Adsorption and photocatalytic degradation of gas-phase UDMH under simulated sunlight by AgBr/TiO2/rGA

Article abstract of DOI:10.1039/d1ra01325d

The degradation of UDMH has long been a concern for its harmful effects on humans and the environment. The current research on gas-phase UDMH treatment is limited and mainly focuses on ultraviolet light and high temperature environments, however the highly toxic substance NDMA is easily produced. In order to investigate the possibility of UDMH degradation in sunlight, AgBr/TiO₂/rGA composites were prepared with the addition of different amounts of silver bromide. The highest UDMH conversion of AgBr/TiO₂/rGA in humid air is 51%, much higher than the control group value of 24%, which can be ascribed to the synergy of adsorption and photocatalysis. The graphene and silver in AgBr/TiO₂/rGA not only enhance the adsorption of light and UDMH, but also inhibit charge recombination and enhance electron-hole separation. More importantly, the temperature of the AgBr/TiO₂/rGA composite was raised by the photothermal effect of graphene with promoted UDMH degradation efficiency. Furthermore, it is noted that NDMA was not detected in the optimal conditions.

Full text of DOI:10.1039/d1ra01325d

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Adsorption and photocatalytic degradation of gas-
phase UDMH under simulated sunlight by AgBr/
TiO2/rGA
Cite this: RSC Adv., 2021, 11, 12583
*
Jia Ying, Lv Xiaomeng, Huang Yuanzheng and Shen Keke
Hou Ruomeng,
The degradation of UDMH has long been a concern for its harmful effects on humans and the environment.
The current research on gas-phase UDMH treatment is limited and mainly focuses on ultraviolet light and
high temperature environments, however the highly toxic substance NDMA is easily produced. In order to
investigate the possibility of UDMH degradation in sunlight, AgBr/TiO2/rGA composites were prepared with
the addition of different amounts of silver bromide. The highest UDMH conversion of AgBr/TiO2/rGA in
humid air is 51%, much higher than the control group value of 24%, which can be ascribed to the
synergy of adsorption and photocatalysis. The graphene and silver in AgBr/TiO2/rGA not only enhance
the adsorption of light and UDMH, but also inhibit charge recombination and enhance electron–hole
separation. More importantly, the temperature of the AgBr/TiO2/rGA composite was raised by the
photothermal effect of graphene with promoted UDMH degradation efficiency. Furthermore, it is noted
that NDMA was not detected in the optimal conditions.
Received 18th February 2021
Accepted 17th March 2021
DOI: 10.1039/d1ra01325d
rsc.li/rsc-advances
In the methods of degradation of gaseous pollutants,
adsorption is effective due to the advantages of easy operation,
1. Introduction
high efficiency, and low cost. Carbon-based materials like gra-
phene, graphene oxide (GO), and reduced graphene oxide (rGO)
have received worldwide attention because of their unique
Unsymmetrical dimethylhydrazine (UDMH) is the main fuel of
liquid propellants in rocket and missile engines widely used in
the national defense and aerospace industry. The UDMH gas
produced during use and storage is in urgent need of treatment
due to its toxic, malodorous, mutagenic and carcinogenic
nature.1,2 In the past years, much research has been done to
solve the problem. Z. R. Ismagilov3 realized the deep catalytic
oxidation of UDMH by CuxMg1ꢀxCr2O4/Al2O3, which produced
high yields of CO2 and low yields of NOx in a temperature range
of 150–400 ꢁC. Kolinko4 studied the gas phase photocatalytic
oxidation of UDMH using a photocatalyst in a batch reactor
under UV light with a stable performance. In the traditional
method, high temperature or pressure is used to treat gas phase
UDMH and the degradation of gas-phase UDMH by photo-
catalysts is carried out under UV light. However, the degrada-
tion of UDMH at normal temperature and pressure under
visible light saves energy and is convenient and therefore
deserves to be studied for practical use. In the degradation
products, NDMA (N-nitrosodiethylamine) is a highly carcino-
genic substance, which can generate spontaneously when
UDMH is just exposed to natural conditions.5 The NDMA level
increases obviously in the common methods for UDMH
degradation, such as the addition of ozone or irradiation under
ultraviolet light. Therefore, the NDMA level deserves the atten-
tion of researchers.
