Available online at website: https://journal.bcrec.id/index.php/bcrec

Bulletin of Chemical Reaction Engineering & Catalysis, 20 (4) 2025, 582-593
Original Research Article

Green Synthesis of Chitosan-Assisted ZnO Nanoparticles and
Their Photocatalytic Application in ZnO/TiO₂ Composites for
Isopropanol Degradation
Le H. Hoang1,a, Ho G. Quynh1,a, Mai H. Nghi1, Nguyen T. T. Phuong1, Ngo T. H. Duong1,2,
Nguyen Q. Long1,2,*
1Faculty of Chemical Engineering, Ho Chi Minh City University of Technology (HCMUT), 268 Ly Thuong Kiet Street,

Ho Chi Minh City, Vietnam
2Vietnam National University Ho Chi Minh City, Ho Chi Minh City, Vietnam.

Received: 2nd July 2025; Revised: 31th August 2025; Accepted: 5th September 2025
Available online: 8th September 2025; Published regularly: December 2025

Abstract
Nano-sized ZnO particles were successfully synthesized via a green, efficient, and chitosan-assisted method, which is
both cost-effective and environmentally friendly. The nanoscale characteristics of the synthesized particles were
confirmed through various analytical techniques, including X-ray diffraction (XRD), scanning electron microscopy
(SEM), nitrogen adsorption–desorption isotherms, diffuse reflectance spectroscopy (DRS), and Fourier-Transform
Infrared Spectroscopy (FTIR). This study primarily investigated the photocatalytic performance of ZnO/TiO₂
composites prepared by a simple mechanical mixing approach for the degradation of isopropanol (IPA) in a continuousflow system under UVA irradiation at room temperature. A range of experimental conditions, including initial IPA
concentrations, gas flow rates, relative humidity levels, and the number of UV lamps, were systematically explored.
The mechanically mixed ZnO/TiO₂ nanomaterial exhibited enhanced photocatalytic activity compared to pure ZnO.
Notably, while commercial TiO₂ showed reduced IPA removal efficiency under humid conditions, the ZnO/TiO₂
composite maintained superior performance, achieving a removal efficiency of 45% over a 3-hour period at 30% relative
humidity with an inlet IPA concentration of about 1200 ppmv, a flow rate of 3 L/h, and illumination by four UV lamps.
Copyright © 2025 by Authors, Published by BCREC Publishing Group. This is an open access article under the CC
BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).

Keywords: Nanoparticle; ZnO; ZnO/TiO2; Photocatalysis; IPA
How to Cite: Hoang, L. H., Nghi, M. H., Quynh, H. G., Phuong, N. T. T., Duong, N. T. H., Long, N. Q. (2025). Green
Synthesis of Chitosan-Assisted ZnO Nanoparticles and Their Photocatalytic Application in ZnO/TiO2 Composites for
Isopropanol Degradation. Bulletin of Chemical Reaction Engineering & Catalysis, 20 (4), 582-593. (doi:
10.9767/bcrec.20431)
Permalink/DOI: https://doi.org/10.9767/bcrec.20431

1. Introduction
The rapid expansion of global industries has
driven excessive consumption of fossil fuels, raw
materials, and industrial chemicals, resulting in
significant
emissions
of
harmful
gases,
particularly volatile organic compounds (VOCs).
Existing VOC control technologies are often
insufficiently effective or poorly implemented,
* Corresponding Author.
Email: nqlong@hcmut.edu.vn (N.Q. Long)
a

Le H. Hoang and Ho G. Quynh contributed equally to this
work as first authors.

worsening air pollution and posing serious risks to
public health. Consequently, substantial research
has focused on developing advanced VOC
mitigation technologies that meet increasingly
stringent environmental standards. Strategies to
reduce VOC emissions follow two main
approaches: source reduction and end-of-pipe
treatment. Early efforts prioritized improving
processes, equipment, and raw materials to
minimize VOC generation, prevent leakage, and
limit volatilization.
VOC treatment methods are generally
classified into recovery-based (adsorption,

