Abstract
Titanium dioxide (TiO2) nanomaterials have been comprehensively studied in recent decades because of their possible practical applications in diverse fields, including photocatalysis, energy storage, sensing and biomedicine. Numerous strategies have been reported on preparing TiO2 nanostructures with regulated sizes, shapes and surface ligands; however, developing a simple, eco-friendly, cost-effective and scalable procedure for the fabrication of TiO2 nanomaterials while controlling those properties is still challenging. Herein, titanium dioxide nanostructures, e-TiO2, are prepared using a scalable green electrochemical approach, in which Ti foils act as precursors. Electrolysis is carried out in an electrolyte solution containing only a surfactant in water. This approach reduces the use of toxic chemicals, and thus it is eco-friendly and cost-effective. Moreover, the electrochemical synthesis of colloidal e-TiO2 can be successfully scaled up to 5 L. Three cationic surfactants, tetrapropylammonium bromide (TPAB), tetrabutylammonium bromide (TBAB) and cetyltrimethylammonium bromide (CTAB), are employed as electrolytes to fabricate e-TiO2, resulting in the formation of three types of nanostructures: negatively charged e-TiO2/TPAB nanospheres, negatively charged e-TiO2/TBAB nano-ellipses and positively charged e-TiO2/CTAB nanorods. They are characterized to clarify the roles of surfactants in controlling sizes, shape and surface properties of e-TiO2 nanostructures. Dispersibility of three types of e-TiO2 is investigated in aqueous and nonpolar solvents. e-TiO2/TPAB and e-TiO2/TBAB are only well-dispersed in aqueous media. In particular, e-TiO2/CTAB can be dissolved in both kinds of solvents. Thanks to the unique rotatable property of the coating agent (CTAB), a “smart” TiO2 nanomaterial (e-TiO2/CTAB nanorod) has been fabricated, which can migrate from an aqueous medium to a nonpolar one and vice versa.
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Introduction
Titanium dioxide (TiO2) nanomaterials have been recognized as one of the most promising semiconductor materials for a wide range of applications, especially in energy and environmental sectors, thanks to their unique photoelectric properties whereby electron–hole pairs are created under proper radiation and driving redox reactions. Therefore, they have been in the forefront of intensive research, much of which reveals that their properties are highly dependent on various factors, including size, shape and surface modification.1,2,3,4 Moreover, the surface properties of TiO2 nanomaterials are also related to the development of several bio-relevant and sensing applications.5,6 Therefore, various approaches have been investigated to prepare TiO2 nanostructures, as well as to control their sizes, shapes and surface properties, including physical, chemical and biological approaches.
Physical methods, such as physical vapor deposition, sputtering and plasma, have been employed to synthesize TiO2 nanomaterials with high purity and controllable phase structure. For example, Xiang et al. designed and fabricated aligned TiO2 nanowire arrays with single-crystalline structure and high purity on Si wafers using a thermal deposition method.7 In another study, Rafieian and co-workers reported on an approach for controlling the formation of anatase and rutile TiO2 thin films by adjusting oxygen concentration during sputtering.8 However, it is undeniable that these top-down methods are not convenient for controlling the size and shape of TiO2 nanostructures. Moreover, they are complex and energy-intensive. Additionally, they required expensive equipment and well-trained operators. Therefore, it is difficult to scale up the synthesis of TiO2 nanomaterials using physical methods. As a simpler and more cost-effective approach, biological methods for TiO2 synthesis have recently attracted increasing interest.9,10,11 Biological synthesis is a green approach for preparing TiO2 nanomaterials, in which a reduction process occurs with the aid of plant extracts or activity of microorganisms. However, it is difficult to regulate the size, shape, and crystalline structure of the nanostructures. Most of the established studies resulted in spherical or irregular shapes of TiO2 nanomaterials.11 In addition, the complexity of chemical contents in plant extracts and microorganism culture media is not ideal for controlling the surface properties of TiO2 nanomaterials during the fabrication process. Compared to those two approaches, the chemical strategy is the most widely studied. Various chemical methods, such as sol–gel, hydrothermal, sonothermal, microemulsion, and sonochemical synthesis, have been developed to prepare TiO2 nanostructures with different sizes and shapes, which are regulated by adjusting experimental parameters including pH, temperature, concentrations of precursors and other chemical reagents.3,12 For instance, distinct shapes of anatase TiO2 nanomaterials, including spherical, rod-like and wire-like nanostructures, were fabricated by Lee et al. while adjusting pH values and varying concentrations of titanium isopropoxide (TTIP) (precursor) and ethylenediamine (shape controller) in a sol–gel synthetic method.13 Also using different pH values, Yang et al. prepared a set of crystal-controlled TiO2 nanorods with different exposed facets using hydrothermal synthesis.14 In an effort to better control the size and shape of the TiO2 nanostructures, many researchers have chosen a surfactant-assisted synthesis strategy.12 Since amphiphilic compounds possess hydrophilic heads and hydrophobic tails, surfactants tend to form micelles in aqueous solutions and reverse micelles in nonpolar solvent media. Surfactant micelles are reported to act as templates for nanostructure formation.4,12 Therefore, Padmanabhan et al. took advantage of the changes in cetyltrimethylammonium bromide (CTAB) micelle shape at different pH values to prepare a set of TiO2 nanostructures with different sizes and shapes.4 In another study, Wang et al. reported that the concentration of the surfactant could affect surface tension and micelle self-assembly. Thus, by increasing the concentration of CTAB from 55 mm to 137 mm, they decreased the diameter of TiO2 nanoparticles from 240 nm to 156 nm.15 The use of surfactants was also reported to provide coating agents for the obtained nanostructures.4,16 Obviously, the chemical approach of TiO2 synthesis exhibits advantage of convenient shape and size control of nanostructures; however, the use of many toxic organic solvents and chemicals is a concern.
