.0 and 2.3%, respectively,
conrmed the successful synthesis of the hollow Rh/Sr-doped
TiO structure.
The X-ray diffraction (XRD) pattern of hollow Sr/Rh–TiO
(
2
2
-900
(line b) (Fig. 3A) indicate the presence of
(line a) and rutile TiO
2
rutile TiO
Rh were detected.
2
in the prepared photocatalysts, as neither SrTiO
3
nor
-900
were slightly shied to lower angles as compared to non-doped
21,22
The XRD peaks of hollow Sr/Rh–TiO
2
3
+
rutile-TiO , which could be due to the substitution of Rh for
2
Ti , as Rh ions (0.805 A) are larger than Ti ions (0.745 A˚ ).23
4
+
11
3+
˚
4+
2
+
˚
Because of the higher ionic radius of Sr (1.12 A) as compared
4
+
2+
˚
2
to Ti (0.61 A), Sr was unable to enter the TiO lattice. Hence,
2
+
24,25
Sr could be present on the TiO
Fig. 3B shows the ultraviolet-visible (UV-vis) spectra of
hollow Sr/Rh–TiO -550, hollow Sr/Rh–TiO -900, and bulk Rh–
2
surface as Sr–O clusters.
2
2
TiO -900. A remarkable enhancement in the light absorption
2
along with a color alteration from white to yellow, as shown in
2
Fig. 2 EDX elemental mapping of hollow Sr/Rh–TiO -900: (A) SEM
image; (B) Rh and (C) Sr at high magnification; (D) Ti; (E) O.
Fig. SI 6,† was observed for hollow Sr/Rh–TiO -900 and Rh–
2
TiO
effect of calcination temperature on the light absorption prop-
erties of Rh-doped TiO materials. Higher calcination temper-
atures may favor the introduction of Rh to TiO crystallites
forming Rh-doped TiO with a smaller bandgap. The UV-vis
2 2
-900 compared to hollow Sr/Rh–TiO -550, indicating the
2
formed due to the combustion of the carbon colloidal spheres.
Broken spheres were observed (Fig. 1B), conrming the
formation of the hollow structure. The morphology and size of
the hollow structures remained unchanged (500 nm to 2 mm)
2
2
spectrum of Rh–TiO -900 revealed a discontinuous level at
2
ꢂ420 nm, which could be attributed to electron excitation from
ꢁ
aer calcination at 900 C, as depicted in Fig. 1C. Fig. SI 2†
3+
the donor levels formed by Rh to the conduction band of rutile
shows a transmission electron microscopy (TEM) image of
11,26
TiO
2
.
In addition, a signicant absorption band around 580–
ꢁ
hollow Sr/Rh-doped TiO
2
aer calcination at 900 C, indicating
7
50 nm appeared in the UV-vis spectrum of this sample, indi-
the formation of particles (100–200 nm in size) on the wall of
4+
cating the presence of an acceptor level formed by Rh in the
TiO
bulk Rh–TiO
the hollow structure. Interestingly, we found that the incorpo-
3+
4+
2
structure. Thus, both Rh and Rh were present in the
-900 sample. Compared to bulk Rh–TiO -900,
2
+
ration of Sr increased the stability of the hollow structure at
2
2
high temperatures. As shown in Fig. SI 3 and SI 4,† the samples
hollow Sr/Rh–TiO -900 (in the presence of Sr) exhibited a simple
2
2
+
prepared using the same procedure in the absence of Sr (Rh–
TiO -900 and TiO -900) exhibited completely destroyed struc-
tures. In contrast, hollow TiO structures were retained at high
temperatures in the presence of Sr with Rh (Fig. 1C) or
absorption prole with an onset at ꢂ420 nm, and the disap-
2
2
pearance of the absorption band at around 580–750 nm. This
2
3+
2
suggested that only Rh was present in hollow Sr/Rh–TiO -
2
+
3+
26–28
3+
900.
