Hollow sphere TiO2: Synthesis and its application for dye-sensitized solar cells

Khusna Arif Rakhman; Nurul Hidayat Aprilita; Rohana Adnan; Indriana Kartini

doi:10.25259/AJC_127_2024

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Review Article

2025

:18;

1272024

doi:

10.25259/AJC_127_2024

Hollow sphere TiO₂: Synthesis and its application for dye-sensitized solar cells

Khusna Arif Rakhman^a, Nurul Hidayat Aprilita^b, Rohana Adnan^c, Indriana Kartini^b,*

aDepartment of Chemistry Education, Universitas Khairun, Ternate, Indonesia

bDepartment of Chemistry, Universitas Gadjah Mada, Yogyakarta, Indonesia

cSchool of Chemical Sciences, Universiti Sains Malaysia, 11800 Gelugor, Penang Malaysia

* Corresponding author: E-mail address: indriana@ugm.ac.id (I. Kartini)

Received: 2024-10-31, Accepted: 2025-02-18, Published: 2025-04-17

Licence

This is an open-access article distributed under the terms of the Creative Commons Attribution-Non Commercial-Share Alike 4.0 License, which allows others to remix, transform, and build upon the work non-commercially, as long as the author is credited and the new creations are licensed under the identical terms.

Abstract

The architecture of semiconductor oxides as the core photoanodes in the dye-sensitized solar cells affecting the energy conversion of the dye-sensitized solar cells (DSSC) has been a slightly neglected parameter. Herein, we study that the morphology of hollow sphere (HS) TiO₂ as the semiconductor photoanode of DSSC has proven to result in the power conversion efficiency (PCE) higher than the solid sphere of TiO₂, and other morphology such as nanotube, nanowire, and nanorod. Affecting the photocurrent density (Jsc) and PCE, efficient electron transfer was achieved in HS TiO₂. The percentage of PCE of DSSC using HS TiO₂ semiconductor is also influenced by physical properties such as particle size and shell thickness, which influence the dye-loading capacity, the rate of electron transfer, and the scattering effects in visible light harvesting. This review discusses the synthesis and characterization of HS TiO₂ from two different synthesis pathways, the one-pot and two-step synthesis, and the modification of HS TiO₂ to improve the efficiency of DSSC.

Keywords

DSSC

Hollow sphere TiO2

Photovoltaic

Solar cells

TiO2

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1. Introduction

The development of renewable energy remains as a great challenge in overcoming the depletion of fossil energy and its negative impact on the environment. Dye-sensitized solar cell (DSSC) is one of the renewable energy devices receiving great attention because it can directly convert sunlight into electrical energy [1]. DSSC is a light and flexible solar cell with simple, low-cost fabrication and can be designed for green devices [2–4]. The main components of DSSC are: a thin layer of semiconductor mesopore, dye molecules as sensitizers, electrolytes, and counter electrodes, which are attached to a conducting glass substrate [4]. A thin layer consists of transparent glass or flexible plastic substrate that can transmit solar radiation into the photoanode. The glass substrate provides a surface for the deposition of semiconductor metal oxides (photoanode) and catalyst material (counter electrode). The photoanode has good electrical conductivity properties to collect the current from the dye-sensitized TiO₂ layer and transfer it to the external circuit [5]. Mesoporous photoanodes are metal oxide semiconductors that play an important role in the loading of dyes and the transfer of electrons to the external circuit [6]. Dye sensitizer (photosensitizer) is a special component in DSSC and is the essence of this type of solar cell. The photosensitizer is attached to the semiconductor through covalent bonds and is responsible for harvesting sunlight, especially in the visible region [7]. Electrolytes are a redox salt solution (e.g. I^-/I₃^-), which is injected between a thin layer of photoanode and the calculating electrode and functions to regenerate the oxidized dye. The last component is the counter electrode (CE), which consists of a thin layer of deposited catalyst. The arrangement of energy levels in all DSSC components must be in harmony to facilitate the transfer of electrons in converting light to electrical energy. Sequentially, the arrangement of energy levels starts from the lowest energy level of the lowest unoccupied molecular orbital (LUMO) of the dye, followed by the conduction band (CB) of the photoanode semiconductor, then the redox potential of the electrolyte, and the energy level of the highest occupied molecular orbital (HOMO) level of the dye [6].

The efficiency of DSSC is influenced by several parameters such as photocurrent (Jsc), photovoltage (Voc), and fill factor (FF) [8–10]. Meanwhile, Jsc relies on the efficiency of light harvesting, injection, and electron collection efficiency [11–13]. According to Ahmad et al. [14], limiting the recombination rate can increase the electron density in the conduction band of the semiconductor as well as increases the quasi-Fermi energy level, and Voc. All of these parameters are closely related to the electron transfer mechanism in the DSSC component. Capturing electrons in a semiconductor is the beginning of photovoltage and the flow of electrons to produce photocurrent [15]. The transfer of electrons in the DSSC component starts from the absorption of photons (hv) by the photosensitizer (dye), which then causes the electron from the dye molecule (D) to experience excitation from the HOMO to the LUMO energy level, as shown in the Eq. (1) [5]:

(1)

D_{adsorbed} + h v \to D *_{adsorbed}

The excited dye molecule then injects electrons into the conduction band (CB) of the semiconductor, leaving the dye molecule oxidized (D⁺) (Eq. 2):

(2)

D_{(adsorbed)}^{*} \to D_{(adsorbed)}^{+} + e_{(injected)}^{-}

The injected electrons are then transmitted through the semiconductor, followed by the transfer of electrical energy through a conductive substrate, external circuit, and a counter electrode (CE, Eq. 3):

(3)

e_{(injected)}^{-} +  CE \to e_{(CE)}^{-} + electrical energy

Transport of electrons through diffusion process is strongly influenced by the intensity of the incident light, electron concentration, and the efficiency of media transport [5]. At the counter electrode, electrons in the electrolyte component are reduce from I₃^- to I^-. At the same time, the oxidized dye (D⁺) returns to the ground state (D) after receiving electrons from I^- and thus completing the regenerative cycle (Eqs. 4 and 5).

(4)

I_{3}^{-} +  2 . e_{(CE)}^{-} \to  3 I_{(CE)}^{-}

(5)

D_{(adsorbed)}^{+} +  3 / {2I}^{-} \to D_{(adsorbed)} +  1 / {2I}_{3}^{-}

However, in its application, not all electrons can be transferred perfectly in a DSSC device. Electron recombination always accompanies the electron diffusion process in this device. Three types of recombination co-occur with the electron transport process [5,16]. The first is direct recombination; it describes the lifetime of the excited state of the dye molecule. In other words, due to the absorption of sunlight, the excited electrons in the conduction band (CB) return directly to the ground state in the dye molecule’s valence band (VB) (Eq. 6):

(6)

D_{(adsorbed)}^{+} + e_{(injected)}^{-} \to D_{(adsorbed)}

Second, recombination in the injection of electrons in the semiconductor (SC) with oxidized dye (Eq. 7):

(7)

D_{(adsorbed)}^{+} + e_{(SC)}^{-} \to D_{(adsorbed)}

The third is the recombination of injected electrons in the surface of the semiconductor (SC) with the acceptor in the electrolyte (Eq. 8).

