Surface Photovoltage Spectroscopy Observes Sub-Bandgap

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C: Energy Conversion and Storage; Energy and Charge Transport

Surface Photovoltage Spectroscopy Observes Sub-Bandgap Defects in Hydrothermally Synthesized SrTiO Nanocrystals 3

Xiaoqing Ma, Zongkai Wu, Emily J. Roberts, Ruirui Han, Guodong Rao, Zeqiong Zhao, Maximilian Lamoth, Xiaoli Cui, R. David Britt, and Frank E Osterloh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b06727 • Publication Date (Web): 06 Sep 2019 Downloaded from pubs.acs.org on September 6, 2019

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The Journal of Physical Chemistry

Surface Photovoltage Spectroscopy Observes Sub-Bandgap Defects in Hydrothermally Synthesized SrTiO3 Nanocrystals Xiaoqing Ma,abc Zongkai Wu,b Emily J. Roberts,d Ruirui Han,b Guodong Rao,b Zeqiong Zhao, b Maximilian Lamoth,be Xiaoli Cui,c R. David Britt,b Frank E. Osterloh*b

a School

of Materials Engineering, Shanghai University of Engineering Science, 333 Longteng Road,

Shanghai, 201620, P.R.China. b Department

of Chemistry, University of California, Davis. One Shields Avenue, Davis, CA, 95616,

USA. Fax: (+1)530 752 8995; E-mail: [email protected]. c

Department of Materials Science, Fudan University, Shanghai, 200433, P. R. China

d Department

of Chemistry, 840 Downey Way, University of Southern California, Los Angeles, CA

90089-0744, USA. e Department

of Chemistry, University of Munich (LMU), Butenandtstrasse 5−13, 81377 Munich,

Germany.

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ABSTRACT

SrTiO3 has been of interest as a photocatalyst for overall water splitting but the energy conversion efficiency of this material is limited by recombination at surface and lattice defects. Here we use surface photovoltage spectroscopy (SPS) to map defects in SrTiO3 nanocrystals made by hydrothermal synthesis, as a function of temperature during a subsequent thermal annealing step. Two types of defects, D1 and D2, can be identified on the basis of their photovoltage contributions at 2.0 eV in as-synthesized particles and at 2.0 and 2.7 eV in 300 ºC annealed particles. Using Rh:SrTiO3 nanocrystals with a defined 2.9 eV photovoltage as internal standard, defect concentrations in SrTiO3 are estimated at 0.47-1.10 atom% (based on Ti) in 25 ºC - 300 ºC annealed samples and at 0.13-0.20% after annealing at 400-500 ºC in air. Based on EPR spectra at 77K, the defects are assigned to Ti(III) states associated with oxygen vacancies and to a Ni(III) ion contamination from the NiFe stainless steel autoclave used in the synthesis of the nanocrystals. Furthermore, 400 and 500 ºC annealed nanocrystal films show small positive photovoltage signals that are attributed to a 0.8 to 2.9 micrometer wide depletion layer with a +0.2 V potential barrier. The ability of SPS to detect photoactive defects in metal oxide particles at concentrations of 0.13 % (atom) is relevant to their use in photocatalytic solar energy conversion.

INTRODUCTION Many properties of refractive metal oxides, incl. their electrical conductivity, optical properties and photocatalytic performance are controlled by lattice defects.1-6 For example, while SrTiO3 and TiO2 are colorless electrical insulators, annealing them under oxygen deficient conditions turns them into deeply colored electric conductors, due to the creation of oxygen vacancy defects.

7-12

The defect chemistry of

SrTiO3 in particular has attracted interest recently in the context of the photocatalytic properties of that material. 13,14-25 It was shown that removing Ti3+ defects through aliovalent dopants increases the overall 2 ACS Paragon Plus Environment

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water splitting activity of this photocatalyst 300 times over the non-doped material.14, 26-29 Of the many techniques for characterizing defects, surface photovoltage spectroscopy (SPS) stands out in terms of ease of application and because of its high sensitivity.30-35 In SPS, changes of the contact potential difference (CPD) of a sample film is measured under illumination with light of variable wavelength. The corresponding photovoltage provides information about the majority carrier type and charge separation properties of semiconductors.22,

36-40

In that way, SPS is complementary to photoelectrochemical

measurements, which provide the open circuit potential of illuminated semiconductor contacts.