structure and properties. Graphene possesses
a
two-
dimensional planar hexagonal structure with a large specic
surface area and good chemical stability, which is suitable for
use as an adsorbent.6 Furthermore, graphene can also serve as
the electron transport medium in the photocatalytic reaction for
high electron mobility.7–9
In practice, the synergistic effect of adsorption and photo-
catalysis should be emphasized to obtain high gas degradation
efficiency. TiO2, as an efficient photocatalyst, is widely used in
environmental applications because of its wide bandgap, low
cost, high chemical stability, and non-toxic properties.10,11
However, its photocatalytic properties are limited by the high
recombination rate of the photogenerated carrier.12 Graphene,
as a cocatalyst, can combine with TiO2 by a Schottky junction as
the Fermi level of graphene is lower than TiO2 and electrons can
transfer from TiO2 to graphene. Jiang13 synthesized a graphite
oxide/TiO2 composite, which showed 7.4 and 5.4 times higher
degradation rates for methyl orange and the photo-reductive
conversion rate of Cr(VI) than P25. Nowadays, the develop-
ment of visible-light photocatalysts is emerging as an important
research direction to make full use of solar energy. Silver
bromide (AgBr) is a kind of semiconductor with a bandgap of
2.6 eV. Metallic Ag0 produced in the process may act as
a cocatalyst to enhance the photocatalytic activity of TiO2. Silver
nanoparticles could have a strong absorption of photon energy,
High-Tech Institute of Xi'an, Xi'an 710025, China. E-mail: jyingsx@163.com; Tel:
+86-9178744251
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and a local plasma resonance phenomenon would occur when
the frequency of incident light matches the vibration frequency
of silver under irradiation. The efficiency of light adsorption
and electron transportation will increase through the SPR effect
of Ag. In previous research, Ag–AgBr/TiO2/rGO was fabricated to
degrade penicillin
G under white LED irradiation. The
composite could be photoexcited by visible light with wave-
lengths extending up to 600 nm.14
Fig. 1 A schematic illustration of the preparation of AgBr/TiO2/rGA.
Here we prepared AgBr/TiO2/reduced graphene oxide aero-
gels (rGA) by hydrothermal reduction and then used the
precipitation method to degrade gas-phase UDMH under
simulated sunlight. The composites were characterized and the
effect of humidity on UDMH degradation was evaluated with
different contents of AgBr. The promotion effect of the photo-
thermal process was also investigated. The possible degrada-
tion mechanisms and routes were proposed based on the
characteristic results and chromatographic data.
2.2 Characterizations
The phase structures of the samples were analyzed by X-ray
diffraction (XRD) with Cu Ka radiation (l ¼ 0.15401 nm) at
a generator voltage of 40 kV and a current of 40 mA (D/max
2600, ISUZU, Japan). Analysis was performed under a 2q
range of 5.0–85.0ꢁ and at a scanning rate of 0.02ꢁ per second.
The surface morphologies were characterized using a trans-
mission electron microscopy (TEM) system (TecnaiG2F20,
FEI, USA) and a scanning electronic microscopy (SEM) system
(VEDAIIXMUINCN, TESCAN, Czech Republic) equipped with
an energy dispersive X-ray spectroscopy (EDS) system. Raman
spectra were taken using Ar+ (532 nm) laser excitation (in Via
Reex, Renishaw, England). The spectra were taken in the
range 100–3200 cmꢀ1. The surface functional groups were
analyzed by Fourier transform infrared (FT-IR) (NEXUS,
Nicolet, USA) in the transmittance mode with the spectral
range of 400–4000 cmꢀ1. X-ray photoelectron spectroscopy
(XPS) measurements were performed by a Thermo Scientic
ESCA Lab250 spectrometer with an Al Ka X-ray source. All
binding energy values were corrected by calibrating the C 1s
peak at 284.8 eV. The Brunauer–Emmett–Teller (BET) specic
surface area and porosity of the samples were evaluated
based on nitrogen adsorption isotherms measured at 77 K
using a gas adsorption apparatus (TriStar II 3020, micro-
meritics, USA). All the samples were degassed at 180 ꢁC
before the nitrogen adsorption measurements. The stability
of the samples and contents of each component were deter-
mined by TGA/DSC (SDT-Q600, TA, USA) in the air with
a heating rate of 20 ꢁC minꢀ1. UV-vis DRS spectra were
recorded on a UV-vis spectrophotometer (UV-3600, Shi-
madzu, Japan) equipped with an integrating sphere
assembly. PL measurements were performed with the exci-
tation wavelength of 325 nm using a uorescence spectro-
photometer (FLS1000, Edinburgh, England).
2. Experimental section
2.1 Synthesis
2.1.1 Preparation of GO. Graphite oxide was prepared
according to the modied Hummers method.15 Briey, 3 g of
natural graphite was mixed with 70 mL of concentrated H2SO4
under stirring, then 1.5 g of NaNO3 and 9 g of KMnO4 were
slowly added to the mixture in an ice bath. Aerwardꢁs, the
solution was removed from the ice bath and kept at 35 C for
90 min under strong stirring. Successively, 140 mL of DI water
was added, and the solution was stirred at 95 ꢁC for 15 min.
