Solvothermally Synthesized Hierarchical Aggregates of Anatase TiO2 Nanoribbons/Nanosheets and Their Photocatalytic–Photocurrent Activities

Hierarchical aggregates of anatase TiO2 nanoribbons/nanosheets (TiO2-NR) and anatase TiO2 nanoparticles (TiO2-NP) were produced through a one-step solvothermal reaction using acetic acid or ethanol and titanium isopropoxide as solvothermal reaction systems. The crystalline structure, crystalline phase, and morphologies of synthesized materials were characterized using several techniques. According to our findings, both TiO2-NR and TiO2-NP were found to have polycrystalline structures, with pure anatase phases. TiO2-NR has a three-dimensional hierarchical structure made up of aggregates of TiO2 nanoribbons/nanosheets, while TiO2-NP has a nanoparticulate structure. The photocatalytic and photocurrent activities for TiO2-NR and TiO2-NP were investigated and compared with the widely used commercial TiO2 (P25), which consists of anatase/rutile TiO2 nanoparticles, as a reference material. Our findings showed that TiO2-NR has higher photocatalytic and photocurrent performance than TiO2-NP, which are both, in turn, higher than those of P25. Our developed solvothermal method was shown to produce a pure anatase TiO2 phase for both synthesized structures, without using any surfactants or any other assisted templates. This developed solvothermal approach, and its anatase TiO2 nanostructure output, has promising potential for a wide range of energy harvesting applications, such as water pollution treatment and solar cells.


Introduction
Titanium dioxide (TiO 2 ) is commonly employed in the fields of energy harvesting and energy storage, due to its unique properties [1,2]. It is widely used as a photocatalyst in various energy-related applications, including wastewater treatment, self-cleaning coatings, and solar energy conversion [2]. In order to achieve higher photocatalytic activity, materials with specific features should be utilized. These materials should have suitable band gap energy, high surface area, a well-defined crystal structure, high charge carrier mobility and lifetime, suitable band edge positions, and optimized catalyst loading [3]. Anatase TiO 2 chemical properties [24]. Nanostructures of titanium dioxide (TiO 2 ) with one-dimensional (1D) characteristics, including nanorods, nanotubes, and elongated cylindrical structures, have been observed to display superior photocatalytic performance when compared to P25 particles. This enhanced activity can be attributed to the improved separation of the electron-hole (e−/h+) pairs and decreased occurrence of charge recombination [25]. Despite the widespread use of TiO 2 photocatalysts, they still have some limitations that affect their photocatalytic activity. Some of these limitations include their poor absorption of sunlight and rapid recombination of photogenerated electrons/holes, which continue to hinder their widespread application [26]. However, research has shown that the porous microstructure of TiO 2 is correlated with improved photocatalytic activity, specifically TiO 2 with a hierarchical structure and multiple levels of nanostructures that are of a particular interest. It has been reported that the hierarchical three-dimensional (3D) TiO 2 nanospheres, which contain one-dimensional (1D) nanorods, exhibit exceptional photocatalytic performance due to their electron transfer capabilities [27]. The synthetic protocols for hierarchical TiO 2 materials have become more advanced, allowing researchers to manipulate and regulate characteristics such as structural arrangement, particle dimensions, shape, and surface characteristics [4]. Designing photocatalysts that possess a hierarchical structure at both micrometer and nanometer dimensions can effectively address numerous obstacles associated with the thermodynamic and kinetic characteristics of a photocatalyst [4]. Hierarchical structures with connected porous networks can help reactants move toward the active sites on the walls of the pores. This allows for better diffusion and enhances various properties, such as improved absorption of light, faster movement of molecules, larger surface area, and more active sites [28]. Nevertheless, achieving a suitable equilibrium among the hydrolysis rate of the titanium precursor, the growth rate of titanium dioxide, and the desired orientation makes it a difficult task during hydrothermal/solvothermal conditions [29].
The precursor is generally prepared by dissolving a suitable Ti precursor in a suitable solvent and then subjecting it to a solvothermal reaction [30]. This process leads to the growth and self-assembly of larger, hierarchical aggregates, which can form complex structures such as hollow spheres, flower-like structures, and dendritic structures. Those structures were found to have enhanced photocatalytic performance due to their larger surface area, enhanced ability to transfer charges, and improved capacity to absorb light [22].