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Bulletin of Chemical Reaction Engineering & Catalysis, 20 (4), 2025, 583
absorption, condensation, membrane separation)
and destruction-based (biofiltration, oxidation)
techniques. On the one hand, recovery-based
approaches can be effective but often face high
operational costs, complex waste management, or
energy-intensive requirements [1,2]. On the other
hand, among destruction methods, biofiltration
offers low-cost and eco-friendly operation but is
limited by slow, selective degradation [3]. Besides,
catalytic oxidation stands out for its high
efficiency, low byproduct formation, and lower
operating temperatures [4]. In particular,
photocatalytic oxidation has emerged as a highly
promising strategy, enabling VOC mineralization
into CO₂ and water under ambient conditions
with minimal secondary pollution. Its widespread
application, however, hinges on the development
of robust, cost-effective catalysts with high
activity and stability.
Numerous studies have demonstrated that
TiO₂ is a highly active semiconductor
photocatalyst suitable for the treatment of a wide
range of contaminants in both liquid and gaseous
phases. The photocatalytic mechanism of TiO₂ can
be described by the following reactions and Figure
1 [5,6].
TiO2 + ℎ𝑣 → 𝑒 − (TiO2 ) + ℎ+ (TiO2 )

(1)

𝐻+
𝑒 − ,𝐻 +
−

(O2 )𝑎𝑞 + 𝑒𝐶𝐵
→ O−
H2 O2
2 → HO2 →
−
−
•
H2 O2 + O2 → OH + OH + O2
ℎѵ
H2 O2 → 2OH 
+
+
•
O2−
s + H + ℎ𝑉𝐵 → OH
•
OH + VOCs + O2 → CO2 + H2 O

(2)
(3)
(4)
(5)
(6)

However, a major drawback of semiconductor
photocatalysis is the rapid recombination of
photogenerated electron–hole pairs, which
significantly limits photocatalytic efficiency. This
limitation can be addressed through various

enhancement strategies, such as doping the
semiconductor with non-metal impurities [7],
incorporating transition metals [8], and using
composite semiconductors [9]. One effective way
to enhance photocatalytic efficiency is by
designing
well-engineered
heterojunction
photocatalysts, where the interface between two
semiconductors with different band structures
promotes charge separation and transfer [10] . In
fact, nano-sized semiconductors have been shown
to effectively enhance the photocatalytic activity
of
heterojunction
systems,
because
nanostructured materials offer a significantly
higher specific surface area compared to their
bulk counterparts. When the size of the
nanoparticles
approaches
the
quantum
confinement limit, the electrons become confined
within extremely small
spatial regions,
comparable to or smaller than the electron’s de
Broglie wavelength. This spatial confinement
alters the electronic energy levels within the
material, resulting in distinct properties
compared to those observed in the bulk state.
Previously, our research team used TiO2 and
carbon quantum dots (CQDs) to deal with the
rapid combination of e-/h+. However, CDs/TiO2
photocatalysts still have certain limitations,
including low surface area per unit mass, low
affinity for organic compounds, and difficulties
with recovery and reuse (the catalysts' tendency
to agglomerate and clump together prevents light
from reaching the active centers on their surface)
[11].
Recently, ZnO nanoparticles (NPs) presents a
promising alternative, offering advantages such
as low capital and operating costs, mild reaction
conditions, and the capability to degrade a wide
range of hazardous organic compounds in water
into CO₂ and H₂O [12]. ZnO has also been
extensively studied due to their outstanding