(8)

I_{3}^{-} + 2 . e_{(SC)}^{-} \to 3 I_{(anode)}^{-}

Both reactions in Eqs. (7) and (8) are undesirable as they may result in the losses in the cell efficiency.

Two of the three factors that affect electron transport in the DSSC mentioned above are dye concentration and the efficiency of the electron transport medium [5]. The high dye concentration allows the release of large amounts of electrons from the dye. The efficiency of the electron transport medium is related to the storage capacity and the speed of transferring electrons from the conduction band of the dye molecule to the conduction band of the TiO₂ semiconductor and subsequently to the external circuit [17]. In DSSC, metal oxide semiconductors are photoelectrodes that play a role in electron collection (or electron capture) and transfer, such that their physical properties and structure determine the performance of the DSSC [18]. The structure and physical properties of semiconductors, such as surface area, morphology, and band gap, are some of the factors that affect the efficiency of DSSC [19]. In principle, the efficiency of DSSC depends on the dye loading concentration, and the electron transport speed of the component is strongly influenced by the surface area and pore size of the semiconductor. Whereas the surface area and pore size of the semiconductor are related to the particle size and morphology of the material [20–22].

Various modifications of the semiconductors have been conducted to enhance the efficiency of DSSC. For instance, Ma et al. [23] reported an enhancement in electron transport from the TiO₂ double-layer formation. The decrease in particle size results in a blue shift in the band gap of the TiO₂ semiconductor. It has an impact on the arrangement of energy levels and electron transfer rates in the device, and the smallest particle size results in the best DSSC efficiency [24]. Examples of the modifications reported in the literature include coating of semiconductor surface with TiO_x or TiCl₄ solution to reduce the excessive recombination rate [3], incorporating plasmon material as a composite to accelerate electron transfer [25], and modifying the surface morphology in order to increase the surface area to increase the dye loading [26] and their electric properties [22]. A semiconductor with a high specific surface area has a high adsorption capacity. Meanwhile, a large pore volume benefits the electrolyte injection and effective contact between the electrolyte and the photoanode, thereby increasing the photoelectric conversion performance of DSSC [1].

TiO₂ nanoparticles with high specific surface area and sufficient dye adsorption capacity are needed to increase the light-harvesting efficiency (LHE) [27]. TiO₂ with specific morphology can increase the scattering effect of incident light [26]. TiO₂ is a suitable material for use in photoanode applications [17] compared to other metal oxides (such as ZnO, Nb₂O₅, SnO₂, and WO₃) because it achieves the highest energy efficiency [6]. TiO₂ has wide bandgap, non-toxic, is stable, ecologically benign, cost-effective, and has long-term stability against photo corrosion and chemical corrosion [28–31]. In recent years, research on the morphology of TiO₂ as a semiconductor in DSSC reported that hollow sphere HS TiO₂ was the best at increasing PCE compared to several other forms, such as nanorods, nanoflowers, and nanotubes. The spherical geometry of these hollow structures facilitates Mie scattering, effectively trapping incident light within the photoanode and prolonging the optical path length. This increased light interaction boosts photon absorption by the dye molecules, thereby enhancing the photocatalytic performance of the DSSC [32]. Moreover, the hollow nature of HS TiO₂ provides a high surface area, offering abundant sites for dye loading [33–36].On the other hand, the hollow structures exhibit good light trapping, charge separation and transport, and surface reactions in photocatalytic applications [37–39]. The improved efficiency in light trapping is attributed to the multiple directions of light scattering and the large surface area photon-retarding effect. Additionally, the hollow structure suppresses the recombination of e^- and h⁺, promoting photocatalytic activity by reducing the charge recombination rate [40], thereby enhancing the light-trapping effect in the photocatalytic process [41]. HS TiO₂ with a high specific surface area and sufficient dye adsorption capacity has the potential to enhance electron transfer rates, LHE, and scattering effects to improve DSSC efficiency.

Fabrication of HS TiO₂ for the development of DSSC has shown promising advancements in efficiency, as reported by various authors [5,28]. For instance, Tamilselvan and Shanmugan [42] reported an efficiency of 11% using bio-nano materials. Key areas of improvement include molecular engineering or synthesis methods, carrier transport materials, efficient sensitizers, and better electrodes [43,44]. Hydrothermal/solvothermal and sol-gel methods have been widely used to prepare HS TiO₂ with various morphology and specific surface area. Exploring efficient HS TiO₂ fabricating methods to obtain products with uniform sizes and stable hollow structures is a lengthy process. Until now, there has been a lack of review covering TiO₂ HS specifically for DSSC applications. Therefore, the present review discusses the HS TiO₂ architecture and explores the synthesis methods, photovoltaic characteristics, and modifications made to improve the efficiency of DSSC. Obtaining precise and significant information about the synthesis pathway and characteristics of HS TiO₂ for photoanode applications is the relevant objective of this review.

2. Morphology Effect

A good photoanode is described by a high surface area and light scattering [45,46]. The development of DSSC using commercial TiO₂ improves surface area; however, it does not enhance light scattering because the particle diameter is smaller than the wavelength of incident light. Previously, mesoporous multilayer DSSC photoanodes with high surface area and high light scattering capability with an incident photon conversion efficiency of 10% have been developed [47]. Mesoporous materials with pore diameters ranging from 2 to 50 nm possess a continuous network of interconnections, leading to a high surface area and light scattering. Due to these advantages, mesoporous materials serve as a standard for the development of DSSC [48]. However, the mesoporous structure often exhibits low conductivity, which can facilitate the recombination of electrons. Consequently, new nanostructures with high conductivity and enhanced dye loading have become the focus of the next generation of photoanodes [47]. To fulfill the two basic requirements of a good photoanode—high dye loading and light scattering capabilities—the development of TiO₂ based photoanodes is still ongoing research. Table 1 compares the different TiO₂ morphologies that have been developed for photoanodes with dye loading using commercial dye N719 at the concentration of 5×10^-4 M and its photovoltaic parameters.

Table 1. Summary of the performance of different TiO₂ nanostructure morphologies for DSSC photoanode reported in the literature.

