Use of a New Non-Pyrophoric Liquid Aluminum Precursor for Atomic Layer Deposition

An Al2O3 thin film has been grown by vapor deposition using different Al precursors. The most commonly used precursor is trimethylaluminum, which is highly reactive and pyrophoric. In the purpose of searching for a more ideal Al source, the non-pyrophoric aluminum tri-sec-butoxide ([Al(OsBu)3], ATSB) was introduced as a novel precursor for atomic layer deposition (ALD). After demonstrating the deposition of Al2O3 via chemical vapor deposition (CVD) and ‘pulsed CVD’ routes, the use of ATSB in an atomic layer deposition (ALD)-like process was investigated and optimized to achieve self-limiting growth. The films were characterized using spectral reflectance, ellipsometry and UV-Vis before their composition was studied. The growth rate of Al2O3 via the ALD-like process was consistently 0.12 nm/cycle on glass, silicon and quartz substrates under the optimized conditions. Scanning electron microscopy and transmission electron microscopy images of the ALD-deposited Al2O3 films deposited on complex nanostructures demonstrated the conformity, uniformity and good thickness control of these films, suggesting a potential of being used as the protection layer in photoelectrochemical water splitting.


Introduction
As a dielectric metal oxide, Al 2 O 3 exhibits a high transparency, large bandgap and excellent electrical insulation properties, hence it is widely applied in electronic devices and electrochemistry, e.g., as a protection layer [1][2][3][4][5][6].Commonly used methods for making Al 2 O 3 films include sol-gel, sputtering, evaporation, physical vapor deposition (PVD), chemical vapor deposition (CVD) and atomic layer deposition (ALD) [7][8][9][10][11].CVD is a widely used thin film deposition technique to provide good quality thin films with control over the chemical composition.ALD uses sequential pulses of precursors which react with the surface in a self-limiting way [12,13], giving better control of material thickness, stoichiometry and conformality than normally afforded via CVD or PVD [14,15].
Careful consideration must be exercised in precursor selection for all vapor deposition processes.Whilst they may be gas, liquid or solid at room temperature, they must be volatile and thermally stable under zationvaporization conditions.Widely used oxygen-containing precursors are H 2 O, O 2 and O 3 , whilst precursors for metals include metal halides, metal alkyls, metal alkoxides, etc [16].Typical precursors for Al include aluminum chloride [AlCl 3 ] [17], trimethylaluminum [TMA, Al(CH 3 ) 3 ] [18], aluminum tri-isopropoxide [AIP, Al(O i Pr) 3 ] [19] and aluminum acetylacetonate [Al(acac) 3 ] [20].Among the above, TMA is the most commonly used Al precursor, but it is highly reactive, moisture sensitive and pyrophoric, which makes handling inconvenient.The most common non-pyrophoric Al precursor is Al(O i Pr) 3 , but as a solid precursor there is significant possibility of condenzation during vapor transport leading to blockages and consequent reactor downtime.Therefore, in this study, the suitability of aluminum tri-sec-butoxide (ATSB, [Al(O s Bu) 3 ], [Al(OCH(CH 3 )CH 2 CH 3 ) 3 ] was investigated as an Al precursor.At room temperature, ATSB is a non-pyrophoric liquid which contains no halogens atoms (Figure 1).ATSB has been previously reported as a metal-organic chemical vapor deposition (MOCVD) precursor [21].However, to the best of our knowledge, it has never been studied for use in ALD before.
Materials 2018, 11, x FOR PEER REVIEW 2 of 14 reactive, moisture sensitive and pyrophoric, which makes handling inconvenient.The most common non-pyrophoric Al precursor is Al(O i Pr)3, but as a solid precursor there is significant possibility of condenzation during vapor transport leading to blockages and consequent reactor downtime.Therefore, in this study, the suitability of aluminum tri-sec-butoxide (ATSB, [Al(O s Bu)3], [Al(OCH(CH3)CH2CH3)3] was investigated as an Al precursor.At room temperature, ATSB is a nonpyrophoric liquid which contains no halogens atoms (Figure 1).ATSB has been previously reported as a metal-organic chemical vapor deposition (MOCVD) precursor [21].However, to the best of our knowledge, it has never been studied for use in ALD before.In this research, ATSB was used in single-source CVD (also in 'pulsed CVD' mode) and ALD to deposit Al2O3 thin films.The deposition parameters (precursor temperature, deposition temperature, carrier gas flow rate, pulse time and purge time) were optimized on different substrates to demonstrate controllable and uniform film deposition.