Electrochemical synthesis can be regarded as a greener branch of chemical synthesis. Taking advantage of electrical energy to drive chemical reactions, it helps reduce the use of chemical reagents, resulting in cost-effective and eco-friendly production of purer nanostructures.17 In addition, different from other chemical methods, such as hydrothermal and sol–gel syntheses, it does not require extreme reaction conditions and long reaction time. More importantly, it does not realize hazardous waste as a byproduct. Unfortunately, there have been few studies conducted to fabricate TiO2 nanomaterials using electrochemical methods. In a 2014 study, Anandgaonker et al. successfully prepared TiO2 nanoparticles via an electrochemical method using Ti foil as a precursor and tetrapropylammonium bromide (TPAB) as an electrolyte in an organic medium.16 In another study, Bezares et al. reported on size-controlled electrochemically synthesized TiO2 nanoparticles via current applied in tetrabutylammonium bromide (TBAB) aqueous solution.18 The electrolytes (i.e.,TPAB and TBAB) employed in those studies are also surfactants; however, their roles were not fully discussed.
In this study, we aim to develop a scalable electrochemical procedure to prepare TiO2 nanoparticles (e-TiO2 NPs), using three surfactants, TPAB, TBAB and CTAB. While fixing the DC voltage and reaction time of the electrolysis, the use of different electrolytes results in different sizes, shapes and surface properties of e-TiO2 in solution. After steps of drying (at 60°C) and annealing (at 500°C), we obtain amorphous and crystal powder of e-TiO2, respectively. Further characterizations are performed on those materials to better understand their physical and chemical properties. Subsequently, the role of the surfactants and their different structures on the formation of e-TiO2 are discussed. Finally, an interesting behavior of e-TiO2/CTAB nanostructure is presented in polar and nonpolar solvents as its coating agents are reversible to adapt to the surrounding environment. It reveals the fabrication of a novel “smart” e-TiO2, which is promising for various applications in the future.
Methods
Chemical
Tetrapropylammonium bromide (C12H28NBr, 99.9%), tetrabutylammonium bromide (C16H36NBr, 99.9%) and cetyltrimethylammonium bromide (C19H42NBr, 99.9%) were purchased from Shanghai Chemical Reagent Co. and used directly without further purification. Two titanium foils (purity: 99.9%) were purchased from Xi’an HST Metal Material Co., Ltd and prepared with dimensions of 100 mm × 20 mm × 1 mm and 500 mm × 50 mm × 1 mm. Double-distilled water was used throughout the experiments.
Electrochemically Synthesized e-TiO2 Nanomaterials and their Characterizations
In this work, e-TiO2 nanomaterials were fabricated using a green electrochemical method as described in Scheme 1, with three main components: (1) direct current (DC) power supply, (2) titanium foils, and (3) electrolyte aqueous solution. The reaction system was carried out in a beaker containing 500 mL of 1% electrolyte (TPAB, TBAB or CTAB) in distilled water. Two titanium foils were employed as two electrodes placed vertically face-to-face. The applied DC voltage was 10 V. Electrolysis was performed at room temperature (~ 25°C) for 2 h obtaining three solutions of colloidal e-TiO2, including e-TiO2/TPAB, e-TiO2/TBAB and e-TiO2/CTAB (Set 1). Subsequently, e-TiO2 colloidal solutions were dried at 60°C for 8 h in an oven for water evaporation, resulting in e-TiO2 amorphous powders (Set 2). Finally, e-TiO2 crystal powder (Set 3) was obtained by annealing e-TiO2 amorphous powder at 500°C for 2 h. The production of e-TiO2 can be scaled up to 5 L by increasing the size of the Ti foils and the volume of electrolyte solution (5 L).
Schematic illustration of the electrochemical synthesis process of e-TiO2 nanomaterials.
The shapes and sizes of the sets of TiO2 were collected using a scanning electron microscope (SEM Hitachi S-4800) operating under an acceleration voltage of 5 kV. The crystal phase was investigated by x-ray diffraction (Bruker D5005 x-ray diffractometer, Cu Kα, λ = 1:5406 Å) under voltage of 40 kV and current of 30 mA. The optical properties of e-TiO2 were studied by ultraviolet–visible (UV-Vis) absorption spectra using a JENWAY 6850 double-beam spectrophotometer. Chemical properties were investigated by Raman spectroscopy using a MacroRAM™ Raman spectrometer (Horiba) with 785-nm laser excitation. Thermal properties were studied by thermogravimetric analysis (TGA) using a TGA 4000 thermogravimetric analyzer (PerkinElmer). The surface properties and stability were analyzed by ζ potential measurements (Zetasizer Ultra Red Label).