Aer that, 500 mL of DI water and 20 mL of 30% H2O2 was
added. Finally, the solution was ltered and washed with HCl
(1 : 10) and DI water several times. The supernatant was then
removed by centrifugation to obtain a brown slurry. Then the
slurry was dissolved in DI water by ultrasonication with
a concentration of 2 g Lꢀ1
.
2.1.2 Preparation of TiO2/RGH (reduced graphene oxide
hydrogel). 40 mL of GO, 0.03 g of glucose and 0.72 g of Ti(SO4)2
were magnetically stirred for 30 min and placed into a Teon-
lined autoclave maintained at 180 ꢁC for 12 h. The hydrogel
was then rinsed twice with DI water. Pure TiO2 was prepared by
the same method without the addition of GO.
2.1.3 Preparation of AgBr/TiO2/rGA. Typically, 0.275 g of
hexadecyl trimethyl ammonium bromide (CTAB) was added to
160 mL of DI water and 40 mL of alcohol. The as-prepared TiO2/
ꢁ
RGH was immersed in a CTAB solution at 60 C for 6 h. CTAB
can provide Brꢀ and charge the surface of the graphene sheets.
2.3 Photocatalytic activity
The excess CTAB solution is ltered out and the hydrogel was
further immersed in 40 mL of 0.0075 mol Lꢀ1 AgNO3 solution The photocatalytic degradation of UDMH was conducted in
for 12 h. The process was operated in dark conditions. The a stainless steel cylindrical reactor with a quartz window
prepared AgBr/TiO2/RGH was washed with alcohol and deion- right on the reactor under the irradiation of Xenon lamp
ized water and dehydrated by freeze-drying to obtain AgBr/TiO2/ (150 W power, AHD). A quartz tank with a sand-plate was put
rGA-1, as shown in Fig. 1. The masses of CTAB were adjusted to in the reactor and 30 mg of catalyst was placed on the sand-
0.55, 1.1, 2.2 and 4.4 g, and the concentrations of AgNO3 were plate. The distance between the sample and lamp was
0.015, 0.03, 0.06, and 0.12 mol Lꢀ1 and the products were 12.7 cm. The reaction device was cooled by circulating water
recorded as AgBr/TiO2/rGA-2, AgBr/TiO2/rGA-3, AgBr/TiO2/rGA- and the temperature was controlled by a heating plate at the
4, and AgBr/TiO2/rGA-5. AgBr/TiO2/rGA represents AgBr/TiO2/ bottom of the reactor. A schematic diagram of reaction
rGA-2 if not specied.
system is shown in Fig. 2. UDMH was put in a refrigerator
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Fig. 2 A schematic diagram of reaction system.
purged by nitrogen. Water vapor was generated by bubbling where UDMHinlet and UDMHoutlet is the concentration of
puried air provided by an air compressor. The ow of UDMH at the inlet and outlet, respectively.
UDMH, water vapor and puried air at a total ow rate of 0.6
L minꢀ1 were adjusted with a mass owmeter and then mixed
3. Results and discussion
3.1 Microstructure and chemical composition
well in a buffer bottle. The duration time of each experiment
is 60 min aer equilibrium. The concentration of UDMH was
measured by a gas chromatograph (Clarus 680, PerkinElmer,
USA)-mass spectrometer (Clarus SQ 8T, PerkinElmer, USA).
The degradation gas was periodically transferred into
a sampling loop (500 mL) via a ten-way valve and separated
through a capillary column (Elite-WAX, PerkinElmer) with an
inner diameter of 0.25 mm and a length of 2 m. The oven
The crystal structures of GO, TiO2/rGA, AgBr/rGA, and AgBr/
TiO2/rGA were characterized by XRD in Fig. 3. The diffraction
peaks at 10.9ꢁ in GO represent oxidized graphite corresponding
to the (001) plane.16 The d-spacing value of GO calculated using
Bragg's law (2d sin q ¼ nl) is 0.76 nm based on the (001)
reection plane. The wide diffraction peak at around 26.5ꢁ in
temperatures wereꢁprogrammed as follows: rstly, the initial
rGA can be attributed to the high stripping of graphene. The d-
temperature of 50 C was maintained for ꢀ11min, then it was
spacing decreased to 0.336 nm due to the reduction of oxygen-
containing groups and the overlap of graphene. The main
diffraction peaks in AgBr/rGA and AgBr/TiO2/rGA at 2q values of
26.7ꢁ, 30.9ꢁ, 44.4ꢁ, 55.0ꢁ, 64.5ꢁ, and 73.3ꢁ can be respectively
indexed to the (111), (200), (220), (222), (400), and (420) crystal
planes of AgBr phase (JCPDS no. 06-0438). What is more, the
diffraction peaks of Ag appeared at 38ꢁ in AgBr/rGA and AgBr/
TiO2/rGA (overlap by TiO2), because the defects on rGA
ꢁ
ꢁ
increased to 100 C at a rate of 20 C min and maintained
for 1 min. Finally, it was increased to 180 ꢁC at a rate of
10 ꢁC minꢀ1 and maintained for 1 min. Conversion of UDMH
was dened as the following:
Conversion (%) ¼ (UDMHinlet ꢀ UDMHoutlet)/UDMHinlet
ꢂ
100%
Fig. 3 (a) XRD patterns of the samples. (b) XRD patterns of AgBr/TiO2/rGA with different contents of AgBr.