Various methods can be used to synthesize hierarchical TiO 2 , including those involving surfactants or templates, and those without either of them. The choice of synthesis method has a profound impact on the produced TiO 2 material's morphology, crystallinity, and surface properties. Extensive research explored the advantages and disadvantages of both surfactant/template-assisted and surfactant/template-free approaches for the synthesis of hierarchical TiO 2 [4,31,32].
Surfactants and templates were found to be essential in directing the growth of TiO 2 nanoparticles and facilitating their self-assembly into hierarchical structures. Surfactants, such as Cetrimonium bromide (CTAB) or Polyvinylpyrrolidone (PVP), assist in stabilizing the nanoparticles and controlling their size and shape. Templates provide a framework for hierarchical assembly, to help in producing specific morphologies such as nanorods, nanotubes, or mesoporous structures [33].
Numerous studies have successfully synthesized hierarchical TiO 2 using surfactant or template methods [34]. For example, in 2017, Bhat et al. utilized a template-assisted solvothermal method in order to synthesize and control mesoporous hierarchical TiO 2 spheres [35]. These spheres exhibited a high surface area and enhanced photocatalytic activity. In the same year, Hu et al. employed a surfactant-templated hydrothermal approach in order to synthesize TiO 2 nanoparticles and nanowires that showed improved photocatalytic performance [36].
On the other hand, the surfactant/template-free synthesis of hierarchical TiO 2 has been considered an alternative method with distinct advantages [37,38]. This approach eliminates the need for additional purification steps and overcomes potential issues associ-Nanomaterials 2023, 13,1940 4 of 13 ated with surfactant or template residues. Moreover, it provides a significant control over the morphology and surface properties of the resulting TiO 2 structures [38].
Recent studies have reported the successful synthesis of hierarchical TiO 2 without surfactants or templates. In 2013, Chen et al. developed self-assembled ultrathin TiO 2 hierarchical nanostructures using a surfactant-free sol-gel method. These hierarchical nanostructures additionally demonstrated improved photocatalytic performance compared to conventional TiO 2 materials [38]. Earlier this year, Yu et al. achieved surfactant-free hydrothermal synthesis of hierarchical TiO 2 nanosheets with exposed (001) facets [39]. These nanosheets exhibited enhanced photocatalytic activity, which was attributed to their unique morphology and increased surface area.
Various synthetic routes were explored for producing hierarchical TiO 2 nanostructures exhibiting both 2D and 3D shapes. Nowadays, researchers are actively investigating a variety of methods for synthesizing hierarchical TiO 2 nanostructures with a wide range of porosity and controlled morphologies. The control of particle size and crystallographic nanostructure orientation is crucial to achieving high performance and reusability [40].
Recently, Vidyasagar et al. and Wang et al. prepared hierarchical meso-macroporous nanoflowers with mesoporous and macroporous structures via a template-assisted sol-gel method, and demonstrated their excellent performance for the degradation of phenol and methylene blue under visible and ultraviolet lights, respectively [41,42]. Moreover, Zhu et al. also synthesized hierarchical mesoporous TiO 2 microspheres with macroporous structures via solvothermal synthesis, with a facile formation mechanism and enhanced photocatalysis [43]. Hongwei Bai et al. presented the synthesis and characterization of 3D dendritic TiO 2 nanospheres with extremely long 1D nanoribbons/wires for the simultaneous purification of water using photocatalytic membranes. These studies highlight the potential of these structures for efficient photocatalysis, offering improved performance due to their unique morphology and enhanced charge separation, as well as their transport properties. Further research is still ongoing to develop advanced hierarchical structures with advanced photocatalysis capabilities [29].
The synthesis of hierarchical TiO 2 with/without surfactants or templates presents distinct advantages and disadvantages. Surfactant/template-assisted methods offer precise morphology control, but may introduce impurities and interfere with photocatalytic properties. Meanwhile, surfactant/template-free methods provide simplicity, cost-effectiveness, and enhanced photocatalytic activity, but they may also have limitations in morphology control. Researchers are actively exploring novel synthesis strategies in order to customize hierarchical TiO 2 structures for diverse applications, aiming to strike a balance between the advantages offered by both approaches.
