TiO2-reduced graphene oxide for the removal of gas-phase unsymmetrical dimethylhydrazine

Article abstract of DOI:10.1039/d0nj04617e

Unsymmetrical dimethylhydrazine (UDMH) contaminated waste gas and related intermediates pose a great threat to human health. TiO2-reduced graphene oxide aerogel (rGA) samples with different graphene content levels were synthetized and characterized for the degradation of UDMH. The effects of GO content, humidity, and temperature were investigated under UV and VUV light, with highest UDMH conversion values of 68% and 95%, respectively. Compared with pure TiO2, the enhanced degradation activity of TiO2-rGA under UV light can be attributed to a synergetic effect between absorption and photocatalysis, while the high UDMH conversion under VUV light relies on photolysis and ozonation. The high oxygen-containing group content, rather than a high SSA, and electron trapping by graphene are key factors determining the outstanding performance of TiO2-rGA with 80 mg of GO. The prepared TiO2-graphene aerogels are promising for the degradation of gas-phase UDMH. This journal is

Full text of DOI:10.1039/d0nj04617e

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TiO2–reduced graphene oxide for the removal of
gas-phase unsymmetrical dimethylhydrazine†
Cite this: New J. Chem., 2021,
45, 394
Hou Ruomeng,
Jia Ying,* Huang Yuanzheng, Shen Keke and Zhu Huixin
Unsymmetrical dimethylhydrazine (UDMH) contaminated waste gas and related intermediates pose a great
threat to human health. TiO2–reduced graphene oxide aerogel (rGA) samples with different graphene content
levels were synthetized and characterized for the degradation of UDMH. The effects of GO content, humidity,
and temperature were investigated under UV and VUV light, with highest UDMH conversion values of 68%
and 95%, respectively. Compared with pure TiO2, the enhanced degradation activity of TiO2–rGA under UV
light can be attributed to a synergetic effect between absorption and photocatalysis, while the high UDMH
conversion under VUV light relies on photolysis and ozonation. The high oxygen-containing group content,
rather than a high SSA, and electron trapping by graphene are key factors determining the outstanding per-
formance of TiO2–rGA with 80 mg of GO. The prepared TiO2–graphene aerogels are promising for the
degradation of gas-phase UDMH.
Received 16th September 2020,
Accepted 5th November 2020
DOI: 10.1039/d0nj04617e
rsc.li/njc
of being low-cost, non-toxic, and environmentally friendly.9
TiO2 can create electron–hole pairs under illumination, fol-
1. Introduction
Unsymmetrical dimethylhydrazine (UDMH) is a commonly lowed by the generation of free radicals that are able to undergo
used propellant in rocket and missile engines because of its secondary reactions. For practical applications, the problem of
high specific impulse. The waste liquid and gas produced high photoinduced electron and hole recombination rates
during its use and storage can cause serious environmental exists.10 To overcome this problem, a Schottky junction can
pollution. UDMH is also seriously toxic to organisms, and it be fabricated on a surface via loading with conductive materials
can cause different degrees of damage to the human central with higher work functions than TiO2.11
nervous system, liver, and kidneys. The treatment of UDMH
Graphene, consisting of super strong sheets of carbon,
waste has long been a research hotspot around the world. conducts heat and electricity extremely well. TiO2 and graphene
Traditionally, waste UDMH gas was treated via water absorption, can be integrated to effectively transport photoinduced carriers
high-altitude emission, and direct combustion methods. How- and prolong carrier lifetimes. What is more, TiO2/graphene can
ever, these methods often have the disadvantages of needing a easily form chemical bonds in the process, and the close
lot of energy, being labor intensive, and having high cost. It is interaction between TiO2 and graphene is conducive to charge
necessary to seek an effective, green, and convenient method for transfer and band-gap reduction.12 During the in situ preparation
UDMH degradation. Photocatalysis and absorption are the most of TiO2/graphene, hydrophilic groups, including hydroxyl and
effective means that can be applied to remove UDMH contami- carboxyl groups on GO, facilitate dispersion in water and the
nants. Gao1 used CdS-sensitized TiO2 nanorod arrays decorated heterogeneous nucleation and growth of titanium dioxide.
with Au nanoparticles to degrade UDMH wastewater with an A. Trapalis13 explored the use of TiO2/graphene composite photo-
efficiency of 51.51%. However, little research has been done on catalysts made from pure graphene (G) and graphene oxide (GO)
the degradation of gas-phase UDMH using photocatalysts.
to remove NOx. The results showed that TiO2/rGO prepared via a
Photocatalysis, as an advanced oxidation technology, has solvothermal method exhibited superior efficiency to TiO2/G. For
been used in the field of waste-gas treatment, such as for TiO2/rGO, the type and number of remaining oxygen-containing
formaldehyde,2,3 benzene,4,5 and toluene.6–8 Titanium oxide groups on rGO vary as a result of different degrees of reduction
(TiO2) is considered to be a distinguished photocatalyst for during the hydrothermal reaction. In addition, it is easy to carry
remediating environmental pollution due to its unique features out the chemical absorption of UDMH molecules on these groups.
