Hydrothermal Synthesis of a Valence State Constant High-Entropy Perovskite Sr(TiZrHfVNb)O3 with Improved Photoresponsiveness

A vanadium ion valence state constant high-entropy perovskite system was synthesized using the hydrothermal method with a trivalent vanadium ion as the vanadium source. The B-site of the perovskite crystal lattice was loaded with five atoms in equal proportions. We tried to synthesize the Sr(TiZrHfVNb)O3 high-entropy system using different methods. However, the valence state of the vanadium ion could only be kept constant using the hydrothermal process in the valence balanced high-entropy composition system. There was significant vanadium element segregation and second phase in the Sr(TiZrHfVNb)O3 system prepared using the solid-state reaction process. Also, obvious vanadium ion valence state ascending from V3+ to V5+ appeared in this high-entropy system with an increase in calcination temperature. Inconspicuous vanadium element segregation appeared at 900 °C, the significant segregation phenomenon and second phase appeared at 1200 °C, and the particle size increased with the temperature. This meant that the high-entropy value could not only stabilize the crystal phase, but also stabilize the ionic valence state. Moreover, the constant trivalent vanadium ion valence state could provide coordinated performance with a wide optical response range and a low band gap for the high-entropy system. This suggests that the system might grow a potential ceramic material for optical applications.

Perovskite is a simple-structure crystal that has a high tolerance factor [14].It is widely researched as a photoelectric material for photovoltaic devices, photo catalysis, and so on [15].Inorganic perovskite (SrTiO 3 , BaTiO 3 , etc.) has its own advantages such as high stability and high-temperature resistance [16].Therefore, it is one of the most popular entropy-engineered ceramics [17].Usually, bandgap widths of inorganic perovskite are often greater than 3 eV, resulting in a response in the UV wavelength range only [18].Many investigations have improved the light absorption range using element doping, but this is not ideal in practical applications [19].The high-entropy component design can reconstruct the electronic structure and modulate the band gap, which is a feasible way to broaden the light absorption range of perovskite materials.For entropy engineering of perovskite, the atomic radius of the design element can be selected in a large range.Even if cube perovskite fails to form, it can be distorted into tetragonal or quadrature phase structures.
So far, the conventional preparation methods for high-entropy ceramic perovskite oxides include the solid-state reaction [20,21], sol-gel [22,23], or co-precipitation processing [24].These methods tend to make the valance states of some elements change during the reaction, such as vanadium, which disorganizes the material structure [25].In contrast, a hydrothermal reaction provides an approach to obtain a sample at a lower temperature.Vanadium-based materials are common optical function materials such as SrVO 3 , Sr 3 (VO 4 ) 2 , and SrV 6 O 11 , etc. [26].Previous researchers have already synthesized a few high-entropy systems with vanadium elements [27,28].The introduction of a vanadium element not only effectively inhibits grain growth and improves the mechanical properties [29], but also eliminates Nb segregation and significantly improves the densification process [30].However, the valence of vanadium is hard to control, which is detrimental to the preparation of HECs.
In the present work, we design a high perovskite system Sr(TiZrHfVNb)O 3 .The hydrothermal process is employed to synthesize this system to improve the photoresponsiveness of perovskite.A somewhat surprising experimental result is that this system could only be synthesized using the hydrothermal method.The most commonly used ceramic powder calcination process cannot achieve the entropy stability of the perovskite phase.This advances our understanding of the relationship between the entropy stability of the phase structure and the ionic valence state of the designed high-entropy elements.

