Utafiti huu unachunguza kama inawezekana kutengeneza large-area titanium dioxide (TiO₂) nanorod photoanodes bila kutumia high-pressure steel autoclave. Researchers waliunganisha Petri-dish-based hydrothermal growth kwenye FTO conductive glass iliyofanywa at 100 °C for 4 hours, approximately 2 nm-thick Al₂O₃ surface passivation, na electrochemical reduction process. Optimized TiO₂/Al₂O₃-ER photoanode ilifikia photocurrent density ya 2,35 mA cm_-2 na surface charge separation efficiency ya %96,7 at 1,23 V^RHE. Hata hivyo, measurements zilifanywa katika three-electrode laboratory cell yenye external potential; unbiased full-cell solar-to-hydrogen efficiency na quantitative hydrogen production rate hazikuamuliwa.

First stage ya production method ilitengeneza rutile TiO₂ nanorod arrays zenye porous and tapered tips. Kulingana na mechanism iliyopendekezwa na authors, high-curvature tips zinaimarisha local electric field, na kurahisisha transport ya OH^- ions kwenda active regions na oxygen evolution reaction. Mesopores zilizoripotiwa ndani ya nanorods katika range 5–35 nm zinaongeza electrode-electrolyte contact na electrochemically active surface area. Local electric-field mechanism haijathibitishwa kwa direct field measurement au numerical electric-field map; imependekezwa kutoka shape na performance results.

Approximately 2 nm amorphous Al₂O₃ coating ilipassivate surface defects na kupunguza recombination ya photogenerated charges kwenye surface. Subsequent electrochemical reduction iliongeza oxygen-vacancy/Ti³⁺ ratios, na kuboresha electron transport na interfacial charge transfer. Photocurrent ya bare TiO₂ photoanode ilikuwa 1,72 mA cm^-2, ikaongezeka hadi 2,03 mA cm_-2 baada ya Al₂O³ coating na hadi 2,35 mA cm^-2 baada ya reduction process.

Optimized photoanode ilihifadhi %96,8 ya initial photocurrent baada ya 50 hours continuous illumination. Pia, 10 × 10 cm² photoanode ilitengenezwa na katika setup yenye three electrodes stacked in parallel total current density ilipimwa 3,82 mA cm^-2. Hata hivyo, photocurrent distribution across full area ya 100 cm² sample, total gas production, large-area stability na mass-production cost hazikupimwa. Kwa hiyo study inatoa important manufacturability demonstration kwa scale-up, lakini bado haithibitishi industrial application.

Tatizo kuu la utafiti ni nini?

Photoelectrochemical water splitting ni energy-conversion method inayolenga semiconductor electrodes kutumia light energy kugawanya water kuwa hydrogen na oxygen. Oxygen evolution reaction hutokea kwenye photoanode, na hydrogen evolution reaction kwenye counter-electrode side.

Photoanode inapokamata light huunda electron-hole pairs. Ili electrons na holes zishiriki katika useful reactions, stages zifuatazo lazima zote ziwe effective:

• Absorption of light by semiconductor,
• Separation of electrons and holes inside material,
• Transport of charges to relevant surface without recombination,
• Oxidation of water or OH^- ions by holes reaching surface,
• Transfer of generated electrons through external circuit to counter electrode.

TiO₂ ni mojawapo ya most common photoanode materials kwa sababu ya low cost, chemical stability na relatively environmentally suitable structure. Kwa upande mwingine, wide band gap yake inalimit utilization ya large portion ya sunlight. Low carrier mobility na surface defects pia zinaweza kusababisha electron-hole recombination kabla ya useful reaction.

Second problem ya study ni production scale. Conventional hydrothermal nanorod production mara nyingi hufanywa ndani ya closed autoclaves zinazostahimili high pressure. Autoclave volume inaweza kulimit size ya FTO glass inayoweza processed. Alternatives kama vapor deposition zinaweza kuhitaji more complex au expensive equipment.

Kwa hiyo researchers wanashughulikia problems mbili together:

1. Je, large-area na uniform TiO₂ nanorod photoanode inaweza kutengenezwa bila autoclave?
2. Charge separation, surface reaction na stability zinawezaje kuboreshwa huku scalable production ikihifadhiwa?

Three-part improvement strategy iliyopendekezwa na study

Study haitumii single modification bali three complementary structural and chemical interventions:

Components hizi tatu hazifanyi same physical effect. Al₂O₃ inalenga mainly surface passivation, wakati reduction process inalenga defect engineering katika bulk na near-surface regions za material. Mesoporous architecture hubadilisha reaction-accessible area na transport pathways.

Petri-dish-based production method

FTO ni fluorine-doped tin oxide-coated conductive glass na ilitumika kama light-facing electrical substrate ya photoanode. FTO samples zilisafishwa kwanza ultrasonically katika deionized water, ethanol na acetone.

Hydrothermal solution iliandaliwa kwa:

• 30 mL deionized water,
• 20 mL ethanol,
• 50 mL hydrochloric acid yenye %36–38 concentration,
• 4,5 mL titanium butoxide.