Therefore, chemical absorption and charge-carrier separation are
proposed as key factors for improving the degradation perfor-
High-Tech Institute of Xi’an, Xi’an 710025, China. E-mail: hourmhjj@163.com,
mance of TiO2/rGO. In a previous report, Ebrahimi14 prepared
jyingsx@163.com
P25–rGO with P25 nanoparticles uniformly well-decorated on
graphene, and the efficiency of the degradation of gaseous
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0nj04617e
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acetaldehyde reached about 70% at a velocity of 17 mL minꢀ1 using Ar+ (532 nm) laser excitation (inVia Reflex, Renishaw,
under visible light.
England) in the range of 100–3200 cmꢀ1. The sample was
Nowadays, another challenge hindering the widespread loaded on a silica wafer and focused using a 50ꢁ objective.
application of granular nanoparticles is that most photocata- The surface functional groups were analysed via Fourier-
lysts are difficult to gather and recover. TiO2/reduced graphene transform infrared (FT-IR) spectroscopy (NEXUS, Nicolet, USA)
oxide aerogel (rGA) obtained via freeze-drying is easy to recycle. in transmittance mode with a spectral range of 400–4000 cmꢀ1
.
Also, the high surface area and countless bonding sites of 3D X-Ray photoelectron spectroscopy (XPS) characterization was
aerogels are favorable for physical and chemical adsorption. performed (Escalab 250Xi, Thermo Scientific) with an Al Ka
In this study, we combine reduced graphene oxide with TiO2 X-ray source. All binding energy values were corrected via cali-
via a one-pot hydrothermal method. The degradation of gaseous brating to the C 1s peak at 284.8 eV. Brunauer–Emmett–Teller
UDMH by TiO2/rGA was investigated under ultraviolet (UV) light (BET) specific surface areas and porosities of samples were
and vacuum ultraviolet (VUV) light in flowing gas. The phase evaluated based on nitrogen adsorption isotherms measured
structure, texture, and optical properties were characterized to at 77 K using gas adsorption apparatus (TriStar II 3020, Micro-
understand the underlying reasons behind the improved UDMH meritics, USA). All samples were degassed at 180 1C before nitrogen
conversion. Also, a possible degradation mechanism and proposed adsorption measurements. The BET surface areas were determined
pathway are discussed based on the degradation products.
using adsorption data in the relative pressure (p/p0) range of 0.06–
0.2. PL measurements were performed at room temperature using
a Hitachi F-4700 fluorescence spectrophotometer.
2. Experimental
2.1. Preparation of TiO2/rGA
2.3. UDMH degradation experiments under UV and VUV light
The photocatalytic degradation of UDMH was investigated in a
continuous fixed-bed reactor, as shown in Fig. 1. UDMH was
put in a refrigerator purged using nitrogen at a flow rate of
1 mL minꢀ1. Water vapor was generated via bubbling purified
air provided by an air compressor. The flows of UDMH, water
vapor, and purified air were adjusted using a mass flowmeter
and then mixed in a buffer bottle. The total flow rate is
controlled at 0.6 L minꢀ1. The gas from the buffer bottle then
flowed into the reactor.
A cylindrical reactor made of stainless steel with a working
volume of 500 mL was used in the experiments. UV and VUV
lamps and a catalyst quartz tube were fixed vertically in the
reactor. The UV and VUV lamp tubes used in the experiment
were both 4 W lamps (Cnlight) with irradiation peaks at 254 nm
and 185 nm, respectively. The distances between the lamps and
the quartz tube were both 2 cm. The as-synthesized nanocom-
posite was loaded in the tube, through which the gas would be
detected by the analysis system.
