Metallic Plants – The Beauty of Crystalline Silver Dendrites
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The following article was originally published in the journal for educators Chemia w Szkole (eng. Chemistry in School) (3/2015):
Silver Dendrites
Metals have played a pivotal role in the development of human civilization, as reflected in the division of prehistory into distinct eras. The Stone Age, which was both the earliest and the longest-lasting period, relied on tools made from various types of stone. Each subsequent age was defined by the mastery of methods for obtaining and utilizing different metals: the Copper, Bronze, and Iron Ages. Of course, metals weren’t used solely for practical tools and equipment. Precious metals like gold and silver, valued for their special properties, beauty, and rarity, were also used to make ornaments and even served as currency.
A glance at the periodic table confirms that metals make up the vast majority of known elements. But what exactly defines a metal? Among other types of substances, metals are distinguished by the presence of free electrons within their crystal lattice. These mobile charge carriers are responsible for metals’ excellent electrical conductivity. Solid metals also typically exhibit luster, ductility, malleability, and good thermal conductivity. From a chemist’s point of view, it’s also worth noting that metals tend to form compounds with more basic and nucleophilic properties, rather than acidic or electrophilic ones.
Among all metals, a special class known as noble metals stands out. This is a conventional term for chemically unreactive metals, including the platinum group (ruthenium, rhodium, palladium, osmium, iridium, and platinum) and two coinage metals: silver and gold. Some definitions also include rhenium and mercury. Due to its slightly higher reactivity, copper is sometimes referred to as a semi-noble metal.
In their solid state, metals exist in a crystalline form. Metal crystals can develop into a wide variety of shapes. One particularly interesting example is the dendrite, a branching structure composed of tiny interconnected crystals that typically resemble plant shoots.
The formation of dendrites requires specific conditions. These structures are not only visually striking but also scientifically valuable, as studying them offers insights into the processes that shape many aspects of the natural world. For these very reasons, we’re going to grow some silver dendrites!
The Experiment
Crystalline silver in the form of dendrites can be obtained in two similar, yet slightly different, ways. Each method yields a distinct result, so I encourage readers to try both versions.
In both experiments, we will use silver(I) nitrate, AgNO3, as the silver ion source. This compound is still sometimes called "lapis", as alchemists once referred to it as lapis infernalis, which is Latin for "infernal stone". Why such a dramatic name for a substance that appears as simple white crystals (Photo 1)? One reason is that when AgNO3 comes into contact with the skin and is exposed to light, it forms metallic silver, leaving behind dark stains that are very difficult to remove. These marks only fade after several days as the skin’s outer layer naturally exfoliates. Let this serve as a warning: always wear proper personal protective equipment when working! A lab coat, gloves, and safety goggles are essential.
Now, let's learn how to perform the proposed experiments.
Version I
To conduct this version of the experiment, prepare an aqueous silver(I) nitrate solution with a concentration of a few percent. It’s crucial to use distilled or demineralized water, as tap water typically contains impurities that react with silver ions, causing noticeable cloudiness. I used a solution with a concentration of about 3%. Different concentrations can yield varied results, so I encourage you to experiment.
The second essential component is copper wire, preferably with a diameter between 0.2 and 0.5 mm (approximately 0.008 to 0.02 inches). It must be uninsulated, and any surface oxides or contaminants should be removed by sanding with fine-grit sandpaper and rinsing with acetone.
Then place a piece of copper wire, a few centimeters (about an inch) long, in a Petri dish positioned on a dark surface, and cover it with a few drops of silver nitrate solution. For best results, observe the reaction under a magnifying glass. Within moments, you’ll notice a shiny, moss-like growth on the surface of the wire (Photo 2A). But don’t stop watching yet! The growth continues, eventually forming beautiful, branched metallic silver structures (Photo 2B).
The growth of these structures is rapid, and the formation and elongation of new branches can be observed with the naked eye, as shown in the sequence of images in Photo 3.
A closer look at the resulting dendrites reveals their delicate, tree-like structure, as shown in the microscopy images (Photo 4).
This experiment can be repeated multiple times by removing the reduced silver with a piece of filter paper and replacing the silver salt solution.