Water na ethanol zilichanganywa na hydrochloric acid, baada ya five minutes stirring titanium butoxide ikaongezwa na stirring ikaendelea kwa another ten minutes. FTO glass iliingizwa katika solution na kuwekwa at 100 °C for four hours katika electric furnace.

Katika stage hii glass Petri dish ilitumika badala ya conventional stainless-steel autoclave. Ingawa process haihitaji high-pressure autoclave, entire production route si low-temperature. Baada ya washing, nanorods zili-anneal at 750 °C for 30 minutes ili kuboresha crystal structure.

Role ya ethanol katika production

Addition ya 20 mL ethanol kwenye precursor solution ilionekana critical kwa photocurrent performance. Value bila ethanol au under unsuitable condition ilikuwa 0,77 mA cm^-2, wakati optimized ethanol amount ilitoa 1,72 mA cm^-2.

Researchers wanasema ethanol huchukua nafasi ya sehemu ya water na kurahisisha dense TiO₂ film growth katika shorter time. Detailed curves kwa different ethanol amounts zimetumwa Figure S9, lakini supporting figure haipo katika uploaded file.

Al₂O₃ passivation layer iliandaliwaje?

TiO₂ nanorod-coated FTO iliingizwa katika 5 mM Al(NO₃)₃ aqueous solution kwa four minutes. Sample kisha ili-anneal katika air at 200 °C for one hour.

Imeelezwa kwamba immersion time ikiwa very short surface inaweza kutofunikwa fully; ikiwa too long, pores zinaweza kufunga, light absorption inaweza kudhoofika na charge-transfer resistance inaweza kuongezeka. Detailed time optimization imetolewa katika Figure S11, lakini figure hiyo haipo katika uploaded PDF.

Electrochemical reduction ilitumikaje?

TiO₂/Al₂O₃ photoanodes zilireducewa katika 1 M KOH kwa constant potential -0,5 V versus Ag/AgCl reference electrode. Times zilizochunguzwa ni 0, 30, 60, 120 na 180 seconds.

Aim ya process ni kuondoa oxygen kutoka TiO₂ lattice ili kuunda oxygen vacancies na kureduce sehemu ya Ti⁴⁺ species kuwa Ti³⁺ state. Imeelezwa excessively long reduction inaweza kusababisha excessive defect formation na deterioration ya crystal integrity.

Exact reduction time ya optimized final sample haijaandikwa explicitly katika main text na imeelekezwa kwenye optimization katika Figure S12. Kwa kuwa supporting figure haipo katika uploaded file, duration haijakisiwa.

Nanorod morphology

SEM images zinaripoti nanorods approximately 400–600 nm long na 100–170 nm diameter. Label kwenye cross-sectional SEM image inaonyesha total film thickness approximately 862 nm. Difference kati ya individual nanorod length na total coating thickness inaweza kuhusiana na growth arrangement kwenye substrate na total layer iliyopimwa kwenye image; study haija-reconcile separately measurements hizi mbili.

Katika top view, nanorods zimepacked densely na zina form array yenye rounded au tapering tips. Cross-sectional image inaonyesha nanorods attached to FTO surface na growing largely in vertical direction.

TEM image inaonyesha nanorod structure iliyoaggregated na kuunda two tapered tips. Authors wanasisitiza geometry hii ni tofauti na conventional quadrangular column-shaped TiO₂ nanorods.

Tapered-tip effect ina maana gani?

Concentration ya electric field lines kwenye sharp high-curvature tips huitwa “tip effect.” Katika mechanism schematic, field hii inapendekezwa kuelekeza OH^- ions kwenda active regions near tip na kuharakisha oxygen evolution reaction.

Proposed process inaweza kusummarizewa:

1. Light hutengeneza electrons na holes ndani ya TiO₂.
2. Holes husafirishwa kwenda photoanode surface.
3. Local field near tapered tip hurahisisha transport ya charged species na accumulation ya OH^-.
4. Surface holes huoxidize OH^- species na kuchangia O₂ formation.

Mechanism hii inaendana na performance increase; hata hivyo, study haijapima directly electric-field intensity around tip, kutoa quantitative field map kutoka finite-element simulation, au compare separately tipped versus untipped nanorods zenye same chemistry. Kwa hiyo contribution ya tip effect haiwezi kutenganishwa fully na total effect ya overall architecture.

Mesoporous structure

HAADF-STEM images zinaonyesha many bright and dark pore regions ndani ya nanorods. Kulingana na supporting pore-distribution analysis, pore diameters zimeripotiwa mostly between 5–35 nm.

Three contributions zimependekezwa kwa mesoporous architecture:

• Increase surface area in contact with electrolyte,
• Create more surface reaction sites,
• Reduce distance over which charges and reaction species must be transported.

Kwa kuwa Figure S3 yenye pore-diameter distribution haipo katika uploaded PDF, histogram shape, sample count na distribution uncertainty hazikuweza kuchunguzwa independently.