Materials and Preparation
For the synthesis of high-entropy ceramic powder using the one-step hydrothermal method, liquid raw material TiCl 4 and powder raw materials ZrCl 4 , HfCl 4 , VCl 3 , FeCl 3 , NbCl 5 were purchased from Shanghai Macklin Biochemical Technology Co., Ltd.(Shanghai, China).These chlorides with the most stable valence state of the elements were used as their source, except for vanadium(III) trichloride which was used as vanadium source.SrCl 2 •6H 2 O and NaOH were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China which were used as strontium source and pH regulator, respectively.All raw materials were analytically pure reagents.The raw materials with ABO 3 atomic ratio were mixed.The A-site was strontium, and the B-site was designed to contain five equimolar elements according to Sr(Ti 0.2 Zr 0.2 Hf 0.2 V 0.2 Fe 0.2 )O 3 and Sr(Ti 0.2 Zr 0.2 Hf 0.2 V 0.2 Nb 0.2 )O 3 .The powder was mixed with deionized water in hydrothermal synthesis reactor and stirring for 10 min to form a suspension.Then, the liquid TiCl 4 was dropwise added into the suspension.Then, 3 mol/L NaOH solution was employed to keep the suspension a high alkaline environment pH = 11.The mixture was then transferred to a homogeneous reactor and heated at 180 • C for 12 h at 20 r/min.After the reaction, the sediment removed from the reactor was repeatedly and alternately cleaned with ethanol and deionized water by centrifugation and suction filtration until the pH value of filtrate reached 7. The high-entropy ceramic powder samples were obtained after drying the wet powder.To investigate the change of element valence state, the powder samples were calcined at different temperatures with a heating rate of 5 • C/min for 12 h.The photographs of the hydrothermal and calcined high-entropy samples are shown in Figure S1 in Supplementary Materials.
For the preparation of high-entropy ceramic powder by solid-state reaction process, submicron analytical reagents TiO 2 , ZrO 2 , HfO 2 , V 2 O 3 , Nb 2 O 5 and SrCO 3 were used as raw materials which were purchased from Sinopharm Chemical Reagent Co., Ltd., China.The mixed powder with Sr(Ti 0.2 Zr 0.2 Hf 0.2 V 0.2 Nb 0.2 )O 3 atomic ratio was ball-milled with ethanol for 8 h.The mass ratio of the grinding media to the mixed powder was 2:1.The obtained powder was dried at 80 • C for 24 h, and then reacted at 1200 • C in air for 2 h with a heating rate of 5 • C/min.

Characterization
X-ray diffraction (XRD) using Cu K α radiation (X'Pert PRO MPD, PANalytical B•V., Almelo, The Netherlands) was utilized for characterizing the phase composition of perovskite powder specimens.The micromorphology of high-entropy powder was observed using Scanning electron microscope (SEM; MERLIN VP Compact, Carl Zeiss, Jena, Germany).Thermoelectric field emission multi-purpose transmission electron microscope (JEM-F200, JEOL Ltd., Akishima, Japan) with Oxford EDS detector techniques was employed to observe the microstructures of the high-entropy perovskite powder.An X-ray photoelectron spectroscopy (XPS) microprobe using a monochromated micro-focused Al Kα radiographic source with a spot size of 200-800 µm (ESCALAB 250Xi, Thermo Fisher Scientific, Waltham, MA, USA) was employed to analyze the change in the valence state at different temperatures.The absorbance of the specimens was characterized using a UV-VIS-NIR spectrophotometer (Cary 7000, Agilent, Santa Clara, CA, USA).

