Highly photocatalytic activity of natural halloysite - Based material for the treatment of dyes in wastewater

specific surface area and 
pore volume as well as pore diameter of HNT, the 
BET nitrogen adsorption - desorption isotherm at 
77.3 K according was carried out for halloysite 
nanotube material samples. 
The curves are exhibited in Figure 3, showing 
that the adsorption - desorption isothermal curve 
of the HNT is of type IV and has a hysteresis curve 
H3 corresponding to the mesoporous material 
according to IUPAC classification. The pore size 
distribution curve illustrates that the mean pore 
diameter of HNT was 4.8 nm. 
Typical parameters of HNT obtained from BET 
measurement results: 
 - Specific surface area (SBET): 28.0186 m2/g 
 - Total pore volume (Vpore): 0.137840 cm3/g 
 - Pore diameter (DBJH): 21.8489 nm. 
3.1.4. Characterization of Ag - TiO2/HNT 
The results of XRD, UV - vis solid, EDX and SEM 
of Ag - TiO2/HNT materials are exhibited in Figures 
4÷7. 
X - ray spectrum of Ag - TiO2/HNT material 
(Figure 4) still confirms the existence of the 
characteristic peaks of TiO2, which proves that 
there is no change in the structure of TiO2 material. 
Figure 3. Nitrogen adsorption - desorption 
isotherm at 77.3 K of HNT. 
Figure 4. X - ray (XRD) spectrum of different 
catalysts. 
Figure 5. Graph of [F(R)hν]2 as a function of hν for 
Ebg calculation. 
Figure 6. EDX spectrum of Ag - TiO2/HNT. 
24 Son Ha Ngo and et al./Journal of Mining and Earth Sciences 62(3), 19 - 28 
There are additional peaks which appear at 2θ = 
38.1 (111) and 64.4 (220) and weaker intensity 
peak at 2θ = 44.3 (200) characterizing the presence 
of metallic silver (JCPDS 65 - 2871). The obtained 
results demonstrate that the insertion of TiO2 and 
Ag - TiO2 on the HNT support causes no effect on 
the halloysite nanotube structure. 
The band - gap energy Eg of the photocatalytic 
materials TiO2, Ag - TiO2, TiO2 - HNT, Ag - TiO2/HNT 
were 3.2 eV, 2.8 eV, 3.0 eV, and 2.6 eV, respectively 
that were indicated in Figure 5. Those values 
appear to verify that the combination of HNT and 
TiO2 has adjusted the band - gap of TiO2 3.2÷3 eV. 
Furthermore, the addition of Ag onto the surface of 
TiO2 has drastically reduced the band - gap of TiO2 
to 2.8 eV that could consequently lead to a 
significant influence on the photocatalytic 
properties of TiO2. Thereby, the excited zone of Ag 
- TiO2/HNT shifted considerably from the 
ultraviolet (UV) to the visible (Vis) region, and the 
material has a narrower band - gap (2.6 eV) 
compared to pure TiO2 (3.2 eV) 
Also, the X - ray energy dispersive spectrum of 
Ag - TiO2/HNT in the binding energy range of 0÷10 
keV in Figure 6 points out the corresponding 
signals of the elements that make up the Ag - 
TiO2/HNT catalyst and no different elements 
appear. Additionally, the mass percentage of the 
elements are quite compatible with the theoretical 
calculation used in the first step to synthesize Ag - 
TiO2/HNT. 
SEM, TEM images (Figure 7) presents that the 
implantation of Ag - TiO2 onto halloysite nanotubes 
did not break the halloysite's tube structure. The 
images show the uniform dispersion of the Ag - 
TiO2 nanoparticles on halloysite (HNT) nanotubes 
and the Ag nanoparticles that are tightly attached 
on the surface of the TiO2 nanorods to form Ag - 
TiO2 nanorods with an average size is about 10÷20 
nm. 
As can be described in Figure 8 and Table 1, Ag 
- TiO2/HNT has twice as much specific surface area 
and significantly increased pore volume as the HNT 
support. It could be explained that Ag - TiO2 also 
has its own specific surface area and has its porous 
system. As a result, the presence of Ag - TiO2 on the 
inner and outer surface of halloysite nanotubes 
could contribute to the number of pores 
representing for the higher surface area as well as 
the total pore volume. The higher pore volume of 
Ag - TiO2/HNT could guarantee the better 
performance of this catalyst in terms of pollutants 
Figure 7. SEM (a and b), TEM (c and d) images of Ag - TiO2/HNT. 
