Controlling the Morphology of Barrel-Shaped Nanostructures Grown via CuZn Electro-Oxidation

Herein, we report a feasible method for forming barrel-like hybrid Cu(OH)2-ZnO structures on α-brass substrate via low-potential electro-oxidation in 1 M NaOH solution. The presented study was conducted to investigate the electrochemical behavior of CuZn in a passive range (−0.2 V–0.5 V) and its morphological changes that occur under these conditions. As found, morphology and phase composition of the grown layer strongly depend on the applied potential, and those material characteristics can be tuned by varying the operating conditions. To the best of our knowledge, the yielded morphology of barrel-like structure has not been previously observed for brass anodizing. Additionally, photoactivity under both UV and daylight irradiation-induced degradation of organic dye (methyl orange) using Cu(OH)2-ZnO composite was explored. Obtained results proved photocatalytic activity of the material that led to degradation of 43% and 36% of the compound in UV and visible light, respectively. The role of Cu(OH)2 in improving ZnO photoactivity was recognized and discussed. As implied by both the undertaken research and the literature on the subject, cupric hydroxide can act as a trap for photoexcited electrons, and thus contributes to stabilizing electron-hole recombination. This resulted in improved light-absorbing properties of the photoactive component, ZnO.


Introduction
Brass-based composites have wide applications in various fields of modern material science. As a zinc-containing alloy a brass-based composite can be used as a substrate for preparation of semiconducting materials for photoinduced catalysis. When oxidized, it forms ZnO, which is a p-type semiconductor with wide band gap of 3.37 eV [1]. ZnO alone indicates absorption of light mainly from the UV region (only~5% of visible light). Moreover, its ability to generate reactive agents during the photocatalytic process is limited due to the high rate of photo-generated charge carrier recombination [2][3][4]. Those drawbacks reduce the applicability of ZnO in solar energy-harvesting devices and motivate further exploration of modified Zn-containing bimetallic systems.
Copper-and zinc-derived oxide materials are used in many catalytic processes due to their accessibility, low price and versatility. It has been reported that CuZn-based systems efficiently catalyze CO 2 reduction [5][6][7][8], organic pollutants degradation [9,10], and are used in electrochemical sensing [11]. Subjecting copper-zinc materials to oxidation yields very diverse oxide structures in terms of both chemical composition and morphological was tested at constant potentials from passive range for 10 min each. In order to investigate the potential influence on morphology of obtained oxide films, eight potential values from −0.2 V to 0.5 V range with 0.1 V increments were selected for this study. Passive range was established from cyclic voltammetry by plotting a polarization curve in typical Tafel's coordinates (E vs. Log|j|), where E stands for applied potential and j stands for current density.

Materials Characterization
Field emission scanning electron microscopy (FE-SEM, Quanta 3D FE-SEM, FEI) was employed to examine morphology of the prepared materials.
X-ray diffraction (XRD) phase analysis of the fabricated materials was made with Rigaku ULTIMA IV diffractometer equipped with Co Kα radiation source within the 2θ range of 20 • -90 • , step size of 0.02 • and an acquisition rate of 1 • per minute. The acquired data were processed using Match! software, and the crystallographic phases were identified using COD crystallographic database.
X-ray photoelectron (XPS) analysis was carried out using a PHI-TFA XPS spectrometer (Physical Electronic Inc.), equipped with an X-ray Al-monochromatic source. The vacuum during XPS analysis was 10 −9 mbar. The analyzed area was 0.4 mm in diameter and the analysis depth was 3-5 nm. Narrow multiplex scans of the peaks were recorded using a pass energy of 23.5 eV with a step size 0.1 eV, at a take-off angle of 45 • with respect to the sample surface. Low energy electron gun was used for surface charge neutralization XPS. Spectra were processed using Multipak v8.0 (Physical Electronics Inc., Chanhassen, MN, USA). The elemental composition was determined from the XPS survey spectra. High-energy resolution spectra of O 1s and Zn 2p and Cu 2p photoelectron peaks were curve-fitted.