Chemicals and Materials
Aluminum tri-sec-butoxide (ATSB, 97%) was purchased from Sigma-Aldrich (St. Louis, MI, USA).Propan-2-ol (laboratory reagent grade), methanol (HPLC grade) and ethanol absolute (HPLC grade) were received from Fisher Scientific (Loughborough, UK).Super premium glass microscope slides were purchased from VWR International, LLC (Radnor, PA, USA).Quartz slides were obtained from Wuxi Crystal and Optical Instrument Co., Ltd (Wuxi, China).P-doped silicon wafers were bought from Suzhou Crystal Silicon Electronics & Technology Co., Ltd (Suzhou, China).Deionized water was obtained from ELGA Purelab Option (ELGA LabWater).Liquid nitrogen and argon gas were provided by the BOC group (Surrey, UK).

Al2O3 Thin Film Deposition via CVD
Aluminum tri-sec-butoxide (maintained at 120 °C) was used as a single-source CVD precursor.ATSB vapor was carried to the reaction chamber via Ar carrier gas.To operate in CVD mode, the inlet valve and outlet valve of the ATSB bubbler were kept open for the entire deposition and no water co-reactant was used.The ATSB bubbler was heated to 115 °C, 120 °C, 125 °C, respectively.The gas flow rate was set to 150 sccm.The substrate was heated to 300 °C, 350 °C or 400 °C for film deposition.Each deposition took 24 hours.A deposition mode called 'pulsed CVD' was used to mimic the metal precursor half cycle in ALD.A cycle in pulsed CVD mode only contained one ATSB pulse (20 s or 1 min) and one argon purge (1 min).For all experiments, the pipework was heated to 170 °C to prevent precursor condenzation.

Al2O3 Thin Film Deposition via ALD
Al2O3 films were grown via ALD between 250 and 1000 deposition cycles.ATSB (maintained at 120 °C) and deionized water (maintained at 5 °C) were held in bubblers as precursors and the precursor vapor was carried to the reaction chamber via Ar carrier gas.Each cycle consisted of an ATSB pulse (2.5-20 s), an argon purge (1-3 min), then followed by a 2 s water vapor pulse and an argon purge of at least 3 min.It is worth noting that the minimum dose time in this reactor is 2 s, In this research, ATSB was used in single-source CVD (also in 'pulsed CVD' mode) and ALD to deposit Al 2 O 3 thin films.The deposition parameters (precursor temperature, deposition temperature, carrier gas flow rate, pulse time and purge time) were optimized on different substrates to demonstrate controllable and uniform film deposition.

Chemicals and Materials
Aluminum tri-sec-butoxide (ATSB, 97%) was purchased from Sigma-Aldrich (St. Louis, MI, USA).Propan-2-ol (laboratory reagent grade), methanol (HPLC grade) and ethanol absolute (HPLC grade) were received from Fisher Scientific (Loughborough, UK).Super premium glass microscope slides were purchased from VWR International, LLC (Radnor, PA, USA).Quartz slides were obtained from Wuxi Crystal and Optical Instrument Co., Ltd (Wuxi, China).P-doped silicon wafers were bought from Suzhou Crystal Silicon Electronics & Technology Co., Ltd (Suzhou, China).Deionized water was obtained from ELGA Purelab Option (ELGA LabWater).Liquid nitrogen and argon gas were provided by the BOC group (Surrey, UK).