Do not pour any leftover AgNO3 solution down the drain, as even trace amounts of silver salts can kill the microorganisms used in wastewater treatment plants. Fortunately, safe disposal is simple: the silver can be reduced by adding a few pieces of zinc sheet or granules to the solution. An astute reader might ask:
— Why use zinc instead of copper?
Of course, copper would work as well. However, reduction with zinc happens much faster. In this case, though, you won’t get those beautiful silver structures; instead, you will obtain only an unremarkable dark precipitate of finely dispersed silver that can be filtered out and used for other purposes.
Version II
In the second version of the experiment, the preparation of the silver salt solution requires not only silver(I) nitrate AgNO3 but also aqueous ammonia NH3(aq) at a concentration of approximately 25% [1].
Ammonia gas released from the solution is irritating to the respiratory system and toxic at higher concentrations! All experiments involving this substance must be carried out under a functioning fume hood or outdoors.
Preparing the solution is slightly more complex [2]. First, prepare 50 cm3 (1.69 fl oz) of a 3% AgNO3 solution. Then, while stirring continuously, add the aqueous ammonia drop by drop. Initially, a brown precipitate of silver(I) oxide Ag2O forms, according to the following reaction:
At a certain point, after adding one more drop of aqueous ammonia, the brown precipitate will dissolve, and the solution will turn clear. This happens because silver(I) oxide Ag2O reacts with excess ammonia to form a soluble complex known as diamminesilver(I), [Ag(NH3)2]+, according to:
Warning: This solution should never be prepared in advance or stored for future use. Over time, it may give rise to what is known as fulminating silver, a substance with a poorly understood composition, likely silver nitride Ag3N or silver imide Ag2NH, both of which are highly explosive. Any leftover solution should be safely disposed of as soon as possible, following the method described later.
For this version of the experiment, you will also need a DC power source that provides a few volts, appropriate connecting wires, and a piece of wire approximately 1 mm (0.04 in) in diameter. Steel is the preferred material. A straightened paperclip can be used effectively for this purpose.
Assembling the setup is fairly straightforward. Pour a shallow layer of the prepared diamminesilver(I) solution, just a few centimeters deep (around an inch), into a flat dish such as a Petri dish. Place the anode, made of steel wire, fully submerged near the edge of the dish. Position the cathode at the center, making sure it touches only the boundary between the liquid and the air. If the cathode is submerged too deeply, dendritic growth may not occur as expected. The complete setup is shown in Photo 5A.
As soon as the electrodes are connected to the power source, silvery, branched structures begin to form at the phase boundary around the cathode (Photo 5B). These dendrites are significantly larger than those produced using the previous method, making magnification unnecessary. Their growth is clearly visible to the naked eye and typically completes within a few dozen seconds. The intricate, tree-like formations can be quite striking, especially when viewed in close detail (Photo 6).
After the experiment, the leftover solution should be neutralized by carefully acidifying it with aqueous hydrochloric acid HClaq. Any remaining silver can then be reduced by adding zinc, just as described earlier. And once again, an crucial reminder: this solution must never be stored!
Explanation
As is often the case, the underlying principles behind the observed phenomena are quite simple. In both versions of the experiment, silver ions are reduced to metallic silver.
In the first case, silver is displaced from the solution through a reaction with metallic copper. As we know, more chemically active metals, those with lower standard electrode potentials (E0), can displace less active ones with higher potentials from their compounds. The standard potential for silver (measured relative to the hydrogen electrode) is 0.80 V, while for copper it is 0.34 V [3]. The reaction can therefore be written as:
Copper dissolves by forming divalent cations, while silver is reduced to its elemental form.
In the second case, the process is electrolytic: silver cations are reduced by electrons supplied by the cathode:
Although the silver cation is presented here in its simple form as Ag+, it's important to remember that the actual reaction involves the complex ion [Ag(NH3)2]+.
While this explains how metallic silver is formed during the experiment, it does not account for the mechanism behind the development of the characteristic dendritic structures.