Photocatalysis Tests
Photocatalytic activity of the α-brass samples electro-oxidized at 0.1 V was determined by quantifying photodecomposition of methyl orange (MO) at room temperature (kept constant by a water recirculation system). The samples were immersed into 10 mL of 8 mg L −1 aqueous MO solution and placed on a perforated holder 5 mm above the bottom of the reactor and 25 cm under the source of irradiation. The solution was constantly stirred (300 rpm) throughout the photocatalysis measurements, with the magnetic stirrer placed underneath the sample holder. Prior to irradiation, the catalyst and solution were stirred in the dark for 30 min until adsorption-desorption equilibrium was achieved. Irradiation of the samples, lasting for 6 h, was conducted by using a Osram Vitalux lamp (300 W) that simulates solar spectrum and Philips HPR (125 W) Mercury lamp that provides UV light. After every hour, a fixed amount (1 mL) of the MO solution was removed for the absorption measurement at 464 nm (the maximum of the absorption spectra) using Agilent Carry 60 UV-Vis spectrophotometer, and after that returned to the solution. The absorbance was converted to MO concentration in accordance with the standard curve, showing a linear relationship between the concentration and the absorbance at this wavelength. Correspondingly, the MO adsorption in darkness with the presence of photocatalyst was also recorded every hour during 6 h. Prior to the measurement, MO solution was tested for photocatalysis in the absence of the photocatalyst to confirm its stability. The absence of change in MO concentration after 6 h of irradiation indicated its stability under the applied conditions and its degradation resulted only from the presence of a photocatalyst. The photocatalytic activity was tested and averaged for 5 samples for each irradiation source.

Materials Characterization
To investigate the electrochemical behavior of a CuZn surface subjected to passivation in highly alkaline media, current density vs. time (j-t) relation curves of each tested potential value were recorded with high resolution. As presented in Figure 1, the shape of j-t curves represents chemical transformations the CuZn surface undergoes during the initial phase of the process. The first decline of current density after electrode polarization can be observed within the first 2 s of the process irrespective of potential applied. This initial current drop represents covering the exposed metal surface with the first atomic layer of oxide, patching surface spots that were not covered with native oxide. The width of the peak representing this initial current decline narrowed down as more oxidative conditions were used. This indicates more rapid formation of the first atomic layer with more oxidative potential. In the next step, current density increases, implying breaking the previously formed native oxide layer. After the j-t curve reaches its peak value once more, current density starts to decline again which results from passive layer formation and its further propagation. The time in which this stage of the process occurs is also dependent on the potential applied. The more noble the value of potential, the more rapid the passive layer formation, that in turn prohibits current flow through the system. When the potential applied was lower than 0.3 V, current density dropped to minimal values within 10 to 15 s. Processes conducted under higher constant potential led to formation of a complete passive layer within 5 to 10 s. tested potential value were recorded with high resolution. As presented i shape of j-t curves represents chemical transformations the CuZn surface u ing the initial phase of the process. The first decline of current density afte larization can be observed within the first 2 s of the process irrespective o plied. This initial current drop represents covering the exposed metal surfac atomic layer of oxide, patching surface spots that were not covered with na width of the peak representing this initial current decline narrowed down a tive conditions were used. This indicates more rapid formation of the firs with more oxidative potential. In the next step, current density increases, im ing the previously formed native oxide layer. After the j-t curve reaches once more, current density starts to decline again which results from pas mation and its further propagation. The time in which this stage of the pr also dependent on the potential applied. The more noble the value of pote rapid the passive layer formation, that in turn prohibits current flow throu When the potential applied was lower than 0.3 V, current density dropp values within 10 to 15 s. Processes conducted under higher constant poten mation of a complete passive layer within 5 to 10 s.  