In this study, we synthesized the hierarchical structures of anatase TiO 2 nanoribbons/nanosheets and anatase TiO 2 nanoparticles using template/surfactant-free solvothermal reaction. Their photocatalytic activities and photocurrents were also compared with those of the commercial (P25) TiO 2 . The anatase TiO 2 nanoribbons/nanosheets showed unique morphology, with elongated shapes and large surface area. The hierarchical anatase TiO 2 nanoribbons or nanosheets are favorable materials for photocatalytic applications, due to their large surface area, high reactivity, and excellent photocatalytic properties. Their photocatalytic and photocurrent performances were found to be improved by aggregating them into hierarchical structures, in comparison to the commercial P25, which is widely used as a benchmark in photocatalysis research as an ideal standard material for measurements.

Experimentation, Materials and Characterizations
TiO 2 -NR and TiO 2 -NP were synthesized using a solvothermal method. A precursor of titanium isopropoxide (TTIP, supplied by Sigma, Macquarie Park, NSW, Australia) was used, while acetic acid (Sigma, Macquarie Park, NSW, Australia) and absolute ethanol (Sigma, Macquarie Park, NSW, Australia) were used as solvents. TTIP (1.5 mL) was slowly added to ethanol or acetic acid, while vigorously stirring at room temperature for one hour.
From this, a white solution was obtained, which was then subsequently moved into a (45 mL) stainless steel autoclave lined with Teflon. (Manufactured by Parr Instrument Company, Moline, IL, USA). After maintaining the autoclave at a temperature of 180 • C for 9 h, a white precipitate formed upon cooling it to ambient temperature. The precipitate was rinsed twice using a mixture of ethanol and distilled water, and subsequently dried overnight at a temperature of 90 • C. Finally, TiO 2 -NR and TiO 2 -NP powders were sintered at 450 • C at a rate of 1 • C per minute in the presence of air for 3 h. The commercial Degussa TiO 2 (P25) (Sigma, Macquarie Park, NSW, Australia), was used as received.
The synthesized materials were characterized using various techniques. The crystalline structure of materials was analyzed using an X-ray diffractometer (manufactured with GBC Scientific Equipment LLC, Hampshire, IL, USA). The X-ray instrument was set at scan range = 20 • -80 • , voltage = 40 kV, current = 30 mA, and a wavelength of Cu Kα radiation = 1.54 Å. The physical characteristics of the samples, including their morphology, internal structure, and element composition, were analyzed using field-emission scanning electron microscopy (FE-SEM) with a JEOL JSM-7500 instrument (Tokyo, Japan), and transmittance electron microscopy (TEM) with a JEOL JEM-6500F instrument (Tokyo, Japan). The Brunauer-Emmet-Teller (BET) surface area, as well as the porosity and pore volume obtained with BHJ (Barrett, Joyner, and Halenda), was determined by collecting the data on Microtrac Belsorp mini equipment (Osaka, Japan). The photocatalytic activity experiments were conducted by dispersing TiO 2 -NR, TiO 2 -NP, and P25 in a water-based solution containing Rhodamine B dye (purchased from Sigma, 95% purity). The catalyst was added at a concentration of around 20 mg per 20 mL of dye solution, with a concentration of 25 µM. The measurements were conducted using simulated sunlight illumination with the Oriel LCS-100 at an intensity of 100 mW/cm 2 . The photocurrents of TiO 2 -NR, TiO 2 -NP, and P25 were measured using 1 V applied voltage, 300 W Xenon light, Na 2 SO 4 electrolyte, 50 s on-off time, and 1 × 1 cm 2 thin film area. Measurements of the optical energy gaps of TiO 2 -NR and TiO 2 -NP were performed utilizing a combination of a Shimadzu UV-3600 spectrophotometer and an attached integrating sphere (ISR-3100) (Tokyo, Japan).

Proposed Synthesis Mechanism
The choice of solvent can significantly affect the formation of hierarchical aggregates of anatase TiO 2 nanoribbons/nanosheets. Ethanol and acetic acid are commonly employed as solvents in solvothermal synthesis due to their capability to dissolve Ti precursor species and effectively control the growth and morphology of the resulting nanostructures [44,45]. Ethanol is a polar solvent with a low boiling point, while acetic acid is a weak acid with a higher boiling point. Acetic acid plays a significant role in facilitating the hydrolysis and condensation of titanium precursor compounds. Additionally, it can also regulate the acidity or alkalinity of the solution involved in the reaction [46][47][48]. In order to achieve the desired morphology and properties for enhanced photocatalytic applications, meticulous selections and optimizations of the solvents and the reaction conditions should be considered [49,50].