Figure 1. Photocatalytic mechanism for TiO2.
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Bulletin of Chemical Reaction Engineering & Catalysis, 20 (4), 2025, 584
properties in fields, such as gas sensors [13],
optical and electronic devices [14], solar cells [15],
drug delivery [16], antibacterial materials [17],
and photocatalysis [18]. Furthermore, in recent
years, numerous studies have demonstrated the
effectiveness of ZnO as a semiconductor
component in heterojunction systems for the
efficient degradation of various types of pollutants
[13,19]. Therefore, this study investigates the
photocatalytic efficiency of a heterojunction
system constructed using nanosized ZnO
particles.
In fact, the similarity in bandgap energies
and the alignment of the conduction and valence
bands between ZnO and TiO₂ facilitate the
efficient transfer of electrons and holes, enabling
the formation of effective heterojunctions between
the two materials [20-22]. The difference in
conduction band positions between ZnO and TiO₂
enables the photogenerated electrons in ZnO to
rapidly transfer to TiO₂ upon light absorption.
This electron migration promotes efficient charge
separation and reduces the recombination rate of
electron–hole pairs at the surface. Additionally,
the incorporation of ZnO into TiO₂ introduces
defects and impurities into the TiO₂ bandgap,
which can effectively narrow its bandgap and
lower the excitation energy required for the
composite material [23].
Unlike most previous studies on ZnO/TiO₂
catalysts, which typically rely on conventional
chemical or energy-intensive synthesis routes,
this study introduces a green and efficient
chitosan-assisted method for producing nanosized
ZnO. To the best of our knowledge, this simple and
novel approach has not been reported before. The
as-synthesized ZnO was mechanically combined
with TiO₂ and the composite’s photocatalytic
performance was tested against isopropanol, a
common VOC, to demonstrate its potential for air
pollution control.
2. Materials and Methods
2.1 Syntheses and Characterization of Nano
ZnO/TiO2
All chemicals were used without further
purification and were purchased from commercial
suppliers as follows: zinc(II) chloride (ZnCl₂,
>98%), acetic acid (>99.5%), and sodium
hydroxide (NaOH, >99%) were obtained from
Xilong Co., while titanium dioxide (TiO₂) and
chitosan were sourced from commercial vendors.
In terms of the catalyst synthesis, initially,
5 g of ZnCl₂ was weighed and dispersed in 100 mL
of 1% (v/v) acetic acid, followed by sonication in an
ultrasonic bath for 15 minutes. Separately, a 1%
(w/v) chitosan solution was prepared by dissolving
1 g of chitosan in 1% acetic acid. The two solutions
were then combined, adjusted to pH of 5, and
stirred at 45 °C for 22 hours. Subsequently, 1 M

NaOH was added dropwise to the mixture until
the pH reached 10, while maintaining stirring at
room temperature. The mixture was aged for an
additional 22 hours at room temperature. The
resulting solution was filtered and thoroughly
washed with distilled water to remove unreacted
chitosan and byproducts such as NaCl. The
obtained white precipitate was dried at 80 °C for
16 hours. Finally, the dried material was calcined
by heating at a rate of 50 °C per 20 minutes up to
550 °C, and then held at this temperature for
3 hours to produce nanosized ZnO.
Next, the mechanical mixing samples were
made by physically mixing nano-ZnO and TiO₂
powders in different weight ratios. The mixture
was dispersed in 2 mL of pure ethanol (>99.5%)
and sonicated for 15 minutes. Subsequently, the
samples were heated at 200 °C for 1 hour to
evaporate the solvent and remove any adsorbed
substances from the surface. Each mixture was
applied in successive layers onto the outer surface
of a glass tube via the rotary spray-coating
technique. During coating, the glass tubes were
positioned horizontally and rotated at 140 rpm.
The coated catalysts were then heated at 200 °C
for 1 hour to activate them prior to the reaction
tests. The samples are denoted as M-a:b, where a
and b represent the weight fractions of ZnO and
TiO₂ in the sample, respectively. Table 1 clearly
presents the masses of ZnO and TiO₂ in the
different composites.
The
structural
and
physicochemical
properties of the prepared catalysts were
thoroughly characterized using a combination of
analytical techniques. X-ray Powder Diffraction
(XRD) analysis was performed using a D2 Phaser
instrument
(Bruker,
Germany)
with
monochromatic Cu-Kα radiation (λ = 1.5406 Å),
scanning in step-scan mode (Δ2θ = 0.05°) over the
2θ range of 10°–80°, to identify crystalline phases
and estimate crystallite size. Furthermore,
surface area and porosity were determined by
nitrogen adsorption–desorption analysis using a
NOVA 2200e Quantachrome 10.0 instrument. The
Brunauer–Emmett–Teller (BET) method was
applied to the adsorption data in the relative
pressure range of 0 to 1 to calculate specific
surface areas. Moreover, the surface morphology
and particle size distribution were examined