Results and Discussion
Characterizations of Set 1: e-TiO2 Colloidal Solutions
Morphology of e-TiO2 Colloidal Solutions
Electrochemical synthesis using three different electrolytes (i.e., TPAB, TBAB and CTAB) resulted in a set of three e-TiO2 colloidal solutions. The obtained solutions exhibited the characteristic bright white color of TiO2 without aggregation. The corrosion process led to a significant transformation of the surface of Ti foils, transitioning it from a smooth to a rough texture (Scheme 1 inset). The morphology of each e-TiO2 sample was then studied by SEM. Figure 1 displays the SEM images of e-TiO2/TPAB (a), e-TiO2/TBAB (b) and e-TiO2/CTAB (c). e-TiO2/TPAB is spherical with an average diameter of about 15 nm (Fig. 1a). The SEM image of e-TiO2/TBAB reveals the formation of elliptical nanostructures, which are marked with dashed red lines in Fig. 1b, with average axes of 75 nm and 30 nm. Surprisingly, the size of e-TiO2/CTAB nanostructures are much larger with average sizes of 300 nm in width and 700 nm in length (Fig. 1c). It is worth stressing that the electrochemical synthesis of all of three samples were carried out under the same conditions of applied voltage, reaction time and temperature. Therefore, this significant difference is due to the difference in the structure of electrolytes.
SEM images of e-TiO2/TPAB (a), e-TiO2/TBAB (b) and e-TiO2/CTAB colloidal solutions (c); Raman spectra (d) and FTIR spectra (e) of e-TiO2/TPAB, e-TiO2/TBAB and e-TiO2/CTAB colloidal solutions (Color figure online).
Chemical Properties of e-TiO2 Colloidal Solutions
Figure 1d shows the Raman spectra of e-TiO2/TPAB, e-TiO2/TBAB and e-TiO2/CTAB colloidal solutions with no signal of crystal TiO2. Instead, the presence of electrolytes is reflected clearly in the Raman spectrum of every sample. The peaks detected in the Raman spectra of e-TiO2 samples are similar to the peaks observed in the Raman spectra of their electrolytes. To be more detailed, both the Raman spectra of e-TiO2/CTAB and CTAB exhibit the bands at 375 cm−1, 730 cm−1, 1004 cm−1 and 1450 cm−1, which are characteristic peaks of CTAB. The bands at 375 cm−1 and 730 cm−1 are assigned to the C4N+ deformation and CH2 rocking mode vibration. The bands at 1004 cm−1 and 1450 cm−1 represent C–C stretching and CH2 scissoring mode, respectively.19 A clear similarity was also observed between the Raman spectra of e-TiO2/TPAB and TPAB as well as e-TiO2/TBAB and TBAB in Fig. 1d. In addition, a peak at 686 cm−1 appears in the Raman spectra of all e-TiO2 samples, corresponding to Ti–O stretching mode that is typically detected in amorphous Ti–O materials.20 Therefore, the Raman spectra suggests the formation of e-TiO2 at amorphous phase after the step of electrochemical synthesis. In addition, unlike the Raman spectrum of e-TiO2/CTAB, those of e-TiO2/TPAB and e-TiO2/TBAB exhibit an obvious peak at 230 cm−1, which might be ascribed to the Ti–Br stretching mode.21 The presence of this band will be discussed in detail in the following sections.
FTIR studies of three e-TiO2 colloidal solutions only showed two characteristic peaks at 3320 cm−1 and 1620 cm−1, which are ascribed to –OH stretching vibration and –CH2 bonds, respectively (Fig. 1e). The wide and strong absorption of the peak at 3320 cm−1 can be due to the aqueous media of the samples. Meanwhile, the appearance of –CH2 bonds can be explained by the presence of the electrolytes in colloidal solutions. With no strange peak detected in either Raman or FTIR spectra, the high purity of the e-TiO2 colloidal solutions is confirmed.
Optical Properties of e-TiO2 Colloidal Solutions
To investigate optical properties of e-TiO2 colloidal solutions, UV-Vis absorption spectra were recorded over a range from 200 nm to 800 nm covering UVC (200–280 nm), UVB (280–315 nm), UVA (315–400 nm) and visible (400–800 nm) regions (Fig. 2). TPAB, TBAB and CTAB solutions only exhibit absorption in a narrow range from 200 nm to 215 nm within the UVC region. In contrast, e-TiO2/TPAB, e-TiO2/TBAB and e-TiO2/CTAB solutions exhibit a broad absorption band with a shoulder at around 325 nm, resulting in a wide range of absorption from 200 nm to 400 nm. Additionally, poor absorbance can be observed within the range of visible light. Notably, the difference among the absorbance of three e-TiO2 samples is negligible, suggesting that the use of different electrolytes during electrochemical synthesis might not affect the optical properties of the e-TiO2 colloidal solutions.
UV-Vis absorption spectra of TPAB and e-TiO2/TPAB colloidal solution (black); TBAB and e-TiO2/TBAB colloidal solution (blue); CTAB and e-TiO2/CTAB colloidal solution (olive) (Color figure online).