Morphology	Jsc (mA cm^-2)	Voc (mV)	FF (%)	PCE (%)	References
Urchin-like	12.72	71.60	67.03	6.10	[35]
Nanowire	3.60	86.80	67.60	2.12	[49]
Nanotubes	10.48	83.50	68.12	5.96	[49]
Nanotubes array	10.15	74.50	62.00	4.69	[33]
Nanorods	8.88	66.00	47.00	2.75	[34]
Hollow nanoparticles	15.80	74.00	71.00	8.30	[36]
Nanoparticles	4.30	69.00	72.00	2.14	[50]
Nanoflower	5.10	79.70	72.60	2.95	[51]

Jsc: short circuit current density, Voc: open circuit voltage, FF: Fill factor, PCE: Power conversion efficiency

Anatase phase TiO₂ nanoparticles with a wide band gap are the most effective materials for photoanodes due to their cost efficiency, good stability, easy availability, and compatible optical and electronic properties [22]. TiO₂ with different morphologies, such as nanotubes, urchin-like, and hollow nanoparticles exhibiting different PCEs, have been reported (Table 1). It can be seen that nanotubes, urchin-like, and hollow nanoparticles (HNPs) produce high PCE, above 5.0%. The different morphology of TiO₂ changes the pore characteristics and the surface area of the material, which also affects the Jsc value. A smaller TiO₂ solid particle size can provide a larger surface area to improve the amount of loading of dyes and their Jsc [24,28]. For example, the use of nanotubes (NTs) TiO₂ with a surface area of 64.22 m² g^-1 and pore size of 3-10 nm produced dye loading of 103.1 nmol cm^-2, Jsc of 10.48 mA cm^-2, and PCE of 5.96% [46]. In another study, the urchin-like TiO₂ (ULT) that has a higher pore size distribution of 3-60 nm resulted in a higher Jsc of 12.72 mA cm^-2 and optimal dye adsorption capacity of about 319.73 nmol cm^-2 and PCE of 6.10% [35]. The hollow sphere has been reported to produce the highest PCE than other morphologies; photovoltaic with a Jsc value of 15.8 mA cm^-2 and PCE of 8.3% has been produced from HNPs with a surface area of 129 m² g^-1 and particle diameter ∼100 nm [36]. The unique structural properties of HNPs allow for enhanced light-harvesting capabilities and reduced electron recombination, which are critical for improving overall efficiency in DSSCs. As a specific type of HNPs, HS TiO₂ represents an evolution in design that retains these benefits while potentially addressing any limitations in conductivity or charge transport. Therefore, HS TiO₂ is positioned as a promising candidate for further development as a semiconductor in DSSCs, leveraging its excellent properties to maximize efficiency and performance in solar energy applications.

The Jsc depends on the light-harvesting efficiency, injection, and electron collection efficiency mentioned in the early section. Recent studies have demonstrated significant improvements in DSSC performance using hierarchical TiO₂ structures. Hierarchical nanostructures are integrated architectures comprising well-ordered nanoscale subunits with zero, one, two, or three-dimensional architectures [52], including nanostructures such as quantum dots, nanofibers, nanorods, nanowires, nanosheets, nanoflowers, nanospheres, or a combination of these architectures [53]. These structures combine the advantages of nanoparticles and larger spheres or rods, offering enhanced dye loading, light scattering, and electron transport [54]. It is reinforced by a hollow structure to improve the surface scattering of light and enhance the incidence of harvesting light [55]. A detailed discussion of the photovoltaic characteristics of hollow spheres TiO₂ has been presented in Section 3.

3. Synthesis of HS TiO₂

Generally, HS TiO₂ can be produced by two synthesis methods: hydrothermal/solvothermal and sol-gel (Figure 1). The hydrothermal/solvothermal method has a single-step synthesis. Meanwhile, the sol-gel method was a two-stage synthesis (Table 2). Synthesis of HS TiO₂ with several methods and templates can produce different particle sizes. Hydrothermal/solvothermal methods produce HS TiO₂ more than 100 nm in diameter [56], the largest among other methods. The sol-gel (carbon-based) template (SGCT) method produced more than a 50 nm diameter. Meanwhile, HS TiO₂, which is produced using a sol-gel hard template (SGHT) method, one of which uses SiO₂, can produce a diameter of more than 25 nm [57]. Based on the findings from the previous studies Table 2 presents the diameters of the HS TiO₂ produced by the SGHT > SGCT > hydrothermal/solvothermal method.

3.1. Hydrothermal/solvothermal method

Hydrothermal/solvothermal methods were used in many nanoparticle fabrications in the ceramic industry using an aqueous or nonaqueous solution in an autoclave at a high temperature and pressure [58]. In this condition, supercritical fluids are formed and significantly improve the solubility of solid precursors resulting in the formation of nanostructures. In particular, wet fabrication by hydrothermal/solvothermal methods utilizing physical phenomena such as the Kirkendall effect and Ostwald ripening has been widely used to produce hollow nanostructures [59,60].

3.1.1. Kirkendall effect

Synthesis of HS TiO₂ by the solvothermal method using ethanol as the solvent at 180°C for 24 hrs resulted in the 1 µm-anatase phase material with a pore size of 5.2 nm and surface area of 242 m² g^-1. Hydrothermal synthesis of HS TiO₂ can take place via two formation mechanisms: the Kirkendall effect or Ostwald ripening [61]. Chen et al. [62] reported the production of a pure anatase phase of HS TiO₂ with 300–400 nm in diameters and a wall thickness of approximately 50 nm, using polyethyleneimine (PEI), which acts as a stabilizing and templating agent via solvothermal method. Time-dependent morphological evolution using transmission electron microscopy (TEM) analysis confirmed the formation of HS TiO₂ via the Kirkendall effect (Figure 2). In the synthesis of HS TiO₂ through the Kirkendall effect, solid nanospheres are formed at the beginning of the reaction (Figure 2a) (before heating). After heating at 180°C for 6 hrs, the nanosphere size and surface roughness increased, and a small cavity began to form (Figure 2b). When the reaction time increased to 16 hrs, the increase in the nanosphere particle size and inner cavity size was observed (Figure 2c). Meanwhile, different quantity of PEI was associated with the different viscosity of the reaction system and the adsorption of PEI molecules on the TiO₂ surface and these controlled the morphology of the TiO₂ produced [62]. Chen et al. [62] and Zhu et al. [63] reported that calcination treatment after the completion of the reaction at 400 to 700°C further enhanced the crystallinity of the TiO₂ without changing the crystalline phase.

TEM images (a-c): the formation of hollow TiO2 at different reaction times of 0, 6, and 16 h. (d) Scheme illustration of the formation of a TiO2 hollow nanosphere via the Kirkendall effect [62]. Licensed by Elsevier.