41

Recently, we employed SPS to study the effect of metal ion doping on the photophysical behavior of SrTiO3 nanocrystals and their ability to catalyze the hydrogen evolution reaction under visible light.42-44 Transition metal ions produce sub-bandgap photovoltage signals, depending on their energetic position in the band gap and depending on their reactivity. That study also revealed the presence of sub-bandgap defects in non-doped SrTiO3 nanocrystals,

42

which are responsible for the weak photocatalytic H2

evolution activity from aqueous methanol under visible light (>400 nm). Here we extend that work to look at the dependence of the intrinsic defect signals in hydrothermally synthesized SrTiO3 nanocrystals as a function of annealing temperature. We make a first attempt to quantify the defect concentration using added Rh-doped SrTiO3 nanocrystals as a reference. And we use XPS and EPR spectroscopy to determine the origin of the defect photovoltage signals. This work reveals annealing temperature as a key parameter for the defect chemistry of SrTiO3 nanocrystals and it establishes SPS as a suitable method to observe sub-bandgap defects at concentrations down to 0.13% (atom). Finally, we show that SrTiO3 nanocrystals made by hydrothermal synthesis incorporate Ni(III) impurities originating from the nickel-iron steel autoclave.



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EXPERIMENTAL SECTION Strontium hydroxide octahydrate (99%, Alfa Aesar), titanium(IV) oxide (Aeroxide P25, Acros Organics), potassium hydroxide (≥ 85%, Sigma-Aldrich), rhodium(III) chloride hydrate (38-41% Rh, Strem Chemicals), and nickel(II) nitrate hexahydrate (Acros Organics, > 99%), were used as received. Water was purified to 18 MΩ cm resistivity using a Nano-pure system. Nanostructured SrTiO3 (STO) particles were synthesized hydrothermally at 150 C. 0.598g (2.25 mmol) Sr(OH)2·8H2O, 0.165g P25 TiO2 (2.06 mmol) and 1.262g (22.5 mmol) KOH were mixed in 23 mL nanopure water, using 9.2 % excess of Sr(OH)2·8H2O to effectively eliminate TiO2 impurity in the final product. The solution was then transferred to 45 mL PTFE lined PARR Instrument Company autoclave (general purpose acid digestion vessel model 4744) after ultrasonicating and stirring for 10 minutes. The autoclave was heated in oven at 423 K for 72 hours. After that, the precipitate was washed with 0.6 M HCl acid to remove residual SrCO3 and possible excess Sr(OH)2, followed by washing with ultrapure water until neutral PH. After drying in vacuum overnight, the yield of the product was approximately 290 mg, 77% when 2.06 mmol starting material of Ti were used. Further calcination step was performed at 100, 200, 300, 400 and 500 C respectively for 2h in air for alternative use. Rh (3%)doped STO was synthesized following the same method by adding 14.1 mg (0.0675 mmol) of RhCl3·xH2O as the dopant precursor, following the published method.22 Films with different thicknesses were prepared by drop-coating SrTiO3 suspended in water at different concentrations on FTO substrate. In detail, FTO glasses were first cut into pieces and pre-cleaned by ultrasonication in acetone, methanol, isopropanol and water respectively for every 20 minutes. Then about 0.1 mL of as-prepared solution were drop coated onto the substrate using a syringe. The area of films was fixed to 0.8×0.8 cm2 using a tape template. After drying in air overnight, the films were annealed at 373, 473, 573, 673 and 773K respectively for 2 h. The STO-FTO pieces were cut into 1.0×1.0 cm2. 4 ACS Paragon Plus Environment