Surface Properties and Stability of e-TiO2 Colloidal Solutions
Surface properties of e-TiO2 were studied via ζ potential measurements (Fig. 3). One day after the fabrication, ζ potentials of e-TiO2/TPAB, e-TiO2/TBAB and e-TiO2/CTAB were measured to be −39 mV, −32 mV and 64 mV, respectively, representing high stability and excellent dispersibility of the nanostructures in solution. e-TiO2/TPAB, e-TiO2/TBAB and e-TiO2/CTAB solutions were all slightly acidic with pH values of 6.3, 6.1 and 5.6, respectively. Therefore, e-TiO2/CTAB could be coated by a positively charged agent while the others could be stabilized by negatively charged agents. It is worth noting that each electrochemical synthesis method involved only water, a surfactant as an electrolyte and Ti foils. Therefore, the coating agents of the e-TiO2 nanostructures that were obtained had to originate from either water or the electrolytes. CTAB is a cationic surfactant with a hydrophilic head (N+) and a hydrophobic tail; thus, it could be responsible for the positively charged surface of e-TiO2/CTAB. However, TPAB and TBAB are also cationic surfactants with N+ heads. Hence, the shorter hydrophobic tails might have prevented them from acting as coating agents for e-TiO2. Instead, e-TiO2/TPAB and e-TiO2/TBAB nanostructures were coated by negatively charged agents. As discussed previously, the peak at 230 cm−1, which is assigned to the symmetric Ti–Br stretching vibration, could only be detected in the Raman spectra of e-TiO2/TPAB and e-TiO2/TBAB but not in that of e-TiO2/CTAB, which could explain the negatively charged surfaces of e-TiO2/TPAB and e-TiO2/TBAB and the presence of Br− through Ti–Br bond formation on the e-TiO2 surface. In contrast, the absence of this band in the Raman spectrum of e-TiO2/CTAB also confirms the stabilization of CTAB on the surface of e-TiO2 nanomaterials. Thanks to the protection of CTAB, Br− in the solution could not access as-prepared e-TiO2 to form Ti–Br bonds on the surface of the nanostructures. The formation of e-TiO2 and the role of surfactants will be further discussed in the section “Roles of Surfactants in Controlling Size, Shape and Dispersibility of e-TiO2 Nanostructures.” ζ potential measurements were also performed 3 months after fabrication. After 3 months of storage in laboratory conditions (25°C), all e-TiO2 samples retained their surface charge. e-TiO2/TPAB, e-TiO2/TBAB and e-TiO2/CTAB were measured to be −25 mV, −32 mV and 39 mV. Moreover, no aggregation was observed in the solution, which confirmed the long-term stability of e-TiO2 nanostructures obtained using electrochemical method.
Zeta potential of e-TiO2/TPAB (a), e-TiO2/TBAB (b) and e-TiO2/CTAB (c) colloidal solutions over a period of 1 day and 3 months (Color figure online).
Characterizations of Set 2: e-TiO2 Amorphous Powders
Morphology of e-TiO2 Amorphous Powders
e-TiO2 colloidal solutions were dried at 60°C for 5 h for water evaporation, resulting in a set of e-TiO2 amorphous powders, including e-TiO2/TPAB, e-TiO2/TBAB and e-TiO2/CTAB. SEM images of e-TiO2/TPAB, e-TiO2/TBAB, and e-TiO2/CTAB amorphous powders are exhibited in Fig. 4a, b and c, respectively. In every sample, e-TiO2 nanostructures are highly uniform. In addition, their sizes and shapes are similar to those shown in Fig. 1a, b and c, respectively, indicating that the drying step did not affect the morphology of the nanostructures.
SEM images of e-TiO2/TPAB (a), e-TiO2/TBAB (b) and e-TiO2/CTAB amorphous powders (c) (Color figure online).
Crystal Properties of e-TiO2 Amorphous Powders
The crystal properties of e-TiO2 amorphous powders were studied via x-ray diffraction measurements. Figure 5 shows the XRD patterns of three powder samples (i.e. e-TiO2/TPAB, e-TiO2/TBAB and e-TiO2/CTAB) in comparison to the reference pattern of TiO2 (ICDD file No. 01-086-1157). The characteristic peaks appear at 2θ = 25.23°, 37.55°, 47.70° and 55.32° in the XRD patterns of three e-TiO2 samples, corresponding to the (100), (004), (200) and (211) planes of the tetragonal crystal structure of the TiO2 anatase phase, respectively. The presence of those bands reveal the crystallinity of the samples. However, with such low intensity, these e-TiO2 samples exhibit low levels of crystallinity. In other words, the main phase composition of these samples is amorphous; thus, in this context, we marked them as amorphous samples to compare with the crystalline ones, which will be discussed in the following section. In addition, the diffractograms also exhibit two strange peaks at 22.29° and 26.98° in the pattern of e-TiO2/TPAB, one at 23.62° in the pattern of e-TiO2/TPAB and two at 21.33° and 24.46° in the pattern of e-TiO2/CTAB which can be indexed to the characteristic peaks of TPAB, TBAB and CTAB, respectively.22,23 The drying temperature of 60°C could be too low to promote crystallization of TiO2 and degrades the surfactants.
XRD patterns of e-TiO2/TPAB, e-TiO2/TBAB and e-TiO2/CTAB amorphous powders (Color figure online).
Chemical, Optical and Surface Properties of e-TiO2 Amorphous Powders
Similar to those of e-TiO2 colloidal solutions, the characteristic peaks of TPAB, TBAB and CTAB can still be detected in the Raman spectra of the e-TiO2 amorphous powders (Fig. 6a). The new peaks arising at 159 cm−1 and 634 cm−1 can be assigned to TiO2 crystal phase (Eg). Thus, heating at 60°C for 5 h might have led to few changes in the crystal structure of TiO2. However, the low intensity of these peaks as well as the absence of other characteristic peaks of TiO2 crystal phase (B1g and A1g) may be attributed to the poor crystallinity of the e-TiO2 amorphous powders. Obviously, 60°C is still too low to promote atomic reorganization for crystallization of TiO2.