In order to understand the mechanism, the synthesis of TiO 2 -NR and TiO 2 -NP was performed without using surfactants. After allowing the solvothermal reaction to proceed for 9 h at 180 • C, amorphous Ti chain precursors were formed. The porosity of the interconnected groups of chains was controlled via the esterification of the organic acid with alcohol in the reaction system, which is released as TTIP dissociates [13]. In the early phases of the process, the amorphous titanium chains are surrounded by water-repellent acetate groups, and the outer surfaces of these chains attach to esters within the chemical reaction setup. The carboxyl groups in the solvent precursor are expected to coordinate with Ti atoms, forming bidentate complexes through the interaction with an organic ligand present in the titanium-containing precursor [51]. The proposed formation process of the TiO 2 -NR includes the process of TTIP acidolysis, facilitated by acetic acid as a catalyst to form Ti chains. Afterward, the esters facilitate interactions that promote the proximity of the chains and allow their organization into regular repeating patterns. In the previous study conducted by our group, the initial stages of formation for amorphous Ti chains were explained when synthesizing anatase single-crystal TiO 2 using a similar titanium alkoxide, specifically titanium butoxide, along with acetic acid [13]. Subsequently, subjecting anatase TiO 2 to calcination at a temperature of 450 • C for 3 h in air leads to the creation of networks comprising Ti-O-Ti and O-Ti-O bonds. Finally, the calcination step leads to the formation of TiO 2 -NR or TiO 2 -NP, where esters serve as an implicitly established self-template during the formation process [45]. It is proposed that the resulting phases that emerged from the initial interaction between the TTIP and acetic acid or ethanol, along with the interplay between the Ti source and solvent, play a crucial role in the formation of the porous structure and morphology of the TiO 2 -NR and TiO 2 -NP. The hierarchical nanostructure of TiO 2 -NR was successfully achieved through a straightforward and precisely controlled solvothermal synthesis method known as "soft", based on acetic acid as a "structure-directing effect".

Structural and Optical Characterizations
The crystalline phases of TiO 2 -NR and TiO 2 -NP were confirmed via X-ray diffraction (XRD) data. The diffraction patterns observed represent the crystal planes of anatase TiO 2 , which was further confirmed in their corresponding selected area electron diffraction patterns (SAED). This implies the successful formation of the anatase phase, with a highly crystalline structure for both TiO 2 -NR and TiO 2 -NP, as shown in Figure 1a,b, (XRD data card, JSPD.21-1272). reaction setup. The carboxyl groups in the solvent precursor are expected to coordinate with Ti atoms, forming bidentate complexes through the interaction with an organic ligand present in the titanium-containing precursor [51]. The proposed formation process of the TiO2-NR includes the process of TTIP acidolysis, facilitated by acetic acid as a catalyst to form Ti chains. Afterward, the esters facilitate interactions that promote the proximity of the chains and allow their organization into regular repeating patterns. In the previous study conducted by our group, the initial stages of formation for amorphous Ti chains were explained when synthesizing anatase single-crystal TiO2 using a similar titanium alkoxide, specifically titanium butoxide, along with acetic acid [13]. Subsequently, subjecting anatase TiO2 to calcination at a temperature of 450 °C for 3 h in air leads to the creation of networks comprising Ti-O-Ti and O-Ti-O bonds. Finally, the calcination step leads to the formation of TiO2-NR or TiO2-NP, where esters serve as an implicitly established self-template during the formation process [45]. It is proposed that the resulting phases that emerged from the initial interaction between the TTIP and acetic acid or ethanol, along with the interplay between the Ti source and solvent, play a crucial role in the formation of the porous structure and morphology of the TiO2-NR and TiO2-NP. The hierarchical nanostructure of TiO2-NR was successfully achieved through a straightforward and precisely controlled solvothermal synthesis method known as "soft", based on acetic acid as a "structure-directing effect".