The Kirkendall effect basically refers to the process of nonequilibrium mutual diffusion through the atoms in a diffusion couple. This causes supersaturation of the vacancies at the interface, and further condensation of the excess vacancies and interdiffusion leads to the formation of a hollow sphere [64,65]. The Kirkendall effect has been used in the fabrication route to form HS. However, not every particle size, shell thickness, or material crystallinity can be obtained from this route [66]. Basically, synthesis through the Kirkendall effect requires a surfactant to produce HS TiO₂. The Kirkendall effect is a one-pot synthesis with a two-step process that can produce HS TiO₂. The first stage is the formation of vacancies in the TiO₂ nanostructure. In this stage, the faster-diffusing species will be confined into a nanocrystal core. The vacancy-injection net rate increased, as evidenced by the high surface-to-volume ratio of the transformed particles and the absence of defects in the TiO₂ nanostructure. In the next stage, supersaturated void clouds will gather together and form a cavity of HS TiO₂ (Figure 2d) [67].

Table 2. Synthesis of HS TiO₂.

Method	Materials		Synthesis condition		Characteristics				References
Method	Template/surfactant	Precursor	Temperature (°C)	Time (hour)	Surface area (m²/g)	Particle size (nm)	Pore size (nm)	Wall thickness (nm)	References
Hydrothermal/solvothermal	Urea, tert-Butyl ammonium hydroxide	Titanium isopropoxide	180	24	78.3	3000			[68]
	Tween 60 & polyethylene glycol	Titanium isopropoxide	180	24	155	1000			[69]
	Oxalic acid	Titanium n-butoxide	180	10		450		30	[70]
	Polyethyleneimine	Tetrabutyl titanate	180	24		100-200	2-50		[63]
	Glucose, polyvinyl pyrrolidone (PVP)	Titanium tetrabutoxide (TBOT)	125	72		75-100		10-14	[71]
	Valeric acid, butyric acid	Titanium isopropoxide	190	18	83	514	7		[72]
	NH4F, CO(NH2)2	Ti(SO4)2 & Zn(NO3)2·6H2O	180	6	55.985	∼2000			[73]
	Octanoic acid	Titanium isopropoxide	190	18	47.5, 52.1, 55.9, 54.0	220, 440, 620, 800			[55]
	Polyethyleneimine (PEI)	Butyl titanate	180	24		300-400		50	[62]
	Tween 20	Titanium isopropoxide	180	24	242		52		[74]
	Acetylacetone	Tetrabutyl titanate	200	10		900	∼700	∼90	[75]
	Urea	Ti plate: TiCl4:H2SO4	200	12		1000		20-30	[76]
		Titanium(IV) butoxide (TBT)	220	5		1000-2000			[56]
		Tetrabutyl titanate	130	48		10		2-4	[67]
	Oxalic acid	Tetrabutyl titanate	180	6	166.2	1000-1500	3-33	150	[77]
	PVP, NaF	Tetrabutyl titanate	110	2, 3, 4, 5		320			[78]
	H3BO3	AHFT: (NH4)2TiF6	90	0.25	102	800-1000			[79]
		Titanium (IV) chloride	220	6	50.2	1000	18.7		[80]
		Tetrabutyl titanate (TBOT)	220	5		1200		100	[81]
Sol-gel (carbon-based template)	Polystyrene	TTIP	50	1.5		∼380, ∼190		∼60, ∼50, ∼100	[82]
	Polystyrene	Tetra-n-butyl titanate (TBT)	RT	0.5	41, 36, 34		17, 21, 18		[83]
	Polystyrene	Titanium tetrabutoxide (TBOT)	80	2		147, 151, 155, 159		9, 14, 17, 23	[84]
	Polystyrene	TBOT	70	2	96.7, 54.2	332, 412	11.3, 8.1	28, 50	[85]
	Polystyrene	Titanium ethoxide	70	12		60-365		100, 110, 120, 130	[86]
	Poly styrenemethacrylic acid (PSA)	Ti(SO4)2	50	1.5	20.821, 52.412, 113.335	1000-120			[87]
	Carbon hollow sphere	Titanium(IV) n-butoxide (Ti(OBu)4)	24.85	24			400-600	20-60	[88]
	Glucose	Titanium tetraisopropoxide (TTIP)		1		250-350			[89]
	Polystyrene	tetrabutyl titanate (TBT), tetrabutylzirconate (TBZ) (1:1)	25	3		190			[84]
	Polystyrene	tetrabutyl titanate (TBT)	80	6	58.3	300		13.3	[90]
	CTAB	TiCl4, (CH3CH3CHO)4Ti		2		94.9	36000		[57]
	Polydopamine (PDA)	Tetrabutyl titanate (TBT)	RT	2		568		105	[91]
	Anhydrous dextrose	Titanium (IV) butoxide	RT	1	138.07, 101.98, 54.53	667.98, 327.03, 170.91		46.98, 28.36, 32.62	[92]
	Polystyrene	Tetrabutyl titanate (TBT)	RT	20		85	59	16	[93]
Sol-gel (hard template)	SiO2: (TEOS)	Titanium tetraisopropoxide (TTIP)	55, 50, 40, 35, 20	6		25, 50, 75, 100, 125			[94]
	SiO2: (TEOS)	Titanium tetraisopropoxide (TTIP)				50			[95]
	SiO2: (TEOS)	Titanium tetraisopropoxide (TTIP)	5	6		25, 50, 100		5.5, 11, 20.5	[96]
	SiO2: (TEOS)	Titanium n-butoxide (TBOT)	85	2		360		Outer: 30, Inner: 35	[97]
	SiO2: (TEOS)	Titanium tetraisopropoxide (TTIP)	4	6	246	120			[98]
	Au: HAuCl4	TiF4	100	1		200		60	[99]
	SiO2: (TEOS)	Titanium tetraisopropoxide (TTIP)	4	6	129	∼100	Cav: 5, Intra: 35		[36]
	K3PW12O40	Ti(SO4)2	125	8		500-1000			[100]
	CaCO3	TEOS, TBOT	RT	3	113			<181	[101]
	Au: HAuCl4	TiF4	180	1		∼200			[102]
	SiO2: (TEOS)	tetrabutyl titanate (TBT)	85	1.7		∼200		∼50	[103]
	SiO2: (TEOS)	Titanium tetraisopropoxide (TTIP)		12		140		20	[104]
	SiO2: (TEOS)	Titanium n-butoxide (TBOT)		2		185		30	[105]
	SiO2: (TEOS)	Titanium n-butoxide (TBOT)	RT	2		455, 470, 494, 530, 606		23, 31, 41, 61, 99	[106]
	SiO2: (TEOS)	Titanium Isopropanol	25	1.5	74.3	204		17	[107]
	SiO2: (TEOS)	Titanium n-butoxide (TBOT)	45	24		480		20-30	[108]

RT: Room temperature, PVP: Polyvinylpyrolidone, NaF: Sodium flouride, TTIP: Titanium isopropoxide, CTAB: Cetyltrimethylammonium Bromide

3.1.2. Ostwald ripening

The formation of HS TiO₂ through Ostwald ripening is a one-pot synthesis without a template, surfactant, and calcination process. This process is a growth from a solid bulk structure, spherical solid structure to a spherical hollow structure [90]. High pressure can affect colloidal crystallite suspensions, causing compression and potentially altering electrostatic properties while also increasing orientational order and crystallite growth [109]. Ostwald ripening is also described as dissolving the core and recrystallizing the shell, resulting in a cavity of the structure (Figure 3) [70].

