Trace Amount of SnO2-Decorated ZnSn(OH)6 as Highly Efficient Photocatalyst for Decomposition of Gaseous Benzene: Synthesis, Photocatalytic Activity, and the Unrevealed Synergistic Effect between ZnSn(OH)6 and SnO2

Article abstract of DOI:10.1021/acscatal.5b02593

It is still a challenge to develop a high-performance photocatalyst for the abatement of aromatic compounds like benzene (C₆H₆), which has been regarded as a priority hazardous volatile organic compound substance in the indoor atmosphere. Zinc hydroxystannate (ZnSn(OH)₆, ZHS) is a promising material for this application. However, the key structural features that are responsible for the high activity are still ambiguous. To address this issue, a series of ZHS with different surface properties were hydrothermally synthesized by varying the treatment temperature and the solution pH. Although ZHS can be readily synthesized under a mild reaction condition (temp 90-120°C, pH 4-10), most of the samples were contaminated by a trace amount of low-crystallized SnO₂. Pristine ZHS can only be produced in a strong alkaline solution (pH 13). The sample prepared at 120°C in a pH 10 solution shows the highest activity for the degradation of gaseous C₆H₆ and an efficiency almost 6 times higher than P25 (TiO₂). More importantly, no obvious deactivation of the sample and the formation of stable deposits were observed in a long-term reaction for 48 h. Although the SnO₂ amount is quite small and bare ZHS shows almost no activity, both of them are indispensable for the degradation of C₆H₆. SnO₂ and ZHS can be understood as photoactive sites to produce charge carriers and preferential sites for the adsorption of O₂, H₂O, and C₆H₆, respectively. A synergistic effect between SnO₂ and ZHS in the formation of active radicals and the degradation of C₆H₆ accounts for the high performance of the SnO₂-decorated ZHS.

Full text of DOI:10.1021/acscatal.5b02593

Subscriber access provided by CMU Libraries - http://library.cmich.edu
Article
Trace Amount of SnO2-decorated ZnSn(OH)6 as Highly Efficient Photocatalyst
for Decomposition of Gaseous Benzene: Synthesis, Photocatalytic Activity
and the Unrevealed Synergistic Effect between ZnSn(OH)6 and SnO2
Xianliang Fu, Jinghui Wang, Danwei Huang, Sugang Meng,
Zizhong Zhang, Longfeng Li, Tifang Miao, and Shifu Chen
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02593 • Publication Date (Web): 30 Dec 2015
Downloaded from http://pubs.acs.org on January 2, 2016
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street
N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 43
ACS Catalysis
1
2
3
4
5
6
7
8
9
Trace Amount of SnO2-decorated ZnSn(OH)6 as
Highly Efficient Photocatalyst for Decomposition of
Gaseous Benzene: Synthesis, Photocatalytic Activity
and the Unrevealed Synergistic Effect between
ZnSn(OH)6 and SnO2
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Xianliang Fu†*, Jinghui Wang†, Danwei Huang†, Sugang Meng†, Zizhong Zhang§, Longfeng Li†,
Tifang Miao†, Shifu Chen†‡*
† College of Chemistry and Material Science, Huaibei Normal University, Huaibei, Anhui,
China, 235000.
‡ Department of Chemistry, Anhui Science and Technology University, Fengyang, Anhui, China,
233100.
§ Research Institute of Photocatalysis, State Key Laboratory of Photocatalysis on Energy and
Environment, Fuzhou University, Fuzhou, China, 350002.
ABSTRACT
1
ACS Paragon Plus Environment

ACS Catalysis
Page 2 of 43
1
2
3
4
5
6
7
8
9
It is still a challenge to develop a high performance photocatalyst for the abatement of aromatic
compounds like benzene (C6H6) that has been regarded as a priority hazardous VOC substance
in the indoor atmosphere. Zinc hydroxystannate (ZnSn(OH)6, ZHS) is a promising material for
the application. However, the key structural features which responsible for the high activity are
still ambiguous. To address this issue, a series of ZHS with different surface properties was
hydrothermally synthesized by varying the treatment temperature and the solution pH. Although
ZHS can be readily synthesized under a mild reaction condition (Tem. 90-120 oC, pH 4-10),
most of the samples were contaminated by trace amount of low-crystalized SnO2. Pristine ZHS
can only be produced in a strong alkaline solution (pH 13). The sample prepared at 120 oC in a
pH 10 solution shows the highest activity for the degradation of gaseous C6H6 and the efficiency
almost six times higher than P25 (TiO2). More importantly, no obvious deactivation of the
sample and the formation of stable deposits were observed in a long-term reaction for 48 h.
Although SnO2 amount is quite small and bare ZHS shows almost no activity, both of them are
indispensable for the degradation of C6H6. SnO2 and ZHS can be understood as photoactive sites
to produce charge carriers and preferential sites for the adsorption of O2, H2O, and C6H6,
respectively. A synergistic effect between SnO2 and ZHS in the formation of active radicals and
the degradation of C6H6 accounts for the high performance of the SnO2 decorated ZHS.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
KEYWORDS: zinc hydroxystannate, tin dioxide, photocatalysis, benzene degradation,
synergistic effect, photoactive sites
1. INTRODUCTION
As an important solvent and raw material, benzene (C6H6) is widely used in industry for
producing drugs, dyes, insecticides, and plastics. It is a widespread volatile organic compound
2
ACS Paragon Plus Environment