Table 1. Mass of ZnO and TiO2 in composite.
Sample
M-1:0
M-1:1
M-1:3
M-3:1
M-0:1

Copyright © 2025, ISSN 1978-2993

Weight (g)
ZnO
TiO2
0.2
0
0.1
0.1
0.05
0.15
0.15
0.05
0
0.2

Bulletin of Chemical Reaction Engineering & Catalysis, 20 (4), 2025, 585
using Scanning Electron Microscopy (SEM) with a
JSM-IT200 instrument (JEOL, Japan). In
addition, optical properties were investigated
using Diffuse Reflectance Spectroscopy (DRS) on
a UV-2600 spectrophotometer (Shimadzu, Japan),
equipped with an integrating sphere accessory
(ISR-2600 Plus). Measurements were conducted
in the wavelength range of 220–1400 nm, using
halogen and deuterium lamps as light sources and
a PMT R928 detector. Besides, Fourier-Transform
Infrared Spectroscopy (FTIR) analysis was carried
out using an Alpha II instrument (Bruker,
Germany) to identify functional groups and
surface interactions. Samples were mixed with
KBr, pressed into pellets, and analyzed in
transmission mode over the spectral range of
4000–500 cm⁻¹.

and an intensity of 1.5 W. The distance between
the UV lamps and the photocatalyst surface was
maintained at 2.5 cm. Under standard operating
conditions with all four lamps activated, the
reaction temperature stabilized at 39 °C, which
was maintained by a continuous stream of cooling
air provided by a fan positioned at the base of the
reactor. The outlet concentration of isopropanol
(IPA) during the photocatalytic process was
continuously monitored using a Hewlett-Packard
5890 Series II Plus gas chromatograph equipped
with a flame ionization detector (FID) and an HPPlot/Q column (30 m × 0.53 mm). The
investigation was carried out to investigate the
effect of the ZnO/TiO₂ mixing ratio. the initial
VOC concentration, relative humidity, and UV
intensity, on the photocatalytic activity of the
material.

2.2 Photocatalytic Activity Measurement
The photocatalytic oxidation system was
designed to simulate atmospheric conditions
suitable for the investigation. Two high-pure gas
sources, nitrogen (N₂) and oxygen (O₂) with
purities above 99.99%, were mixed at a 4:1 volume
ratio to approximate the composition of air. The
nitrogen stream was further divided into three
separate flows: (1) one stream was passed through
a container containing IPA to generate a VOCladen stream at the desired concentration, (2)
another stream was directed through distilled
water to introduce water vapor into the system,
and (3) a third stream of pure nitrogen was used
to dilute the IPA concentration as needed. All gas
streams, including oxygen, were precisely
controlled using flowmeters and then combined in
a mixing vessel to ensure a stable and
homogeneous gas mixture before entering the
photocatalytic reactor. The mixed gas stream was
then passed through the photocatalytic reactor,
which was irradiated by four Sanko Denki
F10T8BLB lamps with a wavelength of 352 nm

3. Results and Discussion
3.1. Characterization of the Materials
Regarding the XRD results, Figure 2.a and
Figure 2.b illustrate the XRD patterns of M-1:0
and M-0:1, respectively. Sample material M-1:0 is
compatible with the ZnO crystalline phase, which
has a hexagonal wurtzite structure (JCPDS
number
036-1451),
because
it
exhibits
characteristic X-ray diffraction peaks at 2θ values
of 31.8°, 34.4°, 36.3°, 47.6°, 56.6°, 62.9°, 66.4°,
68.0°, 69.2°, and 77.0°, which correspond to the
(100), (002), (101), (102), (110), (103), (200), (112),
(201), and (004) planes [24,25]. No additional
peaks corresponding to other crystalline phases
were observed when compared with the standard
diffraction patterns, indicating the purity of the
synthesized materials. For the M-0:1 sample,
which served as the catalyst, the XRD spectrum
exhibited distinct diffraction peaks at 25.5° and
38°, characteristic of the anatase phase of TiO₂,
consistent with the JCPDS reference pattern No.