TEM images of the samples with different reaction times: (a) 3 h at 150°C. If not stated, all the reactions are at 180°C. (b) 2 h; (c) 3 h; (d) 4 h; (e) 5 h; (f) 6 h; [70]. Licensed by American Chemical Society. TEM: Transmission electron microscopy.

The production of HS TiO₂ through Ostwald ripening begins with the etherification of alcohol as a solvent by H₂O. The TiO₂ precursor undergoes hydrolysis and forms a cavity in the hollow structure (Figure 4). Synthesis with this principle is very dependent on the reaction temperature. At temperatures less than 140°C, alcohol etherification is difficult, and hydrolysis of TiO₂ precursors cannot be carried out, so HS TiO₂ is not formed. When the temperature exceeds 140°C, the system allows for the formation of HS TiO₂ from the precursor, but the reaction rate will vary at different temperatures [90]. In a reaction rate investigation, Nguyen and Kim [110] reported that HS TiO₂ was successfully produced through Ostwald ripening at 220°C. HS TiO₂ with anatase crystal phase was formed after the reaction was running for 5 h, and the crystallinity and structure after being extended 7 hrs did not change. Meanwhile, control of the diameter of HS TiO₂ can be done by adjusting the addition of Ti precursors, but this can impact decreasing the pore diameter and surface area [56]. Excess precursors are also thought to reduce the core’s dissolution process in cavity formation.

Scheme to illustrate the formation process for hollow TiO2 spheres [90]. Licensed by Elsevier.

3.2. Sol-gel method

3.2.1. Sol-gel carbon-based template

Fabrication of HS TiO₂ by the sol-gel method has a two-step synthesis. Templating the HS TiO₂ by producing a core@shell structure is the first step, and removing the templates (core) to produce a hollow sphere is the second step. The sol-gel method uses two templates to get core@shell, carbon-based, and hard templates. Polydopamine (PDA) has also shown good performance as a carbon-based template (CBT) in the manufacture of HS TiO₂. The synthesis of HS TiO₂ with a PDA template was reported by Ma et al. [91], resulting in HS TiO₂ with a diameter of ∼500 nm for photocatalytic applications.

In principle, PDAs have demonstrated the ability to effectively adhere to many surface materials such as metals, oxides, ceramics, polymers, carbon nanotubes, and magnetite nanoparticles [111]. The other CBT used to produce HS TiO₂ from the sol-gel process include poly(styrenemethacrylic acid) (PSA) [87], glucose [89], and polystyrene (PS) [83]. As a template, PS is produced by the polymerization of styrene. To be able to deposit TiO₂ on the PS surface, it is necessary to add a cationic initiator such as 2-(methacryloyloxy)ehyl trimethylammonium chloride (DMC) (Figure 5) [112]. Similarly, cationic polymerization initiators like 2,2-azobis-(isobutyramidine) dihydrochloride (AIBA) [85] and potassium persulfate (KPS) [84] have been reported to provide positive charges on the surface of PS and assist the formation of PS@TiO₂ (Figure 5). The formation of HS TiO₂ with different particle sizes can be affected by pH and the ratio of the addition of titania precursor to solvent. For example, Kanjana et al. [92] reported that a reaction at low pH (acidic) resulted in larger particle size compared to reactions conducted under neutral or alkaline conditions. In addition to that, Syoufian et al. [84] reported that adding an excessive amount of precursor increased the particle size of HS TiO₂. The TiO₂ template removal to produce the hollow structure can be carried out simultaneously with calcination. Synthesis with this method can also produce HS TiO₂ at low pressures to yield particle sizes of hundreds to thousands of nm and shell thicknesses of up to hundreds of nm (Table 2).

Synthetic route of PS/TiO2 core@shell particles [112]. Licensed by Elsevier.

3.2.2. Sol-gel hard template

The sol-gel hard template method is a two-step process commonly used to synthesize hollow nanostructures like HS TiO₂ [113]. The first stage is templating core@shell with SiO₂ as the core coated with TiO₂ in the shell. The second stage is the formation of HS TiO₂ by removing the core from the core@shell (Figure 6). The synthesis of HS TiO₂ through the formation of SiO₂@TiO₂ was carried out at alkaline pH to prepare a SiO₂ template with a negative surface charge (-) to interact with the titania precursor prepared in acidic medium (positive surface charge). The synthesis of HS TiO₂ can be conducted by SiO₂ core from SiO₂@TiO₂ material using ammonia or sodium hydroxide solution by etching [95–97, 107]. The fabrication process involves the introduction of ammonium ions (NH₄⁺), which bind to the negatively charged SiO₂ surface through electrostatic interactions with the –OH groups, creating a positively charged surface. This step ensures strong adhesion of the titania precursor to the SiO₂ template, enabling the formation of a uniform TiO₂ shell.

Schematic of two step HS TiO2 synthesis via SiO2@TiO2.

The accumulation of NH₄⁺ on the surface of SiO₂ encourages interaction with the negatively charged –TiOH group and, through a condensation process with an NH₄⁺ catalyst forms SiO₂@TiO₂. The formation of a hollow material can be determined by core@shell (CS) as the initial product of HS TiO₂ synthesis. In manufacturing the SiO₂@TiO₂, SiO₂ is widely used as a core because it is easy and fast to prepare. The Stöber method is a well-known successful method for producing monodisperse SiO₂ spheres [114]. Moreover, SiO₂ has good solubility in based solution, making this compound easy to remove. In the final step, ammonia or sodium hydroxide solution was used to remove the SiO₂ core from SiO₂@TiO₂ by etching [95–97, 107]. Adding a hard alkaline solution to SiO₂@TiO₂ through sonication and stirring pushes high-intensity energy into the voids between the silica-titania, which causes partial etching of the silica core. Under conditions of strict induction reaction, it caused corrosion and etched silica-titania part. The etched silica and titania species are released into the solution and diffused from the nanosphere to the titania shell. Furthermore, the etched fragments undergo condensation and re-deposition on the titania shell’s surface through Ostwald maturation (reverse reaction). This reversible reaction continues until a hollow nanosphere structure is formed [103]. The anatase phase of HS TiO₂ was obtained after calcination of SiO₂@TiO₂ at 500-900°C [97,103,105]. Calcination at 900°C showed the optimal temperature of the anatase phase of HS TiO₂ [103]. At high temperatures, the slowing formation of the crystal structure of anatase HS TiO₂ occurs due to interaction with SiO₂ as a core [103].