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produced in the reduction could reduce Ag+ to metallic Ag.17 positions of the single component, implying the mutually
The wide peak of rGA at around 26.5ꢁ can be recognized in AgBr/ independent relation during the in situ growth.19 AgBr/TiO2/rGA
rGA, which proves the formation of the composite. The peak of composites with different amounts of AgBr are compared in
graphene in TiO2/rGA and AgBr/TiO2/rGA is not obvious due to Fig. 3b. The diffraction peaks of AgBr become sharp as the
the decreased layer-attacking regularity of graphene nano- contents increased, indicating the increase in grain size and
sheets18 and the relative low content compared with TiO2. The a higher degree of crystallinity. Based on the Scherrer equation
existence of graphene is illustrated in the following SEM, TEM (d ¼ kl/b cos Q), the crystallite sizes of the AgBr in AgBr/TiO2/
and FTIR. The peaks in TiO2/rGA and AgBr/TiO2/rGA at values of rGA-1, AgBr/TiO2/rGA-2, AgBr/TiO2/rGA-3, AgBr/TiO2/rGA-4, and
25.3ꢁ, 37.8ꢁ, 48.1ꢁ, 54.4ꢁ, 62.9ꢁ, 69.9ꢁ, and 74.9ꢁ can be respec- AgBr/TiO2/rGA-5 were 27 nm, 28 nm, 30 nm, 34 nm and 37 nm,
tively indexed to the (101), (004), (200), (211), (204), (220), and calculated using the (200) plane of AgBr.
(215) planes of the TiO2 anatase phase (JCPDS-21-1272). The co-
The surface morphologies were characterized by SEM and
existence of AgBr and TiO2 does not change the diffraction peak TEM in Fig. 4. The transparent sheet structure with wrinkles is
Fig. 4 SEM images of (a) GO, (b) rGA, (c) TiO2/rGA (d) AgBr/rGA and (e) AgBr/TiO2/rGA, the EDX pattern of (f) AgBr/TiO2/rGA and TEM images of
(g and h) GO and (i–k) AgBr/TiO2/rGA.
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considered as the surface of GO (Fig. 4a) and only a few layers change shows the reduction of GO as the XRD revealed. The
can be seen (Fig. 4g and h) with the mean d-spacing. The d- peak of the C]C vibration became weak and wide for the low
spacing between graphite layers calculated from the corre- content of rGA and overlapping occurred with the adsorption
sponding diffraction pattern in the TEM image is 0.73 nm, band at 1633 cmꢀ1 for the Ti–O–Ti stretching vibration.23 The
which is similar to the XRD result. The surface of rGA is wide peak between 500–900 cmꢀ1 could be ascribed to Ti–O–Ti
relatively smooth because the oxygen-containing groups were and Ti–O–C, which further indicates that a chemical bond
reduced during the hydrothermal reaction (Fig. 4b). In TiO2/ formed between TiO2 and graphene.24,25 The peak is the same as
rGA (Fig. 4c), individual graphene sheets are not distin- shown in TiO2.