Figure 2 shows diffractograms of α-brass samples subjected to potent tion in 1 M NaOH under selected applied potentials. To facilitate phase a pared materials and enable clear comparison, a reference sample of the b was also included in the analysis. As shown in Figure 2b, the diffraction p substrate represented as a black bottom line, indicating peaks for α-brass, c each oxidized sample. Appearance of additional peaks suggesting formatio talline phases can be spotted within the narrower 2 theta angle range (Figur in a typical range for Cu and Zn oxidation products. As was found, electro-o ment of CuZn under reported conditions led mainly to formation of Cu(II) line phases. Furthermore, there is a visible influence of applied potentia composition of synthesized materials. When lower values of potential Cu(OH)2 (spertiniite) was found to be a dominant crystalline component Two relatively intensive peaks at 27.7° and 39.8°, as well as two more minor and 46.2°, match very well to reflections from (021), (002), (111) and (1 Cu(OH)2, respectively. This pattern appeared in samples prepared under lo tentials (from -0.1 V to 0.3 V). Within this range of applied potential, some tr (Cu2O) can also be detected since minor peaks at 43.2° assigned to reflect  Figure 2 shows diffractograms of α-brass samples subjected to potentiostatic oxidation in 1 M NaOH under selected applied potentials. To facilitate phase analysis of prepared materials and enable clear comparison, a reference sample of the brass substrate was also included in the analysis. As shown in Figure 2b, the diffraction pattern for the substrate represented as a black bottom line, indicating peaks for α-brass, can be found in each oxidized sample. Appearance of additional peaks suggesting formation of new crystalline phases can be spotted within the narrower 2 theta angle range (Figure 2a), therefore in a typical range for Cu and Zn oxidation products. As was found, electro-oxidative treatment of CuZn under reported conditions led mainly to formation of Cu(II)-based crystalline phases. Furthermore, there is a visible influence of applied potential on the phase composition of synthesized materials. When lower values of potential were applied, Cu(OH) 2 (spertiniite) was found to be a dominant crystalline component of grown film. Two relatively intensive peaks at 27.7 • and 39.8 • , as well as two more minor peaks at 41.8 • and 46.2 • , match very well to reflections from (021), (002), (111) and (130) planes of Cu(OH) 2 , respectively. This pattern appeared in samples prepared under lower onset potentials (from -0.1 V to 0.3 V). Within this range of applied potential, some traces of cuprite (Cu 2 O) can also be detected since minor peaks at 43.2 • assigned to reflection from (111) cuprite plane might be included as relevant findings of the analysis. In the case of samples prepared at higher applied potential (0.5 V), crystalline composition differs from other synthesized materials. There are only two peaks at 41.5 • and 45.1 • that match reflections from CuO (tenorite) planes: (111) or (002) and (−111) or (200), respectively. These results suggest valid implications concerning the electro-oxidation sequence of CuZn alloy in highly alkaline media. As XRD analysis proved, Cu(OH) 2 is a dominant phase with a small amount of Cu 2 O in samples prepared when the constantly applied potential was equal or more negative than 0.3 V. Above this value, in samples prepared under more oxidative conditions, solely CuO is present in the oxide layer. These findings represent consecutive oxidation reactions that the CuZn surface was involved in within the presented system. synthesized materials. There are only two peaks at 41.5° and 45.1° from CuO (tenorite) planes: (111) or (002) and (−111) or (200), resp suggest valid implications concerning the electro-oxidation sequ highly alkaline media. As XRD analysis proved, Cu(OH)2 is a domi amount of Cu2O in samples prepared when the constantly applied more negative than 0.3 V. Above this value, in samples prepared conditions, solely CuO is present in the oxide layer. These finding oxidation reactions that the CuZn surface was involved in within Thorough electrochemical analysis using cyclic voltammetry dation sequence of CuZn alloy components when subjected to el NaOH solution. The first anodic peak (A1) that can be noticed in grams ( Figure 3) appears at approx. −1.16 V and originates from d less noble component-zinc and its subsequent bonding with hydro Thorough electrochemical analysis using cyclic voltammetry (CV) revealed the oxidation sequence of CuZn alloy components when subjected to electro-oxidation in 1 M NaOH solution. The first anodic peak (A 1 ) that can be noticed in presented voltammograms ( Figure 3) appears at approx. −1.16 V and originates from dissolution activation of less noble component-zinc and its subsequent bonding with hydroxide ions [25,26]: formation of type I oxidized zinc form to type II is accompanied by color chan surface. Dehydration in highly alkaline media results in increased zinc con within oxide layer and its grey color intensifies towards black as Zn concen creases [28]. While the potential continues to increase towards more noble valu acteristic critical potential (Ecrit) is reached. At this point, preferential dissolution ends and both components of the allo oxidation. It can be observed as significant current increase as both Zn and C and go through further transformations into consecutive forms. Therefore, E mined by a beginning of the first Cu oxidation peak marked as A3 in Figure 3 is related to the first step of Cu oxidation, namely formation of a thin porous lay on the metal surface as represented by Equation (3). As grown pores expand in of Cu2O is dissolved (Equation (4)). This process, together with metal dissolut (Equation (5)) occurring inside the oxide pores, leads to initialization of Cu mation. Therefore, a subsequent anodic peak A4 is assigned to nucleation and Cu(OH)2 phase.