Microscopic evidence ya Al₂O₃ layer

HRTEM image inaonyesha approximately 2 nm-thick amorphous Al₂O₃ layer covering TiO₂ nanorod surface. Coating imetafsiriwa amorphous kwa sababu haionyeshi crystal fringes.

Average lattice-fringe spacing ndani ya TiO₂ ilipimwa 0,313 nm na assigned to rutile TiO₂ (110) plane. Katika peak-intensity profile, sum ya approximately ten lattice spacings imeonyeshwa 3,13 nm.

HAADF-STEM na EDS maps zinaonyesha Ti, O na Al signals along nanorod. Distribution ya Al signal along nanorod surface na general shape ina-support coating over broad scale. Hata hivyo, spatial resolution ya EDS mapping haionyeshi kwamba coating ni exactly 2 nm at every point; thickness ilipimwa kutoka selected local HRTEM region.

Crystal structure na strain relaxation

XRD patterns zilibainisha dominant phase ya all photoanodes kuwa rutile TiO₂. Other prominent peaks ni za tetragonal SnO₂ phase ya FTO substrate. Hakuna separate diffraction peak iliyoonekana kwa amorphous na ultrathin Al₂O₃ layer.

TiO₂ (101) na (211) peaks zilishift kwenda lower 2θ angles baada ya Al₂O₃ coating na low-temperature annealing. Authors wanaeleza shift hii kwa strain relaxation:

1. Initial annealing at 750 °C na rapid cooling vinaweza kutengeneza residual compressive strain kutokana na thermal mismatch na phase transformation kati ya TiO₂ na FTO.
2. Second annealing at 200 °C inaweza kuruhusu lattice atoms kurelax kwenda lower-energy positions.
3. Al–O–Ti bonds zinaweza kubadilisha surface boundary conditions na kuchangia redistribution ya strain.

Study haijapima directly residual stress kwa Raman shift, curvature measurement au mechanical-stress analysis. XRD peak shifts ndiyo main evidence ya structural change.

Light absorption na band gap

Katika UV-vis spectra, reduced TiO₂/Al₂O₃-ER sample ilionyesha highest absorption. Spectra zina absorption tail extending into visible region.

Tauc plots zinaonyesha optical transition value approximately 3,01 eV kwa bare TiO₂ na approximately 2,89 eV kwa reduced photoanode. Authors wana-associate change hii na Ti³⁺ species na oxygen vacancies zilizoundwa baada ya electrochemical reduction, ambazo huunda defect levels ndani ya band gap.

Additional absorption katika visible region haimaanishi all these photons zinabadilishwa kuwa useful current kwa equal efficiency. IPCE results zinaonyesha highest conversion bado iko predominantly katika 350–420 nm ultraviolet na near-ultraviolet region.

Oxygen vacancies na Ti³⁺ species

O 1s XPS spectra ziligawanywa katika three components:

• Approximately 529,6 eV: lattice oxygen,
• Approximately 531,2 eV: oxygen-vacancy-related component,
• Approximately 532,5 eV: chemically adsorbed oxygen.

Baada ya Al₂O₃ coating, oxygen-vacancy-related ratio iliongezeka huku Ti³⁺ ratio ikipungua slightly. Authors wanaeleza hii kwa charge balancing kwenye Al–O–Ti interface na local structural change around Ti⁴⁺ na Al³⁺ species.

Baada ya electrochemical reduction, both oxygen-vacancy na Ti³⁺ ratios ziliongezeka clearly. Shift ya Ti 2p peaks kwenda lower binding energy pia ina-support increased Ti³⁺ content. Al 2p peak at approximately 74,1 eV imeassigned to presence ya Al²O₃ with Al₃₊ state.

Photoelectrochemical measurement setup

Performance measurements zilifanywa katika three-electrode cell:

• Working electrode: TiO² photoanode illuminated over 1 cm₂,
• Reference electrode: Ag/AgCl containing 3,0 M NaCl,
• Counter electrode: Platinum plate,
• Electrolyte: 1 M NaOH,
• Light: AM 1.5G, 100 mW cm^-2 simulated sunlight.

Three-electrode cell ni suitable kwa detailed study ya potential-dependent photoanode behavior. Hata hivyo, setup hii si completed solar-hydrogen device operating without external voltage. Kwa hiyo photocurrent density na applied-bias efficiency hazipaswi kulinganishwa directly na full-system solar-to-hydrogen efficiency.

Conversion ya potentials kwenda RHE scale

Potentials zilizopimwa versus Ag/AgCl reference zilibadilishwa kwenda reversible hydrogen electrode scale kwa:

\[ E_{\mathrm{RHE}}=E_{\mathrm{Ag/AgCl}}+0{,}059\,\mathrm{pH}+E_{\mathrm{Ag/AgCl}}^{0} \]

At 25 °C:

\[ E_{\mathrm{Ag/AgCl}}^{0}=0{,}1976\ \mathrm{V} \]

• E_RHE: Potential versus reversible hydrogen electrode,
• E_Ag/AgCl: Potential measured versus Ag/AgCl reference,
• pH: Acidity-basicity value ya electrolyte,
• E⁰_Ag/AgCl: Standard potential ya reference electrode.

