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NANOSYSTEMS: Dorosheva I.B., et al. Nanosystems:
PHYSICS, CHEMISTRY, MATHEMATICS Phys. Chem. Math., 2023,14 (4), 447-453.
http://nanojournal.ifmo.ru
Original article DOI 10.17586/2220-8054-2023-14-4-447-453
Induced surface photovoltage in TiO2 sol-gel nanoparticles
IrinaB. Dorosheva1'2", Alexanders. Vokhmintsev16, IlyaA. Weinstein1,2'c, AndreyA. Rempel1'2^ 1Ural Federal University, Yekaterinburg, Russia
2 Institute of Metallurgy of the Ural Branch of the Russian Academy of Sciences, Yekaterinburg, Russia [email protected], [email protected], [email protected], [email protected] Corresponding author: I. B. Dorosheva, [email protected]
Abstract TiO2 nanoparticles synthesized by the sol-gel method and modified by annealing in air and hydrogen atmospheres were studied by surface photovoltage spectroscopy (SPS). SPS measurements showed that the modified in air TiO2 nanoparticles have a more intense signal than those treated in hydrogen. A linear correlation was found between the SPS and the diffuse reflectance spectra of the samples. Keywords Titanium dioxide, sol-gel method, nanoparticles, surface photovoltage (SPV). Acknowledgements A.S.V. and I.A.W. thank the Ministry of Science and Higher Education of the Russian Federation (research project FEUZ-2023-0014) for support. Part of synthesis of titanium dioxide carried out within the framework of RF State assignment for IMET UB RAS.
For citation Dorosheva I.B., Vokhmintsev A.S., Weinstein I.A., Rempel A.A. Induced surface photovoltage in TiO2 sol-gel nanoparticles. Nanosystems: Phys. Chem. Math., 2023, 14 (4), 447-453.
1. Introduction
Titanium dioxide has attracted the attention of scientists around the world for many years, because it is a chemically stable, non-toxic, and inexpensive material that has functional properties for use as photocatalysts in the decomposition of toxicants [1-4], photosorbents of rare earth elements [5-7], renewable energy sources [8-10], resistive memory elements [11-13], new organic compounds synthesis for medicine [14-16], etc. There are also various ways for the nanosized titanium dioxide synthesis: solvothermal [17-19], hydrothermal [20-22], anodic oxidation [2,23,24], chemical vapor deposition [25-27], electrodeposition [28-31], sonochemical [32-35], and microwave methods [36-39]. However, TiO2 has a wide bandgap of 3.0-3.3 eV [40,41], which makes it difficult to apply it as a photosorbent and photocatalyst under visible light irradiation. In this regard, the material is modified in various ways: by creating a heterostructure, doping with metal ions, annealing in atmospheres of air, hydrogen, argon, nitrogen, etc. [42-49] to create atomic defects and surface states capable to capture photons with lower energy. One of the instruments for study such states is the surface photovoltage spectroscopy (SPS), which allows a non-contact, non-destructive and highly sensitive way to determine the electronic features in surface of solids and can be used to analyze charges separation and transfer in the mechanisms of photocatalysis, dye sensitization, etc. [50-55]. The method is based on the generation of voltage by a capacitor structure with a sample under study between electrodes upon excitation by light [52] and is a suitable tool for study photophysical processes in nanoscale semiconductors for several reasons. First, the SPS is generated as a result of band-to-band transitions, which makes it possible to estimate the semiconductor bandgap [52]. Second, the method is sensitive to band-to-band transitions due to the surface states of a material, which, as a rule, arise due to impurities or point defects in its structure. Therefore, the aim of this work is to study the titanium dioxide nanoparticles, synthesized by the sol-gel method, via surface photovoltage spectroscopy.
2. Samples and experiment
TiO2 nanoparticles were obtained by the sol-gel method at the initial solution pH of 2,6, and 8, followed by annealing in air in an SNOL 6.7/1300 muffle furnace at a temperature of 350°C for 4 hours [15,43,56,57]. Also, the sample obtained at pH = 6 was annealed in an MPT-2MR tube furnace in a hydrogen atmosphere at temperatures of 200, 400 and 600° C for 60 minutes [43]. The synthesized TiO2 nanoparticles were previously studied using X-ray phase analysis, diffuse reflectance spectroscopy, and the Brunauer-Emmett-Teller (BET) method [15,43,56,57] (see Table 1).
The surface photovoltage (SPV) spectra were investigated using the measuring installation based on an LS 55 luminescent spectrometer (Perkin-Elmer, USA) with the developed electroluminescent cell [58] and a PXI-4071 digital multimeter (National Instruments, USA). The SPS measurements were controlled by the original virtual device developed in the LabVIEW programming environment. The studied TiO2 samples were placed between the transparent ITO glass electrode and the Sn0.61 Cuo.39 electrode (Fig. 1).