Graphite oxide was prepared according to the modified Hummers’
method.15 Briefly, 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. After-
wards, the solution was removed from the ice bath and kept
between 35 and 40 1C for 90 min under strong stirring. Then,
140 mL of deionized (DI) water was added and the solution was
stirred at 95 1C for 15 min. After that, 500 mL of DI water and
20 mL of 30% H2O2 were added. Finally, the solution was filtered
and washed with HCl (1 : 10) and DI water several times. The
supernatant was then removed via centrifugation to obtain a
brown slurry. Then, the slurry was diluted and subjected to
ultrasonic treatment for 1 h before further use. To prepare TiO2/
rGA, titanic sulfate (Ti(SO4)2) was chosen as the precursor, and
glucose acted as the crosslinking agent and reducing agent. A
certain amount (20 mg, 40 mg, 80 mg, 120 mg, or 160 mg) 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 Teflon-lined autoclave, which was
maintained at 180 1C for 12 h. The prepared TiO2/rGO hydrogel
(rGH) samples were washed and dehydrated via freeze-drying to
form TiO2/rGA samples, referred to as TiO2/rGA-1, TiO2/rGA-2,
TiO2/rGA-3, TiO2/rGA-4, and TiO2/rGA-5. In this work, TiO2/rGA
represents TiO2/rGA-3 unless otherwise specified. For comparison,
rGA (without adding Ti(SO4)2) and TiO2 (without adding GO) were
also synthesized via the same procedures.
The concentration of UDMH was measured via a gas chro-
matograph (Clarus 680, PerkinElmer, USA)-mass spectrometer
(Clarus SQ 8T, PerkinElmer, USA). The gas was periodically
2.2. Characterization methods
The phase structures of the samples were analysed via X-ray
diffraction (XRD) with Cu Ka radiation (l = 0.15401 nm) using a
generator voltage of 40 kV and a current of 40 mA. Analysis was
performed over a 2y range of 5.0–85.01 at a scanning rate of
0.021 per second. The surface morphologies were characterized
using a scanning electron microscopy (SEM) system (VEDAIIX-
MUINCN) equipped with an energy-dispersive X-ray spectro-
scopy (EDS) system. Raman spectra of the samples were taken
Fig. 1 A schematic diagram of the reaction system.
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transferred into a sampling loop (500 mL) via a ten-way valve
and separated through a capillary column (Elite-WAX, Perki-
nElmer) with an inner diameter of 0.25 mm and a length of
2 m. The oven temperatures were programmed as follows: the
temperature was initially set to 50 1C and maintained for 1 min,
then increased to 100 1C at a rate of 20 1C minꢀ1 and maintained
for 1 min, and then increased to 180 1C at a rate of 10 1C minꢀ1
and maintained for 1 min. The measurement time for each
sample was 3 hours. UDMH conversion was defined as follows:
UDMH conversion (%) = (UDMHinlet ꢀ UDMHoutlet)/UDMHinlet
ꢁ 100%
where UDMHinlet and UDMHoutlet are the concentrations of
UDMH at the inlet and outlet, respectively. Each experiment
was duplicated under the same conditions three times.
Fig. 3 XPS spectra: full spectrum of TiO2/rGA (a), O 1s spectrum of TiO2/
rGA (b), Ti 2p spectra of TiO2/rGA and of TiO2 (c), and C 1s spectra of TiO2/
rGA and GO (d).
3. Results and discussion
3.1. Phase structure and chemical composition
Fig. 2 displays the XRD patterns of GO, rGA, TiO2, and TiO2/rGA.
The diffraction peaks at 10.91 and 26.51 represent GO and rGA.
The diffraction peak at 261 in the GO pattern is attributed to
unoxidized graphite in small amounts. The d-spacing values of
the graphite layers calculated using Bragg’s law (2d sin y = nl) are
0.804 nm and 0.336 nm. The d-spacing of GO was broadened
compared with natural graphite due to the introduction of
functional groups between the graphite layers. The preparation
of rGA involves the reverse process, namely the removal of
functional groups from GO. The wide peak in rGA represents
the high stripping of graphene and the formation of few-layer
rGO sheets. The peaks at values of 25.31, 37.81, 48.11, 54.41, 62.91,
69.91, and 74.91 in the pure TiO2 and TiO2/rGA patterns can be
respectively indexed to the (101), (004), (200), (211), (204), (220),
and (215) planes of the TiO2 anatase phase (JCPDS-21-1272).
Based on the Scherrer equation, the crystallite sizes of TiO2 in the
pure TiO2 and TiO2/rGA samples were 11 and 14 nm, respec-
tively, via assuming a shape factor of 0.9. Graphene and GO can
hardly be seen in TiO2/rGA due to the low content and the
decreased layer-regularity of graphene nanosheets.16
TiO2/rGA, which demonstrates the coexistence of Ti, O, and C.
The O 1s spectrum of TiO2/rGA is exhibited in Fig. 3b. The O 1s
XPS spectrum can be divided into three peaks at 530.5, 532.25
and 533.55 eV, representing Ti–O–Ti, C–O–Ti, and C–O, respec-
tively, in TiO2/rGA. In Fig. 3c, the Ti 2p peaks, located at
binding energies of 459.2 and 465 eV, in the TiO2 spectrum,
with a separation of 5.8 eV, can be ascribed to pure anatase
phase Ti4+ 17
The peak at 472.7 eV is a satellite peak of Ti 2p1/2.