Mott-Schottky equation na notation issue katika text

Equation iliyotolewa katika study kuhesabu charge-carrier density imeprintiwa katika PDF kama:

\[ \left(\frac{A_S}{C_{\mathrm{bulk}}}\right)= \frac{2}{q\varepsilon_r\varepsilon_0N_D} \left(V-E-\frac{k_BT}{q}\right) \]

• A_S: Surface-area correction,
• C_bulk: Space-charge capacitance,
• N_D: Donor or carrier density,
• V: Applied potential,
• k_B: Boltzmann constant,
• T: 298 K,
• q: Elementary electric charge,
• ε₀: Vacuum permittivity,
• ε_r: Relative permittivity taken as 170 for TiO₂.

Ingawa text inaeleza energy term katika equation kama flat-band potential E_FB, equation inaandika only E. Pia capacitance term imeprintiwa without square katika PDF. Equation haijasahihishwa kimya kimya hapa; internal notation inconsistency hii, ambayo inaweza kuwa muhimu kwa reproducibility, imeelezwa explicitly.

Photon-to-current conversion efficiency

Wavelength-dependent incident photon-to-current efficiency ilihesabiwa kwa:

\[ \mathrm{IPCE}= \frac{1240\,J} {P_{\mathrm{light}}\lambda} \]

• J: Photocurrent density measured under monochromatic light, mA cm^-2,
• P_light: Light power at relevant wavelength, mW cm^-2,
• λ: Wavelength of incident light, nm,
• 1240: Coefficient from conversion between photon energy and wavelength.

IPCE inaonyesha fraction gani ya incident photons at given wavelength inabadilishwa kuwa measurable electrical current. Si hydrogen-production efficiency ya full cell.

Applied-bias photon-to-current efficiency

Study inatoa applied-bias photon-to-current efficiency kama:

\[ \mathrm{ABPE}= \frac{J_{\mathrm{ph}}\left(1{,}23-E\right)} {P_{\mathrm{irradiation}}} \]

• J_ph: Photocurrent density,
• 1,23 V: Thermodynamic potential used for water splitting,
• E: Potential applied to photoanode,
• P_irradiation: Incident light power density.

ABPE ni photoelectrode metric inayozingatia externally applied electrical bias. Si solar-to-hydrogen efficiency ya device operating without external voltage.

Optimization ya synthesis conditions

Highest bare TiO₂ performance kwa Petri-dish method ilipatikana katika conditions zifuatazo:

• 4,5 mL titanium butoxide,
• 20 mL ethanol,
• 100 °C hydrothermal temperature,
• 4 hours growth time,
• Annealing at 750 °C for 30 minutes.

Katika conditions hizi bare TiO₂ photoanode ilitoa 1,72 mA cm_-2 photocurrent at 1,23 V^RHE. Comparison table katika study inatoa values 0,32–0,96 mA cm_-2 kwa baadhi ya TiO² nanorod photoanodes zilizotengenezwa kwa conventional autoclave.

Ingawa title ya Table 1 inarejea photoanodes synthesized by autoclave, first row pia ina current autoclave-free study. Hii ni inconsistency katika table-title scope; haibadilishi numerical values lakini inapaswa kuzingatiwa wakati wa interpretation.

Improvement katika photocurrent density

Bare photoanode iliyotengenezwa kwa Petri-dish method ilitoa approximately 3,2 times higher photocurrent than conventional TiO₂ comparison katika study. Additional increase kutoka Al₂O₃ coating ilikuwa approximately 0,31 mA cm^-2, na additional contribution ya electrochemical reduction approximately 0,32 mA cm^-2.

Comparison hii inaonyesha sequential sample modifications. Kwa sababu separate untapered control yenye same porosity au nonporous control yenye same tip geometry haikuandaliwa, effects za two morphology components hazikutenganishwa fully.

ABPE results

Al₂O₃ na reduction process ziliinua ABPE peak na kuifanya ipatikane at lower applied potential compared with bare TiO₂.

Carrier density na flat-band potential

Increase ya carrier density baada ya Al₂O₃ ilihusishwa na reduction ya surface traps na increase ya available carriers. Electrochemical reduction iliinua carrier density zaidi.

Values hizi zilihesabiwa kutoka Mott-Schottky slope kwa kutumia relative permittivity assumed constant kwa TiO₂ na ideal space-charge approach. Kwa hiyo si direct carrier counts na zinategemea model assumptions.

Impedance results

Photoelectrochemical impedance spectroscopy ilifanywa at 0,25 V_RHE, frequency range 0,1 Hz–100 kHz na chini ya one-sun illumination. Nyquist curves zilifit kwa equivalent circuit yenye two RC units.