guishable and TiO2 nanoparticles are evenly intermixed with
The structures of GO, rGA and AgBr/TiO2/rGA were further
the graphene sheets presumably due to the crosslinking characterized by Raman spectra, as presented in Fig. 6. The
effect of glucose.20,21 In AgBr/rGA (Fig. 4d) and AgBr/TiO2/rGA peaks at about 1350 cmꢀ1and 1590 cmꢀ1 represent the D and G
(Fig. 4e), AgBr particles are distributed uniformly on the band of graphite, respectively. The D and G band reect the
graphene sheets because the graphene aerogel inhibits the disorder and in-plane sp2 carbon structure.22 The higher ID/IG
aggregation of nanoparticles. All the elements including Ag, value shows that the average size of the in-plane sp2 domains is
Br, Ti, O, and C can be found in the EDS of AgBr/TiO2/rGA smaller and the reduction degree of GO is higher.26 The ID/IG
(Fig. 4f). The atomic percentages of C, O, Ti, Br, and Ag on the value of GO, rGA and AgBr/TiO2/rGA is 0.90, 1.01 and 0.93,
surface of AgBr/TiO2/rGA are 47, 33, 14, 3, and 3%, respec- respectively. The increased value in rGA and AgBr/TiO2/rGA
tively. The exposed crystal face can be inferred from the conrmed the reduction of GO, which could suggest the
lattice fringes. The spacings of the lattice fringes in AgBr/ formation of defects or graphene agglomeration during the
TiO2/rGA were 0.352, 0.288 and 0.236 nm (Fig. 4i), which reaction.27 Compared with the spectra of pure graphite crystals
could be assigned to the crystalline planes of anatase TiO2 (1575 cmꢀ1), the peak of the G band shis to 1587.6 cmꢀ1 in
(101), AgBr (200) and Ag (111), respectively. AgBr and TiO2 are AgBr/TiO2/rGA, which suggests some structural imperfections
closely related to each other. The above microscopic study of the carbon shells.28
provides sufficient evidence in support of the fact that AgBr/
TiO2/rGA ternary composites with interfacial contacts were rGA were investigated by XPS spectra. The full-scale XPS survey
successfully fabricated. spectrum of AgBr/TiO2/GA demonstrates the coexistence of Ti,
The surface components and chemical states of AgBr/TiO2/
The Fourier transform infrared spectra of the samples are O, C, Ag and Br, which supports the results of the EDS. The XPS
shown in Fig. 5. The characteristic peaks of the carbonyl C]O spectrum of Ti 2p for AgBr/TiO2/rGA can be divided into three
stretching vibration, C]C vibration of molecular water, C–OH characteristic peaks at the binding energy of 459.5, 465.1 and
bending vibration, and C–O stretching vibrations or epoxy 472.65 eV. The peaks at 459.5 and 465.1 eV with a separation of
C–O–C vibrations appear at 1734, 1616, 1429, and 1080 cmꢀ1 in 5.6 eV can be ascribed to Ti4+ in the pure anatase phase,29 which
GO, respectively. The abundant oxygen-containing groups on indicates the intact TiO2 in the characteristic crystal structure.
the surface indicate the good hydrophilicity of GO,22 which The satellite peak of Ti 2p1/2 at 472.65 eV demonstrates
make it easy to disperse in water and bond with other elements. a chemical linkage between TiO2 and graphene sheets as the
In TiO2/rGA, AgBr/rGA and AgBr/TiO2/rGA, the C]O stretching FTIR revealed. The XPS spectra of the Br species in AgBr/TiO2/
vibration and C–OH bending vibration almost disappear. The rGA display the binding energy of Br 3d3/2 and Br 3d5/2 at 67.9
Fig. 5 FTIR patterns of the samples.
Fig. 6 Raman spectra of rGA, GO and AgBr/TiO2/rGA.
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and 68.9 eV, representing the existence of Brꢀ. The high reso- mesopores of AgBr/TiO2/rGA and the large surface area can provide
lution XPS spectrum of O 1s deconvoluted into three peaks, enough active sites for photocatalysis.
529.4, 530.2 and 532 eV, which correspond to the Ti–O, C]O
and C–OH groups. The energy peaks at 284.8, 285.3 and
286.7 eV for C 1s represent the C–C/C]C, C–OH and C]O
groups in AgBr/TiO2/rGA. Notably, the peak intensities for the
oxygen-containing groups become lower or diminish compared
with the C 1s of GO, which signies the reduction of GO. The
result corresponds with the analysis of Raman and FTIR. The
high resolution XPS spectra of Ag 3d display two individual
bands assigned to the Ag 3d5/2 and Ag 3d3/2 binding energies,
which could be deconvoluted into two new groups of peaks at
367.45 and 368.8 eV and 373.4 and 374.8 eV, respectively. The
peaks at 367.45 and 373.4 eV represent Ag+ and the peaks at
368.8 and 374.8 eV are ascribed to Ag0.
3.2 Optical properties
The UV-vis-NIR DRS spectra shown in Fig. 10 were recorded to
analyze the light adsorption properties of the samples. TiO2
can absorb short-wavelength UV light and the adsorption in
visible light decreases obviously. For AgBr/TiO2, the adsorp-
tion edge exhibits a red shi compared with TiO2 because
silver bromide has a smaller band gap than titanium dioxide
and the SPR effect of silver benets the visible light adsorp-
tion. Compared with AgBr/TiO2, remarkable enhancements
of the adsorption ability were observed in AgBr/rGA in the
visible-light region because graphene can absorb almost the
whole spectrum of solar light due to its black color and zero
band gap. The samples containing graphene, like AgBr/TiO2/
rGA, TiO2/rGA and AgBr/rGA, show a lower adsorption in UV
light, probably because some carbons became doped in the
semiconductor and reduced the bandgap. AgBr/TiO2/rGA and
TiO2/rGA show a higher adsorption intensity in the visible
and infrared light regions for the formation of the Ti–O–C
bond. The enhanced optical absorption benets photo-
catalytic activity in the simulated sunlight.