Raman spectra (a) and UV-Vis spectra (b) of e-TiO2/TPAB, e-TiO2/TBAB and e-TiO2/CTAB amorphous powders (Color figure online).
The step of drying also did not show any clear influence on the optical properties of e-TiO2. Dried amorphous e-TiO2 powders were re-dissolved in water. Their absorption spectra are presented in Fig. 6b. With a strong and broad absorption band in the UV range (200–400 nm) and almost no absorption in the visible light region, the absorption spectra of e-TiO2 amorphous powders show negligible changes compared to those of e-TiO2 colloidal solutions.
Also, using e-TiO2 amorphous powders dissolved in water, ζ potential measurements were performed to investigate the surface charge of e-TiO2 (Fig. 7b) and compare to that of e-TiO2 in the initial colloidal solutions. The slight changes observed in their ζ potential values may be due to differences in ionic forces within water and the initial electrolyte solution. More importantly, after the drying step at 60°C, every e-TiO2 sample was still well dispersed in water while retaining its surface charge.
TGA (a) and zeta potential (b) of e-TiO2/TPAB, e-TiO2/TBAB and e-TiO2/CTAB amorphous powders (Color figure online).
Thermal Properties of e-TiO2 Amorphous Powders
TGA analysis was carried out to further understand the effect of temperature on the crystallization process and to determine the content of components of the e-TiO2 amorphous powders. Figure 7a shows thermograms of weight against temperature for three e-TiO2 amorphous powder samples. Each thermogram can be divided into four stages: (1) amorphous zone—stage 1 and 2: from room temperature to 312°C; (2) atomic reorganization zone—stage 3: from 312°C to 530°C; (3) complete crystallization zone—stage 4: from 530°C to 700°C. In the amorphous zone (stages 1 and 2—from 25°C to 271°C) of e-TiO2/TPAB, the first weight loss of around 15% occurs due to the evaporation of adsorbed water, whereas the dramatic weight loss (~22%) from 216°C to 271°C can be associated with the removal of TPAB (melting point ~ 266–271°C). Stage 3 in the temperature range from 271°C to 530°C can be ascribed to the atomic reorganization process when the atoms are rearranged under high temperature. The arrangement of atoms from an amorphous (short-range order) state to a crystalline (long-range order) state does not reduce the mass of the material. However, the rearrangement of atom position under intensive heat treatment is convenient for desorption of the remaining organic residues in the lattice structure. Therefore, the total weight loss of ~ 10% in the atomic reorganization zone could be explained by the liberation of organic molecules (TPAB) from the lattice structure of TiO2. The complete crystallization zone (stage 4) occurring in the temperature range from 530°C to 700°C shows no change in material weight, indicating that the TiO2 lattice is pure and temperature variations only concern the crystal phase composition of e-TiO2. TGA measurement of e-TiO2/TPAB demonstrated the presence of TPAB in the amorphous powder with content of about 32 wt.%. Similarly, TGA measurements of e-TiO2/TBAB and e-TiO2/CTAB amorphous powders also revealed the presence of TBAB and CTAB, respectively, through the weight loss in amorphous zone and atomic reorganization zone. Stage 2 of e-TiO2/TBAB and e-TiO2/CTAB amorphous powders occurred at the temperature from 102°C to 300°C and 241°C to 312°C, corresponding to the removal of TBAB (melting point ~ 102–106°C) and CTAB (melting point ~ 237–243°C), respectively. The content of TBAB and CTAB in e-TiO2/TBAB and e-TiO2/CTAB amorphous powders was calculated to be 25 wt.% and 45 wt.%, respectively. The TGA analysis, therefore, reconfirmed the presence of two main components in e-TiO2 amorphous powders: amorphous TiO2 (55–75 wt.%) and electrolytes/surfactants (25–45 wt.%).
Characterizations of Set 3: e-TiO2 Crystal Powders
Morphology of e-TiO2 Crystal Powder
The TGA results show that the onset temperature for complete crystallization of e-TiO2 is about 500°C. Hence, thermal treatment at 500°C was chosen to promote crystallization of e-TiO2 to obtain crystal powders. Figure 8 shows the SEM images of e-TiO2/TPAB (a), e-TiO2/TBAB (b) and e-TiO2/CTAB (c) crystal powders, with significant differences in morphology. After calcination, e-TiO2/TPAB crystal nanostructures are still spherical, with average size of 15 nm, which is similar to those in the amorphous powder sample. The elliptical shape of e-TiO2/TPAB and rod-like shape of e-TiO2/TPAB nanostructures can also be observed in the SEM images. However, our measurements showed an increase in the size of these nanostructures. While the minor axis nearly remained constant, the major axis of e-TiO2/TBAB increased to 95 nm. In addition, after crystallization, e-TiO2/CTAB also appeared to be larger, with an average width of 500 nm and length of 1.5 µm. The changes in size could be a consequence of the arrangement of atoms into crystal lattices. Another small change on the surface of the nanostructures after crystallization that was detected in SEM images is the disappearance of the bright layer surrounding them, as shown in Fig. 1a. This could be due to the removal of coating agents on the nanoparticles, leading to the reduction of water adsorbed on their surface.
SEM images of e-TiO2/TPAB (a), e-TiO2/TBAB (b) and e-TiO2/CTAB (c) crystal powders; XRD pattern (d) and Raman spectra (e) of e-TiO2/TPAB, e-TiO2/TBAB and e-TiO2/CTAB crystal powders (Color figure online).