.
The Ti 2p peaks are positively shifted to 459.35 and 465.15 eV in
TiO2/rGA, which is caused by electrons migrating away from
TiO2 to graphene, because the Fermi level of TiO2 is higher than
that of graphene. A comparison of the C 1s spectra of GO and
TiO2/rGA is presented in Fig. 3d. The XPS peaks at 284.8, 286.3,
and 288.95 eV can be assigned to the C–C/CQC, CQO, and
OQC–O bonds of TiO2/rGA and GO. The peak intensities of the
oxygen-containing groups were reduced after the hydrothermal
reaction, indicating that GO had been reduced. Notably, a few
CQO and OQC–O bonds can still be found in TiO2/rGA.
Fourier transform infrared spectra of the samples are shown
in Fig. 4. The characteristic peaks from carbonyl CQO stretching
The surface components and chemical states of TiO2/rGA
were investigated via XPS. Fig. 3a exhibits the full spectrum of
Fig. 2 XRD patterns of the samples.
Fig. 4 FTIR patterns of the samples.
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vibrations, molecular water or CQC vibrations, C–OH bending between TiO2 and graphene. As shown in Fig. 5b, the ID/IG
vibrations, and C–O stretching vibrations or epoxy C–O–C vibra- values of TiO2/rGA-1, TiO2/rGA-2, TiO2/rGA-4, and TiO2/rGA-5
tions in GO appear at 1734, 1633, 1200, and 1080 cmꢀ1
,
were all greater than 1, with only TiO2/rGA-3 being less than 1.
respectively. The abundant oxygen-containing groups on the TiO2/rGA-3 has the lowest reduction degree and some oxygen
surface indicate the good hydrophilicity of GO,18 easing its containing groups remain on the surface, which is similar to
dispersal in water and bonding with other elements. During the FTIR spectra results.
hydrothermal treatment, overheated supercritical water can play
the role of a reducing agent, leading to decreases in the inten-
sities of CQO stretching vibrations and C–O/C–O–C vibrations in
TiO2/rGA. The changes show the reduction of GO, as shown
by XPS, and previous research produced similar results.19 The
C–OH bonds exhibit more intense peaks in TiO2/rGA than GO
due to interactions between graphene and hydroxyl groups after
the hydrolysis of titanous sulfate. The wide peak between 500 and
900 cmꢀ1 could be ascribed to Ti–O–Ti and Ti–O–C, confirming
the chemical interaction between TiO2 and graphene.20,21
Comparing TiO2/rGA with different GO content levels, the
C–OH and C–O/C–O–C peaks at 1200 and 1080 cmꢀ1 in
TiO2/rGA-3 are relatively high. This is due to a combination
of the hydrolysis of titanous sulfate and the reduction of GO,
which vary at different GO content levels and for different
structures.
Fig. 5a presents the Raman spectra of GO, rGA, TiO2, and
TiO2/rGA. The peaks at about 1350 cmꢀ1 and 1590 cmꢀ1
represent the D band and G band of graphene, respectively.
In detail, the D band reflects edge carbon or the disordered sp3
structure,22 while the G band represents the ordered sp2 carbon
structure. With a higher ID/IG ratio, the average size of the
in-plane sp2 domains and the conductivity of the composite will
decrease, while the degree of reduction becomes higher. The
ID/IG ratios of GO, rGA, and TiO2/rGA-3 are 0.89, 1.04, and 0.91,
respectively. The increased ID/IG ratio of rGA confirms the
reduction of GO, as well as the presence of sp3 defects.18,23
Furthermore, bare TiO2 exhibits five intensive peaks from
anatase. The peaks at 144 and 636 cmꢀ1 can be assigned to
the Eg mode of TiO2 (symmetric stretching vibrations of O–Ti–O
bonds), and the peaks at 396 cmꢀ1 and 517 cmꢀ1 correspond to
the B1g mode of TiO2 and an A1g/B1g doublet (symmetric and
asymmetric bending vibrations of O–Ti–O).7 As seen in the
enlarged view shown in Fig. 5a, the peak of TiO2 at 144 cmꢀ1 is
shifted positively to 150 cmꢀ1 in TiO2/rGA, which indicates that
the symmetric O–Ti–O stretching vibration in TiO2 has been
destroyed to some extent due to new bonds being formed
3.2. Microstructure and texture properties
The morphologies and constitutions of GO, rGA, TiO2, and
TiO2/rGA were observed clearly via SEM and EDS studies, as
seen in Fig. 6. As shown in Fig. 6a, the surface of GO is wrinkled
and cross-linked, which can provide enough active sites. After
the hydrolysis reaction, the three-dimensional porous structure
of rGA can be seen in Fig. 6b. The pure TiO2 nanoparticles are
seen to clump together in Fig. 6c. The SEM images of TiO2/rGA
are quite different due to the different amounts of graphene.