Al 2 O 3 Thin Film Deposition via CVD
Aluminum tri-sec-butoxide (maintained at 120 • C) was used as a single-source CVD precursor.ATSB vapor was carried to the reaction chamber via Ar carrier gas.To operate in CVD mode, the inlet valve and outlet valve of the ATSB bubbler were kept open for the entire deposition and no water co-reactant was used.The ATSB bubbler was heated to 115 • C, 120 • C, 125 • C, respectively.The gas flow rate was set to 150 sccm.The substrate was heated to 300 • C, 350 • C or 400 • C for film deposition.Each deposition took 24 hours.A deposition mode called 'pulsed CVD' was used to mimic the metal precursor half cycle in ALD.A cycle in pulsed CVD mode only contained one ATSB pulse (20 s or 1 min) and one argon purge (1 min).For all experiments, the pipework was heated to 170 • C to prevent precursor condenzation.

Al 2 O 3 Thin Film Deposition via ALD
Al 2 O 3 films were grown via ALD between 250 and 1000 deposition cycles.ATSB (maintained at 120 • C) and deionized water (maintained at 5 • C) were held in bubblers as precursors and the precursor vapor was carried to the reaction chamber via Ar carrier gas.Each cycle consisted of an ATSB pulse (2.5-20 s), an argon purge (1-3 min), then followed by a 2 s water vapor pulse and an argon purge of at least 3 min.It is worth noting that the minimum dose time in this reactor is 2 s, which inevitably requires longer purge times than normally observed in an ALD reaction.The gas flow rate through the ATSB bubbler and through the purge line was set to 120 sccm.The substrate was heated to 200 • C, 250 • C, 300 • C or 350 • C for film deposition, respectively.Glass, quartz and silicon substrates were used for the comparative study of film growth and quality.Al 2 O 3 films of different thickness (20-300 cycles) were also deposited on Au NPs/WO 3 NRs system to demonstrate uniformity.

Characterization
The thickness of the films was measured using a Variable Angle Spectroscopic Ellipsometer (VASE, SEMILAB SE-2000, Semilab, Budapest, Hungary) in the wavelength range of 1.25-5 eV with variable measuring angle (56 • , 57 • , 58 • , 59 • and 60 • ).The optical model used for ellipsometric characterization of the ALD-like thin films was an effective medium approximation (EMA) comprising of air and the aluminum oxide (previously reported refractive index) with variable concentration [22,23].Glass substrates were modelled using standard parameters for sodlime glass and the silicon substrates were assumed to have a 1 nm native oxide.Fittings of the ellipsometric data converged without the need for substrate-film and film-air diffusion layers.The crystalline structure of the samples was examined by X-ray diffraction (XRD, Bruker, Billerica, MA, USA, D8 Discover LynxEye) with a current of 40 mA and a voltage of 40 kV.The data was collected with a scanning rate of 0.05 • /s over a 2θ range from 10 • to 66 • counted at 2 s per step.X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, Waltham, MA, USA, Neslab ThermoFlex1400) with a monochromatic Al Kα X-ray source was used to measure the elemental composition and electronic states of the elements.Tapping mode atomic force microscopy (AFM, Bruker, Billerica, MA, USA, Dimension FastScan2-SYS) using a NCLR tip was employed to investigate the surface structure of the sample.The morphologies of the samples were observed by field emission scanning electron microscope (FESEM, JEOL, Tokyo, Japan, JSM-7800F) with a current of 10 mA and an acceleration voltage of 5-15 kV after Au coating.Films deposited on Au/WO 3 films and were examined by transmission electron microscopy (TEM, JEOL, Tokyo, Japan, JEM-2100) after they were sonicated in methanol and dropped on copper grids to show the details.Photoelectrochemical (PEC) measurement was carried out using a homebuilt PEC system with a Xe lamp (75W, USHIO, California, USA) equipped with an AM 1.5 G filter (Newport, California, USA) using a Pt mesh counter electrode, a Ag/AgCl reference electrode and Na 2 SO 4 electrolyte.All the photoelectrochemical tests were performed using a potentiostat (Interface 1000, Gamry, Pennsylvania, USA).