Page 3 of 43
ACS Catalysis
1
2
3
4
5
6
7
8
9
(VOC) in the polluted indoor atmosphere. Due to its high carcinogenicity and environmental
persistence,1 C6H6 has been regarded as a priority hazardous substance.2 Developing a highly
efficient treatment technology for the abatement of highly stable C6H6 remains an unanswered
challenge at ambient condition.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Presently, the methods for the elimination of VOCs are mainly focused on active carbon
adsorption, catalytic combustion, and photocatalytic oxidation (PCO).3 Among them, TiO2 based
PCO is a promising technology for the treatment of low concentration of VOCs in ppb-ppm level.
Compared to a physical adsorption process, the mineralization of VOCs can be achieved by PCO
only through the use of air and light at room temperature. The operating condition is much
milder than the catalytic combustion process which is generally performed at a high temperature
(ca. 300 oC).3c Although numerous VOCs can be readily removed by PCO, unfortunately, limited
success has been achieved in the treatment of aromatic compounds like benzene and toluene.
Due to the deposition of tar or stable carbonaceous by-products,4 the oxidation reactions cannot
be sustained for a long time on some typical photocatalysts such as TiO2 and ZnO.4a, c, d, 5 To deal
with the issue of low efficiency, several approaches have been explored to improve the
decomposition efficiency of C6H6, including modifying a photocatalyst with noble metals (Pt,
Pd, or Rh),6 introducing H2O vapor or H2 into the reaction system,7 and developing new highly
efficient photocatalyst instead of the inefficient TiO2.4d, 8 The third approach is clearly more
attractive than the former two in consideration of the high cost of the noble metals and the
complexity of the hybrid system.
In theory, PCO can be promoted by a hydroxyl material as the fact that photocatalytical
degradations are mainly triggered by hydroxyl radical (·OH) whose formation was favored by a
surface with a high population of OH.9 In addition, PCO can also be induced directly by photo-
3
ACS Paragon Plus Environment

ACS Catalysis
Page 4 of 43
1
2
3
4
5
6
7
8
9
generated hole (h+). Both the routes can lead to the decomposition of the most pollutants.10
However, as for the decomposition of benzene, ·OH mediated degradation route is more crucial
than the latter because the reported works11 indicated that a free radical polymerization would be
induced by the attack of C6H6 with h+, which resulted in the deposition of stable coke. The
beneficial effect of hydroxyl structure for PCO abatement of C6H6 is also supported by reported
hydroxide photocatalysts, such as In(OH)3, ZnSn(OH)6, InOOH, GaOOH, CaSn(OH)6,
SrSn(OH)6.4a, 12 Among them, the low cost Sn-based hydroxides are more realistic for practical
application due to the rarity and high cost of In and Ga. ZnSn(OH)6 (ZHS) is a perovskite-
structured hydroxide. We first found that the degradation of gaseous C6H64a and the reforming of
ethanol to H2 and CH413 can be achieved on ZHS via a photocatalytic process. Its versatility was
further confirmed by PCO of methyl orange (MO), cyclohexane, phenol and nitrogen
monoxide.14 However, some inconsistent results were also observed recently.15 These works
indicated that not all the prepared ZHS samples showed high activity for the degradation of
C6H6. The light absorption, morphology, and activity of ZHS highly depend on the preparation
route. Then a question arises, which structural features are essentially responsible for PCO
activity of ZHS and how the preparation condition affects them? A comprehensive investigation
of these issues will not only benefit the understanding of PCO mechanism of ZHS, but also
contribute to the development of high performance photocatalyst based on hydroxides for
benzene abatement.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Herein, a series of ZHS with different surface properties was hydrothermally synthesized. The
effects of the preparation condition including the hydrothermal temperature and the solution pH
on the formation and the properties of ZHS were investigated systematically. The aim of this
work is to reveal the key structural features of ZHS, which favour the degradation of C6H6. The
4
ACS Paragon Plus Environment

Page 5 of 43
ACS Catalysis
1
2
3
4
5
6
7
8
9
results indicated that the high performance ZHS samples were contaminated by trace amount of
low-crystalized SnO2. A synergistic effect between SnO2 and ZHS in the formation of active
radicals and the degradation of C6H6 accounts for the high performance of SnO2 contaminated
ZHS. Both of them are indispensable for the degradation of C6H6.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
2. EXPERIMENTAL SECTION
2.1 Chemicals and Preparation of ZSH.
Analytically pure Na2SnO3∙4H2O, ZnCl2, NaOH, hydrochloric acid (37%), and MO were
purchased from Sinopharm Chemical Reagent Co. Ltd and used as received. The spin-trap
reagent 5, 5-dimethyl-1-pyrroline-N-oxide (DMPO) was purchased from Sigma Chemical Co.
ZHS samples were prepared by a hydrothermal method. In a typical procedure, 20 mL 2 mM
ZnCl2 solution was first introduced into a 50 mL PTFE vessel. 20 mL 2 mM Na2SnO3 solution
was added drop wise to the solution under vigorous stirring. After the addition, the pH of the
white suspension was adjusted to a desired value (1-13) by 0.15 M HCl or NaOH solution. The
resulted suspension was then transferred into an autoclave and heated at a desired temperature
o
(20-180 C) for 24 h. The product formed after the treatment was carefully recovered by
o
centrifugation, rinsing thoroughly with deionized water, and finally dried at 95 C for 4 h. The
o
effects of the solution pH (with temperature fixed at 120 C) and the hydrothermal temperature
(with solution pH fixed at 10) on the formation of ZHS were investigated, respectively.
For comparison, pristine SnO2 and ZnO were prepared via a similar hydrothermal route by
using only one of the precursors in a neutral solution. The solution pH was adjusted by 0.15 M
HCl or NaOH solution.
2.2 Characterization
5
ACS Paragon Plus Environment