Figure 2. (a) XRD patterns of M-1:0; b) XRD patterns of M-0:1.
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Bulletin of Chemical Reaction Engineering & Catalysis, 20 (4), 2025, 586
021-1272 [26], as well as a peak at 27.6°,
corresponding to the rutile phase of TiO₂ (JCPDS
No. 029-1360) [27]. From the XRD results of the
M-1:0 sample, the average crystal size was
determined using the Scherrer equation [28]
giving a value of 22 nm. While this method is
simple and widely used, it assumes negligible
lattice strain and is best suited for relatively
simple crystal structures.
For the nitrogen adsorption-desorption
analysis, the specific surface area was determined
using the BET model. The M-1:0 sample exhibited
a BET surface area (SBET) of 16.4 m²/g. Based on
the equation: 𝐷 = 6000/(𝑑BET × 𝜌) , where the
density of ZnO is 5.606 g/cm³, the average particle
size was estimated to be approximately 65 nm.
The synthesis process for nano-sized ZnO
particles was effective in producing the M-1:0
material with a significantly smaller particle size
compared to the pristine ZnO sample, which had
a particle size of 454 nm and an SBET of 2.4 m²/g
[29]. The difference between the average grain
size calculated using the BET equation and the
values obtained from XRD analysis can be
attributed to the tendency of crystals to
agglomerate during heat treatment, resulting in
larger grain sizes. Table 2 presents the specific
surface areas of the three investigated materials.
Among them, sample M-0:1 exhibited the highest
specific surface area of 49.44 m²/g, approximately
three times greater than that of sample M-1:0.
This result indicates a superior adsorption

capacity of M-0:1, suggesting its ability to
accommodate a larger quantity of adsorbates on
its surface. In comparison, sample M-1:3 showed
a specific surface area of 31.26 m²/g, which is
higher than that of M-1:0 but lower than M-0:1.
This observation suggests that the mechanical
mixing process may induce aggregation and
rearrangement of particles, increasing the
average particle size and reducing the number of
voids and interparticle contact regions. Such
rearrangement may also promote the formation of
intermetallic phases at specific sites.
In addition, scanning electron microscopy
(SEM) was employed to directly examine the
morphology and crystallite size of the M-1:0
material. SEM analysis offers localized insights
into the surface structure and crystal size. To
ensure objectivity, the sample surface was
analyzed at two different positions. Figures 3.a
and 3.c confirm the uniformity of the material
structure, while Figures 3.b and 3.d highlight the
crystal sizes observed. The morphology visible in

Table 2. Surface area of the three samples M-a:b.
Sample

BET specific surface area (m2/g)

M-1:0
M-1:3
M-0:1

16.40
31.26
49.44

Figure 3. SEM images of sample M-1:0 at different magnifications: (a) ×40.0k, (b) ×120k, (c) ×60.0k, and
(d) ×150k
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Bulletin of Chemical Reaction Engineering & Catalysis, 20 (4), 2025, 587
SEM images is consistent with the hexagonal
wurtzite structure identified via X-ray diffraction
analysis. To further assess the particle size
distribution, the SEM images were processed
using ImageJ software. As shown in Figure 4, the
majority of particles fall within the 30–50 nm
range, with an average diameter of 45.25 nm.
These findings confirm the successful synthesis of
nano-sized ZnO/TiO₂ composite materials.
Furthermore, Fourier-transform infrared
(FTIR) spectroscopy was employed to conduct
qualitative chemical analysis of sample M-1:0.
The FTIR spectrum, presented in Figure 5, shows
a distinct absorption peak at 511 cm⁻¹, which
corresponds to the stretching vibrations of Zn–O
bonds in the ZnO lattice [30]. The absence of
additional peaks associated with organic
functional groups suggests that no residual
organic compounds remained in the sample
following the synthesis process.
In addition, diffuse reflectance spectroscopy
(DRS) was used to investigate the optical
properties of samples M-1:0, M-1:3, and M-0:1, as