The photoluminescence (PL) spectra were used to analyze
the lifetime and the migration efficiency of the photo-
generated charge carrier as shown in Fig. 11. The weaker PL
intensity represents the lower electron–hole recombination
rate. The PL spectra of TiO2/rGA was lower than TiO2 for the
addition of graphene. Graphene, acting as the electron
mediator, can efficiently transport and store the photoelec-
trons produced by TiO2,10 and the separation of the photo-
generated electron–hole pairs can be greatly improved.
When AgBr was added into TiO2/rGA, the PL intensity further
decreased for the formation of a Schottky junction between
silver and TiO2 on the surface of the nanocomposite. What is
more, the PL intensity of AgBr/TiO2 is lower than TiO2/rGA
and AgBr/TiO2/rGA. It can be inferred that the nanosilver
can promote the interfacial charge transfer more efficiently
than rGA.
Thermogravimetric analysis (TGA) was carried out to study the
thermo-stability of the samples, and the test results are shown in
Fig. 8. The mass losses of GO can be divided into two stages: at
ꢁ
temperatures below and above 200 C. This is caused by volatili-
zation of water molecules adsorbed by graphite oxide and the
thermal decomposition of oxygen-containing groups in graphite
oxide, respectively. The residual content of carbon in rGA is higher
than GO. In rGA and AgBr/TiO2/rGA, the transition temperatures
rise to 550 ꢁC representing good thermal stability. When the
temperature is 800 ꢁC, the mass percentage of the AgBr/TiO2/rGA
residue increased to 54% compared with 35% of rGA for the
residue of titanium dioxide and silver bromide in AgBr/TiO2/rGA.
The specic surface area and pore structure were characterized
to investigate the internal morphology.30 As shown in Fig. 9, the
isotherms of AgBr/TiO2/rGA, TiO2/rGA, AgBr/rGA and rGA resem-
bled the type IV isotherm of IUPAC with a clear hysteresis loop,
which indicates the mesoporous structures of the samples.31 In
rGA and AgBr/rGA, the H3 hysteretic loop is not in equilibrium
when the relative pressure is close to the saturated vapor pressure
and the adsorption curve increased sharply, mainly because of the
slit-shaped holes formed by the graphene. The surface area of
AgBr/rGA is obviously lower as the introduction of AgBr blocks up
the holes of the graphene aerogel. TiO2/rGA displays the hysteresis
loop with a combination of H2 and H3, possibly caused by
homogeneous titanium dioxide and graphene. The surface area of
TiO2/rGA is 116.74 m2 gꢀ1, as shown in Table 1, higher than that of
rGA. On the one hand, the mesoporous structure in the TiO2/rGA
particles is similar to rGA,32 as shown in Fig. 4b and c. On the other
3.3 Photocatalytic degradation of UDMH
3.3.1 Samples with different components. The UDMH
hand, graphene aerogels suppress the agglomeration of TiO2.33,34 conversion of samples under different conditions is shown in
AgBr/TiO2/rGA has a triangular-shaped adsorption isotherm with Fig. 12. It is noted that rGA presents higher UDMH conver-
the holes mainly in the shape of an ink bottle with some slits. The sion in dark conditions. As shown in the XRD pattern, some
surface area of AgBr/TiO2/rGA is 91.1082 m2 gꢀ1, and the oxygen groups can still be found in rGA. The surplus graphite
oxide would be further reduced under simulated sunlight,
which leads to an increase in the proportion of sp2-hybrid-
ized carbon atoms to sp3-hybridized ones.35 Therefore, the
Table 1 The specific surface area and pore structure determined by
adsorption for UDMH is reduced in the light. TiO2/rGA shows
the same UDMH conversion in simulated sunlight and dark
conditions. It can be concluded that the degradation mech-
anism by TiO2/rGA is mainly adsorption. The mesoporous
structure and the oxygen containing groups on TiO2/rGA are
benecial for adsorption. The mechanism is described in
detail in earlier research.36 The UDMH conversion by AgBr/
the multipoint BET method
Composite
SBET (m2 gꢀ1
)
Vpore (cm3 gꢀ1
)
Dpore (nm)
rGA
97.6
116.7
5.3
0.08
0.18
0.02
0.14
3.33
4.76
16.84
6.32
TiO2/rGA
AgBr/rGA
AgBr/TiO2/rGA
91.1
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rGA and TiO2/rGA was also compared in the dark with 37.5% spectra that shows that AgBr/rGA has a lower recombination
and 40%. The surface area of AgBr/rGA and TiO2/rGA is 5.3 rate of photoinduced carriers than TiO2/rGA. The AgBr/TiO2/
and 116.7 m2 gꢀ1. AgBr/rGA has a higher UDMH conversion rGA composites show better performance than other binary
of 42% than that of TiO2/rGA with 40% in simulated sunlight, and unitary samples under simulated sunlight. The blank
which is attributed to the extraordinary visible light absorp- group was performed in the same condition without catal-
tion of AgBr and nanosilver. The result corresponds to the PL ysis. By comparing the blank group and AgBr/TiO2/rGA in
Fig. 7 XPS spectra of the full spectrum (a), Ti 2p (b), Br 3d (c), O 1s (d), and Ag 3d (f) of AgBr/TiO2/rGA and C 1s (e) of AgBr/TiO2/rGA and GO.