Al₂O₃ coating ilipunguza interfacial charge-transfer resistance huku trap-related resistance ikibaki largely preserved. Authors wana-interpret result hii kama coating kufanya kazi predominantly kwenye surface.

Baada ya electrochemical reduction, both interfacial charge-transfer resistance na trap resistance zilipungua. Hii ina-support interpretation kwamba reduction process inabadilisha si surface reaction pekee bali pia carrier transport ndani ya material.

Full numerical parameters za equivalent circuit zimetumwa Figure S13, lakini supporting figure haipo katika file. Kwa hiyo numerical values za resistances na capacitances hazikuweza kuhamishwa kutoka main PDF.

Open-circuit photovoltage

Open-circuit potential difference kati ya dark na illuminated states ilipimwa kama:

Larger photovoltage ilihusishwa na greater charge separation under illumination na higher carrier accumulation at surface. Large increase baada ya Al₂O₃ coating inaonyesha limiting surface recombination ni one of main contributions.

Wavelength-dependent IPCE

All photoanodes zilionyesha highest IPCE values katika 350–420 nm range. At approximately 350 nm:

• TiO₂: approximately %42,
• TiO₂/Al₂O₃: approximately %48,
• TiO₂/Al₂O₃-ER: approximately %50.

Katika 360–390 nm range IPCE ya reduced sample katika some points ilikuwa slightly lower than only Al₂O₃-coated sample. Kwa upande mwingine, katika 390–550 nm range oxygen-vacancy na Ti³⁺-related states zinaonekana kuchangia more clearly kwa reduced sample.

Calculated photocurrents kutoka integration ya IPCE spectrum dhidi ya AM 1.5G solar spectrum:

• TiO₂: 1,73 mA cm^-2,
• TiO₂/Al₂O₃: 2,03 mA cm^-2,
• TiO₂/Al₂O₃-ER: 2,36 mA cm^-2.

Values hizi zinaagree closely na J–V measurements za 1,72; 2,03 na 2,35 mA cm^-2.

Surface na bulk charge separation efficiencies

Ili kutenganisha surface na bulk contributions, 0,5 M Na₂SO₃ hole scavenger iliongezwa kwenye 1 M NaOH electrolyte.

Surface charge separation efficiency inaonyesha fraction gani ya holes zinazofika photoanode surface zinashiriki katika water oxidation bila recombining.

Bulk charge separation efficiency inaonyesha fraction gani ya holes generated inside material zinaweza kufika surface bila recombining katika bulk.

Largest improvement ilitokea katika surface efficiency. Hii inaonyesha major part ya performance increase inahusiana na passivation ya surface defects na improved interfacial charge transfer. Increase katika bulk efficiency ni more limited.

Authors wanaeleza %96,7 kama record-level surface charge separation efficiency. Study haitoi comprehensive systematic meta-analysis inayounga mkono “record” claim hii; kwa hiyo phrase inapaswa kusomwa kama authors’ literature assessment.

Electrochemically active surface area

Double-layer capacitance ilihesabiwa kutoka cyclic-voltammetry curves:

ECSA ilihesabiwa kwa:

\[ \mathrm{ECSA}=\frac{C_{\mathrm{dl}}}{C_s} \]

ambapo specific capacitance ilikubaliwa:

\[ C_s=40\ \mu\mathrm{F\,cm^{-2}} \]

.

Double-layer capacitance ya reduced sample ni 1,74 times that of only Al₂O₃-coated sample na 1,88 times that of bare TiO₂. ECSA si absolute geometric surface measurement; ni indirect comparison based on assumed specific capacitance.

50-hour stability

Al₂O₃ layer inapendekezwa partially isolate surface from electrolyte na limit photocorrosion na side reactions; reduction process inapendekezwa kuboresha charge transfer na coating-substrate interface.

Fifty-hour test ni meaningful laboratory-scale stability indicator, lakini haiwakilishi months au years za solar-hydrogen operation. Hakuna detailed degradation analysis iliyopewa ambapo microstructure, Al dissolution, oxygen-vacancy density au gas-production efficiency ziliremeasurewa baada ya test.

Parallel photoanode stack

Kwa kutumia short na partially light-transmitting nanorod films, arrangement yenye one, two na three photoanodes placed in parallel iliandaliwa.

Kila added electrode iliongeza total current, lakini contribution ilipungua progressively. Kwa sababu upper layers zinaabsorb na scatter light, less light inafika second photoanode na still less inafika third.

Wakati triple setup ikioperate, oxygen bubbles kwenye photoanodes na hydrogen bubbles kwenye platinum cathode zilionyeshwa visually. Hata hivyo, gas chromatography, volumetric gas measurement, H₂:O₂ stoichiometry au Faradaic efficiency hazikuripotiwa. Bubble image inaunga mkono gas formation lakini haitoi quantitative hydrogen-production efficiency.

10 × 10 cm² photoanode

Kwa Petri-dish method, TiO²-coated FTO sample yenye 10 × 10 cm₂ ilitengenezwa. Photograph inaonyesha coating imeenea macroscopically across entire surface, huku printed patterns chini zikiendelea kuonekana.