The distance between the electrodes was d « 20 ^m. To generate photovoltage, the sample was excited by monochromatic light from a pulsed Xe lamp in the range of 290-450 nm at a monochromator exit slit of 10 nm and a scanning rate
© Dorosheva I.B., Vokhmintsev A.S., Weinstein I.A., Rempel A.A., 2023
I. B. Dorosheva, A. S. Vokhmintsev, I. A. Weinstein, A. A. Rempel Table 1. Characteristics of titanium dioxide powders [15,43,56,57]
Sample pH of sol Annealing temperature in H2 Degussa P25 Hombifine
2 6 8 200° C 400° C 600° C
Phase composition Anatase Amorphous Anatase Anatase, rutile Anatase
Coherent scattering region ±10 %, nm 8 9 20 - 46 36 25 15
SBET ± 2 %, m2/g 140 140 90 310 120 40 55 70
Eg ± 0.1, eV 3.0 3.2 3.2 3.3 3.2 3.2 3.0 3.3
Fig. 1. Image and scheme of a cell for measuring SPV
of 1 nm/s. The SPS signal was recorded by a digital multimeter with a period of 250 ms. Since electrodes with different materials were used, differing in the work function Aout « 4.8 eV (ITO) and 4.4 eV (Cu, Sn), under the conditions of the induced internal electric field, the background SPS signal was recorded in the dark. Taking into account the emerging contact potential difference of 0.4 eV and the sample thickness d, the induced electric field was E = 2 • 106 V/cm. In this regard, the indicated background SPS signal was subtracted when processing the initial experimental data. The resulting SPS curves were normalized to the ITO transmission spectrum (Fig. 2).
Fig. 2. ITO transmission spectrum
3. Results and discussion
The SPS of the studied samples are shown in Figs. 3-4. Hombifine powder was measured for the first time (Fig. 3), there are no data in the literature on measuring its properties by this method. The absolute changes in induced photovoltage AUmax (Fig. 4) depending on the excitation wavelength for samples annealed in air (AUmax « 150 mV) are almost two times higher than for samples annealed in hydrogen (AUmax « 60 ^ 80 mV).
g 300
~o
>
^ — —" Hombifine
; à - - -
; AU, • ' i ■ i ^max Degussa p25
* II *
« « * §
* « *
360 390
Wavelength, nm
Fig. 3. SPV spectra of Degussa P25 and Hombifine powders
Fig. 4. SPV spectra of TiO2 samples obtained by the sol-gel method
Fig. 5. Normalized surface photovoltage spectra for Degussa P25 and Hombifine powders
Energy, eV 3.8 3.6 3.4
1.00-
0.75-
0.50-
0.25-
D
<
<
0.75-
0.50-
0.25-
0.00-
300 320 340 360 3s0 Wavelength, nm
1 1 1 1 1 1 1 1 CP i 1 i 1 i a).
■y (P 2,98 eV 3.17 eV i-3.22 eV+ ! ■
= 2 ♦ ! " i i 1
pH = 8 ^iffi 1 ' 1 ^J-nrdmrn-^,^.,.,.^
A 3,32 eV
o \ 200 °C \ 13,23 eV i i i i , i
°°° \ 3,24 eV l i . h
OO :, ^fey. , i 11 ^¡¡ate^, ii ly
400 °C ^^ if • iv-i.. j® _ ........
400 420
Fig. 6. Normalized surface photovoltage spectra of TiO2 samples obtained by the sol-gel method
¡s
El 50
<
100
A pH = 2 •
A pH = 6
A pH= 8
V 200 °C
7 400 °C
T 600 °C , *
• Degussa P25 , *
A >'*'
T
10
30 40 50
R, %
60 70 SO 90
Fig. 7. The dependence of the SPS signal AUmax on the reflectance R at a 430 nm wavelength
After annealing TiO2 nanoparticles in air, the SPS spectra have a similar shape, regardless of the sol pH during their synthesis. Comparing the studied samples, it can be noted that the highest value of AUmax appears after annealing in air. This is due to the complete crystallization of the samples to anatase symmetry and a decrease of the vacancies concentration upon annealing in air. Upon annealing in a reducing hydrogen atmosphere, both with partial and complete crystallization, the oxygen vacancies concentration remains high in the sample structure. Charge carriers are localized on these defects, reducing the AUmax value. The highest AUmax among those annealed in hydrogen is demonstrated by the sample treated at 600°C. It is probably due to more efficient separation of electron-hole pairs due to complete crystallization to the anatase phase. A low AUX value indicates inefficient separation of photoinduced electron-hole pairs or their capture by defect states. Upon annealing at 200° C, the amorphous structure is saved and AUmax « 60 mV. When the temperature rises to 400° C - AUmax « 40 mV incomplete crystallization of TiO2 to anatase occurs at this temperature. As a result of the presence of boundaries in such a two-phase structure, the migration of charge carriers to the surface is hindered; therefore, AUmax is smaller than in amorphous TiO2. Annealing at 600°C leads to increase AUmax « 80 mV. This value is higher than that of amorphous TiO2 obtained at 200°C. Complete crystallization of the sample to anatase at 600° C leads to the vacancy defects concentration decrease, a more efficient separation of the electron-hole pair and, consequently, a change in the surface charge.