Crystal and Chemical Properties of e-TiO2 Crystal Powders
XRD patterns of e-TiO2/TPAB, e-TiO2/TBAB and e-TiO2/CTAB crystal powders show sharp peaks that correspond to the (100), (004), (220), (105) (204) and (211) planes, indicating the tetragonal structure of TiO2 (Fig. 8d). All peaks are indexed to the anatase phase, without the appearance of other phases. The Debye–Scherrer formula was employed to calculate the average crystal grain size of e-TiO2/TPAB, e-TiO2/TBAB and e-TiO2/CTAB, revealing size of 10 nm, 12 nm and 9 nm, respectively.24 In addition, the crystal properties of e-TiO2 crystal powder were also confirmed by Raman spectroscopy with sharp and intense characteristic peaks at Eg (147 cm−1), B1g (399 cm−1), A1g (518 cm−1) and Eg (640 cm−1), revealing the tetragonal anatase structure (Fig. 8e). Therefore, after calcination at 500°C, the anatase phase of TiO2 was dominant. Furthermore, the XRD pattern and Raman spectra of e-TiO2 crystal powder no longer reflect the presence of the organic electrolytes in the TiO2 matrix, suggesting their decomposition under the intensive heat treatment.
Optical and Surface Properties of e-TiO2 Crystal Powder
The absorption spectra of e-TiO2 crystal powders are presented in Fig. 9a. In general, strong absorption bands in the UV region and poor absorbance in the visible region of e-TiO2 were still preserved even after the intensive heat treatment, ensuring the optical properties of the semiconductors. Optical band gaps of crystal e-TiO2 samples were determined via a Tauc plot (Fig. 9a, inset) using Kubelka–Munk function,25 resulting in 3.30 eV, 3.35 eV and 3.38 eV for e-TiO2/TPAB, e-TiO2/TBAB and e-TiO2/CTAB, respectively. Once again, it is confirmed that even though the use of different electrolytes during electrochemical synthesis led to differences in e-TiO2 morphology, it showed no significant effect on their optical properties.
e-TiO2 crystal powders were dissolved in water to investigate their surface properties via ζ potential measurements (Fig. 9b). All samples resulted in nearly neutral surface charge, which confirms the removal of coating agents on the surface of e-TiO2 nanostructures after the annealing step at 500°C.
UV-Vis spectra, Tauc plot (a) and zeta potential (b) of e-TiO2/TPAB, e-TiO2/TBAB and e-TiO2/CTAB crystal powder (Color figure online).
Roles of Surfactants in Controlling Size, Shape and Dispersibility of e-TiO2 Nanostructures
By changing only electrolytes in the electrolysis, we have fabricated three types of e-TiO2 different in size, shape and surface properties. Therefore, the effects of these electrolytes on the formation of e-TiO2 is undeniable. TPAB, TBAB and CTAB are all amphiphilic cationic surfactants with hydrophobic and hydrophilic groups. In theory, this allows them to form micelles in aqueous solution, in which the positively charged hydrophilic heads face the outside surface and the hydrophobic tails form the core as shown in Fig. 10. However, only the formation of TBAB and CTAB micelles was reported in previous studies.26,27,28,29,30 TPAB has been employed to facilitate the preparation of NPs; however, the assembly of micelles of this surfactant has been not discussed. The symmetrical structure with four nonpolar alkyl chains might have made it difficult to form micelles. In addition, the alkyl chain of TPAB is shorter than that of TBAB; therefore, the hydrophobic interaction may be not strong enough to form the core of the micelles.
Proposed mechanisms of e-TiO2 formation with the aid of different surfactants as electrolytes (Color figure online).
Concerning the micelle-forming surfactants, the nucleation of TiO2 grains was reported to occur in the solution; subsequently, they adsorbed and assembled on the outer surface of the surfactant micelle, followed by the growth of the grains to form TiO2 nanostructure.12 Thus, the size, shape and behavior of the micelles in aqueous solution can affect the size and shape of the nanoparticles. Meanwhile, all characteristics of the micelles are related to the structure of the surfactants.
Among three surfactants in this study, CTAB molecules possess the longest hydrophobic tail (16 C), allowing them to form the largest micelles. The pH value of CTAB solution prepared for the synthesis was measured as 5.6. It was reported that pH > 2 would lead to the enlargement and elongation of the micelles,26,27 similar to the rod-like shape of e-TiO2/CTAB. This similarity in shape suggests that CTAB micelles would have acted as templates for the assembly of TiO2 grains on their positively charged surfaces. However, this hypothesis is not in agreement with the positive ζ potential value of e-TiO2/CTAB nanostructures. Therefore, the TiO2 grains adsorbed on the micelle surface would have been drawn into the core thanks to hydrophobic interaction between the tails of the surfactants. Moreover, this hydrophobic interaction was reported to strongly promote adsorption, leading to the growth and assembly of the TiO2 grains inside the micelles.4,12 As a soft template, CTAB micelles did not break with the growth of TiO2 up to a size of 700 nm. This could be due to the interactions between the long alkyl chains of the surfactants. Van der Waals interaction between neighboring chains12 and the adsorption of CTAB onto newly assembled TiO2 nanostructures4 might ensure the coating of the surfactant molecules.