 Son Ha Ngo and et al./Journal of Mining and Earth Sciences 62(3), 19 - 28 25 
adsorption. Based on the evidence given by various 
characterization techniques, it could be concluded 
that Ag - TiO2 was successfully grafted on the 
surface of HNT. Moreover, Ag - TiO2 particles in 
nano size, which were distributed evenly on the 
surface, could bring efficient catalytic activity to the 
as - prepared nanocomposite. 
Parameter Value 
Specific surface area (SBET - m2/ g) 57.3668 
Pore diameter (DBJH - nm) 10.6906 
Pore volume (Vpore - cm3/ g) 0.163701 
Micropore volume (cm3/ g) 0.000193 
3.2. Photocatalytic activity of Ag - TiO2/HNT 
3.2.1. Comparison of photocatalytic activity of 
synthesized materials 
The photocatalytic decomposition of RR - 195 
was performed at room temperature with the 
following conditions: the initial concentration of 
RR - 195 in water was 50 ppm, then 1.5 ml of H2O2 
was added with 0.05 g solid catalyst. Next, the 
mixture was stirred for 3 hours to reach 
equilibrium adsorption. After that, the mixture was 
irradiated with UV light for the next 3 hours. The 
results of the catalytic activity evaluation are 
presented in Figure 9. 
The results pointed out that the Ag - TiO2/HNT 
synthesized by the direct method has the highest 
photocatalytic activity. This can be elucidated by 
the existence of Ag - TiO2 on the surface of 
halloysite reduced the band - gap energy of TiO2 
(2.6 eV). In addition, Ago can be a center of charge 
transition and has high ability to trap electrons to 
prevent recombination of photo - generated 
electrons and holes on TiO2 surface. At the same 
time, silver also has a plasmon resonance effect 
resulting in the more OH radical generation and 
increased photocatalytic efficiency. 
3.2.2. Impact of H2O2 concentration on the 
conversion of RR - 195 
H2O2 has an important impact on the 
photocatalytic degradation of pollutants. The effect 
of H2O2 in RR - 195 was investigated, and the 
results are presented in Figure 10. 
As can be observed in Figure 10, when using 2 
ml of H2O2 with the presence of Ag - TiO2/HNT 
catalyst under UV irradiation, the conversion of RR 
- 195 was 79% in a period of 360 minutes. When 
increasing the amount of H2O2 to 4ml, the 
mineralization of RR - 195 after 360 minutes did 
not occur faster but tended to decrease (76%). In 
addition, if the amount of H2O2 is reduced to 1ml 
and 1.5 ml, the rate as well as the decomposition 
efficiency RR - 195 decreased also standing at 66% 
and 76%, respectively. 
This is because more •OH radicals generated 
from H2O2 promote the reaction leading to 
increased decomposition rate and efficiency. 
However, when the amount of H2O2 in the solution 
is too high or too low will reduce the free radicals 
•OH occurs according to the equation (Szczepanik 
Figure 8. Nitrogen adsorption - desorption 
isotherm at 77.3K of Ag - TiO2/HNT catalyst. 
Table 1. Parameters of Ag - TiO2 - HNT obtained 
from BET method. 
Figure 9. Conversion of RR - 195 in photocatalytic 
degradation process using different catalysts. 
26 Son Ha Ngo and et al./Journal of Mining and Earth Sciences 62(3), 19 - 28 
et al., 2017; Abdel Fattah et al., 2016; Natarajan et 
al., 2015): 
H2O2 + •OH •HO2 + H2O 
•HO2 + •OH O2 + H2O 
(2) 
In addition, a high amount of H2O2 also results 
in the saturation of the active sites of the catalyst, 
thereby diminishing the reaction rate. For that 
reason, 1.5 ml of H2O2 was chosen to be applied to 
all remaining research processes to best assess the 
catalytic activity of the catalyst materials. 
3.2.3. Influence of catalyst weight 
The influence of the catalyst content on the 
conversion of dye RR - 195 on Ag - TiO2/HNT 
catalyst was investigated. The results are shown in 
Figure 11. 