ACS Catalysis
Page 6 of 43
1
2
3
4
5
6
7
8
9
X-ray diffraction (XRD) patterns were performed on a Bruker D8 Advance diffractometer by
using Ni filtered Cu Kα radiation λ = 1.5406 Å. The UV–Vis optical absorption spectra (UV-vis
DRS) were recorded on a Shimadzu UV 3600 UV–Vis–NIR spectrometer with BaSO4 as a
reference. The morphologies were examined by using a field emission scanning electron
microscopy (SEM, SU8000, Hitachi) and a transmission electron microscopy (TEM, FEI Tecnai
G2 F20 S-TWIN). N2 adsorption–desorption isotherms were obtained at 77 K by using a
Micromeritics Tristar II 3020 surface area analyzer. Multipoint Brunauer-Emmett-Teller (BET)
specific surface areas were then determined from the linear part of the BET transform of the
adsorption isotherms. Fourier transform infrared (FTIR) spectra were recorded on a Thermo
Nicolet Nexus 6700 FTIR spectrophotometer by using KBr pellets.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
2.3 Photocatalytic Measurements
Both liquid-solid and gas-solid photocatalytic activities of the prepared samples were evaluated
by using MO solution and gaseous C6H6 as a model of pollutant, respectively. The schematic
diagram of the reaction systems had been described in our former works15b and their pictures are
shown in Figure S1 (Supporting information). The degradation of MO reactions was carried out
in a 200 mL tubular reactor which surrounded by a water cooling jacket. A 9 W H-shaped low
pressure mercury UV lamp (254 nm, Philips TUV PLS-9w) was assembled at the center of the
reactor. In a typical experiment, 100 mg ZHS was dispersed in 200 mL 10 ppm MO aqueous
solution and then magnetically stirred for 30 min in the dark to reach an adsorption-desorption
equilibrium. The solution was ventilated with air (20 mL/min) throughout the reaction and its
temperature was controlled at 20 oC by circulating water. During irradiation, a 3 mL solution was
withdrawn through a pipette every 20 min followed by a centrifugation at 10000 rpm for 3 min.
The concentration of the residual MO in the supernatant was monitored by a spectrophotometer
6
ACS Paragon Plus Environment

Page 7 of 43
ACS Catalysis
1
2
3
4
5
6
(Shimadzu UV 3600 UV–Vis–NIR Spectrometer). The experimental error was estimated within
5% by repeating runs.
7
8
9
Photocatalytic degradation of gaseous C6H6 was performed in a fixed-bed tubular mico-reactor
with a continuous flow mode. The reactor is made of a quartz tube with a length of 250 mm and
an inner diameter of 4 mm, which was surrounded by four 6 W UV lamps (254 nm, TUV 6W/G6
T5, Philips). The heat generated by the lamps was removed by a suction fan and the reactor
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
o
temperature was controlled at ca. 30 C. In a typical experiment, 0.1 g ZHS was first mixed
thoroughly with 1.0 g 50-70 mesh high purity quartz sands (as support material). The mixture
was then loaded into the reactor. Benzene vapor was supplied by a gas cylinder at a constant
concentration of 250 ppm with the balance of O2 (Dalian Da’te ga Co., Ltd.). The flow velocity
was kept at 30 mL/min corresponding to a reactor residence time of 1.8 s. Simultaneous
determination of C6H6 and the formed CO2 were performed on an online gas chromatograph
(Agilent 7820A) with a FID and a TCD detector, respectively. The conversion (CE, C%) and the
mineralization efficiency (ME, M%) of C6H6 were calculated according to the steady-state
reaction data by Eq. 1 and Eq. 2, respectively. The degradation (rd, μmol∙g-1∙h-1) and
mineralization (rm, μmol∙g-1∙h-1) rates of C6H6 on the samples were calculated by Eq. 3 and Eq.
4, respectively. The feed gas was treated as ideal gas for the calculations.
C H
- C H
[
]
[
[
]
6
6
6
6
initial
steady
C%=
C H
]
6
6
initial
(Eq. 1)
CO
[
]
2
steady
M% =
( C H
− C H steady )×6
[
]
[
]
6
6
6
6
initial
(Eq. 2)
(Eq. 3)
C H initial ×30(mL/min)×60(min)×C%
[
]
6
6
rD =
22.4(L/mol)×0.1(g)
7
ACS Paragon Plus Environment

ACS Catalysis
Page 8 of 43
1
2
3
4
rM=rD×M%
(Eq. 4)
5
6
2.4 Active Radicals Measurement
7
8
9
The formation of the active radicals in the suspension of selected sample was measured by
electron spin resonance (ESR) technique with the use of DMPO as a spin-trap agent. The ESR
signals of the trapped radicals were recorded at ambient temperature on a Brucker ESR 300 E
spectrometer. All freshly-prepared suspensions (for ∙OH measurement: DMPO: 0.20 M, 20 μL +
0.5 mL water; for O2∙- measurement: DMPO: 0.20 M, 20 μL + 0.5 mL methanol; sample: 10 mg)
were mixed directly and then transferred into a cylindrical quartz cell (length 100 mm, diameter
2 mm). A spot UV-light source (Hamamatsu LC8, equipped with a narrow-band-pass filter of
254 nm) was used in situ as a light source. The signals were taken every 40 s.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
3. RESULTS AND DISCUSSION
3.1 Effects of Preparation Conditions on ZSH Formation
Figure 1 XRD patterns of the products prepared at (a) different solution pH and (b) different
temperature.
8
ACS Paragon Plus Environment

Page 9 of 43
ACS Catalysis
1
2
3
4
5
6
7
8
9
Figure 1a and 1b shows the XRD patterns of the samples prepared at different solution pH and
different hydrothermal temperature, respectively. As shown in Figure 1a, the sample prepared at
pH 1 shows some weak and broad diffraction peaks which can be indexed to the tetragonal SnO2
(JCPDS No. 77-449). The hydrolysis of [SnO3]2- in the acidic solution16 accounts for the
formation of SnO2, while Zn precursor is still in Zn2+ state at pH 1. No any ZHS peak can be
found in the pattern. However, when the solution pH was increased to 4, a well-crystallized ZHS
sample was formed via Eq. 5 due to the hydrolyzation of Na2SnO3 to [Sn(OH)6]2-.17 The pattern
can be ascribed to the cubic phase of ZnSn(OH)6 (JCPDS No. 073-2384). However, a small
broad peak located at 2Ө=26 o is still present (as shown in the enlarged figure in Figure 1a). This
peak can be assigned to the (110) reflection of the tetragonal SnO2 as pure phase tetragonal
SnO2 has been formed at pH 1. Although the intermediated oxidation states tin oxides (i.e.
Sn2O3, and Sn3O4) have a similar diffraction peak around 2Ө=26 o, these oxides are commonly
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
18
formed during the conversion of SnO to SnO2 or a heat treatment of SnO2 at elevated
temperature in the absence of excess oxygen19 (formed by etching of lattice O atoms from
SnO220). Here, as SnCl4 was used as Sn precursor and the hydrothermal temperature is not too
o
high (no higher than 160 C), the formation of these intermediated oxidation samples are
minimal. With further increasing the solution pH from 4 to 13, the intensity of ZHS peaks is
slightly improved. Meanwhile, the inconspicuous SnO2 peak fades away and is hard to be
distinguished at pH 13. Obviously, the solution with a high pH value favors the formation of
ZHS. As shown in Figure 1b, well-crystallized ZHS can be readily synthesized in a temperature
o
range of 20-150 C without adjusting the solution pH (at ca. 10). The (110) peak of SnO2 is
gradually promoted by increasing the temperature. It suggests that part of Sn precursor did not
involve in the formation of ZHS at elevated temperature. With further increasing the temperature
9
ACS Paragon Plus Environment