The final stage of Cu oxidation relies on transformation of both Cu2O an into CuO. This phase is represented as peak A5 in Figure 3. CuO seems to be a st since the current decreased with potential shifting to more noble values. It c cluded that CuO formed a passive layer on the electrode surface. This analysi ported by XRD findings showing dependence of applied potential on phase co of oxide layers grown during potentiostatic oxidation of α-brass conducted in Obtained diffractograms indicated consecutive transformation of Cu2O into Cu finally into CuO with increasing applied potential.
As presented in Figure 4, the morphology of the CuZn alloy surface af oxidation in 1 M NaOH depends on the applied potential. When the brass s subjected to passivation under the least noble potential (−0.2 V) from the tested process yielded nanoneedles morphology. This type of nanostructure is charac As the potential increases, the initially formed first type of oxidized zinc undergoes further oxidation leading to formation of ZnO as represented by Equation (2). This step of brass oxidation can be observed as anodic peak A 2 in recorded cyclic voltammograms ( Figure 3). The potential region limited by those two Zn oxidation peaks can be referred to as preferential dissolution of the less noble component in alloy systems. As has been proven [27], preferential dissolution during brass electro-oxidation causes interdiffusion rearrangement within the alloy as Zn migrates from bulk to surface of the material. Transformation of type I oxidized zinc form to type II is accompanied by color change of brass surface. Dehydration in highly alkaline media results in increased zinc concentration within oxide layer and its grey color intensifies towards black as Zn concentration increases [28]. While the potential continues to increase towards more noble values, a characteristic critical potential (E crit ) is reached.
At this point, preferential dissolution ends and both components of the alloy undergo oxidation. It can be observed as significant current increase as both Zn and Cu dissolve and go through further transformations into consecutive forms. Therefore, E crit is determined by a beginning of the first Cu oxidation peak marked as A 3 in Figure 3. This peak is related to the first step of Cu oxidation, namely formation of a thin porous layer of Cu 2 O on the metal surface as represented by Equation (3). As grown pores expand in time, part of Cu 2 O is dissolved (Equation (4)). This process, together with metal dissolution as Cu 2+ (Equation (5)) occurring inside the oxide pores, leads to initialization of Cu(OH) 2 formation. Therefore, a subsequent anodic peak A 4 is assigned to nucleation and growth of Cu(OH) 2 phase. 2Cu Cu + nOH − → Cu(OH) 2−n n + 2e − (4) The final stage of Cu oxidation relies on transformation of both Cu 2 O and Cu(OH) 2 into CuO. This phase is represented as peak A 5 in Figure 3. CuO seems to be a stable phase since the current decreased with potential shifting to more noble values. It can be concluded that CuO formed a passive layer on the electrode surface. This analysis was supported by XRD findings showing dependence of applied potential on phase composition of oxide layers grown during potentiostatic oxidation of α-brass conducted in this study. Obtained diffractograms indicated consecutive transformation of Cu 2 O into Cu(OH) 2 and finally into CuO with increasing applied potential.
As presented in Figure 4, the morphology of the CuZn alloy surface after electrooxidation in 1 M NaOH depends on the applied potential. When the brass sample was subjected to passivation under the least noble potential (−0.2 V) from the tested range, the process yielded nanoneedles morphology. This type of nanostructure is characteristic for copper electro-oxidation in alkaline media [29]. During the process, crystallization nuclei are formed on the metal surface exposed to electro-oxidation in alkaline media defining spots for nanostructure growth and expansion. As the surface continues to be oxidized, crystallization progresses perpendicularly from the surface plane. At some point, after reaching considerable height, formed Cu(OH) 2 nanoneedles fall down, leaving morphology that can be observed in Figure 4 for the sample oxidized at −0.2 V and schematically represented by Figure 5a. Further experiments conducted at more noble constant potentials revealed the evolution of the formed nanostructure. When applying potential of −0.1 V and higher, Cu(OH) 2 nanoneedle formation is intensified, which results in more densely packed morphology of thicker nanowires. Increased density of superficially grown crystalline structures with wider cross-sectional diameters led to formation of nanowires agglomerates as implied by SEM observations of the sample electrooxidized at −0.1 V shown in Figure 4. copper electro-oxidation in alkaline media [29]. During the process, crystallization nuclei are formed on the metal surface exposed to electro-oxidation in