Structural and Optical Characterizations
The crystalline phases of TiO2-NR and TiO2-NP were confirmed via X-ray diffraction (XRD) data. The diffraction patterns observed represent the crystal planes of anatase TiO2, which was further confirmed in their corresponding selected area electron diffraction patterns (SAED). This implies the successful formation of the anatase phase, with a highly crystalline structure for both TiO2-NR and TiO2-NP, as shown in Figure 1a,b, (XRD data card, JSPD.21-1272).   Figure 2a,b represent the energy dispersive X-ray diffraction (EDX) spectra that confirmed the pure chemical composition of both TiO 2 -NR and TiO 2 -NP (Cu and C peaks refer to the elements of the grid used in the SEM measurements). The inset histograms quantified the average particle size distributions, which were measured as 1.7 ± 0.3 µm and 18 ± 5 nm for TiO 2 -NR and TiO 2 -NP, respectively. rounded TiO2 nanoparticles. The insets in Figure 2a,b represent the energy dispersive X-ray diffraction (EDX) spectra that confirmed the pure chemical composition of both TiO2-NR and TiO2-NP (Cu and C peaks refer to the elements of the grid used in the SEM measurements). The inset histograms quantified the average particle size distributions, which were measured as 1.7 ± 0.3 µm and 18 ± 5 nm for TiO2-NR and TiO2-NP, respectively. Furthermore, the internal structure (i.e., the size and shape of the nanoparticles) for TiO2-NP and TiO2-NR were investigated using TEM and HRTEM images, and the results were presented in Figure 3a-f. Figure 3a,b show that TiO2-NP contained nanoparticles with an average size of 18 ± 5 nm, and the shape of these particles was primarily determined by the extent to which the octahedral structure is shortened during the solvothermal process [52,53]. Moreover, Figure 3c-f exhibited two distinct morphologies within the spherical TiO2-NR, namely nanoribbons that formed aggregated spindle nanoparticles, and nanoparticles that formed aggregated nanosheets, with a size range of 18 ± 5 nm in size. The insets in Figure 3a,e,f showed the shape morphologies of aggregated nanoparticles in TiO2-NP and TiO2-NR. The crystal structures of the primary TiO2 nanoparticles, which aggregated to form TiO2-NP and TiO2-NR, were further confirmed with high-resolution TEM (HRTEM) images in Figure 3b,e,f. This confirmation was achieved by analyzing the interplanar distances and their corresponding Miller indices. HRTEM findings further confirmed the XRD and SAED analyses in Figure 1a,b. Furthermore, the internal structure (i.e., the size and shape of the nanoparticles) for TiO 2 -NP and TiO 2 -NR were investigated using TEM and HRTEM images, and the results were presented in Figure 3a-f. Figure 3a,b show that TiO 2 -NP contained nanoparticles with an average size of 18 ± 5 nm, and the shape of these particles was primarily determined by the extent to which the octahedral structure is shortened during the solvothermal process [52,53]. Moreover, Figure 3c-f exhibited two distinct morphologies within the spherical TiO 2 -NR, namely nanoribbons that formed aggregated spindle nanoparticles, and nanoparticles that formed aggregated nanosheets, with a size range of 18 ± 5 nm in size. The insets in Figure 3a,e,f showed the shape morphologies of aggregated nanoparticles in TiO 2 -NP and TiO 2 -NR. The crystal structures of the primary TiO 2 nanoparticles, which aggregated to form TiO 2 -NP and TiO 2 -NR, were further confirmed with high-resolution TEM (HRTEM) images in Figure 3b,e,f. This confirmation was achieved by analyzing the interplanar distances and their corresponding Miller indices. HRTEM findings further confirmed the XRD and SAED analyses in Figure 1a,b.
It is expected that TiO 2 -NR, with both truncated and spindle-shaped nanoparticles, will have a higher number of exposed energetic (001) facets compared to TiO 2 -NP. These differences in morphology between the synthesized materials were mainly attributed to the differences in the synthesis reaction solvents.
XRD, SEM, and TEM characterizations have clearly shown that TiO 2 -NR has a hierarchical structure made up of tiny anatase TiO 2 nanoribbons/nanosheets, which form a highly interconnected mesoporous structure compared to TiO 2 -NP. Additionally, the BET analysis obtained from N 2 adsorption/desorption isothermal data (illustrated in Figure 4a and Table 1) has also confirmed the formation of a highly mesoporous structure in TiO 2 -NR compared to TiO 2 -NP and P25. Table 1. The porosity (P), surface area (S a ), pore size (P d ), and roughness factor (R f ), of TiO 2 -NR, TiO 2 -NP, and P25. Porosity was calculated using P = P V /(1/ρ + P V ), where (P V ) is the cumulative pore volume, and ρ is the density value of TiO 2 (0.257 cm 3 /g) [54].  It is expected that TiO2-NR, with both truncated and spindle-shaped nanoparticles, will have a higher number of exposed energetic (001) facets compared to TiO2-NP. These differences in morphology between the synthesized materials were mainly attributed to the differences in the synthesis reaction solvents.