2
The presence of Rh ions in the hollow Sr/Rh–TiO -900
3
+
without Rh (Fig. SI 5†). In general, the sintering-induced
phase transformation of TiO from anatase to rutile during
calcination at high temperatures is responsible for the change
2+
further conrm the important role of Sr in this synthesis, as it
2
not only prevented sintering induced by high temperature
19,20
of morphology and the collapse of hollow structure.
The
addition of Sr could lower the surface energy change during
the TiO phase transformation at high calcination temperatures
2
+
2
thus preventing the sintering effect, and as a consequence,
maintaining the hollow structure in the resulting hollow Sr/Rh–
TiO -900.
2
Elemental mapping by energy-dispersive X-ray microscopy
(
EDX) from SEM analysis of a single hollow Sr/Rh–TiO -900
2
sphere clearly conrmed the homogeneous distribution of Ti,
O, Rh, and Sr, as depicted in Fig. 2. The homogeneous distri-
bution of these elements indicates efficient doping of Sr and Rh
into the TiO
2
lattice without a phase separation. In this
approach, using carbon colloidal spheres with a high density of
2
+
Fig. 3 (A) XRD pattern of the hollow Sr/Rh–TiO
2
sample after calci-
OH groups on the surface facilitated the adsorption of Sr and
ꢁ
3
+
nation at 900 C and that of rutile TiO . (B) UV-vis spectra of the
2
Rh cations, which resulted in a homogeneous distribution of
2 2
various hollow samples: Sr/Rh–TiO -550 (black line), Sr/Rh–TiO -900
doping elements in the nal hollow sphere products. The
(
red line), and Rh–TiO -900 (blue line).
2
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+
3+
2
annealing, but also maintained the Rh chemical state in the presence of Rh chemical state in the hollow Sr/Rh–TiO -900.
2
Rh-doped material. Fig. SI 7† shows the plot of (ahn) versus Fig. 4C shows the Ti 2p XPS spectra of the hollow Sr/Rh–TiO2-
photon energy for the bandgap calculation of the samples. The 900 (line a) and Rh–TiO -900 (line b). The Rh–TiO -900 exhibi-
2
2
band gap of hollow Sr/Rh–TiO -900 was calculated to be 2.25 eV, ted two peaks at binding energies of 458.2 and 464.1 eV, which
2
4
+
which was signicantly lower than that of the hollow sample could be assigned to Ti 2p3/2 and Ti 2p1/2 of Ti in the rutile
ꢁ
33
calcined at 550 C (3.37 eV). The increased band gap of hollow phase. These peaks are ꢀ0.1 eV shied compared to those of r-
4
+
3+
4+
Sr/Rh–TiO
2 2 2
-550 as compared to anatase TiO alone (3.2 eV) TiO , suggesting the replacement of Ti by both Rh and Rh ,
2
+
4+
could be ascribed to the presence of Sr on the TiO
which reduced the particle size.
2
surface, which induced a higher electron density around Ti atoms in
29
the lattice, as depicted in Table SI-1.† In the case of hollow Sr/
To understand the surface composition and chemical states Rh–TiO -900, the Ti 2p and Ti 2p1/2 peaks appeared at the
2
3/2
of the elements in the samples, X-ray photoelectron spectros- binding energies of 457.8 and 463.7 eV, respectively, which are
copy (XPS) was carried out for the hollow Sr,Rh-doped TiO -900 ꢀ0.4 eV shied compared to those of Rh–TiO -900. This indi-
2
2
and Rh-doped TiO
Rh–TiO
shown in Fig. SI 8,† for the Sr 3d XP spectrum, the deconvoluted further conrm the replacement of Ti by Rh in the hollow Sr/
peaks centered at 133.1 and 134.87 eV could be attributed to Sr Rh–TiO -900. These results, in combination with the data ob-
surface. Fig. 4B shows tained from UV-vis, suggest that Rh is presented in the hollow
2
-900. The presence of Sr, Rh, Ti, and O in Sr/ cates a signicant increase in the electron density of Ti atoms in