The morphologies of TiO2/rGA-1 and TiO2/rGA-2 are close to
that of titanium dioxide, as shown in Fig. 6d and e. Upon
increasing the graphene content, the skeleton of the graphene
aerogel can be clearly seen in the next three images, and the
TiO2 nanoparticles are evenly coated on graphene due to the
crosslinking effect of glucose and chemical interactions
between TiO2 and graphene. As shown in Fig. 6i, the elements
Ti, O, and C can be detected in the EDS spectrum.
The N2 adsorption–desorption isotherm curves for TiO2/rGA
and TiO2 are shown in Fig. 7, and the corresponding data are
summed up in Table 1. TiO2 displayed a type-IV physisorption
isotherm (based on IUPAC classification) with a H2 hysteresis
loop. The pores are mainly in the shape of an ink bottle, and
they are possibly caused by the accumulation of homogeneous
titanium dioxide. TiO2/rGA displayed type-IV physisorption
isotherms, with a combination of H2 and H3 hysteresis loops.
The hysteresis loops are triangularly shaped, mainly due to
Fig. 5 (a) Raman spectra of rGA, GO, TiO2, and TiO2/rGA. (b) Raman
Fig. 6 SEM images of (a) GO, (b) rGA, (c) TiO2, and (d–h) TiO2/rGA, and
spectra of TiO2/rGA with different amounts of graphene.
the EDX spectrum (i) of TiO2/rGA.
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Fig. 7 BET spectra of (a) TiO2/rGA-1, TiO2/rGA-2, and TiO2, and (b) TiO2/
rGA-3, TiO2/rGA-4, TiO2/rGA-5, and rGA.
Fig. 8 PL spectra of TiO2/rGA-1, TiO2/rGA-2, TiO2/rGA-3, TiO2/rGA-4,
TiO2/rGA-5, and TiO2.
Table 1 Specific surface area and pore structure values obtained via the
multipoint BET method
3.4. The photocatalytic degradation of UDMH under UV light
Composite
SBET (m2 gꢀ1
)
Vpore (cm3 gꢀ1
)
Dpore (nm)
3.4.1 Humidity. Photocatalytic experiments involving the
gas-phase degradation of UDMH were performed at room tem-
perature (25 1C) and under ambient pressure. UDMH conversion
over bare TiO2, rGA, TiO2/rGA-1, TiO2/rGA-2, TiO2/rGA-3, TiO2/
rGA-4, and TiO2/rGA-5 at different humidity levels (a: dry air;
b: 50% H2O) under UV light is shown in Fig. 9. The UDMH
conversion levels when using TiO2/rGA lie in the range of 19–28%,
which are higher than the reference catalyst of pure TiO2 in dry air.
Pure rGA shows better performance than TiO2/rGA-1 and TiO2/
rGA-2 due to its high absorption capacity. The relatively high
efficiency of TiO2/rGA is attributed to a synergetic effect between
photocatalysis and absorption. On one hand, TiO2 can produce
electron–hole pairs under UV irradiation, and graphene can
efficiently transport the carriers, as verified by the Ti 2p XPS
spectra. On the other hand, the Schottky junction formed between
TiO2 and rGA contributes to the degradation of UDMH. The
Schottky barrier serves as an efficient electron trap, thus leading
to a high partial electron density around graphene for photocata-
lysis and preventing the unexpected migration of electrons from
graphene back to the semiconductor.26 In addition, the upshifted
Fermi level of TiO2 under UV light irradiation could decrease the
TiO2–rGA barrier and benefit electron transport.27
rGA
TiO2
TiO2/rGA-1
TiO2/rGA-2
TiO2/rGA-3
TiO2/rGA-4
TiO2/rGA-5
97.5722
34.6566
116.7444
140.3755
13.0532
32.5060
38.9673
0.081287
0.105611
0.185195
0.177772
0.024687
0.051381
0.067024
1.63237
4.58944
4.7602
4.2374
5.1295
4.8364
5.8281
pores arising from aggregated nanoparticles and slit-shaped
mesopores due to the stacking of rGO sheets.24 The hysteresis
loops of TiO2/rGA are closer to TiO2-type at low graphene
content and closer to graphene-type at high graphene content,
which agrees with the SEM results. From the BJH spectra, it can
be seen that TiO2 and TiO2/rGA showed uniform aggregated
mesopores in the range of 3–8 nm. The specific surface areas of
the samples are shown in Table 1. TiO2/rGA-1 and TiO2/rGA-2
exhibited higher specific surface areas than pure TiO2 and rGA,
with surface areas of up to 116 m2 gꢀ1 and 140 m2 gꢀ1, respec-
tively. The addition of graphene in small amounts contributed to
the overall specific surface area and the crystallite growth. When
the graphene content was higher, the surface area sharply
decreased to 13 m2 gꢀ1. As shown in Fig. 6, the morphology of
TiO2/rGA changed from granular to lamellar. Compared with
Fig. 6b, the graphene sheets are stacked together and more closely
linked to titanium dioxide.