ACS Catalysis
Page 10 of 43
1
2
3
4
5
6
7
8
9
o
to 180 C, ZHS cannot sustain anymore and decomposes to the tetragonal SnO2 and cubic
Zn2SnO4 (JCPDS 74-2184) via Eq. 6. Small amount of triclinic Sn2O3 (JCPDS 25-1259) can
also be observed in the product, which is formed by the etching of SnO2 lattice oxygen atoms at
high hydrothermal temperature.20 The XRD results suggest that most of the well-crystallized
ZHS samples are contaminated by some low-crystalized SnO2 (denoted as S-ZHS hereafter).
Low hydrothermal solution pH or high preparation temperature promotes the segregation of
SnO2 component. Although no ZnO diffraction peaks can be found in all the samples, ZnO may
exist in an amorphous state. The possibility will be determined by UV-vis DRS analysis.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Zn2+ + [Sn(OH)6]2- → ZnSn(OH)6
(Eq. 5)
(Eq. 6)
ZnSn(OH)6 → Zn2SnO4+ SnO2 + 3H2O
Figure 2 Yields of the product prepared at different solution pH and different hydrothermal
temperature.
10
ACS Paragon Plus Environment

Page 11 of 43
ACS Catalysis
1
2
3
4
5
6
7
8
9
Although the phase composition and the structure of the prepared ZHS samples are tentatively
investigated by XRD, Sn4+ and Zn2+ may be in form of amorphous oxides or hydroxides states
which are hard to be disclosed by XRD technology, especially under the strong shielding effect
of highly crystalized ZnSn(OH)6. However, if the prepared samples were contaminated by these
components, the yields of the products would be quite different. To assist the XRD analysis, the
yields of the products under different preparation conditions were carefully determined. The
XRD result (Figure 1) suggests that the products may be in form of SnO2, ZHS, or SnO2-
Zn2SnO4. Thus, the corresponding theoretical yields are calculated to be 0.30, 0.57, and 0.48 g,
respectively. As shown in Figure 2, the sample obtained at pH 1 is 0.33 g. It is slightly higher
than the theoretical yield of SnO2 suggesting the product may contain crystal water (in the form
of SnO2∙H2O21). The formation of ZHS can be largely achieved in the pH range of 4-10 as the
yields are almost identical to the theoretical value. A slight drop of the yield is observed with
further increasing the pH to 13. Unrevealed by the XRD result, the finding suggests that a
surface etching process is occurred over ZHS in the strong basic solution for the amphoteric
nature of Sn4+.22 The corrosion results in the disappearance of SnO2 (110) peak in Figure 1a.
These results suggest that the amount of SnO2 (observed in the XRD patterns) is quite small and
should present on ZHS surface rather than in its bulk. As for the effect of the hydrothermal
temperature, a complete formation of ZHS cannot be achieved at room temperature and a
treatment at 90-120 oC is required. Because of the thermal instability of ZHS, a slight decline in
the yield can be observed when the temperature is beyond 120 oC. The yield (0.49 g) achieved at
180 oC is comparable to the theoretical value of SnO2-Zn2SnO4 composite indicating a thorough
breakdown of ZHS is occurred. Thus, the condition for high yield of ZHS is conducting the
hydrothermal treatment in a solution with pH 4-10 at a mild temperature (90-120 oC).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
11
ACS Paragon Plus Environment

ACS Catalysis
Page 12 of 43
1
2
3
4
3.2 UV-Vis DRS Analysis
5
6
7
8
UV-Vis DRS has been extensively used to study the electronic state information of a
photocatalyst, as well as the information of the surface metal ions, such as the coordination
symmetry and the valence state.23 This technology was used to investigate the absorptions of the
prepared samples and to further study their surface structures. The UV-Vis DRS of the prepared
ZHS are shown in Figure 3. Consistent with our former works,13, 15b two absorption bands can be
distinguished in the spectra. Taken the sample pH-4 (Figure 3a) as example, the spectrum can be
deconvoluted into two bands: one is threshold at ca. 250 nm (B-250), and the other is at ca. 350
nm (B-350). Highly similar spectra have been observed in some Sn-containing samples, such as
Sn-Zn layered double hydroxides and Sn-SBA-15.24 The absorptions have been assigned to the
charge-transfer transition from O2- to Sn4+ which located in a different coordination environment.
The absorptions are quite sensitive to the coordination structure of Sn4+. Generally, the intrinsic
absorption of hydroxides occurs in a deep UV region, such as In(OH)3 and GaOOH.12d, 25 Thus,
B-250 can be ascribed to the intrinsic band transition of pristine ZHS. This absorption is hardly
impacted by the preparation condition. As most of the ZHS samples are contaminated by low-
crystallized SnO2, B-350 can be ascribed to the absorption of SnO2. Unlike B-250, the
absorption decreases with the solution pH and vanishes at pH 13 (Figure 3a) due to the etching
of SnO2, but increases with the treatment temperature (Figure 3b). Apparently, the absorption
band of B-350 and the SnO2 (110) peak (Figure 1) share a same variation tendency with the
change of the hydrothermal temperature or solution pH, suggesting the attribution of B-350 to
SnO2 is fairly reasonable.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Tauc plot approach26 was used to determine the band gap energy (Eg) of the prepared ZHS
samples:
12
ACS Paragon Plus Environment