2
2 2
-900 was conrmed by XPS, as depicted in Fig. 4A. As hollow Sr/Rh–TiO -900 compared to that in Rh–TiO -900 which
4
+
3+
2
30
3d5/2 and Sr 3d3/2 of Sr–O-modied TiO
2
3
+
the deconvoluted peaks of Rh 3d XPS spectra in hollow Sr/Rh– Sr/Rh–TiO -900 primarily in the form of Rh . Fig. 4D presents
2
TiO -900 (a) and Rh–TiO -900 (b). It should be noted that iso- the O 1s XP spectra of the hollow Sr/Rh–TiO -900 (line a) and
2
2
2
3
+
4+
lated observation of Rh and Rh is ambiguous by XPS anal- Rh–TiO -900 (line b). Two peaks with binding energies of 529.3
2
ysis. However, it is essential to consider the shiing of their and 530.1 eV were observed in the deconvoluted peaks of O 1s in
binding energies because it would help to evaluate the electron Rh–TiO
density of Rh atoms contributing to conrm the chemical states ascribed to the lattice oxygen in TiO
in each sample. For the Rh–TiO -900, the Rh 3d XPS spectrum 530.1 eV could be attributed to strongly bound Ti–OH. Using the
shows two peaks centered at 309.3 eV and 314.2 eV, which could O 1s of rutile-TiO alone as a reference (Table SI-1†), the O 1s
2
-900 (line b). The peak centered at 529.3 eV could be
2
, while other one at
2
2
3
+
4+
be attributed to 3d
states, respectively.
and 3d3/2 of Rh and Rh chemical peak (at 529.3 eV) of Rh–TiO -900 exhibited a negative shi in
5
1,32
/2
2
3
Interestingly, these peaks were shied comparison to that of pristine rutile-TiO (at 529.6 eV), which
2
4
+
ꢀ
0.3 eV and appeared at 390.0 and 313.9 eV for the hollow Sr/ originated from the introduction of partially lled Rh states
13
Rh–TiO
2
-900, which could be attributed to the higher electron within the band gap. This may cause the downward shiing of
density of Rh cations. In other words, the chemical states of Rh Fermi level towards the valence band, which may decrease
in the hollow Sr/Rh–TiO -900 is lower than that in Rh–TiO -900. energy bands along with the reduction potential of photo-
This is associated with UV-vis analysis, which conrms mainly generated electrons and induce a negative impact on the pho-
2
2
13,34,35
toactivity.
In contrast, a strong positive shi in O1s peak
was observed (at 529.9 eV) for hollow Sr/Rh–TiO -900 in
2
comparison to rutile-TiO alone and Rh–TiO -900, for which the
2
2
O1s peaks were centered at 529.9 and 530.5 eV. The increased
binding energy of O1s in hollow Sr/Rh–TiO -900 could be
2
3
+
assigned to the lling of Rh states within the band gap, which
caused the upward shiing of Fermi level towards the conduc-
tion band, which increase the energy bands along with the
reduction potential of photogenerated electrons and has
3
6,37
a positive impact on the photocatalytic activity.
These
4
+
differences could explain the disadvantages of Rh and the
3
+
critical role of Rh in doped photocatalysts. Therefore, the
introduction of Sr to Rh-doped TiO favors the formation of
2
. In the absence of Sr , Rh oxidizes to
Rh to re-establish the charge balance, resulting in the
2
+
3
+
2+
3+
Rh in the doped TiO
2
4
+
4
+
2+
formation of Rh -doped TiO
2
. In contrast, the presence of Sr
which could function as the critical factor limiting this oxida-
3
+
tion process and leading to the formation of Rh -doped TiO .
2
The photocatalytic activity of a catalyst is strongly dependent
on the number of active sites on its surface. Thus, surface and
porosity properties are the important parameters governing the
performance of a photocatalyst because high specic surface
area and large pore volume facilitate mass transfer and effec-
tively promote the kinetics of the photocatalytic reaction.