alkaline media defining spots for nanostructure growth and expansion. As the surface continues to be oxidized, crystallization progresses perpendicularly from the surface plane. At some point, after reaching considerable height, formed Cu(OH)2 nanoneedles fall down, leaving morphology that can be observed in Figure 4 for the sample oxidized at −0.2 V and schematically represented by Figure 5a. Further experiments conducted at more noble constant potentials revealed the evolution of the formed nanostructure. When applying potential of −0.1 V and higher, Cu(OH)2 nanoneedle formation is intensified, which results in more densely packed morphology of thicker nanowires. Increased density of superficially grown crystalline structures with wider cross-sectional diameters led to formation of nanowires agglomerates as implied by SEM observations of the sample electrooxidized at −0.1 V shown in Figure 4.  As suggested by SEM micrographs for samples prepared at higher potentials (0-0.2 V), the aspect ratio of grown nanowires increases as applied potential increases. More rapid growth of thicker Cu(OH)2 nanowires causes its faster detachment from the surface. Thicker and shorter nanocrystals undergo further crystallization, expanding their sectional diameter (Figure 5c,d). This leads to formation of barrel-like agglomerates that orig- copper electro-oxidation in alkaline media [29]. During the process, crystallization nuclei are formed on the metal surface exposed to electro-oxidation in alkaline media defining spots for nanostructure growth and expansion. As the surface continues to be oxidized, crystallization progresses perpendicularly from the surface plane. At some point, after reaching considerable height, formed Cu(OH)2 nanoneedles fall down, leaving morphology that can be observed in Figure 4 for the sample oxidized at −0.2 V and schematically represented by Figure 5a. Further experiments conducted at more noble constant potentials revealed the evolution of the formed nanostructure. When applying potential of −0.1 V and higher, Cu(OH)2 nanoneedle formation is intensified, which results in more densely packed morphology of thicker nanowires. Increased density of superficially grown crystalline structures with wider cross-sectional diameters led to formation of nanowires agglomerates as implied by SEM observations of the sample electrooxidized at −0.1 V shown in Figure 4.  As suggested by SEM micrographs for samples prepared at higher potentials (0-0.2 V), the aspect ratio of grown nanowires increases as applied potential increases. More rapid growth of thicker Cu(OH)2 nanowires causes its faster detachment from the surface. Thicker and shorter nanocrystals undergo further crystallization, expanding their sec- As suggested by SEM micrographs for samples prepared at higher potentials (0-0.2 V), the aspect ratio of grown nanowires increases as applied potential increases. More rapid growth of thicker Cu(OH) 2 nanowires causes its faster detachment from the surface. Thicker and shorter nanocrystals undergo further crystallization, expanding their sectional diameter (Figure 5c,d). This leads to formation of barrel-like agglomerates that originate from initially grown cupric hydroxide nanostructures. The size of barrel-like agglomerates was found to strongly depend on applied potential. Constant expanding of their diameters along with the potential increase was disrupted with the sample electrooxidized at 0.3 V. Above this value, the size of barrel agglomerates decreased when passivation was carried out at more noble potentials. Such an outcome may be explained by the fact that more oxidative conditions accelerate surface architecture rearranging (Figure 1), and thus nanoneedle growth, their subsequent agglomeration and eventual detachment occurs more rapidly. Therefore, faster rearranging leads to formation of smaller barrel-like agglomerates but in significantly higher quantity. Drastic alteration of the morphology was found in the sample passivated at the most oxidative applied potential tested in this study, i.e., at 0.5 V. In this case, it is clear that the oxide structure followed a different growth mechanism as the surface exhibited initial phase of a flower-petal-like morphology formation (Figure 4, sample prepared at 0.5 V). Such morphology is characterized by polygonal flat nanocrystals resembling rose petals. This finding suggests the contribution of a ZnO crystalline phase in forming the passive layer on the CuZn surface [30]. ZnO crystallizes in a hexagonal wurtzite-type structure [31], which might be recognized as an origin for polygonal morphology obtained during CuZn electro-oxidation at 0.5 V. Taking this justification into account, it can be stated that 0.5 V is a sufficiently high potential value required to trigger ZnO crystallization in the presented system. Furthermore, it implies that at less noble potentials Zn oxidation yields an amorphous phase that cumulates on the brass surface as a compact oxide layer.