Deposition Parameter Optimization for CVD
Aluminum sec-butoxide ([Al(O s Bu) 3 ]) is a high viscosity liquid metal alkoxide, typically preferred for CVD reactions in comparison to solid precursors such as Al(O i Pr) 3 .The vapor pressure of ATSB at different temperatures was calculated based on data from literature [24,25] (Figure S1).Note that the function of vapor pressure in the Antoine equation is fitted as lg(PTorr) = 10.16 − 4177.25/T.
According to our previous study [26], a metal precursor vapor pressure higher than 0.13 Torr would generate enough metal precursor vapor in our reactor for one pulse.Therefore, a temperature above 115 • C would provide sufficient vapor pressure.
Initial experiments focused on optimizing the precursor temperature, gas flow rate and deposition temperature for CVD deposition.After several attempts at 115 • C, 120 • C and 125 • C, the optimum ATSB bubbler temperature was found to be 120 • C.Meanwhile, increasing the precursor temperature further did not produce any significant increase in thickness at a deposition temperature of 350 • C (additional precursor not utilised in deposition).The composition of the films was analyzed by XRD, Raman spectroscopy (inVia Raman microscope, Renishaw, Gloucestershire, UK) and XPS.The crystallization temperature of γ-Al 2 O 3 is at least 600 • C and 1000 • C is needed for it to transform to α-Al 2 O 3 [27][28][29].As the deposition temperature here was much lower than the crystallization temperature of Al 2 O 3 , it is consistent that neither XRD patterns nor Raman spectra showed pronounced peaks (Figures S2 and S3).For a film deposited at 400 • C for 24 h with 120 • C ATSB precursor temperature and 150 sccm gas flow, the Al 2p spectrum (Figure 2a) displayed an intense peak for the Al 2p ionization at 73.6 eV which matches with the binding energy of Al 3+ in Al 2 O 3 [30].The Si 2p peak is of very low intensity (Figure 2b), which indicates that no Si from the glass substrate is observed, thus the Al 2 O 3 film covers the entire area of the analysis spot.These findings are consistent with the films deposited via CVD using ATSB as a single-source precursor as an amorphous Al 2 O 3 .
Materials 2018, 11, x FOR PEER REVIEW 4 of 14 temperature of Al2O3, it is consistent that neither XRD patterns nor Raman spectra showed pronounced peaks (Figures S2 and S3).For a film deposited at 400 °C for 24 h with 120 °C ATSB precursor temperature and 150 sccm gas flow, the Al 2p spectrum (Figure 2a) displayed an intense peak for the Al 2p ionization at 73.6 eV which matches with the binding energy of Al 3+ in Al2O3 [30].
The Si 2p peak is of very low intensity (Figure 2b), which indicates that no Si from the glass substrate is observed, thus the Al2O3 film covers the entire area of the analysis spot.These findings are consistent with the films deposited via CVD using ATSB as a single-source precursor as an amorphous Al2O3.To observe the effect of substrate temperature, the CVD deposition temperatures were set to 300 °C, 350 °C and 400 °C (with a bubbler temperature of 125 °C).AFM scans (Figure 3) show an increase in the roughness of film surface as deposition temperature increases; Rq (300 °C) = 3.45 nm, Rq (350 °C) = 3.76 nm and Rq (400 °C) = 18.21 nm, which can also be seen in SEM images (Figure 3).Increasing the deposition temperature increases film roughness, which is typically unfavorable for protective coatings.Consequently, we concluded that 350 °C provided a good balance between growth rate and surface roughness.To observe the effect of substrate temperature, the CVD deposition temperatures were set to 300 • C, 350 • C and 400 • C (with a bubbler temperature of 125 • C).AFM scans (Figure 3) show an increase in the roughness of film surface as deposition temperature increases; R q (300 • C) = 3.45 nm, R q (350 • C) = 3.76 nm and R q (400 • C) = 18.21 nm, which can also be seen in SEM images (Figure 3).Increasing the deposition temperature increases film roughness, which is typically unfavorable for protective coatings.Consequently, we concluded that 350 • C provided a good balance between growth rate and surface roughness.