With shorter alkyl chains, TBAB facilitates electrochemical synthesis of e-TiO2, resulting in nanoparticles of smaller size. The self-assembly of these two surfactants is not as well studied as CTAB. However, recently, Krisnaveni and Kannan have reported on the assembly of TBAB molecules into micelles as templates for the preparation of AlPO2 and AlSiO4.29 Interestingly, TBAB micellization only occurs at room temperature and pressure. Thus, in this study, the mild conditions of electrolysis reactions might have allowed the formation of TBAB micelles in the electrolyte solution. Moreover, the short tube-like shape of TBAB micelles was also reported,29 which is in close agreement with the shape of the e-TiO2/TBAB nano-ellipses in this study. Therefore, TBAB micelles would have acted as templates for the formation of the nanostructures. However, the differences in size and ζ potential suggest the differences in the formation of TiO2 based on TBAB and CTAB. With shorter alkyl chains (4 C), TBAB molecules would form smaller micelles. Moreover, the hydrophobic interaction between these chains is less strong than that for the longer one.12 Therefore, the absorption of TiO2 grains onto the micelle surface would have been affected, leading to the accumulation of fewer grains onto one assembling center. Hence, the size of TiO2 nanostructures was smaller. Furthermore, TiO2 grains seemed not to migrate into the core of the micelle. In addition to the weak hydrophobic interaction inside the core, short alkyl chains of tetra-alkyl ammonium ions, including TBA+, were reported to be able to penetrate into the interior of surfactant micelles.31,32 Thus, these molecules might have occupied spaces inside TBAB micelles, preventing TiO2 grains from migrating inside. Even if TiO2 grains were drawn into the micelle core, it would not provide stable encapsulation for their growth. In the previous study mentioned above, the nanostructures were built on the positively charged surface the TBAB templates. Therefore, in this study, the formation of the TiO2 nanostructure might have also occurred on the micelle surface. As a result, TBAB micelles were locked inside the TiO2 matrix, which explains the presence of TBAB in e-TiO2/TBAB amorphous powder, reflected in Raman and XRD spectra. In addition, because TiO2 grains assembled and grew on the micelle surface, e-TiO2/TBAB did not exhibit positive ζ potential as e-TiO2/CTAB. Instead, Br− in the solution would have attached to the surface of as-prepared NPs as discussed in the section “Characterizations of Set 1: e-TiO2 Colloidal Solutions.”
Compared to the other surfactants, TPAB was the least widely used in established surfactant-assisted NP synthesis procedures. Anandgaonker et al. utilized TPAB in an electrochemical procedure to synthesize TiO2 NPs. However, it was only mentioned as a supporting electrolyte and a capping agent.16 In this study, the negative ζ potential value of e-TiO2/TPAB contradicts the capping role of TPAB. In several other reports, TPAB was mentioned as a template for fabrication of zeolitic frameworks.33,34 However, in those studies, TPAB molecules did not self-assemble to form micelles. Instead, TPAB acted independently as a center for the development of the framework. However, TPAB did not guide the assembly of silica structures as usual templates; therefore, Huang et al. challenged this role of the surfactant. The explanation for the presence of TPAB in the silica matrix was that the surfactant was pre-surrounded by silica precursor during nucleation and crystalline processes.35 In this study, TPAB is not the coating agent of e-TiO2/TPAB nanostructures; however, its presence in the TiO2 matrix of e-TiO2 before annealing was confirmed by Raman spectroscopy, XRD and TGA. Therefore, TPAB molecules were probably trapped randomly in the TiO2 matrix during nucleation and assembly of TiO2 grains. Without the guide of any surfactant template, TiO2 nanostructures could grow freely in every direction, resulting in the formation of spherical e-TiO2. However, because TPAB did not trigger the assembly of TiO2 grains as TBAB and CTAB, it obtained smaller nanostructures.
The position of surfactant molecules in/on e-TiO2 nanostructures was also reflected in the XRD patterns of the amorphous powders (Fig. 5). It can be observed that the peaks representing the presence of CTAB in the e-TiO2/CTAB pattern is sharp and intense. The presence of CTAB on the surface of e-TiO2/CTAB nanostructures could be more easily detected by x-ray, in comparison to TBAB, which was “hidden” under many layers of atoms in e-TiO2/TBAB nano-objects. Thus, Fig. 5 shows the XRD pattern of e-TiO2/TBAB with a poor signal of TBAB. Being trapped randomly in the TiO2 matrix, the presence of TPAB was also detected in the XRD pattern of e-TiO2/TPAB. However, its characteristic peaks are less intense than those of CTAB in the e-TiO2/CTAB pattern. A similar trend can also be observed in the Raman spectra of e-TiO2 amorphous powders (Fig. 6a).
Being the center for the assembly of TiO2 grain also limited the content of TBAB in e-TiO2 nanostructures, compared to the other surfactants. It was confirmed by TGA analysis that e-TiO2/TBAB had the lowest surfactant content (25 wt.%).
With the assistance of three different surfactants, three kinds of e-TiO2 were fabricated with different sizes and shapes. More importantly, the as-prepared nanoparticles were also distinct in surface properties, which may have significant implications for future applications of the nanomaterials.
Despite the differences in coating agents, all of the newly fabricated e-TiO2 were well-dispersed in water. However, the amphiphilic coating agents on the surface of NPs were reported to be able to rotate to change their orientation to adapt to the surrounding environment. More specifically, in aqueous solutions, which have high polarity, the hydrophilic heads were exposed. In contrast, in nonpolar solvents, the coating agents rotate, and the hydrophobic tails stretched into the solvents, allowing the NPs coated with amphiphilic molecules to be dispersed in a variety of solvents.36 In this study, e-TiO2/CTAB NPs were also highly dispersed in oil. In contrast, two types of e-TiO2 NPs coated with Br− were non-dispersible in oil.