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Films prepared by a mixture with Rh-doped STO and pure STO were used to quantify the defect concentration by using the photovoltage signal from the Rh impurity. In detail, Rh (3.0%) doped STO were synthesized according to reported method, followed by creating a suspension of 1mg/mL kept in continuous ultrasonication.22 After 6h, different amount of Rh-STO suspension and STO powders were mixed in pure water, forming 10 mg/mL solutions with Rh-STO present as 0.5, 1.0, 2.0, 5.0 and 10.0 wt%. These solutions were then ultrasonicated for another 6 hours to create uniform suspension. Films were prepared by drop coating using the same method and then annealed at 773K for 2h. Powder XRD scans were performed on Scintag XRD, at a wavelength of  = 0.154 nm with 2 mm tube slit divergence, 4 mm scatter, 0.5 mm column scatter and 0.2 mm receiving widths. Crystal domain sizes were calculated from the Scherrer equation, D= kλ/β(θ)cos θ, where D is the average grain diameter, k the constant (shape factor about 0.9), λ the x-ray wavelength, β(θ) the full width at half maximum (FWHM) of the diffraction line, and θ the diffraction angle.72 Scanning electron microscopy (SEM) measurements were taken with XL30 FEG (Philips) on films pre-sputtered with Au nanoparticles. TEM images were recorded on a Philips CM-120 at 80 kV accelerating voltage. UV/vis diffuse reflectance spectra were collected on Thermo Scientific Evolution 220 Spectrometer equipped with integrated sphere. Dried thick films were used for testing by dropping slurry of catalysts on a white filter paper. Surface photovoltage (SPV) measurements were conducted in vacuum chamber with gold Kelvin probe and quartz window. As-prepared films were placed into vacuum chamber and illuminated by monochromatic light from a 150 W Xe lamp filtered through an Oriel Cornerstone 130 monochromator (1-10 mW cm-2). Contact potential difference (CPD) signals were recorded and corrected for drift effects by subtracting a dark scan. A Dektak 150 profilometer was used to measure film thicknesses. The reflectance data were converted to the Kubelka-Munk function as f(R) = (1-R)2/(2R) for scattering correction. XPS spectra were acquired on a Kratos Axis Ultra X-ray photoelectron spectrometer with the analyzer lens in hybrid mode



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and using samples drop-coated on FTO substrates, as described above. High resolution scans were performed using a monochromatic aluminum anode with an operating current of 10 mA and voltage of 6 kV using a step size of 0.1 eV, a pass energy of 40 eV, and a pressure range between 1 and 3 × 10–8 Torr. The binding energies for all spectra were referenced to the adventitious C1s core level at 284.7 eV. The electron paramagnetic resonance (EPR) spectroscopy were performed in the CalEPR center in University of California, Davis. Continuous wave EPR spectra were recorded in the Bruker Biospin EleXsys E500 spectrometer equipped with a super high Q resonator (ER4122SHQE). Cryogenic temperature was achieved by using an ESR900 liquid helium cryostat with a temperature controller (Oxford Instrument ITC503) and a gas flow controller. CW EPR spectra were recorded under slow-passage, non-saturating conditions with the following spectrometer settings: conversion time = 40 ms, modulation amplitude = 0.5 mT, modulation frequency = 100 kHz. Sample powders for EPR were prepared as described above, followed by annealing in air for 2h under 100℃, 200℃, 300℃, 400℃, and 500℃, or simply by storage in air at 25℃.


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RESULTS Figure 1A shows the TEM image of SrTiO3 nanocrystals obtained by hydrothermal reaction of TiO2 and Sr(OH2) in water. The nanocrystals are cubic with an average size of 40±11 nm (Fig. 1B). XRD patterns in Fig. S1A confirm the perovskite structure type without any discernible crystalline impurities.

Figure 1. (A) TEM image and (B) crystal size distribution of non-annealed as-prepared SrTiO3 nanocrystals. (C) Top view and (D) cross-sectional view of SEM for as-prepared SrTiO3 films on FTO. Same for 300 C annealed (E and F) and 500 C annealed films (G and H).