Demonstration hii ina-support processing ya larger substrates independent of autoclave volume. Hata hivyo, study haitoi measurements zifuatazo kwa large-area sample:

• Photocurrent density across entire 100 cm² area,
• Center-edge thickness and morphology map,
• Electrical resistance across area,
• Total hydrogen and oxygen production rate,
• Large-area stability test,
• Production yield or defective-sample fraction.

Kwa hiyo 10 × 10 cm² sample inaonyesha physical feasibility ya large-area growth; haithibitishi 100 cm² device inafanya kazi kwa same efficiency kama small sample.

Technical interpretation ya Figure 1

Top row ya Figure 1 inaonyesha sequentially Petri-dish nanorod growth, annealing at 750 °C, Al(NO₃)₃ coating na electrochemical reduction steps.

SEM images zinaonyesha dense nanorod array na approximately 862 nm total film thickness; TEM image inaonyesha tapered-tip, aggregated na porous nanorod. HRTEM inaonyesha rutile (110) lattice spacing pamoja na approximately 2 nm Al₂O₃ boundary. Elemental maps katika bottom row zinavisualize Ti, O na Al distributions.

Technical interpretation ya Figure 2

Katika XRD graphs rutile TiO₂ main phase imehifadhiwa, huku selected peak shifts kwenda lower angle zikihusishwa na strain relaxation. UV-vis na Tauc plots zinaonyesha reduced sample ina broader optical absorption na lower calculated transition energy.

O 1s na Ti 2p XPS deconvolutions zinaonyesha oxygen-vacancy- na Ti³⁺-related components zinaongezeka after electrochemical reduction. Al 2p peak ni one chemical evidence ya Al₂O₃ coating.

Technical interpretation ya Figure 3

Katika J–V curves, black TiO₂, red TiO₂/Al₂O₃ na blue TiO₂/Al₂O₃-ER curves zinaonyesha sequential performance increase.

Tip-effect schematic inaonyesha OH^- ions zikielekezwa toward high-electric-field nanorod tip na O₂ formation. Reduced sample ina highest peak katika ABPE graph. Mott-Schottky, impedance na OCP graphs zinaonyesha complementarily improvements katika carrier density, charge transfer na photovoltage.

Katika literature-comparison graph, bare TiO₂ value ya study, 1,72 mA cm^-2, imeonyeshwa higher than selected references. Comparison hii ni limited kwa studies zilizochaguliwa katika paper na reported under similar conditions; si systematic review ya entire TiO₂ literature.

Technical interpretation ya Figure 4

IPCE graph inaonyesha performance concentrated mainly katika ultraviolet region. Surface-separation-efficiency curves zinaweka reduced sample highest across nearly entire potential range, wakati differences kati ya bulk-efficiency curves ni smaller.

Katika double-layer capacitance graph, slope ya reduced sample ni markedly larger. Katika fifty-hour stability graph, blue curve inabaki high na relatively flat huku bare TiO₂ curve ikiendelea kushuka.

Bottom mechanism schematic inaonyesha residually strained TiO₂ after high-temperature annealing, Al–O–Ti/Al₂O₃ formation after Al(NO₃)₃ treatment, na increase ya Ti³⁺ na oxygen vacancies after electrochemical reduction.

Technical interpretation ya Figure 5

J–V curves za one-, two- na three-layer photoanodes zinaonyesha total current inaongezeka with layer count, lakini increase si linear. Layer-contribution graph inaonyesha clearly progressively declining share ya second na third electrode.

Gas bubbles zinaonekana kwenye electrode surfaces katika photograph ya operating cell. Large-area photograph inaonyesha macroscopic appearance ya coating kwenye 10 × 10 cm² substrate.

Message ya graphical abstract

Graphical abstract inalinganisha 10 cm-size photoanode na Petri-dish method na selected photocurrent values kutoka conventional autoclave studies. Right side inaonyesha tapered TiO₂ nanorod, Al₂O₃ coating na OH^-–O₂ conversion near tip.

Graphical abstract inasisitiza claims mbili kuu together: autoclave-free method suitable for larger-substrate production na enhanced PEC performance kupitia high-area/pore/passivation/defect engineering.

Nguvu za utafiti

• Open na relatively simple growth approach isiyohitaji high-pressure autoclave ilitengenezwa.
• Production parameters zilioptimizewa kwa precursor amount, ethanol, temperature, time na annealing.
• Morphology, surface chemistry, crystal structure na electrochemical performance zilichunguzwa kwa many complementary methods.
• Tapered tips, mesopores, surface passivation na internal defect engineering ziliunganishwa katika same photoanode.
• Approximately 2 nm Al₂O₃ coating iliimagiwa directly kwa HRTEM.
• Increase ya oxygen vacancies na Ti³⁺ ililinganishwa quantitatively kwa XPS.
• Photocurrent, ABPE, IPCE, Mott-Schottky, impedance, OCP, charge-separation efficiency na ECSA zinatoa mutually supporting performance picture.
• Photocurrent calculated from IPCE na J–V measurement zilipatikana very close.
• Surface na bulk charge separation zilitathminiwa separately.
• Fifty-hour stability measurement ilifanywa.
• Total current ya triple parallel stack na contribution ya each added electrode zilitolewa separately.
• 10 × 10 cm² photoanode ilitengenezwa physically.