Results and Discussion
The hydrothermal process is a common method used for the synthesis of inorganic perovskite materials.However, it is rarely used in the preparation of high-entropy ceramic powders.Most high-entropy powders are still prepared using a solid-state reaction.In this study, we synthesized the high-entropy system Sr(TiZrHfVNb)O 3 using the hydrothermal process and calcined the materials at different temperatures.A system with the same composition was also prepared using the solid-state reaction at 1200 • C. The XRD spectra of the high-entropy samples are shown in Figure 1.It can be seen in Figure 1a that the hydrothermal sample obtained a good perovskite structure.Then, an insignificant second phase Sr 5 (VO 4 ) 3 R with an apatite structure appeared in the samples calcined up to 500 • C, and stood out at 800 • C. The second phase significantly transfers to Sr 3 (VO 4 ) 2 with a commonly sintering temperature of about 1150 • C when the temperature reaches 1200 • C. It seems that Sr 5 (VO 4 ) 3 R decomposes at 1200 • C.However, the second phase of the solidstate reaction sample is Sr 5 (VO 4 ) 3 R at the same temperature 1200 • C, which is shown in Figure 1b.The XRD results indicate that the high-entropy Sr(TiZrHfVNb)O 3 system can only be obtained using hydrothermal synthesis, rather than the solid-state reaction method.Thus, the hydrothermal process reveals a remarkable advantage.And, we believe that the formation of high-entropy structures is closely related to the stability of the valence states of the elements.
Materials 2024, 17, x FOR PEER REVIEW 3 of 13 mixed powder with Sr(Ti0.2Zr0.2Hf0.2V0.2Nb0.2)O3atomic ratio was ball-milled with ethanol for 8 h.The mass ratio of the grinding media to the mixed powder was 2:1.The obtained powder was dried at 80 °C for 24 h, and then reacted at 1200 °C in air for 2 h with a heating rate of 5 °C/min.

Characterization
X-ray diffraction (XRD) using Cu K α radiation (X'Pert PRO MPD, PANalytical B⋅V., Almelo, The Netherlands) was utilized for characterizing the phase composition of perovskite powder specimens.The micromorphology of high-entropy powder was observed using Scanning electron microscope (SEM; MERLIN VP Compact, Carl Zeiss, Jena, Germany).Thermoelectric field emission multi-purpose transmission electron microscope (JEM-F200, JEOL Ltd., Akishima, Japan) with Oxford EDS detector techniques was employed to observe the microstructures of the high-entropy perovskite powder.An X-ray photoelectron spectroscopy (XPS) microprobe using a monochromated micro-focused Al Kα radiographic source with a spot size of 200-800 µm (ESCALAB 250Xi, Thermo Fisher Scientific, Waltham, MA, USA) was employed to analyze the change in the valence state at different temperatures.The absorbance of the specimens was characterized using a UV-VIS-NIR spectrophotometer (Cary 7000, Agilent, Santa Clara, CA, USA).