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The grain size is calculated as 43 nm from the Scherrer equation using the width of the (110) diffraction peak (Fig. S1B). For surface photovoltage spectroscopy (SPS) measurements, films of the nanocrystals on FTO substrates were prepared by drop coating of the aqueous particle suspensions, followed by drying and thermal annealing at temperatures between 25 and 500 C. Figures 1C-H display SEM images of selected films. The films are about 1 μm thick and porous. No changes in the morphology of the individual crystals are observed below 300 ºC, but marked densification of the films occurs after heating to 500 ºC. According to XRD, the crystal structure of SrTiO3 is not affected by the heat treatment, but the average grain size increases to 48 nm and 53 nm for the 300 and 500 ºC annealed samples, based on the Scherrer analysis in Fig. S1C. TEM images for the high temperature annealed nanoparticles are shown in Fig. S2. They confirm that particle agglomeration increases in the 400-500 ºC temperature interval but that the cube morphology of the individual SrTiO3 nanocrystals is maintained. This finding agrees with the literature, which states that heating above 600 C is generally required for grain growth and particle fusion.45 For SPS measurements, sample films on FTO substrates were mounted underneath a vibrating Kelvin probe, placed under vacuum (10-4 mBar), and illuminated with monochromatic light from a Xe lamp. Fig. 2A displays the surface photovoltage spectrum of a SrTiO3 film (8.9 μm) annealed at 300 C for 2h together with its UV-vis diffuse reflectance spectrum (DRS). A negative photovoltage develops at photon energies above 2.0 eV. This photovoltage reaches its maximum value of –2.9 V at 3.4 eV and then decays to a residual value of about –1.3 V at 4.75 eV. This decline is attributed to the reduced light penetration depth into the sample and to the diminishing light intensity from the Xe light source (see spectrum in Fig. S3). Based on earlier SPS measurements of films of Fe2O3, 46 Rh:SrTiO3, 22 BiVO4, 37 Al:SrTiO3,

44

and CuBi2O4,

47

the negative photovoltage signal in SrTiO3 corresponds to the diffusive

transfer of majority carriers (electrons) into the FTO substrate and to the trapping of minority charge 8 ACS Paragon Plus Environment

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carriers in states at the particle surfaces. These processes are shown in the energy diagram in Figure 2B. Charge transfer from the SrTiO3 conduction band into the FTO is driven thermodynamically by the energy offset between the SrTiO3 conduction band and the Fermi energy in FTO. At neutral pH, ECB(Donor)EF(Acceptor) can be estimated as –1.7 eV, which constitutes the theoretical maximum photovoltage from electron injection into the FTO substrate. Another contribution to the observed –2.9 V photovoltage

Figure 2. (A) SPV and UV-vis absorption spectra of as-prepared SrTiO3 film annealed at 300 C. The photovoltage is the illumination-induced change of the contact potential difference (CPD) versus the gold Kelvin probe. (B) Energy diagram of defect presented SrTiO3 at pH=7 on electrochemical and vacuum



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scales and photochemical charge transfer pathways contributing to the measured surface photovoltage. D1 and D2 are observed sub-bandgap defects. Surface states are attributed to reduced carbon. therefore has to come from trapping of holes at surface and defect states, as shown schematically in Fig. 2B. A similar surface charge trapping effect had been previously identified for films of Fe2O3 particles and CuWO4 particles.

46, 48

For the SrTiO3 films studied here, surface trap states may originate from

adsorbed H2O, oxygen or carbon species, or from Ti3+ions. According to the Kubelka-Munk absorption spectrum in Figure 2A, the bandgap (EG) of the SrTiO3 particle film is 3.2 eV, which agrees well with the literature value for this material. However, most of the photovoltage in Fig. 2A occurs at lower photon energy. This sub-bandgap voltage contribution is attributed to excitation of photoactive defect states in the sample.49

In surface photovoltage spectra, separate physical excitation phenomena can be

distinguished as changes of the photovoltage slope.50 On this basis, the spectrum in Figure 2A contains three separate excitation pathways. The first excitation involves sub-bandgap states at 2.0 eV (region I) and contributes almost –2 V to the photovoltage. Excitation of a second mid-bandgap state at around 2.7 eV (region II) contributes another –0.6 V, before direct bandgap excitation of SrTiO3 is observed at approximately 3.1 eV (region III) as a minor voltage of –0.3 V. A similar bandgap photovoltage signal had been previously observed in SrTiO3 by Zhang et al.51 Based on the sub-bandgap photovoltage features, SrTiO3 contains at least two types of defects, D1 and D2. Based on their excitation energies, one possible assignment of these states is shown in the energy diagram in Fig. 2B. D1 might correspond to states approximately 2 eV above the valence band, and D2 might correspond to states approximately 2.7 eV below the conduction band. Both states must generate mobile charge carriers under illumination in order to produce a photovoltage. In order to more comprehensively evaluate the defect photovoltage signal in SrTiO3, surface photovoltage spectra were recorded as a function of annealing temperature and film thickness. This data 10 ACS Paragon Plus Environment