illustrated in Figure 6.a. The DRS technique
provides insights into particle size effects based on
the
shift
in
absorption
features.
For
nanomaterials, characteristic absorption edges
typically shift toward shorter wavelengths (a blue
shift) compared to their bulk counterparts. In this
study, the absorption spectrum of sample M-1:0
exhibited a noticeable blue shift, confirming the
formation of ZnO nanoparticles. According to
Singh et al., the maximum absorption peak of bulk
ZnO appears at 374 nm, while ZnO nanoparticles
show a peak at 368 nm, indicating a reduction in
particle size correlates with a decrease in the
absorption wavelength. This observed shift
further supports the successful synthesis of
nanoscale ZnO in the M-1:0 sample [31]. From
Figure 6, the maximum absorption wavelength
(λAE of sample M-1:0 is observed in the range of
327–368 nm. For sample M-1:3, the absorption
peak lies between 220–320 nm, while for sample
M-0:1, it ranges from 220–290 nm. The band gap
energy (Eg values of samples M-1:0, M-1:3, and M0:1, calculated from the Tauc plot in Figure 5.b,

Figure 4. Size distribution graph of sample M-1:0.

Figure 5. FTIR spectrum of sample M-1:0.sample.

Figure 6 a) DRS spectra of the three samples M-a:b; b) Tauc plot of the three samples M-a:b.
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Bulletin of Chemical Reaction Engineering & Catalysis, 20 (4), 2025, 588
are 3.12 eV, 3.20 eV, and 3.31 eV, respectively.
Sample M-1:3 shows improved absorption
intensity in the energy range of 200–1400 nm
compared to TiO₂. Moreover, its lower band gap
value compared to commercial TiO₂ suggests a
higher generation of electron-hole pairs under UV
light, which enhances its photocatalytic activity.
The formed holes can effectively interact with
adsorbates on the catalyst surface.
3.2. Effect of ZnO/TiO2 Ratio on the Catalytic
Performance
Figure 7.a illustrates the IPA adsorption
capacity of the five samples before participating in
the photocatalytic reaction. IPA adsorption on
sample M-0:1 occurs rapidly and shows the
highest adsorption capacity among all samples, as
indicated by the low C/C₀ ratio within the first 15
minutes. This reflects a strong interaction
between the catalyst and the gas molecules. In
contrast, sample M-1:0 exhibits weaker
adsorption, with consistently higher C/C₀ ratios
during the same period, suggesting fewer
interactions between the catalyst and VOC
molecules. This reduced contact lowers the
efficiency of the catalytic process and diminishes
the material's activity. Furthermore, the surface
reactions of adsorbed molecules can alter the
chemical properties of the catalyst and reduce its
performance.
Therefore,
optimizing
the
adsorption step is essential to improve the overall
efficiency of the photocatalytic process.
Figure 7.b presents the effect of different
mixing ratios on the photocatalytic removal of
IPA. The experiments revealed that varying the
mixing ratios significantly influenced the
photocatalytic performance of the composite
materials.
Among
them,
sample
M-0:1
demonstrated the highest photocatalytic activity,

achieving approximately 91.59% IPA removal
after 45 minutes. Sample M-1:3 showed the
second-highest efficiency at around 40%, while the
remaining samples exhibited removal rates below
20%. These differences in activity can be
attributed to the formation of the ZnO/TiO₂
heterojunction and the effect of electron-hole
recombination
within
the
semiconductor
materials. The dispersion between the two phases
is critical in determining photocatalytic
performance, and an optimal ZnO/TiO₂ ratio
exists for mechanically mixed catalysts to
maximize activity. Based on these findings, M-1:3
was selected for further investigation, as it
showed the most effective interaction between the
two materials among the tested samples.
3.3. Effect of IPA Concentration on the Catalytic
Performance of ZnO/TiO2
Figure 8.a and Figure 8.b illustrate the effects
of initial IPA concentration on the adsorption
capacity and photocatalytic performance of the M1:3 sample, respectively. As shown in Figure 8.a,
the adsorption capacity is strongly influenced by
the initial concentration of IPA. With increasing
concentrations, the time required for the material
to reach saturation decreases, as more IPA
molecules quickly occupy and cover the available
surface sites. In Figure 8.b, a clear decline in
photocatalytic efficiency is observed with higher
IPA concentrations. At an initial concentration of
1174 ppmv, the catalyst achieved approximately
40% removal efficiency within one hour. However,
when the initial concentrations increased to 1879
ppmv and 2349 ppmv, the maximum conversion
rates dropped to 27.41% and 15.5%, respectively.
Although a higher concentration leads to more
IPA molecules contacting the catalyst surface, it
can also overwhelm the catalyst’s capacity [32].