Therefore, the photocatalytic activity is signicantly
Fig. 4 XPS spectra of hollow Sr/Rh–TiO
those of Rh–TiO -900 (b) survey spectrum of hollow Sr/Rh–TiO
A); XPS spectra of Rh 3d (B), Ti 2p (C), and O 1s (D).
2
-900 (a) in comparison to
2
2
-900
(
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Table 1 Physicochemical properties of various samples
%
dopants
Band gap energy
(eV)
Specic surface area
3
+
2+
2
ꢀ1
Photocatalyst
TiO
2
phase
Morphology
(Rh , Sr
)
(m g
)
Hollow Sr/Rh–TiO
Hollow Sr/Rh–TiO
2
2
-900
-550
Rutile
Anatase
Rutile
Rutile
Rutile
Hollow spheres
Hollow spheres
Bulk crystals
Bulk crystals
Bulk crystals
Nanoparticles
2.0; 2.3
2.0; 2.3
2.0; 2.3
2.1
0; 0
0; 0
2.25
3.3
2.25
2.0
3.1
3.2
140
190
11
3
10
50
Sr/Rh–TiO
Rh–TiO -900
r-TiO
TiO
2
-900-B
2
2
2
(P25)
Anatase-rutile
38,39
enhanced.
The specic surface areas of various samples
were determined by N physical adsorption measurements at 77
2
K. As shown in Fig. SI 9A,† hollow Sr/Rh–TiO -900 exhibits
2
a high adsorption capacity in high relative pressure (P/P : from
0
40
0
.8 to 1), suggesting a large porous structure. The (Barrett–
Joyner–Halenda) BJH pore size distribution obtained from the
adsorption branch also reveals that the majority of pores are
mainly from 2 to 10 nm, as depicted in Fig. SI 9B.† As shown in
2
Table 1, the specic surface area of hollow Sr/Rh–TiO -900
2
ꢀ1
reached 140 m g , which was ꢂ50 times higher than that of
2
ꢀ1
Rh–TiO -900 (3 m g ). Also, the specic surface area of hollow
2
2
Fig. 5 CO formation from the photodegradation of isopropanol of
Sr/Rh–TiO -900 was signicantly higher than those of r-TiO (10
2
2
different types of photocatalysts with 1% Pt: (A) under visible light, (B)
2
ꢀ1
2
ꢀ1
m g ) and commercial anatase TiO
ꢂ13 times that of bulk Sr/Rh–TiO
of our knowledge, this hollow Sr/Rh–TiO
one of the highest surface areas among doped rutile TiO
2
(P25) (50 m g ), and
ꢀ2
under solar simulator irradiation (AM 1.5 G, intensity 100 mW cm ): (a)
2
-900-B (Table 1). To the best hollow Sr/Rh–TiO
2 2 2
-900; (b) bulk Sr/Rh–TiO -B; (c) rutile TiO ; (d) Rh–
TiO -900. ND: not detected.
2
2
-900 photocatalyst has
2
22,41,42
materials reported to date.
The above analyses afforded
2
+
two important ndings about the roles of Sr : (i) the presence The amount of CO
of Sr functioned as a stabilizer to maintain the hollow struc- under visible light was 27.5 mmol g
ture leading to a high surface area photocatalyst; (ii) the 23 times higher than that of Sr/Rh–TiO
2
produced over the hollow Sr/Rh–TiO
2
-900
2
+
ꢀ1 ꢀ1
h , which was more than
ꢀ1
ꢀ1
-B (1.2 mmol g
h
).
2
3
+
chemical oxidation state of Rh could be controlled in the Under similar reaction conditions, CO
was not detected for the
samples, indicating their negligible
oxidation of Rh to Rh . We believe that these two unique photoactivities under visible light. As shown in Fig. 5B, hollow
2
2
+
doped TiO
2
2 2
with the co-presence of Sr , which limited the Rh–TiO -900 and r-TiO
3
+
4+
properties have a close relation in the resulted photocatalyst.