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is shown in separate plots in Fig. 3A-F. All spectra, except for the thinnest films in Fig. 3F, show negative photovoltage signals. Films that were annealed at 25, 100, 200, and 300ºC have significant sub-bandgap contributions, but these contributions mostly disappear at 400 ºC and above. For these highly annealed films, the major photovoltage results from SrTiO3 excitation across the bandgap. For most films, the observed photovoltage remains below 4.6 V, the theoretical maximum of the energy configuration shown in Figure 2B. The higher voltage (5.3 V) seen in the 200 ºC annealed 8.9 m film in C is tentatively attributed to a weak ferroelectric effect in SrTiO3 nanocrystals, as previously noted in the literature.52-55 Further investigations of this effect are underway.



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Figure 3. SPV spectra of as-prepared SrTiO3 films on FTO with different film thicknesses after annealing at (A) 25C, (B) 100C, (C) 200C, (D) 300C, (E) 400C, (F) 500 C. The low photovoltage of the 1.3 m thick film in (A) is attributed to local variations in film thickness and packing effects. The relative contributions from the sub-bandgap states D1 and D2 and from the bandgap excitation Eg are compared in Fig. 4A (see also Fig. S4 and numerical data in Table S1). It can be seen that for films annealed in the 25-300ºC temperature interval, defect D1 contributes nearly 50% to the overall photovoltage. This contribution increases slightly with increasing annealing temperature. At 300 ºC, a new defect, D2, is observed and contributes about 15% to the overall photovoltage. Annealing at 500 ºC reduces the contributions of both D1 and D2 to less than 10% of the total photovoltage. This shows that the relative defect concentrations in the samples are a strong function of annealing temperature.


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Figure 4. (A) Relative photovoltage contributions of the 8.87±1.26 μm thick films at different temperatures. The spectra are shown in Fig. S4 and the data is listed in Table S1. (B) Plot of photovoltage contributions versus film thickness for the 300 ºC annealed films. (C) Schematic illustration of charge transport before and after annealing. (D) Band bending and space charge layer (w) in 500 oC annealed film. As can be seen from Figure 3, for most of the films, the size of the photovoltage signal correlates with the film thickness (for exceptions see Figure 3 caption). A representative trend is shown in Fig. 4B for the 300 ºC annealed sample (similar plots for non-annealed and 500º annealed samples are shown in Figure S5). Both defect and bandgap induced photovoltage increase until the films reach 5.4 m and then decline. This trend has been previously observed for BiVO4 films and can be attributed to the balance between the finite light absorption depth and electron diffusion length in metal oxides.29, 37, 48 While the photovoltage of thin films is limited by the absorption of photons, the photovoltage of thick films is limited by electron transport. Charge transport also strongly depends on the contact between SrTiO3 particles in the film. As a result, the 500 ºC annealed films do not show a photovoltage reduction even as their thickness approaches 9 m (Figure S5). This is due to sintering of the particulate films and the reduction in grain boundaries between SrTiO3 particles (SEM images in Fig. 1 and schematic model in Fig. 4C). As the electric connection within the films and with the FTO surface improves, charge transfer becomes more efficient. Due to poor electrical contact, the photovoltage trend of non-annealed SrTiO3 films (Fig.S5) mostly resembles that of the 300ºC annealed film, with a photovoltage decline observed for the thickest films. An interesting phenomenon is observed in thin films of the 400-500 ºC annealed samples (Fig.3E, F). Here, a positive photovoltage of up to +0.2 V forms under illumination between 2.0 and 3.0 eV, followed by the familiar negative photovoltage under 3.1 eV bandgap excitation. A similar voltage inversion was 13 ACS Paragon Plus Environment


observed in previous photovoltage studies on CdSe nanocrystal films polymers.

57-58

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56

and semiconducting organic

In these cases, inversion of the photovoltage sign is due to the formation of a depletion

layer at the sample-substrate interface. This is shown in Fig.4D. The depletion layer results from the electrochemical equilibrium between SrTiO3 and the FTO substrate. Based on the size of the positive voltage, the potential drop across the depletion layer is +0.2V, and based on the thickness dependence of the photovoltage, the space charge layer is 2.9 m wide in the 500ºC annealed film, 0.8 m in the 400 ºC annealed film, and negligible for the other films. The associated electric field is strong enough to repel photoelectrons in defect states, but not enough to prevent conduction band electrons generated under bandgap excitation from injecting into the FTO substrate.