Hollow sphere TiO₂ can also be synthesized with a hard template using gold (Au) from HAuCl and several inorganic salts (Table 2). The manufacture of HS TiO₂ with an Au template began with the formation of Au@TiO₂ resulting from the sol-gel method at more than 100°C. The HS TiO₂ was obtained by depositing the Au core with KCN [99]. The fabrication of HS TiO₂ has also been reported by Chen et al. [101] which uses the CaCO₃ template through a two-step reaction. The hollow sphere formation was obtained after etching CaCO₃ using HCl [101]. Lan et al. [100] reported the use of K₃PW₁₂O₄₀, obtained by mixing a solution of KCl and H₃PW₁₂O₄₀, as a template to produce HS TiO₂ with a diameter of 0.5-1 µm. The template was removed using 1 M NaOH solution [100]. Rakhman et al. [115] introduced a novel two-step synthesis method to fabricate HS TiO₂, achieving optimal crystallite sizes of 9.53 to 20.54 nm and micro-strains. The resulting hollow spheres demonstrated excellent photooxidative properties under UV irradiation, highlighting their potential for solar cell applications. Similarly, a one-pot sol-gel method has been applied for synthesizing HS TiO₂ embedded with silver nanoparticles (AgNPs), achieving uniform distribution of AgNPs on the HS TiO₂ and enhanced light harvesting for DSSCs [116].

Several existing synthesis methods have similar techniques for producing HS TiO₂ with different particle sizes. In the hydrothermal carbon-based templates and hard templates method, adding excess titania precursors was adopted to design HS TiO₂ with larger particle sizes. However, in the hydrothermal/solvothermal method, this approach will affect the cavity size of the HS TiO₂. The sol-gel method using a template can improve the shell’s thickness in this situation. The template-assisted method also makes it possible to design the size of the HS TiO₂ cavity. Kanjana et al. [92] reported that the formation of HS TiO₂ with different particle sizes could be influenced by the pH and ratio of addition of the titania precursor to the solvent. The reaction at low pH (acidic) gave a larger particle size compared to neutral or alkaline conditions in the sol-gel carbon-based template synthesis [117].

In addition, other methods like co-precipitation and solid-state methods can also be used to synthesize HS TiO₂ [81,118]. However, this review focuses on the hydrothermal/solvothermal and sol-gel methods, as both these synthesis routes are the most popular among all.

3.3. Characterization of HS TiO₂

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images are the primary data to determine the morphology of HS TiO₂. The data such as diameter size, particle size, grain size, crystal size, cavity, and shell thickness can also be obtained using both. Other data, such as binding energy curve by x-ray photoelectron spectroscopy (XPS), elemental mapping plot by energy dispersive X-ray spectroscopy (EDS), and absorption spectra by IR, were added by several researchers to provide information on reaction products at each stage, especially in the sol-gel method using templates. Interestingly, several articles discussed changes in the structure of the TiO₂ products to show the progress of the reaction. For example, Chen et al. [62] showed TEM images emphasizing the Kirkendall effect process (Figure 2) in producing HS TiO₂. Zhao et al. [119] studied the reaction rate of HS TiO₂ formation through Ostwald ripening (Figure 3) with TEM images.

Most of the characterization of HS TiO₂ by SEM and TEM through two-stage sol-gel synthesis presented different images of templates, intermediate materials, and hollow sphere products. In the carbon-based templates method, Song and Gao [85] presented a TEM images (Figure 7) of polystyrene (PS) sphere, PS@TiO₂, and HS SiO₂/TiO₂.

TEM images: (a) polystyrene (PS) spheres, (b) PS@TiO2, (c) HS SiO2/TiO2 [85]. Licensed by American Chemical Society. TEM: Transmission electron microscopy.

The PS ball template (Figure 7a) looks clear and smooth with a uniform size. The TiO₂ coating forming PS@TiO₂ (Figure 7b) shows a different image of the PS sphere with a rougher surface and larger particle size changes. Meanwhile, the HS SiO₂/TiO₂ formed after the removal of the PS core shows a smoother ring image than PS@TiO₂ (Figure 7c). Karabacak et al. [120] show the PS sphere’s change to PS@TiO₂ with the SEM image (Figure 8). Similar to the TEM image, the SEM image shows a smoother PS sphere surface of uniform size (Figure 8a) compared to PS@TiO₂, which offers a rougher surface (Figure 8b).

SEM images: (a) Polystyrene, (b) PS@TiO2 [120]. Licensed by Elsevier.

The formation of PS@TiO₂ as an intermediate for HS TiO₂ through a sol-gel pathway based on a carbon template was supported by IR spectroscopy. Shi et al. [121] confirmed the formation of PS@TiO₂ by comparing the IR spectra of PS, PS@TiO₂, and TiO₂ anatase. The difference in absorption bands of the three indicates the interaction of PS and TiO₂. The PS microspheres showed absorption bands around 3000 and 1470 cm^-1, representing the C-H and C-C stretchings of the phenyl group [121]. Absorption around 760 and 700 cm^-1 was associated with bending vibrations of phenyl in the C-H and C-C groups. While the absorption at 650 cm^-1 is related to solid Ti-O-Ti vibrations [121]. The production of HS TiO₂ by the sol-gel synthesis pathway using SiO₂ as the template was also confirmed via TEM images (Figure 9). The different solid structure of SiO₂@TiO₂ (Figure 9a) with ring-like HS TiO₂ (Figure 9b) followed by a change in the percentage of silica and titania in the EDS spectra indicated the success of dissolving the SiO₂ core by NaOH [36].

TEM images and EDS spectra (inset) of (a) SiO2@TiO2, (b) HS TiO2 [36]. Licensed by American Chemical Society. TEM: Transmission electron microscopy.

In addition to the primary data from TEM and SEM images, the formation of SiO₂@TiO₂ in several articles is shown by supporting data such as absorption spectra by fourier tranform infrared spectroscopy (FTIR), binding energy curve by XPS, and qualitative data on elemental percent by EDX (Table 3). The FTIR spectra specifically showed Si-O-Ti interaction on the formation of SiO₂@TiO₂ in the absorption area of 940-962 cm^-1. While the analysis of bond energy (B.E) with XPS on O1s measurements showed 2 to 3 peaks, with the characteristics of Ti-O-Ti at 527.6-530.2 eV, Ti-O-Si at 530.7-532.5 eV, and Si-O-Si at 531.5-533.3 eV. The SiO₂@TiO₂ characteristic is also shown in the reflectance and absorbance of UV/Vis spectra (Figure 5). The reflective index (n) of SiO₂@TiO₂ was between SiO₂ (1.47) dan TiO₂ (2.49) nanoparticles at 400-700 nm. Meanwhile, the UV/Vis SiO₂@TiO₂ absorption spectra showed higher adsorption than SiO₂ and TiO₂ nanoparticles at 400-650 nm [122].