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Fig. 8 TGA images of GO, rGA and AgBr/TiO2/rGA.
Fig. 10 UV-vis-NIR DRS spectra of the samples.
humid air, the UDMH conversion increased from 24% to 37%
under dark conditions. The increase of 13% is attributed to
adsorption. The conversion is 51% for AgBr/TiO2/rGA in
simulated sunlight, and the increase of 14% is due to pho-
tocatalysis. Therefore, adsorption and photocatalysis should
both be taken into consideration in the degradation of
UDMH. The 3D aerogel is conducive to the adsorption of
UDMH. Furthermore, AgBr/TiO2/rGA could exhibit great
visible light adsorption, and the high efficiency of carrier
separation is benecial for photocatalysis.
3.3.2 Different contents of AgBr in AgBr/TiO2/rGA. The
UDMH conversion on AgBr/TiO2/rGA with different contents of
AgBr was compared. As the content of AgBr increases, the
UDMH conversion slowly increases and then decreases.
Compared with AgBr/TiO2/rGA-1, AgBr/TiO2/rGA-2 with enough
nano-silver can produce surface plasma resonance under
simulated sunlight. With more silver in AgBr/TiO2/rGA, the
conversion decreases probably because silver nanoparticles
accumulate into clusters of silver.37 The clusters can reect light
and the SPR effect is constrained, which results in low photo-
catalytic efficiency.
In order to further understand the role of AgBr in the pho-
tocatalysis, the spectra of Ag 3d for AgBr/TiO2/rGA aer the
reaction were recorded, as depicted in Fig. 7a. The surface ratio
of metallic Ag0 is 45% and 83% before and aer the reaction,
respectively. The content of nano-silver increases during the
reaction. As shown in Fig. 13, AgBr can produce photogenerated
electrons and holes once receiving simulated sunlight. The
electrons can easily transfer to the graphene and conduction
band (CB) of TiO2. The Fermi level of silver is low, the photo-
induced electrons on silver bromide can also easily transfer to
the surface of the nano-silver. The photoproduced electrons
Fig. 9 N2 adsorption/desorption curves of AgBr/TiO2/rGA, TiO2/rGA,
AgBr/rGA and rGA.
Fig. 11 PL spectra of AgBr/TiO2/rGA, TiO2/rGA, and TiO2.
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Fig. 12 The UDMH conversion of different samples in dry air and 50%
H2O air.
Fig. 14 A gas chromatogram of the degradation products of UDMH
under simulated sunlight.
during the photocatalysis can also be trapped by Ag+ in AgBr to
produce more metallic Ag0. On the other hand, nano-silver will
produce surface plasma resonance under illumination, which
will raise the Fermi level of silver.27 Polarization elds produced
around Ag could promote electrons to stay away from the silver
bromide,38,39 and inject the electrons into rGA. The carriers on
the surface can react with water and oxygen to produce active
groups with improved photocatalytic efficiency.
humidity has a certain effect on the photocatalyst and UDMH.
Water may inuence the adsorption of AgBr/TiO2/rGA as the
water molecules would occupy the sites in the pores competi-
tively.40 Due to the reduction of GO, more sp2 carbon atoms
emerged on the photocatalyst which would weaken the reaction
between water vapor and the photocatalyst.41 Thus, the
adsorption of UDMH is more than water for AgBr/TiO2/rGA. On
the other hand, the hydrogen bonding and electrostatic forces
may be formed on the adsorbed water to UDMH. Furthermore,
hydroxyl radicals can be produced on the surface of the
3.3.3 Humidity. The UDMH conversion of different
samples in dry and humid air are shown in Fig. 12. The
Fig. 13 Possible photocatalytic mechanism of AgBr/TiO2/rGA nanocomposites.