Because metallic silver conducts electricity very well, electrons, whether released by copper atoms in the first version of the experiment or provided by the cathode in the second, can flow through the silver that has already been deposited. This allows further reduction of silver ions at those same sites. Small initial irregularities in deposition caused by impurities, uneven mixing, temperature gradients, diffusion, Brownian motion, or other factors tend to become amplified, leading to a structure that appears random. Understanding this process in detail requires deeper analysis.
Why do the terminal branches of the metallic aggregates grow significantly faster than others? This can be explained by the tendency of electric charge to accumulate in regions of a surface with the greatest curvature. As a result, the charge density is highest at the distal parts of the structure, causing silver ion reduction to occur more rapidly there and leading these branches to elongate faster. This is a classic example of positive feedback. This effect is especially evident in the second version of the experiment, where the longest and most elaborate branches consistently grow toward the anode due to electrostatic interactions. As we know, opposite charges attract.
However, this explanation is still incomplete. Based on these assumptions, one would expect the formation of long, straight needle-like structures. Yet during the experiment, we observed numerous branches. This can be explained by the phenomenon of diffusion-limited aggregation (DLA) [4]. For simplicity, consider a two-dimensional example, although the model can be easily extended to three-dimensional space.
Consider a stationary object called a seed placed on the surface being studied (Fig. 1A). Particles move randomly across the surface and can become immobilized either on the seed’s surface or on other particles that have already been immobilized. This process results in the formation of an initial structure with a random arrangement (Fig. 1B).
Logically, new particles are more likely to attach not near the center, but along the randomly forming branches because of their greater spatial reach. As a result, the structure grows into an increasingly branched, tree-like form (Fig. 1C, D). It’s remarkable that such simple conditions lead to structures with striking geometric beauty (Fig. 2). The resemblance to the experimentally observed silver dendrites (Photos 4 and 6) is truly striking.
This model can be easily applied to the experiments described: mobile silver ions in the solution are reduced on the metal surface, forming insoluble and thus immobilized silver atoms. It’s also worth noting that the diffusion-limited aggregation process is further enhanced by the uneven distribution of electric charge mentioned earlier.
These processes are not just a laboratory curiosity; they also occur in nature. One example is the dendrites of pyrolusite, a common mineral that is a crystalline form of manganese(IV) oxide (MnO2, Photo 7). Pyrolusite aggregates are often mistaken for plant fossils, but they are entirely abiotic in origin.
Similar structures can also be found in plants. The resemblance of a branch of northern white-cedar, Thuja occidentalis, to artificially grown dendrites (such as silver) or natural ones (like pyrolusite) is no coincidence. There is strong evidence that the molecular processes shaping plant organs are similar, though likely more complex.
Dendritic structures are not limited to plants; they also appear in animals. For example, nerve cells have highly branched projections known as dendrites.
The forms we’ve discussed also evoke fractals, which are objects defined by self-similarity [5]. This means that parts of the structure resemble the whole. Due to their vast diversity, these forms defy strict definitions. Interestingly, despite often exhibiting incredible structural complexity, they can usually be described by relatively simple recursive rules [6].
It’s fascinating how, starting from straightforward experiments, we arrive at deeper, more fundamental insights. This shows that you don’t need million-dollar equipment (though it can help) to begin uncovering the laws and principles that shape our world. To me, that’s the true beauty of science!
References:
- [1] Roesky H.W., Möckel K., Niezwykły świat chemii, Wydawnictwo Adamantan, Warszawa, 2001, pp. 7-9 back
- [2] Dobrowolski J., Podręcznik chemii analitycznej, Państwowe Zakłady Wydawnictw Lekarskich, Warszawa, 1964, p. 203 back
- [3] CRC Handbook of Chemistry and Physics 88th, CRC Press, 2008 back
- [4] Witten T. A., Sander L. M., Diffusion-Limited Aggregation, a Kinetic Critical Phenomenon, Physical Review Letters 47, 1981 back
- [5] Barnsley M.F., Fractals Everywhere, Academic Press Professional, Boston, 1993 back
- [6] Falconer K., Techniques in Fractal Geometry, John Willey and Sons, 1997 back
All photographs and illustrations were created by the author.
Addendum
Witnessing the growth of silver dendrites up close is a truly captivating experience. The video below clearly demonstrates how the process unfolds.
Marek Ples