Pulsed CVD
After the optimization of CVD parameters, a pulsed CVD process was introduced to identify the minimum duration required for a metal precursor pulse, thereby preventing over-dosing.In order to obtain vapor transport (film growth), the pulse of ATSB was set to 20 s or 1 min, the deposition temperature was 350 °C and the precursor temperature was fixed at 120 °C with an Ar flow rate of 120 sccm.Comparing the AFM images of Al2O3 films deposited via continuous CVD (24 h), pulse CVD (1 min ATSB pulse, 1 min Ar purge, 1000 cycles) and pulsed CVD (20 s ATSB pulse, 1 min Ar purge, 1000 cycles).Figure 4 shows the surface roughness decrease from Rq (continuous deposition) = 10.19 nm to Rq (1min pulse) = 2.51 nm and Rq (20 s pulse) = 2.27 nm.We concluded from these data that a 20 s pulse produced complete coverage and was therefore a suitable starting point for ATSB precursor dosing in an ALD process.

Pulsed CVD
After the optimization of CVD parameters, a pulsed CVD process was introduced to identify the minimum duration required for a metal precursor pulse, thereby preventing over-dosing.In order to obtain vapor transport (film growth), the pulse of ATSB was set to 20 s or 1 min, the deposition temperature was 350 • C and the precursor temperature was fixed at 120 • C with an Ar flow rate of 120 sccm.Comparing the AFM images of Al 2 O 3 films deposited via continuous CVD (24 h), pulse CVD (1 min ATSB pulse, 1 min Ar purge, 1000 cycles) and pulsed CVD (20 s ATSB pulse, 1 min Ar purge, 1000 cycles).Figure 4 shows the surface roughness decrease from R q (continuous deposition) = 10.19 nm to R q (1min pulse) = 2.51 nm and R q (20 s pulse) = 2.27 nm.We concluded from these data that a 20 s pulse produced complete coverage and was therefore a suitable starting point for ATSB precursor dosing in an ALD process.

Deposition Parameter Optimization for ALD
The reaction mechanism when using ATSB and water as ALD precursors may be expected as [31,32]: with x = 1 being the monodentate, x = 2, bidentate and x = 3, tridentate, respectively, depending on the number of alkoxide ligands.
A full ALD reaction contains two half reactions.In the first half reaction, ATSB reacts with OH groups on the substrate surface to produce (-O-)xAl(O s Bu)3−x, where x is the number of OH surface sites react with ATSB which also determines the geometrical configuration on the surface.The (-O-)xAl(O s Bu)3−x group can react with H2O to form (-O-)xAl(OH)3−x in the second half of the reaction, therefore, the surface layer is covered with OH groups again, allowing the first half reaction to take place again [33].