The photocatalytic activity of hollow Sr/Rh–TiO
-900 was Sr/Rh–TiO
Sr/Rh–TiO
2
-900 also exhibited a higher photoactivity than bulk
-B and r-TiO under simulated solar irradiation. The
evolved for hollow Sr/Rh–TiO -900 was 146 mmol
, which was ꢂ25 times higher than that of Sr/Rh–TiO -B
-900 showed
Pt has been demonstrated to signicantly improve the photo- very low photoactivities. Compared to hollow Sr/Rh–TiO -550,
-900 exhibited ꢂ2.5 times higher CO
was evolved from generation (Fig. SI 11†), despite the higher surface area of
the decomposition of isopropanol for the hollow Sr/Rh–TiO
hollow Sr/Rh–TiO -550 (see Table 1). Moreover, the amount of
00 and no CO
was detected for the rest of samples, as shown CO produced with the hollow Sr/Rh–TiO
in Fig. SI 10.† This indicates the critical role of Pt particles as co- times higher than that of commercial TiO
catalyst. Therefore, the evolution of CO from the decomposi- superior activity of hollow Sr/Rh–TiO
2
2
2
evaluated in the photodecomposition of isopropanol in water. amount of CO
2
2
ꢀ1
ꢀ1
So far, the isopropanol photodegradation is used as a model
g
h
2
ꢀ
1
ꢀ1
43
reaction for photo-oxidation of organic compounds. Notably, (5.68 mmol g
h ), whereas r-TiO and Rh–TiO
2 2
2
44
catalytic activity of TiO
photocatalysts, only small amount of CO
2
-based photocatalysts. For free-Pt hollow Sr/Rh–TiO
2
2
2
2
-
2
9
2
2
2
-900 catalyst was 6.6
(P25). Therefore, the
-900 could be attributed to
2
2
2
+
3
tion of isopropanol under visible light (l $ 420 nm) and solar the synergistic effect of doping Rh into the TiO
structure in
light irradiation (see Experimental section) was monitored and the presence of Sr , which causes strong solar light absorption,
compared with those observed using Sr/Rh–TiO -B, Rh–TiO
and a hollow structure with a high surface area. These features
00 and r-TiO
with the optimized amounts of Pt loading, which signicantly enhanced the generation of electron–hole pairs
were found to be ꢂ1% Pt for the hollow Sr/Rh–TiO
-900, ꢂ0.3% and improved the charge separation during photocatalysis.
for Sr/Rh–TiO -B and r-TiO -900,
The photocatalytic efficiency (PE) for the photocatalytic
2
2
+
2
2
-
9
2
2
2
2
and ꢂ0.1% for Rh–TiO
2
respectively. As shown in Fig. 5A, the photocatalytic activity of decomposition of isopropanol under solar irradiation was
hollow Sr/Rh–TiO -900 was superior to those of Sr/Rh–TiO -B calculated using the following equation:
2
2
and Rh–TiO -900, as indicated by the higher CO generation.
2
2
PE ¼ Nconsumed photons for CO2/Nincident photons
(1)
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contributions of strong solar light absorption and high surface
area. Therefore, our hollow material has signicant potential for
applications in water purication under natural solar illumina-
tion. Furthermore, this synthetic strategy can be extended to
other hollow systems for a wide range of applications in catalysis
and energy conversion. Work into gaining further insight into the
mechanistic aspects of this process is in progress.
Experimental section
Chemicals
Fig. 6 Schematic illustration of the photodecomposition of organic
D-(+)-Glucose, hexachloroplatinic acid hexahydrate, strontium
chloride, rhodium(III)nitrate, and tetraethyl titanium butoxide
were purchased from Sigma-Aldrich. All reagents were used as
received without further purication.
2
isopropanol using hollow Sr/Rh–TiO -900 under solar light irradiation.
where Nconsumed photons for CO2 is the number of photons
consumed for CO generation and N is the total
2
incident photons
number of incident photons of simulated solar light. The
45,46
Material syntheses
details of this calculation were described previously.
for CO generation using hollow Sr/Rh–TiO -900 under simu-
lated solar light was 0.242%, which is signicantly higher than
that using bulk Sr/Rh–TiO (P25,
-900-B (ꢂ0.01%) and TiO
.03%), conrming the promising application for solar-driven
photodecomposition of organic pollutants.