Page 13 of 43
ACS Catalysis
1
2
3
4
K(hv-Eg)1/n =F(R)hv
5
6
7
8
where F(R) is the absorption coefficient, hv is photon energy, K is a constant, and n equals 2 for
direct transition of SnO227and ZHS25b. The plots are shown in the inset of Figure 3. Obviously,
two Eg edges located at 3.5 and 5.0 eV can be observed for most of the samples except the one
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
prepared at pH 13 (denoted as ZHS-13), which shows only the Eg of 5.0 eV. The absorption edge
19a, 21a
energies at 3.5 and 5.0 eV can be ascribed to the band gap transition of SnO2
ZnSn(OH)625b, respectively.
and
Figure 3 UV-Vis DRS of the samples prepared at (a) different pH solution and (b) different
temperature. The insets show the Tauc plots.
For comparison, pristine SnO2 and ZnO were prepared in a similar route by using only one of
the precursors and the light absorptions of the samples were then characterized. The XRD results
(Figure S2) indicated that a hexagonal ZnO (JCPDS No. 75-1526) and a tetragonal SnO2
(JCPDS No. 77-449) samples were successfully synthesized. Figure S3 shows the UV-Vis DRS
of the two samples. The onsets of the absorptions are at ca. 350 and 390 nm for SnO2 and ZnO,
13
ACS Paragon Plus Environment

ACS Catalysis
Page 14 of 43
1
2
3
4
5
6
7
8
9
respectively. The result further confirms that B-350 originates from the band-gap absorption of
SnO2. As the prepared ZHS samples show no absorption around 390 nm and no ZnO diffraction
peak is observed in Figure 1, the existence of ZnO on the prepared ZHS samples thus can be
excluded. These results also rule out the presence of the heterovalent tin oxides as their Eg values
are lower than 2.7 eV.18b, 19a, 28
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
3.3 Morphology
Figure 4 (a) SEM, and (b-d) TEM images of the ZHS prepared at pH 10 and (e) SEM, and (f)
TEM images of the ZHS prepared at pH 13
The representative morphologies of the ZHS prepared at pH 10 (denoted as ZHS-10) and 13
(i.e. ZHS-13) were investigated by SEM and TEM. As the intrinsic structure of ZHS has a cubic
symmetry, the samples are mainly composed of cubic particles (Figure 4). As shown in Figure
4a, the edge length of the cubes in ZHS-10 is in the range of 50 to 100 nm. The surface is coarse
and some small nanoparticles contact tightly on the cubes. This morphology feature is further
14
ACS Paragon Plus Environment

Page 15 of 43
ACS Catalysis
1
2
3
4
5
6
7
8
9
confirmed by the TEM image (Figure 4b and c). HRTEM image (Figure 4d) indicates that the
size of the nanoparticles is no more than 5 nm (as indicated by the cycles). A set of lattice fringes
with the inter-plane spacing ca. 0.33 nm is present in the nanoparticles, which correspond to the
d value of (110) plane of tetragonal SnO2. As a type of hydroxides, the radiation damage of ZHS
under the exposure of high energy of electron beam can be observed in Figure 4b. A layer of
amorphous phase with a thickness about 4 nm is formed during the TEM observation. The angle
between the two adjacent facets is ca. 90o suggesting the cube surface is dominated by six
equivalent {100} facets.29 As for the sample ZHS-13, the size of the cubes is comparable to that
in the ZHS-10 sample (Figure 4e and f). However, the morphology becomes less uniform and the
surface is even and compact. No any SnO2 nanoparticles can be observed, consisting well with
the XRD and UV-Vis DRS results. The smoothness of the surface and the absence of SnO2
component imply that an etching process is occurred on sample ZHS-13 during the hydrothermal
treatment. A similar phenomenon has been reported.15a, 30
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
3.4 Photocatalytic Activities
3.4.1 Decoloration of MO Solution
15
ACS Paragon Plus Environment

ACS Catalysis
Page 16 of 43
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 5 (a) Chemical structure of MO and its UV-Vis absorption spectrum and time-
dependent absorption spectrum of MO on different samples under irradiation: (b-e) ZHS
prepared at different pH, and (f-h) at different temperature, (i) no catalyst, (j) ZnO, (k) SnO2 and
(l) P25.
MO was chosen as a model of pollutant to investigate the liquid-solid degradation activity of
the prepared ZHS samples. Using MO as the model has both practical and theoretical
significances. It is a well-known dye pollutant in effluent from textile and other industrial
processes.31 On the other hand, as shown in Figure 5a, MO has an azo bond (N=N) which
associates with chromophores (benzene rings, BRs) and auxochromes. The azo bond accounts
for the maximum light absorption at 465 nm (B1), while the two BRs lead to the small
absorption at 270 nm (B2).32 Monitoring the variation of these bands, especially the B2, can
disclose whether the sample has a capability to open the highly stable BR. Figure 5b-h shows the
16
ACS Paragon Plus Environment

Page 17 of 43
ACS Catalysis
1
2
3
4
5
6
7
8
9
time dependent photocatalytic decoloration of MO over the prepared ZHS samples. A significant
degradation of MO can be observed on most of the ZHS samples (i.e. S-ZHS) except ZHS-13
and the MO solutions become colorless within ca. 40 min. The activities are comparable to the
famous P25 (Figure 5l). More importantly, the BR can be successfully destroyed over these
samples. A simultaneous decrease of B1 and B2 with illumination time can be observed.
However, ZHS-13 (Figure 5e) shows no apparent activity. The decoloration of MO is slightly
larger than a photolysis process (Figure 5i) and B2 also shows no significant change with
irradiation time. Due to the use of high energy UV-C (254 nm), the breakage of the azo bond
(B1) can be observed in the photolysis process. Similar decoloration process has been observed
under irradiation.15b, 32 However, B2 is not affected by the irradiation suggesting the BR cannot
be cracked by UV-C. The activities of the as prepared ZnO and SnO2 were also investigated and
the results are shown in Figure 5j and k. Apparently, ZnO shows much lower activity than the
prepared S-ZHS samples, while the activity of SnO2 is comparative. However, the capability of
SnO2 to open the BR is substantially lower than that of S-ZHS samples (indicated by the
rectangles in Figure 5g and k). After irradiation for 40 min, although the solution is already
colorless in SnO2 suspension, the absorption band of B2 still can be found. It suggests that SnO2
is more effective for cracking of the azo bond rather than the BR.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
17
ACS Paragon Plus Environment