Deposition Parameter Optimization for ALD
The reaction mechanism when using ATSB and water as ALD precursors may be expected as [31,32]: x -OH (s) with x = 1 being the monodentate, x = 2, bidentate and x = 3, tridentate, respectively, depending on the number of alkoxide ligands.A full ALD reaction contains two half reactions.In the first half reaction, ATSB reacts with OH groups on the substrate surface to produce (-O-) x Al(O s Bu) 3−x , where x is the number of OH surface sites react with ATSB which also determines the geometrical configuration on the surface.The (-O-) x Al(O s Bu) 3−x group can react with H 2 O to form (-O-) x Al(OH) 3−x in the second half of the reaction, therefore, the surface layer is covered with OH groups again, allowing the first half reaction to take place again [33].
The deposition temperature used in ALD should be lower than that used in CVD to avoid uncontrolled CVD-like film growth, therefore the ALD deposition temperature was decreased in comparison to CVD in increments from 350 • C to 300 • C, 250 • C and 200 • C whilst the other parameters were kept unchanged (20 s ATSB pulse, 1 min Ar purge, 2 s H 2 O pulse and 3 min Ar purge with 120 sccm Ar gas flow deposited for 500 cycles).XPS spectra of this series of samples indicate that films deposited at 200 • C, 250 • C and 300 • C show similar binding energy intensity at approximately 74 eV (Figure 5).The growth rate of Al 2 O 3 was between 1.9-2.3nm/cycle in the deposition temperature range of 200 • C-300 • C with a 20 s pulse time, thus 200 • C was selected in further experiments (Figure S4).The thickness and growth rate were found to be much higher than previously reported values (0.1 nm/cycle) [34][35][36][37][38][39] indicating a combination of ALD and CVD-like growth.One likely explanation is that the dose time of ATSB was too long for the purge time used, therefore the ATSB dose time was reduced from 20 s to 2.5 s with the aim to avoid CVD-like growth.Changing the ATSB purge time from 1 min to 3 min reduced the deposition rate, while more than 3 min for a 2.5 s dose time resulted in no change in the growth rate, therefore 3 min purge was considered optimum for a 2.5 s Similarly, for a 2 s H 2 O dose, changing the purge time from 3 min to 5 min had no effect, indicating that a 3 min purge was sufficient to ensure all water was evacuated between cycles.
The deposition temperature used in ALD should be lower than that used in CVD to avoid uncontrolled CVD-like film growth, therefore the ALD deposition temperature was decreased in comparison to CVD in increments from 350 °C to 300 °C, 250 °C and 200 °C whilst the other parameters were kept unchanged (20 s ATSB pulse, 1 min Ar purge, 2 s H2O pulse and 3 min Ar purge with 120 sccm Ar gas flow deposited for 500 cycles).XPS spectra of this series of samples indicate that films deposited at 200 °C, 250 °C and 300 °C show similar binding energy intensity at approximately 74 eV (Figure 5).The growth rate of Al2O3 was between 1.9-2.3nm/cycle in the deposition temperature range of 200 °C-300 °C with a 20 s pulse time, thus 200 °C was selected in further experiments (Figure S4).The thickness and growth rate were found to be much higher than previously reported values (0.1 nm/cycle) [34][35][36][37][38][39] indicating a combination of ALD and CVD-like growth.One likely explanation is that the dose time of ATSB was too long for the purge time used, therefore the ATSB dose time was reduced from 20 s to 2.5 s with the aim to avoid CVD-like growth.Changing the ATSB purge time from 1 min to 3 min reduced the deposition rate, while more than 3 min for a 2.5 s dose time resulted in no change in the growth rate, therefore 3 min purge was considered optimum for a 2.5 s Similarly, for a 2 s H2O dose, changing the purge time from 3 min to 5 min had no effect, indicating that a 3 min purge was sufficient to ensure all water was evacuated between cycles.With a 2.5 s dose time, the thickness reduced to approx.50 nm after 500 cycles , which gave growth rates in the range of 0.12 to 0.15 nm/cycle with successful deposition on silicon, quartz and glass substrates (Table S1).The growth rate using ATSB as an ALD precursor is similar to that using TMA or AlCl3 (0.08-0.2 nm/cycle) [38,[40][41][42].Thus, the ALD-like parameters were fixed at an ATSB bubbler temperature of 120 °C, deposition temperature of 200 °C and gas flow rate of 120 sccm (a full cycle includes 2.5 s ATSB pulse, 3 min Ar purge, 2 s H2O pulse and 3 min Ar purge).
The density of the Al2O3 component in the ellipsometric EMA model varied between 0.720 and 0.757 (Table S1 and Figure S5).The films deposited with short 2.5 s ATSB pulse times showed lower growth rates (0.12-0.15 nm/cycle) and higher densities than those grown with longer 20 s pulses.The root mean square roughness of Al2O3 film on three substrates were Rq(silicon) = 1.48 nm, Rq(quartz) = 1.16 nm and Rq(glass) = 0.96 nm, suggesting an applicability of Al2O3 on a variety of substrates (Figure 6).With a 2.5 s dose time, the thickness reduced to approx.50 nm after 500 cycles, which gave growth rates in the range of 0.12 to 0.15 nm/cycle with successful deposition on silicon, quartz and glass substrates (Table S1).The growth rate using ATSB as an ALD precursor is similar to that using TMA or AlCl 3 (0.08-0.2 nm/cycle) [38,[40][41][42].Thus, the ALD-like parameters were fixed at an ATSB bubbler temperature of 120 • C, deposition temperature of 200 • C and gas flow rate of 120 sccm (a full cycle includes 2.5 s ATSB pulse, 3 min Ar purge, 2 s H 2 O pulse and 3 min Ar purge).
The density of the Al 2 O 3 component in the ellipsometric EMA model varied between 0.720 and 0.757 (Table S1 and Figure S5).The films deposited with short 2.5 s ATSB pulse times showed lower growth rates (0.12-0.15 nm/cycle) and higher densities than those grown with longer 20 s pulses.The root mean square roughness of Al 2 O 3 film on three substrates were R q(silicon) = 1.48 nm, R q(quartz) = 1.16 nm and R q(glass) = 0.96 nm, suggesting an applicability of Al 2 O 3 on a variety of substrates (Figure 6).