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Figure 5. (A) SPV spectra of SrTiO3 films with variable amount of Rh (3.0%)-SrTiO3. The Rh:SrTiO3 and SrTiO3 nanocrystals were mixed in water and drop coated onto FTO substrates, followed by annealing at 500 C to eliminate residual sub-bandgap defects in SrTiO3. (B) Sub-bandgap photovoltage at 2.74 eV versus the percentage of Rh (3.0%)-SrTiO3 (obtained via Gaussian fitting, see Figure S6). (C) Total defect concentration (atom % with regard to Ti) in SrTiO3 nanocrystal films versus annealing temperature.

In order to estimate the absolute defect concentration in the samples, Rh(3%)-doped SrTiO3 nanocrystals were applied as a photovoltage label. As shown previously, Rh-doped SrTiO3 nanocrystal films produce a strong photovoltage signal at 2.87 eV that results from excitation of the Rh3+ ions in the lattice of the nanocrystals.22, 59 Fig. 5A shows SPV spectra on sample films containing known amounts of Rh:SrTiO3 mixed together with regular SrTiO3 nanocrystals. A Rh:SrTiO3 concentration-dependent photovoltage signal at 2.87 eV can be clearly identified. The defect signal concentration photovoltage trend in Figure 5B was obtained by modeling the rhodium photovoltage contribution using Gaussian fits, as shown in Figure S6. The resulting trend line in Fig 5B is nearly linear with regard to the Rh concentration. Deviations at the lowest Rh:SrTiO3 concentration are attributed to residual sub-bandgap photovoltage from the SrTiO3 sample. Using the correlation in Fig. 5B, the concentration of photochemically active defects in SrTiO3 nanocrystal films can be estimated for each temperature. This is shown in Figure 5C. For as prepared nanocrystals, the concentration of defects is 0.47 atom% (based on Ti). This value increases to 1.10 atom% in the 300 ºC annealed film and then falls to 0.13 atom% in the 400ºC annealed one. In order to identify the chemical origin of the defects, X-ray photoelectron spectra were collected for non-annealed, 300C and 500 C annealed SrTiO3 films. The corresponding data is shown in Figures 6 and S7 with fitting parameters given in Table S2. Overall, the spectra resemble those in previous reports 15 ACS Paragon Plus Environment


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for SrTiO3. 11, 51 In the Ti 2p XPS spectral region (Fig. 6A), two symmetrical peaks at 463.6 eV (Ti 2p1/2) and 457.9 eV (Ti 2p3/2) demonstrate the presence of Ti4+ without any sign for Ti3+ states, which would appear as doublet shifted to lower energy. 51 The two peaks at 132.7 and 134.6 eV in Fig. 6B can be clearly assigned to divalent Sr2+. The O 1s spectra (Fig. 6C) show evidence for several species. The de-convoluted peaks at 528.9 and 529.8 eV binding energy are assigned to bridging O2- ions on the surface and lattice O2− respectively.60 The additional peak at 531.9 eV has been previously associated with absorbed molecular oxygen and oxygen-vacancy species in the lattice,23, 51 but might also have contributions from O associated with organic carbon. Signals in the carbon energy region (Fig. S7b) can be assigned to elemental carbon, C-O, C=O and satellite peaks caused by the X-ray source.

11, 61-62

Because carbon

compounds are common contaminants, no clear relation of these species with annealing conditions of the SrTiO3 samples can be established.

Figure 6. High resolution XPS spectra of (A) Ti 2p, (B) Sr 3d, (C) O 1s on SrTiO3 films under different annealing temperatures. Detailed fitting parameters are shown in Table S2. 16 ACS Paragon Plus Environment

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Because of the intrinsic sensitivity limits, XPS is not able to resolve the impurities responsible for the sub-bandgap surface photovoltage signals. For this reason, EPR spectra at 77K were recorded for hydrothermally synthesized and 100 - 500 C annealed SrTiO3 nanoparticle powders. The spectra in Figure 7 contain distinct peaks at g values of 2.35, 2.19, 2.14, 2.06, 2.01, and 1.99 with the relative signal intensities being a strong function of the annealing temperatures. Signal assignments are given in Table 1.