When the humidity was raised to 50%, the UDMH conver-
sion of TiO2/rGA-3 can reach 68%, which is much higher than
in dry air. More reactive oxygen species can be produced in
humid air. As a strong oxidizing agent, hydroxide radicals may be
generated on the surface of TiO2. The UDMH conversion levels of
TiO2/rGA with different GO content vary greatly, following the
3.3. Optical properties and band gaps
The PL intensity can reflect the photogenerated hole and
electron separation efficiency.6 The PL spectra of TiO2 and
TiO2/rGA are shown in Fig. 8. There are strong emission peaks
for TiO2 at around 460 nm and 425 nm, which can be ascribed
to self-trapped excitons in TiO2.25 TiO2 and TiO2/rGA show
similar PL signal shapes, however, the TiO2/rGA signals were
obviously weaker than that of TiO2. This is mainly because the
addition of graphene can decrease the radiative recombination
of carriers at the rGO–TiO2 interface, henceforth quenching
the PL intensity. The excited electrons are transferred from the
conduction band of TiO2 to the rGO sheets, prolonging the
carrier lifetimes.
Fig. 9 UDMH conversion under UV light in (a) dry air and (b) air at 50%
humidity.
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Fig. 10 The FTIR patterns of fresh and used TiO2/rGA.
Fig. 12 A schematic diagram of the photocatalytic degradation mecha-
nism on TiO2/rGA.
order: TiO2/rGA-3 4 TiO2/rGA-4 4 TiO2/rGA-5 4 TiO2/rGA-2 4
TiO2/rGA-1 4 rGA 4 TiO2. The material with the highest
efficiency is TiO2/rGA-3, with a relatively low SSA and moderate
carrier density based on the previous characterization results.
From the obtained PL spectra, we can infer that TiO2/rGA-5 has a
relatively high catalytic effect, as it had the lowest fluorescence
intensity. However, the degradation efficiency of TiO2/rGA-5 is
lower because the degradation of UDMH under UV light is
affected by a combination of absorption and catalysis. In order
to analyse the degradation mechanism, the FTIR patterns of fresh
and used TiO2/rGA-3 are shown in Fig. 10. The characteristic peaks
of CQO, C–OH, and C–O–C decreased after degradation, and two
new peaks due to C–H and N–H appeared. Degradation may arise
due to chemical interactions between reducing UDMH and oxygen-
containing groups. As shown in Fig. 4 and 5, the reduction degree
of TiO2/rGA-3 is weaker and the number of oxygen-containing
groups on TiO2/rGA-3 is higher than the other samples. It can be
deduced that the increased activity of TiO2/rGA-3 can mainly be
accredited to chemical absorption on oxygen-containing groups.
3.4.2 Temperature. Fig. 11 demonstrates the effects of tem-
perature on the degradation of UDMH in the range of 20–140 1C.
UDMH conversion decreased as the temperature increased from
20 to 80 1C. The results suggest that UDMH adsorption on TiO2/
rGA is an exothermic process. Elevating the temperature can
promote molecular diffusion, enhancing the adsorption rate.28
Cation–p bonding and hydrogen bonding between UDMH and
rGO also contribute to absorption.29 When the temperature was
further increased, the UDMH conversion to other organics slightly
increased. These results are similar to a previous report.28
3.4.3 Reaction mechanism and the proposed degradation
pathway of UDMH under UV light. The UDMH degradation
mechanism includes both chemical adsorption and a photocata-
lytic process. Fig. 12 schematically shows the degradation mecha-
nism on TiO2/rGA. Because most of the oxygen-containing groups
contribute to the adhesion of hydrophilic VOCs onto the carbon
surface, the adsorption of UDMH on graphene can provide more
opportunities for TiO2 to come into contact with UDMH. This
allows further photocatalysis on TiO2/rGA. Electron–hole pairs can
be generated by TiO2 when it absorbs photons with energy higher
than the intrinsic bandgap energy. Graphene can effectively
transfer the photogenerated electrons and prevent the recombina-
tion of h+ and eꢀ on TiO2. Holes on the surface of TiO2 can rapidly
react with water to form ꢂOH. Also, oxygen radical will be
generated upon the interaction of electrons with O2. The reactive
oxygen species (ROS) can then react with UDMH.