It was found (Fig. 5) that Degussa P25 powder has a maximum photoinduced response in the region of 310 nm, which corresponds to band-to-band optical transitions [59,60]. The Degussa P25 SPS spectra measured in this work and in [61] differ in the peak position (see Fig. 5). In the cited work, the maximum is shifted to the long wavelength region of 340 nm. This is due to the difference in the materials used for the photovoltaic cell electrodes: two ITO glasses in [61]; in our work, one of the cell electrodes is ITO glass, and the second one is Sn0.61Cu0.39 alloy. The work function of these materials differs by « 0.4 eV. Therefore, in our case, additional energy is required to separate the electron-hole pair and release the charge to the surface for signal registration. The corresponding shift in the SPS spectra (Fig. 5) clearly demonstrates this fact. It was also found that Hombifine powder has a maximum photoinduced response in the excitation region of 340 nm. It means that for the separation of electron-hole pairs and the release of charge carriers to the surface, less energy is required than for Degussa P25. This is probably due to the single-phase anatase structure of the sample, i.e. there is no charge carriers scattering at the interphase boundaries of rutile and anatase, which can be characteristic for Degussa P25. The arrows in Figs. 5-6 indicate the bandgap energies of the samples. In addition, a linear relationship was found between the SPS signal and the diffuse reflectance at a wavelength of 430 nm (see Fig. 7) with a Pearson's correlation coefficient of r = 0.88.
Thus, the structural and phase features of stoichiometric TiO2 nanoparticles, which are characterized by a singlephase composition, as well as a low concentration of vacancy and surface defects, significantly affect the efficiency of charge carrier separation processes during band-to-band optical excitation and the formation of SPV. Note that a controlled change in the nanostructures surface charge is great importance for optimizing their photovoltaic properties.
4. Conclusions
In this work, we study the photovoltaic properties of titanium dioxide nanoparticles synthesized by the sol-gel method and modified by annealing in air and in hydrogen atmospheres. According to the increment SPV spectra AUX, it was found that the nanoparticles modified in air have a more intense signal compared to those treated in hydrogen. Due to the high concentration of oxygen vacancies formed as a result of annealing in a reducing atmosphere, defect states appeared, on which excited electrons are localized, which 2 times weakens the SPV signal. A linear correlation of the SPS data with the diffuse reflection spectra (Pearson's correlation coefficient r = 0.88) was found, which also confirms the efficiency of electron-hole pairs separation and changes in the surface charge, provided that the structure is stoichiometric and with the least number of defects.
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Submitted 26 May 2023; revised 21 June 2023; accepted 22 June 2023
Information about the authors:
Irina B. Dorosheva - Ural Federal University, Mira str., 19, Yekaterinburg, 620002, Russia; Institute of Metallurgy of the Ural Branch of the Russian Academy of Sciences, Amundsena str., 101, Yekaterinburg, 620016, Russia; ORCID 0000-0001-9229-6758; [email protected]
Alexander S. Vokhmintsev - Ural Federal University, Mira str., 19, Yekaterinburg, 620002, Russia; ORCID 0000-00032529-3770; [email protected]
IlyaA. Weinstein - Ural Federal University, Mira str., 19, Yekaterinburg, 620002, Russia; Institute of Metallurgy of the Ural Branch of the Russian Academy of Sciences, Amundsena str., 101, Yekaterinburg, 620016, Russia; ORCID 00000002-5573-7128; [email protected]
AndreyA. Rempel - Ural Federal University, Mira str., 19, Yekaterinburg, 620002, Russia; Institute of Metallurgy of the Ural Branch of the Russian Academy of Sciences, Amundsena str., 101, Yekaterinburg, 620016, Russia; ORCID 0000-0002-0543-9982; [email protected]
Conflict of interest: the authors declare no conflict of interest.