Material Pv (cm 3 /g) P (%) S a (m 2 /g) R f (µ/m) P d (nm)
XRD, SEM, and TEM characterizations have clearly shown that TiO2-NR has a hierarchical structure made up of tiny anatase TiO2 nanoribbons/nanosheets, which form a highly interconnected mesoporous structure compared to TiO2-NP. Additionally, the BET analysis obtained from N2 adsorption/desorption isothermal data (illustrated in Figure 4a and Table 1) has also confirmed the formation of a highly mesoporous structure in TiO2-NR compared to TiO2-NP and P25.  Table 1. The porosity (P), surface area (Sa), pore size (Pd), and roughness factor (Rf), of TiO2-NR, TiO2-NP, and P25. Porosity was calculated using P = PV/(1/ρ + PV), where (PV) is the cumulative pore volume, and ρ is the density value of TiO2 (0.257 cm 3 /g) [54]. The structure of TiO2-NR has a greater surface area and cumulative pore volume, due to the presence of a higher amount of condensed nitrogen in the large voids and pores of TiO2-NR. Furthermore, the average pore size distributions obtained with the BJH analysis (inset in Figure 4a) have revealed that the internal pore size of TiO2-NR is slightly smaller than that of TiO2-NP due to the slightly smaller size of their aggregated primary nanoparticles. The voids among the hierarchical structures of TiO2-NR are expected to be approximately submicrosized; however, this is not observable due to the measurement limit of the BET equipment. The BET calculations shown in Table 1 suggest that TiO2-NR will be expected to accommodate a greater amount of dye, due to higher surface area and porosity compared to both TiO2-NP and P25. This indicates enhanced surface reactivity, as well as the potential for improved photocatalytic activity. Moreover, the unique morphology of TiO2-NR, characterized by highly interconnected nanoribbons/nanosheets, forms a hierarchical structure. This structure serves as an efficient pathway for charge diffusion, both within the material's internal pores and along its external surface. Consequently, the presence of these pores is expected to enhance the photocurrent. Figure 4a and Table 1 show the higher surface area, cumulative pore volume, porosity, and roughness factor of TiO2-NR in comparison to TiO2-NP and P25. On the other hand, the average pore size was observed to be similar to those of both TiO2-NR and TiO2-NP. This indicates the increased surface reactivity of TiO2-NR compared to TiO2-NP and P25. It should be noted that performing Hg intrusion in addition to N2 adsorption/desorption isotherms is recommended in order to detect larger pore sizes. The structure of TiO 2 -NR has a greater surface area and cumulative pore volume, due to the presence of a higher amount of condensed nitrogen in the large voids and pores of TiO 2 -NR. Furthermore, the average pore size distributions obtained with the BJH analysis (inset in Figure 4a) have revealed that the internal pore size of TiO 2 -NR is slightly smaller than that of TiO 2 -NP due to the slightly smaller size of their aggregated primary nanoparticles. The voids among the hierarchical structures of TiO 2 -NR are expected to be approximately submicrosized; however, this is not observable due to the measurement limit of the BET equipment. The BET calculations shown in Table 1 suggest that TiO 2 -NR will be expected to accommodate a greater amount of dye, due to higher surface area and porosity compared to both TiO 2 -NP and P25. This indicates enhanced surface reactivity, as well as the potential for improved photocatalytic activity. Moreover, the unique morphology of TiO 2 -NR, characterized by highly interconnected nanoribbons/nanosheets, forms a hierarchical structure. This structure serves as an efficient pathway for charge diffusion, both within the material's internal pores and along its external surface. Consequently, the presence of these pores is expected to enhance the photocurrent. Figure 4a and Table 1 show the higher surface area, cumulative pore volume, porosity, and roughness factor of TiO 2 -NR in comparison to TiO 2 -NP and P25. On the other hand, the average pore size was observed to be similar to those of both TiO 2 -NR and TiO 2 -NP. This indicates the increased surface reactivity of TiO 2 -NR compared to TiO 2 -NP and P25. It should be noted that performing Hg intrusion in addition to N 2 adsorption/desorption isotherms is recommended in order to detect larger pore sizes. Figure 4b showed the calculated energy band gaps (E g ) of TiO 2 -NR and TiO 2 -NP using the Tauc plots. The E g values of TiO 2 -NR and TiO 2 -NP were found to be 3.23 eV and 3.21 eV, respectively, which are almost the same. These values confirm the ultraviolet absorption region of anatase TiO 2 , which can serve as a photocatalyst.