Mapungufu ya utafiti

• Study ni preprint ambayo haijapitia peer review.
• Supporting Figures S1–S16 hazipo katika uploaded file.
• Optimized electrochemical reduction time haijaelezwa explicitly katika main text.
• Number ya independent synthesis repeats na sample-to-sample standard deviations hazijaripotiwa.
• Main performance graphs hazina error bars au statistical significance tests.
• Measurements zilifanywa katika three-electrode laboratory cell yenye external potential applied.
• Unbiased tandem full cell au direct solar-to-hydrogen efficiency haikupimwa.
• Hydrogen na oxygen amounts hazikupimwa quantitatively kwa gas chromatography.
• Faradaic efficiency na H₂:O₂ stoichiometry hazikutolewa.
• Electric field kwenye tapered tip haijapimwa directly au modeled quantitatively.
• Hakuna controlled samples zinazotenganisha effects za tapered tip na mesoporosity.
• Al₂O₃ coating thickness iliamuliwa kutoka selected HRTEM region; large-area thickness distribution haikutolewa.
• Operando change ya oxygen-vacancy na Ti³⁺ species during reaction haikufuatiliwa.
• Mott-Schottky equation ina internal notation issue kuhusu capacitance square na flat-band symbol.
• ECSA ni indirect calculation based on assumed constant specific capacitance.
• Fifty-hour stability haiwakilishi industrial operating lifetime.
• Long-term stability ya triple-photoanode stack haikutestwa.
• Photocurrent na thickness homogeneity across 10 × 10 cm² sample hazikupimwa quantitatively.
• Ingawa autoclave imeondolewa, process inahitaji 750 °C annealing.
• Acid use, furnace energy consumption, production-cycle duration na cost per unit area hazikuhesabiwa.
• Hakuna economic analysis kwa FTO cost, platinum counter electrode na large-scale device components.
• Life-cycle impact na net energy payback time hazikutathminiwa.

Study inaunga mkono nini?

• Rutile TiO₂ nanorods zinaweza grown kwenye FTO kwa Petri-dish-based hydrothermal method.
• Method ilitumika kwenye 10 × 10 cm² substrate.
• Produced nanorods zinaonyesha tapered-tip na mesoporous morphology.
• Approximately 2 nm amorphous Al₂O₃ layer inaweza applied kwenye TiO₂ surface.
• Al₂O₃ coating ilitoa improvement consistent with surface charge separation na stability.
• Electrochemical reduction iliongeza XPS signals zinazohusiana na oxygen vacancies na Ti³⁺.
• Combined treatment ilitoa 2,35 mA cm_-2 photocurrent at 1,23 V^RHE.
• Surface charge separation efficiency ilifikia %96,7.
• Reduced photoanode ilihifadhi %96,8 ya photocurrent after 50 hours.
• Triple parallel stack iliongeza total photocurrent hadi 3,82 mA cm^-2.
• Contribution ya second na third layers ina diminishing returns kutokana na light attenuation.

Study haithibitishi nini?

• Commercial-scale economical hydrogen production haijathibitishwa.
• Haijaonyeshwa kwamba 10 × 10 cm² photoanode ina same photocurrent density kama small sample.
• Continuous water splitting without external electrical bias haijaonyeshwa.
• ABPE si full-cell solar-to-hydrogen efficiency.
• Haijathibitishwa kwa Faradaic measurement kwamba all photocurrent inakwenda kwa hydrogen na oxygen production.
• Independent contribution ya tapered tips katika performance increase haijakwantifiwa.
• Independent contribution ya mesopores haikupimwa kwa separate control electrode.
• Haijathibitishwa independently kwamba %96,7 surface efficiency ni definitive world record katika entire TiO₂ photoanode literature.
• Haijaonyeshwa kwa comprehensive cost analysis kwamba three layers ni definite economic optimum.
• Outdoor durability over thousands of hours haijabainishwa.
• Safety, emissions na waste management ya large-scale acidic production process hazikutathminiwa.

Maana kwa energy na hydrogen technologies

Main technological contribution ni approach inayoondoa physical size limitation ya pressure autoclave katika TiO₂ nanorod photoanode production. Hii inaweza kuruhusu larger flat-glass substrates processed katika same solution.

Combined use ya surface passivation na defect engineering inaboresha charge arrival na utilization at surface badala ya only increasing light absorption. Especially much larger increase katika surface-separation efficiency kuliko bulk-separation efficiency inaonyesha interface losses ni muhimu katika TiO₂ photoanodes.

Parallel stacking inaonyesha light transmitted through upper photoanode inaweza kureused katika lower electrodes. Hata hivyo, declining layer contribution inaonyesha optical design, electrode spacing na light management zitakuwa decisive katika future devices.