In order to further understand the possible UDMH transforma-
tion path, different intermediate products observed via GC-MS
are listed in Fig. 13 and Table 2. The organic products vary
considerably under different conditions. In dry air, the main
components other than UDMH are formaldehyde dimethyl-
hydrazone (FDMH), dimethylamine (DMA), and nitrosodimethyl-
amine (NDMA).
Fig. 13 Gas chromatograms of the degradation products of UDMH under
Fig. 11 The effects of temperature on UDMH conversion under UV light.
UV light.
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Table 2 Products observed during the reaction of 1,1-dimethyldrazine
with TiO2–rGA under UV light
Product
Peak/min
CAS no.
Dry
50% H2O
FDMH
UDMH
Butanedial
DMA
2.2
2035-89-4
57-14-7
638-37-9
124-40-3
62-75-9
+
+
+
+
+
+
+
2.42
3.09
3.77
5.19
+
+
NDMA
The high yields of FDMH observed during the transforma-
tion of UDMH were initiated by H-abstraction from C–H bonds
Fig. 15 The effects of temperature on UDMH conversion under UV light.
ꢂ
of UDMH. Firstly, –CH2 is formed in the presence of oxygen.
ꢂ
Then, oxygen inserts into R–CH2 to form HCHO and methyl-
hydrazine (MMH) (eqn (1)). Lastly, FDMH is formed when
UDMH reacts with HCHO30 (eqn (2)). MMH can further be
attacked by ozone to form methyl radicals (eqn (5)). The formed
methyl radicals account for the observed DMA groups. When
the humidity of UDMH gas is adjusted to 50%, a new peak
appeared at 3.09, which was correspondingly assigned to
butanedial. The formation of butanedial may involve the parti-
cipation of hydroxide radicals. The hydroxide radicals can react
with methyl radicals (eqn (5)) to form aldehydes.
wavelength of 185 nm, generates energetic photons that can
activate oxygen and water vapor to produce numerous reactive
species, such as O(1D), O(3P), hydroxyl radicals (ꢂOH), and O3.32
VUV photolysis has been used to destroy VOCs like benzene and
formaldehyde.33,34 The bonds of UDMH are initially destroyed by
VUV photolysis and are then oxidized by residual O3 from
photolysis. Due to the abundant reactive species generated in
the gas phase, the oxidation degree of UDMH increases and the
number of intermediates increases. When water was added, the
UDMH conversion increased slightly, as more hydroxyl radicals
(ꢂOH) were generated in the system.
O2
!
O2
!
ꢂ
R1ꢀCH3
R ꢀCH
R ꢀðOÞCH ! R H þ HCHO (1)
1
2
1
1
2
3.5.2 Temperature. The UDMH conversion at different
temperatures at 50% humidity is shown in Fig. 15. With an increase
in temperature, the UDMH conversion is almost unchanged.
Absorption is an exothermic reaction under UV light, while UDMH
conversion increased slightly under VUV light. Therefore, adsorp-
tion does not play the main role and temperature has little effect on
UDMH conversion, while photocatalysis and ozonation play the
main role in the process.
R2–NH2 + HCHO - R3–NQCH2 + H2O
(2)
(3)
O2 + Oꢂ - O3
hn
O2
Oꢂ þ Oꢂ
(4)
ƒ!
O3
!
O3
!
H3CꢀNHNH2
O3
H CꢀNꢂꢀNH
O3
H CꢀN ¼ NH
3
2
3
(5)
H CꢀN ¼ Nꢂ
CH þN ꢃ N
ꢂ
!
!