ALD Al2O3 Films on Complex Structured Au/WO3
Our aim is to use this ALD Al2O3 film as a passivation layer for photoelectrode.In former studies of ALD thin films that include Al2O3 and other oxides [43][44][45], it has been mentioned that either an ultrathin film of several nanometers or a thin film with a thickness above 20 nm showed different degrees of current density enhancement during a photoelectrochemical test.
Al2O3 films were deposited via the ALD-like process on a complex nanostructure consisting of Au nanoparticles on needle-like WO3 nanostructures (Au/WO3, Figures 7 and 8).Au/WO3 is an electrode prepared in house on an FTO glass for water splitting.However, it is not stable in any kind of electrolyte.Therefore, Al2O3 films were deposited with the aim to protect the electrode without severely affecting the photocurrent required for water splitting.The SEM image of the Au/WO3 shows a plane of nanoneedles which has a high surface area and high porosity.Due to the complex structure, the thickness of the ALD layer was difficult to measure using an optical method, therefore film thickness during deposition was recorded using a 'witness' planar substrate inserted in the reactor at the same time with the sample.Growth rate as a function of number of cycles was plotted  Our aim is to use this ALD Al 2 O 3 film as a passivation layer for photoelectrode.In former studies of ALD thin films that include Al 2 O 3 and other oxides [43][44][45], it has been mentioned that either an ultrathin film of several nanometers or a thin film with a thickness above 20 nm showed different degrees of current density enhancement during a photoelectrochemical test.
Al 2 O 3 films were deposited via the ALD-like process on a complex nanostructure consisting of Au nanoparticles on needle-like WO 3 nanostructures (Au/WO 3 , Figures 7 and 8).Au/WO 3 is an electrode prepared in house on an FTO glass for water splitting.However, it is not stable in any kind of electrolyte.Therefore, Al 2 O 3 films were deposited with the aim to protect the electrode without severely affecting the photocurrent required for water splitting.The SEM image of the Au/WO 3 shows a plane of nanoneedles which has a high surface area and high porosity.Due to the complex structure, the thickness of the ALD layer was difficult to measure using an optical method, therefore film thickness during deposition was recorded using a 'witness' planar substrate inserted in the reactor at the same time with the sample.Growth rate as a function of number of cycles was plotted (Figure 8a).The growth rate of Al 2 O 3 on both the witness plate and the nanostructures was between 0.1 nm/cycle and 0.13 nm/cycle (shown in Figure S6).For a given number of cycles, similar thickness were achieved on both surfaces.There is a strong correlation between the film thickness and the number of ALD cycles (Table 1), e.g., 50 cycles and 300 cycles gave 4 nm and 40 nm respectively on both the planar substrate (measured using ellipsometry) and the complex nanomaterial architecture (measured by TEM, Figure 8c,d).
Materials 2018, 11, x FOR PEER REVIEW 9 of 14 (Figure 8a).The growth rate of Al2O3 on both the witness plate and the nanostructures was between 0.1 nm/cycle and 0.13 nm/cycle (shown in Figure S6).For a given number of cycles, similar thickness were achieved on both surfaces.There is a strong correlation between the film thickness and the number of ALD cycles (Table 1), e.g., 50 cycles and 300 cycles gave 4 nm and 40 nm respectively on both the planar substrate (measured using ellipsometry) and the complex nanomaterial architecture (measured by TEM, Figure 8c,d).The XPS spectrum of Al 2p shows that the intensity of Al 2p peak increases with the increase of the number of cycles (Figure 9a).In contrast, the intensity of Au 4f peak decreases with the increase of the number of cycles, with very little Au 4f signal intensity remaining after 50 cycles (~6 nm Al2O3) and no Au signal detected after 150 cycles (~18 nm) (Figure 9b).Our interest in the conformal Al2O3  The XPS spectrum of Al 2p shows that the intensity of Al 2p peak increases with the increase of the number of cycles (Figure 9a).In contrast, the intensity of Au 4f peak decreases with the increase of the number of cycles, with very little Au 4f signal intensity remaining after 50 cycles (~6 nm Al 2 O 3 ) and no Au signal detected after 150 cycles (~18 nm) (Figure 9b).Our interest in the conformal Al 2 O 3 is as a barrier/corrosion resistant layer for electrochemistry/photoelectrochemistry.To evaluate the quality of the coating, the photoelectrochemical performance was tested (Figure 10).The results demonstrate that increasing the Al 2 O 3 film thickness beyond 50 deposition cycles (~6 nm) lead to a significant decrease in photocurrent density, as expected for high dieletric material.Consequently, both the XPS and PEC data indicate that even for a 50 cycles coating, the Al 2 O 3 film was conformal and without pinholes even on these high aspect ratio, porous nanostructures.