Photocatalytic and Photocurrent Characterizations
To assess the photocatalytic activity of TiO 2 -NR and TiO 2 -NP, photocatalytic degradation experiments were conducted, using a model organic pollutant, such as organic Rhodamine B, as a standard dye compound. The photocatalytic performances of TiO 2 -NR and TiO 2 -NP were determined by measuring the degradation rate, or measuring the percentage of Rhodamine B degradation over a certain time, and then comparing it against that of the standard material, TiO 2 (P25). This is typically measured under ultraviolet and visible lights. Photocurrent characterizations were also performed, in order to evaluate the charge transport properties of TiO 2 -NR and TiO 2 -NP using photocurrent transient measurements. As shown in Figure 5a-d, TiO 2 -NR and TiO 2 -NP exhibited higher photocatalytic performance than that of the standard, P25; however, TiO 2 -NR still showed better performance compared to TiO 2 -NP. The unique morphology of TiO 2 -NR, in addition to its high surface area and hierarchical shape (i.e., 3D hierarchically aggregated 1D nanoribbons/2D nanosheets structure) provided increased light absorption and efficient charge transfer. This results in enhanced photocatalytic performance. The standard P25 exhibited the lowest photocatalytic/photocurrent activities, due to its lowest surface area, cumulative pore volume, porosity, roughness factor, and pore size. These photocatalytic measurements were performed on P25 under similar conditions as a catalyst reference material, and were recorded and listed in Table 1. and TiO2-NP were determined by measuring the degradation rate, or measuring the percentage of Rhodamine B degradation over a certain time, and then comparing it against that of the standard material, TiO2 (P25). This is typically measured under ultraviolet and visible lights. Photocurrent characterizations were also performed, in order to evaluate the charge transport properties of TiO2-NR and TiO2-NP using photocurrent transient measurements. As shown in Figure 5a-d, TiO2-NR and TiO2-NP exhibited higher photocatalytic performance than that of the standard, P25; however, TiO2-NR still showed better performance compared to TiO2-NP. The unique morphology of TiO2-NR, in addition to its high surface area and hierarchical shape (i.e., 3D hierarchically aggregated 1D nanoribbons/2D nanosheets structure) provided increased light absorption and efficient charge transfer. This results in enhanced photocatalytic performance. The standard P25 exhibited the lowest photocatalytic/photocurrent activities, due to its lowest surface area, cumulative pore volume, porosity, roughness factor, and pore size. These photocatalytic measurements were performed on P25 under similar conditions as a catalyst reference material, and were recorded and listed in Table 1.

Conclusions
Our developed solvothermal synthesis technique, utilizing TTIP as a precursor and acetic acid or ethanol as solvents, without using templates or surfactants, was discovered to generate a hierarchical aggregation of anatase TiO 2 nanoribbons/nanosheets (TiO 2 -NR) and anatase TiO 2 nanoparticles (TiO 2 -NP). Remarkably, these synthesized materials exhibited superior photocatalytic and photocurrent performances, compared to the commercial TiO 2 (P25). Additionally, the synthesis process allowed for the production of two anatase TiO 2 nanostructures with different sizes, shapes, and morphologies, which, in turn, optimized photocatalytic performance. The enhanced photocatalytic and photocurrent activities of TiO 2 -NR were found to be attributed to multiple factors. Firstly, the unique morphology of TiO 2 -NR provides a larger surface area, promoting improved light absorption and increasing the number of active sites (such as the exposed facets 001 and 101) for efficient photocatalytic reactions. Secondly, the hierarchical aggregation of TiO 2 -NR results in interconnected networks, facilitating the efficient transport of photo-generated charges and minimizing recombination losses. On the other hand, TiO 2 -NP exhibited lower photocatalytic and photocurrent activities because of its reduced surface area and increased recombination losses. These effects can be attributed to the spherical nanoparticulate shape of TiO 2 -NP and the increased boundaries among the aggregated nanoparticles. This further substantiated the superior photocatalytic performance of TiO 2 -NR. The photocurrent measurements of the materials also indicated that the anatase TiO 2 -NR and TiO 2 -NP still exhibited improved charge transport properties compared to the standard P25, suggesting their potential for many energy applications, such as solar energy harvesting, water purification, and other energy-related applications.