The PE
2
2
Synthesis of carbon colloidal spheres@Sr/Rh(C@Sr/Rh).
D-(+)-Glucose (17 g) was rst dissolved in deionized water (160
mL). SrCl (3.2 g) and an aqueous solution of Rh(NO ) (2 g, 1 mg
2
2
2
3 3
ꢀ
1
0
mL ) were then added to the obtained solution. The resulting
ꢁ
mixture was heated in a Teon-lined autoclave at 180 C for 18 h.
Subsequently, the solid product was collected by centrifugation at
The stability of the hollow Sr/Rh–TiO -900 was also studied
2
by performing multi-recycling experiments under the same
conditions, as shown in Fig. SI 12.† No signicant change in the
6000 ppm for 10 min, and was washed several times with water
ꢁ
and ethanol before being dried overnight at 80 C.
production of CO
morphology was retained aer the h cycle indicating the
stability of the hollow Sr/Rh–TiO -900, as shown in Fig. SI 13.†
It has been demonstrated that rutile-TiO contains a large
2
was observed aer 5 cycles. Also, the
2
Synthesis of hollow Sr/Rh-doped TiO . The obtained carbon
colloidal spheres C@Sr/Rh were loaded with a titanium alkoxide
precursor as follows: C@Sr,Rh (2 g) was dispersed in anhydrous
ethanol (30 mL) and then stirred for 1 h, followed by sonication
for 15 min to ensure good dispersion. A solution of tetraethyl
titanium butoxide in ethanol (15 mL, 50 mg mL ) was then
added dropwise to the above solution with stirring for 1 h.
Subsequently, a solution of water in ethanol (3.5 mg mL water)
was slowly added dropwise to the above solution to accelerate the
hydrolysis of the titanium precursor. Aer stirring for 6 h, the
resulting material was recovered by centrifugation, washed with
2
2
47–51
amount of adsorbed oxygen on its surface.
Therefore, the
ꢀ1
photodecomposition of organic pollutants should be mainly
directed toward the electron usage route, which produces strong
ꢀ
ꢀ
1
oxidants (O c , H O ) (E ¼ ꢀ0.7 V vs. standard hydrogen
2
2
2
CB
ꢀ
ꢀ
electrode (SHE); O
+ 2OH , +0.28 vs. SHE).
generation with Sr/Rh-doped TiO
2
+ e ¼ O
2
c , ꢀ0.28 V vs. SHE; O
2 2
+ 2H O + 2e
ꢀ
47–51
¼
H O
2 2
A mechanism for CO
2
2
is proposed in Fig. 6. Hollow
Sr/Rh–TiO -900 exhibited high photoactivity, which could be due
2
ꢁ
anhydrous ethanol, and dried overnight at 80 C in a vacuum
oven to yield carbon colloidal spheres@Sr,Rh–TiO (C@Sr/Rh–
3
+
to the synergistic contribution of efficient Rh doping and high
surface area. Indeed, efficient absorption of sunlight appeared to
generate additional electron–hole pairs for the photodegradation
2
2 2
TiO ). The as-prepared C@Sr/Rh–TiO was loaded in an
aluminum crucible and subjected to two-step calcination at
ꢁ
different temperatures. The rst step was carried out at 550 C for
of isopropanol into CO . Moreover, the high surface area of the
2
ꢁ
ꢀ1
hollow structure increases the number of exposed active sites for
5
h (heating rate of 1 C min ) to obtain anatase TiO
2
(denoted
48
the photoreactions. As a result, both as-photogenerated elec-
tron–hole pairs could contribute to produce strong oxidants
inducing the efficient degradation of isopropanol.
as hollow Rh/Sr–TiO
2
-550). The second step was carried out at
ꢁ
ꢁ
ꢀ
1
9
00 C for 10 h (heating rate of 3 C min ) to achieve the rutile
phase (denoted as hollow Sr/Rh–TiO -900).