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Amino oxidation would generate NDMA (eqn (1)).5 DMA
(dimethylamine) and NH2$ can be produced by cleavage of
the N–N bond. NH2$ can be further oxidized to form NO,
which would react with DMA to form NDMA42 (eqn (2)). In
humid air, NDMA cannot be detected, probably because of
the hydrogen bonds formed between water and nitrogen in
UDMH, hindering the amino oxidation process (eqn (3)). In
addition, the methyl group on the UDMH can be oxidized to
form HCHO (formaldehyde) and MMH (methylhydrazine)
(eqn (4)). FDMH (formaldehyde dimethylhydrazone) can be
produced by UDMH reacting with HCHO (eqn (5)). The
hydrogen atom on the methyl group of UDMH can also be
extracted (eqn (6)). The formed (CH2$) (CH3) NNH2 can
combine with oxygen to generate a stable aldehyde. An aldehyde
can also be formed by the hydroxyl radical reacting with a methyl
radical so more butanedial was produced in humid air. What
is more, ethanol and acetic acid would be produced in the pres-
ence of water (eqn (7)),43 which further led to the formation of an
ester. On the other hand, the amino group, as a nucleophile,
may attack the carbonyl group on the ester to form ethyl ethani-
midate (eqn (8)).
Table 2 Products of the reaction of UDMH with AgBr/TiO2/rGA under
simulated sunlight
50%
Product
Peak/min
CAS No.
Dry
H2O
FDMH
UDMH
2.12
2.37
2035-89-4
57-14-7
+
++
+
+
Ethyl ethanimidate
Butanedial
DMA
2.87
3.10
3.77/3.63
5.19
1000-84-6
638-37-9
124-40-3
62-75-9
+
++
+
+
+
+
NDMA
3.3.4 Photothermal experiment. As shown in Fig. 12, the
UDMH conversion of AgBr/TiO2/rGA is higher in the simulated
sunlight than that in the dark. It is worth considering that the
catalytic reaction is photocatalysis or a thermocatalytic
process,44,45 which is initiated by hot electrons under light or
phonons produced by hot electrons. The photothermal
experiment of AgBr/TiO2/rGA was studied by an IR thermal
camera, as depicted in Fig. 16. 30 mg of AgBr/TiO2/rGA was
irradiated under a Xenon lamp for 5 minutes with the same
distance of the UDMH degradation experiment. The rGA and
blank group were conducted for comparison. The weight of
rGA was the same as that of rGA in the AgBr/TiO2/rGA sample.
Before irradiation, the initial temperature was controlled at
Fig. 15 The formation pathways of the main products.
ꢁ
about 16 C. Aer 5 minutes irradiation, the temperature of
photocatalyst, which benet photocatalysis and chemisorption.
As a result, UDMH can easily transform to other species in the
conditions of water and air. In order to investigate the different
paths of UDMH transformation under dry and humid air, the
chromatograms and intermediates are compared in Fig. 14 and
Table 2.
According to the transition state theory, the abstraction of
hydrogen or the addition of oxygen can take place on UDMH
aer UDMH is exposed to oxygen. The main conversion path
is shown in Fig. 15. The reaction barrier of the N–H bond is
the lowest so the intermediate (CH3)2NNH$ is easily formed.
the blank group remained relatively stable. The temperature
of AgBr/TiO2/rGA rose to 51.7 ꢁC, higher than the blank
control group of 29.3 ꢁC. The temperature could reach 48.1 ꢁC
in rGA, which means that the rise of temperature in AgBr/
TiO2/rGA mainly comes from graphene. The possible mech-
anism is that the photoexcitation under light irradiation
heats up the graphene sheets and promotes the plasmon
resonance of the surface silver via its photothermal effect.
The elevated temperature on graphene also benets the
carrier transportation, which in turn enhances the electron
Fig. 16 The IR images of AgBr/TiO2/rGA and the blank group.
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In this work, we synthesized AgBr/TiO2/rGA ternary nano-
composites via a facile solvothermal method with a uniform
distribution of AgBr. The nanocomposites exhibit great thermal
stability and visible light adsorption. The UDMH degradation
by AgBr/TiO2/rGA in the owing gas is enhanced with an
optimal conversion of 51% compared with other binary and
unitary samples, which is attributed to the high specic surface
area, the signicant enhanced light adsorption and the efficient
separation and transmission of carriers. The photothermal
effect also contributes to the degradation of UDMH. The species
and quantity of products vary with different humidity levels and
the carcinogenic substance NDMA is not detected in humid air.
Further studies are required to optimize the experimental
equipment to further improve the efficiency. The work could
also pave the way for UDMH waste gas treatment in sunlight.
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Author contributions
HOU Ruomeng: methodology, soware, validation, and writing-
original dra preparation. JIA Ying: conceptualization and
supervision. LV Xiaomeng: resources. Hung Yuanzheng: inves-
tigation. Shen Keke: writing-reviewing and editing.
Conflicts of interest
There are no conicts to declare.
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380–386.
26 J. Luo, Z. Yan, R. Liu, J. Xu and X. Wang, RSC Adv., 2017, 7,
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Acknowledgements
This project is supported by the National Natural Science
Foundation of China (21875281).
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