4. Photovoltaic Characteristics

4.1. Scattering light ability of HS TiO₂

TEM image showing HS TiO₂ appears as a ring, indicating the presence of a cavity. It is a unique feature of HS TiO₂ in addition to the pores of the interface (intra-shell). This feature allows a strong internal light scattering in the HS TiO₂ compared to solid particles [98]. Lei et al. [123] investigated the light scattering properties of HS TiO₂ as indicated by the UV-Vis reflectance spectrum (Figure 10a). HS TiO₂ exhibits strong reflectance in the 400-800 nm range to more than 50%, while the TiO₂P25 is lower.

(a) DR UV-Vis spectra of the HS TiO2 and TiO2 P25 films. (b) A schematic light pathway illustration of [1] HS TiO2 and [2] TiO2 P25 nanoparticles [123]. Licensed by Elsevier.

In addition, HS TiO₂ also shows a stronger visible light absorption ability than TiO₂ P25 [69]. It is attributed to the ability of the hollow structure to absorb more photons than TiO₂ P25 [124]. The dye loading on HS TiO₂ also showed a higher amount than TiO₂ P25. However, dye-sensitized TiO₂ HS was able to maintain a higher reflectance than TiO₂ P25 [77]. This condition is associated with a greater opportunity for the dye to absorb photons [26]. The multi-reflection effect arises from HS TiO₂ to scatter incident light at different wavelengths in the visible area, and the scattering effect can increase the optical path of the incident light (Figure 10b). This condition contributes to higher light-harvesting efficiency to increase photocurrent density (Jsc) [123].

4.2. Photovoltaic performance

As mentioned earlier, the unique feature of HS TiO₂ is a distinct advantage in photovoltaic characteristics. Lee et al. [125] reported that the structural effect of HS TiO₂ was important, where HS TiO₂ showed better light absorption compared to the same particles after crushing into powders with a size of 10 nm. Despite having the same band gap at 3.2 eV as anatase TiO₂, the diameter and thickness of the HS TiO₂ shell were different, indicating different visible light absorption activities in the 400-800 nm area. This condition confirms that the morphology of HS TiO₂ has a different effect on its photoactivity [106]. The HS TiO₂ with a diameter of ∼100 nm provided the best PCE [36]. In addition to that, the diameter of the HS TiO₂ also affects the dye loading ability (Figure 11), where a larger HS TiO₂ diameter caused a reduction in the amount of dye loading [138]. This condition is thought to occur when light incident on the surface of the material experiences an extension of the wavelength path in the cavity, enhancing photon capture by dye molecules [36,99]. The large surface area of particles allows the dye loading to be higher than the solid particles, thus encouraging more optimal photon harvesting [98].

Table 3. Characterization of SiO₂@TiO₂.

FTIR	XPS B.E. (eV)						SEM/FESEM/TEM/HRTEM				EDX	References
Ti-O-Si	Si 2p	O 1s			Ti 2p		Particle size (nm)	Crystallite size (nm)	Core size (nm)	Shell thickness (nm)	Si:Ti (%)
(cm^-1)	Si-O-Si	Ti-O-Ti	Ti-O-Si	Si-O-Si	Ti 2p1/2	Ti 2p3/2	Particle size (nm)	Crystallite size (nm)	Core size (nm)	Shell thickness (nm)	Si:Ti (%)
940		529.9	532.5	533			∼440 -∼560					[126]
							40					[114]
							110, 240, 530				46:54, 80:20, 92:8	[122]
							200, 260, 300, 320, 370					[127]
							81.2, 101.6, 124.4, 152.5				55.54:44.46	[128]
	101.8			531.5			∼130					[129]
							225, 245, 250, 265	4.4, 4.5, 4.3, 4.6	200	23, 25, 34, 33		[130]
962		530.1	531.9	532.9	464.4	458.8		14.85	∼310			[131]
943		530.2	531.9	532.6	464.2	458.6	450-550		∼310			[132]
402								∼7			38.60:1.15, 42.09:7.53, 25.29:14.49	[133]
	102.7				464.2	458.8	390			65		[23]
955								18.18, 18.26, 22.75, 28.98, 38.18	221	7, 17, 25	36.6:0.92, 36.02:1.56, 28.22:2.91	[134]
	101.41	527.63	530.73		462.13	456.18						[135]
	104.2	530.2	531.6	533.3			222		193			[136]
	103.9			533	464.1	458.4			100	0.35		[137]

Effect of HS TiO2 diameter on photovoltaic properties [30,36,70,124,139-141].

The higher photon absorption of HS TiO₂ is indicated by a decrease in wavelength (350-800 nm) in the transmittance spectrum [99]. This phenomenon is associated with the efficiency of current conduction (electron transfer) from the dye by HS TiO₂ to the DSSC system [26]. This indication is seen in the pore characteristics and surface area of the material that affect the loading ability of the dye molecules and the electron transfer speed of the dye molecules [26].

4.3. Enhancing photovoltaic characteristic

Modifications of HS TiO₂ as a semiconductor component in DSSC have been reported to enhance the Jsc and PCE (Table 4). It is reported that the combination of HS TiO₂ and TiO₂ nanosheets enhanced Jsc by 2.44% and PCE by 0.96% [119]. Meanwhile, HS TiO₂ composite films with TiO₂ nanotube arrays enhanced Jsc by 3.05%, with a 1.31% improvement in PCE [140]. Adding non-semiconductor materials such as carbon nanotubes enhanced Jsc by 1.6% and PCE by 0.77% [141]. Meanwhile, 0.25 mol% Fe doping on HS TiO₂ increased Jsc by 4.84% and PCE by 2.39% at the optimum conditions, which have the surface area and dye loading of 122.84 m² g^-1 and 22.12×10^-8 mol cm^-2, respectively [142]. Additionally, a study reported that DSSCs fabricated with Fe/S–TiO₂ photoanodes exhibited an improved solar-to-electrical energy conversion efficiency of 6.46%, compared to 3.43% for pure TiO₂, under full sunlight illumination [143].

Table 4. Modifications of HS TiO₂ for DSSC.