To confirm the presence of amorphous ZnO phase in samples passivated at lower potentials in the tested range, a CuZn sample electro-oxidized at 0.1 V was analyzed using X-ray photoelectron spectroscopy. High-resolution spectra for Zn 2p region and O 1s region are presented in Figure 6. Analyses of fitted spectra revealed the presence of zinc oxide in the tested sample (Figure 6a). Considering that XRD analysis did not show any traces of crystalline ZnO and SEM observation did not provide any structural suggestions on wurtzite formation at potentials below 0.5 V, XPS findings clearly indicate that an amorphous form of a less noble Zn component in the system is generated. As shown in Figure 6b, oxygen was found to contribute mainly as hydroxide. However, part of the oxygen species was identified as oxide. This outcome is in line with the conclusion that the passive film created under studied conditions is composed of two components: crystalline Cu(OH) 2 and amorphous ZnO, as suggested by XRD and XPS techniques, respectively. Therefore, brass electro-oxidation within the passive range (from −0.2 V to 0.5 V) in highly alkaline media leads to formation of a semi-crystalline hybrid oxide film. As proven by various reports (Table 1), to achieve complete copper oxidation and form crystalline ZnO, brass electro-oxidation has to be performed in more aggressive conditions. X-ray photoelectron spectroscopy. High-resolution spectra for Zn gion are presented in Figure 6. Analyses of fitted spectra reveale oxide in the tested sample (Figure 6a). Considering that XRD ana traces of crystalline ZnO and SEM observation did not provide any on wurtzite formation at potentials below 0.5 V, XPS findings c amorphous form of a less noble Zn component in the system is g Figure 6b, oxygen was found to contribute mainly as hydroxide oxygen species was identified as oxide. This outcome is in line w the passive film created under studied conditions is composed of talline Cu(OH)2 and amorphous ZnO, as suggested by XRD and X tively. Therefore, brass electro-oxidation within the passive range in highly alkaline media leads to formation of a semi-crystalline proven by various reports (Table 1), to achieve complete copper ox talline ZnO, brass electro-oxidation has to be performed in more a   Figure 7 shows the absorption and degradation behavior of MO with the electrooxidized α-brass samples as catalyst. The results are given as ((C 0 -C)/C 0 ), where C 0 was the initial MO concentration and C was the MO concentration after a given time in the dark, and under two different irradiation sources. The samples show decent photocatalytic activity, which increases with time. However, the usage of the UV lamp increases the photocatalytic activity from the value of 36% to the value of 43%, both at 6 h of irradiation. Photocatalytic activity of the bare α-brass samples was also tested and is presented in Figure 7a for reference. The absorption was quite small compared to the photoactivity, i.e., the increase in MO photodegradation is the result of the photocatalytic reactions in the presence of the formed nanostructures and the change in surface morphology, an increase in surface area accessible to the photocatalysis and number of photocatalitically active sites that facilitate dye molecules and reactive oxygen species (ROS) diffusion and transportation [35].

Photocatalytic Activity
The mechanism of MO photodegradation with Cu(OH) 2 -ZnO material can be understood from Figure 8. Zinc oxide as an n-type semiconductor with a band gap of 3.37 eV was found to be active in photoinduced degradation of organic compounds [36]. Because of its wide band gap, ZnO exhibits photoactivity practically only under UV light irradiation. Commonly used methods for enhancing the photocatalytic efficiency of ZnO are based on extending its optical absorption to the visible region as well as inhibition of the recombination of photogenerated electron-hole pairs [37]. Photoactivity of ZnO originates from electron-hole separation that occurs due to light excitation. By that, excited electrons (e − ) are transferred to and occupy conducting band (CB), whereas holes (h + ) are formed on valance band (VB) [38]. Both of these species are involved in the photodegradation process as they are responsible for generation of radicals causing degradation of organic compounds. Thus, under light irradiation, excited electrons and formed holes trigger generation of superoxide anion radicals ( • O 2 − ) and hydroxyl radicals ( • OH), respectively. Those consecutive steps of photoinduced MO degradation by ZnO can be represented by Equations (6)- (9):  Figure 7 shows the absorption and degradation behavior of MO with the electrooxidized α-brass samples as catalyst. The results are given as ((C0-C)/C0), where C0 was the initial MO concentration and C was the MO concentration after a given time in the dark, and under two different irradiation sources. The samples show decent photocatalytic activity, which increases with time. However, the usage of the UV lamp increases the photocatalytic activity from the value of 36% to the value of 43%, both at 6 h of irradiation. Photocatalytic activity of the bare α-brass samples was also tested and is presented in Figure 7a for reference. The absorption was quite small compared to the photoactivity, i.e., the increase in MO photodegradation is the result of the photocatalytic reactions in the presence of the formed nanostructures and the change in surface morphology, an increase in surface area accessible to the photocatalysis and number of photocatalitically active sites that facilitate dye molecules and reactive oxygen species (ROS) diffusion and transportation [35].  