3
3
3.5.3 Reaction mechanism and the proposed degradation
pathway of UDMH under VUV light. The possible reaction
mechanism of UDMH transformation is reliant on direct
photolysis and photocatalysis reactions between UDMH and
photogenerated ROS under VUV light. Different intermediate
products are shown in Fig. 16, and the product details observed
via GC-MS are listed in Table 3. The proportion of UDMH
converted to other organics is almost 95%, much higher than
under UV light. One of the degradation products, NDMA, has
toxic mutagenic, teratogenic, and carcinogenic properties. It is
noted that the yield of NDMA is obviously more than under UV
light. Because more ozone can be produced under VUV light,
the N–H bonds of UDMH could easily be attacked by ozone, and
H-abstraction reactions can take place. Then, ozone attacks
intermediates containing NHꢂ free radicals to form NDMA30
(eqn (6)). NDMA can also be synthesized via combining FDMH
with ozone (eqn (7)). The high yields of NDMA are confirmed in
EC Tuzon’s study.35 FDMH can also react with oxygen to
produce (CH3ꢂ)2NN and (CH3ꢂ)2NN, which quickly combine to
form 1,1,4,4-tetramethyl-1,2-tetrazene (TMT). Furthermore,
NDMA and dimethylnitramine (NTDMA) can be formed in the
presence of TMT upon adding ozone (eqn (8)). When water is
involved in the reaction, the NQCH2 species in FDMH will
In eqn (1) and (2), R1 refers to (CH3)NNH2; R1H refers to
MMH; R2 refers to (CH3)(CH3)N.
3.5. The photocatalytic degradation of UDMH under VUV light
3.5.1 Humidity. The UDMH conversion levels under VUV
light in both dry and moist air are about 95%, as shown in
Fig. 14. The differences in UDMH conversion are negligible at
different GO content levels. Presumably, most UDMH molecules
were degraded in the gas phase and not on the photocatalyst
surface.31 The VUV light used in the experiment, with a
Fig. 14 UDMH conversion under VUV light in (a) dry air and (b) air at
50% humidity.
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times in deionized water and dried at 110 1C prior to the next
reaction cycle. As shown in Fig. 17, the UDMH conversion
slightly decreased in UV and VUV light during the first cycle,
due to partial sample loss. After five cycles, the samples still
show stable UDMH conversion levels, implying good reusability
during the degradation of UDMH.
4. Conclusions
In summary, a series of TiO2/rGA samples was prepared using
titanium sulfate and GO as precursors. The morphologies and
number of surface oxygen-containing groups of the TiO2/rGA sam-
ples varied considerably upon the addition of different amounts of
GO, as determined through characterization studies. TiO2/rGA-3
exhibited extraordinary UDMH degradation performance under UV
light. Moist conditions were conducive to the removal of gas-phase
UDMH, and absorption on TiO2–rGA was an exothermic process.
UDMH conversion could reach about 68% and 95% at 50%
humidity under UV and VUV light, respectively. Compared with
pure TiO2 and graphene, chemical interactions and the Schottky
junction formed in TiO2–rGA accounted for the high charge separa-
tion and transmission efficiencies, which are helpful for improving
the catalytic performance. TiO2/rGA-3 showed higher UDMH con-
version under UV light, which was mainly due to chemical absorp-
tion between reducing UDMH and oxygen-containing groups. The
mechanism of UDMH degradation under VUV light was attributed
to photocatalysis and ozonation. The high UDMH conversion in a
continuous fixed bed reactor shows that this photocatalyst system is
useful for removing gas-phase UDMH in practice.
Fig. 16 Gas chromatograms of the degradation products of UDMH under
VUV light.
Table 3 Products observed during the reaction of 1,1-dimethyldrazine
with TiO2/rGA under VUV light
Product
Peak/min CAS no.
Dry 50% H2O
FDMH
2.2
2035-89-4
57-14-7
638-37-9
124-40-3
926-64-7
62-75-9
+
+
+
+
+
+
+
+
+
UDMH
Butanedial
DMA
(Dimethylamino)acetonitrile 4.34
NDMA 5.19
2.42
3.09
3.77
+
+
+
Dimethylnitramine (NTDMA) 7.8
4164-28-7
ꢂ
ꢂ
convert to CN and then react with CH3 and UDMH to form
(dimethylamino)acetonitrile.
O3
!
O3
!
ðCH Þ NNH2
ðCH Þ NNHꢂ
ðCH Þ NNO
(6)
(7)
3
2
3
2
3 2
Conflicts of interest
O3
!
ðCH Þ NN ¼ CH2
ðCH Þ NNO
3
2
3 2
There are no conflicts to declare.
O2
!
ꢂ
ðCH Þ NN ¼ CH2
O3
ðCH Þ NN ! ðCH Þ NNNNðCH Þ
3
2
3
2
3
2
3 2
Acknowledgements
ðCH Þ NNO þ ðCH Þ NNO
!
2
3
2
3 2
This project is supported by the National Natural Science
Foundation of China (21875281).
(8)
3.6. Reusability of TiO2/rGA under UV light and VUV light
Notes and references
TiO2/rGA-3 was selected to investigate the stability and reusa-
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exposed to flowing UDMH gas for 3 h. Then it was washed three
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