Results and Discussion
The hydrothermal process is a common method used for the synthesis of inorganic perovskite materials.However, it is rarely used in the preparation of high-entropy ceramic powders.Most high-entropy powders are still prepared using a solid-state reaction.In this study, we synthesized the high-entropy system Sr(TiZrHfVNb)O3 using the hydrothermal process and calcined the materials at different temperatures.A system with the same composition was also prepared using the solid-state reaction at 1200 °C.The XRD spectra of the high-entropy samples are shown in Figure 1.It can be seen in Figure 1a that the hydrothermal sample obtained a good perovskite structure.Then, an insignificant second phase Sr5(VO4)3R with an apatite structure appeared in the samples calcined up to 500 °C, and stood out at 800 °C.The second phase significantly transfers to Sr3(VO4)2 with a commonly sintering temperature of about 1150 °C when the temperature reaches 1200 °C.It seems that Sr5(VO4)3R decomposes at 1200 °C.However, the second phase of the solidstate reaction sample is Sr5(VO4)3R at the same temperature 1200 °C, which is shown in Figure 1b.The XRD results indicate that the high-entropy Sr(TiZrHfVNb)O3 system can only be obtained using hydrothermal synthesis, rather than the solid-state reaction method.Thus, the hydrothermal process reveals a remarkable advantage.And, we believe that the formation of high-entropy structures is closely related to the stability of the valence states of the elements.Figure 2 shows the micromorphology of the high-entropy Sr(TiZrHfVNb)O 3 samples with the EDS.The homogeneously distributed elements can be seen clearly in the hydrothermal and calcined 500~700 • C samples (Figures S2-S4).And, the strength of the vanadium element was weakened very slightly at 800~900 • C.Then, significant vanadium segregation appears at 1200 • C, and the strontium and oxygen elements are also clearly present in the segregation position.This is roughly consistent with the XRD results, which show a large amount of phase separation at 1200 • C.Moreover, the above phase separation phenomenon is more obviously observed in the samples prepared using the solid-state reaction (Figure S5).This indicates that the vanadium precipitates from the perovskite lattice and is combined with a portion of strontium.Furthermore, the other elements are still homogeneously distributed in the same position.This proves that other elements can still form perovskite crystals, even without the involvement of vanadium.In order to explore the reason for the vanadium segregation, the valence states of the designed high-entropy elements are systematically analyzed using XPS.The fitted core level spectrums of Ti2p, Zr3d, Hf4f, Nb3d and O1s from the high-entropy samples before and after calcining at 1200 • C are shown in Figure 3.The assignment of the XPS peaks of these elements is listed in Table 1.It can be seen that tetravalent and pentavalent elements keep their initial valence states in the hydrothermal samples.However, all the tetravalent elements Ti, Zr and Hf with a small percentage of trivalent peaks were detected in the high-entropy sample calcined at 1200 • C. The Nb remains constant at pentavalence.Moreover, the original percentage balance of the lattice oxygen and the vacancy oxygen is broken.The concentration of oxygen defects increased to 81.2%.The fitted core level spectrums of different elements calcined at 500~900 • C are shown in Figures S6-S10.Also, the fitted core level spectrums of V2p from the high-entropy samples before and after calcining at different temperatures are shown in Figure 3.The assignment of XPS peaks of V2p is listed in Table 2.It is clear that the vanadium(III) trichloride (VCl 3 ) is used as a vanadium source in the synthesis process.The valence state of the vanadium element almost remains unchanged after the hydrothermal process.Most importantly, the percentage of V 3+ gradually decreases from 60.9% (500 • C), 49.3% (600 • C), 25.7% (700 • C), 20.2% (800 • C) to 6.9% (900 • C), and then, all V 3+ ascends to V 4+ and V 5+ at 1200 • C. The fundamental reason for the phase separation of the calcined samples is that the oxygen partial pressure is high enough to oxidize the V element.This indicates that vanadium will be gradually oxidized to a higher stable valence state with an increase in temperature.The valence stability of the vanadium leads to the valence instability of other high-entropy atoms in the perovskite lattice.That is to say, in order to stabilize the perovskite lattice with Nb 5+ , a part of the tetravalent elements is forced to be loaded with electrons from the oxygen.Thereby, the valence state of the tetravalent elements decreases, and the oxygen vacancy increases greatly.Therefore, we believe that in order to achieve the entropy stability of the perovskite lattice, it is important and necessary to maintain the valence equilibrium of the high-entropy system.
Figure 4 shows the microstructure and particle size distribution of the Sr(TiZrHfVNb)O 3 samples synthesized at different temperatures.It can be observed that the average particle size of the hydrothermal samples is about 150 nm with a concentrated distribution.The particle size gradually increases when the temperature increases.After the temperature exceeds 900 • C, the particle distribution becomes more dispersed.In general, the particles are clustered in the range of 100~500 nm.