ACS Catalysis
Page 18 of 43
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 6 Decoloration efficiency of MO after irradiation for 20 min over the prepared ZHS
samples and the compared photocatalysts.
To compare the photocatalytic performance of the ZHS samples, the decoloration efficiencies
of MO after irradiation for 20 min are summarized in Figure 6 based on the variation of B1. It
indicates that increasing the preparation solution pH from 4 to 10 brings a slight decrease of the
decoloration efficiency, from 73.3% to 69.1%. With further increasing the pH to 13, the
efficiency of the resulted sample (ZHS-13) drops sharply to only 7.2%. It is almost identical to
the efficiency of the photolysis process (7.0%). As for the effect of the preparation temperature,
the efficiency is improved considerably with the temperature, from 39.3% (20 oC) to 69.1% (120
oC), and then remains almost constant. The samples prepared at low pH (4-7) or at mild
o
temperature (120-150 C) show no remarkable difference for the degradation of MO. On these
samples, the cleavage of the azo bond efficiency is comparable to that of SnO2 (69.2%) and
famous P25 (73.6%), but much larger than that of ZnO (only 8.5%).
18
ACS Paragon Plus Environment

Page 19 of 43
ACS Catalysis
1
2
3
4
5
6
7
8
9
The N2 adsorption–desorption isotherms of the prepared ZHS were determined at 77 K (Figure
S4) and the BET surface areas were then calculated from the BET transform plot of 1/Q[(P0/P)-
1] versus P/P0 (the insert of Figure S4). The samples have IV-type isotherms with a small
hysteretic loop according to the IUPAC classification. For the samples prepared at different pH,
the areas decrease in the order 27.0 (pH 4) > 15.1 (7) > 10.1 (10) ≈ 10.6 m2/g (13). For the
samples prepared at different temperature, the area increases slightly from 12.8 to 14.2 m2/g with
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
o
o
o
o
temperature from 20 C to 90 C, and then reduces from 10.1 (120 C) to 9.5 m2/g (150 C). An
association analysis has been made to disclose the relationship between the activity of ZHS and
its surface area. As shown in Figure 6, there is no consistent relationship between them
suggesting the different activity should be mainly controlled by other factors rather than only the
surface area. It is more likely to be the presence of SnO2 considering the degradation efficiency
exhibits almost the same varying trend as that of the SnO2 amount (estimated by the SnO2 (110)
peak intensity shown in Figure 1) with change of the preparation conditions. The assumption is
also supported by the fact that, after removing SnO2 component, no obvious photocatalytic
activity was observed on the sample ZHS-13.
3.4.2 Decomposition of Gaseous Benzene
The degradation of MO indicates that S-ZHS samples have a great capability to cleave the BR,
as well as P25. However, the decomposition and the mineralization of benzene in the gas
atmosphere may be quite different to that in a liquid medium for the deficiency of water which is
crucial for the elimination of VOCs.11a, 33 To verify the validity of S-ZHS and to study the effects
of the preparation conditions on the activity, PCO of gaseous C6H6 was further evaluated in a
plug flow mico-reactor with a continuous flow mode. As shown in Figure S5-A and B, excluding
ZHS-13 (Figure S5-A-d), the degradation of C6H6 can be achieved on the prepared ZHS samples
19
ACS Paragon Plus Environment

ACS Catalysis
Page 20 of 43
1
2
3
4
5
6
7
8
9
(i.e. S-ZHS samples). C6H6 concentration drops immediately after turning on the UV lamps.
Meanwhile, the mineralization product of CO2 is present. The reactions gradually approach a
steady state after illumination for no more than 4 h. The performances of the prepared ZnO and
SnO2, the support SiO2, and the benchmark P25 were also investigated (Figure S5-C). SiO2 and
ZnO showed no activity for the degradation of C6H6. After purging with the reaction gas for
about 180 min, an equilibrium concentration ca. 250 ppm C6H6 (the exact value of the feed gas)
can be reached and no CO2 was generated even being illuminated for about 4 h (Figure S5-C-a
and c). It precludes the possibility that the support (SiO2) contributes to the degradation of C6H6.
Although the decomposition of C6H6 can be realized on SnO2 and P25, the activities are much
lower than that of the prepared S-ZHS samples. What is worse, unlike S-ZHS, the color of SnO2
and P25 turn brown (Figure 10) due to the deposition of some reaction intermediates. It suggests
that the decomposition of C6H6 on these samples cannot sustain for a long time. In fact, as
shown in Figure S5-C-d, the deactivation of SnO2 with irradiation time can be perceived.
According to the steady-state profiles shown in Figure S5, the CE and ME of C6H6 on the
different samples can be calculated. The results are summarized in Figure 7. It indicates that the
conversion of C6H6 on the ZHS samples is favored by raising the solution pH, from 50.1% (pH
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
o
o
3) to 66% (pH 10), or the temperature, from 25.4 % (20 C) to 66% (120 C). By further
increasing the pH to 13, the CE of ZHS-13 is only 4.1% and almost identical to the value of
o
SnO2. A decay of CE can be found on the sample prepared at a high temperature (150 C). The
sample prepared at 120 oC exhibits the highest CE. Although P25 shows high capability to crack
BR in the degradation of MO (Figure 5l), the CE of gaseous C6H6 is only 9.7%. Unlike CE, the
ME of C6H6 on the prepared S-ZHS seems less impacted (around 68%) by the solution pH, but
o
o
decreases with the hydrothermal temperature, from 77.5% (20 C) to 42.5% (150 C). A much
20
ACS Paragon Plus Environment

Page 21 of 43
ACS Catalysis
1
2
3
4
5
6
7
8
9
higher ME of C6H6 is achieved on ZHS-13, SnO2, and P25 as only minor amounts of C6H6 are
involved in the degradation reactions. Although S-ZHS samples show lower mineralization
efficiency of C6H6 than SnO2 and P25, it should be noted that the efficiencies were achieved at a
quite high conversion of C6H6 in a short contact time (1.8 s). High ME value can be expected by
increasing the loading amounts of the photocatalysts or the contact time. This assumption was
confirmed by a control test with 0.3 g ZHS-10 sample was loaded. The results indicated that
C6H6 could not be detected anymore, which suggests a complete conversion of C6H6.
Meanwhile, ca. 1200 ppm CO2 was produced indicating a ME value of ca. 80%.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
21
ACS Paragon Plus Environment