2
Synthesis of Rh-doped TiO -900 and TiO -900. Rh-doped
2
2
TiO
2
-900 (denoted as Rh–TiO
2
-900) and TiO
2
-900 were synthe-
Conclusions
sized using the same procedure as described above except that
3
+
We successfully prepared a novel type of hollow Sr/Rh-doped only Rh was added to the mixture of colloidal spheres for the
2
+
3+
TiO
For the rst time, we demonstrated that the introduction of Sr
to Rh-doped TiO stabilizes the hollow structure and maintains
2
using carbon colloidal spheres as the sacricial template. Rh–TiO
2
-900 and neither Sr or Rh was used for undoped
-900 samples.
Synthesis of bulk Sr/Rh–TiO -900. Bulk Sr/Rh-doped TiO -
2 2
2
+
TiO
2
2
3
+
the Rh oxidation state of Rh in the doped material at high 900 (Sr/Rh–TiO -900-B) was prepared via a solid-state reaction
2
temperatures. The obtained photocatalyst exhibited high photo- for comparison. The starting materials (SrCl , Rh(NO ) , and
2
3 3
activity in the degradation of isopropanol under visible light and TiO
simulated solar irradiation, owing to the synergistic TiO
2
) were mixed in the same atomic ratio as for hollow Sr/Rh–
-900. The mixture was calcined at 1000 C for 10 h.
ꢁ
2
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Characterization
The amount of CO
2
generated under simulated solar light
was determined to be 8.8 mmol. Therefore, the number of
Transmission electron microscopy (TEM) images of the samples
were obtained using a JOEL JEM 1230 operated at 120 kV. Scan-
ning electron microscopy (SEM) images were obtained using
a JEOL 6360 operated at 15 kV. Powder X-ray diffraction (XRD)
patterns of the samples were obtained using a Bruker SMART
APEXII X-ray diffractometer equipped with a Cu Ka radiation
photons consumed to produce CO under solar light illumina-
2
tion over 3 h was calculated as follows:
ꢀ6
23
consumed photons for CO2 ¼ 6 ꢃ (8.8 ꢃ 10 ) ꢃ (6.023 ꢃ 10 )
N
19
¼
3.18 ꢃ 10
(3)
˚
source (l ¼ 1.5418 A). X-ray photoelectron spectroscopy (XPS)
PE ¼ Nconsumed photons for CO2/Nincident photons
measurements were carried out in the ion-pumped chamber
evacuated to 10 Torr) of a photoelectron spectrometer (Kratos
19
22
¼
(3.18 ꢃ 10 /1.31 ꢃ 10 ) ꢃ 100 ¼ 0.242%
(4)
ꢀ
9
(
Axis-Ultra) equipped with a focused X-ray source (Al Ka, hn ¼
The same procedure was used to determine the PE of
1
486.6 eV). UV-vis spectra were recorded on a Cary 300 Bio UV- commercial TiO (P25) and Sr/Rh–TiO -900-B.
2
2
2
visible spectrophotometer. N adsorption–desorption isotherms
of the samples were obtained at 77 K using a Quantachrome
Autosorb-1 MP analyzer. Prior to the measurements, the samples
were outgassed under vacuum for 6 h at 150 C.
Acknowledgements
ꢁ
This work was supported by the Natural Science and Engi-
neering Research Council of Canada (NSERC) through the
Collaborative Research and Development with EXP Inc. (CRD)
and Discovery Grants.
Photocatalytic tests
The photocatalytic decomposition of isopropanol was carried out
in a top-irradiated reactor at ambient temperature and pressure.
Prior to the photocatalytic tests, Pt was photodeposited on the
catalyst. In a typical photocatalytic experiment, 20 mg of the
photocatalyst, the optimum amount of photocatalyst for this
photocatalytic reactor system, was dispersed in a 200 ppm solu-
tion of isopropanol in water. The reaction cell was then lled with
fresh synthetic air (Prax air), stirred for 1 h to get steady-state
regime prior to be illuminated with a solar simulator 150 W Xe
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