To prove the above view, a high-entropy system Sr(TiZrHfVFe)O 3 with an obviously unbalanced valence state is also designed and attempted to be synthesized using a hydrothermal process and calcined at 1200 • C. The XRD spectra of the high-entropy Sr(TiZrHfVFe)O 3 and the fitted core level spectrum of the elements in this system are shown in Figure 5.The assignment of XPS peaks of high-entropy elements is listed in Table 3.It can be seen clearly that both the hydrothermal and the calcined samples are not pure perovskite.For the hydrothermal sample, the phase separation is easy to understand.There are no pentavalent elements in the system during the hydrothermal process.Excess trivalent elements reduce the supply of electrons.The experiment temperature also failed to provide enough energy to snatch electrons from the elements' more stable orbits.There are too many holes without electron filling and the trivalent elements are squeezed out.In other words, the entropy value of a five-element high-entropy system is not enough to stabilize the lattice.However, the 1200 • C calcined sample should be in a balanced valence state.The single phase is still unable to be achieved.This is due to the fact that the hydroxyapatite structure already formed around the hydrothermal process is stable.It will not decompose at high temperatures to release pentavalent vanadium and refill into the perovskite lattice.Meanwhile, we believe that valence equilibrium is not the only condition to realize the entropy stability of the phase; the electronic structures of the elements must also match each other.This work is ongoing and will be published in the near future.Figure 4 shows the microstructure and particle size distribution of the Sr(TiZrHfVNb)O3 samples synthesized at different temperatures.It can be observed that the average particle size of the hydrothermal samples is about 150 nm with a concentrated distribution.The particle size gradually increases when the temperature increases.After the temperature exceeds 900 °C, the particle distribution becomes more dispersed.In general, the particles are clustered in the range of 100~500 nm.To prove the above view, a high-entropy system Sr(TiZrHfVFe)O3 with an obviously unbalanced valence state is also designed and attempted to be synthesized using a hydrothermal process and calcined at 1200 °C.The XRD spectra of the high-entropy Sr(TiZrHfVFe)O3 and the fitted core level spectrum of the elements in this system are shown in Figure 5.The assignment of XPS peaks of high-entropy elements is listed in Table 3.It can be seen clearly that both the hydrothermal and the calcined samples are not pure perovskite.For the hydrothermal sample, the phase separation is easy to understand.There are no pentavalent elements in the system during the hydrothermal process.Excess trivalent elements reduce the supply of electrons.The experiment temperature also failed to provide enough energy to snatch electrons from the elements' more stable orbits.There are too many holes without electron filling and the trivalent elements are squeezed out.In other words, the entropy value of a five-element high-entropy system is not enough to stabilize the lattice.However, the 1200 °C calcined sample should be in a balanced valence state.The single phase is still unable to be achieved.This is due to the fact that the hydroxyapatite structure already formed around the hydrothermal process is stable.It will not decompose at high temperatures to release pentavalent vanadium and refill into the perovskite lattice.Meanwhile, we believe that valence equilibrium is not the only condition to realize the entropy stability of the phase; the electronic structures of the elements must also match each other.This work is ongoing and will be published in the near future.To investigate the photoresponsiveness of the high-entropy Sr(TiZrHfVNb)O 3 system, UV-Vis spectra and the (αhv) 2 ~hv curves of the samples synthesized at different temperatures are shown in Figure 6.It can be seen that all the samples can absorb light in the ultraviolet wavelength range.However, only the hydrothermal and 500 • C calcined samples are well absorbent to the visible light wave range.The absorption of the visible range rapidly decreases with an increase in temperature, and the absorption peak of the UV range narrows gradually.Obviously, the 1200 • C calcined sample is signally absorbent to the UV range and has almost no absorption in the visible range.This is similar to most inorganic perovskites such as SrTiO 3 , BaTiO 3 , SrZrO 3 , PbTiO 3, and so on [18].The band gap of the samples synthesized using hydrothermal and calcined from extrapolating (αhv) 2 ~hv curves are shown in Figure 7.The hydrothermal sample has a narrow band gap of 2.06 eV.The variation trend of the band gap increases with the increase in temperature.Expressly, the 900 • C calcined sample seems to appear to have two band gaps.This is caused by the phase separation of the sample.Due to the segregation of the vanadium, part of the perovskite lattice is missing a vanadium ion, and the chemical formula shall be Sr(TiZrHfNb)O 3 with a band gap of 3.33 eV.Thus, 900 • C can be seen as the transition temperature affecting photoresponsiveness.The species composition of the Sr(TiZrHfVFe)O 3 system at different temperatures is listed in Table 4.To investigate the photoresponsiveness of the high-entropy Sr(TiZrHfVNb)O3 system, UV-Vis spectra and the (αhv) 2 ~hv curves of the samples synthesized at different temperatures are shown in Figure 6.It can be seen that all the samples can absorb light in the ultraviolet wavelength range.However, only the hydrothermal and 500 °C calcined samples are well absorbent to the visible light wave range.The absorption of the visible range rapidly decreases with an increase in temperature, and the absorption peak of the UV range narrows gradually.Obviously, the 1200 °C calcined sample is signally absorbent to the UV range and has almost no absorption in the visible range.This is similar to most inorganic perovskites such as SrTiO3, BaTiO3, SrZrO3, PbTiO3, and so on [18].The band gap of the samples synthesized using hydrothermal and calcined from extrapolating (αhv) 2 ~hv curves are shown in Figure 7.The hydrothermal sample has a narrow band gap of 2.06 eV.The variation trend of the band gap increases with the increase in temperature.Expressly, the 900 °C calcined sample seems to appear to have two band gaps.This is caused by the phase separation of the sample.Due to the segregation of the vanadium, part of the perovskite lattice is missing a vanadium ion, and the chemical formula shall be Sr(TiZrHfNb)O3 with a band gap of 3.33 eV.Thus, 900 °C can be seen as the transition temperature affecting photoresponsiveness.The species composition of the Sr(TiZrHfVFe)O3 system at different temperatures is listed in Table 4.