Conclusions
In the study, the parameters of Al2O3 thin films atomic layer deposition using aluminum tri-secbutoxide as a new Al precursor and water for ALD were investigated.The CVD, pulsed CVD and

Conclusions
In the study, the parameters of Al2O3 thin films atomic layer deposition using aluminum tri-secbutoxide as a new Al precursor and water for ALD were investigated.The CVD, pulsed CVD and ALD process was optimized, with the best deposition conditions for ALD as follows: ATSB bubbler

Conclusions
In the study, the parameters of Al 2 O 3 thin films atomic layer deposition using aluminum tri-sec-butoxide as a new Al precursor and water for ALD were investigated.The CVD, pulsed CVD and ALD process was optimized, with the best deposition conditions for ALD as follows: ATSB bubbler temperature 120 • C; deposition temperature 200 • C; gas flow rate 120 sccm; ATSB pulse time 2.5 s; Ar purge time 3 min; H 2 O pulse time 2 s; Ar purge time 3 min.A stable ALD process with a growth rate of 0.12-0.15nm/cycle was observed by measuring the Al 2 O 3 film thickness on different substrates using ellipsometry.The composition and morphology of the as-synthesized films were analyzed and followed by the comparison of film deposition under different conditions.SEM, AFM and XPS data indicated that the obtained films were dense and continuous with a low concentration of impurities.TEM images of Al 2 O 3 thin films prepared from ATSB and water on complex nanostructures show uniformity, conformality and good control of thickness, strongly suggesting the potential of using this new ALD precursor for Al 2 O 3 thin films.These Al 2 O 3 films are likely to be used as a protection layer on the Au/WO 3 electrode for photoelectrochemical water splitting.

Figure 2 .
Figure 2. High resolution XPS spectra of (a) Al 2p and (b) Si 2p of Al2O3 film deposited on glass via CVD.

Figure 2 .
Figure 2. High resolution XPS spectra of (a) Al 2p and (b) Si 2p of Al 2 O 3 film deposited on glass via CVD.

Figure 8 .
Figure 8.(a) Al 2 O 3 film growth rate (blue) and film thickness (black) as a function of reaction cycles, (b) Au/WO 3 structure, (c) Au/WO 3 with 50 cycles ALD Al 2 O 3 growth on top, (d) Au/WO 3 with 300 cycles ALD Al 2 O 3 growth on top, (e) Schematic procedure of ALD Al 2 O 3 growth.
Figure S2: XRD pattern of Al 2 O 3 film deposited via CVD.This example was made under condition: precursor temperature 120 • C; deposition temperature 350 • C; gas flow rate 150 sccm; deposition time 24 h. Figure S3: Raman spectra of Al 2 O 3 film deposited via CVD.This example was made under condition: precursor temperature 120 • C; deposition temperature 350 • C; gas flow rate 150 sccm; deposition time 24 h. Figure S4: Growth rates of the Al 2 O 3 films as a function of deposition temperature from 200 • C to 300 • C established via ellipsometry.The deposition conditions were 20 s ATSB pulse, 1 min Ar purge, 2 s H 2 O pulse and 3 min Ar pulse for 500 cycles.

Table 1 .
Summary of the ALD Al 2 O 3 thin film deposition parameters and the film thickness on Au/WO 3 , FTO and silicon.