To better demonstrate the dispersibility of e-TiO2 nanostructures, we performed a phase transfer experiment. Three amorphous powders were dissolved in water, resulting in a white color in the aqueous solutions. Subsequently, oil was added gently into the tubes containing e-TiO2 solutions, forming two separate layers (Fig. 11a, b and c). The upper layer in pale yellow was oil while the lower one in white was e-TiO2 in water. These two solvents were then mixed together by shaking well, after which the mixture was left still for 30 min. After 30 min, e-TiO2/TPAB and e-TiO2/TBAB solutions completely restored to their initial state (Fig. 11a and b), which means all e-TiO2 nanoparticles remain in the aqueous solutions. The mixture of e-TiO2/CTAB aqueous solution and oil also formed two layers after 30 min. However, the color of each layer obviously changed (Fig. 11c). The nonpolar layer turned yellowish white, implying the presence of e-TiO2 nanostructures in this solvent. In contrast, the aqueous layer lost its bright white color and turned grayish, representing a decrease in e-TiO2 concentration in the solution. Therefore, a portion of the e-TiO2 nanostructures migrated from water to oil. The migration of e-TiO2 can also occur from oil to water even without shaking (Fig. 11d). Therefore, it is reversible. A narrow zone in milky white can be observed between two layers in Fig. 11c, in which the phase transfer process of e-TiO2/CTAB nanorods might still occur from water to face and vice versa, thanks to their rotatable coating molecules. This phase transfer process was also the reason for the formation of emulsion-like solution in the e-TiO2/CTAB sample during shaking as presented in Fig. 12. Hence, we have fabricated a smart TiO2 nanorod with rotatable coating agents that can change their orientation, depending on the solvent. This surface property opens the opportunity to develop more applications, which may need to take place in diverse environments, either polar or nonpolar. For example, Liu et al. reported on antibacterial application of silver nanoparticles coated with sodium dodecyl sulfate (SDS), another amphiphilic compound. Thanks to the reversibility of the coating agent, the NPs were able to pass through the nonpolar membrane effectively.36
Phase transfer experiments of e-TiO2/TPAB (a), e-TiO2/TBAB (b) and e-TiO2/CTAB (c) from water to oil; (d) phase transfer experiment of e-TiO2/CTAB from oil to water (Color figure online).
Schematic illustrations of dispersibility of different kinds of e-TiO2 in water and in oil (Color figure online).
All three kinds of e-TiO2 exhibited the excellent optical properties of TiO2 as good semiconductors for future industrial applications such as photocatalysis, energy storage, sensing and biomedicine. Controlling the size and shape of e-TiO2 has provided more diverse choices for researchers to select, depending on their specific aims. Moreover, “smart” e-TiO2 could be convenient for various technological applications demanding material phase transfer.
Conclusions
In this study, three kinds of e-TiO2 nanostructures were fabricated using a simple and green electrochemical synthesis procedure, with the aid of three surfactants as electrolytes, TPAB, TBAB and CTAB. The use of different electrolytes resulted in different sizes, shapes and surface properties of e-TiO2 nanomaterials but not their optical properties. All negatively charged e-TiO2/TPAB nanospheres, negatively charged e-TiO2/TBAB nano-ellipses and positively charged e-TiO2/CTAB nanorods exhibited a strong absorption band in the UV range and poor absorbance in the visible light range. Therefore, all three kinds of e-TiO2 exhibited the excellent optical properties of TiO2 as good semiconductors for future industrial applications. Meanwhile, the differences in surface properties of e-TiO2 nanostructures obviously influence their dispersibility. All e-TiO2/TPAB, e-TiO2/TBAB and e-TiO2/CTAB are well-dispersed in aqueous solution; however, only e-TiO2/CTAB can be dissolved in nonpolar solvents. Thanks to the rotatable coating agents, “smart” e-TiO2/CTAB can also migrate between those two kinds of solvents. By controlling their sizes, shapes and surface properties, we varied the selection of e-TiO2 for various technological applications. Moreover, the production of e-TiO2 can be scaled up to 5 L; thus, these newly prepared nanostructures are promising for industrial use.
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Acknowledgments
This research was supported by the Vicostone Joint Stock Company & Phenikaa Group. The authors would like to acknowledge the support for Raman and UV-vis measurements from NEB Lab, and zeta potential, DLS measurements from DKSH Lab at the Phenikaa University; FTIR and TGA measurements from VICOSTONE company.
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QDM Conceptualization, Methodology, Validation, Formal Analysis, Writing-Original Draft; HAN Formal Analysis, Writing-review & editing; NNH Methodology, Validation, Investigation; PCT Validation, Investigation; DQT Validation, Formal Analysis; NAS Validation, Investigation; PAT Methodology, Validation, Supervision, Writing-review & editing; ATL Conceptualization, Methodology, Supervision, Project administration, Writing-review & editing.
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Mai, Q.D., Nguyen, H.A., Huyen, N.N. et al. Large-Scale Green Electrochemical Synthesis of Smart Titanium Dioxide Nanomaterials: Controlled Morphology and Rotatable Surface Ligands via Tuning Electrolyte Structures. J. Electron. Mater. 52, 5884–5900 (2023). https://doi.org/10.1007/s11664-023-10550-3
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DOI: https://doi.org/10.1007/s11664-023-10550-3