The results showed that after 360 minutes of 
UV irradiation using 0.15 g of Ag - TiO2/HNT, the 
efficiency in RR - 195 degradation was 96%. This 
value is considerably higher than the ones obtained 
when 0.05g and 0.1g catalyst was applied, which 
only reached the conversion of 66% and 90%, 
respectively. This can be explained as follows: 
when the amount of catalyst increases, the amount 
of catalytic activity sites increases, causing the 
diffusion rate of the anions RR - 195 to the active 
sites on the surface, leading to an increase the 
number of 96% shows the almost complete 
decomposition of contaminants under normal 
conditions. 
4. Conclusions 
This study has obtained some remarkable 
results with a relatively new photocatalytic 
material system based on the halloysite natural 
mineral. Specifically, purified halloysite (HNT) 
nanotubes have been processed from raw 
halloysite sources in kaolin mines in a specific 
surface area of 28.0186 m2/g with an average 
length of 1.3 μm and average diameter of 130 nm. 
Ag - TiO2/HNT photocatalytic material was then 
synthesized by the direct method. The data showed 
that the doping of Ag significantly narrowed the 
band - gap energy of pure TiO2. Thus, the material 
could be easily excited in the visible region, which 
makes it more efficient in the photocatalytic 
process. In addition, the specific area and pore 
volume of Ag - TiO2/HNT were significantly 
improved in comparison with HNT and TiO2. The 
photocatalytic activity evaluation indicated that 
the Ag - TiO2/HNT had superior efficiency 
compared to the other catalysts namely TiO2; Ag - 
TiO2; TiO2/HNT thanks to the presence of Ag and 
the better distribution of active phase on HNT 
support. As a result, Ag - TiO2/HNT could almost 
completely decomposed RR - 195 at room 
temperature and neutral pH environment. This 
suggests that Ag - TiO2/HNT material could offer 
extreme potential in the treatment of polluted 
wastewater. 
Author contributions 
Son Ha Ngo and Nui Xuan Pham conceived and 
planned the experiments; Tuan Ngoc Tran carried 
out the experiments; Son Ha Ngo and Tuan Ngoc 
Tran contributed to sample preparation. Son Ha 
Ngo and Nui Xuan Pham contributed to the 
interpretation of the results; Son Ha Ngo and Tuan 
Figure 10. The conversion of RR - 195 when using 
Ag - TiO2/HNT at different H2O2 concentrations. 
Figure 11. Influence of catalyst weight in the 
degradation process of RR - 195. 
 Son Ha Ngo and et al./Journal of Mining and Earth Sciences 62(3), 19 - 28 27 
Ngoc Tran performed the calculations; Son Ha Ngo 
took the lead in writing the manuscript. All authors 
provided critical feedback and helped shape the 
research, analysis and manuscript. 
References 
Ansari, S. A., Khan, M. M., Ansari, M. O., Lee, J. & 
Cho, M. H., (2013). Biogenic synthesis, 
photocatalytic, and photoelectrochemical 
performance of Ag - ZnO nanocomposite. J. 
Phys. Chem. C 117, 27023 - 27030. 
B. Szczepanik, P. Rogala, P.M. Słomkiewicz, D. 
Banaś, A. Kubala - Kukuś, I. Stabrawa, (2017). 
Synthesis, characterization and photocatalytic 
activity of TiO2 - halloysite and Fe2O3 - 
halloysite nanocomposites for 
photodegradation of chloroanilines in water, 
Appl. Clay Sci. 149, 118 - 126. 
G. Guo, B. Yu, P. Yu, X. Chen, (2009). Synthesis and 
photocatalytic applications of Ag/TiO2 - 
nanotubes. Talanta. 79, 570 - 575. 
Grabowska, E., Zaleska, A., Sorgues, S., Kunst, M., 
Etcheberry, A., Colbeau - Justin, C., Remita, H., 
(2013). Modification of titanium (IV) dioxide 
with small silver nanoparticles: application in 
photocatalysis. J. Phys. Chem. C 117, 1955 - 
1962. 
H. Dong, Zeng, Tang, . Fan, Zhang, He, He, (2015). 
An overview on limitations of TiO2 - based 
particles for photocatalytic degradation of 
organic pollutants and the corresponding 
countermeasures, Water Res. 79, 128 - 146. 