Modifications	Enhancement of *Jsc* (%)	Enhancement of *Voc* (%)	Enhancement of PCE (%)	References
Film composite HS TiO₂/carbon nanotubes	1.6	0.013	0.77	[141]
Double light-scattering layer HS TiO2 and nanosheet TiO2	2.44	0	0.96	[119]
TiO2 HS/TiO2 nanotube array composite films	3.05	-0.01	1.31	[140]
TiO2 HS assembly from nanosheets	3.99	-0.01	2.22	[30]
Fe-doped TiO2 HS	4.845	0.038	2.39	[142]
HS TiO2 decorated small Ag NP	3.1	0	1.2	[98]
HS TiO₂ decorated large Ag NP	4.3	0	1.9	[98]
HS TiO₂ decorated Ag NP	1	0.01	0.7	[36]
HS TiO₂ decorated Au NP	0.5	0.01	0.5
HS TiO₂ decorated Au@Ag NP	1.5	0.01	1.1

Another modification is the addition of localized surface plasmon resonance material (LSPRM), such as Au, Ag, or alloys of both, which increased the PCE by 0.5, 0.7, and 1.1 %, respectively [141]. The addition of Ag with a larger particle size gave an even higher PCE of 1.9 % [98]. According to Song et al. [144], LSPRM in the DSSC system increased the number of electrons released from the dye and the electron density of the semiconductor. The interfacial interaction between LSPRM and the semiconductor causes a fast electron transfer from the dye molecule [145]. The plasmon effect can be formed when the light comes in and stimulates free electron oscillations in LSPRM [146]. When the LSPRM on the surface of the semiconductor material is excited by light, a strong electromagnetic field is generated. A strong electromagnetic field can enhance the photocurrent and regeneration of photoelectrons [147]. However, the addition of a higher concentration of LSPRM could attenuate the efficiency of the DSSC due to a decrease in dye loading on the TiO₂ semiconductor [146]. Enhancing the local electromagnetic field around the nanostructure is the key to increasing dye adsorption in the DSSC system [148]. LSPRM plays multiple roles in the performance of DSSCs, including an enhancement in the dye adsorption coefficient and optical absorption due to surface plasmonic resonance [149]. LSPRM also acts as an electron sink for charge carriers on photoinduction, enhancing the interfacial electron transfer process and minimizing electron transfer recombination, thereby enhancing the electron transfer process in DSSC [148].

HS TiO₂ has demonstrated excellent photovoltaic performance under laboratory conditions. However, several challenges might hinder its practical applications. One significant challenge is the stability and durability of HS TiO₂ structures. Over time, environmental factors such as moisture, UV radiation, and temperature fluctuations can lead to degradation, resulting in a decline in photovoltaic performance. Implementing protective coatings or encapsulation techniques can shield HS TiO₂ structures from these environmental stressors [150]. Manufacturing scalability is another concern. The synthesis of HS TiO₂ structures often involves complex processes that may not be easily scalable for large-scale production [151]. Therefore, scaled-up productions of HS TiO₂ in order to establish the best performance and cost-effective synthesis route is required.

The addition of LSPRM to HS TiO₂ provides an opportunity to enhance the efficiency of the DSSC compared to the single semiconductor material in the photoanode. It provides a better prospect for DSSC development [36]. Thus, it raises several new challenges to conduct morphological tracing and suitable LSPRM arrangement on HS TiO₂-based photoanode components to increase the work conversion efficiency (PCE) of DSSC.

5. Conclusions and Outlooks

Developing HS TiO₂ as a semiconductor material for DSSC components is important for improving the efficiency of DSSC. HS TiO₂ offers significant morphological advantages, which can improve the PCE of DSSC. The unique morphology of HS TiO₂ contributes to improved short-circuit current density (Jsc) and, consequently, a higher PCE. This review highlights the synthesis of HS TiO₂ and its photovoltaic properties as key areas of innovation.

This review presents several HS TiO₂ synthesis methods, such as hydrothermal/solvothermal, sol-gel by carbon-based template, and hard template. Hydrothermal/solvothermal has the advantages over these methods because it has the shortest synthesis route compared to the sol-gel method for the formation of the HS TiO₂ structure as well as the arrangement of the crystal phase. Meanwhile, the fabrication of HS TiO₂ through the hard template sol-gel method can provide a smaller pore size than other synthesis pathways. The uniqueness of HS TiO₂ pores, smaller particle size, and large surface area allow the acceleration of electron transfer from the dye to the device and show a higher Jsc than other morphologies. Although the HS TiO₂ from the SiO₂@TiO₂ produced smaller particle sizes and higher PCE compared to other pathways, the synthesis of this pathway requires the longest fabrication time starting from the formation of SiO₂@TiO₂, then the growth of the crystal phase by calcination, and followed by the removal of the SiO₂ core through etching.

Hollow sphere TiO₂ can scatter light very well due to the influence of its micropore particles and the uniqueness of its cavity. This property is the advantage of HS TiO₂ for photocatalyst applications and solar cells. Especially in DSSC applications, semiconductor HS TiO₂ has several challenges, including the loading dye capacity and the electron transfer rate, which still depend on the particle size of the HS TiO₂. The particle size of HS TiO₂ is one of the variables studied for its ability as a dye adsorbent, incident light scattering properties, and electron transfer speed in DSSC applications. So far, the thickness of the HS TiO₂ shell has not received similar attention in these studies. According to Mie’s theory, the thickness of the HS TiO₂ shell can affect the scattering of incident light. In the future, therefore, the thickness of the HS TiO₂ shell would be the other key parameter affecting the conversion of light into electricity in DSSC.

Recent advancements in DSSCs have focused on enhancing the dye loading and light-harvesting efficiency through various material modifications. Incorporating carbon nanotubes (CNTs) into the photoanode has been shown to improve electron transport and reduce recombination rates. For instance, research demonstrated that integrating multi-walled carbon nanotubes (MWCNTs) into the BaTiO₃ photoelectrode led to improved photovoltaic performance. Additionally, doping TiO₂ with elements like iron (Fe) has been found to enhance the material’s surface area and electronic properties.

The incorporation of LSPRM, such as silver or gold nanoparticles, has been extensively investigated to enhance light absorption and facilitate faster electron injection in DSSCs. These nanoparticles generate strong localized surface plasmon resonance (LSPR) fields, which can significantly enhance the photonic absorption of dyes. In the future, the composition and construction of the thin layer system, elemental doping, and the addition of LSPRM to HS TiO₂-based photoanode will become promising studies in developing more efficient DSSC.

Credit authorship contribution statement

Khusna Arif Rakhman: Data curation, Investigation, Methodology, Writing – original draft. Nurul Hidayat Aprilita: Formal analysis, Supervision, Writing – review & editing. Rohana Adnan: Formal analysis, Methodology, Writing – review & editing. Indriana Kartini: Funding acquisition, Methodology, Supervision, Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known financial interests in the work reported in this paper.

Declaration of generative AI and AI-assisted technologies in the writing process

The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.

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1. Introduction

2. Morphology Effect

3. Synthesis of HS TiO2

3.1. Hydrothermal/solvothermal method
3.2. Sol-gel method
3.3. Characterization of HS TiO2

4. Photovoltaic Characteristics

4.1. Scattering light ability of HS TiO2
4.2. Photovoltaic performance
4.3. Enhancing photovoltaic characteristic

5. Conclusions and Outlooks

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