Figure 7 a) IPA adsorption capacity over ZnO/TiO 2 samples with different ratios (CIPA: 1174 ppmv,
RH=0%, N2:O2 = 8:2, Gas flow rate: 3 L/h); b) IPA removal over ZnO/TiO2 samples with different ratios
(CIPA: 1174 ppmv, RH=0%, N2:O2 = 8:2, Gas flow rate: 3 L/h, 4 UV lamps).
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Bulletin of Chemical Reaction Engineering & Catalysis, 20 (4), 2025, 589
This overload may reduce active site availability
and lead to the formation of intermediate byproducts that inhibit further adsorption and
reaction. As a result, the overall efficiency of VOC
degradation decreases, and the photocatalytic
performance, when normalized by catalyst mass,
significantly declines.
3.4. Effect of Humidity
Performance of ZnO/TiO2

on

the

Catalytic

The effects of relative humidity on the IPA
adsorption capacity and photocatalytic removal
efficiency of the M-1:3 sample are shown in Figure
9.a and Figure 9.b, respectively. Similar to the
impact of initial IPA concentration, an increase in
humidity leads to a reduction in the time required
to reach adsorption saturation. This can be
attributed to the competitive adsorption of water
vapor on the catalyst surface. Previous studies on

humidity’s influence in photocatalytic VOC
degradation have reported that moderate
humidity levels can enhance photocatalytic
activity by promoting the formation of hydroxyl
(▪OH) radicals on the catalyst surface, which
actively participate in VOC degradation [33].
However, when humidity becomes excessive,
water molecules begin to dominate the available
active sites, hindering the adsorption of IPA. This
competitive effect reduces the number of IPA
molecules interacting with the catalyst,
ultimately
lowering
its
photocatalytic
performance.
3.5. Effect of UV Intensity on the Catalytic
Performance of ZnO/TiO2
The effect of UV light intensity on IPA
removal efficiency is illustrated in Figure 10.

Figure 8. a) Effect of VOC concentration on IPA adsorption capacity of M-1:3 (Gas flow rate: 3 L/h, N2:O2 =
8:2, RH=0%); b) Effect of VOC concentration on IPA treatment over M-1:3 (N2:O2 = 8:2, RH=0%, Gas flow
rate: 3 L/h, 4 UV lamps).

Figure 9. a) Effect of relative humidity on IPA adsorption capacity of M-1:3 (CIPA = 1174 ppmv, N2:O2 =
8:2, Gas flow rate: 3 L/h); b) Effect of relative humidity on IPA treatment over M-1:3 (CIPA = 1174 ppmv,
N2:O2 = 8:2, Gas flow rate: 3 L/h, 4 UV lamps).
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Bulletin of Chemical Reaction Engineering & Catalysis, 20 (4), 2025, 590
Light intensity is a critical parameter influencing
the rate of VOC degradation. As shown in the
Figure 10, increasing the number of UV lamps
from two to four leads to a moderate enhancement
in photocatalytic efficiency. Higher UV intensity
increases the excitation of electrons (e⁻) from the
valence band to the conduction band, thereby
generating more electron-hole pairs (h⁺). These
charge carriers are essential for initiating redox
reactions involved in VOC degradation. In the
presence of H₂O and O₂ in the gas stream, the
photocatalyst facilitates their conversion into
reactive oxygen species. Specifically, H₂O is
oxidized by holes to form hydroxyl radicals (▪OH),
while O₂ is reduced by electrons to produce
superoxide radicals (▪O₂⁻). These highly reactive
species play a key role in breaking down VOCs
into benign end-products such as CO₂ and H₂O
[34]. However, excessive light intensity can induce
an overproduction of free radicals that surpass the
catalytic capacity of the material. In such cases,
the excess radicals may non-selectively oxidize
other substances in the gas stream, including nonharmful compounds, thus diminishing the
treatment selectivity and potentially causing
secondary environmental impacts. Therefore,
while increased UV intensity enhances radical
formation and improves VOC removal, an optimal
light intensity must be determined to balance
treatment efficiency and energy consumption,
particularly for practical applications.