Ili technology ibadilike kuwa industrial hydrogen system, large-area photocurrent mapping, gas efficiency, unbiased tandem cell, long-term outdoor testing, material and energy cost, acid recovery na production automation lazima zivalidatewe separately.

Mbinu na Matokeo ya Utafiti

Technical summary ya production method

Material characterization

Photoelectrochemical measurement conditions

Main microstructural results

Main performance results

Scale-up results

Technical conclusion

Experimental results zinaonyesha autoclave-free Petri-dish growth inaweza kutengeneza high-photocurrent TiO₂ nanorod photoanodes. Largest performance improvement inaonekana katika surface separation efficiency. Hii ina-support central role ya Al₂O₃ passivation na altered interfacial electronic structure after electrochemical reduction.

10 × 10 cm² sample na triple stack zinaonyesha feasibility katika production area na optical layering. Hata hivyo, claims za commercial scale zinahitaji validation ya large-area electrochemical performance, unbiased full cell, gas efficiency, long-term stability na production economics.

Maelezo ya Chanzo na Mbinu

Full original title: Scalable Synthesis of Tip-Enhanced and Mesoporous TiO2 Nanorod Photoanodes for Efficient Solar Water Splitting

Authors and order: Yuanming Zhang; Yingrong Li; Ran Jing; Haoyun Lei; Jae Sung Lee; Hemin Zhang.

Equal-contribution authors: Yuanming Zhang na Yingrong Li. PDF states “These authors contributed equally to this work”.

Corresponding author: Hemin Zhang.

Corresponding-author email: hmzhang@scu.edu.cn

Institutional affiliations

1. Yuanming Zhang, Yingrong Li, Ran Jing, Haoyun Lei na Hemin Zhang: College of Materials Science and Engineering, Sichuan University, Chengdu 610065, China.
2. Hemin Zhang: Engineering Research Center of Alternative Energy Materials and Devices, Ministry of Education, Chengdu 610065, China.
3. Jae Sung Lee: School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology, Ulsan 44919, South Korea.

Source type: Preprint research article including experimental nanomaterial production na photoelectrochemical characterization.

Research areas: Photoelectrochemical water splitting, TiO₂ nanorods, hydrogen production, surface passivation, oxygen-vacancy engineering na large-area photoelectrode fabrication.

Page count: 37.

Publication platform: SSRN.

Upload date: 15 June 2026.

DOI: 10.2139/ssrn.6945787

Official link:SSRN study record

Peer-review status: Study haijapitia peer review. Pages za PDF zina warning “Preprint not peer reviewed”.

Peer-reviewed journal: As of 27 July 2026, no verified peer-reviewed journal version was found.

Original journal publisher: Haiwezi kubainishwa kwa sababu peer-reviewed journal publication haijathibitishwa. Current publication platform ni SSRN.

Author contributions

PDF states Yuanming Zhang na Yingrong Li contributed equally. Beyond this, hakuna CRediT statement inayoorodhesha separately conceptualization, experiments, analysis, software, visualization au writing roles.

Funding

Study iliungwa mkono na National Natural Science Foundation of China grants 52572250 na 52276207.

Researchers pia wanamshukuru Suilin Liu kutoka Analysis and Testing Center of Sichuan University kwa support with XPS analysis.

Conflict of interest

Authors declared no competing financial interests.

Data na supporting information

PDF states supporting information is available online. Hata hivyo, user-uploaded file haina Figures S1–S16, detailed optimization plots, equivalent-circuit parameters au additional characterizations.

Hakuna open raw-data repository, production-protocol video au analysis-code link iliyotolewa katika PDF.

Article preparation method

Makala hii ya Kituruki iliandaliwa kwa kuchunguza title na author information, experimental section, equations, main-text tables, SEM/TEM/HRTEM images, elemental maps, XRD–UV-vis–XPS spectra, photoelectrochemical curves, charge-separation graphs, stability test, triple-stack results, 10 × 10 cm² photoanode image, graphical abstract na references za uploaded 37-page preprint.

Scientific content inategemea only uploaded study. External sources zilitumika only kuverify bibliographically title, author list, DOI, SSRN upload date na current publication status. Hakuna new experimental finding kutoka outside PDF iliyoongezwa.

Main methodological limit

Strongest direct result ya study ni achievement ya 2,35 mA cm^-2 photocurrent, %96,7 surface charge separation na %96,8 current retention after 50 hours katika 1 cm² illuminated laboratory photoanode.

10 × 10 cm² production na 3,82 mA cm^-2 triple-stack result zina-support scale-up potential. Hata hivyo, large-area device efficiency, unbiased solar-hydrogen cell, quantitative gas production, long-term field durability, cost na environmental impact hazijavalidatewa.

Study hii ni preprint ambayo haijapitia peer review. Findings zinaonyesha method ni promising kwa large-area TiO₂ photoanode production; bado hazithibitishi commercial au industrial water-splitting success.