Conclusions
In summary, we designed a high-entropy Sr(TiZrHfVNb)O 3 system, and successfully synthesized a ceramic powder with a perovskite structure using a vanadium(III) source.This system cannot be prepared using calcined or solid-state reactions in air due to the valence change of the vanadium ion.Significant phase separation and element segregation appeared in the calcined and solid phase reacted samples.The ionic valence of the vanadium ascends from V 3+ to V 4+ and V 5+ with an increasing calcine temperature.The precipitated V ionic combines with a part of the Sr.It was proved that the high-entropy value could not only stabilize the crystal phase but also stabilize the ionic valence state.Also, a Sr(TiZrHfVFe)O 3 system cannot be synthesized using the hydrothermal method.It is proposed that, for the synthesis of high-entropy perovskite, the design of entropy components to maintain the valence equilibrium is an important factor in achieving entropy stability of the phase structure.The valence state of the raw material should be carefully selected for the variable valence element sources.The ionic source valence state could be maintained during the hydrothermal process.The high-entropy Sr(TiZrHfVNb)O 3 ceramic powder showed a wide absorption in the visible range.The high-entropy system with the vanadium element might achieve a promising material for optical applications.

Figure 2 .
Figure 2. Micromorphology of Sr(TiZrHfVNb)O 3 with EDS synthesized using hydrothermal and calcined at different temperatures.

Figure 3 .
Figure 3. Fitted core level spectrum of the high-entropy elements in the Sr(TiZrHfVNb)O3 system.Figure 3. Fitted core level spectrum of the high-entropy elements in the Sr(TiZrHfVNb)O 3 system.

Figure 3 .
Figure 3. Fitted core level spectrum of the high-entropy elements in the Sr(TiZrHfVNb)O3 system.Figure 3. Fitted core level spectrum of the high-entropy elements in the Sr(TiZrHfVNb)O 3 system.

Figure 4 .
Figure 4. SEM images and particle size distribution of the Sr(TiZrHfVNb)O3 samples synthesized at different temperatures.

Figure 4 . 13 Figure 5 .
Figure 4. SEM images and particle size distribution of the Sr(TiZrHfVNb)O 3 samples synthesized at different temperatures.Materials 2024, 17, x FOR PEER REVIEW 9 of 13

Figure 5 .
Figure 5. XRD spectra of the high-entropy Sr(TiZrHfVFe)O 3 and fitted core level spectrum of the elements in this system.

Table 1 .
Assignment of XPS peaks of different elements in Sr(TiZrHfVNb)O 3 system before and after calcining at 1200 • C.

Table 2 .
Assignment of XPS peaks of V2p in Sr(TiZrHfVNb)O 3 system before and after calcining at different temperatures.

Table 3 .
Assignment of XPS peaks of different elements in Sr(TiZrHfVFe)O3 system.

Table 3 .
Assignment of XPS peaks of different elements in Sr(TiZrHfVFe)O 3 system.