K. Imamura, E. Ikeda, T. Nagayasu, T. Sakiyama, K. 
Nakanishi, (2002). Adsorption behavior of 
methylene blue and its congeners on a 
stainless steel surface. J. Colloid Interface Sci. 
245, 50 - 57. 
Khan, M. M., Ansari, S. A., Amal, M. I., Lee, J. & Cho, 
M. H., (2013). Highly visible light active 
Ag/TiO2 nanocomposites synthesized using an 
electrochemically active biofilm: a novel 
biogenic approach. Nanoscale 5, 4427 - 4435. 
N. Cotolan, M. Rak, M. Bele, A. Cör, L.M. Muresan, I. 
Milošev, (2016). Sol - gel synthesis, 
characterization and properties of TiO2 and Ag 
- TiO2 coatings on titanium substrate. Surf. 
Coatings Technol. A 307, 790 - 799. 
Ohtani, B., Iwai, K., Nishimoto, S. & Sato, S., (1997). 
Role of platinum deposits on titanium (IV) 
oxide particles: structural and kinetic analyses 
of photocatalytic reaction in aqueous alcohol 
and amino acid solutions. J. Phys. Chem. B 101, 
3349 - 3359. 
Oros - Ruiz, S., Zanella, R., López, R., Hernández - 
Gordillo, A. & Gómez, R., (2013). Photocatalytic 
hydrogen production by water/methanol 
decomposition using Au/TiO2 prepared by 
deposition - precipitation with urea. J. Hazard. 
Mater. 263, 2 - 10. 
R. Kamble, M. Ghag, S. Gaikawad, B. Panda, 2012. 
Halloysite Nanotubes and Applications: A 
Review. J. Adv. Sci. Res. 3, 25 - 29. 
S. Bagane, M., Guiza, (2000). Removal of a dye 
from textile effluents by adsorption. Ann. Chim. 
Sci. Mater 25, 615 - 626. 
S. M., K. Natarajan, (2015). Antibiofilm Activity of 
Epoxy/Ag - TiO2 Polymer Nanocomposite 
Coatings against Staphylococcus Aureus and 
Escherichia Coli. Coatings. 5 (2), 95 - 114. 
S. Rooj, A. Das, V. Thakur, R.N. Mahaling, A.K. 
Bhowmick, G. Heinrich, (2010). Preparation 
and properties of natural nanocomposites 
based on natural rubber and naturally 
occurring halloysite nanotubes. Mater. Des. 31, 
2151 - 2156 
S. A. Amin, M. Pazouki, A. Hosseinnia, (2009). 
Synthesis of TiO2 - Ag nanocomposite with sol 
- gel method and investigation of its 
antibacterial activity against E. coli. Powder 
Technol. 196, 241 - 245. 
Shan, Z., Wu, J., Xu, F., Huang, F. - Q. & Ding, H., 
(2008). Highly effective silver/semiconductor 
photocatalytic composites prepared by a silver 
mirror reaction. J. Phys. Chem. C 112, 15423 - 
15428. 
W.I. Abdel Fattah, M.M. Gobara, W. El - Hotaby, 
S.F.M. Mostafa, G.W. Ali, (2016). Coating 
stainless steel plates with Ag/TiO2 for 
chlorpyrifos decontamination. Mater. Res. 
Express. 3 (5). 
Y. Zhang, A. Tang, H. Yang, J. Ouyang, (2016). 
Applications and interfaces of halloysite 
nanocomposites. Appl. Clay Sci. 119, 8 - 17. 
28 Son Ha Ngo and et al./Journal of Mining and Earth Sciences 62(3), 19 - 28 
Zhou, X., Liu, G., Yu, J. & Fan, W. Surface plasmon 
resonance - mediated photocatalysis by noble 
metal - based composites under visible light. J. 
Mater. Chem. 22, 21337 - 21354 (2012). 
Zielińska, A., Kowalska, E., Sobczak, J. W., Łącka, I.,
Gazda, M., Ohtani, B., Hupka, J., Zaleska, A., 
(2010), Silver - doped TiO2 prepared by 
microemulsion method: surface properties, 
bio - and photoactivity. Sep. Purif. Technol. 72, 
309 - 318.