Figure 7. A) EPR spectra at 77 K of non-annealed (25 C), 100, 200, 300, 400 and 500 C annealed SrTiO3 nanoparticle powders, incl. magnified graphs in the inset. B) variation of the signals with annealing temperature. Conditions: temperature = 77 K, microwave power = 0.2 mW.


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Table 1. EPR signal assignments Peak (g-value)

Assignment

2.35, 2.19 and 2.14, 2.35 (77 K)

Ni3+ in octahedral environment in SrTiO3 This work

2.01 (77 K)

Superoxide on SrTiO3

This work

1.99 (77 K)

Ti3+ in SrTiO3

This work

2.05-2.06 (77 K)

Ni3+

This work

2.110, 2.180, 2.213

Ni3+ in SrTiO3

63

2.003 (77 K)

O2– in Cr:SrTiO3

64

2.003 (77 K)

O2– in Mn:SrTiO3

65

2.026 and 2.010 (90 K)

O2– on TiO2

6

2.008 (77 K)

O3– on SrTiO3

66

2.002 (77 K)

Ti3+ in SrTiO3

66

1.94 (77K)

Ti3+ in SrTiO3

65

1.99 (77 K)

Ti3+ in N-doped SrTiO3

67

1.99 (90 K)

Ti3+ in TiO2

6

1.975 (120 K)

Ti3+ in

SrTiO3

68

2.002 (120K)

O2– on SrTiO3

68

1.975 and 1.944 (75 K)

Ti3+ in TiO2

69

in spin aggregates

Reference

Based on the literature, the signal at g = 2.01 is most likely from adsorbed superoxide (O2–) on the particle surface. Superoxide chemisorption can result from reaction of O2 with surface Ti3+.64, 69 The peak at g = 1.99 is assigned to Ti3+ states, in accordance with previous EPR spectra on various SrTiO3 and TiO2 samples (see Table 1). The Ti3+ ions typically accompany the formation of oxygen vacancies in SrTiO3 29 when samples are treated at higher temperature, under oxygen deficient conditions, or when reduced chemically. 64 The signals at 2.14, 2.19 and 2.35 are assigned to Ni3+ species in an octahedral environment, similar to what was reported by Davidson et al. for Ni3+-doped SrTiO3.63 The presence of nickel in our samples is surprising because the synthesis of the SrTiO3 nanoparticles did not involve added nickel compounds. We speculated that the nickel contamination might stem from either the P-25 TiO2 precursor or from the nickel steel material of the autoclave. The latter hypothesis was



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verified by recording EPR spectra for as-purchased TiO2, for TiO2 heated in two different steel autoclaves (PARR Instrument Company), and on SrTiO3 deliberately made using 0.1% and 3% (atom) Ni(NO3)2·6 H2O, using a previously reported hydrothermal method.42 The EPR spectra are shown in Figure S8. Clearly, no Ni3+ signal is present in commercial P25 TiO2, but nickel-related EPR signals at 2.06-2.08 appear in P25 TiO2 particles heated in the small autoclave, and to a lesser extent in the big autoclave. This establishes the small autoclave as the source of the nickel contamination. The new EPR signals at 2.14, 2.19, and 2.35 in deliberately prepared 0.1% (atom) Ni doped SrTiO3 match those of the 400 oC annealed sample. That confirms the presence of nickel in hydrothermally synthesized SrTiO3. When the added Ni2+ salt amount is 3% (atom), these signals become weaker and a new group of signals arises at 2.06-2.08. These signals are attributed to Ni3+ clusters inside of the SrTiO3. The majority of the Ni in 3% atom doped SrTiO3 is expected to be diamagnetic, EPR silent, Ni2+ ions. Of these species, Ni3+ and Ti3+, appear responsible for the sub-bandgap signals observed in the surface photovoltage spectra. In our previous photovoltage study, Ni2+/3+ ions in SrTiO3 were assigned an energy state 2.75 eV below the conduction band of SrTiO3. 42 This coincides well with the 2.7 eV D2 signal in the SPS data in Figure 2. The appearance of the D2 photovoltage at 300 C and above is attributed to the air-oxidation of EPR silent Ni2+ to EPR active Ni3+ at these higher temperatures. On the basis of the SPS data, the concentration of Ni3+ is

Surface Photovoltage Spectroscopy Observes Sub-Bandgap

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