operation period. However, a slight reduction in
efficiency is observed for M-0:1 under 30% relative
humidity compared to the results under dry
conditions (as shown in Figure 6.b), where the
efficiency after 60 minutes decreased from 92% to
82%. This decline can be attributed to the
increased water vapor content in the gas stream
at 30% humidity, which likely exceeds the optimal
threshold and leads to competitive adsorption
between water molecules and IPA on the catalyst
surface, thereby reducing catalytic performance.
In contrast, the M-1:3 sample also demonstrates
stable activity throughout the reaction period.
Notably, under humid conditions, M-1:3 shows a
slight improvement in photocatalytic efficiency,
increasing from approximately 40% to 45%
compared to the dry condition. Despite this
improvement,
the
overall
photocatalytic
performance of M-1:3 remains lower than that of
M-0:1 under the same conditions.
4. Conclusion

The comparison of photocatalytic activity
between M-1:3 and M-0:1 under humid conditions
is presented in Figure 11. Under identical reaction
conditions, the M-0:1 sample maintains high
photocatalytic efficiency and stability, achieving
approximately 85% IPA removal after a 3-hour

In conclusion, the successful synthesis of ZnO
nanoparticles on a chitosan substrate via the
precipitation method has been achieved. This
approach is relatively simple and environmentally
friendly compared to conventional synthesis
techniques. Characterization by XRD and SEM
confirmed that the particle size was within the
nanoscale range of 30–50 nm. Diffuse Reflectance
Spectroscopy (DRS) further revealed that the ZnO
nanoparticles exhibited a maximum absorption
wavelength (λAE) between 327–368 nm.
Additionally, ZnO/TiO₂ nanocomposites were
successfully prepared through mechanical mixing,
demonstrating
enhanced
photocatalytic
performance relative to pure ZnO. Among the
tested ratios, the ZnO/TiO₂ composite with a 1:3
ratio showed improved photocatalytic activity
under humid conditions, maintaining a stable

Figure 10. Effect of UV intensity on IPA
treatment over M-1:3 (CIPA: 1174 ppmv, N2:O2 =
8:2, RH=0%, Gas flow rate: 3 L/h).

Figure 11. Comparison of IPA removal
performance of M-1:3 and M-0:1 samples under
humid condition (CIPA: 1174 ppmv, N2:O2 = 8:2,
RH=30%, Gas flow rate: 3 L/h, 4 UV lamps).

3.6 Investigation the Photocatalytic Activity
Duration over 3 hours

Copyright © 2025, ISSN 1978-2993

Bulletin of Chemical Reaction Engineering & Catalysis, 20 (4), 2025, 591
efficiency of approximately 45% over a 3-hour
period. However, this performance was still lower
than that of pure TiO₂, which achieved up to 88%
efficiency under the same conditions. Notably,
TiO₂ experienced a decline in activity under
humidity compared to dry environments. These
findings highlight the importance of further
optimizing the mixing ratio to improve
photocatalytic efficiency, particularly under high
humidity conditions.

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Acknowledgments
We acknowledge Ho Chi Minh City
University of Technology (HCMUT), VNU-HCM
for supporting this study.
Declaration of Generative AI and AIAssisted Technologies in the Writing
Process
During the preparation of this work the
authors used ChatGPT by OpenAI in order to
assist in language refinement and grammar
correction during manuscript preparation. After
using this tool, the authors reviewed and edited
the content as needed and took full responsibility
for the content of the published article.
CRedit Author Statement
Author Contributions: Le H. Hoang:
Methodology, Investigation, Formal analysis,
Writing – Reviewing and Editing; Mai H. Nghi:
Investigation, Formal Analysis, Data Curation;
Ho G. Quynh: Writing, Review and Editing;
Nguyen T. T. Phuong: Conceptualization,
Methodology;
Ngo
T.
H.
Duong:
Conceptualization, Methodology, Writing Review
&
Editing;
Nguyen
Q.
Long:
Conceptualization, Methodology, Supervision,
Formal analysis, Writing - Review & Editing. All
authors have read and agreed to the published
version of the manuscript.
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