ACS Catalysis
Page 22 of 43
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 7 (a) Conversion (C%) and mineralization (M%) efficiency of C6H6 achieved on the
prepared ZHS, SnO2, and commercial P25; (b) Degradation (rD) and mineralization (rM) rates of
C6H6 on the prepared ZHS, SnO2, and commercial P25.
The degradation (rD) and mineralization (rM) rates of C6H6 on the photocatalysts are
calculated by Eq. 3 and 4. The result is more straightforward than Figure 7a to disclose the
degradation efficiency of C6H6 on the prepared ZHS. As shown in Figure 7b, with the change of
the ZHS preparation conditions, both rD and rM show a same variation tendency to that of the CE
shown in Figure 7a. Specifically, the rates can be improved by increasing the solution pH from 4
to 10 and then decrease sharply with further increasing the pH to 13. The improvements can also
be achieved by increasing the hydrothermal temperature from 20 to 120 oC. However, compared
to the solution pH, the enhancements are more distinct with the change of the temperature. A
o
significant reduction of the rates is observed with further increasing the temperature to 150 C,
especially rM. Apparently, the rD and rM of C6H6 obtained on the S-ZHS samples are
substantially larger than that on P25, SnO2 and the SnO2-removed ZHS sample (i.e. ZHS-13). It
22
ACS Paragon Plus Environment

Page 23 of 43
ACS Catalysis
1
2
3
4
5
6
7
8
9
seems that the decomposition of gaseous C6H6 depends on both the presence of SnO2 and ZHS
components. Thus, a synergistic effect can be expected between them. The highest rD and rM are
132.6 and 89.6 umol∙h-1∙g-1, respectively and they are achieved on the ZHS-10. The rates are
almost six times higher than that of P25. Here, we must highlight that the activity is quite
remarkable because only 0.1 g photocatalyst was used and the concentration of C6H6 is as high
as 250 ppm. More importantly, the degradation was performed in a flow mode. In most reported
works, the level of C6H6 concentration is only several ppb or few ppm and several grams of
photocatalysts were used or the degradation reaction was performed in a batch mode.8c, 34
3.4.3 Photocatalytic Stability of ZHS
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The stability of S-ZHS was evaluated by degradation of gaseous C6H6 on the selected ZHS-
10. The reaction was lasted for 48 h. During the reaction, the lamps were turned off intentionally
for a while. The decomposition of C6H6 then ceases immediately as shown in Figure 8. The
concentration of C6H6 then returns to the initial value and no CO2 can be detected. The activity
can be reactivated by switching on the lamps again. It clearly indicates that the degradation of
C6H6 is driven by photon. No deactivation of the sample can be observed with the reaction
proceeded. On contrary to the SnO2 and P25 samples, the color of ZHS-10 is unchanged after a
long-term reaction (as shown in Figure 9). XRD result (Figure S6) indicates that the crystal
structure of the used sample is also not affected by the reaction.
23
ACS Paragon Plus Environment

ACS Catalysis
Page 24 of 43
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 8 The long-term degradation of gaseous C6H6 on the ZHS-10 sample. (0.08 g sample
was used).
Generally, the deactivation of a photocatalyst is mainly caused by the decomposition of itself
via a photo-corrosion process, such as CdS and ZnO, or the deposition of some stable
intermediates. To further confirm the stability of the ZHS-10, the depositions of the
intermediates were studied by FTIR analysis. As shown in Figure 9a, the spectrum of the used
sample is almost identical to the fresh one. No new absorption band can be found suggesting no
stable byproduct is formed on the sample. However, as for the used SnO2 and P25 (Figure 9b
and c), some new absorption bands are present at 1482, 1400 and 1170 cm-1. The band at 1480
cm-1 can be assigned to the C–C vibrations of the BR.35 The other bands can be attributed to the
symmetric vibration of carboxylates (-COO-, 1400 cm-1)33, 35a and C-C stretching (1170 cm-1) of
BR35a or the phenolic group vibration36. The formation of these intermediates leads to the color
change of SnO2 and P25 from white to brown. Thus, the stability of ZHS can be largely
approved by these results.
24
ACS Paragon Plus Environment

Page 25 of 43
ACS Catalysis
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 9 FTIR spectra of the samples before and after the degradation of C6H6: (a) sample
ZHS-10, (b) SnO2, and (c) P25. The right inset shows the digital pictures of the used samples.
3.5. Proposed Reaction Mechanism
The photocatalytic performances suggest that a synergistic effect between SnO2 and ZHS,
which accounts for the high PCO activity of the S-ZHS samples, especially for the degradation
of gaseous C6H6. As PCO reaction is composed of a generation of active radicals and a
subsequent surface degradation of C6H6, the synergistic effect should be understood from these
two aspects. With this consideration, the active radicals formed on the ZHS-10 (as a
representative of S-ZHS samples) were then determined and compared with that of the pristine
SnO2 and ZHS (i.e. ZHS-13) by ESR techniques using DMPO as a trapping reagent. The
evolution of the trapped protonated O2∙- (DMPO-∙OOH, labeled with *) and ∙OH (DMPO-∙OH,
25
ACS Paragon Plus Environment