was found to be active in photoinduced degradation of organic compounds [36]. Because of its wide band gap, ZnO exhibits photoactivity practically only under UV light irradiation. Commonly used methods for enhancing the photocatalytic efficiency of ZnO are based on extending its optical absorption to the visible region as well as inhibition of the recombination of photogenerated electron-hole pairs [37]. Photoactivity of ZnO originates from electron-hole separation that occurs due to light excitation. By that, excited electrons (e − ) are transferred to and occupy conducting band (CB), whereas holes (h + ) are formed on valance band (VB) [38]. Both of these species are involved in the photodegradation process as they are responsible for generation of radicals causing degradation of organic compounds. Thus, under light irradiation, excited electrons and formed holes trigger generation of superoxide anion radicals ( • O2 − ) and hydroxyl radicals ( • OH), respectively. Those consecutive steps of photoinduced MO degradation by ZnO can be represented by Equations (6)- (9):

Photocatalytic Activity
O2 + e − → • O2 − H2O + h + → • OH + H + (8) h + + • O2 − + • OH + MO → CO2 + H2O (9) The presence of Cu(OH)2 should also be recognized as beneficial for ZnO photoactivity in the process. On the one hand, neighboring cupric hydroxide structures may act as electron acceptors stabilizing photoinduced electron-hole separation [39,40]. As Yu and Ran [41] confirmed, by using fluorescence quenching experiments, photogenerated electrons can be transferred from TiO2 to Cu(OH)2 formed in a close surrounding to photoactive semiconducting oxide. Thus, cupric hydroxide traps photoexcited electrons sustaining and facilitating radical generation on valance band. Simultaneously, by accepting electrons, Cu(OH)2 undergoes partial reduction to metallic copper, yielding mixed Cu 2+ /Cu 0 The presence of Cu(OH) 2 should also be recognized as beneficial for ZnO photoactivity in the process. On the one hand, neighboring cupric hydroxide structures may act as electron acceptors stabilizing photoinduced electron-hole separation [39,40]. As Yu and Ran [41] confirmed, by using fluorescence quenching experiments, photogenerated electrons can be transferred from TiO 2 to Cu(OH) 2 formed in a close surrounding to photoactive semiconducting oxide. Thus, cupric hydroxide traps photoexcited electrons sustaining and facilitating radical generation on valance band. Simultaneously, by accepting electrons, Cu(OH) 2 undergoes partial reduction to metallic copper, yielding mixed Cu 2+ /Cu 0 clusters. On the other hand, the surface of the Cu(OH) 2 structure may adsorb -OH and, therefore, act as an heterogenous catalyst providing and consequently accelerating formation of hydroxyl radicals as main agents triggering MO decomposition [42]. Moreover, this effect can be intensified, as cupric hydroxide reduction initiated by photoelectrons releases OH − ions that can be included as an additional source for radical formation.

Conclusions
Electrochemical passivation of commercially available α-brass in highly alkaline media can be used as an inexpensive and facile method for preparing a nanostructured material active in photodegradation of an organic dye (methyl orange). Thorough analysis of electrochemical behavior and phase composition provided interesting findings on the oxidation sequence that brass surface follows when subjected to passivation in 1 M NaOH. It was found that potentiostatic oxidation at mild conditions results in formation of a thin semi-crystalline Cu(OH) 2 -ZnO layer on brass surface at potentials lower than 0.5 V vs. Ag|AgCl|KCl sat . The grown oxide films indicated novel morphology of barrel-like structures observed for the first time in the brass surface oxidation. Additionally, the size of these structures can be adjusted by changing the applied potential value varying surface area of the material, which can find application in preparing brass-based electrodes for electrocatalytic purposes. By tuning the applied potential, the presented method can be used for formation of a nanostructured material of three different morphologies on α-brass surface. Nanowire morphology consisting mainly of Cu(OH) 2

Conflicts of Interest:
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.