ACS Catalysis
Page 26 of 43
1
2
3
4
5
6
7
8
9
labeled with ♦) signals with irradiation time are shown in Figure 10A and 11B, respectively. The
spectra recorded at 0 s indicate that O2∙- and ∙OH cannot be formed without irradiation. On
pristine SnO2 and ZnSn(OH)6 (Figure 10A-a and b), the trapped O2∙- signal is still hard to be
perceived even after irradiation for 160 s. Their time-dependent spectra are almost identical to
the corresponding background spectrum. However, the characteristic sextet peaks of the DMPO-
∙OOH adduct37 can be easily identified on ZHS-10 (Figure 10A-c). As for the measurements of
∙OH, with irradiation time prolonged, the characteristic quartet peaks of DMPO-∙OH adduct4a, 38
with intensity ratio ca. 1:2:2:1 can be observed on SnO2 and ZHS-10 (Figure 10B-a and c), but
hard to be distinguished on pristine ZnSn(OH)6 (Figure 10B-b). Apparently, the formation of
O2∙- and ∙OH on SnO2 or ZHS-10 is induced by photons because their signals increase steadily
with irradiation time. Although pristine SnO2 and ZnSn(OH)6 show low or no capability for the
formation of active radicals, amazingly, a pronounced formation of O2∙- and ∙OH can be
achieved on the SnO2-decorated ZnSn(OH)6, i.e. ZHS-10.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
26
ACS Paragon Plus Environment

Page 27 of 43
ACS Catalysis
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 10 ESR spectra of (A) DMPO-∙OOH and (B) DMPO-∙OH adducts formed with
irradiation time of 254 nm light in the suspension of (a) SnO2, (b) ZHS-13, and (c) ZHS-10
samples.
The ESR results indicate that both O2∙- and ∙OH are hard to be generated on pristine
ZnSn(OH)6, while O2∙- cannot be formed on SnO2. The phenomena can be interpreted by their
band structures. As shown in Figure 3, the Eg of ZnSn(OH)6 is 5.0 eV, corresponding to a light
wavelength of 248 nm. Therefore, ZnSn(OH)6 is essentially hard to be activated by the UV
lamps because the emission is mainly centered at 254 nm. This is the reason why ZHS-13 shows
almost no activity for the degradation of MO and C6H6. As for SnO2, although it can be
27
ACS Paragon Plus Environment

ACS Catalysis
Page 28 of 43
1
2
3
4
5
6
7
8
9
activated by the lamps, the conduction band (CB) edge of SnO2 locates at ca. 0 V (vs. NHE)39
and is more positive than the reduction potential of O2 (O2 + e- → O2∙-, -0.33 V vs. NHE40).
Thus, O2 cannot be directly reduced to O2∙- by the photo-induced e- in SnO2. However, the
formation of ∙OH can be achieved as its valence band edge (ca. 3.5 V) is larger than the stand
redox potential of ∙OH/-OH (1.99 V, vs. NHE41). In the degradation of MO solution, since a
significant activity has been observed (Figure 5k), e- in SnO2 is more likely to be scavenged by
O2 through a two-electro reduction process: O2 + 2e- + 2H+→ H2O2, 0.69 V vs. NHE.40b, 42
As ZnSn(OH)6 is hard to be activated by the lamps, the pronounced formation of active
radicals on ZHS-10 should be originated from SnO2 component, which serves as a photoactive
center to provide charge carriers. Thus, the role of ZHS component should be understood as a
promoter for the generation of ∙OH and O2∙- rather than a photoactive center. This capability is
afforded by its hydroxyl structure. ∙OH is generally derived through the oxidation of surface –
OH or adsorbed H2O by h+. The hydroxyl structure of ZHS can provide large quantities of M–
OH (M=Sn or Zn) and consequently facilitates its formation. Furthermore, a sustainable
formation of ∙OH also can be guaranteed by the structure. In a photocatalytic process, the surface
–OH will be consumed with the reaction processed (the vacancy of –OH group labeled as M+-□).
Thus the regeneration of surface –OH is an important prerequisite for a sustainable formation of
∙OH. Due to its polyhydroxy surface, the adsorption of H2O and the subsequent regeneration of –
OH group can be favored by the structure. The trapping of H2O is extremely important for the
degradation of gaseous C6H6 as the reaction is performed in a water-deficient environment and
most of the moisture comes from the degradation product of C6H6. Thus, an efficient and
sustainable generation of ∙OH can be expected on ZHS, which has been observed in our previous
work.4a
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
28
ACS Paragon Plus Environment

Page 29 of 43
ACS Catalysis
1
2
3
4
5
6
7
8
9
Although O2 cannot be directly reduced to O2∙- on pristine SnO2 (O2 + e- → O2∙-), the
formation of O2∙- on ZHS-10 is more likely via a proton-involved reduction process O2 + H+ + e-
→ HO2∙- (-0.046 V vs. NHE42-43) as the CB edge of SnO2 (ca. 0 V) almost locates at the
potential. The proton may come from the regeneration of –OH group: M+-□ + H2O →M–OH +
H+. Previous works indicated that a negative shift of the SnO2 CB edge (from 0.06 V to -0.27 V)
can be achieved by doping Zn2+ 44 or decoration of an insulating component like MgO39d, 45. In
ZHS-10 sample, a similar shift of the CB edge of SnO2 component may occur to satisfy the
potential deficiency. The dopant Zn2+ can be provided by ZHS as some Zn2+ can diffuse into
SnO246 during the hydrothermal treatment. Here, to confirm the negative shift of the CB edge, a
comparison test was performed between pure SnO2 and Zn doped SnO2. The samples were
prepared by the similar hydrothermal methods as that of ZHS samples. As shown in Figure S7-a,
both of the samples can be indexed to the tetragonal SnO2. However, a redshift of the absorption
can be observed in the Zn doped SnO2 in Figure S7-b. Consistent with the reported work, Mott-
Schottky analysis (Figure S7-c) indicate that the flat-band potential of SnO2 moves from -0.04 to
-0.11 V after doping with Zn. Besides, ZHS component may serve a similar function as that of
MgO and also leads to the shift of SnO2 CB. Although the exact reaction mechanism is still
unknown, one thing is sure that HO2∙- can be efficiently produced in the SnO2 decorated ZHS as
shown in Figure 10A-c. Furthermore, it has been demonstrated that the adsorption of O2 is
proportional to the amount of surface OH group.7b, 47 That means the polyhydroxy structure of
ZHS also favors the formation of HO2∙-.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
According to the discussion, the formation processes of ∙OH, HO2∙- and the regeneration of –
OH in the ZHS-10 sample are briefly described in Scheme 1.
29
ACS Paragon Plus Environment